Chemistry of arsenic in acid sulphate soils of Northern Alberta

Abstract

Acid sulphate soils from central Alberta were analyzed for mineralogical composition, pore water chemistry, and content of arsenic in order to determine the stability and weathering of minerals and the form, distribution, and mechanisms involved in the accumulation of native arsenic within the soil solum and acid shale parent materials. Weathering of pyrite in the soil solum resulted in strong acidity (pH values near 3) and formation of natrojarosite, selenite, and iron oxyhydroxides. Analyses for total As in soil and segregated mineral samples indicated the element partitioned into iron rich phases. Content of As in some samples of iron enriched soil were as high as 500 mg/kg. Use of selective extradants for estimation of Ca‐, Al‐, and Fe‐arsenate forms provide limited information in this study. Results of thermodynamic modeling of pore water chemistry using the MINTEQ computer code were compared to the empirical data on mineral association of As and mineral alteration during the genesis of the acid sulfate soil. The thermodynamic analyses indicated arsenate precipitates would not be stable under present soil solution conditions.

Key words:

• Trace element
• thermodynamic modeling
• form
• mineralogy