Arsenic Speciation in a Fly Ash Settling Basin System

Abstract

The sluicing of coal fly ash to settling basins is a major method for disposal of this industrial by-product. Fly ash often contains elevated concentrations of trace elements such as As, Se, and Mo, which can be solubilized upon contact with water and also become elevated in the surficial sediments. Both the soluble and sediment-sorbed trace elements can be bioavailable and potentially toxic to animals inhabiting the ash basins. This study examines the aqueous speciation of As in the surface and interstitial waters and the solid phase As speciation in the sediments of a fly ash basin system. Ion chromatography coupled to inductively coupled plasma mass spectrometry (IC-ICP-MS) was used to determine arsenite As(III), arsenate As(V), dimethylarsenate (DMA), and momomethylarsenate (MMA) in the aqueous samples. Hydoxylamine hydrochloride and oxalic acid extractions were used to assess the proportion of amorphous Fe, amorphous Al and amorphous aluminosilicates in depth sectioned samples of a sediment core taken from the ash basins. The concentration of As solubilized by these extractants was also measured. Surface water As concentrations were low with an average of 13 and 3 μg 1⁻¹ determined in the summer and fall 2000. Arsenate was the major As species in the surface waters; DMA and As(III) were detected in the summer sampling but no DMA was detected in the fall sampling. Pore water As concentrations were much higher than the surface waters, reaching a maximum of 110 μg 1⁻¹ at a sediment depth of 8–12cm. Arsenate was the major dissolved species at the sediment-water interface but decreased with depth, while the proportion of AS(III) increased to a maximum at a depth of 8–12 cm. The increase in total dissolved As with depth was mirrored by an increase in soluble Mo and an increase in pH, and the depth of maximum As concentration marked the onset of an increase in soluble Fe. This suggests that the observed increased As solubility may result from the decrease in sorption by amorphous Fe phases due to the onset of reductive dissolution, coupled with the prevalence of As(III), that may be poorly sorbed by the remaining mineral phases in the sediment. This observation was supported by the selective extraction data of the sediment core sections, which indicated that As was mostly bound to amorphous Fe phases in the sediment. The oxalate extraction also showed that a significant proportion of total Al was present as amorphous phases and that < 20% of amorphous Al was present as amorphous aluminosilicates.