Accelerated two-step arsenic photoredox sedimentation in the sequential UV/SO32− reduction and UV/MnO2 oxidation processes

Abstract

Heavy metals in groundwater are concerned due to their potential negative impact on human such as persistent toxicity, conversation to more toxic compounds, possibility of condensation, entry into the food chain and non-biodegradability. In this study, removal of arsenic in a novel process includes reduction and complexion, adsorption and oxidation in the sequential UV/SO32- reduction and UV/MnO2 particle oxidation processes investigated. In our study, in the optimal condition MnO2 0.1 mM, Na2SO3 0.3 mM, 6 min reaction time (synthetic sample) and at pH 6, 10 mg L−1 arsenic was removed. After that, the real sample examined and anions increase the time to reach in blew the World Health Organization standard. In the first stage, the sulfite-sulfate cycle converts arsenate to arsenite, and then both substances are removed from the solution by forming a complex on the MnO2 surface advantages of this method against others include less time, higher efficiency, less use of reactive materials, and no need for large pH changes without release of sulfite or sulfate. At pH 8 about 60% and 40% of reaction species were oxidative and reductive species respectively. However, at pH 6 the number of oxidative species is much less than the reactive species. Considering the better efficiency at pH 6, it shows that reducing species have a more important and primary role in arsenic removal. Also, the amount of energy consumed decreases from 15.23 to 5.37 kWh per cubic meter, Kobs (min−1) 0.0748 to 0.1218 and robs (mg/L.min) increase from 3.74 to 24.36 with change of As concentration from 50 to 250 µg L−1. This method investigated in a real well sample with anions, and more effective anions were phosphate and sulfate. Phosphate reacts with eaq− and react with sulfite radical and does not allow the formation of complexes, on the other hand Sulfate help to convert Arsenite to arsenate and arsenate direct removal.