**7.2 Removal of Chromium**

*Emerging Contaminants*

metals [28].

**metal ions**

treatment to increase their sorption capacities.

**7.1 Removal of cadmium**

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

**7. Role of agro/food waste as biosorbents for the eradication of toxic** 

Utilization of agro and food waste as biosorbents for the eradication of toxic metal ions from the wastewater and aqueous streams is a promising and innovative technology [35]. In the past several years, this promising technology has gained more attention because these waste materials have shown a higher efficacy towards the removal of heavy metals and the other reasons include low cost, and easy availability. The efficacy of agro/food waste depends upon its phsico-chemical nature, capacity of adsorption and affinity. A lot of research has already been carried out for the removal of heavy metals such as arsenic, lead, mercury, cadmium, chromium, nickel, cobalt, etc. by using various types of biosorbents namely rice husk, wheat bran, peels of apple and banana, etc. [14, 46–48]. They have used biosorbents either in the natural form or in modified form by thermal and chemical

Cadmium metal and its ions are the most severe pollutants because they are highly soluble in water as compared to the other toxic metal ions. Therefore, they are easily mobile in soil and have a higher tendency to bioaccumulate. Some of the basic sources that produce cadmium into the environment include solid waste, sewage irrigation, plastics, application of fertilizer, mining, plating on steel, etc. [49]. If the human body is exposed to cadmium for a long time it may result in severe diseases such as bone damage, kidney and lung cancer [50]. Rice husk, rice bran, wheat bran, black gram husk, rice polish, fig leaves, jack fruit, and orange peels, are some of the waste materials that have shown the excellent efficiency towards the removal of Cd. Several studies have been tried using these materials for the eradication of Cd either in their natural form or in the modified form. In addition, potato peels, olive branches, Musa paradisiacal peels, and coconut waste have also been extensively used for the removal of Cd. The adsorption capacity of coconut waste for the removal of Cd (II) was found to be 285.70 mg/g while, the adsorption capacities of other material such as potato peels, olive branches, and Musa paradisiacal peels, were found to be 125, 38.17, 10.0 mg/g, respectively [15]. Therefore, coconut waste has shown the highest adsorption capacity as compared to the potato peels, olive branches, and Musa paradisiacal peels. Some of the waste materials have shown higher adsorption capacity when they were used at acidic pH namely, fig leaves, medlar peels, beans, and jack fruits [51, 52]. Some of the research work that

**136**

Chromium is a naturally occurring heavy metal found in the earth's crust. It can be released into the environment either through the natural process or by manmade industrial activities. Industrial activities include wood preservation, tanning, textile manufacturing, pigments, paints, and dyes manufacturing. Chromium is generally found in a number of oxidation states but Cr (III), and Cr (VI) are the largest threat to the environment [42, 84]. In the last two decades, a lot of researches have been done by using various agricultural wastes such as orange, lemon and banana peels, soybean and rice hulls, hazelnut and peanut shells for the removal of chromium metals and ions from different types of wastewater [26, 85]. These waste materials have shown significant chromium removal efficiency as shown in **Table 1**. Other than these waste materials, neem leaves powder, cactus leaves, coconut shell fiber, and pine needles have also shown a promising efficiency in the range 90–100% for the removal of chromium at optimum pH [71]. Wheat brans have showed the highest adsorption capacity for Cr (VI) at 310.58 mg/g whereas, rice bran is a lesser effective adsorbent for the removal of Cr with only up to 50% efficiency [86]. Saw dust, bagasse, rice husk, and mustard oil cake have also been tried by numerous researchers for the removal of chromium and they have reported significant efficiencies in literature [85].
