Quantification of kinetic rate law parameters for the dissolution of natural autunite in the presence of aqueous bicarbonate ions at high concentrations

Uranium is a key contaminant of concern in the groundwater at U.S. Department of Energy (DOE) facilities within the United States and is a potential source of groundwater contamination and a risk to human health and the environment through discharges to surface water. Dissolved inorganic carbon (bicarbonate/carbonate) has a high affinity for complexing with uranium that is present as sorbed or unique uranium-bearing mineral phases within the sedimentary matrix. This process can result in the formation of soluble uranyl carbonate aqueous species, which are mobile under circumneutral pH conditions. This study was conducted to quantify the rate of release of uranium from the autunite mineral, (Ca[(UO₂)(PO₄)]₂•3H₂O), that was formed during polyphosphate injection to remediate uranium; the dissolution of uranium was studied as a function of the aqueous bicarbonate concentration, ranging from 25 to 100 mM. Experiments were carried out in the pH range from 7 to 11 in the temperature range of 23–90 °C via single-pass flow-through testing. Consistent with the results of previous studies (Gudavalli et al., 2013a, 2013b), the rate of uranium release from autunite exhibited minimal dependency on temperature, but was strongly dependent on pH and increasing concentrations of bicarbonate in the solution. Data obtained during these experiments were compared with results of previous experiments conducted using a low-concentration range of bicarbonate solutions (0.5–3.0 mM). An 8- to 30-fold increase in the rate of uranium release was observed in the presence of high bicarbonate concentrations at pH 7–8 compared to low bicarbonate values, while at pH 9–11, there was only a 5-fold increase in uranium rate of release with an increase in bicarbonate concentrations. The rate of uranium release was calculated to be between 5.18 × 10⁻⁸ and 1.69 × 10⁻⁷ mol m⁻² s⁻¹. The activation energy values at high and low bicarbonate concentrations were similar, with ratio values in the range of 0.6–1.0.