Environmental availability of uranium from secondary uranyl minerals: Experimental determination of the dissolution kinetics of natural and synthetic autunite minerals

Autunite minerals are the targeted product of in-situ immobilization efforts for uranium remediation using phosphate amendments. They have been shown to form in contaminated areas and have been suggested as a long-term controlling phase of aqueous uranium concentrations. In order to assess the efficacy of precipitating uranium-phosphate minerals in-situ for uranium immobilization, the kinetic dissolution rate of autunite must be quantified under relevant environmental conditions to evaluate the longevity of precipitated minerals.; Single-pass flow-through (SPFT) tests were conducted on well-characterized natural and synthetic autunite minerals, natural calcium meta-autunite, Ca[(UO 2)(PO4)]2 · 3H2O, and synthetic sodium meta-autunite, Na[(UO2)(PO4)] · 3H 2O, to quantify the dissolution rate as a function of pH (7--10) and temperature (5--70°C) in the presence and absence of aqueous organic material. Results show: (1) the dissolution of autunite minerals increases as a function of temperature and pH (i.e., pH = 7--10 and temperature = 5 to 70°C) in 0.01 and 0.05 TRIS buffer solutions; (2) in the absence of aqueous organic material, solubility limits for secondary minerals were rapidly met and had a pronounced affect on the apparent dissolution; and (3) aqueous organic material forms complexes with dissolved uranium, lowering its activity within the system, precluding secondary phase solubility limits and enhancing the dissolution rate. The mechanistic dissolution of autunite occurs through basal plane cleavage resulting in the physical release of the interlayer cation. Dissolution of the uranyl-phosphate polyhedreal backbone occurs through hydrolysis of the uranyl polyhedra. The results further demonstrate the significance of structure in understanding the mechanism of mineral dissolution and the limitations of current mechanistic models of dissolution (i.e., Transition-State Theory and water-ligand exchange).; The ability of autunite minerals to persist under conditions highly conducive to dissolution supports phosphate amendment strategies for remediation of uranium contaminated groundwater. Autunite mineral precipitation presents the distinct advantage of not relying on redox manipulation to immobilize uranium. Moreover, solubility limits for secondary uranyl phases under seemingly dilute conditions will pose an additional limiting factor for the long-term control of uranium within the environment.