ARSENIC SPECIATION IN A FLY ASH SETTLING BASIN
SYSTEM
Brian P. Jackson, John C. Seaman, and William Hopkins
Savannah River Ecology Laboratory University of Georgia P.O. Drawer
E Aiken, SC 29802
1. ABSTARCT
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 detem1ine 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 A1 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 ddepth 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
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 porrly sorbed 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.
SREL Reprint #2752
Jackson, B. P., J. C. Seaman and W. A. Hopkins. 2003. Arsenic speciation in a fly ash settling basin system, p. 203-218. In Chemistry of Trace Elements in Fly Ash, edited by K. S. Sajwan, A. K. Alva and R. F. Keefer. Kluwer Academic/Plenum Publishers.
