Biogeochemical Controls of Uranium Bioavailability from the Dissolved Phase in Natural Freshwaters

Abstract

To gain insights into the risks associated with uranium (U) mining and processing, we investigated the biogeochemical controls of U bioavailability in the model freshwater species Lymnaea stagnalis (Gastropoda). Bioavailability of dissolved U(VI) was characterized in controlled laboratory experiments over a range of water hardness, pH, and in the presence of complexing ligands in the form of dissolved natural organic matter (DOM). Results show that dissolved U is bioavailable under all the geochemical conditions tested. Uranium uptake rates follow first order kinetics over a range encompassing most environmental concentrations. Uranium uptake rates in L. stagnalis ultimately demonstrate saturation uptake kinetics when exposure concentrations exceed 100 nM, suggesting uptake via a finite number of carriers or ion channels. The lack of a relationship between U uptake rate constants and Ca uptake rates suggest that U does not exclusively use Ca membrane transporters. In general, U bioavailability decreases with increasing pH, increasing Ca and Mg concentrations, and when DOM is present. Competing ions did not affect U uptake rates. Speciation modeling that includes formation constants for U ternary complexes reveals that the aqueous concentration of dicarbonato U species (UO2(CO3)2(-2)) best predicts U bioavailability to L. stagnalis, challenging the free-ion activity model postulate.

pH‐dependent speciation and hydrogen (H2) control U(VI) respiration by Desulfovibrio vulgaris

“…Contrary to the conventional wisdom that mono-nuclear complexes are more bioavailable (Bernhard et al, 1998;Croteau et al, 2016;Drobot et al, 2015;Du et al, 2011;Guillauont et al, 2003;Jones et al, 2015;Kulkarni et al, 2016), we found that all of the mono-nuclear, and poly-nuclear species of U(VI) were about equally bioavailable. This means that pH should not affect U(VI) respiration as long as pH itself is not inhibitory.…”

“…Cytochrome c3 has a mid-point redox potential near −300 mV versus SHE (standard hydrogen electrode) at 25°C (Niki et al, 1984), while the U(VI)-U(IV) couple has a mid-point potential of +334 mV versus SHE of dissolved inorganic carbon (DIC) and with a wide range of pH is of paramount importance for understanding microbial reduction of U(VI), since different U(VI) species could have different susceptibilities of being reduced based on their bioavailability. A decrease in U(VI) reduction rate with increasing DIC concentration, along with sensitivity towards pH, has been well documented in the literature (Belli et al, 2015;Bernhard, Geipel, Brendler, & Nitsche, 1998;Brooks et al, 2003;Croteau, Fuller, Cain, Campbell, & Aiken, 2016;Drobot et al, 2015;Guillauont et al, 2003;Jones et al, 2015;Sheng & Fein, 2014;Ulrich et al, 2011).…”

“…Figure classifies U(VI) speciation based the level of uranium complexation: Free UO 2 2+ , the mono‐nuclear complexes (UO 2 CO 3 , UO 2 (CO 3 ) 2 2− , UO 2 (CO 3 ) 3 4− , UO 2 OH + , UO 2 (OH) 4 2− ), and the poly‐nuclear complexes (UO 2 ) 3 (CO 3 ) 3 6− , (UO 2 ) 2 (OH) 2 2+ , (UO 2 ) 3 (OH) 5 + , (UO 2 ) 2 (OH) 3+ , (UO 2 ) 3 (OH) 4 2+ , (UO 2 ) 3 (OH) 7 − , and (UO 2 ) 4 (OH) 7 + ). If the poly‐nuclear species are not bioavailable (Bernhard et al, ; Croteau et al, ; Guillauont et al, ; Jones et al, ; Lovley et al, ; McCarty, ; Natrajan et al, ), U(VI) respiration ought to be slow in the pH range of 5–8, where the poly‐nuclear species dominate U(VI) speciation. If poly‐nuclear species are bioavailable, U(VI) reduction should be largely independent of pH as long as an extreme pH is not itself inhibitory (Wall, ).…”

“…All trends in Figure 2 agree with trends in previous speciation modeling of a similar system (Krestou & Panias, 2004 Figure S2). (Bernhard et al, 1998;Croteau et al, 2016;Guillauont et al, 2003;Jones et al, 2015;Lovley et al, 1991;McCarty, 2007;Natrajan et al, 2014), U(VI) respiration ought to be slow in the pH range of 5-8, where the poly-nuclear species dominate U(VI) speciation. If poly-nuclear species are bioavailable, U(VI) reduction should be largely independent of pH as long as an extreme pH is not itself inhibitory (Wall, 2007).…”

“…The conventional thinking is that free UO 2 2+ and its mono-nuclear complexes with hydroxides and carbonates are bioavailable to microorganisms (Tutem, Apak, Turgut, & Apak, 1991). Poly-nuclear complexes are known to have lesser cell permeability and, hence, may have lower bioavailability (Bernhard et al, 1998;Croteau et al, 2016;Drobot et al, 2015;Guillauont et al, 2003;Jones et al, 2015;Kulkarni, Misra, Gupta, Ballal, & Apte, 2016). In addition, adsorption of U(VI) to colloidal or particulate ligands should render it even less bioavailable (Du et al, 2011).…”