Sustainable valorization of chili pepper stems into biochar for cadmium removal from real zinc smelter wastewater: Adsorption mechanism and optimization study

Cadmium contamination in industrial wastewater remains a persistent environmental concern, particularly in effluents discharged from zinc smelter facilities. This study investigates the feasibility of converting chili pepper ( Capsicum annuum L.) stems, a lignin-rich agricultural residue with high carbon retention potential during pyrolysis, into biochar for Cd removal from real zinc smelter wastewater. Biochars were produced at 300–750 °C and systematically characterized to evaluate the influence of pyrolysis temperature on physicochemical properties and adsorption performance. Among the prepared biochars, CPSB–450 exhibited the highest Cd adsorption amount (28.7 mg/g). Adsorption kinetics followed the pseudo-second-order and Elovich models, indicating a chemisorption-controlled process. Equilibrium data were better described by the Freundlich model, suggesting multilayer adsorption. The maximum adsorption capacity estimated from the Langmuir model was 63.0 mg/g. The adsorption mechanism involved surface complexation with oxygen-containing and aromatic functional groups, coupled with ion exchange of K + , Ca 2+ , and Mg 2+ and surface precipitation as CdCO3. Response surface methodology applied to real zinc smelter wastewater identified optimal removal conditions at a CPSB–450 dose of 44.0 g/L, temperature of 34.4 °C, contact time of 6.5 h, and pH 6.9, achieving a maximum removal efficiency of 92.6%. These findings demonstrate that chili pepper stems can be transformed into an effective adsorbent for Cd remediation, offering a practical strategy for simultaneous agricultural residue valorization and heavy metal removal in complex wastewater systems.