Dissolution kinetics of meta-torbernite under circum-neutral to alkaline conditions

Abstract

Environmental context. Uranium-phosphate minerals have been identified as a long-term controlling phase that limit the mobility of uranium to groundwater in many contaminated subsurface environments. Complex, coupled processes confound the ability to isolate the rates attributed to individual processes. Results of this investigation provide the necessary information to refine current prediction on the release and long-term fate of uranium in subsurface environments. Abstract. The purpose of this investigation was to conduct a series of single-pass flow-through (SPFT) tests to (1) quantify the effect of temperature (23–90°C) and pH (6–10) on meta-torbernite dissolution; (2) compare the dissolution of meta-torbernite to other autunite-group minerals; and (3) evaluate the effect of aqueous phosphate on the dissolution kinetics of meta-torbernite. Results presented here illustrate meta-torbernite dissolution rates increase by ~100× over the pH interval of 6 to 10, irrespective of temperature. The power law coefficient for meta-torbernite, η = 0.59 ± 0.07, is greater than that quantified for Ca-meta-autunite, η = 0.42 ± 0.12. This suggests the stability of meta-torbernite is greater than that of meta-autunite, which is reflected in the predicted stability constants. The rate equation for the dissolution of meta-torbernite as a function of aqueous phosphate concentration is log rdissol (mol m–2 s–1) = –4.7 × 10–13 + 4.1 × 10–10[PO43–].

Dissolved Carbonate and pH Control the Dissolution of Uranyl Phosphate Minerals in Flow-Through Porous Media

“…Effluent cation concentrations indicated ion exchange with the BPW solution, especially in the K-ankoleite and Na-autunite systems, which account for nonstoichiometric release of counter cations. This has been previously seen during the dissolution of synthetic and natural uranyl phosphate ,,, and oxyhydroxide minerals, where the nonstoichiometric release of interlayer cations was independent of pH and temperature and orders-of-magnitude faster with respect to U release. In our study, monovalent Na, the most labile cation, was strongly depleted, while bivalent Ca was exchanged with interlayer cations and/or incorporated into the mineral structures of K-ankoleite and Na-autunite during dissolution.…”

“…Although the identity of the interlayer cation does not strongly influence the dissolution rate, bivalent Ca in interlayer sites has stronger bonds to uranyl-phosphate layers than monovalent interlayer cations, which may impart greater structural stability (Figure S5b). A similar stabilization mechanism was noted in dissolution studies of meta-torbernite (Cu­(UO 2 ) 2 (PO 4 ) 2 ·6.5H 2 O) compared to meta-ankoleite (K) and meta-autunite (Ca, Na). , …”

“…Dissolution rates (log R s , mol m –2 s –1 ) of uranyl phosphate (circles), oxyhydroxide (diamonds) and silicate (squares) minerals obtained from this work (full symbols) and the literature (empty symbols) as a function of (A) pH or (B) dissolved total carbonate (log TC). Dissolution rate were obtained from: (i) flow-through column (FTC) experiments of synthetic K-ankoleite, Na-autunite, Ca-autunite and chernikovite (this work), and K-compreignacite and Na-compreignacite (K 2 (UO 2 ) 6 O 4 (OH) 6 ·8H 2 O and Na 2 (UO 2 ) 6 O 4 (OH) 6 ·8H 2 O; Reinoso-Maset et al); (ii) single-pass flow-through (SPFT) experiments of a natural Ca-autunite (meta-autunite, Ca­[(UO 2 )­(PO 4 )] 2 ·3H 2 O) and a synthetic Na-autunite (Na­[(UO 2 )­(PO 4 )]·3H 2 O) (Gudavalli et al; , Wellman et al), and a natural meta-torbenite (Cu­[(UO 2 )­(PO 4 )] 2 ·6.5H 2 O; Wellman et al ); and (iv) batch experiments of synthetic uranophane and soddyite ((UO 2 ) 2 SiO 4 ·2H 2 O; Pérez et al , ).…”

Effect of pH and temperature on the carbonate promoted dissolution of sodium meta-autunite

“…Detailed procedures for error calculations are presented in Wellman et al (2006Wellman et al ( , 2009.…”

“…The error analysis determining the standard deviations of the dissolution rates was performed by following the procedures described in Wellman et al (2009). The experimental results were correlated by linear regression using SigmaPlot-11.2 (Systat Software Inc.).…”