Nano-zerovalent copper biochar composite for treating selenium oxyanions in water: synthesis, evaluation, removal mechanism, density functional theory, and molecular dynamics simulations

This study focused on the synthesis and applicability of nano-zerovalent copper biochar composite (nCu0-BC) for treating selenium oxyanions in water. In batch experiments, 2 g/L of nCu0-BC efficiently removed 98.96 % of selenite [Se(IV)] and 98.78 % of selenate [Se(VI)] in 10 mL of water (10 mg/L for each Se oxyanion) in 12 h. Both Se(IV) and Se(VI) removal followed the pseudo-second-order kinetic model, and their removal rates increased by 3.02 and 7.44-folds, respectively, by increasing the nCu0-BC dose from 2 to 3 g/L. Co-existing ions had minimal effect on Se(VI) removal except PO43- which significantly reduced the Se(VI) removal; while the presence of calcium, sodium, nitrate, and bicarbonate lowered Se(IV) removal by 7–10 %. Spectroscopic analyses were employed to characterize virgin and spent nCu0-BC. XRD analysis revealed that nCu0-BC contains copper in two crystalline phases, i.e., zerovalent copper (Cu0) and cuprous oxide. XPS analysis showed that Cu0 was the main source of electrons for the reduction of Se oxyanions; Se(IV) was reduced to Se(0) while Se(VI) was reduced to Se(IV) and Se(0). The density functional theory simulation indicated that both Se(VI) and Se(IV) form stable complexes with nCu0-BC with the calculated adsorption energies of −3,657 kcal/mol and −3,669 kcal/mol, respectively. The molecular dynamic simulation revealed a high adsorption affinity of Se oxyanions towards nCu0-BC owing to the presence of Cu0 on its surface, resulting in stable bidentate binuclear complexes formation between Se oxyanions and Cu0.