Calcium-Uranyl-Carbonato Species Kinetically Limit U(VI) Reduction by Fe(II) and Lead to U(V)-Bearing Ferrihydrite

Abstract

Reaction conditions and mechanisms promoting or inhibiting U reduction exert a central control on U solubility and, therefore, U transport and its associated risks. Here, we vary and track common aqueous uranium species to show that a kinetic restriction inhibits homogeneous reduction of the calcium-uranyl-carbonato species (CaUO2(CO3)3 2– and Ca2UO2(CO3)3) by Fe­(II)(aq), while ferrihydrite surface-catalyzed reduction of all aqueous uranyl by Fe­(II) proceeds. Using U L3 high energy resolution fluorescence detection (HERFD) X-ray absorption near edge structure (XANES) spectroscopy, U L3 extended X-ray absorption fine structure (EXAFS) spectroscopy, and transmission electron microscopy (TEM), we also show that U­(V) is generated and incorporated into ferrihydrite formed from homogeneous U­(VI) reduction by Fe­(II)(aq). Through elucidation of the mechanisms that inhibit reduction of the calcium-uranyl-carbonato species and promote stabilization of U­(V), we advance our understanding of the controls on U solubility and thus improve prediction of U transport in surface and subsurface systems.

Multi‐Isotope Based Identification and Quantification of Oxygen Consuming Processes in Uranium Hosting Aquifers With CO2 + O2 In Situ Leaching

“…The formation of the uncharged Ca 2 UO 2 (CO 3 ) 3 (aq) species can inhibit U(VI) adsorption onto sediments at circumneutral to weakly alkaline pH conditions (W. Dong & Brooks, 2006; W. M. Dong et al., 2005; J. Liu et al., 2018; Stewart et al., 2010). Ca 2+ inhibited the reduction of U(VI) due to the presence of Ca 2 UO 2 (CO 3 ) 3 (aq) complex, which is a less effective electron acceptor than the other U(VI) forms (Brooks et al., 2003; Dewey et al., 2020). Therefore, the aqueous species of U in the groundwater of the mining area decreased the adsorption onto sediments and enhanced its migration.…”

Soil and Aquifer Properties Combine as Predictors of Groundwater Uranium Concentrations within the Central Valley, California

“…Likewise, modeling aqueous U concentrations with geochemical reactions for U­(VI) adsorption on ferrihydrite illustrated that increasing Ca concentrations promoted U mobilization under increasing bicarbonate concentrations (Figure C). Ternary uranyl–Ca–carbonato species have a low affinity for soil/sediment solids, limiting adsorption. , The Ca 2 UO 2 (CO 3 ) 3 0 complex is both uncharged, limiting electrostatic retention, and chemically inert with respect to chemisorption due to Ca atoms bonded to equatorial carbonate anions. , The Ca position within the ternary complex blocks access to the equatorial ligands and limits chemical reactivity with respect to both adsorption and reduction by Fe­(II) . Fox et al noted that predictions of U­(VI) adsorption were improved in a surface complexation model when Ca 2+ was included, and Stewart et al explained their adsorption data by modeling ternary uranyl–Ca–carbonato species as unreactive with mineral surfaces.…”

“…34,75 The Ca position within the ternary complex blocks access to the equatorial ligands and limits chemical reactivity with respect to both adsorption and reduction by Fe(II). 76 Fox et al 34 noted that predictions of U(VI) adsorption were improved in a surface complexation model when Ca 2+ was included, and Stewart et al 33 explained their adsorption data by modeling ternary uranyl−Ca− carbonato species as unreactive with mineral surfaces.…”

Deep anoxic aquifers could act as sinks for uranium through microbial-assisted mineral trapping

“…Although the presence of calcite and Ca in groundwater is reported to suppress the removal of U(VI)-carbonates by silicate minerals 65 , Fe-sulfides are expected to hinder U remobilization by fast reduction of U(VI). While UO 2 (CO 3 ) 3 4-(aq) in a Ca-free system can be readily reduced by Fe(II) (aq) species 66 in Ca-rich settings, the dominating Ca 2 UO 2 (CO 3 ) 3(aq) is a more stable complex that is directly identified in deep aquifer settings even under reducing conditions due to the occurrence of Fe(II) (aq) 67 . A minor effect from U(VI) reduction on FeS might be due to the fact that it is formed later in stagnant water under nearly steady-state conditions 47 .…”