IN SITU HYDROXYAPATITE PERMEABLE REACTIVE BARRIER PERFORMANCE AT THE OLD RIFLE, CO URANIUM PROCESSING MILL SITE

An ongoing field test is being used to evaluate the long term efficacy of an in situ hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) permeable reactive barrier (PRB) and source area treatment (SAT) technology to remediate uranium at the former US Department of Energy Old Rifle uranium mill processing site near Rifle, Colorado (USA). Uranium ore was processed at the site from the 1940s through the late 1950s. Mill facilities and associated uranium mill tailings previously stored there have all been removed. However, groundwater in the alluvial aquifer beneath the site still contains slightly elevated concentrations of uranium (150-300 ppb) and the site is currently used for field tests to study uranium behavior in groundwater and to investigate potential uranium remediation technologies. The technology being investigated in the current work relies upon the in situ precipitation of hydroxyapatite in subsurface sediments, which serves as a strong sorbent for aqueous uranium. The process involves co-injecting two solutions: the first is a mixture of calcium complexed by citrate and the second is a sodium phosphate solution. After the solutions are co-injected, indigenous aquifer microorganisms degrade the citrate thereby releasing the calcium, which is then able to react with sodium phosphate to form a poorly crystalline hydroxyapatite precipitate. Subsequently, this precipitate serves two roles: it serves as a SAT by preventing uranium leaching from uraniferous minerals in the sediment and acts as a PRB by sorbing and removing uranium from solution. Here we report on the performance of this technology at the Old Rifle site nearly three years after its initial deployment including the solid-phase analysis of finely particulate hydroxyapatite minerals recovered from monitoring wells within the treatment zone. Total extractable uranium associated with these recovered materials suggest an uptake capacity capable of decades of additional uranium sequestration in the absence of additional injections.