Kinetic studies on arsenic release from geogenically enriched soils under oxidized and reduced conditions

Arsenic (As) release under natural conditions controls As mobility, bioavailability, and toxicity in soils. However, batch kinetic investigations on As release are often done using As-spiked soils at wide soil to water ratios of 1: 10 to 1:50, which are far from the typical field conditions. In the present work, the kinetics of As release was studied in five naturally enriched soils under two moisture or redox regimes of oxidized (50 % of saturation moisture) and reduced (saturation moisture) using phosphate as an extractant over 60 days. The rate of As release was fast initially and then gradually decreased until the end of reaction time in all soils under both redox conditions, beyond 60 days in most soils. The cumulative amount of As released from soils under oxidized conditions ranged from 3.2 to 60.1 mg/kg. However, only a small fraction of total As (3.3 to 8.5 %) was released. Arsenic release increased significantly in soils with increasing phosphate concentration. Arsenic release under reduced condi-tions was much higher than under oxidized conditions, ranging from 8.0 to 21 % of total As. Among six kinetic models, the power function model best described As release from all soils under both redox conditions and under low or high concentration of extracting solution. Our results imply that extensive use of phosphate fertilizers in these soils would increase the release of As and augment its environmental cycling. The higher As release under reduced conditions also suggests that environmental risks of these soils will significantly increase if they are utilized under near-saturation conditions or if they are poorly drained.