Indigenous Sulfate-reducing Bacteria Research Articles

Recovery of copper-dominated resources from copper mine drainage by chemical oxidation and sulfur biocycling: A pilot-scale study

The copper mine drainage (CMD) formed during copper mining and smelting has the characteristics of being low pH, with a high sulfate concentration and large amounts of metal ions such as Fe, Cu, and Mn. In order to recover copper-dominated many resources from CMD, realize effective control of environmental pollution and avoid the loss of precious resources. The combined process of Fe pretreatment recovery, Cu treatment recovery, and S treatment recovery was established based on H2O2 oxidation, sulfate-reducing bacteria (SRB) reduction, and sulfur-oxidizing bacteria oxidation. A pilot-scale study was carried out with a CMD inlet flow rate of 12 m³/d. Results show that the average Cu2+ removal rate was 99.66%, and the recovered dry matter precipitated (mainly was CuS) copper content was 35.6%. In addition, the average total Fe, Mn and Mg removal rate was 99.46%, 99.10% and 84.08%. And the S2− oxidation rate was above 90% (mainly converted to sulfur). 27.52% Fe precipitated dry matter and 42.9% S, 32.2% Mg, and 13.8% Mn precipitated dry matter were also recovered as by-products. Through high-throughput analysis of the functional and structural changes in the SRB community, showed that indigenous SRB from the copper mine exhibit good adaptability to CMD conditions, with the enrichment and acclimation of SRBs achieving a high level of expression of bioactive. The bottleneck of SRB applied to CMD treatment was solved. Finally, the economic feasibility analysis shows that the combined process treatment CMD can save 58.6% of the cost compared with traditional lime neutralization method, and avoid the sludge treatment cost of 29.03 million USD per year. The above provides engineering practical reference for CMD treatment and resource recovery.

Bioremediasi air asam tambang batubara dengan pengayaan bakteri pereduksi sulfat dan penambahan substrat organik

Acid mine drainage (AMD) has a negative impact that must be managed because it can cause environmental pollution and disrupt mining operations. Acid mine drainage contains dissolved heavy metals that are harmful to the ecosystem. Sulfate-reducing bacteria (SRB) can be used in acid mine drainage bioremediation. This study aims to analyze pH, C-organic and BPS growing in acid water sediments of coal mines and several other ecosystems, analyze C/N ratio and pH of potential organic substrates, and recover AMD by enrichment of indigenous SRB and addition of organic substrates. The ecosystems observed were AMD sediment, paddy field mud, mangrove sediment, in situ sediment, ditch sediment, and rhizosphere soil. The organic materials tested included water hyacinth compost, a mixture of paddy field mud with wood sawdust, oil palm empty fruit bunch (EFB) compost, chicken manure, cow manure, dairy factory waste, and mixtures thereof. The results showed that SRB could be found in all studied ecosystems in the range of 5.8E+2 – 1.26E+5 MPN-unit/dry weight. The value of the C/N ratio with variations from 9.23-89.5 had a negative correlation with increasing pH. The application of organic matter with concentrations of 3%, 5%, 7%, and 10% helped SRB to raise the pH of AMD to more than 6 and reduced Fe by 88.18 – 97.27%. The highest SRB population was obtained in the mixed treatment of EFB and chicken manure with a population of 2.41E+12 MPN- unit/dry weight.

Microbial Diversity of Bacteria Involved in Biomineralization Processes in Mine-Impacted Freshwaters.

In order to increase the knowledge about geo-bio interactions in extreme metal-polluted mine waters, we combined microbiological, mineralogical, and geochemical analyses to study the indigenous sulfate-reducing bacteria (SRB) involved in the heavy metal (HM) biomineralization processes occurring in Iglesiente and Arburese districts (SW Sardinia, Italy). Anaerobic cultures from sediments of two different mining-affected streams of this regional framework were enriched and analyzed by 16S rRNA next-generation sequencing (NGS) technique, showing sequences closely related to SRB classified in taxa typical of environments with high concentrations of metals (Desulfovibrionaceae, Desulfosporosinus). Nevertheless, the most abundant genera found in our samples did not belong to the traditional SRB groups (i.e., Rahnella, Acinetobacter). The bio-precipitation process mediated by these selected cultures was assessed by anaerobic batch tests performed with polluted river water showing a dramatic (more than 97%) Zn decrease. Scanning electron microscopy (SEM) analysis revealed the occurrence of Zn sulfide with tubular morphology, suggesting a bacteria-mediated bio-precipitation. The inocula represent two distinct communities of microorganisms, each adapted to peculiar environmental conditions. However, both the communities were able to use pollutants in their metabolism and tolerating HMs by detoxification mechanisms. The Zn precipitation mediated by the different enriched cultures suggests that SRB inocula selected in this study have great potentialities for the development of biotechnological techniques to reduce contaminant dispersion and for metal recovery.

Gene Sets and Mechanisms of Sulfate-Reducing Bacteria Biofilm Formation and Quorum Sensing With Impact on Corrosion.

Sulfate-reducing bacteria (SRB) have a unique ability to respire under anaerobic conditions using sulfate as a terminal electron acceptor, reducing it to hydrogen sulfide. SRB thrives in many natural environments (freshwater sediments and salty marshes), deep subsurface environments (oil wells and hydrothermal vents), and processing facilities in an industrial setting. Owing to their ability to alter the physicochemical properties of underlying metals, SRB can induce fouling, corrosion, and pipeline clogging challenges. Indigenous SRB causes oil souring and associated product loss and, subsequently, the abandonment of impacted oil wells. The sessile cells in biofilms are 1,000 times more resistant to biocides and induce 100-fold greater corrosion than their planktonic counterparts. To effectively combat the challenges posed by SRB, it is essential to understand their molecular mechanisms of biofilm formation and corrosion. Here, we examine the critical genes involved in biofilm formation and microbiologically influenced corrosion and categorize them into various functional categories. The current effort also discusses chemical and biological methods for controlling the SRB biofilms. Finally, we highlight the importance of surface engineering approaches for controlling biofilm formation on underlying metal surfaces.

Bioremediation of heavy metals pollution from acidic coal gangue with sulfate-reducing bacteria

Acid mine drainage (AMD) generated by acidic coal gangue has caused coal mine area environmental degradation. Sulfate-reducing bacteria (SRB) was used to inhibit the oxidation of metal sulfide minerals from acidic coal gangue in Kaili city, Guizhou province. Four experimental groups (named as CK, R-M, R-S, R-SCa) were carried out, the results showed that SRB can effectively raise the pH (2.3 in CK, 7.8 in R-SCa) and inhibit heavy metals leachability. The low initial pH of the system can retard the start-up of sulfate reduction process. The leaching toxicity test indicated that the leachate of heavy metals in coal gangue after bioremediation was significantly reduced. High-throughput sequencing analysis showed the diversity of SRB genera in R-SCa (e.g., Desulfotomaculum and Desulfobulbus was detected) was enhanced in comparation to R-M and R-S. The results demonstrated that SRB significantly inhibit acidic coal gangue contamination and the biostimulation is also a potential solution due to indigenous SRB existence.

Ferric iron stimulation in marine SMFCs: Impact on the microbial structure evolution in contaminated sediments with low and high molecular weight PAHs

The impact of ferric iron stimulation on the evolution of microbial structure in marine sediment microbial fuel cells (SMFCs), operated for the bioremediation of a complex mixture of low and high molecular weight PAHs (naphthalene, fluorene, pyrene and benzo(a)pyrene), was assessed. Microbial evolution profiles showed high relative abundances of exoelectrogenic iron-reducing bacteria throughout the biodegradation, namely Geoalkalibacter, under ferric iron stimulation and anode reducing conditions, irrespective of sulfate reducing bacteria (SRB) inhibition. Highest PAHs removal was measured in the absence of anode reduction, under Fe stimulation and SRB inhibition, reaching 40.85% for benzo(a)pyrene, the most persistent PAH used in this study. Results suggest that amendment of contaminated sediment with ferric iron could constitute a better bioremediation strategy than using SMFCs. This becomes significant when considering the well-established and dominant indigenous SRB population in marine sediments that usually limits the performance of the anode as a terminal electron acceptor in marine SMFCs.

Sulfate-reducing bacterial community shifts in response to acid mine drainage in the sediment of the Hengshi watershed, South China.

Sulfate-reducing bacteria (SRB) are an attractive option for treating acid mine drainage (AMD) and are considered to be of great significance in the natural attenuation of AMD, but the available information regarding the highly diverse SRB community in AMD sites is not comprehensive. The Hengshi River, which is continually contaminated by AMD from upstream mining areas, was selected as a study site for investigation of the distribution, diversity, and abundance of SRB. Overall, high-throughput sequencing of the 16S rRNA and dsrB genes revealed the high diversity, richness, and OTU numbers of SRB communities, suggesting the existence of active sulfate reduction in the study area. Further analysis demonstrated that AMD contamination decreased the richness and diversity of the microbial community and SRB community, and led to spatiotemporal shifts in the overall composition and structure of sediment microbial and SRB communities along the Hengshi watershed. However, the sulfate reduction activity was high in the midstream, even though AMD pollution remained heavy in this area. Spatial distributions of SRB community indicated that species of Clostridia may be more tolerant of AMD contamination than other species, because of their predominance in the SRB communities. In addition, the results of CCA revealed that environmental parameters, such as pH, TS content, and Fe content, can significantly influence total microbial and SRB community structure, and dissolved organic carbon was another important factor structuring the SRB community. This study extends our knowledge of the distribution of indigenous SRB communities and their potential roles in natural AMD attenuation.

Biosafety of cadmium contaminated sediments after treated by indigenous sulfate reducing bacteria: Based on biotic experiments and DGT technique

Sulfate reducing bacteria (SRB) biostabilization has attracted particular attention due to its ability to prevent and control heavy metal pollution. In this study, biotic experiments (immobilisation test of Daphnia (D.) magna, germination experiment of cucumber seeds, and in vitro experiment using gut juices of Sipunculus (S.) nudus) and diffusive gradients in thin films (DGT) technique were performed to investigate the biosafety of cadmium (Cd) contaminated sediments after being treated by indigenous SRB. Results showed that SRB treatment reduced Cd bioaccessibility of sediment to S. nudus, Cd levels in the overlying water and Cd bioavailability to D. magna. However, the treatment increased the biotoxicity of overlying water due to significant reduction in the root length and germination index of cucumber seeds. DGT results confirmed that SRB treatment increased Cd stability in sediment, and reduced its release from the sediment into the overlying water. The biotoxicity of overlying water was not caused by Cd, but possibly by the added culture medium, SRB itself, or its metabolites. More attention is required to assess the safety of SRB treatment when it is used to remediate environmental matrix contaminated by heavy metals.

Microbiological analysis of cadmium-contaminated sediments during biostabilization with indigenous sulfate-reducing bacteria

Sulfate-reducing bacteria (SRB) have received particular attention in the bioremediation of sediments contaminated with heavy metals. In this study, indigenous SRB were used to stabilize Cd in sediments spiked with different Cd concentrations (≤ 600 mg kg−1). The study investigated the Cd leaching efficiency from sediments after 166 days (d) of biotreatment and assessed the bacterial community and bacteria relationship in sediments during SRB biostabilization. The study found that the Cd leaching efficiency of sediments was reduced by 18.1–40.3% (29.4 ± 8.7%) after 166 days of biotreatment. During the biostabilization, the bacterial community in sediments significantly changed, particularly after 61 days of biotreatment. At the family level, the identified dominant bacteria (mean abundance > 3%) included Bacillaceae, norank Nitrospira, Anaerolineaceae, Nitrospinaceae, Streptococcaceae, and Hydrogenophilaceae. The study also speculated the complex relationships between these bacteria. The relative abundance of Desulfobacteraceae and Desulfobulbaceae in sediments was enhanced after biotreatment. Bacillaceae and Streptococcaceae may play a negative role in Cd biostabilization and inhibited SRB biological activity. However, Anaerolineaceae and Hydrogenophilaceae may have commensalism and mutualism relationships, respectively, with typical SRB. The presence of Nitrospinacea and norank Nitrospira may reduce the inhibitive effect of denitrifying bacteria on SRB, thereby exhibiting a positive effect on biologic sulfate reduction and Cd biostabilization. Indigenous SRB treatment increased Cd stability in sediments and changed bacterial community. During SRB biostabilization, complex relationships between bacteria in sediments were speculated, including competition, syntrophism, and antagonism. These results provide insights for better regulating and controlling SRB biostabilization.

Reductive dissolution of jarosite by a sulfate reducing bacterial community: Secondary mineralization and microflora development

Jarosite is an iron-hydroxysulfate mineral commonly found in acid mine drainage (AMD). Given its strong adsorption capacity and its ability to co-precipitation with heavy metals, jarosite is considered a potent scavenger of contaminants in AMD-impacted environments. Sulfate-reducing bacteria (SRB) play an important role in the reductive dissolution of jarosite; however, the mechanism involved has yet to be elucidated. In this study, an indigenous SRB community enriched from the Dabaoshan mine area (Guangdong, China) was employed to explore the mechanism of the microbial reduction of jarosite. Different cultures, with or without dissolved sulfate and the physical separation of jarosite from bacteria by dialysis bags, were examined. Results indicate that the reduction of jarosite by SRB occurred via an indirect mechanism. In systems with dissolved sulfate, lactate was incompletely oxidized to acetate coupled with the reduction of SO42− to S2−, which subsequently reduced the Fe3+ in jarosite, forming secondary minerals including vivianite, mackinawite and pyrite. In systems without dissolved sulfate, jarosite dissolution occurred prior to reduction, and similar secondary minerals formed as well. Extracellular polymeric substances secreted by SRB appeared to facilitate the release of sulfate from jarosite. Structural sulfate in the solid phase of jarosite may not be available for SRB respiration. Although direct contact between SRB and jarosite is not necessary for mineral reduction, wrapping jarosite into dialysis bags suppressed the reduction to a certain extent. Microbial community composition differed in direct contact treatments and physical separation treatments. Physical separation of the SRB community from jarosite mineral supported the growth of Citrobacter, while Desulfosporosinus dominated in direct contact treatments.

Field-scale bioremediation of arsenic-contaminated groundwater using sulfate-reducing bacteria and biogenic pyrite

This research demonstrates that biogenic pyrite formed by stimulation of indigenous sulfate-reducing bacteria (SRB) in a natural aquifer can remove dissolved arsenic from contaminated groundwater under strongly reducing conditions. SRB metabolism led to the precipitation of biogenic pyrite nanoparticles capable of sorbing and co-precipitating arsenic. The field site is an industrial site where shallow groundwater in an unconfined sandy aquifer is contaminated by arsenic. Therefore, biodegradable organic carbon, ferrous iron, sulfate, and fertilizer were injected into groundwater and SRB metabolism began about 1 week later. Microscopic, X-ray diffraction, X-ray fluorescence, and electron microprobe analyses confirm the bio-mineralization of pyrite and over time, pyrite nanoparticles grew to form well-formed crystals (1–10 µm in diameter) or spherical aggregates that contain 0.05–0.4 wt. % arsenic, indicative of their capacity to sequester arsenic. Consequently, dissolved arsenic decreased from its initial concentration of 0.3–0.5 mg/L to below the regulatory clean-up standard for the site of 0.05 mg/L in three downgradient wells in a matter of weeks after injection. The main sequestration stage, with total arsenic removal rates greater than 90%, lasted for at least 6 months until the arrival and mixing of untreated groundwater from upgradient. Treated groundwater with most active bacterial sulfate reduction became enriched in heavy 34S (range from 2.02 to 4.00 ‰) compared to unaffected well water (0.40–0.61 ‰). One to three orders of magnitude increases in SRB cells were observed in treated wells for at least 2 months after injection. For a full-scale remediation, the injection of solution should start at positions hydrologically upgradient from the major plume and proceed downgradient. If needed, aquifers may be repeatedly amended with biodegradable organic carbon to reestablish the reducing conditions that favor arsenic sequestration.

Complete removal of arsenic and zinc from a heavily contaminated acid mine drainage via an indigenous SRB consortium

Acid mine drainages (AMD) are major sources of pollution to the environment. Passive bio-remediation technologies involving sulfate-reducing bacteria (SRB) are promising for treating arsenic contaminated waters. However, mechanisms of biogenic As-sulfide formation need to be better understood to decontaminate AMDs in acidic conditions. Here, we show that a high-As AMD effluent can be decontaminated by an indigenous SRB consortium. AMD water from the Carnoulès mine (Gard, France) was incubated with the consortium under anoxic conditions and As, Zn and Fe concentrations, pH and microbial activity were monitored during 94days. Precipitated solids were analyzed using electron microscopy (SEM/TEM-EDXS), and Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopy at the As K-edge. Total removal of arsenic and zinc from solution (1.06 and 0.23mmol/L, respectively) was observed in two of the triplicates. While Zn precipitated as ZnS nanoparticles, As precipitated as amorphous orpiment (am-AsIII2S3) (33–73%), and realgar (AsIIS) (0–34%), the latter phase exhibiting a particular nanowire morphology. A minor fraction of As is also found as thiol-bound AsIII (14–23%). We propose that the formation of the AsIIS nanowires results from AsIII2S3 reduction by biogenic H2S, enhancing the efficiency of As removal. The present description of As immobilization may help to set the basis for bioremediation strategies using SRB.

Hydrogen Sulfide Induces Direct Radical-Associated DNA Damage

Hydrogen sulfide (H(2)S) is produced by indigenous sulfate-reducing bacteria in the large intestine and represents an environmental insult to the colonic epithelium. Clinical studies have linked the presence of either sulfate-reducing bacteria or H(2)S in the colon with chronic disorders such as ulcerative colitis and colorectal cancer, although at this point, the evidence is circumstantial and underlying mechanisms remain undefined. We showed previously that sulfide at concentrations similar to those found in the human colon induced genomic DNA damage in mammalian cells. The present study addressed the nature of the DNA damage by determining if sulfide is directly genotoxic or if genotoxicity requires cellular metabolism. We also questioned if sulfide genotoxicity is mediated by free radicals and if DNA base oxidation is involved. Naked nuclei from untreated Chinese hamster ovary cells were treated with sulfide; DNA damage was induced by concentrations as low as 1 micromol/L. This damage was effectively quenched by cotreatment with butylhydroxyanisole. Furthermore, sulfide treatment increased the number of oxidized bases recognized by formamidopyrimidine [fapy]-DNA glycosylase. These results confirm the genotoxicity of sulfide and strongly implicate that this genotoxicity is mediated by free radicals. These observations highlight the possible role of sulfide as an environmental insult that, given a predisposing genetic background, may lead to genomic instability or the cumulative mutations characteristic of colorectal cancer.

Bioremediation of benzene, ethylbenzene, and xylenes in groundwater under iron‐amended, sulfate‐reducing conditions

Elevated concentrations of sulfide in groundwater (approximately 63 mg S(2-)/L in water and 500 mg dissolved H2S/L dissipating from the wellhead) at a field site near South Lovedale (OK, USA) were inhibiting the activity of sulfate-reducing bacteria (SRB) that are known to degrade contaminants, including benzene, toluene, ethylbenzene, and xylenes. Elevated concentrations of these contaminants, except for toluene, also were present in this groundwater. Microcosms were established in the laboratory using groundwater and sediment collected from the field site and amended with various nutrient, substrate, and inhibitor treatments. All microcosms initially were amended with FeCl2 to induce FeS precipitation and, thereby, to reduce aqueous sulfide concentrations. Complete removal of benzene, ethylbenzene, and m+p-xylenes (BEX; o-xylene not detected) was observed within 39 d in treatments with various combinations of nutrient and substrate amendments, including treatments with no amendments (other than FeCl2). This indicates that the elevated concentration of sulfide is the only limiting factor to BEX biodegradation at this site under anaerobic conditions and that treating the groundwater with FeCl2 may be a simple remedy to both facilitate and enhance BEX degradation by the indigenous SRB population.

Dissimilatory Reduction of Sulfate in Black Layer

Black layer has been associated with a severe decline in the quality of turf on putting greens. It was suggested that the black layer results from dissimilatory sulfate (SO42–) reduction. This study was done to determine if SO42– reduction occurs in an existing black layer. Radioactive 35SO42– was used to calculate the rate of SO42– reduction in intact soil cores taken from an existing black layer in a `Penncross' creeping bentgrass (Agrostis palustris Huds. `Penncross') putting green. When 10–3 M 35SO42– with a specific activity of 1.554 × 105 Bq·mg–1 SO42– was injected into a core it reduced to sulfide (35S2–) at a mean rate of 7.1 nmol sulfur (S)/cm3 soil/d. Injecting azide (N3–) or molybdate (MoO42–) at 10% w/v with the label reduced the rate of SO42– reduction to 0.03 and 0.01 nmol S/cm3 soil/d, respectively. The effect of N3– confirmed that reduction of SO42– was biological, while the effect of MoO42– confirmed that the entities responsible for the reductive cycling were sulfate-reducing bacteria (SRBs). This was the first proof that biological reduction of SO42– produces S2– in a black layer from a creeping bentgrass putting green. It was concluded that the respiration of indigenous SRBs was linked to development of this black layer. Thus, a key to successfully controlling black layer in putting greens must involve regulating the respiratory activities of SRBs.

Column experiments to assess the effects of electron donors on the efficiency of in situ precipitation of Zn, Cd, Co and Ni in contaminated groundwater applying the biological sulfate removal technology

In a previous study, we explored the use of acetate, lactate, molasses, Hydrogen Release Compound (HRC, which is based on a biodegradable poly-lactate ester), methanol and ethanol as carbon source and electron donor to promote bacterial sulfate reduction in batch experiments, this with regards to applying an in situ metal precipitation (ISMP) process as a remediation tool to treat heavy metal contaminated groundwater at the site of a nonferrous metal work company. Based on the results of these batch tests, column experiments were conducted with lactate, molasses and HRCI as the next step in our preliminary study for a go-no go decision for dimensioning an on site application of the ISMP process that applies the activity of the endogenous population of sulfate-reducing bacteria (SRB). Special attention was given to the sustainability of the metal precipitation process under circumstances of changing chemical oxygen demand (COD) to [SO4(2-)] ratios or disrupted substrate supply. To optimize the ISMP process, an insight is needed in the composition and activity of the indigenous SRB community, as well as information on the way its composition and activity are affected by process conditions such as the added type of C-source/ electron donor, or the presence of other prokaryotes (e.g. fermenting bacteria, methane producing Archaea, acetogens). Therefore, the biological sulfate reduction process in the column experiments was evaluated by combining classical analytical methods [measuring heavy metal concentration, SO4(2-)-concentration, pH, dissolved organic carbon (DOC)] with molecular methods [denaturing gradient gel electrophoresis (DGGE) fingerprinting and phylogenetic sequence analysis] based on either the 16S rRNA-gene or the dsr (dissimilatory sulfite reductase) gene, the latter being a specific biomarker for SRB. All carbon sources tested promoted SRB activity, which resulted within 8 weeks in a drastic reduction of the sulfate and heavy metal contents in the column effluents. However, unexpected temporal decreases in the efficiency of the ISMP process, accompanied by the release of precipitated metals, were observed for most conditions tested. The most dramatic observation of the failing ISMP process was observed within 12 weeks for the molasses amended column. Subsequent lowering the COD/ SO4(2-) ratio from 1.9 to 0.4 did not alter the outcome of sulfate reduction and metal precipitation efficiency in this set-up. Remarkably, after 6 months of inactivity, bacterial sulfate reduction was recovered in the molasses set up when the original COD/ SO4(2-) ratio of 1.9 was applied again. Intentional disruption of the lactate and HRC supplies resulted in an immediate stagnation of the ISMP processes and in a rapid release of precipitated metals into the column effluents. However, the ISMP process could be restored after substrate amendment. 16S rDNA-based DGGE analysis revealed that the SRB population, in accordance with the results of the previously performed batch experiments, consisted exclusively of members of the genus Desulfosporosinus. The community of Archaea was characterized by sequencing amplicons of archaeal and methanogen-specific PCR reactions. This approach only revealed the presence of non-thermophilic Crenarchaeota, a novel group of organisms which is only distantly related to methane producing Euryarchaeota. DGGE on the dsrB genes was successfully used to link the results of the ISMP process to the community composition of the sulfate reducing bacteria. In the case of an intentional disruption of substrate supply, the ISMP process failed most likely because the growth and activity of the indigenous SRB community stopped due to a lack of a carbon and electron donor. On the other hand, the cause of the sudden temporal shortcomings of the ISMP process in the presence of different substrates was not immediately clear. It was first thought to be the result of competition between methanogenic prokaryotes (MP) and sulfate reducers, since the formation of small amounts of CH4 (0.01-0.03 ppm ml(-1) was detected. However, the results of molecular analyzes indicate that methanogens do not constitute a major fraction of the microbial communities that were enriched in the column experiments. Therefore, we postulate that the SRB population becomes inhibited by the formed metal sulfides. Our results indicate that the ISMP process is highly dependent on SRB-stimulation by substrate amendments and suggest that this remedial approach might not be viable for long-term application unless substrate amendments are continued and environmental conditions are strictly controlled. This will include the removal of affected aquifer material from the metal precipitation zone at the end of the remediation process, or removal of metal precipitates when the microbial activity decreases. Additional tests are necessary to investigate what will happen when clear groundwater passes through the reactive zone while no more C-sources are amended and all indigenous carbon is consumed. Also, the effects of dramatic increases in sulfate- or HM-concentrations on the SRB-community and the concomitant ISMP process need to be studied in more detail.

Reduction of chromate (CrO 42−) by an enrichment consortium and an isolate of marine sulfate-reducing bacteria

An enrichment consortium and an isolate (isolate TKW) of sulfate-reducing bacteria (SRB) have been obtained from metal-contaminated marine sediments of Tokwawan, Hong Kong SAR. These bacteria are capable of reducing highly toxic and soluble hexavalent chromium (Cr 6+) enzymatically into less toxic and insoluble trivalent chromium (Cr 3+) under anaerobic conditions. The enrichment consortium almost completely (98.5%) reduced 0.6 mM Cr 6+ in 168 h and the rate of reduction was 0.5 g (Cr 6+) g (protein) −1 h −1. In comparison, with Cr 6+ as the sole electron acceptor (as a surrogate for SO 4 2−), isolate TKW reduced 94.5% of the initially added Cr 6+ (0.36 mM) in 288 h, with the rate of 0.26 g (Cr 6+) g (protein) −1 h −1. Adsorption by these bacteria was not the major mechanism contributing to the transformation or removal of Cr 6+. The biomass and Cr 3+ in the cultures increased simultaneously with the reduction of Cr 6+. These indigenous SRB might have potential application in bioremediation of metal contaminated sediments.

Microbial release of 226Ra2+ from (Ba,Ra)SO4 sludges from uranium mine wastes.

226Ra2+ is removed from uranium mine effluents by coprecipitation with BaSO4. (Ba,Ra)SO4 sludge samples from two Canadian mine sites were found to contain active heterotrophic populations of aerobic, anaerobic, denitrifying, and sulfate-reducing bacteria. Under laboratory conditions, sulfate reduction occurred in batch cultures when carbon sources such as acetate, glucose, glycollate, lactate, or pyruvate were added to samples of (Ba,Ra)SO4 sludge. No external sources of nitrogen or phosphate were required for this activity. Further studies with lactate supplementation showed that once the soluble SO4(2-) in the overlying water was depleted, Ba2+ and 226Ra2+ were dissolved from the (Ba,Ra)SO4 sludge, with the concurrent production of S2-. Levels of dissolved 226Ra2+ reached approximately 400 Bq/liter after 10 weeks of incubation. Results suggest that the ultimate disposal of these sludges must maintain conditions to minimize the activity of the indigenous sulfate-reducing bacteria to ensure that unacceptably high levels of 226Ra2+ are not released to the environment.