| Abstract
| - Although humic substances occur ubiquitously in soil andgroundwater, their effect on the biological reduction ofuranium(VI) and subsequent reoxidation of U(IV) is poorlyunderstood. This study investigated the role of humicsin enhancing the bioreduction of U(VI) in laboratory kineticstudies, in field push−pull tests, and in the presence orabsence of metal ions such as Ca2+ and Ni2+, which areknown to inhibit the biological reduction of U(VI). Resultsfrom laboratory experiments indicate that, under strictanaerobic conditions, the presence of humic materialsenhanced the U(VI) reduction rates (up to 10-fold) andalleviated the toxicity effect of Ni2+ on microorganisms.Humic acid was found to be more effective than fulvic acidin enhancing the reduction of U(VI). Such an enhancementeffect is attributed to the ability of these humics infacilitating electron-transfer reactions and/or in complexingCa2+ and Ni2+ ions. Similarly, field push−pull testsdemonstrated a substantially increased rate of U(VI)reduction when humic acid was introduced into the sitegroundwater. However, humics were also found to formcomplexes with reduced U(IV) and increased the oxidationof U(IV) (when exposed to oxygen) with an oxidation half-life on the order of a few minutes. Both of these processesrender uranium soluble and potentially mobile in groundwater,depending on site-specific and dynamic geochemicalconditions. Future studies must address the stability andretention of reduced U(IV) under realistic field conditions(e.g., in the presence of dissolved oxygen and lowconcentrations of complexing organics).
|
