Watermelon rind: agro-waste or superior biosorbent?

Biosorption of copper (Cu), zinc (Zn), and lead (Pb) on watermelon rind in a well-stirred batch system was investigated. pH showed significant influence on the biosorption process. Optimal pH for Cu, Zn, and Pb biosorption was found to be 5.0, 6.8 and 6.8, respectively. Watermelon rind was in favor of Pb and it could remove up to 99% Pb between pH ranges of 5 and 6.8 when Pb concentration is lower than 100 mg/L. The biosorptive capacity of watermelon on Cu, Zn, and Pb was 6.281, 6.845, and 98.063 mg/g, respectively. The equilibrium data fitted well to Langmuir adsorption isotherm while pseudo-second-order kinetic model exhibited more advantages for describing kinetic data than pseudo-first-order kinetic model. NaOH was found to be a suitable eluent. After desorption in NaOH solution, the resorption efficiency reached as high as 99% of these three metals either in a single-component or multi-component system. From the characterization study, ion exchange and micro-precipitation were estimated to be the main mechanisms. Due to its high metal uptake capacity, reusability, and metal recovery, watermelon rind can be considered as an eco-friendly and economic biosorbent for removing Pb from water and wastewater.