**5.3 Chemisorption**

Chemical adsorption is also known as chemisorption. It is a process of adsorption which involves a chemical reaction between the surface and the adsorbate. Mainly, it occurs when the adsorbate and adsorbent are attracted via a chemical bond or due to the chemical forces of attraction [39]. During the process of chemisorption only a single layer of adsorbate on adsorbent is formed and this process has high enthalpy of adsorption. With a rise in temperature, the rate of the process of chemisorption first increases and then decreases.

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

This process also plays a vital role in adsorption. During the biosorption process it exchanges the binary metal ions with the counter ions present on the surface of the biosorbent. Various systems that are readily available for the purification of water, work on the ion exchange mechanism. The process of ion exchange generally takes place either by cation or anion exchange. Amino groups are one of the good examples of anion exchangers while carboxyl groups represent the cation exchangers [28]. Several studies have reported the ion exchange mechanism of biosorption using various agro/food waste such as watermelon rind, rice straw etc. for the eradication of toxic metal ions such as chromium, Cu (II), Zn (II), Pb (II),

It is a process in which the metal ions present in the aqueous solution form precipitates with the functional groups that are present on the surface of the microbial cells due to which the metal ions remain intact with the microbial cell. Organic and inorganic metal precipitates are generally formed during the process of adsorption. Use of microbial cells forms the organic metal precipitates, that occur due to the excretion of extracellular polymeric substances. In several other cases, insoluble inorganic metal precipitates are also formed. Several studies have reported the involvement of surface precipitation mechanism using the husk of green tomato, soybean meal, and watermelon rind for the eradication of Cu (II), Cu (III), Pb (II),

Reduction is also an important mechanism of adsorption that plays a vital role in the biosorption of various heavy metals such as gold and palladium. During the process of reduction the metal connects with the functional group, gets reduced, and undergoes the growth of crystals. The metal that is reduced binds the biosorbent at various places. The eradication of numerous heavy metals such as chromium, gold, palladium etc. can be done easily by the process of reduction. For example, by using the process of biosorption, removal of Cr (VI) can be done

easily by reducing it into Cr (III) from the aqueous solution [17].

**6. Involvement of functional groups in adsorption process**

Several types of functional groups are generally involved in the process of adsorption namely, hydroxyl, amino and carboxyl groups [43]. These groups play an influential role in the metal adsorption process. The heavy metals are more efficiently absorbed by the phenolic, lactonic, and oxygen functional groups as compared to the other groups. During the formation of adsorbent the temperature and high degree carbonization majorly affects the mechanism of these functional groups but the other factors such as porosity, surface area, increase in pH, etc. do not alter the mechanism of these groups [28]. Several authors have confirmed the potential role of functional groups in the adsorption process by using the FTIR (Fourier Transform Infrared Spectroscopy) [44]. Low temperature pyrolysis retains the functional groups inside the sample whereas, an increase in temperature during the pyrolysis may lead to the loss of the functional group. For example,

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

**5.4 Ion exchange**

and cadmium [17, 40].

**5.5 Surface precipitation**

Zn (II), Fe and Mn [41, 42].

**5.6 Reduction**

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

#### **5.4 Ion exchange**

*Emerging Contaminants*

**5.2 Complexation**

*Factor affecting the mechanism of bioadsorption.*

**Figure 3.**

It is a process in which two or more species are associated and they form a complex. When the complexes of metal ions and the ligands are formed in such a way that the single metal atom enjoys the central position then it is called a mononuclear complexes. When more than one metal ion is present in the centre they form a polynuclear complex [38]. During the formation of polynuclear complexes, on the basis of number of binding ligands involved the metal atom may hold a positive, negative, or neutral charge. Several studies have confirmed that the formation of complexes by the mononuclear ligands is more desirable than polynuclear ligands because the latter contains multiple ligands results multiple species

Chemical adsorption is also known as chemisorption. It is a process of adsorption which involves a chemical reaction between the surface and the adsorbate. Mainly, it occurs when the adsorbate and adsorbent are attracted via a chemical bond or due to the chemical forces of attraction [39]. During the process of chemisorption only a single layer of adsorbate on adsorbent is formed and this process has high enthalpy of adsorption. With a rise in temperature, the rate of the process of

**134**

binding.

**5.3 Chemisorption**

chemisorption first increases and then decreases.

This process also plays a vital role in adsorption. During the biosorption process it exchanges the binary metal ions with the counter ions present on the surface of the biosorbent. Various systems that are readily available for the purification of water, work on the ion exchange mechanism. The process of ion exchange generally takes place either by cation or anion exchange. Amino groups are one of the good examples of anion exchangers while carboxyl groups represent the cation exchangers [28]. Several studies have reported the ion exchange mechanism of biosorption using various agro/food waste such as watermelon rind, rice straw etc. for the eradication of toxic metal ions such as chromium, Cu (II), Zn (II), Pb (II), and cadmium [17, 40].
