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Published literature for reported sorption coefficients (K(d)) of eight anionic per- and polyfluoroalkyl substances (PFAS) in soil was reviewed. K(d) values spanned three to five log units indicating that no single value would be appropriate for use in estimating PFAS impacts to groundwater using existing soil-water partition equations. Regression analysis was used to determine if the soil or solution parameters might be used to predict K(d) values. None of the 15 experimental parameters collected could individually explain variability in reported K(d) values. Significant associations between K(d) and soil calcium and sodium content were found for many of the selected PFAS, suggesting that soil cation content may be critical to PFAS sorption, as previously noted in sources like Higgins and Luthy (2006), while organic carbon content was significant only at elevated levels (>5%). Unexplained discrepancies between the results from studies where PFAS were introduced to soil and desorbed in the laboratory and those that used material from PFAS-impacted sites suggest that laboratory experiments may be overlooking some aspects critical to PFAS sorption. Future studies would benefit from the development and use of standardized analytical methods to improve data quality and the establishment of soil parameters appropriate for collection to produce more complete data sets for predictive analysis.

Per- and polyfluoroalkyl substances (PFAS) are a family of fluorinated organic compounds of anthropogenic origin. Due to their unique chemical properties, widespread production, ubiquitous environmental distribution, long-term persistence, bioaccumulative potential, and associated risks for human health, PFAS have been classified as persistent organic pollutants of significant concern. Scientific evidence from the last several decades suggests that their widespread occurrence in the environment correlates with adverse effects on human health and ecology. The presence of PFAS in the aquatic environment demonstrates a close link between the anthroposphere and the hydrological cycle, and concentrations of PFAS in surface and groundwater range in value along the ng L−1–μg L−1 scale. Here, we critically reviewed the research published in the last decade on the global occurrence and distribution of PFAS in the aquatic environment. This is the first study to critically evaluate the occurrence of PFAS at continental scale and the evolving global regulatory responses to manage and mitigate the adverse human health risks posed by PFAS. The review reports that PFAS are ubiquitous despite being phased out—they have been detected in different continents irrespective of the level of industrial development. Their occurrence far from the potential sources suggests that long-range atmospheric transport is an important pathway of PFAS distribution. Recently, several studies have investigated the health impacts of PFAS exposure—they have been detected in biota, drinking water, food, air, and human serum. In response to the emerging information about PFAS toxicity, several countries have provided administrative guidelines for PFAS in water, including Canada, the United Kingdom, Sweden, Norway, Germany, and Australia. In the US, additional regulatory measures are under consideration. Further, many PFAS have now been listed as persistent organic pollutants. This comprehensive review provides crucial baseline information on the global occurrence, distribution, and regulatory framework of PFAS.

The utility of rare-earth elements (REEs) as natural geochemical tracers for the analysis of groundwater remediation was examined in several example permeable reactive barriers (PRBs). The PRBs utilize zero-valent iron and organic carbon plus limestone mixtures for contaminant treatment. Zero-valent iron removed REEs from groundwater to below detection levels (2–4 ng/L) and subsequent rebound of REE concentrations in regions down-gradient of the treatment zones was not observed. In addition, REE concentrations within and down-gradient of an organic carbon/limestone PRB were significantly reduced to

Hexavalent chromium Cr(VI) is toxic and can be highly mobile in many aquifer systems. Redox reactions with naturally occurring minerals and organic compounds can reduce Cr(VI) to Cr(III), forming labile Cr(III) oxyhydroxide precipitates, which is a natural attenuation process. In fractured bedrock aquifers, reduction of Cr(VI) in the rock matrix can enhance attenuation beyond that from matrix diffusion only, and potentially reduce back diffusion if concentrations in fractures decline following source reduction via natural processes or engineered remediation. In this study, we develop an extraction method for labile Cr(III) precipitates from Cr(VI) reduction using 5% hydrogen peroxide (H 2 O 2 ). Combining Cr(III) extractions with an established sodium hydroxide (NaOH) method for determination of Cr(VI) concentrations in rock porewater, a measure of the labile Cr(III) and Cr(VI) fractions in geologic samples is achieved. The methods were applied to cores from a contaminated groundwater system in fractured porous bedrock in order to assess the effectiveness of natural attenuation and whether Cr(VI) mass that diffused into the bedrock matrix was undergoing reduction. Detailed vertical distributions display two depth intervals with corresponding elevated concentrations of Cr(VI) in the porewater and extractable total Cr. The correspondence of Cr(VI) and labile Cr(III) provides evidence for reduction of Cr(VI) contamination in the bedrock matrix. Mineralogical analysis suggests that Fe(II)-bearing minerals, chlorite and biotite are the most likely candidates for natural reductants. This study provides evidence for the natural attenuation of anthropogenic Cr(VI) contamination in the porewater of a fractured bedrock aquifer, and it outlines a quantitative method for evaluating the effectiveness of natural attenuation in groundwater systems.

Cadmium (Cd) is a non-essential trace element that is widely distributed in the environment. Both geogenic and anthropogenic sources can elevate Cd concentrations in soils and groundwater, which are important for maintaining healthy supplies of food and safe drinking water. Elevated Cd doses are carcinogenic to humans. The WHO Guidelines for Drinking-Water Quality recommend a guideline value for Cd of 3 μg/L. Important anthropogenic Cd sources include mining, atmospheric deposition of combustion emissions, and the use of Cd-containing fertilizers. We document several cases of Cd pollution in soil and groundwater based on worldwide accounts. Besides anthropogenic Cd sources, Cd is also incorporated into sulfides, carbonates, and phosphorites resulting in elevated Cd concentrations in associated rock types. The crustal median Cd content is 0.2 mg/kg. In soils, Cd occurs at concentrations of 0.01 to 1 mg/kg with a worldwide mean of 0.36 mg/kg. Weathering can lead to Cd concentrations up to 5 μg/L in soil water and up to 1 μg/L in groundwater. In aqueous solutions, Cd generally occurs as the divalent Cd2+ and it is mobilized mainly in oxic, acidic conditions. Cadmium sorption is enhanced by the presence of high amounts of hydrous oxides, clay minerals, and organic matter, and its mobility is further influenced by pH, the redox state, and ionic strength of the solution. However, Cd can remain in solution as water-soluble complexes with anions, such as CdCl+ and Cd(SO4)22-, and dissolved organic matter while sorption and precipitation decrease the aqueous concentration of most other heavy metals. As a consequence, Cd is one of the most mobile heavy metals in the environment. The elevated mobilization potential, e.g., through competition and ligand induced desorption, is the reason for faster Cd release from soil into groundwater than other heavy metals. The goal of this study was to present a broad overview of the origin and concentration of Cd in groundwater, and its reaction pathways in aquatic environments. To gain an overview of the hydrochemical behavior of Cd, cases of Cd pollution in soil and groundwater, studies investigating Cd release, and information about the legal framework were compiled.

Co-contaminant behavior of arsenic (As) and selenium (Se) in groundwater is examined in this study at a former lead and zinc smelting facility. We collected water quality data, including concentrations of trace metals, major ions, and metalloid speciation, over a 15-year period to document long-term trends and relationships between As, Se, geochemical parameters, and other redox-sensitive trace metals. Concentrations of dissolved As and Se were negatively correlated (Kendall’s Tau B correlation coefficient, r = −0.72) and showed a distinctive L-shaped relationship. High-concentration arsenic wells (>5 mg L(−1)) were characterized by intermediate oxidation-reduction conditions (75 < Eh < 275 mV), near-neutral pH (6.1–7.9), low Ca/Na ratios, elevated Fe and Mn concentrations, and high proportions of As(III) relative to total dissolved As. High-concentration Se wells (>500 μg L(−1)) were characterized by more positive Eh (305–500 mV), low Fe concentrations, and high proportions of As(V). Batch micocosm experiments showed that aquifer solids contain mineral surfaces and/or microbial communities capable of removing selenate from groundwater. Electron microprobe and Se K-edge X-ray absorption near-edge spectroscopic analyses demonstrated that Se was predominantly associated with elemental Se in the reduced aquifer solids. Factor analysis revealed three discernible groupings of trace metals. Group I includes U, Se, and nitrate-N, all of which are mobile under oxygenated to moderately oxygenated conditions. Group II includes elements that are mobile under Fe(III)-reducing conditions: Fe, total dissolved As, As(III), and ammonium-N. Group III elements (Mo, Sb, and V) showed mobility across the entire range of redox conditions encountered in site groundwater; As(V) clustered with this group of elements. Geochemical modeling suggests that As and Se species were in a state of disequilibrium with respect to measured parameters indicative of redox conditions, although predicted patterns of redox-controlled mobility and attenuation were confirmed. This analysis is important to better understand groundwater contaminant behavior in response to redox conditions ranging from oxic/suboxic to Fe(III)-reducing, but excluding sulfate-reducing conditions.

The uptake of aqueous Ni(II) by synthetic mackinawite (FeS) was examined in anaerobic batch experiments at near-neutral pH (5.2 to 8.4). Initial molar ratios of Ni(II) to FeS ranged from 0.008 to 0.83 and maximum Ni concentrations in mackinawite, expressed as the cation mol fraction, were as high as XNi = 0.56 (Fe1 - x Ni x S; 0 ≤ x ≤ 1). Greater than 99% Ni removal from solution occurred when Ni loading remained below 0.13 ± 0.03 (1σ) mol Ni per mol FeS due to sorption of Ni at the mackinawite surface. Characterization of experimental solids using X-ray diffraction and Raman spectroscopy showed patterns characteristic of nanocrystalline mackinawite; no evidence of nickel monosulfide (α-NiS or millerite), polydymite (Ni3S4), or godlevskite [(Ni,Fe)9S8] formation was indicated regardless of the amount of Ni loading. Slight expansion of the c-axis correlated with increasing Ni content in synthetic mackinawite, from c = 5.07 ± 0.01 A at XNi = 0.02 to c = 5.10 ± 0.01 A at XNi = 0.38. Ni K-edge extended X-ray absorption fine structure (EXAFS) spectra of synthetic Ni-bearing mackinawite are similar in phase and amplitude to the Fe K-edge EXAFS spectrum of Ni-free mackinawite, indicating that the molecular environment of Ni2+ in Ni-bearing mackinawite is similar to that of Fe2+ in Ni-free mackinawite. EXAFS data fitting of Ni-bearing mackinawite with XNi = 0.42 indicated a coordination number of 4.04 ± 0.30 and an average Ni_S bond distance of 2.28 A, in good agreement with the Fe_S bond distance of 2.26 A in mackinawite, tetrahedral Fe coordination, and slight lattice expansion along the c-axis. At lower Ni loadings (XNi = 0.05-0.11), EXAFS analysis showed a decrease in Ni_S coordination towards CN = 3, which reflects the influence of sorbed Ni. Continued Ni uptake, past the maximum amount of sorption, was accompanied by proportional molar release of Fe to solution. Interstitial occupancy of Ni within the mackinawite interlayer may be transitional to structural substitution of Fe. The Ni-mackinawite solid-solution is described by a one-site binary mixing model: LnKd=lnKe-(W/RT)(1-2XNi) where Kd is the distribution coefficient, Ke is the ratio of equilibrium constants for Ni-mackinawite and mackinawite (14.4 ± 1.3), W is an ion interaction parameter, and XNi is the mole fraction of end-member NiS in the solid solution. The experimentally determined value of W is 17.74 ± 1.15 kJ/mol and indicates significant non-ideality of the solid solution. Transformation processes were evaluated by aging Ni-mackinawite with polysulfides and solutions saturated with air. Reaction of Ni-mackinawite with polysulfides led to the formation of pyrite (FeS2) and Ni retention in the solid phase. When Ni-mackinawite was aged in the presence of dissolved oxygen, transformation to goethite (FeOOH) and violarite (FeNi2S4) was observed.

Arsenic toxicity and mobility in groundwater depend on its aqueous speciation. Uncertainty about the methods used for measuring arsenic speciation in sulfate-reducing environments hampers transport and fate analyses and the development of in-situ remediation approaches for treating impacted aquifers. New anion-exchange chromatography methods linked to inductively coupled plasma mass spectrometry (ICP-MS) are presented that allow for sample/eluent pH matching. Sample/eluent pH matching is advantageous to prevent thioarsenic species transformation during chromatographic separation because: species protonation states remain unaffected, hydroxyl-for-bisulfide ligand substitution is avoided, and oxidation of reduced arsenic species is minimized. We characterized model and natural solutions containing mixtures of arsenic oxyanions with dissolved sulfide and solutions derived from the dissolution of thioarsenite and thioarsenate solids. In sulfidic solutions containing arsenite, two thioarsenic species with S/As ratios of 2:1 and 3:1 were important over the pH range from 5.5 to 8.5. The 3:1 thioarsenic species dominated when disordered As(2)S(3) dissolved into sulfide-containing solution at pH 5.4. Together with the preferential formation of arsenite following sample dilution, these data provide evidence for the formation and detection of thioarsenite species. This study helps resolve inconsistencies between spectroscopic and chromatographic evidence regarding the nature of arsenic in sulfidic waters.

Understanding the factors that govern aqueous solubility of uranyl minerals is important for predicting uranium mobility in groundwater and for designing effective remediation strategies. The uranyl-containing minerals metaschoepite [UO3∙(2H2O)] and uranophane [Ca(UO2)2(SiO3OH)2·5H2O] were synthesized and evaluated in batch solubility experiments conducted in the presence of common groundwater ions: calcium, bicarbonate/carbonate, and dissolved silica. Solid-phase characterization revealed the expected structural and thermogravimetric properties of metaschoepite and uranophane. Metaschoepite solubility in carbonate-free water followed a u-shaped pH dependency with minimum solubility near pH 8.5; uranium concentrations at pH ≳ 8.5 were approximately equivalent to the reference value for safe drinking water established by the EPA (30 μg/L). With increasing bicarbonate/carbonate concentration (1 mM – 50 mM) the solubility of metaschoepite increased, presumably due to the formation of uranyl-carbonate complexes. However, the experimental concentrations of uranium were lower than concentrations predicted from accepted complexation constants. For uranophane, equilibrium uranium concentrations were < 75 μg/L at typical groundwater concentrations of calcium and dissolved silica (pH > 7). The diversity of uranyl minerals that possibly form in the presence of common groundwater species: Ca-Mg-Na-K-Si-bicarbonate/carbonate-sulfate-chloride, has not been fully explored with respect to understanding potential mineral transformations and impacts on uranium solubility and mobility.

This study provides a twenty-two-year record of in situ degradation of chlorinated organic compounds by a granular iron permeable reactive barrier (PRB). Groundwater concentrations of trichloroethene (TCE) entering the PRB were as high as 10,670 μg/L. Treatment efficiency ranged from 81 to >99% and TCE concentrations from C4 compounds, and possibly CO(2(aq)) and methane. Abiotic patterns of TCE degradation were indicated by compound-specific stable isotope data and the distribution of degradation products. δ(13)C values of methane within and down-gradient of the PRB varied widely from −94‰ to −16‰; these values cover most of the isotopic range encountered in natural methanogenic systems. Methanogenesis is a sink for inorganic carbon in zero-valent iron PRBs that competes with carbonate mineralization and this process is important for understanding pore-space clogging and longevity of iron-based PRBs. The carbon isotope signatures of methane and inorganic carbon were consistent with open-system behavior and 22% molar conversion of CO(2(aq)) to methane.

This study uses a combination of conventional and high resolution field and laboratory methods to investigate processes causing attenuation of a hexavalent chromium (Cr(VI)) plume in sedimentary bedrock at a former industrial facility. Groundwater plume Cr(VI) concentrations decline by more than three orders of magnitude over a 900 m distance down gradient from the site. Internal plume concentrations generally exhibit stable to declining trends due to diffusive and reactive transport in the low permeability matrix as fluxes from the contamination source dissipate due to natural depletion processes and active remediation efforts. The strong attenuation is attributed to diffusion from mobile groundwater in fractures to immobile porewater in the rock matrix, and reactions causing transformation of aqueous Cr(VI) to low-solubility Cr(III) precipitates, confirmed by high spatial resolution rock matrix contaminant concentrations and comparisons with groundwater concentrations from multi-level sampling within the plume. Field characterization data for the fracture network and matrix properties were used to inform 2-D discrete-fracture matrix (DFM) numerical model simulations that quantify attenuation due to diffusion and reaction processes, which show consistency with field datasets, and provide insights regarding future plume conditions. The combination of field, laboratory and modeling evidence demonstrates effects of matrix diffusion and reaction processes causing strong attenuation of a Cr(VI) plume in a sedimentary bedrock aquifer. This approach has important implications for characterization of sites with Cr(VI) contamination for improved site conceptual models and remediation decision-making.

A new method for quantification of hexavalent chromium (Cr(VI)) in the porewater of rock core samples from contaminated sedimentary bedrock has been developed here. The method combines alkaline extraction with cation exchange column separation followed by determination of Cr concentrations by inductively coupled plasma mass spectrometry (ICP-MS). A porewater detection limit of 45 μg/L was determined by performing extractions on uncontaminated samples, and accounts for dilution of porewater volumes by the extraction solution. Recoveries of Cr(VI) in quality control (QC) samples were greater than 90% and there was no significant interference from Cr(III). Relative standard deviations (RSD) were less than 10% for QC samples spiked with Cr(VI), and 2 to 47% (average of 21%) for replicate analyses of core samples. Cr(VI) analyses were conducted on depth-discrete core samples collected at intervals of

Abiotic degradation of chlorinated solvents by reactive iron minerals such as iron sulfides, magnetite, green rust, and other Fe(II)-containing minerals has been observed in both laboratory and field studies. These reactive iron minerals form under iron- and sulfate-reducing conditions which are commonly found in permeable reactive barriers (PRBs), enhanced reductive dechlorination (ERD) treatment locations, landfills, and aquifers that are chemically reducing. The objective of this review is to synthesize current understanding of abiotic degradation of chlorinated solvents by reactive iron minerals, with special focus on how abiotic processes relate to groundwater remediation. Degradation of chlorinated solvents by reactive minerals can proceed through reductive elimination, hydrogenolysis, dehydrohalogenation, and hydrolysis reactions. Degradation products of abiotic reactions depend on degradation pathways and parent compounds. Some degradation products (e.g., acetylene) have the potential to serve as a signature product for demonstrating abiotic reactions. Laboratory and field studies show that various minerals have a range of reactivity toward chlorinated solvents. A general trend of mineral reactivity for degradation of chlorinated solvents can be approximated as follows: disordered FeS > FeS > Fe(0) > FeS2 > sorbed Fe2+ > green rust = magnetite > biotite = vermiculite. Reaction kinetics are also influenced by factors such as pH, natural organic matter (NOM), coexisting metal ions, and sulfide concentration in the system. In practice, abiotic reactions can be engineered to stimulate reactive mineral formation for groundwater remediation. Under appropriate site geochemical conditions, abiotic reactions can occur naturally, and can be incorporated into remedial strategies such as monitored natural attenuation.

We evaluated water quality characteristics in the northern Raton Basin of Colorado and documented the response of the Poison Canyon aquifer system several years after upward migration of methane gas occurred from the deeper Vermejo Formation coalbed production zone. Results show persistent secondary water quality impacts related to the biodegradation of methane. We identify four distinct characteristics of groundwater–methane attenuation in the Poison Canyon aquifer: (i) consumption of methane and sulfate and production of sulfide and bicarbonate, (ii) methane loss coupled to production of higher molecular weight (C(2+)) gaseous hydrocarbons, (iii) patterns of (13)C enrichment and depletion in methane and dissolved inorganic carbon, and (iv) a systematic shift in sulfur and oxygen isotope ratios of sulfate, indicative of microbial sulfate reduction. We also show that the biogeochemical response of the aquifer system has not mobilized naturally occurring trace metals, including arsenic, chromium, cobalt, nickel, and lead, likely due to the microbial production of hydrogen sulfide which favors stabilization of metals in aquifer solids.

Predicting the long-term potential of permeable reactive barriers for treating contaminated groundwater relies on understanding the endpoints of biogeochemical reactions between influent groundwater and the reactive medium. Iron hydroxy carbonate (chukanovite) is frequently observed as a secondary mineral precipitate in granular iron PRBs. Mineralogical characterization was carried out using X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, and X-ray absorption spectroscopy on materials collected from three field-based PRBs in the US (East Helena, MT; Elizabeth City, NC; Denver Federal Center, CO). These PRBs were installed to treat a range of contaminants, including chlorinated organics, hexavalent chromium, and arsenic. Results obtained indicate that chukanovite is a prevalent secondary precipitate in the PRBs. Laboratory experiments on high-purity chukanovite separates were carried out to constrain the room-temperature solubility for this mineral. An estimated Gibbs energy of formation ({Delta}{sub f}G{sup o}) for chukanovite is - 1174.4 {+-} 6 kJ/mol. A mineral stability diagram is consistent with observations from the field. Water chemistry from the three reactive barriers falls inside the predicted stability field for chukanovite, at inorganic carbon concentrations intermediate to the stability fields of siderite and ferrous hydroxide. These new data will aid in developing better predictive models of mineral accumulation in zerovalent ironmore » PRBs.« less

Trichloroethylene (TCE) is one of the most common and persistent groundwater contaminants encountered at hazardous waste sites around the world. A growing body of evidence indicates that iron sulfides play an important role in degrading TCE in natural environments and in engineered systems designed for groundwater cleanup. In this study, we investigate transformation processes of iron sulfides and consequent impacts on TCE degradation using batch experimental techniques, transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). Our results show that mackinawite is highly reactive toward TCE and no detectable mineralogical changes were detected during the course of reaction. However, freeze-dried FeS transformed to a mixture of mackinawite and greigite during the freeze drying process, with further mineralogical changes during reaction with TCE to lepidocrocite, goethite and pyrite. Newly formed lepidocrocite is a transient phase, with conversion to goethite over time. TCE transformation kinetics show that freeze-dried FeS is 20–50 times less reactive in degrading TCE than non-freeze-dried FeS, and the TCE degradation rate increases with pH (from 5.4 to 8.3), possibly due to an increase of surface deprotonation or electron transfer at higher pH. Results suggest that freeze drying could cause FeS particle aggregation, decreased surface area and availability of reactive sites; it also could change FeS mineralogy and accelerate mineral transformation. These aspects could contribute to the lower reactivity of freeze-dried FeS toward TCE degradation. Modeling results show that FeS transformation in natural environments depends on specific biogeochemical conditions, and natural FeS transformation may affect mineral reactivity in a similar way as compared to the freeze drying process. Rapid transformation of FeS to FeS2 could significantly slow down TCE degradation in both natural and engineered systems.

Sequential extractions can provide analytical constraints on the identification of mineral phases that control arsenic speciation in sediments. Model solids were used in this study to evaluate different solutions designed to extract arsenic from relatively labile solid phases. Sequential and single-step extractions were conducted to evaluate the selectivity of these extraction solutions for differentiating arsenic associated with iron (hydr)oxides and iron sulfides. Extraction phases were categorized as 1) MgCl2-extractable, 2) phosphate-extractable, 3) ascorbate-extractable, and 4) HCl-extractable when the procedure is applied in sequence. Arsenic, iron, and sulfur concentrations were compared between the single-step and sequential extractions. Additionally, phosphate extractions were conducted at three pH values for comparison purposes. Application of the extraction protocol to orpiment of varying crystallinity and crystalline arsenopyrite indicated that these phases have limited solubility in the extractant solutions. Extraction with ascorbate at circumneutral pH appears to target dissolution of poorly crystalline iron (hydr)oxides and is largely selective for extracting arsenic associated with this phase. Tests of the ascorbate extraction solution on sediment samples previously characterized for mineralogy and solid phase arsenic speciation by X-ray absorption spectroscopy reveal the utility of ascorbate for differentiating arsenic associated with poorly crystalline iron (hydr)oxides and iron sulfides. Results from tests in which samples were dried by different approaches or were subjected to air exposure prior to or during extraction demonstrate the importance of sample handling on the outcome of extraction protocols.

The long-term effectiveness of an FeSO4 + Na2 S2 O4 reductant solution blend for in situ saturated zone treatment of dissolved and solid phase Cr(VI) in a high pH chromite ore processing solid waste fill material was investigated. Two field pilot injection studies were conducted that showed sustained treatment of Cr(VI) over evaluation periods of more than 1,000 days. No well or aquifer formation clogging was observed during injection although treatment was limited to the pore volume displacement radius of the injected reductant. Analysis of posttreatment core samples suggested >85% treatment effectiveness of solid phase Cr(VI) , whereas lab tests suggested treatment of the solid phase Cr(VI) can be complete provided the chromite ore processing solid waste sediments are sufficiently dosed with the reductant. The sustained treatment of dissolved phase Cr(VI) migrating through the treatment zones for more than 1,000 days following injection provided strong evidence of a residual treatment capacity having be...

A field study was conducted to evaluate the performance of a ferrous iron based in situ redox zone for the treatment of a dissolved phase Cr(VI) plume at a former industrial site. The ferrous iron based in situ redox zone was created by injecting a blend of 0.2 M ferrous sulfate and 0.2 M sodium dithionite into the path of a dissolved Cr(VI) plume within a shallow medium to fine sand unconfined aquifer formation. Monitoring data collected over a period of 1020 days after more than 100 m of linear groundwater flow through the treatment zone indicated sustained treatment of dissolved phase Cr(VI) from initial concentrations between 4 and 8 mg/L to less than 0.015 mg/L. Sustained treatment is assumed to be primarily due to the reduction of Cr(VI) to Cr(III) by ferrous iron adsorbed to, precipitated on, and/or incorporated into aquifer iron (hydr)oxide solid surfaces within the treatment zone. Precipitated phases likely include FeCO3 and FeS based on saturation index considerations and SEM/EDS analysis. The detection of solid phase sulfites and thiosulfates in aquifer sediments collected from the treatment zone more than 2 years following injection suggests dithionite decomposition products may also play a significant role in the long-term treatment of the dissolved phase Cr(VI).

Perchlorate salts of potassium and ammonium are the primary oxidants in pyrotechnic mixtures, yet insufficient information is available regarding the relationship between fireworks displays and the environmental occurrence of perchlorate. Here we document changes in perchlorate concentrations in surface water adjacent to a site of fireworks displays from 2004 to 2006. Preceding fireworks displays, perchlorate concentrations in surface water ranged from 0.005 to 0.081 microg/L, with a mean value of 0.043 microg/L. Within 14 h after the fireworks, perchlorate concentrations spiked to values ranging from 24 to 1028x the mean baseline value. A maximum perchlorate concentration of 44.2 microg/L was determined following the July 4th event in 2006. After the fireworks displays, perchlorate concentrations decreased toward the background level within 20 to 80 days, with the rate of attenuation correlating to surface water temperature. Adsorption tests indicate that sediments underlying the water column have limited (100 nmol/g) capacity to remove perchlorate via chemical adsorption. Microcosms showed comparatively rapid intrinsic perchlorate degradation in the absence of nitrate consistent with the observed disappearance of perchlorate from the study site. This suggests that at sites with appropriate biogeochemical conditions, natural attenuation may be an important factor affecting the fate of perchlorate following fireworks displays.

Objective To establish whether metformin has a significant action in reducing visceral fat and improving other metabolic parameters in women with polycystic ovary syndrome (PCOS). Design Randomised, double-blind, placebo-controlled trial. Setting Reproductive medicine clinic. Population Forty women with anovulatory PCOS. Methods Participants were randomised into receiving metformin 500 mg three times a day or placebo for 3 months. Main outcome measures Fat distribution was measured by computed tomography scan. Secondary outcome measures included serum indices of the metabolic syndrome and evidence of ovulation. Results We found no significant differences in any of the measures of fat distribution between the placebo and metformin groups. The metformin group had significantly lower total cholesterol (P= 0.02), low-density lipoprotein cholesterol (P= 0.02) and cholesterol:high-density lipoprotein cholesterol ratio (P= 0.05), but there was no statistically significant treatment effect on androgens, insulin, insulin resistance, triglycerides, ovulation or pregnancy. Conclusions Metformin has no clinically significant effect in reducing visceral fat mass, although it does have a beneficial effect on lipids. This trial lends support to the growing evidence that metformin is not a weight loss drug. Metformin might therefore be used as an adjunct to lifestyle modification in women with PCOS, but not as a substitute for it.

The geochemical partitioning of arsenic in organic-rich sediments from a contaminated wetland is examined using X-ray absorption spectroscopy and selective chemical extraction procedures, and evaluated in context to the anoxic diagenesis of iron and sulfur. The interaction between ground water and surface water has a significant influence on iron sulfide formation in the wetland sediments. Ground-water seeps supply concentrations of sulfate, dissolved hydrocarbons, ferrous iron, and arsenic, and sediments located near seeps are anomalously enriched in arsenic, reactive iron, and acid-volatile sulfides. Degree-of-sulfidation (DOS) values are high in sediments adjacent to sites of ground-water discharge, ranging from 0.57 to 1.0. Pyrite (FeS{sub 2}) formation is apparently not limited by the abundance of any one primary reactant, e.g., organic carbon, sulfate, or reactive iron; instead, persistence of precursor iron monosulfides is attributed to slow pyrite formation kinetics due to low concentrations of reactive intermediate sulfur species or possibly due to high concentrations of arsenite, dissolved organic-carbon, or other solutes that adsorb to iron monosulfides surfaces and impede transformation reactions to pyrite. Greigite (Fe{sub 3}S{sub 4}) accounts for > 80% of total reduced sulfur in sediments rich in acid-volatile sulfide and X-ray absorption spectroscopy data for magnetic separates provide direct evidence thatmore » As(III) is, at least in part, associated with reduced sulfur in the form of greigite. However, pyrite can only account for a small percentage, < 20%, of the total arsenic budget in the reduced sediments. Although pyrite is the predicted stable endpoint for reactive iron and sulfur, it appears that within a 30 y time period pyrite is a relatively unimportant host for arsenic in the system investigated here. The abundance of reactive iron in the sediments prevents accumulation of dissolved sulfide and thus prevents formation of soluble thioarsenic species. X-ray absorption near-edge structure (XANES) spectroscopy indicates only the presence of As(III) in the reduced sediments. Results of linear combination fitting of reference spectra to sediment spectra are consistent with sulfur- and/or oxygen-coordinated As(III) in association with iron monosulfides and ferrous-bearing carbonates or hydroxides.« less

The fate of arsenic discharged from contaminated ground water to a small, shallow lake at a hazardous waste site was examined to understand the role of iron (hydr)oxide precipitation–dissolution processes within the water column. Field and laboratory observations indicate that arsenic solubility was controlled, in part, by the extent of ferrous iron oxidation–precipitation and arsenic sorption occurring near the lake chemocline. Laboratory experiments were conducted using site-derived water to assess the impact of these coupled processes on the removal of dissolved arsenic from the water column. The measured concentration of organic carbon from epilimnetic and hypolimnetic water sampled from the lake was approximately 1.3 mM and 17.0 mM, respectively. Experiments conducted with these samples along with synthetic controls containing no organic carbon demonstrated that observed rates of formation and crystallinity of the precipitated iron (hydr)oxide were dependent on the concentration of organic carbon in the lake water. Increasing dissolved organic matter concentration did not significantly interfere with ferrous iron oxidation, but inhibited iron (hydr)oxide precipitation and subsequent sorption of arsenic. For experiments using water sampled from the lake hypolimnion there was a strong relationship between the fraction of precipitated iron and the fraction of sorbed arsenic. Laboratory- and field-derived iron (hydr)oxide precipitates were characterized to evaluate mineralogy and arsenic distribution. In-situ suspended solids and precipitates formed in laboratory experiments using hypolimnetic water were identified as poorly crystalline 2-line ferrihydrite. These solids were readily dissolved in the presence of dithionite indicating that elevated dissolved iron and arsenic observed in the hypolimnion resulted, in part, from in-situ reductive dissolution of settling 2-line ferrihydrite near the sediment–water interface. These observations support the contention that the levels of dissolved arsenic observed in the shallow lake can be attributed to ground-water discharge and internal recycling of arsenic within the water column. The efficiency of the process resulting in iron (hydr)oxide precipitation and arsenic sorption limits the downgradient export of arsenic derived from ground-water discharge.

Solid-phase associations of chromium were examined in core materials collected from a full-scale, zerovalent iron permeable reactive barrier (PRB) at the U.S. Coast Guard Support Center located near Elizabeth City, NC. The PRB was installed in 1996 to treat groundwater contaminated with hexavalent chromium. After eight years of operation, the PRB remains effective at reducing concentrations of Cr from average values1500 microg L(-1) in groundwater hydraulically upgradient of the PRB to values1 microg L(-1) in groundwater within and hydraulically downgradient of the PRB. Chromium removal from groundwater occurs at the leading edge of the PRB and also within the aquifer immediately upgradient of the PRB. These regions also witness the greatest amount of secondary mineral formation due to steep geochemical gradients that result from the corrosion of zerovalent iron. X-ray absorption near-edge structure (XANES) spectroscopy indicated that chromium is predominantly in the trivalent oxidation state, confirming that reductive processes are responsible for Cr sequestration. XANES spectra and microscopy results suggest that Cr is, in part, associated with iron sulfide grains formed as a consequence of microbially mediated sulfate reduction in and around the PRB. Results of this study provide evidence that secondary iron-bearing mineral products may enhance the capacity of zerovalent iron systems to remediate Cr in groundwater, either through redox reactions at the mineral-water interface or by the release of Fe(II) to solution via mineral dissolution and/or metal corrosion.

The objectives of this study were to conduct batch and column studies to (i) assess the effectiveness of zero-valent iron for arsenic remediation in groundwater, (ii) determine removal mechanisms of arsenic, and (iii) evaluate implications of these processes with regard to the stability of arsenic and long-term remedial performance of the permeable reactive barrier (PRB) technology. A high concentration arsenic solution (50 mg l(-1)) was prepared by using sodium arsenite (arsenic (III)) to simulate groundwater at a heavily contaminated Superfund site in the USA. Batch studies indicate that the removal of arsenic is a two-step reaction with fast initial disappearance of arsenite followed by a slow subsequent removal process. Flow-through columns were conducted at a flow rate of 17 ml h(-1) under reducing conditions for 6.6 mo. Kinetic analysis suggested that arsenic removal behaves as a zero-order reaction at high arsenic concentrations. Arsenic removal rate constants decreased with time and arsenic breakthrough was observed in the column study. Arsenic removal capacity of zero-valent iron was determined to be approximately 7.5 mg As/g Fe. Carbonate green rust was identified from the analysis of surface precipitates; arsenite uptake by green rust may be a major mechanism responsible for arsenic remediation by zero-valent iron. Analysis of HCl-extractable arsenic from iron samples indicated that approximately 28% of arsenic was in the form of arsenate suggesting that a surface oxidation process was involved in the arsenic removal with zero-valent iron.

Aims and methods It has recently been hypothesized that weight gain in childhood accelerates the onset of Type 1 diabetes, as well as increasing its risk, and that Type 1 diabetes and Type 2 diabetes may be one and the same disorder of insulin resistance. An explanation is needed for the rising incidence of childhood diabetes and, to test the Accelerator Hypothesis, we examined the anthropometric measurements recorded from birth in 168 young people presenting with Type 1 diabetes between 1980 and 2002. Pre-onset as well as peri- and post-onset measurements of height and weight were available, and waist circumference was recorded at various intervals after onset. Results The mean birth weight of the children and their height, weight and body mass index (BMI) at diagnosis lay close to the population mean. However, pre-onset and post-onset BMI were both well above the population mean, were closely correlated with each other (r = 0.79, P

Ferrihydrite, which is known to form in the presence of oxygen and to be stabilized by the adsorption of Si, PO4 and SO4, is ubiquitous in the fine-grained fractions of permeable reactive barrier (PRB) samples from the U.S. Coast Guard Support Center (Elizabeth City, NC) and the Denver Federal Center (Lakewood, CO) studied by high-resolution transmission electron microscopy and selected area electron diffraction. The concurrent energy-dispersive X-ray data indicate a strong association between ferrihydrite and metals such as Si, Ca, and Cr. Magnetite, green rust 1, aragonite, calcite, mackinawite, greigite and lepidocrocite were also present, indicative of a geochemical environment that is temporally and spatially heterogeneous. Whereas magnetite, which is known to form due to anaerobic Fe0 corrosion, passivates the Fe0 surface, ferrihydrite precipitation occurs away from the immediate Fe0 surface, forming small (

We examined the use of room-temperature hydrochloric acid (1-6 M) and salt solutions of magnesium chloride, sodium carbonate, and sodium sulfide for the removal of arsenic from synthetic iron monosulfides and contaminated sediments containing acid-volatile sulfides (AVS). Results indicate that acid-soluble arsenic reacts with H2S released from AVS phases and precipitates at low pH as disordered orpiment or alacranite. Arsenic sulfide precipitation is consistent with geochemical modeling in that conditions during acid extraction are predicted to be oversaturated with respect to orpiment, realgar, or both. Binding of arsenic with sulfide at low pH is sufficiently strong that 6 M HCl will not keep spiked arsenic in the dissolved fraction. Over a wide range of AVS concentrations and molar [As]/[AVS] ratios, acid extraction of arsenic from sulfide-bearing sediments will give biased results that overestimate the stability or underestimate the bioavailability of sediment-bound arsenic. Alkaline solutions of sodium sulfide and sodium carbonate are efficient in removing arsenic from arsenic sulfides and mixed iron-arsenic sulfides because of the high solubility of arsenic at alkaline pH, the formation of stable arsenic complexes with sulfide or carbonate, or both.

Bifunctional aluminum is prepared by sulfating aluminum metal with sulfuric acid. The use of bifunctional aluminum to degrade methyl tert-butyl ether (MTBE) in the presence of dioxygen has been examined using batch systems. Primary degradation products were tert-butyl alcohol, tert-butyl formate, acetone, and methyl acetate. The initial rate of MTBE degradation exhibited pseudo-first-order behavior, and the half-life of reaction was less than 6 h. XPS analysis indicates the formation of sulfate at the surface of bifunctional aluminum. The concentration of surface sulfate varies linearly with increasing strength of the sulfuric acid used during bifunctional aluminum preparation. The rate of MTBE degradation is a function of the concentration of the surface sulfate. MTBE degradation rates increased by a factor of 2 as surface sulfate concentrations increased from 233 to 641 micromol/m2. This relationship implies that sulfate at the surface of bifunctional aluminum acts as a reactive site.

The ability to confidently measure the concentration of dissolved oxygen (D.O.) in ground water is a key aspect of remedy selection and assessment. Presented here is a comparison of the commonly practiced methods for determining D.O. concentrations in ground water, including colorimetric, membrane-covered electrode, and modified Winkler techniques. The Winkler titration (azide and permanganate modifications) is the most accurate and precise technique for determining D.O. and is appropriate for applications with stringent data quality objectives. In addition, excellent correlation over a wide range of D.O. concentrations was found between Winkler titrations and colorimetric tests using the rhodazine D (below 1 part per million [ppm]) and indigo carmine reagents (above 1 ppm). Electrode measurements represent the simplest method for determining D.O. concentrations on a continuous basis and electrode determinations positively correlate with Winkler results above 1 ppm. Below 1 ppm, electrodes provide only a qualitative measure of low D.O. level, apparently due to slow electrode response.

—Time-dependent variations in the physicochemical properties of pyrite were investigated in four sediment cores from the Black Sea. In the laminated, deep-basin sediments of Unit I and Unit II, >86% of pyrite particles are present as fine-grained framboidal aggregates. In these sediments, the dominance of pyrite framboids, with a narrow size distribution and maximum size < 18 μm, is evidence of syngenetic (water column) pyrite formation subjacent to the O2-H2S boundary. Sediments from the basin margin collected below the impingement of the O2-H2S boundary contain an increased proportion of fine-grained euhedral grains of pyrite relative to framboidal aggregates, suggesting increased additions of diagenetic pyrite below the sediment-water interface. The more efficient reduction and sulfidation of iron in the water column of the central region of the basin implies a more reactive source of iron there compared to the basin margins, and is tied to increasing %pyrite as framboids and greater degrees of pyritization (DOP) in the basin center relative to the basin margins. An evaluation of Fe-S-C relations indicates that pyrite formation was limited by sulfate availability during deposition of the oldest lacustrine sediments of Unit III, C-limited for upper Unit III due to the input of sulfate-rich Mediterranean seawater, and Fe-limited in the laminated, organic carbon-rich sediments of Units I and II. These changes in pyrite-limiting factors occurred within 4 m of compacted burial representing

Natural clinoptilolite (Cpt: Na 0.085 K 0.037 Ca 0.010 Mg 0.020 Al 0.182 Si 0.818 O 2 ·0.528H 2 O) from Castle Creek, Idaho, and its cation-exchanged variants (Na-Cpt, NaK-Cpt, K-Cpt, and Ca-Cpt) were studied by high-temperature calorimetry. The hydration enthalpy for all the clinoptilolites is about −30 kJ/mol H 2 O (liquid water reference state) at 25 °C. The energetic stabilization effect of hydration on each clinoptilolite can be largely correlated to its hydration capacity. The higher the average ionic potential of the extra-framework cations, the larger the hydration capacity of the clinoptilolite. This trend may be attributed to the small size as well as the efficient water-cation packing of high field strength cations in the zeolite structure. The hydration properties of these clinoptilolites are compared with those previously reported in the literature. The dehydration conditions as well as the measurement direction (dehydration of the initially hydrated sample or rehydration of the dehydrated zeolites) are important factors to control to obtain consistent thermodynamic properties for hydration. The standard enthalpy for formation of the clinoptilolites from the constituent elements at 25 °C based on two framework O atoms was obtained from the calorimetric data: −1117.57 ± 0.95 kJ/mol Cpt, −1130.05 ± 1.00 kJ/mol Na-Cpt, −1109.49 ± 1.04 kJ/mol NaK-Cpt, −1094.21 ± 1.12 kJ/mol K-Cpt, and −1153.78 ± 1.07 kJ/mol Ca-Cpt. Their molar entropy was determined by a summation method based on the thermodynamic properties of the component oxides. Thus the standard free energy based on two framework O atoms was derived: −1034.01 ± 1.05 kJ/mol Cpt, −1044.19 ± 1.10 kJ/mol Na-Cpt, −1027.26 ± 1.13 kJ/mol NaK-Cpt, −1014.89 ± 1.21 kJ/mol K-Cpt, and −1064.95 ± 1.16 kJ/mol Ca-Cpt.

We measured d 13 C values of free and sulfur-bound lipids and framboidal pyrite-size distributions in three sediment cores from the southern margins of the Black Sea. The margin cores show a marked diAerence in the occurrence of biomarkers from green sulfur bacteria compared with the deep-basin cores, as a result of deepening of the chemocline resulting from enhanced mixing and/or decreased light-penetration as a consequence of high turbidity and productivity in shelf waters. Quantitation of biomarkers suggests that photic-zone anoxia along the shelf margin was generally absent during the deposition of unit I, although occurred during the deposition of Unit IIb at two sites. # 2000 Elsevier Science Ltd. All rights reserved.

The kinetics and mechanism of analcime formation from precursor Na-clinoptilolite (Cpt-Na) and Na-mordenite (Mor-Na) were investigated from 125–225 °C, pressures up to 34.5 MPa, and pH = 9.2–10.7. By using batch and flow-through experimental methodologies, compositions of solids and solutions were monitored to track reaction progress, determine rates of analcime nucleation and growth, and evaluate the rate dependency of these processes on aqueous supersaturation. Analcime formation proceeds via a clinoptilolite (or mordenite) dissolution→analcime nucleation→analcime growth sequence. Synthetic analcime crystals are similar in morphology (trapezohedron to cubic trapezohedron) and composition (Si/Al = 2.1–2.7) to sedimentary analcimes formed during the low-grade alteration of crustal rocks, evidence that the experimental reaction mechanism is similar to that in natural environments. Rates of analcime nucleation were approximated by evaluating the time-dependence of the size and number of particles and range between 109.80 and 1011.88 per h per cm3 at 150° and 225 °C, respectively. The nucleation rate is a function of temperature and degree of supersaturation: ln rate = 56.76 – 15978.9/ T + 2.99 × 10−4·Δ G r, where Δ G r is the free energy change of analcime precipitation and T is temperature in Kelvins. This rate equation is consistent with an apparent activation energy of analcime nucleation ( E a,n) of 132.8 ± 8.3 kJ/mol. Although conditions were thermodynamically favorable for analcime formation at 125 °C, nucleation was not detected after 144 hours. These data suggest that diagenetic timing of the clinoptilolite to analcime transformation is principally controlled by kinetics rather than by thermodynamic equilibrium. Rates of analcime growth were estimated by measuring particle size distributions. Average growth rates ranged from 0.15 μm/h at 150 °C to 0.396 μm/h at 225 °C, and are consistent with an apparent activation energy of analcime growth ( E a,g) of 77.1 ± 9.4 kJ/mol. These nucleation and growth parameters are combined to successfully model the evolution of analcime particle size distributions. The experimental methods developed in this study demonstrate the use of hydrothermal flow-through methods in the study of zeolite transformations.

The formation pathways of pyrite are controversial. Time resolved experiments show that in reduced sulphur solutions at low temperature, the iron monosulphide mackinawite is stable for up to 4 months. Below 100°C, the rate of pyrite formation from a precursor mackinawite is insignificant in solutions equilibrated solely with H2S(aq). Mackinawite serves as a precursor to pyrite formation only in more oxidised solutions. Controlled, intentional oxidation experiments below 100°C and over a wide range of pH (3.3–12) confirm that the mackinawite to pyrite transformation occurs in slightly oxidising environments. The conversion to pyrite is a multi-step reaction process involving changes in aqueous sulphur species causing solid state transformation of mackinawite to pyrite via the intermediate monosulphide greigite. Oxidised surfaces of precursors or of pyrite seeds speed up the transformation reaction. Solution compositions from the ageing experiments were used to derive stability constants for mackinawite from 25°C to 95°C for the reaction: FeS(s) +2 H + ⇔ Fe 2+ + H 2 S The values of the equilibrium constant, logKFeS, varied from 3.1 at 25°C to 1.2 at 95°C and fit a linear, temperature-dependent equation: logKFeS=2848.779/T−6.347, with T in Kelvin. From these constants, the thermodynamic functions were derived. These are the first high temperature data for the solubility of mackinawite, where Fe2+ is the dominant aqueous ferrous species in reduced, weakly acidic to acidic solutions.

The hydration state of Na- and K-exchanged clinoptilolite from Castle Creek (Idaho, U.S.A.) has been measured by a pressure titration method to 300 °C and PH2O

In a shallow, fjord-like estuary, the Upper and Lower Basins of the Pettaquamscutt River (Rhode Island) each contain water columns stratified into oxic and anoxic plus sulfidic layers. Both water columns contain iron monosulfides and pyrite from just below the oxic-anoxic interface down to the sediment-water interface. Concentrations of iron monosulfides increase with increasing depth in the water columns, but concentrations of pyrite are relatively constant with depth. Suspended pyrite is present only as framboids, whereas the underlying sediments contain dominantly framboids but also euhedral grains and infilled framboids. Based on pyrite textures, S-isotopic compositions of water-column and sediment sulfides, and settling rate calculations, about 70% of the pyrite burial flux to the Pettaquamscutt sediments is accounted for by framboids forming at the oxic-anoxic interface and settling to the sediment-water interface. The remaining pyrite burial flux is secondary pyrite nucleation and growth within the sediments represented by euhedral grains and infilled framboids. The S-isotopic composition of pyrite in the Pettaquamscutt sediments, and presumably of pyrite in the sediments of other modern anoxic basins and analogous ancient sediments, therefore, contains a component originating near the oxic-anoxic interface and a component added below the sediment-water interface. Pyrite framboids form in the water column where there is high supersaturation with respect to pyrite and slight undersaturation with respect to the iron monosulfides. We propose that framboid nucleation and growth occurs in the water column, adjacent to, and below, the oxic-anoxic interface, where supplies of ferrous species, sulfide species, and suitable electron acceptors are available. Infilled and overgrown framboids from in the sediments as a result of surface nucleation and continued pyrite growth on framboids that originate above the sediment-water interface.

A detailed study of size distributions of framboidal pyrite in Holocene Black Sea sediments establishes the timing of a change from deposition under an oxic water column to deposition under an anoxic and sulfidic water column. In the most recent carbonate-rich sediments (Unit I) and in the organic carbon-rich sapropel (Unit II), framboid size distributions are remarkably uniform (mean diameter = 5 μm); over 95% of the framboids in Unit I and Unit II are

Pyrite framboid formation may be the result of four consecutive processes: (1) nucleation and growth of initial iron monosulfide microcrystals; (2) reaction of the microcrystals to greigite (Fe 3 S 4 ; (3) aggregation of uniformly sized greigite microcrystals, i.e., framboid growth; and (4) replacement of greigite framboids by pyrite. The uniform morphology, uniform size range, and ordering of the microcrystals in individual framboids, as well as the range of observed framboid structures from irregular aggregates to densely packed spherical aggregates and polyframboids, are consequences of these processes. Using DLVO theory (Derjaguin, Landau, Verwey, and Overbeek), we have evaluated the stability of colloidal, iron monosulfide suspensions with ionic strengths typical of marine and lacustrine waters. In addition to van der Waals attractive and double-layer repulsive forces, a term is included to account for the ferrimagnetic properties of greigite. Numerical models predict that magnetically saturated greigite particles >0.1 μm in diameter will rapidly aggregate in either marine or fresh water. The aggregation model is in agreement with the sequence of greigite formation followed by pyrite framboid formation established in a previous experimental study (Sweeney and Kaplan, 1973) and is consistent with the occurrence of framboids composed of other magnetic minerals, e.g., greigite, magnetite, and magnesioferrite. Based on the temperature-dependent magnetic properties of greigite and aging experiments in hydrothermal solutions, this mechanism for framboid formation via precursor greigite could operate to temperatures of ∼200°C, consistent with the occasional occurrence of pyrite framboids in the paragenesis of metalliferous ore deposits.

The fifteen-year performance of a granular iron, permeable reactive barrier (PRB; Elizabeth City, North Carolina) is reviewed with respect to contaminant treatment (hexavalent chromium and trichloroethylene) and hydraulic performance. Due to in-situ treatment of the chromium source zone, reactive and hydraulic longevity of the PRB has outlived the mobile chromate plume. Chromium concentrations exceeding 3 μg/L have not been detected in regions located hydraulically down-gradient of the PRB. Trichloroethylene treatment has also been effective, although non-constant influent concentrations of trichloroethylene have at times resulted in incomplete dechlorination. Daughter products: cis-1,2-dichloroethylene, vinyl chloride, ethene, and ethane have been observed within and down-gradient of the PRB at levels

Pyrite formation has been investigated at 70°C and pH 6–8 by aging precipitated, disordered mackinawite, Fe9S8, and greigite, Fe3S4, in solutions containing aqueous H2S, HS−, Sx2−, S2O32−, SO32−, colloidal elemental sulfur, and the organic sulfur species thiol, disulfide, and sulfonate. Pyrite formed in all experiments where unoxidized iron monosulfides were aged with species containing zero-valent sulfur, i.e., polysulfides and colloidal elemental sulfur, but not with hydrogen sulfide (or bisulfide), the sulfoxy anions, or the organic sulfur species. Pyrite formation also occurred in experiments where the starting monosulfides were air-exposed prior to aging in sulfide solutions, or when air was bubbled through a reaction vessel containing iron monosulfides suspended in a sulfid sulfide solution. The experiments indicate the rate of conversion from iron monosulfides to pyrite is not only a function of solution chemistry (i.e., pH and aqueous speciation), but also depends on the surface oxidation state of the precursor iron monosulfides. Measurements of δ34S of reactants and products from pyrite-forming experiments suggest that the conversion from iron monosulfides to pyrite may proceed via loss of ferrous iron from, rather than via addition of zero-valent sulfur to, the precursor monosulfides. The sulfur isotopic composition of pyrite in sedimentary environments should reflect the sulfur isotopic composition of the precursor iron monosulfide plus sulfur sources incorporated during surface-controlled growth processes. Pyrite forms produced in this study ranged from poorly developed octahedral grains, in experiments where initial pyritization rates were the slowest, to framboidal aggregates in experiments where initial pyritization rates were the fastest. Although greigite formation occurred in experiments that produced framboids, not all experiments that produced greigite led to framboid formation. The formation of pyrite with framboidal texture is apparently favored when iron monosulfides rapidly convert to pyrite.