EVALUATION OF THE EFFICACY OF POLYPHOSPHATE REMEDIATION TECHNOLOGY: DIRECT AND INDIRECT REMEDIATION OF URANIUM UNDER ALKALINE CONDITIONS

Uranium in soluble form is of concern for chemical toxicity and for radiological exposure. Despite the cessation of uranium releases and the removal of shallow vadose zone source materials, groundwater beneath the 300 Area at the Hanford Site in Southeastern Washington State continues to contain uranium at concentrations that exceed US Environmental Protection Agency (EPA) maximum contaminant level (MCL). Polyphosphate remediation technology was optimized through a site-specific treatability test for enhanced monitored natural attenuation of the uranium plume within the 300 Area aquifer. The objective was to demonstrate the efficacy of polyphosphate to: 1) reduce the inventory of available uranium that contributes to the groundwater plume through direct precipitation of uranyl-phosphate (autunite) solids, and 2) provide secondary containment to influent uranium through the precipitation of apatite that can serve as a long-term sorbent for uranium. The field-scale demonstration test site contained 15 monitoring wells and an injection well near the process trenches that had previously received uranium-bearing effluents. The results indicated that direct formation of autunite appears to have been successful; however, the formation of apatite during the test was limited. On the basis of this study, we can conclude that two separate overarching issues impact the efficacy of apatite remediation for uranium sequestration within the 300 Area: 1) efficacy of apatite for sequestering uranium under the present geochemical and hydrodynamic conditions, and 2) formation and emplacement of apatite via polyphosphate technology.