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Environmental Factors Affecting Rates of Arsine Evolution from and Mineralization of Arsenicals in Soil

Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616.

^* Corresponding author (rgburau{at}ucdavis.edu).

ABSTRACT

Arsenic (As) compounds are used as pesticides; As accumulation in soil is a concern. Arsenicals are subject to microbial reduction and methylation leading to volatilization as arsines while organic arsenicals can be demethylated or mineralized to inorganic arsenic. This laboratory study systematically examines the effects of arsenic form, arsenic concentration, soil moisture, soil temperature, and cellulose amendment on these transformations in samples of a Sacramento silty day soil. Volatilization and mineralization rates were both greatly affected by these factors. Arsine evolution rate followed the order: sodium cacodylate (CA) > methanearsonic acid (MAA) > sodium arsenite [As(III)] = sodium arsenate [As(V)]. Evolution rate increased linearly with cacodylate concentration in the range of 0 to 100 mg As ka^–1 soil. The optimum soil moisture level was between 250 and 350 g H₂O kg^–1 soil (–0.3-–0.03 MPa). More arsines were produced at 25°C than at 5°C. The data suggested that CA demethylates directly to As(V) rather than through MAA as an intermediate. Cellulose addition enhanced arsine evolution. Arsenate was the main metabolite from degradation of MAA and CA. The rate of MAA mineralization was slower than that of CA under the same conditions. Methylation of arsenic in soil, that is, transformation of arsenate to MAA or of MAA to CA, was negligible. The amount of CA mineralized was linearly related to the concentration of CA amended into the soil indicating that the rate is first order. Mineralization of CA increased as soil moisture increased from 50 to 550 g H₂O kg^–1 soil but a strong effect was shown only when soil moisture was >350 g H₂O kg^–1 soil (–0.03 MPa). At 25°C mineralization of CA followed a rate decreasing function but at 5°C, it followed zero-order kinetics. Cellulose addition (0–5%, w/w) depressed CA degradation. The overall percentage of CA and MAA mineralized after 70 d ranged from 3 to 87% that was much higher than arsenic loss as volatile arsines (0.001–0.4%) suggesting that loss of arsenic from some soils to the atmosphere may not be a major pathway and inorganic arsenic may accumulate in soil from arsenical usage.

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