Investigation of arsenic speciation on drinking water treatment media utilizing automated sequential continuous flow extraction with IC-ICP-MS detection

Abstract

Three treatment media, used for the removal of arsenic from drinking water, were sequentially extracted using 10 mM MgCl₂ (pH 8), 10 mM NaH₂PO₄ (pH 7) followed by 10 mM (NH₄)₂C₂O₄ (pH 3). The media were extracted using an on-line automated continuous extraction system which allowed the arsenic in each of the extraction fluids to be speciated on-line using IC-ICP-MS. The 10 mM MgCl₂ preferentially extracted As(III) from each of the media. The percentage of the arsenic extracted by the MgCl₂, relative to a HNO₃/H₂O₂ digestion of the media, ranged from 0.1–2.3% for the three solids. The next sequential extraction fluid, 10 mM NaH₂PO₄, extracted some of the residual As(III) remaining on each of the media but the predominant species extracted was As(V). The 10 mM NaH₂PO₄ extracted 15.3 to 42.8% of the total arsenic relative to a total digested concentration for each of the media. The As(III) and As(V) stability studies conducted in these two extraction fluids indicated that conversion between As(III) and As(V) was not significant for the short extraction fluid sample contact time associated with the on-line continuous flow extraction cell. Finally, the 10 mM (NH₄)₂C₂O₄ extraction fluid was utilized in an off-line analysis mode because the Fe and As concentrations extracted from the media were not compatible with direct ICP-MS detection. The (NH₄)₂C₂O₄ extracted 2.9–29% As(III) for all three media and caused an oxidation of As(III) to As(V) during the extraction period for one of the three media. The sum of the arsenic from each of the three extraction fluids represented 92%, 44% and 53% of the available total arsenic for the three media, respectively. The speciation results for each media were obtained by adding all the speciation results from all three extraction fluids together and the resulting distribution of As(III)/As(V) compared well with the speciation results obtained via XANES.