**4. Biosorbents from agro and food waste**

Agricultural waste, pulp, peels and seeds of fruits and vegetables are the discarded waste material and due to its several properties it may have a wide range of application in the removal of heavy metal ions. Generally, the agricultural waste having cellulose shows a high metal biosorption capacity. The ingredients of agricultural waste such as proteins, lipids, lignin, hemicelluloses, starch, hydrocarbons, and functional groups facilitate metal complexation which helps in the removal of heavy metals [29]. Several studies reveal [20, 26, 30] that this low cost, readily available, renewable, efficient, and eco-friendly waste material seems to be a feasible option for the removal of toxic metal ions. In the past several years, research has been carried out for the eradication of toxic metal ions using several bioadsorbents that come from agro and food waste such as wheat bran, rice bran, coconut shells, wheat husk, rice husk, saw dust of various plant, maize corn cob, arjun nuts, black gram husk, sugar cane bagasse, coffee beans, apple peels, banana peels, orange peels, sugar beet pulp, and grapes stalks etc. [12, 14, 18, 28, 31]. They have been used either in their natural form or after some modification (may be physical or chemical).

#### **5. Biosorption mechanisms**

The mechanism of biosorption is a complex process which involves the binding of sorbate onto the biosorbent [17]. Various materials that are found natural including agricultural wastes can be used as biosorbents which involve the binding of metal ions by several mechanisms including chelation, complexation, ion exchange, chemisorption, reduction, precipitation, and adsorption on the surface and pores [32, 33]. **Figure 2** shows the different mechanisms involved in the biosorption phenomenon. Bisorbents especially made from agro and food waste contain several compounds

**133**

**5.1 Chelation**

**Figure 2.**

*Potential Use of Agro/Food Wastes as Biosorbents in the Removal of Heavy Metals*

such as starches, cellulose, simple sugar, hemicellulose, lignin, proteins, hydrocarbons and various functional groups such as carbonyl, amine, amide, sulfonate, carbonyl, phenolic, carboxyl groups, alcohols, and esters that can attract and isolate the metal ions. In the past several years many researchers have confirmed the presence of the aforementioned functional groups in the biosorbents and furthermore, they have also reported their complexation with heavy metals during the biosorption process [34, 35]. Some of the factors that help in controlling and characterizing the

It is a mechanism in which an organic complex agent (chelate) binds the metal ions at more than one place at a time in order to form a ring structure. The molecules on an organic compound that form these types of coordination are called as ligands and the ligands—metal association is referred as coordination complex [36]. An increase in the coordination complexes on mineral surfaces weakens the bond of the metal or cation to the crystal lattice resulting in the dissolution. Chelates form several binding with the metal ion at more than one place as compared to the complexes, therefore, chelates are more stable. In the past several years various studies have reported the application of this mechanism for the eradication of toxic metal ions from the waste water that was obtained from different sources [17]. For example, in one study, for the eradication of Cd (II) from the wastewater rice straw

mechanism of biosorption are given in **Figure 3**.

*Various mechanisms involved in bioadsorption process.*

has been used successfully as a potential biosorbent [37].

*DOI: http://dx.doi.org/10.5772/intechopen.94175*

*Potential Use of Agro/Food Wastes as Biosorbents in the Removal of Heavy Metals DOI: http://dx.doi.org/10.5772/intechopen.94175*

**Figure 2.** *Various mechanisms involved in bioadsorption process.*

such as starches, cellulose, simple sugar, hemicellulose, lignin, proteins, hydrocarbons and various functional groups such as carbonyl, amine, amide, sulfonate, carbonyl, phenolic, carboxyl groups, alcohols, and esters that can attract and isolate the metal ions. In the past several years many researchers have confirmed the presence of the aforementioned functional groups in the biosorbents and furthermore, they have also reported their complexation with heavy metals during the biosorption process [34, 35]. Some of the factors that help in controlling and characterizing the mechanism of biosorption are given in **Figure 3**.
