H2O2-Enhanced As(III) Removal from Natural Waters by Fe(III) Coagulation at Neutral pH Values and Comparison with the Conventional Fe(II)-H2O2 Fenton Process

Arsenic is a naturally occurring contaminant in waters, which is toxic and adversely affects human health. Therefore, treatment of water for arsenic removal is very important production of safe drinking water. Coagulation using Fe(III) salts is the most frequently applied technology for arsenic removal, but is efficient mostly for As(V) removal. As(III) removal usually requires the application of a pre-oxidation step, which is mainly conducted by chemical or biological means. In this study, we show that Fe(III) coagulation in the presence of H2O2 can be a very efficient treatment process for As(III) removal, which has been never been shown before in the literature. The results showed that addition of 8.7-43.7 mM hydrogen peroxide to Fe(III) coagulation process was able to increase the effectiveness of As(III) removal in synthetic groundwater by 15-20% providing residual concentrations well below the regulatory limit of 10 mu g/L from initial As(III) concentrations of 100 mu g/L, at pH 7. The enhanced coagulation process was affected by the solution pH. The removal efficiency substantially declined at alkaline pH values (pH > 8). Addition of EDTA in the absence of H2O2 had a strong inhibiting effect where the As(III) removal was almost zero when 88.38 mu M EDTA were used. Radical quenching experiments with 50, 100 and 200 mM DMSO, methanol and 2-propanol in the H2O2-coagulation process had a slightly adverse effect on the removal efficiency. This is considered as indicative of an adsorption/oxidation of As(III) process onto or very near the surface of iron oxide particles, formed by the hydrolysis of Ferric iron ions. In practice, the results suggest that addition of H2O2 increases the As(III) removal efficiency for Fe(III) coagulation systems. This is an important finding because the pre-oxidation step can be omitted with the addition of H2O2 while treating water contaminated with As(III).