Concentration Of Electron Acceptor Research Articles

A kinetic model based on non-linear regression analysis is proposed for the degradation of phenol under UV/solar light using nitrogen doped TiO2

Nitrogen was doped into the TiO2 matrix in the concentration range of 0.05–0.20at.% and the photocatalytic activities were tested for the degradation of phenol (Ph) under UV/solar light using hydrogen peroxide (HP) and ammonium persulfate (APS) as electron acceptors. The prepared photocatalysts characterized by various analytical techniques confirm the incorporation of nitrogen in the TiO2 lattice. The 0.15at.% dopant concentration shows higher photocatalytic activity compared with sol–gel TiO2 (SG) and Degauss P25 TiO2 (P25) for the degradation process. Photoluminescence technique was used for studying the extent of hydroxyl radicals produced on TiO2 and TiO2−xNx (NT) surface in the presence of oxidants under UV/solar light irradiation. The kinetic rate equation obtained for the best system TiO2−xNx (x=0.15at.%)/APS/under solar irradiation is found to be: rap,solar=0.0041[NT]0.54[APS]0.71[Ph]−0.70. A kinetic/mathematical model was developed based on the nonlinear regression analysis for the various processes and the validity of the model was tested by comparing the experimental values with the theoretically calculated data as a function of variable parameters like catalyst dosage, concentration of electron acceptors and initial concentration of Ph.

Compendium and synthesis of bacterial manganese reduction rates

We have compiled time-series concentration data for the biological reduction of manganese(III/IV) published between 1985 and 2004 and fit these data with a simple hyperbolic rate expression or, when appropriate, one of its limiting forms. The compiled data and rate constants are available in Electronic Annex EA-1. The zero- and first-order rate constants appear to follow a log–normal distribution that could be used, for example, in predictive modeling of Mn-oxide reduction in a reactive transport scenario. We have also included details of the experimental procedures used to generate each time-series data-set in our compilation. These meta-data—mostly pertaining to the type and concentration of micro-organism, electron donor, and electron acceptor—enable us to examine the rate data for trends. We have computed a number of rudimentary, mono-variate statistics on the compiled data with the hope of stimulating both more detailed statistical analyses of the data and new experiments to fill gaps in the existing data-set. We have also analyzed the data with parametric models based on the log–normal distribution and rate equations that are hyperbolic in the concentration of cells and Mn available for reduction. This parametric analysis allows us to provide best estimates of zero- and first-order rate constants both ignoring and accounting for the meta-data.

Geobacter sulfurreducens can protect 304L stainless steel against pitting in conditions of low electron acceptor concentrations

The effect of Geobacter sulfurreducens cells was studied on the electrochemical behaviour of 304L stainless steel, emphasizing the role of the soluble electron acceptor (fumarate). In fumarate-lacking media, the presence of G. sulfurreducens induced free potential ennoblement in a few hours. This ennoblement has already been observed in standard media that contained fumarate. Our previous studies have shown that G. sulfurreducens shifted the pitting potential toward the positive values. The pits induced by the presence of the bacteria were wider and deeper than in the absence of bacteria. Here, in fumarate-lacking media, similar shift in pitting potential was observed, but the repassivation phase was strongly improved. AFM analysis showed that pits were identical with those observed in the absence of bacteria at lower potential. In contrast with all the previous work where G. sulfurreducens enhanced corrosion, here at a low concentration of electron acceptor, the presence of the bacteria protected the steel against pitting.

Metabolic Modeling of Spatial Heterogeneity of Biofilms in Microbial Fuel Cells

Microbial fuel cells (MFCs) provide an alternative approach to generate electricity from organic matter. In these MFCs, microorganisms such as the Geobacter species oxidize organic waste and transfer electrons to an electrode. Mathematical models have been used to study and optimize biofilm processes, in developing MFCs into commercial applications. Existing biofilm models are based on Nernst-Monod type expressions, and are restricted to studying extracellular behavior such as electrochemical and microbiological components, separated from the metabolic behavior of microorganisms or vice versa. This paper presents a model that addresses the spatial heterogeneity across the biofilm, while incorporating the metabolic fluxes in each individual cell as extracellular conditions (electron donor and acceptor concentrations) vary across the biofilm. This model shows the effect of different maintenance energy requirements on maximum current production and the thickness of the biofilms, and predicts the variations in biofilm thicknesses in DL1 and KN400 strains under similar biofilm conditions, successfully.

Effect of different types of electron acceptors on the anoxic phosphorus uptake activity of denitrifying phosphorus removing bacteria

The effect of different types of electron acceptors (O 2, NO 3 - and NO 2 - ) on anoxic phosphorus uptake was investigated to obtain a better understanding of the metabolic behavior of denitrifying phosphorus removing bacteria (DPB). Two sets of sequencing batch reactor (SBR) were used to enrich phosphorus accumulation organisms (PAOs) by alternative anaerobic and aerobic operations (oxygen was used as the electron acceptor), batch experiments under two different electron acceptor conditions, nitrate and nitrite, were performed by using activated sludge generated from the above-operated SBR reactors. Anoxic phosphorus uptake achieved successful both by using nitrate or nitrite as the electron acceptor. However, the relative small amount of anoxic phosphorus was taken up by microorganisms with nitrite used as the electron acceptor in comparison with nitrate. The optimal stoichiometric loading amount of electron acceptors of nitrate and nitrite to phosphorus (N/P ratio) were 0.89 and 0.60, respectively. If the initial concentration of electron acceptors was controlled below the optimal ratio, the amount of phosphorus taken up increased with the concentration, otherwise it would decrease. It was also found that DPB lost the ability of phosphorus release and uptake rapidly under continuous anaerobic–anoxic operation condition.

Dynamics of redox processes in a minerotrophic fen exposed to a water table manipulation

Climate change studies foresee substantial changes in precipitation patterns in northern temperate regions, including an increased frequency of drought and intense rain events. To elucidate the impact of drying and rewetting on redox processes in peatlands we conducted a field scale manipulation experiment, tracing the below ground dynamics of dissolved inorganic carbon (DIC), methane (CH 4), and electron acceptors in a minerotrophic fen. On three treatment plots, a drought phase of ~ 4 weeks was initiated by means of a drainage system and a mountable roof. Subsequently we simulated a heavy rainfall by irrigation, which raised the water table by 20–35 cm. Three plots served as control. Solute electron acceptor concentrations increased during drought and decreased after rewetting, consistent with treatment and weather. Changes in solid phase electron acceptor contents were not detected. Drought induced aeration regenerated electron acceptors and suppressed methanogenic activity. This suppressive effect persisted during and some 1–4 weeks after rewetting, dependent on depth. Hydrogen- and acetate-dependent iron and sulfate reduction were thermodynamically viable processes in the peat as their energy gain exceeded the threshold for microbial metabolism of − 23 kJ mol − 1 . Hydrogenotrophic methanogenesis was apparently thermodynamically inhibited in most of the peat, but proceeded presumably in microenvironments. In the uppermost peat layers, partly above the water table level, iron and sulfate reduction and occasionally methanogenesis overlapped and aeration was limited. Thus, in these layers reductive processes could partly proceed even during drought. As all processes, including methanogenesis, responded quickly to wetting events, the microbial community seemed to be well adapted to fluctuating redox conditions. This study demonstrated that the dense and highly decomposed material could still provide anaerobic habitats for microorganisms during drought and the depth distribution of redox processes was quite insensitive towards the water table manipulations, presumably due to formation of microenvironments.

Concentration-Dependent Front Velocity of the Autocatalytic Hydrogenase Reaction

HynSL hydrogenase from Thiocapsa roseopersicina was applied to catalyze the oxidation of molecular hydrogen in a new, improved, thin-layer reaction chamber. Investigation of the nature of this catalysis via the development of reduced benzyl viologen showed clearly the typical characteristics of an autocatalytic reaction: propagation of a reaction front originating from a single point, with a constant velocity of front propagation. The dependence of the reaction velocity on enzyme concentration was a power function with a positive enzyme concentration threshold, with an exponent of 0.4 ± 0.05. This indicates that the autocatalyst is an enzyme form. The front velocity decreased on increase of the electron acceptor concentration, as a sign that the autocatalyst interacts directly with the final electron acceptor. Overall, it may be concluded that the autocatalyst is an enzyme form in which [FeS] distal is reduced. Model calculations corroborate this. Because the reduction of all [FeS] clusters would be possible in a nonautocatalytic reaction, we hypothesize a small conformational change in the enzyme, catalyzed by the autocatalyst, which removes a block in the electron flow in either [NiFe] → [FeS] proximal or the [FeS] proximal → [FeS] distal reaction step, or removes a block of the penetration of gaseous hydrogen from the surface to the [NiFe] cluster.

Modeling the Biodegradation Kinetics of Perchlorate in the Presence of Oxygen and Nitrate as Competing Electron Acceptors

ABSTRACT A mathematical model was developed to describe the biodegradation kinetics of perchlorate in the presence of nitrate and oxygen as competing electron acceptors. The rate of perchlorate degradation is described as a function of the electron donor (acetate) degradation rate, the concentration of the alternate electron acceptors, and rates of biomass growth and decay. The kinetics of biomass growth are described using a modified Monod model, and inhibition factors are incorporated to describe the influence of oxygen and nitrate on perchlorate degradation. In order to develop input parameters for the model, a series of batch biodegradation studies were performed using Azospira suillum JPLRND, a perchlorate-degrading strain isolated from groundwater. This strain is capable of utilizing oxygen, nitrate, or perchlorate as terminal electron acceptors. The maximum specific growth rate (μmax) and half-saturation constant (K S don) for the bacterium when utilizing either perchlorate or nitrate were similar; 0.16 per h and 158 mg acetate/L, respectively. However, these parameters were different when the strain was growing on oxygen. In this case, μmax and K S don were 0.22 per h and 119 mg acetate/L, respectively. The batch experiments also revealed that nitrate inhibits perchlorate biodegradation by this strain. This finding was incorporated into the model by applying an inhibition coefficient (K i nit) value of 25 mg nitrate/L. Combined with appropriate groundwater transport models, this model can be used to predict perchlorate biodegradation during in situ remediation efforts.

Determination of dehydrogenase activity in sediment of shallow lakes and its ecological significance

以武汉市浅水湖泊(月湖)为实验对象确定了沉积物脱氢酶活性(DHA)测定的最佳条件:以经甲醛灭菌的沉积物为对照,以氯化三苯基四氮唑为电子受体(0.4%),沉积物用量为0.5g,pH值为7.5,培养时间为3h.据上述条件测定的DHA比未优化条件下的相应值高3-5倍,且表现出与沉积物有机质含量更为显著的正相关关系.武汉东湖3个子湖共23个采样点沉积物样品的测试结果进一步证实了上述方法的可行性,同时初步揭示了湖泊沉积物脱氢酶反映微生物活性与有机质状态的生态学意义.;The samples were taken from the shallow urban lakes(Lake Yuehu) in Wuhan city to optimize the determination of dehydrogenase activity (DHA) in sediment, which gave the following suitable conditions: the sediment sterilized by formaldehyde was used as the control, the concentration of electron acceptor (2,3,5-triphenyl-2H-tetrazolium chloride) was 0.4%, the sediment amount used was 0.5g, pH value was 7.5 and the reaction time was 3h. Under these conditions, the DHA assayed was 3-5 times higher, and its positive relation to the content of organic matter in sediment was more significant, compared to those determined by the unmodified method. The data based on the 23 sites from 3 basins of Lake Donghu demonstrated the feasibility of the method. In the same time, the ecological significance of DHA was further shown in terms of the measurements of microbial activities and organic matter status in lake sediment

Effect of loaded silver nanoparticles on TiO 2 for photocatalytic degradation of Acid Red 88

Nanometer sized Ag–TiO 2 nanoparticles were prepared (by photoreduction of Ag + ions) in order to assess its photocatalytic degradation ability of target pollutant (textile dye; Acid red 88) upon visible light irradiation. Furthermore, oxidative reagents such as peroxomonosulfate (PMS), peroxodisulfate (PDS) and hydrogen peroxide (H 2O 2) were added to the photocatalytic system, which may act as an alternative electron acceptor and result in a notably enhanced degradation rate (seven-fold increase with PMS as oxidant) of pollutant destruction. Mineralization of target pollutant was also performed by total organic carbon (TOC) analysis and from the results, it was confirmed that 65% mineralization was achieved in 7 h using PMS as electron acceptor. Overall, this system is relatively inexpensive, reproducible, extremely stable and efficient in complete degradation of dye in aqueous solution. In order to obtain maximum information about the performance of Ag–TiO 2 photocatalyst, we did experiments under different operating conditions, i.e., variation of amount of catalyst, concentration of dye and electron acceptors. In addition to the above, a comparative study on the photocatalytic activities of TiO 2 was also made.

Biocatalysis of theophylline oxidation by microbial theophylline oxidase in the presence of non-physiological electron acceptors

Microbial theophylline oxidase (ThOx) is a redox enzyme catalysing 8-hydroxylation of theophylline to form 1,3-dimethyluric acid. In this work, ThOx has been characterized as a fragile haem-containing protein complex composed of several non-covalently bound dynamic domains with molecular weights of around 60 and 210 kDa, and capable of formation of 1.5 MDa assemblies as well. The rate of theophylline oxidation by ThOx with the non-physiological electron acceptor ferricyanide was 0.17 s−1, approaching that with cytochrome c, 0.33 s−1. The apparent catalytic constant depended on the electron acceptor concentration. At concentrations lower than 0.2 mM the reaction did not fit the Michaelis–Menten scheme, and some non-catalytic processes dominated in the overall reaction. The kinetics of ThOx catalysis were also studied at electrodes modified with self-assembled monolayers (SAM) of hydroxyl- and amine-terminated alkanethiols. Different compositions of the SAM provide different orientations of ThOx on these layers. Depending on the orientation of ThOx onto the SAM-modified electrodes, the heterogeneous electron transfer (ET) constant, ks, which characterizes the ET reaction between the electrodes and the haem of ThOx (Eo/ of 87 mV (NHE)) was 0.4 s−1 and 3.2 s−1. Only the low-ET-rate orientation appeared to be productive for the electrocatalytic function of ThOx, giving a reaction similar to that with ferricyanide and cytochrome c. The apparent efficiency of ThOx bioelectrocatalysis in the absence of mediators was substantially lower than that mediated by ferricyanide or cytochrome c. This lower efficiency is consistent with a correspondingly lower amount of ThOx being in direct ET contact with the electrodes and thus involved in electrocatalysis.

Biogeochemical reactive–diffusive transport of heavy metals in Lake Coeur d’Alene sediments

Abstract Decades of runoff from precious-metal mining operations in the Lake Coeur d’Alene Basin, Idaho, have left the sediments in this lake heavily enriched with toxic metals, most notably Zn, Pb and Cu, together with As. The bioavailability, fate and transport of these metals in the sediments are governed by complex biogeochemical processes. In particular, indigenous microbes are capable of catalyzing reactions that detoxify their environments, and thus constitute an important driving component in the biogeochemical cycling of these metals. Here, the development of a quantitative model to evaluate the transport and fate of Zn, Pb and Cu in Lake Coeur d’Alene sediments is reported. The current focus is on the investigation and understanding of local-scale processes, rather than the larger-scale dynamics of sedimentation and diagenesis, with particular emphasis on metal transport through reductive dissolution of Fe hydroxides. The model includes 1-D inorganic diffusive transport coupled to a biotic reaction network including consortium biodegradation kinetics with multiple terminal electron acceptors and syntrophic consortium biotransformation dynamics of redox front. The model captures the mobilization of metals initially sorbed onto hydrous ferric oxides, through bacterial reduction of Fe(III) near the top of the sediment column, coupled with the precipitation of metal sulfides at depth due to biogenic sulfide production. Key chemical reactions involve the dissolution of ferrihydrite and precipitation of siderite and Fe sulfide. The relative rates of these reactions play an important role in the evolution of the sediment pore-water chemistry, notably pH, and directly depend on the relative activity of Fe and SO4 reducers. The model captures fairly well the observed trends of increased alkalinity, sulfide, Fe and heavy metal concentrations below the sediment–water interface, together with decreasing terminal electron acceptor concentrations with depth, including the development of anoxic conditions within about a centimeter below the lake bottom. This effort provides insights on important biogeochemical processes affecting the cycling of metals in Lake Coeur d’Alene and similar metal-impacted lacustrine environments.

Use of Surfactants to Improve the Biological Degradation of Petroleum Hydrocarbons in a Field Site Study

Mineral oil products are produced and utilized to a large extent. During the production, storage, filling up and utilization many spills occurred during the last decades. As a result, mineral oil contamination is recently one of the main problems in soil remediation. The predominant portion of the different oil components is easily degradable by microorganisms. But, its biodegradation in natural environments is often limited by the availability of the substrate to the microorganisms or by limited concentrations of nutrients and electron acceptors. Thus, the optimization of the environmental conditions can significantly improve the efficiency of degradation in both soil and groundwater (Enhanced Natural Attenuation approach). One major limiting factor in terms of the bioavailability of mineral oil is its limited solubility in water. Enhancing the solubility of the contaminants e.g. by the use of surfactants can significantly improve the efficiency of biodegradation. But, for the purpose of bioremediation only those surfactants should be used that are themselves completely biodegradable. Moreover, they have to be compatible to the surfaces of the bacteria. These requirements are met by biosurfactants and by those surfactants that contain structures comparable to naturally occurring microbial surfactants. The efficiency of such a close-to-nature surfactant (BioVersal FW) has been demonstrated for the in situ remediation of a highly contaminated site at Halle/S. (Germany). During the field scale investigation up to 50 g hydrocarbons per kg soil were eliminated over a period of 15 months.

Oxidation of sulfamethoxazole and related antimicrobial agents by TiO 2 photocatalysis

The widespread detection of pharmaceutically active compounds, including many synthetic antimicrobial agents, in aquatic environments is raising public health concerns. As a result, there is growing interest in the development of innovative technologies to efficiently transform these compounds to non-toxic and pharmaceutically inactive byproducts. This work examines the photocatalytic degradation of sulfamethoxazole (SMX) and related sulfonamide antimicrobial agents in aqueous suspensions of nanophase titanium dioxide (TiO 2). Experimental results demonstrate that SMX is mineralized by TiO 2 irradiated with ultraviolet-A light (UVA: 324⩽ λ⩽400 nm). Rates of UVA-TiO 2 photocatalyzed SMX degradation are dependent upon several variables, including the initial SMX concentration, catalyst phase identity and concentration, electron acceptor identity and concentration, and the presence of non-target water constituents. In contrast, reaction rates are not sensitive to changes in sulfonamide structure. Although pH has little direct effect on reaction rates, the presence of natural organic matter (NOM) inhibits photocatalytic degradation of SMX to a much greater extent at pH 5 than pH 9. In addition, the presence of bicarbonate leads to enhanced SMX photocatalysis at pH 9. Kinetic trends are consistent with a mechanism involving sulfonamide oxidation by hydroxyl radicals ( OH) generated via TiO 2 band gap excitation by UVA radiation. Identified transformation intermediates and products are consistent with SMX mineralization initiated by OH attack on either the aromatic or heterocyclic rings or the sulfonamide bond. Results demonstrate that UVA-TiO 2 photocatalysis can be a very effective approach for degrading sulfonamide micropollutants, particularly in natural waters exhibiting either alkaline pH or low concentrations of NOM, or both conditions.

Photocatalytic effects of titania supported nanoporous MCM-41 on degradation of methyl orange in the presence of electron acceptors

This study reports on the synthesis and photocatalytic evaluation of nanometer sized titania supported MCM-41 (TiMCM-41) by photodegrading methyl orange in the presence of electron acceptors such as peroxomonosulphate (PMS), peroxodisulphate (PDS) and H 2O 2 using visible-light irradiation to prove it is a better photocatalyst for advanced oxidation technologies (AOTs). This system is relatively inexpensive, reproducible, extremely stable and efficient in complete degradation of dye in aqueous solution. In order to obtain maximum information about the performance of TiMCM-41 catalyst, we did experiments under different operating conditions, i.e., variation of amount of catalyst, concentration of dye and electron acceptors. In addition to the above, a comparative study on the photocatalytic activities of colloidal TiO 2 was also made.

Two-dimensional concentration distribution for mixing-controlled bioreactive transport in steady state

Under steady-state conditions, the degradation of contaminant plumes introduced continuously into an aquifer is controlled by transverse dispersion when the other reacting compound is provided from ambient groundwater. Given that the reaction is instantaneous and longitudinal dispersion can be neglected, the length of the plume is inversely proportional to the transverse dispersion coefficient. In typical scenarios of natural attenuation, however, the considered reaction is biotic and kinetic. The standard model of bioreactive transport relies on double-Monod kinetics and pseudo first-order biomass decay. Under these conditions, a fraction of the injected mass flux remains beyond the length of the plume determined for the instantaneous reaction. We present an analytical framework to derive the steady-state concentration distributions of the dissolved compounds and the biomass from the concentration distribution of a conservative compound, assuming double-Monod kinetics and two different models describing biomass decay. The first biomass-decay model assumes a constant first-order decay coefficient, while the second assumes that the decay coefficient depends upon the electron-acceptor concentration. We apply the method to the case of a line-injection in two-dimensional uniform flow. In general, the bioreactive concentration distributions are similar to the distributions computed for an instantaneous reaction. The similarity is greater when the biomass decay coefficient is assumed to depend on the electron-acceptor concentration rather than being constant.

Une approche innovante pour modéliser la biodégradation des composés organochlorés volatils en aquifères poreux

The goal of this study was to develop an innovative chloroethene biodegradation module based on biological, thermodynamical and mechanistic concepts. The biodegradation scheme was based on the postulate that in each part of an aquifer only one degradation mechanism is dominant: the one involving the most energetic electron acceptor. Thus, the selection of the active degradation mechanism was a function of the concentration of different electron acceptors. Modified Monod-type kinetics was used in order to take into account the possible influence of some compounds on the biodegradation of a given organic compound. The numerical model developed was applied to a simple test case, whose results are presented here. To cite this article: F. Nex et al., C. R. Geoscience 338 (2006).

Numerical experiments and field results on the size of steady state plumes

Contaminated groundwater poses a serious risk for drinking water supplies. Under certain conditions, however, groundwater contamination remains restricted to a tolerable extent because of natural attenuation processes. We present an innovative approach to evaluate the size of these so-called steady-state plumes by 2-D and 1-D modelling in homogeneous aquifers. If longitudinal mixing is negligible, scenarios can be modelled in a simplified way using a 1-D domain vertical to the direction of flow. We analysed the sensitivity of the plume length with respect to biodegradation kinetics, flow velocity, transverse vertical dispersivity α t, the source and aquifer geometry and reaction stoichiometry. Our findings indicate that for many readily biodegradable compounds transverse–dispersive mixing rather than reaction kinetics is the limiting factor for natural attenuation. Therefore, if α t, aquifer and source geometry and concentrations of electron acceptors and donors are known, the length of the steady state contaminant plume can be predicted. The approach is validated under field conditions for an ammonium plume at a former landfill site in SW Germany.

Transport and degradation of ethanol in groundwater

Ethanol is rapidly replacing methyl tert-butyl ether (M tBE), the primary fuel oxygenate in the US, and ethanol releases from spills and leaky underground storage tanks (LUSTs) are anticipated. Ethanol has received little attention as a potential groundwater contaminant. This study investigates the fate and transport of ethanol under transient conditions in a sand and gravel aquifer. A pulse containing approximately 220 mg L − 1 ethanol and 16 mg L − 1 bromide was injected into the shallow sand and gravel aquifer and monitored to estimate its persistence and transport. The plume was monitored for 2.5 months using downgradient multilevel samplers (MLSs). Values for ethanol retardation were measured from ethanol and bromide breakthrough data and compared to estimates using published K oc values for low carbon aquifer sediments ( f oc = 10 μg C g − 1 sediment). Ethanol transport was not retarded ( R = 0.99). A 3-dimensional model reasonably simulated bromide and ethanol breakthrough curves. An average first-order decay constant was estimated to be 0.32 d − 1 ( t 1 / 2 = 2.2 d). At the second fence, 75% of the injected bromide and less than 3% of ethanol remained in the plume. Monitored terminal electron acceptor concentrations demonstrated that the majority of the ethanol was transformed by anaerobic processes other than denitrification and sulfate reduction.

A revised isotope fractionation model for dissimilatory sulfate reduction in sulfate reducing bacteria

Sulfur isotope fractionation during dissimilatory sulfate reduction has been conceptually described by the widely accepted Rees model as related to the stepwise reduction of sulfate to sulfide within the cells of bacteria. The magnitude of isotope fractionation is determined by the interplay between different reduction steps in a chain of reactions. Here we present a revision of Rees’ model for bacterial sulfate reduction that includes revised fractionation factors for the sulfite-sulfide step and incorporates new forward and reverse steps in the reduction of sulfite to sulfide, as well as exchange of sulfide between the cell and ambient water. With this model we show that in contrast to the Rees model, isotope fractionations well in excess of −46‰ are possible. Therefore, some of the large sulfur isotope fractionations observed in nature can be explained without the need of alternate pathways involving the oxidative sulfur cycle. We use this model to predict that large fractionations should occur under hypersulfidic conditions and where electron acceptor concentrations are limiting.