Biochar-Based Sorbents for the Extraction of Emerging Organic Contaminants from Environmental Matrices: A Review

Abstract

The increasing occurrence of emerging organic contaminants (EOCs) in environmental matrices has intensified the demand for sensitive, selective, and sustainable analytical methods. Sample preparation plays a pivotal role, particularly for trace-level determination in complex matrices. Biochar is a promising sorbent for sample preparation because of its porous structure, tunable surface chemistry, low cost, and potential to be produced from renewable biomass waste. This review examines the use of biochar-based sorbents to extract EOCs from environmental matrices since 2020, with special emphasis on the relationships among precursor biomass, production conditions, surface chemistry, sorption mechanisms, and extraction performance. Agricultural waste was the predominant biomass class, while solid-phase extraction (SPE), magnetic solid-phase extraction (MSPE), and solid-phase microextraction (SPME) were the most frequently reported approaches. Biochar-based extraction has been applied to a wide range of EOCs, with pesticides being the most investigated analyte class. The authors discuss how pyrolysis conditions, activation, surface chemistry, and other parameters affect analyte retention and desorption. The authors compare different extraction approaches in terms of extraction efficiency, precision, and other analytical parameters. Finally, the authors discuss current limitations and research opportunities to guide the development of biochar-based sorbents as sustainable materials for sample preparation and environmental monitoring.

1. Introduction

The intensification of industrial activities has increased chemical consumption for industrial, agricultural, and domestic applications, contributing to the widespread presence of a wide range of chemical compounds in the environment. Among these, emerging organic contaminants (EOCs) have attracted growing scientific attention because of their continuous release, environmental persistence, and limited understanding of their long-term environmental and health impacts. Emerging contaminants comprise a broad spectrum of compounds, including pharmaceuticals, pesticides, plasticizers, additives, disinfection by-products, and personal care products. These compounds have distinct physicochemical properties, such as solubility, volatility, toxicity, and chemical stability, that directly influence their environmental fate, occurrence, and bioaccumulation in aquatic and terrestrial ecosystems [1,2].

The presence of these contaminants at trace and ultratrace concentrations may create the misconception that their environmental and health risks are negligible. However, detection and assessment should not rely solely on the concentration of individual compounds; they should also consider continuous release and potential combined effects within complex chemical mixtures. Consequently, conventional wastewater treatment processes are often insufficient to remove these micropollutants effectively, as they were primarily designed to eliminate biodegradable organic matter, suspended solids, and pathogenic microorganisms rather than trace organic contaminants [3,4]. A global survey conducted by Jiang et al. [5], based on more than 1000 studies published over the past two decades, identified pharmaceuticals such as carbamazepine, ibuprofen, triclosan, sulfamethoxazole, and caffeine among the contaminants most frequently detected in wastewater treatment plants, despite their occurrence at trace concentrations. According to the Food and Agriculture Organization (FAO) [6], global pesticide consumption exceeds 3.5 million tons annually, substantially increasing the likelihood of these compounds reaching aquatic environments through leaching and surface runoff. These findings highlight the need for increasingly sensitive and selective analytical methods to determine EOCs across a wide range of environmental matrices.

Determining EOCs remains a major analytical challenge because they occur at trace concentrations and in complex environmental matrices, including surface water, wastewater, soils, sediments, and biota samples [7,8]. Under these conditions, dissolved organic matter, humic substances, salts, and other matrix constituents can interfere with analyte recovery, reduce analytical sensitivity, and intensify matrix effects. Consequently, sample preparation is considered one of the most critical steps in analytical chemistry, as it minimizes matrix interferences while improving method selectivity, sensitivity, and the reliability of analytical results. Therefore, efficient sample preparation is essential for accurately determining trace contaminants in complex environmental matrices [9,10].

Among the factors influencing the performance of sample preparation techniques, sorbent selection is one of the most critical, as it directly affects extraction efficiency, analyte recovery, and method selectivity. In this context, biochar-based sorbents have attracted considerable attention owing to their renewable origin and favorable physicochemical properties, including a highly porous structure, large specific surface area, abundant surface functional groups, and tunable surface chemistry. Depending on the feedstock, production conditions, and potential activation or surface modification, biochars may exhibit high specific surface areas, providing numerous active sites for analyte–sorbent interactions and potentially enhancing extraction performance. Furthermore, their chemical stability and potential for regeneration and reuse make biochar an attractive sorbent for analytical sample preparation [10,11,12,13].

Several reviews have addressed the use of biochar in environmental applications. However, their objectives and analytical perspectives differ from those of the present review. Reviews focused on biochar for remediating environments contaminated with EOCs have primarily examined adsorption capacity, removal efficiency, adsorption mechanisms, and the factors governing contaminant–biochar interactions in aqueous and soil systems, with limited emphasis on analytical extraction performance and method validation [3,4,9,14]. Other reviews have addressed sample preparation strategies for EOCs in environmental matrices more broadly, including solid-phase extraction (SPE) and microextraction approaches, but without a specific focus on biochar as the sorbent material [8,10,13]. More recently, reviews of biomass-based sorbents have discussed their application in analytical extraction across environmental, food, and biological matrices, while broader reviews of food-waste-derived sorbents have also highlighted the increasing contribution of biochar-based materials to sample preparation [15,16]. In addition, a recent review specifically examined the greenness of extraction procedures involving biochar-based sorbents using Analytical Greenness Metric for Sample Preparation (AGREEprep) and related metrics [17]. Despite these important contributions, a comprehensive assessment linking biochar production strategies to its application as an extraction sorbent for EOCs in environmental matrices remains lacking. In particular, the existing literature has not been comprehensively organized by the analytical extraction techniques employed, nor has it systematically related biochar preparation and surface properties to extraction configuration, sorption mechanisms, and analytical performance.

Building on this gap, this review provides a comprehensive overview of recent advances in using biochar as a sorbent material for the analytical extraction of EOCs, with particular emphasis on natural waters, drinking water, wastewater, and related aqueous matrices, as well as soil samples. Particular attention is given to the biochar production routes, surface modification strategies, and the physicochemical properties that govern sorbent performance. The main sorption mechanisms involved in analyte extraction are discussed in relation to the properties of both the biochar and the target contaminants. The analytical performance, advantages, limitations, reusability, and sustainability of biochar-based sorbents are also critically discussed, together with the current challenges and future perspectives for their application in the extraction and determination of EOCs.

This review is organized as follows: Section 2 presents an overview of biochar production, its physicochemical properties, and functionalization strategies, including the key adsorption and extraction mechanisms governing analyte retention. Section 3 first presents a systematic classification of the EOCs covered in this review and discusses how their physicochemical properties influence the design and selection of biochar-based sorbents. It then presents a comprehensive discussion of biochar-based extraction techniques applied to these contaminants, including SPE, magnetic solid-phase extraction (MSPE), dispersive solid-phase extraction (DSPE), solid-phase microextraction (SPME), rotating disk sorptive extraction (RDSE), and pipette-tip solid-phase extraction (PT-SPE). Section 4 addresses the main challenges and prospects of biochar-based sample preparation, highlighting current limitations and promising directions for the field. Finally, this review summarizes its main conclusions.

2. Methodology

The literature search was conducted in the Web of Science Core Collection database to identify studies on the application of biochar-based materials for the extraction and analysis of EOCs. The search was restricted to original research articles published between 2020 and 2026. In the first step, the keyword ‘biochar’ was searched in the title field, yielding 25,434 records. To refine the search toward studies addressing organic contaminants, a second step combined the terms ‘biochar’ (title field) and ‘Organic’ (all fields), under the same restrictions of publication period and document type, resulting in 900 records. Finally, a third step added the term ‘Sample preparation’ (all fields) to the previous combination, maintaining the same restrictions, which narrowed the results to 55 records. No duplicate studies were identified. After full-text screening, 21 articles were excluded because they did not meet the scope of this review. Therefore, 34 studies were included in the present review (Figure 1).

Figure 1. Diagram of the literature search and study selection process retrieved from the Web of Science database.

We performed keyword co-occurrence analysis using VOSviewer (version 1.6.21) to identify the main research hotspots and thematic relationships within the selected literature. Author keywords were analyzed using full counting, and terms occurring below the selected minimum threshold were excluded. Synonymous and closely related terms were standardized using a thesaurus file prior to visualization. The resulting network was visualized using network maps, allowing the identification of the major thematic clusters and research hotspots in biochar-based extraction of organic contaminants.

Figure 2 presents the keyword co-occurrence network map generated from the author keywords of the studies included in this review. The network is clearly dominated by the term “biochar”, which acts as the central node with the highest number of connections, linking terms such as “adsorption”, “removal” and “water”. This central position reinforces biochar’s role as the core adsorbent material in the field and highlights the strong practical focus on remediating water resources. The term “adsorption” appears as a critical hub connecting most of the remaining nodes, confirming that adsorption is the predominant mechanism governing the extraction of organic contaminants by biochar-based materials. The map also reveals a temporal evolution of the research themes: terms such as “degradation”, “composite” and “activated carbon” form a cluster of more established research lines, whereas terms such as “nanoparticles”, “mechanism” and “adsorbents” emerge as more recent hotspots, indicating a growing interest in the modification of biochar (e.g., through combination with nanomaterials) and in the elucidation of the underlying sorption mechanisms. Overall, the analysis shows that biochar-based adsorption is a consolidated yet still evolving strategy for extracting organic contaminants, with biochar modification and mechanism-oriented studies representing the most active frontiers of current research.

Figure 2. Keyword co-occurrence network map of the studies included in this review, generated with VOSviewer based on the author keywords of publications retrieved from the Web of Science database.

3. Biochar: Production, Properties and Functionalization

3.1. Biomass Precursors and Biochar Production Routes

Biochar can be produced from a wide variety of biomass sources, including agricultural residues, agro-industrial by-products, lignocellulosic waste, medicinal plant and herb residues, mushroom-derived materials, aquatic biomass, and animal-derived by-products. The choice of precursor determines the physicochemical properties of the resulting carbonaceous material, as variations in lignin, cellulose, hemicellulose, protein, mineral, and inorganic ash contents can influence behavior during carbonization, as well as aromaticity, porosity, surface functionality, and elemental composition. Consequently, the precursor biomass should not be viewed merely as a renewable carbon source, but rather as a determinant of the chemical and structural characteristics that can influence analyte-sorbent interactions [18,19].

To evaluate the diversity of biomass precursors employed in analytical extraction, the studies compiled in this review were classified according to the primary biomass source used to prepare the biochar. The literature review revealed a clear predominance of agricultural residues (19 studies, 55.9%), as shown in Figure 3A. Coconut husks and fibers represented the second most frequent class, accounting for four studies (11.8%), followed by medicinal herbs (3 studies, 8.8%), mushroom waste (2 studies, 5.9%), and animal-derived by-products (1 study, 2.9%). The remaining five studies (14.7%) comprised a heterogeneous group of precursors, including sewage sludge, seaweed, avocado pits, and other less frequently investigated biomass sources. This distribution indicates that developing biochar-based sorbents for analytical extraction is strongly linked to valorizing abundant, low-cost biomass residues, particularly those generated by agricultural activities.

Figure 3. Overview of the studies included in the review: (A) distribution of biomass precursor classes used for biochar preparation; (B) distribution of the extraction techniques employed among the studies included in this review. Abbreviations: MSPE, magnetic solid-phase extraction; SPME, solid-phase microextraction; SPE, solid-phase extraction; PT-SPE, pipette-tip solid-phase extraction; RDSE, rotating disk sorptive extraction.

The predominance of agricultural residues is consistent with their broad availability, low economic value, and relatively high lignocellulosic content, which favors their conversion into carbon-rich materials through thermochemical processes. The studies reviewed included agricultural residues such as straw, corn cob, garlic straw, peanut shells, fruit peels, seeds, and other crop-derived materials. For example, garlic straw was used to prepare magnetic biochar for MSPE of multiple pesticide residues in environmental water samples, demonstrating how an abundant agricultural residue can be converted into a functional sorbent for trace analysis. The resulting magnetic material exhibited rapid adsorption and enabled efficient separation from aqueous samples, illustrating the additional analytical advantages of combining biomass valorization with magnetic functionality [20]. Similarly, corn straw, corncob, peanut shell, pomegranate peel, and other agricultural residues have been explored as precursors for applications in various extraction techniques.

Nevertheless, the predominance of agricultural residues in the reviewed literature should not be interpreted as evidence that these precursors inherently provide superior analytical performance. Most studies evaluate a single biomass-derived biochar under individually optimized extraction conditions, whereas direct comparisons among different feedstocks prepared and tested under equivalent conditions remain scarce. Consequently, the current literature does not allow the identification of a universally preferable biomass class for analytical extraction. Rather, the high representation of agricultural residues appears to reflect their availability and suitability for valorization, while extraction performance ultimately depends on the combined effects of precursor composition, production conditions, surface modification, target analyte, and extraction configuration.

Coconut shells and fibers constituted the second most represented biomass class in the reviewed studies (11.8%). This group is particularly relevant because coconut-derived residues are lignocellulosic materials with relatively high carbon content and structural rigidity, characteristics that favor the development of carbonaceous matrices with suitable porosity after thermal treatment. Coconut-shell-derived biochars have been investigated both in conventional and modified forms, including chemically activated materials and functionalized sorbents. The recurrence of coconut-derived materials reflects not only their availability but also their versatility as precursors for subsequent application as adsorbents [21,22,23]. These coconut-derived biochar sorbents have been successfully applied to extract pesticides and antibiotics from different matrices.

Medicinal and herbal residues represented 8.8% of the studies. Although less frequently investigated than agricultural residues, these materials are particularly interesting because their composition may provide precursor-specific functional groups and mineral constituents that can be preserved or transformed during carbonization. Porias cocos herbal residue, for example, has been converted into activated biochar and incorporated into extraction devices [24]. In another study, Gynostemma pentaphyllum has recently been transformed into a highly engineered biochar through K₂CO₃ activation, ball milling, and plasma treatment. The latter material was used to extract bisphenol compounds from marine water and showed superior extraction performance compared with polydimethylsiloxane, carboxen/polydimethylsiloxane, and polyacrylate commercial fibers. In addition, its low production cost and stable extraction performance over 100 adsorption/desorption cycles highlight its potential as a cost-effective and reusable alternative to commercial SPME fibers. The environmental profile of the proposed method was also evaluated using AGREEprep, indicating a favorable level of greenness [25].

Mushroom residues and animal-derived by-products accounted for 5.9% and 2.9% of the studies, respectively. The increasing use of mushroom residues is particularly relevant from a circular-economy perspective because spent mushroom substrates and cultivation residues represent an abundant secondary biomass stream that can be converted into functional carbonaceous sorbents [26,27]. Animal by-products, although less frequently represented, offer distinctive chemical compositions. Shrimp shells, for example, contain chitin, proteins, and inorganic components, which can contribute nitrogen-containing functionalities and mineral phases to the resulting carbonaceous material. A shrimp-shell-derived magnetic biochar prepared at 600 °C showed high adsorption capacity toward tetracyclines and retained adequate magnetic separation performance after repeated use, supporting its application in magnetic sample preparation [28].

Among the different production routes, pyrolysis remains the predominant approach for converting biomass into biochar. Conventional pyrolysis involves heating the precursor under oxygen-limited or inert conditions, commonly using nitrogen or argon, with the final properties being strongly dependent on temperature, heating rate, residence time, and atmosphere. In the studies compiled in this review, pyrolysis temperatures ranged from approximately 300 to 800 °C, although 400–600 °C was used most often. For example, Japanese knotweed biochar was produced at 500 °C under continuous N₂ flow, whereas shrimp-shell-derived magnetic biochar was carbonized at 600 °C for 2 h and garlic straw was subjected to high-temperature pyrolysis before magnetic functionalization [20,28,29,30]. These differences in thermal conditions directly influence the degree of carbonization, volatile matter content, aromaticity, pore development, surface functional groups, and mineral composition.

Pyrolysis temperature is therefore one of the most important parameters controlling the properties of biochar-based sorbents. In general, increasing temperature promotes dehydration, decarboxylation, devolatilization, and aromatization, resulting in higher fixed-carbon content and greater structural condensation, while progressively decreasing volatile matter and the abundance of some oxygen-containing functional groups. Depending on the precursor, higher temperatures may also enhance pore development and specific surface area; however, this behavior is not universal because mineral content, ash formation, pore blockage, and precursor composition can substantially influence the final structure [19]. An additional consequence of increasing pyrolysis temperature is the reduction in biochar yield [31]. For extracted cashew nut shell, for example, increasing the pyrolysis temperature from 450 to 650 °C decreased the biochar yield from approximately 48% to 40%, which was attributed to enhanced devolatilization and secondary decomposition reactions [32]. Thus, the optimum pyrolysis temperature should balance biochar yield with the physicochemical characteristics required for the intended analytical application, rather than being defined solely by surface area development or degree of carbonization.

From an analytical extraction perspective, this balance matters because stronger analyte–sorbent interactions do not necessarily translate into better extraction performance. Highly carbonized biochars may favor hydrophobic and π–π interactions, but excessive retention can compromise analyte desorption and increase solvent requirements. Therefore, pyrolysis temperature should ultimately be optimized considering the complete extraction process, including analyte retention, matrix discrimination, desorption efficiency, and, when applicable, sorbent regeneration. This perspective highlights that the temperature producing the most developed porous or carbonaceous structure is not necessarily the temperature providing the best analytical performance.

Although conventional pyrolysis remains the predominant approach, alternative thermochemical routes are also represented in the literature. Hydrothermal approaches can convert wet biomass or biomass-derived precursors at relatively mild temperatures, without the need for complete drying. In the reviewed studies, hydrothermal processing was employed for fungal biomass and other carbonaceous materials [33,34], while microwave-assisted thermocracking was used to prepare spent-mushroom-derived biochar [26]. Other less conventional approaches include flame-curtain pyrolysis of sewage sludge and laser-assisted carbonization of biomass-derived materials [35]. These alternative routes can offer advantages in processing time, energy transfer, precursor moisture tolerance, or control over carbon structure, although their application to analytical sample preparation remains far less explored than conventional pyrolysis.

Beyond carbonization, activation and surface modification are frequently used to tailor biochar for analytical extraction. Chemical activation using KOH, K₂CO₃, ZnCl₂, H₃PO₄, or related reagents can increase pore development and alter surface chemistry, whereas physical or mechanical treatments can reduce particle size, expose new active sites, and introduce structural defects. More recently, researchers have combined plasma treatment with chemical activation and ball milling to simultaneously increase oxygen-containing functional groups, defect density, and surface area. For example, Gynostemma pentaphyllum-derived biochar underwent K₂CO₃ activation, ball milling, and plasma treatment before being deposited onto an SPME fiber, illustrating a multistep strategy for tailoring the sorbent to polar EOCs [25]. Such approaches demonstrate that the final sorbent properties result from the combined effects of biomass composition and subsequent thermal, chemical, and physical treatments.

Magnetization represents another important modification route, particularly for MSPE applications. Magnetic functionality can be introduced through different strategies, including impregnation–pyrolysis, in situ formation of iron oxides during carbonization, post-pyrolysis coprecipitation, or solvothermal deposition. The resulting magnetic biochars combine the adsorption properties of the carbonaceous matrix with rapid magnetic separation, facilitating sorbent recovery from complex samples without conventional filtration or centrifugation [36]. This characteristic is particularly advantageous for analytical sample preparation because it can simplify handling, reduce processing time, and potentially facilitate sorbent reuse. Recent studies illustrate this strategy using agricultural residues and animal by-products, including garlic straw and shrimp shells, for MSPE applications [20,28].

Overall, the literature compiled in this review shows that biochar-based sorbents for analytical sample preparation are derived predominantly from low-cost, readily available residues, particularly agricultural biomass. However, the increasing diversity of precursors and production routes indicates a shift from simple conversion of biomass into carbonaceous materials to the development of materials with specific structural, chemical, and magnetic properties. This evolution is particularly relevant for EOCs, whose diverse physicochemical characteristics require sorbents with tunable polarity, porosity, surface functionality, and affinity. Consequently, the relationship among precursor composition, production conditions, surface modification, and extraction performance is central to evaluating the potential of biochar-based sorbents for analytical sample preparation.

3.2. Physicochemical Properties Relevant to Extraction

The physicochemical properties of biochar determine its performance as a sorbent in extraction procedures for EOCs. Sorption behavior is governed by the combined effects of textural properties and surface chemistry, which are strongly influenced by the biomass precursor and production conditions [19,37]. For analytical sample preparation, these properties determine not only the sorbent’s capacity and affinity for target analytes but also its selectivity, retention, and subsequent desorption.

Specific surface area (SSA) and pore structure are among the most important textural properties of biochar. Pyrolysis at relatively high temperatures generally promotes the development of surface area and pore volume through decomposition of biomass components, although the magnitude of these changes depends strongly on feedstock composition and pyrolysis conditions [38]. Biochar commonly exhibits a hierarchical pore structure comprising micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm). Micropores can contribute substantially to surface area and pore-filling interactions, whereas mesopores and larger pores can facilitate analyte diffusion and accessibility to internal sorption sites [38,39]. Consequently, an appropriate pore-size distribution is particularly important for analytical extraction because excessive microporosity may increase retention while limiting the accessibility and desorption of relatively large molecules.

Surface chemistry is another fundamental factor governing biochar–analyte interactions. Oxygen-containing functional groups, including carboxyl, hydroxyl, carbonyl, phenolic, and ether groups, are generally more abundant in biochars produced at relatively low temperatures, whereas increasing pyrolysis temperature promotes deoxygenation, dehydration, and aromatization, leading to more condensed and aromatic carbon structures [40,41,42]. These alterations modify surface polarity and hydrophilicity and can directly affect the type of interaction during analyte extraction. Therefore, the chemical characteristics of the target contaminant should be considered together with the surface chemistry of the biochar.

Surface charge and acid–base properties are particularly relevant for extracting ionizable emerging organic contaminants. The surface charge of biochar depends on its functional groups, mineral composition, and the pH of the surrounding solution, while the point of zero charge (pH_pzc) provides an indication of the pH at which the net surface charge approaches zero. Changes in sample pH can therefore modify both the ionization state of the analyte and the charge of the biochar surface, potentially enhancing or suppressing electrostatic attraction [43,44].

Aromaticity, hydrophobicity, and polarity also contribute to the selectivity of biochar-based extraction. Increasing pyrolysis temperature generally decreases the H/C and O/C atomic ratios, reflecting progressive carbonization, aromatic condensation, and loss of oxygen-containing functionalities [41]. Lower H/C ratios are commonly associated with greater aromaticity and structural stability, whereas O/C provides information related to oxygenation and surface polarity. However, these ratios should be interpreted alongside direct measurements of surface chemistry and mineral composition, particularly because O/C determination may be affected by inorganic oxygen associated with the ash fraction [45].

Ash content and mineral composition may also substantially influence extraction performance. The inorganic fraction originates largely from the biomass precursor and becomes concentrated during thermal conversion. Depending on its composition and distribution, ash may block pores and decrease the accessible surface area or, conversely, contribute mineral phases and ion-exchange or surface-complexation sites [46]. Therefore, the effect of ash should not be interpreted solely as detrimental, particularly for biochars derived from mineral-rich feedstocks or animal by-products.

From a sample-preparation perspective, the evidence reviewed suggests that no single physicochemical property can reliably predict biochar performance in extraction. Although specific surface area is frequently highlighted as an indicator of sorbent quality, this parameter alone cannot explain differences in recovery, selectivity, or desorption efficiency. A biochar with a lower surface area may outperform a highly porous material if its surface chemistry better matches the ionization state, polarity, and aromaticity of the target analytes. Therefore, biochar extraction performance should be understood as the result of the interplay among surface area, pore-size distribution, surface functionality, charge, aromaticity, polarity, and mineral composition. Together, these properties determine sorbent–analyte affinity and ultimately influence extraction recovery, selectivity, matrix effects, and reusability. Accordingly, characterization of biochar-based sorbents should ideally combine textural, chemical, and surface-charge measurements to establish meaningful relationships between material properties and analytical performance.

The elemental composition of biochar, typically expressed by the carbon, hydrogen, nitrogen, and sulfur (CHNS) contents, is a key descriptor of its surface chemistry and directly governs its sorption performance [47,48]. The H/C ratio reflects the degree of aromaticity and carbonization: lower H/C values indicate highly condensed aromatic domains that favor π–π electron donor–acceptor interactions with aromatic contaminants [41,49]. Conversely, the O/C ratio reflects biochar surface polarity, with higher values indicating abundant oxygen-containing functional groups that enhance hydrogen bonding and electrostatic interactions, especially toward polar and ionizable compounds [50]. Although usually present in lower amounts, nitrogen contributes basic functional groups that modulate surface charge and the sorbent’s affinity for acidic or negatively charged analytes [47,51]. Sulfur, when present, may also participate in specific interactions with certain contaminants, although it is generally a minor constituent of biochars. Therefore, tailoring the biochar’s elemental stoichiometry (e.g., through feedstock selection and pyrolysis temperature) is a straightforward strategy to tune its performance toward specific classes of EOCs [50,52].

3.3. Surface Engineering and Modification Strategies

Surface engineering and modification strategies have become important approaches for tailoring biochar-based sorbents to specific analytical extraction requirements. Although pristine biochar possesses a porous carbonaceous structure and diverse surface functionalities, its extraction performance can be further adjusted by modifying its textural properties, surface chemistry, charge, polarity, or magnetic behavior. Such modifications may increase the accessibility of sorption sites, introduce or enrich specific functional groups, enhance affinity toward selected analytes, improve extraction kinetics, or facilitate sorbent recovery and reuse. Common strategies include chemical activation, physical and mechanical treatment, acid/base washing, incorporation of magnetic phases, plasma treatment, and combinations of these approaches [20,25,28,30,32,53].

Magnetization is one of the most relevant modification strategies for analytical sample preparation because it combines sorption with facile phase separation. In shrimp-shell-derived biochar, iron oxide phases were introduced using FeCl₃·6H₂O and FeSO₄·7H₂O followed by coprecipitation, producing a magnetic sorbent with a specific surface area of 71.30 m² g⁻¹ and high adsorption capacities toward tetracycline compounds [28]. The observed sorption behavior involved multiple interactions, including surface complexation, hydrogen bonding, hydrophobic interactions, electrostatic attraction, and π–π interactions. Beyond enabling magnetic recovery, incorporation of iron-containing phases can modify the surface chemistry and textural properties of biochar, potentially introducing additional sorption sites and changing the relative contribution of different analyte–sorbent interactions [28].

A comparable approach was reported for garlic straw biochar, in which Fe³⁺ and Fe²⁺ salts were incorporated through an in situ magnetization strategy. The resulting material exhibited a porous morphology with strong magnetic responsiveness and was applied to the MSPE of multiple pesticide residues from environmental water samples. Under optimized conditions, adsorption equilibrium was reached within 30 s, illustrating the potential of magnetic dispersion to accelerate mass transfer between the aqueous phase and the sorbent [20]. For analytical sample preparation, the main advantage of magnetization is therefore not necessarily an intrinsic increase in sorption capacity, but the ability to disperse the sorbent directly in the sample and recover it rapidly with an external magnetic field, simplifying phase separation and reducing filtration or centrifugation steps.

Chemical activation represents another major strategy for tailoring the textural properties of biochar. Activation with alkaline agents such as KOH or K₂CO₃ can promote the development of micro- and mesoporosity and increase the accessible surface area [20,25]. Garlic straw pretreated with KOH before pyrolysis produced a highly porous magnetic biochar for pesticide extraction [20], whereas Gynostemma pentaphyllum biochar activated with K₂CO₃ at 900 °C exhibited enhanced surface area and active-site availability for the extraction of bisphenol compounds from marine water [25]. Chemical activation can therefore increase the accessibility of sorption sites. However, surface area should not be regarded as an independent predictor of analytical performance because activation simultaneously modifies surface polarity, functional-group density, pore accessibility, and the strength of analyte retention.

Recent studies have also explored sequential modification strategies to tailor textural and chemical properties simultaneously. In Gynostemma pentaphyllum-derived biochar, activation combined ball milling with air-plasma treatment. Ball milling increased surface-site exposure and the abundance of oxygen-containing functionalities, whereas plasma treatment introduced additional structural defects and reactive surface groups. Together, these treatments produced a sorbent with enhanced extraction performance compared with conventional commercial SPME fibers [25]. This sequential approach illustrates how complementary modification mechanisms can be combined to optimize surface area, pore accessibility, surface functionality, and analyte–sorbent interactions rather than relying on a single modification step.

Post-pyrolysis acid or alkaline treatments provide a simpler route for controlling surface chemistry and mineral composition. In shrimp-shell-derived biochar, treatment with 5% HCl removed mineral phases before the subsequent analytical application. Such treatments can decrease ash content, dissolve carbonate and other mineral phases, expose previously inaccessible surface functionalities, and modify surface acidity and charge [28]. These changes may substantially affect analyte retention, particularly for ionizable compounds for which electrostatic and acid–base interactions contribute to extraction.

An important distinction in analytical sample preparation is that surface engineering should be designed not only to maximize sorption but also to balance retention and desorption. A sorbent with very high affinity or extensive microporosity may provide excellent extraction efficiency but may require stronger or longer desorption conditions. Conversely, excessive modification may introduce functional groups that increase matrix interactions or reduce sorbent stability. Therefore, the optimal modification strategy should be selected according to the target analytes, sample matrix, extraction configuration, and subsequent desorption/detection conditions [25,53]. Surface engineering may also have implications beyond extraction efficiency. Modification can alter the release of dissolved organic matter and other soluble components from biochar, potentially affecting blank levels, matrix effects, and the introduction of sorbent-derived compounds into the analytical system [30].

Overall, surface engineering provides a versatile means of transforming biochar into a material with tailored textural, chemical, and magnetic properties. The most promising strategies for analytical sample preparation therefore simultaneously improve analyte accessibility and selectivity while maintaining rapid kinetics, efficient desorption, low matrix contribution, and convenient sorbent recovery.

3.4. Adsorption and Extraction Mechanisms

The extraction of EOCs by biochar-based sorbents results from several physicochemical interactions. Depending on the biomass precursor, pyrolysis conditions, surface chemistry, pore structure, and target-analyte properties, biochar can retain organic compounds through hydrophobic interactions, van der Waals forces, pore filling, π–π interactions, hydrogen bonding, electrostatic interactions, ion exchange, and surface complexation. The relative contribution of these mechanisms strongly depends on the molecular structure, polarity, aromaticity, and ionization state of the analyte, as well as on the physicochemical characteristics of the biochar [40,54,55,56]. Figure 4 presents a schematic overview of the main physicochemical mechanisms involved in the interaction between EOCs and biochar-based sorbents.

Figure 4. Schematic representation of the main physicochemical mechanisms involved in the extraction of EOCs by biochar-based sorbents: (1) hydrophobic interactions; (2) pore filling; (3) π–π interactions; (4) hydrogen bonding; (5) electrostatic interactions; and (6) surface complexation.

Pore structure plays an important role in retaining organic contaminants. Micropore filling can contribute substantially to the adsorption of molecules whose dimensions fit the microporous network, whereas mesopores can facilitate mass transfer and provide access to internal adsorption sites [40,56]. Consequently, a high specific surface area does not necessarily guarantee superior extraction performance, because pore accessibility, pore size distribution, and the relationship between pore dimensions and analyte molecular size are also critical. In addition, highly microporous structures may increase retention while simultaneously restricting mass transfer and subsequent analyte desorption.

Hydrophobic and aromatic interactions are particularly important for many EOCs. Hydrophobic domains within the carbonaceous structure can promote the partitioning of nonpolar or weakly polar compounds from aqueous phases into the biochar, while aromatic contaminants can interact with the condensed aromatic structure through π–π interactions and related electron donor–acceptor interactions [54,56].

Hydrogen bonding and other polar interactions are also relevant to emerging contaminant extraction. Oxygen-containing functional groups, including hydroxyl, carboxyl, carbonyl, and phenolic groups, can interact with polar analytes through hydrogen bonding and other specific surface interactions [40,56]. The abundance and accessibility of these functional groups depend on the biomass precursor, pyrolysis temperature, and post-treatment procedures. Electrostatic interactions become particularly important for ionizable emerging organic contaminants. The surface charge of biochar depends on its functional groups, mineral composition, and the pH of solution, whereas the charge state of the analyte is governed by its acid–base properties [55]. Changes in sample pH can therefore simultaneously modify the analyte’s ionization state and the sorbent’s surface charge, potentially enhancing or suppressing extraction through electrostatic attraction or repulsion. Ion exchange and surface complexation may provide additional retention pathways, particularly for biochars containing substantial mineral fractions or abundant ionizable surface groups. Mineral phases derived from the biomass precursor can contribute exchangeable ions and reactive sites, while oxygen-containing functional groups may participate in specific interactions with analytes [40,54]. These mechanisms are particularly relevant for biochars produced from animal-derived or mineral-rich feedstocks, in which inorganic constituents may contribute significantly to overall surface reactivity.

Operational parameters also influence the analyte extraction. Contact time determines the progression toward sorption equilibrium and is affected by particle size, pore structure, analyte diffusion, and the accessibility of sorption sites [40]. Kinetic models, such as pseudo-first-order and pseudo-second-order equations, are frequently employed to describe experimental adsorption rates. Similarly, Langmuir, Freundlich, and related isotherm models can describe equilibrium sorption behavior [40,54].

Overall, biochar extraction performance is governed by the balance among pore accessibility, surface chemistry, analyte properties, solution conditions, matrix composition, and desorption behavior. Understanding these interactions is essential for designing biochar-based sorbents with appropriate selectivity, extraction efficiency, and analytical recovery for different classes of EOCs.

4. Biochar-Based Extraction Techniques for Emerging Organic Contaminants

4.1. Classification of Emerging Organic Contaminants

Emerging organic contaminants (EOCs) comprise a broad and heterogeneous category of compounds, including pharmaceuticals and personal care products (PPCPs), pesticides, plasticizers, veterinary drugs, per- and polyfluoroalkyl substances (PFAS), food additives, polycyclic aromatic hydrocarbons (PAHs), among others [57]. These compounds differ markedly in their molecular size, hydrophobicity, charge, and functional groups, and such differences impose distinct requirements on the biochar sorbents employed for their extraction [52]. For instance, hydrophobic and aromatic contaminants are preferentially retained through hydrophobic partitioning and π–π electron-donor–acceptor interactions with the aromatic domains of biochar, whereas polar and ionizable compounds rely more heavily on hydrogen bonding and electrostatic interactions with surface functional groups [51,52]. Likewise, the molecular dimensions of the analyte dictate the accessibility of the pore network, so that the porosity and pore-size distribution of the biochar must be matched to the size of the target compounds. Consequently, the selection and functionalization of biochar for sample preparation should be guided by the physicochemical properties of the target EOC class, rather than by a one-size-fits-all approach [51,58]. The following subsections discuss the biochar-based extraction techniques applied to these different classes of EOCs.

4.2. Biochar-Based Extraction Techniques

Biochar-based sorbents have been incorporated into a wide range of sample preparation strategies to extract and preconcentrate EOCs from environmental matrices. The studies identified in this review encompass conventional and miniaturized SPE approaches, as well as magnetic, dispersive, microextraction, and other configurations that exploit biochar’s physicochemical properties. Table 1 summarizes the studies considered in this review, including the biochar precursor, production and modification strategies, extraction technique, target analytes, environmental matrices, and relevant analytical characteristics. Thus, it provides an overview of how biochar has been incorporated into different extraction configurations and highlights the diversity of materials and analytical applications reported in the literature.

Table 1. Overview of studies reporting the use of biochar-based sorbents for the extraction of EOCs from environmental matrices.

Biomass	Biochar Production	Analytes	Matrix	Extraction Technique	Solvent	Determination Technique	Recovery (%)	Precision (%)	LOQ (μg/L)	Authors
Corn straw-derived	Fe2+/Fe3+ impregnation at pH 10–11, followed by pyrolysis under N2 at 700 °C for 40 min (20 °C min−1)	ethoprophos, dimethoate, isazofos, phosphamidon, pirimiphos-methyl, fenitrothion, chlorpyrifos, parathion, medathion, profenofos, ethion, and triazophos	Water and soil	MSPμE	Acetone	GC-MS	72.4–96.8	1.0–8.6	0.08–0.36	[59]
Pomelo peel	Synthes under different pyrolytic temperatures	Morphine, codeine, and 6-MAM	Wastewaters	MSPE	Methanol	LC-MS	71.6–84.4	2.4–8.6	0.021–0.033	[60]
Pine Needle	Pyrolysis at 500 °C for 2 h under an N2 atmosphere	Sulfadiazine	Natural waters	MSPE	Methanol	LC-MS/MS	96.3–99.8	7.7	35.5	[61]
Date stone	Pyrolysis at 500 °C for 2 h under an argon atmosphere and subsequently magnetized via a solvothermal process using FeCl3·6H2O in ethylene glycol.	nonsteroid anti-inflammatory drugs	Aqueous solutions	MSPE	Acetonitrile	LC-MS/MS	>92.60	<3.2	0.20–0.37	[62]
Pumpkin peel	Fe2+/Fe3+ modification by co-precipitation with NH3 at 80 °C for 30 min	Naproxen	Water samples	MSPE	Methanol and water	HPLC-UV	100.0	0.6	37	[63]
Coconut clothing	KOH activation (2 mol L−1; 1:2, w/v) followed by pyrolysis under N2 at 600 °C for 4 h	acetamiprid, thiacloprid, imidacloprid, thiamethoxam	Water samples	MSPE	Acetonitrile and formic acid	LC-MS/MS	68.6–104.0	0.4–9.3	0.005–0.020	[21]
Fungal hyphae	Hydrothermal synthesis with FeCl3·6H2O and urea at 200 °C for 10 h	pentachloronitrobenzene, α-endosulfan, β-BHC, γ-BHC, δ-BHC, heptachlor, mirex, α-BHC, p,p’-DDD, and p,p’-DDE	Water samples	MSPE	Acetonitrile	GC-MS/MS	80.5–117.2	<8.9	0.0009–0.0462	[33]
Garlic straw	KOH activation (1:2, w/v) followed by pyrolysis under N2 at 800 °C for 4.5 h (3 °C min−1)	imidacloprid, acetamiprid, thiacloprid and orysastrobin	Water samples	MSPE	Acetone	HPLC-UV	70.0–99.5	0.1–6.1	1.7–2.6	[20]
Shrimp shell (MBC-600)	Pyrolysis under N2 at 600 °C for 2 h (10 °C min−1)	tetracycline, doxycycline, oxytetracycline, chloromycin, methacycline, demeclocycline, meclocycline, minocycline, anhydrotetracycline, 4-epi chlortetracycline, 4-epi tetracycline, 4-epi oxytetracycline	Aquatic products	MSPE	Acetonitrile and oxalic acid	UHPLC-MS/MS	86.6–98.9	2.41–11.6	0.50–1.63 *	[28]
Spent mushroom	Microwave-assisted pyrolysis under N2 at 700 °C for 2 h (5 °C min−1)	Tetramethrin, Flucythrinate, Betacypermethrin, Fenvalerate, Permethrin, Bifenthrin	Water samples	MSPE	Acetonitrile	HPLC-DAD	82.02–91.80	0.39–1.57	0.5	[27]
Peanut shells	Pyrolysis under N2 at 400 °C for 3 h, followed by HCl treatment	Bisphenol A, diethylstilbestrol, Hexafluorobisphenol, 4,4-sulfonyldiphenol, 4,4-methylenebisphenol, and 4-cumylphenol	Water samples	PT-SPE	Water	HPLC-UV	83.5–117.1	0.3–13.2	0.5–5	[64]
Corn cob-derived	Pyrolysis under N2 without further chemical modification	methylparaben, bisphenol A, 17α-ethinylestradiol, and triclosan	Water samples	PT-SPE	Acetonitrile and ethanol	HPLC-DAD	81.2–116.5	1.1–11.3	1.44–3.28	[65]
Coconut shell (CSB)	Pre-carbonization at 200 °C for 2 h, followed by anaerobic pyrolysis at 700 °C for 4 h (10 °C min−1)	dicamba, 4- chlorophenoxyacetic acid, 2,4-dichlorophenoxyacetic acid, and 4-(2,4-dichlorophenoxy) butyric acid	Water and soils samples	QuEChERS	Acetonitrile	UPLC-MS/MS	74.9–109.7	1.0–8.9	Not reported	[22]
peanut shell	Pyrolysis under N2 at 600 °C (BC) or ZnCl2 impregnation (75%, w/w) followed by pyrolysis at 400 °C (BAC), for 30 min (10 °C min−1)	ethyl paraben, diclofenac, triclosan, bisphenol A, and 17-α-ethinylestradiol	Influent and effluent of wastewater treatment plant and river water	RDSE	ethyl acetate	GC-MS	16–87	<10	0.03–2.09	[66]
avocado seed	ZnCl2 impregnation (20, 30, and 40%, w/v) followed by pyrolysis under N2 at 400, 600, or 800 °C for 30 min (10 °C min−1)	Ibuprofen and 1-hydroxy-ibuprofen	Influent and effluent of wastewater treatment plant and river water	RDSE	methanol	GC-MS	81–91	<9	0.25–0.77	[67]
sewage-sludge	Flame-curtain pyrolysis at ~600 °C for 60 min, followed by KOH, MeOH, KOH/MeOH, or Fe3O4 modification	bisphenol-A	aqueous solution	SPE	acetonitrile	HPLC-UV	85%	<4	not reported	[35]
corncob	Pyrolysis under N2 at 400 °C, followed by KOH and HCl activation	2,4-D, alachlor, ametryn, atrazine, bifenthrin, carbosulfan, diuron, flutriafol, imidacloprid, metolachlor, simazine, thiacloprid, and thiamethoxam.	water samples	SPE	dichloromethane/methanol (50:50, v/v)	LC-MS	70.4–114.6	<13.8	0.1–0.5	[68]
Wolfberry biomass waste	Hydrothermal carbonization at 180 °C for 8 h	14 fluoroquinolones, 12 sulfonamides	aquatic products	SPE	methanol 1% formic acid	HPLC-HRMS	75–119	<10	1–4.5 *	[69]
Pomegranate	Carbonization at 300 °C, followed by surface modification with pomegranate peel acetone extract	Tetracycline and Pb	aqueous samples	SPE	acidified methanol (1:1 v/v HCl:methanol	UV-Vis spectrophotometer	99.3		5230	[70]
coconut shell	H3PO4 activation followed by pyrolysis at 500 °C and functionalization with betaine ionic liquid	nrofloxacin, ciprofloxacin, sarafloxacin, danofloxacin, norfloxacin, ofloxacin, orbfloxacin, nalidixic, and flumequine	fish-farming water and milk samples	SPE	acetonitrile:methanol: ammonia (3:2:2, v/v/v)	LC-MS/MS	82.1–106.7%	<7	0.45–0.95	[23]
spent coffee grounds	H3PO4 activation (3:1, w/w) followed by pyrolysis under vacuum at 500 °C for 4 h	tetracycline, oxytetracycline, chlortetracycline, and doxycycline	synthetic wastewater	SPE	methanol/TFA (99:1, v/v)	LC-MS/MS	49.7–66.4%	7.6–16.4	not reported	[71]
Corncob	Carbonization at 200–500 °C for 1–4 h	naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, and pyrene	Purified, natural, rain and river waters	SPME	Acetonitrile and water	HPLC-DAD	81.7–116.8	<6.7	0.010–0.099	[72]
Marine algae	Carbonization under N2 at 500 °C for 4 h (5 °C min−1), followed by K2CO3 activation (1:4, w/w) at 900 °C for 1 h	1,3,5-trichlorobenzene, 1,2,4-trichlorobenzene, 1,2,3-trichlorobenzene, 1,2,3,4-tetrachlorobenzene, 1,2,4,5-tetrachlorobenzene, pentachlorobenzene, and hexachlorobenzene	Lake water	SPME	Not used	GC-ECD	89.55–105.19	3.5–6.3	0.000023–0.000261	[73]
biochar nanospheres	Hydrothermal carbonization of glucose with PSSMA at 180 °C for 8 h onto TiO2 nanorod arrays/carbon fibers	eight polycyclic aromatic hydrocarbons, four estrogens, four bisphenols and seven phthalate esters	Tap, river and rainwater samples	SPME	Acetonitrile, methanol and water	HPLC-DAD	75.0–140.2	0.2–15.2	not reported	[34]
lotus pedicel, watermelon rind and litchi rind	Pyrolysis under N2 at 500 or 800 °C for 2 h (5 °C min−1), followed by ball milling at 500 rpm for 12 h	Benzene, Toluene, Ethylbenzene, m-Xylene, Chlorobenzene, 2-Chlorotoluene, 4-Chlorotoluene, 1,4-Dichlorobenzene, 1,2-Dichlorobenzene	Tap water	SPME	Not used	GC-MS	91–109	<8.9	not reported	[74]
Gynostemma pentaphyllum	Carbonization under Ar at 450 °C for 2 h (5 °C min−1), followed by K2CO3 activation (1:4, w/w) at 900 °C for 2 h; ball milling (400 rpm, 12 h) and air-plasma treatment (48.8 W, 15 min)	bisphenol F, bisphenol B, bisphenol S, bisphenol C, bisphenol Z, bisphenol AP, bisphenol AF, bisphenol M, and bisphenol FL	Marine water samples	SPME	0.1% NH4OH in methanol	UPLC-MS/MS	80.4–119.9	<9.9	0.00039–0.0029	[25]
Cucumis metuliferus	ZnCl2 activation (1:2, w/w) followed by pyrolysis under Ar at 800 °C for 4 h (5 °C min−1)	naphthalene, acenaphthylene, acenaphthene, fluorene, and anthracene, 2,2′,5-trichlorobiphenyl, tetrachlorobiphenyl, hexachlorobenzene and 1,1-dichloro-2,2-bis(4-chlorophenyl)ethylene	Marine water samples	SPME	Not used	GC-MS	80.2–119.9	<9.9	0.001–0.0033	[75]
Porias cocos herbal residue	ZnCl2 activation followed by pyrolysis under N2 at 550 °C for 4 h	Metronidazole, Secnidazole, Dimetridazole, and Ornidazole	drinking, tap, and river water samples	in-syringe SPE	acetonitrile	UHPLC-MS/MS	79.02–99.1	<8.2	0.021–0.048	[24]
orange peel	pyrolysis of orange peel at 650 °C for 6 h under a nitrogen atmosphere	hydroxyprogesterone, progesterone, medroxyprogesterone acetate, 17- β-estradiol, 17-α-ethynylestradiol, estrone, and testosterone	tap, river, and lake water, wastewater treatment plant effluent	in-vial SPE	ethanol	HPLC-MS/MS	60–123	<18	not reported	[76]
Poria cocos residue	ZnCl2 activation followed by pyrolysis under N2 at 550 °C for 4 h and incorporation into a CaCl2-crosslinked sodium alginate membrane	bisphenol AF, bisphenol A, bisphenol F, bisphenol C, and bisphenol B	tap, drinking and river waters	stir-disc SPE	acetone	HPLC-FLD	71–108	0.9–13.3	0.21–0.57	[77]
rich cork sheets	CO2-laser irradiation at 9 W and 250 mm s−1 under ambient conditions	diazinon, parathion-methyl, chlorpyrifos-methyl, pirimiphos-methyl, fenitrothion, malathion, chlorpyrifos, profenofos, ethion	water samples	u-SPE	acetonitrile	GC-MS	64.8–126.6	1.44–8.99	0.08–0.42	[78]
Straw	Pyrolysis at 350 °C for 35 min (10 °C min−1)	Chlorpyrifos	Aqueous samples	dSPE	Not used	HPLC-DAD	81.2–103.6	≤9.2	4	[79]
Waste mushroom sticks	Fe(NO3)3·9H2O impregnation followed by one-step pyrolysis to obtain magnetic biochar	cyanazine, atrazine, propazine, and terbuthylazine	Water	MD-μSPE-MDLLME	Deep eutectic solvent	HPLC	91.8–97	5.4	1	[27]
Coconut shell	Pre-carbonization at 200 °C for 2 h, followed by anaerobic pyrolysis at 700 °C for 4 h (10 °C min−1)	prometryn, propazine and atrazine	Lettuce, cucumber, seawater and soil	SALLE-MSC-SPME	acetonitrile/acetic acid 70:30 (v:v)	UPLC-MS/MS	69–124.72	<0.43	0.00699–0.011	[80]

* value in μg/kg; SPE: Solid-phase extraction; MSPE: Magnetic Solid-Phase Extraction; dSPE: Dispersive solid-phase extraction; SPME: Solid-Phase Microextraction; RDSE: Rotating Disk Sorptive Extraction; PT-SPE: Pipette-tip solid-phase extraction; MSPμE: magnetic solid-phase microextraction; QuEChERS: Quick, Easy, Cheap, Effective, Rugged, and Safe; MD-μSPE: magnetic dispersive micro-solid phase extraction; MDLLME: magnetic dispersive liquid–liquid microextraction; SALLE: salting-out assisted liquid–liquid extraction; MSC-SPME: monolithic spin columns for solid-phase microextraction; GC-MS: Gas Chromatography-Mass Spectrometry; HPLC: High-Performance Liquid Chromatography; UPLC-MS/MS: Ultra-Performance Liquid Chromatography-Tandem Mass Spectrometry; LC-MS: Liquid Chromatography-Mass Spectrometry; HPLC-UV: High-Performance Liquid Chromatography with Ultraviolet Detector; LC-MS/MS: Liquid Chromatography-Tandem Mass Spectrometry; GC-MS/MS: Gas Chromatography-Tandem Mass Spectrometry; UHPLC-MS/MS: Ultra-High Performance Liquid Chromatography-Tandem Mass Spectrometry; HPLC-DAD: High-Performance Liquid Chromatography with Diode Array Detector; UPLC-MS/MS: Ultra-Performance Liquid Chromatography-Tandem Mass Spectrometry; UV-VIS: Ultraviolet-Visible; HPLC-HRMS: High-Performance Liquid Chromatography-High-Resolution Mass Spectrometry; HPLC-FLD: High-Performance Liquid Chromatography with Fluorescence Detector; GC-ECD: Gas Chromatography with Electron Capture Detector.

Figure 3B presents the distribution of extraction techniques among the studies included in this review. MSPE represented the most frequently reported approach, followed by SPME and conventional SPE. PT-SPE and RDSE were comparatively less represented. Additional studies used alternative extraction techniques, which were grouped into the “Other techniques” category.

The prevalence of pesticides (mainly insecticides) in the reviewed literature indicates that biochar-based extraction has been particularly explored for contaminants with diverse aromatic and hydrophobic characteristics, which can interact favorably with biochar’s carbonaceous structure through various surface interactions. The strong representation of antibiotics and other therapeutic-class compounds further demonstrates biochar’s applicability to chemically diverse contaminants. However, the comparatively small number of studies addressing pharmaceuticals and other emerging contaminants suggests significant research opportunities.

The prevalence of MSPE highlights the growing interest in magnetically responsive biochar as an analytical sorbent. Combining biochar’s adsorption properties with magnetic responsiveness enables rapid separation of the sorbent from the sample matrix using an external magnetic field; this can simplify the extraction procedure and reduce the need for conventional separation steps, such as filtration or centrifugation. The relatively high number of studies employing SPME and conventional SPE also indicates that biochar has been successfully used in both miniaturized and conventional extraction formats. Conversely, the smaller number of studies involving PT-SPE and RDSE suggests that these configurations remain comparatively underexplored for biochar-based extraction, representing potential opportunities for further methodological development.

The distribution of target analytes provides another perspective on the development of biochar-based extraction methods. Pesticides were the most frequently investigated class, followed by antibiotics and other therapeutic-class compounds. Fewer studies addressed other EOC classes.

The predominance of pesticides in the reviewed literature indicates that biochar-based extraction has been particularly explored for contaminants with diverse aromatic and hydrophobic characteristics, which can interact favorably with biochar’s carbonaceous structure through hydrophobic, π–π, and other surface interactions. The substantial representation of antibiotics and other therapeutic-class compounds further demonstrates biochar’s applicability to chemically diverse contaminants. Nevertheless, the comparatively smaller number of studies addressing pharmaceuticals and other emerging contaminants suggests that important research opportunities remain.

Overall, the studies summarized in Table 1 demonstrate the versatility of biochar as a sorbent platform for analytical sample preparation.

Among the reviewed studies, relatively few addressed the simultaneous extraction of multiple contaminant classes, highlighting the need for further research on multiclass extraction approaches.

In terms of geographical distribution (Figure 5), a marked concentration of research on using biochar as a sorbent in sample preparation was observed in Asia. Of the 34 studies included in this review, 26 (76.5%) originated from Asian countries, demonstrating this region’s prominent contribution to the development and application of biochar-based extraction approaches. China was by far the most represented country, accounting for 21 studies (61.8% of the total), followed by Turkey and Vietnam, with two studies each, and Thailand, with one. South America accounted for four studies (11.8%), equally distributed between Brazil and Chile. Europe and Africa each contributed two studies (5.9%), with studies from Greece and Italy in Europe and Tunisia and Egypt in Africa. Overall, these findings reveal a pronounced geographical imbalance in the literature, with research strongly concentrated in Asia, particularly China, while applications of biochar-based sorbents in sample preparation remain comparatively less explored in other regions.

Figure 5. Representative map of the geographical distribution of studies employing biochar in sample preparation techniques for the extraction of EOCs from environmental matrices published between 2020 and 2026.

The following sections discuss the main extraction techniques individually, highlighting the different types of biochar-based sorbents and their applications in the analytical extraction of EOCs.

4.3. Solid-Phase Extraction Approaches

Solid-phase extraction (SPE) is a widely used sample-preparation technique for isolating, cleaning up, and preconcentrating analytes before instrumental analysis. It operates based on the distribution of target compounds between a liquid sample and a solid sorbent, according to their relative affinities for both phases [81]. In SPE, the sample is brought into contact with a solid sorbent that selectively retains the target analytes, while matrix constituents are removed during loading and washing. The retained analytes are subsequently desorbed using an appropriate elution solvent and subjected to instrumental determination. Retention efficiency depends on the physicochemical properties of the analytes and sorbent, as well as operational variables such as sample pH, ionic strength, loading flow rate, sorbent mass, and eluent composition [82].

Because emerging organic contaminants commonly occur at trace or ultra-trace concentrations in chemically complex matrices, SPE is one of the most widely applied preparation techniques for their determination in environmental samples [83]. The analytical relevance of SPE is further demonstrated by its incorporation into official methods issued by regulatory agencies. Notable examples include U.S. EPA Method 542 [84] which employs SPE to determine pharmaceuticals and personal care products in finished drinking water, and the U.S. EPA Method 537.1 [85], which uses SPE to determine selected per- and polyfluoroalkyl substances (PFAS) in drinking water. Together, these methods illustrate the importance of SPE in achieving the sensitivity and selectivity required for monitoring trace-level contaminants.

The evolution of SPE has been closely associated with advances in sorbent materials. Early applications relied on carbonaceous materials, such as animal charcoal, primarily to remove pigments from reaction mixtures. Subsequently, synthetic polymeric sorbents, particularly styrene–divinylbenzene resins, were introduced to extract organic compounds. The development of chemically bonded silica phases, especially octadecyl-bonded silica (C18), represented a major advance and contributed to the expansion of SPE into environmental, clinical, and pharmaceutical applications. Collectively, these developments have progressively improved the applicability, selectivity, and extraction efficiency of SPE for analytes with diverse physicochemical properties [86]. Because of the technique’s versatility and the ongoing demand for more efficient and sustainable extraction approaches, researchers have investigated a growing variety of materials as sorbent phases. Among these, biochar has emerged as a promising low-cost and sustainable alternative because of its porous structure, tunable surface chemistry, and capacity to interact with organic contaminants through multiple adsorption mechanisms.

Birer et al. (2021) [35] evaluated sewage-sludge-derived biochar and its chemically modified derivatives in SPE for the extraction of bisphenol A (BPA). The original biochar was modified with KOH/methanol (KOH/MeOH-SSB). The study used model aqueous BPA solutions prepared in ultrapure water. For SPE, 0.1 g of KOH/MeOH-SSB was packed into a 6 mL cartridge, conditioned with 10 mL of methanol followed by 15 mL of ultrapure water, loaded with the BPA solution, washed with 5 mL of ultrapure water, and eluted with 3 mL of acetonitrile. BPA was determined by HPLC–UV. At the natural sample pH of 6.5, the optimized sorbent provided BPA recoveries of approximately 85% over BPA concentrations of 1–100 mg/L, and RSDs were below 4%. However, the study was designed primarily to assess sorbent performance, and it did not report analytical validation parameters such as linear range, limit of detection (LOD), limit of quantification (LOQ), or selectivity. Sorbent reuse was investigated after regeneration with a 50:50 methanol/acetic acid mixture. The BPA recovery decreased only slightly from 88.1% to 87% after three cycles. The authors showed that sewage sludge can be converted into biochar using a simple and cost-effective pyrolysis system, producing a carbonaceous material suitable for the SPE of hydrophobic analytes [35].

Wolfberry biomass waste-derived biochar, obtained as a by-product of carbon quantum dot synthesis by hydrothermal carbonization, was proposed as a sustainable sorbent for SPE of fluoroquinolone and sulfonamide antibiotics from aquatic products (fish and shellfish muscle). The extracts were analyzed by HPLC coupled to high-resolution mass spectrometry (HPLC-HRMS, Orbitrap) (Thermo Fisher, Bremen, Germany). The optimized SPE procedure used 200 mg of biochar packed into a cartridge, conditioned with 5 mL methanol and 5 mL EDTA-phosphate buffer, then loaded with 4 mL of sample extract at 2 mL/min. The cartridge was washed with 2 mL ultrapure water, and analytes were eluted with 8 mL methanol containing 1% formic acid. The method showed excellent linearity (R² > 0.990) over the range of 1–320 μg/kg, LODs of 1.0–4.5 μg/kg, recoveries of 75–119%, and RSDs < 10%. The LOQ was not reported. Sorbent reusability was investigated, demonstrating stable adsorption efficiency for up to three adsorption–desorption cycles. The authors showed that biochar-packed SPE cartridges effectively removed lipids and other matrix interferences, enabling the accurate and precise determination of multiple antibiotic residues by HPLC–HRMS [69].

Abdelwahab et al. (2026) [70] developed a sustainable SPE sorbent based on modified pomegranate peel biochar (POM), produced by pyrolysis of pomegranate peel at 300 °C followed by surface modification with phytochemicals extracted from the peel. The method was applied to extract tetracycline from aqueous samples using 10 mg of sorbent, sample pH 4, and 30 min of shaking at 200 rpm as the optimized adsorption conditions. Desorption was achieved with 5 mL of acidified methanol (methanol:HCl, 1:1, v/v) for 30 min, providing a 99.3% recovery. The modified biochar exhibited a maximum tetracycline removal efficiency of 87.6%, significantly higher than the unmodified biochar (45.9%). The method presented LOD and LOQ values of 1.73 and 5.23 mg/L [70].

Nguyen et al. (2026) [23] developed a betaine ionic liquid-functionalized biochar (BC-Si) synthesized from phosphoric acid-activated coconut shell biochar pyrolyzed at 500 °C and functionalized with N-(3-triethoxysilylpropyl)-N,N-dimethylcarbamoylbetaine chloride. The material was used as an SPE sorbent to extract nine fluoroquinolone antibiotics from fish-farming water and milk samples, followed by determination using HPLC-MS/MS. The optimized SPE conditions consisted of 100 mg of BC-Si, 50 mL of sample at pH 6.0, cartridge conditioning with 3 mL methanol and 3 mL water, sample loading at 3 mL min⁻¹, washing with 5 mL of water, and elution with 2 mL of acetonitrile:methanol:amonia (3:2:2, v/v/v). The method showed recoveries of 82.1–106.7%, LOQs of 1.57–2.68 μg/L, and RSDs below 7% for fish-farming water. In addition, the sorbent was evaluated for reusability over five adsorption–desorption cycles. The authors showed that the modified biochar was an efficient SPE sorbent and had considerable potential for monitoring aquatic micropollutants, supporting efforts to protect ecosystem integrity [23].

Huyen et al. (2026) [71] developed a sustainable biochar derived from spent coffee grounds (SCG) activated with phosphoric acid (SCG-PBC) as a sorbent for SPE of tetracycline antibiotics from synthetic wastewater. The SPE cartridges were packed with 200 mg of biochar (63–100 μm), conditioned with 5 mL methanol and 10 mL water, then loaded with 50 mL sample, washed with 3 mL acetonitrile, and eluted with 10 mL methanol/TFA (99:1, v/v). Tetracyclines were determined by HPLC-MS/MS after SPE (HPLC-UV was used during optimization studies). The method provided recoveries of 49.7–66.4% for synthetic wastewater samples, with RSDs between 7.6 and 16.4%, and linearity higher than 0.99. The biochar exhibited a high adsorption capacity for tetracycline (95.3–97.4 mg/g) and maintained acceptable performance over five adsorption–desorption cycles. The authors demonstrated that phosphoric-acid-activated biochar derived from coffee grounds is a feasible and sustainable sorbent for sample pretreatment and analyte preconcentration, offering a promising alternative to conventional SPE materials [71].

Overall, biochar-based SPE has shown promising performance for EOC extraction, with satisfactory recoveries, precision, and potential for sorbent reuse.

4.4. Magnetic Solid-Phase Extraction Approaches

Magnetic Solid-Phase Extraction (MSPE) has emerged as a promising sample preparation technique owing to its simplicity, speed, high extraction efficiency, and straightforward magnetic separation. MSPE is one of several advanced extraction approaches derived from conventional SPE, alongside SPME, stir bar sorptive extraction (SBSE), and dispersive solid-phase extraction (d-SPE). All these techniques were developed to overcome the inherent limitations of traditional cartridge-based SPE [87].

MSPE has attracted considerable attention because magnetic sorbents are dispersed directly into the sample solution, maximizing the contact between the sorbent and the target analytes and promoting efficient extraction. After adsorption onto the surface of the magnetic material, the analytes are desorbed using a small volume of an appropriate eluent before instrumental determination.

The main advantages of MSPE are the rapid phase separation achieved by applying an external magnetic field, which eliminates conventional, time-consuming steps such as filtration and centrifugation. Compared with conventional SPE, MSPE also requires lower amounts of sorbents and organic solvents, reduces extraction time and analytical costs, and simplifies the overall extraction procedure, since the sorbent does not need to be packed into cartridges or other extraction devices [87]. Furthermore, advances in materials science and nanotechnology have enabled the development of a wide variety of magnetic sorbents with high surface area, excellent adsorption capacity, and tailored surface functionalities, including biochar-based materials. These developments have considerably expanded the applicability of MSPE for extracting and preconcentrating a broad range of organic compounds from complex matrices, mainly environmental water samples [88].

In brief, the MSPE procedure involves three key steps: the magnetic sorbent is dispersed in the sample solution or suspension to retain the target analytes; the sorbent is then separated from the sample using an external magnetic field; and, finally, the retained analytes are desorbed from the sorbent using an appropriate elution solvent. Compared with traditional SPE processes, the magnetic adsorbents used in MSPE do not require a packing step for the SPE column, minimizing column blocking or leakage in SPE [89].

Safarikova and Safarik were the first authors to publish a study on MSPE in 1999. They developed a new SPE procedure based on magnetic sorbents, which they called MSPE. In their experiments, they attached reactive copper phthalocyanine dye to silanized magnetite (blue magnetite). They used the biochar as a magnetic adsorbent to preconcentrate safranin O and crystal violet dyes. Samples containing different amounts of dyes were subjected to extraction, and recoveries ranged from 14.97 to 29.97% and from 56.06 to 77.32% when blue and biochar magnetites were used, respectively [90].

Starting in 2010, MSPE gained popularity through synthesis, modification, and application of magnetic nanomaterials for preconcentration of organic compounds in environmental samples. Subsequently, other studies employed MSPE in combination with biochar for the analysis of organic contaminants in environmental samples, and main parameters such as pH, extraction time, amount of adsorbent, and sample volume, which affect magnetic solid phase extraction, are the variables most studied and used in MSPE [20].

For water samples, numerous studies have highlighted the increasing use of biochar owing to its high capacity to adsorb organic pollutants from aqueous media. As an example, Karatas et al., (2024) [63] obtained a new MSPE based on a magnetic composite modified with biochar obtained from pumpkin peel. The authors developed it for enriching and extracting naproxen from pharmaceuticals, lake water, milk, and artificial urine. The main parameters, such as pH, extraction time, amount of adsorbent, and sample volume, which affect magnetic solid-phase extraction, were investigated. The authors obtained 100% recovery, 0.6% RSD, and an LOQ of 37.0 ng/mL [63].

In 2023, the potential use of by-products as sorbents for the MSPE of nonsteroidal anti-inflammatory drugs (NSAIDs) from aqueous samples, followed by liquid chromatography–mass spectrometry analysis, was investigated. An experimental design was used to optimize the technique’s main variables. As a result, the LOQ and LOD ranged from 0.20 to 0.37 μg/L and from 0.061 to 0.111 μg/L, with inter-day and intra-day precision below 3.2%. The recoveries of NSAIDS were higher than 92.60%. The authors concluded that the biochar can be reused five times without any decrease in adsorption capacity. The results demonstrate the potential of biochar as a new adsorbent in MSPE devices for removing NSAIDs from aqueous solutions [62].

Wang et al. (2024) [33] produced a magnetic biochar derived from fungal hyphae for Organochlorine pesticides (OCPs) extraction in water samples. The extraction conditions were optimized as 20 mL of sample solution with 70 mg of sorbent and 2.0 g of NaCl oscillated at 50 °C for 5 min. The optimum desorption was performed by oscillating the sorbent in 1.0 mL of acetonitrile for 5 min. LOQ: 0.9 to 46.2 ng/L; recoveries of 80.5–117.2% and RSDs < 8.9%. The method was suitable for the sensitive determination of OCPs from simple to complex matrix samples [33].

Lv et al. (2024) [21] developed magnetic coconut-shell biochar as an adsorbent in MSPE for neonicotinoid insecticide extraction from environmental water samples. Under the optimum conditions (adsorption for 15 min, 5% formic acid-acetonitrile as elution condition, 50 mL of sample volume, 15 mg of magnetic biochar, pH at 5.0), recoveries were in the range of 68.6–104.0% with RSDs from 0.4% to 9.3% and LOQ between 0.005 and 0.020 ng/mL. These results demonstrated that the established method was highly sensitive, convenient, and repeatable for directly analyzing trace neonicotinoid insecticides in environmental water samples. More importantly, the novel biochar from coconut clothing was introduced into sample pretreatment to enrich trace organic pollutants [21].

Wang et al. (2025) [20] obtained biochar prepared from garlic straw for multipesticide extraction residues (imidacloprid, acetamiprid, thiacloprid, orysastrobin) in environmental water samples. The material presented excellent magnetic properties, and optimized conditions, such as sample pH, adsorption time, elution conditions, and sample volume, were studied. The proposed method showed an LOQ range of 1.7 and 2.6 ng/mL, with recovery between 70.0% and 99.5%, and RSDs from 0.1% to 6.1%. Notably, the biochar material possessed ultra-fast adsorption efficiency in the MSPE process. Also, the novel biochar from garlic straw was introduced into the sample pretreatment process and could be potentially explored for enriching other trace organic pollutants [20].

In 2025, Li et al. (2025) [28] used the magnetic biochar derived from shrimp shell (MBC-600) in MSPE for the extraction of 12 tetracyclines in aquatic products. Under optimized conditions, the MSPE based on MBC-600 coupled with UHPLC-MS/MS was established for highly sensitive determination of TCs from aquatic products. The proposed method showed LOQs between 0.50 and 1.63 μg/kg, acceptable recoveries between 86.6% and 98.9%, and RSDs ranging from 2.41% to 11.6%. The developed green, recyclable magnetic biochar material has a facile preparation process with minimal reagent consumption, adheres to green chemistry principles, and enables sustainable valorization of seafood processing by-products while expanding biochar’s applicability in advanced sample pretreatment and ultrasensitive trace analysis systems [28].

An MSPE method based on a deep eutectic solvent-functionalized magnetic biochar synthesized from spent mushroom substrate, providing a sustainable and environmentally friendly adsorbent for determining pyrethroid pesticides in environmental water samples, was also developed. The method was established in water samples; briefly, the adsorbent was added to a 10 mL centrifuge tube containing 6 mL ultrapure water and 100 μL of a 0.17 mg/mL mixed standard solution. The mixture was vortexed for 12 min; after adsorption, the adsorbent was collected using a magnet under an external magnetic field, and the solution was discarded. Thereafter, acetonitrile and 25 min of continuous ultrasound bath assistance were applied to elute pyrethroids from the adsorbents. Finally, the desorption solution was filtered through a 0.22 μm organic filter membrane, and pyrethroid concentrations were measured by HPLC-DAD. All experiments were repeated three times to ensure accuracy. The authors obtained 82.02–91.80% recovery, 0.39–1.57% precision, and an LOQ of 0.5 μg/mL. In conclusion, the authors stated that the MSPE adsorbent offers high efficiency, environmental friendliness, and time-saving operation, while also providing a new strategy for removing pyrethroids from water [26].

To further improve the extraction capacity for organic contaminants using biochar and MSPE, researchers continue to develop new methods, including combinations of extraction techniques. As an example of the expansion and miniaturization of MSPE, some studies have developed a miniaturized version of the technique—namely, magnetic solid-phase microextraction (MSPμE). In the study developed by Zhang et al. (2021) [59], the authors used corn straw-derived biochar in environmental sample pretreatment. Results showed that the magnetic biochar pyrolyzed at 700 °C exhibited the best extraction performance, with enrichment factors ranging from 52 to 210. The magnetic biochar-based extraction was further combined with gas chromatography coupled to mass spectrometry (GC-MS) to extract organophosphorus pesticides from environmental samples. The method demonstrated low LODs (0.02–0.11 μg/L) and high recoveries (72.4–96.8%) from spiked water and soil samples. The results suggest the promising potential of using corn straw-derived biochar to efficiently enrich organophosphorus pesticides from complex environmental samples [59].

In another example, in 2025, Zheng et al. [27] developed a new, environmentally friendly, and efficient method for triazine herbicide determination in water (and other matrices such as tea and juice). The study combined magnetic dispersive micro-solid phase extraction (MD-μSPE) with magnetic dispersive liquid–liquid microextraction (MDLLME), followed by HPLC determination. The pretreatment process utilized magnetic biochar and magnetic deep eutectic solvent as the adsorbent and eluent, respectively. Fe(NO₃)₃ was loaded onto waste mushroom sticks to prepare the biochar via impregnation-pyrolysis. The method demonstrated excellent linearity (1–100 μg/L with R² > 0.999), an LOD of 0.3 μg/L, recoveries in water ranging from 91.8 to 97%, and RSDs < 6.5%. The MD-μSPE-MDLLME approach offers a greener, more efficient extraction process for separating and concentrating pesticides from complex samples [27].

Overall, the main analytical advantage of MSPE arises from the combination of sorption and simplified phase separation, which reduces the need for filtration or centrifugation and facilitates the use of dispersed sorbents. At the same time, the literature reveals substantial variability in recovery, precision, and detection limits among different magnetic biochars, indicating that magnetization alone does not guarantee improved analytical performance. In some cases, incorporating magnetic phases may alter pore accessibility and surface chemistry and may therefore improve handling without necessarily improving analyte affinity. Thus, the principal value of magnetic biochar in sample preparation appears to be operational rather than purely sorptive: it enables efficient contact between sorbent and sample while simplifying sorbent recovery. This distinction should be considered when comparing MSPE with conventional SPE and other extraction configurations.

4.5. Dispersive Solid-Phase Extraction Approaches

Dispersive solid-phase extraction (dSPE) has gained widespread recognition since its introduction by Anastassiades et al. in 2003 as a key cleanup step in the QuEChERS (Quick, Easy, Cheap, Effective, Rugged, and Safe) method for multi-residue pesticide analysis in high-water food matrices. In the original protocol, a 1 mL aliquot of the upper acetonitrile layer is transferred into a vial containing 25 mg of primary secondary amine (PSA) and 150 mg of anhydrous MgSO₄, vortexed for 30 s, and centrifuged for 1 min at 6000 rpm to isolate the supernatant for GC-MS analysis [91,92,93].

As a versatile sample preparation approach, dSPE has been conventionally applied in two main formats: as a clean-up step in the QuEChERS protocol and as an independent extraction technique using micro- and nanosorbents for the direct separation and preconcentration of organic compounds. This approach has proven highly effective for isolating diverse analyte classes from environmental, food, and biological matrices of varying complexity, including veterinary drugs, polycyclic aromatic hydrocarbons (PAHs), polyphenols, pesticides, and bisphenols. Following extraction, these targets are typically quantified via gas chromatography (GC), high-performance liquid chromatography (HPLC), or capillary electrophoresis (CE) [91,93].

To improve sorbent dispersion beyond conventional mechanical shaking, researchers have developed alternative advanced mechanisms. Vortex-assisted dSPE (VA-dSPE) utilizes vortex mixers at 2500–3200 rpm to induce an unsteady turbulent flow, promoting efficient mixing and, in some cases, reducing the particle size of readily dispersible sorbents. Ultrasonic-assisted dSPE (UA-dSPE) employs acoustic cavitation generated by ultrasonic baths or probes to promote sorbent dispersion, particularly for nanomaterials prone to agglomeration. However, prolonged sonication may heat the sample, potentially degrading thermolabile analytes, or form stable suspensions that hinder subsequent phase separation during centrifugation. Effervescence-assisted dSPE (EA-dSPE) drives dispersion in situ without mechanical devices by compressing the sorbent into a tablet with sodium carbonate and a solid acid, which releases CO₂ upon contact with the aqueous sample [93].

The extraction efficiency of these methods depends on multi-parametric factors, including sorbent mass, particle size, dispersion mode, pH, and ionic strength. Sorbent affinity remains paramount, driving analyte capture through intermolecular interactions such as van der Waals forces, hydrophobic effects, electrostatic interactions, and hydrogen bonding. Currently, five primary sorbent groups dominate dSPE applications: carbon-based nanomaterials or composites (including biochar, carbon nanotubes (CNTs), and graphene oxide), metal and nonmetal oxides, metal–organic frameworks (MOFs), polymeric materials, and molecularly imprinted polymers (MIPs) [92,93].

Chen and Tian (2025) [79] developed a biochar-functionalized molecularly imprinted polymer (BC-MIPs) for the determination of chlorpyrifos (CPF) in aqueous samples using dSPE. The methodology involved three main stages: producing straw biochar, synthesizing the CPF-BC-MIPs sorbent, and optimizing dSPE parameters, including extraction and desorption times, sample pH, and desorption solvents. The dSPE method was coupled with high-performance liquid chromatography with diode array detection (HPLC-DAD) and validated using laboratory tap water and river water samples from the Baisha River in Qingdao. This approach demonstrated high selectivity, reusability, and sensitivity for CPF, achieving LOD and LOQ of 1.0 ng/mL and 4.0 ng/mL, respectively. Furthermore, the method required 75 min for analyte extraction and 12 min for chromatographic analysis, while delivering satisfactory accuracy in river samples with recoveries ranging from 81.2% to 103.6% and RSDs ≤ 9.2% [79].

Niu et al. (2025) [22] investigated the presence of four phenoxyacetic acid herbicides (PAAs), including dicamba (DICA), 4-chlorophenoxyacetic acid (4-CPA), 2,4-dichlorophenoxyacetic acid (2,4-D), and 4-(2,4-dichlorophenoxy) butyric acid (2,4-DB), in environmental water and soil samples collected from 20 sites in Haikou City. The study utilized a sustainable coconut shell biochar (CSB) as a dSPE sorbent within a modified QuEChERS framework. The procedure comprised three main stages: CSB production, optimization of the adsorption process, and standardization of QuEChERS parameters, specifically sample pH and CSB dosage. Separation and quantification were performed using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The validated method achieved LODs ranging from 0.083 to 8.333 ng/mL. Method accuracy was demonstrated in environmental matrices with recoveries between 74.9% and 109.7% and standard deviations (SD) spanning 1.0% to 8.9%. Overall, the approach offers a highly efficient, low-cost, sensitive, and eco-friendly alternative by employing a green adsorbent [22].

As the range of potential applications broadens, exploring novel materials ensures that dSPE aligns with Green Analytical Chemistry (GAC) principles. The technique satisfies key green metrics by lowering LOD/LOQ and maintaining high accuracy and reproducibility while reducing analysis time, solvent consumption, and operational costs through miniaturization and automation [91,93].

4.6. Solid-Phase Microextraction Approaches

Solid-Phase Microextraction (SPME) is a miniaturized, solvent-free (or minimal solvent) sample preparation technique developed in the early 1990s [94]. The technique is based on the partition equilibrium of analytes between the sample matrix and a thin layer of extracting phase, typically a liquid polymer or solid adsorbent, immobilized on the surface of a fused silica fiber or metallic support. This process integrates sampling, extraction, concentration, and sample introduction into a single step, making it widely used for trace analysis of volatile and semi-volatile organic compounds because it is compatible with both GC and LC systems [95].

Depending on analyte volatility and matrix complexity, SPME can be operated using two main extraction modes: Direct Immersion (DI-SPME) and Headspace (HS-SPME). In DI-SPME, the fiber is immersed directly into the liquid sample, ideal for less volatile compounds or clean matrices. In HS-SPME, the fiber is exposed to the vapor phase above the sample in a sealed vial. This approach is highly advantageous for volatile analytes as it protects the extracting phase from macromolecules and non-volatile matrix interferences. Additionally, variations such as membrane-protected SPME expand the method’s applicability to complex environmental matrices with high particulate loads [95].

Beyond its operational versatility, SPME stands out for the high robustness and reusability of its devices, allowing a single fiber to be used for hundreds of extraction and desorption cycles without significant loss of efficiency, which reduces the cost per analysis. Another important advantage of the technique is the flexibility of its extracting phase: while conventional commercial polymers (such as PDMS and Carboxen) are well established, the ability to incorporate and test different materials as sorbents in the fiber composition has enabled the development of customized phases with enhanced selectivity and thermal stability. In this context, biochar has emerged as a particularly promising material for advancing SPME due to its low cost, high sorption capacity, tunable surface properties, and potential for sustainable production.

An example of utilizing biochar in SPME was reported by Chen et al. (2026) [25], by the synthesis of a novel biochar derived from Gynostemma pentaphyllum waste (designated as PBAGB). To maximize extraction efficiency, the authors employed a comprehensive “waste-to-wealth” modification strategy integrating K₂CO₃ activation, ball-milling, and plasma treatment. This multi-step modification yielded a carbonaceous material with elevated specific surface area, abundant oxygen-containing functional groups, and numerous structural defect sites, which greatly increased the available active sites for analyte adsorption. When applied as an SPME fiber coating for bisphenol (BP) extraction, the PBAGB-coated fiber outperformed commercial counterparts, demonstrating superior extraction performance, remarkable stability, and significantly lower production cost. Coupled with ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS), the developed method achieved low LODs (0.39–2.9 ng/L) and relative recoveries ranging from 80.4% to 119.9%. The practical feasibility of the biochar-based device was demonstrated by determining BPs in 21 complex marine samples collected from the Beibu Gulf, China. The concentration of total detected BPs ranged from 3.8 to 92.5 ng/L. Specifically, bisphenol A, one of the main plasticizers used worldwide, was successfully detected in all 21 samples [25].

In another study, the same research group demonstrates the cost-effectiveness and high performance of engineered biochars for environmental monitoring using an activated Cucumis metuliferus biochar (ACMB) for the trace analysis of typical persistent organic pollutants in marine water. Through a straightforward activation process, the resulting ACMB exhibited a hierarchical porous structure, large pore volume, and an expansive specific surface area. The material was successfully fabricated into highly uniform SPME fibers with a low production cost. When applied to the extraction of five PAHs and four OCPs, the ACMB-coated fiber yielded enrichment factors ranging from 1601 to 6732. It significantly outperformed conventional commercial fibers regarding extraction capacity, stability, and affordability. Coupled with GC-MS, this automated method achieved low LODs (0.30–0.99 ng/L). The ACMB-SPME fiber was used to determine PAHs and OCPs across 18 marine water samples from the South China Sea, providing crucial spatial distribution data for contaminants such as acenaphthene, fluorene, and anthracene [75].

Expanding the scope of biochar-based microextraction formats, a recent study reported a rapid, green method using coconut shell biochar (CSB) packed into self-assembled monolithic spin columns for solid-phase microextraction (MSC-SPME). To handle multi-media environmental and food matrices while effectively eliminating matrix interferences, the authors integrated salting-out-assisted liquid–liquid extraction with MSC-SPME (SALLE-MSC-SPME). Fundamental adsorption kinetics and isotherms confirmed strong interactions between the CSB sorbent and target triazine herbicides. Through orthogonal design optimization of critical extraction parameters, the entire extraction and cleanup procedure was achieved in under 10 min. Coupled with UPLC-MS/MS, the green SALLE-MSC-SPME protocol provided low LODs (6.99–11.00 ng/L), and satisfactory accuracy and precision (R% between 69.00% and 124.72%; RSD ≤ 0.43%). This work highlights how combining biochar with alternative miniaturized formats (like spin columns) can drastically streamline sample throughput without requiring tedious matrix-cleanup steps [80].

Over the past five years, the practical utility of biochar-based SPME has been extensively demonstrated across a remarkably diverse range of sustainable feedstocks, chemical modification strategies, and target organic pollutants. Agricultural and industrial wastes, including microalgal biomass [73], corncob [72], fruit rinds such as lotus pedicel, watermelon, and litchi [74], and even animal by-products like bovine bone [29], have been successfully converted into high-value sorptive coatings. By applying tailored activation protocols such as K₂CO₃ treatment, ball-milling, heteroatom doping, or integration with TiO₂ nanorods and covalent organic frameworks [34], researchers have generated engineered biochars with hierarchical porosity, expansive specific surface areas, and abundant heteroatoms (nitrogen and oxygen). In addition to traditional fiber configurations, these materials have been configured into innovative microextraction formats, such as in-tube SPME assemblies. These biochar-functionalized devices have yielded extraordinary enrichment factors (ranging from thousands up to 155,000) and ultra-trace detection capabilities (from ng to pg/L) for a wide spectrum of environmental contaminants, including chlorobenzenes, PAHs, estrogens, phthalates, and aromatic hydrocarbon metabolites in complex water matrices.

4.7. Rotating Disk Sorptive Extraction Approaches

Rotating Disk Sorptive Extraction (RDSE) is an innovative, disk-based miniaturized sample preparation technique introduced in 2009 [96]. Developed to overcome the mass-transfer limitations and fragility associated with conventional sorptive microextraction devices, RDSE uses a small Teflon or glass disk containing an embedded magnetic core, with one surface coated or packed with a sorbent material. The disk is placed directly into the liquid sample and rotated at high speeds using a standard magnetic stirrer, combining sample agitation and analyte extraction into a single, highly efficient step. The forced convection generated by the rotating motion drastically reduces the boundary layer thickness at the sorbent-water interface, significantly accelerating extraction kinetics and reducing overall sample preparation times [97].

Depending on the physical setup and analyte desorption strategy, RDSE can operate in two primary modes. In direct immersion mode, the coated disk rotates freely within the aqueous matrix, maximizing surface contact with the bulk solution and making it well suited for extracting organic contaminants from larger sample volumes (typically 100–1000 mL) to achieve ultra-trace enrichment factors. In the cavity-packed or disk-sleeve configuration, the disk features a small cavity packed with loose sorbent particles secured by a porous membrane, protecting the sorbent from particulate matter while permitting free diffusion of target analytes [97].

Following extraction, analytes can be desorbed either by immersing the disk in a small volume of organic solvent for subsequent chromatographic analysis (HPLC, LC-MS/MS, or GC-MS), or by direct thermal desorption units coupled to GC. Beyond its rapid mass transfer, RDSE stands out for its high phase-volume ratio, good mechanical stability, and reduced susceptibility to breakage and clogging compared with fiber-based extraction formats [97].

A key strength of RDSE lies in the remarkable diversity of phase chemistries that can be coated onto or packed within the disk, catering to a broad spectrum of physicochemical properties. Conventional commercial phases include polydimethylsiloxane (PDMS) for non-polar volatile and semi-volatile compounds, octadecylsilane (C18), and hydrophilic-lipophilic balance polymers like Oasis^® HLB. To enhance selectivity toward specific contaminant classes, researchers have investigated other materials, including styrene-divinylbenzene polymers, cation-exchange resins, chelating phases for metalorganics, chiral sorbents for enantiomeric separations, and MIPs for target-tailored recognition. More recent developments have explored alternative materials and extraction media, such as nylon, cork particles, agarose gel, deep eutectic solvents (DES), and ionic liquid-modified clays such as montmorillonite modified with ionic liquid (MMT-IL) [97]. Within this expanding range of materials, engineered biochar emerges as a promising sorptive phase for RDSE. Its surface functional groups, tunable porosity, potentially high specific surface area, and eco-friendly nature fit seamlessly into the rotating disk format, offering a low-cost, sustainable alternative with high sorption capacity for emerging organic pollutants in complex environmental matrices.

Demonstrating the viability of agricultural byproducts as sorptive phases in RDSE, untreated peanut shells, peanut shell biochar, and activated carbon were evaluated for the microextraction of emerging contaminants spanning a broad polarity range (ethyl paraben, diclofenac, triclosan, bisphenol A, and 17-α-ethinylestradiol) prior to GC-MS determination. Physicochemical characterization revealed that untreated peanut shells lacked porosity, whereas carbonization and subsequent activation generated a highly porous network; the resulting activated phase exhibited a high BET surface area (516 m²/g), a total pore volume of 0.27 cm³/g, and a mean pore diameter of 2.12 nm. FTIR and morphological analyses indicated that pore filling and π–π stacking served as the primary driving mechanisms governing analyte retention. Under optimized RDSE conditions (10 mg sorbent, 15 mL sample at pH 2, 2000 rpm for 70 min, and ethyl acetate elution), the method yielded LODs between 0.003 and 0.729 µg/L, absolute recoveries ranging from 16 to 87%, and precision (RSD < 10%). Notably, comparative benchmarking against conventional commercial phases confirmed that the biomass-derived activated carbon achieved equal or superior extraction efficiencies, proving the potential of pyrolyzed agricultural waste as a cost-effective, high-performing replacement for commercial RDSE sorbents [66].

In a further demonstration of sustainable material design for RDSE, avocado seed waste and its derived carbonaceous sorbents were investigated for the trace extraction of ibuprofen and its primary metabolite, 1-hydroxy-ibuprofen, followed by GC-MS determination. The authors evaluated pyrolyzed biochar alongside chemical activation strategies and found that avocado seed activated carbon prepared at 600 °C with a ZnCl₂ impregnation ratio of 1:1.2 (w/w) yielded optimal extraction performance. Physicochemical characterization confirmed that pyrolysis and chemical activation progressively eliminated the native lignocellulosic functional groups of the raw seed, drastically increasing specific surface area and pore volume. This structural transformation established pore filling and π–π stacking as the dominant retention mechanisms governing analyte uptake. Under optimized RDSE parameters (50 mg sorbent, 30 mL sample volume at pH 4, 2000 rpm for 90 min, and methanol elution), the biochar-based platform achieved low LODs (0.23 µg/L for ibuprofen and 0.07 µg/L for 1-hydroxy-ibuprofen) and high recoveries of 81% and 91%, respectively. Benchmarking against established commercial phases (Oasis^® HLB and C18) revealed no statistically significant differences in extraction efficiency, confirming that avocado seed-derived activated biochar serves as an eco-friendly and cost-effective substitute for commercial sorbents in disk-based microextraction technologies [67].

4.8. Pipette Tip Solid-Phase Extraction Approaches

Pipette-tip solid-phase extraction (PT-SPE) is a sample preparation technique that uses only two pipette tips, a small amount of cotton, and a small quantity of sorbent, and has shown promising potential for analyte extraction and preconcentration. The selection and optimization of highly efficient adsorbents are crucial to PTSPE effectiveness, emphasizing the need to develop new adsorbents and establish simpler, faster, and more economical sample pretreatment techniques [98]. This method has gained popularity because of its advantages, including low sample and solvent consumption, rapid on-site processing, and selective extraction of proteins, peptides, lipids, and small molecules. Adsorbent selection is a decisive factor in PT-SPE; in this sense, biochar has been widely used [99].

The first study of PT-SPE was in 1999 by van Hout et al., to miniaturize conventional SPE by packing or immobilizing the sorbent inside a disposable pipette tip. Although the technique was introduced commercially in 1998 with sorbent-immobilized pipette tips developed by Ansys Technologies, van Hout et al. (1999) demonstrated the extraction of drugs using PT-SPE coupled to GC [100].

Since its emergence, PT-SPE has undergone continuous development, evolving from an initial miniaturized format of conventional SPE into a versatile and powerful sample preparation strategy for a wide range of analytical applications [101]. In the early 2000s, the technique began to expand beyond its initial applications, driven by advances in sorbent chemistry, device design, and automation. These developments enabled its successful integration with different chromatographic platforms, including GC, LC, and MS techniques, enhancing its applicability for analyzing complex samples. Consequently, PT-SPE has been increasingly applied for extracting and preconcentrating organic compounds in diverse matrices, particularly environmental samples such as surface water, groundwater, wastewater, soils, and sediments [101].

The growing demand for more sustainable analytical approaches has stimulated the development of alternative sorbent materials that improve extraction performance while reducing environmental impacts for a broad range of organic contaminants, including pesticides, pharmaceuticals, personal care products, endocrine-disrupting chemicals, and other emerging pollutants. Biochar, produced by pyrolyzing lignocellulosic biomass, has shown considerable potential as a sorbent in sample preparation techniques. Its use can add value to biomass-derived materials while providing an efficient and environmentally friendly alternative for extraction and preconcentration procedures, including PT-SPE [65].

For example, Sun et al. (2022) [64] developed a biochar material derived from carbonizing peanut shells and used it as the adsorbent in PT-SPE to enrich and determine six endocrine-disrupting phenols (EDPs) in combination with HPLC-UV. Abundant aliphatic and aromatic carbon structures and functional groups from polar heteroatoms (N, O, S) were distributed in the low-cost and eco-friendly peanut shells-derived biochar materials and were favorable for the enrichment of target EDPs. The experimental factors influencing the extraction efficiency were investigated. Under the optimal conditions, the proposed PT-SPE method exhibited a good linear relationship (R² > 0.993) in the range of 0.5–400 μg/L and low LODs from 0.25 to 2.5 μg/L, as well as good precision and accuracy with RSDs of 0.3–13.2% and recoveries of 83.5–117.1%. Finally, the biochar-based miniaturized pretreatment method was used to determine six EDPs in bottled water, milk, tea beverage, and disposal plastic bag-soaked solution, with recoveries from 77.5% to 116.5% [64].

Cardoso et al. (2025) [65] developed a simple sample preparation approach combining dispersive pipette extraction (DPX) and a biochar-based biosorbent derived from corn cob to extract endocrine-disrupting compounds (EDCs) from surface water samples, followed by HPLC-DAD determination. The recoveries ranged from 81.2% to 116.5%, and LOQs were 1.44–3.28 μg/L. The green nature of the method was evaluated using the Analytical Greenness Metric (AGREE) and the AGREEprep. The metrics indicated excellent green performance, highlighting low analyst exposure, use of reusable tips, a biologically derived extracting phase, and process miniaturization. Based on the results, the proposed method proved to be a fast, efficient, and eco-friendly approach for determining EDCs in surface water samples, effectively detecting these organic compounds in real samples [65].

4.9. Other Techniques

Zhang et al. (2023) [24] developed an in-syringe SPE method using a biochar/sodium alginate mixed-matrix membrane (biochar/SA MMM) for the determination of four nitroimidazole antibiotics in drinking, tap, and river water samples. The biochar was produced from Poria cocos herbal residue after ZnCl₂ activation, followed by pyrolysis at 550 °C for 4 h under a nitrogen atmosphere. The resulting biochar was incorporated into a sodium alginate matrix and cross-linked with CaCl₂, producing a hydrophilic membrane with a specific surface area of 150.2 m²/g. The membrane was cut into 25 mm disks and placed inside a syringe filter. Under the optimized conditions, 10 mL of water sample, without pH adjustment, was passed through the membrane using six pull–push extraction cycles. The retained analytes were eluted with 1.5 mL of acetonitrile through six additional pull–push cycles. Analytes were quantified by UHPLC–MS/MS. The method showed linear ranges with coefficients of determination between 0.9995 and 0.9997. The LODs and LOQs ranged from 0.006 to 0.014 ng/mL and from 0.021 to 0.048 ng/mL, respectively. Recoveries obtained for spiked real water samples ranged from 79.02% to 99.1%, with RSDs below 8.0%; intra- and inter-day RSDs were below 7.1% and 8.2%, respectively. The membrane was reusable for at least five extraction cycles after washing three times with methanol and water, without a significant decrease in extraction performance and with recoveries remaining above 70%. The authors demonstrated that a biochar/sodium alginate mixed-matrix membrane prepared from Poria cocos residue can serve as an effective sorbent for in-syringe SPE, enabling the determination of trace nitroimidazoles in water samples, highlighting the potential of this sustainable material for advanced sample-preparation applications [24].

Bianchini et al. (2024) [76] developed an in-vial solid-phase extraction method using orange-peel-derived biochar immobilized on the inner walls of sampling tubes to extract seven steroid hormones. The method was evaluated in tap, river, and lake water, as well as wastewater treatment plant effluent. The biochar was produced by pyrolysis of orange peel at 650 °C for 6 h under a nitrogen atmosphere. Approximately 151 ± 10 mg of biochar was immobilized on the tube wall using silicone. Under the optimized conditions, the sorbent was conditioned with 6 mL of ethanol followed by 6 mL of blank water. A 25 mL water sample at its native pH was extracted for 20 min at room temperature on a roller mixer operating at 40 rpm. The analytes were eluted with 6 mL of ethanol for 15 min, after which the extract was evaporated under nitrogen at 50 °C and reconstituted in 0.5 mL of ethanol. Determination was performed by HPLC–ESI–MS/MS. The method provided recoveries of 60–110% in tap water, with RSDs below 10%. Inter-day, intra-batch, and inter-batch RSDs were below 12% and 15%, respectively. In river water, lake water, and wastewater effluent spiked at environmentally relevant concentrations of 0.2–1.0 µg/L, the authors reported overall recoveries of approximately 60–123% and RSDs of 10–18%. Because this was a proof-of-concept study focused on the extraction device, the authors did not report analytical parameters such as linear range, LOD, and LOQ, and indicated that fully validated methods remain to be developed. The biochar-coated tubes maintained extraction performance for at least ten consecutive sorption–desorption cycles, with no detectable carry-over or loss of extraction capability [76].

Zhang et al. (2025) [77] developed a portable stir-disc solid-phase extraction method using a biochar/sodium alginate mixed-matrix membrane to enrich five bisphenols—BPF, BPA, BPB, BPAF, and BPC—from tap, drinking, and river water samples. The biochar was prepared from ZnCl₂-activated Poria cocos residue by pyrolysis at 550 °C for 4 h under a nitrogen atmosphere and subsequently incorporated into a sodium alginate membrane crosslinked with CaCl₂. The optimized extraction procedure used 200 mL of sample adjusted to pH 4, a 25 mm sorptive membrane, and a 10 min extraction at 8000 rpm. The analytes were desorbed with 2 mL of acetone for 15 min, and the extracts were analyzed by HPLC-FLD. The method showed determination coefficients of 0.9959–0.9994. The LODs and LOQs ranged from 0.06 to 0.17 ng/mL and from 0.21 to 0.57 ng/mL, respectively. Spiked recoveries ranged from 71.0% to 108%, with RSDs of 0.9–13.3%. The membrane was successfully reused for five adsorption–desorption cycles without a significant decrease in recovery. The proposed biochar/SA mixed-matrix membrane-based stir-disc SPE method offers several advantages, including low cost, environmental friendliness, reduced sorbent and organic solvent consumption, and the ability to process relatively large sample volumes [77].

Lu and Varanusupakul (2025) [78] developed a laser-induced biochar sorbent from lignin-rich cork sheets for the micro-solid-phase extraction of eight OPPs pesticides—diazinon, parathion-methyl, chlorpyrifos-methyl, fenitrothion, pirimiphos-methyl, malathion, chlorpyrifos, and profenofos—from water samples. The sorbent was produced under ambient conditions by a single CO₂-laser irradiation at 9 W and 250 mm/s. The optimized μSPE procedure employed two 5 × 20 mm sorbent pieces, 40 mL of sample adjusted to pH 7, extraction at 500 rpm for 90 min, and desorption with 800 μL of acetonitrile for 20 min. Determination was performed by GC–MS in selected-ion-monitoring mode. The method provided linear ranges within 0.08–2.1 μg/L, coefficients of determination of 0.974–0.999, LODs of 0.03–0.14 μg/L, and LOQs of 0.08–0.42 μg/L. Recoveries from ultrapure water ranged from 64.8% to 126.6%, with intraday and interday RSDs of 1.44–8.99% and 1.55–7.57%, respectively. Relative recoveries in canal water ranged from 44.0% to 102.3%. Sorbent reuse over successive extraction cycles was not evaluated; only a second desorption was conducted to verify negligible carryover. The authors highlighted laser-derived biochar as a promising sustainable sorbent for miniaturized SPE, noting that its production by laser irradiation may require up to 114 times less energy per extraction than conventional pyrolysis-based preparation [78].

The analytical figures of merit summarized in Table 1 also highlight an important limitation when comparing biochar-based extraction methods across different studies. Although several methods exhibit low LODs and LOQs, these values cannot be attributed solely to biochar sorbent performance, as analytical sensitivity is also strongly influenced by factors such as sample volume, enrichment factor, elution volume, extraction configuration, instrumental technique, detector sensitivity, and calibration strategy. Therefore, LOD and LOQ values should not be considered in isolation when assessing the analytical contribution of a biochar-based sorbent. A more meaningful comparison should simultaneously consider recovery, precision, enrichment factor, matrix effects, sorbent mass, extraction time, solvent consumption, and desorption conditions. This broader perspective matters because the studies reviewed generally optimize and validate individual analytical procedures rather than evaluate different biochars under equivalent experimental and instrumental conditions. Consequently, the current literature demonstrates the analytical potential of biochar-based sorbents but does not yet provide sufficient evidence to rank different biochars solely by their reported detection limits.

The comparison of extraction configurations suggests that the choice of technique should be guided by the analytical objective rather than by the sorbent material alone. Conventional SPE provides a robust, readily transferable platform but generally requires larger sorbent and solvent volumes. MSPE offers a particularly attractive balance between extraction efficiency and operational simplicity because magnetic separation facilitates dispersive extraction and sorbent recovery. SPME offers the strongest potential for miniaturization and solvent reduction, but requires integrating the biochar into a mechanically stable extraction device or coating. Conversely, dSPE and other miniaturized formats remain less explored, despite their potential to reduce sorbent consumption and simplify sample handling. Thus, the current predominance of MSPE and SPE should not necessarily be interpreted as evidence that these are intrinsically superior approaches; rather, it may reflect their greater methodological maturity and easier adaptation to conventional laboratory workflows.

5. Challenges and Future Perspectives

Despite notable progress in using biochar-based sorbents to extract EOCs from environmental samples, several challenges still limit their widespread application in routine analytical laboratories. One of the most critical limitations is biochar’s inherent heterogeneity. Since its properties depend heavily on the precursor biomass, pyrolysis temperature, heating rate, residence time, and post-synthesis treatments, sorbents produced under slightly different conditions can exhibit distinct surface chemistry, porosity, and sorption behavior [102]. This variability compromises reproducibility and hinders the standardization of biochar-based methods. Selectivity represents a second limitation. Although biochar offers a wide range of sorption mechanisms, non-target compounds present in complex environmental matrices (e.g., natural organic matter, humic substances, and coexisting ions) can compete for the same sorption sites, reducing extraction efficiency and generating matrix effects that impair accuracy and precision. In many cases, additional cleanup steps or careful optimization of washing and elution conditions are still required. The mechanical and physical stability of biochar also warrants attention. In dispersive systems, particularly in MSPE, the stability of the magnetic material and the sorbent’s resistance to repeated use are not always fully characterized. Furthermore, sorbent mass loss, leaching of magnetic nanoparticles, and progressive saturation of active sites can occur over multiple extraction cycles, affecting reusability and long-term performance [103]. Furthermore, quantitatively recovering strongly retained analytes can be challenging, as the high affinity of certain organic contaminants for biochar surfaces may require large solvent volumes or harsh elution conditions; this conflicts with the principles of GAC and may introduce additional sources of error. Finally, most studies reported to date have been conducted at the laboratory scale using spiked samples and well-defined matrices; therefore, transferring these methods to real-world monitoring programs, where contaminant concentrations are extremely low and matrices are highly variable, remains a challenge.

Studies reported in the literature show that activation steps allow tuning of biochar surface chemistry, increasing selectivity for specific classes of contaminants, such as pesticides and pharmaceuticals. In this context, combining biochar with MIPs has emerged as a particularly promising strategy, as the imprinted cavities provide recognition sites that substantially enhance selectivity while retaining the advantages of the carbonaceous support [104]. Magnetization further expands the applicability of biochar in sample preparation; magnetic biochar composites combine the high sorption capacity of the carbonaceous matrix with the ease of phase separation afforded by an external magnetic field, eliminating the need for centrifugation or filtration and enabling faster, simpler, and automatable workflows [103]. Miniaturized extraction techniques (such as MSPμE) are gaining momentum because they require only small amounts of sorbent and solvent, aligning with the principles of GAC [105]. Using agricultural and food processing residues as precursors is another rapidly growing trend. As highlighted throughout this review, agricultural residues represent the majority of the biomass sources investigated, reflecting a circular-economy logic in which abundant, renewable, low-cost waste materials are converted into high-value analytical sorbents [106]. This approach offers an economically attractive alternative to commercial sorbents. Furthermore, exploring novel and underutilized raw materials, combined with developing more energy-efficient production routes, is expected to further consolidate this trend. Finally, integrating biochar-based extraction with advanced detection and automation techniques is opening new possibilities. Coupling biochar-based sample preparation with high-resolution mass spectrometry results in methods with lower LODs and higher sample throughput.

In the future, establishing standardized production and characterization protocols is expected to be essential to ensure the reproducibility and comparability of biochar-based methods. Furthermore, the correlation between precursor type, production conditions, and surface properties and extraction performance should evolve into predictive models that guide the rational design of sorbents for specific applications. Another important issue is scaling up and validating biochar-based methods under real-world conditions. Demonstrating the robustness of these techniques in diverse environmental matrices (such as surface water, wastewater, soil, sediments, and biota) under realistic conditions and for extended periods will be crucial to consolidating their reliability. Developing certified reference materials and participating in interlaboratory studies would contribute significantly to this effort.

In addition, the sustainability and life cycle assessment of biochar-based sample preparation also deserve attention. Although biochar is generally considered a green and renewable sorbent, quantitative assessments of the energy consumed in its production, the solvents used in extraction, the sorbent mass required, the material’s reusability, and the waste generated remain scarce. At the production level, life cycle assessments of biochar systems have shown that the pyrolysis of residual biomass can be energetically favorable and can even result in net negative greenhouse gas emissions [107] which supports the renewable character of the biochar precursor. However, quantitative data for the analytical application of biochar are still emerging, and production-level results cannot be directly extrapolated to the sample preparation step. To render the sustainability comparison of extraction techniques objective, established greenness metrics, such as the AGREE and AGREEprep, can be employed [108,109]. These tools quantify exactly the parameters raised above, namely solvent type and volume, sorbent mass and reusability, energy demand, and waste generation. In the studies compiled in this review, the miniaturized and dispersive formats (MSPE, dSPE, SPME, RDSE and PT-SPE) typically require only a few milligrams of sorbent and solvent volumes in the range of a few hundred microliters to a few milliliters, whereas conventional SPE generally consumes tens of milliliters of solvent and higher sorbent amounts, resulting in proportionally greater waste generation. In addition, several biochar-based sorbents compiled in this review were successfully reused for multiple extraction cycles. A comprehensive evaluation of these factors, based on explicitly reported numerical values (solvent volumes, sorbent masses, number of reuse cycles, energy inputs and waste amounts), will demonstrate the sustainable nature of biochar-based methods compared to conventional sorbents.

6. Conclusions

This review provided a comprehensive overview of the recent advances in the use of biochar-based sorbents for the extraction and preconcentration of EOCs from environmental matrices. The literature analysis showed that agricultural residues are the most widely used feedstocks, with pyrolysis as the predominant production route, and that activation and surface functionalization strategies are essential to tailor sorbent properties to specific analytes. Biochar-based sorbents have been successfully applied in a wide range of sample preparation techniques, including SPE, MSPE, SPME, dSPE, PT-SPE, and RDSE, generally providing satisfactory recoveries, good precision, and low limits of quantification. The sorption of EOCs onto biochar is governed by multiple mechanisms, whose relative contribution depends on the physicochemical properties of both the sorbent and the target contaminants, including the biochar’s elemental composition.

Despite these advances, important limitations remain. The heterogeneity of biochar, arising from diverse feedstocks and production conditions, hinders the standardization and comparison of results across studies. Selectivity toward specific analytes in complex matrices is still challenging, and the long-term stability and reusability of biochar-based sorbents require further investigation. Moreover, most applications have been demonstrated at the laboratory scale, and validation under real-world conditions, as well as the scalability of biochar production, remains limited.

Future research should therefore focus on the development of standardized production protocols and on the rational design of biochar tailored to the properties of specific EOC classes, for instance through feedstock selection, pyrolysis conditions, and targeted functionalization. Finally, life cycle assessments and sustainability analyses are needed to consolidate biochar-based sample preparation as a green and economically viable alternative for routine environmental analysis.