Simultaneous redox conversion and sequestration of chromate(VI) and arsenite(III) by iron(III)-alginate based photocatalysis

Multiple heavy-metal ions (e.g. Cr(VI) and As(III)) normally co-exist in acid wastewater, making the wastewater treatment complicated. Herein, the synergistic redox conversion and removal of Cr(VI)/As(III) were effectively achieved by applying iron(III) cross-linking alginate hydrogel beads (Fe-SA) as photocatalyst under simulated sunlight. Results show that not only 100% of the Cr(VI)/As(III) redox conversion was obtained in 150 min at pH 3.0, but also the removal efficiency of the transformed products (Cr(III) and As(V)) was greatly enhanced to above 80% in a wide pH range of 3–7. The CO2− radicals, produced by the ligand to metal charge transfer (LMCT) excitations on Fe-SA, together with the phtocatalysis-generated Fe(II), was responsible for the Cr(VI) reduction. Meanwhile, OH originating from CO2− mainly contributed to the As(III) oxidation, as confirmed by electron paramagnetic resonance and the by-product of CO2. Moreover, Fe-SA composite presented excellent reusability and performance in treatment of Cr(VI)/As(III) in real waters.