Biological Perchlorate Reduction Research Articles

Biological Reduction of Perchlorate from Spent Modified Reed Mediated by an Enriched Microbial Culture

Disposal of untreated perchlorate-laden modified reed (MR) generated in the treatment of contaminated water risks further contamination issues. The primary aim of this study was to assess the enriched microbial culture (EMC)-mediated biodegradation of perchlorate bound to spent quaternary amine-MR adsorbent. The culture was enriched to utilize perchlorate as an electron acceptor and brewer’s yeast as an electron donor. Kinetics experiments were performed to determine biodegradation rates and the total time for complete removal. The secondary aim of this research was the development and testing of a microwave-assisted extraction (MAE)-facilitated accurate perchlorate quantitation method. Extraction efficiency was optimized by adjusting the nitric acid concentration, resulting in 90 ± 5% recovery. Results show that EMC can degrade aqueous perchlorate effectively and consistently. EMC can also effectively degrade adsorbed perchlorate from spent MR. In conclusion, anaerobic biodegradation of perchlorate from spent MR can be used as an efficient, cost-effective, and environmentally friendly alternative or supplement to other treatment options, such as ion exchange and incineration. This approach can also be an effective solution to the waste generated by using MR to remove perchlorate from contaminated water.

Enabling the use of seawater for hydrogen gas production in water electrolyzers

Hydrogen gas will play an increasingly critical role in developing a carbon-neutral energy infrastructure, but it will need to be produced by water splitting using renewable electricity sources. A recent study by Veroneau and Nocera in Proceedings of the National Academy of Sciences (PNAS) suggests an electrolyzer that produces desalinated water from abundant seawater by using forward osmosis.

Biological perchlorate reduction: which electron donor we can choose?

Biological reduction is an effective method for removal of perchlorate (ClO4-), where perchlorate is transformed into chloride by perchlorate-reducing bacteria (PRB). An external electron donor is required for autotrophic and heterotrophic reduction of perchlorate. Therefore, plenty of suitable electron donors including organic (e.g., acetate, ethanol, carbohydrate, glycerol, methane) and inorganic (e.g., hydrogen, zero-valent iron, element sulfur, anthrahydroquinone) as well as the cathode have been used in biological reduction of perchlorate. This paper reviews the application of various electron donors in biological perchlorate reduction and their influences on treatment efficiency of perchlorate and biological activity of PRB. We discussed the criteria for selection of appropriate electron donor to provide a flexible strategy of electron donor choice for the bioremediation of perchlorate-contaminated water.

Biological reduction of perchlorate in domesticated activated sludge considering interaction effects of temperature, pH, electron donors and acceptors

This study investigated the direct microbial reduction of perchlorate using domesticated activated sludge and showed the interaction effect of water quality factors on perchlorate bio-reduction. The dominant perchlorate-reducing bacteria belong to Dechloromonas sp., which was successfully acclimated from regular activated sludge after 20 days domestication. Batch reduction experiments indicated that high bio-reduction efficiency of perchlorate could be achieved at 303–313 K and pH = 6.5–7.5. Perchlorate-reducing bacteria can more efficiently utilize acetate and butanedioic to reduce perchlorate compared with the other electron donors that significantly affect the microbial community structure and dominant bacteria of activated sludge. The three-level and three-variable Box-Behnken Design response surface method predicted a maximum perchlorate reduction efficiency of 100% under the following optimal conditions: pH = 7.5, 308 K, and 0.51 mg/L of acetate. Furthermore, the influence of two factors on perchlorate reduction was in this order: interaction between pH and acetate > interaction between pH and temperature > interaction between temperature and acetate.

Perchlorate Reductase Is Distinguished by Active Site Aromatic Gate Residues

Perchlorate is an important ion on both Earth and Mars. Perchlorate reductase (PcrAB), a specialized member of the dimethylsulfoxide reductase superfamily, catalyzes the first step of microbial perchlorate respiration, but little is known about the biochemistry, specificity, structure, and mechanism of PcrAB. Here we characterize the biophysics and phylogeny of this enzyme and report the 1.86-Å resolution PcrAB complex crystal structure. Biochemical analysis revealed a relatively high perchlorate affinity (Km = 6 μm) and a characteristic substrate inhibition compared with the highly similar respiratory nitrate reductase NarGHI, which has a relatively much lower affinity for perchlorate (Km = 1.1 mm) and no substrate inhibition. Structural analysis of oxidized and reduced PcrAB with and without the substrate analog SeO3 (2-) bound to the active site identified key residues in the positively charged and funnel-shaped substrate access tunnel that gated substrate entrance and product release while trapping transiently produced chlorate. The structures suggest gating was associated with shifts of a Phe residue between open and closed conformations plus an Asp residue carboxylate shift between monodentate and bidentate coordination to the active site molybdenum atom. Taken together, structural and mutational analyses of gate residues suggest key roles of these gate residues for substrate entrance and product release. Our combined results provide the first detailed structural insight into the mechanism of biological perchlorate reduction, a critical component of the chlorine redox cycle on Earth.

Influence of pH and Nitrate on Perchlorate Biological Reduction

The pH and nitrate effect on the anaerobic biological reduction of perchlorate using acetate as carbon source was studied. Perchlorate reduction occurred throughout the pH range from 6.0 to 9.0. However, initiation of perchlorate reduction was rapid under neutral pH conditions. At initial pH 7, perchlorate reduction followed pseudo-first-order with the reaction rate constants was 0.3732 d-1 and the half-life time was 1.85 d. In the absence and presence of nitrate, the kinetics of perchlorate reduction with the reaction rate constants were 0.3229 d-1 and 0.2307 d-1 (after a lag phase for 3 d), the half-life time were 2.1 d and 6 d. With multiple additions of nitrate, perchlorate removal was observed with a longer lag period, the consequent production of high level nitrite may delay perchlorate degradation.

A comparative study of perchlorate degradation in acute toxicity and chronic toxicity

Since the discovery of perchlorate in water system, the public has been concerned about its human health effect. In practice it was reported that chronic exposure to perchlorate may lead to damage in thyroid hormone activity. This study introduced a method of perchlorate reduction, using autotrophic bacteria which utilise hydrogen as an electron donor. Two experiments were conducted to compare the effects of acute and chronic perchlorate toxicity on bacterial perchlorate reduction potential. One was a batch-fed operation generating acute toxicity and another was a continuous-fed operation generating chronic toxicity. Acclimation period of the batch-fed operation was 14 days while that of the continuous-fed operation was 31 days as commensurate with double. Lots of batch tests using the mixed culture passing through acclimation were conducted to figure out kinetics of biological perchlorate reduction. The maximum perchlorate utilisation rate (q(max)) of the mixed culture acclimated by acute toxicity was 2.92 mg ClO(4)(-)/mg dry-weight (DW)/d, while that of chronic toxicity was 0.27 mg ClO(4)(-)/mg DW/d. Half-maximum rate constants (K(s)) of acute and chronic toxicity were 567.3 and 25.6 mg ClO(4)(-)/L respectively. This result showed that acute toxicity acclimated the mixed culture more rapidly and produced a higher activity for biological perchlorate reduction than chronic toxicity.

In Situ Bioremediation of Perchlorate in Vadose Zone Soil Using Gaseous Electron Donors: Microcosm Treatability Study

A series of microcosm experiments were performed to determine the effectiveness of various gaseous electron donors (including hydrogen, 1-hexene, ethyl acetate, and liquefied petroleum gas [LPG]) for supporting biological perchlorate reduction under different electron donor concentrations and soil moistures. Under high soil moisture (16% w/w) conditions, complete or partial perchlorate degradation was achieved with all of the tested electron donors, except for ethyl acetate. Hydrogen was the most promising of the tested electron donors, achieving complete perchlorate degradation with first-order rate constants ranging from 0.13 to 0.20 day(-1) and reducing concentrations to non-detectable levels within 35 to 42 days. The LPG and 1-hexene each promoted partial perchlorate reduction, with average first-order rate constants of 0.05 and 0.11 day(-1), respectively. Although significant perchlorate reduction was observed with as little as 13% moisture, the moisture content for complete perchlorate degradation in this particular soil was determined to be 17%.

Biological Perchlorate Reduction in Packed Bed Reactors Using Elemental Sulfur

Sulfur-utilizing perchlorate (ClO4-)-reducing bacteria were enriched from a denitrifying wastewater seed with elemental sulfur (S0) as an electron donor. The enrichment was composed of a diverse microbial community, with the majority identified as members of the phylum Proteobacteria. Cultures were inoculated into bench-scale packed bed reactors (PBR) with S0 and crushed oyster shell packing media. High ClO4-concentrations (5-8 mg/L) were reduced to < 0.5 mg/L at an empty bed contact time (EBCT) of 13 h. Low C1O4- concentrations (60-120 microg/L), more typical of contaminated groundwater sites, were reduced to < 4 microg/L at an EBCT of 7.5 h. PBR performance decreased when effluent recirculation was applied or when smaller S0 particle sizes were used, indicating that mass transfer of ClO4- to the attached biofilm was not the limiting mechanism in this process, and that biofilm acclimation and growth were key factors in overall reactor performance. The presence of nitrate (6.5 mg N/L) inhibited ClO4- reduction. The microbial community composition was found to change with ClO4- availability from a majority of Beta-Proteobacteria near the influent end of the reactor to primarily sulfur-oxidizing bacteria near the effluent end of the reactor.

Fractionation of stable isotopes in perchlorate and nitrate during in situ biodegradation in a sandy aquifer

Environmental context. Perchlorate (ClO4–) and nitrate (NO3–) are common co-contaminants in groundwater, with both natural and anthropogenic sources. Each of these compounds is biodegradable, so in situ enhanced bioremediation is one alternative for treating them in groundwater. Because bacteria typically fractionate isotopes during biodegradation, stable isotope analysis is increasingly used to distinguish this process from transport or mixing-related decreases in contaminant concentrations. However, for this technique to be useful in the field to monitor bioremediation progress, isotope fractionation must be quantified under relevant environmental conditions. In the present study, we quantify the apparent in situ fractionation effects for stable isotopes in ClO4– (Cl and O) and NO3– (N and O) resulting from biodegradation in an aquifer. Abstract. An in situ experiment was performed in a shallow alluvial aquifer in Maryland to quantify the fractionation of stable isotopes in perchlorate (Cl and O) and nitrate (N and O) during biodegradation. An emulsified soybean oil substrate that was previously injected into this aquifer provided the electron donor necessary for biological perchlorate reduction and denitrification. During the field experiment, groundwater extracted from an upgradient well was pumped into an injection well located within the in situ oil barrier, and then groundwater samples were withdrawn for the next 30 h. After correction for dilution (using Br– as a conservative tracer of the injectate), perchlorate concentrations decreased by 78% and nitrate concentrations decreased by 82% during the initial 8.6 h after the injection. The observed ratio of fractionation effects of O and Cl isotopes in perchlorate (ϵ18O/ϵ37Cl) was 2.6, which is similar to that observed in the laboratory using pure cultures (2.5). Denitrification by indigenous bacteria fractionated O and N isotopes in nitrate at a ratio of ~0.8 (ϵ18O/ϵ15N), which is within the range of values reported previously for denitrification. However, the magnitudes of the individual apparent in situ isotope fractionation effects for perchlorate and nitrate were appreciably smaller than those reported in homogeneous closed systems (0.2 to 0.6 times), even after adjustment for dilution. These results indicate that (1) isotope fractionation factor ratios (ϵ18O/ϵ37Cl, ϵ18O/ϵ15N) derived from homogeneous laboratory systems (e.g. pure culture studies) can be used qualitatively to confirm the occurrence of in situ biodegradation of both perchlorate and nitrate, but (2) the magnitudes of the individual apparent ϵ values cannot be used quantitatively to estimate the in situ extent of biodegradation of either anion.

Chemisorption of oxygen onto activated carbon can enhance the stability of biological perchlorate reduction in fixed bed biofilm reactors

Fixed bed biofilm reactors with granular activated carbon (GAC) or glass beads as support media were used to evaluate the influence of short-term (12 h) and long-term (23 days) increases of influent dissolved oxygen (DO) concentrations on biological perchlorate removal. The goal was to evaluate the extent by which chemisorption of oxygen to GAC can enhance the stability of biological perchlorate reduction. Baseline influent concentrations were 50 μg/L of perchlorate, 2 mg/L of acetate as C, and 1 mg/L of DO. Perchlorate removal in the glass bead reactor seized immediately after increasing influent DO concentrations from 1 to 4 mg/L since glass beads have no sorptive capacity. In the biologically active carbon (BAC) reactor, chemisorption of oxygen to GAC removed a substantial fraction of the influent DO, and perchlorate removal was maintained during short-term increases of influent DO levels up to 8 mg/L. During long-term exposure to influent DO concentrations of 8.5 mg/L, effluent perchlorate and DO concentrations increased slowly. Subsequent exposure of the BAC reactor bed to low DO concentrations partially regenerated the capacity for oxygen chemisorption. Microbial analyses indicated similar microbial communities in both reactors, which confirmed that the differences in reactor performance during dynamic loading conditions could be attributed to the sorptive properties of GAC. Using a sorptive biofilm support medium can enhance biological perchlorate removal under dynamic loading conditions.

Kinetics of biological perchlorate reduction and pH effect

Batch experiments were conducted to investigate the kinetics of perchlorate reduction by heterotrophic and mixed perchlorate-reducing bacteria. Substrate-utilizing and cellular maintenance models were employed to fit the experimental data for microbial perchlorate reduction. The half saturation constant, K s, obtained in this study was below 0.1 mg/L, which indicated that perchlorate-reducing bacteria are effective at utilizing low concentrations of perchlorate. The effect of pH on the kinetics of microbial perchlorate reduction was also studied. Perchlorate reduction occurred throughout the pH range from 5.0 to 9.0. Nevertheless, the rates of perchlorate removal by a unit mass of bacteria were significantly different at various pHs with a maximum rate at pH 7.0. The variation of q max with pH was described well with a Gaussian peak equation. This equation is expected to be applicable for practical purposes when pH effects need to be considered.

Effect of backwashing on perchlorate removal in fixed bed biofilm reactors

The influence of backwashing on biological perchlorate reduction was evaluated in two laboratory scale fixed bed biofilm reactors using 1- or 3-mm glass beads as support media. Influent perchlorate concentrations were 50 μg/L and acetate was added as the electron donor at a concentration of 2 mg C/L. Perchlorate removal was evaluated at various influent dissolved oxygen (DO) concentrations. Complete perchlorate removal was achieved with an influent DO concentration of 1 mg/L resulting in bulk phase DO concentrations below the detection limit of 0.01 mg/L. The influence of increasing influent DO concentrations for 12 h periods was evaluated before and after individual backwash events. Partial perchlorate removal was achieved with an influent DO concentration of 3.5 mg/L before a strong backwash (bulk phase DO concentrations of approximately 0.2 mg/L), while no perchlorate removal was observed after the strong backwash at the same influent DO level (bulk phase DO concentrations of approximately 0.8 mg/L). The immediate effect of backwashing depended on influent DO concentrations. With influent DO concentrations of 1 mg/L, strong backwashing resulted in a brief (<12 h) increase of effluent perchlorate concentrations up to 20 μg/L; more pronounced effects were observed with influent DO concentrations of 3 mg/L. Daily weak backwashing had a small and, over time, decreasing negative influence on perchlorate reduction, while daily strong backwashing ultimately resulted in the breakdown of perchlorate removal with influent DO concentrations of 3 mg/L.

Perchlorate Reduction by Autotrophic Bacteria Attached to Zerovalent Iron in a Flow-Through Reactor

Biological reduction of perchlorate by autotrophic microorganisms attached to zerovalent iron (ZVI) was studied in flow-through columns. The effects of pH, flow rate, and influent perchlorate and nitrate concentrations on perchlorate reduction were investigated. Excellent perchlorate removal performance (> or = 99%) was achieved at empty bed residence times (EBRTs) ranging from 0.3 to 63 h and an influent perchlorate concentration of 40-600 microg L(-1). At the longest liquid residence times, when the influent pH was above 7.5, a significant increase of the effluent pH was observed (pH > 10.0), which led to a decrease of perchlorate removal. Experiments at short residence times revealed that the ZVI column inoculated with local soil (Colton, CA) containing a mixed culture of denitrifiers exhibited much better performance than the columns inoculated with Dechloromonas sp. HZ for reduction of both perchlorate and nitrate. As the flow rate was varied between 2 and 50 mL min(-1), corresponding to empty bed contact times of 0.15-3.8 h, a maximum perchlorate elimination capacity of 3.0 +/- 0.7 g m(-3) h(-1) was obtained in a soil-inoculated column. At an EBRT of 0.3 h and an influent perchlorate concentration of 30 microg L(-1), breakthrough (> 6 ppb) of perchlorate in the effluent did not occur until the nitrate concentration in the influent was 1500 times (molar) greater than that of perchlorate. The mass of microorganisms attached on the solid ZVI/sand was found to be 3 orders of magnitude greater than that in the pore liquid, indicating that perchlorate was primarily reduced by bacteria attached to ZVI. Overall, the process appears to be a promising alternative for perchlorate remediation.

Chemolithotrophic perchlorate reduction linked to the oxidation of elemental sulfur

Perchlorate (ClO(4)(-)) contamination of ground and surface water has been recently recognized as a widespread environmental problem. Biological methods offer promising perspectives of perchlorate remediation. Facultative anaerobic bacteria couple the oxidation of organic and inorganic electron-donating substrates to the reduction of perchlorate as a terminal electron acceptor, converting it completely to the benign end-product, chloride. Insoluble inorganic substrates are of interest for low maintenance bioreactor or permeable reactive barrier systems because they can provide a long-term supply of electron donor without generating organic residuals. The main objective of this research was to investigate the feasibility of utilizing elemental sulfur (S(0)) as an insoluble electron donor for the biological reduction of perchlorate. A chemolithotrophic enrichment culture derived from aerobic activated sludge was obtained which effectively coupled the oxidation of elemental sulfur to sulfate with the reduction of perchlorate to chloride and gained energy from the process for cell growth. The enrichment culture grew at a rate of 0.41 or 0.81 1/d in the absence and presence of added organic carbon for cell growth, respectively. The enrichment culture was also shown to carry out sulfur disproportionation to a limited extent as evidenced by the formation of sulfide and sulfate in the absence of added electron acceptor. When nitrate and perchlorate were added together, the two electron acceptors were removed simultaneously after an initial partial decrease in the nitrate concentration.

Microbial reduction of perchlorate with zero-valent iron

Microbial reduction of perchlorate in the presence of zero-valent iron was examined in both batch and column reactors to assess the potential of iron as the electron donor for biological perchlorate reduction process. Iron-supported mixed cultures completely removed 65 mg/L of perchlorate in batch reactors in 8 days. The removal rate was similar to that observed with hydrogen gas (5%) and acetate (173 mg/L) as electron donors. Repeated spiking of perchlorate to batch reactors containing iron granules and microorganisms showed that complete perchlorate reduction by the iron-supported culture was sustained over a long period. Complete removal of perchlorate by iron-supported anaerobic culture was also achieved in a bench-scale iron column with a hydraulic residence time of 2 days. This study demonstrated the potential applicability of zero-valent iron as a source of electrons for biological perchlorate reduction. Use of zero-valent iron may eliminate the need to continually supply electron donors such as organic substrates or explosive hydrogen gas. In addition, iron is inexpensive, safe to handle, and does not leave organic residuals in the treated water.

Perchlorate reduction in a HYDROGEN‐BASED MEMBRANE–BIOFILM REACTOR

The perchlorate anion (ClO4–) has been found in potentially harmful concentrations in numerous water sources. Because perchlorate is not removed by conventional water treatment processes, new treatment processes are needed. Biological perchlorate reduction is a promising alternative. The authors investigated a hydrogen‐oxidizing hollow‐fiber membrane–biofilm reactor system for perchlorate removal. Hydrogen is an ideal electron donor for biological drinking water treatment because it presents no toxicity, is inexpensive, and is unlikely to persist as a source of biological instability in distributions systems. The reactor delivers hydrogen in an efficient and safe manner. Results showed that biological perchlorate reduction takes place concurrently with nitrate reduction, no specialized inoculation is required, and perchlorate can be removed to below the preliminary regulatory standards with no chemical addition other than hydrogen gas. The optimal pH is 8, and the accumulation of intermediates is unlikely. Full denitrification and pH control may be required for excellent perchlorate removal.

Biological reduction of perchlorate in ion exchange regenerant solutions containing high salinity and ammonium levels.

The most promising technologies to remove perchlorate from water are ion exchange and biological reduction. Although successful, ion exchange only separates perchlorate from water; it does not eliminate it from the environment. The waste streams from these systems contain the caustic or saline regenerant solutions used in the process as well as high levels of perchlorate. Biological reduction could be used to treat the regenerant waste solutions from the ion exchange process. A treatment scheme, combining ion exchange and biodegradation, is proposed to completely remove perchlorate from the environment. Perchlorate-laden resins generate brines containing salt concentrations up to 6% or caustic solutions containing up to 0.5% ammonium. Both, high salt and ammonium hydroxide concentrations are potentially toxic to microorganisms. Therefore, the challenge of the proposed system is to find perchlorate reducing microorganisms that are effective under such stressful conditions. Preliminary results have shown that salt concentrations as low as 0.5% reduced the perchlorate biodegradation rate by 30%; salt concentrations greater than 1% decreased this rate to 40%. Although biodegradation was seen in ammonium levels of 0.4%, 0.6% and 1%, the perchlorate biodegradation rate was 90% of that at 0% ammonium hydroxide. Further research will focus on the isolation and/or acclimation of microorganisms that are able to biodegrade perchlorate under these stressful conditions.

Biological Perchlorate Reduction in High-Salinity Solutions

Perchlorate (ClO 4 −) has been detected in numerous ground and surface waters, and has recently been added to the drinking water Candidate Contaminant List in the United States. Perchlorate can be removed from drinking water using ion exchange, but this results in the production of highly saline (7–12%) perchlorate-contaminated brines. Perchlorate-degrading microbial enrichments capable of growth in highly saline water were obtained by screening six salt water environments including marine and lake surface waters, salt marshes, subtidal sediments, and a biofilm/sludge from a seawater filter. Perchlorate reduction was obtained in three of these samples (seawater, saline lake water, and biofilm/sludge) at a salinity of 3%. The salinity range of two of these cultures was extended through serial transfers into media having higher salt concentrations (3–7%). Growth rates were measured over a salinity range of 1–15%. The maximum growth rate measured for the saline lake-water enrichment was 0.060±0.003 d −1 (doubling time of 11.6±0.8 d) at a salinity of 5%. Growth rates decreased to 0.037±0.002 d −1 at a salinity of 11%, and no growth was observed at salinities of 13 or 15%. These results demonstrate for the first time that biological perchlorate reduction is possible in solutions having a salinity typical of ion exchange brines.