Controlled electrochemical design of activated carbon surface chemistry: enhanced copper recovery using functionalized walnut shell-derived sorbents

Heavy metal contamination from industrial effluents presents a significant environmental challenge worldwide. This study presents a novel electrochemical approach to control the surface functional groups of walnut shell-based activated carbon (AC) to enhance the adsorption capacity of Cu²⁺. Unlike conventional chemical activation methods, our electrochemical technique employing NaOH and HNO₃ electrolytes offers a more sustainable and cost-effective alternative. Cyclic voltammetry revealed that NaOH modification generated strong redox reactions, significantly improving porosity and specific surface area compared to unmodified carbon. The adsorption indicator method used for the first time showed a 10-fold and 7-fold increase in adsorption centers after NaOH and HNO₃ electrochemical treatments, respectively, providing insights into the distribution of functional groups on the carbon surface. The maximum Cu²⁺ adsorption capacity reached 41.61 mg/g for NaOH-modified carbon, substantially outperforming the 24.44 mg/g capacity of conventional activated carbon. The process involves monolayer adsorption through electrostatic interactions and chemical bonding between copper ions and oxygen-containing functional groups (OCFG). Adsorption isotherm studies demonstrated that modification methods can be tailored to achieve either physical or chemical adsorption mechanisms. Desorption studies confirmed the feasibility of reusing these sorbents for up to three cycles, demonstrating practical applicability for sustainable water treatment applications.