DEPENDENCE OF NA-AUTUNITE DISSOLUTION KINETICS ON PH

Activities related to the nuclear weapons program have resulted in widespread uranium contamination of soils and sediments at DOE and military sites across the U.S.  Mobility of the uranyl cation, UO₂²⁺, is of concern under neutral to alkaline conditions where UO₂²⁺ is complexed by carbonate to form anionic species.  Under these circumstances the mobility of uranium is not likely to be sorption limited.  Autunite hydrates {(X^1-2+)_2-1(UO₂)₂(PO₄)₂ ·x(H₂O)}(i.e. autunite and its dehydration products) have been found to precipitate from natural and anthropogenically altered groundwaters possessing minimum uranium concentrations of 10^-8 to 10^-9 M.  Additionally, they are thermodynamically stable and generally insoluble over a wide range of geochemical conditions.  Because autunite saturation limits the mobility and controls groundwater concentrations of uranium, it is an important solid phase in the geochemical cycling of uranium. Precipitation of discrete uranyl-phosphate mineral phases indicates that the concentration of uranium is solubility, rather than sorption, limited.  However, previous efforts to quantify key stability data pertinent to understanding the migration of uranium in the subsurface have been limited.

Na-Autunite, Na₂[(UO₂)(PO₄)]₂ •x(H₂O), was synthesized via an ion-exchange process from chernikovite.  Once synthesized, the Na-autunite was characterized using XRD, SEM, and EDS.  Analyses confirmed that the synthesized Na-autunite was the lower hydration state Na₂[(UO₂)(PO₄)]₂ •3(H₂O) with a morphology of non-fractured, well-formed platelets. Synthetic Na-autunite was reacted in Single-Pass Flow-Through (SPFT) experiments, under dilute conditions, at a flow rate of 20 mL d^-1, T = 40^oC, and pH 7, 8, and 9.  Steady-state uranium release rates were achieved between approximately 5 to 8 reactor volumes.  Element release rates increase linearly by nearly three orders of magnitude from pH 7 to 9.  Measured log₁₀ rates at pH 7, 8, and 9, respectively, are -4.2 ± 0.09, -2.6 ± 0.07, and -1.5 ± 0.04 g m^-2 d¹.  Uranium release rates quantified in this study represent the first known experimental measurements reported for the dissolution kinetics of Na-autunite at a single hydration state.  Results presented here highlight the dependence of sodium autunite, Na₂[(UO₂)(PO₄)]₂ •3(H₂O), dissolution on pH.