FeS Precipitation Research Articles

Hydrology and oxygen addition drive nutrients, metals, and methylmercury cycling in a hypereutrophic water supply reservoir

Impaired water quality in Mediterranean climate reservoirs is mainly associated with eutrophication and internal nutrient loading. To improve water quality in hypereutrophic Hodges Reservoir, California, United States, a hypolimnetic oxygenation system (HOS), using pure oxygen gas, was implemented in 2020. This study encompasses 3 years of pre-oxygenation data (2017–2019) and 2 years of post-oxygenation data (2020–2021) to understand the cycling of nutrients, metals, and mercury in the reservoir. During the wet year of 2017, mildly reduced conditions lasted until mid-summer in the enlarged reservoir. Nutrients and metals were seen in the hypolimnion including ammonia (~2 mg-N/L), manganese (~0.5 mg/L), phosphate (~0.5 mg-P/L), and sulfide (~10 mg/L). Production of methylmercury (MeHg), an important bioaccumulative toxin, was favored from April to June with a hypolimnetic accumulation rate of around 200 ng/m2·d. In contrast, the dry year of 2018 exhibited higher hypolimnetic concentrations of ammonia (~4 mg-N/L), manganese (~1 mg/L), phosphate (>0.5 mg-P/L), and sulfide (>15 mg/L). The rapid onset of highly reduced conditions in 2018 corresponded with low MeHg hypolimnetic accumulation (~50 ng/m2·d). It seems that mildly reduced conditions were associated with higher MeHg accumulation, while sulfidic, reduced conditions impaired inorganic mercury bioavailability for MeHg production and/or promoted microbial demethylation. Sulfide also appeared to act as a sink for iron via FeS precipitation, and potentially for manganese via MnS precipitation or manganese coprecipitation with FeS. Mass flux estimates for 2017–2019 indicate that much of the nutrients that accumulated in the hypolimnion moved via turbulent diffusion into the epilimnion at loading rates far exceeding thresholds predicting eutrophic conditions. After oxygenation in 2020–2021, the reservoir water column was highly oxidized but showed a lack of thermal stratification, suggesting reservoir operations in combination with HOS implementation inadvertently mixed the water column in this relatively shallow reservoir. Post-oxygenation, concentrations of ammonia, phosphate, manganese, and mercury in bottom waters all decreased, likely in response to oxidized conditions. Oxygenated bottom waters exhibited elevated nitrate, a byproduct of ammonia nitrification, and iron, a byproduct of FeS oxidation, indicating a lake-wide response to oxygenation.

Rapid formation of iron monosulfide on Indicator of Reduction in Soil devices in S‐rich hydric soils

AbstractIndicator of Reduction in Soil (IRIS) devices are used to identify anaerobic conditions in soils for hydric soil and wetland identification purposes. IRIS devices quantify anaerobic conditions via the visualization of iron (Fe) reduction (e.g., orange Fe3+ paint reduces to soluble Fe2+, leaving behind the bare, white polyvinyl chloride device). Under stronger reducing conditions, sulfate (SO42−) reduction can occur, and sulfides (S2− and/or H2S) can react with the Fe3+ paint, resulting in the precipitation of black‐colored, reduced iron monosulfide (FeS). While these processes are well known, the rate of FeS formation remains relatively understudied and current IRIS methods may not capture these data accurately. This study investigated FeS formation on IRIS films to identify deployment times that capture the minimum and maximum precipitation of FeS. To determine the timing and magnitude of FeS‐precipitation on IRIS films, five replicate films were deployed in a wet, S‐rich soil, across 11 periods ranging from 2 min to 30 days. Results show that FeS precipitated on IRIS films in just 2 min, and the highest average amount of FeS (82%) precipitated in 1 day. After 1 day, the percentage of FeS decreased and a white color change became more apparent on IRIS films. Our results suggest that the recommended 30‐day deployment period is too long for accurately measuring FeS‐precipitation on IRIS devices deployed in wet, alkaline soils. These considerations should be incorporated into standard IRIS protocols used to quantify anaerobic conditions and other biogeochemical conditions (e.g., pore‐water sulfide levels) in S‐rich soils.

Phosphorus immobilisation in sediment by using iron rich by-product as affected by water pH and sulphate concentrations

Iron (Fe) rich by-product from drinking water treatment plants can be added to rivers and lakes to immobilise phosphorus (P) in sediment and lower eutrophication risks. This study was set up to investigate the P immobilisation efficiency of an Fe rich by-product as affected by the pH and sulphate (SO4) concentration in the overlying water. Both factors are known to inhibit long-term P immobilisation under anoxic conditions. A static sediment-water incubation was conducted at varying buffered water pH values (6, 7 and 8) and different initial SO4 concentrations (0–170 mg SO4 L−1) with or without Fe rich by-product amendment to the sediment. In the unamended sediment, the P release to the overlying water was highest, and up to 6 mg P L−1, at lowest water pH due to higher reductive dissolution of Fe(III) oxyhydroxides. The Fe rich by-product amendment to the sediment largely reduced P release from sediment by factors 50–160 depending on pH, with slightly lowest immobilisation at highest pH 8, likely because of pH dependent P sorption. The total sulphur (S) concentrations in the overlying water reduced during incubation. The P release in unamended sediments increased from 2.7 mg L−1 to 4.2 mg L−1 with higher initial SO4 concentrations, suggesting sulphide formation during incubation and FeS precipitation that facilitates release of P. However, no such SO4 effects were found where Fe rich by-product was applied that lowered P release to <0.1 mg L−1 illustrating high stability of immobilised P in amended sediments. This study suggests that Fe rich by-product is efficient for P immobilisation but that loss of Fe in low pH water may lower its long-term effect.

Do bacterial viruses affect framboid-like mineral formation?

Abstract. Framboidal pyrite has been a matter of interest of many studies due to its abundance in a wide range of environments and being a marker of redox conditions. However, the clear origin of framboidal pyrite remains unresolved. Our studies are preliminary laboratory investigations on the influence of the shape and physicochemical properties of bacteriophages on the synthesis of framboid-like structures. This paper discusses the possible role of bacteriophages (bacterial viruses) in the precipitation of sulfide minerals (FeS and CuS) and their impact on the formation of framboid-like structures. Here, two bacteriophages (Escherichia phage P1 and Pseudomonas phage Φ6), which differ significantly in shape and physicochemical properties, were used. Our observations suggest that viruses may bind ions from the solution. Moreover, we showed that bacteriophages P1 can lead to the formation of finer mineral particles of FeS and CuS, whereas the framboid-like structures were found only in experiments with precipitation of FeS. However, the lipid-enveloped Pseudomonas phage Φ6 did not cause the formation of similar structures. It is assumed that Escherichia phage P1 can promote the formation of FeS-based framboid-like or spherical structures. The proposed four-step conceptualized mechanism facilitating the framboid-like structure synthesis via viruses is as follows: (i) binding of ions by capsids, (ii) bacteriophages behaving like a crystallization surface, (iii) destabilization of the colloid (ζ potential ± 0), and (iv) formation of fine agglomerates and subsequent formation of small crystallites. Further studies are required to find all factors that may be affected by bacteriophages during sulfide precipitation. In addition, it is important to consider viruses present in sedimentation environments, despite possible difficulties in laboratory culturing. The consideration of such viruses may make laboratory testing more valid in terms of sedimentation environments.

Drainage chemistry of mine tailings from a carbonatite-hosted Nb-REE deposit, Oka, Québec, Canada

Potential environmental issues associated with the mining of carbonatites are receiving increased attention due to the importance of critical metals for green technologies. This study investigates the chemistry of tailings seepage at the former Saint Lawrence Columbium mine near Oka, Québec, Canada, which produced pyrochlore concentrate and ferroniobium from a carbonatite-hosted Nb-REE deposit. Detailed field sampling and laboratory methods were used to characterize the hydraulic properties of the tailings, their bulk chemistry, mineralogy, pore water and effluent chemistries. The tailings are composed of REE-enriched calcite (64–89 wt %) and fluorapatite (2–22 wt %), as well as biotite (6–17 wt %) and chlorite (0–7 wt %). Minor minerals include ankerite, pyrite, sphalerite, molybdenite, magnetite and unrecovered pyrochlore. Secondary minerals include gypsum, barite, strontianite and rhodochrosite. Geochemical mass balance modeling, constrained by speciation modeling, was used to identify dissolution, precipitation and exchange reactions controlling the chemical evolution of pore water along its flow path through the tailings impoundment. In the unsaturated zone, these reactions include sulfide oxidation and calcite dissolution with acid neutralization. Below the water table, gypsum dissolution is followed by sulfate reduction and FeS precipitation driven by the oxidation of organic carbon in the tailings. Incongruent dissolution of biotite and chlorite releases K, Mg, Fe, Mn, Ba and F and forms kaolinite and Ca-smectite. Cation exchange reactions further remove Ca from solution, increasing concentrations of Na and K. Fluoride concentrations reach 23 mg/L and 8 mg/L in tailings pore water and effluent, respectively. These values exceed Canadian guidelines for the protection of aquatic life. In the mildly alkaline (pH 8.3) pore waters, Mo is highly mobile and reaches an average concentration of 83 μg/L in tailings effluent, which slightly exceeds environmental guidelines. Concentrations (unfiltered) of Zn reach 1702 μg/L in tailings pore water although values in effluent are usually less than 20 μg/L. At the ambient pH, Zn is strongly adsorbed by Fe–Mn oxyhydroxides. Although U forms mobile complexes in tailings pore water, concentrations do not exceed 16 μg/L due to the low solubility of its pyrochlore host. Adsorption and the low solubility of pyrochlore limit concentrations of Nb to less than 49 μg/L. Cerium, from calcite dissolution, is strongly adsorbed although it reaches concentrations (unfiltered) in excess of 1 mg/L and 100 μg/L in pore water and effluent, respectively. Results of this study show that mine tailings from carbonatite deposits are enriched in a wide variety of incompatible elements with multiple mineral hosts of varying solubility. Some of these elements, such as F and Mo, may represent contaminants of concern because of their mobility in alkaline tailings waters.

Ratiometric fluorescence detection of sulfide ions based on lanthanide coordination polymer using guanosine diphosphate as ligand

The efficiency of energy transfer from guanine nucleotide to terbium ion (Tb3+) is affected by the phosphate group significantly. Compared with the biomolecules 5′-GMP (guanosine monophosphate), guanosine diphosphate (GDP) exhibits better sensitize ability to Tb3+ ions luminescence. Assisted with the carboxycoumarin ligand, we synthesized a more stable optical Coumarin@GDP-Tb polymer with the characteristic emission peaks located on 440 nm and 545 nm in this work. The Coumarin@GDP-Tb polymer is not only rich in metal binding sites, but also maintains a moderate ionic binding force, which helps metal ions to bind or leave it easily. Experiment result shows that Coumarin@GDP-Tb polymer has the appropriate binding force for Fe2+ ions, which can be destroyed by sulfur ions (S2−) as the formation of FeS precipitation. Based on this, Coumarin@GDP-Tb was designed as the ratio fluorescence probe for sulfur ions detection, where the fluorescence at 545 nm can be selectively quenched by Fe2+ ions, while that at 440 nm was unaffected, in the presence of S2− ions, the quenched fluorescence can be recovered remarkably. With the increasing S2− ions from 0.1–45 μM, the ratio of fluorescence intensity at 545 nm to 440 nm (F545/F440) is linear to S2− concentration, and the detection limit of S2− was calculated to be 0.073 μM. Contrast to those fluorescence probes with single wavelength emission, Coumarin@GDP-Tb displays a comparable sensitivity, the introduced self-adjust wavelength improved the detection accuracy efficiently. The above 98.1 % recovery rates of S2− ions in the actual water sample demonstrated the practicability of Coumarin@GDP-Tb fluorescence probe.

Orange leads to black: evaluating the efficacy of co‐culturing iron‐oxidizing and sulfate‐reducing bacteria to discern ecological relationships

Two global cycles, iron and sulfur, are critically interconnected in estuarine environments by microbiological actors. To this point, the methods of laboratory study of this interaction have been limited. Here we propose a methodology for co-culturing from numerous coastal environments, from the same source inocula, iron-oxidizing and sulfate-reducing bacteria. The use of same source inocula is largely beneficial to understand real-world interactions that are likely occurring in situ. Through the use of this methodology, the ecological interactions between these groups can be studied in a more controlled environment. Here, we characterize the oxygen and hydrogen sulfide concentrations using microelectrode depth profiling in the co-cultures of iron-oxidizing bacteria and sulfate-reducing bacteria. These results suggest that while oxygen drives the relationship between these organisms and sulfate-reducers are reliant on iron-oxidizers in this culture to create an anoxic environment, there is likely another environmental driver that also influences the interaction as the two remain spatially distinct, as trends in FeS precipitation changed within the anoxic zone relative to the presence of Fe(III) oxyhydroxides. Understanding the relationship between iron-oxidizing and sulfate-reducing bacteria will ultimately have implications for understanding microbial cycling in estuarine environments as well as in processes such as controlling microbially influenced corrosion.

Concentration of Iron(II) in Fresh Groundwater Controlled by Siderite, Field Evidence

Iron(II) concentrations in fresh groundwater in Dutch aquifers range from absent up to 50 mg/l. Evaluation of extensive chemical data sets learned that the maximum logarithmic concentration of iron(II) in aquifers, between ± 6.5 < pH < ± 8, is a linear function of pH, governed by Siderite. It is a broad relation due to oversaturation with respect to Siderite and to variation in alkalinity. Iron(II) is continuously supplied to groundwater by reduction of hydrous ferric oxides (HFO), until becoming saturated with respect to Siderite, and from then on, HFO reduction and Siderite precipitation occur simultaneously. In Dutch aquifers, the electron supply rate (equivalent to the organic matter oxidation rate) apparently exceeds the HFO electron uptake rate (equivalent to the HFO reduction rate) and the excess supply is taken up by sulfate (equivalent to the sulfate reduction rate): HFO reduction, sulfate reduction and FeS precipitation occurring simultaneously, where the presence of Siderite prevents a dip in the iron(II) concentration. After sulfate becomes exhausted, the excess electron supply is transferred to methane production: HFO reduction and methane production occurring simultaneously. This evaluation also demonstrated that the organic matter oxidation rate and the HFO reduction rate decrease over time. The results of this study are also relevant for the behavior of As and of Co, Ni and Zn in groundwater, as HFO, Pyrite and Siderite may contain variable contents of these elements.

Remarkable promotion in particle dispersion and electron transfer capacity of sulfidated nano zerovalent iron by coating alginate polymer

Alginate has been widely employed to increase the performance of nanoscale zerovalent iron (nZVI)-based materials for site remediation. Yet, the effects of alginate on reactivity of sulfidated nZVI (an efficient reductant material) towards contaminants have been understood poorly. In this study, we have developed a one-step synthesis of alginate-coated sulfidated nZVI (S-nZVI@alginate) under air atmosphere and evaluated the reactivity of S-nZVI@alginate towards tetrabromobisphenol A (TBBPA) debromination. Surface analysis shows that S-nZVI has been successfully coated by alginate through the interaction of OH and COO− groups of alginate with Fe species. The coating of alginate increases particle stability and dispersion under various conditions and facilitates FeS precipitation on the particle surface. Reactivity experiments show that the coating of alginate significantly enhances TBBPA debromination by S-nZVI. The optimized alginate to Fe weight ratio of S-nZVI@alginate is 0.06, with ~3-fold greater TBBPA debromination rate than S-nZVI. S-nZVI@alginate can completely debrominate TBBPA into bisphenol A via a four-sequential step debromination pathway while S-nZVI not. Its superior reactivity may be attributed to that the formation of alginate-Fe complex can lower the redox potential of Fe species to accelerate electron transfer on the particle surface. The TBBPA debromination rate by S-nZVI@alginate is initially enhanced followed by a decrease with an increase in TBBPA concentration, while it can increase 3.3-, 8.9- and 5.6-fold by increasing S-nZVI@alginate dosage, decreasing pH and adding co-contaminant Cd2+, respectively. S-nZVI@alginate has greater performance in aging and reusability tests than S-nZVI, and achieves rapid TBBPA removal from wastewater, which may be due to the role of alginate on inhibiting surface oxidation of Fe and S species. Taken together, these results suggest that S-nZVI@alginate provides better reactivity, longevity and reusability than S-nZVI, having the great potential for application into site remediation.

Versatile zero valent iron applied in anaerobic membrane reactor for treating municipal wastewater: Performances and mechanisms

Comprehensive insight into the long-term impact of applying zero valent iron (ZVI) in anaerobic membrane reactor (AnMBR) was conducted here. ZVI showed its versatility for enhancing the performances of AnMBR including COD and TP removal, H2S removal and membrane fouling suppression in this study, and the iron ions releasing rate of ZVI had great influence on the enhancing performances. Two iron ions releasing states were used including “with electric field” and “without electric field”. Applying electric field could boost iron ions releasing rate by 12 times here. COD removal rate increased by 3% via coagulation effect with and without electric field. H2S in biogas was decreased from >500 ppm to <60 ppm via the formation of FeS precipitation with and without electric field. Applying ZVI with electric field increased the removal rate of TP by 50% via the formation of Fe3(PO4)2, but applying ZVI without electric field did not change the TP removal due to the insufficient iron ions. Utilizing ZVI without electric field suppressed the membrane fouling rate by 20% via the improvement of mixed liquor filterability, however utilizing ZVI with electric field accelerated the membrane fouling rate by 17 times due to the formation of dense Fe-enriched fouling layer with server inorganic and organic fouling. This study provided a novel approach to improve the performances of AnMBR by utilizing versatile ZVI. Meanwhile, this study enclosed that the iron ions releasing rate of ZVI is vital technical parameter for its application.

Rapid formation of iron sulfides alters soil morphology and chemistry following simulated marsh restoration

Many marshes show signs of degradation due to fragmentation, lack of sediment inputs, and erosion which may be exacerbated by sea level rise and increasing storm frequency/intensity. As a result, resource managers seek to restore marshes via introduction of sediment to increase elevation and stabilize the marsh platform. Recent field observations suggest the rapid formation of iron sulfide (FeS) materials following restoration in several marshes. To investigate, a laboratory microcosm study evaluated the formation of FeS following simulated restoration activities under continually inundated, simulated drought, and simulated tidal conditions. Results indicate that FeS horizon development initiated within 16 days, expanding to encompass >30% of the soil profile after 120 days under continuously inundated and simulated tidal conditions. Continuously inundated conditions supported higher FeS content compared to other treatments. Dissolved and total Fe and S measurements suggest the movement and diffusion of chemical constituents from native marsh soil upwards into the overlying sediments, driving FeS precipitation. The study highlights the need to consider biogeochemical factors resulting in FeS formation during salt marsh restoration activities. Additional field research is required to link laboratory studies, which may represent a worst case scenario, with in-situ conditions.

Application of iron-crosslinked sodium alginate for efficient sulfide control and reduction of oilfield produced water

Sulfide production and oilfield produced water are considered as environmental challenges in the oil industry. Iron-crosslinked sodium alginate (SA–Fe) was used to address these problems simultaneously. A pair of columns containing one coarse-sand column and one fine-sand column was designed to simulate heterogeneous rock layers and evaluate the plugging effect of SA–Fe. Generation of FeS precipitates led to decreases of sulfide in the gas phase by 45 ± 3.2% and in the aqueous solution by 75 ± 4.7%. The generated FeS nanoparticles and sulfate-reducing bacteria attached on the surface of the sand in the coarse-sand column to plug the pores that caused the water flow to switch from the coarse-sand column to the fine-sand column. Analysis of FeS distribution indicated that the column inlet was effectively plugged by FeS. The theoretical amount of FeS (1.19 mmol) that was determined based on sulfur balance was nearly equal to the actual amount of FeS precipitation (1.11 mmol). Additionally, water viscosity increased from 0.9 mPa s to 342 mPa s, induced by the collapse of SA–Fe gels, which reduced the difference in viscosity between oil and water to avoid viscous fingering. As a consequence, the oil recovery improved from 46 ± 2.6% to 85 ± 3.0% in the sand column oil-saturated recovery experiment, which contributed to the decrease of oil-normalized produced water from 70.1 ± 4.0 to 37.5 ± 1.3 mL water/mL oil. Therefore, this study shows that SA–Fe exhibits potential for application in controlling sulfide as well as reducing produced water.

Response and mechanisms of the performance and fate of antibiotic resistance genes to nano-magnetite during anaerobic digestion of swine manure.

Swine manure is an important reservoir of environmental antibiotic resistance genes (ARGs), and anaerobic digestion (AD) is a commonly used method for swine manure treatment. In this study, the optimized dosage of nano-magnetite to enhance methane production was figured out, the changes of the fate of ARGs response to nano-magnetite were investigated, and the microbial mechanisms were deciphered through the microbial community analysis and key functional genes quantification. Results showed that nano-magnetite could improve the methane production by maximum 6.0%, the maximum daily methane production could be increased by 47.8%, and the AD time could be shortened by above 20.0% at the addition of 75 mmol. The improved performance could be associated with the enhancement of direct interspecies electron transfer (DIET) and the inhibition release due to the formation of Fe-S precipitation not the nutrition elements role of nano-magnetite, and nano-magnetite did not significantly influence the dynamics of microbial community. Nano-magnetite could enhance the methanogenesis instead of the acetogenesis reflected by the functional genes analysis, and the limited effects of nano-magnetite on the fate of ARGs could be associated with its limited influence on the microbial community which determined the fate of ARGs during AD of swine manure.

Iron sulfide formation on root surfaces controlled by the life cycle of wild rice (Zizania palustris)

Iron sulfide plaques have been observed on roots of wild rice (Zizania palustris) and other wetland plants grown in sulfur-impacted freshwater ecosystems, but the mechanism of their formation and ramifications for plants have not been investigated. We exposed a model annual wetland plant, Zizania palustris, to elevated sulfate concentrations (3.1 mM) and quantified the development of iron oxide and iron sulfide precipitates on root surfaces throughout the plant life cycle. During the onset of seed production, root surfaces amended with sulfate transitioned within 1 week from iron (hydr)oxide plaques to iron sulfide plaques. During the same week, Fe(III) decreased on roots of plants not amended with sulfate but FeS did not accumulate. Prior to FeS accumulation, sulfate-amended plants had taken up the same amount of N as unamended plants. After FeS accumulation, total plant nitrogen did not increase further on sulfate-amended plants, indicating a cessation in nitrogen uptake, whereas total plant N continued to increase in unamended plants. Sulfate-amended plants produced fewer and lighter seeds with less nitrogen than unamended plants. FeS precipitation on roots may be associated with elevated sulfide and inhibited nitrogen uptake before the end of the plant’s life cycle, thus affecting the populations of this annual aquatic plant. We propose a mechanism by which a physiologically-induced decline in radial oxygen loss near the end of a plant’s life cycle initiates a precipitous decline in redox potential at the root surface and in adjacent porewater, initiating accumulation of iron sulfide plaques. These plaques could be an important locus for iron sulfide accumulation in wetland sediments.

Bioremoval of arsenic and antimony from wastewater by a mixed culture of sulfate-reducing bacteria using lactate and ethanol as carbon sources

We investigated the remediation of wastewater containing As and Sb through the application of a mixed culture of sulfate-reducing bacteria (SRB). The effect of Fe(II) and different carbon sources on the removal of As and Sb was evaluated. The wastewater initially contained 5 mg L−1 of both As(V) and Sb(V), and the treatment was conducted over a 12-d period. The produced precipitates were characterized by TEM and XRD to elucidate the metalloid removal mechanism. In the absence of Fe(II), Sb was efficiently removed (97.6% and 97.8% with lactate and ethanol as carbon sources, respectively, hereinafter the same), whereas only a relatively small fraction (27.8% and 26.4%) of As was removed. The addition of 200 mg L−1 Fe(II) greatly improved the removal of As (78% and 98.2%) and further increased the removal of Sb (98.8% and 99.4%). We hypothesized that As was removed through sorption/co-precipitation by FeS instead of the formation of As2S3. The use of ethanol as a carbon source generated a relatively lower yield of sulfide compared to the use of lactate, but it resulted in a higher removal of As and Sb. This may be attributed to the low production of sulfide, which possibly resulted in the slow precipitation of FeS that enhanced the sorption/co-precipitation of ions. This work demonstrates the high application potential of ethanol as a carbon source and the addition of Fe(II) in the bioremoval of As and Sb from wastewater by SRB.

Arsenite and arsenate immobilization by preformed and concurrently formed disordered mackinawite (FeS)

Arsenic (As) immobilization in aqueous environments is controlled by sorption and/or coprecipitation with iron (Fe) oxyhydroxides under oxic conditions and with ferrous iron (Fe(II)) and sulfide (S(-II)) minerals under anoxic conditions. Shifts to reducing conditions trigger the release of dissolved As by reductive dissolution of Fe oxyhydroxides. More stringent reducing conditions in sulfur (S) rich environments form solid S(-II) phases that immobilize As. This work studied the association of arsenite and arsenate with freshly-formed disordered mackinawite (FeS), a model Fe and reduced S mineral of environmental relevance. The association of As to preformed FeS (Set P experiments) was contrasted to the case when As is present during FeS precipitation (Set C experiments). Most abiotic studies of As reactivity under reducing conditions have been done under Set P setting, despite that redox shifts are likely to induce environments analogous to Set C experiments. Dissolved As removal was initially higher for Set C compared to Set P experiments at least by a factor of two, except for arsenate with low initial redox potential. Set C experiments had a rapid initial As sorption, followed by some As desorption. When stringent reducing conditions ensue, the extent and rate of As removal by FeS is misrepresented by sorption experiments to preformed FeS. A continuous release of As concurrent with the formation of thioarsenates - a dissolved form of As with weaker binding to Fe minerals - was observed in Set C experiments with arsenite. The formation of thioarsenates occurred preferably in Set C experiments at low redox potential, while it was negligible for arsenate experiments. Remarkably, the occurrence of thioarsenates disputes a widespread notion that their formation is inhibited by Fe(II)-S(-II) affinity and precipitation as FeS. This study shows the potential of FeS to scavenge arsenite and to decrease its mobility in reduced sediments and groundwater, and highlights the role of thioarsenates in As mobility in systems with changing redox conditions where the reactive solid forms in presence of the contaminant.

New Approach to Study Iron Sulfide Precipitation Kinetics, Solubility, and Phase Transformation

A reliable anoxic plug flow reactor has been developed to study iron sulfide (FeS) precipitation kinetics, solubility, and phase transformation simultaneously. The effects of temperature (23–125 °C), ionic strength (0.00886–5.03 mol/kg), and ferrous iron [Fe(II)] to sulfide [S(-II)] concentration ratio (1:20 to 1:5) were studied. The kinetics of FeS precipitation was found to be a pseudo-first-order reaction with respect to Fe(II) concentration, when Fe(II) concentration is significantly lower than the S(-II) concentration. FeS precipitation kinetics can be accelerated by high temperature and high ionic strength but not be affected by the Fe(II) to S(-II) concentration ratio at the tested ratio range. A model for predicting FeS solubility and precipitation was developed by using FeS solubility data measured in this study and data from literature. At a temperature ≤100 °C, freshly precipitated FeS was found to be mackinawite. Mackinawite can transform to troilite at a temperature ≥50 °C, and low ionic stre...

Early diagenesis in the sediments of the Congo deep-sea fan dominated by massive terrigenous deposits: Part II – Iron–sulfur coupling

Deep-sea fans are well known depot centers for organic carbon that should promote sulfate reduction. At the same time, the high rates of deposition of unconsolidated metal oxides from terrigenous origin may also promote metal-reducing microbial activity. To investigate the eventual coupling between the iron and sulfur cycles in these environments, shallow sediment cores (< 50cm) across various channels and levees in the Congo River deep-sea fan (~5000m) were profiled using a combination of geochemical methods. Interestingly, metal reduction dominated suboxic carbon remineralization processes in most of these sediments, while dissolved sulfide was absent. In some ‘hotspot’ patches, however, sulfate reduction produced large sulfide concentrations which supported chemosynthetic-based benthic megafauna. These environments were characterized by sharp geochemical boundaries compared to the iron-rich background environment, suggesting that FeS precipitation efficiently titrated iron and sulfide from the pore waters. A companion study demonstrated that methanogenesis was active in the deep sediment layers of these patchy ecosystems, suggesting that sulfate reduction was promoted by alternative anaerobic processes. These highly reduced habitats could be fueled by discrete, excess inputs of highly labile natural organic matter from Congo River turbidites or by exhumation of buried sulfide during channel flank erosion and slumping. Sulfidic conditions may be maintained by the mineralization of decomposition products from local benthic macrofauna or bacterial symbionts or by the production of more crystalline Fe(III) oxide phases that are less thermodynamically favorable than sulfate reduction in these bioturbated sediments. Overall, the iron and sulfur biogeochemical cycling in this environment is unique and much more similar to a coastal ecosystem than a deep-sea environment.

Microbial Sulfide Filter along a Benthic Redox Gradient in the Eastern Gotland Basin, Baltic Sea

The sediment-water interface is an important site for material exchange in marine systems and harbor unique microbial habitats. The flux of nutrients, metals, and greenhouse gases at this interface may be severely dampened by the activity of microorganisms and abiotic redox processes, leading to the “benthic filter” concept. In this study, we investigate the spatial variability, mechanisms and quantitative importance of a microbially-dominated benthic filter for dissolved sulfide in the Eastern Gotland Basin (Baltic Sea) that is located along a dynamic redox gradient between 65 and 173 m water depth. In August-September 2013, high resolution (0.25 mm minimum) vertical microprofiles of redox-sensitive species were measured in surface sediments with solid-state gold-amalgam voltammetric microelectrodes. The highest sulfide consumption (2.73–3.38 mmol m−2 day−1) occurred within the top 5 mm in sediments beneath a pelagic hypoxic transition zone (HTZ, 80–120 m water depth) covered by conspicuous white bacterial mats of genus Beggiatoa. A distinct voltammetric signal for polysulfides, a transient sulfur oxidation intermediate, was consistently observed within the mats. In sediments under anoxic waters (>140 m depth), signals for Fe(II) and aqueous FeS appeared below a subsurface maximum in dissolved sulfide, indicating a Fe(II) flux originating from older sediments presumably deposited during the freshwater Ancylus Lake that preceded the modern Baltic Sea. Our results point to a dynamic benthic sulfur cycling in Gotland Basin where benthic sulfide accumulation is moderated by microbial sulfide oxidation at the sediment surface and FeS precipitation in deeper sediment layers. Upscaling our fluxes to the Baltic Proper; we find that up to 70% of the sulfide flux (2281 kton yr−1) toward the sediment-seawater interface in the entire basin can be consumed at the microbial mats under the HTZ (80–120 m water depth) while only about 30% the sulfide flux effuses to the bottom waters (>120 m depth). This newly described benthic filter for the Gotland Basin must play a major role in limiting the accumulation of sulfide in and around the deep basins of the Baltic Sea.

Influence of drill mud on the microbial communities of sandstone rocks and well fluids at the Ketzin pilot site for CO2 storage

At a pilot site for CO2 storage in Ketzin (Germany), a drastic decrease in injectivity occurred in a well intended for injection. This was attributed to an obstruction of the pore throats due to microbial degradation of the organic drill mud and subsequent iron sulfide (FeS) precipitation in the highly saline brine (240 g L−1). To better understand the biogeochemical processes, the response of the autochthonous microbial community to drill mud exposure was investigated. Pristine cores of two aquifers with different salinity were incubated under simulated in situ conditions (50 bar, 40 °C and 45 bar, 25 °C, respectively) and CO2 atmosphere. For the first time, rock cores obtained from the CO2 plume of the storage formation were investigated. The influence of acetate as a biodegradation product of drill mud polymers and the effectiveness of a biocide were additionally tested. Increased microbial diversities were observed in all long-term (8–20 weeks) incubations, even including biocide. Biofilm-like structures and small round-shaped minerals of probable microbiological origin were found. The results indicate that the microbial community remains viable after long-term CO2 exposure. Microorganisms hydrolyzing cellulose polymers (e.g., Burkholderia spp., Variovorax spp.) biodegraded organic components of the drill mud and most likely produced low molecular weight acids. Although the effects of drill mud were less strong as observed in situ, it was demonstrated that acetate supports the growth of sulfate-reducing bacteria (i.e., Desulfotomaculum spp.). The microbial-induced precipitation of amorphous FeS reduced the injectivity in the near-well area. Therefore, when using organic drill mud, the well must be cleaned intensively to minimize the hazards of bacterial stimulation.