A critical review on the separation of heavy metal(loid)s from the contaminated water using various agricultural wastes

Abstract Wastewater contamination with heavy metal(loids)s has become a worldwide environmental and public health problem due to their toxic and non-degradable nature. Different methods and technologies have been applied for water/wastewater treatment to mitigate heavy metal(loid)-induced toxicity threat to humans. Among various treatment methods, adsorption is considered the most attractive method because of its high ability and efficiency to remove contaminants from wastewater. Agricultural waste-based adsorbents have gained great attention because of high efficiency to heavy metal(loids)s removal from contaminated water. Chemically modified biosorbents can significantly enhance the stability and adsorption ability of the sorbents. The two mathematical models of sorption, Freundlich and Langmuir isotherm models, have mostly been studied. In kinetic modeling, pseudo-second-order model proved better in most of the studies compared to pseudo-first-order model. The ion exchange and electrostatic attraction are the main mechanisms for adsorption of heavy metal(loid)s on biosorbents. The regeneration has allowed various biosorbents to be recycled and reused up to 4-5 time. Most effective eluents used for regeneration are dilute acids. For practical perspective, biosorbent removal efficiency has been elucidated using various types of wastewater and economic analysis studies. Economic analysis of adsorption process using agricultural waste-based biosorbents proved this approach cheaper compared to traditional commercial adsorbents, such as chemically activated carbon. The review also highlights key research gaps to advance the scope and application of waste peels for the remediation of heavy metal(loid)s-contaminated wastewater. Novelty statement This review provides new information and insights on the potential utilization of agriculture-based biosorbents for the removal of contaminants, especially heavy metal(loid)s from toxic water/wastewater, as well as their mechanisms, adsorption efficiency, and regeneration ability. For practical perspective, biosorbent adsorption efficiency was elucidated by using various types of wastewater and economic analysis studies.


Introduction
The uncontrolled discharge of wastewater from various pointand non-point sources has become a major global environmental threat over the last 3-4 decades (Luong Nguyen 2021;Sharma et al. 2021;Xue et al. 2022;Younas, Niazi, et al. 2022).Heavy metal(loid)s contaminated (e.g., arsenic (As), chromium (Cr), cadmium (Cd), mercury (Hg), and lead (Pb)) wastewater has developed a major environmental and health threat (Masindi and Muedi 2018;Rajendran et al. 2022;Wang et al. 2022).There are some permissible limits set by different environmental regularity authorities like the World Health Organization (WHO) and the United States Environmental Protection Agency (USEPA) for heavy metal(loid) contamination in contaminated water (Table 1).The heavy metal(loid) toxicity and tendency toward bioaccumulation endangers the health of all life forms (Ali and Khan 2019;Younas, Bibi, et al. 2022).These heavy metal(loid)s are toxic even in very trace concentrations when they enter the food chain (Guan et al. 2019;Rashid et al. 2023;Hussain et al. 2023).
Accumulation of heavy metal(loid)s in the tissues and cells of human being can be toxic, neurotoxic, carcinogenic, and mutagenic (Wang et al. 2022;Xue et al. 2022).Environmental exposure to heavy metal(loid)s can be caused by natural and human activities (Younas et al. 2023).Natural processes include volcanic eruptions, weathering of parent material, and geothermal waters.The anthropogenic activities included oil refineries, coal-fired power plants, high-voltage power lines, textiles, plastics, microelectronics, wood preservation, paper mills, mining and smelting, leather tanning industry and pesticides application in agriculture (Rahman and Singh 2019;Alsafran et al. 2023).However, the most efficient ways for heavy metal(loid)s remediation from wastewater or drinking water are chemical precipitation, membrane separation, activated carbon, ion exchange, reverse osmosis, clay minerals, biosorbents, and biochar (Ambashta and Sillanp€ a€ a 2012; Wenten and Khoiruddin 2016;Figueiredo et al. 2018;Mudila et al. 2019).
In the last few decades, various organic (bark, vegetable and fruit peels, marine algae, tea waste, pulp and paper waster, straw leaves, and sugarcane bagasse, citrus peel, risk husk) (Jobby et al. 2018;Liu et al. 2019) and inorganic sorbents (iron oxides, clay minerals, modified zeolite, nano materials, and carbon nanotubes) have been used as potential adsorbent to immobilize the heavy metal(loid)s from wastewater (Widyawati et al. 2019;Younas et al. 2021;El Nemr et al. 2023).Different studies have been conducted using various sorbents and their carbon and ash, and different experimental approaches have been developed on various types of metals that were very effective in sorption conditions and showed a high removal efficiency (Rosales-Landeros et al. 2013;Khashei Siuki et al. 2021;Sheikhi and Rezaei 2021).
Biosorption is a process that involve removing toxic heavy metal(loid)s from contaminated water/wastewater on the biomass of different organic materials, such as agricultural and food industry biowastes and algal biomass.It is considered as an efficient, less time consuming and sustainable approach with no toxic byproducts or secondary pollution causing agents (Zhang et al. 2014;Fawzy, Nasr, Adel, et al. 2018;Fawzy et al. 2019; Mohy-Ud-Din 2020).In literature, it is studied that waste materials are cost effective compared to commercial sorbents (Fawzy, Nasr, Nagy, et al. 2018).Sharma and Ayub (2019) have performed the cost analysis of adsorption processes using agricultural waste materials (tea and ginger mix, pea pod peels and banana peels (BPs)) to remove hexavalent Cr (Cr(VI)) from contaminated water.As per cost analysis study, the cost of removing 1 g of hexavalent Cr from contaminated water is PKR 4.71, 9.14, and 3.11, respectively, for selected agricultural waste materials.Cost analysis of this study showed that cost of biosorption is significantly lower than using commercial modified carbon that was around PKR 142.Bhatnagar et al. (2010) reported that cost of lemon peel for heavy metal removal from wastewater was 10 times cheaper compared to commercial activated carbon.Based on economic analysis, several recent studies proved that biosorbents are cost effective compared to commercial sorbents (Ali et al. 2016;Fawzy et al. 2019;Hamdy et al. 2019;Kumar et al. 2019;Syeda et al. 2022).
Agricultural or food industry derived organic waste peels could be promising biosorbents and potentially used for the efficient removal of various water pollutants including toxic heavy metal(loid)s ions (such as As, Cd, Pb, Cr, and nickel (Ni)).This review elaborates the agricultural waste peels with their potential to adsorb specific pollutants through their synthesis and modification processes.Also, elucidates the various aspects such as economic analysis, desorption/regeneration of several agricultural wastes and performance of sorbents using industrial wastewater.This review also gives better explanation on what is preventing adsorbents to replace commercial sorbents such as activated carbon (C).In inclusion to this review, recommendations had given to makeable lab scale adsorption/biosorption to be eventually used at industrial level to remove pollutants from wastewater.

Biosorbents efficiency
Biosorption is the ability of organic material to sorb/adsorb contaminants such as heavy metal(loid)s from the wastewater or contaminated drinking water by metabolic or physicochemical methods (Acheampong et al. 2010;Yadav et al. 2021).The mechanism of bioadsorption includes two types, namely metabolism-dependent and metabolism-independent.  (Babel and Kurniawan 2003;Pourrut et al. 2011;Shahid et al. 2017;Hussain et al. 2019).

Heavy metal(loid)s
Health hazards Permissible limits for potable water (mg L The uptake of metal ions on the cell wall or membrane varies depending on cellular metabolism.These metabolic processes often depend on how metal ions react with active defense systems in living cells (Siddiquee et al. 2015).In the metabolism independent biological sorption mechanism, metal(loid)s sorption takes place through ion exchange, physical or chemical adsorption.
The microbial biomass cell wall consists of lipids, polysaccharides, and proteins with various functional groups and can interact with metallic ions such as aldehydes, ketones, amides, carboxyl groups, sulfates, phosphates, and amino groups.This type of biosorption is typically faster compared to metabolically dependent mechanisms (Durve and Chandra 2014;Javanbakht et al. 2014;Lin et al. 2019).The amount of metal(loid)s sorbed by the biomaterial is quantified through the q e parameter, which represents the ions concentration stored per unit mass of a biosorbents.The biosorption process is influenced by different parameters, namely the metal solution pH, the amount of the biosorbent, the temperature, and metal ion concentration (Das et al. 2008;Durve and Chandra 2014;Masindi and Muedi 2018).
The best condition to eliminate heavy metal(loid)s from the high volume of wastewater through biological adsorption includes a lower metal ion concentration, a higher dosage of the biosorbent, and a moderate temperature (Kanamarlapudi et al. 2018).The bio-sorbent must have the following characteristics: i.e., high adsorption capacity, easy availability, low cost, and no issue of toxic secondary products generation.

Importance of biosorption process compared to other conventional approaches
The traditional metal ion removal method is not effective, costly, and the removal process takes a long time.If the removal is not complete, toxic compounds and sludge could possibly be generated, which need to be disposed safely and cannot be applied on a large scale (Rajasulochana and Preethy 2016).Compared with traditional methods, biosorption has the following advantages: 1.The process is based on the selection of biosorbents, which are often abundant.Therefore, the biosorption method is cost-effective.2. Regeneration is possible during biosorption.Biosorbents can also be reused, making the process more economical.3.After adsorption from wastewater, metals can be recovered from the surface of the biosorbents.4. By applying optimized conditions, the efficiency of the process can be increased, i.e., high acidic pH, low metal ion concentration, high sorbent loading, moderate temperature, and proper contact time. 5.The activity of biosorbents can be improved by reducing their particle size, chemical modification, and using some synthetic adsorbents.6.The competitive performance of biosorption is very similar to the ion exchange method.
Although biosorption is an effective removal process, several disadvantages affect its performance.The main disadvantage is early saturation and desorption of metal(s).Improvement of biosorbents is not possible through genetic engineering, nor is it possible to change the valence state of metals (Won et al. 2014;Zhu et al. 2016).

Processes involved in removal of heavy metal(loid)s via biosorption
The use of agricultural waste peels for remediating toxic heavy metal(loid)s from water and wastewater is based on adsorption capacity (Gupta et al. 2015;Kari c et al. 2022).The higher affinity of the adsorbent to the contaminants, ions attached and attracted toward the adsorption site present on the biosorbents via complicated processes are illustrated in Figure 1.These biomaterials usually contain cellulose and lignin contents primarily.This may be altered by a variety of mechanisms, including chemical adsorption, surface and pore adsorption, ion exchange and chelation, complexation and physical adsorption, and entrapment by structural polysaccharides (Farooq et al. 2010;Carolin et al. 2017;Younas et al. 2021).
Cellulose is the glucose homo-polymer with b1-4 with four glycosidic bonds as well as intramolecular and intermolecular hydrogen bonds (Dhillon et al. 2013;Ali and Khan 2019).Hemicellulose consists of xylose and b1 ! 3 glycosidic bonds to other ingredients like acetyl-ferulic acid groups and uranyl acid group (Garg et al. 2008;Witek-Krowiak et al. 2014).Lignin consists of aromatic compounds that are bound covalently to xylem in hardwood and also to galactomannan in softwood (Garg et al. 2007;Gupta et al. 2010).Some biosorbent can adsorb several heavy metal(loid)s without special priority while others are specific to certain kinds of ions due to their chemical composition.

Efficacy of agricultural waste peels as potential biosorbents
In kitchen waste containers, various wastes are normally generated for the period of processing (selection, sorting, and cooking) of the fruits and vegetables in the market/industry (Figure S1, Supplementary Information).The bulk amount of fruit and vegetable peels are prepared in the garbage or maybe supplied to farm animals.These waste and by-products that are produced in bulk quantities through industrial processing should be recycled and managed because of their environmental effects.Another way, they are abundant in biologically active constituents and appraise health-promoting (Chanda et al. 2010;Bhatnagar et al. 2015).
Over the past decade, researchers have devoted efforts to improve recycling of the fruit and vegetable wastes.One of the most important methods is to add value to the biological components of the by-products of the food industry.Plant residues can also be easily decomposed by microorganisms (Hussain 2021).So far, industrial wastes have been used as animal feed or organic matter sources in soil.These are high economic value products with their recycling, which will be desirable (Bhatnagar et al. 2015;Vardhan et al. 2019).In recent years, some new value-added goods have been made from food waste, including edible oils, essential oils, food additives, polyphenol compounds, pigments, cancer drugs, fiber, enzymes, bioethanol, biodegradable plastics, and other various products (Wadhwa et al. 2015;Ravindran and Jaiswal 2016).Agricultural waste and residue are naturals, renewable resources, environment friendly, promising environmental resources, and cost effective sources of biosorbents that can eliminate various kinds of contaminants from water/wastewater and reduce environmental contamination (Vardhan et al. 2019;Farooqi 2021).

Impact of citrus waste peel on removal of organic and inorganic pollutants
In recent decades, the worldwide production and consumption of citrus fruits have increased substantially.Statistics of Food and Agriculture Organization (FAO) shows that the worldwide citrus production exceeded 100 million tons in 2010 (Lin et al. 2013;FAOSTAT 2016).After consumption by humans or in industrial processing, there is a lot of citrus peel waste that can make up to 50% of the weight of the fruits.Until now, most citrus peel waste has been accumulated or disposed of by incineration for the production of active ingredients and the production of animal feed, which leads to serious pollution and wasted resources (Nikodinovic-Runic et al. 2013;Kari c et al. 2022).
Therefore, there is a great need for the development of environmentally friendly and economical methods for treating citrus waste.More research is also warranted to explore the efficiency of citrus peel for remediation of different heavy metal(loids).Also, more advanced techniques are needed to be used for making better an understanding about interaction and mechanisms with possible bonding with sorption sites.

Potential of pomelo peel in biosorption of toxic elements
Grapefruit peels have segmented membranes other than that of citrus fruits.In the laboratory, the capability to eliminate Pb from contaminated-wastewater was tested by taking the ZnCl 2 activation method as a bio-sorbent (Patel 2012).The optimal adsorption conditions are: the initial pH of the wastewater is 5.3-6.5, the exposure time is 1.5 h, the adsorbent doses are about 10 g/L and the early Pb concentration of the wastewater is 100 mg/L at 30 C. At given investigational parameters, the adsorbent can remediate !90% of Pb from the wastewater (Rwiza et al. 2018).Tasaso (2014) investigated the Cu adsorption and remediation ability of pomelo peel (PP) in an aqueous solution.Factors that influence adsorption were analyzed, such as the concentration, pH, contact time, and temperature.Highest Cu adsorption capacity of PP was also found to be 19.7 mg/g in these circumstances: pH ¼ 4, initial concentration ¼ 125 mg/L, temperature ¼ 25 C, and equilibrium time ¼ 60 min.A method for treating dye wastewater by using biowaste an adsorbent simulated with methylene blue (MB) was investigated by Saikaew et al. (2009).Results revealed that the greatest conditions like 0.4 g PP powder, pH ¼ 8, temperature ¼ 30 C, oscillation time ¼ 60 min, and 140 mg/L methylene-blue solution can remove !83% contaminants.The theoretical saturation adsorption capacity of PP at 30 C is 133 mg/g (Saikaew et al. 2009).

Effect of banana waste peel as biosorbent
Banana is a tropical fruit that is consumed worldwide, including various varieties.Banana peels are the key residue with a share of 30-40%(w/w) and it is utilized for composting, animal feeding, production of protein, methane, pectin, ethanol, and also enzymes (Silva et al. 2013;Kari c et al. 2022).Its main components are cellulose, pectin, chlorophyll, hemicellulose, and other hydrocarbons.Banana peels have the highest adsorption capability for the metal ions and organic toxins, mainly that of the availability of carboxyl and hydroxyl groups in pectin.Adsorption of Pb and Cd in BP was investigated in batches (Anwar et al. 2010).Maximum adsorption capability of Langmuir isotherms on BP shows that 1 g BP can adsorb 5.71 mg Cd and 2.18 mg Pb.Un-treated bananas peel (UTBPs), acid hydrolyzed banana peels (AcBPs), alkaline hydrolyzed banana peels (AlBPs), and bleached banana peels (BBPs) were utilized as the adsorbents to remove Cr(VI) and Mn, respectively, extracted from contaminated water in a batch experiment (Ali and Saeed 2015).
Adsorption of Zn, Cu, Ni, Co, and Pb from BP treated with alkali, acid, and also the water was studied (Ngah and Hanafiah 2008).The adsorption capacities of 6.88 mg/g (Ni), 5.80 mg/g (Zn), 4.75 mg/g, (Cu), and 2.55 mg/g (Co) were found using BP.Being bio-sorbent, the possible use of BP, for the removal of phenolics compounds from the wastewater was examined (Achak et al. 2009).Banana peel showed the highest adsorption capability for phenolic compounds (689 mg/g).The adsorption procedure is speedy and reaches a balance within a contact time of 3 h.Equilibrium solid-phase concentrations of phenol reduced by increasing the concentration of the adsorbent (PP), which is due to the unsaturation of the adsorptions site.Desorption experiments show that there is chemical adsorption among the natural phenolic formaldehyde and adsorption site on the BP (Bhatnagar et al. 2015;Wadhwa et al. 2015).

Cassava peel as a new biosorbent for remediation of heavy metal(loid)s
In various countries (such as Indonesia, Nigeria, etc.), cassava is considered as one of the most important agricultural products.Cassava is used as a raw material for the manufacture of cassava starch, traditional foods, and cake.Leaves of cassava are used as a vegetable and as a natural medicine because cassava comprises a lot of proteins and many other biologically active ingredients and wood is used as fuel for cooking.When processing cassava starch, a massive quantity of solids waste (cassava shells) is produced, and the immediate discharge of this solid waste leads to ecological problems.
The use of cassava shells as activated C with a high surface area has been proven by many investigators.The modified activated C has been produced using cassava waste skin by physical and chemical methods and its effectiveness in removing dyes and ions of metal from aqueous solutions has been tested (Rajeshwarisivaraj et al. 2001;Crini 2006).Both sorbents have been proven to be efficient for metal ions and dyes, but the material soaked with H 3 PO 4 was more efficient than the heat-treated material.Cassava peel carbon (PVPCC) coated with polyvinylpyrrolidone (PVP) K25 was used to adsorb arsenic from the aqueous solution (Suharso et al. 2019;Farooqi 2020).
Absorption of the methyl-azo dye methyl red on the NaOH-activated cassava dish in an aqueous solution was studied.This study showed that the maximum removal rate via adsorption was 78.62%.The ability of cassava bark to remove copper from aqueous solutions was evaluated with the highest adsorption capacity of 41.77 mg/g (Kosasih et al. 2010).The probability of Ni adsorption through cassava peel was also assessed.The Sips model showed the best fit at the pH 4.5, the maximum absorption capacity of Ni ion was 57 mg/g (0.971 mmol/g) (Kurniawan et al. 2011).

Biosorption potential of pomegranate peel explored
Pomegranate is the most popular fruit around the world because of its pleasant flavor, numerous medicinal characteristics, and high nutritional quality.Pomegranate fruits are consumed fresh in large quantities in the form of juice, jam, and wine.Its peels are a byproduct of the food industry and makeup 5-15% of their total weight.Peel of pomegranate consists of a variety of ingredients, including polyphenols, ellagitannins, gallic acid, and ellagic acid.Pomegranate peel residues may be used as inexpensive and renewable biosorbents.
Removal of Pb and Cu from the aqueous solution was investigated using residues of pomegranate (raw material), activated-C from residue of pomegranate (AC1), and modified activated-C from chemically treated residue of pomegranate (AC2 and AC3) (El-Ashtoukhy et al. 2008;Lesmana et al. 2009).The optimal pH required for maximum adsorption was 5.8 and 5.6 for Cu and Pb.The removal of Ni from water using pomegranate peel waste was also investigated (Bhatnagar and Minocha 2010b).The higher adsorption ability of the biosorbent to remove Ni was also measured to be approximately 52 mg/g.Cu, Ni, Cd, Zn, and Cr(VI) ions in pomegranate peels were also tested (Rao and Rehman 2010).
Maximum adsorption was also observed for Cu ion, followed by Cd, Zn, Cr(VI), and Ni ions.Try using column methods and batch to desorb Ni, Cu, Cd, Zn, and Cr(VI) ions from synthetic wastewater and wastewater from the electroplating activities.When these ions are used in combination, the penetration capabilities of Cu, Ni, Zn, Cd, and Cr(VI) ions are 6, 2, 2, 2, and 6, respectively, 0.5 mg/g.The adsorbent is used to recover Cr(VI) ions from galvanic wastewater (Amin 2009).The adsorption potential of pomegranate peel to remove the 2,4-dichlorophenol (2,4-DCP) from aqueous solutions was investigated (Bhatnagar and Minocha 2009).Adsorption capacity of pomegranate peel adsorbent for 2,4-DCP is 65.7 mg/g.The ability of pomegranate peel carbon to adsorb Fe ions from the solution was also tested.At pH 6.0, the maximum adsorption potential of pomegranate peel activated carbon on Fe was 18.52 mg/g (calculated according to the Langmuir model), and the bio-sorbent concentration is 1 g/L at 29 C (Moghadam et al. 2013).

Orange peel as a potential biosorbent for contaminated-water cleanup
Orange peel is thrown away worldwide from soft drink and orange juice industry without treatment.The decomposition of orange residue has caused two important problems for the orange industry, namely the occupation of the land area and the pollution of the phenolic compounds.It consists of chlorophyll pigments, cellulose, pectin, hemicellulose, lignin, and hydrocarbons with low molecular weight compounds.These hydrocarbons contain numerous functional groups and thus become a potential adsorbent for several pollutants.Since it is freely available in the processing industry of orange, many scientists have examined its capability likewise adsorbent for removing different water-pollutants.To remediate Zn, Cu, Pb, Ni, and Cr in an aqueous solution by adsorption, the capability of orange peel has been examined (Lugo-Lugo et al. 2009;G€ onen and Serin 2012;Boumediene et al. 2018).
Order of the adsorption was Ni > Cu > Pb > Zn > Cr.Ni adsorption was affected by pH value and maximum removal of Ni is possible at pH 6.It can be desorbed with 0.05 M HCl and was found 76% in the batch and 95.83% in the column process.The waste adsorbent can be restored and recycled three times.The removal and recovery rates in the wastewater were 89% and 93.33%, respectively.It has also been reported to be reformed with various chemical substances that were used as biological adsorbents to remove Cd ions from wastewater (Ajmal et al. 2000;Acharya et al. 2018).Also, the process of acid oxidation after saponification involves the generation of an-hydride.Natural, aldehyde-treated, and co-polymer grafted orange peel was assessed as per adsorbent for removing Pb ions from the aqueous solutions (Sayed Ahmed et al. 2012).The maximum pH value for lead sorption is 5.The activity of adsorption was quick and the natural and treated biomass reached 99% of the adsorption capacity within 10 min and the grafted material within 20 min (Abd-Talib et al. 2020).

Jackfruit peel application as a biosorbent
Jackfruit is widely distributed in India, Thailand, Malaysia, Indonesia, the Philippines, and Myanmar.Jackfruit usually reaches a weight of 10-25 kg when ripe, but large jackfruit sometimes reaches up to 50 kg.The jackfruit peel waste has no economic cost and usually causes serious disposal problems for the local environment.Therefore, the use of jackfruit peels as an cost effective adsorbent increases the economic value and contributes to lowering the disposal cost.In addition, the pollution problem can be significantly reduced.The carbonaceous product (JPC), which was prepared by treating jackfruit peels with sulfuric acid, has been used to test the effectiveness as an biosorbent for eliminating the cadmium from the contaminated water (Inbaraj and Sulochana 2004).
By increasing pH from 2.0 to 5.0, the removal rate of Cd ion increases from 13.1% to 98.7% and then remained in the pH ranges from 5.0 to 10.0 (98.7-99.4%)quantitatively.After adsorption experimentation with a 40 mg/L cadmium solution and a 0.7 g JPC dose, the Cd-containing wastecarbon was removed and carefully washed with the distilled water to remove all non-absorbed Cd to remediate.The carbon-containing Cd was stirred with 100 mL of hydrochloric acid (0.01-0.1 M) of various concentrations for one hour, and then cadmium ions were sorbed into the aqueous solution.The adsorption of cadmium can be completely recovered using hydrochloric acid at a concentration of only 0.01 M. Probability of the using jack-fruit (JFP) to adsorb MB has been examined (Hameed 2009;Bhatnagar and Sillanp€ a€ a 2010).
Batch adsorption studies were performed to estimate the effect of contact time at 30 C, initial concentration (35-400 mg/L), pH (2-11), and the adsorbent dose (0.05-1.20 g) to evaluate dye removal.The experimental data are very suitable for the Langmuir model type 2. The Jackfruit peel capability to remove dyes from the contaminated water was examined (Jayarajan et al. 2011).Several experiments were carried out to determine adsorption isotherms at different adsorption doses, temperature (30-60 C), and the pH (4.95-9.74).
The single-layer adsorption ability was found as 1.98-4.361mg/g.Efficacy of adsorbents made with jackfruit peels to remove 4-chlorophenol, 2,4-DCP phenol, and 2chlorophenol from the aqueous solutions was investigated (Jain and Jayaram 2007).The ratio of C and H was found to decrease after mercerizing the dish.Adsorption equilibrium reached within 5 h.The highest adsorption abilities of 2-chlorophenol, phenol, 4-chlorophenol, and 2,4-DCP were 243.9 mg/g, 144.9 mg/g, 277.7 mg/g, and 400.0 mg/g.Reduction of phenol is efficient at a lower pH (Mu'Azu et al. 2017).

Garlic peel for the remediation of pollutants
Garlic peels (GPs) can be useful for the treatment of wastewater.Due to more consumption, a large amount of the peel is discarded, which leads to serious problems in the community.The feasibility of removing Pb, Cu, and Ni from GP was explored (Liang et al. 2013).The findings suggest that the biosorption can reach at equilibrium within 20 minutes.The highest biosorption capability of GP is 209 mg/g; however, adsorption affinity of GP for Pb is significantly higher than that of Cu, and Ni.Adsorption efficacy and absorption ability of one metal are decreased due to the existence of another ions of metals.Natural GP and the mercerized GP have also been investigated as the adsorbents for the removal of Pb (Liu et al. 2014).After mercerization, the adsorption capacity of the GP increased 2.1 times and reached a maximum of 109.05 mg/g.FTIR and the scanning electron microscope (SEM) results show that the mercerized GP has many more pores than the adsorption sites than natural GP and has a lower degree of polymerization and crystallinity as well as higher functional OH group.Garlic peel has a higher adsorption capacity for remediation.FT-IR and X-ray photoelectron spectroscopy analysis before and after the addition of Pb to the GP also showed that Pb was adsorbed on a surface by the chelation between oxygen and Pb atoms on the surface of the GP.The capability of GP to eliminate the MB from the contaminated water was also assessed in a batch method (Hameed and Ahmad 2009;Tan et al. 2009).
The experiments were conducted based on pH (4-12), initial concentration (25-200 mg/L), contact time and temperature (303, 313, and 323 K).At 303, 313, and 323 K, the highest ability of single layer adsorption was 82.64, 123.45, and 142.86 mg/g.The capability of GP biosorbents for the elimination of direct red 12B (DR12B) from contaminated wastewater was studied (Asfaram et al. 2014).Having an initial dye concentration of 50 mg/L, the removal efficiency of over 99% can be achieved in 25 min with an adsorption dose of 0.2 g per 50 mL.The maximum DR12B adsorption capacity of the adsorbent was 37.96 mg/g (Asfaram et al. 2014).Muthamilselvi et al. (2016) also conducted optimization studies on the adsorption of phenol on GP.It was found that the best optimal conditions for the highest phenol reduction from a contaminated water of 50 mg/L were: pH: 2, adsorption dose: 2.1 g/L, contact time: 7 h, and stirring speed: 135 rpm.The research result shows that under optimal conditions mentioned above, the removal rate of phenol can reach more than 80%.The highest adsorption efficiency was assessed to be 14.49mg/g (determined by Langmuir isotherm).

Other agricultural waste peels for remediating the heavy metal(loid)s in wastewater
The removal of Hg ions from Egyptian citrus peel has been examined (Husein 2013).Three separate adsorbents were developed.Raw leather (MP), mandarin peel pretreated with NaOH (MNa), and carbonized mandarin peel (CMP) were used as the first three adsorbents (Hoseinzadeh et al. 2014).MNa, MP, and MC had gross monolayer adsorption capacities of 23.26, 19.01, and 34.84 mg/g, respectively.The use of potato peel coal (PPC) as an adsorbent for the removal of Cu from an aqueous solution was studied (Aman et al. 2008;Fu and Wang 2011).Potato peels' biosorption ability for removing Pb, cadmium, and zinc from aqueous solutions was also explored (Taha et al. 2011).The pH value, contact time, initial metal ion concentration, and temperature all have a strong impact on metal ion adsorption, whereas the particle size of the adsorbent has little effect.The reduction percentages of Pb, cadmium, and zinc were 92, 75, and 42%, respectively, at an 100 mg/L initial concentration and normal temperature.
The study has been done on adsorption of MB in aqueous solution by potato peel (PP) ( € Oktem et al. 2012).After 1 h, the highest biosorption capability was decided to be 33.87 mg/g at pH 8.The PP can also be used for making C samples after pyrolysis hydrothermal treatment (Kyzas and Deliyanni 2015).The resulting substance was used to strip two medicinal substances such as pramipexole and dorzolamide from artificial water waste after activation with KOH and alteration with an oxidizing agent.According to studies, the adsorbent generated has a sorption ability of 52-66 mg/g.The possibility of utilizing biomass from waste to adsorb two acid dyes (Acid Black and Acid Blue 113) from an water was tested, and satisfactory results were obtained (Hoseinzadeh et al. 2014).Chemically prepared and heat-treated watermelon peel (TWMP) is used to extract methylparathione (MP) pesticides (Mohy-Ud-Din 2020).The surface area of the untreated watermelon peel (UTWMP) was 15.1 m 2 /g and the adsorbent rose surface area to 23.4 m 2 /g after application.The overall adsorption of the MP solution (0.38-3.80) 10 -4 mol dm -3 is 99%.A 20 mL solution is mixed in for 60 min at a pH of 6, and 0.1 g of adsorbent is used (Memon et al. 2008).Melon peel (MP) (Cucumis melo L.) is used to remove Cd from the aqueous phase (Hamdaoui et al. 2010).The median monolayer adsorption of Cd by MP, according to the researchers, was 81.97 mg/g.Pb is often removed using a modified adsorbent for watermelon peels (Huang et al. 2012).
Ponkan peel capability to eliminate lead ions in an contaminated water via biosorption has been investigated (Pavan et al. 2008).Ponkan shell waste had a pore volume of 0.30 cm 3 /g and also had a particular surface area of 115.3 m 2 /g according to the BET method.The BJH approach determines a pore diameter of 5 nm, which is typical of mesoporous materials.For optimum adsorption power, a pH of 5.0 is ideal.Biosorption is fast, taking around 60 min.The overall absorption of lead ions on Ponkan dishes, according to the Langmuir isotherm, is 112.1 mg/g.In dishes from Mozambique (C.limetta), the absorption of Cr(VI) from contaminated water was analyzed (Saha et al. 2013).The maximum absorption of Cr(VI) from polluted water is 250 mg/g at pH 2.0 and temperature 40 C. Cr binding on the cell surface is mediated by -OH, -NH, C, O, and CO classes, according to FTIR spectroscopy (Saha et al. 2013).
Mango peel waste (MPW) has been reported to remove Cu, Ni, and Zn from the formed metal solution and the actual wastewater from the electroplating industry as an adsorbent (Iqbal et al. 2009).The adsorption mechanism was discovered to be pH based, with maximal adsorption occurring at pH 5-6 and equilibrium occurring within 60 min.Cu, Ni, and Zn have maximal adsorption capacities of 46.09, 39.75, and 28.21 mg/g, respectively, in equilibrium.During the EDX study of the Cu, Ni, Zn, and MPX absorption, MPW releases alkaline and alkali metal cations (K, Mg, Na, and Ca) and protons before and after the metal adsorption phase, indicating that ion exchange is the key mechanism of adsorption.The functional carboxyl and hydroxyl groups are linked to the adsorption of Cu, Ni, and Zn, according to FTIR research.For the elimination of acid blue 25-dye from aqueous solutions, the adsorption ability of the lychee peel was studied (Bhatnagar and Minocha 2010a).The adsorption capacity is about 200 mg/g for the 25-acid blue dye found in the adsorbent for lychee residue.The litchi shell also showed a significant adsorption capacity for Cr(VI) ions (Ali Khan Rao et al. 2012).

Modification to enhance biosorption potential of agricultural waste peels
There are many kinds of adsorbents, activated carbon is one of them and can be used as example to explain adsorbents synthesis and their modifications (Figure S2, Supplementary Information).It can be produced from naturally occurring and synthetic carbonaceous solid precursors, and it has been classified according to its source material (Figure 2).The form of starting material or precursor has a significant impact on the consistency, features, and properties of the activated carbon that results (Othman 2008;Yahya et al. 2015;Jain et al. 2016).Cagnon et al. (2009) have added that the composition of the starting material can also have an effect on the activated carbon properties.In addition, the properties of the activated carbon generated will be determined by the types of triggering reagents used, as well as time, inorganic impurities, impregnation state, and carbonization temperature (Jain et al. 2016;Joo et al. 2021).

Possible activated carbon modifications
There are two different processes for processing activated carbon: chemical and physical processing (Yahya et al. 2015(Yahya et al. , 2018)).Both treatment results in changes in size and shape.Precursors would be carbonized first and activated with steam or carbon dioxide during physical therapy.This suggests that there are two phases to this physical activation: carbonization and activation.Chemical therapy, on the other side, involves impregnating precursors with an activating reagent and then boiling them in an inert environment.The triggering reagents were able to dissolve the precursor's cellulosic components, facilitating the forming of cross-links (Lima and Marshall 2005;Yahya et al. 2015).
Nevertheless, chemical activation has shown other advantages over physical activation.Compared to physical activation, it requires lower temperatures, produces higher yields, has a larger surface area, requires only one step, produces well-developed micropores (Cruz et al. 2012) and reduces mineral content.However, there are certain drawbacks of chemical activation, such as the need for cleaning to clear impurities from the triggering chemicals, as well as the agents' corrosive properties (Din et al. 2017;Yahya et al. 2018).

Modification of agricultural waste peels by activated carbon physically
Thermal and physical treatment is a two-step method, i.e., activation and carbonization (Aguilar-Rosero et al. 2022).The interaction between the samples and gaseous (CO 2 and air), steam, or a combination of gaseous and steam is referred to as dry oxidation at temperature reaching above 700 C (Al-Qodah and Shawabkah 2009; Aguilar-Rosero et al. 2022).Because of its slow reaction rate, CO 2 has been widely used because it is safe, simple to manage, and the activation mechanism can be easily managed at temperatures about 800 C. When comparing CO 2 activation to steam activation, a retreater uniformity of pore can be obtained (Abioye and Ani 2015).
Carbonization is the process of pyrolyzing precursors and removing non-carbon species.Low molecular weight volatiles are released first, followed by light aromatics and hydrogen.The result of this process is solid carbonaceous char.The tarry pyrolysis residue fills the pores created during the carbonization process.This carbonaceous type is triggered by an activation process (Ali 2010;Abioye and Ani 2015).Due to the entry of oxidizing gases into the char and the elimination of reaction products by particles, pores, and vessels shape during the process through extracting the more volatile carbon atoms first, the gasification process will create porosity.Further gasification will result in the production of activated carbon with a large porosity (Abioye and Ani 2015).The type and degree of activation could affect the physical and chemical properties of the activated carbon (Li et al. 2008;Kosheleva et al. 2019).The aim of activation is to increase porosity and order the structure, resulting in a highly porous solid of activated carbon (Guo et al. 2009).The activation mechanism divides pore production into three phases: opening previously inaccessible pores, new pore development through selective activation, and widening of established pores (Li et al. 2008;Kosheleva et al. 2019).

Chemical modification of activated carbon to boast the biosorption potential
Chemical activation is also called as wet oxidation (Srivastava et al. 2021).To synthesize activated carbon, the catalyst must be impregnated into the precursor and cleaned (Kalderis et al. 2008;Vargas et al. 2011).This activation basically involves a relatively low temperature range from 300 to 700 C (Giraldo and Moreno-Piraj an 2012) or 400 to 700 C or 400 to 800 C (Alhamed 2006) or 500 to 800 C (Pragya 2013) and is mostly reliant on the inorganic additives' ability to dissolve and dehydrate the cellulosic materials present in the precursor (Bello and Ahmad 2011).

Regeneration and disposal of biosorbents
Adsorbents that have been used (saturated adsorbents) are typically discarded.Handling used adsorbents can be difficult because some are hazardous and involve oxidation, whereas some are poisonous.In addition to emptying used adsorbents, a new approach for recovering adsorbents is being created called regeneration.In this approach, used adsorbents are recycled for possible reuse by using techno economic approaches (Momina et al. 2018).
The adsorbate may be a source of nutrients and metals that must be recovered or calculated to earn recycling credits in a variety of situations (Figure 3).This method is costeffective due to the reuse of excess adsorbent and the revival of adsorbate.As a result, desorbing the adsorbed ions and stimulating the adsorbent for usage as an additional cycle are recommended.The correct choice of eluent is critical for a successful deployment desorption operation, which is primarily determined by the type of bio-sorbent used and the biosorption system used (Hossain et al. 2012;Li et al. 2016).
Study indicate that the 0.05 M HCl is the most effective in desorbing 87.23% of SCA adsorbed Zn.A maximum removal rate of 93.72% for Cd can be achieved by adding 0.15 M HCl from Cd-loaded SCA.Using 0.1 M HCl to desorb Co and Ni from metal-filled SPA, 81.06% and 80.11% of these metals were recovered, made a change in the adsorption efficiency of Cu and Cd with the number of adsorption-desorption cycles (Tan et al. 2015;Awual 2019).It has been found that, relative to the cycle's numbers (>5 cycles), the adsorption efficiency of these two metals remains almost constant, i.e., the adsorbent could possibly be reusable after the metal has eluted, also carried out desorption experiments at various HCl concentrations (Lu et al. 2012).Percentage of desorb raises as the hydrochloric acid level rises (from 0.005 to 0.1 M), but then stays relatively constant.It is almost 100% for HCl concentrations ranging from 0.1 to 0.2 M. As a result, the optimum desorption concentration is 0.1 M HCl.Six cycles were used to test the recycled bio-adsorption sorbent's capability.After six cycles, the adsorption capacity of 272SCO was found to be just marginally reduced.Lead adsorption rate on 272SCO was already 89.61% after the sixth period (Lu et al. 2012).Desorption experiments were conducted in 0.1 M HNO 3 with Cd containing MNP-OPP, and the results of this study reveal that up to 98% of the sorbed cadmium was desorbed (Lata et al. 2015).The desorption rate was 98.19-98.66% in all cycles.After five consecutive adsorption-desorption cycles, the adsorption efficiency decreased by 4.74%.In the desorption experiment, when the desorption pH enhanced from 2 to 12, the desorbed % of DR23 enhanced from 37.5% to 97.7% and DR80 from 2.2% to 93% at a dye initial level of 50 mg L À1 (Arami et al. 2005).
The ZCOW reusability test was carried out on dry ZCOW (Jha et al. 2015).After the 1st to 8th cycle operation sequence, the ZCOW adsorption percentage of fluoride was found to decrease from 97% to 83%.The regeneration of crystal violet (CV) and grape fruit peel residues GFP, which were obtained after exhaustion of the biomass of the GFP was tested by taking 1 M NaOH aqueous solution (Saeed al. 2010).The desorption rate of CV was found to be very fast, with a maximum amount of elution of 82% achieved in just 82 min.However, the rate of desorption slowed considerably after this contact time, reaching a steady level after 60 min, signaling that the mechanism had achieved equilibrium.The quality of desorption is up to 98%.Regenerated grapefruit peel biomass was taken to strip CV again, with efficiencies of 81% in third cycle and 87% in second cycle.Thirumavalavan et al. (2011) described that the efficiency of desorption of sorbed metals were up to 97%.The recyclability of the adsorbent was measured by repeating the metal ion adsorption-desorption cycle four to five times using the same adsorbent.Another desorption study was performed to recover cadmium and sorbent and treated water (Kurniawan et al. 2011).The adsorption of Cd ions can be completely recovered using HCl at a concentration of only 0.01 M.
The main aim of an excellent biosorbent is to apply at industrial scale for treatment of wastewater, so for largescale economic aspects, biosorbents not only should have excellent adsorption ability, but also have long-term stability and renderability for reuse of adsorbents (Chen et al. 2018;Li et al. 2020).Biosorbents do not completely regenerate; however, its ability down during subsequent desorbing cycles.So, the adsorbent regeneration is an important factor of consideration (Li et al. 2020).
Limited research has been done on regeneration and sustainable management of biosorbents used for wastewater treatment.Disposal of used biosorbents rich with contaminants often poses environmental and public risk.In developing countries, it becomes more risk due to the lack of incinerators and landfills.Used biomaterials could be disposed by using phytoremediation cells, incineration, used in constructions, but all these approaches have high environmental risk and could be very expensive (Chaukura et al. 2016).In future, research should be focused on recovery/regenerability and safe disposal of biosorbents loaded with contaminants.
Modeling of biosorption: kinetic, isotherm, and thermodynamic models Adsorption data obtained are fitted into many isotherm, kinetic, thermodynamic, or breakthrough empirical models that help in taking the insights by adsorption process (Kolluru et al. 2021;Sheth et al. 2021).All these models can figure out the adsorption rate and adsorption feasibility at a particular heat by determining Gibbs free energy, exothermic/endothermic nature and the type of interaction within adsorbate and adsorbent (Wang and Chen 2009).Matouq et al. (2015) explained the isotherm models for single-component adsorption using moringa as an adsorbent.Several researchers provide a description of the mathematic modeling of biosorption in detail (Wang and Chen 2009;Qiu et al. 2018;Sheth et al. 2021;Chen et al. 2022;Syeda et al. 2022).In an experiment, the authors fitted the adsorption data into the Freundlich, Langmuir, Temkin, and Dubinin-Radushkevich models to investigate the adsorption of Pb ions on the agricultural waste material such as coffee waste (Edathil et al. 2018).
Researchers reviewed a detailed introduction of biosorption modeling such as single-sorbate isotherms and multisorbate sorption equilibrium (Sheth et al. 2021).Kulkarni et al. (2022) explained the kinetic modeling for biosorption in a batch and continuous system.In a study conducted by Akar et al. (2013), biosorption dynamics were summarized.Despite extensive research on the various biosorption models in heavy metal adsorption, a specific and comprehensive literature review on this content is still needed in the future.

Removal of different pollutants using natural and modified agriculture waste peels
Heavy metal ion separation from wastewater by using agricultural peels has been reported to be sophisticated and capable approach.The capability, affinity, and specificity of waste, which are comprised of physical and chemical properties, determine its efficacy.Dispersion of metal ions such as Cd, Ni, As, Hg, Cr, Pb, and other metals was examined using various biological adsorbents.The adsorbent may be used in its natural state or adjusted chemically and thermally to improve its adsorption capability.

Chromium sorption onto various agricultural-based waste peels
Chromium is a toxic heavy metal that can enter the environment through leather tanning, plastic paints, wood preservatives, pigments, paints, and textiles.Chromium exists in different oxidation states, but Cr(VI) and Cr(III) are the biggest environmental concerns (Bashir et al. 2020(Bashir et al. , 2021;;Yu et al. 2000).The use of agricultural waste to remove Cr has been reported to be a heavy workload.Many agricultural wastes such as hazelnut peel, orange peel, corn on the cob, peanut peel, soybean peel, jackfruit, natural or modified forms of soybean peel have been investigated and substantial removal effectiveness has been reported (Kurniawan et al. 2006b).
Also, a variety of plant residues, such as bark, coconut shell fiber, pine needles, coconut cactus leaves fiber pulp, and Indian leaf powder tries to remove Cr, the efficiency is more than 90% at the optimal pH of solution containing 100 ppm Cr(VI) (Dakiky et al. 2002;Sarin and Pant 2006;Venkateswarlu et al. 2007).Rice bran and wheat are found to be less effective as adsorbents since they are reported to have only 50% removal efficiency (Sud et al. 2008;Younas et al. 2021).Gardea-Torresdey et al. (2004) reported that Avena monida (total plant biomass) can remove 90% of Cr(VI) at an optimal pH of 6.0.The effects of removing Cr(VI) using the natural type of rice husk and active rice husk carbon were compared to commercial activated carbon and other adsorbents.Treatment of Indian rosewood sawdust with formaldehyde and sulfuric acid resulted in successful Cr(VI) elimination (Burakov et al. 2018).Attempts have also been made to remove Cr using beech sawdust and rubber-wood sawdust (Mohan and Pittman 2006).Bagasse can be used in both natural and modified forms, and the chrome removal efficiency of the two forms is compared (Krishnani et al. 2004;Miretzky and Cirelli 2010).
The use of mustard cake oil has been described as having a remarkable detoxifying effect.Results of activated carbon from sugar industry waste were compared with commercial granular activated carbon for sequestration of metal ions in aqueous solutions (Saha and Orvig 2010).Bagasse enzymes, corn cobs, and jatropha oil cakes have recently been used to remove Cr under optimized conditions (Garg et al. 2007).Many studies show that agricultural waste has a high biosorption rate of Cr, which is between 50% and 100%.Biosorption takes place mainly in the acidic range, especially at pH 2.0.Therefore, the form of Cr plays a leading role in determining the removal efficiency, since Cr is in the form of Cr(III) at a pH of 2.

Lead removal via organic-based waste peels
Plastics, cathode ray tubes, finishing materials, ceramics, solder, Pb flash, and other secondary goods, steel, and cables are the primary sources of Pb in the environment (Sardar et al. 2018).Lead may have a variety of biological consequences depending on the degree and extent of exposure (Lalor 2008).In the environment, Pb is firmly bound to particles such as sediment, oil, and sewage sludge.Therefore, the removal of Pb has received a lot of attention.Various agricultural wastes have been reported to be used to remove Pb from straw, soybean shells, bagasse, natural peanut shells, and walnut shells (Cao and Harris 2010).Kumar et al. (2017) examined the natural form of the bark.The removal effect of Pb in petioles linoleum palms (PFP), agricultural waste from rye peel, hum grass flowers, tea, and water hyacinth waste were examined.The usage rate of these materials varied from 70% to 98% (Saeed et al. 2005).The removal of Pb and other metal ions by phosphorus oxychloride-modified apple pomace was investigated and compared in a batch study and a column study.NaOHpretreated rose petals, calcium-treated sargassum and succinic anhydride-modified sugarcane also significantly removed Pb (Karnitz et al. 2007;Hubbe et al. 2011).
Activated carbon from agricultural waste has also been studied by various workers, and it has been reported that the efficacy of Pb removal is very high (Wilson et al. 2006;Mohan et al. 2007).Using bagasse fly ash to remove Pb, the removal rate reached upto 65%.The removal efficiency of maple sawdust, Pinus sylvestris var.removal efficiency.Literature research shows that the best Pb biosorption value can be found when the pH is 5-6 (Ahluwalia and Goyal 2007;Yagub et al. 2014).

Remediation of cadmium using agricultural waste peels
Cadmium and its compounds are relatively water-soluble compared to other heavy metal(loid)s so that they can move in the soil and tend to bioaccumulate.Durable PVC window frames, coatings on plastic and steel are the basic sources of Cd in the environment.Cadmium, especially in the kidneys, accumulates in the human body and leads to kidney dysfunction (Bernard 2008;Latif et al. 2020;Naveed et al. 2020;Sabir et al. 2020).
Attempts have been made to chelate Cd with rice bran and wheat bran, and removal efficiency is reported to be very high (Farajzadeh and Monji 2004;Montanher et al. 2005).Methods for removing Cd using natural and modified forms of rice oil, rice husk, and black husk have also been studied and their relative efficiencies have been described (Iqbal and Edyvean 2005;Ahmaruzzaman and Gupta 2011).
To remove Cd, it has been tried in the bark of plants such as Picea glehnii and Abies sachalinensis, and dry dicots plants (Sharma et al. 2006;Ajmal et al. 2020).
The use of other plant parts such as pea peels, fig leaves, faba beans, orange peels, wolfberry peels, and jackfruit as adsorbents has been reported to have high removal efficiencies at acidic pH (Benaissa 2006).Adsorption experiments on hazelnut shells, peanut shells, walnut shells, and green coconut shells provided important results for Cd removal (Kurniawan et al. 2006a;Barakat 2011).Activated carbons from bagasse, coconut shells, peanut shells and dates have been studied with removal efficiencies ranging from 50% to 98% (Krishnan and Anirudhan 2003;Kannan and Rengasamy 2005;Kula et al. 2008).
Studies have also been carried out with chemically treated agricultural waste such as rice husks after basic treatment, juniper fiber-treated corn cobs, citric acid-modified corn cobs, modified peanut shells, sugar cane treated with succinic anhydride, etc. (Karnitz et al. 2007;Hamdaoui et al. 2010).Most studies have shown that agricultural waste, in its natural or improved form, is very effective in removing cadmium metal ions.

Removal of nickel via biosorption process
Nickel and its compounds have no distinctive smell or taste.The environmental sources for Ni are Ni plating, batteries, non-ferrous ceramics, alloys or furnaces as well as waste incineration plants from power plants.The most harmful effects of Ni are allergic reactions (Radenovi c et al. 2011).Nickel removal experiments were performed on the cassia fistula biomass in its natural form, and the results showed that the removal efficiency was 99-100% (Boji c et al. 2016).Attempts have also been made to chelate Ni from aqueous solutions with tea waste leaves (Ahluwalia and Goyal 2005).
Maple, oak, and grasshopper sawdust are reported to contain a capable bio-sorbent for Ni removal (Sciban et al. 2006).Agricultural waste in natural or modified forms such as peanuts, pecans, walnuts, hazelnuts, and peanut shells are also used for biosorption (Shukla and Pai 2005;Kurniawan et al. 2006a).Other agricultural wastes such as modified coconut fiber, cottonseed, soybeans, and corn cobs were also examined to remove Ni (Shukla and Pai 2005;Ngah and Hanafiah 2008).Bagasse in its raw form shows a removal efficiency of over 80% (Garg et al. 2007).

Removal of other toxic metal(loid)s
Other metal ions that are present in various industrial wastewater such as Cu, Zn, As, Hg, and Co have environmental problems due to high toxicity.The entrance of all these metals into the water environment is due to different human and industrial applications (Kar and Misra 2004;Aguayo-Villarreal et al. 2011;Liang et al. 2020).Several studies have been performed on rice husks, water hyacinths, and other inexpensive adsorbents with efficiencies between 71% and 96% (Mohan and Pittman 2006).
Various agricultural wastes such as weeds, bamboo pulp, dry pine needles, modified cotton fibers, and sawdust are used to remove Hg ions (Kim et al. 2020).Attempts have also been made to impregnate Cu and chemically modified sawdust to remove As with significant efficiency (Roy et al. 2013;Carolin et al. 2017).The use of sawdust also plays an important role in the removal of Cu ions (Larous et al. 2005;Ahmad et al. 2009).Other wastes such as wheat husks and carbonated coconut shell pulp are also highly efficient in chelating Cu ions.The modified bark of mustard oil cake and pine bark has also proven to be a potential bio-sorbent (Dhir 2014;Afroze and Sen 2018).Table 2 indicates the previous studies with reported efficacies in removing heavy metals from aqueous solutions.

Cost analysis of adsorption processes
Economic analysis is an important factor to investigate the practical applicability of the sorption process using biosorbents.In literature, it is often mentioned that waste materials are very cheap and cost effective compared to commercial sorbents (Fawzy, Nasr, Nagy, et al. 2018).Sharma and Ayub (2019) showed the cost of adsorption processes using agricultural waste materials (tea and ginger mix, pea pod peels and BPs) to remove hexavalent Cr (Cr(VI)) from contaminated water.As per cost analysis, the cost to remove 1 g of Cr(VI) from contaminated water was PKR 4.71, 9.14, and 3.11, respectively.Cost analysis of this study showed that cost of biosorption is significantly lower compared to adsorption using commercial activated carbon, which was PKR 142.145.
Rice husk is a very common biosorbent in the adsorption process.In an experiment studied by Sakhiya et al. (2021), the authors estimated the cost of a chemically modified risk tray sorbent at $2,579, which is very cheap.The total economic cost of producing 1 kg of bagasse, alkaline and acid activated bagasse, etc. sorbents was reported to be estimated at INR 49.28,INR 103.40,and INR 309.65.
Modifications enhance the total cost of the adsorbents.The cost for removal of I g Cu(II) from contaminatedwastewater using sugarcane bagasse, base and acid modified sugarcane bagasse was INR 10.18, INR 50.19, and 57.88, respectively.However, modification enhances the cost, it was still less expensive compared to commercial modified/activated carbon taken by the authors that costed INR 649.47 (Gupta et al. 2018).Bhatnagar et al. (2010) reported that cost of lemon peel for heavy metals removal from wastewater was 10 times cheaper compared to commercial activated carbon.Based on economic analysis, several recent studies proved that biosorbents are cost effective compared to commercial sorbents (Ali et al. 2016;Fawzy et al. 2019;Hamdy et al. 2019;Kumar et al. 2019;Syeda et al. 2022).

Future directions and conclusions
Biosorption is an emerging way of treating wastewater without producing any toxic secondary products.As agricultural waste peels are available in bulk, affordable, inexpensive, easily accessible, reusable, and have a high adsorption capacity for heavy metal(loid)s.This review concluded that agricultural waste peels are most efficient in removal of contaminants from wastewater.Also, modification and treatment with other elements or carbonaceous compounds enhanced the adsorption capacity of these agricultural peels.Based on economic analysis, several recent studies proved that biosorbents are cost effective compared to commercial sorbents From mathematical models of adsorption processes, Freundlich and Langmuir isotherm models have been mostly studied.In kinetic modeling, pseudo-second-order model best fitted in many studies.In literature, chelation, ion exchange and electrostatic attraction were the main adsorption/biosorption processes.Despite extensive research, and the various biosorption modeling in heavy metals adsorption, a specific and comprehensive literature review on this content is still needed in future.
Biosorbent regeneration had allowed many adsorbents to be recycled and reused efficiently up to 10 times.Limited research has been done on regeneration and sustainable management of biosorbents used for wastewater treatment.Disposal of used biosorbents rich in contaminants often poses environmental and public risk.Used biomaterials could be disposed by using phytoremediation cells, incineration, used in constructions, but all these approaches have high environmental risk and could be very expensive.In future, research should be focused on recovery/regenerability and safe disposal of biosorbents loaded with contaminants.Most of the research published has been conducted in batch processes, but more field-based research trials are needed to do so that its practical application can be warranted, and it may help in controlling/remediating wastewater contamination at large scale.

Figure 1 .
Figure 1.Mechanisms of heavy metal(loid)s removal by waste peel derived biosorbents from contaminated water.

Figure 3 .
Figure 3. Adsorption and desorption pathways of biosorbents to remove heavy metal(loid)s from wastewater.

Table 1 .
Permissible limits for different heavy metal(loid)s in potable water

Table 2 .
Efficacy of various adsorbents for remediation of heavy metal(loid)s in contaminated water.