Removal of nitrate and hexavalent uranium from groundwater by sequential treatment in bioreactors packed with elemental sulfur and zero‐valent iron

Abstract

The bioreduction of soluble hexavalent uranium (U(VI)) to insoluble tetravalent uranium (U(IV)) is an attractive bioremediation strategy for the clean-up of contaminated groundwater. High levels of the common occurring co-contaminant, nitrate (NO3(-)), can potentially interfere with uranium bioremediation. In this study, treatment of a synthetic groundwater containing a mixture of NO3(-) and U(VI) was investigated in a sulfur-limestone autotrophic denitrifying (SLAD) bioreactor that was coupled in series with a bioreactor packed with zero-valent iron (Fe(0), ZVI) and sand. An additional aim of the study was to explore the possible role of biological activity in enhancing the reduction of U(VI) by Fe(0). The SLAD reactor removed NO3(-) efficiently (99.8%) at loadings of up to 20 mmol NO3(-) L(r)(-1) d(-1), with near stoichiometric conversion to benign dinitrogen gas (N(2)). The ZVI bioreactor subsequently removed uranium (99.8%) at high (0.22 mM) and low (0.02 mM) influent concentrations of the radionuclide. Aqueous uranium was reliably eliminated to below the maximum contaminant level of 30 µg L(-1) (0.13 µM) when the ZVI reactor was operated at average empty bed hydraulic retention times as low as 2.3 h, demonstrating the feasibility of the sequential treatment strategy in packed bed bioreactors. Sequential extraction of the ZVI reactor packing confirmed that uranium was immobilized as U(IV). Uranium removal was enhanced by microbial activity as confirmed by the increased rate of uranium removal in batch assays inoculated with effluent from the ZVI bioreactor and spiked with Fe(0) compared to abiotic controls.

Sulfur-based autotrophic denitrification of drinking water using a membrane bioreactor

“…The second one is the contamination of treated effluent by microorganisms sloughing off attachment media, sulfur and/or limestone particles. Last one is the generation of sulfide due to sulfur disproportionation process [25] during which elemental sulfur is transformed to sulfide in the absence of electron donor and acceptor, corresponding to low ORP values (<À100 mV). Therefore, presence of elemental sulfur at high concentrations, like in column bioreactors, may increase the possibility of sulfide generation.…”

Characterization of uranium in the extracellular polymeric substances of anaerobic granular sludge used to treat uranium-contaminated groundwater

“…26 The extraction efficiency was analyzed by the total concentration of carbohydrates, proteins, humic substances, and nucleic acids in the EPS extracts, which was determined as previously described. 24 The fractionation of uranium in the AnGS before and aer uranium immobilization reaction was performed according to the literature, 6,7 with successive extractions of anaerobic MilliQ water (overnight), anaerobic NaHCO 3 (1 M, overnight), and HNO 3 (10%, 4 h), representing the water soluble U(VI), adsorbed/complexed U(VI), and insoluble U(IV), respectively.…”

“…[3][4][5] Besides the abovementioned pure cultures, mixed cultures, such as anaerobic granular sludge (AnGS), have also been proven longterm effective for the reduction of U(VI), which have been inoculated to sand column reactors. 6,7 Hence, a permeable reactive barrier inoculated with AnGS will be very benecial for future in situ remediation of uranium-contaminated groundwater due to its convenient maintenance and cost. Moreover, AnGS will also act as an inhabitant in groundwater systems, affecting the U(VI) migration behavior.…”

Designing laboratory metallic iron columns for better result comparability

“…For example, only shaking intensity lower than 50 m −1 could enable the formation of a universal oxide-scale in the vicinity of the Fe 0 surface as observed in column studies and in full-scale barriers [25]. A careful look behind published data on laboratory column experiments ( [27][28][29][30][31][32]; see Table 1) also demonstrates large variability in the experimental design. General design procedures are not available.…”