Enhanced As(III) oxidation and removal by combined use of zero valent iron and hydrogen peroxide in aerated waters at neutral pH values

The oxidation and removal of As(III) by commercially available micro-scale zero-valent iron (mZVI) was studied in aerated synthetic groundwater with initially 6.7 μM As(III) at neutral pH values. Batch experiments were performed to investigate the influence of ZVI and H2O2 concentrations on As(III) oxidation and removal. Oxidation and removal kinetics was significantly increased by increasing ZVI concentration or by adding H2O2 in micromolar concentrations slightly higher than that of initial As(III). Observed half-lifes for arsenic removal without added H2O2 were 81-17 min at ZVI concentrations of 0.15-2.5 g/L, respectively. X-ray absorption spectroscopy (XAS) confirmed that almost all As(III) was converted to As(V) after 2 h of reaction in the pH range 5-9. Addition of 9.6 μM H2O2 to 0.15 g/L ZVI suspensions diminished half-lifes for arsenic removal from 81 to 32 min and for As(III) oxidation from 77 to 8 min, i.e., by approximately a factor of 10. The increased rate of As(III) oxidation is attributable to enhanced formation of oxidants by the Fenton reaction with higher initial concentrations of H2O2. In practice, results of this study suggest that addition of small amounts (<1 mg/L) of H2O2 in various forms (e.g. stable and widely available Na-percarbonate) to water prior to treatment could significantly enhance As(III) oxidation and removal with ZVI.