Physicochemical Properties and Adsorption Mechanisms of Bentonite–Sawdust-Derived Carbon Composites

Abstract

The escalating global water crisis necessitates the development of efficient, sustainable, and cost-effective remediation technologies. This review highlights bentonite–sawdust-derived carbon composites as a promising class of adsorbents for the removal of diverse water pollutants. The synthesis strategies, physicochemical properties, key interfacial adsorption mechanisms, and adsorption performance toward different pollutant categories are systematically discussed. These hybrid materials exhibit synergistically enhanced properties, including increased surface area, optimized porosity, abundant functional groups, tunable surface charge, and improved structural stability, often outperforming the individual components. Their effectiveness has been demonstrated for both heavy metals (e.g., Cd and Pb) and organic contaminants (e.g., dyes and pharmaceuticals), governed by a combination of ion exchange, electrostatic attraction, π–π interactions, and pore-filling mechanisms. Current challenges related to large-scale production, long-term stability, and regeneration are critically evaluated, and future research directions for the sustainable application of these composites in advanced water treatment systems are outlined.

1. Introduction

The rapid growth of the global population, coupled with accelerated industrialization and agricultural intensification, has placed unprecedented pressure on freshwater resources, making water pollution one of the most critical environmental challenges of the 21st century [1,2]. Aquatic systems are increasingly contaminated by a wide range of pollutants, including toxic heavy metals, recalcitrant organic compounds (e.g., dyes, phenols, and pharmaceuticals), and emerging micropollutants, posing severe risks to ecosystems and human health [3,4,5,6]. Consequently, the development of efficient, cost-effective, and sustainable water remediation technologies has become a major research priority [2,7].

Among available physicochemical treatment methods, adsorption has emerged as a particularly attractive approach due to its simplicity, high efficiency, operational flexibility, and relatively low cost [8,9]. Adsorption relies on the accumulation of pollutants on the surface of solid materials through various physical and chemical interactions [10]. Although conventional adsorbents such as activated carbon are widely applied, their high production costs, limited reusability, and non-selective adsorption behavior often hinder large-scale and sustainable implementation [11,12]. These limitations have stimulated intensive research into alternative adsorbents that are environmentally benign, economically viable, and derived from abundant or waste resources [13,14].

In this context, composite adsorbents that combine complementary materials have attracted growing attention, as they can overcome the intrinsic drawbacks of individual components through synergistic effects [1,15,16,17]. Composites based on natural clays and biomass-derived carbons have emerged as promising candidates owing to their low cost, abundance, and tunable physicochemical properties [12,17,18]. Bentonite, a widely available smectite clay, exhibits a layered structure, high cation exchange capacity, and considerable surface area, making it effective for the removal of cationic contaminants, especially heavy metals [19,20,21]. However, its tendency to swell and disperse in aqueous media, together with limited affinity toward hydrophobic organic pollutants, restricts its standalone application [1,21].

Conversely, biomass-derived carbonaceous materials such as biochar and activated carbon are well known for their porous structures, large surface areas, and diverse surface functional groups, which enable efficient adsorption of a broad spectrum of organic and inorganic pollutants [22,23,24]. Sawdust, an abundant lignocellulosic by-product of the timber industry, represents an attractive and sustainable precursor for producing such carbons, contributing to waste valorization and circular economy strategies [25,26,27,28]. Nevertheless, biochar materials often suffer from fine particle size, variable surface chemistry depending on pyrolysis conditions, and challenges in post-treatment recovery from aqueous systems [29,30].

The integration of bentonite with sawdust-derived carbon into composite materials provides an effective strategy to exploit the strengths of both constituents while mitigating their individual limitations [15,16]. Bentonite–sawdust-derived carbon composites are engineered to exhibit enhanced surface area and porosity, abundant and diverse functional groups, tunable surface charge, and improved structural stability [1,15]. As a result, these composites demonstrate superior adsorption performance toward a wide range of contaminants, including heavy metals, dyes, phenols, and pharmaceutical residues [31,32,33,34]. Furthermore, additional functionalization strategies, such as magnetic modification, enable facile separation and regeneration, addressing key operational challenges associated with practical water treatment applications [30,33,35,36].

The present review provides a comprehensive and critical overview of bentonite–sawdust-derived carbon composites for water purification. It systematically covers: (i) synthesis and characterization approaches, including carbonization, activation, and composite integration strategies; (ii) physicochemical properties governing adsorption behavior; (iii) adsorption mechanisms and performance toward heavy metals and organic pollutants; and (iv) current challenges, limitations, and future research perspectives related to regeneration, stability, scalability, and real-world applicability.

The scope and novelty of this review, unlike general reviews of biochar-based adsorbents, is focused specifically on carbon composites derived from bentonite and sawdust, with an emphasis on preparation methods, interfacial integration strategies, and key physicochemical parameters controlling adsorption. Particular attention is given to contaminant-specific adsorption mechanisms, linking clay-mediated ion exchange and electrostatic interactions with carbon pore filling, π–π interactions, and hydrogen bonding. Practical aspects, including separation, regeneration, long-term stability, and scalability, are critically discussed to facilitate the translation from laboratory research to water treatment practice.

2. Materials and Methods

Generative Artificial Intelligence (GenAI) tools were used in this review exclusively for the preparation of schematic illustrations intended to support the visual presentation of conceptual frameworks. Specifically, the AI-assisted tool ChatGPT (GPT-4) was employed to assist in generating draft graphical layouts based on author-provided descriptions.

3. Synthesis and Characterization of Bentonite–Sawdust-Derived Carbon Composites

The rational design and controlled synthesis of bentonite–sawdust-derived carbon composites are critical for achieving high adsorption efficiency in water treatment applications. These materials are engineered hybrids rather than simple physical mixtures, intentionally combining the complementary properties of clay minerals and biomass-derived carbon. Typical synthesis routes involve the thermochemical conversion of sawdust into a carbonaceous phase, followed by its integration with bentonite through in situ or post-synthesis approaches. Additional treatments, such as chemical activation, surface functionalization, or magnetic modification, are often applied to further tailor textural properties, surface chemistry, and interfacial interactions toward specific classes of pollutants [7,8,15]. An overview of the principal synthesis pathways and the associated interfacial adsorption interactions is schematically illustrated in Figure 1.

3.1. Synthesis Methodologies for Composite Fabrication

The synthesis of bentonite–sawdust-derived carbon composites involve a sequence of interrelated steps aimed at combining the structural and chemical advantages of clay minerals with those of biomass-derived carbon. Unlike simple physical mixtures, these composites are designed to promote intimate contact between phases, enabling cooperative effects in porosity development, surface chemistry, and adsorption behavior. Typical fabrication routes include thermochemical conversion of sawdust into a carbonaceous phase followed by its integration with bentonite through in situ or post-synthesis approaches, often complemented by chemical activation or surface modification to further tailor interfacial properties [7,8,15].

3.1.1. Sawdust-Derived Carbon Production

The carbonaceous phase in bentonite–sawdust-derived composites is commonly generated via thermochemical conversion of lignocellulosic biomass, where the choice of process governs the resulting pore architecture, surface chemistry, and interfacial compatibility with the clay fraction [23,24,25,26]. Among the available routes, pyrolysis and hydrothermal carbonization are the most frequently applied, although they lead to markedly different carbon structures and adsorption-relevant properties [37,38,39]. At relatively low pyrolysis temperatures, the resulting carbon retains a higher density of oxygen-containing functional groups, including hydroxyl and carboxyl moieties. These functionalities enhance surface polarity and favor interactions with metal ions through electrostatic attraction and surface complexation, which is particularly relevant for the removal of cationic contaminants [36,40]. When combined with bentonite, these changes directly influence the balance between clay-driven ion exchange and carbon-mediated adsorption pathways, indicating that pyrolysis conditions should be selected in relation to the target contaminant class rather than treating biochar as a generic sorbent component [41,42,43,44,45].

Hydrothermal carbonization represents an alternative thermochemical pathway that proceeds in aqueous media at comparatively lower temperatures. HTC-derived carbons generally exhibit a higher proportion of oxygenated surface groups and a less developed pore network than pyrolytic biochar’s, which can influence adsorption behavior and interfacial interactions with bentonite. While such features may limit adsorption capacity for bulky organic molecules, they can enhance hydrogen bonding and surface anchoring between the carbon phase and the clay matrix, potentially improving composite stability and dispersion [15].

Overall, neither pyrolysis nor hydrothermal carbonization is universally superior; their suitability depends on the required balance between porosity, surface chemistry, and composite integrity. A recurring limitation in the current literature is the lack of systematic comparisons between pyrolysis- and HTC-derived composites under equivalent conditions, which hampers clear identification of structure–property–performance relationships [15,37,38,39,40].

3.1.2. Integration Strategies for Bentonite and Sawdust-Derived Carbon

Once the carbonaceous phase is obtained, various strategies are employed to integrate it with bentonite to enhance interfacial contact and exploit synergistic effects. The simplest approach involves physically mixing pre-formed bentonite and biochar. While operationally straightforward, this method often results in limited interaction between the phases, leading to aggregation and adsorption behavior that largely reflects the individual components rather than true composite functionality [15].

More effective integration can be achieved through co-pyrolysis, where bentonite is combined with sawdust or partially carbonized biomass prior to thermal treatment. This method promotes the formation of more integrated structures in which the clay influences carbon development, and in turn, the carbon phase enhances the pore volume and average pore width of the composite. Such effects have been reported in systems where biomass feedstocks were pretreated with clay suspensions before pyrolysis [15]. In situ carbon-bed pyrolysis methods, incorporating minerals such as montmorillonite with ball-milled biochar, further demonstrate that intimate clay–carbon contact can be achieved under appropriate processing conditions [46].

Impregnation and precipitation methods aim to improve phase integration by promoting the direct deposition or formation of the carbon phase on the bentonite surface. In these approaches, bentonite is either impregnated with a carbon precursor followed by carbonization or carbonaceous species are precipitated directly onto the clay matrix. Compared to physical mixing, these strategies generally provide better dispersion of the carbon phase and improved structural stability, although they require careful control of precursor concentration and processing conditions [47]. Low-temperature catalytic carbonization represents a related approach that enables controlled composite formation while maintaining relatively mild processing conditions, offering a potentially cost-effective pathway for producing composites with enhanced adsorption performance [48].

3.1.3. Chemical Activation and Surface Modification

Beyond the initial formation and integration of the clay and carbon phases, the properties of bentonite–sawdust-derived carbon composites are often adjusted through chemical activation and surface modification. These post-synthesis treatments primarily regulate pore structure, surface chemistry, and interfacial reactivity, thereby enhancing adsorption performance and operational stability under realistic water treatment conditions.

Acid activation of bentonite is one of the most used modification strategies. Treatment with mineral acids such as H₂SO₄, HNO₃, or HCl selectively leaches octahedral cations from the clay structure, leading to partial delamination, increased internal surface area, and enhanced surface acidity [15]. These changes generally improve affinity toward cationic pollutants by increasing the availability of acidic sites and strengthening electrostatic interactions. However, excessive acid treatment may compromise structural integrity and reduce cation exchange capacity, underscoring the need for controlled activation rather than aggressive leaching.

Organic and polymer modifications provide an alternative route for tailoring surface functionality and adsorption selectivity. Grafting or coating bentonite with organic molecules or polymers alters surface polarity, hydrophilicity, and the accessibility of adsorption sites. For example, dually organic-modified bentonite has demonstrated improved adsorption–desorption behavior toward pharmaceutical compounds such as tetracycline, highlighting the role of tailored surface chemistry in controlling affinity and reversibility [39]. Similarly, carboxymethyl cellulose sodium–bentonite composites have enhanced heavy metal uptake by introducing additional carboxylate groups capable of metal coordination, while simultaneously improving dispersion stability in aqueous media [21].

Activation of the sawdust-derived carbon fraction is equally important for composite performance. Chemical activation using agents such as K₂CO₃, H₃PO₄, or KOH is widely employed to increase specific surface area and generate a higher density of oxygen-containing functional groups, which contribute to adsorption through surface complexation and electrostatic interactions [25]. In addition to conventional chemical activation, mechanochemical approaches based on intensive grinding have been reported to significantly enhance microporosity in otherwise poorly porous biocarbon’s, leading to substantial increases in surface area and pore volume without the need for high-temperature treatment [49].

Magnetic modification has gained attention as a strategy to facilitate the recovery and reuse of sorbents. Incorporating magnetic nanoparticles, typically iron oxides, enables rapid separation of the composite from aqueous suspensions using an external magnetic field [35]. Magnetic bentonite and magnetic clay–biochar composites combine adsorption efficiency with improved operational handling, addressing one of the major practical limitations of powdered adsorbents [32]. However, the introduction of magnetic phases may partially occupy active sites or alter pore structure, which can reduce adsorption capacity if not carefully controlled. Beyond magnetic separation, incorporating metal oxides and additional clay phases into the carbon matrix has also been reported to enhance adsorption selectivity toward specific contaminants, giving rise to biochar-supported metal-inorganic nanocomposites as a promising class of environmentally benign sorbents for heavy metal removal [1,50].

Overall, chemical activation and surface modification provide versatile tools for tuning the physicochemical properties of bentonite–sawdust-derived carbon composites. As summarized in

Table 1. Overview of Key Synthesis Methods for Bentonite–sawdust-derived carbon Composites.

Synthesis Method	Core Principle	Advantages	Disadvantages/Challenges	References
Co-pyrolysis	Simultaneous carbonization of bentonite–biomass mixtures	Improved clay–carbon integration	Complex process control; pore blocking risk	[15,46]
Physical Mixing	Blending of bentonite with pre-formed biochar/activated carbon	Simple; low cost	Weak interfacial contact; aggregation	[15]
Impregnation/ Precipitation	Carbon precursor deposition on bentonite followed by carbonization	Enhanced dispersion; interfacial control	Multi-step synthesis; process complexity	[47]
Chemical Activation (of components)	Post-treatment of biochar or bentonite with acids/bases	High surface area; tailored functionality	Corrosive reagents; secondary waste	[15,25]
Organic/Polymer Modification	Grafting of organic or polymeric species	Improved selectivity and stability	Polymer leaching; high cost	[21,39]
Magnetic Modification	Incorporation of magnetic nanoparticles	Easy separation and reuse	Reduced capacity; pore occupation	[32,35]

, different modification strategies—ranging from acid activation and organic or polymer grafting to magnetic functionalization—offer distinct advantages but are also associated with specific limitations related to pore blockage, structural alteration, or regeneration efficiency. However, many studies focus primarily on maximizing adsorption capacity, while systematically evaluating trade-offs between surface area, functional group density, structural stability, and reusability remains limited. More comparative and mechanism-oriented investigations are therefore needed to identify modification strategies that provide balanced performance under application-relevant conditions.

Table 1 summarizes the principal synthesis, activation, and modification strategies employed for the preparation of bentonite–sawdust-derived carbon composites, highlighting their underlying principles, key advantages, and inherent limitations. The comparison illustrates that no single approach is universally optimal: while co-pyrolysis and impregnation-based methods favor intimate clay–carbon integration, they may require careful control to avoid pore blockage or structural degradation. Chemical activation and organic or polymer modification enable targeted tuning of surface chemistry and adsorption selectivity, often at the expense of increased processing complexity. Magnetic functionalization, although beneficial for adsorbent recovery and reuse, may partially compromise adsorption capacity due to occupation of active sites. Overall, the table emphasizes the need to balance textural properties, surface functionality, and structural stability when selecting or designing composite fabrication strategies for specific water treatment applications.

3.2. Characterization Techniques for Bentonite–Sawdust-Derived Carbon Composites

A consistent set of characterization techniques is required to verify the successful synthesis of bentonite–sawdust-derived carbon composites and to relate their properties to adsorption behavior. Structural and morphological methods are used to examine how bentonite and sawdust-derived carbon are combined; textural measurements quantify surface area and porosity; spectroscopic and elemental methods describe surface chemistry; and thermal and electrokinetic analyses provide information on stability and surface charge.

3.2.1. Structural and Morphological Investigations

Structural and morphological characterization is essential for understanding how bentonite and sawdust-derived carbon are assembled within the composite and how this architecture controls the accessibility of adsorption sites and mass transport. Key questions include whether the smectite framework is preserved or modified during processing, how uniformly the carbon phase is distributed, and whether the preparation route generates additional micro-/mesostructures that facilitate diffusion and up-take.

X-ray diffraction (XRD) is routinely employed to identify the mineralogical phases of bentonite and to track structural changes during composite formation [20,51]. The position and intensity of basal reflections (e.g., the (001) smectite peak) provide evidence on whether the interlayer structure remains intact, partially collapses, or expands after treatment and interaction with the carbonaceous phase. Shifts in basal spacing are commonly associated with intercalation of organic/inorganic species into the interlayer galleries, whereas peak broadening and intensity changes may indicate partial disordering, delamination, or heterogeneous stacking. The presence of an amorphous carbon contribution is often reflected by a broad diffraction feature at characteristic 2θ regions, and additional reflections may arise from metal oxides or other phases introduced through activation or magnetic modification [46,50,52]. Comparative analysis of XRD patterns of raw bentonite, sawdust-derived carbon (biochar/activated carbon), and the resulting composite therefore supports evaluation of clay–carbon interactions and overall structural integrity.

Microscopic techniques, primarily scanning electron microscopy (SEM), including field emission SEM (FESEM) and transmission electron microscopy (TEM), complement XRD by resolving morphology and phase distribution across different length scales. SEM is widely used to assess particle size and shape, surface roughness, fissures and pore development, and the extent to which the carbon phase coats or bridges bentonite platelets [21,24,46,53]. Many studies report rougher, more irregular composite surfaces relative to the parent mate-rials, consistent with partial carbon coverage and/or carbon occupying interparticle voids. Such textural changes can enhance accessible surface area and improve external mass transfer, although pronounced aggregation may reduce effective diffusion into the composite interior.

TEM provides nanoscale information on the arrangement of smectite layers and the spatial relationship between carbon domains and clay lamellae [46,53]. Depending on the synthesis route, TEM observations may indicate partially delaminate-ed/exfoliated structures, interlayer separation by carbonaceous matter, or the presence of carbon nanoparticles/thin films attached to clay surfaces. These features are frequently interpreted as indicators of improved accessibility of interlayer regions and the formation of micro-/mesoporous transport pathways, which can be beneficial for adsorption performance and mechanical stability.

Representative FESEM and TEM micrographs reported by Barakat et al. [32] clearly illustrate the interfacial integration of bentonite (BE) platelets with fibrous sawdust-derived carbon (SD), forming the bentonite/sawdust composite (BE/SD), and the subsequent decoration of the composite matrix by spherical Fe₃O₄ magnetic nanoparticles (MNPs), resulting in the BE/SD–MNPs system. The images reveal that magnetic nanoparticles are preferentially anchored on clay surfaces and within surface fissures and pores, resulting in a more heterogeneous but well-connected microstructure. This morphology is commonly associated with increased surface roughness, improved dispersion of the carbonaceous phase, and enhanced accessibility of adsorption sites (Figure 2a–i).

3.2.2. Textural Properties

Textural properties, including specific surface area, pore volume, and pore-size distribution, play a central role in controlling the adsorption capacity and kinetics of bentonite–sawdust-derived carbon composites. These parameters determine the accessibility of active sites, the diffusion of solutes within the pore network, and the extent to which different pollutant classes (e.g., bulky dye molecules versus hydrated metal ions) can penetrate and interact with internal surfaces.

Nitrogen adsorption–desorption measurements at 77 K are most used to evaluate these characteristics. The overall shape of the physisorption isotherm and the type of hysteresis loop provide qualitative insight into pore architecture and can be interpreted using IUPAC recommendations [21,22]. Quantitatively, the Brunauer–Emmett–Teller (BET) method is typically applied to estimate specific surface area, while Barrett–Joyner–Halenda (BJH) analysis or density functional theory (DFT) approaches are used to derive pore-size distributions and pore volumes, depending on the pore regime and model assumptions [21,22,53]. In many reported systems, composite formation increases accessible surface area and promotes the development of mesoporosity relative to raw bentonite or the carbon precursor alone.

Additional methods can complement N₂ adsorption. CO₂ adsorption at 273 K is commonly used to probe narrow micropores that may be less accessible to N₂, whereas mercury intrusion porosimetry can provide information on macropores and interparticle voids. Combining multiple techniques enables a more complete pore-size profile and helps distinguish intraparticle porosity from intergranular transport pathways.

Correlating textural descriptors with adsorption performance is essential for assessing composite quality. While higher surface area and total pore volume often support increased uptake, pore-size distribution and pore-network connectivity can be equally critical, particularly for larger organic molecules. Accordingly, composites exhibiting hierarchical micro–mesoporous structures are frequently reported to display faster adsorption kinetics and improved capacities compared with single-component counterparts, highlighting the importance of textural optimization in composite design [21,22,53].

3.2.3. Chemical Composition and Surface Chemistry

Chemical composition and surface chemistry govern the types and strengths of interactions between bentonite–sawdust-derived carbon composites and dissolved pollutants. Surface functional groups, heteroatom content, and the chemical states of near-surface species collectively influence electrostatic attraction/repulsion, surface complexation, hydrogen bonding, hydrophobic effects, and π–π interactions, and they are therefore closely linked to pH-dependent charge behavior and selectivity.

Fourier-transform infrared spectroscopy (FTIR) is widely used to identify surface functionalities. Typical spectra contain bands attributed to hydroxyl, carboxyl, phenolic, and other oxygen-containing groups associated with the carbonaceous phase, together with silanol and siloxane vibrations characteristic of bentonite [21,24,46,50,53]. Changes in band positions or intensities after composite formation or post-modification can indicate the introduction of new functionalities, removal of labile groups, or the formation of interfacial interactions (e.g., hydrogen bonding) between the clay and carbon components. In some studies, FTIR recorded before and after adsorption is used as qualitative evidence of functional-group involvement in pollutant binding, although assignments should be interpreted in combination with complementary techniques.

X-ray photoelectron spectroscopy (XPS) provides complementary near-surface information on elemental composition and oxidation states. Deconvolution of C 1s, O 1s, N 1s, and relevant metal core-level spectra enables identification of bonding environments such as carboxyl/carbonyl species, phenolic groups, nitrogen functionalities, and metal–oxygen motifs [53]. XPS is particularly valuable for confirming successful surface modification (e.g., introduction of metal oxides or grafted moieties) and for assessing interfacial chemistry between the bentonite fraction and the carbon phase. In adsorption studies, XPS may also reveal retained metal ions or surface-associated organic species and thereby support mechanistic interpretation.

Elemental analysis (CHNS/O) provides bulk contents of carbon, hydrogen, nitrogen, sulfur, and oxygen. These data assist in assessing the degree of carbonization of the sawdust-derived carbon, estimating the relative contribution of the carbon fraction in the composite, and identifying heteroatoms that can participate in adsorption via specific interactions. When interpreted alongside spectroscopic data, elemental analysis helps track composition changes arising from activation or chemical modification.

Raman spectroscopy is frequently applied to characterize the structural order of the carbon phase. The positions and relative intensities of the D and G bands provide information on defect density and the balance between ordered and disordered carbon domains [24,46]. These characteristics can influence surface polarity/hydrophobicity, electron-transfer behavior, and the propensity of the carbon surface to engage in π–π interactions with aromatic pollutants. Because Raman metrics can be sensitive to measurement conditions and fitting approaches, comparisons are most robust when evaluated within consistent experimental frameworks.

Overall, the combined use of FTIR, XPS, CHNS/O, and Raman spectroscopy enables a coherent description of composite surface chemistry and provides the basis for linking synthesis/modification choices to adsorption mechanisms and pollutant-specific selectivity trends discussed in the subsequent sections.

3.2.4. Thermal Stability and Surface Charge

Thermal stability and surface charge behavior are important parameters for evaluating the operational robustness of bentonite–sawdust-derived carbon composites, particularly about regeneration and reuse.

Thermogravimetric analysis (TGA) is commonly employed to assess thermal stability and decomposition behavior. Typical TGA profiles exhibit mass-loss steps associated with the removal of physically adsorbed water, decomposition of residual organic matter, and dehydroxylation of the clay structure [53]. Comparing the TGA curves of raw bentonite, sawdust-derived carbon, and the composite can help estimate the relative contribution of each fraction and reveal stability changes induced by composite formation or post-synthesis treatments. For materials intended for thermal regeneration, TGA provides a practical indication of temperature ranges that may be applied without substantial structural degradation.

Surface charge is typically evaluated through zeta potential measurements as a function of pH. The pH dependence of the zeta potential reflects the acid–base behavior of surface functional groups and enables estimation of the point of zero charge (pH_PZC) [49]. Knowledge of pH_PZC is important for anticipating electrostatic attraction or repulsion toward charged pollutants and for selecting pH conditions that favor adsorption of cationic versus anionic species. Post-modification treatments (e.g., acid activation, organic grafting, or deposition of metal oxides) often shift the zeta potential–pH relationship, indicating changes in surface chemistry and charge distribution; these shifts can therefore support interpretation of performance differences across operating conditions.

Taken together, TGA and zeta potential measurements complement structural, textural, and spectroscopic characterization by providing information on thermal robustness and electrostatic behavior. These descriptors are particularly relevant when assessing performance retention over repeated adsorption–regeneration cycles and when translating batch results to realistic water matrices.

4. Physicochemical Properties of Bentonite–Sawdust-Derived Carbon Composites

The adsorption behavior of bentonite–sawdust-derived carbon composites is governed by physicochemical descriptors that can differ substantially from those of the individual components [1,16]. Composite formation commonly alters textural characteristics (surface area, pore volume, and pore-size distribution), surface chemistry (type and abundance of functional groups), and surface charge; importantly, these descriptors can be tuned through the synthesis and modification strategies discussed in Section 3 [1,15]. The key properties relevant to pollutant removal are summarized in Section 4.1, Section 4.2 and Section 4.3.

4.1. Surface Area and Porosity

Specific surface area and pore-structure characteristics are among the most important factors controlling adsorption capacity and uptake kinetics [1]. Natural bentonite exhibits a layered smectite structure and a moderate specific surface area; however, its hydration/swelling behavior and colloidal dispersion in water can complicate effective site accessibility and practical solid–liquid separation during adsorption processes [1,21]. In contrast, sawdust-derived biochar typically provides a porous carbon framework with substantially higher surface area and a broader pore network [22,23,24]. When combined, bentonite–carbon composites frequently exhibit increased total surface area and an altered pore architecture relative to either precursor alone, reflecting the interplay between clay packing and carbon-derived porosity [19,46].

Multiple studies report that incorporation of biochar into clay matrices can increase pore volume and average pore width in the resulting composites [15,54]. For example, increases in pore volume and average pore width by factors of 1.6 and 1.73, respectively (relative to pristine biochar), have been attributed to biochar–clay integration effects [15]. Textural enhancement can also be achieved via bentonite modification itself. Intercalation with Fe/Al polyhydroxycations has been reported to increase the specific surface area from 37–51 m² g⁻¹ to 68–82 m² g⁻¹, accompanied by a shift toward a higher proportion of micro- and mesopores [19,20,54]. The resulting nanostructured materials commonly feature pores in the 1.5–8.0 nm range, which is favorable for adsorption of many dissolved species [19,20,54].

In composite systems, macroporosity contributed by the carbon phase can facilitate transport, whereas micro- and mesopores provide a high density of accessible adsorption sites and can support stronger interactions with solutes [21,23]. In magnetic sawdust-biochar systems, incorporation of Fe₃O₄ may partially block or shift the original porosity (e.g., from predominantly microporous to mesopore-dominated structures); nevertheless, adsorption performance can be maintained or improved due to changes in pore accessibility and the introduction of additional active sites [40]. Overall, hierarchical pore architectures can support both capacity and kinetics by combining fast diffusion pathways with a dense network of adsorption sites [26]. Accordingly, pyrolysis temperature, activation protocol, and related processing parameters strongly influence the structure and textural properties of the carbon fraction and, consequently, the performance-relevant descriptors of the final composite [24,55].

4.2. Functional Groups and Surface Chemistry

The surface chemistry of bentonite–sawdust-derived carbon composites is governed by functional groups contributed by both the clay and carbon fractions, and these functionalities largely determine adsorption selectivity and binding strength toward dissolved pollutants [15,16]. Bentonite provides edge-site hydroxyl groups, commonly described as silanol (–SiOH) and aluminol (–AlOH), together with exchangeable interlayer cations that underpin its cation-exchange capacity [18,19]. The sawdust-derived carbon fraction (biochar/activated carbon) contributes oxygen-containing functionalities whose abundance depends on thermochemical history; materials produced at relatively lower pyrolysis temperatures or subjected to mild oxidation typically retain higher densities of carboxyl (–COOH), hydroxyl (–OH), phenolic, and other C–O moieties [22,23,24,49,55]. When nitrogen-containing precursors or modifiers are used, additional N functionalities (e.g., pyridinic, pyrrolic, quaternary, or oxidized nitrogen) may be introduced and can enhance adsorption in certain systems by providing basic sites and altered surface polarity [22].

These functional groups enable multiple interaction pathways. Acidic oxygen-containing groups (e.g., –COOH and –OH) can promote uptake of metal cations via surface complexation and, depending on solution conditions, electrostatic attraction [15,31]. Because these groups undergo protonation–deprotonation equilibria, pH strongly influences surface charge and, consequently, adsorption affinity and mechanism [31,56]. In polymer-modified composites, the coexistence of bentonite hydroxyl sites with carboxyl and/or amino groups introduced by the organic modifier has been reported to increase binding toward specific metals (e.g., Cd²⁺), consistent with the formation of additional coordination/complexation sites [21].

Spectroscopic techniques are routinely used to relate surface chemistry to performance. FTIR provides qualitative identification of functional groups and can track changes in band positions/intensities during composite formation and post-modification [21,24,46,50,53]. XPS offers complementary near-surface information on elemental composition and chemical states, enabling more detailed assessment of introduced functionalities and clay–carbon interfacial chemistry [53,57]. As an illustrative modification strategy, sulfur doping has been reported to introduce sulfur-containing surface species (e.g., C–S and related motifs) into sawdust-derived biochar, which in turn improved cadmium adsorption capacity in the tested systems [33].

4.3. Surface Charge Characteristics

Surface charge is an important property of bentonite–sawdust-derived carbon composites and is strongly governed by solution pH and the type and abundance of surface functional groups [58]. Pristine bentonite typically exhibits a net negative charge arising from isomorphic substitution in the clay lattice, which underpins its high cation-exchange capacity [18,19]. The carbon fraction (sawdust-derived biochar/activated carbon) also displays pronounced pH-dependent charging behavior; as pH increases, deprotonation of acidic surface groups generally shifts the surface toward more negative values [31,58].

In composite systems, the combined contributions of bentonite, the carbon phase, and any additional modifiers enable tuning of the surface-charge profile. For example, carboxymethyl cellulose sodium/bentonite composites have been reported to maintain negative surface potentials across a broad pH range, which can favor adsorption of cationic metals (e.g., Cd²⁺) via electrostatic attraction in addition to other binding pathways [21]. Mineral phases associated with the biochar fraction may also influence surface electrochemistry and ion-exchange behavior and, consequently, the net charge response of the composite [31]. Control of surface charge is therefore important for optimizing electrostatic contributions, which often play a substantial role in the removal of charged contaminants from water [47,58].

Zeta potential measurements as a function of pH provide experimental quantification of surface charge behavior and enable estimation of the point of zero charge (pH_PZC) [49,55]. Such data support prediction of adsorption trends under varying solution conditions and help identify pH windows that favor uptake of cationic versus anionic species. Surface charge can also influence adsorption of ionizable organic molecules by modulating electrostatic attraction/repulsion and is sensitive to factors such as the degree of carbonization and solution composition (e.g., background electrolyte) [58].

4.4. Enhanced Structural Stability

An important advantage of bentonite–sawdust-derived carbon composites is their improved structural stability relative to the individual constituents, particularly pristine bentonite. Natural bentonite tends to swell and disperse in aqueous media, which can complicate solid–liquid separation and limit practical reuse [1,21]. Incorporation of a sawdust-derived carbon phase—especially via co-pyrolysis, impregnation, or related integration routes—can mitigate these limitations by constraining clay swelling and promoting the formation of mechanically robust composite particles [51].

In many systems, the carbon fraction acts as a reinforcing framework that interlocks with bentonite platelets and reduces their propensity for dispersion [21]. When polymer-assisted routes are used, additional interfacial interactions between bentonite and the organic/carbon phase may further enhance integrity by promoting network formation and improving thermal robustness, depending on the chemistry of the modifier [21]. More continuous composite architectures generally exhibit greater resistance to mechanical stress and can facilitate handling under operational conditions [18,51]. These improvements are relevant for process implementation because they support easier recovery of the adsorbent and more stable performance over repeated adsorption–desorption cycles [15].

Magnetic functionalization provides an additional practical advantage by enabling rapid separation from aqueous suspensions using an external magnetic field, as reported for magnetic clay–biochar composites [32,35,40,59,60]. More broadly, combining the shaping/binding characteristics of bentonite with the rigidity of the carbon fraction can enable preparation of granular sorbents with improved physical and mechanical properties [18,51,59]. A qualitative comparison of structural stability and related physicochemical characteristics of raw bentonite, sawdust-derived biochar, and representative composites is summarized in

Table 2. Comparative physicochemical properties of pristine bentonite, sawdust-derived carbon and their composites.

Property	Pristine Bentonite	Sawdust Biochar	Bentonite–Sawdust-Derived Carbon Composite	Significance for Adsorption	References
Specific Surface Area	Moderate; swelling-prone	High; pyrolysis-dependent	Elevated; hierarchical	Increased active site availability	[15,19,20,21,22,23,24,54]
Total Pore Volume	Limited accessibility	High micro-/mesoporosity	Improved pore network	Enhanced diffusion and entrapment	[15,21,54]
Pore Size Distribution	Mainly micro-/mesopores	Micro-/mesopores	Tunable; hierarchical	Size-selective adsorption	[19,21,23,54]
Major Functional Groups	Silanol, aluminol, exchangeable cations	–COOH, –OH, C–O	Combined surface chemistry	Multiple binding mechanisms	[15,21,22,24,33,49,55]
Surface Charge	Net negative	pH-dependent	Tunable; pH-responsive	Electrostatic attraction/repulsion	[21,31,49,58]
Cation Exchange Capacity	High	Low–moderate	Enhanced via synergy	Metal ion removal	[18,19,31]
Structural Stability	Swelling-prone	Brittle	Reduced swelling; robust	Reusability and lifespan	[1,15,18,21,40,51]
Magnetic Properties	Non-magnetic	Non-magnetic	Magnetically modifiable	Easy separation	[32,35,40]

.

Finally, representative examples illustrating how acid activation, organic/polymer modification, heteroatom doping, and magnetic functionalization influence surface area/porosity, surface chemistry, and surface charge in bentonite–sawdust-derived carbon composites are summarized in

Table 3. Influence of Specific Modifications on Physicochemical Properties of Bentonite–sawdust-derived carbon Composites.

Modification Type	Example	Impact on Specific Surface Area	Impact on Functional Groups	Impact on Structural Stability	References
Pyrolysis Conditions	High-temperature biochar	Increased; higher aromaticity	Reduced O-groups; stable C–C	Improved thermal stability	[23,24,41,55]
Acid Activation	H2SO4-treated bentonite/biochar	Increased internal area	Enhanced acidity (–OH, –COOH)	Stability depends on severity	[15,61]
Organic/Polymer Grafting	CMC, organic surfactants	Coverage-dependent	New specific binding sites	Improved mechanical integrity	[15,21,39]
Magnetic Modification	Fe3O4 co-precipitation	Slight decrease (pore blocking)	Fe–O surface complexes	Improved recoverability	[32,35,40]
Mechanochemical Activation	Ball milling	Increased micro-/mesoporosity	Oxygen functionalities	Improved dispersion	[46,49]
Sulfur Doping	H2SO4, Na2S2O3 treatment	Modified pore structure	Sulfur-containing sites	Enhanced chemical stability	[33]

.

Table 2 compares key physicochemical descriptors of pristine bentonite, sawdust-derived biochar, and their composites. Pristine bentonite is characterized by a layered (smectitic) structure, moderate specific surface area, and comparatively low pore volume, while exhibiting high cation-exchange capacity associated with permanent structural charge and exchangeable interlayer cations. Sawdust-derived biochar, in contrast, typically shows substantially higher BET surface area and a more developed micro–mesoporous network, together with abundant oxygen-containing surface functionalities; however, its intrinsic cation-exchange capacity is generally low compared with that of smectitic clays. The composite materials integrate contributions from both phases: in most cases, their specific surface area and total pore volume exceed those of bentonite and are comparable to, or slightly lower than, those of the biochar fraction, while the pore-size distribution becomes more hierarchical due to combined micro-, meso-, and macropore contributions. Importantly, a significant fraction of the clay’s cation-exchange capacity can be retained, and the overall density/diversity of surface functional groups is often enhanced due to the presence of the carbon phase. The composites may also exhibit improved thermal robustness and shifted surface-charge behavior (e.g., pH-dependent zeta potential trends) relative to the parent materials. Overall, the data summarized in Table 2 support a synergistic modification of texture, surface chemistry, and stability that is frequently associated with improved adsorption performance in bentonite–sawdust-derived carbon composites. Building on this general comparison, Table 3 summarizes how acid activation, organic/polymer modification, heteroatom doping, and magnetic functionalization further tailor these descriptors.

Table 3 summarizes how different modification strategies alter the physicochemical properties of bentonite–sawdust-derived carbon composites. Acid activation of the clay phase generally increases BET surface area and microporosity and enhances surface acidity, although partial leaching of structural cations may reduce cation exchange capacity. Organic and polymer modifications typically introduce additional functional groups (e.g., carboxyl, amino, quaternary ammonium), which can improve affinity and, in some cases, selectivity toward metal ions or organic molecules, even when the overall surface area remains unchanged or decreases slightly due to partial pore blocking. Heteroatom doping of the carbon phase (for example, with nitrogen or sulfur) modifies the electronic environment and creates new donor or soft-ligand sites, strengthening complexation with heavy metals and π–π or n–π interactions with aromatic pollutants. Magnetic functionalization, most often via deposition of iron oxides, tends to reduce BET surface area to some extent and alter pore-size distribution, but introduces additional metal-oxide active sites and, importantly, enables rapid magnetic separation of the adsorbent from water. Overall, the trends presented in Table 3 illustrate that targeted modification allows textural properties, surface chemistry and surface charge to be tuned in a complementary manner, providing a flexible basis for tailoring composites to specific contaminant types and operating conditions.

5. Adsorption Mechanisms and Performance for Pollutant Removal

The adsorption performance of bentonite–sawdust-derived carbon composites is governed by multiple mechanisms that can operate concurrently and can be tuned toward different pollutant classes. Integrating bentonite—providing cation-exchange capacity and a layered structure—with sawdust-derived biochar—offering a porous carbon matrix and diverse surface functionalities—creates complementary uptake pathways and frequently results in improved removal performance compared with the individual components [1,15,16]. The dominant mechanisms and their relevance to pollutant removal are outlined below.

5.1. Adsorption Mechanisms

Pollutant uptake by bentonite–sawdust-derived carbon composites is typically controlled by several concurrent processes, the relative importance of which depends on pollutant type, composite descriptors (texture, surface chemistry, and charge), and solution conditions (notably pH, ionic strength, and temperature) [62,63]. Clarifying these contributions is essential for rational adsorbent design and optimization.

Ion exchange. Ion exchange is a primary mechanism associated with the bentonite fraction, particularly for cationic heavy metals. Isomorphic substitution within the clay lattice generates a permanent negative structural charge that is compensated by exchangeable interlayer cations (e.g., Na⁺, Ca²⁺), which can be replaced by dissolved metal ions [18,19,20]. In some systems, the carbon fraction may also contribute to exchange-like behavior via oxygenated surface sites and/or mineral (ash) phases, especially when the carbon is modified to increase its ion-binding functionality [31].

Electrostatic interactions. Both bentonite and the carbon phase exhibit pH-dependent charging behavior due to protonation/deprotonation of surface functional groups (e.g., carboxyl and hydroxyl groups on the carbon fraction and edge hydroxyls on the clay) [31,58,63]. Electrostatic attraction therefore contributes to uptake of oppositely charged solutes. Negatively charged composites tend to favor adsorption of metal cations (e.g., Cd²⁺, Pb²⁺) and cationic dyes (e.g., methylene blue), whereas positively charged surfaces can promote binding of anionic contaminants under appropriate pH conditions [15,21,47]. Because pH simultaneously controls surface charge and pollutant speciation, it is a critical parameter for modulating electrostatic contributions [31,38].

Surface complexation and (co-)precipitation. Surface functional groups (e.g., hydroxyl, carboxyl, and phenolic groups) can bind metal ions through inner- or outer-sphere complexation and may contribute substantially to metal removal alongside ion exchange [33,64]. At elevated pH, hydrolysis and precipitation of metal hydroxides (or co-precipitation with surface-associated species) can occur on or near the adsorbent surface, providing an additional removal route under specific conditions [36]. These processes commonly coexist with electrostatic attraction and ion exchange.

π–π and hydrophobic interactions. Aromatic domains within the biochar fraction can support π–π electron donor–acceptor interactions with aromatic organic pollutants (e.g., dyes, PAHs, pharmaceuticals) [22,63]. For nonpolar or weakly polar compounds, hydrophobic interactions may become increasingly important, particularly for composites with higher carbonization degree or organic modification that increases surface hydrophobicity [15,63]. The presence of extended aromatic surfaces and accessible porosity in clay–biochar composites are therefore favorable for these mechanisms [15]. In addition to π–π interactions with aromatic organic molecules, π-electron-rich domains on the carbon surface may also participate in interactions with metal centers. Such interactions are conceptually analogous to π–metal coordination observed in classical organometallic systems, such as Zeise’s salt, where electron donation from π orbitals stabilizes metal complexes. Although these interactions are expected to be weaker and more heterogeneous on carbonaceous surfaces, they may contribute to the retention of certain metal species, particularly under conditions where oxygen-containing surface functional groups are limited or partially blocked.

Pore filling and physical adsorption. Micro-, meso-, and macropores associated with the carbon fraction provide numerous sites for physical adsorption (van der Waals interactions) and, in some cases, capillary condensation [23]. Pollutants can diffuse into and be retained within the pore network, a behavior often described using pore-filling concepts, especially for small molecules capable of accessing narrow micropores [22,63].

Organic partitioning. For poorly soluble organic contaminants, partitioning into the organic/carbonaceous fraction can represent an additional uptake pathway [15]. In such cases, distribution between the aqueous phase and the carbon matrix may be described using partition coefficients reflecting contaminant affinity for the organic phase.

5.2. Performance for Heavy Metal Removal

Bentonite–sawdust-derived carbon composites have been extensively studied for the removal of heavy metals from aqueous systems, owing to the complementary effects of clay-driven ion exchange, pH-dependent electrostatic interactions, and surface complexation involving oxygen- (and, where applicable, heteroatom-) containing functional groups. The reported adsorption performance is strongly influenced by solution chemistry, including pH, ionic strength, the presence of competing ions, and the specific synthesis or modification strategy used.

• Cadmium (Cd).

Cadmium removal is typically governed by a combination of cation exchange on the bentonite fraction and coordination with oxygen-containing functional groups associated with the carbon or organic component. For instance, carboxymethyl cellulose sodium–bentonite composites have been reported to achieve a maximum adsorption capacity of 42.43 mg g⁻¹ for Cd²⁺, highlighting the role of carboxylate functionalities in metal binding [21]. Magnetic clay–biochar composites have also demonstrated effective Cd uptake, with reported capacities of up to 26.22 mg g⁻¹, while offering the practical advantage of easy magnetic separation [32]. Further enhancement of Cd binding has been achieved through sulfur functionalization of sawdust-derived biochar, introducing high-affinity sulfur-containing sites capable of selectively coordinating Cd²⁺ ions [33].

• Lead (Pb).

Lead removal is generally facilitated by negatively charged surfaces and strong surface complexation with oxygenated functional groups, while magnetic functionalization aids in the post-treatment recovery of the adsorbent. For example, magnetic biochar prepared from eucalyptus leaf residue was reported to reach an adsorption capacity of 52.4 mg g⁻¹ for Pb²⁺, underscoring the broader potential of magnetically responsive carbon-based sorbents for lead removal [34].

• Chromium (Cr(VI)) and Arsenic (As(V)).

The removal of oxyanionic species such as chromate and arsenate differ fundamentally from that of divalent metal cations and is largely governed by surface charge and ligand-exchange mechanisms. Positively charged hydroxylated surfaces, particularly those modified with Fe- or Al-polyhydroxycations, promote effective binding of both bichromate and arsenate species through inner-sphere surface complexation [19]. In addition to adsorption, redox transformations may contribute to chromium removal, especially in systems containing redox-active components, complicating the interpretation of removal mechanisms [19]. For arsenic, the coexistence of multiple oxidation states and oxyanionic forms renders adsorption behavior highly sensitive to surface chemistry and aqueous speciation.

• Mercury (Hg).

Mercury removal presents additional challenges due to its complex environmental chemistry and speciation. Magnetic bentonite has been reported as an effective sorbent for Hg removal, combining clay-based adsorption sites with rapid magnetic recovery from suspension [35]. However, mercury may exist in inorganic, organometallic, or neutral forms, each exhibiting distinct affinities toward ion-exchange sites, oxygen-containing functional groups, and π-electron-rich carbon domains. Consequently, reported removal efficiencies should be interpreted with caution, as adsorption performance may vary significantly based on mercury speciation and transformation pathways in different environmental contexts.

• Iron (Fe).

Iron is not typically classified as a toxic metal; however, its mobility and reactivity can significantly influence certain aquatic and geochemical environments. Baseline removal performance has been reported for unmodified bentonite, with natural bentonite achieving Fe removal efficiencies of up to 85.5% under optimized conditions [36]. Such data are best regarded as reference behavior, providing a benchmark for assessing the added value of composite formation and surface functionalization strategies rather than as evidence of detoxification performance.

Across these examples, heavy-metal uptake by bentonite–sawdust-derived carbon composites are governed by the interplay between bentonite’s cation-exchange capacity and the complexing functionality of the carbon or organic fraction (e.g., –COOH, –OH). The relative contribution of each pathway is dictated by pH-dependent surface charge and metal speciation [15,31]. While the adsorption of metals such as Pb²⁺ and Cd²⁺ can often be interpreted using relatively simple ion-exchange and surface-complexation models, elements like mercury and arsenic require a more nuanced, speciation-aware perspective. Therefore, evaluation of composite performance for these elements should be framed within a broader biogeochemical context rather than relying solely on batch adsorption metrics.

5.3. Performance for Organic Pollutant Removal

For organic contaminants, the sawdust-derived carbon fraction is commonly the dominant contributor because it provides accessible porosity and aromatic/functionalized surfaces that support multi-interaction adsorption. Incorporation of bentonite can further improve performance by adding additional adsorption domains, modifying surface charge behavior, and improving handling/structural stability.

Dyes (e.g., methylene blue, methyl orange). Clay–biochar composites frequently show enhanced uptake of dyes, with performance governed by a combination of π–π interactions (between aromatic dye structures and carbon domains), electrostatic attraction/repulsion (depending on dye charge and pH), hydrogen bonding, and pore-filling within the micro–mesoporous network [24,63]. In some studies, methylene blue uptake increased by up to five-fold relative to pristine biochars [15]. Magnetic clay–biochar materials have been reported to achieve maximum adsorption capacities of 63.34 mg g⁻¹ for methyl orange, while enabling magnetic recovery [32].

Phenols and related compounds. Activated sawdust-based adsorbents have been reported to remove phenol with efficiencies up to 78.3% under optimized conditions [37]. Bentonite can also adsorb phenol and bisphenol A, with uptake strongly influenced by pH due to changes in both phenol speciation and surface charge [38]. In composite systems, complementary interactions—including hydrogen bonding, π–π interactions, and hydrophobic effects—can operate simultaneously, with relative contributions depending on carbonization degree and surface functionality.

Pharmaceuticals (e.g., tetracycline, norfloxacin). Biomass-derived carbons have shown substantial affinity toward antibiotics such as tetracycline, where uptake is commonly attributed to π–π electron donor–acceptor interactions, hydrogen bonding, and pore filling [22]. In parallel, organically modified bentonites can improve adsorption of tetracycline and ciprofloxacin by introducing tailored organic functionalities at the clay surface [39]. Clay–biochar composites have also been investigated for immobilization of norfloxacin, indicating applicability to a broader range of pharmaceutically active compounds [15].

Other organic contaminants and complex effluents. Acid-modified sawdust materials have demonstrated decolorization capability for tanning wastewater, illustrating relevance for complex industrial matrices [65]. More broadly, composite adsorbents are being developed for emerging micropollutants, including activated mineral–biochar systems targeting pharmaceutical residues [66].

Representative adsorption capacities and operating conditions for key heavy metals and organic pollutants are summarized in

Table 4. Adsorption Performance of Bentonite–sawdust-derived carbon Composites for Key Pollutants.

Pollutant Type	Pollutant	Adsorbent Type	Adsorption Performance	Dominant Mechanism	References
Heavy metals	Cd2+	CMC–bentonite composite	42.43 mg g−1	Ion exchange/complexation	[21]
	Pb2+	Magnetic biochar	52.4 mg g−1	Surface complexation	[34]
	Cr(VI)	Fe/Al-modified bentonite	Effective removal	Electrostatic/ligand exchange	[19]
	As(V)	Fe/Al-modified bentonite	Effective removal	Electrostatic/ligand exchange	[19]
	Hg	Magnetic bentonite	Effective removal	Complexation/magnetic separation	[35]
	Fe	Natural bentonite	85.5% removal	Ion exchange	[36]
Organic pollutants	Methylene blue	Clay–biochar composite	11.94 mg g−1	π–π interactions	[15]
	Methyl orange	Magnetic clay–biochar	63.34 mg g−1	Electrostatic/pore filling	[32]
	Phenol	Activated sawdust	78.3% removal	Hydrophobic adsorption	[37]
	Tetracycline	Waste fiberboard biochar	Significant capacity	π–π/H-bonding	[22]

.

5.4. Factors Influencing Adsorption Efficiency

The efficiency of pollutant removal by bentonite–sawdust-derived carbon composites depend on a combination of solution chemistry and material/process parameters, as consistently reported for clay–biochar and bentonite-based adsorbents [37,63]. The most frequently discussed factors include solution pH, initial pollutant concentration, adsorbent dosage, contact time, temperature, the presence of coexisting ions, and the particle size of the carbon fraction.

Solution pH. pH is typically the most influential operational parameter because it simultaneously controls surface charge (via protonation/deprotonation of functional groups) and pollutant speciation/charge state [31,38,58]. For many metal cations, adsorption increases from moderately acidic toward near-neutral conditions, which is commonly attributed to reduced competition with H⁺ and increased availability of deprotonated binding sites on the carbon/organic fraction, together with more negative surface potentials that favor electrostatic attraction [21]. In contrast, adsorption of anionic species often decreases at higher pH due to increased electrostatic repulsion from negatively charged surfaces and shifts in oxyanion speciation [19,38]. These qualitative trends are consistent with the pH optima summarized for different metals and oxyanions in Table 4.

Initial pollutant concentration. The influence of initial concentration is commonly evaluated using equilibrium isotherms. Higher initial concentrations increase the mass-transfer driving force and can yield higher measured uptake until adsorption sites approach saturation [15]. Many systems exhibit a transition from a concentration-dependent regime to a plateau regime where site availability and intraparticle diffusion become increasingly important [37]. The Langmuir/Freundlich parameters compiled in Table 4 illustrate how composite composition and modification strategy influence apparent maximum capacity and affinity.

Adsorbent dosage. Increasing adsorbent dosage typically improves overall removal efficiency because more binding sites are introduced into the system [37]. However, the adsorption capacity normalized per unit mass often decreases at high dosages, which is commonly attributed to particle aggregation, overlap of available sites, and incomplete utilization of internal porosity [37,63]. Optimization of dosage is therefore necessary to balance removal efficiency with material consumption in practical deployment.

Contact time. Kinetic studies frequently show a two-stage behavior: rapid initial uptake associated with readily accessible external sites followed by a slower stage controlled by diffusion into pores and gradual approach to equilibrium [36,38]. Experimental datasets are commonly fitted by pseudo-first-order or pseudo-second-order models; the latter is often reported to provide a better empirical description when chemisorption-like contributions are significant [37,63]. The characteristic equilibration times summarized in Table 4 reflect the combined effects of pore architecture, particle size, and mass-transfer limitations.

Temperature. Reported temperature effects vary across systems. In some cases, increased temperature enhances uptake, consistent with improved diffusion and/or endothermic adsorption contributions; in other cases, adsorption decreases with temperature, which is consistent with exothermic behavior [37]. Thermodynamic analyses (often based on van ’t Hoff relationships) are used to estimate apparent enthalpy/entropy changes and to support mechanistic interpretation, although these parameters may be sensitive to the chosen model and experimental range.

Coexisting ions. In real waters, coexisting ions can substantially affect adsorption through competition for binding sites and modification of the electrical double layer [63]. High concentrations of common ions (e.g., Na⁺, Ca²⁺, Cl⁻, SO₄²⁻) often suppress uptake of target contaminants, whereas moderate ionic strength can sometimes enhance removal by screening electrostatic repulsion, depending on pollutant speciation and the dominant binding pathway [63]. Such matrix effects are pollutant-specific and should be considered in process design.

Carbon particle size. Particle size of the biochar/carbon fraction influences both kinetics and operational handling. Smaller particles provide higher external surface area and shorter diffusion pathways, which can accelerate adsorption and may increase apparent uptake under diffusion-limited conditions [63]. However, very fine particles can complicate solid–liquid separation and increase pressure drop in fixed-bed systems. Consequently, many studies recommend intermediate particle-size ranges as a compromise between kinetic performance and practical operability.

An overview of how these factors affect adsorption capacities, kinetic parameters, and isotherm descriptors for representative pollutants is provided in Table 4.

Table 4 summarizes reported adsorption performances of bentonite–sawdust-derived carbon composites and related clay–biochar systems toward representative inorganic and organic pollutants. For heavy metals such as Cd²⁺, Pb²⁺ and Hg²⁺, capacities in the range of tens of milligrams per gram are commonly observed under optimized conditions, with ion exchange, electrostatic attraction and surface complexation identified as the dominant mechanisms. For organic contaminants, including dyes (e.g., methylene blue, methyl orange), phenolic compounds and selected pharmaceuticals, composites typically show higher uptake than the individual components, reflecting the contribution of π–π interactions, hydrophobic effects, hydrogen bonding and pore filling in the carbon phase. The table also highlights the influence of experimental parameters such as pH, contact time and temperature, as well as the impact of magnetic or chemical modification on performance. Overall, the data compiled in Table 4 illustrate the broad applicability of bentonite–sawdust-derived carbon composites for the removal of both inorganic and organic pollutants and underline the importance of matching composite design and operating conditions to the properties of target contaminants.

5.5. Regeneration and Reusability

For sustainable and cost-effective deployment in wastewater treatment, regeneration and repeated use of adsorbents are essential considerations [11,67]. Studies on bentonite–sawdust-derived carbon composites and closely related clay–biochar systems indicate that these materials can retain a substantial fraction of their adsorption performance over multiple cycles; however, outcomes depend strongly on the selected regeneration protocol and on the dominant binding pathways (e.g., electrostatic/ion-exchange versus inner-sphere complexation or precipitation) [11,15,34,67].

Chemical regeneration. Chemical regeneration is commonly performed using acidic, alkaline, or chelating solutions to desorb retained pollutants. For bentonite-based systems, significant desorption of methylene blue has been reported using aqueous HCl, with effective regeneration maintained over several consecutive cycles (up to seven in one study) [68]. In general, uptake dominated by ion exchange and electrostatic interactions is more readily reversible than adsorption governed by inner-sphere complexation or surface precipitation [11]. Nevertheless, repeated exposure to aggressive regenerants may alter surface functional groups, promote leaching of exchangeable/structural cations, or change pore accessibility, leading to gradual shifts in capacity and selectivity.

Thermal regeneration. Thermal regeneration relies on heating spent adsorbents to remove or decompose retained species. The carbon fraction is typically stable under moderate heating, and several studies have explored temperature windows that enable partial restoration of adsorption performance while limiting damage to the porous carbon structure [66,67]. However, thermal approaches are energy-intensive and, at elevated temperatures, may induce loss of labile surface functionalities and/or changes in pore structure. Optimization of temperature and treatment duration is therefore required to balance regeneration efficiency against material stability.

Magnetic recovery and reuse. For magnetically functionalized bentonite–sawdust-derived carbon composites, recovery from treated water can be simplified using an external magnetic field, improving operational handling and reducing solids-management losses during cycling [32,40]. Magnetic biochar-based systems have shown good reusability over multiple cycles; for example, Pb²⁺ adsorption capacity remained relatively stable over six regeneration cycles while maintaining efficient magnetic separation [34]. Similar cycling behavior has been reported for magnetic clay–biochar sorbents used in dye removal, where methylene blue removal efficiencies of ~70% were still observed after six adsorption–desorption cycles [15].

Electrochemical and advanced oxidation approaches. Electrochemical advanced oxidation processes (EAOPs) have been proposed as alternative “green” regeneration routes for biochar-based adsorbents [67]. In these systems, oxidizing species generated in situ can degrade adsorbed organic pollutants and may partially restore active sites without requiring concentrated chemical regenerants. Although still emerging, such approaches align with circular-economy concepts and may complement conventional chemical and thermal protocols.

Overall, available evidence suggests that bentonite–sawdust-derived carbon composites can be reused across multiple cycles with acceptable performance losses when regeneration conditions are matched to the prevailing adsorption mechanisms. Combined with the low cost and availability of precursor materials, this reusability supports the economic and environmental attractiveness of these composites as alternatives or complements to conventional activated carbons.

6. Challenges, Limitations, and Future Perspectives in Bentonite–Sawdust-Derived Carbon Composite Application

Despite substantial progress in the development and laboratory validation of bentonite–sawdust-derived carbon composites for water purification, several challenges continue to limit broader adoption and commercial implementation [8]. While many studies report high removal efficiencies under controlled conditions, translation to long-term operation and field-relevant water matrices remains insufficiently demonstrated [4]. This section summarizes key obstacles highlighted in the literature and outlines future research directions aimed at improving robustness, reproducibility, and practical deployability of these hybrid sorbents.

6.1. Current Challenges and Limitations

The optimization and implementation of bentonite–sawdust-derived carbon composites are constrained by recurring scientific and engineering issues.

Recovery and regeneration. Efficient recovery of spent sorbent from treated water remains a practical bottleneck, particularly for fine powdered biochar fractions that can remain dispersed, complicating solid–liquid separation and potentially contributing to secondary turbidity [29,30]. Magnetic functionalization offers a promising route to facilitate separation using external magnets; however, performance can be affected by aggregation, changes in magnetic response over repeated cycling, and process-dependent losses during handling [32,69]. In parallel, chemical and thermal regeneration methods may require substantial reagent or energy inputs and can gradually alter surface chemistry or pore accessibility, raising concerns regarding operating cost and long-term stability [3,11]. Achieving high regeneration efficiency while maintaining structural integrity and adsorption capacity over extended cycling therefore remains a key challenge [12].

Performance variability and consistency. Variability in sawdust feedstock (composition, particle size, moisture content) and differences in bentonite mineralogy can translate into significant batch-to-batch variation in composite texture, surface chemistry, and adsorption performance [15,18]. Controlling synthesis parameters—such as pyrolysis temperature, activation strategy, phase ratio, and residence time—to obtain reproducible materials with targeted surface area, pore hierarchy, and functional-group distributions at scale is non-trivial [5,70]. In the absence of standardized production protocols and quality-control criteria, consistent performance across batches and across different water matrices remains difficult to guarantee [8].

Scale-up and commercialization constraints. Moving from laboratory studies to industrial deployment introduces additional constraints. Large-scale biomass-to-biochar conversion can be logistically demanding, requiring stable feedstock supply, pretreatment, and controlled thermochemical processing [5,71]. Practical implementation also requires appropriate sorbent handling (transport, dosing, and containment) and compatibility with existing treatment trains. Moreover, real wastewaters typically contain multiple competing species and fluctuating pH, ionic strength, and natural organic matter, which can reduce uptake relative to simplified laboratory systems [8,15]. Consequently, techno-economic assessment, life-cycle considerations, and compliance with regulatory requirements are essential components of commercialization pathways [4,72,73].

Long-term stability and leaching risks. Although composite formation often improves structural stability, long-term integrity under realistic operating conditions and the potential release of incorporated components (e.g., magnetic nanoparticles, organic modifiers) or previously adsorbed pollutants remain concerns [6,74]. Biochar itself may contain ash constituents and trace metals and can release residual organics under specific conditions, potentially creating secondary environmental risks [74,75,76]. Comprehensive toxicity/ecotoxicity evaluation across the full life cycle (use, regeneration, and end-of-life management) is therefore necessary to support environmentally acceptable deployment.

Lack of standardized assessment and benchmarking. A frequently noted limitation is the lack of harmonized procedures for comparing bentonite–sawdust-derived carbon composites with each other and with commercial adsorbents [12]. Differences in experimental conditions, reported performance metrics, model fitting approaches, and data presentation complicate direct comparison and hinder objective assessment of competitive advantages in application-relevant scenarios. More systematic benchmarking against established technologies and standardized testing protocols would support clearer performance evaluation and accelerate translation.

The challenges outlined above, together with representative mitigation strategies proposed in recent studies, are summarized in

Table 5. Key challenges and proposed mitigation strategies for bentonite–sawdust-derived carbon composites in water treatment.

Challenge	Description	Proposed Solutions/Research Directions	References
Recovery and Regeneration	Difficult separation; capacity loss	Magnetic modification; green regeneration; durability-oriented design	[3,11,12,29,30,32,34,69]
Performance Variability	Raw material heterogeneity	Feedstock standardization; synthesis control; QA characterization	[5,15,18,70]
ScalingUp and Commercialization	High cost; lab-to-field gap	Low-cost synthesis; modular reactors; pilot studies; TEA/LCA	[3,4,5,8,12,17,69,71,72,77,78,79]
Long-TermStability and Leaching	Leaching; degradation risks	Chemically stable composites; leaching tests; ecotoxicity assessment	[6,74,75,76]
Complex Wastewater Matrices	Competitive adsorption	Selectivity tuning; hybrid treatment systems	[15,80,81]
Economic Viability	High overall cost	Waste-derived feedstocks; adsorbent valorization; TEA	[1,3,4,7,13,15,18,27,28,72,82,83,84,85,86]

.

Table 5 organizes the main challenges associated with bentonite–sawdust-derived carbon composites into several categories and links them to potential mitigation strategies suggested in the literature. For recovery and regeneration, proposed solutions include magnetic modification to simplify solid–liquid separation, optimization of mild chemical and thermal regeneration protocols and exploration of electrochemical or advanced oxidation approaches to reduce reagent use and secondary waste generation. To address performance variability, authors emphasize the need for better control of feedstock quality, standardized synthesis conditions and implementation of quality-control criteria for key physicochemical parameters. At the scale-up and commercialization level, strategies focus on integrating biochar production with existing biomass or waste-management chains, conducting realistic pilot-scale tests in complex wastewater matrices and performing techno-economic and life-cycle assessments. Concerns about long-term stability and leaching have led to calls for more comprehensive ecotoxicological testing and the development of guidelines for safe use and disposal. Finally, several studies recommend the development of harmonized testing protocols and systematic benchmarking against commercial adsorbents to clarify the relative advantages and limitations of these composites in practical water-treatment scenarios.

6.2. Future Perspectives and Research Directions

Although the challenges outlined above remain substantial, recent literature points to several research directions that may accelerate the practical implementation of bentonite–sawdust-derived carbon composites for environmental remediation.

Facile and sustainable synthesis. Increasing attention is being directed toward simplified, resource-efficient, and low-cost fabrication routes that utilize abundant waste-derived precursors and reduce processing steps and chemical consumption [7,18,26,27]. Direct co-pyrolysis of sawdust with clay and mechanochemical approaches (e.g., ball milling) are being explored to improve phase integration while streamlining synthesis [17,78,87]. Further progress will require systematic optimization of process conditions (e.g., heating profiles, residence time, and activation strategies) and, where relevant, identification of suitable catalysts to tune pore hierarchy and surface chemistry toward target applications [15,87].

Advanced functionalization and tailored selectivity. Next-generation bentonite–sawdust-derived carbon composites are expected to increasingly rely on functionalization strategies that enhance selectivity for specific pollutant classes, including emerging contaminants (e.g., pharmaceuticals, pesticides, and microplastics) [66,81,88,89,90,91]. Promising approaches include grafting of polymers/biomolecules/specific ligands and heteroatom doping of the carbon fraction (e.g., N, S, P) to introduce additional binding motifs and modulate electronic and interfacial properties [16,33]. The development of “smart” or stimuli-responsive sorbents—where external triggers such as magnetic fields or controlled pH/temperature shifts facilitate pollutant capture and release—represents another active direction, particularly for improving recoverability and reusability [32,80].

Hybrid treatment trains and reactor design. To address limitations of adsorption as a stand-alone unit operation, there is growing interest in incorporating these composites into hybrid treatment trains. Integration with membrane filtration, photocatalysis, biological processes, and electrochemical methods can enable multi-barrier configurations with broader contaminant coverage and improved robustness under complex water matrices [71,72,77,79]. In parallel, increased emphasis is being placed on practical reactor configurations—such as fixed-bed columns, reactive filtration units, and other flow-through systems—that move beyond batch testing and provide more realistic pathways for scale-up and implementation [71,77].

A conceptual framework that situates these developments within a circular-economy context—covering sustainable feedstock sourcing, green synthesis, application, regeneration, and valorization of spent sorbents—is illustrated in Figure 3.

Life cycle assessment and techno-economic analysis. Comprehensive life cycle assessment (LCA) and techno-economic analysis (TEA) are increasingly recognized as necessary to quantify the overall sustainability and cost competitiveness of composite adsorbents and to benchmark them against conventional sorbents and treatment technologies [13,28,72,92]. Such studies typically account for feedstock logistics and preprocessing, energy and reagent inputs during production and regeneration, associated emissions, and end-of-life scenarios [1,3,72,83,84,85]. In a circular-economy context, particular interest is directed toward valorization routes for spent composites—for example, incorporation into building materials or use as soil amendments—if leaching and ecotoxicity risks are appropriately managed [72,86]. Under specific system boundaries and operating scenarios, biochar-containing treatment units may also contribute to improved greenhouse-gas balances; however, these outcomes are highly case-dependent and require transparent assumptions and sensitivity analyses [72].

Mechanistic understanding, modelling, and field validation. Further progress will also depend on deeper mechanistic insight into pollutant–sorbent interactions and on predictive modelling frameworks that can guide rational composite design. Advanced characterization (including high-resolution microscopy and complementary spectroscopic methods) can elucidate structural evolution and binding environments at nano- and molecular scales [93,94,95]. Computational approaches (e.g., molecular dynamics simulations and related modelling tools) offer complementary routes to explore clay–carbon interfacial structure and to rationalize adsorption pathways at the atomistic level [94]. Finally, more pilot-scale and field studies are required to validate long-term stability, regeneration performance, and operational practicality under realistic conditions in municipal, industrial, and agricultural wastewater matrices [60].

6.3. Summary of Advantages and Research Needs

Bentonite–sawdust-derived carbon composites combine several beneficial features, including the use of abundant and low-cost raw materials, enhanced adsorption capacities for both inorganic and organic pollutants, tunable physicochemical properties and, in many cases, promising regeneration and reuse performance. At the same time, several critical research needs remain, particularly in relation to process standardization, selectivity toward emerging contaminants, integration into practical treatment trains, life cycle and techno-economic assessment, long-term stability and environmental safety and large-scale validation. The main advantages and key research priorities are summarized in

Table 6. Main advantages of bentonite–sawdust-derived carbon composites for water treatment and key research needs for their broader implementation.

Aspect	Key Advantages	Priority Research Needs	References
Resource Utilization	Waste valorization; low-cost feedstocks	Sustainable synthesis routes; alternative waste sources	[17,26,27,28,96,97]
Adsorption Performance	Synergistic multi-mechanistic uptake	Selectivity tuning; testing in complex matrices	[15,19,21,22,32,33,34,35,36,37,38,39,98,99,100,101,102]
Operational Efficiency	Improved stability; magnetic recoverability	Regeneration efficiency; long-term cycling stability	[11,32,34,67,98]
Environmental Impact	Reduced footprint; circular pathways	Leaching assessment; standardized ecotoxicity tests	[6,74,75,76,103,104]
Commercial Viability	Low-cost raw materials; scalability potential	Production standardization; TEA; pilot-scale validation	[4,5,7,8,12,72,83,84,85,92]

.

Table 6 consolidates the main strengths of bentonite–sawdust-derived carbon composites alongside the most frequently cited research gaps. Key advantages include valorization of waste sawdust and abundant bentonite, high and tunable adsorption performance, and the ability to tailor texture and surface chemistry through comparatively simple synthesis and functionalization routes. In many systems, these materials also show promising regeneration and reuse potential. At the same time, Table 6 underscores that further progress relies on standardized synthesis, characterization, and performance-testing protocols; improved selectivity toward emerging contaminants under complex water matrices; implementation in realistic reactor configurations and hybrid treatment schemes; and robust life cycle and techno-economic evidence to substantiate sustainability and cost competitiveness. Long-term stability, leaching behavior, and ecotoxicological impacts remain critical uncertainties, and pilot- and field-scale validation is required to translate laboratory performance into reliable real-world applications.

If the limitations summarized in Section 6.1 are addressed and the research directions outlined above are pursued, bentonite–sawdust-derived carbon composites may become increasingly relevant components of sustainable water-treatment and environmental-remediation strategies.

7. Conclusions

The growing demand for clean water and the increasing complexity of aquatic pollution continue to drive interest in low-cost, efficient, and sustainable sorbent materials. In this context, bentonite–sawdust-derived carbon composites have emerged as a promising class of adsorbents that integrate the high cation-exchange capacity and layered structure of bentonite with the developed porosity and versatile surface chemistry of biomass-derived carbon. The existing body of literature provides a solid foundation for evaluating their synthesis routes, physicochemical characteristics, adsorption mechanisms, and performance toward a broad range of contaminants.

Available studies consistently demonstrate that composite formation enhances physicochemical properties and, in many cases, leads to superior adsorption performance compared with the individual components. The diversity of synthesis and modification strategies enables effective tuning of surface area, pore architecture, and accessibility of active sites, while advanced characterization techniques have been essential for confirming composite formation and elucidating structure–property relationships. As a result, these materials exhibit multi-mechanistic adsorption behavior, allowing efficient removal of both inorganic and organic pollutants under a wide range of conditions.

Beyond adsorption performance, bentonite–sawdust-derived carbon composites align well with circular-economy and sustainability principles by valorizing abundant waste resources and offering potentially low-cost treatment solutions. Composite formation generally improves mechanical robustness and handling properties, while magnetic modification can further facilitate solid–liquid separation and reduce secondary pollution risks. Together, these attributes highlight the technical attractiveness of such composites for advanced water-treatment applications.

Despite significant progress, several challenges must be addressed before widespread practical implementation can be achieved. These include the need for standardized and scalable synthesis protocols, consistent material performance, and rigorous benchmarking against commercial adsorbents. Future research should focus on improving selectivity toward emerging contaminants, evaluating performance in complex wastewater matrices, and integrating these composites into hybrid and continuous-flow treatment systems. In parallel, comprehensive life cycle assessments and techno-economic analyses, together with long-term stability, leaching, and ecotoxicity studies, are essential to support reliable scale-up and real-world deployment.