Microbial metabolism of triethylphosphate, a potential phosphate source for radionuclide mineralization

Significant quantities of metals and radionuclides contaminate unsaturated zones at several sites in the western U.S. In many cases, this contamination has migrated to groundwater, sometimes decades after being released into the subsurface. A potentially useful approach for immobilizing radionuclides such as uranium and strontium in the vadose zone is precipitation with microbially-generated phosphate. Triethylphosphate (TEP) is a low-toxicity organophosphate that can be vaporized and delivered to the vadose zone. Microbes can catalyze TEP degradation, leading to the release of inorganic phosphate that can then lead to the precipitation of phosphate minerals. These minerals are typically highly stable and poorly soluble under environmental conditions. Sequestration in phosphate minerals is a promising strategy for mitigating radionuclide transport in the environment. To examine the feasibility of this strategy, we set up lab-scale incubation experiments with TEP-amended synthetic groundwater inoculated with vadose zone-derived mixed cultures from the Idaho National Laboratory (INL), and sediment slurries using solids from the Hanford Reservation in Washington (U.S. Department of Energy facilities with significant radionuclide contamination in the vadose zone). The amount of phosphate released in the cultures was monitored, and the microbial communities were characterized with a high-density microarray (PhyloChip). Significant biodegradation of TEP was observed in the experiments with the synthetic groundwater amended with 5 mM TEP. Phosphate concentrations in live cultures steadily increased to >0.25 mM after 13 months with no phosphate accumulated in killed controls. Surprisingly, no evidence for phosphate mineral precipitation was observed, contrary to expectations based on equilibrium considerations. Studies are underway to investigate potential kinetic inhibition of precipitation under these conditions. Cell counts increased by approximately one order of magnitude during that period. Significant decreases in the d13C values of dissolved inorganic carbon in the live cultures were observed, indicating the microbial community was respiring the carbon in the TEP. In contrast, no significant accumulation of phosphate was observed in the sediment slurries with 5 mM TEP, most likely due to phosphate adsorption to the solids. Microbial community identification indicated that organisms in the families of Xanthomonadaceae, Crenotrichaceae and Comamonadaceae were enriched by the addition of TEP. Further characterization of radionuclide-biota interactions would lead to enhanced understanding of the fate and transport of these contaminants in the subsurface.