Biogeochemistry of pyrite and iron sulfide oxidation in marine sediments

Pyrite (FeS₂) and iron monosulfide (FeS) play a central role in the sulfur and iron cycles of marine sediments. They may be buried in the sediment or oxidized by O₂ after transport by bioturbation to the sediment surface. FeS₂ and FeS may also be oxidized within the anoxic sediment in which NO₃⁻, Fe(III) oxides, or MnO₂ are available as potential electron acceptors. In chemical experiments, FeS₂ and FeS were oxidized by MnO₂ but not with NO₃⁻ or amorphous Fe(III) oxide (Schippers and Jørgensen, 2001). Here we also show that in experiments with anoxic sediment slurries, a dissolution of tracer-marked ⁵⁵FeS₂ occurred with MnO₂ but not with NO₃⁻ or amorphous Fe(III) oxide as electron acceptor. To study a thermodynamically possible anaerobic microbial FeS₂ and FeS oxidation with NO₃⁻ or amorphous Fe(III) oxide as electron acceptor, more than 300 assays were inoculated with material from several marine sediments and incubated at different temperatures for > 1 yr. Bacteria could not be enriched with FeS₂ as substrate or with FeS and amorphous Fe(III) oxide. With FeS and NO₃⁻, 14 enrichments were obtained. One of these enrichments was further cultivated anaerobically with Fe²⁺ and S⁰ as substrates and NO₃⁻ as electron acceptor, in the presence of ⁵⁵FeS₂, to test for co-oxidation of FeS₂, but an anaerobic microbial dissolution of ⁵⁵FeS₂ could not been detected. FeS₂ and FeS were not oxidized by amorphous Fe(III) oxide in the presence of Fe-complexing organic compounds in a carbonate-buffered solution at pH 8. Despite many different experiments, an anaerobic microbial dissolution of FeS₂ could not be detected; thus, we conclude that this process does not have a significant role in marine sediments. FeS can be oxidized microbially with NO₃⁻ as electron acceptor. O₂ and MnO₂, but not NO₃⁻ or amorphous Fe(III) oxide, are chemical oxidants for both FeS₂ and FeS.     

The solubility of FeS

The solubility of FeS_m, synthetic nanoparticulate mackinawite, in aqueous solution was measured at 23 °C from pH 3-10 using an in situ precipitation and dissolution procedure and the solution species was investigated voltammetrically. The solubility is described by a pH-dependent reaction and a pH-independent reaction. The pH-dependent dissolution reaction can be described by

FeS_m+2H⁺→Fe²⁺+H₂S     

Determination of the Electrochemical Properties of a Soluble Aqueous FeS Species Present in Sulfidic Solutions

Field and laboratory data are presented that show a soluble FeS species(FeS_aq) exists in sulfidic seawater solutions, and is observedwhen the IAP exceeds the K_sp of amorphous FeS. TheFeS_aq yields a discrete signal (double peak) using square-wavevoltammetry and two one-electron waves in sampled DC polarographyexperiments at the Hg electrode. The aqueous FeS species reacts irreversiblyat the electrode as a single FeS subunit and not as a polymeric entity. Thepeak potential of FeS_aq occurs at -1.1 V whereas the peakpotential of Fe occurs at-1.45 V; the positive shift for Fe²⁺ reduction inFeS_aq indicates a change in geometry for Fe²⁺from octahedral to tetrahedral. The kinetics of electron transfer at theelectrode are determined to be similar for both Fe²⁺ andFeS_aq. Molecular orbital energy diagrams, further indicatethat Fe(II) does change from octahedral to tetrahedral geometry in solution.First, Fe(II) exists as octahedralFe in solution whichundergoes a substitution reaction of bisulfide for water. The resultingcomplex, Fe(H₂O)₅(HS)⁺, thentransforms to a tetrahedral complex on further addition of sulfide. Thisgeometry change is consistent with the formation of amorphous FeS thatconverts to mackinawite which has tetrahedral Fe(II). The process is entropydriven because of the water loss that occurs. The overall sequence can berepresented as: Soluble FeS species are important asreactants in the formation of iron-sulfide minerals including pyrite.     

The post-depositional reactivity of iron and managanese in the sediments of a macrotidal estuarine system

Four cores of anoxic sediments were collected from the Seine estuary to assess the early diagenesis pathways leading to the formation of previously reactive phase. Pore waters were analyzed for dissolved iron (Fe) and manganese (Mn) and different ligands (e.g., sulfate, chloride, total inorganic carbon). The anoxic zone is present up to the first centimeter depth, in these conditions the reduction of Mn and Fe oxides and SO₄ ²⁻ was verified. The sulfate reduction was well established with a subsequent carbon mineralization in the NORMAI94 core. The chemical speciation of Mn and Fe in the dissolved and solid phases was determined. For the dissolved phase, thermodynamic calculations were used to characterize and illustrate the importance of carbonate and phosphate phases as sinks for Fe and Mn. The ion activity product (IAP) of Fe and Mn species was compared to the solubility products (Ks) of these species. In the solid phase, the presence of higher concentration of calcium carbonate in the Seine sediments is an important factor controlling Mn cycle. The carbonate-bound Mn can reach more than 75% of the total concentration. This result is confirmed by the use of electron spin resonance (ESR) spectroscopy. The reduction of Fe is closely coupled to the sulfate reduction by the formation of new solid phases such as FeS and FeS₂, which can be regarded as temporal sinks for sulfides. These forms were quantified in all cores as acid volatile sulfide (AVS: FeS+ free sulfide) and chromium reducible sulfide (CRS: FeS₂+elemental sulfur S⁰).     

Arsenite sorption on troilite (FeS) and pyrite (FeS2)

Arsenic is a toxic metalloid whose mobility and availability are largely controlled by sorption on sulfide minerals in anoxic environments. Accordingly, we investigated reactions of As(III) with iron sulfide (FeS) and pyrite (FeS₂) as a function of total arsenic concentration, suspension density, sulfide concentration, pH, and ionic strength. Arsenite partitioned strongly on both FeS and FeS₂ under a range of conditions and conformed to a Langmuir isotherm at low surface coverages; a calculated site density of near 2.6 and 3.7 sites/nm² for FeS and FeS₂, respectively, was obtained. Arsenite sorbed most strongly at elevated pH (>5 to 6). Although solution data suggested the formation of surface precipitates only at elevated solution concentrations, surface precipitates were identified using X-ray absorption spectroscopy (XAS) at all coverages. Sorbed As was coordinated to both sulfur [d(As-S) = 2.35 Å] and iron [d(As-Fe) = 2.40 Å], characteristic of As coordination in arsenopyrite (FeAsS). The absorption edge of sorbed As was also shifted relative to arsenite and orpiment (As₂S₃), revealing As(III) reduction and a complete change in As local structure. Arsenic reduction was accompanied by oxidation of both surface S and Fe(II); the FeAsS-like surface precipitate was also susceptible to oxidation, possibly influencing the stability of As sorbed to sulfide minerals in the environment. Sulfide additions inhibit sorption despite the formation of a sulfide phase, suggesting that precipitation of arsenic sulfide is not occurring. Surface precipitation of As on FeS and FeS₂ supports the observed correlation of arsenic and pyrite and other iron sulfides in anoxic sediments.     

Low-oxygen and chemical kinetic constraints on the geochemical niche of neutrophilic iron(II) oxidizing microorganisms

Neutrophilic iron oxidizing bacteria (FeOB) must actively compete with rapid abiotic processes governing Fe(II) oxidation and as a result have adapted to primarily inhabit low-O₂ environments where they can more successfully compete with abiotic Fe(II) oxidation. The spatial distribution of these microorganisms can be observed through the chemical gradients they affect, as measured using in situ voltammetric analysis for dissolved Fe(II), Fe(III), O₂, and FeS_(aq). Field and laboratory determination of the chemical environments inhabited by the FeOB were coupled with detailed kinetic competition studies for abiotic and biotic oxidation processes using a pure culture of FeOB to quantify the geochemical niche these organisms inhabit. In gradient culture tubes, the maximum oxygen levels, which were associated with growth bands of Sideroxydans lithotrophicus (ES-1, a novel FeOB), were 15-50 μM. Kinetic measurements made on S. lithotrophicus compared biotic/abiotic (killed control) Fe oxidation rates. The biotic rate can be a significant and measurable fraction of the total Fe oxidation rate below O₂ concentrations of approximately 50 μM, but biotic Fe(II) oxidation (via the biotic/abiotic rate comparison) becomes difficult to detect at higher O₂ levels. These results are further supported by observations of conditions supporting FeOB communities in field settings. Variablity in cell densities and cellular activity as well as variations in hydrous ferrous oxide mineral quantities significantly affect the laboratory kinetic rates. The microbial habitat (or geochemical niche) where FeOB are active is thus largely controlled by the competition between abiotic and biotic kinetics, which are dependent on Fe(II) concentration, P_O2, temperature and pH in addition to the surface area of hydrous ferric oxide minerals and the cell density/activity of FeOB. Additional field and lab culture observations suggest a potentially important role for the iron-sulfide aqueous molecular cluster, FeS_(aq), in the overall cycling of iron associated with the environments these microorganisms inhabit.     

Intense pyrite formation under low-sulfate conditions in the Achterwasser lagoon, SW Baltic Sea

In comparison to similar low-sulfate coastal environments with anoxic-sulfidic sediments, the Achterwasser lagoon, which is part of the Oder estuary in the SW Baltic Sea, reveals unexpectedly high pyrite concentrations of up to 7.5 wt%. Pyrite occurs mainly as framboidal grains variable in size with diameters between 1 and 20 μm. Pyritization is not uniform down to the investigated sediment depth of 50 cm. The consumption of reactive-Fe is most efficient in the upper 20 cm of the sediment column, leading to degrees of pyritization (DOP) as high as 80 to 95%.Sediment accumulation in the Achterwasser takes place in high productivity waters. The content of organic carbon reaches values of up to 10 wt%, indicating that pyrite formation is not limited by the availability of organic matter. Although dissolved sulfate concentration is relatively low (<2 mmol/L) in the Achterwasser, the presence of H₂S in the pore water suggests that sulfate is unlikely to limit pyrite authigenesis. The lack of free Fe(II) in the pore waters combined with the possibility of a very efficient transformation of Fe-monosulfides to pyrite near the sediment/water interface suggests that pyrite formation is rather controlled by (i) the availability of reactive-Fe, which limits the FeS formation, and by (ii) the availability of an oxidant, which limits the transformation of FeS into pyrite. The ultimate source for reactive-Fe is the river Oder, which provides a high portion of reactive-Fe (∼65% of the total-Fe) in the form of suspended particulate matter. The surficial sediments of the Achterwasser are reduced, but are subject to oxidation from the overlying water by resuspension. Oxidation of the sediments produces sulfur species with oxidation states intermediate between sulfide and sulfate (e.g., thiosulfate and polysulfides), which transform FeS to FeS₂ at a significant rate. This process of FeS-recycling is suggested to be responsible for the formation of pyrite in high concentrations near the sediment surface, with DOP values between 80 and 95% even under low sulfate conditions.A postdepositional sulfidization takes place in the deeper part of the sediment column, at ∼22 cm depth, where the downward diffusion of H₂S is balanced by the upward migration of Fe(II). The vertical fluctuation of the diffusion front intensifies the pyritization of sediments. We suggest that the processes described may occur preferentially in shallow water lagoons with average net-sedimentation rates close to zero. Such environments are prone to surficial sediment resuspension, initiating oxidation of Fe-sulfides near the sediment/water interface. Subsequent FeS₂ formation as well as postdepositional sulfidization leads to a major pyrite spike at depth within the sediment profile.     

Greigite: a true intermediate on the polysulfide pathway to pyrite

The formation of pyrite (FeS₂) from iron monosulfide precursors in anoxic sediments has been suggested to proceed via mackinawite (FeS) and greigite (Fe₃S₄). Despite decades of research, the mechanisms of pyrite formation are not sufficiently understood because solid and dissolved intermediates are oxygen-sensitive and poorly crystalline and therefore notoriously difficult to characterize and quantify.     

Reduced sulfur and iron species in anoxic water column of meromictic crater Lake Pavin (Massif Central, France)

The vertical distribution of reduced sulfur species (RSS including H₂S/HS⁻, S⁰, electroactive FeS) and dissolved Fe(II) was studied in the anoxic water column of meromictic Lake Pavin. Sulfide concentrations were determined by two different analytical techniques, i.e. spectophotometry (methylene blue technique) and voltammetry (HMDE electrode). Total sulfide concentrations determined with methylene blue method (∑H₂S_MBRS) were in the range from 0.6 µM to 16.7 µM and were substantially higher than total reduced sulfur species (RSS_V) concentrations determined by voltammetry, which ranged from 0.1 to 5.6 μM. The observed difference in the sulfide concentrations between the two methods can be assigned to the presence of FeS colloidal species.Dissolved Fe was high (> 1000 µM), whereas dissolved Mn was only 25 µM, in the anoxic water column. This indicates that Fe is the dominant metal involved in sulfur redox cycling and precipitation. Consequently, in the anoxic deep layer of Lake Pavin, “free” sulfide, H₂S/HS⁻, was low; and about 80% of total sulfide detected was in the electroactive FeS colloidal form. IAP calculations showed that the Lake Pavin water column is saturated with respect to FeS_am phase. The upper part of monimolimnion layer is characterized by higher concentrations of S(0) (up to 3.4 µM) in comparison to the bottom of the lake. This behavior is probably influenced by sulfide oxidation with Fe(III) oxyhydroxide species.     

Chemical and structural characterization of As immobilization by nanoparticles of mackinawite (FeSm)

The mobility and availability of arsenite, As(III), in anoxic environments is largely controlled by adsorption onto iron sulfides and/or precipitation of arsenic in solid phases. The interaction of As(III) with synthetic mackinawite (FeS_m) in pH 5 and 9 suspensions was investigated using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), STEM elemental mapping, high resolution TEM, and X-ray photoelectron spectroscopy (XPS). At pH 5, arsenic sulfide phases precipitate among the FeS_m particles as discrete particles that are an amorphous hydrous phase of arsenic sulfide. The oxidation state of As in the surface layers of the arsenic sulfide precipitates is ‘realgar-like’ based on XPS results showing that > 75% of the As 3d peak area is due to As with oxidation states between 0 and 2+. Discrete, arsenic sulfide precipitates are absent at pH 9, but elemental mapping in STEM-EDX mode shows that arsenic is uniformly distributed on the FeS_m, suggesting that uptake is caused by the sorption of As(III) oxyanions and/or the precipitation of highly dispersed arsenic sulfides on FeS_m. XPS also revealed that the FeS_m that equilibrated without As(III) has a more oxidized surface composition than the sample at pH 9, as indicated by the higher concentration of O ( three times greater than that at pH 9) and the larger fraction of Fe(III) species making up the total Fe (2p_3/2) peak. These findings provide a better understanding of redox processes and phase transitions upon As(III) adsorption on iron sulfide substrates.     

Reductive transformation of iron and sulfur in schwertmannite-rich accumulations associated with acidified coastal lowlands

We examined the transformations of Fe and S associated with schwertmannite (Fe₈O₈(OH)₆SO₄) reduction in acidified coastal lowlands. This was achieved by conducting a 91 day diffusive-flux column experiment, which involved waterlogging of natural schwertmannite- and organic-rich soil material. This experiment was complemented by short-term batch experiments utilizing synthetic schwertmannite. Waterlogging readily induced bacterial reduction of schwertmannite-derived Fe(III), producing abundant pore-water Fe^II, SO₄ and alkalinity. Production of alkalinity increased pH from pH 3.4 to pH ∼6.5 within the initial 14 days, facilitating the precipitation of siderite (FeCO₃). Interactions between schwertmannite and Fe^II at pH ∼6.5 were found, for the first time, to catalyse the transformation of schwertmannite to goethite (αFeOOH). Thermodynamic calculations indicate that this Fe^II-catalysed transformation shifted the biogeochemical regime from an initial dominance of Fe(III)-reduction to a subsequent co-occurrence of both Fe(III)- and SO₄-reduction. This lead firstly to the formation of elemental S via H₂S oxidation by goethite, and later also to formation of nanoparticulate mackinawite (FeS) via H₂S precipitation with Fe^II. Pyrite (FeS₂) was a quantitatively insignificant product of reductive Fe and S mineralization. This study provides important new insights into Fe and S geochemistry in settings where schwertmannite is subjected to reducing conditions.     

Surface chemistry and structural properties of mackinawite prepared by reaction of sulfide ions with metallic iron

Tetragonal FeS_1−x mackinawite, has been synthesized by reacting metallic iron with a sodium sulfide solution and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), transmission Mössbauer spectroscopy (TMS) and X-ray photoelectron spectroscopy (XPS). Based on XRD and TEM analyses, synthetic mackinawite exhibits crystallization and is identical to the natural mineral. Unit cell parameters derived from XRD data are a = b = 0.3670 nm and c = 0.5049 nm. The bulk Fe:S ratio derived from the quantitative dispersive energy analysis is practically 1. XPS analyses, however, showed that mackinawite surface is composed of both Fe(II) and Fe(III) species bound to monosulfide. Accordingly, monosulfide is the dominant S species observed at the surface with lesser amount of polysulfides and elemental sulfur. TMS analysis revealed the presence of both Fe(II) and Fe(III) in the mackinawite structure, thus supporting the XPS analysis. We propose that the iron monosulfide phase synthesized by reacting metallic iron and dissolved sulfide is composed of Fe(II) and S(-II) atoms with the presence of a weathered thin layer covering the bulk material that consists of both Fe(II) and Fe(III) bound to S(-II) atoms and in a less extent of polysulfide and elemental sulfur.     

A comparison of the early diagenetic environment in intertidal sands and muds of the Manukau Harbour,New Zealand

The early diagenetic environment of intertidal sandy sediments (sands) and muddy sediments (muds) is described and compared from two cores taken from an unpolluted part of the Manukau Harbour, New Zealand. Extraction techniques characterized the form of the trace elements (Fe, Mn, S, C, Pb, Zn, Cu) at different depths in the sediment. Dissolved forms of Fe, Mn, and S were measured in interstitial water. Nonresidual metal concentrations, humic acid, FeS, and FeS₂ are an order of magnitude higher in the muds than in the sands because of dilution by unreactive sand particles. Muds contain a larger proportion of metals in the mobile fractions; exchangeable (Mn), carbonate (Mn, Fe, Zn), and easily-reducible oxide (Fe, Mn, Zn, Pb). This is due to greater surface area (for Mn adsorption); the favorable conditions for MnCO₃, FeCO₃, and FeS precipitation; and higher concentrations of easily reducible iron oxide and humic acid. Therefore, compared to the sands, muds are more important as reservoirs for toxic metals, both in terms of quantity and availability. At either site there was very little difference between the forms of Zn, Pb or Cu identified by sequential extraction as sediments changed from oxic to anoxic conditions. One reason for this is that the amounts and proportions of some of the important components that bind metals, viz., amorphous iron hydrous oxides, humic acids, and FeS₂, do not change much. Other components that do change with redox conditions, for example, manganese phases and FeS, are only minor components of the sediment. Redox conditions, then, have relatively little effect on trace-metal partitioning in the sediment matrix of these unpolluted sediments.     

Geochemistry and mineralogy of arsenic in (natural) anaerobic groundwaters

Here new data from field bioremediation experiments and geochemical modeling are reported to illustrate the principal geochemical behavior of As in anaerobic groundwaters. In the field bioremediation experiments, groundwater in Holocene alluvial aquifers in Bangladesh was amended with labile water-soluble organic C (molasses) and MgSO₄ to stimulate metabolism of indigenous SO₄-reducing bacteria (SRB). In the USA, the groundwater was contaminated by Zn, Cd and SO₄, and contained <10 μg/L As under oxidized conditions, and a mixture of sucrose and methanol were injected to stimulate SRB metabolism. In Bangladesh, groundwater was under moderately reducing conditions and contained ∼10 mg/L Fe and ∼100 μg/L As. In the USA experiment, groundwater rapidly became anaerobic, and dissolved Fe and As increased dramatically (As > 1000 μg/L) under geochemical conditions consistent with bacterial Fe-reducing conditions. With time, groundwater became more reducing and biogenic SO₄ reduction began, and Cd and Zn were virtually completely removed due to precipitation of sphalerite (ZnS) and other metal sulfide mineral(s). Following precipitation of chalcophile elements Zn and Cd, the concentrations of Fe and As both began to decrease in groundwater, presumably due to formation of As-bearing FeS/FeS₂. By the end of the six-month experiment, dissolved As had returned to below background levels. In the initial Bangladesh experiment, As decreased to virtually zero once biogenic SO₄ reduction commenced but increased to pre-experiment level once SO₄ reduction ended. In the ongoing experiment, both SO₄ and Fe(II) were amended to groundwater to evaluate if FeS/FeS₂ formation causes longer-lived As removal. Because As-bearing pyrite is the common product of SRB metabolism in Holocene alluvial aquifers in both the USA and Southeast Asia, it was endeavored to derive thermodynamic data for arsenian pyrite to better predict geochemical processes in naturally reducing groundwaters. Including the new data for arsenian pyrite into Geochemist’s Workbench, its stability field completely dominates in reducing Eh–pH space and “displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling exercises and reported in rare field conditions.     

Low temperature anaerobic bacterial diagenesis of ferrous monosulfide to pyrite

In vitro enrichment cultures of dissimilatory sulfate-reducing bacteria precipitated FeS and catalyzed its transformation into FeS₂ at ambient temperature and pressure under anaerobic conditions. When compared to purely abiotic processes, the bacterially mediated transformation was shown to be more efficient in transforming FeS into FeS₂. This occurred due to the large, reactive surface area available for bacterially catalyzed diagenesis, where the biogenic FeS precursor was immobilized as a thin film (∼25 nm thick) on the μm-scale bacteria. The bacteria also contained the source(s) of sulfur for diagenesis to occur. Using a radiolabeled organic-sulfur tracer study, sulfur was released during cell autolysis and was immobilized at the bacterial cell surface forming FeS₂. The formation of FeS₂ occurred on both the inner and outer surfaces of the cell envelope and represented the first step of bacterial mineral diagenesis. Pyrite crystals, having linear dimensions of ∼1 μm, grew outward from the bacterial cell surfaces. These minerals were several orders of magnitude larger in volume than those originating abiotically.     

Geochemistry of Flooded Underground Mine Workings Influenced by Bacterial Sulfate Reduction

Unlike the majority of the water in the flooded mine complex of Butte Montana, which includes the highly acidic Berkeley pit lake, groundwater in the flooded West Camp underground mine workings has a circum-neutral pH and contains at least 8 μM aqueous sulfide. This article examines the geochemistry and stable isotope composition of this unusual H₂S-rich mine water, and also discusses problems related to the colorimetric analysis of sulfide in waters that contain FeS_(aq) cluster compounds. The West Camp mine pool is maintained at a constant elevation by continuous pumping, with discharge water that contains elevated Mn (90 μM), Fe (16 μM), and As (1.3 μM) but otherwise low metal concentrations. Dissolved inorganic carbon in the mine water is in chemical and isotopic equilibrium with rhodochrosite in the mineralized veins. The mine water is under-saturated with mackinawite and amorphous FeS, but is supersaturated with Cu- and Zn-sulfides. However, voltammetry studies show that much of the dissolved sulfide and ferrous iron are present as FeS_(aq) cluster molecules: as a result, the free concentration of the West Camp water is poorly constrained. Concentrations of dissolved sulfide determined by colorimetry were lower than gravimetric assays obtained by AgNO₃ addition, implying that the FeS_(aq) clusters are not completely extracted by the Methylene Blue reagent. In contrast, the clusters are quantitatively extracted as Ag₂S after addition of AgNO₃. Isotopic analysis of co-existing aqueous sulfide and sulfate confirms that the sulfide was produced by sulfate-reducing bacteria (SRB). The H₂S-rich mine water is not confined to the immediate vicinity of the extraction well, but is also present in flooded mine shafts up to 3 km away, and in samples bailed from mine shafts at depths up to 300 m below static water level. This illustrates that SRB are well established throughout the southwestern portion of the extensive (>15 km³) Butte flooded mine complex.     

Monitoring and Modeling of Metal Concentration Distributions in Anoxic Basins: Aitoliko Lagoon, Greece

The balance between physicochemical processes, influencing vertical and temporal distributions of metal compounds in one relatively isolated anoxic environment, constitutes the objective of the present work. Ion activity product (IAP) was calculated for manganese and iron sulfides, in order to define the metal sulfide forms that control Fe and Mn solubility in the bottom waters of anoxic lagoons. Iron solubility depended on amorphous FeS formation, while manganese sulfides were a minor component in a solid solution lowering its solid-phase activity. A theoretical physicochemical model was developed for the iron speciation, based on experimental pH and redox potential data. A very good match was achieved for the measured and the theoretical total dissolved iron, at all depths. The dominance of oxidant iron species Fe(OH) ₃ ^? in the surface waters and their sequence by FeSH⁺ and FeS_aq in the deeper layers brings out the influence of physicochemical parameters (dissolved oxygen, sulfide, pH and E_h) in vertical distribution of dissolved metal species, in anoxic/hypoxic basins. Based on these findings, we can conclude that the distribution of manganese and iron is of special interest, not only because these are the indicators of redox conditions but also for the role of their oxidized/reduced forms in the formation of the biogeochemical structure of redox zone.     

Factors Controlling Sulfide Geochemistry in Sub-tropical Estuarine and Bay Sediments

The primary factors that control the concentration of total reduced (inorganic) sulfide in coastal sediments are believed to be the availability of reactive iron, dissolved sulfate and metabolizable organic carbon. We selected nine sites in shallow (<3 m), close to sub-tropical, estuaries and bays along the central Texas coast that represented a range in sediment grain size (a proxy for reactive iron), salinity (a proxy for dissolved sulfate), and total organic carbon (a proxy for metabolizable organic carbon). Based on these parameters a prediction was made of which factor was likely to control total reduced sulfide at each site and what the relative total reduced sulfide concentration was likely to be. To test the prediction, the sediments were analyzed for total reduced sulfide, acid volatile sulfide, and citrate dithionate-extractable, HCl-extractable and total Fe in the solid phase. Using solid-state gold–mercury amalgam microelectrodes and voltammetry, we determined pore water depth profiles of Fe(II) and ΣH₂S and presence or absence of FeS_(aq). At five of the nine sites the calculated degree of sufildization of citrate dithionite-reactive-iron was close to or greater than 1 indicating that rapidly reactive iron was probably the limiting factor for iron sulfide mineral formation. At one site (salinity = 0.9) dissolved Fe(II) was high, ΣH₂S was undetectable and the total reduced sulfide concentration was low indicating sulfate limitation. At the last three sites a low degree of sulfidization and modest total reduced (inorganic) sulfide concentrations appeared to be the result of a limited supply of metabolizable organic carbon. Fe(II)–S(-II) clusters (FeS_(aq)) were undetectable in 10 out of 12 bay sediment profiles where ΣH₂S was close to or below detection limits, but was observed in all other porewater profiles. Acid volatile sulfide, but not total reduced sulfide, was well correlated with total organic carbon and ranged from being undetectable in some cores to representing a major portion of total reduced sulfide in other cores. Although predicted controls on total reduced sulfide were good for very low salinity water or sandy sediments, they were only right about half the time for the other sediments. The likely reasons for the wrong predictions are the poor correlation of total organic carbon with grain size and differing fractions of metabolizable organic carbon in different sedimentary environments. Differences in sediment accumulation rates may also play a role, but these are difficult to determine in this region where hurricanes often resuspend and move sediments. This study demonstrates the need to examine more complex and often difficult to determine parameters in anoxic “normal marine” sediments if we are to understand what controls the concentration and distribution of sulfides.     

Experimental concentration-time curves for the iron(II) sulphide precipitation process in aqueous solutions and their interpretation

The first precipitate formed through the reaction between aqueous Fe(II) salts and dissolved sulphide at ambient temperatures and pH < 9, appears to be a highly disordered gel approaching the composition Fe(HS)₂ on a water-free basis. After 0.4 s this precipitate loses sulphide and amorphous FeS begins to appear. The mackinawite structure begins to develop after several hours.

The rate of formation of the initial precipitate can be approximated by a pseudo first-order reaction, directly dependent on total sulphide concentration and with an apparent pseudo first-order rate constant of 48 ± 9 s⁻¹. Dissolved Fe concentration does not appear to be rate limiting.

The estimated solubility of the initial phase is variable but consistently one to two orders of magnitude greater than the measured solubilities for amorphous FeS. In natural systems this may lead to variable Fe(II) solubilities in sulphidic environments. This initial material may play a more central role in iron sulphide reaction pathways than either mackinawite or amorphous FeS.     

The diagenetic geochemistry and contamination assessment of iron,cadmium, and lead in the sediments from the Shuangtaizi estuary,China

Sediment and pore water samples have been collected from the coastal tidal flat in the Shuangtaizi estuary, China, in order to investigate the geochemical behavior of iron, cadmium, and lead during diagenesis and to assess the degree of contamination. The calculated enrichment factors and geoaccumulation indices for separate elements show that anthropogenic activities have had no significant influence on the distribution of Fe and Pb in the study area, whereas the distribution of Cd has been closely influenced in this way. The high percentage of exchangeable Cd (average of 56.34%) suggests that Cd represents a potential hazard to benthic organisms in the estuary. The calculated diffusive fluxes of metals show that the most mobilized metal is Fe (9.22 mg m^?2 a^?1), followed by Cd (0.54 mg m^?2 a^?1) and Pb (0.42 mg m^?2 a^?1). Low Fe²⁺ contents in surface pore water, alongside high chromium-reducible sulfur contents, and low acid-volatile sulfur, and elemental sulfur contents at 0–25 cm depth in sediments show that Fe²⁺ is formed by the reduction of Fe oxides and is transformed first to a solid phase of iron monosulfides (FeS) and eventually to pyrite (FeS₂). The release of adsorbed Pb due to reductive dissolution of Fe/Mn oxides during early diagenesis could be a source of Pb²⁺ in pore water. From the relatively low total organic carbon contents measured in sediments (0.46–1.28%, with an average of 0.94%) and the vertical variation of Cd²⁺ in pore water, sulfide or Fe/Mn oxides (instead of organic matter) are presumed to exert a significant influence on carrying or releasing Cd by the sediments.