Range Of Sulfide Concentrations Research Articles

Caracterización del proceso biológico de sulfuro oxidación: un enfoque de respirometría de pulsos in situ y microrespirometria de pulsos ex situ

During the optimization of environmental biotechnology processes, it is important to count on proper and reliable information about the kinetic and stoichiometric parameters of the microorganisms. In this work, the biological sulfide oxidation process was assessed under two novel dynamic pulse respirometric approaches: in-situ pulse respirometry conducted in airlift bioreactor and ex-situ pulse respirometry carried out in microreactors (microrespirometry) with samples taken from the airlift bioreactor. The process was characterized in terms of the growth yield, substrate oxidation yield, maximum oxygen uptake rate, affinity constant, and mass transfer coefficient. The in-situ pulse respirometry showed to be a reproducible technique that allowed the determination of the kinetic and stoichiometric parameters besides the detection of mass transfer limitations in the airlift bioreactor, however, its use in biological sulfide oxidation is limited to a few experiments and experimental conditions with significant time investment. On the other hand, the ex-situ pulse microrespirometry allowed the acquisition of a higher amount of information under a broader range of sulfide concentrations (from 5 to 60 mg H2S L-1) and experimental conditions such as different pH values. The results obtained showed that the ex-situ microrespirometry technique would be preferable over in-situ pulse respirometry for the proper and reliable characterization of the sulfide oxidation process.

A novel selective detection method for sulfide in food systems based on the GMP-Cu nanozyme with laccase activity

A selective and sensitive colorimetric strategy for sulfide analysis was developed using GMP-Cu nanozymes with a laccase-like activity. This research discovered for the first time that sulfide could significantly enhance the catalytic activity of the GMP-Cu nanozymes by about 3.5 folds. The enhanced laccase activities duo to two reasons. First, Cu2+ in GMP-Cu nanozymes was reduced to Cu+. The other reason was the formation of Cu–S bond which was beneficial to accelerate the electron transfer rate to improve catalytic activity. Therefore, this method showed an excellent selectivity for sulfide. And it had a linear relationship in the sulfide concentration range of 0–220 μmol/L with a detection limit of 0.67 μmol/L. Furthermore, the proposed method was successfully applied to examine sulfide in the food systems. This new method may be used in sulfide detection to improve food quality and safety.

A turn-on fluorescent sulfide probe prepared from carbon dots and MnO2 nanosheets

A turn-on fluorescent sulfide probe was prepared from carbon dots (CDs; synthesized using an environmentally friendly method) and MnO2 nanosheets. In this composite probe, the fluorescence of the CDs (with excitation/emission peaks at 330/430nm) is quenched by the MnO2 nanosheets through an inner filter effect. Introducing sulfide causes the MnO2 nanosheets to be disintegrated through a redox reaction between sulfide and MnO2. Hence, the blue fluorescence of the CDs is restored. Under the optimum conditions, fluorescence increases linearly in the 2-25μM sulfide concentration range. The detection limit is 0.8μM. The method was used to analyze spiked real water samples, and satisfactory results were achieved. Graphical abstract Schematic of the detecting sulfide ions using the carbon dots/MnO2 fluorescence probe based on the recovery of carbon dot fluorescence when sulfide ions are added.

A dual emission nanocomposite prepared from copper nanoclusters and carbon dots as a ratiometric fluorescent probe for sulfide and gaseousH2S.

A series of dual-emission fluorescent probes was prepared from copper nanoclusters (Cu NCs) and carbon dots (CDs). They show two emission peaks (blue at 469nm and red at 622nm) when photoexcited at 365nm. Upon exposure to sulfide, the Cu NCs will be deteriorated because they react with sulfide to form CuS. This results in the quenching of the red fluorescence of the Cu NCs, while the blue fluorescence of the CDs remains constant. Thus, the color of the nanocomposite changes from red to blue. The ratio of the fluorescences at the two wavelengths decreases linearly in the 2-10ppb (26-128nM) sulfide concentration range, and the limit of detection is 0.33ppb (4.3nM). The nanocomposite also was placed in an agar gel and then incorporated into a paper strip for fluorometric monitoring of gaseous hydrogen sulfide. Graphical abstract Schematic presentation of the synthesis of Cu NCs (copper nanoclusters)-CDs (carbon dots) dual-emission nano-assembly, Cu NCs-CDs-agar fluorescent film and their application for the detection of sulfide and H2S.

Mercury transformations in resuspended contaminated sediment controlled by redox conditions, chemical speciation and sources of organic matter

Mercury (Hg) contaminated sediments can be significant sources of Hg in aquatic ecosystems and, through re-emission processes, to the atmosphere. Transformation and release of Hg may be enhanced by various sediment perturbation processes, and controlling biogeochemical factors largely remain unclear. We investigated how rates of Hg transformations in pulp-fiber enriched sediment contaminated by Hg from chlor-alkali industry were controlled by (i) transient redox-changes in sulfur and iron chemistry, (ii) the chemical speciation and solubility of Hg, and (iii) the sources and characteristics of organic matter (OM). Sediment-bottom water microcosm systems were exposed to four combinations of air and nitrogen gas for a total time of 24 h. The treatments were: 24 h N2, 0.5 h air + 23.5 h N2, 4 h air + 20 h N2 and 24 h of air exposure. As a result of these treatments, microcosms spanned a wide range of redox potential, as reflected by the dissolved sulfide concentration range of ≤0.3–97 µM. Four different chemical species of inorganic divalent Hg (HgII) and methyl mercury (MeHg), enriched in different Hg isotope tracers, were added to the microcosms: 201Hg(NO3)2(aq), 202HgII adsorbed to OM (202HgII-OM(ads)), 198HgII as microcrystalline metacinnabar (β-198HgS(s)) and Me204HgCl(aq). Microcosm systems were composed of bottom water mixed with sediment taken at 0–2, 0–5 and 0–10 cm depth intervals. The composition of OM varied with sediment depth such that compared to deeper sediment, the 0–2 cm depth-interval had a 2-fold higher contribution of labile OM originating from algal and terrestrial inputs, serving as metabolic electron-donors for microorganisms. The potential methylation rate constant (kmeth) of Hg tracers and net formation of ambient MeHg (MeHg/THg molar ratio) increased up to 50% and 400%, respectively at intermediate oxidative conditions, likely because of an observed 2-fold increase in sulfate concentration stimulating the activity of sulfate reducing bacteria with a capability of methylating HgII. Due to differences in HgII water-sediment partitioning, kmeth varied by a factor of 11–70 for the different isotope-enriched Hg tracers. The chemical form of HgII was a major controlling factor for kmeth and its response to the resuspension-oxidation of the system. The β-198HgS(s) tracer had the lowest kmeth and it was mainly constrained by redox-driven HgII solubility. The 202HgII-OM(ads) tracer showed an intermediate value on kmeth. It was controlled by both HgII solubility and availability of electron donors and acceptors, regulating bacterial activity. The 201Hg(NO3)2(aq) tracer had the highest value on kmeth which was limited mainly by bacterial activity. The kmeth was up to a factor of 3 higher in the 0–2 cm sediment depth-interval than in 0–5 and 0–10 cm intervals due to a larger contribution of labile OM in the 0–2 cm sediment. Reduction of HgII to Hg0 followed by volatilization exclusively occurred at high sulfidic conditions in the top 0–2 cm sediment. Aromatic moieties of terrestrial OM, present mainly in the top sediment, is suggested to control the reduction of HgII. The Hg0 volatilization rate constant for the 202HgII-OM(ads) tracer exceeded that for β-198HgS(s) by one order of magnitude. Our results suggest that contaminated sediments posing a high risk for reactivation of legacy Hg following transient redox resuspension events are characterized by depletion of sulfate in the sediment porewater prior to resuspension, predominance of HgII species with solubility exceeding that of crystalline β-HgS(s), and conditions promoting in situ formation and/or import of labile OM from algal and terrestrial sources.

Visible light photoelectrochemical sulfide sensor based the use of TiO2 nanotube arrays loaded with Cu2O

This work describes a photoelectrochemical for sensing sulfide method that is based on TiO2 nanotubes loaded with Cu2O particles (Cu2O/TiNTs). The narrow band gap of Cu2O and the heterostructure at the interface between Cu2O and TiO2 in the Cu2O/TiNTs significantly enhance visible light absorption and charge separation, and these results in an improved photocurrent of the Cu2O/TiNT electrode under irradiation with visible light (> 400 nm). In the presence of sulfide, Cu7S4 is formed on the surface of the Cu2O/TiNT, this leading to a decrease in the photocurrent on the electrode. At a potential of 0.0 V, the photocurrent of the Cu2O/TiNTs before and after immersing it into 300 μM sulfide solution is 1086 and 310 μA cm−2, respectively. The findings were used to design a photoelectrochemical sulfide sensor. Under optimum conditions and at a pH value of 7.0, the change in photocurrent varies linearly in the 1 to 300 μM sulfide concentration range with a 0.6 μM detection limit. The sensor was applied to the determination of sulfide in spiked real water samples.

Sulfide effects on the anaerobic metabolism of polyphosphate-accumulating organisms

Sulfate rich wastewaters can be generated from industry, use of seawater in urban environments, or by saline water infiltration into the sewerage. Under anaerobic conditions sulfate can be converted to sulfide, which may affect micro-organisms performing biological nutrient removal. The objective of this study was to evaluate the effect of sulfide on the activity of polyphosphate-accumulating organisms (PAO) in the anaerobic stage of the enhanced biological phosphorus removal process (EBPR). In this regard, a highly enriched culture of PAO was exposed in short-term activity tests to a range of sulfide concentrations at different operational pH values. The PAO activity was mainly affected by un-dissociated H2S. The specific acetate uptake rate was inhibited by 50% at around 60mg H2S.L−1. With increasing H2S concentrations, higher phosphate release rate to acetate uptake rate ratios were observed, possibly due to increased energy requirements for cell detoxification. Mathematical expressions were developed, which satisfactorily described the sulfide effects on the acetate uptake rate and phosphate release rate. The results show that, dependent on the pH, EBPR might be negatively affected by total sulfide concentrations exceeding 275mg SO4.L−1 at pH 6.5 or 1200mg SO4.L−1 at pH 7.8mg SO4.L−1 or when freshwater is partially replaced by seawater more than 45% (pH 7.8) or 10% (pH 6.5) used as secondary quality water. The findings of this study imply that sulfide, which is commonly found in different type of wastewaters, affects the anaerobic metabolism of PAO and may play an important role in the process performance of treatment plants treating wastewaters with high sulfide content.

Organic semiconductor polymer nanodots as a new kind of off-on fluorescent probe for sulfide

The authors describe a fluorescent probe for sulfide that is based on carboxy-functionalized semiconducting polymer dots (P-dots). The dots were prepared from carboxy-functionalized poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,1′-3-thiadiazole)] (referred to as COOH-PFBT) via co-precipitation. The P-dots aggregate on addition of Cu(II) ions and their green fluorescence (with excitation/emission peaks at 455/540 nm) is then quenched. Fluorescence is restored on addition of sulfide to the aggregates due to the formation of CuS. This quenching-recovery (“off-on”) mechanism forms the basis for a new sulfide detection scheme. Fluorescence increases linearly in the 1.25 to 75.0 μM sulfide concentration range, with a 0.45 μM detection limit. Good selectivity over other anions is demonstrated. The method shows recoveries ranging between 98.6% and 105.7% when applied to the determination of sulfide in spiked real water samples.

Determination of trace hydrogen sulfide by using the permanganate induced chemiluminescence of carbon dots

Carbon dots (C-dots) were prepared in aqueous solution by a one-step ultrasonic approach, and the absorption and fluorescence spectra were acquired. It is found that acidic solutions of potassium permanganate cause C-dots to produce green and red chemiluminescence (CL) peaking at 520 nm and 620 nm, respectively, and that CL intensity is further enhanced on addition of sulfide. A mechanism leading to this enhancement is suggested. After optimizing the reaction conditions, the CL-based calibration plot is linear in the 1.0 to 100 μM sulfide concentration range, with a 0.2 μM detection limit (at a signal-to-noise ratio of 3). The relative standard deviation is 1.7 % for the determination of a 10 μM solution of sulfide. The method was employed to quantify trace levels of sulfide in water samples.

Effects of Acid Volatile Sulfides (AVS) from Na2S-Amended Sediment on Hyalella azteca

Ratios of acid volatile sulfides (AVS) and simultaneously extracted metals (SEM) have been used extensively for predicting bioavailability of divalent metals (i.e., Cd, Cu, Pb, Ni, Zn) in sediments of aquatic environments. However, the role of sulfides (as AVS) as a toxicant has been largely ignored. The aim of this research was to measure relationships of AVS (as sodium sulfide [Na2S]-amended sediment) and toxicity to a sensitive benthic amphipod Hyalella azteca to evaluate the exposure-response relationships among a series of sulfide exposures. The specific objectives were to (1) measure SEM/AVS ratios in a series of sodium sulfide (Na2S·9H2O)-amended sediments producing a range of sulfide concentrations and (2) measure responses of H. azteca (as mortality) in 96-h static sediment toxicity tests to exposures of Na2S-amended sediments. Amended sediments had a predictable increase in AVS concentrations and a concomitant decrease in ∑SEM/AVS ratios. Increasing concentrations of AVS resulted in a range of ∑SEM/AVS ratios that varied over more than an order of magnitude from 0.185 to 0.006. H. azteca survival decreased with increasing concentrations of “excess” AVS, with 96-h no observable effect concentration (NOEC) and LC50 of 0.041 and 0.019 ∑SEM/AVS, respectively. Clearly, the SEM/AVS model provides a useful tool for evaluating potential bioavailability of divalent metals and predicting ecological risk; however, this study demonstrates the need to consider sulfide (as AVS) as a potential source of toxicity in situations with low [<<1] ∑SEM/AVS ratios.

Sensitivity of Ruppia maritima and Zostera marina to sulfide exposure around roots

Ruppia maritima and Zostera marina often inhabit the same coastal areas and are often exposed to fairly high sulfide concentrations, with the former being most resilient. Our aim was to investigate differences in sulfide tolerance between the two species. Sensitivity towards sulfide in R. maritima and Z. marina was investigated in split chambers that allowed exposure of roots to a range of sulfide concentrations, while the leaves were kept under optimal light and oxygen conditions. Roots of R. maritima show a higher threshold towards sulfide (1.8–5.9mM H2S) than those of Z. marina (0–0.5mM H2S) after two weeks of incubation. The difference in sulfide sensitivity is attributed to different capacity of root oxygen leakage. Light and oxygen exposure enables oxygen transport from leaves down to the roots during both day and night. The oxic micro-shield around roots has a capacity to withstand sulfide intrusion until the sensitivity level is reached, when no other stressors are present. Leaves of exposed R. maritima plants are affected at similar sulfide concentrations as roots, while leaves of Z. marina are affected at higher sulfide concentrations (1–3.5mM H2S), reflecting different pathways of sulfide intrusion, root permeability and the ability to allocate energy away from exposed plant parts. Sensitivity towards sulfide occurs gradually and probably as a linear response to increasing sulfide concentration in R. maritima, but the exact pattern in Z. marina is still unclear. Other external stressors, such as anoxia, shading, or increased temperature may change the thresholds of the species by affecting radial oxygen loss from roots, and make especially R. maritima more prone to sulfide toxicity due to its highly permeable roots.

Examination and Refinement of the Determination of Aqueous Hydrogen Sulfide by the Methylene Blue Method

The accurate and precise measurement of total sulfide has been of major interest for well over a century. The most commonly used method involves the formation of a methylene blue–sulfide complex and spectrophotometric measurement of its concentration. The study presented herein compares the two most commonly used methods as outlined in Standard Methods for the Examination of Water and Wastewater (in APHA, Standard methods for the examination of water and wastewater, Washington, 1960) and by Cline (Limnol Oceanogr 14:454–458, 1969). In addition, this study clarifies the existing confusion of Cline’s reagent preparation procedure, as it is apparent that various interpretations exist among research groups regarding reagent preparation. After evaluating both methods with respect to precision and accuracy, detection limit, sample storage time, and ease of use, the method outlined in Cline was determined to be superior. Furthermore, we suggest that the reagent concentration has to be optimized depending on the range of sulfide concentrations to increase the accuracy and precision of the method.

Investigating the viability of electrodeposited vanadium pentoxide as a suitable electrode material for in vivo amperometric hydrogen sulfide detection

A vanadium pentoxide (V2O5) film was electrodeposited onto the end of a PtIr wire (d=125μm) via potential cycling in order to preliminarily assess its viability as a prospective material for in vivo H2S monitoring. The deposited oxide film exhibited catalytic activity towards the oxidation of H2S in a physiologically relevant pH buffer solution, which resulted in a large increase in peak current compared to that exhibited by the bare PtIr surface. The modified electrode exhibited a linear response over a total sulfide concentration range of 0.5–15μM. The catalytic activity of the V2O5-modified wire towards the oxidation of upper physiological levels of NO and CO, two gaseous interferences found in vivo, was investigated using amperometry. While the electrode demonstrated some activity towards NO, it exhibited significant activity towards CO in the potential region of interest. However, the response towards NO and CO was much slower than the response towards H2S and the linear range remained unchanged despite the presence of the interfering gases.

Performance of An Enzyme Electrode Designed for a Sulfide Monitoring Biosensor

Abstract The construction and evaluation of an enzyme electrode designed for a sulfide monitoring biosensor, are reported in this work. The enzyme, which is immobilized in a carbon (graphite) paste with 1,1′ dimethylferrocene, catalyzes the oxidation of sulfide to sulfur. Amperometric measurements were carried out at room temperature, and a fixed potential of 0.3 V with reference to a saturated calomel electrode, using Tris-HCl buffer (pH 7.5) as the electrolyte, and sodium sulfide solution as the substrate. Under deaerated conditions, the biosensor responded linearly to the tested sulfide concentration range of 0–4mM, giving an average apparent Michaelis constant of 3mM, and maximum current responses between 0.7 and 3.6 μA. The sensitivity of the electrodes tested ranged from 0.2–0.3 μA mM−1.

Competition among thiols and inorganic sulfides and polysulfides for Hg and MeHg in wetland soils and sediments under suboxic conditions: Illumination of controversies and implications for MeHg net production

Current research focus in mercury biogeochemistry is on the net production and accumulation of methyl mercury (MeHg) in organisms. The activity of iron‐ and sulfate‐reducing bacteria (FeRB and SRB) has been identified as important for MeHg production. There are indications of a passive uptake of neutral Hg‐sulfides by SRB, as well as of a facilitated bacterial uptake of Hg complexed by small organic molecules. In order to understand these processes, the chemical speciation of Hg and MeHg, and most important, the competition among organic thiols and inorganic sulfides and polysulfides, needs to be clarified under suboxic conditions (nM to low μM range of total sulfide concentrations) in wetland soils and sediments. In this paper the chemical speciation of Hg and MeHg is modeled at pH 4.0 and 7.0 in a conceptual wetland soil/sediment with typical concentrations of thiols, sulfides, Hg, and MeHg. Effects of precipitated HgS(s), the formation of Hg‐polysulfides, and the size of the controversial stability constant for the formation of HOHgSH0 (aq) are emphasized. The outcome of the modeling is discussed in light of chosen stability constants for Hg complexes with thiols, sulfides, and polysulfides. It is concluded that organic thiols are competitive with inorganic sulfides in the approximate total sulfide concentration range 0–1 μm. It is also concluded that increases in absolute aqueous concentrations of MeHg, or the molar ratio of dissolved MeHg/Hg, are not appropriate as indirect measures of MeHg net production, unless changes and differences in solubility of MeHg and Hg are corrected for.

Modification of sediment geochemistry by the hydrocarbon seep tubeworm Lamellibrachia luymesi: A combined empirical and modeling approach

The sulfide-oxidizing symbiotic tubeworm Lamellibrachia luymesi is a dominant member of deep-sea hydrocarbon seep ecosystems on the Gulf of Mexico seafloor. This tubeworm forms large aggregations that can live for centuries and provide habitat for an assortment of associated fauna. Previous studies have suggested that persistence of these tubeworms for such long time periods is contingent upon their ability to supply sediments with sulfate. To examine this hypothesis, we characterized the tubeworm’s geochemical environment using pore water peepers and compared the measured depth profiles with those predicted by a sulfur diffusion–reaction–supply model. We found a large range of sulfide concentrations in the tubeworm habitat, indicating that this species can live under conditions of both high and low sulfide availability. In sediments rich in hydrocarbons, we found compelling evidence that tubeworms enhance microbial sulfide production, likely through a combination of sulfide uptake and sulfate release through their root-like structures buried in the sediment. Our in situ empirical data combined with the results of the geochemical model corroborate previous physiological studies that indicate that tubeworms release between 70% and 90% of the sulfate produced during sulfide oxidation by their symbionts across their roots into the surrounding sediment. In sediments low in hydrocarbon content, sulfide production is hydrocarbon-limited rather than sulfate-limited, and our model predicts that tubeworm growth could be limited by low sulfide availability.

A comparison of flow injection methods for sulfide determination based on phenothiazine dyes produced from diverse aromatic amines

In the present work a systematic comparison among spectrophotometric flow injection methods for sulfide determination based on phenothiazine dye production from diverse aromatic p-substituted amines were performed. The behavior of N, N-dimethylphenyl- p-diamine (DMPD), N, N-diethylphenyl- p-diamine (DEPD), phenyl- p-diamine (PPD), p-aminophenol (PAP) and other three aromatic amines was investigated and the chemical parameters of proposed flow methods were optimized by applying central composite design. For each evaluated method the concentration of amine, Fe 3+ and H 2SO 4 was optimized and after the evaluation of reagents addition order the flow parameters were independently ascertained. Analytical signal was strongly diminished in the presence of iodide for flow methods based on the reaction of sulfide with DMPD, PPD or DEPD while fluoride was considered as an important interference for methods based on the reaction with PPD or PAP. The evaluated aromatic amines have permitted sulfide determination in a wide concentration range from 0.05 to 3.0 mg L − 1 and limits of detection (3 σ) varying from 18.8 to 51.0 μg L − 1 , for DEPD and PPD, respectively. The sensitivity of flow methods based on PPD and DMPD was higher even as PAP has permitted sulfide determination in a large sulfide concentration range. In addition, higher throughput was attained for DMPD method. The proposed methods were applied for sulfide determination in industrial wastewater and the obtained results were in agreement with reference method at 95% confidence level.

Survival and growth of two heterotrophic hydrothermal vent archaea, Pyrococcus strain GB-D and Thermococcus fumicolans, under low pH and high sulfide concentrations in combination with high temperature and pressure regimes

Growth and survival of hyperthermophilic archaea in their extreme hydrothermal vent and subsurface environments are controlled by chemical and physical key parameters. This study examined the effects of elevated sulfide concentrations, temperature, and acidic pH on growth and survival of two hydrothermal vent archaea (Pyrococcus strain GB-D and Thermococcus fumicolans) under high temperature and pressure regimes. These two strains are members of the Thermococcales, a family of hyperthermophilic, heterotrophic, sulfur-reducing archaea that occur in high densities at vent sites. As actively growing cells, these two strains tolerated regimes of pH, pressure, and temperature that were in most cases not tolerated under severe substrate limitation. A moderate pH of 5.5-7 extends their survival and growth range over a wider range of sulfide concentrations, temperature and pressure, relative to lower pH conditions. T. fumicolans and Pyrococcus strain GB-D grew under very high pressures that exceeded in-situ pressures typical of hydrothermal vent depths, and included deep subsurface pressures. However, under the same conditions, but in the absence of carbon substrates and electron acceptors, survival was generally lower, and decreased rapidly when low pH stress was combined with high pressure and high temperature.

Fate and transport of metals in H2S-rich waters at a treatment wetland

The aqueous geochemistry of Zn, Cu, Cd, Fe, Mn and As is discussed within the context of an anaerobic treatment wetland in Butte, Montana. The water being treated had a circum-neutral pH with high concentrations of trace metals and sulfate. Reducing conditions in the wetland substrate promoted bacterial sulfate reduction (BSR) and precipitation of dissolved metal as sulfide minerals. ZnS was the most common sulfide phase found, and consisted of framboidal clusters of individual spheres with diameters in the submicron range. Some of the ZnS particles passed through the subsurface flow, anaerobic cells in suspended form. The concentration of "dissolved" trace metals (passing through a 0.45 μm filter) was monitored as a function of H2S concentration, and compared to predicted solubilities based on experimental studies of aqueous metal complexation with dissolved sulfide. Whereas the theoretical predictions produce "U-shaped" solubility curves as a function of H2S, the field data show a flat dependence of metal concentration on H2S. Observed metal concentrations for Zn, Cu and Cd were greater than the predicted values, particularly at low H2S concentration, whereas Mn and As were undersaturated with their respective metal sulfides. Results from this study show that water treatment facilities employing BSR have the potential to mobilize arsenic out of mineral substrates at levels that may exceed regulatory criteria. Dissolved iron was close to equilibrium saturation with amorphous FeS at the higher range of sulfide concentrations observed (>0.1 mmol H2S), but was more likely constrained by goethite at lower H2S levels. Inconsistencies between our field results and theoretical predictions may be due to several problems, including: (i) a lack of understanding of the form, valence, and thermodynamic stability of poorly crystalline metal sulfide precipitates; (ii) the possible influence of metal sulfide colloids imparting an erroneously high "dissolved" metal concentration; (iii) inaccurate or incomplete thermodynamic data for aqueous metal complexes at the conditions of the treatment facility; and (iv) difficulties in accurately measuring low concentrations of dissolved sulfide in the field.

The influence of sulfide on solid-phase mercury bioavailability for methylation by pure cultures of Desulfobulbus propionicus (1pr3).

To help understand the mechanism and control of Hg uptake in Hg-methylating bacteria, we investigated the effect of sulfide on Hg methylation by pure cultures of the sulfate-reducing bacterium Desulfobulbus propionicus (1pr3). Our previous research in natural sediments has suggested that Hg methylation occurs most rapidly when sulfide concentrations favor formation of neutral dissolved Hg-S species. In this study, the chemical speciation of Hg in culture media was manipulated by growing D. propionicus across a range of sulfide concentrations, with inorganic Hg (HgI) added in the form of ground ores. A solid-phase, rather than a dissolved source of Hg, was used to simulate the controls on Hg partitioning between solid and aqueous phases found in natural sediments. Methylmercury (MeHg) production by cultures was not related to the absolute solid-phase concentration of Hg in the ores, and it was only weakly related to the dissolved HgI concentration in the medium. However, MeHg production was linearly related to the calculated concentration of the dominant neutral complex in solution, HgS degrees. Furthermore, the diffusive membrane permeability of HgS degrees, as estimated from its octanol-water partitioning coefficient, was found to be sufficient to support MeHg production by cells. The present paper expands on our previous work by providing experimental support of our hypothesis that sulfide influences methylation by affecting the speciation of dissolved HgI and its uptake via passive diffusion.