Superoxide-mediated Fe(II) formation from organically complexed Fe(III) in coastal waters

Fe(III) complexed by organic ligands (Fe(III)L) is the primary form of dissolved Fe in marine and coastal environments. Superoxide, typically produced in biological and photochemical processes, is one of the reducing agents that contributes to transformation of Fe(III)L to bioavailable, free dissolved Fe(II) (Fe(II)′). In this work, the kinetics of superoxide-mediated Fe(II)′ formation from Fe(III)L in a simulated coastal water system were investigated and a comprehensive kinetic model was developed using citrate and fulvic acid as exemplar Fe-binding ligands. To simulate a coastal environment in laboratory experiments, Fe(III)L samples with various ligand/Fe ratios were incubated for 5 min to 1 week in seawater medium. At each ratio and incubation time, the rate of superoxide-mediated Fe(II)′ formation was determined in the presence of the strong Fe(II) binding ligand ferrozine by spectrophotometrically measuring the ferrous-ferrozine complex generated at a constant concentration of superoxide. The Fe(II)′ formation rate generally decreased with incubation time, as Fe(III)L gradually dissociated to form less reactive Fe(III) oxyhydroxide. However, when the ligand/Fe ratio was sufficiently high, the dissociation of Fe(III)L (and subsequent Fe precipitation) was suppressed and Fe(II)′ was formed at a higher rate. The rate of Fe(II)′ produced during the experiment was explained by the kinetic model. The model confirmed that both the ligand/Fe ratio and incubation time have a significant effect on the pathway via which Fe(II)′ is formed from Fe(III)-fulvic acid complexes.     

Reaction-based modeling of quinone-mediated bacterial iron(III) reduction

This paper presents and validates a new paradigm for modeling complex biogeochemical systems using a diagonalized reaction-based approach. The bioreduction kinetics of hematite (α-Fe₂O₃) by the dissimilatory metal-reducing bacterium (DMRB) Shewanella putrefaciens strain CN32 in the presence of the soluble electron shuttling compound anthraquinone-2,6-disulfonate (AQDS) is used for presentation/validation purposes. Experiments were conducted under nongrowth conditions with H₂ as the electron donor. In the presence of AQDS, both direct biological reduction and indirect chemical reduction of hematite by bioreduced anthrahydroquinone-2,6-disulfonate (AH₂DS) can produce Fe(II). Separate experiments were performed to describe the bioreduction of hematite, bioreduction of AQDS, chemical reduction of hematite by AH₂DS, Fe(II) sorption to hematite, and Fe(II) biosorption to DMRB. The independently determined rate parameters and equilibrium constants were then used to simulate the parallel kinetic reactions of Fe(II) production in the hematite-with-AQDS experiments. Previously determined rate formulations/parameters for the bioreduction of hematite and Fe(II) sorption to hematite were systematically tested by conducting experiments with different initial conditions. As a result, the rate formulation/parameter for hematite bioreduction was not modified, but the rate parameters for Fe(II) sorption to hematite were modified slightly. The hematite bioreduction rate formulation was first-order with respect to hematite ”free“ surface sites and zero-order with respect to DMRB based on experiments conducted with variable concentrations of hematite and DMRB. The AQDS bioreduction rate formulation was first-order with respect to AQDS and first-order with respect to DMRB based on experiments conducted with variable concentrations of AQDS and DMRB. The chemical reduction of hematite by AH₂DS was fast and considered to be an equilibrium reaction. The simulations of hematite-with-AQDS experiments were very sensitive to the equilibrium constant for the hematite-AH₂DS reaction. The model simulated the hematite-with-AQDS experiments well if it was assumed that the ferric oxide “surface” phase was more disordered than pure hematite. This is the first reported study where a diagonalized reaction-based model was used to simulate parallel kinetic reactions based on rate formulations/parameters independently obtained from segregated experiments.     

Microbial reduction of iron(III)-rich nontronite and uranium(VI)

To assess the dynamics of microbially mediated U-clay redox reactions, we examined the reduction of iron(III)-rich nontronite NAu-2 and uranium(VI) by Shewanella oneidensis MR-1. Bioreduction experiments were conducted with combinations and varied concentrations of MR-1, nontronite, U(VI) and the electron shuttle anthraquinone-2,6-disulfonate (AQDS). Abiotic experiments were conducted to quantify U(VI) sorption to NAu-2, the reduction of U(VI) by chemically-reduced nontronite-Fe(II), and the oxidation of uraninite, U^(IV)O_2(s), by nontronite-Fe(III). When we incubated S. oneidensis MR-1 at lower concentration (0.5 × 10⁸ cell mL⁻¹) with nontronite (5.0 g L⁻¹) and U(VI) (1.0 mM), little U(VI) reduction occurred compared to nontronite-free incubations, despite the production of abundant Fe(II). The addition of AQDS to U(VI)- and nontronite-containing incubations enhanced both U(VI) and nontronite-Fe(III) reduction. While U(VI) was completely reduced by S. oneidensis MR-1 at higher concentration (1.0 × 10⁸ cell mL⁻¹) in the presence of nontronite, increasing concentrations of nontronite led to progressively slower rates of U(VI) reduction. U(VI) enhanced nontronite-Fe(III) reduction and uraninite was oxidized by nontronite-Fe(III), demonstrating that U served as an effective electron shuttle from S. oneidensis MR-1 to nontronite-Fe(III). The electron-shuttling activity of U can explain the lack or delay of U(VI) reduction observed in the bulk solution. Little U(VI) reduction was observed in incubations that contained chemically-reduced nontronite-Fe(II), suggesting that biologic U(VI) reduction drove U valence cycling in these systems. Under the conditions used in these experiments, we demonstrate that iron-rich smectite may inhibit or delay U(VI) bioreduction.     

Solubility and dissimilatory reduction kinetics of iron(III) oxyhydroxides: A linear free energy relationship

Rates of reduction of Fe(III) oxyhydroxides by the bacterium Shewanella putrefaciens were measured as a function of the bacterial density and the Fe(III) substrate concentration. The results show that an earlier reported positive correlation between the solubility products (^*K_so) and the maximum cell-specific reduction rates (v_max) of predominantly poorly crystalline Fe(III) oxyhydroxides also applies to insoluble and crystalline Fe(III) oxyhydroxides. The mineral solubilities were measured by a dialysis bag technique under acidic conditions (pH 1 up to 2.5) at 25 °C. Initial iron reduction rates by S. putrefaciens were determined in the presence of excess lactate as electron donor. In all cases, the microbial reduction rate exhibited saturation behavior with respect to the Fe(III) oxyhydroxide concentration. On a double logarithmic scale, the maximum rates v_max and the solubility products defined a single linear free energy relationship (LFER) for all the Fe(III) oxyhydroxides considered. The solubility provided a better predictor of v_max than the specific surface area of the mineral phase. A rate limitation by the electron transfer between an iron reductase and a Fe(III) center, or by the subsequent desorption of Fe²⁺ from the iron oxide mineral surface, are both consistent with the observed LFER.     

Effect of hydrolysed aluminum(III) and chromium(III) cations on the lipophilicity of talc

Liquid-liquid extraction and electrokinetic studies show that both the talc particles and the isooctane droplets are affected by the presence of hydrolysable metal species of Al(III) and Cr(III). The inherent lipophilicity of talc is unchanged by the hydroxo complexes of these metal species, while positive shifts in zeta potentials of talc and isooctane take place due partly to the strong adsorption of these hydroxo complexes. In the pH range where the hydroxide precipitates, the talc particles are rendered hydrophillic between the precipitation pH and the point-of-zero charge of the metal hydroxide. This hydrophile-lipophile transition of talc in the isooctane-water system relates to heterocoagulation among talc particles, isooctane droplets and hydroxide precipitates.     

Densification of iron(III) sludge in neutralization

Acid mine drainage (AMD), of which iron is a substantial component, is a potential by-product in the mining industry. Conventional neutralization is a common approach to treat AMD, although it creates a major disposal problem due to the generation of voluminous sludge. Sludge recirculation improves solid density by slowing down the rate of neutralization and allowing the growth of precipitates, while existing solids act as seed particles by providing necessary surface area for precipitation. The mechanisms of iron sludge densification are not fully understood, mainly because of the complex nature of iron chemistry, and the variety of amorphous, polymeric oxides that could be formed. In this work, the effects of alkaline reagents, flocculant addition, and dosing sequence, on the precipitation of iron (III) hydroxide and densification of the recycled sludge were investigated. Slowly dissolving lime (Ca(OH)₂) was found to be more effective than caustic (NaOH) in producing sludge with higher solid contents. Polymers addition created stronger aggregates that could withstand shearing without significant size reduction, but the overall sludge density was lower than those produced without flocculant. Conditioning the sludge at pH between 3.5 and 4.5 by adding fresh lime in a specific dosing manner appeared to be conducive to the growth of large agglomerates. The final sludge solid content of ∼15 wt.% was considerably higher than others produced under different conditions. The plate-like structures of precipitates generated with more recycles in this instance, possibly helped ease the release of entrapped water between solids during shearing, thus producing sludge with higher solid density.     

Role of Iron(III) and Aluminum Hydroxides in Concentration/reduction of Au(III) Complexes

Abstract: The adsorption of gold on iron(III) and aluminum hydroxides from solutions containing Au(III) complexes has been studied as a function of pH and chloride concentration at 30C. Iron(III) hydroxide was more effective in adsorbing gold from solution than aluminum hydroxide. However, both hydroxides controlled the behavior of Au(III) complex with very similar manner. The most effective gold adsorption occurred in aqueous solution with near neutral pH and low Cl concentration. In this solution condition, Au(III) complexes were mainly dissolved as AuCl₂(OH)₂^- and AuCl(OH)₃^-, and the surface charge for both hydroxides was positive. In addition, the adsorbed Au(III) complexes were spontaneously reduced to elemental gold in spite of the absence of a specific reducing agent.
The results of this study suggest that adsorption and spontaneous reduction of gold complexes on the surface of hydrous metal oxides with positive charge play an important role in gold precipitation in subsurface environment.     

Reactive iron(III) in sediments: Chemical versus microbial extractions

The availability of particulate Fe(III) to iron reducing microbial communities in sediments and soils is generally inferred indirectly by performing chemical extractions. In this study, the bioavailability of mineral-bound Fe(III) in intertidal sediments of a eutrophic estuary is assessed directly by measuring the kinetics and extent of Fe(III) utilization by the iron reducing microorganism Shewanella putrefaciens, in the presence of excess electron donor. Microbial Fe(III) reduction is compared to chemical dissolution of iron from the same sediments in buffered ascorbate-citrate solution (pH 7.5), ascorbic acid (pH 2), and 1 M HCl. The results confirm that ascorbate at near-neutral pH selectively reduces the reactive Fe(III) pool, while the acid extractants mobilize additional Fe(II) and less reactive Fe(III) mineral phases. Furthermore, the maximum concentrations of Fe(III) reducible by S. putrefaciens correlate linearly with the iron concentrations extracted by buffered ascorbate-citrate solution, but not with those of the acid extractions. However, on average, only 65% of the Fe(III) reduced in buffered ascorbate-citrate solution can be utilized by S. putrefaciens, probably due to physical inaccessibility of the remaining fraction of reactive Fe(III) to the cells. While the microbial and abiotic reaction kinetics further indicate that reduction by ascorbate at near-neutral pH most closely resembles microbial reduction of the sediment Fe(III) pool by S. putrefaciens, the results also highlight fundamental differences between chemical reductive dissolution and microbial utilization of mineral-bound ferric iron.     

The effects of pH, ionic strength, and iron-fulvic acid interactions on the kinetics of non-photochemical iron transformations. I. Iron(II) oxidation and iron(III) colloid formation

Flow injection analysis was used to study the effect of a fulvic acid on the kinetics of iron(II) oxidation and iron colloid formation under conditions approximating fresh natural waters. While iron(II) oxidation in high-carbonate inorganic solutions is predicted well by a recently proposed homogeneous model, it overestimates the oxidation rate in low-carbonate solutions, possibly due to the formation of an intermediate iron(II) colloid or surface species. Results in fulvic acid solutions are consistent with the formation of an iron(II)-fulvic acid complex at both pH 6.0 and 8.0 which accelerates the overall oxidation rate relative to inorganic solutions. However, iron(III) complexation by fulvic acid greatly slows the formation of iron colloids, stabilizing dissolved iron(III). Decreased pH and increased ionic strength slow and decrease iron colloid formation. Evidence of a kinetic control on the distribution of iron(III) between organically complexed and colloidal forms is presented.     

Complexing of silica by iron(III) in natural waters

Determination of amorphous silica solubility in acidified ferric nitrate solutions confirms the presence of ferric silicate complexing. A dissociation constant for the reaction:

$\text{FeH}{}_3\text{SiO}{}_4{}^{\mathrm{2+}}\text{→}\text{Fe}{}^{\mathrm{3+}}\text{+}\text{H}{}_3\text{SiO}{}_4{}^{\mathrm{?}}$

of 10^?9.8 ± 0.3 pK units at room temperature (22 ± 3°C) is obtained, in close agreement with reported values at 25°C corrected to zero ionic strength of 10^?9.9 by Weber and Stumm and 10^?9.5 by Olson and O'Melia. Iron-silicate complexing may be of significance to the mobilization of silica in acid waters associated with oxidizing sulphide deposits and coal strip mining and the precipitation of secondary silicate mineral phases.     

Relative contributions of sulfate- and iron(III) reduction to organic matter mineralization and process controls in contrasting habitats of the Georgia saltmarsh

The objectives of this study were to partition out the predominant anaerobic respiration pathways coupled to C oxidation and to further elucidate the controls of anaerobic C respiration in three major saltmarsh habitats at Skidaway Island, GA; the short form of Spartina alterniflora (SS), the tall form of S. alterniflora (TS), and unvegetated, bioturbated creekbank (CB). Geochemical analysis of pore water and solid phase constituents revealed that the SS site experienced highly reducing conditions with two orders of magnitude higher pore water sulfide inventories (1.884 mmol m⁻²) than TS (0.003 mmol m⁻²) and CB (0.005 mmol m⁻²), respectively. Conversely, reactive Fe(III) inventories at TS (2208 mmol m⁻²) and CB (2881 mmol m⁻²) were up to 7–9 times higher than at SS (338 mmol m⁻²). Incubations and intact core experiments indicated that reduction accounted for 95% (SS), 37% (TS) and 66% (CB) of total anaerobic respiration. There was no detectable Fe(III) reduction at SS, while Fe(III) reduction accounted for up to 70% of C oxidation in the 3–6 cm depth interval at TS and 0–3 cm depth of CB, and on average, approximately 55% of C oxidation over two-thirds of marsh surface area. Laboratory manipulations provided further evidence for the importance of Fe(III) reduction as the accumulation rates of fermentation products were high when Fe(III) reduction was inhibited by removing the Fe(III) minerals from highly bioturbated CB sediments with higher Fe(III) mineral contents. Anaerobic C oxidation, - and Fe(III)-reduction rates appeared to be highest at the TS site during active plant growth in summer. Overall results suggest that bioturbation by macrofauna is the overriding factor in modulating the pathway of C mineralization in the saltmarsh, whereas availability of organic substrates from plants is a key factor in controlling the C oxidation rate.     

Control of Fe(III) site occupancy on the rate and extent of microbial reduction of Fe(III) in nontronite

A quantitative study was performed to understand how Fe(III) site occupancy controls Fe(III) bioreduction in nontronite by Shewanella putrefaciens CN32. NAu-1 and NAu-2 were nontronites and contained Fe(III) in different structural sites with 16 and 23% total iron (w/w), respectively, with almost all iron as Fe(III). Mössbauer spectroscopy showed that Fe(III) was present in the octahedral site in NAu-1 (with a small amount of goethite), but in both the tetrahedral and the octahedral sites in NAu-2. Mössbauer data further showed that the octahedral Fe(III) in NAu-2 existed in at least two environments- trans (M1) and cis (M2) sites. The microbial Fe(III) reduction in NAu-1 and NAu-2 was studied in batch cultures at a nontronite concentration of 5 mg/mL in bicarbonate buffer with lactate as the electron donor. The unreduced and bioreduced nontronites were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy, and transmission electron microscopy (TEM). In the presence of an electron shuttle, anthraquinone-2,6-disulfonate (AQDS), the extent of bioreduction was 11%-16% for NAu-1 but 28%-32% for NAu-2. The extent of reduction in the absence of AQDS was only 5%-7% for NAu-1 but 14%-18% for NAu-2. The control experiments with heat killed cells and without cells did not show any appreciable reduction (<2%). The extent of reduction in experiments performed with a dialysis membrane to separate cells from clays (without AQDS) was 2%-3% for NAu-1 but 5%-7% for NAu-2, suggesting that cells probably released an electron shuttling compound and/or Fe(III) chelator. The reduction rate was also faster in NAu-2 than that in NAu-1. Mössbauer data of the bioreduced nontronite materials indicated that the Fe(III) reduction in NAu-1 was mostly from the presence of goethite, whereas the reduction in NAu-2 was due to the presence of the tetrahedral and trans-octahedral Fe(III) in the structure. The measured aqueous Fe(II) was negligible. As a result of bioreduction, the average nontronite particle thickness remained nearly the same (from 2.1 to 2.5 nm) for NAu-1, but decreased significantly from 6 to 3.5 nm for NAu-2 with a concomitant change in crystal size distribution. The decrease in crystal size suggests reductive dissolution of nontronite NAu-2, which was supported by aqueous solution chemistry (i.e., aqueous Si). These data suggest that the more extensive Fe(III) bioreduction in NAu-2 was due to the presence of the tetrahedral and the trans-octahedral Fe(III), which was presumed to be more reducible. The biogenic Fe(II) was not associated with biogenic solids or in the aqueous solution. We infer that it may be either adsorbed onto surfaces of nontronite particles/bacteria or in the structure of nontronite. Furthermore, we have demonstrated that natural nontronite clays were capable of supporting cell growth even in medium without added nutrients, possibly due to presence of naturally existing nutrients in the nontronite clays. These results suggest that crystal chemical environment of Fe(III) is an important determinant in controlling the rate and extent of microbial reduction of Fe(III) in nontronite.     

Biogenic uraninite precipitation and its reoxidation by iron(III) (hydr)oxides: A reaction modeling approach

One option for immobilizing uranium present in subsurface contaminated groundwater is in situ bioremediation, whereby dissimilatory metal-reducing bacteria and/or sulfate-reducing bacteria are stimulated to catalyze the reduction of soluble U(VI) and precipitate it as uraninite (UO₂). This is typically accomplished by amending groundwater with an organic electron donor. It has been shown, however, that once the electron donor is entirely consumed, Fe(III) (hydr)oxides can reoxidize biogenically produced UO₂, thus potentially impeding cleanup efforts. On the basis of published experiments showing that such reoxidation takes place even under highly reducing conditions (e.g., sulfate-reducing conditions), thermodynamic and kinetic constraints affecting this reoxidation are examined using multicomponent biogeochemical simulations, with particular focus on the role of sulfide and Fe(II) in solution. The solubility of UO₂ and Fe(III) (hydr)oxides are presented, and the effect of nanoscale particle size on stability is discussed. Thermodynamically, sulfide is preferentially oxidized by Fe(III) (hydr)oxides, compared to biogenic UO₂, and for this reason the relative rates of sulfide and UO₂ oxidation play a key role on whether or not UO₂ reoxidizes. The amount of Fe(II) in solution is another important factor, with the precipitation of Fe(II) minerals lowering the Fe⁺² activity in solution and increasing the potential for both sulfide and UO₂ reoxidation. The greater (and unintuitive) UO₂ reoxidation by hematite compared to ferrihydrite previously reported in some experiments can be explained by the exhaustion of this mineral from reaction with sulfide. Simulations also confirm previous studies suggesting that carbonate produced by the degradation of organic electron donors used for bioreduction may significantly increase the potential for UO₂ reoxidation through formation of uranyl carbonate aqueous complexes.     

Reduction of crystalline iron(III) oxyhydroxides using hydroquinone: Influence of phase and particle size

Iron oxides and oxyhydroxides are common and important materials in the environment, and they strongly impact the biogeochemical cycle of iron and other species at the Earth's surface. These materials commonly occur as nanoparticles in the 3–10 nm size range. This paper presents quantitative results demonstrating that iron oxide reactivity is particle size dependent. The rate and extent of the reductive dissolution of iron oxyhydroxide nanoparticles by hydroquinone in batch experiments were measured as a function of particle identity, particle loading, and hydroquinone concentration. Rates were normalized to surface areas determined by both transmission electron microscopy and Braunauer-Emmett-Teller surface. Results show that surface-area-normalized rates of reductive dissolution are fastest (by as much as 100 times) in experiments using six-line ferrihydrite versus goethite. Furthermore, the surface-area-normalized rates for 4 nm ferrihydrite nanoparticles are up to 20 times faster than the rates for 6 nm ferrihydrite nanoparticles, and the surface-area-normalized rates for 5 × 64 nm goethite nanoparticles are up to two times faster than the rates for 22 × 367 nm goethite nanoparticles.     

Biotite dissolution and Cr(VI) reduction at elevated pH and ionic strength

The effects of elevated pH, ionic strength, and temperature on sediments in the vadose zone are of primary importance in modeling contaminant transport and understanding the environmental impact of tank leakage at nuclear waste storage facilities like those of the Hanford site. This study was designed to investigate biotite dissolution under simulated high level waste (HLW) conditions and its impact on Cr(VI) reduction and immobilization. Biotite dissolution increased with NaOH concentrations in the range of 0.1 to 2 mol L^-1. There was a corresponding release of K, Fe, Si, and Al to solution, with Si and Al showing a complex pattern due to the formation of secondary zeolite minerals. Dissolved Fe concentrations were an order of magnitude lower than the other elements, possibly due to the formation of green rust and Fe(OH)₂. The reduction of Cr(VI) to Cr(III) also increased with increased NaOH concentration. A homogeneous reduction of chromate by Fe(II)_aq released through biotite dissolution was probably the primary pathway responsible for this reaction. Greater ionic strengths increased biotite dissolution and consequently increased Fe(II)_aq release and Cr(VI) removal. The results indicated that HLW would cause phyllosilicate dissolution and the formation of secondary precipitates that would have a major impact on radionuclide and contaminant transport in the vadose zone at the Hanford site.     

Iron(III) reduction and phosphorous solubilization in humid tropical forest soils

Phosphorus is one of the nutrients most commonly limiting net primary production in soils of humid tropical forests, mainly because insoluble Al and Fe phosphates and strong sorption to Fe(III) (hydr)oxides remove P from the bioavailable pool. Recent field studies have suggested, however, that this loss may be balanced by organic P accumulation under a wet moisture regime (>3350 mm annual precipitation). It has been hypothesized that, as the moisture regime changes from dry to mesic to wet, periods of anoxic soil conditions increase in intensity and duration, depleting Fe(III) (hydr)oxides and releasing sorbed P, but also slowing organic matter turnover, thus shifting the repository of soil P from minerals to humus. Almost no quantitative information is available concerning the coupled biogeochemical behavior of Fe and P in highly weathered forest soils that would allow examination of this hypothesis. In this paper, we report a laboratory incubation study of the effects of biotic Fe(III) (hydr)oxide reduction on P solubilization in a humid tropical forest soil (Ultisol) under a wet moisture regime (3000-4000 mm annual rainfall). The objectives of our study were: (1) to quantify Fe(III) reduction and P solubilization processes in a highly weathered forest soil expected to typify the hypothesized mineral dissolution-organic matter accumulation balance; (2) to examine the influence of electron shuttling on these processes using anthraquinone-2,6-disulfonate (AQDS), a well-known surrogate for the semiquinone electron shuttles in humic substances, as an experimental probe; and (3) to characterize the chemical forms of Fe(II) and P produced under anoxic conditions, both with and without AQDS. Two series of short-term incubation experiments were carried out, one without AQDS and another with an initial AQDS concentration of 150 μM. We measured pH, pE, and the production of Fe(II), total Fe [Fe(II) + Fe(III)], inorganic P, total P (inorganic P + organic P), and biogenic gases (CO₂, H₂ and CH₄). The same positive correlation was found between soluble P release and soluble Fe(II) production throughout incubation, implying that reduction of Fe(III) solubilized P. The Fe(II) produced was mainly particulate, evidently due to the formation of Fe(II) solid phases. Thermodynamic calculations indicated that precipitation of siderite and, in the presence of AQDS, vivianite was favored under the anoxic conditions that developed rapidly in the soil suspensions. Inorganic soluble P released during incubation was very small, indicating that the soluble P produced was mainly in organic form, which is consistent with the hypothesis that P accumulates in soil humus. Our net CO₂ production, H₂ consumption, and Fe(II) production data all suggested that reductive dissolution of Fe(III) (hydr)oxides was a terminal electron-accepting process coupled both to H₂ consumption and organic C oxidation by the native population of microorganisms in the soil. Addition of AQDS accelerated the production of Fe(II) and the release of soluble P, while hastening the decline in H₂ gas levels and suppressing CH₄ production. However, throughout incubation, the same quantitative relationships between soluble Fe(II) and P, and between pE and pH, were found, irrespective of AQDS addition. Thus we conclude that, in our soil incubation experiments, added AQDS functioned with the native microbial population solely as an electron shuttle catalyzing Fe(III) reduction. Whether humic substances in the soil also can act as electron shuttles in this way is a matter for future investigation.     

Electronic structures of iron(III) and manganese(IV) (hydr)oxide minerals: Thermodynamics of photochemical reductive dissolution in aquatic environments

Sunlight-induced reduction and dissolution of colloidal Fe-Mn (hydr)oxide minerals yields elevated concentrations of Fe²⁺ and Mn²⁺ in natural waters. Since these elements may be biolimiting micronutrients, photochemical reactions might play a significant role in biogeochemical cycles. Reductive photodissolution of Fe (hydr)oxide minerals may also release sorbed metals. The reactivity of Fe-Mn (hydr)oxide minerals to sunlight-induced photochemical dissolution is determined by the electronic structure of the mineral-water interface. In this work, oxygen K-edge absorption and emission spectra were used to determine the electronic structures of iron(III) (hydr)oxides (hematite, goethite, lepidocrocite, akaganeite and schwertmannite) and manganese(IV) oxides (pyrolusite, birnessite, cryptomelane). The band gaps in the iron(III) (hydr)oxide minerals are near 2.0-2.5 eV; the band gaps in the manganese (IV) oxide phases are 1.0-1.8 eV. Using published values for the electrochemical flat-band potential for hematite together with experimental pH_pzc values for the (hydr)oxides, it is possible to predict the electrochemical potentials of the conduction and valence bands in aqueous solutions as a function of pH. The band potentials enable semiquantitative predictions of the susceptibilities of these minerals to photochemical dissolution in aqueous solutions. At pH 2 (e.g., acid-mine waters), photoreduction of iron(III) (hydr)oxides could yield millimolal concentrations of aqueous Fe²⁺ (assuming surface detachment of Fe²⁺ is not rate limiting). In seawater (pH 8.3), however, the direct photo-reduction of colloidal iron(III) (hydr)oxides to give nanomolal concentrations of dissolved, uncomplexed, Fe²⁺ is not thermodynamically feasible. This supports the hypothesis that the apparent photodissolution of iron(III) (hydr)oxides in marines systems results from Fe³⁺ reduction by photochemically produced superoxide. In contrast, the direct photoreduction of manganese oxides should be energetically feasible at pH 2 and 8.3.     

Contrasting effects of Al substitution on microbial reduction of Fe(III) (hydr)oxides

Aluminum, one of the most abundant elements in soils and sediments, is commonly found co-precipitated with Fe in natural Fe(III) (hydr)oxides; yet, little is known about how Al substitution impacts bacterial Fe(III) reduction. Accordingly, we investigated the reduction of Al substituted (0-13 mol% Al) goethite, lepidocrocite, and ferrihydrite by the model dissimilatory Fe(III)-reducing bacterium (DIRB), Shewanella putrefaciens CN32. Here we reveal that the impact of Al on microbial reduction varies with Fe(III) (hydr)oxide type. No significant difference in Fe(III) reduction was observed for either goethite or lepidocrocite as a function of Al substitution. In contrast, Fe(III) reduction rates significantly decreased with increasing Al substitution of ferrihydrite, with reduction rates of 13% Al-ferrihydrite more than 50% lower than pure ferrihydrite. Although Al substitution changed the minerals’ surface area, particle size, structural disorder, and abiotic dissolution rates, we did not observe a direct correlation between any of these physiochemical properties and the trends in bacterial Fe(III) reduction. Based on projected Al-dependent Fe(III) reduction rates, reduction rates of ferrihydrite fall below those of lepidocrocite and goethite at substitution levels equal to or greater than 18 mol% Al. Given the prevalence of Al substitution in natural Fe(III) (hydr)oxides, our results bring into question the conventional assumptions about Fe (hydr)oxide bioavailability and suggest a more prominent role of natural lepidocrocite and goethite phases in impacting DIRB activity in soils and sediments.     

Reduction of iron(III) minerals by natural organic matter in groundwater

Construction of the entrance tunnel to the Äspö Hard Rock Laboratory, a prototype repository in Sweden for research into the geological disposal of spent nuclear fuel, has resulted in increased transport of organic carbon from the surface into the groundwater. This increased input of organic matter has induced accelerated oxidation of organic carbon associated with reduction of iron(III) minerals as the terminal electron acceptor in microbial respiration. Hydrochemical modeling of major solute ions at the site indicates an apparent first-order decay constant for organic carbon of 3.7 ± 2.6/yr. This rapid turnover is not accompanied by an equivalent mobilization of ferrous iron. Thermodynamic calculation of iron mineral solubility suggests that ferrous clay minerals may form in hydraulically transmissive fractures. The conditional potentials for the oxidation–reduction of such phases coincide with measured redox potentials at the site. The calculated potential is sufficiently low so that such phases would provide reducing capacity against future intrusion of O₂ into the groundwater, thus buffering a repository against oxic corrosion of the engineered barriers.