Adsorption of Heavy Metals: Mechanisms, Kinetics, and Applications of Various Adsorbents in Wastewater Remediation—A Review

: Heavy metal contamination in wastewater is a signiﬁcant concern for human health and the environment, prompting increased efforts to develop efﬁcient and sustainable removal methods. Despite signiﬁcant efforts in the last few decades, further research initiatives remain vital to comprehensively address the long-term performance and practical scalability of various adsorption methods and adsorbents for heavy metal remediation. This article aims to provide an overview of the mechanisms, kinetics, and applications of diverse adsorbents in remediating heavy metal-contaminated efﬂuents. Physical and chemical processes, including ion exchange, complexation, electrostatic attraction, and surface precipitation, play essential roles in heavy metal adsorption. The kinetics of adsorption, inﬂuenced by factors such as contact time, temperature, and concentration, directly impact the rate and effectiveness of metal removal. This review presents an exhaustive analysis of the various adsorbents, categorized as activated carbon, biological adsorbents, agricultural waste-based materials, and nanomaterials, which possess distinct advantages and disadvantages that are linked to their surface area, porosity, surface chemistry, and metal ion concentration. To overcome challenges posed by heavy metal contamination, additional research is necessary to optimize adsorbent performance, explore novel materials, and devise cost-effective and sustainable solutions. This comprehensive overview of adsorption mechanisms, kinetics, and diverse adsorbents lays the foundation for further research and innovation in designing optimized adsorption systems and discovering new materials for sustainable heavy metal remediation in wastewater.


Introduction
The discharge of toxic heavy metals into the environment poses a significant threat to the quality of water and aquatic ecosystems, endangering human health [1,2].Trace elements such as arsenic (As), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), lead (Pb), mercury (Hg), nickel (Ni), and zinc (Zn) are recognized as metallic pollutants in wastewater, industrial effluent, and sewage sludge [3][4][5][6].In surficial environments, the most inorganic stable forms of As and Cr occur as inorganic oxyanions but are frequently referred to as cationic species, e.g., As 3+ , As 5+ , Cr 3+ , and Cr 6+ [7][8][9][10].In general, complete or partial toxic heavy metals removal from wastewater and polluted water is essential to prevent potential health and environmental problems and ensure ecosystem sustainability [11,12].The World Health Organization has established maximum contaminant or permissible limits for As, Cu, Pb, Cd, Cr, Co, Hg, Ni, and Zn in drinking water at 0.01, 2.5, 0.05, 0.003, 0.05, 0.1, 0.001, 2.0, and 5.0 mg/L, respectively [13].Traditional methods for removing metal ions from effluents, such as chemical precipitation, lime coagulation, ion exchange, reverse osmosis, and solvent extraction, have limitations, such as insufficient metal removal, high reagent and energy requirements, and the production of noxious sludge or waste products that require proper disposal [13].Therefore, it is necessary to devise efficient and environmentally friendly methods for reducing heavy metal content.
Among various methods for removing metal ions, adsorption is regarded as the most promising due to its simplicity of use, high removal efficiency across a wide pH range, and low cost [14].However, the production of suitable adsorbent materials can be costly, and certain materials, such as commercial activated carbons, cannot be regenerated after use, rendering large-scale applications unsustainable [15].In order to promote sustainable treatment methods, it is crucial to develop and implement readily available, inexpensive, and renewable adsorbents [16].The conversion of agricultural waste and residues into value-added sorbents aligns with the circular bioeconomy and green chemistry principles, providing a renewable and environmentally beneficial approach [17].In this context, the investigation of agricultural, biological, and industrial byproducts as potential metal adsorbents has been motivated by the search for inexpensive and readily available sorbents.
Bioadsorbents have several advantages over traditional techniques.These inexpensive biofilter materials have a high affinity and capacity for metal ions, and they are readily available.Some bioadsorbents exhibit a broad spectrum of metal ion-binding capabilities, whereas others are selective towards particular metal ion types [18].The use of biological organisms as adsorbents is limited by their intolerance for low pH or high concentrations of toxic metal ions [19].Plant fibers, in contrast, are chemically and physically more robust, making them appropriate for sorption applications [20].Metal ions can be effectively adsorbed by plant fibers, which are predominantly made up of cellulose, hemicelluloses, lignin, pectin, and other plant extracts.Metal ions are bonded predominantly to chemical functional groups, such as carboxylic (predominant in hemicelluloses, pectin, and lignin), phenolic (lignin and extractives), hydroxylic (cellulose, hemicelluloses, lignin, and pectin), and carbonyl (lignin and extractives) groups.Through complexation and ion exchange, hydroxyl, carboxyl, and phenolic groups frequently form strong bonds with metal ions [21,22].
Though there are several prospects of bioadsorbents and nanomaterials, such as environmental sustainability, biodegradability, eco-friendliness, high adsorption capacity, etc., issues such as regeneration and reusability, selectivity and specificity, scale-up and practical applications in real-world scenarios, competitive adsorption, and long-term stability and durability still need to be addressed.Moreover, understanding the adsorption process and mechanism, evaluating adsorption kinetics and thermodynamics, designing effective adsorbents, and assessing adsorption capacity are also important in biosorption studies.
This study summarizes the mechanisms, kinetics, and applications of various adsorbents for the removal of heavy metals from an effluent.Bioadsorbents, which include both non-living detritus and living plants and microorganisms, are also studied as alternatives to conventional practices.A schematic representation of this article is presented in Figure 1.

Adsorption Mechanisms
The adsorption process forms a layer of adsorbate (metal ions) on the surface of adsorbents.Adsorption can be reproduced for multiple applications via a desorption method (reverse adsorption in which adsorbate ions are transported from the adsorbent surface) because adsorption is a reversible process in certain circumstances [23].Adsorption onto a solid adsorbent includes three major steps: transportation of the pollutant to the adsorbent surface from aqueous solution, adsorption onto the solid surface, and transport within the adsorbent particle.Generally, electrostatic attraction causes charged pollutants to adsorb on differently charged adsorbents because heavy metals have a vigorous affinity for hydroxyl (OH − ) or other functional group surfaces [24].
Adsorption is mainly classified into two types: physical adsorption and chemisorption (described as activated adsorption as well).Physical adsorption is the adhesion of an adsorbent to the surface of an adsorbate because of the nonspecific (i.e., independent of the nature of the material) van der Waals force, whereas chemisorption occurs while chemical bonding creates strong attractive forces, i.e., chemical adsorption constructs ionic or covalent bonds through chemical reactions.Nevertheless, physical adsorption is a reversible process but less specific, whereas chemisorption is irreversible but more specific [25].When adsorption occurs over biological systems, the process is referred to as biosorption.Biosorption is a process that combines metal removal and recovery.Biosorption is effective due to the adsorbents' low cost and ease of regeneration.Bacteria, fungi, algae, industrial waste, agricultural waste, natural residues, and other biological materials have

Adsorption Mechanisms
The adsorption process forms a layer of adsorbate (metal ions) on the surface of adsorbents.Adsorption can be reproduced for multiple applications via a desorption method (reverse adsorption in which adsorbate ions are transported from the adsorbent surface) because adsorption is a reversible process in certain circumstances [23].Adsorption onto a solid adsorbent includes three major steps: transportation of the pollutant to the adsorbent surface from aqueous solution, adsorption onto the solid surface, and transport within the adsorbent particle.Generally, electrostatic attraction causes charged pollutants to adsorb on differently charged adsorbents because heavy metals have a vigorous affinity for hydroxyl (OH − ) or other functional group surfaces [24].
Adsorption is mainly classified into two types: physical adsorption and chemisorption (described as activated adsorption as well).Physical adsorption is the adhesion of an adsorbent to the surface of an adsorbate because of the nonspecific (i.e., independent of the nature of the material) van der Waals force, whereas chemisorption occurs while chemical bonding creates strong attractive forces, i.e., chemical adsorption constructs ionic or covalent bonds through chemical reactions.Nevertheless, physical adsorption is a reversible process but less specific, whereas chemisorption is irreversible but more specific [25].When adsorption occurs over biological systems, the process is referred to as biosorption.Biosorption is a process that combines metal removal and recovery.Biosorption is effective due to the adsorbents' low cost and ease of regeneration.Bacteria, fungi, algae, industrial waste, agricultural waste, natural residues, and other biological materials have all been widely used to adsorb heavy metals from wastewater [26].Physical adsorption, chemisorption, electrostatic interactions, simple diffusion, intra-particle diffusion, hydrogen bonding, redox interactions, complexation, ion exchange, precipitation, and pore adsorption are all possible mechanisms to adsorb heavy metal ions onto bioadsorbents [27,28].Figure 2 illustrates several possible mechanisms of metal adsorption onto the biosorbents.
Biowaste materials contain a variety of functional groups, the majority of which are negatively charged, such as hydroxyl, carboxyl, carbonyl, and amino groups.Bioadsorbents, such as plant fibers or other biomass-based materials, often have a porous structure with various cavities and surface sites where metal ions can bind.The presence of these pores and cavities increases the surface area available for adsorption, providing more opportunities for metal ions to interact with and be retained by the biosorbent [31].As a result, the porous nature of bioadsorbents contributes to their high metal ion adsorption capacity and efficiency in wastewater treatment applications.Adsorption of contaminants from effluents is a process that involves the diffusion of pollutant molecules and their electrostatic attractions to the surface.The adsorption of potentially toxic elements onto wood biochar, for example, can be described by a variety of mechanisms [32].Possibly, the electrostatic attractions between positively charged metal ions and negatively charged functional groups of bioadsorbents successfully promote the adsorption capacity.Additionally, attractive forces such as hydrophobic interactions, van der Waal forces, and hydrogen bonding could be involved in the process of metal adsorption on the surface of biosorbents [33].Additionally, complexation and chelation are other known mechanisms Adapted from [29,30].
Biowaste materials contain a variety of functional groups, the majority of which are negatively charged, such as hydroxyl, carboxyl, carbonyl, and amino groups.Bioadsorbents, such as plant fibers or other biomass-based materials, often have a porous structure with various cavities and surface sites where metal ions can bind.The presence of these pores and cavities increases the surface area available for adsorption, providing more opportunities for metal ions to interact with and be retained by the biosorbent [31].As a result, the porous nature of bioadsorbents contributes to their high metal ion adsorption capacity and efficiency in wastewater treatment applications.Adsorption of contaminants from effluents is a process that involves the diffusion of pollutant molecules and their electrostatic attractions to the surface.The adsorption of potentially toxic elements onto wood biochar, for example, can be described by a variety of mechanisms [32].Possibly, the electrostatic attractions between positively charged metal ions and negatively charged functional groups of bioadsorbents successfully promote the adsorption capacity.Additionally, attractive forces such as hydrophobic interactions, van der Waal forces, and hydrogen bonding could be involved in the process of metal adsorption on the surface of biosorbents [33].Additionally, complexation and chelation are other known mechanisms in the adsorption process.Generally, complexation is a process that occurs when multiple species combine, whereas chelation is a specific case of complexation that results in the development of rings [34].A metal surrounded by ligands takes the central position in the complexation process and forms mononuclear complexes.Polynuclear complexes are created when two or more metals are bound together by ligands in the central position.Likewise, polydentate ligands could be used in the chelation to aid a stable structure formation via multiple bonding [31].

Equilibrium Models
Adsorption at a given temperature can be quantified using mathematical equations in the form of an adsorption isotherm that relates the amount of adsorbate retained by the adsorbent (q e ) to the concentration in solution at equilibrium (C e ) (Table 1).The two empirical models that are most frequently used for describing the heavy metal adsorption process at a certain temperature and on different bioadsorbent materials are the Freundlich and Langmuir isotherms [35][36][37].In addition, Temkin, Dubinin-Radushkevich, Redlich-Peterson, Koble-Corrigan, and Toth isotherms are used to describe how toxic pollutants interact with adsorbent materials [38][39][40][41].Adsorption isotherms play a vital role in interpreting the mechanism of metal ion adsorption onto different adsorbents [42].These models shed light on the surface properties of adsorbents and the intermolecular interactions between adsorbed molecules and the adsorbent matrix [43].Isotherm and kinetic models contribute to understanding the adsorption process, relying on various factors, including the adsorbent's structure and the physical and chemical characteristics of the solute [42].The Langmuir model finds application in solid-liquid systems, elucidating that all sites on the surface of the adsorbent have equal opportunities to be occupied by heavy metals.On the contrary, the Freundlich model characterizes a non-ideal process occurring on heterogeneous surfaces, often involving multilayer formation [44].

•
No interaction between the adsorbed molecules.

•
Shows a linear decrease in heat of adsorption of all molecules with an increase in bioadsorbent surface coverage. [49] Dubinin-Radushkevich q e = q m exp(−βε 2 ) q e : the amount of adsorbed metal ions per unit mass of adsorbent at equilibrium (mg/g) q m : theoretical adsorption capacity/the maximum amount of ion that can be adsorbed by unit weight of adsorbent (mg/g) β: Dubinin-Radushkevich constant related to the mean free energy of adsorption (mol 2 /J 2 ) ε: Polanyi potential = RTln(1 + 1 Ce ) R: universal gas constant (8.314J/mol K) T: absolute temperature (

Redlich-Peterson
q e : amount of metal adsorbed per unit weight of adsorbent at equilibrium (mg/g) K R K K R (L/g), a R (L/mg) and β (between 0 and 1) are empirical parameters of the R-P isotherm without physical meaning.When
[ 64,65] In a study on Pb 2+ adsorption from aqueous solutions by raw and activated charcoals of Melocanna baccifera Roxburgh (bamboo), Lalhruaitluanga et al. utilized a combined Langmuir-Freundlich equation (L-F) to describe the equilibrium relationships between sorbent and Pb 2+ ions in solution [66].Reddy, Seshaiah, Reddy, Rao, and Wang evaluated the experimental equilibrium adsorption data using four widely employed twoparameter equations: the Langmuir, Freundlich, Dubinin-Radushkevich (D-R), and Temkin isotherms [38].The findings indicated that the Freundlich model provided the best fit for the Pb 2+ adsorption data on Moringa oleifera bark.Chen, Li, Li, Chen, Chen, Yang, Zhang, and Liu [36] observed that the chitosan fibers had better adsorption of Cu 2+ ions than Cr 4+ .This might be due to the amino groups in chitosan fibers, which may have good chelation with Cu 2+ ions.The Cu 2+ ion adsorption process followed the quasi-second-order kinetic equation, and was compatible with the Langmuir isotherm.

Kinetic Models
Kinetic adsorption models describe the mechanism of adsorption of heavy metal ions by biosorbents and particularly determine the rate of biosorption during the removal of heavy metals from wastewater on an industrial scale to optimize the design parameters, including the adsorbate residence time and reactor dimension [67].In experimental works, the kinetic description of adsorption processes and the removal efficiency of heavy metals by various adsorbents have frequently been evaluated by several mathematical models [68][69][70], most of which are empirical equations [71][72][73].The common kinetics models (pseudo-first order, pseudo-second order, intra-particle diffusion kinetic, and Elovich), which were used to fit experimental data, are expressed in Table 2.The choice between these equations is frequently based on the goodness-of-fit, as judged by the coefficient of determination (R 2 ) [73,74].The use and misuse of adsorption kinetic data and linear forms of pseudofirst-order and pseudo-second-order equations were discussed in [75,76].Recently, Bullen et al. revised the pseudo-order models and proposed the following equation (Equation ( 1)), called the 'revised rate equation' (rPSO) [77]: where k is the revised pseudo-order rate constant (k' = k 2 q 2 /C 0 ), C 0 is the initial adsorbate concentration in the solution, and C t is the adsorbate concentration at time t.They stipulated that this rPSO provides the first-order and zero-order dependencies upon C 0 and C t (amount of adsorbate adsorbed onto adsorbent).
Table 2.The kinetic models used for heavy metal adsorption.

Models Adsorbents Heavy Metals Main Findings References
Pseudo-first order Nonlinear form: q t = q e (1 − e −k 1 t ) Linear form: ln(q e −q t )=lnq e −k 1 t q e (mg/g): amount of adsorbed metal ions per unit mass of adsorbent at equilibrium q t (mg/g): amount of adsorbed metal ions per unit mass of adsorbent at time t (min) k 1 (min −1 ): pseudo-first-order rate constant Palm kernel shell Cd 2+ • Known also as the Lagergren model.

•
Assumes one metal ion is adsorbed onto one adsorption site.

•
Considers the rate of adsorption site occupation is proportional to the number of unoccupied sites.
[ 54,69,71,78] Chickpea Husk Pb 2+ [56] Black tea leaves Pb 2+ [79] Cherry leaves Cr 6+ [80] Pseudo-second order Nonlinear form: Linear form: ) q e (mg/g): adsorption capacity at equilibrium q t (mg/g):  Wattanakornsiri et al. investigated the removal of Pb 2+ and Cd 2+ ions from waste aqueous solutions using naturally modified biosorbents derived from three local fruit peels: dragon fruit peel, rambutan peel, and passion fruit peel [88].The study found that adsorption followed the pseudo-second-order kinetic model, and the adsorption data fit well with both the Freundlich and Langmuir isotherm models.However, the Langmuir model demonstrated the best fit.

Thermodynamic Parameters
A thermodynamic study offers insights into the minimum kinetic energy necessary for the adsorbate to become bound to the adsorption site [89].The nature of the adsorption process (spontaneity, randomness, endothermicity, or exothermicity) can be evaluated by estimating thermodynamic parameters [72,90] such as the Gibbs free energy change (∆G • , kJ/mol), the standard enthalpy change (∆H • , kJ/mol), and the standard entropy change (∆S • , J/mol K −1 ) [91].These parameters can be calculated by the following equations [72,78,92,93] stated as (Equations ( 2)-( 5)): where R represents the universal (ideal) gas constant (8.314J/mol K), T is the temperature in Kelvin (K); K c is the apparent equilibrium adsorption [72] or the Langmuir isothermal constant [92]; C ad,e and C e are the concentration (mg/L) of heavy metal in adsorbent and in solution, respectively [78].∆H 0 and ∆S 0 values can be calculated at different temperatures, assuming these parameters to be independent of temperature [90].A negative or positive value ∆G • at a known temperature confirms the spontaneity (or non-spontaneity) of the adsorption process, a positive value of ∆H • suggests the endothermic nature of the adsorption of a pollutant by the sorbent, and a positive value of ∆S • illustrates an increase in the randomness of the adsorption process [92,93].
The thermodynamic properties associated with the removal of metal ions exhibit variation due to the adsorbent's composition, structure, and surface characteristics, leading to differing affinities for metal ion removal.Enhanced surface area and a porous structure can promote interactions and subsequent adsorption [91].Factors like the presence of competing ions and shifts in pH can modify the charge distribution on both the metal ion and adsorbent surfaces, thereby influencing thermodynamic equilibrium.Additionally, distinct metal ions possess varying thermodynamic affinities due to their unique electronic configurations and charge densities [22,94].

Different Adsorbents for Heavy Metal Removal from Wastewater
Numerous adsorbents were used in their native or modified forms to remove metallic trace elements (e.g., heavy metals and metalloids) from wastewater (Table 3).Activated carbons (AC), zeolites, clay minerals, nanosized metal oxides, animal-based wastes, agricultural and food wastes, industrial waste materials, and various advanced adsorbents have been tested for multi-metal removal from wastewaters and aqueous solutions [23].EDTA: Ethylenediamine tetra-acetic acid.

Industrial Solid Wastes
Industrial waste can encompass a wide range of materials, including leftover raw materials, production residues, processed by-products, and pollutants generated during manufacturing, processing, or other industrial activities.Various industrial solid wastes showed a significant capacity for adsorption, which could be used to remove metal ions from wastewater.Due to their by-product status, they are readily available and particularly cost-effective.Traditionally, the byproducts like fly ash [109,110], blast furnace sludge [111], waste slurry [112], lignin [113], Fe(OH) 3 [114], and red mud [115] have been utilized as effective adsorbents due to their technological viability to remove heavy metal ions from polluted water [116].Among the industrial by-products, red mud is considered inexpensive and readily available, as well as highly efficient in metal adsorption.However, the difficulty of disposing of wastewater that is generated during its activation prior to application and its recovery after application has limited its practicability [117].Several other industrial wastes, including sawdust [118], areca waste [119], tea factory waste [120], battery industry waste [121], waste biogas residual slurry [122], sea nodule residue [123], and grape stalk waste [124] have been used as low-cost adsorbents to remove heavy metals from contaminated water.
Recently, micro-and nano-plastics such as polyamide (nylon), polyester, polypropylene, polyethylene, and polyvinyl chloride were reported to have the ability of heavy metal adsorption.Polypropylene, among the other polymers, showed a higher capacity for adsorption [125].Godoy et al. studied the adsorption of several heavy metals (Cd, Co, Cu, Cr, Ni, Pb, and Zn) on different types of microplastics [126].They found that the polyethylene and polyvinyl chloride showed a higher ability to adsorb Pb, Cr, and Zn, while the low adsorption capacity was related to polyethylene terephthalate.Fu et al. described that microplastics have the ability to adsorb heavy metals, and the adsorption process can be influenced by many factors, such as particle size, type of microplastics, and the type and concentration of metal ions [127].Zon et al. studied the adsorption of Cr by polyethylene micro-beads in seawater [128].The process was influenced by surface area, reactivity of metals, particle size, and pH.Dong et al. reported the adsorption mechanism of As 3+ by polystyrene microplastic particles [129].They described that electrostatic forces and non-covalent interactions might be the key mechanisms for As 3+ adsorption.

Biomaterials as Metal Biosorbents
Biosorbents, where the biological matrix acts as an active binding site [130,131], are typically obtained from three sources: (i) non-living biomass such as bark, crab shells, shrimp shells, fish scales, krill, lignin, and squid, (ii) algal biomass including micro-and macro-algae, and algal-derived biochar [41], and (iii) living or dead microbial biomass like bacteria, fungi, and yeast [23,132].
In a study, Strychnos potatorum seeds with chemically modified surfaces (SMSP) were examined for their ability to remove Pb 2+ ions from aqueous solutions [164].The experimental adsorption isotherm data were analyzed, and it was determined that the Freundlich adsorption isotherm model provided a better fit.SMSP exhibited a maximal adsorption capacity of 166.67 mg/g for Pb 2+ ions under optimal conditions comprising a pH of 5.0, a contact time of 30 min, a dosage of 2 g/L, and a temperature of 30 • C. In addition, the adsorption kinetics of SMSP for the removal of Pb 2+ ions was consistent with the pseudosecond-order kinetic model.In a separate study, Shukla et al. assessed the ability of coir, a low-cost lignocellulosic fiber, to remove heavy metal ions such as Ni 2+ , Zn 2+ , and Fe 2+ from aqueous solutions [165].Langmuir-type adsorption was accomplished using coir fibers.The modified coir fibers (oxidized with hydrogen peroxide) adsorbed 4.33, 7.88, and 7.49 mg/g of Ni 2+ , Zn 2+ , and Fe 2+ , respectively, whereas the unmodified coir fibers adsorbed 2.51, 1.83, and 2.84 mg/g [165].
Algae, a renewable biomass that grows universally and amply in the world's littoral zones, have piqued the interest of numerous researchers as potential new adsorbents for metal ion removal.Several advantages of algae involve their widespread accessibility, low cost, and relatively consistent features [23].Bioadsorption of Cu 2+ and Zn 2+ using dried marine green macroalgae, Chaetomorpha linum [166]; Cu 2+ , Zn 2+ , Cd 2+ , and Pb 2+ by Caulerpa lentillifera [167] are examples demonstrating the effectiveness of algae as heavy metal adsorbents (Table 5).Nowadays, metal ion removal by microorganisms has been considered extremely efficient.For instance, Bacillus cereus [168], Escherichia coli [169], Pseudomonas aeruginosa [170], etc., have all been studied for their ability to bind heavy metals in aqueous solutions.Additionally, several species of bacteria, including Bacillus sp., Micrococcus luteus, Pseudomonas cepacia, Bacillus subtilis, and Streptomyces coelicolor have been successfully employed to remove Cu 2+ , Zn 2+ , Cd 2+ , and Ni 2+ from an effluent [171][172][173].Metal biosorption by bacteria involves physicochemical interactions between the metal and the functional groups on the cell surface. [182]

Escherichia coli
Ni 2+ : 55.31 Cd 2+ : 45.37 The biosorption of Cd 2+ and Ni 2+ was dependent on the concentrations of metal ions.Adsorption efficiency followed the Freundlich adsorption isotherm. [183] Gluconoacetobacter hansenii Pb 2+ : 82 * Cd 2+ : 41 * Ni 2+ : 33 * Bio-filtration: utilized heavy metals rapidly due to their small size and high surface-to-volume ratio.Biosorption: included covalent bonding, electrostatic interaction, redox interaction, van der Waals forces.[184]  Fungi are another frequently used biosorbent, which is easy to grow to produce a high yield of biomass in a short time and can be simply manipulated genetically and morphologically [190,191].Among the fungi used as biosorbents, Aspergillus niger [192], Saccharomyces cerevisiae [193], Lentinus edodes [193], etc., are the most common species.Bhainsa and D'souza [194] studied Cu 2+ ion removal using Rhizopus oryzae biomass modified with NaOH and achieved a maximum adsorption of 43.7 mg Cu 2+ /g.Although fungal biosorbents have a wide range of sources, are inexpensive, and exhibit rapid adsorption, their separation following the process can be difficult [23].Yeast is also used as an adsorbent, which is a fungus larger than bacteria.Like other eukaryotic organisms, it contains a nucleus and related cytoplasmic organelles [190,195].For living cells, the cytoplasm is critical because it interacts with metal ions and separates into compartments to remove them once they enter the cells [23].Han et al. described that waste beer yeast from the brewing industry could be a promising adsorbent to remove Cu 2+ (1.45 mg/g) within 30 min [196].
Apart from the aforementioned biosorbents, biochar prepared from lignocellulosic materials and microalgae can prevent pollutants from reaching organisms via soil or water and reduce bioavailability through adsorption because of its graphene-like carbon matrix, large surface area, high porosity, and increased cation and anion exchange capacity [24].Biochar has been widely used in anaerobic digestion and in wastewater treatment processes for eliminating pathogens, trace metals, and suspended matter [197].The adsorption mechanism of biochar depends on the chemical properties of the biochar surface and the nature of pollutants.Generally, three major types of adsorption for biochar are: (i) physical passage, in which pollutants settle on the adsorbent surface; (ii) pore filling, in which adsorbate condenses into the pores of biochar; and (iii) precipitation, in which adsorbate forms layers on the adsorbent surfaces [198].In fact, the dissociation of O 2 -containing functional groups creates a negatively charged biochar surface, which facilitates electrostatic attraction between cations and biochar.Removal of Cu 2+ from aqueous media was studied using rice straw biochar, which indicated that the ion exchange of native cations with Cu 2+ might be the dominant mechanism for heavy metal adsorption [199].Microalgae-derived biochar possesses an irregular porosity of 1 mm, which enables it to act as an adsorbent; however, it has a lower cation exchange capacity (CEC) compared to lignocellulose-derived biochar [200].Van Hien, Valsami-Jones, Vinh, Phu, Tam, and Lynch applied biochar from biomass residue for remediating Zn-contaminated water and observed that the biomass dose, contact time, and metal concentration had a great effect on heavy metal uptake [59].

Activated Carbon
In recent times, the wastewater treatment industry has shown significant interest in activated carbon (AC) due to its remarkably high adsorption capacity for heavy metals.AC's porous structure, small particle size, high surface area, active free valences, appropriate surface functional groups, and affinity for adsorbing various substances make it a valuable resource with significant adsorption potential in diverse applications [201].AC is derived from various naturally produced waste materials, including rubber wood sawdust [202], rice husk [203], coconut shell [204], hazelnut shell [205], palm shell [206], apricot stone [207], eucalyptus bark [208], soybean hulls [209], bamboo [210], etc., and has been investigated to extract metal ions from wastewater.Additionally, AC that is altered with alginate [211], tannic acid [212], magnesium nitrate [213], and surfactants [214] might be a viable alternative to remove heavy metals from wastewaters and aqueous solutions.
The mechanism of heavy metal adsorption by AC involves a combination of multiple mechanisms, including physical adsorption, electrostatic adsorption, ion exchange, reduction, complexation, and precipitation [215].Ongoing advancements have focused on augmenting the adsorption efficacy of AC through modifications employing physical, chemical, organic, and inorganic loading techniques.Physical modification techniques, including microwave heating, ultrasound irradiation, steam activation, and non-thermal plasma technology, have been explored to enhance active sites [215].Furthermore, chemical modification methods involving acidic and alkaline treatments have also been used [215,216].While AC is primarily utilized to eliminate unpleasant color, odor, taste, and other organic impurities from water/wastewater, the adsorption capacity of AC is limited by several factors, including loss of adsorption efficacy upon regeneration, the need for regeneration following exhaustion, and the possibility of secondary contamination due to pollutants being separated from the AC but not eliminated [24].Therefore, research and the use of alternative adsorbents are imperative.

Plant Fiber Components
The use of plant fiber-based food wastes, particularly agro-waste materials, as biosorbents for the removal of heavy metals from aqueous solutions is gaining momentum.Numerous studies have examined the potential use of plant fiber components such as hemicelluloses, cellulose, pectin, and lignin in heavy metal adsorption from an effluent (Table 6).Cellulose is the most prevalent organic compound on Earth, and its high surface area, hydroxyl groups, and porous structure allow for efficient adsorption of heavy metals [217].Heavy metal removal has been studied using cellulose-based materials such as cellulose nanofibers, cellulose derivatives, and cellulose-based composites.Through surface complexation, ion exchange, and electrostatic interactions, they can absorb heavy metals [218].SLR: solvent liquid ratio (g/mL) * adsorption efficiency in %.
Several agro-waste materials (agave bagasse, sorghum straw, oats straw) and their fractions to identify functional groups with hydroxyl, carboxyl, and nitrogen-containing compounds were observed [230,231].They observed that the lignin exhibited a higher contribution than hemicelluloses regarding the adsorption capacity of Cr 3+ in sorghum straw and oats straw.In the case of agave bagasse, lignin was found to be the primary fraction responsible for Cr 3+ adsorption.In their study, the primary contributors to Cr 3+ removal from an aqueous solution were identified as hemicelluloses and lignin, whereas cellulose present in the studied agro-waste adsorbents did not appear to play a significant role in this process, although cellulose constitutes the largest proportion (greater than 46%) of the agro-waste materials compared to hemicelluloses (12-26%), lignin (3-10%), and other compounds (22-30%).In contrast, Pejic et al. investigated the sorption capacity of waste short hemp fibers for Pb 2+ , Cd 2+, and Zn 2+ ions in aqueous mediums [232].They demonstrated that by gradually reducing the amount of lignin or hemicelluloses in hemp fibers via chemical treatment, the sorption characteristics of hemp fibers improved.Short hemp fibers can sorb metal ions (Pb 2+ , Cd 2+, and Zn 2+ ) from both individual and combined metal solutions.The maximum total adsorption capacities for Pb 2+ , Cd 2+, and Zn 2+ ions were the same in single solutions, which was 0.078 mmol/g.However, in ternary mixtures, their adsorption capabilities differed, with values of 0.074 mmol/g for Pb 2+ and 0.035 mmol/g for both Cd 2+ and Zn 2+ [232].Hu et al. reported that the amount of metal ions attached to rice bran fibers varies [233].The maximum metal ion (Cd 2+ , Cu 2+, and Pb 2+ ) binding capacity was demonstrated by soluble hemicellulose: 76.3 mg/g for Pb, 68.5 mg/g for Cu, and 59.1 mg/g for Cd.In contrast, insoluble fiber removed 32.5 mg/g of Pb, 10.6 mg/g of Cu, and 18.3 mg/g of Cd.Rice bran cellulose exhibited low binding capacities: 20.5 mg/g for Pb, 13.6 mg/g for Cu, and 9.9 mg/g for Cd.Al-Ghouti et al. discovered that raw date pits (RDP) could be used as a solid adsorbent to remove copper ions, cadmium ions, and methylene blue (MB) [234].They discovered two methods for MB adsorption: hydrogen bonding and electrostatic attraction.They observed that the most common method for Cd 2+ to bind in the cellulose/lignin unit was via two OHgroups.In another study, the ability of spruce, coconut coir, sugarcane bagasse, kenaf bast, kenaf core, and cotton to remove Cu 2+ , Ni 2+ , and Zn 2+ ions from aqueous solutions was investigated [235].The purpose of the study was to determine the relationship between the lignin content of these substances and their capacity to absorb these metal ions.Kartel et al. observed that beet pectin had a high affinity for Pb 2+ and Cu 2+ , citrus pectin had a high affinity for Ni 2+ , and apple pectin had a high affinity for Co 2+ [236].This could be due to the substantial structural differences between pectin sources.According to Khotimchenko et al., pectin with an esterification level close to "0" binds the most Zn 2+ .The degree of methylation, which varies between 1 and 60%, is the primary factor influencing heavy metal adsorption [237].
It has been well-established that plant fiber components exhibit effective heavy metal removal from wastewater.The adsorption capacities are influenced by various factors, such as contact time, pH, concentration of heavy metals, adsorbent dosage, and temperature [13,22,238].To better understand the adsorption behavior of these components, models of adsorption isotherms, kinetics, and thermodynamics have been utilized.Furthermore, numerous modification techniques, including chemical modification, surface functionalization, and composite formation, have been investigated to enhance the adsorption efficiency of these plant fiber materials.

Nanomaterials
Conventional adsorbents, some of which are depicted above, typically have an insufficient capacity for metal adsorption and thus cannot efficiently remove the majority of heavy metals in wastewater treatment [23].In this concern, researchers investigated how to develop novel adsorbents with enhanced properties and functionalities.These days, there is a high demand for highly porous nanostructures such as graphene [239], fullerene [240], nanosized metal oxides and MXene [24], graphitic carbon nitride (g-C 3 N 4 ) and metalorganic frameworks [23], halloysite particles [216] and especially, carbon nanotubes [241] have been reported as substitute adsorbents to remove metal ions from wastewater considering the advantages of nanotechnology (Table 7).They are usually strong, resistant, electrically conductive, non-corrosive, and thermally stable.Additionally, high surface areas and large pore volumes of these nanomaterials, when associated with different types of intermolecular interactions, enable effective adsorption in a variety of systems [24].By and large, these nanomaterials outperform conventional adsorbents such as titanium dioxide, activated carbon, and iron oxide.Nevertheless, novel materials also have shortcomings, and therefore, scientific and technological research must address issues of durability and functionalities, which are critical in environmental applications [24].The highest adsorption of Cr 6+ was at pH 4. Adsorption of As 3+ , As 5+ was observed at a pH range of 2-8.Initial ion concentration showed a positive correlation with metal adsorption efficiency. [106] Table 7. Cont.

Nano-Adsorbents Heavy Metals Removed Main Findings References
Carbon nanotube (multi-walled carbon nanotube functionalized and sulfonated) Co 2+ , Zn 2+ Using nanofiltration.98% removal of Zn 2+ due to the reduction of membranes' pore size.Functionalization was critical.Removal efficiency was dependent on zeta potential, hydrophilicity of the fillers, and oxygen functional groups on the surface of this membrane. [242] Cellulose (micellar-enhanced ultrafiltration) As 3+ , Cd 2+ Using nanofiltration.>90% removal of As 3+ at pH > 7.
Higher negative zeta potential.Higher Pb 2+ removal than the precipitation method.
The data best fit a pseudo-second-order model.[252] Nanosheets composed of two-dimensional nanomaterials, comprised of Ca 2+ (Ca) and Y 3+ (Y) cations along with carbonate [CO 3 2− ] anions, referred to as CaY-CO 3 2− layered double-hydroxide (LDH) materials, exhibit exceptional affinity and selectivity for toxic transition metal ions such as Cr 3+ , Ni 2+ , Cu 2+ , Zn 2+ , Pb 2+ , Cd 2+ , as well as metalloid As 3+ [10].Furthermore, there is an emerging focus on the use of nanoparticles derived from food wastes, particularly nano-cellulose, as potential adsorbents for heavy metal removal [253,254].A review by Fayaz et al. explored the utilization of nano-cellulose obtained from food waste through various treatment processes [255].When subjected to specific treatments, nano-cellulose can be modified to enhance its adsorption capacity for metal ions in wastewater treatment [256].These modified nano-cellulose materials show promise as they possess inherent advantages like renewability, biodegradability, and low cost, given their origin from food waste.However, to fully harness their potential, further research and development efforts are required to optimize their durability and functionalities in environmental applications.The exploration of nano-cellulose and its derivatives from food waste as adsorbents represents a sustainable and innovative approach toward mitigating heavy metal pollution in water resources.

Conclusions and Outlook
The use of various non-toxic adsorbents has proven to be a successful approach for the simultaneous removal of multiple heavy metals, metalloids, and other pollutants from polluted water and wastewater.This method is highly effective, practical, economical, and environmentally friendly [39,257].To achieve optimal results, mathematical tools [258], such as machine learning algorithms [259], can be employed to assess and optimize the adsorption processes' isotherms, thermodynamics, kinetics, and operational parameters.In this regard, low-cost and locally available agro-biowastes have emerged as promising candidates for heavy metal and metalloid removal on an industrial scale.These biowastes, such as plant fibers, fruit and vegetable peels, and byproducts from food processing industries, possess large multi-chemical functional groups, surface area, cation-exchange capacity, and controllable pore structures [30,88].Their physical preparation as adsorbents adheres to the principles of green chemistry.Additionally, these bioadsorbents can be reused for multiple cycles, further enhancing their cost-effectiveness and sustainability [39,260,261].Despite these advancements, further research is necessary to optimize the performance of adsorbents, explore new materials, gain deeper insights into the underlying mechanisms, and develop more cost-effective and sustainable methods for heavy metal removal from wastewater.An outline diagram for the challenges and potential opportunities associated with the present topic is presented in Figure 3 and described as follows: 1.
Biological adsorbents, including microbial biomass, algae, and fungi, utilize living or nonliving biomass to bind heavy metals.These bioadsorbents are eco-friendly, easily accessible, and can be regenerated through biomass regeneration or metal recovery procedures.However, their adsorption capacities and selectivity may vary depending on the biomass source and pretreatment techniques.In contrast, nanomaterials, such as nanoparticles and nanocomposites, offer distinct advantages for heavy metal adsorption due to their small dimension, large surface area, and enhanced reactivity.Through magnetic or functionalized modifications, their recyclability and reusability can be improved.Nevertheless, concerns about potential environmental impacts and long-term stability must be addressed;

2.
Inexpensive adsorbents like agricultural waste materials, such as vegetable and fruit peels, have gained popularity due to their abundant surface functional groups, leading to high metal adsorption capacities.These readily accessible plant-based materials, including plant fibers and other detritus, offer cost-effective and efficient sorbents for metal ions.Utilizing plant-based byproducts as metal sorbents aligns with circular bioeconomy and green chemistry principles, offering economically viable and ecofriendly solutions.However, their application may require pretreatment to enhance adsorption efficiency and stability.A practical application involves using plant fibers as low-cost biowaste for adsorbing heavy metals from polluted water or acid mine drainage generated by mining industries.In this method, fibers can act as adsorbents in a bioreactor, attracting and retaining heavy metal ions (Figure 4); 3.
Current research in the field of heavy metal adsorption and removal from water primarily focuses on the adsorption of heavy metal cations.However, there is a noticeable lack of specific studies on the adsorption of hydroxy compound ions or other complex ions.In most documents, the adsorption of complex ions is only briefly mentioned or addressed in a limited manner; 4.
Modeling the biosorption process is challenging due to the diverse physical and chemical processes involved.The nature of active sites in bioadsorbents varies significantly based on their source, making characterization difficult.While successful in validating experimental data for single-component biosorption, real-life scenarios often involve multiple components adsorbing simultaneously on a heterogeneous biosorbent surface.This complexity leads to dynamic interactions between metal ions and functional groups, making the modeling of such systems more intricate; 5.
Plant-based adsorbents require surface modifications to enhance their surface functionalities and develop suitable pores for effective adsorption.Surface oxidation, sulfonation, amination, and adjustments to pore structures are crucial processes in achieving improved adsorption performance.Various modification procedures, such as chemical, mechanical, thermal, gasification, and combinations of these techniques, are available to tailor the adsorbent properties according to specific contaminant removal needs.By exploring and optimizing these modification methods, plantbased adsorbents can be fine-tuned to be efficient and versatile tools for water and wastewater treatment, contributing to sustainable and eco-friendly solutions; 6.
Modified carbonaceous materials have shown great promise in achieving high adsorption capacity and efficient removal of heavy metals.However, the process of modification, especially through chemical means, can be quite intricate.This complexity, along with considerations of cost, yield, and operational practicality, poses challenges for their application on an industrial scale.Additionally, the use of some novel modifiers might introduce new sources of pollution, making it essential to carefully assess their environmental impact; 7.
Further study of selective adsorption and competitive adsorption behavior among heavy metal ions is highly valuable.Understanding the mechanisms that govern the preferential adsorption of specific metal ions and how different ions interact and compete for adsorption sites can significantly enhance the development of efficient and targeted remediation strategies for contaminated water and wastewater; 8.
In future research, the regeneration of carbon-based materials, particularly plant fibers, holds significant practical importance.Exploring the properties of these materials and understanding the optimal operational conditions for the regeneration process are essential steps.Additionally, the development of effective desorption solutions is crucial for ensuring the full utilization and reusability of carbon-based adsorbents.

Figure 1 .
Figure 1.A schematic representation of the current review.

Figure 1 .
Figure 1.A schematic representation of the current review.

Figure 2 .
Figure 2. Different mechanisms of cationic heavy metals adsorption by several types of biosorbents.

Figure 3 .
Figure 3. Outline diagram for the challenges and potential opportunities of heavy metal removal from wastewater.

Figure 4 .
Figure 4. Biofilter for removing heavy metals from contaminated water.Author Contributions: Z.R.: writing-original draft, writing-review and editing; A.K. (Ahasanul Karim): conceptualization, writing-original draft, writing-review and editing; A.K. (Antoine Karam): supervision, writing-review and editing; S.K.: conceptualization, supervision, writingreview and editing.All authors have read and agreed to the published version of the manuscript.

Figure 3 .
Figure 3. Outline diagram for the challenges and potential opportunities of heavy metal removal from wastewater.

Figure 3 .
Figure 3. Outline diagram for the challenges and potential opportunities of heavy metal removal from wastewater.

Figure 4 .
Figure 4. Biofilter for removing heavy metals from contaminated water.Author Contributions: Z.R.: writing-original draft, writing-review and editing; A.K. (Ahasanul Karim): conceptualization, writing-original draft, writing-review and editing; A.K. (Antoine Karam): supervision, writing-review and editing; S.K.: conceptualization, supervision, writingreview and editing.All authors have read and agreed to the published version of the manuscript.

Figure 4 .
Figure 4. Biofilter for removing heavy metals from contaminated water.

Table 1 .
Commonly used isotherm models for heavy metal adsorption.

Table 3 .
Commonly used adsorbents and their performance.

Table 4 .
Commonly used agricultural wastes for heavy metal removal from wastewater.

Table 5 .
Biosorption of heavy metal ions using microorganisms.

Table 6 .
Plant fiber components used for removing heavy metals from wastewater.

Table 7 .
Some commonly used nano-adsorbents for heavy metal removal.