Preservation of co-occurring As, Sb and Se species in water samples with EDTA and acidification

The simultaneous preservation of the following redox couples was studied: As(III, V), Sb(III, V) and Se(IV, VI). Over a time period of 11 weeks the stability of these three redox couples was assessed in water samples with different dissolved oxygen content, i.e., groundwater, lake water and river water. High concentrations of Fe (25.0 mg L−1) and Mn (25.0 mg L−1) were added to each of the different matrices to simulate a Fe and Mn rich environment. In addition to their natural concentration, each sample was spiked with 5.0 μg L−1 As(III and V) and Sb(III and V) and 15.0 μg L−1 Se(IV and VI). As potential preservation strategies, EDTA alone and EDTA combined with HCl, HNO3, formic acid or acetic acid were investigated and compared to unpreserved samples. In addition, preserved samples were stored at 4°C in the dark, while unpreserved samples were stored at room temperature in the presence of light. The results showed that the addition of EDTA combined with acidification to a pH of 3 successfully preserved all three redox couples for at least 11 weeks stored at 4°C in the dark. EDTA alone (pH = 6) failed to preserve the As and Sb species, although it successfully preserved the Se species. Primarily based on observations made for the unpreserved samples, it was concluded that Sb(III) could be oxidized easier than As(III) and Se(IV) at neutral pH, and that the Se species in general were most stable. The formation of Fe-(oxy) hydroxide and possibly Mn-(oxy) hydroxide in the unpreserved samples also allowed an estimation of the relative adsorption behaviour. Arsenic(III), Sb(III), Se(IV)) and As(V) showed a strong adsorption affinity for Fe-(oxy) hydroxide and Mn-(oxy) hydroxide probably due to the fact that they all form inner sphere complexes, whereas Sb(V) and Se(VI) rarely adsorbed because they form outer sphere complexes and thus bond via weak electrostatic adsorption. Antimony(III) could chelate with EDTA and formed several complexes according to pH. The most stable species of Sb(III)Y− (Y = EDTA) existed at a pH range of 1.8 to 3.0. Apparently Sb(V), on the other hand, did not chelate with EDTA and thus should exist mainly in the form of Sb(OH)6− and minor Sb(OH)5 in this pH range.