Surface modification of biochars for efficient Cu2+ adsorption: Effects of microwave-assisted oxygen and nitrogen doping via hydrogen peroxide and nitric acid

In this study, microwave-assisted heteroatom doping was employed to synthesize oxygen-functionalized (MW@O) and nitrogen-oxygen dual-functionalized (MW@N, O) biochars, which were evaluated for Cu2+ removal from aqueous solutions. Comprehensive adsorption analyses, including isotherm modeling, kinetic evaluation, thermodynamic assessment, and point of zero charge (pH(p)zc) determination, were performed to clarify the adsorption behavior. The pH(p)zc values of MW@O and MW@N, O doped biochars were 6.15 and 4.27, respectively, indicating enhanced Cu2+ uptake at pH values above these thresholds due to favorable electrostatic interactions. At 303 K, Langmuir monolayer capacities (Q(m)) were 109.89 mg/g for MW@O doped biochar and 140.84 mg/g for MW@N, O doped biochar. Although surface area decreased after modification, adsorption capacity increased, suggesting that Cu2+ uptake was primarily governed by the enrichment of oxygen- and nitrogen-containing functional groups (e.g., -OH, -COOH, -NH2) rather than porosity. Adsorption followed the Langmuir model and proceeded through combined electrostatic attraction and surface complexation mechanisms. These results demonstrate that microwave-assisted heteroatom modification effectively tailors' biochar surface chemistry and enhances Cu2+ adsorption performance under controlled experimental conditions, providing mechanistic insight into the role of surface functionalization in heavy metal removal.