Effect of Nitrate, Acetate and Hydrogen on Native Perchlorate-reducing Microbial Communities and Their Activity in Vadose Soil

The effect of nitrate, acetate, and hydrogen on native perchlorate-reducing bacteria (PRB) was examined by conducting microcosm tests using vadose soil collected from a perchlorate-contaminated site. The rate of perchlorate reduction was enhanced by hydrogen amendment and inhibited by acetate amendment, compared with unamendment. Nitrate was reduced before perchlorate in all amendments. In hydrogen-amended and unamended soils, nitrate delayed perchlorate reduction, suggesting that the PRB preferentially use nitrate as an electron acceptor. In contrast, nitrate eliminated the inhibitory effect of acetate amendment on perchlorate reduction and increased the rate and the extent, possibly because the preceding nitrate reduction/denitrification decreased the acetate concentration that was inhibitory to the native PRB. In hydrogen-amended and unamended soils, perchlorate reductase gene (pcrA) copies, representing PRB densities, increased with either perchlorate or nitrate reduction, suggesting that either perchlorate or nitrate stimulates the growth of the PRB. In contrast, in acetate-amended soil pcrA increased only when perchlorate was depleted: a large portion of the PRB may have not utilized nitrate in this amendment. Nitrate addition did not alter the distribution of the dominant pcrA clones in hydrogen-amended soil, likely because of the functional redundancy of PRB as nitrate-reducers/denitrifiers, whereas acetate selected different pcrA clones from those with hydrogen amendment

Perchlorate Reduction Using Fine Media Fluidized Bed Bioreactor with Oxidation-Reduction Potential-Based Feed Control

Certain bacteria, prevalent in the environment, use perchlorate as an electron acceptor and reduce it to chloride under anaerobic conditions. To develop an ex-situ treatment system for perchlorate-contaminated groundwater, we performed bench-scale test using a fine media fluidized bed reactor (FMFBR; 0.5-ft diameter, 8-ft high) inoculated with a perchlorate-reducing culture. The system was operated under anaerobic conditions. A perchlorate-water solution was introduced into a recirculating stream in the FMFBR at an upward velocity of 16 cm/min. Acetate (acetic acid) was fed as an electron donor. The objective of this study was to establish a minimal acetate feed ratio for sufficient perchlorate reduction by monitoring oxidation-reduction potential (ORP) and, consequently, to prevent ORP from falling to a range of sulfate reduction, and to limit the biomass growth from excess acetate. The FMFBR was able to degrade 3000 - 5000 μg/l perchlorate to less than 4 μg/l in a single pass (16 min empty bed contact time) without excessive hydrogen sulfide production, when effluent ORP (vs. Ag/AgCl) was -290 - -410 mV. Accurate feed control is essential since an imbalance in acetate feed ratio results in unreacted perchlorate or sulfide production. A base feed pump was used to provide 80 % of the acetate required and an ORP controller was used to trim and balance the feed rate using a second pump. The second feed pump was activated when effluent ORP rose to or above -315 mV and deactivated when it fell to or below -320 mV. Some oscillation of effluent ORP was observed, but perchlorate was not detected in the effluent when the oscillation was kept relatively small. Average acetate feed ratio was approximately 1.1-times stoichiometry. For more stable perchlorate degradation, we will examine an earlier ORP detection in the bioreactor column and a more flexible control method for acetate feed

Reduction of Perchlorate and Nitrate by Microbial Communities in Vadose Soil

Perchlorate contamination is a concern because of the increasing frequency of its detection in soils and groundwater and its presumed inhibitory effect on human thyroid hormone production. Although significant perchlorate contamination occurs in the vadose (unsaturated) zone, little is known about perchlorate biodegradation potential by indigenous microorganisms in these soils. We measured the effects of electron donor (acetate and hydrogen) and nitrate addition on perchlorate reduction rates and microbial community composition in microcosm incubations of vadose soil. Acetate and hydrogen addition enhanced perchlorate reduction, and a longer lag period was observed for hydrogen (41 days) than for acetate (14 days). Initially, nitrate suppressed perchlorate reduction, but once perchlorate started to be degraded, the process was stimulated by nitrate. Changes in the bacterial community composition were observed in microcosms enriched with perchlorate and either acetate or hydrogen. Denaturing gradient gel electrophoresis analysis and partial sequencing of 16S rRNA genes recovered from these microcosms indicated that formerly reported perchlorate-reducing bacteria were present in the soil and that microbial community compositions were different between acetate- and hydrogen-amended microcosms. These results indicate that there is potential for perchlorate bioremediation by native microbial communities in vadose soil