Although humic substances occur ubiquitously in soil and groundwater, their effect on the biological reduction of uranium(VI) and subsequent reoxidation of U(IV) is poorly understood. This study investigated the role of humics in enhancing the bioreduction of U(VI) in laboratory kinetic studies, in field push-pull tests, and in the presence or absence of metal ions such as Ca2+ and Ni2+, which are known to inhibit the biological reduction of U(VI). Results from laboratory experiments indicate that, under strict anaerobic conditions, the presence of humic materials enhanced the U(VI) reduction rates (up to 10-fold) and alleviated the toxicity effect of Ni2+ on microorganisms. Humic acid was found to be more effective than fulvic acid in enhancing the reduction of U(VI). Such an enhancement effect is attributed to the ability of these humics in facilitating electron-transfer reactions and/or in complexing Ca2+ and Ni2+ ions. Similarly, field push-pull tests demonstrated a substantially increased rate of U(VI) reduction when humic acid was introduced into the site groundwater. However, humics were also found to form complexes with reduced U(IV) and increased the oxidation of U(IV) (when exposed to oxygen) with an oxidation halflife on the order of a few minutes. Both of these processes render uranium soluble and potentially mobile in groundwater, depending on site-specific and dynamic geochemical conditions. Future studies must address the stability and retention of reduced U(IV) under realistic field conditions (e.g., in the presence of dissolved oxygen and low concentrations of complexing organics).

Using empirical observations, it is shown that the available potential energy of fluctuations (the APEF) is proportional to the product of the Cox number C,,, the local buoyancy frequency N, and the molecular diffusivity of temperature D. The nondimensional number Cpe = APEF/(ND) is shown to be a good predictor of the Cox number and can be used as an indicator of mixing in stratified fluids. Analysis of the APEF at ocean station P shows that turbulence in the seasonal thermocline responds to strong surface forcing.1.

Reoxidation and mobilization of previously reduced and immobilized uranium by dissolved-phase oxidants poses a significant challenge for remediating uranium-contaminated groundwater. Preferential oxidation of reduced sulfur-bearing species, as opposed to reduced uranium-bearing species, has been demonstrated to limit the mobility of uranium at the laboratory scale yet field-scale investigations are lacking. In this study, the mobility of uranium in the presence of nitrate oxidant was investigated in a shallow groundwater system after establishing conditions conducive to uranium reduction and the formation of reduced sulfur-bearing species. A series of three injections of groundwater (200 L) containing U(VI) (5 μM) and amended with ethanol (40 mM) and sulfate (20 mM) were conducted in ten test wells in order to stimulate microbial-mediated reduction of uranium and the formation of reduced sulfur-bearing species. Simultaneous push-pull tests were then conducted in triplicate well clusters to investigate the mobility of U(VI) under three conditions: 1) high nitrate (120 mM), 2) high nitrate (120 mM) with ethanol (30 mM), and 3) low nitrate (2 mM) with ethanol (30 mM). Dilution-adjusted breakthrough curves of ethanol, nitrate, nitrite, sulfate, and U(VI) suggested that nitrate reduction was predominantly coupled to the oxidation of reduced-sulfur bearing species, as opposed to the reoxidation of U(IV), under all three conditions for the duration of the 36-day tests. The amount of sulfate, but not U(VI), recovered during the push-pull tests was substantially more than injected, relative to bromide tracer, under all three conditions and further suggested that reduced sulfur-bearing species were preferentially oxidized under nitrate-reducing conditions. However, some reoxidation of U(IV) was observed under nitrate-reducing conditions and in the absence of detectable nitrate and/or nitrite. This suggested that reduced sulfur-bearing species may not be fully effective at limiting the mobility of uranium in the presence of dissolved and/or solid-phase oxidants. The results of this field study confirmed those of previous laboratory studies which suggested that reoxidation of uranium under nitrate-reducing conditions can be substantially limited by preferential oxidation of reduced sulfur-bearing species.

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