PIMS with Apatite II for the Remediation of U and Pu
Judith Wright, PIMS NW, Inc., Carlsbad, NM 88220, 505-628-0916, firstname.lastname@example.org and James Conca, EES-12, Earth and Environmental Science Division, LANL-Carlsbad Office, MS A141, Los Alamos National Laboratory, Carlsbad, NM 88220, 505-628-3018, email@example.com
Uranium (U) and plutonium (Pu) are the two actinide elements most likely to occur as contaminants in the environment. Phosphate-Induced Metal Stabilization (PIMS) has been shown to be effective both in sequestering dissolved U, Pu and other metals, and in transforming soil-bound metals to less soluble phases. The reactive media developed for these applications is Apatite II™, an inexpensive, amorphous form of a carbonated hydroxy-apatite with random embedded apatite nanocrystals, resulting in rapid precipitation of various phosphate phases, e.g., autunite (Ksp=10-49) and Pu(HPO4)2 (Ksp=10-24). Apatite II is also an efficient non-specific surface adsorber. The nominal composition of Apatite II is Ca10-xNax(PO4)6-x(CO3)x(OH)2 where x < 1. For U, the overall reaction is actually a dissolution reaction of the Apatite II followed by precipitation of autunite, Ca(UO2)2(PO4)2· 10H2O, on existing Apatite II surfaces, simplified as:
Ca10-xNax(PO4)6-x(CO3)x(OH)2 + 14H+ à (10-x)Ca2+ + xNa+ + (6-x)[H2(PO4)]- + xH2CO3 + 2H2O(1)
2UO22+ + Ca2+ + 2H2(PO4)- + 10H2O à Ca(UO2)2(PO4)2· 10H2O + 4H+(2)
Similar reactions occur for Pu and Pb, among other metals. The degree of protonation of the phosphate and carbonate, and the exact actinide species involved, depend upon the pH and aqueous chemistry. The above example is for the range of pH between 3 and 6 in Oak Ridge groundwater in equilibrium with air. Reaction (1) does not usually lead to reaction (2), but provides a constant supply of phosphate to solution to induce reaction (2) whenever UO22+ is in solutions contacting the apatite. This excess dissolution leads to the strong pH buffering (to pH 6.5 to 7) exhibited by Apatite II from reaction (1). However, the low solubility of Apatite II prevents excess P from remaining in solution thus no phosphate loading occurs. PIMS with Apatite II has been successfully applied at Oak Ridge and Paducah for U, and feasibility studies have been successful at Los Alamos for Pu and U. In these studies, Apatite II sequestered up to 50% of its weight in U that subsequently passed EPA TCLP leach tests.