Modeling in-situ uranium(VI) bioreduction by sulfate-reducing bacteria

We present a travel-time based reactive transport model to simulate an in-situ bioremediation experiment for demonstrating enhanced bioreduction of uranium(VI). The model considers aquatic equilibrium chemistry of uranium and other groundwater constituents, uranium sorption and precipitation, and the microbial reduction of nitrate, sulfate and U(VI). Kinetic sorption/desorption of U(VI) is characterized by mass transfer between stagnant micro-pores and mobile flow zones. The model describes the succession of terminal electron accepting processes and the growth and decay of sulfate-reducing bacteria, concurrent with the enzymatic reduction of aqueous U(VI) species. The effective U(VI) reduction rate and sorption site distributions are determined by fitting the model simulation to an in-situ experiment at Oak Ridge, TN. Results show that (1) the presence of nitrate inhibits U(VI) reduction at the site; (2) the fitted effective rate of in-situ U(VI) reduction is much smaller than the values reported for laboratory experiments; (3) U(VI) sorption/desorption, which affects U(VI) bioavailability at the site, is strongly controlled by kinetics; (4) both pH and bicarbonate concentration significantly influence the sorption/desorption of U(VI), which therefore cannot be characterized by empirical isotherms; and (5) calcium-uranyl-carbonate complexes significantly influence the model performance of U(VI) reduction.     

Uranium removal from groundwater via in situ biostimulation: Field-scale modeling of transport and biological processes

During 2002 and 2003, bioremediation experiments in the unconfined aquifer of the Old Rifle UMTRA field site in western Colorado provided evidence for the immobilization of hexavalent uranium in groundwater by iron-reducing Geobacter sp. stimulated by acetate amendment. As the bioavailable Fe(III) terminal electron acceptor was depleted in the zone just downgradient of the acetate injection gallery, sulfate-reducing organisms came to dominate the microbial community. In the present study, we use multicomponent reactive transport modeling to analyze data from the 2002 field experiment to identify the dominant transport and biological processes controlling uranium mobility during biostimulation, and determine field-scale parameters for these modeled processes. The coupled process simulation approach was able to establish a quantitative characterization of the principal flow, transport, and reaction processes based on the 2002 field experiment, that could be applied without modification to describe the 2003 field experiment. Insights gained from this analysis include field-scale estimates of the bioavailable Fe(III) mineral threshold for the onset of sulfate reduction, and rates for the Fe(III), U(VI), and sulfate terminal electron accepting processes.     

Mixing-controlled biodegradation in a toluene plume--results from two-dimensional laboratory experiments

Various abiotic and biotic processes such as sorption, dilution, and degradation are known to affect the fate of organic contaminants, such as petroleum hydrocarbons in saturated porous media. Reactive transport modeling of such plumes indicates that the biodegradation of organic pollutants is, in many cases, controlled by mixing and therefore occurs locally at the plume's fringes, where electron donors and electron-acceptors mix. Herein, we aim to test whether this hypothesis can be verified by experimental results obtained from aerobic and anaerobic degradation experiments in two-dimensional sediment microcosms. Toluene was selected as a model compound for oxidizable contaminants. The two-dimensional microcosm was filled with quartz sand and operated under controlled flow conditions simulating a contaminant plume in otherwise uncontaminated groundwater. Aerobic degradation of toluene by Pseudomonas putida mt-2 reduced a continuous 8.7 mg L(-1) toluene concentration by 35% over a transport distance of 78 cm in 15.5 h. In comparison, under similar conditions Aromatoleum aromaticum strain EbN1 degraded 98% of the toluene infiltrated using nitrate (68.5+/-6.2 mg L(-1)) as electron acceptor. A major part of the biodegradation activity was located at the plume fringes and the slope of the electron-acceptor gradient was steeper during periods of active biodegradation. The distribution of toluene and the significant overlap of nitrate at the plume's fringe indicate that biokinetic and/or microscale transport processes may constitute additional limiting factors. Experimental data is corroborated with results from a reactive transport model using double Monod kinetics. The outcome of the study shows that in order to simulate degradation in contaminant plumes, detailed data sets are required to test the applicability of models. These will have to deal with the incorporation of existing parameters coding for substrate conversion kinetics and microbial growth.     

In situ assessment of microbial sulfate reduction in a petroleum-contaminated aquifer using push-pull tests and stable sulfur isotope analyses

Anaerobic microbial activities such as sulfate reduction are important for the degradation of petroleum hydrocarbons (PHC) in contaminated aquifers. The objective of this study was to evaluate the feasibility of single-well push-pull tests in combination with stable sulfur isotope analyses for the in situ quantification of microbial sulfate reduction. A series of push-pull tests was performed in an existing monitoring well of a PHC-contaminated aquifer in Studen (Switzerland). Sulfate transport behavior was evaluated in a first test. In three subsequent tests, we injected anoxic test solutions (up to 1000 l), which contained 0.5 mM bromide (Br-) as conservative tracer and 1 mM sulfate (SO4(2-)) as reactant. After an initial incubation period of 42.5 to 67.9 h, up to 1100 l of test solution/groundwater mixture was extracted in each test from the same location. During the extraction phases, we measured concentrations of relevant species including Br-, SO4(2-) and sulfide (S(-II)), as well as stable sulfur isotope ratios (delta 34S) of extracted, unconsumed SO4(2-) and extracted S(-II). Results indicated sulfate reduction activity in the vicinity of the test well. Computed first-order rate coefficients for sulfate reduction ranged from 0.043 +/- 0.013 to 0.130 +/- 0.015 day-1. Isotope enrichment factors (epsilon) computed from sulfur isotope fractionation of extracted, unconsumed SO4(2-) ranged from 20.2 +/- 5.5@1000 to 22.8 +/- 3.4@1000. Together with observed fractionation in extracted S(-II), isotope enrichment factors provided strong evidence for microbially mediated sulfate reduction. Thus, push-pull tests combined with stable sulfur isotope analyses proved useful for the in situ quantification of microbial sulfate reduction in a PHC-contaminated aquifer.     

Spotting zones of dissimilatory sulfate reduction in a forested catchment: the 34S-35S approach

The localization of sulfate reducing sites in forested catchments is of major importance, because dissimilatory sulfate reduction can be a considerable sink for deposited sulfate. To localize dissimilatory sulfate reduction sites in a forested catchment (northeastern Bavaria, Germany), three sites within the catchment (upland site, intermittent seep, fen) were investigated for delta 34S depth profiles of soil sulfur and potential sulfate reduction rates were measured with 35S radiolabeling. Stable sulfur isotopes indicate that aerobic metabolism is the dominant process on the upland site and the intermittent seep (delta 34S of soil sulfur between +1.6 and +9.0@1000) and dissimilatory reduction is not a significant sink for sulfate. However, results of the 35S radiolabeling indicated for the upland site that the soil has potentially high sulfate reduction rates under laboratory conditions. Soil sulfur of the fen was markedly depleted in 34S (delta 34S between -6 and +2.6@1000). Both, 34S and 35S data indicated that dissimilatory sulfate reduction is an ongoing process on this site. The 34S and 35S approaches are complementary. While measurements using 35S can show momentary potential for dissimilatory bacterial sulfate reduction, delta 34S data reflect long-term predominance of either assimilatory or dissimilatory S metabolism at a particular site.     

生物强化处理石油污染土壤理化性质和微生物学特性的纵向分布特征

采用原位强化生物修复技术对某区块石油污染土壤进行为期16个月的生物修复,考察了处置后污染土壤理化性质、微生物学特性以及石油烃组成的纵向分布特征。实验结果表明,经过修复后各土层的石油烃去除率是表层土IN-3(50.42%)中层土IN-2(23.54%)底层土IN-1(10.51%);IN-1处于缺氧环境,存在硫酸盐还原和反硝化作用,使得土壤pH值从7.86±0.03降低至7.27±0.03,土壤总氮从2.53±0.13 g/kg降低至0.77±0.04 g/kg;厌氧菌的种群数量是IN-1(10.43±0.71×104CFU/g)IN-3(6.74±0.39×104CFU/g)IN-2(5.15±0.42×104CFU/g),放线菌数量与石油烃含量显著负相关(r=-0.989,p=0.0110.05);IN-3对饱和份和芳香份的降解率最高,分别达到了70.27%和54.52%,远高于IN-2和IN-1;模拟蒸馏结果表明,IN-3正构烷烃得到了很大程度的去除,缺氧的IN-1对正构烷烃去除得较少;厌氧菌数量与胶质和沥青质去除率之间成正相关关系,对于污染源较为分散的污染区域,采用原位生物强化修复时可以考虑引入厌氧修复。     

Natural attenuation of a plume from an emplaced coal tar creosote source over 14 years

An emplaced source of coal tar creosote within the sandy Borden research aquifer has documented the long-term (5140 days) natural attenuation for this complex mixture. Plumes of dissolved chemicals were produced by the essentially horizontal groundwater flowing at about 9 cm/day. Eleven chemicals have been extensively sampled seven times using a monitoring network of approximately 280, 14-point multilevel samplers. A model of source dissolution using Raoult's Law adequately predicted the dissolution of 9 of 11 compounds. Mass transformation has limited the extent of the plumes as groundwater has flowed more than 500 m, yet the plumes are no longer than 50 m. Phenol and xylenes have been removed and naphthalene has attenuated from its maximum extent on day 1357. Some compound plumes have reached an apparent steady state and the plumes of other compounds (dibenzofuran and phenanthrene) are expected to continue to expand due to an increasing mass flux and limited degradation potential. Biotransformation is the major process controlling natural attenuation at the site. The greatest organic mass lost is associated with the high solubility compounds. However, the majority of the mass loss for most compounds has occurred in the source zone. Oxygen is the main electron acceptor, yet the amount of organics lost cannot be accounted for by aerobic mineralization or partial mineralization alone. The complex evolution of these plumes has been well documented but understanding the controlling biotransformation processes is still elusive. This study has shown that anticipating bioattenuation patterns should only be considered at the broadest scale. Generally, the greatest mass loss is associated with those compounds that have a high solubility and low partitioning coefficients.     

Liability Theories in Contaminated Groundwater Litigation

Disputes regarding contaminated sites have given rise to an explosion of federal and state court litigation. Congress enacted the Resource Conservation and Recovery Act of 1976 ("RCRA"), 42 U.S.C. §§6901 et seq. , as a contemplated "cradle to grave" regulatory scheme for solid waste, and passed the Comprehensive Environmental Response, Compensation, and Liability Act ("CERCLA"), 42 U.S.C. §§9601 et seq. , to address the investigation and remediation of contaminated sites. RCRA and CERCLA each contemplate a strict, joint and several regulatory liability scheme to address sites that have required billions of dollars to investigate and remediate, and which will require additional billions in order to complete the job. Federal and state law cases involving groundwater contamination cases have presented special challenges to litigants and the Courts. Determining the source of groundwater contamination is often a matter of technical dispute, particularly in the case of commingled plumes or older plumes where original point source locations can no longer be located or where there has been a significant amount of contaminant degradation over time. Contaminated plumes derived in whole or in part from potential continuing sources can present difficult remedy selection issues and uncertainty regarding future cost obligations. While a variety of federal and state law theories of liability are available to address groundwater contamination, each presents certain benefits and limitations. For example, CERCLA claims can cover a wide spectrum of potentially responsible parties and are based on strict liability; however, CERCLA excludes petroleum from the scope of its liability scheme and limits recovery to necessary response costs consistent with the National Contingency Plan. RCRA claims can include petroleum contamination, but cannot seek recovery of past costs. Common law theories such as nuisance, trespass and negligence can permit recovery of a broader scope of potential damages and provide for joint and several liability, but they are not based on notions of strict liability and sometimes present difficult statute of limitations problems. This article identifies federal and California state law liability theories addressing responsibility for groundwater contamination, and reviews the elements of liability, potential limitations on available relief, and recent case law developments under these theories.     

Investigating the role of gas bubble formation and entrapment in contaminated aquifers: Reactive transport modelling

In many natural and contaminated aquifers, geochemical processes result in the production or consumption of dissolved gases. In cases where methanogenesis or denitrification occurs, the production of gases may result in the formation and growth of gas bubbles below the water table. Near the water table, entrapment of atmospheric gases during water table rise may provide a significant source of O(2) to waters otherwise depleted in O(2). Furthermore, the presence of bubbles will affect the hydraulic conductivity of an aquifer, resulting in changes to the groundwater flow regime. The interactions between physical transport, biogeochemical processes, and gas bubble formation, entrapment and release is complex and requires suitable analysis tools. The objective of the present work is the development of a numerical model capable of quantitatively assessing these processes. The multicomponent reactive transport code MIN3P has been enhanced to simulate bubble growth and contraction due to in-situ gas production or consumption, bubble entrapment due to water table rise and subsequent re-equilibration of the bubble with ambient groundwater, and permeability changes due to trapped gas phase saturation. The resulting formulation allows for the investigation of complex geochemical systems where microbially mediated redox reactions both produce and consume gases as well as affect solution chemistry, alkalinity, and pH. The enhanced model has been used to simulate processes in a petroleum hydrocarbon contaminated aquifer where methanogenesis is an important redox process. The simulations are constrained by data from a crude oil spill site near Bemidji, MN. Our results suggest that permeability reduction in the methanogenic zone due to in-situ formation of gas bubbles, and dissolution of entrapped atmospheric bubbles near the water table, both work to attenuate the dissolved gas plume emanating from the source zone. Furthermore, the simulations demonstrate that under the given conditions more than 50% of all produced CH(4) partitions to the gas phase or is aerobically oxidised near the water table, suggesting that these processes should be accounted for when assessing the rate and extent of methanogenic degradation of hydrocarbons.     

Landfill Gas Effects on Groundwater Samples at a Municipal Solid Waste Facility

A study was performed to determine the source of low concentrations of volatile organic compounds (VOCs) detected in groundwater samples at a solid waste management facility. The affected wells were identified as hydraulically upgradient of an old unlined facility, but downgradient of a new clay-lined landfill. These monitoring wells are close to both sites. Subsurface landfill gas migration was identified after a low permeability cap was installed on the older site. Subsurface gas pressure was monitored to identify horizontal landfill gas migration. Monitoring well headspace gases were evaluated to identify depressed oxygen concentrations and methane because of landfill gas migration into the well. Monitoring well headspace gas VOC concentrations were compared to groundwater VOC concentrations to determine the direction of phase transfer. A ratio above 1.0 of the observed well headspace gas concentration of a VOC to the concentration that would be in equilibrium with the groundwater concentration indicates gas-to-water phase transfer within the well. For the major gas-phase and aqueous-phase VOC, cis-1,2-dichloroethene, gas-to-water phase transfer is clearly indicated from the data for two of the four wells. Fifteen other VOCs were detected in monitoring well headspace gases but not in groundwater samples from the four wells studied. Only one compound in one well was detected in the groundwater sample but not in the headspace gases, and only one compound in one well was detected in both matrices at concentrations that suggested water-to-gas phase transfer. This study suggests that if landfill gas is suspected as the source of detected VOCs, monitoring well construction and stratigraphy are important considerations when attempting to differentiate between groundwater contamination by landfill gas and contamination from other sources.     

Effects of nutrient and temperature on degradation of petroleum hydrocarbons in contaminated sub-Antarctic soil

Mesocosm studies using sub-Antarctic soil artificially contaminated with diesel or crude oil were conducted in Kerguelen Archipelago (49 degrees 21' S, 70 degrees 13' E) in an attempt to evaluate the potential of a bioremediation approach in high latitude environments. All mesocosms were sampled on a regular basis over six months period. Soils responded positively to temperature increase from 4 degrees C to 20 degrees C, and to the addition of a commercial oleophilic fertilizer containing N and P. Both factors increased the hydrocarbon-degrading microbial abundance and total petroleum hydrocarbons (TPH) degradation. In general, alkanes were faster degraded than polyaromatic hydrocarbons (PAHs). After 180 days, total alkane losses of both oils reached 77-95% whereas total PAHs never exceeded 80% with optimal conditions at 10 degrees C and fertilizer added. Detailed analysis of naphthalenes, dibenzothiophenes, phenanthrenes, and pyrenes showed a clear decrease of their degradation rate as a function of the size of the PAH molecules. During the experiment there was only a slight decrease in the toxicity, whereas the concentration of TPH decreased significantly during the same time. The most significant reduction in toxicity occurred at 4 degrees C. Therefore, bioremediation of hydrocarbon-contaminated sub-Antarctic soil appears to be feasible, and various engineering strategies, such as heating or amending the soil can accelerate hydrocarbon degradation. However, the residual toxicity of contaminated soil remained drastically high before the desired cleanup is complete and it can represent a limiting factor in the bioremediation of sub-Antarctic soil.     

Determination of Henry's law constant for methyl tert-butyl ether (MTBE) at groundwater temperatures

The dimensionless Henry's law constant was determined for methyl tert-butyl ether (MTBE) at six temperatures (3, 5, 10, 15, 20 and 25 degrees C) by using a thermostatted flask (430 ml) containing an aqueous MTBE solution. The ratio between the gas phase and the water phase in the flask was approximately 1.7:1. The aim of this study was to acquire data needed to model the behaviour of MTBE at groundwater conditions. The dimensionless Henry's law constant at 10 degrees C is approximately 0.01 but is 0.03 at 25 degrees C. This is important for modelling MTBE because the variation of vaporization cannot be disregarded at groundwater temperatures. In a second experiment the water solubility of MTBE was determined to be 62.1 g/l at 5 degrees C and 35.5 g/l at 20 degrees C). The high solubility at low temperatures could cause MTBE plumes from spills (fuel accidents etc.) to spread rapidly.     

Characterization of redox conditions in groundwater contaminant plumes

Evaluation of redox conditions in groundwater pollution plumes is often a prerequisite for understanding the behaviour of the pollutants in the plume and for selecting remediation approaches. Measuring of redox conditions in pollution plumes is, however, a fairly recent issue and yet relative few cases have been reported. No standardised or generally accepted approach exists. Slow electrode kinetics and the common lack of internal equilibrium of redox processes in pollution plumes make, with a few exceptions, direct electrochemical measurement and rigorous interpretation of redox potentials dubious, if not erroneous. Several other approaches have been used in addressing redox conditions in pollution plumes: redox-sensitive compounds in groundwater samples, hydrogen concentrations in groundwater, concentrations of volatile fatty acids in groundwater, sediment characteristics and microbial tools, such as MPN counts, PLFA biomarkers and redox bioassays. This paper reviews the principles behind the different approaches, summarizes methods used and evaluates the approaches based on the experience from the reported applications.     

Microbial community response to addition of polylactate compounds to stimulate hexavalent chromium reduction in groundwater

To evaluate the efficacy of bioimmobilization of Cr(VI) in groundwater at the Department of Energy Hanford site, we conducted a series of microcosm experiments using a range of commercial electron donors with varying degrees of lactate polymerization (polylactate). These experiments were conducted using Hanford Formation sediments (coarse sand and gravel) immersed in Hanford groundwater, which were amended with Cr(VI) and several types of lactate-based electron donors (Hydrogen Release Compound, HRC; primer-HRC, pHRC; extended release HRC) and the polylactate-cysteine form (Metal Remediation Compound, MRC). The results showed that polylactate compounds stimulated an increase in bacterial biomass and activity to a greater extent than sodium lactate when applied at equivalent carbon concentrations. At the same time, concentrations of headspace hydrogen and methane increased and correlated with changes in the microbial community structure. Enrichment of Pseudomonas spp. occurred with all lactate additions, and enrichment of sulfate-reducing Desulfosporosinus spp. occurred with almost complete sulfate reduction. The results of these experiments demonstrate that amendment with the pHRC and MRC forms result in effective removal of Cr(VI) from solution most likely by both direct (enzymatic) and indirect (microbially generated reductant) mechanisms.     

A strategy for aromatic hydrocarbon bioremediation under anaerobic conditions and the impacts of ethanol: a microcosm study

Increased use of ethanol-blended gasoline (gasohol) and its potential release into the subsurface have spurred interest in studying the biodegradation of and interactions between ethanol and gasoline components such as benzene, toluene, ethylbenzene and xylene isomers (BTEX) in groundwater plumes. The preferred substrate status and the high biological oxygen demand (BOD) posed by ethanol and its biodegradation products suggests that anaerobic electron acceptors (EAs) will be required to support in situ bioremediation of BTEX. To develop a strategy for aromatic hydrocarbon bioremediation and to understand the impacts of ethanol on BTEX biodegradation under strictly anaerobic conditions, a microcosm experiment was conducted using pristine aquifer sand and groundwater obtained from Canadian Forces Base Borden, Canada. The initial electron accepter pool included nitrate, sulfate and/or ferric iron. The microcosms typically contained 400 g of sediment, 600 approximately 800 ml of groundwater, and with differing EAs added, and were run under anaerobic conditions. Ethanol was added to some at concentrations of 500 and 5000 mg/L. Trends for biodegradation of aromatic hydrocarbons for the Borden aquifer material were first developed in the absence of ethanol, The results showed that indigenous microorganisms could degrade all aromatic hydrocarbons (BTEX and trimethylbenzene isomers-TMB) under nitrate- and ferric iron-combined conditions, but not under sulfate-reducing conditions. Toluene, ethylbenzene and m/p-xylene were biodegraded under denitrifying conditions. However, the persistence of benzene indicated that enhancing denitrification alone was insufficient. Both benzene and o-xylene biodegraded significantly under iron-reducing conditions, but only after denitrification had removed other aromatics. For the trimethylbenzene isomers, 1,3,5-TMB biodegradation was found under denitrifying and then iron-reducing conditions. Biodegradation of 1,2,3-TMB or 1,2,4-TMB was slower under iron-reducing conditions. This study suggests that addition of excess ferric iron combined with limited nitrate has promise for in situ bioremediation of BTEX and TMB in the Borden aquifer and possibly for other sites contaminated by hydrocarbons. This study is the first to report 1,2,3-TMB biodegradation under strictly anaerobic condition. With the addition of 500 mg/L ethanol but without EA addition, ethanol and its main intermediate, acetate, were quickly biodegraded within 41 d with methane as a major product. Ethanol initially present at 5000 mg/L without EA addition declined slowly with the persistence of unidentified volatile fatty acids, likely propionate and butyrate, but less methane. In contrast, all ethanol disappeared with repeated additions of either nitrate or ferric iron, but acetate and unidentified intermediates persisted under iron-enhanced conditions. With the addition of 500 mg/L ethanol and nitrate, only minor toluene biodegradation was observed under denitrifying conditions and only after ethanol and acetate were utilized. The higher ethanol concentration (5000 mg/L) essentially shut down BTEX biodegradation likely due to high EA demand provided by ethanol and its intermediates. The negative findings for anaerobic BTEX biodegradation in the presence of ethanol and/or its biodegradation products are in contrast to recent research reported by Da Silva et al. [Da Silva, M.L.B., Ruiz-Aguilar, G.M.L., Alvarez, P.J.J., 2005. Enhanced anaerobic biodegradation of BTEX-ethanol mixtures in aquifer columns amended with sulfate, chelated ferric iron or nitrate. Biodegradation. 16, 105-114]. Our results suggest that the apparent conservation of high residual labile carbon as biodegradation products such as acetate makes natural attenuation of aromatics less effective, and makes subsequent addition of EAs to promote in situ BTEX biodegradation problematic.     

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

BACKGROUND, AIMS AND SCOPE: 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. METHODS: 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. RESULTS AND DISCUSSION: 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. CONCLUSIONS: 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. RECOMMENDATION AND PERSPECTIVE: 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.     

Kinetics of chemical decolorization of the azo dye C.I. Reactive Orange 96 by sulfide

The mechanism of decolorization of azo dyes based on the extracellular chemical reduction with sulfide (H2S, HS-, S2-) was postulated for sulfate reducing environments. To design technical decolorization processes of textile wastewater treatment with sulfide produced by sulfate reducing bacteria (SRB), kinetics is of great significance. Batch experiments were made in order to investigate the kinetics of abiotic decolorization of the reactive mono-azo dye C.I. Reactive Orange 96 (RO 96) with sulfide, with varying pH. The decolorization of RO 96 by sulfide under the exclusion of O2 corresponded to first-order kinetics with respect to both dye and sulfide concentration. The decolorization of RO 96 with sulfide at neutral pH (7.1) was advantageous compared with that at pH for 4.1, 6.3, and 6.5. This is attributed to an increase in the fraction of HS- of total sulfide species at neutral pH. The rate constants k for the decolorization at 37 degrees C were obtained as 0.01 for pH = 4.1, 0.06 for pH = 6.3, 0.08 for pH = 6.5, and 0.09 for pH = 7.1 in mM(-1) min(-1). The high rate constants for sulfide at pH 6.5-7.1 support that the decolorization through SRB (i.e. by bio-sulfide) can be effective in anaerobic bacterial systems with sulfate.     

Aerobic biodegradation of dichloroethenes by indigenous bacteria isolated from contaminated sites in Africa

The widespread use of tetrachloroethene (PCE) and trichloroethene (TCE) as dry cleaning solvents and degreasing agents for military and industrial applications has resulted in significant environmental contamination worldwide. Anaerobic biotransformation of PCE and TCE through reductive dechlorination frequently lead to the accumulation of dichloroethenes (DCEs), thus limiting the use of reductive dechlorination for the biotransformation of the compounds. In this study, seven bacteria indigenous to contaminated sites in Africa were characterized for DCE degradation under aerobic conditions. The specific growth rate constants of the bacterial isolates ranged between 0.346-0.552d(-1) and 0.461-0.667d(-1) in cis-DCE and trans-DCE, respectively. Gas chromatographic analysis revealed that up to 75% of the compounds were degraded within seven days with the degradation rate constants ranging between 0.167 and 0.198d(-1). The two compounds were also observed to be significantly degraded, simultaneously, rather than sequentially, when present as a mixture. Phylogenetic analysis of the 16S rRNA gene sequences of the bacterial isolates revealed their identity as well as their relation to other environmentally-important bacteria. The observed biodegradation of DCEs may contribute to PCE and TCE removal at the aerobic fringe of groundwater plumes undergoing reductive dechlorination in contaminated sites.     

电动力学技术强化原位生物修复研究进展

介绍了利用电动力学技术强化土壤及地下水原位生物修复的原理和最新进展。电动力学强化的基本原理是利用电渗析、电迁移和电泳等电动力学效应加速污染环境中有机污染物和微生物运动,注入营养物、电子受体或活性微生物,或者利用电极反应和电流热效应为地下生物降解创造有利条件。研究表明．电动力学技术能有效地强化原位生物修复,而且该技术不破环生态环境．安装和操作简单,成本低廉．有广泛的应用前景。     

Tracer test for the measurement of gas diffusion and non-aqueous phase liquid (NAPL) saturation in soil

During soil bioremediation, the diffusion of oxygen into the soil is an important prerequisite for aerobic biodegradation, and the decrease of petroleum products is the ultimate goal. Both processes need to be monitored. The aim of this work was to develop a gas tracer test that yields information on both, gas diffusion and residual saturation with non-aqueous phase liquids (NAPLs) in unsaturated soil heaps. One conservative tracer (methane) and 4 partitioning gas tracers (diethylether, methyl tert-butyl ether, chloroform and n-heptane) were injected as vapors into laboratory columns filled with unsaturated sand with increasing NAPL saturation. Breakthrough curves of gaseous compounds were measured at two points and compared to analytical solutions of an analytical diffusive-reactive transport equation. By fitting of methane data, robust results for effective diffusivity (tortuosity) were obtained. NAPL saturation was most accurately measured by the moderately water soluble tracers (ethers and chloroform). The hydrophobic tracer n-heptane did not partition into water-immersed NAPL. An easy and accurate way to assess air-NAPL partitioning constants from gas chromatography retention times is furthermore reported. It is concluded that gas tracer tests have the potential for measuring two important properties in soil bioremediation systems easily and quickly.