Generation of Alkylarsines from Soil

Products volatilized from arsenical-treated soil were identified and their rates of formation measured to determine if generation of alkylarsines by soils is part of a natural arsenic cycle. 74As-sodium arsenate, 14C-MSMA (monosodium methanearsonate), or 14C-cacodylic acid (hydroxydimethylarsine oxide), were added at 13.3 μMoles (10 ppmw As) to individual flasks containing 100 g of Mattapeake silt loam. Either air or N2 was used to sweep gases from the flasks and through trapping solutions. Arsenical content in the solutions was monitored by liquid scintillation and assayed for arsenic by atomic absorption spectroscopy. Volatile arsine gases were produced under all conditions. The organo-arsenicals formed volatile compounds most quickly under aerobic conditions. In 160 days, the trapping solutions contained 18.0%, 12.5%, and 1.0% of the activity from soils treated with cacodylic acid, MSMA, or sodium arsenate respectively, when flushed continuously with air, and 7.8%, 0.8%, and 1.8% when flushed with N2. The volatile gases were identified as dimethyl- and trimethylarsine (DMA and TMA) by ion exchange and gas chromatography, and mass spectroscopy. Methylarsine (MA) was not detected. Most of the extractable arsenic remaining in the soil was present as a trimethyl arsenical compound regardless of the treatment. Since arsenic was transferred to air regardless of arsenical treatment, the transfer of arsenic from soil to air is part of the natural arsenic cycle in soils.

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