Enhancing Cd²⁺ Removal from Aqueous Solution: ZnCl₂-Modified Citrus Peel Carbon Optimization via Box-Behnken Response Surface Methodology

Abstract This study explores the efficiency of removing Cd²⁺ ions from aqueous solutions using citrus peel carbon (CPC) activated with ZnCl₂. The CPC was characterized through proximate analysis, elemental composition, surface textural characteristics, FT-IR, and FE-SEM. Using a Box-Behnken Design matrix of response surface methodology (RSM), we analysed the effects of process variables such as pH (2–8), CPC dose (0.1-5.0 g∙L⁻¹), initial Cd²⁺ concentration (25–100 mg∙L⁻¹), and temperature (313–323 K). A second-order polynomial regression model was developed to maximize Cd²⁺ removal efficiency. The high regression coefficient (R² = 0.9845) indicates a strong correlation between the response and the factors. Analysis of variance confirmed the statistical significance of the model. Optimal conditions (pH = 8, CPC dose = 2.81 g∙L⁻¹, Cd²⁺ concentration = 100 mg∙L⁻¹, temperature = 303 K) led to a maximum Cd²⁺ removal of 80.27%. Desorption studies showed that HCl was most effective for Cd²⁺ recovery from CPC. By reducing the desorbing acid volume to a quarter, Cd²⁺ re-concentration nearly tripled. Reusability tests indicated an 11% reduction in adsorption capacity, demonstrating CPC's potential for heavy metal removal.