Agricultural Byproducts Used as Low-Cost Adsorbents for Removal of Potentially Toxic Elements from Wastewater: A Comprehensive Review

Abstract

Potentially toxic elements (PTEs) are ubiquitous chemical compounds in the environment due to contamination of air, water, or soil. They are primarily sourced from fossil fuel combustion, mining and smelting, electroplating, dyes and pigments, agricultural treatments, and plastic and metallic industries. These chemical contaminants can produce various adverse effects when they enter the human body and can also affect crops and aquatic ecosystems. To address these issues, researchers are developing various techniques, including ion exchange, membrane filtration, photocatalysis, electrochemical methods, bioadsorption, and combinations of these processes, to reduce the levels of these contaminants, especially from wastewater. Among these methods, bioadsorption has gained much attention due to its high efficiency, low cost, and abundance of adsorbent materials. Agricultural byproducts used as biosorbents include rice husk and bran, citrus peel, banana peel, coconut husk, sugarcane bagasse, soybean hulls, walnut and almond shells, coconut fiber, barley straws, and many others. Biosorption capacity can be described using adsorption kinetic models such as Elovich, Ritchie’s, and pseudo-second-order models, as well as different adsorption isotherm models such as Freundlich, Langmuir, Temkin isotherm, and BET models. Both conventional processes and adsorption models are influenced by parameters such as pH, agitation speed, contact time, particle size, concentration of the adsorbent material, initial concentration of the contaminant, and the type of modifying agent used. This review paper aims to examine the low-cost adsorbents and their removal efficiency and bioadsorption capacity for different PTEs present in wastewater, and their potential as decontamination methods.

1. Introduction

Potentially toxic elements encompass both nonmetals, such as arsenic (As) and selenium (Se), and metals, including cadmium (Cd), lead (Pb), mercury (Hg), aluminum (Al), chromium (Cr), cobalt (Co), and antimony (Sb), as well as copper (Cu), nickel (Ni), and zinc (Zn), which are micronutrients capable of inducing adverse effects in living organisms when present in excessive concentrations [1]. Despite being present in the Earth’s crust since its formation, pollution with potentially toxic elements, originating from both natural and anthropogenic sources, poses a significant concern. Anthropogenic sources of contamination, such as metal industries (mining, smelting, foundries), automobile exhaust, industrial waste, agricultural treatments (inorganic fertilizers, pesticides, insecticides), wastewater irrigation, and others, represent the primary contributors to pollution. Natural sources of pollution include volcanic activity, geological weathering, soil erosion, metal corrosion, metal evaporation from contaminated water, and sediment resuspension [2].

Wastewaters are the primary source of pollution for both soil and surface waters. They are often used for irrigation without adequate treatment, leading to contamination. Even treated wastewaters can still contain substances that have passed through biological treatment plants or were discharged into water bodies, contributing to pollution [1].

Approximately 20 million hectares of arable land is being irrigated with wastewater, which has the potential to cause the accumulation of toxic metals in soil, resulting in soil contamination [1]. Additionally, this practice can lead to pollution of crops, which may compromise food quality and safety [3]. Large quantities of metals can reach the human body through the cultivation of plants and crops on contaminated soil, making this pathway a major route of exposure to potentially toxic elements [3]. In addition, toxic metals can enter the body in smaller amounts through ingestion of contaminated water, dermal contact, or inhalation [4].

Toxic metals may accumulate in vital organs (such as the liver, brain, kidney, and heart) of the human body, leading to disturbances in their normal functioning and blocking vital activity. Continuous exposure to potentially toxic elements can cause an internal imbalance, as they may be used as substitutes for the body’s essential elements [4].

Table 1. Health effects and permissible limits of potentially toxic metals.

Toxic Metal	Health Hazard	Permissible Limit	Reference
Lead (Pb)	Hypertension, anorexia, renal impairment, senile dementia, reduced fertility, cognitive impairment, abdominal pain	WHO—0.05 mg/L USEPA—0.005 mg/L	[2,5,6,7,8]
Cadmium (Cd)	Bone disease, lung and prostate cancer, pulmonary fibrosis, Itai–Itai disease, hypertension, renal toxicity, testicular atrophy	WHO—0.003 mg/L USEPA—0.005 mg/L	[2,5,6,7,8]
Mercury (Hg)	Ataxia, dermatitis, kidney damage, Minamata disease, miscarriages, DNA damage, pulmonary edema, dementia, gingivitis	WHO—0.001 mg/L USEPA—0.002 mg/L UE—0.001 mg/L	[2,5,7,8]
Arsenic (As)	Brain damage, liver tumor, melanosis, lung irritation, infertility and miscarriage, hemolysis, hepatomegaly, conjunctivitis	WHO—0.01 mg/L USEPA—0.05 mg/L	[2,5,6,7,8]
Chromium (Cr)	Chronic bronchitis, vomiting, lung cancer, pneumonia, liver damage, nausea and vomiting, renal failure, reproductive toxicity	WHO—0.05 mg/L USEPA—0.1 mg/L	[2,5,6,7,8]
Cobalt (Co)	Skin problems, congestion, edema, allergic dermatitis, respiratory problems, pneumonia and fibrosis, cardiac problems, asthma, nausea and vomiting, liver disorders	WHO—0.1 mg/L	[2,5]
Copper (Cu)	Anemia, metabolic disorders, metal fiver, Wilson disease, hepatic and kidney disease, reproductive and developmental toxicity, diabetes, dizziness	WHO—2.5 mg/L USEPA—1.3 mg/L	[2,5,6,7,8]
Manganese (Mn)	Hypertension, pneumonia, neurological problems (dullness, lethargy, weakness), mimicry of Parkinson’s disease, infertility problems, pneumonia	WHO—0.5 mg/L	[2,5]
Nickel (Ni)	Lung and nasal cancer, dermatitis, nausea and vomiting, kidney diseases, nausea and vomiting, dizziness, heart disorders, asthma	WHO—2.0 mg/L	[2,5,7,8]
Iron (Fe)	Neurodegenerative diseases (Alzheimer’s disease, Parkinson’s disease), effects on cell proliferation and differentiation, affects cognition and memory	WHO—0.3 mg/L	[5,9]
Zinc (Zn)	Ataxia, depression, gastrointestinal problems, respiratory disorders, anemia, impaired immune function, metal fume fever, prostate cancer	WHO—5.0 mg/L	[2,5,7,8]
Aluminum (Al)	Dementia, lung damage, pulmonary fibrosis, colitis, neurological diseases (Alzheimer’s, sclerosis, Parkinson’s disease), kidney issues	WHO—0.2 mg/L USEPA—0.2 mg/L	[2,10]

WHO = World Health Organization; USEPA = United States Environmental Protection Agency.

presents the health effects of the most important potentially toxic metals.

The contamination of air, water, and food with potentially toxic elements is a global issue, and researchers are working on developing new and effective methods for removing them from wastewater sources. The elimination of these elements from water is crucial to protecting human health due to their negative impact [11].

Agricultural byproducts are environmentally friendly and exhibit potential biosorption capacity for potentially toxic metals, but also for other chemical contaminants (nitrates, pesticides, colorants, bisphenol A) harmful to living organisms. The major advantages of agricultural waste include (i) high regeneration and reusability of the spent adsorbents, (ii) reduced sludge generation, (iii) high efficiency, (iv) low cost, (v) availability of many adsorbent materials (husks, shells, peels, vegetable waste, leaves, stalks), (vi) strong affinity, and (vii) high selectivity for many metal ions (due to the availability of functional groups from the adsorbent surface). Agricultural adsorbents have demonstrated high removal performances and adsorption capacities due to their porous structure and high amount of various functional groups (hydroxyl, carboxyl) capable of binding ionic compounds. Also, agricultural adsorbents can be recovered and reused without posing severe impact on environment, being consistent with concepts of effective, innovative and suitable waste management. All these aspects make them superior to conventional removal processes [12].

The primary objectives of this review study are to (1) characterize the mitigation techniques used for the removal of potentially toxic elements from wastewaters; (2) present the effects of various parameters, such as pH, adsorbent dosage, contact time, initial concentration of the contaminant, ionic strength, temperature, and modifying agents on adsorption capacity and removal efficiency; and (3) investigate the adsorption capacities and removal percentages of different agricultural byproducts for various potentially toxic elements.

2. Estimation of Removal Percentage and Adsorption Capacity

“Removal percent means a percentage expression of the removal efficiency across a treatment facility for a given pollutant or contaminant parameter as determined from a comparison of influent and effluent concentrations for a given time period” [13].

Removal efficiency can be estimated using Equation (1).

$$Removalefficiency\left(\%\right)=\frac{C_i-C_e}{C_i}\times 100$$

(1)

where C_i is the initial concentration of the adsorbate (mg L⁻¹) and C_e is the final concentration of the adsorbate (mg L⁻¹) [14].

“Adsorption capacity is the amount of adsorbate taken up by the adsorbent per unit mass (or volume) of the adsorbent” [15] and can be calculated using Equation (2).

$$Adsorptioncapacity,q_e\left(\mathrm{m}\mathrm{g}{\mathrm{g}}^{-1}\right)=\frac{C_i-C_e}{M_s}\times V$$

(2)

where C_i is the initial concentration of the adsorbate (mg L⁻¹) and C_e is the final concentration of the adsorbate (mg L⁻¹), V is the volume of the adsorbate solution (L), M_s is the adsorbent dose (g) [14].

The adsorption capacity of an adsorbent material can be assessed using various adsorption isotherms or kinetic models, which estimate the quantity of target metal adsorbed onto the surface of the material per unit weight, under equilibrium conditions and at a constant temperature [16]. The most commonly used isotherms are Langmuir and Freundlich, but there are several other isotherms and models available, such as pseudo-first-order (PFO), pseudo-second-order (PSO), Dubinin–Radushkevich, Elovich, Redlich–Peterson, Flory–Huggins, Henry, Temkin, Radke and Prausnitz, Halsey, Sips, Toth, Harkins–Jura, Koble and Corrigan, and many others [16,17,18].

Table 2. Equation of the most important isotherm and kinetic models used in adsorption studies.

Isotherm/Kinetic Model	Equation	Reference
Langmuir isotherm	$q_e=\frac{q_m{K}_L{C}_e}{1+K_L{C}_e}$	[18]
Freundlich isotherm	$q_e=K_F{C}_e^{1/n}$
Redlich–Peterson isotherm	$q_e=\frac{K_R{C}_e}{1+{\alpha }_R{C}_e^g}$
Sips isotherm	$q_e=\frac{K_S{C}_e^{{\beta }_S}}{1+{\alpha }_S{C}_e^{{\beta }_S}}$
PFO model	$q_t=q_e(1-e^{-k_{1}t}$)
PSO model	$q_t=\frac{k_2{q}_e^{2}t}{1+k_2{q}_{e}t}$
Henry’s linear model	$q_e=K_p{C}_e$	[19]
Temkin isotherm	$q_e=\frac{RT}{b}\ln \left(A{C}_e\right)$
Toth isotherm	$q_e=\frac{K_T{C}_e}{{({\alpha }_T+C_e^z)}^{1/z}}$

presents the equations for the most commonly used isotherms and models for bioadsorption studies.

3. Removal Techniques of Potentially Toxic Metals

Wastewater from different industrial sectors contains various chemical contaminants, such as dyes, pigments, potentially toxic elements, and pesticides, among many other hazardous chemicals. The elimination of these compounds is a necessary step for wastewater treatment [20]. Techniques available for mitigating potentially toxic elements from wastewater include conventional methods such as membrane filtration, chemical precipitation, ion exchange, chemical coagulation, bioremediation, flocculation, and adsorption [21], as well as modern methods based on nanotechnology [11]. Adsorption-based techniques are more effective and economical than other conventional methods, due to the large surface area of solid adsorbents, the large number of active sites, small size, porosity, easy preparation, amorphous nature, and low cost [11].

Conventional methods can be grouped into membrane-based filtration methods, chemical-based separation methods, electricity-based separation methods, photocatalytic-based separation methods, and adsorption-based separation methods.

An overview of the most important techniques for heavy metals removal is presented in Figure 1.

The membrane-based filtration process is a separation technique that uses a porous membrane and a driving force applied across the membrane to separate contaminants from solution, based on the substrate size, membrane pore size, solution concentration, and applied pressure [23]. It is a pressure-based removal process that can be enhanced by treating membranes with various chemical compounds [24]. These treatments, based on use of membranes in the form of a complex structure that contains elements at a nanometric scale, showed excellent results for mitigation of potentially toxic elements from wastewater [21]. Depending on the size of the pores, membrane filtration processes can be grouped as ultrafiltration, nanofiltration, or microfiltration. This category also includes reverse osmosis, forward osmosis, and electrodialysis [24,25].

Chemical-based separation techniques involve the transformation of toxic metals into solid precipitates and separation from the liquid phase [23]. These chemical methods for mitigation of heavy metals include chemical precipitation, coagulation–flocculation, and flotation [24]. In chemical precipitation, metal ions are combined with precipitants, Ca(OH)₂, NaOH, Na₂CO₃, Na(HCO₃)₂, Na₂S, and NaHS, to obtain less soluble compounds (hydroxides, carbonates, sulfides), which can be removed from the solution by using physical processes such as sedimentation, flotation, or filtration [26].

Electricity-based separation methods are extremely effective treatments for toxic metal removal based on their recovery in the elemental state by using anodic and cathodic reactions in the electrochemical cell [21]. They include electrochemical methods (electrodeposition, electrocoagulation, electroflotation, electrooxidation, electrodestabilization) and ion exchange [24,25]. Electrochemical separation is an advanced treatment involving the application of a constant voltage to a pair of electrodes immersed in a solution containing metal ions for their partial or complete decomposition [27].

In photocatalytic separation, a photocatalyst accelerates the process by increasing the necessary activation energy and reaction rate without participating in the reaction [28]. Photocatalytic-based separation has gained considerable attention for toxic metal removal and involves the separation of potentially toxic metals by using light and semiconductors (e.g., UV-irradiated TiO₂) to remove chemical contaminants [24,29]. The strategies used for photochemical degradation of toxic metals include direct photolysis, oxidation induced through photolysis, oxidation mediated by photosensitive molecules, and photocatalytic oxidation [30].

Interfacial solar steam generation (ISSG) is a promising and sustainable solution that directly utilizes the solar energy to obtain fresh water by locally heating water at the evaporator–air interface [31]. For this purpose, various interfacial solar evaporators, obtained from different photothermal materials, such as metal nanoparticles, carbonaceous materials (supercapacitor, lithium-ion battery), hydrogels, polymers, or semiconductors, were assessed to obtain high performance solar steam generation [31,32]. Using this technique, light-to-heat conversion takes place on the surface of solar evaporators, the energy loss being converted into electricity using a photovoltaic, thermoelectric (TE), piezoelectric, or triboelectric module [32].

Adsorption-based separation is a solid–liquid mass transfer based on the migration of the adsorbate (toxic metal) to the surface of the solid adsorbent and their bonding over the adsorbent surface due to the chemical and physical interactions [21]. This process involves physisorption; when the adsorbate is linked to the adsorbent using van de Waals forces and chemisorption, chemical reactions take place between them. The physisorption process is reversible, weak, and endothermic compared with chemisorption, which is irreversible, selective, and exothermic [16].

The advantages and drawbacks of the processes mentioned above are presented in

Table 3. Advantages and drawbacks of various removal processes.

Removal Technique	Advantages	Drawbacks	Reference
Ultrafiltration	Cost effective, low-pressure demand, great performance	Secondary pollutants, requirement of posttreatments	[23]
Nanofiltration	Easy to operate, high removal performances, reliability, suitable for industrial practice	High energy consumption, lower water permeability	[24,25]
Microfiltration	Low operation pressure, cost effective, easy to operate, low energy demand	Low removal performances	[29,33]
Reverse osmosis	High removal performances	Membrane fouling, membrane degradation, high power consumption	[24,29]
Forward osmosis	Energy-saving because it does not need hydraulic pressure, environmentally friendly, easy cleaning, low fouling	Internal and external concentration polarization, draw solution reconcentration,	[29]
Electrodialysis	High water recovery, can operate in a wide range of pH, no phase change, no reaction products	Membrane fouling, high operation cost, high cost of membranes, need of electric potential, periodic maintenance	[24,29]
Chemical precipitation	Simple operation, low cost of precipitant, ease of automatic control, inexpensive and simple, high removal performances, less dissolve solids	Requires a large quantity of precipitates, sludge generation, can produce hydrogen sulfide gas, extra cost for sludge disposal	[25,29,34]
Coagulation–flocculation	Suitable for broader pH range, simplicity in operation, low energy consumption, high versatility	Production of sludge, transfer of toxic compound into solid phase, toxicity, hazardousness of inorganic coagulants, results in a high quantity of sludge, selective technique for metals, inefficient for some emerging chemical contaminants, the flocculants are nonbiodegradable	[25,29,35]
Flotation	Low-energy demand, rapid and compact process, reduced volumes of sludge, moderate cost	Demand of selective treatments, requirement of efficient and nontoxic surfactants	[29]
Electro-deposition	Selective method, cost-effective, high capacity to recovery of valuable materials	High sensitivity, side reactions affect the removal capacity	[29]
Electrocoagulation	Sludge resulted is stable, easily removable and nontoxic, easy to operate	High energy consumption, resulting of harmful byproducts	[29]
Electroflotation	Short process time, resulting of stable sludge	Difficulty in controlling system pH	[29]
Electrooxidation	Oxidizes high toxic compounds, does not result in secondary pollutants	Corrosion of electrodes	[29]
Ion exchange	High removal efficiency, low cost, large applicability in a wide range of treatments, high selectivity, rapid kinetics, high treatment capacity, recovery of heavy metals, fast kinetics	Low longevity, high cost of reduction phase, resins pollution, residual sludge production, demand of resins	[25,33]
Photocatalysis	High efficiency, rapid destruction, less harmful byproducts	High cost, long duration time, limited applications	[24,34]
Solar steam generation	High efficiency, low cost, no consumption of additional energy, low heat loss, improves energy utilization of elaborated infrastructures	Difficulty of precise customization of conical arrays, extended processing time	[31,32,36]
Adsorption	Easy to operate and use, simplicity, low cost, high effect on contaminants removal, lack of sludge, recycling of adsorbed materials, usage of particles with different sizes, availability of adsorbent materials	Low selectivity	[34,37]

.

Adsorption process can be achieved by using different adsorbents, such as carbon-based adsorbents, mineral adsorbents, magnetic adsorbents, metal–organic frameworks adsorbents (MOFs), agricultural waste adsorbents, and others. Each type of adsorbent has its advantages and disadvantages, and their efficacy for heavy metal removal is influenced by various factors such as surface area, pore size and volume, pH, temperature, and initial concentration of the contaminant. Different modification methods can enhance the adsorption capacity and removal performance of the adsorbents. However, some adsorbents have drawbacks such as low stability, poor chemical stability, low mechanical strength, and demand for pretreatments to enhance their adsorption capacity [24,25,29,37,38].

In the adsorption process, metal ions from solvent are attached to the high area surface of solid adsorbent (sorbent) and bond with the functional groups (-COOH, -NH₂, -OH, -CONH₂, -SH, -OCH₃) presented on the adsorbent surface through different processes, such as chemisorption, chelation, complexation, adsorption by physical forces, adsorption on surface and pores, and diffusion through the cell wall and membrane [39].

The adsorption process includes three steps: (i) physical adsorption of ions on adsorbent surface; (ii) precipitation, complexation, and deposition of adsorbate on the adsorbent surface; (iii) pore filling—the adsorbate condensed into the pore of the adsorbent [40].

The adsorbed molecules create a molecular of atomic film on the surface of adsorbent, due to the imbalance and residual forces that continue to attract and retain the adsorbate on the adsorbent. Van der Waals forces and covalent bonds take place between adsorbent and adsorbent material [41].

Adsorption process can be divided into physical adsorption, when the metal ions are adsorbed on the surface of adsorbent using van de Waals forces, hydrogen bonding, dipole–dipole interactions, and chemical adsorption based on the irreversible attachment of adsorbate to adsorbent through chemical bonding or electron transfer [41].

4. Key Factors That Affect the Adsorption Process

The factors that affect the removal of heavy metals through adsorption include the characteristics of the toxic metals, the adsorbent material, the water quality conditions, and the adsorption process. The pH value of the solution affects the speciation and ionization of the metal, as well as the surface charge of the adsorbent. The initial concentration of the metal ions, the adsorbent dose, and the temperature also affect the removal efficiency. Increasing the adsorbent dose and temperature generally increases removal efficiency, while increasing the initial concentration of the metal ions can initially increase adsorption capacity [20,38].

The optimum parameters used to obtain higher removal efficiency and/or adsorption capacity for different types of agricultural waste are presented in

Table 4. The optimum parameters of adsorption with different agricultural wastes.

Adsorbent	Metal Ion	Optimum Parameters					R.E. * (%)	qe* (mg g−1)	Reference
		M (g)	pH	C0 (mg/L)	t (min)	T (°C)
Brewed tea waste	Pb2+, Cd2+, Ni2+, Zn2+	1.0; 1.0; 2.0; 2.0	4.0; 4.0; 5.0; 5.0	100	2; 5; 30; 10	20 °C	97.97; 84.74; 82; 76	1.197; 1.457; 1.163; 2.468	[42]
Tea waste	Pb2+, Cd2+, Cu2+	0.1	5.0	100; 20	60	25 °C	-	33.49; 16.87; 21.02	[43]
Walnut shell	Pb2+	1.0	4.0	100	2	20 °C	90	9.912	[44]
Tea waste	Cd2+	10	5.0	5	90	20 °C	99.5	1.76	[45]
Waste black tea	Zn2+	20	5.0	100	250	20 °C	80	166.7	[46]
Coffee grounds	Zn2+, Cu2+, Pb2+	2.5	5.0	100	300	55 °C	84.55; 68.73; 57.23	5.25; 24.28; 48.13	[47]
Modified green tea waste	Ni2+, As3+	0.3	3.0	5	10	33 °C	-	0.3116; 0.4212	[48]
Potato peel	Cr6+	4.0	2.5	20	48	-	100	3.28	[49]
Potato peels charcoal	Fe2+	0.8	-	50	20	-	90.4	121.95	[50]
Peanut shells	Cr3+, Cu2+	10	5.0	-	60	20 °C	-	27.86; 25.39	[51]
Peanut husk	Pb2+, Cd2+, Co2+, Mn2+, Ni2+	5.0	6.0	100	180; 240 (Mn)	-	99; 62; 30; 45; 38	27.03; 11.36; 6.10; 14.29; 56.82	[52]
Peanut shells	Cd2+, Hg2+	0.2	3	10	30	-	37 (Hg)	6.0; 1.90	[53]
Peanut shells							100 (Hg)	14.17; 30.72
Tea waste	Zn2+	0.2	4.2	25	30	-	-	8.90	[54]

* R.E.—removal efficiency; q_e—adsorption capacity.

.

5. Surface Modification of Agricultural Wastes

Modification of adsorbent surface represents adjusting of its physical, chemical, or biological characteristics to obtain different forms on the surface, making it suitable for a desired purpose. This process involves the removal of surface impurities to produce physical and chemical changes, such as surface energy, reactivity, surface charge, hydrophobicity, surface area using mechanical, thermal, and chemical methods, or using a combination of methods, such as mechanochemical and thermochemical processes. Mechanical and thermal processes are used to create pores on the surface, while chemical methods are used to improve the characteristics of the surface. In the case of chemical modification, the skeleton of the adsorbent is affected, leading to improvement of the functional groups and properties of the adsorbent’s surface due to chemical interactions between the modifying agent and the surface of the adsorbent material. Chemical modification can be performed by using acid, alkaline, or neutral solutions [55]. Chemical modification involves processes such as acidification, alkalization, esterification, etherification, carbonization, magnetization, and grafting, which are more effective concerning the surface area, functional groups, and adsorption performances compared with physical modification processes [56].

Acid modification is a wet oxidation process that involves the use of organic acids (acetic, carboxylic, formic, and oxalic acid), mineral acids, or other oxidants such as HNO₃, HClO, HCl, SO₄, and PO₄. Acidification of the surface improves the adsorption of positively charged metal ions due to the positive charge of the surface [55].

During this process, acids can dissolve the constituents from agricultural byproducts with a simultaneous increase in O₂ content and decrease in the tortuosity of the pores. Also, this treatment can enhance the hydrolysis of cellulose, resulting a more reactive adsorbent material, compared with untreated waste [56].

Huang et al., 2019 [57], observed an increase in the content of polar functional groups (such as hydroxyl, carboxyl, and carbonyl) and the appearance of nitro groups on the surface of the modified adsorbent. This treatment with nitric acid improved the adsorbent’s polarity and negatively-charged properties.

According to Lesoana et al., 2019 [58], acidification of macadamia-activated carbon with sulfuric, nitric, and phosphoric acids increases the surface area of modified adsorbent from 545 to 824 m² g⁻¹. Acidification of the material surface reduces the mineral content and increases the adsorption capacity of toxic metals from solutions because the competition between cations and active sites decreases. In the case of dilute acids, the amount of C–H, O–H, and C–O functional groups increase compared with concentrated acids, where the hydroxyl and aldehyde functional groups are transformed in carboxyl groups, which are more oxidized [56].

Boeykens et al., 2019 [59], used H₃PO₄ as modifying agent for avocado seeds and demonstrated an increase in the adsorption capacity of modified adsorbent for Pb²⁺ and Cr⁶⁺. Kim et al., 2020 [60] and Thabede et al., 2020 [61] also used H₃PO₄ to improve banana peel biochar and black cumin seeds, respectively. They observed an increase in the adsorption capacity for Mn²⁺ and Fe²⁺ [60], and Cd²⁺ [61]. When H₂SO₄ was used as modifying agent, Wanja et al., 2016 [62]; and Abid et al., 2016 [63]; reported an improvement in the adsorption capacity of avocado seeds and orange peel for Pb²⁺, Cd²⁺, Cu²⁺ [62], and As⁶⁺ [63]. Tejada-Tovar et al., 2015 [64], demonstrated a higher adsorption capacity by using citric acid as modifying agent for raw palm bagasse (from 162 to 451 mg g⁻¹). Ding et al., 2014 [65], and Shooto et al., 2019 [66], reported a higher adsorption capacity of peanut hull and black cumin seeds by using mercaptoacetic acid and HCl as modifying agents for the adsorption of Hg²⁺ [65] and Co²⁺ [66].

Alkaline modification improves the adsorption capacity of negatively charged species due to the increase in alkali functional groups that adsorb a positive charge on the surface. This process can be performed by treating adsorbents with NaOH, KOH, LiOH, Na₂SiO₃, Na₂CO₃, and other oxides [55].

Alkalization reduces the content of lignin and hemicellulose in the adsorbent material and improves its surface area, mechanical properties, and thermal stability through H-bonding, ion exchange, and complexation reactions between the alkaline-treated agricultural material and pollutants [56].

Zheng et al., 2013 [67], obtained an increase in specific area and pore volume with the increase in NaOH concentration, which involves a higher adsorption capacity of contaminants. In the case of Gao et al., 2013 [68], after treating of Enteromorpha prolifera-activated carbon with KOH, the surface area increased up to 3500 m² g⁻¹ and the total pore volume up to 2872 cm³ g⁻¹, while the adsorption capacity increased from 454 mg g⁻¹ to 2500 mg g⁻¹.

Research studies have demonstrated that KOH alkalization unlocks structural pores from adsorbent materials, obtaining more visible pores compared with unmodified material, which improves the retention capacity for chemical contaminants. Also, alkaline treatments can enhance the release of cations, such as K⁺ and Ca²⁺ from the treated adsorbent materials and improve ion exchange capacity and the retention of cationic pollutants [56].

Ali and Saeed, 2015 [65], who investigated the effect of NaOH as a modifying agent for banana peel, demonstrated that this treatment improved the adsorption capacity of the biosorbent. Higher adsorption capacity (897.54 mg g⁻¹) was reported by Shooto et al., 2019 [66], by treating black cumin seeds with NaOH. Most probably, these differences are also due to the concentration in which the modifying agent is added.

Other chemical methods (oxidants, organic and metal ions): Another chemical process used for modification of the surface is the use of oxidizing agents (KMnO₄, H₂O₂), neutral agents (ZnCl₂ and NaCl), metal impregnation (ferric chloride, cerium, zirconium, hydroxides, carbonates, chromates, or nitrates), and organic agents (ethanol) [55].

According to Enaime et al., 2020 [69], oxidizing agents are responsible for increasing the number of oxygen functional groups on adsorbent materials. Wang et al., 2015 [70], also used KMnO₄ to modify the surface of hickory wood, and observed an increase in the number of oxygen-containing functional groups as well as an increase in surface area.

Ahmed et al., 2021 [71], and Thabede et al., 2020 [61], used oxidizing agents, such as H₂O₂ and KMnO₄, to modify the surface of watermelon seeds biochar and black cumin seeds. The authors demonstrated that the maximum adsorption capacity of 60.87 mg g⁻¹ for Pb²⁺ [71] and 23.87 mg g⁻¹ for Cd²⁺ [61] are higher than of the unmodified adsorbent. Contrarily, Tejada-Tovar et al., 2018 [72], who used a neutral agent (CaCl₂) to improve the sorption capacity of orange peel, demonstrated that this pretreatment negatively influences the absorbability of the adsorbent, and the adsorption capacity of Cr⁶⁺ decreased from 12.29 mg g⁻¹ for unmodified orange peel to 4.950 mg g⁻¹ in treated orange peel.

Esterification: Esters result from the reaction of hydroxyl groups presented in cellulose with carboxyl groups (anhydrides), which improve the hydrophobicity and mechanical strength of agricultural adsorbents, in this way enhancing the adsorption performances of the waste material. The most used compounds for the esterification of an adsorbent surface are represented by succinic anhydride, EDTA (ethylenediaminetetraacetic acid) dianhydride, citric acid anhydride, and maleic anhydride. The -OH bound with cellulose reacts with citric acid-resulting carboxyl groups on the straw surface, which creates complexation reactions with metal ions. In the case of EDTA, the esterification reaction generates amino and carboxyl groups, which bind with metal ions. Carboxyl groups resulting after esterification are involved in the roughness of the absorbent surface, increasing the surface area or enhancing the porosity of the adsorbent. Also, the type of the esterification reagent strongly influences the adsorption capacity of the adsorbent material due to differences in molecular structure. For example, citric acid, which contains more carboxyl groups compared with tartaric acid, can generate more active sites after treatment of adsorbent and implicitly has a higher adsorption capacity. The esterification reaction between citric acid and hydroxyl groups can be enhanced by the addition of a mild catalyst such as NaH₂PO₂ · H₂O [56].

Etherification: Etherification of agricultural waste involves the substitution of -OH groups with other functional groups from modifying agent, resulting in ethers that can bind potentially toxic metal ions. The most used modifying agents are triethylenetetramine, diethylenetriamine, and ethylenediamine, which can generate amino groups for adsorbent material, able to bind various ions from polluted water.

In addition to amino groups, after the etherification process, the adsorbent material also contains carboxyl and thiol functional groups, which can adsorb chemical pollutants. Carboxyl groups enhance the charge transfer to the oxygen to attract metal ions and promote pollutant retention, while thiol groups can bind to the cation pollutants [56].

Magnetization: Modification of agricultural waste using magnetization involves the introduction of transition metals or their oxides into the adsorbent material to obtain a magnetic material, which are easily removed with an external magnet, making them very effective for the retention of chemical pollutants from aqueous solutions. FeCl₃, Fe₃O₄, Fe₂O₃, and zero-valent iron nanoparticles are the most used compounds for magnetic modification of adsorbent materials. In the case of nanoscale zero-valent iron nanoparticles, Shao et al., 2020 [73], demonstrated that modified adsorbent revealed enhanced removal performance compared with raw adsorbent. Also, they found that a portion of the metal ions was directly adsorbed by the waste material, while the other part was first reduced to zero-valent Cu and Cu₂O, and then adsorbed on the material surface due to the Fe-oxide structure. The addition of Fe₃O₄ to the surface of agricultural waste is the most used magnetization technique, which can be applied for ferrofluids, microwave-assisted, and mechanochemical techniques for toxic metal removal. Fe₃O₄ from the adsorbent surface can be obtained using coprecipitation, the functional groups OH and Fe–O from modified adsorbent resulting from the interaction between O and Fe from Fe₃O₄. This magnetization with Fe₃O₄ can increase surface area, enhance porosity, activate functional groups, and stabilize surface properties. Magnetization can enhance adsorption properties through specific actions, such as modification of lignocellulose using magnetic materials, easy recovery, and reuse of the adsorbent during the removal process [56].

Surfactant modification: Surfactants are compounds that contain various functional groups, such as phosphates, sulphonates, and quaternary ammonium salts, involved in selective adsorption of the pollutants from the solution. Surfactant modification improves the adsorption capacity of modified adsorbent through the improvement of its surface hydrophobic/hydrophilic properties and enrichment of the variety and quantity of functional groups in the structure of the modified material. The most used compounds for surfactant modification are linear alkyl benzene sulphonates, secondary alkane sulphonates, alkyl trimethyl ammonium halides, and quaternary ammonium compounds. Agricultural byproducts treated with cationic surfactants have revealed good applicability for anionic contaminants removal from contaminated wastewater. These surfactants create numerous positive charges on the material surface and improve the retention of negatively charged pollutants [56]. Soldatkina and Zavrichko, 2019 [74], observed that cationic surfactants can reduce the surface area of the adsorbent material due to the blockage of pores and reduction in access to the internal surface area without influencing adsorption performance.

Carbonization: Carbonization is a thermal decomposition process that involves the heating of agricultural waste in a low-oxygen or anaerobic environment and production of carbonaceous residues, such as biochar, biooil, and non-condensable gas products [56,75]. The main purpose of carbonization is the conversion of agricultural byproducts into solid biochar, which is a heterogeneous charcoal material [56,75]. Synthesis of biochar can be performed using various processes, such as traditional pyrolysis carbonization, hydrothermal carbonization, and microwave carbonization, traditional pyrolysis being the most used process for biochar products due to its high efficiency, simple operation, and large output [75]. Compared with raw agricultural byproducts, obtained biochar products are characterized by a higher specific area, improved porosity and an increased number of functional groups capable of retaining a higher number of pollutants [56].

The yield, performance, structural characteristics, and chemical and physical properties of the biochar obtained using pyrolysis carbonization depend on reaction time and, especially, carbonization temperature [56].

Acidic or alkaline compounds can be added to agricultural products, resulting in an effect on the characteristics, structure, and properties of the obtained biochar. Also, post-optimization of biochar is useful for improving its potential value after pyrolysis [56]. For this purpose, Goswami and Phukan, 2017 [76], added H₂SO₄ during the carbonization process, resulting in -SO₃ groups on the biochar surface that increased the adsorption of adsorbent for chemical contaminants through covalent bonding. Also, Yu et al., 2018 [77], treated biochar obtained with HNO₃, generating N− and ≡N+ groups, which can interact with negative metal ions.

Adsorption capacities of different agricultural biosorbents, modified by various processes or unmodified are presented in

Table 5. The absorption capacities of unmodified and modified adsorbents.

Adsorbent	Metal Ions	Modifying Agent	Adsorption Capacity (mg g−1)		Reference
			Unmodified Form	Modified Form
Acidification
Avocado seeds	Pb2+, Cr6+	H3PO4	18.9; 3.39	26.6; 5.10	[59]
Banana peel biochar	Mn2+, Fe2+	H3PO4	0.796; 27.355	2.319; 29.550	[60]
Black cumin seeds	Cd2+	H3PO4	18.16	19.48	[61]
Avocado seeds	Pb2+, Cd2+, Cu2+	H2SO4	21.48; 11.78; 46.02	271.02; 237.14; 102.57	[62]
Orange peel	As6+	H2SO4	86.95	98.03	[63]
Peanut hull	Hg2+	Mercaptoacetic acid	30.3	83.3	[65]
Black cumin seeds	Co2+	HCl	28.05	78.58	[66]
Banana peel	Mn2+	HCl	2.864	4.736	[78]
Alkalization
Black cumin seeds	Co2+	NaOH	28.05	87.54	[66]
Banana peel	Mn2+	NaOH	2.864	3.601	[78]
Wheat straw	Cu2+	NaOH	10.11	10.59	[79]
Peanut hulls	Pb2+	NaOH	19.7	49.6	[80]
Oxidizing agents
Watermelon seeds biochar	Pb2+	H2O2	44.32	60.87	[71]
Orange peel	Cr6+	CaCl2	12.29	4.950	[72]
Black cumin seeds	Cd2+	KMnO4	18.16	23.87	[61]
Esterification
Raw palm bagasse	Pb2+	Citric acid	162	451	[64]
Barley straw	Cu2+	Citric acid	4.64	31.71	[81]
Rice husk	Cu2+, Pb2+, Ni2+	N-isopropylacrylamide and citric acid	86.81; 79.75; 69.98	84.67; 118.3; 79.87	[82]
Etherification
Sugarcane bagasse	Pb2+, Cu2+, Zn2+	Ethylenediamine, CS2	-	558.9; 446.2; 363.3	[83]
Corn stalk	Cd2+	Acrylonitrile	3.39	12.73	[84]
Rice straw	Cr6+, Ni2+	Triethylamine	-	15.82; 3.95	[85]
Corn stalk	Cr6+	Diethylenetriamine	-	200.0	[86]
Magnetization
Sugarcane bagasse	Pb2+, Cd2+	Citric acid and Fe2O3	-	33; 117	[56]
Rice straw	Cu2+, Pb2+	Fe3O4	-	16.31; 19.45	[87]
Sugarcane bagasse	As5+	Fe oxides	-	22.1	[88]
Wheat straw	Cu2+	Nanoscale zero-valent		376.4	[73]
Surfactant modification
Peanut husk	Cr6+	Epichlorohydrin	-	138.34	[89]
Coconut coir pith	Cr6+	Hexadecyltrimethylammonium bromide	-	76.3	[90]
Cotton stalk	Ni2+, Cd2+, Cr3+	Sodium dodecyl sulphate	31.91; 19.58; 38.29	38.23; 24.93; 43.33	[91]
Carbonization
Peanut hull biochar	Pb2+	H2O2	-	22.82	[92]
Rice straw biochar	Cd2+	H3PO4	12.17	41.9	[93]
Nut shell biochar	Cd2+	SO2	104.17	142.86	[94]

.

6. Adsorption Capacity and Removal Performances of Different Types of Agricultural Waste

According to Karic et al., 2022 [95], the global levels of various agricultural waste are represented by 80 million tons of husk, 70–140 thousand tons of potato peel, 4.24 million tons of banana peel (40% of global produce of 10.6 million tons), 55 million tons of corn cob, 300 million tons of rice straw, 10 million tons of medicinal plant waste, 23–28 million tons of citrus peel (50–60% of global production of 47 million tons), 25 million tons of coconut waste, and 1436.1 thousand tons of shell waste (20% of global production of 7180.5 thousand tons).

Agricultural waste can be grouped into four groups, namely (i) cereal waste (straw, husks), (ii) fruit waste (pits, peels, shells), (iii) plant waste (cobs, beet, stems), and (4) biosorbents (represented by living and nonliving biomass: algae, microorganisms) [95].

For the mitigation of potentially toxic elements, various types of plants or different parts of plants, such as stems, barks, straws, husks, stalks, roots, leaves, and others, can be utilized, as these byproducts are readily and abundantly available [96].

Husks are a byproduct widely used for removing toxic metals from wastewater sources. Rice husks are the most commonly used agricultural waste for metal mitigation processes [96], but research studies have revealed the use of other husks such as peanut husk [97], sunflower husk [98], corn husk [99], coffee husk [100], or walnut husk [101]. As can be seen from

Table 6. Removal efficiency and adsorption capacity of some unmodified agricultural biosorbents.

Agricultural Waste	Metal Ions	Removal Efficiency (%)	Adsorption Capacity (mg g−1)	Reference
Husks
Peanut husk	Cr6+	-	33.1	[102]
Rice husk	Cr3+, Cu2+	-	22.5; 30.0	[105]
Rice husk	Cr6+	-	52.1	[103]
Rice husk ash	Cr6+, Pb2+, Zn2+	87.12; 88.63; 99.28	-	[104]
Sunflower husk	Pb2+, Cu2+, Cd2+, Ni2+	98.7; 90.3; 80.0; 94.0	-	[98]
Corn husk	Pb2+, Cu2+, Ni2+	99.66; 96.01; 92.36	-	[99]
Coffee husk	Pb2+, Cd2+, Cr3+, Cu2+	-	1.470; 0.174; 0.188; 0.259	[100]
Walnut husk	Cd2+	96.11	-	[101]
Walnut seed husk	Pb2+	-	4.0	[106]
Straw
Wheat straw	Cd2+, Cu2+	-	14.56; 11.43	[107]
Wheat straw	Cr6+, Ni2+	-	47.16; 41.84	[108]
Wheat straw	Cd2+	-	39.22	[109]
Barley straw	Cu2+, Pb2+	-	4.64; 23.2	[110]
Barley straw	Cu2+, Ni2+, Co2+, Cd2+	-	17.8; 8.25; 6.58; 1.42	[111]
Rice straw	Pb2+, Zn2+	-	17.93; 25.73	[112]
Corn straw	Pb2+	-	15.0236	[113]
Corn straw	Cd2+, Pb2+	99.24; 98.62	-	[114]
Rape straw	Cd2+	-	32.737	[115]
Rape straw	Ni2+	99.7	-	[116]
Peel
Pomegranate peel	Cr6+	98.95	-	[117]
Grapefruit peel	Cd2+	-	42.09	[118]
Banana peel	Cd2+, Cr6+, Pb2+	-	3.66; 6.85; 20.90	[119]
Sweet lime peel	Cr6+	-	250	[120]
Orange peel	Cu2+, Pb2+	-	15.27; 73.53	[121]
Orange peel	Cu2+, Cd2+	96.9; 98.1	2.78; 2.57	[122]
Potato peel	Cu2+	-	84.74	[123]
Tangerine peel	Cr3+, Cu2+, Mn2+, Co2+, Ni2+, Pb2+, Cd2+, Zn2+	88.92; 97.04; 92.48; 94.70; 93.50; 93.0; 97.90; 96.80	-	[124]
Lemon peel	Cd2+, Co2+, Cr6+, Cu2+, Mn2+, Ni2+, Pb2+	-	7.34; 5.63; 7.56; 7.17; 5.17; 5.73; 8.17	[125]
Melon peel	Cu2+, Cd2+, Pb2+	-	77.76; 76.16; 191.93	[126]
Shells
Cashew nut shell	Cd2+	-	22.11	[127]
Almond shell	Pb2+	-	8.08;	[128]
Hazelnut shell			28.18
Walnut shell	Cu2+, Pb2+, Cd2+, Zn2+	-	30.18; 70.37; 44.94; 58.96	[129]
Walnut shell	Cr6+	-	200	[130]
Cashew nut shell	Cu2+, Cd2+, Zn2+, Ni2+	-	406.6; 436.7; 455.7; 456.3
Bael fruit shell	Cr6+	-	17.27	[131]
Fluted pumpkin seed shell	Pb2+	-	14.286	[132]
Coconut shell	Cd2+	-	14.22	[18]
Cocoa shell	Cr6+	-	23.36	[133]
Stalk
Sunflower stalk	Pb2+, Cd2+	97; 87	182.90; 69.80	[134]
Corn stalk	Cd2+	-	12.73	[84]
Corn stalk	Cd2+	-	21.37	[135]
Corn stalk	Cr6+	28.67	-	[136]
Corn stalk	Cu2+	-	54.05	[137]
Banana stalk	Cu2+, Mn2+, Zn2+	-	134.88; 109.10; 108.10	[138]
Cotton stalk	Pb2+	-	146.78	[139]
Cotton stalk	Cr3+, Ni2+, Cd2+	100; >90; >90	31.91; 19.58; 38.29	[91]
Grape stalk	Cd2+	-	21.5	[140]
Sweet sorghum stalk	Cu2+	-	13.32	[141]
Sesame stalk	Ni2+, Zn2+	-	47.62; 100	[142]
Cassava stalks	Pb2+, Zn2+	-	163.93; 84.74	[143]
Tea stalk	Cu2+, Zn2+	-	50.34; 37.87	[144]
Stalks of tobacco species	Cr6+	99.13; 98.33; 95.0	-	[145]
Stone
Apricot stone	Pb2+	-	111.11	[146]
Apricot stone	Al3+, Zn2+	-	333.3; 500	[147]
Olive stone	Cu2+, Cd2+, Pb2+, Cr6+	77.4; 80.5; 94.5; 46	0.557; 0.3; 0.581; 2.345	[148]
Peach stone	Pb2+, Cd2+, Cu2+	-	118.76; 37.48; 32.22	[149]
Peach stone	Cr6+	97	-	[150]
Lime stone	Pb2+, Cu2+, Cr6+, As3+	30.8; 16.5; 11.5; 8.9	-	[151]
Activated plum stone	Cu2+, Pb2+	-	48.31; 80.65	[152]
Raw plum stone	Pb2+, Cd2+, Ni2+	-	9.93; 12.45; 5.63	[153]
Mango stone	Cd2+, Pb2+	-	21.05; 1.90	[154]
Cherry stones	Cr6+	81.3	-	[155]
Vegetable waste
Cabbage waste	Pb2+	-	54.945	[156]
Cabbage waste	Pb2+, Cd2+	-	60.57; 20.57	[157]
Cauliflower waste	Pb2+, Cd2+	-	47.63; 21.32
Tomato waste	Cu2+	-	25	[158]
Tomato waste	Cu2+	92.08	34.48	[159]
Tomato waste	Pb2+	-	152	[160]
Carrot waste	Cr3+, Cr6+	-	86.65; 88.27	[161]
Onion waste	Pb2+, Cd2+	92.05; 94.89	9.74; 14.17	[162]
Onion waste	As3+, Fe2+, Pb2+, Sn2+, Cd2+, Hg2+	-	2.56; 8.012; 9.957; 7.812; 1.36; 4.95	[163]
Garlic waste	As3+, Fe2+, Pb2+, Sn2+, Cd2+, Hg2+	-	2.304; 8.459; 10.496; 7.132; 1.47; 5.12
Seeds
Moringa seeds	Fe2+, Mn2+	80.5; 93	10.28; 11.641	[164]
Avocado seeds	Cd2+, Cu2+, Pb2+	98.23; 99.12; 99.29	-	[165]
Avocado seeds	Pb2+, Cr6+	-	18.9; 3.39	[59]
Mangifera indica seeds	As3+	94	0.365	[166]
Artocarpus heterophyllus seeds	As3+	93	29.25
Schizizium commune seeds	As3+	92	0.360
Grape seeds biochar	As3+	-	5.082	[167]
Allium Cepa seeds	Cr6+, Cd2+, Zn2+, Cu2+, Pb2+	-	1.78; 1.52; 1.40; 1.62; 1.68	[168]
Watermelon seeds biochar	Pb2+	-	44.32	[71]
Papaya seeds	Cu2+, Pb2+	-	97.55; 99.96	[169]
Cakes
Gingelly oil cake	Pb2+	-	105.26	[170]
Neem cake	Cu2+, Cr6+, Ni2+	93.3; 85.4; 96.6	-	[171]
Moringa seed cake	Cr6+	-	3.191	[172]
Oil palm cake	Cu2+, Pb2+, Zn2+	-	45.01; 125.51; 39.21	[173]
Cottom seed cake	Cu2+	88	-	[174]
Mustard oil cake	Cr6+, Hg2+	-	29; 48	[175]
Rapeseed oil cake	Pb2+, Ni2+	-	129.87; 133.33	[176]
Rapeseed cake	Cu2+, Zn2+	-	52.196; 29.043	[177]
Olive cake	Cu2+, Mn2+, Zn2+,Ni2+, Pb2+, Cr3+	-	30.031; 3.571; 12.693; 5.851; 41.539; 22.193	[141]
Black cumin cake	Cu2+	-	106.38	[178]
Leaves
Bamboo leaf powder	Hg2+	-	27.11	[179]
Castor leaf powder	Hg2+	-	37.20	[180]
Pine leaf powder	As5+	-	3.27	[181]
Cauliflower leaf biochar	Cu2+, Pb2+	-	81.43; 224.60	[182]
Cabbage leaves	Pb2+	95.67	6.307	[183]
Aloe vera leaf powder	Pb2+	96.2	-	[184]
Tea leaves	Cr6+	95.42	10.64	[185]
Mango leaves	Cd2+	-	4.08	[186]
Eucalyptus leaves	As3+, Hg2+	> 94%	84.03; 129.87	[187]
Mangrove leaf powder	Cr6+	-	60.24	[188]
Bagasse
Sugarcane bagasse	Ni2+	-	2.0	[189]
Grape bagasse	Cu2+	-	43.47	[190]
Agave bagasse	Pb2+, Cd2+, Zn2+	-	93.14; 28.50; 24.66	[191]
Agave bagasse	Zn2+, Cd2+, Pb2+	-	8; 14; 36	[192]
Sugar beet bagasse	Cr6+	-	52.87	[193]
Mango bagasse	As3+	-	1.35	[194]
Oil palm bagasse	Cr6+	-	111.45	[195]
Palm bagasse	Pb2+	-	162	[64]
Hulls
Peanut hull	Cu2+	-	14.13	[196]
Rice hull	Zn2+, Cd2+, Fe3+	91.017; 84.848; 94.667	1.3367; 0.137; 25.403	[197]
Groundnut hull	Cr6+	-	90	[198]
Maize hull	Cu2+	50	-	[199]
Pistachio hull	Ni2+	> 75	14	[200]
Soybean hulls	Cr6+	91.991	-	[201]
Hazelnut hull	Fe3+	83.5	13.59	[202]
Cotton hull	Pb2+	-	27.65	[203]
Almond hull	Cr6+	> 94.14	-	[204]
Buckwheat hulls	Hg2+	-	243.90	[205]
Other agricultural wastes
Green tea	Cr6+	99.98	-	[117]
Tea waste biochar	Cr6+	-	198.0	[206]
Brewed tea waste	Pb2+, Cd2+, Ni2+, Zn2+	-	1.1947; 1.457; 1.163; 2.468	[42]
Hemp fiber	Co2+	-	13.58	[207]
Corn cob	Cd2+, Cr3+, Pb2+	-	13.577; 18.782; 29.168	[119]
Sunflower achene head			11.404; 12.206; 22.644
Cocoa pod	Cd2+, Pb2+	-	12.15; 5.31	[154]
Apple juice residue	Pb2+	-	108	[160]
Coffee waste	Cu2+, Pb2+, Zn2+	-	8.2; 27.6; 8.0	[208]
Mango bark	As3+	-	1.25	[194]
Fig sawdust	Pb2+	95.8	80.645	[209]
Coconut coir	Cu2+	13.43–41.56	0.5–1.15	[210]
Coconut husk-activated carbon	Cr6+	95.28	173.9	[211]
Agricultural waste-activated carbon	As3+, Pb2+	-	200; 250	[212]

, husks can be used to remove various toxic metals such as Cr⁶⁺ [102], Pb²⁺, Cd²⁺, Cu²⁺, and Cr³⁺ [100], Ni²⁺ [103], or Zn²⁺ [104]. Good absorbability was demonstrated in studies conducted by Sugashini et al., 2015 [103], for Cr⁶⁺, with a retention capacity of 52.1 mg g⁻¹; by Olguin et al., 2013 [102], for Cr⁶⁺, with a retention capacity of 33.1 mg g⁻¹; and by Sobhanardakani et al., 2013 [105], for Cr³⁺ and Cu²⁺, with retention capacities of 22.5 mg g⁻¹ and 30.0 mg g⁻¹, respectively. However, a lower retention capacity of 1.470 mg g⁻¹, 0.174 mg g⁻¹, 0.188 mg g⁻¹, and 0.259 mg g⁻¹ was demonstrated by Guevara-Bernal et al., 2022 [100], for Pb²⁺, Cd²⁺, Cr³⁺, and Cu²⁺, using coffee husk as the adsorbent material.

Concerning the removal efficiency, the results obtained by Priya et al., 2022 [103]; Radenkovic et al., 2022 [98]; Ismail et al., 2022 [99]; and Dalali and Hagghi, 2015 [101]; revealed removal performances higher than 80% for Pb (98.7%; 81.76%), Cu (90.3%), Cd (80.0%; 90.3%), Cr (90.84%), and Ni (94.0 %).

Straws are lignocellulosic waste materials that result from the cereal industry. The most commonly used adsorbent materials are those obtained from wheat straws and barley straws [96], but research studies have shown improved absorbability for other types of straw, such as rice straw [112], corn straw [113], or rape straw [115]. Isotherm and kinetic models reveal that these byproducts are suitable for various metals, such as Cu²⁺, Ni²⁺, Co²⁺, Cd²⁺ [111], Pb²⁺, Zn²⁺ [112], and Cr⁶⁺, Ni²⁺ [108]. Higher retention capacity was obtained for Cr⁶⁺ and Ni²⁺ at 47.16 mg g⁻¹ and 41.84 mg g⁻¹, respectively [108], and for Cd²⁺ at 39.22 mg g⁻¹ [109] and 32.737 mg g⁻¹ [115]. Conversely, Pehlivan et al., 2009 [110], and Arshadi et al., 2014 [111], obtained reduced absorbability for Cu²⁺ (4.64 mg g⁻¹) [110], Cd²⁺ (1.42 mg g⁻¹), Co²⁺ (6.58 mg g⁻¹), and Ni²⁺ (8.25 mg g⁻¹) [111]. As shown in Table 6, there are significant differences in the adsorption capacity of Ni²⁺ in the studies of Dhir and Kumar, 2010 [108], and Arshadi et al., 2014 [111], but these differences may be caused by the adsorbent used or the modification of the adsorption capacity using pretreatment. With regard to the removal efficiency, Chi et al., 2017 [114], and Wu and Wang, 2016 [116], reported values higher than 98% for Cd²⁺ and Pb²⁺ [114] and Ni²⁺ [116].

Fruit peel is a waste material from fruits, which is abundant and has high potential for mitigating toxic metals due to its high content of carbon-rich organic compounds such as cellulose, hemicellulose, pectin, chlorophyll, and other low molecular weight compounds. Orange peel is the most commonly used adsorbent material from this category, and several researchers have reported the removal of toxic compounds after chemically modifying the peel surface using acid or alkaline treatments [213]. In addition to orange peel, pomegranate peel [117], grapefruit peel [118], banana peel [119], potato peel [123], tangerine peel [124], lemon peel [125], and melon peel [126] have been used as adsorbent materials. Concerning the adsorption capacity, sweet lime peel revealed a higher absorbability for Cr⁶⁺ (250 mg g⁻¹) [120], but also melon peel for Pb²⁺ (191.93 mg g⁻¹) [126] and potato peel for Cu²⁺ (84.74 mg g⁻¹) [123]. Lower retention capacity was obtained by Yirga et al., 2022 [122], for Cu²⁺ and Cd²⁺ adsorption using orange peel, of 2.78 mg g⁻¹, respectively 2.57 mg g⁻¹, but also by Sabanovic et al., 2020 [125], for retention of Cd²⁺, Co²⁺, Cr⁶⁺, Cu²⁺, Mn²⁺, Ni²⁺, and Pb²⁺ using lemon peel, and by Mahmood-ul-Hassan et al., 2015 [119], for adsorption of Cd²⁺ and Cr⁶⁺ using banana peel. In the case of removal efficiency, Abdic et al., 2017 [124], demonstrated performances higher than 88% for Cr³⁺, Cu²⁺, Mn²⁺, Co²⁺, Ni²⁺, Pb²⁺, Cd²⁺, and Zn²⁺ by using tangerine peel as adsorbent material.

Shells are highly porous materials with a large surface area and a high affinity for water and cell wall components [96]. Research studies have demonstrated the capacity of various materials, such as cashew nutshell, walnut shell [127], almond shell, hazelnut shell [128], Bael fruit shell [131], coconut shell [18], and cocoa shell [133], for the removal of toxic metals. As can be seen from Table 6, higher adsorption capacities were obtained for cashew nutshells of 406.6 mg g⁻¹, 436.7 mg g⁻¹, 455.7 mg g⁻¹, 456.3 mg g⁻¹ for Cu²⁺, Cd²⁺, Zn²⁺, Ni²⁺, respectively, and for walnut shell, for Cr⁶⁺, of 200 mg g⁻¹ [127]. Contrarily, low absorbability was demonstrated by Pehlivan et al., 2009 [128], for almond shell, of 8.08 mg g⁻¹, for Pb²⁺.

The use of stalks as an adsorbent material is owed to their high content in cellulose, hemicellulose, and lignin [96], which can allow potentially toxic metals to bind to these compounds. Jalali and Aboulghaez, 2013 [134], investigated the feasibility of sunflower stalks as an adsorbent material for Pb²⁺ and Cd²⁺. They obtained a high removal efficiency of 97% and 87%, respectively, and a good adsorption capacity of 182.9 mg g⁻¹ and 69.8 mg g⁻¹. The kinetic models demonstrated improved retention capacity for banana stalks [138], cotton stalks [139], and cassava stalks [143]. Lower uptake capacities were obtained for corn stalks [84], sweet sorghum stalks [141], and grape stalks [140]. As concerning mitigation performance, Yang et al., 2022 [136], demonstrated that 28.67% of Cr⁶⁺ ions were removed using corn stalk, while Dehgani et al., 2014 [91], revealed that cotton stalk removed between 90–100% of Cr⁶⁺, Mn²⁺, and Zn²⁺ ions. Also, Prokopov et al., 2019 [145], reported a removal efficiency of higher than 95% for Cr⁶⁺ by using the stalks of three species of tobacco.

Various fruit stones have been used as natural adsorbents in many research studies, after removing their central fleshy parts [96], such as apricot stones [146,147], olive stones [148], peach stones [149,150], lime stones [151], plum stones [152], mango stones [154], and cherry stones [155]. These byproducts are rich in elemental carbon (40–45 wt%) [214]. El-Saharty et al., 2018 [147], reported, by using isotherm and kinetic models, adsorption capacities of apricot stones, for Al³⁺ and Zn²⁺, of 333.3 mg g⁻¹ and 500 mg g⁻¹, respectively. Good results were revealed by Abbas et al., 2014 [146], for apricot stone, of 111.11 mg g⁻¹ (Pb²⁺); Yan et al., 2018 [149], for peach stone, of 118.76 mg g⁻¹ (Pb²⁺); and by Parlayici and Pehlivan, 2017 [152], of 80.65 mg g⁻¹ (Pb²⁺). Low uptake capacities were obtained in the studies conducted by Amar et al., 2020 [148], of 0.557 mg g⁻¹, 0.3 mg g⁻¹, 0.581 mg g⁻¹, 2.345 mg g⁻¹, for Cu²⁺, Cd²⁺, Pb²⁺, and Cr⁶⁺ (olive stone); Pap et al., 2017 [153], for Pb²⁺, Cd²⁺, Ni²⁺, of 9.93 mg g⁻¹, 12.45 mg g⁻¹, 5.63 mg g⁻¹ (raw plum stone); and Olu-Qwolabi et al., 2012 [154], of 1.90 mg g⁻¹ for Pb²⁺ (mango stone). Regarding the removal efficiencies, Ku et al., 2012 [151], reported percentages of 8.9–30.8% for Pb²⁺, Cu²⁺, Cr⁶⁺, and As³⁺ by using limestone; Amar et al., 2020 [147], of 46–94.5% for Cr⁶⁺, Cu²⁺, Cd²⁺, and Pb²⁺ and 81.3% for Cr⁶⁺ removal using cherry stones [155]. Improved mitigation performance was demonstrated by Khemmari and Benrachedi, 2019 [150], for Cr⁶⁺ by using peach stone.

Vegetable waste is used as a low-cost adsorbent material because it is easily available and has no economic use, leading to zero waste discharge to the environment [96]. Kinetic models have demonstrated higher uptake capacities for tomato waste (150 mg g⁻¹ for Pb²⁺) [160], carrot waste (86.65 mg g⁻¹ and 88.27 mg g⁻¹ for Cr³⁺, and Cr⁶⁺ respectively) [161], and cabbage waste (54.945 mg g⁻¹ for Pb²⁺) [156]. Lower retention capacities were obtained for onion waste and garlic waste for As³⁺, Fe²⁺, Pb²⁺, Sn²⁺, Cd²⁺, and Hg²⁺ [163]. Yusuff et al., 2019 [162], obtained removal efficiencies higher than 92% for Pb²⁺ and Cd²⁺ by using onion waste, while Yargic et al., 2016 [159], reported uptake efficiency higher than 92% for Cu²⁺, using tomato waste as an adsorbent material.

Seeds are used as an adsorbent material due to their high lignin content, which is a highly branched polymer insoluble in water and capable of binding toxic metal ions [96]. Many researchers have used moringa seeds as adsorbent materials, but good results have also been obtained with avocado seeds, grape seeds, and papaya seeds. As shown in Table 6, kinetic models have demonstrated higher adsorption capacities for papaya seeds, with values of 97.55 mg g⁻¹ and 99.96 mg g⁻¹ for Cu²⁺ and Pb²⁺, respectively [169]. Watermelon seed biochar has also shown good results, with an adsorption capacity of 44.32 mg g⁻¹ for Pb²⁺ [71]. Contrarily, low absorbability was obtained for Mangifera indica seeds (0.365 mg g⁻¹ for As³⁺), for Schizizium commune seeds (0.360 mg g⁻¹ for As³⁺) [166], and for Allium cepa seeds, between 1.40 mg g⁻¹ and 1.78 mg g⁻¹, in the case of Cr⁶⁺, Cd²⁺, Zn²⁺, Cu²⁺, and Pb²⁺ [168].

In case of cakes, many studies on using of olive or cotton cake for potentially toxic metal removal are reported in the literature, due to their high cellulose content [96]. Ucar et al., 2015 [176], examined the applicability of rapeseed oil cake for adsorption of Pb²⁺ and Ni²⁺, obtaining an adsorption capacity of 129.87 mg g⁻¹ and 133.33 mg g⁻¹, respectively. Contrarily, Mazurek et al., 2021 [177], obtained lower uptake capacity for Cu²⁺ and Zn²⁺ by using rapeseed cake. Kinetic models revealed good sorption capacity for black cumin seeds (106.38 mg g⁻¹ for Cu²⁺) [178], oil palm cake (125.51 mg g⁻¹ for Pb²⁺) [173], and gingelly oil cake (105.26 mg g⁻¹ for Pb²⁺) [170]. Lower adsorption capacity was reported by Meneghel et al., 2013 [172], for moringa seed cake (3.191 mg g⁻¹ for Cr⁶⁺) and by Fernandez-Gonzalez et al., 2018 [171], for olive cake (3.571 mg g⁻¹ and 5.851 mg g⁻¹, for Mn²⁺ and Ni²⁺, respectively). Regarding removal efficiency, Sireesha and Sreedhar, 2022 [174], demonstrated a capacity of removal higher than 85% for Cu²⁺, Cr⁶⁺, and Ni²⁺, while Thirugnanasambandham and Sivakumar, 2015 [215], obtained a mitigation performance of 88% for Cu²⁺.

Leaves are used as natural adsorbent materials due to their abundance in nature and their high lignocellulosic compound content [96]. Ge et al. (2020) [182] demonstrated adsorption capacities of 81.03 mg g⁻¹ and 129.87 mg g⁻¹ for Cu²⁺ and Pb²⁺, respectively, using kinetic models. However, low adsorption capacities were observed for pine leaf powder (3.27 mg g⁻¹ for As⁵⁺) [181], mango leaves (4.08 mg g⁻¹ for Cd²⁺) [186], and cabbage leaves (6.307 mg g⁻¹ for Pb²⁺) [183]. The removal performances reported by Alimohammadi et al., 2017 [187]; Nur-E-Alam et al., 2018 [185]; Malik et al., 2015 [184]; and Kamar et al., 2015 [183] were higher than 94% for As³⁺ and Hg²⁺ [187], 95.42% for Cr⁶⁺ [185], 95.2% for Pb²⁺ [184], and 95.67% for Pb²⁺ [183] using eucalyptus leaves, tea leaves, aloe vera leaf powder, and cauliflower leaves biochar, respectively, as natural adsorbents.

Bagasse is a common waste material resulting from the sugarcane industry and is used for adsorption of various heavy metal ions from wastewater [96]. In addition to sugarcane bagasse, grape bagasse [190], agave bagasse [191,192], mango bagasse [194], and palm bagasse [64] are also used for this purpose. Villabona-Ortiz et al., 2022 [195], and Tejada-Tovar et al., 2015 [64], reported improved uptake capacities for the removal of Cr⁶⁺ and Pb²⁺ using oil palm bagasse and palm bagasse, respectively, with values of 111.45 mg g⁻¹ and 162 mg g⁻¹. However, low sorption capacities of 1.35 mg g⁻¹ and 2.0 mg g⁻¹ were obtained by Mohan et al., 2019 [194], and Aloma et al., 2012 [189], for the mitigation of As³⁺ and Ni²⁺ using mango bagasse and sugarcane bagasse, respectively.

The use of hulls has been investigated in many research studies, including peanut hulls [196], rice hulls [197], groundnut hulls [198], maize hulls [199], pistachio hulls [200], soybean hulls [201], hazelnut hulls [202], cotton hulls [203], almond hulls [204], and buckwheat hulls [205], due to their high cellulose and hemicellulose contents, which are suitable for binding toxic metal ions. Wang et al., 2013 [205], and Owalude and Tella, 2016 [198], reported good adsorption capacities for Hg²⁺ and Cr⁶⁺ of 243.9 mg g⁻¹ and 90 mg g⁻¹, respectively, using buckwheat hulls and groundnut hulls. In contrast, Osman et al., 2010 [197], reported very low uptake capacities of 1.3367 mg g⁻¹ and 0.137 mg g⁻¹ for Zn²⁺ and Cd²⁺, respectively, in the case of rice hulls, as revealed by using isotherm and kinetic models. As for removal performances, the percentages obtained by Ghasemi et al., 2017 [199]; Beidokhti et al., 2019 [200]; Sheibani et al., 2012 [202]; Sheng-Quan et al., 2012 [201]; and Sahranavard et al., 2011 [204] were 50% for Cu²⁺ (maize hull), >75% for Ni²⁺ (pistachio hull), 83.5% for Fe³⁺ (hazelnut hull), 91.99% for Cr⁶⁺ (soybean hull), and >94.14% for Cr⁶⁺ (almond hull).

Generally, the physical characteristics of the adsorbent are influenced by the chemical composition and physical structure of the precursor materials. For example, the surface area of an adsorbent may increase as the porosity of the precursor material increases. This can occur when using materials such as straw, which typically have high porosity and therefore can result in an adsorbent with a large surface area. In contrast, using more compact materials such as peels may lead to adsorbents with lower surface area. Furthermore, the size and shape of the precursor materials can also influence the physical characteristics of the prepared adsorbents. For instance, husks with irregular shapes and varying sizes may result in an adsorbent with a less uniform surface area than straw, which has a more uniform shape and size. Overall, the choice of precursor material can have a significant impact on the physical characteristics of the prepared adsorbent, including surface area, porosity, and morphology. Therefore, it is essential to carefully consider the precursor materials used when preparing adsorbents for specific applications.

In addition to these categories of agricultural waste, studies have reported the use of other natural materials, such as green tea waste [117], tea waste biochar [206], hemp fibers [207], corn cob, sunflower achene head [119], cocoa pod [154], coffee waste [208], and mango bark [194]. Khalil et al., 2020 [206], reported higher uptake capacities of 198 mg g⁻¹ using tea waste biochar to remove Cr⁶⁺ ions, and Heraldy et al., 2018 [160], reported 108 mg g⁻¹ for Pb²⁺ (apple juice residue). Celebi et al., 2020 [42], reported reduced absorbability between 1.163 and 2.468 mg g⁻¹ for the removal of Pb²⁺, Cd²⁺, Ni²⁺, and Zn²⁺ using brewed tea waste. Also, Mohan et al., 2019 [194], obtained a lower uptake capacity of 1.25 mg g⁻¹ for As³⁺ ions using mango bark. The values of adsorption capacities and removal performances presented in Table 6 depend, first of all, on the type of adsorbent used, the modification of the material (e.g., using alkaline, acidic, or other agents), but also on the parameters of the process (initial concentration, temperature, pH value, and adsorbent dosage). However, these parameters can also be influenced by chemical and physical properties of the adsorbent surface, as well as the presence of soluble substances [55].

Optimal characteristics of a suitable adsorbent material must include a large surface area, small pore diameter, high porosity, good mechanical strength, and good chemical and thermal stability, which can enhance the adsorption capacity due to a large exposed surface area and a suitable surface chemistry [55].

7. Recyclability and Regeneration of Agricultural Adsorbents

The regeneration ability of the adsorbent materials prove that the adsorbents are sustainable and economically viable. For this reason, the circularity of adsorbents has received significant attention. If the adsorbed pollutants can be desorbed from the absorbent surface, the process becomes more feasible due to the repeated use of the adsorbent and pollutants [216]. The regeneration capacity of adsorbents is influenced by the pH of the solution, complexation, and rate of oxidation and degradation [217].

The main regeneration techniques are decomposition and desorption methods. Decomposition techniques include oxidation, electrochemistry, ultrasound, and microbiological methods. Desorption methods can be grouped into thermal methods (steam, inert gas, hot water, microwave) and nonthermal methods (surfactants, chemical, supercritical) [218]. The advantages and drawbacks of these techniques are presented in Table 7.

Desorption mechanisms involve the treatment of exhaust adsorbent with eluting agents (solvents or solutions, or a mixture of both), which breaks the bond formed between adsorbent–adsorbate and liberates the adsorbate from the solution. This technique is represented by the following steps: (i) washing the spent absorbent with water; (ii) treating the washed adsorbent with eluting agent in batch/column treatment, stirring the spent adsorbent with eluting agent at different speed, time, and temperature (for batch process) or passing the eluting agent through the spent adsorbent at a predefined flow rate (column process); (iii) washing of treated adsorbent with water to remove the eluting agent [219].

Table 7. Advantages and disadvantages of different regeneration methods.

Process	Advantage	Disadvantage	Reference
Microbiological regeneration	Ecofriendly, efficient, time effective, cost effective	Very slow, blocking of the pore, slow regeneration, not applicable for surfactant-modified adsorbents, low efficiency	[217,220,221]
Thermal regeneration	Efficient, useful for adsorbents loaded with heterogeneous mixture of adsorbates	Expensive, time-consuming, modification of structure adsorption pore, high energy consumes, air pollution problems associated with gases released during the process, loss of carbon surface area	[217,220,221]
Oxidative regeneration	Efficient	Expensive, degradation of pores, time-consuming	[217,221]
Microwave regeneration	Ecofriendly, efficient, time effective, short regeneration time, suitable for multicomponent adsorbents, energy-saving process	Expensive, degradation of pores, further secondary treatments are needed	[217,221]
Ultrasound regeneration	Ecofriendly, efficient, time effective, low energy requirement, low carbon loss	Expensive, degradation of pores	[217,220,221]
Chemical regeneration	Cost effective, can be coupled easily with other techniques, fast regeneration, high regeneration efficiency, almost zero carbon loss, quick regeneration, possible adsorbate recovery	Modification of structure of the adsorbent, oxidant wastage, low solubility of the adsorbate, occasional sludge generation, toxicity issues, require of further treatment	[217,220,221]
Ozonation	Efficient	Surface modification of adsorbents, increasing costs due to calcination of adsorbents before use, acidification of adsorption surface which interfere with anionic pollutants	[219,220]
Photo-assisted oxidation	Ecofriendly, fast degradation of organic pollutants	Release of some harmful byproducts, long regeneration required, decreasing of efficiency with increasing of regeneration cycles	[220]
Supercritical fluids regeneration	Very fast regeneration times, efficient, less time consuming	Cost of pressure vessels, degradation of pores of adsorbent material	[219,220]
Ultrasonic regeneration	Higher desorption efficiency, low energy consumption, simple equipment required, efficient, eco-friendly, less time consuming	Highly expensive, degradation of pores of adsorbent	[219,220]

Table 7. Advantages and disadvantages of different regeneration methods.

Process	Advantage	Disadvantage	Reference
Microbiological regeneration	Ecofriendly, efficient, time effective, cost effective	Very slow, blocking of the pore, slow regeneration, not applicable for surfactant-modified adsorbents, low efficiency	[217,220,221]
Thermal regeneration	Efficient, useful for adsorbents loaded with heterogeneous mixture of adsorbates	Expensive, time-consuming, modification of structure adsorption pore, high energy consumes, air pollution problems associated with gases released during the process, loss of carbon surface area	[217,220,221]
Oxidative regeneration	Efficient	Expensive, degradation of pores, time-consuming	[217,221]
Microwave regeneration	Ecofriendly, efficient, time effective, short regeneration time, suitable for multicomponent adsorbents, energy-saving process	Expensive, degradation of pores, further secondary treatments are needed	[217,221]
Ultrasound regeneration	Ecofriendly, efficient, time effective, low energy requirement, low carbon loss	Expensive, degradation of pores	[217,220,221]
Chemical regeneration	Cost effective, can be coupled easily with other techniques, fast regeneration, high regeneration efficiency, almost zero carbon loss, quick regeneration, possible adsorbate recovery	Modification of structure of the adsorbent, oxidant wastage, low solubility of the adsorbate, occasional sludge generation, toxicity issues, require of further treatment	[217,220,221]
Ozonation	Efficient	Surface modification of adsorbents, increasing costs due to calcination of adsorbents before use, acidification of adsorption surface which interfere with anionic pollutants	[219,220]
Photo-assisted oxidation	Ecofriendly, fast degradation of organic pollutants	Release of some harmful byproducts, long regeneration required, decreasing of efficiency with increasing of regeneration cycles	[220]
Supercritical fluids regeneration	Very fast regeneration times, efficient, less time consuming	Cost of pressure vessels, degradation of pores of adsorbent material	[219,220]
Ultrasonic regeneration	Higher desorption efficiency, low energy consumption, simple equipment required, efficient, eco-friendly, less time consuming	Highly expensive, degradation of pores of adsorbent	[219,220]

Chemical regeneration is one of the most used techniques for the regeneration of adsorbent materials, which uses solvents and chemical reagents for desorption of the adsorbate from the adsorbent [218], or by decomposition of adsorbed species using chemicals that are oxidants under supercritical or subcritical conditions [221]. The regeneration efficiency of chemical regeneration is influenced by the solubility of adsorbates in the solvents, as the solubility of an adsorbent in a solvent is low, the regeneration efficiency is minimal. This process involves three principles: the change in pH of the solution, degradation of adsorbent via oxidation, and complexation (useful for inorganic pollutants) [221]. Menia et al., 2018 [222], demonstrated that HCl 0.5 M is more effective for pea peel compared with NaOH 0.5 M. The authors concluded that the elimination of Zn²⁺ by desorption is carried out using ion exchange processes. Kalavathy and Miranda, 2010 [223], used different desorbing agents, such as demineralized H₂O, HCl, H₂SO₄, HNO₃, Na₂CO₃, EDTA, NaHCO₃, NH₄Cl, and CH₃COOH. A lower desorption capacity was obtained for demineralized H₂O (<1%), followed by Na₂CO₃, NaHCO₃, and NH₄Cl (<25%). In the case of CH₃COOH, the desorption efficiencies were 45.32% for Cu²⁺, 47.12% for Ni²⁺, and 49.56% for Zn²⁺. Higher recovery efficiency, between 82.11 % and 99.03% for Cu²⁺, Zn²⁺, and Ni²⁺, were obtained after treating the adsorbent with HCl, HNO₃, and H₂SO₄. Complexing agent EDTA revealed higher desorption efficiency (99%), most probably due to the strong metal ion complexing ability to. Contrary to high regeneration capacity, adsorbent treated with EDTA presented the disadvantages of high cost, high stability of metal–EDTA complex, and difficulty in recovering the metal ions from the EDTA solution. In the study of Kyzas, 2012 [224], coffee waste was treated with strong acid and alkaline desorption agents, and the results obtained demonstrated that the desorption percent decreased with the alkalinization of aqueous solution. The decline in adsorption efficiency is a result of both the gradual saturation of the active sites and the deterioration of the material caused by the highly acidic or alkaline pH conditions. Also, Ezeonuegbu et al., 2021 [225], demonstrated that the most effective desorption agent was HNO₃, followed by HCl and NaOH. Acidic media produce protonation of the sorbent surface, which allows for the desorption of positively charged metal ions from the adsorbent.

Adsorption capacities of different agricultural byproducts after adsorption-desorption processes are presented in

Table 8. Regeneration of adsorbents produced from agricultural byproducts. The symbol ↘ means “decreasing”.

Adsorbent	Metal Ions	Amount Desorbed	No. of Cycles	Desorbing Agent	Reference
Corn stalk fibers	Cr6+	-	20	NaOH 0.1 M	[226]
Pea peel	Zn2+	10 mg g−1	5	HCl 0.5 M	[222]
		5 mg g−1	5	NaOH 0.5 M
Moringa oleifera biochar	Cu2+, Zn2+, Ni2+	↘ with 9.15%, ↘ with 10.3%, ↘ with 11.78%	6; 7; 7	H2SO4 0.1 M	[223]
Coffee waste	Cu2+, Cr6+	70–75%, 55–60%	10	-	[224]
Modified orange peel	Pb2+, Cd2+, Ni2+	43.5 mg L−1, 43.1 mg L−1, 41.3 mg L−1	3	0.05 mol/L HCl	[227]
Millet husk	Cr6+	74.48%	6	0.1 M NaOH, followed by 0.1 M HCl	[228]
Green bean husk	Sb3+	83.7%	7	0.1 M HCl	[229]
Sugarcane bagasse	Pb2+, Ni2+	89.9%, 96.11%	1	0.1 M HNO3	[225]
		75.28%, 79.6%	1	HCl
		44.7%, 55.3%	1	NaOH
Sugarcane bagasse	Pb2+	90.05%; 83.97%; 77.92 %; 70.01%	1	0.1 M HNO3, 0.1 M HCl, 0.1 M H2SO4, 0.1 M NaOH	[230]
Tamarind fruit seed powder	Cu2+	90%	-	0.5 N HCl	[231]
Pongamia oil cake	Zn2+	90.14%	6	0.05–0.1 mM HCl; 0.05–0.1 mM H2SO4; 0.01–0.1 mM EDTA	[232]

.

8. Challenges and Future Perspectives on Agricultural Waste Adsorbents

China is currently the country with the highest waste resulting from its agricultural sector in the world. Through the reuse of agricultural byproducts, the air pollution caused by plant incineration can be reduced. Also, the soil and water contamination caused by the residues of heavy metals and pesticides in plants, respectively, by the long-term decay of wastes, can be prevented, in this way improving the soil quality of cultivated land and the ecological environment [233].

This sector faces challenges related to the generation and refinement of surface-modified agricultural waste due to the difficulty of surface-modified procedures. In this manner, Vardhan et al., 2019 [21], recommended the production of techniques fitted to obtain agricultural waste at substantial scale production. An important research step concerning agricultural waste adsorbent is the prediction of the structure of surface-modified adsorbent and enhancing its adsorption capacity. Research on the impact of functional groups on the stability of modified adsorbent, on number of its active sites, and the relation between functional groups and adsorption capacity was considered. They also considered the necessary incorporation of more functional groups, which can retain a higher amount of metal ions, and validation of obtained adsorbent for contaminated industrial wastewaters [21].

Dai et al., 2018 [233], proposed, as future directions, (i) the research and develop of agricultural waste biomass carbonization technology to promote the industrialization of biomass carbon; (ii) development of a green modifier and modification process; and (iii) extending the use of agricultural waste adsorbents in concordance with the engineering problems of pollution scaling.

In addition to the challenges and prospects discussed earlier, several future perspectives must be mentioned. One such perspective involves researching and developing a modification method that can be applied to a blend of two or more agricultural adsorbents. The goal is to create a synergetic effect between the functional groups of the agricultural waste used, resulting in improved adsorption capacities. Another perspective is focused on the development of filter cartridges based on agricultural waste adsorbents. The process would begin with pilot plant trials and then progress to large-scale manufacturing. A third perspective involves researching and developing a green eluent and a green method of regeneration. This would entail finding environmentally friendly solutions for regeneration processes. Another area of research is focused on creating an agricultural adsorbent that can maintain the improved adsorption capacity after many adsorption–desorption cycles. It is also suggested to test the agricultural waste capacity of other harmful substances, such as endocrine disruptor chemicals, e.g., phthalates, plasticizers, flame retardants, parabens, and microplastics. Finally, we propose testing the adsorption capability of agricultural waste adsorbents for solutions that contain a mixture of pollutants. These future perspectives will help in advancing the use of agricultural waste adsorbents and improve their effectiveness in treating contaminated water.

Implementing these perspectives in an efficient and economical manner would require careful planning and the consideration of various factors. Some potential strategies that could be employed are collaboration, optimization, recycling, scaling up, and regulatory support.

Collaboration between academic researchers, industry experts, and policymakers can help to create a more cohesive approach to implementing these perspectives. Working together, stakeholders can leverage their respective areas of expertise to develop efficient and cost-effective methods for implementing these perspectives.

Optimizing the existing processes and technologies used in agricultural waste adsorption can help to reduce costs and improve efficiency. For example, using machine learning algorithms to optimize the design of filter cartridges can help to reduce material waste and improve the performance of the adsorbents.

Developing a closed-loop system for the production and use of agricultural waste adsorbents can help to reduce waste and improve efficiency. For example, spent adsorbents can be recycled and used to produce new filter cartridges or other products, reducing the need for new materials.

Scaling up the production of agricultural waste adsorbents can help to reduce costs by taking advantage of economies of scale. For example, investing in large-scale manufacturing equipment can help to reduce the cost per unit of filter cartridges.

Providing regulatory support for the development and use of agricultural waste adsorbents can help to create a more favorable environment for these technologies. This can include funding for research and development, tax incentives for companies that use these technologies, and streamlined regulatory approval processes.

Implementing these strategies can help to create a more efficient and economical approach to the development and use of agricultural waste adsorbents. By working together and leveraging existing technologies, we can help to create a more sustainable and environmentally friendly approach to water treatment.

9. Conclusions

Agricultural byproducts such as fruit and vegetable peels, hulls, husks, straws, bagasse, stalks, leaves, vegetable and fruit stones, cakes, and vegetable waste are commonly used for chemical contaminants retention due to their low cost, ease of operation, and large surface area. However, the adsorption process is influenced by various factors related to target ions, adsorbent material, wastewater characteristics, and adsorption parameters. Modified adsorbents have remarkable metal uptake capacities compared to unprocessed biosorbents. Agricultural byproducts are suitable for the retention of various heavy metals. There is a need for further research to develop versatile biosorbents for the simultaneous retention of toxic metals from wastewater sources.

This article’s findings indicate that the adsorption capacity of adsorbents is dependent on both the type of agricultural waste utilized and the modifying agent employed to enhance the adsorbent’s surface area. Unprocessed biosorbents contain various functional groups that allow them to bind only a small amount of toxic metal ions in comparison to modified adsorbents, which exhibit significant metal uptake capacities after undergoing chemical pretreatments. Most biosorption studies have demonstrated the suitability of agricultural byproducts for various heavy metals (e.g., Pb²⁺, Cd²⁺, Cu²⁺, Hg²⁺, Cr³⁺, Cr⁶⁺, As³⁺, As⁵⁺, Mn²⁺, Sn²⁺, Co²⁺, Zn²⁺, Ni²⁺, Fe³⁺). There is a pressing need to research and develop versatile biosorbents based on agricultural byproducts that can effectively remove toxic metals from wastewater sources.