Adsorption of metals by watermelon-peel-derived biochar and mechanism in aqueous solution

The large amount of watermelon feedstocks could bring extra pressure on solid waste management. Thus, the present study selected watermelon as a precursor material for biochar production. The properties of biochar derived from watermelon peel pyrolysis (WBC) under three different temperatures and its absorption capacity for heavy metals were investigated. The three types of biochar, WBC400, WBC500 and WBC600, showed high adsorption capacity for copper (Cu), cadmium (Cd) and nickel (Ni) in the present study. The adsorption data were best represented by the Langmuir isotherm model instead of the Freundlich one. The largest sorption quantities were 101, 139 and 59 mg/g of cadmium (Cd²⁺), copper (II) (Cu²⁺) and nickel (II) (Ni²⁺), respectively, according to the Langmuir model. The results from Fourier transform spectroscopy, scanning electron microscopy and X-ray diffractometry all indicated that the biochar adsorption mechanisms involved complexation, precipitation and ion exchange in the process. The results indicate that biochars derived from watermelon peel are potential candidates for efficient sorption of heavy metals from water.