Influence of Uranyl Speciation and Iron Oxides on Uranium Biogeochemical Redox Reactions

Abstract

Uranium is a pollutant of concern to both human and ecosystem health. Uranium's redox state often dictates its partitioning between the aqueous-and solid-phases, and thus controls its dissolved concentration and, coupled with groundwater flow, its migration within the environment. In anaerobic environments, the more oxidized and mobile form of uranium (UO 2 2+ and associated species) may be reduced, directly or indirectly, by microorganisms to U(IV) with subsequent precipitation of UO 2 . However, various factors within soils and sediments may limit biological reduction of U(VI), inclusive of alterations in U(VI) speciation and competitive electron acceptors. Here we elucidate the impact of U(VI) speciation on the extent and rate of reduction with specific emphasis on speciation changes induced by dissolved Ca, and we examine the impact of Fe(III) (hydr)oxides (ferrihydrite, goethite and hematite) varying in free energies of formation on U reduction. The amount of uranium removed from solution during 100 h of incubation with S. putrefaciens was 77% with no Ca or ferrihydrite present but only 24% (with ferrihydrite) and 14% (no ferrihydrite) were removed for systems with 0.8 mM Ca.Imparting an important criterion on uranium reduction, goethite and hematite decrease the dissolved concentration of calcium through adsorption and thus tend to diminish the effect of calcium on uranium reduction. Dissimilatory reduction of Fe(III) and U(VI) can proceed through different enzyme pathways, even within a single organism, thus providing a potential second means by which Fe(III) bearing minerals may impact U(VI) reduction. We quantify rate coefficients for simultaneous dissimilatory reduction of Fe(III) and U(VI) in systems varying in Ca concentration (0 to 0.8 mM), and using a mathematical construct implemented with the reactive transport code MIN3P, we reveal the predominant influence of uranyl speciation, 2 specifically the formation of uranyl-calcium-carbonato complexes, and ferrihydrite on the rate and extent of uranium reduction in complex geochemical systems.3

Sorption coefficients and molecular mechanisms of Pu, U, Np, Am and Tc to Fe (hydr)oxides: A review

“…These observations revealed that, in carbonatebearing waters, neutral to slightly acidic pH values (∼5) and limited dissolved Ca were optimal for uranium sorption. Steward et al [77,78] further investigated the impact of U(VI) speciation on the extent and rate of biological reduction, with specific emphasis on the impact of dissolved Ca and the FHOs (ferrihydrite, goethite and hematite) on reduction. The amount of U removed from solution during 100 h of incubation with a microbial S. putrefaciens was 77% with no Ca or ferrihydrite present, but only 24% (with ferrihydrite) and 14% (no ferrihydrite) were removed from 0.8 mM Ca systems.…”

Reduction of jarosite by Shewanella oneidensis MR-1 and secondary mineralization

“…This process can result in the formation of various Fe-containing secondary phases, such as green rust, goethite, magnetite, siderite, vivianite, and so on (Nealson and Saffarini, 1994;Fredrickson et al, 1998;Dong et al, 2000;Kukkadapu et al, 2001;Parma et al, 2001;Zachara et al, 2002;Hansel et al, 2003). Besides, microbial redox reactions of iron in soils and sediments can significantly affect the geochemistry of other elements or minerals, e.g., decomposition of organic matters (Lovley and Phillips, 1986;Roden et al, 2010), decomposition and dissolution of various minerals (Dong et al, 2003a,b;Seabaugh et al, 2006), and the mobilization or immobilization of many anions and cations including radionuclides and toxic metals (Lack et al, 2002;Stewart et al, 2011;Masue-Slowey et al, 2011;Lu and Wang, 2012). In the last decades, the DIRB reduction of Fe(III) oxides/oxyhydroxides, such as ferrihydrite (Hansel et al, 2003), goethite (Kukkadapu et al, 2001), amorphous hydrous ferric oxide (Fredrickson et al, 1998), and magnetite (Dong et al, 2000) as well as clay minerals containing Fe(III) (Kostka et al, 1999a,b;Dong et al, 2003a), have been well investigated.…”

Effects of aqueous uranyl speciation on the kinetics of microbial uranium reduction

“…Decreased bioreduction rates in the presence of dissolved inorganic carbon (DIC) and Ca 2+ are attributed to the formation of stable uranyl carbonate and ternary Ca uranyl carbonate complexes which are less thermodynamically favorable for reduction and may be inaccessible to bacteria due to steric hindrance or poor affinity for adsorption to the cell membrane (Brooks et al, 2003;Sheng and Fein, 2014;Sheng et al, 2011;Stewart et al, 2011;Ulrich et al, 2011). Similarly, low molecular weight organic acids (Francis and Dodge, 2008;Haas and Northup, 2004;Sheng et al, 2011;Suzuki et al, 2010) and soil humics (Burgos et al, 2007;Gu et al, 2005) may prevent microbial reductive precipitation of uranium by decreasing the bioavailability of U(VI) (e.g., imparting steric hindrance) and/or increasing the solubility of U(IV).…”