Distribution of Inorganic As Species in Groundwater Samples with the Presence of Fe

Abstract

The distribution of inorganic arsenic (As) species in a 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 three catchments in Taiwan. After filtration of samples in the field, inorganic As species were immediately preserved by adding EDTA/acetate acid. Analytical results showed that arsenite was the dominant species of inorganic As in most of samples. However, the distribution of Fe(II) varied widely and was not consistent with the equilibrium calculation by the geochemical program, PHREEQC, as a result of which the Fe(II) should be the primary Fe species under reducing conditions. The positive saturated index (SI) of Fe oxyhydroxides indicated that the Fe precipitation may occur. Indeed, 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.