Combined effects of humic substances and clay minerals on U(VI) bioreduction

In the last several decades, microbial reduction of U(VI) to U(IV) has been applied as a method to remediate uranium contamination in situ. The U(VI) bioreduction kinetics and resulting U(IV) speciation are affected by various environmental factors, such as humic substances (HS) and Fe-bearing clay minerals. Previous studies have largely focused on their individual effects on U(VI) bioreduction. However, HS and Fe(III)-bearing clay minerals often co-exist at uranium contaminated sites, their joint effects may be different from the sum of their individual ones, due to either synergistic or antagonistic interactions between the two. In this work, we investigated the combined effects of HS and Fe(III)-bearing clay minerals (Fe-rich nontronite and Fe-poor montmorillonite) on U(VI) reduction by Shewanella putrefaciens and the resulting U(IV) speciation. HS alone exhibited an inhibitory effect on U(VI) bioreduction, likely because HS retained a fraction of electrons from microbial oxidation of lactate. The resulting U(IV) was in the form of aqueous U(IV)-HS complex. Clay minerals alone also inhibited U(VI) bioreduction because a fraction of reduced U(IV) was likely re-oxidized by structural Fe(III) and thus electrons were partially diverged from U(VI) to structural Fe(III). Bioreduced U(IV) was in a solid phase. However, in the presence of both HS and clay minerals, their combined effect of inhibition on U(VI) bioreduction was not a simple addition of the two individual ones. In the co-presence of clay minerals and a low concentration of HS (10 and 20 mg C/L), their combined inhibition effect on U(VI) bioreduction was stronger than that of HS but weaker than that of clays. The stronger inhibition than that of HS alone was due to electron diversion by clay minerals. The weaker inhibition than that of clays alone was because HS and/or Fe(III)-HS/Fe(II)-HS served as electron shuttles to accelerate the rate of Fe(III) reduction. These double shuttles decreased the ability of structural Fe(III) in clays to diverge electrons from U(VI) and thus partially negated the inhibitory effect of clay minerals. In the co-presence of clay minerals and a high concentration of HS (100 mg C/L), their combined effect on U(VI) bioreduction was initially similar to that in the low concentration of HS scenario. However, over extended time, clay minerals partially negated the inhibitory effect of HS, likely because clay minerals adsorbed a fraction of HS and thus lowered the ability of residual aqueous HS to retain electrons. Biogenic U(IV) was sequestered in solid in the presence of Fe-rich nontronite and HS but partially remained soluble in the presence of Fe-poor montmorillonite and HS. This study highlights the importance of the complex interaction between HS and Fe-bearing clays in bioreduction of U(VI) and speciation of the resulting U(IV).