Chemical Immobilization of Uranium in Contaminated Soil By Phosphate Amendments.

Anthropogenic activities associated with the production of nuclear materials have resulted in uranium (U) contaminated soil and groundwater. The carcinogenic and toxic effects of U contamination pose a significant risk to the environment and human health. Phosphate addition to U-contaminated subsurface environments has been proposed as a strategy for in situ remediation. Saturated and unsaturated batch experiments were performed to investigate the ability of three different phosphate treatments: hydroxyapatite (HA), phytic acid (IP6) and sodium tripolyphosphate (TPP) to chemically immobilize U in contaminated Savannah River Site (SRS) soil. Amendments treatments ranging from 925 to 4620 mg P /kg soil were added to the soil Unsaturated samples, were equilibrated for 3 weeks at 60% of the field capacity, followed by a pore-water extraction by centrifugation. Saturated microcosm experiments were placed on an orbital shaker and equilibrated for 30 days with aliquots taken at specific intervals, in both oxic and anoxic environments.  For the saturated microcosms, HA decreased the mobile U concentration by 98% in both oxic and anoxic environments at all treatment levels. IP6 and TPP were able to decrease the soluble U concentration at low treatment levels, but tended to release U for high treatment level compared to the control. Unsaturated microcosms also showed HA to be the most effective treatment for immobilizing U, but IP6 and TPP were as effective as HA at the lowest treatment level. The poor rates of contaminant immobilization following TPP and IP6 amendments correlate with the dispersion of organic matter and organo-mineral colloids, which probably contain sorbed U. For both experiment types, TPP and IP6 samples showed a very limited PO₄^- in the solution, indicating a slow dissolution of TPP and IP6.