PNNL

Abstract

Autunite minerals have been frequently identified in contaminated sediments as the long-term controlling phase of uranium. Under these conditions the mobility of uranium in subsurface pore waters is limited by the rate of dissolution of autunite and meta-autunite group minerals, [(UO2)(PO4)]2 ? xH2O. Single-pass flow-through (SPFT) tests were conducted to quantify the dissolution kinetics of natural calcium meta-autunite, Ca[(UO2)2(PO4)2]2 ? 3H2O, and synthetic sodium meta-autunite, Na2[(UO2)(PO4)]2 ? 3H2O, as a function of pH (7 -10) and temperature (5 – 70oC) in the presence and absence of aqueous organic material. The data indicate that release of uranium and phosphorus are non-stoichiometric over the range of experimental conditions investigated. In a 0.1 M NH4OH buffer solution, acquisition of valid dissolution rate data was limited by uramphite solubility, NH4[(UO2)(PO4)]2 • xH2O. Dissolution rates obtained in a 0.01 M TRIS [tris (hydroxymethyl) aminomethane] buffered solution increased by a factor of ~100X over the pH interval of 7 to 10 (? = 0.90±0.08), irrespective of temperature. At constant pH the rate data showed a minor increase with temperature. Data from experiments using a more concentrated 0.05 M TRIS buffer exhibited a ~35-fold increase in rates compared to those in a 0.01 M TRIS buffer at constant temperature and pH. The difference in rate between interlayer cation (Na+ or Ca2+) and uranium release is ~10,000 in neutral solutions; however, the difference diminishes to ~10 at higher pH values. The combination of structural dissolution and ion exchange explain these trends in interlayer cation behavior. Data presented here illustrate the significance of pH and dissolved organic material on the dissolution of autunite minerals.