Abstract
Copper contamination in industrial wastewater poses significant environmental and health concerns, particularly in the electroplating and electronics manufacturing industries. Conventional treatment methods often suffer from limited efficiency at low metal concentrations and generate large volumes of secondary waste. This study examines the utilization of waste rice husks as a sustainable precursor for producing low-cost adsorbents for the removal of Cu2+ from aqueous solutions. Rice husk char produced by pyrolysis was processed into three adsorbents: washed rice husk char (B-RHB), impurity-removed rice husk char via acid and alcohol washing (T-RHB), and acrylic acid–grafted, sodium-treated rice husk char (M-RHB). Batch adsorption experiments, supported by isotherm and kinetic modeling, were conducted to compare adsorption performance and mechanisms. All adsorbents exhibited copper removal capability; however, their adsorption behaviors differed significantly. B-RHB showed relatively high apparent adsorption due to combined adsorption–precipitation effects caused by residual inorganic impurities. Removal of these impurities in T-RHB reduced precipitation contributions and resulted in Freundlich-type adsorption behavior. Acrylic acid modification substantially enhanced adsorption performance by introducing sodium carboxylate (–COO⁻Na⁺) functional groups, which promoted ion exchange and coordination with Cu²⁺. The modified adsorbent achieved the highest adsorption capacity (17.31 mg g⁻¹), exhibited faster kinetics, and displayed a Langmuir-type adsorption behavior. M-RHB maintained stable adsorption capacity under acidic conditions, demonstrating improved applicability in industrial wastewater treatment. This study highlights the potential of chemically modified rice husk char as an effective and sustainable adsorbent for copper removal.