A Preliminary Investigation into the Stability of Inorganic Arsenic Species in Laboratory Solutions Simulating Sediment Pore Water

Abstract

A simple method to preserve arsenic species in simulated pore water was investigated. Synthetic pore water containing high levels of Fe, Mn and S (as sulfide, S^2–) were synthesized and spiked with different arsenic species. Arsenite [As(III)], arsenate [As(V)], dimethylarsinate [(CH₃)₂AsO^2–, DMA], monomethylarsonate [(CH3)AsO₃ ^2–, MMA], monothioarsenate(V) species (AsO₃S^3–,MTA) and tetrathioarsenate(v) species (AsS₄^3–, TTA) in different combinations were used. Disodium ethylenediamine tetraacetate (EDTA) and temperature was used to preserve speciation. Solutions were analyzed at intervals of one day, week, month and 2 months after preparation. Samples were analyzed by HPLC-ICP-MS using an ion exchange column and ammonium carbonate buffer. Samples containing Fe and Mn spiked with As(III), As(V), DMA,MMAshowed adequate species preservation for two months when EDTA was added. The total As in samples containing Fe,Mn and S^2– was preserved over 60 days however, speciation was not preserved. Samples spiked with synthesized mono- and tetra-thioarsenate(V) species showed immediate degradation of the mono- and tetra-thioarsenic species into unidentified As-S species. The results show that while EDTA may be adequate to preserve As species containing high Fe and Mn concentrations, the combination of EDTA and temperature was not successful in preserving As speciation in samples containing sulfides.

KEYWORDS Arsenic speciation, EDTA, HPLC-ICP-MS, thioarsenic species.