Physicochemical Characterization of Biochar Derived From the Pyrolysis of Cotton Gin Waste and Walnut Shells

Abstract

Highlights Cotton gin waste and walnut shells can be beneficially transformed into alkaline biochars. The resulting biochars have properties supporting effective adsorption of cationic contaminants. The biochars exhibit irregular particle morphologies, with cotton gin biochar maintaining fibrous structures. Results can help inform appropriate applications of biochar produced from cotton gin waste and walnut shells. Abstract. The sustainable management of agricultural waste has gained increasing attention worldwide, especially regarding the production of value-added products that are renewable and carbon-rich. Further, there is a need to provide low-cost, lower-energy alternatives to materials such as activated carbon for removing contaminants from water. The goal of this study was to characterize various physicochemical properties of biochar produced from cotton gin waste (pyrolyzed for 2 h at 700C, CG700) and walnut shells (pyrolyzed for 2 h at 800C, WS800) to better understand their potential to be effective in various environmental applications. The properties that were characterized are the following: (i) biochar pH; (ii) specific surface area (SSA); (iii) surface functional groups; (iv) surface elemental composition; (v) surface charge; and (vi) surface morphology. Pyrolysis led to the destruction of acidic functional groups within the parent biomass and an increase in ash content, resulting in alkaline biochars with pH values of 9.8 and 10.9 for WS800 and CG700 biochar, respectively. Zeta potential measurements demonstrated that both biochars were negatively charged at environmentally relevant pH ranges. The FT-IR spectrum and XPS results for the CG700 biochar showed the presence of several functional groups, including the OH, C=C, and C-O groups within the biochar samples. BET results demonstrated that CG700 had a low SSA (8.57–22.31 m2 g-1), and the biochar was dominated by fibrous, irregular shaped particles, according to the results from the SEM imaging. The FT-IR spectrum for the WS800 biochar showed the presence of the carbonyl group, which was inherited from the parent biomass. BET measurements for the WS800 showed a decline in SSA with a reduction in particle size, likely due to a collapse of the honeycomb structure of the WS800 biochar with crushing to reduce the particle size, as revealed by the SEM images. The results of this research will help to inform the applications of biochar produced from cotton gin waste and walnut shells, two large sources of agricultural waste materials, and promote sustainable alternatives to extend the life cycle of these materials into value-added products.