A comprehensive review on the decontamination of lead(ii) from water and wastewater by low-cost biosorbents

The disadvantages of conventional methods in water and wastewater management including the demand for high energy consumption, the creation of secondary toxic sludge, and operation cost are much too high for developing countries. However, adsorption using low-cost biosorbents is the most efficient non-conventional technique for heavy metals removal. The high adsorption capacities, cost-effectiveness, and the abundance of agricultural waste materials in nature are the important parameters that explain why these biosorbents are economical for heavy metals removal. The present investigation sought to review the biosorption of lead [Pb(ii)] onto low-cost biosorbents to understand their adsorption mechanism. The review shows that biosorption using low-cost biosorbents is eco-friendly, cost-effective, and is a simple technique for water and wastewater treatment containing lead(ii) ions. The batch biosorption tests carried out in most studies show that Pb(ii) biosorption by the low-cost biosorbents is dependent on biosorption variables such as pH of the aqueous solution, contact time, biosorbent dose, Pb(ii) initial concentration, and temperature. Furthermore, batch equilibrium data have been explored in many studies by evaluating the kinetics, isothermal and thermodynamic variables. Most of the studies on the adsorptive removal of Pb(ii) were found to follow the pseudo-second kinetic and Langmuir isotherm models with the thermodynamics variables suggesting the feasibility and spontaneous nature of Pb(ii) sequestration. However, gaps exist to increase biosorption ability, economic feasibility, optimization of the biosorption system, and desorption and regeneration of the used agricultural biosorbents.


Introduction
Agricultural and industrial activities have become pollutant sources in recent times, primarily concerning the bioaccumulation of toxic metals in the ecosystem. This results in a signicant escalation of the amount of these metals in surface and ground waters thereby contaminating the aquatic bodies. Heavy metals are hazardous and detrimental to living species especially when they could be distributed through the food chain. 1 The manifestation of toxic ions in the aquatic environment is a fundamental universal concern attributable to their continuous-release, venomous nature and other deadly effects on the receiving aqueous media.
Lead(II) is commonly present in effluents and sewages from industries such as paint, pesticides, battery, mine, and smelting. The various sources of Pb(II) contamination in the environment are depicted in Fig. 1. Reports indicate that grown-ups engross 5-15% of lead(II) and nearly 5% of it is being retained and the existence of 0.5-0.8 mg per mL of lead(II) in the blood of living organisms leads to numerous health conditions. 2 The toxicity of lead(II) leads to widespread health disorders including pregnancy miscarriages in women, severe stomachache, hypertension, impaired blood synthesis, brain, and kidney damage. 3 Besides, lead(II) is a universal pollutant on earth, and aqueous media and its exposure cause feebleness in ankles, ngers, and wrists. 4 Some of the toxic effects of Pb(II) on plants, animals, and humans are shown in Fig. 2.
Many treatment systems have been probed for heavy metals reduction from wastewater and water; however, many of these management processes are signicantly pricy and incompetent of eliminating low-level concentrations of heavy metals. In comparison, adsorption (biosorption) appears to be more resourceful and used worldwide owing to the greater yield and cost-effectiveness of the biosorbents. 5 Biosorption is a simple and efficient reversible reaction that takes place between heavy metals and biomass functional groups. Several studies have demonstrated Pb(II) adsorption potentials by natural and agrobased materials leading to multiple sorption mechanisms involving ion exchange, precipitation with inorganic components, interactions with p electrons, and complexation with oxygen-containing functional groups. 6 The objective of the present study is to conduct a review on the biosorption of lead(II) ions from water media on low-cost biosorbents especially those obtained from natural and agrobased materials. The effects of biosorption variables as well as biosorption isotherms, kinetics, and thermodynamic variables evaluated from the adsorption equilibrium data were also reviewed.

Heavy metals
In the environment, trace metals exist naturally, particularly in the Earth's crust, and add to the balancing of the planet. 9 At the non-anthropogenic origin, heavy metals always exist at background concentration levels where their presence in soils could be due to weathering of parent rocks. In the ecosystem, heavy  Kelvin Mtei is an associate professor of agriculture and environmental sciences with vast experience in research and academics for about eighteen years. He has been delivering lectures in various universities both at undergraduate and postgraduate levels. His research emphasizes application of science and technology in socialecological context with a focus on sustainable farming systems; remediation of degraded land; agricultural water management and re-use. He has received several research grants through which he has produced more than 70 publications in peer review journals, conference proceedings and books. metals occur naturally with large variations in concentration. Heavy metals pollution emanates from metals purication including various sources. Especially, copper smelting, nuclear fuels preparation, and electroplating of metals have led to the production of chromium and cadmium. 10 Other sources of metal ions pollution include wet and dry fallouts of atmospheric particulate matter; dead and decomposing vegetation, animal matter, as well as from man's activities. 9,11 Anthropogenic activities that result in toxic metal pollution are cement industry and mining operations, smelting procedures, steelworks, burning of coal and oil. 12 Wuana and Okieimen 13 indicated that agricultural elds could be contaminated due to improper treatments and disposal of mine drainages. Heavy metal ions can be in existence for a lengthy time in different inorganic and organic colloids before being exposed to living species. 14 They are indestructible and do not decay with time. Most metal ions are present at trace levels in soils, vegetation, and in humans, plants, and other living species as microelements. 15 Traces of heavy metals are not lethal in plants and animals at low concentration levels. However, there are few exemptions, cadmium, chromium, mercury, and lead are poisonous at very low concentrations. 16 Toxic metals can enter the human system via food, air, and dermal contact absorption. 17 They also accumulate in aquatic biota, soils, and plants. 9 Biomagnication of metal ions may occur when an organism ejects it slower than it ingests and this can be detrimental to living organisms. 18 The bioaccumulation of heavy metal ions tends to be very hazardous as a result of their extended biological half-lives. 19 As shown in Fig. 3, heavy metals can pollute the ecosystem due to various pathways through natural processes [1], and anthropogenic activities [2], leading to detrimental effects in the environment including direct pollution of the aquatic environment [3], plants [4], as well as triggering toxic effects on humans [5] through different intake paths such as food chain (drinking water and food), inhalation, and skin contact [6].

Heavy metals presence and pollution in the ecosystem
The discharge of metal ions into the ecosystem occurs over wide-ranging processes and pathways including to the surface waters via runoff, transport to soil; hence, into groundwaters and crops as well as to air during combustion. 20,21 Heavy metal ions occurrence in the soils could be accredited to different environmental factors, including, but not restricted to soil characteristics, parent materials, and human activities for example farming and irrigation. 22 Toxic metals in soil can inuence crop growth negatively and as well as interferes with the degeneration of major cell organelles, metabolic processes in plants, inhibition of respiration and photosynthesis. 23 Furthermore, the production and usage of synthetic products such as paints, pesticides, and batteries can lead to heavy metal pollution of agricultural soils and the environment at large. 24 Contamination of soils by heavy metals is primarily due to the use of wastewaters obtained from mining operations to irrigate paddy elds. 25 Heavy metals have the potential of polluting groundwater because of their high rate of transfer in soil proles. The potentiality of metal ions accumulation and bioaccumulation in soils leads to serious food chain contamination. 26 In Ghana, organochlorine pesticides are widely been used for public health and agricultural purposes for more than 40 years with their remains found in sediments, water, humans, and crops. 27 For instance, crop and vegetable farming had led to misuse and excessive use of pesticides. In Ghana, pesticides form part of agriculture and it has been estimated that about 87.00% of farmers depend on pesticides to prevent diseases and pests on vegetable farms, rice elds, cotton, and cocoa plantations and among others. 28 It has also been reported that several toxic metals are found to be present in drinking water media in some mining areas in the country.
Global worries have suddenly increased over toxic metals discharged from various sources into the environment due to their inability to degrade and so they linger in the environment. 22 Metal circulation between soil and vegetation is a very important issue in evaluating the effect of metals on the environment. 29 The main sources of heavy metals in the environment from anthropogenic activities and their implication on human beings and other life forms are presented in Fig. 4.

Effect of heavy metals
The toxicity of metal is mainly dened based on its concentration required to cause an acute response generally a sub-lethal response or death. Several trace elements are recognized to be noxious to human beings and other living species when consumed beyond their concentration required for healthy growth. Some of these heavy metals [arsenic (As), chromium (Cr), cadmium (Cd), lead (Pb), nickel (Ni), mercury (Hg), and zinc (Zn)] and their harmful effects in living organisms when bio-accumulated in higher concentration than their acceptable levels are presented in Fig. 5. There could be a high tendency of metal ions concentration build-up in living tissues once they become part of the food chain and cause severe health disorders when ingestion exceeds the permitted concentration. 30 Heavy metal toxicity has been found to have an inhibitory inuence on enzymatic functions, plant development, stoma roles, photosynthesis processes as well as the destruction of the root system. 31 The health risks assessment of metal ions pollution of potable water bodies are most imperative due to their lethal effects and capability of bioaccumulation via the food chain. 32 Different metals show alterations in the level of harmfulness, some are carcinogenic though other effects may include cardiovascular, 33 epidermal systems alteration, 34 and hematological, neurological, gastrointestinal, immunological, and musculoskeletal effect. 35 Fig. 6 depicts the human organs usually affected as a result of the discharge of excessive heavy metal ions into the environment from different sources. Highly toxic metal ions cause destructive effects even at small concentration levels. 36 The dangers of heavy metals have become the most current environmental health concern because they are potentially dangerous due to bioaccumulation through the food chain. 25 Mostly, heavy metals are distributed, concentrated, and chemically modied through human activities that may increase their toxicity. These activities result in greater concentrations of metal ions relative to their normal background concentrations. 25 Contamination with metal ions poses a serious threat to agriculture and other sources of food for humanity, reducing plant growth and reducing the resistance to pests and diseases. This affects the quality of groundwater, plant growth, food, and microbes. 10 The effects of heavy metals on microbes can lead to reduced waste degradation and nitrogen xation, inefficient nutrient circulation, and impaired enzyme synthesis. 25 Poisonous ions persist in the ecosystem for a prolonged time and do not decompose. 37 Rapid exposure to toxic trace elements in marine and terrestrial organisms can have a negative toxic effect. 38 For example, coastal sh and seabirds are oen observed for the presence of toxins, such as exposure to poisonous metals due to the food chain. It is generally difficult to predict heavy metals exposure effects on species since these metals may be essential or non-essential and very low concentrations of essential metals can be as harmful as high concentrations.

Application of conventional adsorbents in heavy metals decontamination
Earlier adsorbents used for metal ions removal were commercial adsorbents and have been used globally and extensively for   controlling water contamination. In literature, the commonly utilized conventional adsorbents are activated carbons, ionexchange resins (polymeric organic resins), and inorganic materials such as activated alumina, silica gel, zeolites, and molecular sieves. However, only four types of generic adsorbents have enormous surface areas and have dominated the commercial use of adsorption: activated carbons > zeolites > silica gel > activated aluminas. 41 Silica gel is worldwide recognized as an inorganic polymer that is used in chromatographic columns. Its high porous surface topology and enormous specic surface area are key for effective heavy metal ion interaction. In furtherance, silica gel is easily available as a substrate for immobilizing various chemical functional groups. 42 Several silicate-derived gels have been explored to extract, stabilize, and/or decontaminate arsenic and other heavy metal ions from water and wastewater. 43 Due to its strong physical strength, endurance, and high chemical stability, silica gel has been utilized in the sequestration of certain contaminants including solid wastes. 43,44 El-Moselhy et al. 44 were successful in developing hydrated iron oxide nanoparticles using silicate matrix, resulting in high arsenic detoxication. Mesoporous silica materials of MCM-41 type were efficiently utilized by Zhu et al. 45 in the removal of Cu(II), Pb(II), and Cd(II) from aqueous solutions. More so, a silica gel material modied with nitrilotriacetic acid (NTA-silica gel) was applied for the elimination of Cu(II), Cd(II), and Pb(II) from the aqueous system, which indicates high removal efficiency of the three heavy metal ions. 42 Furthermore, silica gel polymer was proven to be effective in Ni(II) removal from aqueous solution. 46 Zeolites are micro-porous crystal-like solids with denite structures and are naturally-occurring materials comprising hydrated aluminosilicate with maximum cation exchangeability with metal ions. 47 Zeolite has a highly porous structure in three-dimensional crystal lattices that display strong cation interaction and ion exchange ability with heavy metal ions. 48 Due to their high porosity and sieving characteristics, zeolites are a good choice for removing heavy metal ions from untreated wastewater. They also have a high ion exchange ability, and the exchangeable ions (Na + , Ca 2+ , and K + ions) are largely harmless. 49 Based on their quick advancements in properties, zeolites are gaining in popularity and are being used in a variety of innovative applications. Biomedical, radioprotection, wastewater purication, biosensor, pure, and applied chemical applications are all driving interest in them.
When using zeolites in sorption, the surface area is not a matter of concern because it is a selective process and it is reversible. 50 Zeolites are used practically in the fuel industries as water soeners, manufacture of detergents, catalysts, and making molecular sieves. Quite a lot of zeolites have been employed as adsorbents for the treatment of contaminants in water and wastewater. A zeolite produced from y ash was capable of decontaminating Hg(II) and Pb(II) ions from aqueous media. 51 Chen et al. 52 designed and prepared zeolite cotton as a form of lter for household water treatment. More so, zeolite composite adsorbents were applied to simultaneously detoxicate heavy metals and total coliforms from wastewater. 53 A highquality zeolite type A was produced and utilized successfully in removing heavy metals from water systems. 54 Besides, zeolitealginate composites were found to be promising in the Pb(II) removal from contaminated water solutions. 55 Makki 56 employed zeolite A4 for Cd(II) and Pb(II) decontamination in water. Also, Cu(II), Ni(II), and Pb(II) ions in an acid mine drainage were removed using Philippine natural zeolite. 57 Activated alumina is one of the most extensively utilized adsorbents for heavy metals sequestration due to its high affinity for metal ions. 58 It consists of porous aluminum oxide with a wide surface area that is resilient to thermal stress and abrasion. It does not swell, shrink, soen, or disintegrate throughout the adsorption process. 59 Furthermore, activated alumina has outstanding adsorption capabilities and a high ion uptake capacity, as well as being cost-effective, safe, and ecologically friendly, making it suitable for use in wastewater treatment. 60 Activated alumina has a good surface area and is frequently applied to get rid of oxygenates and mercaptans of hydrocarbons and especially, uorides in water. 61 A nanoalumina was produced and used effectively in As(V) decontamination from an aqueous solution. 62 Furthermore, novel galumina nanoparticles were developed for Ni(II) decontamination from the solvent phase and found procient. 63 In a study, a mesoporous carbon stabilized alumina was employed in removing Cd(II) and Pb(II) ions from aqueous media. 64 Besides, an alumina composite was shown to be effective in the removal of both Cr(VI) and methylene blue from aqueous solutions. 65 Globally, the elimination of noxious pollutants in water has been attributed to activated carbons. The well-known and lasting adsorbent in water and wastewater treatment is charcoal. The preparation of activated charcoal involves three stages; dehydration, carbonization, and activation. Due to the high surface area of activated carbons, they are highly recognized and recommended as procient adsorbents for water contaminants treatment and gas purication. 66,67 Activated carbons are black and made up of solid carbonaceous materials with high porosity, internal surface area, and mechanical strength. Granular activated carbon, powdered activated carbon, and activated carbon cloth are among the different forms of activated carbon used to remove contaminants from water and wastewater. 68 The constant utilization of activated carbons has produced fruitful outcomes in contaminant management processes. 41 Hu et al. 69 report the applicability of humic acid-impregnated activated carbon in removing Cu(II). Meanwhile, Jjagwe et al. 70 previously reviewed the synthesis and application of granular activated carbon from biomass waste materials for water treatment. Besides, Manjuladevi et al. 71 investigated Cr(VI), Ni(II), Cd(II), and Pb(II) ions removal from aqueous solutions using activated carbon prepared from Cucumis melo peel. Fan and Anderson 72 carried out a related study using granulated activated carbon to eradicate Cu(II) and Cd(II). Also, activated carbon has been applied in the decontamination of Pb(II) as recounted by many studies. 73,74 Regardless of the capabilities and prolic application of commercial activated carbons, there are some constraints on usage attributable to its high cost. For that reason, attention has been directed towards the preparation and production of costeffective adsorbents from natural materials that are readily available, carbonaceous, and easy to activate. 75 To produce activated carbon, an organic material undergoes thermal degradation for subsequent decomposition into carbon granules. 70

Application of agricultural waste materials in heavy metals removal
Commercially activated carbons are expensive adsorbents regardless of their widespread use in water and wastewater treatments. This has generated an increased interest in the production of safe, cheap, and economical adsorbents, which are more economical than commercially available activated carbon aimed at removing heavy metals from polluted resources. 76,77 Natural and waste materials of agriculture and industries are extensively being modied and converted into activated carbons aimed at heavy metals cleansing from wastewater. 75 For instance, Fig. 7 demonstrates the application of activated carbons as adsorbents derived from agricultural wastes for water and wastewater treatment. An effective adsorbent should have a wide adsorption capability, a fast rate of sorption, be easy to regenerate or retrieve from water, have high porosity, and have a small pore diameter, because adsorption ability is proportional to the amount of surface area available. 78 Typical adsorption properties of these materials are derived polymers complexes such as lignin, simple sugars, hemicellulose, proteins, lipids, and starch with diverse functional groups. These constituents play a vital role in the adsorption of heavy metal ions as they can form complexes and chelates with heavy metal ions. They are capable of replacing hydrogen ions with hazardous heavy metal ions dissolved in the aqueous media or donating a pair of electrons to bind these metal ions. 79 The functional groups that are present in agricultural wastes include carbonyl, acetamido, phenolic, amido, carboxyl, amino, alcohols, esters, and sulphydryl. 80 It is these groups, which possess an affinity for complexation with metals. During adsorption studies, various studies have conrmed the existence of functional groups and the binding ability of these groups with metal ions through spectroscopy analysis. [81][82][83] Toxic metals elimination by dint of low-cost biosorbents is regarded as more promising and cost-effective since there are numerous natural resources available locally and plentifully, which could be used as low-cost biosorbents. 84 The development of agro-based materials into less expensive biosorbents is well acknowledged as a prospective and economical for the treatments of pollutants in water. The sources of agro-based biosorbents are presented in Fig. 8. The increasing number of studies or researches and publications on the adsorption Fig. 7 Application of low-cost activated carbons derived from agricultural wastes for environmental remediation (this figure has been adapted from Heidarinejad et al. 85 with permission from Springer, copyright 2020). process (as shown in Fig. 9) using low-cost adsorbents conclude and recommend the increasing interest in the search for more low-cost adsorbents that are suitable for removing pollutants. As a result, several studies have reported the uses of agro-based adsorbents in eliminating heavy metals from water, wastewater, or effluents. Some of these studies are summarized in Table 2.

Elimination of Pb(II) ions using low-cost adsorbents
Bayuo et al. 86 studied the optimization of adsorption factors to eliminate lead(II) from an aqueous medium by groundnut husk. The inuences of three factors (contact time, pH, and initial metal Pb(II) concentration) on two responses (removal efficiency and adsorption capacity) were explored by Central Composite Design. The optimal operative conditions for Pb(II) removal were established at 90 min contact time, pH of 8 and 75 mg L À1 Pb(II) initial concentration by a 0.966 desirability. At this operating condition, the optimal percent removal of Pb(II) and adsorption capacities were 90.26% and 3.428 mg g À1 respectively. The equilibrium data agreed with the Langmuir isotherm and pseudo-second-order kinetic model. Hydroxyapatite nanostructures were synthesized by microwave irradiation from eggshells by Safatian et al. 87 This nano adsorbent was applied to eliminate lead(II) from wastewater. The impacts of solution pH, contact time, dosage, Pb(II) initial concentration, and temperature on the elimination of lead(II) were examined revealing Pb(II) depollution dependence on these parameters. Langmuir and Freundlich's models were tried and the results show good tness to Langmuir isotherm. The synthesized nano adsorbent displays great adsorption abilities for removing lead(II) from water media.
Mustapha et al. 88 produced a biosorbent using Albizia lebbeck pods to remove Pb(II), Cd(II), Zn(II), and Cu(II) ions from aqueous media. The inuences of many parameters including solution pH, contact time, dosage, initial concentration, and temperature were studied by batch mode. Equilibrium and kinetic experiments were conducted as well as change in enthalpy entropy and free energy was determined. The study conrmed that Albizia lebbeck pods are capable of removing metals from the aqueous environment. The experimental data tted the Langmuir model with a high correlation coefficient (R 2 > 0.94); however, the kinetic data obeyed the pseudo-second-order model. The adsorption thermodynamic variables attested that the biosorption of these metals was endothermic, nonspontaneous, and spontaneous at lower and higher temperatures respectively.
Boulaiche et al. 89 reported the use of Posidonia oceanica in removing Pb(II), Cd(II), Ni(II), Cu(II), and Zn(II) using the batch procedure. The inuence of pH, contact time, initial metal ion concentration, and the temperature was considered. Sorption kinetics and isotherm models were veried for the biosorption of the metals. The concentrations of the metals adsorbed were found to be 48.33, 43.9, 41.02, 37.90, and 30.22 mg g À1 for Pb(II), Cu(II), Ni(II), Zn(II), and Cd(II), respectively. The biosorption kinetic data conformed to the pseudo-second-order model while the isotherm equilibrium data rightly tted Langmuir models for all the metals investigated. The biosorption of Pb(II) was endothermic and spontaneous as suggested by the thermodynamic variables evaluated.
A novel natural adsorbent obtained from thorns was utilized by Alatabe and Kariem 90 to remove lead(II) from wastewater. The inuence of diverse factors (pH, contact time, initial concentration, and different electrolytes) was investigated by batch technique to optimize conditions for which optimal biosorption can be achieved. The experimental data were scrutinized with isotherm and kinetic equations. The study revealed that the adsorbent has a high ability to eliminate Pb(II) and at a pH of 6, optimum biosorption was accomplished. The removal concentration was dependent on the time of equilibrium. At higher temperatures of the isotherm studies, Langmuir model was obeyed. The Pb(II) ions adsorption was inuenced by raising electrolyte concentrations. According to the experimental data, the pseudo-second-order model illustrates better applicability indicating chemical sorption. The thermodynamic data point out Pb(II) depollution to be endothermic and spontaneous.
Manjuladevi et al. 71 examined Pb(II), Ni(II), Cr(VI), and Cd(II) eradication from aqueous media using Cucumis melo peelactivated carbon (CMAC). The biosorption characteristics of  Bagasse is agro-based biomass that was employed by Poonam et al. 91 for investigating its possibility in removing lead(II) from effluents of the battery industry. Kinetics and batch experiments comprising several factors (pH, contact time, and dosage) were investigated using biochar produced from the bagasse waste. The maximum removal was attained at a pH of 5, 140 min contact time, and room temperature (25 AE 3 C) using 5 g biochar. At the optimum pH of 5, the optimum percent removal and the amount of Pb(II) adsorbed were 75.38% and 12.741 mg g À1 correspondingly. The equilibrium data were well represented by Langmuir suggesting monolayer and homogenous biosorption of Pb(II). The pseudo-second-order model indicates conformity to the experimental data implying chemisorption.
Mahdi et al. 92 studied lead(II) removal from aqueous media onto biochars obtained from the biomass of date seed. The impacts of solution pH, contact time, and initial Pb(II) concentration were conducted. Aqueous solution pH exhibited an inuence on the biosorption capability of the biochar and maximum removal occurred at a pH of 6.0. The equilibrium data were effectively described by both Freundlich and Langmuir isotherms. The experimental data conformed well to the pseudo-second-order kinetic model. The study indicated that date seed-based biochar has high removal potential for Pb(II) in comparison to other plant-derived biochars.
Gaur et al. 93 examined the ability of soya bean as a biosorbent in the purication of lead (Pb) and arsenic (As) in wastewater. Batch tests were conducted and the inuence of some biosorption parameters (pH, contact time, adsorbent dose, and temperature) was determined. It was found that the maximum elimination of Pb(II) and As(III) was achieved at 37 C and 3.0 g per 100 mL. At a pH of 2.0 and 4.0 AE 0.26, maximum decontamination of As(III) and Pb(II) were attained respectively at a shaking time of 60 min. Langmuir and Freundlich's models were tested and Langmuir suitably t equilibrium data. Thermodynamics studies suggest the biosorption of Pb(II) using the soya bean biosorbent was exothermic. Ferreira et al. 94 studied the application of a modied cashew nutshell (Anacardium occidentale) (CNS) in the depollution of Pb(II), Cd(II), and Cr(III) from an aqueous environment. Batch tests were conducted to determine the optimal conditions of adsorption parameters. The cashew nutshell displayed high decontamination of Pb(II), Cd(II), and Cr(III). The study discovered that the maximum biosorption took place at a pH of 5.0, 4 g of the biosorbent, and a contact time of 40 min. The equilibrium data demonstrated a good agreement with Langmuir and Freundlich isotherms suggestive of the biosorption taking place on mono and multilayers.
Klapiszewski et al. 95 combined lignin and silica to produce a multifunctional adsorbent with substantial sorption abilities. Batch experiments were conducted using silica/lignin hybrid in eliminating lead(II) from water media. It was found that the efficiency of the biosorption rest on removal time, pH, and adsorbent dosage with a maximum adsorption capacity of 89.02 mg g À1 . The experimental data of lead(II) obtained by SiO 2 /lignin biosorbent show the best t for Langmuir and pseudo-second-order models.
Ghogomu et al. 96 utilized activated carbons produced from Raphia hookeri fruit epicarps using ZnCl 2 (CAZn), H 3 PO 4 (CAH), and KOH (CAK) as activation agents to remove lead(II) from wastewater. Batch tests were performed to determine the impacts of contact time, solution pH, dosage, and initial Pb(II) concentration at 27 C. The results disclosed that optimum removal of Pb(II) occurred at 80 min contact time for CAZn and CAH and 100 min for CAK, pH of 6 for all samples with biosorption capacities of 66.37 mg g À1 for CAZn, 56.30 mg g À1 for CAH and 28.0 mg g À1 for CAK, respectively. The experimental data illustrate a good t using the Langmuir model with monolayer biosorption capacities of 72.62, 58.62, and 12.16 mg g À1 for CAZn, CAH, and CAK correspondingly. The kinetic investigations have shown that the biosorption system ts the pseudo-second-order model. Raphia hookeri based activated carbons (CAZn and CAH) were revealed to be capable of removing lead(II) from an aqueous environment.
Nnaji and Emefu 97 conducted experiments to investigate lead(II) decontamination using different mesh sizes of activated sawdust of two species of timber. The best particle sizes were 0.85 and 1.18 mm for Khaya ivorensis and Pycanthus angolensis, respectively. The equilibrium data were applied to isotherm and kinetic models. The biosorption of lead(II) using Khaya ivorensis and Pycanthus angolensis followed Langmuir and Freundlich models while experimental data conformed to the pseudo-rstorder model.
Nordine et al. 98 studied lead(II) elimination from synthetic wastewater using natural materials such as pine, beech, and r sawdust. Several biosorption variables (pH, sawdust dosage, contact time, initial Pb(II) concentration, stirring speed, and temperature) were investigated. The study established that the pine sawdust show higher lead(II) removal (15.5 mg g À1 ) at pH of 5.45 AE 0.05, 10 g L À1 of pine sawdust, initial Pb(II) concentration of 100 mg L À1 , and 23 AE 2 C temperature. The Langmuir model showed a good representation of the experimental data describing the Pb(II) biosorption mechanism as a monolayer.
Yahaya and Akinlabi 99 investigated cocoa pod husk in lead(II) decontamination. The adsorbent was activated chemically by treating it with dilute HNO 3 for 24 hours. The adsorbent was modied with 7% (v/v) sulphuric acid (H 2 SO 4 ) for 90 minutes. An additional modication of the adsorbent was done using 0.3 dm 3 of pyridine and 5.67 g EDTA under reux for 24 hours. At the maximum time of 50 min, the removal percentages of lead were 94.6% and 98.2%, respectively for the original and improved cocoa pod husk. The best-tted model was pseudosecond-order with a high correlation coefficient ranging from 0.9776 to 1.0000. From the thermodynamic studies, the adsorption process was feasible, increase in randomness and endothermic.
Activated carbon produced using sugarcane bagasse (SCBA) and its possibility to adsorb lead(II) from synthetic water was studied by Kane et al. 100 Batch mode trials were conducted to probe the impacts of pH and dosage of SCBA on the biosorption of Pb(II). The batch results demonstrated Pb(II) biosorption dependence on the pH and SCBA dosage. The optimum depollution of Pb(II) was obtained at pH 5.0 and 10 g L À1 SCBA dose with a removal efficiency of 87.3%. The isotherm data indicated good agreement with the Langmuir model and the maximal biosorption capacity determined from the model was 23.4 mg g À1 .
Khandanlou et al. 5 prepared nanocomposites (RS/Fe 3 O 4 -NCs) using rice straw to eliminate Pb(II) and Cu(II) from water. The inuences of three parameters, removal time, adsorbent dose, and initial ion concentration were examined on Pb(II) and Cu(II) removal possibilities. The optimum removal conditions of Pb(II) and Cu(II) were found at removal times of 41.96 and 59.35 min; initial ion concentrations of 100 and 60 mg L À1 and 0.13 g of the biosorbent for both ions, respectively. The optimum percent removal of Pb(II) and Cu(II) was attained at 96.25% and 75.54%, respectively. The isotherm and kinetic data obeyed the Langmuir isotherm and pseudo-second-order models for both ions.
Shi et al. 101 investigated heavy metals [Pb(II), Co(II), and Cu(II)] elimination in water media using arborvitae leaves. Single and competitive component absorption experiments were performed and the adsorption capability of the leaf was found to be dependent on the pore diameter and surface. The optimal uptake capacities of Pb(II), Co(II), and Cu(II) at an equilibrium time of 120 min were 7.64, 1.84, and 3.38 mg g À1 , respectively. The single-component adsorption showed that the decontamination of the ions depended on increasing order of electronegativity, Pb (2.33) > Cu (1.90) > Co (1.88). Conversely, the ternary component biosorption process revealed that the uptake of Pb(II), Co(II), and Cu(II) ions onto the biosorbent declined from 35.84 to 9.32 mg g À1 , 6.78 to 1.54 mg g À1 , and 7.94 to 3.07 mg g À1 , respectively. At pH 5.5, maximum decontamination of the ions was achieved and the equilibrium data correlated well with the Langmuir model.
Paliulis 102 conducted a study using peat to remove Pb(II) from aqueous solutions. The study revealed an increase in the uptake capability of peat with the rising contact time, initial concentration of Pb(II) and was remarkably affected at a pH range of 4.0-7.0. At pH of 6.0, a contact time of 360 min and Pb(II) initial concentration of 100 mg L À1 , maximum Pb(II) biosorption (9.489 mg g À1 ) was achieved by the peat. The Langmuir and Freundlich isotherms were employed in the analysis of the biosorption properties of Pb(II) with Freundlich being the best t model.
Misihairabgwi et al. 103 studied heavy metals decontamination from water media utilizing activated carbon obtained from agroforestry wastes including rice husk, baobab shells, pigeon pea husk, macadamia nut shells, marula stones, and Moringa oleifera husks. The uptake capacity was increased when the pH was raised from 4.0 to 6.0 but at a pH of 6.0, 60% uptake of Hg(II) was achieved for all the activated carbons. The elimination of Pb(II) was observed to increase by 22% in removal efficiency in comparison to activated carbons of Moringa oleifera husks, pigeon pea husk, baobab shells, and marula stones. The well-correlated isotherm was the Langmuir model and the adsorbents with the best adsorption capacity were baobab shells and pigeon pea husk as compared to the others.
A chemically modied adsorbent derived from okra was explored in removing toxic metals [Pb(II), Cd(II), Cu(II), and Zn(II)] from synthetic wastewater. 104 Batch technique tests were used to determine the impact of removal time, temperature, pH, and metal particle xation. The experimental data conformed to Dubinin-Radushkevich and Langmuir isotherms. The values of DH 0 and DG 0 obtained from the thermodynamic study showed that the metals uptake on the chemically modied cellulosic biopolymer prepared from okro is unrestricted and exothermic.
Ogunleye et al. 105 explored banana stalk usage in the decontamination of Pb(II) ions from wastewater. Phosphoric acid (H 3 PO 4 ) was used to modify the adsorbent at 1 : 1 (w/v). Besides, the mixture was dehydrated at 105 C for 12 hours and further at 800 C without air for 2 hours leading to the occurrence of carbonization. A 0.1 M HCl was applied to treat the carbonized biosorbent for 1 h. At 120 min equilibrium time, the optimum percentage removal was 97.9% however; at a pH of 8.0 and 250 mg L À1 Pb(II) initial concentration, the optimum capacity attained was 200 mg g À1 . The best kinetic model applied in analyzing the kinetic data was pseudo-second-order while the isotherm model that depicted good agreement to the equilibrium data was the Langmuir model.
Hikmat et al. 106 examined Pb(II) biosorption by absorbents obtained from agro-based materials including petiole and palm tree ber. The uptake of Pb(II) unto the bio-sorbents exhibited dependence on shaking time, pH, biosorbent weight, and Pb(II) initial concentration. The isotherm data best tted Langmuir than that of the Freundlich with a high correlation coefficient of 0.9801 and 0.9974 for ber and petiole correspondingly. The kinetic data indicated that the pseudo-rst-order tted well with high coefficients of determination for ber (0.9773) and petiole (0.9720), respectively.
The potential of natural bentonite and activated carbon for Pb(II) depollution from water media was studied by Yarkandi. 107 The study revealed that the amount of Pb(II) removed upsurges with pH, contact time, and initial concentration but declines with adsorbent dose and temperature. The pseudo-secondorder was the best tted kinetic model while both Langmuir and Freundlich isotherms show good conformity to equilibrium data. Adsorption thermodynamic parameters suggested endothermic, spontaneous, and increase in randomness. The study concluded that bentonite is more efficient in removing Pb(II) than activated carbon.
Vieira et al. 108 conducted kinetic studies on Pb(II) and Cu(II) ions utilizing rice husk ash (RHA) in a xed column bed. The study has shown that RHA could be applied as a cost-effective biosorbent for eliminating Pb(II) and Cu(II) ions from effluents. The optimal removal capacities attained for Pb(II) and Cu(II) were 0.0561 and 0.0682 mmol g À1 respectively at a temperature of 20 C. Biosorption isotherm data were wellrepresented using Langmuir isotherm. The thermodynamic parameters evaluated showed exothermic and spontaneous adsorption of Pb(II) whereas the adsorption system of Cu(II) was endothermic and spontaneous.
Chandrasekaran and Jeyakumar 4 prepared activated carbons using green Ulva fasciata sp. to decontaminate Pb(II) ions from the marine. The impact in the variation of parameters such as pH, contact time, Pb(II) concentration, and biosorbent dose was investigated. Isotherm and kinetic models have been employed and the study indicated that the Langmuir isotherm and pseudo-second-order as the best t biosorption models.
The depollution of Pb(II) from the water was investigated in an acidic medium (pH ¼ 4) by Kouakou et al. 109 using commercial activated carbons produced from wood and coconut shells. The study disclosed that the powdered activated carbon obtained from the wood showed a remarkably high uptake capability than the activated carbon of coconut shells. The powdered and granular activated carbons exhibited diverse sorption abilities. At 25 C, the maximum Pb(II) adsorbed on powdered, 830-granular, and 123-granular activated carbons were 44.58, 38.96, and 39.06 mg g À1 , respectively. Langmuir's model showed a good representation of the isotherm data.
Hikmat et al. 106 examined Pb(II) elimination from the aqueous environment using leaf and ber of a palm tree. The biosorption factors indicated that the removal time for Pb(II) was a bit fast and equilibrium was accomplished at approximately 30 and 40 min by the leaf and ber, respectively. At a pH range of 6.5-7.0, optimal Pb(II) adsorption was attained using 0.5 g of the adsorbent. However, at high temperatures, Pb(II) removal by the adsorbents declined to signify exothermic biosorption. The isotherm and kinetic data of Pb(II) obtained using the two biosorbents conformed to Langmuir and pseudo-rstorder than other models. It was found that the adsorption system was exothermic, feasible, and spontaneous.
Ghasemi and Gholami 110 studied Pb(II) elimination from synthetic water making use of Myrtaceae sawdust. The inuence of pH, contact time, dosage, and initial Pb(II) concentration was investigated using the batch technique. At a pH of 7.0, with the contact time of 30 min and 10 g L À1 dosage, the maximal Table 3 Adsorption potential of some agricultural bioadsorbents for Pb(II) ions removal from aqueous solution Pseudosecondorder 1.000  111 reported Pb(II) sorption from aqueous media using agar beads. The impact of solution pH, biomass dosage, contact time, and initial lead(II) concentration was examined. The study recommended agar beads for the decontamination of Pb(II) from aqueous solutions. The experimental results have shown that biosorption equilibrium was reached below 120 min at a pH of 5.0. The Freundlich isotherm was found to show best t the isotherm data.
Haloi and Chakravarty 112 conducted a study to examine the ability of heartwood charcoal of Areca catechu (HCAC) in depolluting Pb(II) ions from water. Batch tests were piloted to nd out the biosorption characteristics at varied pH, contact times, adsorbent doses, and initial concentrations. At an equilibration time of 25 min, optimal biosorption of Pb(II) was attained at a pH of 5.0 and 0.5 g biomass dosage. The experimental data was found to be well-tted to the Freundlich isotherm model implying that the HCAC surface was heterogeneous while the kinetic data was better described by the pseudo-second-order model.
Athar et al. 113 considered biomass obtained from Trifolium resupinatum in removing Pb(II). The inuences of several factors for example removal time, initial Pb(II) concentration and pH have been explored. The study discovered that 25 min and 3.0 as optimum removal time and pH, respectively. Also, increasing the biomass dosage increased the binding aptitude of the biosorbent. The equilibrium data were analyzed with isotherm and kinetic models for which, the Langmuir and pseudo-secondorder models showed the best representation of the data.
Maize husk was utilized by Adeogun et al. 114 for efficient removal of Pb(II) and Mn(II) using a batch system. The study has shown that the elimination of Pb(II) and Mn(II) depended on pH, biomass dose, initial concentration, and temperature, respectively. The equilibrium data showed a good t to the Langmuir isotherm with 7.38 and 8.52 mg g À1 maximum sorption capacities for Pb(II) and Mn(II) correspondingly on the raw biomass. However, maximum sorption capacities obtained by acid-modied biomass were 9.33 and 9.00 mg g À1 for Pb(II) and Mn(II), respectively. The adsorption thermodynamics revealed that Pb(II) and Mn(II) biosorption onto maize husk is feasible, spontaneous, and exothermic.
Azouaou et al. 115 examined Pb(II) removal from synthetic by using untreated orange barks. The impacts of initial pH, dosage, removal time, and initial Pb(II) concentration were considered. It was advocated that the orange barks are efficient and economical biosorbent in eliminating Pb(II). The study indicated that sorption equilibrium was realized in <30 min. The analysis of the equilibrium data suggested that the Langmuir isotherm correlated best.
Mitic-Stojanovic et al. 116 studied Pb(II), Cd(II), and Zn(II) ions decontamination from water using Lagenaria vulgaris shell. The study employed batch experiments and the impacts of contact time, initial pH, temperature, and stirring speed were investigated. The biosorption of the metals ions considered was rapid,  attaining equilibrium aer around 5 to 10 min. The biosorption showed dependence on pH with optimal pH ranging at 4.5-6.0. The inuence of temperature established that the biosorption of the metal is a chemical process. Qaiser et al. 117 utilized groundnut hulls in the decontamination of Pb(II) from wastewater. Batch experiments were piloted to determine the inuence of biosorption factors including pH, initial Pb(II) concentration, and temperature. At an optimal pH 5 AE 0.1 and 20 AE 2 C temperature, the optimum Pb(II) biosorption was 31.54 AE 0.63 mg g À1 . The equilibrium data was better described by the Langmuir model with a coefficient of determination >0.98. The kinetics of biosorption followed the pseudo-second-order kinetic model. Bhattacharyya and Sharma 118 conducted a study to remove Pb(II) ion from water media by well-developed Azadirachta indica leaves. The biosorbent gave a maximum removal of 93% with a 300 min contact time. The pseudo-second-order model was the well-tted than the pseudo-rst-order model while the two isotherms (Langmuir and Freundlich) agreed better to the experimental data with a coefficient of determination of 0.99.
Abdulkarim and Al-Rub 119 produced activated carbon and chemically modied activated carbon from date pits to remove Pb(II) from aqueous media. The variation of different factors was considered. The study indicated that both carbons exhibited high Pb(II) uptake. Lead(II) biosorption was found to rise by increasing pH and initial Pb(II) concentration until at pH of 5.2 where maximum elimination of Pb(II) was attained. The Langmuir and Freundlich isotherm models obeyed the experimental data using both biosorbents while the kinetic data followed the pseudo-second-order equation.
Wong et al. 120 examined lead (Pb) elimination using batch experiments by tartaric acid modied rice husk. The adsorption system showed dependence on pH and demonstrated characteristics of exothermal adsorption. The pseudo-second-order model showed a good relationship with the equilibrium data than that of the pseudo-rst-order model.
The adsorption potential of some agricultural adsorbents in removing Pb(II) ions from an aqueous solution is summarized in Table 3.

Adsorption kinetic studies
Adsorption kinetic modeling is conducted to determine the rate of biosorption and expressions for a certain reaction. Adsorption kinetics are signicant in the treatment of wastewater as it provides valued information on the reaction pathways and in the mechanism of the adsorption process. 114 The adsorption kinetics has key parameters for describing the basic properties of a good adsorbent. It offers cherished information about the system and regulates the adsorbate reactions in the solid-solute interface at different times. 1,91 Many kinetic models had been applied in the elimination of pollutants in water and wastewater including pseudo-rst-order, pseudo-second-order, Elovich, and intraparticle diffusion models.
For the present review, the linearized adsorption kinetic models commonly utilized in the elimination of Pb(II) from water and wastewater are presented in Table 4. Table 4 Linearized adsorption kinetics model equations  Table 5 Linearized adsorption isotherm model equations

Adsorption isotherm studies
Adsorption isotherms are investigated to describe the adsorption system. Adsorption isotherm models signify the link between the concentration of adsorbate adsorbed on the biosorbent and the concentration of adsorbate retained in the adsorption medium. 86,113,121 Many isotherm models are frequently used to t the adsorption equilibrium data to get a linear regression for the prediction of the maximal sorption capacity of the employed adsorbent. 91 In most of the numerous studies reviewed, the adsorption processes were justied by applying linear forms of one-parameter adsorption isotherm model (Henry), twoparameter adsorption isotherm models (Langmuir, Freundlich, Dubinin-Radushkevich, Temkin, Harkin-Jura, and Elovich), and three-parameter adsorption isotherm models (Redlich-Peterson and Jossens) as summarized in Table 5.

Conclusions
The review of the various studies on Pb(II) biosorption indicates that low-cost biosorbents are eco-friendly, cost-effective, and simple techniques for water and wastewater treatment containing lead(II) ions. Pb(II) biosorption with these low-cost biosorbents was found to show dependence on several adsorption factors such as pH, contact time, adsorbent dose, initial Pb(II) concentration, and temperature. Furthermore, the efficiency of the biosorption procedures was subjected to physicochemical characteristics of the biomass used as the biosorbent. In most of the studies, batch adsorption data have been modeled by determining the kinetic, isothermal, and thermodynamic parameters to elucidate the mechanism of the adsorption system. Most of the studies on the adsorptive removal of Pb(II) were found to follow the pseudo-second kinetic and Langmuir isotherm models with the thermodynamics variables suggesting the feasibility and spontaneous nature of Pb(II) sequestration. However, gaps exist to increase biosorption ability, economic feasibility, optimization of the biosorption system, and desorption and regeneration of the used agricultural biosorbents.

Future perspectives
Various laboratory-scale investigations have demonstrated that the capability of unconventional biosorbents in terms of effectiveness for the removal of Pb(II) from aqueous systems is extremely high. However, because of the lack of condence in the engineering of these materials and practicality, they are not utilized on an industrial scale. Therefore, it is critical to reconsider multidimensional and multidisciplinary research approaches in biosorbent production for commercial and industrial applications in the near future to minimize metal burdens in aquatic systems.
Another consideration is that industrial effluents do not contain a single contaminant, but rather a large mixture of heterogeneous contaminants that may have a larger inuence on adsorption systems and selectivity. As a result, signicant measures should be taken to develop and apply novel solid materials, such as biomass, synthetic nanomaterials, and chitosan as low-cost biosorbents, for the decontamination of various toxic heavy metals from water systems. Therefore, deeper studies into the adsorption mechanisms are required, with a particular emphasis on the simultaneous detoxication of multiple heavy metals ions using cheap and single adsorbents. Besides, future research should look into different combinations of agro-industrial wastes and by-products that can effectively remove heavy metal ions from aqueous media.
The literature review also uncovered a current trend in this area of study, with a signicant shi toward adopting more sustainable techniques and new technologies that prioritize sustainability over relying solely on the efficiency and performance of the technology in treating wastewater containing heavy metals ions. Finding the right balance among heavy metals decontamination and recovery would be one of those sustainability issues. As a result of this development, the attention and motivation have shied away from simple wastewater treatment to the retrieval and recycling of heavy metals as well as the spent biosorbents for further use. Therefore, to avoid further environmental pollution, other feasible alternatives and dependable procedures should be employed for heavy metals recovery, regeneration of exhausted biosorbents, and safe disposal of spent biosorbents.

Conflicts of interest
The authors declare that there is no conict of interest.