Competitive Sorption of Arsenic and Phosphorus On Low and High Phosphorus Ultisols of Alabama.

Arsenic (As) contamination of soil and water is of great concern because As is known to be a carcinogen and mutagen.  Competition between As and P in soils is crucial to understand long-term mobility and transport of As.  This study investigates interactions between P and As (as arsenite and arsenate) in Ultisols with different long-term P management strategies that resulted in 5.13 and 82.7 mg P kg^­‑1 in the low and high P soil, respectively.  Batch experiments were conducted to evaluate the effect of soil P, pH, and As species on As sorption to a highly-weathered Ultisol.  Prior to As sorption experiments, the P sorption capacity of the soil was evaluated.  Phosphate sorption ranged from 50-80% in low P soils and 13-20% in high P soils.  Arsenic concentrations were added from 0 to 13 mmol kg^-1 to both high and low P soils at pH 4.5, 5.5, and 6.5.  Results showed a much greater sorption capacity for arsenate (40-80%) than arsenite (25-50%), which was affected by soil P and pH.  Under high soil P conditions and low pH, arsenate addition resulted in 30-40% P desorption indicating there is some competition for binding between arsenate and soil P.  However, almost no P was desorbed from addition of arsenate to the low P soil or addition of arsenite to either soil.  Highest As sorption potentials were observed for arsenate in low P soils at pH 4.5.  In contrast, arsenite adsorption increased with increasing pH.  Results suggest residual soil P does not have a large impact on As sorption except with arsenate under low pH.