Kinetics of arsenic release from naturally contaminated soils at half saturation moisture by various extractants

Abstract

Kinetic studies on arsenic (As) release from soils are commonly performed using soils spiked or artificially contaminated with As at soil to solution ratios of 1:10 to 1:50 by mass, which make the conditions of study very different from the prevailing field situations. In this study, the kinetics of As release were investigated using different extractants including 1MK2HPO4, 0.5MNa3C6H5O7, 1MNaOH, 1MNH4F, and 1M (NH4)2SO4 in five naturally As-contaminated soils (80–1680 mg total As/kg) at half saturation moisture over a long period of time (15 min–60 d). Six kinetic models were employed to fit the data. Results indicated that the trend of As release was initially rapid followed by a slow release with K2HPO4, (NH4)2SO4 and Na3C6H5O7 solutions, whereas with NaOH and NH4F an increasing As release was followed by a decreasing trend at longer times, and a considerable amount of the released As was re-adsorbed or precipitated. The order of the cumulative amounts of As released by extractants was K2HPO4>Na3C6H5O7>NaOH>NH4F>(NH4)2SO4. The percentage of total As released during the reaction time was less than 3% for all extractants except K2HPO4, which was between 4.0 and 8.5% in different soils. Therefore, much lower extractability of As occurred in the naturally As-contaminated soils at half saturation moisture than has previously been reported for soils spiked or artificially contaminated with As at lower soil to solution ratios (1:10 to 1:50) suggests that caution should be exercised in extending or applying the results of As release studies on artificially contaminated soils to naturally contaminated soils for remediation purposes. Finally among six kinetic models, only the power function and the simplified Elovich models described the release data satisfactorily.