Biological Reduction of Mixed Oxyanions in Wastewater Brine

Treatment of contaminated brine water is an area of active research spurred by the use of advanced treatment technologies, such as membrane separation, reverse osmosis, ion exchange, etc. for water reuse and recycling, which has become critical due to the impact of climate change on water resources throughout the globe. Biological reduction of harmful organic and inorganic pollutants in water has been demonstrated as an effective method for water and wastewater treatment and would be a potential technology for brine treatment. However, biodegradation of contaminants in high salinity waters is challenging because most microbial communities cannot survive brine environments. The biotreatment of high-salinity brines and wastewater is dependent on the identification and isolation of robust halophilic microbes (microbes that require high salinity environments for survival) that have the specific enzymatic structures to reduce targeted organic and inorganic contaminants under extreme salinity conditions.Two halophilic microbes, Halomonas elongata and Haloferax mediterranei were used to biologically reduce co-occurring oxyanion contaminants (nitrate, perchlorate, and chromate) in brines ranging from 8% (w/v) to 20% (w/v) NaCl salinity. Batch microcosm testing was performed with different salt concentrations and varying amounts of contaminants, mimicking concentrations found in industrial waste brines. Previous studies have indicated that these halophilic microbes have specialized nitrate and perchlorate reductase enzymes and are capable of reducing chromium, perchlorate, and nitrate individually. This research sought to determine the mechanism behind chromium reduction from toxic hexavalent Cr(VI) to the less toxic form, Cr(III) in the presence of halophilic microbes and extended the insight into the ability of halophilic microbes to reduce a co-occurring consortium of oxyanions, consisting of nitrate, perchlorate, and chromium. The results show that chromium speciation analysis is a good tool to differentiate Cr(VI) and total(Cr) in the samples, which provided a main line of evidence that Cr(VI) was being reduced to Cr(III) by the halophilic microbes. Microbial growth and abiotic controls supported the hypothesis. The experimental results also showed that Halomonas can reduce a mixture of Cr(VI) and NO3- and Haloferax can reduce a mixture of Cr(VI), NO3-, and ClO4-. The reduction sequence was observed to be chromate>nitrate>perchlorate for both halophilic microbes, which is supported by previous studies and stoichiometry. Long acclimation periods, 4 days to 33 days, were observed between Cr(VI) and subsequent reduction of the other oxyanions, and faster reductions and acclimation periods were observed for lower concentrations of oxyanion contaminants. The results show that promise in the ability of Halomonas elongata and Haloferax mediterranei to successfully bio-reduce chromium and a mixed consortium of oxyanions in brine. An investigation into halophilic biodegradation as a means to treat brine wastes can assist with developing a cost-effective, complete reduction method to treat heavily contaminated brines, resulting in a contaminant-free brine and the potential recovery of salt and water in the process.