Electron Donor Supply Research Articles

Potential Remediation Approach for Uranium-Contaminated Groundwaters Through Potassium Uranyl Vanadate Precipitation

Methods for remediating groundwaters contaminated with uranium (U) through precipitation under oxidizing conditions are needed because bioreduction-based approaches require indefinite supply of electron donor. Although strategies based on precipitation of some phosphate minerals within the (meta)autunite group have been considered for this purpose, thermodynamic calculations for K- and Ca-uranyl phopsphates, meta-ankoleite and autunite, predict that U concentrations will exceed the Maximum Contaminant Level (MCL = 0.13 microM for U) at any pH and pCO2, unless phosphate is maintained at much higher concentrations than the sub-microM levels typically found in groundwaters. We hypothesized that potassium uranyl vanadate will control U(VI) concentrations below regulatory levels in slightly acidic to neutral solutions based on thermodynamic data available for carnotite, K2(UO2)2V2O8. The calculations indicate that maintaining U concentrations below the MCL through precipitation of carnotite will be sustainable in some oxidizing waters having pH in the range of 5.5 to 7, even when dissolution of this solid phase becomes the sole supply of sub-microM levels of V. Batch experiments were conducted in solutions at pH 6.0 and 7.8, chosen because of their very different predicted extents of U(VI) removal. Conditions were identified where U concentrations dropped below the MCL within 1-5 days of contact with oxidizing solutions containing 0.2-10 mM K, and 0.1-20 microM V(V). This method may also have application in extracting (mining) U and V from groundwaters where they both occur at elevated concentrations.

Support of Source Zone Bioremediation through Endogenous Biomass Decay and Electron Donor Recycling

ABSTRACT Enhanced bioremediation strategies employ intensive electron donor amendments that can be successful in generating high biomass concentrations within the targeted area, and this technology is increasingly being applied within source zones to address non–aqueous phase contaminants. An unintended consequence is potential electron donor recycling via the slow endogenous decay of these newly grown cells, which may persist in the source zone even after the enhanced bioremediation project is completed and the introduced electron donor is exhausted. This paper presents a conceptual model that outlines the endogenous decay process within source zones and identifies several key scenarios where it is an important contributor to long-term attenuation. A key concept is that this reservoir represented by decaying biomass is both potentially large and capable of several rounds of turnover before becoming exhausted. Thus, the slow decay of biomass and the recycling of these decay products within the source zone extended the duration of the treatment period. A recent survey on the performance of source depletion technologies strongly suggests that this process is being observed at sites where enhanced bioremediation has been implemented (McGuire et al., 2006, Ground Water Monitor. Remediat. 26:73–84). Concentrations continued to decline several years after treatment, providing a strong indication that there is an endogenous electron donor supply that is contributing to continued contaminant reduction over time. Little evidence of concentration rebound was observed relative to sites where other technologies were used, suggesting long-term benefits associated with enhanced bioremediation that appear to partially offset processes that can contribute to rebound (e.g., matrix diffusion). Because initial colonization can occur near the non-aqueous phase liquid (NAPL)-water interface when exogenous electron donor is readily available the endogenous cell decay occurs in an optimal location to continue to support reductive dechlorination. This electron donor recycling process is ideally suited to favor growth of dechlorinating organisms relative to competing populations because of the slow release rates associated with decay, and it should preferentially stimulate polychloroethylene (PCE) and trichloroethylene (TCE) source removal over metabolites. Endogenous decay can be directly employed as part of the remediation design through groundwater recirculation or by the construction of a carbon-based in situ biowall to generate large amounts of biomass. In the case of an in situ biowall, a key consideration is the placement, either as a permeable reactive barrier filled with fermentable carbon within the source zone, or as a barrier located upgradient of a source zone to ensure that groundwater is reduced before entering the targeted area. Regardless of the approach, the likely impact of electron donor recycling through endogenous decay is to extend low-level activity for up to several years, making enhanced bioremediation a promising technology in terms of initial performance and for long-term polishing.

Stable isotope characterization of fluids from the Lake Chany complex, western Siberia, Russian Federation

Abstract The Lake Chany complex and nearby lakes in western Siberia (Russian Federation) were studied to constrain the S cycle in these terrestrial lake environments. Surface water chemistry was characterized by Na–SO4–Cl composition, comparable to other inland basins in semi-arid climatic zones associated with marine evaporite-bearing formations at depth. Dissolved sulfates showed elevated δ34S (up to +32.3‰). These values are quite distinct from those in similar saline lakes in northern Kazakhstan, the Aral Sea, Lake Barhashi, and a gypsum deposit in the Altai Mountains. The localized distribution of such a unique S isotopic signature in dissolved SO4 negates both aeolian and catastrophic flooding hypotheses previously suggested for the genesis of the dissolved salts. The probable source of the dissolved SO4 in Lake Chany basin is inherited from hidden saline groundwaters (whose location and origins remain unclear) from eastern Paleozoic ranges with Upper Devonian formations with heavy S isotope values. Post-depositional enrichment of heavy S in the dissolved SO4 from saline sediments may be caused by local activity of SO4-reducing bacteria under the ambient supply of electron donors (dissolved river load organic matter and decaying bacterial mats) in the lake complex. Such microbial processes can remove up to ca. 60% of SO4 from the system. Extensive and intensive evaporation of lake fluids, ca. 40%, was indicated by the progressive enrichment of δ18O values in meteoric water samples collected along the river and lake system. This evaporation process compensates the microbial loss of SO4 dissolved in the incoming river water.

Reduction of Cr(VI) by indigenous bacteria in Cr-contaminated sediment under aerobic condition

The geomicrobiological control of chromium behavior in Cr-contaminated sediment by indigenous bacteria was investigated under aerobic conditions with respect to the initial Cr(VI) concentration, electron donor, and pH condition. Sediment samples were collected from a stream which was contaminated with chromium from nearby pigment manufacturing. In experiments using enriched cultures of the indigenous bacteria from the sediment samples, 31% of an initial concentration of Cr(VI) of 13 mg/L, equivalent to the exchangeable Cr(VI) concentration in the sediment, was reduced by the bacteria. Furthermore, the removal efficiency of Cr(VI) by indigenous bacteria was increased with external supply of electron donor in the form of glucose, lactate or acetate. In the range of pH 3–9, the removal efficiencies of Cr(VI) were 31% to 41%. The removal efficiency of Cr(VI) was highest at pH 3, while at pH 7, the pH of the stream sediment in the field, 34% of dissolved Cr(VI) was reduced. The indigenous bacteria can mediate reduction of Cr(VI) to Cr(III), and chromate/dichromate pollution in the field. Environmental factors such as pH and the type of electron donor were found to influence the microbial control of Cr(VI) reduction.

Impaired metabolism–secretion coupling in pancreatic β-cells: Role of determinants of mitochondrial ATP production

Glucose-induced insulin secretion from β-cells is often impaired in diabetic condition and by exposure to diabetogenic pharmacological agents. In pancreatic β-cells, intracellular glucose metabolism regulates exocytosis of insulin granules, according to metabolism–secretion coupling in which glucose-induced mitochondrial ATP production plays an essential role. Impaired glucose-induced insulin secretion often results from impaired glucose-induced ATP elevation in β-cells. Mitochondrial ATP production is driven by the proton-motive force including mitochondrial membrane potential (Δ Ψ m) generated by the electron transport chain. These electrons are derived from reducing equivalents, generated in the Krebs cycle and transferred from cytosol by the shuttles. Here, roles of the determinants of mitochondrial ATP production in impaired glucose-induced insulin secretion are discussed. Cytosolic alkalization, H + leak in the inner membrane by uncoupler (e.g. free fatty acid exposure), decrease in the supply of electron donors including NADH and FADH 2 to the respiratory chain, and endogenous mitochondrial ROS (e.g. Na +/K +-ATPase inhibition) all reduce hyperpolarlization of Δ Ψ m and ATP production, causing decresed glucose-induced insulin release. The decrease in the supply of NADH and FADH 2 to the respiratory chain derives from impairments in glucose metabolism including glycolysis (e.g. MODY2 and exposure to NO) and the shuttles (e.g. diabetic state and exposure to ketone body).

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.

Bioremediation of cis‐DCE at a Sulfidogenic Site by Amendment with Propionate

AbstractThe technical feasibility of in situ reductive dechlorination of cis‐dichloroethene (cDCE) and vinyl chloride (VC) was demonstrated at a sulfidogenic ground water site. Preceding laboratory studies had indicated that dechlorination at the site was limited by the supply of electron donors, that dechlorinating activity was sparsely and heterogeneously distributed, and that dechlorination was strongly enhanced by mixing sediments from multiple locations at the site. Based on these observations, the remediation strategy consisted of amending the ground water with sodium propionate solution using a pair of recirculation wells as a mixing device. This strategy was able to overcome the sparse initial distribution of biological activity by creating a treatment zone. Dechlorination and sulfate reduction commenced within 10 and 4 d, respectively, after propionate amendment began. Dechlorination efficiency increased during the 2 months of continuous operation. By the end of the 2 months, treatment converted ∼1000 μg/L cDCE nearly stoichiometrically into ethene, with only low concentrations of VC remaining. More than 90% of chlorinated ethenes were removed as the ground water traveled from one well to the other (travel time of ∼1 to 2 weeks). Near‐complete removal of ∼250 mg/L sulfate accompanied the rapid dechlorination, but no methanogenesis was observed. Aquifer clogging in the vicinity of the propionate injection wells became evident after ∼40 d of propionate amendment and was attributed to the growth of sulfate‐reducing bacteria and/or the formation of insoluble metal sulfides. Clogging was mitigated by pulsed amendment of the propionate solution.

Identification of groundwater flowpaths and denitrification zones in a dynamic floodpain aquifier

The groundwater flow system in the thick sand and gravel aquifer of the wide forested floodplain along the Lower Wisconsin River was characterized using major ion and oxygen and deuterium stable isotope analyses to explain nitrate attenuation in the floodplain. Critical to the groundwater flow system are the 60–90 m bluffs, the Pleistocene river terrace, and the modern floodplain with ridge and swale microtopography. Upland nitrate concentrations reached 12 mg NO 3–N/L, but were largely below the detection limit in the downgradient floodplain wetlands. A transect of monitoring wells and multilevel samplers, with depths of up to 6 m, were installed to characterize spatial and temporal variability in the floodplain groundwater flowpaths. Results showed that the topography induces deep, intermediate, and shallow flow systems and that temporal variability causes mixing between water from different sources. Nitrate attenuation in this system is likely a combination of dilution, plant uptake and denitrification. The absence of dissolved oxygen and nitrate and presence of high soluble iron concentrations in the lower floodplain indicate an ample supply of electron donors for oxygen consumption, denitrification and subsequent iron reduction. The data indicate that conditions created by the topography and temporal variability of the system are suitable for denitrification to occur at depth.

Variations in soil N cycling and trace gas emissions in wet tropical forests

We used a previously described precipitation gradient in a tropical montane ecosystem of Hawai'i to evaluate how changes in mean annual precipitation (MAP) affect the processes resulting in the loss of N via trace gases. We evaluated three Hawaiian forests ranging from 2200 to 4050 mm year-1 MAP with constant temperature, parent material, ecosystem age, and vegetation. In situ fluxes of N2O and NO, soil inorganic nitrogen pools (NH4+ and NO3-), net nitrification, and net mineralization were quantified four times over 2 years. In addition, we performed 15N-labeling experiments to partition sources of N2O between nitrification and denitrification, along with assays of nitrification potential and denitrification enzyme activity (DEA). Mean NO and N2O emissions were highest at the mesic end of the gradient (8.7+/-4.6 and 1.1+/-0.3 ng N cm-2 h-1, respectively) and total oxidized N emitted decreased with increased MAP. At the wettest site, mean trace gas fluxes were at or below detection limit (<or=0.2 ng N cm-2 h-1). Isotopic labeling showed that with increasing MAP, the source of N2O changed from predominately nitrification to predominately denitrification. There was an increase in extractible NH4+ and decline in NO3- , while mean net mineralization and nitrification did not change from the mesic to intermediate sites but decreased dramatically at the wettest site. Nitrification potential and DEA were highest at the mesic site and lowest at the wet site. MAP exerts strong control N cycling processes and the magnitude and source of N trace gas flux from soil through soil redox conditions and the supply of electron donors and acceptors.

Continuous denitrification by external electron-donor supply utilizing an algorithm-based software controller

This paper presents the development and application of a knowledge-based software controller for the external electron-donor supply in the anaerobic denitrification stage. The basic principle of the control concept is based on the continuous measurement of the methane concentration in the off-gas. Since under anoxic conditions and incomplete NO x − removal the denitrifying species out-compete efficiently the methanogens for the carbon source, the appearance of methane at a significant concentration (≥0.3%) in the gas phase indicates the complete denitrification and the increasing consumption of the C-source by the methanogens. Thus, a software controller that uses the methane concentration as variable was designed for process control and tested in different reactor types (CSTR, Fluidized bed). The necessary amount of the external electron-donor can be kept at the optimal level without any undesired accumulation of nitrite or nitrate.

NOx removal from flue gas by an integrated physicochemical absorption and biological denitrification process

An integrated physicochemical and biological technique for NO(x) removal from flue gas, the so-called BioDeNO(x) process, combines the principles of wet absorption of NO in an aqueous Fe(II)EDTA(2-) solution with biological reduction of the sorbed NO in a bioreactor. The biological reduction of NO to di-nitrogen gas (N(2)) takes place under thermophilic conditions (55 degrees C). This study demonstrates the technical feasibility of this BioDeNO(x) concept in a bench-scale installation with a continuous flue gas flow of 650 l.h(-1) (70-500 ppm NO; 0.8-3.3% O(2)). Stable NO removal with an efficiency of at least 70% was obtained in case the artificial flue gas contained 300 ppm NO and 1% O(2) when the bioreactor was inoculated with a denitrifying sludge. An increase of the O(2) concentration of only 0.3% resulted in a rapid elevation of the redox potential (ORP) in the bioreactor, accompanied by a drastic decline of the NO removal efficiency. This was not due to a limitation or inhibition of the NO reduction, but to a limited biological iron reduction capacity. The latter leads to a depletion of the NO absorption capacity of the scrubber liquor, and thus to a poor NO removal efficiency. Bio-augmentation of the reactor mixed liquor with an anaerobic granular sludge with a high Fe(III) reduction capacity successfully improved the bioreactor efficiency and enabled to treat a flue gas containing at least 3.3% O(2) and 500 ppm NO with an NO removal efficiency of over 80%. The ORP in the bioreactor was found to be a proper parameter for the control of the ethanol supply, needed as electron donor for the biological regeneration process. The NO removal efficiency as well as the Fe(III)EDTA(-) reduction rate were found to decline at ORP values higher than -140 mV (pH 7.0). For stable BioDeNO(x) operation, the supply of electron donor (ethanol) can be used to control the ORP below that critical value.

The controls on the composition of biodegraded oils in the deep subsurface—part 1: biodegradation rates in petroleum reservoirs

Biodegradation rates in oilfields have been assessed conservatively using whole oil-column minimum rate estimates, diffusion-controlled oil column compositional gradient modelling and mixed oil kinetic models. Biodegradation rate constants (first order) are around 10−6–10−7 yr−1 for hydrocarbons in the degradation zones these corresponding well with zero order field-wide minimum rate estimates of about 10−8 kg hydrocarbons/kg oil/year for the whole oil column. With biodegradation induction times of around 1–2 Ma to perturb an entire oil column for light oil reservoirs and 10–20 Ma for heavy oil reservoir degradation the results indicate that where we see continuous gradients in the oil columns, degradation must have been occurring episodically for many millions of years. To remove the n-alkanes from an oil (i.e. about 10% of an oil) around ca 15 Ma is needed for a heavy oil (ca 5 Ma for a light N. Sea oil). The timescales of oilfield degradation and filling are thus very similar and consequently the degree of biodegradation will be substantially controlled by oilfield charge history. Assessment of mixed degraded/non-degraded oil occurrence provides an independent confirmation that these rates are realistic and that timescales of degradation and field charging are similar. The maximum effective rate constant of degradation, ultimately controlled by the limiting effect of diffusion of alkanes to the oil water contact (OWC) (ca 10−4 yr−1 for a 130 m thick oil column first order rate constant) is well above the estimated rate constants indicating oil biodegradation rate is not limited by electron donor supply (i.e. hydrocarbons) but by supply of nutrients or oxidants. This suggests that diffusive transport of nutrients and electron acceptors in the aquifer to the site of biodegradation may be adequate to maintain the low rate biosphere.

Spanish treatment facilities gain funding

Enhanced In situ Biodenitrification (EIB) is a capable technology for nitrate removal in subsurface water resources. Optimizing the performance of EIB implies devising an appropriate feeding strategy involving two design parameters: carbon injection frequency and C:N ratio of the organic substrate nitrate mixture. Here we model data on the spatial and temporal evolution of nitrate (up to 1.2 mM), organic carbon (ethanol), and biomass measured during a 342 day-long laboratory column experiment (published in Vidal-Gavilan et al., 2014). Effective porosity was 3% lower and dispersivity had a sevenfold increase at the end of the experiment as compared to those at the beginning. These changes in transport parameters were attributed to the development of a biofilm. A reactive transport model explored the EIB performance in response to daily and weekly feeding strategies. The latter resulted in significant temporal variation in nitrate and ethanol concentrations at the outlet of the column. On the contrary, a daily feeding strategy resulted in quite stable and low concentrations at the outlet and complete denitrification. At intermediate times (six months of experiment), it was possible to reduce the carbon load and consequently the C:N ratio (from 2.5 to 1), partly because biomass decay acted as endogenous carbon to respiration, keeping the denitrification rates, and partly due to the induced dispersivity caused by the well-developed biofilm, resulting in enhancement of mixing between the ethanol and nitrate and the corresponding improvement of denitrification rates. The inclusion of a dual-domain model improved the fit at the last days of the experiment as well as in the tracer test performed at day 342, demonstrating a potential transition to anomalous transport that may be caused by the development of biofilm. This modeling work is a step forward to devising optimal injection conditions and substrate rates to enhance EIB performance by minimizing the overall supply of electron donor, and thus the cost of the remediation strategy.

Hydrogen Release Compound: A Cost‐Effective Alternative to PCE Remediation at Dry Cleaning Facilities

AbstractTetrachloroethylene, also known as perchloroethylene or PCE, is one of the most difficult to treat chlorinated solvents when present in groundwater. Unfortunately, this elusive and recalcitrant compound is also the most commonly used dry cleaning solvent. As a result, releases of PCE at dry cleaning sites are somewhat common. Regenesis Bioremediation Products, of San Clemente, California, has developed Hydrogen Release Compound (HRC), which has been successfully used to promote bioremediation of PCE in groundwater. This product is directly injected into contaminated groundwater to speed up the natural attenuation of PCE through an anaerobic, natural process known as reductive dechlorination. A key benefit of HRC is its ability to slowly release hydrogen over extended periods of time. Reductive dechlorination relies on a steady source and readily available supply of electron donors as part of the degradation process. Hydrogen is one of the best electron donors available, and thus, the application of HRC significantly enhances the rate of PCE degradation. For dry cleaners, this technology can substantially reduce major design, capital, and operating costs, allowing the implementation of a low‐impact application and remediation solution. This article discusses the use of the HRC to remediate PCE contamination and presents the results of two specific HRC‐treated dry cleaner sites. © 2002 Wiley Periodicals, Inc.

Plant carbohydrate limitation on nitrate reduction in wetland microcosms

Although nitrate limitation in most natural wetlands results in pseudo-first-order reductions, large site-to-site variations in apparent denitrification rates cannot be easily explained by water quality (e.g., pH, Temp, DOC) or plant productivity. Our microcosm results show increasing nitrate removal efficiencies at higher ratios of total applied plant carbon to nitrate reduced, suggesting that denitrification rates may be limited by the rates of supply of both electron donor or acceptor, described by an applied carbon to nitrate (C App : N Red) ratio. However, the observed first-order rate constants varied more strongly ( r 2=0.77, p⪡0.0001) with the acid-soluble carbohydrates to nitrate (CH 2O App : N Red) ratio than the total C App : N Red ratio. Although observed rate constants for bulrush ( Scirpus sp .) were significantly lower (0.01< p<0.06) than for other plant sources ( Hydrocotyle sp., Lemna sp., Typha sp .), there were no significant differences in the plant-specific rate constants when compared at the same CH 2O App : N Red ratio. In full-scale wetlands, this suggests that either high plant productivity or low NO 3 − loading (high CH 2O App : N Red) may contribute to a high effective denitrification rate. In contrast, low productivity or a high NO 3 − loading (low CH 2O App : N Red) would promote a lower denitrification rate. This co-limitation between plant carbon and nitrate may confound first-order rate comparisons in full scale denitrification wetlands since highly N-loaded systems may become carbon limited, requiring higher order reaction kinetics to better describe performance variations. In addition to hydraulic residence time, temperature and nitrate removal data, extending these results to compare large wetlands will require estimation of the CH 2O App : N Red ratio from an inventory of plant species, productivity estimates and carbon quality.

Cross-epithelial hydrogen transfer from the midgut compartment drives methanogenesis in the hindgut of cockroaches.

In the intestinal tracts of animals, methanogenesis from CO(2) and other C(1) compounds strictly depends on the supply of electron donors by fermenting bacteria, but sources and sinks of reducing equivalents may be spatially separated. Microsensor measurements in the intestinal tract of the omnivorous cockroach Blaberus sp. showed that molecular hydrogen strongly accumulated in the midgut (H(2) partial pressures of 3 to 26 kPa), whereas it was not detectable (<0.1 kPa) in the posterior hindgut. Moreover, living cockroaches emitted large quantities of CH(4) [105 +/- 49 nmol (g of cockroach)(-1) h(-1)] but only traces of H(2). In vitro incubation of isolated gut compartments, however, revealed that the midguts produced considerable amounts of H(2), whereas hindguts emitted only CH(4) [106 +/- 58 and 71 +/- 50 nmol (g of cockroach)(-1) h(-1), respectively]. When ligated midgut and hindgut segments were incubated in the same vials, methane emission increased by 28% over that of isolated hindguts, whereas only traces of H(2) accumulated in the headspace. Radial hydrogen profiles obtained under air enriched with H(2) (20 kPa) identified the hindgut as an efficient sink for externally supplied H(2). A cross-epithelial transfer of hydrogen from the midgut to the hindgut compartment was clearly evidenced by the steep H(2) concentration gradients which developed when ligated fragments of midgut and hindgut were placed on top of each other-a configuration that simulates the situation in vivo. These findings emphasize that it is essential to analyze the compartmentalization of the gut and the spatial organization of its microbiota in order to understand the functional interactions among different microbial populations during digestion.

Aerobic and Anaerobic Transformations of Pentachlorophenol in Wetland Soils

Strategies to enhance biotransformation of pentachlorophenol (PCP) in a spectrum of wetland soils were investigated under laboratory conditions, which included manipulations of electron acceptors and donors, and PCP concentrations. Maximum transformation rates were found at PCP concentrations &lt;10 μM (methanogenic conditions) and &gt;6 μM to &gt;23 μM (aerobic conditions). Differences in PCP toxicity and sorption among soils and treatments were largely governed by the activities of microbial groups. Within this concentration range, transformation was observed in soils under aerobic and methanogenic conditions, but was inhibited under denitrifying and SO2−4–reducing conditions. Aerobic PCP transformation initially produced small amounts of pentachloroanisole (PCA). However &gt;75% of both chemicals disappeared in 30 d from five soils. Measured soil properties were not significantly correlated to aerobic transformation rates. Under methanogenic conditions, PCP was reductively dechlorinated to yield a mixture of tetra‐, tri‐, and dichlorophenols in eight soils, with rates strongly correlated to measures of electron donor supply (total C, N, organic C mineralization rates) and microbial biomass. Addition of protein‐based electron donors enhanced reductive dechlorination in a soil low in organic matter and microbial biomass. Results demonstrated the widespread occurrence of PCP transforming microorganisms in soils, which may be promoted by manipulating environmental conditions.

Nitrous oxide (N 2O) production by Alcaligenes faecalis during feast and famine regimes

The environmentally harmful compound, nitrous oxide (N 2O), can accumulate as an intermediate in the process of denitrification. One important parameter, which can influence this accumulation, is the feeding regime sensed by the bacteria involved, in which an unbalanced supply of electron donor and acceptor may occur. When pulse additions of C-compounds (acetate, butyrate and malate) were given to denitrifying cultures of Alcaligenes faecalis strain TUD, the production rate of N 2O was reduced from 9.9–18.5% to 1.8–10.4% of the total nitrite converted, as long as the C-substrate was in excess. However, as soon as the availability of carbon compounds became exhausted and the culture entered starvation, N 2O was one of the main products of denitrification and production increased to 32–64% of the total N-feed. Under dynamic feeding conditions, the culture was able to adapt to the fluctuating conditions and the ratio of N 2O to nitrite decreased. However, during starvation the ratio of N 2O to nitrite was still high (± 27%), indicating that with prolonged starvation, the overall N 2O emission will increase. Competition between the enzymes of denitrification for electrons from the cytochrome c pool could explain the emission of N 2O, if the enzyme N 2O-reductase has a lower affinity for the electron-donor than the other reductases.

Subsurface denitrification in a forest riparian zone: interactions between hydrology and supplies of nitrate and organic carbon.

The influence of hydrology andpatterns of supply of electron donors and acceptors onsubsurface denitrification was studied in a forestriparian zone along the Boyne River in southernOntario that received high nitrogen inputs from a sandaquifer. Two hypotheses were tested: (1) subsurfacedenitrification is restricted to localized zones ofhigh activity; (2) denitrification zones occur atsites where groundwater flow paths transportNO3 − to supplies of available organiccarbon. A plume of nitrate-rich groundwater withconcentrations of 10–30 mg N L−1 flowed laterallyat depths of 1.5–5 m in sands beneath peat for ahorizontal distance of 100–140 m across the riparianzone to within 30–50 m of the river. In situ acetyleneinjections to piezometers revealed that significantdenitrification was restricted to a narrow zone ofsteep NO3 − and N2O decline at theplume margins. The location of these denitrificationsites in areas with steep gradients of groundwater DOCincrease supported hypothesis 2. Many of thesedenitrification “hotspots” occurred near interfacesbetween sands and either peats or buried river channeldeposits. Field experiments involving in situadditions of either glucose or NO3 − topiezometers indicated that denitrification wasC-limited in a large subsurface area of the riparianzone, and became N-limited beyond the narrow zone ofNO3 − consumption. These data suggest thatdenitrification may not effectively removeNO3 − from groundwater transported at depththrough permeable riparian sediments unlessinteraction occurs with localized supplies of organicmatter.

Extension of logarithmic growth of Thiobacillus ferrooxidans by potential controlled electrochemical reduction of Fe(III)

In this study, we demonstrated that the period of logarithmic growth for Thiobacillus ferrooxidans could be extended when optimal conditions for cell growth were maintained using potential controlled electrochemical cultivation with sufficient aeration. The optimal pH and Fe(II) concentration for the electrolytic cultivation were determined to be 2.0 and 150 mM, respectively. When the potential was set to 0.0V vs Ag/AgCl, the Pt electrode reduced Fe(III) to Fe(II) with an efficiency of 95%. A porous glass microbubble generator was used to maintain adequate levels of dissolved oxygen, which was the electron acceptor for T. ferrooxidans when the cell density in the medium was high. Under these conditions, cells at an initial density of 107 cells/mL grew logarithmically for 4days until the cell density was 4 × 109 cells/mL. This corresponded to a period of logarithmic growth that was 3 times longer than was observed in batch cultures without electrolysis. In addition, the final cell density reached 1010 cells/mL after 6 days of electrochemical cultivation, which was a 50-fold increase over conventional batch culture. Under conditions of increasing cell density, potentiostatic electrolysis made it possible to remove Fe(III), which causes product inhibition, at an increasing rate and to correspondingly increase the production rate of Fe(II), which is the electron donor for T. ferrooxidans. Thus, our cultivation system provides a sufficient supply of electron donor and acceptor for T. ferrooxidans, thereby elongating the period of logarithmic growth and producing very high cell densities. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 64: 716–721, 1999.