Valorization of Corn Processing Waste as Adsorbents for Soil and Water Remediation: A Systematic and Comparative Review of Native Biomass, Hydrochar, and Biochar

Abstract

Corn processing waste represents an abundant, renewable, and low-cost lignocellulosic resource with considerable potential for environmental remediation applications. Large quantities of residues generated during corn processing, including cobs, husks, bran, and other by-products, are produced annually and can be utilized directly as native biomass or converted through thermochemical processes into hydrochars and biochars. This systematic review provides a comparative analysis of native corn processing biomass, hydrochars produced via hydrothermal carbonization, and biochars obtained through pyrolysis, with a focus on their potential as adsorbents for the removal of organic and inorganic pollutants from soil and water systems. Particular attention is given to the influence of thermochemical conversion processes on the physicochemical properties of the materials, including surface chemistry, porosity, functional groups, and structural characteristics, which govern adsorption mechanisms such as ion exchange, electrostatic interactions, surface complexation, hydrogen bonding, and π–π interactions. Furthermore, the advantages and limitations of each material type are discussed, together with key environmental and techno-economic considerations related to their production and practical application, including indicative production costs (USD per kg of adsorbent) and cost–performance relationships in terms of adsorption capacity. By linking biomass conversion processes, material properties, and adsorption performance, this review aims to provide a comprehensive overview of corn processing waste valorization and to support the development of sustainable adsorbent materials for soil and water remediation. A total of 36 studies were included in the qualitative synthesis following PRISMA guidelines.

1. Introduction

The contamination of soil and water by anthropogenic pollutants represents a critical challenge for environmental sustainability. Industrial effluents, agricultural runoff, and inadequately treated wastewater introduce heavy metals, pesticides, pharmaceuticals, and other persistent contaminants into the environment [1,2,3]. These pollutants are often chemically stable, toxic, and prone to bioaccumulation, posing risks to ecosystem health, crop productivity, and human safety. Addressing these challenges requires remediation strategies that are effective, sustainable, and aligned with circular economy principles.

Adsorption is widely recognized as an efficient and versatile method for pollutant removal due to its simplicity, low energy requirements, and adaptability to various contaminants [4,5,6,7]. Conventional adsorbents, such as coal-based activated carbons or synthetic resins, exhibit good performance but are often limited by high production costs and energy-intensive manufacturing processes. Consequently, there is growing interest in exploiting agro-industrial residues as carbon-rich feedstocks for sustainable adsorbent development [8,9,10,11,12,13,14].

Corn processing waste, such as straw or stover, stalks, cobs, husks, bran, silk and other residues generated during milling and other corn processing operations, represents an abundant, renewable, and low-cost lignocellulosic resource [15,16]. This biomass is rich in cellulose, hemicellulose, lignin, and inorganic constituents, and its surface functional groups, including hydroxyl, carboxyl, and phenolic, enable pollutant sequestration through ion exchange, surface complexation, hydrogen bonding, electrostatic interactions, and π–π interactions [15,16,17]. Such residues are widely available and cost-effective, making them particularly attractive for soil and water remediation applications.

Direct application of untreated corn processing biomass is often constrained by limited surface area, poor porosity, structural instability in aqueous environments, and the leaching of soluble organics [18]. To overcome these limitations, thermochemical conversion processes—including hydrothermal carbonization (HTC) and pyrolysis—have been increasingly employed. HTC performed in subcritical water at moderate temperatures (180–260 °C), facilitates dehydration and decarboxylation while preserving oxygen-containing functional groups, whereas pyrolysis at higher temperatures (>400 °C) promotes extensive carbonization, aromaticity, and micropore formation [19,20]. These processes modify the structural and surface properties of the biomass, enhancing adsorption performance, stability, and reusability.

Although individual hydrochars and biochars derived from corn processing waste have been studied for pollutant removal, few works systematically correlate thermochemical processing conditions with material properties and adsorption mechanisms [17,21]. In this context, bibliometric and visualization tools can provide valuable insights into the development of this research field and help identify key trends and knowledge gaps.

The literature selection process followed PRISMA guidelines, and the workflow is presented in Figure 1a. Additionally, keyword co-occurrence analysis using VOSviewer 1.6.20 (Figure 1b) was performed to identify research trends.

This review provides a comprehensive analysis of native corn processing biomass, hydrochars, and biochars, emphasizing the effects of thermochemical conversion on material characteristics, adsorption mechanisms, and practical applicability. Unlike previous studies, which typically focus on a single material type or isolated adsorption performance, this review provides a systematic comparative evaluation linking thermochemical processing routes with physicochemical properties and underlying adsorption mechanisms. Furthermore, a clear research gap is addressed by integrating insights across native biomass, hydrochar, and biochar systems, which are rarely discussed within a unified framework. Environmental feasibility, carbon yield, and techno-economic considerations are also discussed, highlighting the potential of corn processing waste valorization for sustainable soil and water remediation.

2. Methodology

This review was conducted in accordance with the PRISMA 2020 guidelines. A systematic literature search was performed using the Scopus database covering the period from 2021 to 2026. The search strategy included combinations of keywords such as (“corn” OR “maize”) AND (“biochar” OR “hydrochar”) AND (“adsorption” OR “pollutant removal”).

The inclusion criteria were: (i) experimental studies, (ii) use of corn-derived biomass, biochar, or hydrochar, and (iii) evaluation of adsorption performance for environmental remediation. The exclusion criteria included review articles, studies not related to adsorption processes, and studies not involving corn-based materials.

The literature selection process consisted of identification, screening, eligibility, and inclusion stages, as illustrated in Figure 1a. A total of 138 records were initially identified. After title and abstract screening, 74 records were excluded as irrelevant. The remaining 64 articles were assessed for eligibility through full-text analysis, of which 28 were excluded due to lack of experimental adsorption data or insufficient relevance. Finally, 36 studies were included in the qualitative synthesis.

Data extracted from the selected studies included adsorbent type, preparation method, physicochemical properties, type of pollutants, and adsorption performance.

A formal risk-of-bias assessment was not conducted, as this review focuses on qualitative comparison and trend analysis. The review protocol was not registered.

The completed PRISMA 2020 checklist is provided in the Supplementary Materials (Table S1) to ensure transparency and completeness of reporting [22]. The PRISMA flow diagram is presented in Figure 1a.

3. Corn Processing Waste: Origin, Composition, and Potential as a Sustainable Feedstock

Corn processing generates substantial quantities of lignocellulosic residues as by-products of agricultural harvesting and industrial processing operations, including milling, starch extraction, and bioethanol production. These residues comprise various structural components of the corn plant, such as straw (stover), stalks, cobs, husks, and other processing fractions generated during grain separation and refining [13,14].

Due to the large global production of corn, these residues represent one of the most abundant agricultural biomass resources. Traditionally, they are used for low-value applications such as animal bedding, combustion for energy, or are simply discarded in the field [23]. However, their lignocellulosic composition and inherent carbon content make them suitable precursors for the preparation of carbon-based materials through thermochemical conversion processes [24].

The physicochemical properties of corn residues depend strongly on the specific plant fraction and processing stage from which they originate. Differences in cellulose, hemicellulose, lignin, and inorganic components influence thermal behavior, carbon yield, and structural evolution during thermochemical treatments such as hydrothermal carbonization and pyrolysis [25,26]. Therefore, understanding the origin and composition of these residues is essential for selecting appropriate feedstocks and optimizing material synthesis routes.

3.1. Classification and Global Availability of Corn Processing Residue

Corn processing generates a diverse range of lignocellulosic residues, the characteristics of which depend on both the anatomical part of the plant and the specific industrial process involved. Among the most prominent are cobs, husks, bran, germ fractions, straw, and stalks [27]. Corn cobs are structurally rigid and rich in cellulose, with moderate lignin content, which confers stability and makes them well suited for the production of carbon materials. In contrast, corn husks contain relatively higher proportions of cellulose and hemicellulose but lower lignin levels, promoting thermal reactivity during thermochemical conversion and facilitating the development of porosity. Bran and germ residues, generated primarily during milling, are enriched in proteins, minerals, and minor lipid fractions, which can influence the surface chemistry and functionalization potential of the resulting carbon-based adsorbents. Straw and stalk residues are generally more fibrous and exhibit variability in their lignocellulosic composition, depending on the harvest and processing conditions [28].

These residues are produced in substantial quantities worldwide as by-products of food processing, animal feed production, and biofuel industries [13,14]. Their widespread availability and low economic value render them accessible and attractive for biomass valorization. However, differences in composition among the various residue types can significantly affect thermal degradation behavior, pore development, and mineral content in derived carbon materials. The main lignocellulosic constituents—cellulose, hemicellulose, and lignin—determine the fundamental pathways of thermal conversion and the physicochemical characteristics of the resulting adsorbents. Naturally occurring minerals, including calcium, potassium, and silica, contribute to ash formation and may impact pore structure during carbonization, highlighting the importance of residue selection in designing effective adsorbent materials.

3.2. Physico-Chemical Properties from Macromolecules to Functional Groups

The physicochemical properties of corn residues are largely dictated by their lignocellulosic composition, which governs structural integrity, surface chemistry, and reactivity [26]. Cellulose and hemicellulose constitute the carbohydrate-rich framework of the plant material, providing abundant hydroxyl and ether groups that contribute to surface polarity and offer potential binding sites for a wide range of contaminants [28]. Hemicellulose, being more thermally labile, decomposes at lower temperatures, promoting volatile formation and initiating pore development during thermochemical treatment. Lignin, on the other hand, is a highly aromatic polymer containing phenolic and methoxy groups; its thermal transformation leads to the formation of stable aromatic carbon domains, which can engage in π–π interactions and surface complexation with organic pollutants [29].

In addition to these organic components, corn residues naturally contain inorganic constituents such as calcium, potassium, and silica, which can further enhance pollutant sequestration through ion exchange or surface precipitation. The lignocellulosic composition of different corn processing residues varies considerably depending on the plant part, as summarized in

Table 1. Lignocellulosic composition of corn processing residues.

Feedstock	Cellulose (%)	Hemicellulose (%)	Lignin (%)	Reference
Corn cobs	40	28	12	[30]
Corn husks	36	24	13	[30]
Corn stalk	29–35	24–26	13–15	[31]
Corn stover	30–38	26	11–19	[32]
Corn stover	38.6	22.7	16.8	[33]
Corn leaves	40	28	20	[34]
Corn straw	42	29	21	[35]
Corn silk	42	24	22	[36]

. However, high ash content may obstruct pore formation and reduce the accessible surface area of the final material [15,18]. Thermal degradation of these residues occurs in distinct stages: hemicellulose typically decomposes around 200–300 °C, cellulose at 300–400 °C, and lignin over a broader temperature range due to its complex aromatic structure [30]. These sequential transformations influence char formation, porosity development, and the distribution of surface functional groups, ultimately shaping the adsorption performance and stability of the derived carbon materials.

The individual biopolymers in corn processing residues contribute differently to adsorption behavior and to their transformation during thermochemical conversion. Cellulose, as a crystalline polysaccharide rich in hydroxyl groups, primarily supports hydrogen bonding interactions and provides polar sites for the adsorption of hydrated metal ions and polar organic molecules, although accessibility of these sites is partially restricted in native biomass. Hemicellulose, due to its amorphous structure and lower thermal stability, decomposes at relatively low temperatures and facilitates the development of porosity and oxygen-containing functional groups, thereby improving adsorption accessibility after thermal treatment. Lignin, as an aromatic and highly recalcitrant polymer, serves as the main precursor for the formation of stable carbon frameworks and is primarily responsible for π–π interactions with aromatic organic pollutants. The combined and synergistic behavior of these three biopolymers governs the adsorption performance of corn-derived materials and strongly influences the evolution of hydrochars and biochars with distinct surface chemistry and pollutant affinity.

3.3. Native Biomass for Remediation: Key Advantages and Intrinsic Characteristics

Native corn processing residues can be directly applied as low-cost adsorbents due to the presence of oxygen-containing functional groups, including hydroxyl, carboxyl, and phenolic moieties [12]. These functional groups enable various interactions with pollutants, such as ion exchange, hydrogen bonding, and electrostatic attraction, allowing the removal of both organic and inorganic contaminants from environmental matrices [12,15]. In addition to organic functional groups, native corn processing residues also contain naturally occurring inorganic minerals such as calcium (Ca), potassium (K), magnesium (Mg), sodium (Na), and silicon (Si). These mineral constituents can significantly contribute to the adsorption of inorganic pollutants, particularly heavy metal ions, through mechanisms such as ion exchange, surface complexation, and precipitation. For example, calcium and magnesium sites may participate in cation exchange with toxic metal ions (e.g., Pb²⁺, Cd²⁺), while silicate and carbonate-related surface species can facilitate surface precipitation and immobilization of metals. Therefore, the inherent mineral composition of raw biomass plays an important synergistic role, alongside oxygen-containing functional groups, in governing the removal efficiency of inorganic contaminants. The direct use of untreated biomass also minimizes processing requirements and energy consumption, offering a simple and resource-efficient approach for pollutant removal. Despite these advantages, the practical application of native biomass is often limited by several intrinsic constraints. Raw corn residues typically exhibit relatively low specific surface area and poorly developed internal porosity, which restrict the number of available adsorption sites, particularly for larger organic molecules [14]. Furthermore, lignocellulosic materials may exhibit limited structural stability in aqueous environments, where swelling, microbial degradation, or gradual disintegration can occur over time. Another potential limitation involves the leaching of soluble organic compounds from untreated biomass, which may increase chemical oxygen demand in treated water and potentially lead to secondary contamination [37]. In addition, variations in surface charge and pH-dependent behavior can influence adsorption efficiency and complicate process optimization under different environmental conditions.

These limitations highlight the importance of thermochemical conversion processes, such as hydrothermal carbonization and pyrolysis, for improving the structural stability and adsorption properties of biomass-derived materials. Controlled thermal treatment promotes pore development, enhances structural integrity, and modifies surface chemistry, enabling the production of hydrochars and biochars with improved adsorption performance and greater applicability in soil and water remediation systems.

4. Hydrothermal Carbonization: Engineering Functional Hydrochars

Hydrothermal carbonization (HTC) has emerged as a promising thermochemical pathway for the valorization of corn processing residues into carbon-rich materials known as hydrochars. Compared to conventional pyrolysis, HTC operates in an aqueous environment at relatively moderate temperatures, typically between 180 and 260 °C, under autogenous pressure [16]. This processing environment eliminates the need for energy-intensive feedstock drying, making HTC particularly suitable for biomass streams with high intrinsic moisture content, such as residues generated during corn processing.

By accelerating reactions analogous to natural coalification processes, HTC converts the heterogeneous lignocellulosic structure of corn residues into a more homogeneous carbonaceous matrix enriched with functional surface groups. As shown in Figure 2, increasing HTC temperature from 175 to 200 °C promotes the transformation from a relatively compact structure to a more porous and fragmented morphology with visible microscale cavities, reflecting progressive biomass decomposition and structural reorganization [38]. The resulting hydrochars exhibit tunable physicochemical properties, which can be adjusted through careful control of process parameters such as temperature, residence time, and biomass-to-water ratio. These characteristics make hydrochars derived from corn processing waste promising candidates for adsorption-based remediation of contaminated soil and water.

4.1. Reaction Pathways and Process Dynamics

During HTC process, the conversion of corn processing biomass is governed by a sequence of hydrolysis, dehydration, decarboxylation, polymerization, and condensation reactions [39]. Initially, hemicellulose and cellulose components within corn residues undergo hydrolytic cleavage in subcritical water, producing soluble sugars and oligomeric intermediates. These intermediates subsequently undergo dehydration and fragmentation reactions, generating reactive compounds such as furfurals, hydroxymethylfurfural (HMF), and low-molecular organic acids [40].

Further polymerization and condensation of these intermediates leads to the formation of carbonaceous microspheres that gradually deposit onto the remaining solid biomass structure. This process increases the aromatic character of the material while simultaneously forming a surface enriched with oxygen-containing functional groups. The final physicochemical properties of the resulting hydrochar are highly dependent on operational conditions. Increasing reaction temperature or residence time generally promotes a higher degree of carbonization, resulting in lower atomic H/C and O/C ratios [41,42].

4.2. Structural and Surface Functionality

A surface rich in oxygen-containing functional groups, including hydroxyl, carbonyl, and carboxyl moieties, which originate primarily from the partial decomposition of cellulose and hemicellulose during HTC [41], typically characterize hydrochars produced from corn processing residues. Due to the relatively moderate processing temperatures employed in HTC, these groups remain preserved, in contrast to high-temperature pyrolysis biochars that have more condensed aromatic structures and fewer oxygenated functionalities [42]. The presence of these oxygenated groups enhances the material’s affinity for polar organic molecules and inorganic ions through mechanisms including hydrogen bonding, surface complexation, and electrostatic interactions [43,44].

Although the specific surface area (S_BET) of hydrochars is typically lower than that of high-temperature biochars, their adsorption performance can remain highly effective for polar organics and heavy metal ions due to the abundance of reactive oxygen functional groups and surface sites [45]. Additionally, inorganic constituents naturally present in corn residues, such as calcium, potassium, and silica, may remain partially incorporated into the hydrochar structure and contribute to pollutant removal through ion exchange reactions or surface precipitation mechanisms, particularly in the case of heavy metal ions [12,13,40].

4.3. Adsorption Performance and Dominant Mechanisms

The adsorption performance of corn-derived hydrochars is governed by the interplay of key physicochemical properties, including specific surface area (S_BET), pore morphology and distribution, surface charge (commonly expressed as p_HPZC-dependent behavior), and the nature and density of surface functional groups. A higher specific surface area and well-developed pore structure enhance accessibility of active sites and facilitate mass transfer of pollutants, while surface morphology further influences diffusion pathways and adsorption efficiency. Surface charge plays a crucial role in determining electrostatic interactions with ionic species and is strongly dependent on solution pH relative to the point of zero charge (pH_PZC). In addition, oxygen-containing functional groups such as hydroxyl, carboxyl, and carbonyl moieties provide active binding sites for both metal ions and organic molecules through ion exchange, complexation, and hydrogen bonding. The overall adsorption behavior results from the synergistic interaction of these structural and chemical characteristics, which can be tuned by thermochemical conversion conditions applied to corn processing residues. For inorganic pollutants, particularly heavy metal cations, adsorption is primarily driven by ion exchange and complexation with surface functional groups such as carboxyl and phenolic moieties [17]. For example, Pb²⁺ adsorption on hydro-pyrochars synthesized from corn cob has been shown to follow pseudo-second-order kinetics and Sips isotherms, indicating significant chemisorption contributions from surface functional groups and reactive sites [18].

In the case of organic pollutants, including dyes and other organics, adsorption may occur through hydrogen bonding, π–π interactions between aromatic structures, and electrostatic interactions depending on pollutant speciation and solution chemistry. Hydrochars derived from corn cob have been shown to efficiently remove methylene blue from aqueous solutions, with adsorption behavior well described by Langmuir isotherms and pseudo-second-order kinetics, reflecting the role of abundant oxygenated surface functional groups in binding organic molecules [46].

Environmental parameters such as pH, ionic strength, and competing ions can significantly influence adsorption performance, as changes in pH alter the protonation state of hydrochar surface functional groups, thereby modifying electrostatic attraction or repulsion toward charged pollutants. These aspects have been introduced at the beginning of Section 3.3 to provide a clearer framework for adsorption mechanisms.

4.4. Strategic Advantages and Operational Constraints

HTC proces offers several strategic advantages for the valorization corn processing waste. The carbonization of wet biomass in HTC does not require prior drying, which reduces overall energy input compared to high-temperature dry processes and makes this pathway particularly suitable for industrial residues with high moisture contents. HTC converts biomass into carbon-rich solids under moderate temperatures (180–260 °C) and water as reaction media, enabling energetic and cost-effective conversion of diverse lignocellulosic wastes into functional hydrochars [47]. Furthermore, hydrochars tend to retain abundant surface functional groups that are favorable for adsorption applications, while the aqueous, mild processing conditions limit the formation of gas emissions and reduce energy demand [44].

However, certain limitations remain. Hydrochars generally exhibit lower intrinsic porosity and specific surface area than biochars produced by high-temperature pyrolysis, which can limit their adsorption performance for certain contaminants without further activation [43]. Additionally, variations in the composition of corn processing residues—e.g., between cobs, husks, and bran—can influence the physicochemical properties of the hydrochar and thus its adsorption behavior, highlighting the importance of careful optimization of HTC process parameters (e.g., temperature, residence time, water/biomass ratio) to tailor hydrochar characteristics to particular applications [44].

When properly engineered through adjustment of reaction conditions and, if necessary, post-treatment activation, hydrochars derived from corn residues represent a viable intermediate in biomass valorization, bridging the gap between unprocessed agricultural waste and highly carbonized adsorbents such as activated biochars [48].

5. Pyrolysis: Engineered Biochars with High Aromaticity and Porosity

Pyrolysis is the most established thermochemical route for converting corn processing residues into stable, carbon-rich adsorbents known as biochars. In contrast to hydrothermal carbonization, pyrolysis is performed under oxygen-limited or inert atmospheres at significantly higher temperatures, typically ranging from 400 °C to over 900 °C [16,49]. This high-temperature, dry thermal treatment induces profound structural transformations within the lignocellulosic matrix of corn residues, leading to the evolution of bio-oil and syngas while leaving behind a solid carbon scaffold. The resulting biochars exhibit exceptional chemical recalcitrance and a highly developed pore network, which distinguishes them from both native biomass and hydrochars.

5.1. Thermal Decomposition and Pore Evolution and Surface Chemistry

The conversion of corn residues into biochar is governed by the thermal stability and decomposition behavior of their major macromolecular components. During pyrolysis (thermal treatment in the absence of oxygen), hemicellulose and cellulose decompose at relatively low temperatures (approximately 220–450 °C), while lignin degrades over a broader temperature range due to its complex aromatic structure, which ultimately forms the structural backbone of the resulting char. Devolatilisation—the release of volatile compounds—occurs sequentially, beginning with hemicellulose, followed by cellulose, and finally the slower decomposition of lignin. This staged degradation plays a crucial role in the formation of carbon-rich structures and the development of porosity within the biochar matrix [49,50].

As illustrated in Figure 3, the evolution of functional groups during pyrolysis involves dehydration, oxidation, and cyclization reactions that progressively transform oxygen- and nitrogen-containing groups (e.g., –OH, –COOH, –NH₂) into more stable aromatic and heterocyclic structures [50]. With increasing temperature, aliphatic carbon structures condense into polycyclic aromatic configurations, enhancing the aromaticity and structural stability of the biochar. At the same time, the release of volatile compounds and the breakdown of organic matter generate an interconnected system of micro-, meso-, and macropores, resulting in a highly developed pore network and increased specific surface area.

Higher pyrolysis temperatures are strongly associated with increased porosity and surface area due to the more extensive removal of volatile components and progressive carbonization. Consequently, the resulting biochar exhibits a predominantly microporous structure, which is particularly effective for adsorption processes [51]. Biochars derived from lignocellulosic residues such as corn can therefore achieve specific surface areas (S_BET) several orders of magnitude higher than those of the original biomass, significantly enhancing adsorption capacity through pore filling and surface interactions [52,53].

The FTIR spectra presented in Figure 4 provide direct evidence of the functional groups initially present in native corn residues and their transformation during pyrolysis. The raw corn-cob feedstock exhibits characteristic absorption bands corresponding to hydroxyl (–OH, ~3400 cm⁻¹), aliphatic C–H (~2925 cm⁻¹), and oxygen-containing functional groups such as carbonyl and carboxyl moieties (C=O and C–O vibrations in the range of 1705–1050 cm⁻¹), reflecting the lignocellulosic nature of the biomass. Following pyrolysis at 500 °C, a marked reduction in the intensity of –OH and aliphatic C–H bands is observed, indicating dehydration, demethoxylation, and devolatilization processes. At the same time, the relative increase in aromatic C=C vibrations (~1586 cm⁻¹) and out-of-plane aromatic C–H deformation bands (876 and 820 cm⁻¹) confirms the progressive formation of condensed aromatic structures. The persistence of low-intensity bands associated with C=O and C–O groups suggests the presence of residual oxygen functionalities in partially carbonized domains [54]. These spectral changes are consistent with the transformation pathways illustrated in Figure 3, where labile oxygenated functional groups are progressively eliminated and replaced by more stable aromatic and heterocyclic carbon structures. In addition, the presence of mineral-related bands (e.g., CO₃²⁻ and Si–O vibrations) highlights the role of the inorganic fraction in shaping the surface chemistry of the resulting biochar.

In parallel, these structural transformations are accompanied by changes in surface chemistry. The progressive loss of oxygen-containing functional groups and the increase in aromatic carbon content lead to greater hydrophobicity and surface alkalinity [54]. Lower-temperature biochars retain functional groups such as hydroxyl and carboxyl moieties, whereas higher-temperature treatment promotes the formation of graphene-like and turbostratic carbon structures, improving the adsorption of non-polar organic contaminants [56]. As also indicated in Figure 3, these chemical transformations directly influence key surface properties such as wettability, surface charge, and the availability of acidic functional sites [50].

Additionally, the inorganic fraction of corn residues—including silica, calcium, and potassium—becomes concentrated in the biochar as ash. These mineral components influence surface pH, provide additional binding sites, and facilitate the immobilization of heavy metals through precipitation and complexation mechanisms. Together with the developed pore structure and aromatic carbon framework, these properties make corn-derived biochar an efficient adsorbent for a wide range of organic and inorganic pollutants [57].

5.2. Mechanisms of Pollutant Sequestration

Overall, corn-derived biochars provide a versatile platform capable of simultaneously removing both hydrophobic organics and heavy metals, with performance largely dictated by pore architecture, surface chemistry, and mineral content.

The high adsorption capacity of biochars derived from corn processing residues arises from the interplay between their hierarchical pore structure, highly aromatic carbon matrix, and residual mineral content. Organic pollutants, such as dyes, pesticides, and pharmaceutical residues, are effectively captured through π–π electron donor–acceptor interactions with the graphene-like surfaces of the biochar, while the microporous network physically entraps larger molecules [56,58]. Inorganic contaminants, particularly heavy metal cations, are sequestered via surface complexation with remaining functional groups, electrostatic interactions, and coordination or precipitation with mineral residues inherited from the corn feedstock [57,59]. The efficiency of these processes is strongly modulated by environmental factors, including pH, ionic strength, and competing ions, which affect surface charge, protonation states, and accessibility of active sites.

5.3. Stability and Strategic Application

One of the key advantages of pyrolytic biochars is their exceptional chemical and biological stability. The highly aromatic and recalcitrant carbon framework resists microbial degradation, making these materials suitable for long-term environmental applications, such as in situ soil remediation and persistent pollutant stabilization [57,60,61].

However, the process requires high energy input and feedstock drying, which may increase production costs compared to hydrothermal carbonization. In practice, hydrochars derived from corn residues are optimized for polar and functional-group-driven adsorption, whereas pyrolytic biochars excel in applications requiring high surface area, aromaticity, and long-term stability, particularly for hydrophobic organics and persistent contaminants [22,62].

6. Comparative Analysis of Adsorption Performance and Mechanisms

The valorization of corn processing residues into adsorbents involves a progressive enhancement of adsorption performance from native biomass to hydrochar and biochar, reflecting structural and chemical transformations during thermochemical conversion. While native corn residues rely on the inherent reactivity of lignocellulosic components, hydrochars and biochars provide tunable properties tailored for specific classes of environmental pollutants. Understanding these differences is essential for selecting the most suitable adsorbent for soil and water remediation applications. The adsorption performance of corn-derived materials is governed by a combination of interdependent physicochemical parameters, including specific surface area (S_BET), pore morphology, surface charge (commonly expressed as pH_PZC), and surface functional groups. These properties collectively determine the accessibility and density of active sites and control the dominant adsorption pathways. Higher surface area and well-developed porosity enhance pore-filling mechanisms, while surface functionality (e.g., hydroxyl, carboxyl, and phenolic groups) governs hydrogen bonding, ion exchange, and surface complexation processes. In addition, surface charge plays a critical role in electrostatic interactions, particularly for ionic pollutants, and is strongly dependent on solution pH relative to the pH_PZC of the adsorbent. The interplay between these parameters varies significantly among native biomass, hydrochar, and biochar, leading to distinct adsorption behaviors and pollutant selectivities across different thermochemical conversion routes.

To strengthen the mechanistic interpretation of adsorption behavior, FTIR (vibrational spectroscopy) data reported in the literature for native biomass, hydrochar, and biochar derived from corn residues provide key insights into surface functional group evolution. Native biomass typically exhibits broad bands corresponding to hydroxyl (–OH) stretching (~3200–3600 cm⁻¹), C–H stretching (~2900 cm⁻¹), and carbonyl/carboxyl vibrations (~1700 cm⁻¹), confirming the abundance of oxygenated functional groups associated with cellulose, hemicellulose, and lignin. After hydrothermal carbonization, these bands are partially retained, particularly oxygen-containing groups, indicating preservation of polar functionalities responsible for hydrogen bonding and ion exchange mechanisms. In contrast, pyrolysis-derived biochars show a marked decrease in O–H and C=O bands and an increase in aromatic C=C vibrations (~1580–1620 cm⁻¹), reflecting aromatization and carbon condensation, which supports π–π interactions and hydrophobic adsorption mechanisms. These FTIR-detected transformations directly correlate with the shift in adsorption mechanisms from functionality-driven (hydrogen bonding and complexation in hydrochar) to structure-driven processes (π–π interaction and pore filling in biochar). These spectral changes provide direct experimental evidence supporting the evolution of adsorption mechanisms across thermochemical conversion stages.

The adsorption performance of native corn biomass, hydrochar, and biochar is fundamentally governed by three key interrelated parameters: S_BET, pH_PZC, and surface functional group composition. Native biomass exhibits very low surface area and limited porosity, resulting in restricted access to adsorption sites despite the presence of oxygen-containing functional groups such as hydroxyl, carboxyl, and phenolic moieties. Hydrothermal carbonization significantly increases surface reactivity while maintaining moderate surface area and a relatively oxygen-rich surface, leading to lower pH_PZC values and enhanced affinity for cationic species through ion exchange and surface complexation. In contrast, pyrolysis-derived biochars exhibit substantially higher surface areas and well-developed porous structures, but show reduced oxygenated functional group density and higher pH_PZC values, resulting in more hydrophobic and electron-rich surfaces. These changes shift the dominant adsorption mechanisms from functionality-driven processes in hydrochars (ion exchange, hydrogen bonding, complexation) to structure-driven mechanisms in biochars (pore filling, π–π interactions, and hydrophobic partitioning). Therefore, the combined variation in BET surface area, pH_PZC, and surface chemistry provides a unified explanation for the observed differences in adsorption behavior across corn-derived carbon materials.

6.1. Sequestration of Inorganic Pollutants: Heavy Metals and Metalloids

The removal of cationic heavy metals (e.g., Pb, Cd, Cu) by corn cob-derived adsorbents depends on the availability of reactive sites generated during processing, such as oxygen-containing functional groups formed during hydrothermal carbonization or pyrolysis, as well as surface charge and mineral content [18].

Native corn residues contain carboxylic, hydroxyl, and phenolic groups that allow moderate metal binding, but their low structural stability limits practical applications [12]. On the other hand, hydrochars derived from corn residues generally outperform both native biomass and pyrolytic biochars for inorganic cations. HTC at moderate temperatures preserves oxygen-containing functional groups, enabling surface complexation, ion exchange, and pH-dependent adsorption, making these materials particularly effective for heavy metal ions (Figure 5) [18,62,63,64]. Pyrolytic corn cob biochars exhibit higher surface area, porosity, and long-term stability but lose a significant fraction of oxygenated groups at high temperatures. Metal sequestration is largely governed by surface complexation with residual functional groups, precipitation with mineral residues (e.g., Ca, K, Si), and cation–π interactions with aromatic carbon domains. Chemical activation may be applied to further enhance cation adsorption [64,65,66].

The efficiency of these mechanisms is strongly influenced by environmental conditions, including pH, ionic strength, and competing ions, which modulate the protonation state of surface groups, surface charge, and availability of active binding sites. Overall, corn-derived hydrochars and pyrochars provide complementary strategies: hydrochars are optimal for heavy metals through functional-group-driven adsorption, while pyrochars are suited for persistent contaminants and metals in long-term applications due to their stability, porosity, and mineral-mediated sequestration. A schematic comparison of the dominant adsorption mechanisms and structural characteristics of corn-derived hydrochar and biochar is presented in Figure 5.

The removal of cationic heavy metals is primarily governed by the abundance of oxygen-containing functional groups and the ion exchange capacity of the adsorbent. Native corn residues contain carboxylic, hydroxyl, and phenolic groups that enable moderate metal uptake; however, their practical application is limited by low structural stability in aqueous systems. Hydrochars generally exhibit superior performance compared to both native biomass and pyrolytic biochars for the removal of inorganic cations. This behavior is attributed to hydrothermal carbonization at moderate temperatures, which preserves oxygenated functional groups and promotes adsorption mechanisms such as surface complexation and ion exchange. In contrast, pyrolytic biochars typically possess higher surface area and porosity but undergo a significant loss of oxygen-containing functional groups during high-temperature treatment. As a result, heavy metal removal by biochars is more often governed by mineral precipitation involving residual ash components (e.g., Ca, K, Si) and surface complexation rather than ion exchange. In some cases, additional chemical activation is required to further enhance their adsorption capacity for cationic species.

6.2. Sequestration of Organic Pollutants: Dyes, Pesticides, and Pharmaceuticals

The adsorption efficiency of organic contaminants on corn-derived materials is strongly dictated by the physicochemical properties of the adsorbent, including surface area, pore architecture, surface chemistry, and the organization of the carbon matrix, all of which collectively determine the accessibility of active sites and the extent of mass transfer from the aqueous phase to the sorbent surface. Native corn biomass exhibits limited adsorption performance due to its compact lignocellulosic structure, low specific surface area, and poorly developed porosity, which restrict intraparticle diffusion and limit the accessibility of internal sorption sites. Although some oxygen-containing functional groups are inherently present, their contribution to adsorption is limited because many remain embedded within the cellulose–hemicellulose–lignin matrix, rendering them partially inaccessible to dissolved contaminants [12,67].

HTC converts corn cob into hydrochar while preserving a significant fraction of surface oxygenated functional groups, including hydroxyl (–OH), carbonyl (C=O), and carboxyl (–COOH) moieties [18,44]. Adsorption on hydrochar is therefore primarily governed by specific polar interactions, notably hydrogen bonding and dipole–dipole interactions. Hydrogen bonds form between proton-donating or proton-accepting groups on the hydrochar surface and complementary functional groups in organic molecules (e.g., –NH, –OH, –C=O), while dipole–dipole interactions contribute via electrostatic attraction between polar surface functionalities and polar regions of contaminants [43,45]. Residual aliphatic domains may further contribute weak hydrophobic interactions and van der Waals forces, but the moderate degree of carbonization limits extended π-conjugated structures, reducing strong π–π interactions with highly aromatic compounds. As a result, hydrochars are particularly effective for polar and moderately polar organic pollutants (Figure 6).

In contrast, high-temperature pyrolysis produces corn-derived biochar with enhanced aromatic condensation, increased carbon ordering, reduced surface polarity, and a well-developed micro- and mesoporous network. These structural features favor π–π electron donor–acceptor interactions between delocalized aromatic domains of the biochar surface and aromatic pollutants, while expanded porosity facilitates pore-filling and intraparticle diffusion, critical for larger organic molecules. Hydrophobic interactions between nonpolar domains of the biochar and hydrophobic contaminants further strengthen adsorption, enabling efficient retention of dyes, selected pesticides, and pharmaceutical residues [55,68].

Overall, the thermal conversion pathway dictates the dominant adsorption mechanisms of corn-derived materials. HTC yields a functionality-driven sorbent, where hydrogen bonding and dipolar interactions dominate, whereas high-temperature pyrolysis produces a structure-driven sorbent, in which aromaticity, π–π interactions, hydrophobic partitioning, and pore architecture collectively govern organic contaminant removal [69,70]. This mechanistic differentiation allows for the tailored application of hydrochar and biochar in the remediation of pollutants with distinct polarity and aromaticity profiles, complementing their respective strengths in targeted adsorption.

6.3. Integrated Process–Property–Performance Framework and Scope Limitations

While the previous sections discuss native biomass, hydrothermal carbonization (HTC), and pyrolysis-derived biochars separately in terms of structure, properties, and adsorption performance, a more integrated, process-oriented comparison is essential for evaluating their practical applicability. In this context, native corn residues, hydrochars, and biochars represent consecutive stages in biomass valorization, each characterized by distinct processing requirements, physicochemical properties, and adsorption behaviors.

To address this, a comparative analysis linking key process parameters (e.g., temperature, reaction medium, residence time, and energy demand) with resulting material properties, adsorption mechanisms, and application potential is presented in

Table 2. Integrated comparison of native corn biomass, hydrochar, and biochar: process parameters, properties, and application potential.

Parameter	Native Biomass	Hydrochar (HTC)	Biochar (Pyrolysis)
Processing	None (raw material)	Hydrothermal carbonization	Pyrolysis (thermal decomposition)
Reaction medium	–	Aqueous (subcritical water)	Dry, inert atmosphere
Temperature range	Ambient	180–260 °C	400–900 °C
Pressure	–	Autogenous	Atmospheric/inert
Feedstock condition	Wet/dry	Wet biomass preferred	Dry biomass required
Energy demand	Very low	Moderate	High
Residence time	–	Hours	Minutes–hours
Solid yield	–	High	Moderate
Surface area (SBET)	Very low	Low–moderate	High
Porosity	Poorly developed	Limited	Highly developed
Surface functionality	High (oxygen-rich)	Very high (oxygen-rich)	Low (aromatic carbon-rich)
Dominant mechanisms	Ion exchange, H-bonding	Complexation, ion exchange	π–π interactions, pore filling
pH dependence	Low (limited control)	Strong influence on adsorption and surface charge	Strong influence via pHPZC and surface charge
Target pollutants	Heavy metals (limited efficiency)	Heavy metals, polar organics	Hydrophobic organics, persistent pollutants
Structural stability	Low	Moderate	Very high
Regeneration potential	Low	Moderate	High
Need for activation	Not applicable	Often required	Sometimes required
Co-products	–	Process water	Bio-oil, syngas

. This integrated approach provides a clearer framework for selecting the most appropriate material depending on pollutant type, operational conditions, and scale-up considerations.

This comparison highlights that native biomass, hydrochar, and biochar should not be viewed as competing materials, but rather as complementary stages within a unified biomass valorization framework. The selection of the optimal material depends on the balance between process intensity, desired surface functionality, structural properties, and the nature of the target pollutants, thereby enabling more rational design of sustainable remediation strategies.

It should be noted that the adsorption performance data and mechanistic interpretations discussed in this section are primarily derived from laboratory-scale batch adsorption studies, which represent the most commonly reported experimental approach in the literature [71,72]. In comparison, fewer studies have investigated more application-relevant systems such as continuous-flow (column) experiments, soil incubation tests, regeneration and reuse cycles, or real wastewater and field-scale applications. Therefore, the comparative conclusions presented herein mainly reflect model-system behavior under controlled conditions. The interpretation of hydrochar and biochar performance should thus be considered within the context of the specific experimental design, as adsorption efficiency may vary significantly depending on system complexity and operating conditions.

6.4. Influence of Environmental Factors

The adsorption performance of corn residue-derived materials is strongly governed by environmental conditions, particularly solution pH, ionic strength, and the presence of competing ions. Solution pH plays a decisive role in controlling both pollutant speciation and adsorbent surface charge through its relationship with the point of zero charge (pH_PZC). When the solution pH exceeds the pH_PZC, biochar and hydrochar surfaces become predominantly negatively charged, thereby enhancing the electrostatic attraction of cationic species such as heavy metal ions [18,73]. Conversely, under acidic conditions, protonation of surface functional groups may reduce cation adsorption and, at the same time, promote the removal of anionic contaminants through surface charge reversal and modified surface interactions [74].

In addition to pH effects, ionic strength significantly influences adsorption efficiency by modulating electrostatic interactions and compressing the electrical double layer at the adsorbent–solution interface. Increased ionic strength can either suppress adsorption of charged species through competition for active sites or alter surface complexation equilibria, depending on the contaminant chemistry and adsorbent structure [75]. Furthermore, coexisting ions may compete with target pollutants for binding sites, particularly in systems where adsorption is governed by ion exchange and surface complexation mechanisms [73]. These interactions highlight the necessity of tailoring corn residue-derived adsorbents to specific pollutant classes and site-specific water chemistry to ensure optimal performance under realistic environmental conditions.

6.5. Emerging Strategies: Surface Functionalization and Tailored Composites

While the intrinsic properties of native biomass, hydrochars, and biochars offer significant remediation potential, recent advancements in materials science have shifted toward the development of “designer” or engineered adsorbents. The primary objective of these modifications is to overcome the inherent limitations of raw thermochemical products, such as the low porosity of hydrochars or the diminished surface functionality of high-temperature biochars [18,70,73].

Surface activation strategies are generally categorized into physical and chemical pathways. Physical activation involves the use of oxidizing agents like steam or CO₂ at elevated temperatures to etch the carbon scaffold, thereby drastically increasing the specific surface area and pore volume. Chemical activation, utilizing agents such as KOH, ZnCl₂, or H₃PO₄, facilitates the simultaneous carbonization and development of a microporous network, often resulting in adsorbents that rival commercial activated carbons [18,72].

Beyond pore engineering, the introduction of specific chemical moieties through heteroatom doping (e.g., N, S, or P) or the fabrication of magnetic composites (e.g., impregnation with Fe₃O₄ nanoparticles) has emerged as a high-performance frontier. Nitrogen doping, for instance, enhances the electron density of the carbon framework, significantly boosting π–π interactions for organic micropollutants. Meanwhile, the development of magnetic carbonaceous composites addresses the critical challenge of adsorbent recovery in large-scale water treatment, allowing for efficient separation via external magnetic [18,73]. These sophisticated modification routes enable the rational design of agro-industrial-derived materials that are not only sustainable but also highly selective for recalcitrant environmental contaminants.

7. Applications in Water Treatment and Soil Remediation

The practical implementation of corn processing residue-derived adsorbents requires consideration of the complexity of natural and engineered matrices. The performance of native biomass, hydrochars, and pyrolytic biochars is influenced not only by intrinsic adsorption capacity but also by long-term stability, interaction with the environment, and regenerability, which are critical for sustainable remediation strategies. Corn residue-derived materials have shown versatility for both aquatic and terrestrial applications, with their efficacy strongly dependent on feedstock composition, thermochemical conversion method, and surface properties.

Strategic Implementation in Aquatic Systems

Corn-derived adsorbents are employed across a range of configurations for wastewater treatment and in situ soil remediation, targeting both inorganic and organic contaminants.

Hydrochars from corn cobs or husks, rich in oxygenated functional groups, are highly effective for tertiary treatment of effluents containing heavy metals and polar organic micropollutants (

Table 3. Corn Cob-Derived Adsorbents for Water Treatment (Metals and Organic Pollutants).

Adsorbent	Metal	qmax (mg/g)	Reference
Corn cob	Pb	5.95	[21]
Corn silk	Pb	84.2	[15]
Corn stalk BC *	Pb	54.73	[76]
Corn straw	Pb	56.91	[77]
Corn stalk BC *	Cu	172.41	[78]
Corn stower Si-Mn-modified BC *	Cu	167.88	[79]
Corn cob	Cu	2.62	[80]
Corn silk	Cu	14.42	[16]
Corn silk	Zn	12.56	[16]
Corn cob	Zn	1.23	[80]
H3PO4 CM *** corn cob	Zn	79.21	[81]
Corn silk	Cd	21.96	[18]
KOH CM *** corn silk	Cd	49.06	[18]
Corn cob	Ni	7.5	[82]
Polyethylene imine CM *** corn cob HC **	Ni	29.06	[83]
Zn-Al-LDH/BC * composite	As(V)	16.1	[84]
Fe-Mn-La/corn steam BC * composite	As(III)	15.34	[85]
CM corn stalk BC *	Hg	268.45	[86]
Corn stalk/diatomite gel porous material	Methylene blue	657.89	[87]
H3PO4-modified corn stalks	Methylene blue	129	[88]
Corn stower HC **	Rhdamine B	51.6	[35]
Corn silk	Reactive red	30.7	[89]
* BC biochar
** HC hydrochar
*** CM chemically modified

).

The surface chemistry allows rapid complexation and ion exchange, making them suitable for high-flow treatment conditions. Pyrolytic biochars, with a highly aromatic and porous framework, serve as a sustainable alternative to commercial activated carbons for persistent organic pollutants, such as pharmaceuticals, endocrine disruptors, and aromatic pesticides [90]. Their combination of pore-filling mechanisms and π–π electron donor–acceptor interactions ensures retention of hydrophobic compounds that may escape conventional treatments. Additionally, the recalcitrant aromatic carbon matrix of biochars provides superior mechanical integrity and structural stability under cyclic use compared to hydrochars and native biomass.

Corn residue-derived biochars are particularly valuable for in situ stabilization of soils (

Table 4. Corn Cob-Derived Adsorbents for Soil Remediation (Metals and Organic Pollutants).

Adsorbent	Pollutant Type	Matrix	Key Findings	Reference
Corn cob biochar (400–600 °C) + apatite	Pb, Zn	Contaminated soil	Significant reduction in exchangeable Pb and Zn fractions; transformation to stable fractions (Tessier method)	[93]
Corn cob biochar (300–500 °C), Fe-Mn	F−	Groundwater system	Enhanced fluoride adsorption via surface modification and increased reactivity	[94]
Corn cob biochar (700 °C)	PAHs (naphthalene, fluorene, pyrene, fluoranthene)	Aqueous system (soil-relevant contaminant model)	High adsorption capacity; π–π interactions and pore filling dominant	[55]
Corn cob biochar (400 °C)	Soil structure improvement	Sandy loam soil	Increased water retention and microporosity; improved soil physical properties	[95]
Corn cob biochar (300–400 °C)	Soil fertility under drought	Agricultural soil	Increased aggregate stability, microbial biomass C and N, improved soil quality	[96]
Corn cob biochar (incubation study)	Pb, Zn	Multi-metal contaminated soil	Decreased bioavailable fractions by up to 26–33% when combined with amendments	[97]
Corn cob biochar	Cd, As	Yellow and cinnamon soils	Transformation of labile fractions into residual fraction; reduced leaching	[98]
Corn cob biochar (heat-treated/washed)	Phytotoxicity assessment	Soil application	Washing significantly reduced phytotoxic effects; improved germination	[99]

). Their microporous architecture and aromatic carbon framework enable long-term sequestration of hydrophobic organics and certain heavy metals/metalloids, while also providing microhabitats for indigenous microbes to facilitate coupled adsorption–biodegradation pathways. Hydrochars offer rapid immobilization of cationic pollutants through high ion-exchange capacities but are more susceptible to mineralization and gradual re-release under environmental conditions. The choice between hydrochar and biochar should consider both pollutant type and remediation timescale. Beyond pollutant removal, these materials support circular economy goals by improving soil water retention, nutrient cycling, and overall ecosystem services [91,92].

Corn-derived adsorbents with heterogeneous surface chemistry are capable of simultaneously sequestering metals and organic pollutants, making them suitable for multi-contaminant systems. Mineral ash promotes precipitation of metals, while aromatic carbon domains enable π–π interactions with organics. The balance between surface functionality and porosity, dictated by feedstock type and thermochemical conversion, determines the adsorbent’s efficacy in complex matrices, supporting the engineering of tailored, cost-effective, and sustainable remediation strategies.

8. Techno-Economic and Environmental Considerations

The large-scale implementation of adsorbents derived from corn processing residues necessitates a thorough assessment of economic viability, energy requirements, and environmental sustainability. To compete with commercial activated carbons, a comprehensive evaluation encompassing production costs, life-cycle impacts, and integration into the circular economy is essential.

To ensure transparency of the techno-economic and environmental assessment, it is important to clarify the system boundaries and underlying assumptions adopted in this analysis. The evaluation presented in this section primarily considers the core production chain, including feedstock availability and transport, thermochemical conversion (HTC or pyrolysis), and basic downstream handling of the produced adsorbents. Energy demand, production cost, and carbon footprint estimates are therefore based on these system boundaries.

However, certain process-specific aspects are not explicitly quantified in the comparative metrics, including large-scale logistics optimization, detailed reactor design variations, industrial heat integration schemes, long-term adsorbent regeneration infrastructure, and advanced treatment of HTC process water or liquid by-products. While these factors are discussed qualitatively where relevant, their full integration into life-cycle or techno-economic modeling is beyond the scope of this review. This distinction is essential for interpreting the reported cost and environmental indicators as indicative rather than absolute values, and for avoiding overgeneralization when translating laboratory and literature data to industrial scale applications.

Corn processing residues, including cobs, husks, and bran, are often available at low or even negative cost. Their utilization reduces expenses related to waste handling, transport, and disposal. This creates a strong economic incentive for their valorization into adsorbents. However, this advantage is strongly dependent on local logistics and availability of waste streams, which may limit scalability in certain regions. Moreover, the overall production cost depends on the selected thermochemical conversion pathway, as different processes exhibit varying energy demands and operational requirements. Key factors include energy input, process conditions, and the potential for co-product recovery. HTC is particularly cost-effective for high-moisture feedstocks. It processes biomass in the aqueous phase and eliminates the need for energy-intensive pre-drying. HTC operates at relatively moderate temperatures (180–260 °C) and generally requires lower thermal energy input. This simplifies process integration for wet residues [61,100,101,102]. In contrast, pyrolysis requires prior drying of the feedstock and operates at higher temperatures (>500 °C). These conditions increase energy demand and operational costs. However, part of these costs can be offset through the valorization of co-products such as syngas and bio-oil. These streams can be used for heat and power generation or upgraded into value-added chemicals [44,48,103].

The long-term economic performance is strongly influenced by adsorbent stability and reusability, as these parameters directly determine replacement frequency and overall operational costs. Biochar derived from corn residues maintain structural integrity over multiple adsorption–desorption cycles due to their highly carbonized, aromatic framework and stable pore structure formed at elevated pyrolysis temperatures. This structural robustness enables effective regeneration with minimal loss of adsorption capacity, thereby extending service life and reducing material replacement costs [104]. In contrast, hydrochars and native biomass are less carbonized and contain a higher proportion of oxygen-containing functional groups and labile carbon. While this composition can enhance initial adsorption performance, it also makes these materials more susceptible to structural degradation during regeneration. Common issues include pore collapse, surface oxidation, and mass loss, particularly under thermal or chemical treatment conditions. Consequently, these adsorbents exhibit a faster decline in adsorption efficiency and shorter operational lifetimes. This necessitates more frequent replacement, which offsets their lower initial production cost and negatively affects overall lifecycle economics [105].

Utilizing corn waste as adsorbents contributes to carbon sequestration and mitigates greenhouse gas emissions. Biochar act as long-term carbon sinks due to their highly aromatic, recalcitrant structure, potentially storing carbon in soils for decades to centuries [106]. Hydrochars also demonstrate favorable environmental profiles, efficiently converting waste to a resource with low gaseous emissions. However, their aqueous process byproducts contain organic acids and nutrients that require proper management (e.g., anaerobic digestion) to avoid secondary pollution [40,107].

Compared to coal-based activated carbons, all corn residue-derived adsorbents present substantially lower carbon footprints and eliminate the need for destructive mining and high-energy activation. Selecting between HTC and pyrolysis should consider feedstock moisture content, local energy infrastructure, and intended application, balancing both environmental and economic factors (

Table 5. Comparative techno-economic and environmental metrics of adsorbent materials.

Material	Energy Intensity	Pre-Treatment	Estimated Cost (USD/kg)	Carbon Footprint
Native Corn Biomass	Minimal	Drying	0.02–0.1	Neutral
Hydrochar (HTC 180–260 °C)	Moderate	None (Wet)	0.2–0.5	Negative (C-sequestration)
Biochar (Pyrolysis 500–700 °C)	High	Drying	0.3–0.8	Highly Negative (Carbon Sink)

).

Integrating corn residue-derived adsorbents into a circular economy transforms remediation from a linear “take-make-dispose” model into a closed-loop system. Spent adsorbents can be repurposed as soil amendments, particularly after recovery of heavy metals or nutrients, provided pollutant concentrations comply with safety thresholds. This approach synergistically links agro-waste management, water treatment, and soil enhancement, improving resource efficiency and supporting sustainable agriculture.

Despite the promising techno-economic and environmental advantages of corn residue-derived adsorbents, several limitations must be critically considered. While hydrochars offer lower energy requirements and cost advantages, their relatively lower thermal stability and limited structural robustness may restrict long-term performance in repeated adsorption–desorption cycles. Conversely, biochars exhibit superior stability and reusability; however, their higher production temperatures result in increased energy consumption and operational costs. Moreover, the variability of feedstock composition can significantly affect the reproducibility and consistency of adsorbent properties, which remains a challenge for large-scale implementation. Therefore, the selection of the most suitable material should be application-specific, requiring a balanced assessment of cost, performance, and environmental impact rather than relying on a single dominant criterion.

9. Conclusions and Future Perspectives

The valorization of corn processing residues into adsorbents represents a transformative approach to sustainable environmental remediation, bridging the gap between agricultural waste management and pollutant sequestration. This review demonstrates that the strategic selection of thermochemical conversion pathways—from native biomass to hydrochars and pyrolytic biochars—enables the design of materials with tailored properties suitable for specific soil and water remediation challenges.

9.1. Concluding Remarks

Native corn biomass provides a low-cost and readily available adsorbent; however, its limited surface area, low porosity, and structural instability restrict its practical application. Nevertheless, it serves as a valuable precursor for both hydrochars and biochars. Hydrothermal carbonization efficiently transforms high-moisture corn residues into hydrochars enriched with oxygenated functional groups, making them highly effective for the removal of heavy metals and polar contaminants in aqueous systems. Pyrolysis generates highly aromatic, microporous biochars that are particularly effective in sequestering persistent organic pollutants through π–π interactions and pore-filling mechanisms, while also providing long-term structural stability suitable for soil amendment applications.

The choice of adsorbent material should therefore be guided not solely by general performance metrics but by a careful alignment between the physicochemical properties of the adsorbent, the nature of the target pollutants, and the specific environmental conditions of application.

9.2. Future Research Directions

Despite the considerable progress in the development of corn-derived adsorbents, several challenges remain for their practical implementation at larger scales. One of the key issues is the long-term stability of hydrochars and biochars under realistic environmental conditions. Their aging behavior in complex soil and aqueous systems, as well as the potential release of previously adsorbed pollutants under changing pH, redox conditions, and microbial activity, requires further systematic investigation.

Another important aspect is the performance of these materials in multi-component systems. Real effluents typically contain mixtures of heavy metals, organic pollutants, and natural organic matter, which may compete for active sites and reduce adsorption efficiency. Therefore, future studies should focus on the development of tailored corn residue-based adsorbents with improved selectivity and performance in complex matrices.

The management of hydrothermal carbonization process water also represents an important research direction. This byproduct contains dissolved organic compounds and nutrients, and its proper treatment or reuse—for example, in agriculture or energy recovery—could significantly enhance the overall sustainability of the process.

In addition, variability in corn processing residues can affect the reproducibility of adsorbent properties and performance. The establishment of standardized protocols for feedstock characterization and thermochemical processing is therefore essential for ensuring consistent material quality and enabling scale-up.

Finally, the integration of these materials into circular bioeconomy concepts should be further explored. The reuse of spent adsorbents as soil amendments offers opportunities for nutrient recycling and soil improvement, contributing to more sustainable waste management strategies.

9.3. Conclusions

In conclusion, corn processing waste represents a valuable and sustainable resource for the production of low-cost adsorbent materials. Native biomass, hydrochars, and biochars exhibit significant potential for the removal of a wide range of pollutants from soil and water systems, with their performance strongly governed by physicochemical properties influenced by thermochemical conversion processes.

Although promising results have been achieved, further research is required to address challenges related to material stability, selectivity in complex systems, process optimization, and standardization. Advancing these aspects will facilitate the transition from laboratory-scale studies to practical applications and support the integration of corn residue valorization into circular bioeconomy frameworks.