The reduction of soluble hexavalent uranium to tetravalent uranium can be catalyzed by bacteria and minerals. The end-product of this reduction is often the mineral uraninite, which was long assumed to be the only product of U(VI) reduction. However, recent studies report the formation of other species including an adsorbed U(IV) species, operationally referred to as monomeric U(IV). The discovery of monomeric U(IV) is important because the species is likely to be more labile and more susceptible to reoxidation than uraninite. Because there is a need to distinguish between these two U(IV) species, we propose here a wet chemical method of differentiating monomeric U(IV) from uraninite in environmental samples. To calibrate the method, U(IV) was extracted from known mixtures of uraninite and monomeric U(IV) and testted using X-ray absorption spectroscopy (XAS). Monomeric U(IV) was efficiently removed from biomass and Fe(II)-bearing phases by bicarbonate extraction, without affecting uraninite stability. After confirming that the method effectively separates monomeric U(IV) and uraninite, it is further evaluated for a system containing those reduced U species and adsorbed U(VI). The method provides a rapid complement, and in some cases alternative, to XAS analyses for quantifying monomeric U(IV), uraninite, and adsorbed U(VI) species in environmental samples.

UraniumLIII-edge X-ray absorption spectroscopy is often used to probe the oxidation state and coordination of uranium in environmental samples, and micrometre-sized beams can be used to spatially map the distribution of uranium relative to other elements. Here a variety of uranium-containing environmental samples are analyzed at both microbeam and larger beam sizes to determine whether reoxidation of U(IV) occurred. Monomeric U(IV), a recently discovered product of U(VI) reduction by microbes and certain iron-bearing minerals at uranium-contaminated field sites, was found to be reoxidized during microbeam (3 µm × 2 µm) analysis of biomass and sediments containing the species but not at larger beam sizes. Thus, care must be taken when using X-ray microprobes to analyze samples containing monomeric U(IV).

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The purpose of this study was to investigate the effect of two different sources of alkalinity source on the mechanisms of metal removal in sulfate-reducing bioreactors. Four upward-flow sulfate-reducing bioreactors each containing a 23 L mixture of organic waste materials and either waste mussel shells or limestone as an alkaline amendment were tested at hydraulic retentions of 3.3 and 10 days to treat acidic mine drainage (pH 2.9, 30 mg/L Fe, 16 mg/L Mn, 5 mg/L Zn) for ten months. A combination of methods was used to examine the effect of alkalinity source on the fate of these metals. Consistent with the monitoring data of the effluent that showed circumneutral pH and low metal concentrations, higher concentrations of Fe, Zn and Mn were found in the spent than the initial substrate, with greater metal and acidity removal in l reactors containing mussel shells (at similar residence times). Sequential extraction procedures found that Fe was mainly in the oxidizable and the residual fractions, Zn in the reducible and residual, and Mn in the exchangeable, reducible and acid extractable fractions. SEM analyses confirmed the presence of pyrite in the substrate, and the use of PHREEQC supported the interpretation that precipitation of iron sulfide and oxyhydroxide minerals, manganese carbonates and zinc sulfide occurred within the substrate for both alkalinity sources. Adsorption edge experiments on the initial substrates confirmed the potential for Zn and Mn to adsorb onto organic materials. Alkalinity source greatly affected system performance with the mussel shell reactors outperforming limestone on a volumetric basis, with the inner surfaces of the mussel shells appearing to be important for greater ongoing alkalinity release, and the outer shells important as metal sorption sites not available in limestone reactors.

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