High-efficient removal of Cu(II) using biochar/ZnS composite: optimized by response surface methodology

Rice hull biochar modified by ZnS nanocrystals (RBZ) was developed as an efficient absorbent for the removal of Cu(II) in aqueous solution. Response surface methodology (RSM) was applied for modeling and optimizing the performance of RBZ. The maximum adsorption capacity of RBZ toward Cu(II) through the Langmuir model fitting was 248.98 mg g−1, which was four times larger than that of rice hull biochar (RB). The adsorption kinetics of Cu(II) and XPS analysis revealed that the chemisorption process played the leading role when using RBZ. Through competition ions study, Pb(II), Cd(II), or Ca(II) can slightly reduce the adsorption of Cu(II). According to the Box Behnken design method, the analysis of variance results showed that the pH and adsorbent dosage had significant effects on the predicted responses of adsorption capacity and removal efficiency. When using the optimum conditions, pH 6, temperature 27 °C, and dosage 0.20 mg mL−1 for removing Cu(II) in solution, the maximum adsorption capacity and relatively higher removal efficiency could be obtained through the 3D response surface plots. The results demonstrated that RSM is feasible to achieve optimal conditions for adsorption capacity and removal efficiency simultaneously toward heavy metals from aqueous medium, and the interaction between them when using the biochar/ZnS composite.