Sustainable sewage sludge biosorbent activated carbon for remediation of heavy metals: Optimization by response surface methodology

This study investigated the potential use of sewage sludge-based activated carbon in the removal of copper (Cu) and cadmium (Cd) ions from aqueous solutions. The activated carbon (AC) was prepared by chemical activation using potassium hydroxide KOH and heating in a tube furnace at 600 °C for 2 h. The produced activated carbon was further subject to a sulfurization process to enhance its removal efficiency. The study used Box-Behnken Design (BBD) in response surface methodology (RSM) to optimize several parameters, including initial metal concentration, activated carbon dose, pH, and contact time. Results showed that the sulfurization process introduced sulfur functional groups (S[dbnd]O) to the surface of the activated carbon and decreased its specific area from 190 to 173 m²/g. The quadratic model was suggested as the best model to describe the effect of the adsorption parameters on the removal percentage with high R² values (≥0.9) and p-value less than 0.05. Second-order polynomial equations, analysis of variance (ANOVA), and three-dimensional surface plots were developed to assess the interaction between the parameters and the optimal conditions to remove copper and cadmium ions. The optimum removal parameters for copper ions were 300 min, 100 ppm initial concentration, 20 g/L activated carbon dose, and pH 6, with a removal efficiency of 83.9%. Cadmium removal of 87.5% was achieved at 11 ppm, 300 minutes contact time, pH 6, and 2.5 g/L adsorbent dose. The activated carbon capacity in removing copper and cadmium was 16 mg/g and 17.6 mg/g respectively. The sulfurization process increased the removal percentage for copper and cadmium ions by 14.5 and 18.7% respectively. Advanced analytical techniques such as FITR, SEM-EDX, BET surface area, and elemental analysis were used to characterize the materials.