**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,

when the wood and grass feedstock biomass is heated below 100–200°C then its FTIR spectra shows no prominent change in their functional groups [45]. Some of the external parameters such as pH change the complexity of functional groups when they undergo certain mechanisms. The carboxyl group works efficiently in the adsorption process at the pH range 3–4. Beyond this pH the carboxyl group of biosorbent forms a complex with the positively charged metals after deprotonation. Several authors [17, 33, 35] have observed the changes in the functional groups of adsorbent before and after the adsorption of metals. Various interactions such as complexation, precipitation, cation exchange, electrostatic interaction and chemical reduction have an intrinsic effect to make the process of eradication of heavy metal successful. The functional groups like hydroxyl (-OH) and carboxylate (-COOH) available on the surface of bio char have strong interaction with heavy metals [28].
