The thermodynamics and kinetics of uranophane dissolution in bicarbonate test solutions

Rates and mechanisms of uranyl oxyhydroxide mineral dissolution

“…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.…”

“…This increase agrees with previous observations for the dissolution of other silicate and phosphate uranyl minerals. The dissolution rate of uranophane (Ca(H 3 O) 2 (UO 2 ) 2 (-SiO 4 ) 2 Á3H 2 O) in flow experiments increased from 2.98 Á 10 À11 to 5.89 Á 10 À11 mol m À2 s À1 from pH 8 to 9 and 1 order-of-magnitude higher dissolved bicarbonate concentration (Pérez et al, 2000). 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).…”

“…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.…”

“…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. Thus, in some prior studies, dissolution rate laws were written as a function of bicarbonate concentration in order to capture the dependence of the dissolution rate on the proposed mechanism (Pérez et al, 2000).…”

“…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).…”