Comparison of the kinetic rate law parameters for the dissolution of natural and synthetic autunite in the presence of aqueous bicarbonate ions

Rates and mechanisms of uranyl oxyhydroxide mineral dissolution

“…The overall dissolution rates calculated here for K-and Na-compreignacite in low carbonate BPW solutions are lower by about 2 orders-of-magnitude than those reported by Gudavalli et al (2013) for Ca-autunite at pH 7, but rates are similar or higher for K-and Nacompreignacite in high carbonate BPW solution experiments at pH 8.40. These contrasting rates may be a consequence of intrinsic surface and structural differences between uranyl phosphate and oxyhydroxide minerals, as well as differences between measurements in batch flow versus porous media.…”

“…For soddyite ((UO 2 ) 2 -SiO 4 Á2H 2 O), a uranyl silicate without interlayer cations, rates were on the order of 10 À14 mol m À2 s À1 with a very slight increase in the dissolution rate even with 10 times higher carbonate concentration (Pérez et al, 1996). In a study of Ca-autunite (Ca(UO 2 ) 2 (PO 4 ) 2 Á3H 2 O) dissolution in batch flow-through experiments, overall U dissolution rates on the order of 10 À11 mol m À2 s À1 at pH 7-8 were reported, and increased by 2-3 orders-of-magnitude at pH 10-11 and 6-fold higher carbonate concentration (Gudavalli et al, 2013). The Na end-member mineral (Na Table 5 Linear combination and shell-by-shell fits of U L III -edge EXAFS for K-compreignacite and Na-compreignacite reacted with low and high carbonate background porewater (BPW).…”

“…Dissolution rates have not been reported for any uranyl oxyhydroxide minerals, and differences in rates at different pH and carbonate concentrations have not been investigated. Dissolution rate studies of other uranyl mineral families included three uranyl silicates (Pérez et al, 1996(Pérez et al, , 2000Ilton et al, 2006) and two uranyl phosphates (Gudavalli et al, 2013). In these studies, higher concentration of dissolved carbonate and pH enhanced apparent mineral solubility or dissolution rate by 1-3 orders of magnitude, with the mechanism for U(VI) detachment attributed to the formation of aqueous complexes of uranyl-carbonate and Cauranyl-carbonate species.…”

“…Similar rate equations were used for flow reactor experiments in Pérez et al (2000) and Gudavalli et al (2013), but without considering the mass in the reactor or the decrease of particle size and solid concentration as the mineral dissolved. Here, Eq.…”

“…Most prior dissolution studies of uranyl minerals were carried out as batch experiments, although some studies employed continuously stirred tank flow-through reactors (Pérez et al, 2000) or single-pass flow-through batch reactors (Gudavalli et al, 2013). An advantage of dynamic over static experimental systems is the removal of reaction products that may influence reaction rate or solubility of mineral phases (Handley-Sidhu et al, 2009).…”