**3. Removal of heavy metal**

A significant deal of interest has been focused in the research for the removal of heavy metals from industrial effluent using agricultural by-products as bio-adsorbents. The use of agro waste in bioremediation of heavy metal ions, i.e., biosorption utilizes inactive (nonliving) microbial biomass to bind and aggregates heavy metals from waste water by physicochemical pathways (mainly chelation and adsorption) of uptake [98]. Agro waste such as hazelnut shell, rice husk, pecan shells, jackfruit, maize cob, or husk can be used as bioadsorbent for heavy metal removal after chemical modification or conversion of these agro wastes into activated carbon.. Orange peel was employed for Ni(II) removal from simulated wastewater and was found maximum metal removal occurred at pH 6.0 [99]. Coconut shell charcoal (CSC) modified with oxidizing agents and/or chitosan was used for Cr(VI) removal was investigated well by Babel and Kurniawan [100]. Further, Cu(II) and Zn(II) were removed from real wastewater using pecan-shells-activated carbon [101] and potato peels charcoal [102]. The Cr(VI) removal from an aqueous solution by rice-huskactivated carbon has been studied extensively [103]. It was found that the maximum metal removal by rice husk took place at pH 2.0. Rice husk, containing cellulose, lignin, carbohydrate, and silica, was investigated for Cr(VI) removal from simulated solution [104]. To enhance its metal removal, the adsorbent was modified with ethylenediamine. The maximum Cr(VI) adsorption of 23.4 mg/g was reported to take place at pH 2. Other types of biosorbents, such as the biomass of marine dried green alga (biological materials) [21–25], were investigated for uptake of some heavy metals from aqueous solution. Some of the used alga wastes were *Spirogyra* species [105], Ecklonia maxima [106], *Ulva lactuca* [107], *Oedogonium* sp. and *Nostoc* sp. [108], and brown alga *Fucs serratus* [109]. On the whole, an acidic pH ranging 2–6 is effective for metal removal by adsorbents from biological wastes. The mechanism of uptaking heavy metal ions can take place by metabolism-independent metal binding to the cell walls and external surfaces [110]. This involves adsorption processes such as ionic, chemical, and physical adsorption. A variety of ligands located on the fungal walls are known to be involved in metal chelation. These include carboxyl, amine, hydroxyl,

phosphate, and sulfhydryl groups. Metal ions could be adsorbed by complexing with negatively charged reaction sites on the cell surface shows the adsorption capacities of different biosorbents. Several studies have demonstrated the ability of rice husk to remove heavy metals from water sources. A study of the removal efficiencies of nine different heavy metals using rice husk observed maximum adsorption capacities ranging from 5.5 to 58.1 mg/g, with the values increasing in the following order: Ni(II) < Zn(II) < Cd(II) < Mn(II) < Co(II) < Cu(II) < Hg(II) < Pb(II) [111]. The rice straw and rice bran have been shown to remove Cu(II) with maximum adsorption capacities of 18.4 and 21.0 mg/g, respectively [112]. In a study on the use of rice husks for the adsorption of Cr(VI), significant removal (>95%) occurred in the case of low pH (<3.0), primarily due to the speciation of the Cr(VI) ions [113]. Bansal et al. [114] evaluated the removal of Cr(VI) using rice husk and achieved a maximum adsorption capacity of 8.5 mg/g; they also found that treating rice husk with formaldehyde enhanced removal by approximately 23%. Another study used phosphate-treated rice husk to evaluate the removal of Cd(II) from wastewater and achieved a high maximum adsorption capacity (103 mg/g at 20°C) [115]. Residuals from peanuts were also found to be an effective adsorbent for the removal of heavy metals. A maximum adsorption capacity of 39 mg/g was achieved for the removal of Pb(II) using peanut shells; significant removal was observed at various temperatures and pH conditions [116]. Peanut shells were also shown to removal of Cr(VI) at low pH values, achieving a maximum adsorption capacity of 4.3 mg/g [117]. Moreover, researchers achieved effective removal of Cr(III) and Cu(II) using peanut shells with maximum adsorption capacities of 27.9 and 25.4 mg/g, respectively [118]. Researchers also observed significant heavy metal removal with peanut husks, achieving maximum adsorption capacities of 7.7, 10.2, and 29.1 mg/g for Cr(III), Cu(II), and Pb(II), respectively [119]. Peanut hull, which is an abundant agricultural by-product, has also been shown to remove Cu(II) with a maximum adsorption capacity of 21.3 mg/g [120]. Wastes from other nuts have also been shown to remove heavy metals from different water sources. Several studies have investigated the ability of cashew nut shells to remove heavy metals from aqueous solutions. When evaluating the removal of Cu(II), researchers achieved significant removal (>85%) and a maximum adsorption capacity of 20 mg/g with cashew nut shells [121]. Another study evaluated the removal of Ni(II) using cashew nut shells and achieved 60–75% and a maximum adsorption capacity of 18.9 mg/g [122]. The removal of these heavy metals using cashew nut shells has been attributed primarily to its high surface area, which allows for significant number of active sites for adsorption to occur [121, 122]. Sunflower-derived adsorbents were efficiently applied against heavy in water [123]. The activated carbons generated, from chickpea (Cicer arietinum) husks by chemical treatment with KOH and K2CO3, efficiently removed heavy metals from aqueous solutions [37]. Pistachio hull waste also demonstrated significant removal (>98%) of Cr(VI) from various water sources, achieving a maximum adsorption capacity of 116.3 mg/g [124]. The high adsorption capacity of Cr(VI) by pistachio hull waste was attributed to the electrostatic attraction, as well as binding to various functional groups on the surface of the adsorbent [124]. Another study investigated the use of pecan shells to remove Cu(II), Pb(II), and Zn(II) by utilizing a variety of modification techniques to enhance removal, including acid, steam, and carbon dioxide activation [101]. In this study, Pb(II) was removed at the highest rate, followed by Cu(II) and Zn(II), for each type of modified pecan shell, with maximum adsorption observed for acid-activated pecan shells [101]. Tangerine peel can be used as a potential adsorbent of heavy metal ions, such as Cd, Co, Cr, Cu, Mn, Ni, Pb, and Zn, from aqueous solution [125]. Almond shells also

### *Perspective Chapter: Environmental-Friendly Agro Waste Management DOI: http://dx.doi.org/10.5772/intechopen.107505*

demonstrated approximately 20–40% removal of Cr(VI) when adjusting the pH and the adsorbent dose in the solution [126]. Hazelnut shells also demonstrated effective removal of Cu(II), achieving a maximum adsorption capacity of 58.3 mg/g [127]. Groundnut shells were also used as an adsorbent in the removal of heavy metals [128]. Shukla and Pai achieved maximum adsorption capacities of 4.9, 8.05, and 11.0 mg/g for Cu(II), Ni(II), and Zn(II), respectively, with groundnut shells. These adsorption capacities were also enhanced by 40–70% with chemical modifications to the groundnut shells using reactive dye. Various fruit wastes have been shown to effectively remove heavy metals from aqueous solutions. For instance, lemon peel was shown to effectively remove Zn(II), Pb(II), Cd(II), Cu(II), and Ni(II), achieving maximum adsorption capacities of 27.9, 37.9, 54.6, 71.0, and 80.0 mg/g, respectively [129]. Orange peel also demonstrated effective heavy metal removal in a variety of studies. Ajmal et al. [99] achieved significant removal of Ni(II) (97.5%) with orange peel, along with lower removal efficiencies of Cu(II), Pb(II), Zn(II), and Cr(VI). Thirumavalavan et al. [46] investigated the adsorption of Cd(II), Cu(II), Ni(II), Pb(II), and Zn(II) with orange peel and demonstrated significant removal, achieving maximum adsorption capacities of 41.8, 63.3, 81.3, 27.1, and 24.1 mg/g, respectively. The biochars derived from agricultural wastes were utilized to remove Cd(II) and Cu(II) from aqueous [130]. Lucerne biochar had the highest Langmuir sorption capacity of Cd(II) (6.28 mg/g), and vetch-derived biochar had the highest Cu(II) sorption capacity (18.0 mg/g) at pH 5.5. Another study demonstrated similar removal of Pb(II) using orange peel, achieving a maximum adsorption capacity of 27.9 mg/g [131]. Annadurai et al. [132] also achieved much lower removal of five different heavy metals using orange peel with maximum adsorption capacities ranging from 1.9 to 7.8 mg/g in the following order of adsorption: Pb(II) > Ni(II) > Zn(II) > Cu(II) > Co(II). Significant removal of Cd(II), Cu(II), Pb(II), and Ni(II) was also achieved with chemically modified orange peel with maximum adsorption capacities of 293, 289, 476, and 162 mg/g, respectively [133, 134]. Banana peel also exhibited varying degrees of heavy metal removal in aqueous solution. Thirumavalavan et al. [129] demonstrated significant removal of a variety of heavy metals, achieving maximum adsorption capacities of 21.9, 25.9, 34.1, 52.4, and 54.4 mg/g for Zn(II), Pb(II), Cd(II), Cu(II), and Ni(II), respectively. A study conducted by DeMessie et al. [135] achieved a maximum adsorption capacity of 7.4 mg/g for Cu(II) using banana peel, which increased to 38.3 and 38.4 mg/g after the banana peel was pyrolyzed at 500 and 600°C, respectively. Banana peel, watermelon peel, and grape waste reported to be the most efficient adsorbents for the removal of heavy metal from wastewater over pH 2.0 and 5.5 [136]. Melia et al. [137] had investigated over agricultural wastes and by-products (AWBs) from grape, wheat, barley, and flax production, to reduce the concentration of Cd in contaminated water. Another study observed relatively low removal for several heavy metals using banana peel, achieving maximum adsorption capacities ranging from 2.6 to 7.9 mg/gin the following order of adsorption: Pb(II) > N i(II) > Zn(II) > Cu(II) > Co(II) [132]. Grapefruit peel was also found to be an effective adsorbent for the removal of Cd(II) and Ni(II) from aqueous solution, achieving maximum adsorption capacities of 42.1 and 46.1 mg/g, respectively [138]. The adsorption onto the grapefruit peel was attributed to the ion-exchange mechanism and, to a lesser extent, complexation with –OH functional groups [138]. Grape stalk wastes have also demonstrated the ability to remove heavy metals, achieving maximum adsorption capacities of 10.1 and 10.6 mg/g for Cu(II) and Ni(II), respectively [139]. Other types of vegetable wastes have been shown to remove heavy metals from water source. Mushroom residues were shown to be effective in the removal of heavy

metals. Based on an evaluation of four different types of mushroom residues, removal efficiencies for Cu(II), Zn(II), and Hg(II) ranged from 39.7 to 81.7% [140]. Another study investigated the removal of Cd(II) and Pb(II) using three different mushrooms and achieved maximum adsorption capacities of 35.0 and 33.8 mg/g, respectively [141]. Corncob was also shown to remove heavy metals from aqueous solutions. When investigating its removal of Cd(II), researchers achieved a maximum adsorption capacity of 5.1 mg/g, along with an 4–10-fold increase in removal when the corncob was chemically modified using nitric and citric acid [142]. Moreover, corncob successfully removed Pb(II), with a maximum adsorption capacity of 16.2 mg/g. The adsorption capacity for the removal of Pb(II) using corncob increased significantly (43.4 mg/g) when the corncob was treated with sodium hydroxide. As summarized in **Table 2**, the different bioadsorbents made from different kinds of agro wastes such as orange peel, coconut shell, potato peel, rice waste, spirogyra, peanut shell, cashew nut shell, which are potentially used for the removal of various heavy metals including Cr(VI), Ni(II), Cu(II), Pb(II), Zn(II), etc. Nevertheless, the adsorption capacities


