Simultaneous removal of Arsenic(III) and Chromium(VI) over ZnFe2O4 {100}/{111} Z-scheme photocatalyst: Facet-dependent active site and overlooked As(III)/Cr(VI) complex

Arsenic(III) and chromium(VI) are typical toxic ions in mine drainage, and the co-existence greatly increases treatment difficulty. In this paper, ZnFe2O4 {100}/{111} facet Z-scheme photocatalyst was prepared and applied in simultaneous purity of As(III) and Cr(VI) containing wastewater. Extended X-ray absorption fine structure (EXAFS) analysis and density functional theory (DFT) calculations reveal {100} plane is more likely to produce •OH and Fe(II) by occurring ligand-to-metal charge transfer (LMCT) under light irradiation. By measuring intermediate active species, we found Fe(II) and electrons are responsible for Cr(VI) reduction, and As(III) oxidation is attributed to holes and •OH. The complexation between As(III) and Cr(VI) is firstly proposed and confirmed by time-dependent density functional theory (TDDFT) simulations, and the combination of arsenic and chromium facilitates the synergistic removal. These findings in this paper propel the development and application of Z-scheme photocatalyst, and assist in the understanding of As and Cr synergistic treatment.