Efficacy of soluble sodium tripolyphosphate amendments for the in-situ immobilisation of uranium

Abstract

Environmental context. Contamination of groundwater and sediments by heavy metals and radioactive metals is a significant problem within the United States Department of Energy complex as a result of past nuclear operations. One way to remediate these metals is through reaction with phosphate compounds, which can immobilise the metals by forming highly stable metal phosphate compounds. Long-chain, water-soluble phosphate compounds provide a means to inject phosphate into subsurface contaminant plumes, to precipitate metal ions from solution. Results presented here illustrate that application of a soluble sodium tripolyphosphate to sediment contaminated with uranium will rapidly reduce the concentration of uranium in the pore water to concentrations near or below drinking water limits under water-saturated and unsaturated conditions. Abstract. A series of conventional water-saturated and pressurised unsaturated flow column experiments were conducted to evaluate the effects of using soluble polyphosphate amendments for in-situ, subsurface remediation of uranium. Experiments were conducted under mildly alkaline, calcareous conditions, representative of conditions commonly encountered at sites across the arid western United States. Results presented here illustrate that application of a soluble polyphosphate amendment to sediment contaminated with uranium will rapidly reduce the concentration of uranium released to the porewater to near or below drinking water limits under water-saturated and -unsaturated conditions. Column experiments conducted in the absence of polyphosphate illustrate sustained release of aqueous uranium at concentrations well above drinking water standards in excess of over 25 pore volumes under water-saturated conditions and over 50 pore volumes under unsaturated conditions. In the presence of tripolyphosphate, the concentration of aqueous uranium released from the sediment was below drinking water limits within 10 to 35 pore volumes under water-saturated and unsaturated conditions, respectively. Moreover, results indicate the necessity of conducting site-specific dynamic tests to tailor phosphate-based remediation technology to site specific geochemical and hydrological conditions.