Nano-zerovalent copper biochar composite for treating selenium oxyanions in water: synthesis, evaluation, removal mechanism, density functional theory, and molecular dynamics simulations
Mahvish Abbasi, Osama Shaheen Rizvi, Syed Zajif Hussain, Asim Jilani, Faisal A. Osra, George William Kajjumba, Amit Bhatnagar, Eakalak Khan, Tauqeer Abbas
Abstract
This study focused on the synthesis and applicability of nano-zerovalent copper biochar composite (nCu 0 -BC) for treating selenium oxyanions in water. In batch experiments, 2 g/L of nCu 0 -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 nCu 0 -BC dose from 2 to 3 g/L. Co-existing ions had minimal effect on Se(VI) removal except PO 4 3- 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 nCu 0 -BC. XRD analysis revealed that nCu 0 -BC contains copper in two crystalline phases, i.e., zerovalent copper (Cu 0 ) and cuprous oxide. XPS analysis showed that Cu 0 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 nCu 0 -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 nCu 0 -BC owing to the presence of Cu 0 on its surface, resulting in stable bidentate binuclear complexes formation between Se oxyanions and Cu 0 .