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

The Role of Manganese Carbonate Precipitation in Controlling Fluoride and Uranium Mobilization in Groundwater

“…At the measured E H and pH, the most dominant U­(VI) species predicted was Ca 2 UO 2 (CO 3 ) 3(aq) (Figure S3a). Overall, formation of uranyl carbonate complexes, Ca 2 (UO 2 ) 2 CO 3(aq) , UO 2 (CO 3 ) 3 4– , and UO 2 (CO 3 ) 2 , accounted for >90% of all dissolved U in both the aquifers, as DIC is known to be a strong mobilizing agent of U from solid phases. − These U-carbonate aqueous species can even act as precursors to their incorporated forms in carbonate minerals, as determined in past X-ray absorption fine structure spectroscopic investigations. ,,,, For instance, the U carbonate aqueous complex, UO 2 (CO 3 ) 3 4– , could be integrated into the lattice structure of aragonite without any change to the uranyl coordination . Similarly, UO 2 2+ could form a stable tetrahedral coordination within natural calcite by replacing Ca 2+ and two carbonate groups. , An initial mobilization of U, most likely from the residual or Fe/Mn oxide fractions, followed by U reassociation with carbonate and weakly extractable fractions, could explain the relative distribution of U across various fractions identified in the sequential extraction study.…”

“…Although lower DIC (Figure g) and lower U concentrations in aquifer sediments (Figure c) could also reduce dissolution of U in the deep aquifer, reduced pH would likely have a predominant effect. Studies have reported multifold increase in U mineral dissolution kinetics from increase in pH (from 7 to 8) than from increase in DIC concentrations …”

Molecular Mechanisms Underlying Bacterial Uranium Resistance

“…Metal-phosphate complexation and biomineralization are common mechanisms for limiting metal bioavailability and toxicity ( Gudavalli et al, 2018 ; Zhang et al, 2021 ), including uranium ( Table 1 ; Wufuer et al, 2017 ). Since phosphorus is an essential element ( Smil, 2000 ) and soluble phosphate can be scarce in some environments like soil and water bodies, many bacteria use phosphatases to liberate phosphate ions from mineral or organic phosphorus.…”

“…For instance, if long-term immobilization is preferential or if the removal of uranium without recovery is the main goal, uranium-phosphate precipitation could be the best option. Uranium-phosphate minerals, such as autunite or meta-autunite, have generally low aqueous solubility ( Lobeck et al, 2020 ), are stable over a wide temperature and pH range ( Dzik et al, 2017 ; Wufuer et al, 2017 ; Gudavalli et al, 2018 ) and are not prone to remobilization through reoxidation ( Williamson et al, 2014 ; Romanchuk et al, 2020 ). Abiotic remediation with Pi has been tested, but resulted rapidly in phosphate mineral precipitation not linked with uranium and clogged pore spaces that inhibited further diffusion, which was alleviated by using microbial activity to release Pi continuously from polyphosphates or phytate ( Wellman et al, 2006 ).…”

“…Partial protonation of inorganic phosphate starting below pH 4 could hamper abiotic uranyl phosphate mineralization ( Hinsinger, 2001 ). Above circumneutral pH and in the presence of high (bi)carbonate concentrations, highly soluble uranium-carbonate complexes are formed, which can prevent uranyl phosphate precipitation or can solubilize autunite minerals ( Pablo et al, 1999 ; Gudavalli et al, 2018 ). On the other hand, uranyl-hydroxide formation could allow precipitation ( Chandwadkar et al, 2018 ).…”