Modeling the influence of decomposing organic solids on sulfate reduction rates for iron precipitation

The influence of decomposing organic solids on sulfate (S04(2-)) reduction rates for metals precipitation in sulfate-reducing systems, such as in bioreactors and permeable reactive barriers for treatment of acid mine drainage, is modeled. The results are evaluated by comparing the model simulations with published experimental data for two single-substrate and two multiple-substrate batch equilibrium experiments. The comparisons are based on the temporal trends in SO4(2-), ferrous iron (Fe2+), and hydrogen sulfide (H2S) concentrations, as well as on rates of sulfate reduction. The temporal behaviors of organic solid materials, dissolved organic substrates, and different bacterial populations also are simulated. The simulated results using Contois kinetics for polysaccharide decomposition, Monod kinetics for lactate-based sulfate reduction, instantaneous or kinetically controlled precipitation of ferrous iron mono-sulfide (FeS), and partial volatilization of H2S to the gas phase compare favorably with the experimental data. When Contois kinetics of polysaccharide decomposition is replaced by first-order kinetics to simulate one of the single-substrate batch experiments, a comparatively poorer approximation of the rates of sulfate reduction is obtained. The effect of sewage sludge in boosting the short-term rate of sulfate reduction in one of the multiple-substrate experiments also is approximated reasonably well. The results illustrate the importance of the type of kinetics used to describe the decomposition of organic solids on metals precipitation in sulfate-reducing systems as well as the potential application of the model as a predictive tool for assisting in the design of similar biochemical systems.     

34S/32S fractionation during sulfate reduction in groundwater treatment systems: reactive transport modeling

Isotope ratio measurements provide a tool for indicating the relative significance of biogeochemical reactions and for constraining estimates of the extent and rate of reactions in passive treatment systems. In this paper, the reactive transport model MIN3P is used to evaluate sulfur isotope fractionation in column experiments designed to simulate treatment of contaminated water by microbially mediated sulfate reduction occurring within organic carbon-based and iron and carbon-based permeable reactive barriers. A mass dependent fractionation model was used to determine reaction rates for 32S and 34S compounds during reduction, precipitation, and dissolution reactions and to track isotope-dependent mass transfer during SO4 removal. The δ34S values obtained from the MIN3P model were similar to those obtained from the Rayleigh equation, indicating that there was not a significant difference between the conceptual models. Differences between the MIN3P derived α value and the Rayleigh equation derived value were attributed to minor changes in the dissolution and precipitation rate of gypsum and mathematical differences in the fitting models. The results indicated that the prediction of δ34S was fairly insensitive to differences in the fractionation factor at the concentration ranges measured in the current study. However, more significant differences would be expected at low sulfate conditions.     

Processes at the sediment water interface after addition of organic matter and lime to an Acid Mine Pit Lake mesocosm

A strategy to neutralize acidic pit lakes was tested in a field mesocosm of 4500 m(3) volume in the Acidic Pit Mine Lake 111 in Germany. Carbokalk, a byproduct from sugar production, and wheat straw was applied near to the sediment surface to stimulate in lake microbial alkalinity generation by sulfate and iron reduction. The biogeochemical processes at the sediment-water interface were studied over 3 years by geochemical monitoring and an in situ microprofiler. Substrate addition generated a reactive zone at the sediment surface where sulfate and iron reduction proceeded. Gross sulfate reduction reached values up to 10 mmol m(-2) d(-1). The neutralization rates between 27 and 0 meq m(-2) d(-1) were considerably lower than in previous laboratory experiments. The precipitation of ferric iron minerals resulted in a growing acidic sediment layer on top of the neutral sediment. In this layer sulfate reduction was observed but iron sulfides could not precipitate. In the anoxic sediment H2S was oxidized by ferric iron minerals. H2S partly diffused to the water column where it was oxidized. As a result the net formation of iron sulfides decreased after 1 year although gross sulfate reduction rates continued to be high. The rate of iron reduction exceeded the sulfate reduction rate, which resulted in high fluxes of ferrous iron out of the sediment.     

Uranium(VI) reduction by iron(II) monosulfide mackinawite

Reaction of aqueous uranium(VI) with iron(II) monosulfide mackinawite in an O(2) and CO(2) free model system was studied by batch uptake measurements, equilibrium modeling, and L(III) edge U X-ray absorption spectroscopy (XAS). Batch uptake measurements showed that U(VI) removal was almost complete over the wide pH range between 5 and 11 at the initial U(VI) concentration of 5 × 10(-5) M. Extraction by a carbonate/bicarbonate solution indicated that most of the U(VI) removed from solution was reduced to nonextractable U(IV). Equilibrium modeling using Visual MINTEQ suggested that U was in equilibrium with uraninite under the experimental conditions. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy showed that the U(IV) phase associated with mackinawite was uraninite. Oxidation experiments with dissolved O(2) were performed by injecting air into the sealed reaction bottles containing mackinawite samples reacted with U(VI). Dissolved U measurement and XAS confirmed that the uraninite formed from the U(VI) reduction by mackinawite did not oxidize or dissolve under the experimental conditions. This study shows that redox reactions between U(VI) and mackinawite may occur to a significant extent, implying an important role of the ferrous sulfide mineral in the redox cycling of U under sulfate reducing conditions. This study also shows that the presence of mackinawite protects uraninite from oxidation by dissolved O(2). The findings of this study suggest that uraninite formation by abiotic reduction by the iron sulfide mineral under low temperature conditions is an important process in the redistribution and sequestration of U in the subsurface environments at U contaminated sites.     

Reactivity of Fe(II)-bearing minerals toward reductive transformation of organic contaminants

Fe(II) present at surfaces of iron-containing minerals can play a significant role in the overall attenuation of reducible contaminants in the subsurface. As the chemical environment, i.e., the type and arrangement of ligands, strongly affects the redox potential of Fe(II), the presence of various mineral sorbents is expected to modulate the reactivity of surficial Fe(II)-species in aqueous systems. In a comparative study we evaluated the reactivity of ferrous iron in aqueous suspensions of siderite (FeCO3), nontronite (ferruginous smectite SWa-1), hematite (alpha-Fe2O3), lepidocrocite (gamma-FeOOH), goethite (alpha-FeOOH), magnetite (Fe3O4), sulfate green rust (Fe(II)4Fe(III)2(OH)12SO4 x 4H2O), pyrite (FeS2), and mackinawite (FeS) under similar conditions (pH 7.2, 25 m2 mineral/L, 1 mM Fe(II)aq, O2 (aq) < 0.1 g/L). Surface-area-normalized pseudo first-order rate constants are reported for the reduction of hexachloroethane and 4-chloronitrobenzene representing two classes of environmentally relevant transformation reactions of pollutants, i.e., dehalogenation and nitroaryl reduction. The reactivities of the different Fe(II) mineral systems varied greatly and systematically both within and between the two data sets obtained with the two probe compounds. As a general trend, surface-area-normalized reaction rates increased in the order Fe(II) + siderite < Fe(II) + iron oxides < Fe(II) + iron sulfides. 4-Chloronitrobenzene was transformed by mineral-bound Fe(II) much more rapidly than hexachloroethane, except for suspensions of hematite, pyrite, and nontronite. The results demonstrate that abiotic reactions with surface-bound Fe(II) may affect or even dominate the long-term behavior of reducible pollutants in the subsurface, particularly in the presence of Fe(III) bearing minerals. As such reactions can be dominated by specific interactions of the oxidant with the surface, care must be taken in extrapolating reactivity data of surface-bound Fe(II) between different compound classes.     

Experimentally altered groundwater inflow remobilizes acidity from sediments of an iron rich and acidic lake

To study the impact of changes in groundwater flow and chemistry on acidity export from sediments in acid mine drainage (AMD) polluted lakes, a column experiment was carried out. Schwertmannite rich sediment was subjected to three different flow rates (0, 5, and 20 L m(-2) a(-1)), two percolate chemistries (1/1 mmol L(-1) vs 10/15 mmol L(-1) sulfate/ferrous iron, pH 5), and DOC input (approximately 2.5 mmol C L(-1)). Percolation induced acidity export in all percolated treatments (8.8-40.4 mol m(-2) a(-1)) by accelerated proton generation from schwertmannite transformation (18.0-35.9 mol m(-2) a(-1)) and ferrous iron release (3.8-11.6 mol m(-2) a(-1)) from the sediment matrix. Mobilization increased with flow rate and decreased with sulfate and iron concentrations. Unspecifically bound ferrous iron contents increased within the sediment (up to 40.5 mol m(-2) a(-1)) when iron concentrations in the percolate were high. Reduced sulfur species formed following raises in pH, but acidity consumption through this process (0.3-6.6 mol m(-2) a(-1)) and the formation of carbonates (0.11-0.45 mol m(-2) a(-1)) remained small. The study thus suggests that increases in groundwater inflow remobilize acidity from AMD polluted sediments.     

Trichloroethylene removal from groundwater in flow-through columns simulating a permeable reactive barrier constructed with plant mulch

Groundwater contaminated with TCE is commonly treated with a permeable reactive barrier (PRB) constructed with zero-valence iron. The cost of iron has driven a search for less costly alternatives, and composted plant mulch has been used as an alternative at several sites. A column study was conducted that simulated conditions in a PRB at Altus Air Force Base, Oklahoma. The reactive matrix was 50% (v/v) shredded tree mulch, 10% cotton gin trash, and 40% sand. The mean residence time of groundwater in the columns was 17 days. The estimated retardation factor for TCE was 12. TCE was supplied at concentrations near 20 microM. Over 793 days of operation, concentrations of TCE in the column effluents varied from 0.1% to 2% of the column influents. Concentrations of cis-DCE, vinyl chloride, ethylene, ethane, and acetylene could account for 1% of the TCE that was removed; however, up to 56% of 13C added as [1,2-13C] TCE in the column influents was recovered as 13C in carbon dioxide. After 383 and 793 d of operation, approximately one-half of the TCE removal was associated with abiotic reactions with FeS that accumulated in the reactive matrix.     

Microcosm studies for neutralization of hypolimnic acid mine pit lake water (pH 2.6)

Ten microcosms of 0.088 m3 water volume (0.3 m i.d. and 1.20 m height) were designed for neutralization studies representing hypolimnic ecosystem models for acid mine pit lakes. Sediment and water were collected from an acid lignite mine pit lake (Brandenburg, Germany) and filled into the microcosms. To determine the efficacy of controlled in situ organic carbon amendments as a possible neutralization method, sediment and water were treated with ethanol and Carbokalk with and without wheat straw. The water chemistry was monitored for 1 yr. At start-up and end of the experiments, the sedimentwas characterized. Iron and sulfate were removed with varying intensity from the water phase as a result of microbial iron and sulfate reduction together with a subsequent precipitation of unsoluble sulfide minerals to the sediment. The pH rose, and alkalinity generation and bacterial growth were observed. Neutralization rates were calculated using equivalents of accumulated total reduced inorganic sulfur together with the nonsulfidic reactive ferrous iron in the sediment. In the treated microcosms, the neutralization rates were between 6 and 15 equiv m(-2) a(-1). Carbokalk was most effective in stimulating growth of sulfate-reducing bacteria and probably also served as inoculum. With Carbokalk together with wheat straw, the pH increased from 2.6 to around 6.5 within the whole microcosm. The critical revision of the results indicates that the application of Carbokalk (approximately 3.9 kg m(-2)) together with the application of wheat straw (approximately 9.3 kg m(-2)) is most suitable for further experiments in outdoor enclosures (mesocosms). For that case, the prediction of the water quality for a lake water column after multiple lake turnover events is presented based on batch reaction simulation using the geochemical model PHREEQC.     

Microbial and nutrient investigations into the use of in situ layers for treatment of tailings effluent

The release of acidic drainage, containing high concentrations of dissolved metals, is associated with mining districts throughout the world. Remediation of acidic drainage at active and abandoned mines remains a significant challenge. A potential alternative technique to prevent the release of acidic drainage is the addition of labile organic carbon to mine wastes during deposition, creating large in situ treatment systems. Organic carbon can enhance bacterially mediated sulfate reduction and subsequent metal sulfide precipitation, treating metal-contaminated water prior to discharge from the impoundment. Two laboratory column experiments were conducted using simulated mine drainage water. The columns contained tailings derived from the Kidd Creek Metallurgical site in Timmins, Ontario, and reactive materials mixed to a 4:1 volumetric ratio. The average sulfate reduction rate observed in the woodchip column was 0.009 mmol L(-1) day(-1) g(-1) organic matter and in the pulp waste column 0.018 mmol L(-1) day(-1) g(-1) organic matter. Residence times were 14 days in the woodchip column, resulting in the average removal of 500 mg L(-1) (5.2 mmol L(-1)) SO4 and 60 mg L(-1) (1.1 mmol L(-1)) Fe, and 13 days in the pulp waste column, resulting in the average removal of 600 mg L(-1) (6.2 mmol L(-1)) SO4 and the complete removal of 100 mg L(-1) (1.8 mmol L(-1)) Fe. In both columns, sulfate reduction was coupled with an increase in alkalinity and pH and the complete removal of 80 mg L(-1) (1.2 mmol L(-1)) Zn and other metals. Populations of sulfate-reducing bacteria within both columns increased by 3-4 orders of magnitude, and bacterial activity was up to 5 times greater than in the unamended tailings. The woodchip material contained lower concentrations of labile C, N, and P than the pulp waste, possibly accounting for the lower sulfate reduction rates and metal removal capacity observed.     

Multicomponent reactive transport in an in situ zero-valent iron cell

Data collected from a field study of in situ zero-valent iron treatment for TCE were analyzed in the context of coupled transport and reaction processes. The focus of this analysis was to understand the behavior of chemical components, including contaminants, in groundwater transported through the iron cell of a pilot-scale funnel and gate treatment system. A multicomponent reactive transport simulator was used to simultaneously model mobile and nonmobile components undergoing equilibrium and kinetic reactions including TCE degradation, parallel iron dissolution reactions, precipitation of secondary minerals, and complexation reactions. The resulting mechanistic model of coupled processes reproduced solution chemistry behavior observed in the iron cell with a minimum of calibration. These observations included the destruction of TCE and cis-1,2-DCE; increases in pH and hydrocarbons; and decreases in EH, alkalinity, dissolved O2 and CO2, and major ions (i.e., Ca, Mg, Cl, sulfate, nitrate). Mineral precipitation in the iron zone was critical to correctly predicting these behaviors. The dominant precipitation products were ferrous hydroxide, siderite, aragonite, brucite, and iron sulfide. In the first few centimeters of the reactive iron cell, these precipitation products are predicted to account for a 3% increase in mineral volume per year, which could have implications for the longevity of favorable barrier hydraulics and reactivity. The inclusion of transport was key to understanding the interplay between rates of transport and rates of reaction in the field.     

Effect of precipitation on low frequency electrical properties of zerovalent iron columns

We conducted column studies to investigate the application of a noninvasive electrical method to monitor precipitation in Fe0 columns using (a) Na2SO4 (0.01 M, dissolved oxygen (DO) = 8.8 ppm), and (b) Na2CO3 (0.01 M, DO = 2.3 ppm) solutions. An increase in complex conductivity terms (maximum 40% in sulfate column and 23% in carbonate column) occurred over 25 days. Scanning electron microscopy (SEM) identified mineral surface alteration, with greater changes in the high DO sulfate column relative to the low DO carbonate column. X-ray diffractometry (XRD) identified reduced amounts of hematite/maghemite in both columns, precipitation of goethite/akaganeite in the sulfate column, and precipitation of siderite in the carbonate column. Nitrogen adsorption measurements showed increases in specific surface area of iron minerals (27.5% for sulfate column and 8.2% for carbonate column). As variations in electrolytic conductivity and porosity were minimal, electrical changes are attributed to (1) higher complex interfacial conductivity due to increased surface area and mineralogical alteration and (2) increased electronic conduction due to enhanced electron transfer across the iron-fluid interface. Our results show that electrical measurements are a proxy indicator of Fe0 surface alteration.     

In situ chemical reduction of aquifer sediments: enhancement of reactive iron phases and TCE dechlorination

In situ chemical reduction of aquifer sediments is currently being used for chromate and TCE remediation by forming a permeable reactive barrier. The chemical and physical processes that occur during abiotic reduction of natural sediments during flow by sodium dithionite were investigated. In different aquifer sediments, 10-22% of amorphous and crystalline FeIII-oxides were dissolved/reduced, which produced primarily adsorbed FeII, and some siderite. Sediment oxidation showed predominantly one FeII phase, with a second phase being oxidized more slowly. The sediment reduction rate (3.3 h batch half-life) was chemically controlled (58 kJ mol(-1)), with some additional diffusion control during reduction in sediment columns (8.0 h half-life). It was necessary to maintain neutral to high pH to maintain reduction efficiency and prevent iron mobilization, as reduction generated H+. Sequential extractions on reduced sediment showed that adsorbed ferrous iron controlled TCE reactivity. The mass and rate of field-scale reduction of aquifer sediments were generally predicted with laboratory data using a single reduction reaction.     

Selenite reduction by mackinawite, magnetite and siderite: XAS characterization of nanosized redox products

Suboxic soils and sediments often contain the Fe(II)-bearing minerals mackinawite (FeS), siderite (FeCO3) or magnetite (FesO4), which should be able to reduce aqueous selenite, thereby forming solids of low solubility. While the reduction of selenate or selenite to Se(O) by green rust, pyrite and by Fe2+ sorbed to montmorillonite is a slow (weeks), kinetically limited redox reaction as demonstrated earlier, we show here that selenite is rapidly reduced within one day by nanoparticulate mackinawite and magnetite, while only one third of selenite is reduced by micrometer-sized siderite. Depending on Fe(II)-bearing phase and pH, we observed four different reaction products, red and gray elemental Se, and two iron selenides with structures similar to Fe7Se8 and FeSe. The thermodynamically most stable iron selenide, ferroselite (FeSe2), was not observed. The local structures of the reaction products suggest formation of nanoscale clusters, which may be prone to colloid-facilitated transport, and may have a higher than expected solubility.     

In situ metal precipitation in a zinc-contaminated, aerobic sandy aquifer by means of biological sulfate reduction

The applicability of in situ metal precipitation (ISMP) based on bacterial sulfate reduction (BSR) with molasses as carbon source was tested for the immobilization of a zinc plume in an aquifer with highly unsuitable initial conditions (high Eh, low pH, low organic matter content, and low sulfate concentrations), using deep wells for substrate injection. Batch experiments revealed an optimal molasses concentration range of 1-5 g/L and demonstrated the necessity of adding a specific growth medium to the groundwater. Without this growth medium, even sulfate, nitrogen, phosphorus, and potassium addition combined with pH optimization could not trigger biological sulfate reduction. In column experiments, precipitation of ZnS(s) was induced biologically as well as chemically (by adding Na2S). In both systems, zinc concentrations dropped from about 30 mg/L to below 0.02 mg/L. After termination of substrate addition the biological system showed continuation of BSR for at least 2 months, suggesting the insensitivity of the sulfate reducing system for short stagnations of nutrient supply, whereas in the chemical system an immediate increase of Zn concentrations was observed. A pilot experiment conducted in situ at the zinc-contaminated site showed a reduction of zinc concentrations from around 40 mg/L to below 0.01 mg/L. Termination of substrate supply did not result in an immediate stagnation of the BSR process, but continuation of BSR was observed for at least 5 weeks.     

Iron Availability from Soy, Meat and Soy/Meat Samples in Anemic Rats With and Without Prevention of Coprophagy

The biological availability of iron from samples of soy proteins (nontextured, extruded and spun), meat (chicken and beef) and spun soy/meat combination products was compared to ferrous sulfate using a hemoglobin regeneration bioassay. Compared to ferrous sulfate (55%) iron availability from the various soy proteins ranged from 29-57%, for the meat samples 32-39% and for soy/meat combination products 61-92%. There was no significant improvement in iron availability by fortification with ferrous sulfate or ascorbic acid. Prevention of coprophagy in the anemic rats during the bioassay using aluminum anal cups produced varying degrees of reduction in iron availability for various samples and this effect clearly needs further investigation.     

Sulfide species as a sink for mercury in lake sediments

The interaction between two contrasting examples of lake sediments and small concentrations of mercury added to the sediments in solution has been studied using X-ray absorption spectroscopy. Whereas one lake (Esthwaite Water) is biologically productive, with a seasonal cycle of phytoplankton activity, including stratification and Fe(III) reduction, and a mineralogy involving quartz, muscovite, and clinochlore, the other (Botany Pond) remains oxic throughout the year. In the latter case, the sediment is predominantly quartz and calcite. Chemical analyses of these two lake sediments reflectthe differences in mineralogy and showthat both contain significant organic carbon (approximately 10-12 wt %) and smaller amounts of S (approximately 0.2-1.7 wt %) and Cl (approximately 0.4-1.1 wt %). Despite the substantial amounts of organic matter in both sediments, the spectroscopic data show that the mercury occurs as a sulfide phase with a local structural environment akin to that in cinnabar. Parallel spectroscopic studies conducted on Hg either coprecipitated or sorbed onto FeS (mackinawite), and on oxidized mackinawite, provide supporting information; the possibility of Hg forming a chloride was eliminated by careful mapping of the relevant elements by an electron microprobe. It appears, therefore, that the high affinity of Hg for S predominates even in substantially oxic environments.     

Influence of high dietary iron as ferrous carbonate and ferrous sulfate on iron metabolism in young calves

Twelve intact male Holstein calves averaging 90 kg and 12 wk of age were fed one of three dietary treatments for 28 d. The diets were A) control, B) control plus 1000 ppm iron as ferrous carbonate, and C) control plus 1000 ppm iron as ferrous sulfate monohydrate. Calves were dosed orally on d 15 of the treatment period with 1 mCi of iron-59. Neither source of added iron had a significant effect on weight gains, feed consumption, hemoglobin, packed cell volume, serum total iron, serum total iron-binding capacity, unbound iron-binding capacity, serum copper, tissue copper, fecal dry matter, or a consistent effect on fecal pH. The ferrous carbonate had no significant effect on stable zinc or stable iron in any tissue studied. Calves fed ferrous sulfate had higher average stable iron in most tissues and significantly more in the small intestine. Tissue zinc was lower in spleen and pancreas of ferrous sulfate-fed calves. Both sources of added iron sharply reduced iron-59 in serum, whole blood, and body tissues. The reduction was substantially greater in calves fed the ferrous sulfate iron. Iron in ferrous sulfate had a higher biological availability than that in the ferrous carbonate; however, bioavailability of the ferrous carbonate iron appeared to be substantial and considerably more than that noted in previous studies in which a different source of ferrous carbonate was used. The maximum safe level of dietary iron is materially influenced by the source of iron with a higher tolerance indicated for ferrous carbonated than ferrous sulfate monohydrate.     

Carbon isotope fractionation in the reductive dehalogenation of carbon tetrachloride at iron (hydr)oxide and iron sulfide minerals

Compound-specific isotope analysis (CSIA) is used increasingly in contaminant hydrology in the attempt to assess the nature as well as the extent of in situ transformation reactions. Potentially, variations of stable isotope ratios along a contaminant plume may be used to quantify in situ degradation. In the present study, the abiotic dehalogenation of CCl4 by Fe(II) present at the surface of different iron minerals has been characterized in terms of the reaction rates and carbon isotopic fractionation (delta13C) of carbon tetrachloride (CCl4) as well as the yields and isotopic signatures of chloroform (CHCl3), one of the main transformation products. The abiotic reductive dehalogenation of CCl4 was associated with substantial carbon isotopic enrichment effects. The observed enrichment factors, e, correlated neither with the surface-normalized reaction rate constants nor with the type of products formed but fell into two distinctly different ranges for the two principal groups of minerals studied. With iron (hydr)oxide minerals (goethite, hematite, lepidocrocite, and magnetite) and with siderite, the e-values for CCl4 dehalogenation were remarkably similar (-29 +/- 3 per thousand). Because this value matches well with the theoretical estimates for the cleavage of an aliphatic C-Cl bond, we suggest that dissociative electron transfer to CCl4 controls the reaction rates for this group of iron minerals. Conversely, CCl4 transformation by different preparations of the iron sulfide mackinawite was accompanied by a significantly lower carbon istotopic fractionation (e = -15.9 +/- 0.3 per thousand), possibly due to the presence of nonfractionating rate-determining steps or a significantly different transition state structure of the reaction. Isotopically sensitive branching of the reaction pathways (i.e., the effect of different product distributions on isotope fractionation of CCl4) did not play a significant role in our systems. The extensive data set presented in this study opens new perspectives toward an improved understanding of the factors that determine reaction mechanisms and isotopic fractionation of dehalogenation reactions by Fe(II) at iron containing minerals.     

Reduction of organically complexed ferric iron by superoxide in a simulated natural water

Superoxide (and potentially its conjugate acid hydroperoxyl) is unique among the reactive oxygen species in that its standard redox potential in circumneutral natural waters potentially allows it to reduce ferric iron to the more soluble ferrous state. Here we have observed the superoxide/ hydroperoxyl-mediated reduction of ferric complexes with a variety of synthetic organic ligands and several complexes with natural organic matter (NOM), as well as freshly precipitated amorphous ferric oxyhydroxide, in bicarbonate buffered solutions at pH 8.1. From measurements of superoxide decay in the presence of the complexes, we calculated second-order rate constants for superoxide/ hydroperoxyl-mediated reduction that vary from (9.3+/-0.2) x 10(3) M(-1) s(-1) for the complex between Fe(III) and desferrioxamine B up to (1.9+/-0.2) x 10(5) M(-1) s(-1) for Fe(III)-salicylate and (2.3+/-0.1) x 10(5) M(-1) s(-1) for one of the Fe(III)-NOM complexes. We also verified that ferrous iron was produced from superoxide/hydroperoxyl-mediated Fe(III) reduction using ferrozine to trap free Fe(II). Low yields of the ferrozine complex when compared to the measured rates of superoxide decay suggest that ferric complexes are reduced directlyto corresponding ferrous complexes, with much of the ferrous complex reoxidizing before it is able to release free ferrous iron. This is an important consideration for microorganisms, as the kinetics of trace metal uptake is typically governed by free ion activity.     

Distribution and reactivity of O2-reducing components in sediments from a layered aquifer

The redox status of subsurface aqueous systems is controlled by the reactivity of solid redox-sensitive species and by the inflow of such species dissolved in groundwater. The reactivity toward molecular oxygen (O2) of solid reductants present in three particle size fractions of sediments from a pristine aquifer was characterized during 54 days. The stoichiometric relationships between carbon dioxide (CO2) production and O2 consumption was used in combination with sulfate production to discriminate between the contributions of sedimentary organic matter (0-87%), pyrite (6-100%), and siderite (0-43%) as the dominant reductants. The observed simultaneous oxidation of these reductants indicates that they are reactive on the same time scales. The measured reduction capacity 18-84 micromol O2/g) ranged from 8 to 42% of the total reduction capacity present as pyrite and organic carbon in the total sediment fraction (<2 mm). Fine fractions (<63 microm) were 10-250 times more reactive than their corresponding total fractions. Oxygen consumption rates decreased continuously during carbonate buffered conditions, due to a decreasing reactivity of reductants. Acidification accelerated pyrite oxidation but impeded SOM respiration. Our findings indicate that the geological history of aquifer sediments affects the amounts of organic matter, pyrite and siderite present, while environmental conditions, such as pH and microbial activity, are important in controlling the reactivity of these reductants. These controls should be considered when assessing the natural reduction activity of aquifer sediments in either natural or polluted systems.