Low-Cost Adsorbents for Water Treatment: A Sustainable Alternative for Pollutant Removal

Abstract

This review addresses the potential of low-cost adsorbents (LCAds) derived from agro-industrial and marine residues as sustainable alternatives for water purification. Although raw biomass offers economic advantages, its application is often limited by low surface area and reactivity. Consequently, this paper examined physicochemical modifications—such as pyrolysis, acid/alkali activation, and surface grafting—that enhance adsorptive properties. The superior performance of these modified materials in removing heavy metals, dyes, pesticides, and pharmaceuticals is highlighted. Furthermore, the transition from laboratory scale to industrial application faces key hurdles, such as biomass variability, reactor engineering, and regulatory gaps. Finally, future perspectives are presented, focusing on the integration of LCAds into hybrid treatment systems and their pivotal role in the circular economy for decentralized water management.

1. Introduction

Water contamination has become one of the most critical global environmental issues, as all living organisms depend directly on natural freshwater resources for survival. Despite the existence of environmental regulations, vast quantities of toxic substances are still discharged into water bodies on a daily basis, significantly compromising water quality. The accelerating pace of urbanization, industrial development, and population growth further intensifies the burden on water resources and infrastructure. In addition, the continuous introduction of new synthetic chemicals into the environment—many of which are not adequately addressed by conventional treatment systems—poses increasing challenges for water reuse and effective water management strategies [1,2,3].

Although efficient water treatment technologies exist—such as membrane filtration, advanced chemical treatments, and biological processes—their high operational and energy demands often render them economically unfeasible for implementation in low-income or resource-constrained regions [4].

Adsorption is a separation process that occurs when a substance adheres to the surface of a solid, known as the adsorbent. This mechanism can take place through physical interactions (physisorption) or chemical interactions (chemisorption), depending on the nature of the forces between the adsorbent and the adsorbate. Due to its operational simplicity, low cost, and high efficiency, adsorption is widely employed in water treatment processes [5,6].

Several natural materials and agro-industrial wastes have proven effective as adsorbents. Among them are fruit peels, sugarcane bagasse, sawdust, rice husk, clays, peat, and shells, among others [7,8]. These materials are abundant, biodegradable, and often improperly discarded, which enhances their environmental appeal and classifies them as low-cost adsorbents (LCAds). These adsorbents have been widely investigated for the removal of a broad range of contaminants, including heavy metals, dyes, pharmaceuticals, and others [9,10,11].

In many cases, however, these materials cannot be effectively applied in their raw form. Consequently, their modification through physical and/or chemical methods has been extensively explored as a strategy to enhance the adsorptive performance. Such modifications often aim to increase the surface area and porosity, as well as introduce functional groups or components (e.g., nanoparticles) that can improve the interaction with and removal efficiency of target contaminants.

The following sections provide a comprehensive review of the different modification methods employed in the production of LCAds, followed by an overview of studies on the application of various adsorbents for the removal of major water contaminants. The review concludes with a discussion of the current challenges and future perspectives for adsorption processes using LCAds.

2. Low Cost Adsorbents

Low-cost adsorbents (LCAds) derived from abundant and renewable sources offer a sustainable alternative to conventional water treatment materials, promoting the principles of the circular economy by valorizing waste that would otherwise be improperly discarded. The most frequently investigated precursors include agricultural residues such as rice husk, sugarcane bagasse, sawdust, and fruit peels (e.g., banana, mango, and jackfruit) [7,12,13]. Additionally, lignocellulosic biomass like Moringa oleifera and Eichhornia crassipes, as well as marine and animal wastes such as fish scales, chitosan, and algal biomass, are widely utilized due to their high availability and biodegradability [14,15].

However, when used in their raw (in natura) form, these natural adsorbents often exhibit limited specific surface area, restricted porosity, and fewer active sites, which compromises their efficiency in practical applications. To overcome these drawbacks, various modification strategies are employed to enhance adsorptive performance. Physical treatments, such as thermal activation (pyrolysis), milling, and microwave irradiation, are used to modify morphology and increase pore volume. Concurrently, chemical treatments involving acid or alkaline activation introduce oxygenated functional groups and remove impurities. Furthermore, surface modifications, including the impregnation of metal oxides (e.g., Fe₃O₄, ZnO) and the grafting of biopolymers like chitosan, are effective for increasing selectivity and enabling magnetic recovery of the material.

To improve the material’s performance, the most common modification processes applied to LCAds are described in sequence.

3. Treatments and Modifications to Enhance the Adsorption Capacity of Low-Cost Adsorbents

The modification of LCAds has been consolidated as an essential strategy to enhance the efficiency, selectivity, and stability of biological or mineral materials employed in the removal of emerging pollutants [16]. Among the most promising materials are biochars, clays, zeolites, lignocellulosic residues, and natural biopolymers, which are characterized by their low cost, wide availability, high biodegradability, and potential for the valorization of agro-industrial wastes [17,18].

However, when used in their raw form, natural adsorbents often exhibit limited adsorption capacities, a direct consequence of their low specific surface area, restricted porosity, and reduced number of active sites available for interaction with contaminants. According to Joshi et al. [19] and Wang et al. [20], these structural and chemical limitations compromise the efficiency of adsorption processes and restrict the practical applicability of such materials in real wastewater treatment systems.

To overcome these drawbacks and improve the performance of these materials, various physicochemical treatments have been successfully developed, including thermal activation, chemical functionalization, metal impregnation, and polymer grafting, which can increase the surface area, develop more accessible porous structures, and introduce active functional groups with high chemical affinity toward specific contaminants. Recent reviews, such as those by Aziz et al. [21] and Murtaza et al. [22], emphasize that these modifications promote significant changes in both the morphology and surface chemistry of the materials, resulting in adsorbents with enhanced capacity, selectivity, and stability under different operational conditions.

Furthermore, as highlighted by Yaashikaa et al. [23], the structural and chemical transformations resulting from these modifications make LCAds more efficient and competitive when compared to high-cost synthetic materials such as activated carbons and polymeric resins, with the additional advantage of low environmental impact and high economic feasibility, thus meeting the principles of green chemistry and circular economy.

In general, treatments applied to natural adsorbents can be classified into three main categories. The first comprises physical treatments, which modify the morphological and structural properties of the material, enhancing accessibility to adsorption sites and improving internal diffusion. The second includes chemical treatments, which alter the surface composition and introduce new functional groups capable of selectively interacting with different classes of contaminants. Finally, surface modifications encompass the incorporation of external phases, such as metal oxides, surfactants, or biopolymers, which provide additional properties like stability, regenerability, and multifunctionality [24,25].

Recent studies have shown that combined approaches, involving two or more of these methods applied sequentially, generate significant synergistic effects, simultaneously increasing the adsorptive capacity, selectivity, and operational stability of the materials. Kumar et al. [26] and Velkova et al. [11] emphasize that integrating thermal activation, chemical functionalization, and metal impregnation allows the production of high-performance hybrid materials capable of removing multiple emerging pollutants under complex environmental conditions. This trend reflects the technological transition from conventional natural adsorbents to the development of advanced and sustainable materials aligned with the principles of green chemistry and the circular economy, consolidating modified bioadsorbents as competitive and environmentally responsible alternatives for the treatment of contaminated waters [18].

Research has demonstrated that modification processes significantly improve the adsorptive capacity of low-cost adsorbents.

Table 1. Overview of studies on physical and chemical modifications of low-cost adsorbents.

Category	Modification Method	Main Effects	Impact on Adsorption	Article
Physical Treatments	Thermal activation (pyrolysis)	Increased surface area; development of micro-/mesopores	Higher adsorption capacity and improved intraparticle diffusion	[27,28,29]
	Ultrasound treatment	Pore opening via acoustic cavitation	Enhanced adsorption of dyes and pharmaceuticals	[26]
	Microwave activation	Rapid pore formation through dielectric heating	Improved adsorption of diclofenac and tetracycline	[30]
	Particle size reduction	Increased external surface; reduced diffusion resistance	Higher removal efficiency of metals, pesticides, and drugs	[12,31,32,33,34]
Chemical Treatments	Acid activation	Removal of impurities; introduction of –COOH/–OH groups	Improved adsorption of dyes and Cr(VI)	[7,15,35,36,37]
	Alkaline activation	Increased basicity; structural opening	Enhanced adsorption of metals and pharmaceuticals	[38,39]
	Oxidative treatments	Enrichment of reactive oxygenated groups	Improved hydrogen bonding, complexation, and electrostatic interactions	[40,41,42,43]
	Covalent modifications	Introduction of selective functional groups (–COOH, –NH2, –SO3H)	High selectivity for metals and dyes	[44,45,46]
Surface Modifications	Metal oxide impregnation (Fe3O4, TiO2, ZnO, Al2O3)	Formation of complexation sites; magnetic properties	Higher efficiency + magnetic regeneration	[47,48,49]
	Polymer/biopolymer grafting (e.g., chitosan)	Addition of functional groups; increased active site density	Improved adsorption of pharmaceuticals and metal ions	[50,51]

summarizes relevant studies on these modifications, which are further explained in the subsequent sections.

3.1. Physical Treatments

Physical treatments constitute the first line of modification for LCAds, acting mainly by altering the material’s morphology, surface area, and porous structure, thereby facilitating access to adsorption sites without introducing significant chemical modifications to the original composition. These methods are particularly valuable due to their operational simplicity and lower generation of chemical waste [52,53].

3.1.1. Thermal Activation and Pyrolysis

Among the physical methods, thermal activation stands out as one of the most established and effective strategies for improving natural LCAd. This process involves controlled carbonization of biomass in an inert atmosphere or under limited oxygen (generally between 300 and 800 °C), promoting partial degradation of organic components and the formation of a highly porous carbonaceous matrix. According to Elnour et al. [27], increasing the pyrolysis temperature results in a higher degree of carbon aromatization and a substantial rise in surface area and pore volume, with typical increments from less than 50 m² g⁻¹ to more than 700 m² g⁻¹, depending on operational conditions and the type of precursor used.

Complementary studies, such as Muzyka et al. [28], confirm that variations in pyrolysis conditions including temperature, heating rate, and residence time directly influence the texture, porosity, and distribution of micro- and mesopores, ultimately impacting the adsorptive performance of biochar. Similar results were reported by Barszcz et al. [54], who observed that higher temperatures (above 600 °C) promoted the formation of interconnected porous structures, expanding accessibility to active sites and favoring internal diffusion during adsorption.

These findings are consistent with the results of Tripathi et al. [29] and Suliman et al. [55], who also reported that increasing pyrolysis temperature significantly enhances the BET area and structural stability of biochars derived from lignocellulosic wastes.

Beyond increasing surface area, pyrolysis induces important chemical modifications, such as the formation of oxygenated groups (–OH, –COOH, and –C=O), which are responsible for π–π interactions and hydrogen bonding with organic and ionic contaminants. Recent studies demonstrate the efficiency of this process for the adsorption of nitrogen species, especially nitrate. Mood et al. [56] developed an iron- and nitrogen-modified biochar from Douglas fir biomass and observed a maximum adsorption capacity of 20.67 mg g⁻¹ for NO₃⁻, attributed to the synergy between an increased porous area and the presence of Fe–O–N coordinating groups. Similarly, Qi & Sato [57] reported that lignocellulosic waste biochar modified with magnesium showed a capacity of 19.1 mg g⁻¹ for nitrate, approximately 85% higher than unmodified biochar, due to surface enrichment in –Mg–OH groups that favor anion exchange.

3.1.2. Ultrasound- and Microwave-Assisted Processes

Ultrasound- and microwave-assisted processes have been widely studied as innovative and environmentally safe alternatives for improving the structural properties of natural LCAds. These techniques promote significant physical changes—such as increased surface area, pore opening, and impurity removal—without the use of aggressive reagents, making them compatible with the principles of green chemistry [53].

Ultrasonic treatment acts via acoustic cavitation, in which micro-explosions generated in the liquid medium clear obstructed pores and create microstructures, improving diffusion and access to active sites. Kumar et al. [58] observed that bamboo biochar under both conventional and ultrasound-assisted conditions, showed good adsorption performance when assessed for single, binary, and ternary dye systems with an adsorption capacity of 139.34 mg g⁻¹ for methylene blue (MB), 75.09 mg g⁻¹ for methyl orange (MO), and 98.13 mg g⁻¹ for basic fuchsin (BF) dyes.

Microwave irradiation, in turn, relies on rapid and uniform dielectric heating, promoting intense formation of micro- and mesopores within minutes. Nayak et al. [30] used microwave assisted fabrication to produce a magnetite-biochar nanocomposite adsorbent from jackfruit peel for the removal of nutrients in water treatment; according to the authors, microwave irradiation employed during the adsorbent fabrication involved less energy consumption, high yields and good adsorption performance with a maximum adsorption efficiency of 7.94 mg g⁻¹ and 5.26 mg g⁻¹ for phosphates and nitrates, respectively.

Wang et al. [20] used microwave-assisted synthesis to produce porous graphitized biochar from waste coconut shell with a combination of KOH and KCl as activators; according to the authors, the activators facilitated the creation of a porous and graphitized structure with a high specific surface area of 2047 m²·g⁻¹ and excellent adsorption capacity for tetracycline, as high as of 1033.06 mg·g⁻¹.

Taken together, these studies show that applying ultrasound and microwaves is highly effective for improving the textural properties and adsorptive performance of LCAds, in addition to reducing the process time and energy consumption. These approaches have been consolidated as sustainable tools that complement chemical and metallic modifications, contributing to the development of high-performance hybrid materials designed for the removal of pharmaceuticals and other emerging pollutants.

3.1.3. Milling and Particle Size Reduction

Particle size reduction by milling, crushing, or sieving is a widely employed physical strategy to improve the adsorptive performance of natural materials by directly acting on morphological parameters such as specific surface area and diffusion length [59]. Decreasing granulometry reduces intraparticle diffusion resistance, favors contact between the adsorbent and the adsorbate, and accelerates mass transfer rates.

Chouchane et al. [31] observed that reducing the particle size from 500 µm to 300 µm in a metallurgical residue increased the Ni(II) adsorption capacity from approximately 19.7 to 27.7 mg g⁻¹, raising the removal efficiency from 65.7% to 92.3%. Similar results were obtained by Kamarudin et al. [12], who compared rice husk and palm kernel biochar particles in the ranges <0.3 mm, 0.3–1 mm, and >1 mm, finding that finer fractions delivered better performance in metaldehyde adsorption.

In studies with biosorbents applied to pharmaceutical removal, Ndankou et al. [32] verified that decreasing particle size resulted in a significant increase in the amount of phenobarbital adsorbed, an effect attributed to larger contact area and reduced diffusion resistance.

Similarly, Agoe et al. [33] showed that using ≤100 µm particles of pre-pyrolyzed sewage sludge modified the structural properties of the resulting biochar and improved the removal of methylene blue and mercury. Amutova et al. [34] also confirmed that sargassum-derived biochar particles with granulometry < 150 µm exhibited higher efficiency in immobilizing Dichlorodiphenyltrichloroethane (DDT) and its metabolites in soil, reinforcing the role of size reduction in enhancing adsorptive performance.

3.2. Chemical Treatments

Chemical treatments constitute one of the most effective and widely explored approaches to enhance the adsorptive properties of natural materials, acting directly on the adsorbent’s composition and surface chemistry [60]. These modifications aim to introduce active functional groups, alter the surface acid–base character, remove mineral impurities, and increase the density of reactive sites, resulting in materials with greater selectivity and capacity to interact with target contaminants [61]. In general, the main methods include acid activation, alkaline activation, and oxidative or covalent modifications, which can be applied individually or in combination, depending on the nature of the feedstock and the type of pollutant to be removed [62].

3.2.1. Acid Activation

Acid activation is one of the most traditional and established strategies to improve the adsorptive properties of natural materials. In general, solutions of HNO₃, H₂SO₄, or HCl at different concentrations are used to promote significant chemical transformations on the adsorbent surface. This process operates through synergistic mechanisms, including the removal of inorganic impurities and soluble organic compounds, selective dissolution of structural components, increased porosity, and the introduction of oxygenated functional groups such as carboxyls (–COOH), hydroxyls (–OH), and carbonyls (–C=O). Such modifications favor adsorption interactions based on hydrogen bonding, complexation, and ion exchange, increasing the affinity for metal ions and polar compounds [59].

Recent studies have demonstrated that acid treatment of biomaterials can markedly increase the adsorptive capacity. For example, activated carbon obtained from rice husk via H₂SO₄ activation showed high performance for methylene blue removal compared with the untreated material [35]. Similarly, biochars activated by organic and mineral acids have exhibited significant increases in the surface area and density of oxygenated groups, reflected in higher efficiency for heavy metal removal, such as Cr(VI), lead, copper, and nickel adsorption [36,37]. In the case of lignocellulosic residues such as sugarcane bagasse, acid modification associated with impregnation with metal oxides raised the Cr(VI) adsorption capacity to around 55 mg g⁻¹, a value considerably higher than that of non-functionalized biochar [7]. Moreover, studies with marine biomass such as Posidonia oceanica reported exceptional capacities reaching up to 2681.9 mg g⁻¹ for methylene blue (MB) removal, highlighting the potential of acid activation to modulate surface acidity and maximize adsorptive affinity [15].

3.2.2. Alkaline Activation

Alkaline activation has proven particularly effective in increasing the surface basicity of adsorbents and improving interaction with anionic contaminants or acidic pharmaceuticals. Common reagents such as NaOH, KOH, and Ca(OH)₂ can promote the selective leaching of lignin and hemicellulose, expose the internal structure and generate active sites and basic functional groups. For example, NaOH modifications have shown good results for metals such as methylene blue and Pb(II) [15]. For pharmaceuticals, a study with fique biochar activated with NaOH reported high adsorption capacity for diclofenac under competitive conditions [38]. Likewise, activated biochars have been successfully applied for diclofenac removal using different chemical routes [39]. These examples reinforce that alkaline activation can favor the diffusion and retention of organic and metallic contaminants by improving pore accessibility and introducing more basic surfaces.

3.2.3. Oxidative Treatments

Oxidative treatments constitute an advanced route of chemical modification, based on the use of agents such as H₂O₂, KMnO₄, or diluted acids (e.g., HNO₃) to introduce highly reactive oxygenated groups and increase surface acidity in the adsorbent. Controlled oxidation favors the generation of specific sites suitable for interactions via hydrogen bonding, metal complexation, and electrostatic attraction, being especially effective for the removal of emerging contaminants. For example, Kohira et al. [40] modified water hyacinth biochar with KOH followed by oxidation with H₂O₂, observing significant improvements in adsorptive performance for organic compounds and metals. In another work, Di Vincenzo et al. [41] applied different oxidative systems to poplar biochar, demonstrating that appropriate oxidation conditions increase the density of carboxylic and phenolic groups and improve the adsorption of model dyes. In the context of pharmaceuticals, combined oxidation and metallic modification have also proven effective: Ding et al. [42] used Mn-enriched biochar activated by H₂O₂ to promote synergistic adsorption/oxidation of ciprofloxacin. Additionally, a classic study on HNO₃ oxidation of weed-derived biochar showed an increase in methylene blue adsorption capacity from ~39.68 to ~161.29 mg g⁻¹, illustrating the potential of oxidative surface modulation [43]. These examples confirm that when well controlled, moderate oxidation can increase the density of oxygenated functional sites, enhancing affinity for organic and metallic contaminants.

3.2.4. Specific Covalent Modifications

Carboxylation, generally conducted with organic anhydrides such as succinic or phthalic anhydride, introduces carboxylic groups (–COOH) capable of coordinating metal ions, increasing the density of active sites and the chemical stability of modified materials. This type of modification has proven particularly effective in algal-derived biomaterials. Li et al. [44] reported that the carboxylation of algal residues with succinic anhydride raised the adsorptive capacity three- to sixfold compared with the unmodified material, reaching values of 52.4–78.1 mg g⁻¹ for Cu²⁺ and up to 108 mg g⁻¹ for Pb²⁺. The gain was attributed to the formation of stable complexes between –COOH groups and metal ions, promoting more efficient chelation interactions.

Complementarily, amination and sulfonation have been explored to increase affinity for ionic contaminants. The introduction of amino groups (–NH₂) on carbonaceous surfaces favors electrostatic interaction and the reduction of Cr(VI) to Cr(III) during adsorption. Flores et al. [45] showed that ethylenediamine-modified biochar obtained from agro-industrial residues exhibited selective adsorption of Cr(VI) (46.5 mg g⁻¹) and Cr(III) (27.1 mg g⁻¹), confirming the relevance of nitrogen functionalities for transition metals. In turn, the incorporation of sulfonic groups (–SO₃H) increases polarity and ion-exchange capacity, resulting in the greater removal of cationic dyes. Toy et al. [46] observed that sulfonation of cellulose significantly improved methylene blue adsorption, evidencing the contribution of strong acid groups to interactions with positively charged molecules.

These results reinforce that targeted chemical modifications—especially carboxylation, amination, and sulfonation—are effective routes to adjust the selectivity and capacity of natural and carbonaceous adsorbents toward heavy metals and organic dyes, expanding their potential in water treatment systems.

3.3. Surface Modifications

Surface modifications represent an advanced step in improving natural adsorbents and are widely used to incorporate new components or specific functionalities that are not present in the original structure of the material [60]. Unlike classical chemical treatments, which alter the intrinsic composition of the adsorbent, surface modifications aim to coat, impregnate, or graft active substances onto the base matrix, creating hybrid materials with optimized chemical, structural, and functional properties [63]. This strategy has been widely explored for the development of multifunctional bioadsorbents with applications in the removal of pharmaceuticals, dyes, pesticides, and heavy metals [10].

3.3.1. Impregnation with Metal Oxides

Among the most explored modifications, impregnation with metal oxides such as Fe₂O₃, Fe₃O₄, TiO₂, Al₂O₃, and ZnO stands out, as they can generate selective adsorptive sites and reactive hydroxyl groups that favor complexation, ion exchange, and specific interactions with contaminants. In addition, some of these oxides introduce photocatalytic activity, enabling the simultaneous degradation of organic compounds after adsorption.

Luo et al. [47] demonstrated that impregnating sludge biochar with Fe₃O₄ nanoparticles (~14.3 nm) increased the tetracycline adsorption capacity from 115.3 mg g⁻¹ to 184.5 mg g⁻¹, representing an increase of approximately 60%. The gain was attributed to the formation of additional active sites associated with iron oxide surfaces and to the increase in specific surface area.

Similarly, Azizzadeh et al. [48] produced a magnetic biochar derived from orange leaves (MBC-OL) for removal of the antiviral favipiravir. The unmodified material showed a removal efficiency of 62.4%, whereas the magnetized biochar reached 97.5%, maintaining >90% efficiency after ten regeneration cycles. These results confirm the stability and reusability associated with Fe₃O₄ magnetization.

Regarding TiO₂ modification, Cai et al. [49] used the ramie bars as biomass modified by impregnation with TiO₂ under UV irradiation and verified that the modified biomass exhibited an increase in safranine T removal 2.3 times better than that of the raw biochar, evidencing the combined effect of adsorption and photocatalysis.

These results indicate that impregnation with metal oxides, especially magnetization with Fe₃O₄, constitutes an efficient route to increase adsorptive capacity, enable rapid magnetic regeneration, and reduce operational costs, consolidating itself as a sustainable strategy for the treatment of contaminated waters.

3.3.2. Grafting of Functional Polymers and Biopolymers

Another widely explored route is the grafting of functional polymers or biopolymers with the aim of introducing active groups—such as amines, carboxyls, or hydroxyls onto the adsorbent surface, thereby improving the affinity for organic compounds and metal ions. Among the most used biopolymers is chitosan, a cationic polymer rich in –NH₂/–NH₃⁺ and –OH, with strong affinity for anions and polar molecules. Its combination with mineral or carbonaceous matrices has generated high-performance hybrid adsorbents.

For example, Zhang et al. [50] constructed a magnetic composite, Fe₃O₄@CTS/SBC (sludge biochar with chitosan and magnetite particles). The modified material reached a maximum capacity of 55.16 mg g⁻¹ for Cu²⁺ and ~99.77% removal under specific conditions (30 mg/L, pH 5, 180 min), whereas the raw, unmodified biochar showed inferior performance (lower removal and capacity).

Another noteworthy study is that of Mojiri et al. [51], who developed the adsorbent CMCAB (cross-linked magnetic chitosan/activated biochar) for the removal of pharmaceuticals such as ibuprofen, diclofenac, and naproxen. In tests at ~pH 6 and an initial concentration of ~2.5 mg/L, the modified material removed ~98.8% of ibuprofen, ~96.4% of diclofenac, and ~95.2% of naproxen. After up to eight regeneration cycles, efficiency remained high, demonstrating good stability.

Beyond chitosan, grafts of functionalized synthetic polymers such as polyethyleneimine (PEI) or polyaniline are used to multiply active sites and tailor selectivity. Combining modifications (e.g., magnetization + polymer grafting) leads to multifunctional hybrid systems in which chemical adsorption, magnetic recovery, and even catalytic action can coexist.

In summary, polymer grafting, especially with chitosan, is an effective strategy for tuning adsorbent surfaces, increasing performance, and enhancing practical relevance, especially when compared with unmodified material.

Modification processes, whether applied individually or in combination, enhance the adsorptive capacity of low-cost adsorbents used for the removal of various water contaminants. The following sections examine different studies that utilize these adsorbents for the removal of pharmaceuticals, pesticides, dyes, and heavy metals.

4. Removal of Pharmaceuticals Using Low-Cost Adsorbents

The use of LCAds has emerged as a sustainable and low-cost alternative for the removal of pharmaceuticals from aqueous effluents, particularly considering the limitations associated with commercial adsorbents such as activated carbon. Materials derived from natural biomass such as agricultural, lignocellulosic, and microbial residues offer high availability, the presence of reactive functional groups (hydroxyl, carboxyl, and amine), and the possibility of chemical modification. These features enhance their adsorption capacity and selectivity toward complex organic molecules.

Table 2. Literature studies on the removal of pharmaceuticals using low-cost adsorbents: modifications and maximum adsorption capacities.

Pharmaceutical	Adsorbent/Modification	Adsorption Capacity (qe)	Reference
Diclofenac (DCF)	Surfactant-Modified Guava Seeds	38.0 mg g−1	[64]
	Escherichia coli biomass coated with magnetite (Fe3O4)	46.01 ± 0.12 mg g−1	[65]
	Porous wood sponges (Nanocellulose)	321.3 mg g−1	[66]
	Biohybrid Aerogel Beads	321.3 mg g−1	[63]
	Sugarcane bagasse activated carbon (H3PO4)	233.6 mg g−1	[67]
Tetracycline (TC)	Porous wood sponges (Bamboo cellulose-derived activated carbon aerogel)	863.8 mg g−1	[68]
Ivermectin	Moringa oleifera seed husks functionalized with iron oxide nanoparticles	143.76 mg g−1	[69]

summarizes literature studies utilizing low-cost adsorbents for the removal of pharmaceuticals, particularly diclofenac due to its frequent detection in aquatic environments, highlighting the modifications (when applicable) and the maximum adsorption capacity obtained.

In a recent study, Coria-Zamudio et al. [64] investigated guava seeds (Psidium guajava L.) modified with cetyltrimethylammonium bromide (CTAB) for the adsorption of diclofenac (DCF). The surface modification increased the positive charge of the material, favoring electrostatic and hydrophobic interactions with the anionic drug. The modified bioadsorbent (MGS-2) exhibited a maximum adsorption capacity of 38.0 mg g⁻¹ at 45 °C, surpassing the untreated material (29.7 mg g⁻¹), and fitted well to the Langmuir model (R² = 0.992).

Strategies that incorporate magnetic properties into LCAds have also been explored to facilitate post-treatment separation. Ertaş and Tural [65] developed a magnetic bioadsorbent based on Escherichia coli cells coated with magnetite (Fe₃O₄) nanoparticles for diclofenac removal. The material showed high colloidal stability, efficient magnetic recovery, and significant affinity for the pharmaceutical, demonstrating the potential of modified microbial biomass as an adsorbent matrix.

Lignocellulosic materials, in turn, have exhibited outstanding performance after physical or chemical activation processes. Functionalized porous wood sponges are used for the removal of diclofenac (DCF) [66] and tetracycline (TC) [68], achieving maximum capacities of 321.3 mg g⁻¹ and 863.8 mg g⁻¹, respectively. The kinetic data were best described by the pseudo-second-order model, indicating dominant chemisorption mechanisms, and the Langmuir isotherms confirmed the formation of stable monolayers on the adsorbent surface.

Similarly, El Naga et al. [67] prepared activated carbon from sugarcane bagasse and observed high efficiency in the removal of diclofenac from aqueous media. The adsorption capacity reached 233.6 mg g⁻¹ at pH 6 and 25 °C, a result attributed to the increased surface area and the presence of reactive oxygenated groups on the material surface after H₃PO₄ activation.

Cusioli et al. [69] utilized Moringa oleifera seed husks functionalized with iron oxide nanoparticles to remove ivermectin—used as an insecticide, anti-parasitic, and anti-cancer therapy—from aqueous solutions. Regarding the kinetic study, equilibrium was reached in 400 min with an adsorption capacity of 89.41 mg g⁻¹. The experimental data were best described by the pseudo-first-order and Langmuir models, yielding a maximum capacity of 143.76 mg g⁻¹, leading the authors to conclude that the material demonstrates high potential for the removal of emerging contaminants.

Overall, recent studies show that combining natural biomass with controlled chemical modifications can produce materials with high adsorption capacity, low cost, and strong potential for application in wastewater treatment systems.

5. Using Low-Cost Adsorbents to Remove Pesticides

The extensive use of pesticides in agriculture has been essential for pest control and enhancing productivity, but it also represents one of the major sources of environmental contamination. These chemical compounds, applied to eliminate insects, microorganisms, invasive plants, and other undesirable organisms, also serve complementary functions as plant growth regulators, defoliants, desiccants, and nitrogen stabilizers [70]. Despite their agricultural significance, the intensive and often improper use of these products has contributed to the pollution of soils and water bodies. This underscores the urgent need to develop clean, efficient, and economically viable technologies for removing pesticide residues from the environment [71].

Table 3. Application of LCAds for pesticide removal from aqueous environments.

Pesticides	Adsorbent/Modification	Adsorption Capacity (qe)	Removal Efficiency (%)	Reference
Atrazine	Moringa oleifera—seed husk, seed pulp, pod husk/ (Particle sized reduction)	2.99, 0.86 and 0.31 mg g−1	85%/73%/60%	[72]
Methomyl Carbamate	Peanut husk activated carbon/ (HNO3)	56.62 mg g−1	94.06%	[73]
Diuron	Moringa oleifera—seed husk/ (HNO3 and pyrolysis)	25.36 mg g−1	84.56%	[14]

summarizes studies utilizing LCAds for the removal of pesticides from aqueous media.

Among the sustainable alternatives studied, Moringa oleifera has emerged as a promising natural biosorbent due to its lignocellulosic composition and wide availability [74]. In a recent study, different parts of the plant (seed pulp, seed husk, and pod husk) were evaluated for their potential to adsorb atrazine, a pesticide widely used in agricultural crops. Physicochemical analyses indicated similar compositions among the parts; however, the seeds exhibited higher protein and fatty acid content, and lower amounts of cellulose, hemicellulose, and lignin compared to the husks. In the study by Coldebella et al. [72], they reported maximum ATZ biosorption capacities of 2.99, 0.86 and 0.31 mg g⁻¹ for seeds husks, seeds pulp and pods husks, respectively. These results demonstrate the potential of Moringa oleifera as an ecological, low-cost material that requires simple preparation for removing organic pesticides from aqueous systems.

Another study evaluated activated carbon derived from peanut husks as an adsorbent for the removal of the pesticide methomyl carbamate in aqueous solution. The material, prepared by chemical modification with nitric acid, exhibited notable performance in adsorbing the contaminant. Process optimization, conducted using a Box–Behnken design, identified ideal conditions: an initial concentration of 562 ppm, an adsorbent dosage of 0.50 g, and a contact time of 55 min, achieving a removal efficiency of 94.06%. The process followed the Freundlich isotherm model (R² = 0.9928), suggesting heterogeneous, multilayer adsorption, and conformed to pseudo-second-order kinetics (R² = 0.9988), indicating that chemisorption was the predominant mechanism. The maximum adsorption capacity was 56.62 mg g⁻¹, demonstrating the adsorbent’s high potential for treating pesticide-contaminated water [73].

Cusioli et al. [14] employed Moringa oleifera seed husks modified with nitric acid followed by pyrolysis for the removal of diuron from aqueous media, achieving a diuron adsorption capacity (qmax) of 25.36 mg g⁻¹. The authors concluded that the modified LCAd presented a high total volume of mesopores, becoming efficient after the modification. Furthermore, the material was highlighted as low-cost, easy to obtain, and presenting good potential as an adsorbent.

Beyond batch experiments, LCAds have been successfully employed in continuous fixed-bed column studies, exhibiting significant pesticide removal rates under optimal operating parameters. Atrazine adsorption studied in a fixed-bed column using Moringa oleifera Lam. seeds as a low-cost adsorbent exhibited a 37% increase in specific surface area compared to the in natura (raw) material. Ideal conditions—a bed height of 13 cm, a flow rate of 1 mL min⁻¹, an inlet concentration of 2.0 mg L⁻¹, and a pH of 5.0—resulted in 50% atrazine removal, with a breakthrough time of 25 min and saturation at 420 min. The results demonstrate the viability of the biosorbent’s continuous use in packed-bed systems, reinforcing its potential for industrial-scale applications [75].

Further studies compared the use of activated carbon and biochar in fixed-bed column systems for the adsorption of different pesticides, including atrazine, chlorothalonil, and endosulfan. Increasing the bed height from 10 to 15 cm extended the breakthrough time from approximately 3000 to 5800 min. Conversely, increasing the flow rate (0.5–1.5 mL min⁻¹ and the initial contaminant concentration (50–100 µg L⁻¹) reduced the exhaustion time. The Yoon–Nelson (R² = 0.9427) and Thomas (R² = 0.9921) models adequately described the adsorptive behavior, indicating the high efficiency of the materials under optimized conditions. The observed adsorption sequence was atrazine > chlorothalonil > β-endossulfan > α-endossulfan, confirming the applicability of activated carbon and biochar for the simultaneous removal of pesticides from contaminated water [76].

6. Use of Low-Cost Adsorbents for Dye Removal

Synthetic dyes are widely employed in sectors such as textiles, food, cosmetics, pharmaceuticals, and paper due to their high stability and chromatic diversity. However, the improper disposal of effluents containing these compounds constitutes a severe environmental problem, as many dyes possess complex aromatic structures and high resistance to biological degradation [77].

Even at low concentrations, these contaminants reduce light penetration in the water column, compromising photosynthesis and the aquatic ecological balance [78]. Furthermore, several dyes and their by-products exhibit mutagenic, carcinogenic, and toxic potential to aquatic organisms and humans, reinforcing the need to develop efficient, sustainable, and economically viable technologies for their removal [79].

Among the alternative materials studied, agricultural residues have shown promise as low-cost adsorbents [80].

Table 4. Summary of studies utilizing agro-industrial residues for the removal of dyes from aqueous solutions.

Dye	Adsorbent/Modification	Adsorption Capacity (qe)	Reference
Methylene Blue (MB)	Iron-modified banana peels	28.1 mg g−1	[81]
	Okara (soymilk residue)	93.20 mg g−1	[82]
Safranin Orange	Okara (soymilk residue)	184.59 mg g−1	[82]
	Soybean hulls	221.74 mg g−1	[83]
Neutral Red	Soybean hulls	287.30 mg g−1	[83]
Reactive Blue 19	Walnut shells carbon derived (KOH)	1227.17 mg g−1	[84]
Reactive Red 195	Walnut shells activated carbon (KOH)	235.74 mg g−1	[84]
Malachite green dye	Catha edulis stems activated carbon	5.62 mg g−1	[85]

summarizes studies utilizing agro-industrial wastes for dye removal from aqueous media.

Iron-modified banana peels, for example, achieved a removal efficiency of 91.89% for the dye methylene blue under optimized conditions (50 min, 45 °C, 2.5 g of adsorbent, and 5 mg L⁻¹ of dye). The material exhibited a maximum adsorption capacity of 28.1 mg g⁻¹ according to the Langmuir model, with the adsorbent dosage being the main influencing factor [81].

Other agro-industrial residues are also being explored. Okara, a by-product of soymilk processing, demonstrated a high adsorption capacity of 93.20 mg g⁻¹ for methylene blue and 184.59 mg g⁻¹ for Safranin orange, even without prior treatment, highlighting its applicability as a natural, low-cost biosorbent [82]. Similarly, soybean hulls showed good performance in removing the dyes safranin-O and neutral red, with maximum adsorption capacities of 221.74 mg g⁻¹ and 287.30 mg g⁻¹, respectively. The process was well-described by the Langmuir and pseudo-second-order kinetic models. It was also found to be endothermic, spontaneous, and reversible in nature, and allowed for regeneration for up to six cycles without significant loss of efficiency [83].

Activated carbons have also been widely employed for dye removal due to their high surface area and good chemical stability. Carbon derived from walnut shells, activated with KOH (1:3 ratio), exhibited a surface area of 2347.4 m² g⁻¹ and high efficiency in removing the dyes Reactive Blue 19 and Reactive Red 195. It showed maximum adsorption capacities of 1227.17 and 235.74 mg g⁻¹, respectively, at a near-neutral pH [84].

Activated carbon produced from Catha edulis stems, which displayed a qmax of 5.62 mg g⁻¹ and removal efficiency of 98.8% at pH 10 and an initial concentration of 10 mg L⁻¹. The process followed pseudo-second-order kinetics and the Freundlich isotherm model, indicating adsorption onto heterogeneous surfaces [85].

In conclusion, the reviewed studies highlight the versatility of agro-industrial residues as effective precursors for dye remediation. While activated carbons derived from biomass (such as walnut shells) exhibit superior surface areas and exceptional adsorption capacities, untreated or minimally modified by-products like okara and soybean hulls offer a compelling balance between high performance and cost-effectiveness. The successful application of these materials, supported by favorable kinetic and thermodynamic behaviors, reinforces their viability as sustainable alternatives within the circular economy framework.

7. Low-Cost Adsorbents for Heavy Metal Removal

Heavy metals are naturally present in the biogeochemical cycle of chemical elements. They can be released into the air and water through volcanic eruptions and rock weathering and dispersed into the environment [86]. However, the most significant source of heavy metal pollution stems from anthropogenic activities. Metals such as zinc (Zn), chromium (Cr), nickel (Ni), lead (Pb), cadmium (Cd), mercury (Hg), copper (Cu), and arsenic (As) are used in numerous modern industrial processes, including electroplating, metallurgy, tanneries, battery manufacturing, chemical industries, and even fertilizers [87]. Due to growing industrial expansion and a lack of regulatory oversight, untreated industrial effluents from the massive, large-scale production of these goods are ultimately released into the environment, causing pollution in various environmental compartments [88].

Water contamination by heavy metals is a challenge in terms of ensuring and maintaining water quality as a universal right for all citizens [89]. The presence of these contaminants has been detected in both groundwater and surface waters [90] as well as in sediments, which can either retain or disperse them, depending on the environmental conditions [9].

Considering the extent of the damage caused, the presence of these contaminants in the environment constitutes a serious threat to aquatic and terrestrial life forms, including humans [91]. These compounds possess a high degree of persistence, toxicity, and bioaccumulation in ecosystems [92], and therefore, all efforts to prevent their release into water bodies or mitigate their presence are of utmost importance currently [93]. The consumption of food and water contaminated with heavy metals can lead to severe chronic diseases, such as renal failure, reproductive problems, and neurological damage [94].

In this context, several technologies have been explored for the removal of heavy metals from water, including electrocoagulation, reverse osmosis, chemical precipitation, and adsorption [95]. From the perspective of economic viability and implementation, adsorption has proven to be a promising path as it is a versatile and low-cost process compared to others available on the market [96]. Its advantages also include the possibility of reusing the biosorbent material, recovering the adsorbed metal, and its high removal efficiency even at low concentrations [97].

Besides the conventional materials used in the adsorptive process, such as activated carbon and zeolites, the scientific community has investigated a wide range of organic materials—often derived from food industry, forestry, and agro-industry wastes—for the removal of micropollutants, like heavy metals, from contaminated waters [98]. Therefore, the so-called biosorbents undergo a series of characterizations to evaluate their adsorptive potential under different conditions. The choice of a suitable biosorbent is influenced by parameters such as: satisfactory chemical stability, large surface area, adequate pore distribution, surface functional groups, and its affinity for the contaminant in question [99,100].

The matrix of biosorbents is basically composed of cellulose, proteins, hemicellulose, lignin, and sugars [96]. These molecules present surface groups such as hydroxyls, esters, amines, alcohols, carboxyls, and phosphate groups on the surface of the biosorbents, promoting the interaction between the metallic species and their active adsorptive sites [101].

Several mechanisms can be used to describe this interaction, such as chemisorption, precipitation, ion exchange, physisorption, chelation, and complexation [97,102].

Various studies have explored the most different types of materials as biosorbents: vegetable fibers, peels, stems, stalks, and seeds of fruits and vegetables, fish scale residues, and algal biomass (

Table 5. Summary of studies utilizing diverse biomass sources for the removal of heavy metals from aqueous solutions.

Heavy Metal	Adsorbent/Modification	Adsorption Capacity (qe)	Reference
Pb(II)	Mango peel	9.65 mg g−1	[13]
	Modified banana peduncle (acid/base)	2.49 mg g−1	[103]
	Moringa oleifera + HNO3	208.12 mg g−1	[104]
	Tomato leaves	45.77 mg g−1	[105]
	Eichhornia crassipes + H3PO4	3.99 mg g−1	[106]
	Coffee husk biochar	116.3 mg g−1	[107]
	Persimmon residues	68.79 mg g−1	[108]
	Opuntia ficus-indica + chitosan	102.00 mg g−1	[109]
	Pistachio shells	29.00 mg g−1	[110]
	Honey waste + HNO3	17.24 mg g−1	[111]
Cd(II)	Coffee husk biochar	139.5 mg g−1	[107]
	Opuntia ficus-indica + chitosan	20.30 mg g−1	[109]
	Honey waste + HNO3	16.13 mg g−1	[111]
Cr(VI)	Persimmon residues	139.40 mg g−1	[108]
	Pinus radiata + NaOH	13.95 mg g−1	[112]
Ni(II)	Opuntia ficus-indica + chitosan	26.00 mg g−1	[109]
	Pistachio shells	29.30 mg g−1	[110]
	Almond shells chemically modified	5.67 mg g−1	[113]
	Date pit + ZnO	71.90 mg g−1	[114]
Cu(II)	Opuntia ficus-indica + chitosan	34.70 mg g−1	[109]
	Pistachio shells	24.50 mg g−1	[110]
	Honey waste + HNO3	16.96 mg g−1	[111]
	Date pit + ZnO	82.40 mg g−1	[114]
Cr(III)	Catla catla fish scale waste	304.88 mg g−1	[115]
Co(II)	Pistachio shells	23.00 mg g−1	[110]
	Corn stalk activated carbon	202.68 mg g−1	[116]
	Catla catla fish scale waste	383.14 mg g−1	[115]
Zn(II)	Honey waste + HNO3	16.67 mg g−1	[111]
	Date pit + ZnO	66.30 mg g−1	[114]
Mn(II)	Rice husk + HNO3	60.24 mg g−1	[117]
As(III)	Chlorella sorokiniana + kaolin + FeCl3	17 mg g−1	[118]

).

For example, in a study by Souza et al. [13], mango peel (Mangifera indica) demonstrated an adsorptive capacity (qe) of 9.65 mg g⁻¹ for Pb(II) in 60 min. Banana peduncle, modified and chemically treated with acid and basic solutions, was also used for Pb(II) removal and showed 94% removal of the contaminant (2.49 mg g⁻¹) [103]. The removal of Pb(II) was also investigated using a biosorbent obtained from Moringa oleifera leaves modified with nitric acid. The removal efficiency reached 86.72% with an adsorptive capacity of 208.12 mg g⁻¹ in 60 min [104]. Tomato leaves were used for the removal of Pb(II) from contaminated waters and showed a removal efficiency of 78.4% and an adsorptive capacity of 45.77 mg g⁻¹ [105]. The use of the Eichhornia crassipes plant modified with phosphoric acid for Pb(II) removal from contaminated water was also successful, with a qe of 3.99 mg g⁻¹ [106].

Quyen et al. [107] synthesized a biochar from coffee husk residues for the removal of Pb(II) and Cd(II). The results showed adsorptive capacities of 116.3 and 139.5 mg g⁻¹, respectively, demonstrating the high efficiency of the proposed biosorbent. On the other hand, persimmon residues were used in the synthesis of a biosorbent for the removal of Pb(II) and Cr(VI) from aqueous solutions. The adsorptive capacities reached were 68.79 mg g⁻¹ and 139.40 mg g⁻¹, respectively, due to the strong affinity of the tannins present in its composition for the metallic ions [108].

In the study by Mili et al. [109], a biosorbent composite obtained by combining Opuntia ficus-indica cactus cladodes and chitosan, in a 1:1 (w/w) ratio, was used for the removal of Pb(II), Ni(II), Cu(II), and Cd(II). The data showed a higher affinity of the biosorbent for Pb(II) ions, removing 102 mg g⁻¹, followed by 34.7 mg g⁻¹ for Cu(II), 26.00 mg g⁻¹ for Ni(II), and 20.3 mg g⁻¹ for Cd(II).

Ganji et al. [110] utilized pistachio shells for the removal of Pb, Cu, Co, and Ni in a batch system, using 45 mg of the adsorbent, pH 7, and an initial contaminant concentration of 100 mg L⁻¹. The results showed an adsorption capacity of 29, 24.5, 23, and 29.3 mg g⁻¹, representing 87%, 73%, 69%, and 88% removal, respectively

Biosorbents synthesized from raw honey waste, originating from the fermentation of an alcoholic beverage in Ethiopia, were employed for the simultaneous removal of Cd(II), Cu(II), Pb(II), and Zn(II). The study investigated the effect of acid and base modification on the biosorbent regarding its contaminant removal efficiency. The nitric acid treatment showed the highest removal efficiency, achieving 16.13 mg g⁻¹ for Cd(II), 16.96 mg g⁻¹ for Cu(II), 17.24 mg g⁻¹ for Pb(II), and 16.67 mg g⁻¹ for Zn(II), which was attributed to the increase in porosity and exposure of active sites [111].

Agroforestry residues, such as Pinus radiata, were chemically modified with NaOH and used for the removal of hexavalent chromium (Cr(VI)). The study demonstrated an adsorptive capacity of 13.95 mg g⁻¹ in 45 min of contact, representing 99.12% removal efficiency [112].

Almond shells chemically modified with hexane showed better removal efficiency for Ni(II) compared to in natura almond shells, being 3.60 and 5.67 mg g⁻¹, respectively, with an equilibrium time of 10 min [113].

Date pit waste was modified with zinc oxide (ZnO) for the removal of divalent heavy metals, namely Cu²⁺, Ni²⁺, and Zn²⁺, from aqueous solution. The evaluated biosorbent demonstrated a highly satisfactory adsorption capacity, being 82.4, 71.9, and 66.3 mg g⁻¹, respectively. The study also addressed the material’s reusability, which reached 4 cycles [114].

Another study synthesized activated carbon from corn stalk residues for the removal of Co(II) ions from aqueous solutions, where it was found that the adsorptive capacity of the material increased with the increase in the initial contaminant concentration, reaching 202.68 mg g⁻¹ at 250 mg L⁻¹ [116].

Rice husk modified with nitric acid (0.6 g), in turn, was applied for the removal of Mn(II) and showed an adsorptive capacity of 60.24 mg g⁻¹ with a contact time of 60 min in the work of [117].

Another alternative for heavy metal removal that has been explored as biosorbents is marine materials, due to their availability and biocompatibility. The presence of chitin and chitosan in their composition is also attractive for the process, as is their insolubility [119]. Catla catla fish scale waste was utilized as a biosorbent for the removal of Cr(III) and Co(II). The material was prepared and chemically treated in an acidic medium and applied in batch adsorption, demonstrating a satisfactory maximum adsorption capacity of 304.88 and 383.14 mg g⁻¹ for chromium (Cr) and cobalt (Co), respectively [115].

Algal biomass from Chlorella sorokiniana (60%) was utilized to synthesize a composite, along with kaolin (12%) and ferric chloride (FeCl3) (28%), for the removal of As(III). Considering the recalcitrant nature and difficulty in removing arsenic, the evaluated biosorbent achieved 89% efficiency at pH 6 (17 mg g⁻¹) [118].

It can be observed that the reviewed studies underscore the versatility of agricultural, forestry, and marine residues as effective biosorbents for a wide range of heavy metals, including Pb(II), Cd(II), Cr(VI), and As(III). While raw residues demonstrate potential, the highest adsorption capacities were notably associated with chemically modified materials and composites, such as nitric acid-treated Moringa oleifera leaves and Catla catla fish scales. These treatments typically enhance surface porosity and expose functional groups—such as tannins in persimmon residues and chitin in marine biomass—significantly boosting removal efficiencies. Consequently, these waste-derived biosorbents present a sustainable and competitive alternative to conventional treatment methods.

8. Advantages, Limitations, and Comparison with Conventional Adsorbents

Various types of conventional adsorbents have been used as effective solutions for water decontamination through adsorptive processes, such as activated carbon, ion-exchange resins, alumina, silica gel, and zeolites [120]. These materials are manufactured on a large scale and sold commercially as filtering media [121].

Despite their high efficiency in removing contaminants, there are some limitations related to high production costs stemming from elevated energy consumption [122]. Activated carbon, for example, has a high cost, which hinders its application and reinforces the search for new, efficient materials for the adsorption of emerging contaminants from contaminated water [96]. While conventional activated carbon remains the market standard due to its high efficiency, its widespread application is frequently restricted by high manufacturing costs and significant energy consumption. In contrast, adsorbents derived from agro-industrial residues (LCAd) present a compelling economic advantage. Since these materials originate from abundant biomass and waste by-products—which would otherwise require disposal—the cost of the raw material is virtually negligible. Consequently, even when processing steps are involved, the valorization of this biomass consolidates LCAds as a truly low-cost and sustainable alternative, shifting the paradigm from expensive commercial media to accessible, waste-derived solutions. For example, NaOH modifications.

In this context, the use of organic waste as biosorbents has been widely explored by the scientific community as an alternative to conventional adsorbents, and their excellent performance has attracted significant market attention [123]. The regenerative capacity of biosorbents, which prolongs their life cycle, and the generation of low sludge volumes in effluent treatment processes make them more competitive for applications in industrial plants [97].

The high availability of biosorbents contributes to their widespread application, given the enormous generation of agro-industrial waste and plant and animal biomass by modern industry [124]. Therefore, biosorbents present themselves as a sustainable and plausible alternative for the current global moment, in which the bioeconomy and circular economy must be central to corporate thinking [125].

The conversion of local waste, which is often abundant, into biosorbents favors developing communities and/or those remote from large urban centers, as it strengthens the local bioeconomy through technological solutions that improve quality of life via water and effluent treatment [126].

However, it is important that the adsorption processes for pollutants from contaminated water using biosorbents are designed based on the type of waste generated locally, to avoid transportation and storage costs. Furthermore, it must also be considered that the supply of these materials may vary over time, as their generation is linked to production processes and harvest seasonality [127].

The limitations and disadvantages of applying biosorbents in real systems with high concentrations of multiple contaminants can involve low selectivity due to interference from competing ions in the solution. Some biosorbents are also not chemically and thermally stable, which can compromise their technical viability and efficiency. The heterogeneity in the composition of biosorbents may also be another factor hindering their application, as some applications require quality control and standardization of the raw material to ensure process scalability [125]. Moreover, the environmental footprint of modification processes represents a significant drawback. Although chemical treatments using acidic or alkaline reagents and energy-intensive physical processes improve adsorption capacity, they introduce environmental risks regarding reagent toxicity, energy consumption, and waste disposal. Therefore, proper management of the process and the correct disposal of generated waste are essential considerations. Nevertheless, this issue is not unique to biosorbents; conventional adsorbents also entail significant environmental burdens, including high energy expenditure and waste production during manufacturing.

Indeed, the real-scale utilization of biosorbents is still quite limited at present, and there is an enormous need to commercialize this technology, given its importance in the current scenario where waste management and water/effluent treatment are urgent [95]. Many advances still need to be made in generating computational models and pilot-scale processes for an economic viability analysis of implementing biosorbents on a large scale.

The application of biosorbents in treatment systems is an approach that reduces the carbon footprint of the remediation process via two pathways: waste management and efficient purification of contaminated water [103].

In summary, the utilization of organic/biological waste as biosorbent materials is an approach that offers several economic and environmental advantages. By extending a material’s life cycle through a new applicability, its disposal is postponed, and consequently, its premature deposition in landfills is also avoided. From an environmental standpoint, this is one of the global priorities regarding sustainable initiatives for solid waste management [126]. Furthermore, these materials are considered highly versatile and can undergo surface modifications to further optimize their contaminant removal efficiency [52].

The scientific community has directed many investigations toward evaluating the adsorption capacity of biosorbents, which highlights the need for sustainable materials for removing emerging contaminants from water. However, many results still originate from exploratory laboratory tests in batch or column systems, and few studies have explored industrial-scale projects, where economic feasibility studies and life cycle assessments would greatly contribute to the advancement of this technology [128].

9. Future Perspectives and Emerging Challenges in the Development of Low-Cost Adsorbents for Water Treatment

The global shift toward low-cost natural adsorbents derived from agricultural waste, like fruit biomass or lignocellulosic residues, represents a critical move toward sustainable water treatment [129]. While laboratory studies confirm their robust capacity to remove various pollutants, the goal of large-scale implementation is currently obstructed by a triad of technical, regulatory, and socioeconomic barriers. To transition this science into real-world solutions, these hurdles must be systematically addressed.

9.1. Technical and Engineering Challenges in Scaling Up

9.1.1. Performance Consistency and Advanced Modification

A key challenge for natural adsorbents is ensuring consistent performance when scaled up, given the inherent variability of natural biomass [130]. This contrasts with synthetic materials, which offer high reproducibility. To overcome this, focus must be placed on advanced modification techniques that enhance material stability and selectivity.

Studies have already demonstrated the effectiveness of targeted modification. For instance, the research on diclofenac adsorption using a low-cost adsorbent derived from Guazuma ulmifolia Lam. fruit shows that combined chemical and thermal treatment is crucial to developing specific adsorption sites for complex contaminants [131]. This work demonstrates that low-cost materials can be engineered to target emerging contaminants, traditionally only removed by expensive commercial media. Furthermore, a rigorous study comparing the new adsorbent material from agro-industrial waste with a commercial adsorbent confirmed that optimized natural materials achieve performances competitive with commercial options, validating their economic and technical feasibility [14].

9.1.2. Reactor Design, Flow Dynamics, and Regeneration

Translating batch results into efficient continuous flow operations remains a major engineering challenge. Industrial deployment requires robust designs, such as packed-bed columns or fluidized-bed systems, which must account for the lower mechanical strength and potential for bed compaction typical of biomass-derived adsorbents. The rigor applied in modeling and optimizing column operations for waste-derived adsorbents is essential for predicting breakthrough curves and ensuring hydraulic efficiency at industrial scales, which is a focus of advanced chemical engineering studies [82].

Furthermore, the reusability and regeneration cycle requires rigorous attention. While the adsorption of contaminants has been successfully modeled in column studies using waste-derived materials [82], the subsequent regeneration method must be energy-efficient and environmentally benign. The disposal of the spent adsorbent, now concentrated with pollutants, must also follow a sustainable pathway to maintain the ecological integrity of the entire treatment process.

9.2. Perspectives on Integration and Socioeconomic Impact

9.2.1. Hybrid Systems and Selective Removal

Given the diverse chemical composition of real-world wastewater, relying solely on adsorption is often insufficient. The future points toward hybrid systems where natural adsorbents are integrated into a larger framework. They can serve as a highly effective pre-treatment step or as a polishing step. Integrating them with membrane technologies or Advanced Oxidation Processes (AOPs) maximizes overall efficiency and tackles a broader spectrum of pollutants.

9.2.2. Regulatory Compliance and Vulnerable Communities

A significant barrier to large-scale adoption is the lack of specific regulatory standards for biosorbents. Regulatory bodies demand proof of consistency and safety, which is challenging due to the inherent variability of natural raw materials. Therefore, research must prioritize detailed field validation and transparent toxicological assessments. Ultimately, the most profound impact of this research is socioeconomics. Low-cost adsorbents provide a feasible, decentralized water treatment solution for vulnerable and resource-scarce communities. The use of agricultural waste promotes the circular economy and fosters local autonomy in water management. By focusing on cost-effective modification methods and performance comparable to commercial materials [14], the technology can offer high performance at a fraction of the cost, as demonstrated by the removal efficiencies of modified materials against pharmaceuticals.

10. Conclusions

In conclusion, the reviewed studies demonstrate that low-cost natural adsorbents derived from agricultural, forestry, and marine residues offer a promising and sustainable approach for water treatment, effectively removing a wide range of contaminants such as heavy metals, dyes, and pharmaceuticals. However, the transition from laboratory-scale success to industrial implementation remains restricted by significant technical, regulatory, and socioeconomic challenges. Technically, the inherent variability of natural biomass compromises performance consistency compared to synthetic materials, requiring advanced modification techniques to ensure stability and selectivity. Furthermore, engineering hurdles regarding reactor design for continuous flow systems—specifically issues related to mechanical strength and bed compaction—along with the need for efficient regeneration and sustainable disposal of spent adsorbents, must be addressed.

From a regulatory perspective, the lack of specific standards and the necessity for rigorous toxicological validation hinder commercial adoption. Future perspectives point toward the integration of these biosorbents into hybrid systems, serving as pre-treatment or polishing steps alongside technologies like membrane filtration or Advanced Oxidation Processes (AOPs) to maximize overall efficiency. Ultimately, to consolidate these materials as viable solutions within the circular economy—particularly for decentralized treatment in resource-scarce communities—research must prioritize pilot-scale validations, life cycle assessments, and detailed economic feasibility studies.