ARSENATE FORMS TERNARY AQUEOUS METAL-BRIDGED COMPLEXES WITH NATURAL ORGANIC MATTER

Inorganic arsenic forms stable complexes with dissolved natural organic matter-bound metals. Arsenate was incubated with raw filtered river water that had been pretreated with various concentrations of Al(NO₃)₃ at pH 5. The extent of arsenate complexation with dissolved NOM was then determined by the chromatographic separation of free arsenite and arsenate from NOM-complexed arsenic by anion exchange HPLC with ICP-MS detection. Typically, NOM-complexed arsenic eluted after free arsenite and arsenate. Its peak was consistently very broad and the concentration of the NOM-arsenic species was evenly distributed around the axis of the peak’s maximum intensity. The peak intensity of this NOM-arsenic complex increased with increasing Al(NO₃)₃ concentration and the presence of competing ligands such as phosphate and EDTA reduced the extent of arsenic complexation with Al-NOM. The broadness of the NOM-arsenic peak indicated heterogeneity of the arsenic species as expected for a mixture of NOM-As complexes. The elution of the unknown arsenic species after the free inorganic arsenic species suggests a strong anionic character, which is consistent with the organic acid nature of the NOM molecules. In contrast to the smooth chromatographic elution of the NOM-As complex was the elution of arsenate solutions that had been incubated with Al(NO₃)₃ in Milli-Q deionized water. These demonstrated a prolonged and irregular tailing of arsenic in the chromatogram that was attributed to the desorption/dissolution of adsorbed or coprecipitated arsenate from aluminum oxide colloids. Infrared spectroscopy was used to confirm the formation of ternary aqueous complexes of arsenate with aluminum(III) and salicylic or citric acids as model compounds of NOM. In addition to the observed complexation, reduction of arsenate to arsenite occurred under a variety of oxic conditions in the presence of NOM. The mechanism of arsenate reduction was not readily apparent and is the subject of future research.