Influence of Iron on the Distribution of Inorganic Arsenic Species in Groundwater Samples

The distribution of inorganic arsenic (As) species in groundwater sample, including As(III) and As(V), requires adequate preservation to prevent the adsorption caused by precipitates of Fe oxyhydroxides. Twenty-two groundwater samples with varied As and Fe concentrations were collected from high-As catchments in Taiwan. After filtration of samples in the field, inorganic As species were immediately preserved by adding EDTA/acetate acid. The effectiveness of EDTA was also assessed by long-term experiment to determine the representative preservation method of inorganic As species in groundwater samples. Analytical results showed that the oxidation of Fe(II) rapidly forms Fe oxyhydroxides in controlled samples, causing the adsorption and/or co-precipitation of inorganic As and the decrease of aqueous As concentrations. EDTA/acetate acid can effectively slow the rate of As(III) oxidation and eliminates the precipitation of Fe and As, especially for high-Fe (>6.45 mg/L) groundwater samples. However, for low-Fe groundwater, other preservation methods should be considered, such as the phosphoric acid and hydrochloric acid. For long-term experiment, the EDTA/acetic acid preservation stabilize the distribution of inorganic As species of groundwater samples within 6 months. Keywords—Groundwater, Inorganic arsenic, Iron, Preservation