Interactions among phosphate amendments, microbes and uranium mobility in contaminated sediments

Abstract

The use of sequestering agents for the transformation of radionuclides in low concentrations in contaminated soils/sediments offers considerable potential for environmental cleanup. This study evaluated the influence of three types of phosphate (rock phosphate, biological phosphate, and calcium phytate) and two microbial amendments ( Alcaligenes piechaudii and Pseudomonas putida) on U mobility. All tested phosphate amendments reduced aqueous U concentrations more than 90%, likely due to formation of insoluble phosphate precipitates. The addition of A. piechaudii and P. putida alone were found to reduce U concentrations 63% and 31%, respectively. Uranium removal in phosphate treatments was significantly reduced in the presence of the two microbes. Two sediments were evaluated in experiments on the effects of phosphate amendments on U mobility, one from a stream on the Department of Energy's Savannah River Site near Aiken, SC and the other from the Hanford Site, a Department of Energy facility in Washington state. Increased microbial activity in the treated sediment led to a reduction in phosphate effectiveness. The average U concentration in 1 M MgCl 2 extract from U contaminated sediment was 437 μg/kg, but in the same sediment without microbes (autoclaved), the extractable U concentration was only 103 μg/kg. The U concentration in the 1 M MgCl 2 extract was ∼ 0 μg/kg in autoclaved amended sediment treated with autoclaved biological apatite. These results suggest that microbes may reduce phosphate amendment remedial effectiveness.