Heterogeneous oxidation of Fe(II) on ferric oxide at neutral pH and a low partial pressure of O2

The objective of this study was to identify the rate and mechanism of abiotic oxidation of ferrous iron at the water-ferric oxide interface (heterogeneous oxidation) at neutral pH. Oxidation was conducted at a low partial pressure of O2 to slow the reactions and to represent very low dissolved oxygen (DO) conditions that can occur at oxic/anoxic fronts. Hydrous ferric oxide (HFO) was partially converted to goethite after 24 h of anoxic contact with Fe(II), consistent with previous results. This resulted in a significant decrease in sorption of Fe(II). No conversion to goethite was observed after 25 min of anoxic contact between HFO and Fe(II). O2 was then introduced into the chamber and sparged (transfer half-time of 1.6 min) into the previously anoxic suspension, and the rate of oxidation of Fe(II) and the distribution between sorbed and dissolved Fe(II) were measured with time. The concentration of sorbed Fe(II) remained steady during each experiment, despite removal of all measurable dissolved Fe(II) in some experiments. The rate of oxidation of Fe(II) was proportional to the concentration of DO and both sorbed and dissolved Fe(II) up to a surface density of 0.02 mol Fe(II) per mol Fe(III), i.e., approximately 0.2 Fe(II) per nm2 of ferric oxide surface area. This result differs from previous studies of heterogeneous oxidation, which found that the rate was proportional to sorbed Fe(II) and DO but did not find a dependence on dissolved Fe(II). Most previous experiments were autocatalytic; i.e., the initial concentration of ferric oxide was low or none, and sorbed Fe(II) was not measured. The results were consistentwith an anode/cathode mechanism, with O2 reduced at electron-deficient sites with strongly sorbed Fe(II) and Fe(II) oxidized at electron-rich sites without sorbed Fe(II). The pseudo-first-order rate constants for oxidation of dissolved Fe(II) were about 10 times faster than those previously predicted for heterogeneous oxidation of Fe(II).     

Reoxidation of reduced uranium with iron(III) (hydr)oxides under sulfate-reducing conditions

In cultures of Desulfovibrio desulfuricans 620 the effects of iron(III) (hydr)oxides (hematite, goethite, and ferrihydrite) on microbial reduction and reoxidation of uranium (U) were evaluated under lactate-limited sulfate-reducing conditions. With lactate present, G20 reduced U(VI) in both 1,4-piperazinediethanesulfonate (PIPES) and bicarbonate buffer. Once lactate was depleted, however, microbially reduced U served as an electron donor to reduce Fe(III) present in iron(III) (hydr)oxides. With the same initial amount of Fe(III) (10 mmol/L) for each iron(III) (hydr)oxide, reoxidation of U(IV) was greater with hematite than with goethite orferrihydrite. As the initial mass loading of hematite increased from 0 to 20 mmol of Fe(III)/L, the rate and extent of U(IV) reoxidation increased. Subsequent addition of hematite [15 mmol of Fe(III)/L] to stationary-phase cultures containing microbially reduced U(IV) also resulted in rapid reoxidation to U(VI). Analysis by U L3-edge X-ray absorption near-edge spectroscopy (XANES) of microbially reduced U particles yielded spectra similar to that of natural uraninite. Observations by high-resolution transmission electron microscopy, selected area electron diffraction, and energy-dispersive X-ray spectroscopic analysis confirmed that precipitated U associated with cells was uraninite with particle diameters of 3-5 nm. By the same techniques, iron sulfide precipitates were found to have a variable Fe and S stoichiometry and were not associated with cells.     

Reductive dissolution and biomineralization of iron hydroxide under dynamic flow conditions

Iron cycling and the associated changes in solid phase have dramatic implications for trace element mobility and bioavailability. Here we explore the formation of secondary iron phases during microbially mediated reductive dissolution of ferrihydrite-coated sand under dynamic flow conditions. An initial period (10 d) of rapid reduction, indicated by consumption of lactate and production of acetate and Fe-(II) to the pore water in association with a darkening of the column material, is followed by much lower rate of reduction to the termination of the experiment after 48 d. Although some Fe (<25%) is lost to the effluent pore water, the majority remains within the column as ferrihydrite (20-70%) and the secondary mineral phases magnetite (0-70%) and goethite (0-25%). Ferrihydrite converts to goethite in the influent end of the column where dissolved Fe(II) concentrations are low and converts to magnetite toward the effluent end where Fe(III) concentrations are elevated. A decline in the rate of Fe(II) production occurs concurrent with the formation of goethite and magnetite; at the termination of the experiment, the rate of reduction is <5% the initial rate. Despite the dramatic decrease in the rate of reduction, greater than 80% of the residual Fe remains in the ferric state. These results highlight the importance of coupled flow and water chemistry in controlling the rate and solid-phase products of iron (hydr)oxide reduction.     

Heterogeneous reduction of PuO₂ with Fe(II): importance of the Fe(III) reaction product

Heterogeneous reduction of actinides in higher, more soluble oxidation states to lower, more insoluble oxidation states by reductants such as Fe(II) has been the subject of intensive study for more than two decades. However, Fe(II)-induced reduction of sparingly soluble Pu(IV) to the more soluble lower oxidation state Pu(III) has been much less studied, even though such reactions can potentially increase the mobility of Pu in the subsurface. Thermodynamic calculations are presented that show how differences in the free energy of various possible solid-phase Fe(III) reaction products can greatly influence aqueous Pu(III) concentrations resulting from reduction of PuO?(am) by Fe(II). We present the first experimental evidence that reduction of PuO?(am) to Pu(III) by Fe(II) was enhanced when the Fe(III) mineral goethite was spiked into the reaction. The effect of goethite on reduction of Pu(IV) was demonstrated by measuring the time dependence of total aqueous Pu concentration, its oxidation state, and system pe/pH. We also re-evaluated established protocols for determining Pu(III) {[Pu(III) + Pu(IV)] - Pu(IV)} by using thenoyltrifluoroacetone (TTA) in toluene extractions; the study showed that it is important to eliminate dissolved oxygen from the TTA solutions for accurate determinations. More broadly, this study highlights the importance of the Fe(III) reaction product in actinide reduction rate and extent by Fe(II).     

Thermodynamic constraints on the oxidation of biogenic UO2 by Fe(III) (Hydr)oxides

Uranium mobility in the environment is partially controlled by its oxidation state, where it exists as either U(VI) or U(IV). In aerobic environments, uranium is generally found in the hexavalent form, is quite soluble, and readily forms complexes with carbonate and calcium. Under anaerobic conditions, common metal respiring bacteria can reduce soluble U(VI) species to sparingly soluble UO2 (uraninite); stimulation of these bacteria, in fact, is being explored as an in situ uranium remediation technique. However, the stability of biologically precipitated uraninite within soils and sediments is not well characterized. Here we demonstrate that uraninite oxidation by Fe(III) (hydr)oxides is thermodynamically favorable under limited geochemical conditions. Our analysis reveals that goethite and hematite have a limited capacity to oxidize UO2(biogenic) while ferrihydrite can lead to UO2(biogenic) oxidation. The extent of UO2(biogenic) oxidation by ferrihydrite increases with increasing bicarbonate and calcium concentration, but decreases with elevated Fe(II)(aq) and U(VI)(aq) concentrations. Thus, our results demonstrate that the oxidation of UO2(biogenic) by Fe(III) (hydr)oxides may transpire under mildly reducing conditions when ferrihydrite is present.     

Influence of sediment components on the immobilization of Zn during microbial Fe-(hydr)oxide reduction

The fate of Zn and other sorbed heavy metals during microbial reduction of iron oxides is different when comparing synthetic Fe-(hydr)oxides and natural sediments undergoing a similar degree of iron reduction. Batch experiments with the iron-reducing organism Shewanella putrefaciens were conducted to examine the effects of an aqueous complexant (nitrilotriacetic acid or NTA), two solid-phase complexants (kaolinite and montmorillonite), an electron carrier (anthraquinone disulfonic acid or AQDS), and a humic acid on the speciation of Zn during microbial reduction of synthetic goethite. Compared to systems containing only goethite and Zn, microbial Fe(III) reduction in the presence of clay resulted in up to a 50% reduction in Zn immobilization (insoluble in a 2 h 0.5 M HCl extraction) without affecting Fe(II) production. NTA (3 mM) increased Fe(II) production 2-fold and resulted in recovery of nearly 75% of Zn in the aqueous fraction. AQDS (50 microM) resulted in a 12.5% decrease in Fe(II) production and a 44% reduction in Zn immobilization. Humic acid additions resulted in up to a 25% decrease in Fe(II) production and 51% decrease in Zn immobilization. The results suggest that all the components examined here as either complexing agents or electron shuttles reduce the degree of Zn immobilization by limiting the availability of Zn for incorporation into newly formed biogenic minerals. These results have implications for the remediation of heavy metals in a variety of natural sediments.     

Chemical reduction of U(VI) by Fe(II) at the solid-water interface using natural and synthetic Fe(III) oxides

Abiotic reduction of 0.1 mM U(VI) by Fe(II) in the presence of synthetic iron oxides (biogenic magnetite, goethite, and hematite) and natural Fe(III) oxide-containing solids was investigated in pH 6.8 artificial groundwater containing 10 mM NaHCO3. In most experiments, more than 95% of added U(VI) was sorbed to solids. U(VI) was rapidly and extensively (> or = 80%) reduced in the presence of synthetic Fe(III) oxides and highly Fe(II) oxide-enriched (18-35 wt % Fe) Atlantic coastal plain sediments. In contrast, long-term (20-60 d) U(VI) reduction was less than 30% in suspensions of six other natural solids with relatively low Fe(III) oxide content (1-5 wt % Fe). Fe(II) sorption site density was severalfold lower on these natural solids (0.2-1.1 Fe(II) nm(-2)) compared tothe synthetic Fe(lII) oxides (1.6-3.2 Fe(II) nm(-2)), which may explain the poor U(VI) reduction in the natural solid-containing systems. Addition of the reduced form of the electron shuttling compound anthrahydroquinone-2,6-disulfonate (AH2DS; final concentration 2.5 mM) to the natural solid suspensions enhanced the rate and extent of U(VI) reduction, suggesting that AH2DS reduced U(VI) at surface sites where reaction of U(VI) with sorbed Fe(II) was limited. This study demonstrates that abiotic, Fe(II)-driven U(VI) reduction is likely to be less efficient in natural soils and sediments than would be inferred from studies with synthetic Fe(III) oxides.     

Determination of hexavalent chromium reduction using Cr stable isotopes: isotopic fractionation factors for permeable reactive barrier materials

Cr stable isotope measurements can provide improved estimates of the extent of Cr(VI) reduction to less toxic Cr(III). The relationship between observed (53)Cr/(52)Cr ratio shifts and the extent of reduction can be calibrated by determining the isotopic fractionation factor for relevant reactions. Permeable reactive barriers (PRB) made of Fe(0) and in situ redox manipulation (ISRM) zones effectively remediate Cr-contaminated aquifers. Here, we determine the isotopic fractionations for dominant reductants in reactive barriers and reduced sediments obtained from an ISRM zone at the US DOE's Hanford site. In all cases, significant isotopic fractionation was observed; fractionation (expressed as ε) was -3.91‰ for Fe(II)-doped goethite, -2.11‰ for FeS, -2.65‰ for green rust, -2.67‰ for FeCO(3), and -3.18‰ for ISRM zone sediments. These results provide a better calibration of the relationship between Cr isotope ratios and the extent of Cr(VI) reduction and aid in interpretation of Cr isotope data from systems with reactive barriers.     

Role for Fe(III) minerals in nitrate-dependent microbial U(IV) oxidation

Microbiological reduction of soluble U(VI) to insoluble U(IV) is a means of preventing the migration of that element in groundwater, but the presence of nitrate in U(IV)-containing sediments leads to U(IV) oxidation and remobilizaton. Nitrite or iron(III) oxyhydroxides may oxidize U(IV) under nitrate-reducing conditions, and we determined the rate and extent of U(IV) oxidation by these compounds. Fe(III) oxidized U(IV) at a greater rate than nitrite (130 and 10 microM U(IV)/day, respectively). In aquifer sediments, Fe(III) may be produced during microbial nitrate reduction by oxidation of Fe(II) with nitrite, or by enzymatic Fe(II) oxidation coupled to nitrate reduction. To determine which of these mechanisms was dominant, we isolated a nitrate-dependent acetate- and Fe(ll)-oxidizing bacterium from a U(VI)- and nitrate-contaminated aquifer. This organism oxidized U(IV) at a greater rate and to a greater extent under acetate-oxidizing (where nitrite accumulated to 50 mM)than under Fe(II)-oxidizing conditions. We showthatthe observed differences in rate and extent of U(IV) oxidation are due to mineralogical differences between Fe(III) produced by reaction of Fe(II) with nitrite (amorphous) and Fe(III) produced enzymatically (goethite or lepidocrocite). Our results suggest the mineralogy and surface area of Fe(III) minerals produced under nitrate-reducing conditions affect the rate and extent of U(IV) oxidation. These results may be useful for predicting the stability of U(IV) in aquifers.     

Effect of natural organic matter on the reduction of nitroaromatics by Fe(II) species

Uncertainty still exists regarding the role(s) of natural organic matter in the reduction of chemicals in anoxic environments. This work studied the effect of Suwannee river humic acid (SRHA) on the reduction of nitrobenzenes in goethite suspensions by Fe(II) species. The pseudo-first-order rate constant for the reduction of p-cyanonitrobenzene (k(CNNB)) was different for the first 3 half-lives in systems where Fe(II)aq and dissolved SRHA were equilibrated in reverse orders with goethite in suspensions. k(CNNB) and the reduction capacity of the system having SRHA added after Fe(II)aq was equilibrated with goethite was lower than that of the system for which the components were added in the reverse order. SRHA decreased the reduction capacity of the former system by oxidizing and/or complexing the surface-associated Fe(II), Fe(II)(surf), and/or hindering the access of CNNB to Fe(II)(surf). The log k(CNNB) increased linearlywith increasing concentrations of Fe(II)aq, which decreased as a result of increasing concentrations of SRHA in the system. Different k(CNNB)'s were observed for systems in which Fe(II)aq was equilibrated with goethite/SRHA suspensions for 24 and 48 h, suggesting sorbed SRHA oxidized and/or complexed Fe(II)aq. Findings suggest the concentration of Fe(II)aq and accessible Fe(ll)(surf) will influence the reduction rates of nitroaromatics in anoxic environments.     

Effects of Zwitterionic buffers on sorption of ferrous iron at goethite and its oxidation by CCl4

A major factor which controls sorption and oxidation of Fe(II) at the mineral-water interface is pH, hence buffers are commonly used to control pH in experimental studies. Here, we examined the effects of widely used organic buffers (3-morpholinopropane-1-sulfonic acid (MOPS) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)) on Fe(II) uptake and oxidation by CCl(4) in aqueous suspensions of goethite. Significant sorption of these zwitterionic buffers occurred only at Fe(II)-loaded goethite but not at native goethite. The addition of MOPS and HEPES caused substantial release of Fe(II) from goethite, retarded the oxidation of surface-bound Fe(II) by CCl(4) and changed the reaction pathway as indicated by lower yields of CHCl(3). To explore electrostatic and steric contributions of MOPS and HEPES to the observed phenomena we studied sorption and competitive effects of model sorbates (Ca(2+), sulfonates) which suggest the formation of a complex between surface-bound Fe(II) and MOPS or HEPES. Our study shows for the first time that these frequently used zwitterionic organic buffers may interfere significantly with the surface chemistry and thus with redox reactions of Fe(II) at goethite. Hence, kinetic or mechanistic information obtained in such systems requires careful interpretation.     

Operation of marine diesel engines on biogenic fuels: modification of emissions and resulting climate effects

The modification of emissions of climate-sensitive exhaust compounds such as CO(2), NO(x), hydrocarbons, and particulate matter from medium-speed marine diesel engines was studied for a set of fossil and biogenic fuels. Applied fossil fuels were the reference heavy fuel oil (HFO) and the low-sulfur marine gas oil (MGO); biogenic fuels were palm oil, soybean oil, sunflower oil, and animal fat. Greenhouse gas (GHG) emissions related to the production of biogenic fuels were treated by means of a fuel life cycle analysis which included land use changes associated with the growth of energy plants. Emissions of CO(2) and NO(x) per kWh were found to be similar for fossil fuels and biogenic fuels. PM mass emission was reduced to 10-15% of HFO emissions for all low-sulfur fuels including MGO as a fossil fuel. Black carbon emissions were reduced significantly to 13-30% of HFO. Changes in emissions were predominantly related to particulate sulfate, while differences between low-sulfur fossil fuels and low-sulfur biogenic fuels were of minor significance. GHG emissions from the biogenic fuel life cycle (FLC) depend crucially on energy plant production conditions and have the potential of shifting the overall GHG budget from positive to negative compared to fossil fuels.     

Influence of magnetite stoichiometry on U(VI) reduction

Hexavalent uranium (U(VI)) can be reduced enzymatically by various microbes and abiotically by Fe(2+)-bearing minerals, including magnetite, of interest because of its formation from Fe(3+) (oxy)hydroxides via dissimilatory iron reduction. Magnetite is also a corrosion product of iron metal in suboxic and anoxic conditions and is likely to form during corrosion of steel waste containers holding uranium-containing spent nuclear fuel. Previous work indicated discrepancies in the extent of U(VI) reduction by magnetite. Here, we demonstrate that the stoichiometry (the bulk Fe(2+)/Fe(3+) ratio, x) of magnetite can, in part, explain the observed discrepancies. In our studies, magnetite stoichiometry significantly influenced the extent of U(VI) reduction by magnetite. Stoichiometric and partially oxidized magnetites with x ≥ 0.38 reduced U(VI) to U(IV) in UO(2) (uraninite) nanoparticles, whereas with more oxidized magnetites (x < 0.38) and maghemite (x = 0), sorbed U(VI) was the dominant phase observed. Furthermore, as with our chemically synthesized magnetites (x ≥ 0.38), nanoparticulate UO(2) was formed from reduction of U(VI) in a heat-killed suspension of biogenic magnetite (x = 0.43). X-ray absorption and M?ssbauer spectroscopy results indicate that reduction of U(VI) to U(IV) is coupled to oxidation of Fe(2+) in magnetite. The addition of aqueous Fe(2+) to suspensions of oxidized magnetite resulted in reduction of U(VI) to UO(2), consistent with our previous finding that Fe(2+) taken up from solution increased the magnetite stoichiometry. Our results suggest that magnetite stoichiometry and the ability of aqueous Fe(2+) to recharge magnetite are important factors in reduction of U(VI) in the subsurface.     

Potential function of added minerals as nucleation sites and effect of humic substances on mineral formation by the nitrate-reducing Fe(II)-oxidizer Acidovorax sp. BoFeN1

The mobility of toxic metals and the transformation of organic pollutants in the environment are influenced and in many cases even controlled by iron minerals. Therefore knowing the factors influencing iron mineral formation and transformation by Fe(II)-oxidizing and Fe(III)-reducing bacteria is crucial for understanding the fate of contaminants and for the development of remediation technologies. In this study we followed mineral formation by the nitrate-reducing Fe(II)-oxidizing strain Acidovorax sp. BoFeN1 in the presence of the crystalline Fe(III) (oxyhydr)oxides goethite, magnetite and hematite added as potential nucleation sites. M?ssbauer spectroscopy analysis of minerals precipitated by BoFeN1 in (57)Fe(II)-spiked microbial growth medium showed that goethite was formed in the absence of mineral additions as well as in the presence of goethite or hematite. The presence of magnetite minerals during Fe(II) oxidation induced the formation of magnetite in addition to goethite, while the addition of humic substances along with magnetite also led to goethite but no magnetite. This study showed that mineral formation not only depends on the aqueous geochemical conditions but can also be affected by the presence of mineral nucleation sites that initiate precipitation of the same underlying mineral phases.     

Coupled Fe(II)-Fe(III) electron and atom exchange as a mechanism for Fe isotope fractionation during dissimilatory iron oxide reduction

Microbial dissimilatory iron reduction (DIR) is an important pathway for carbon oxidation in anoxic sediments, and iron isotopes may distinguish between iron produced by DIR and other sources of aqueous Fe(II). Previous studies have shown that aqueous Fe(II) produced during the earliest stages of DIR has delta56Fe values that are 0.5-2.0%o lowerthan the initial Fe(III) substrate. The new experiments reported here suggest that this fractionation is controlled by coupled electron and Fe atom exchange between Fe(II) and Fe(III) at iron oxide surfaces. In hematite and goethite reduction experiments with Geobacter sulfurreducens, the 56Fe/54Fe isotopic fractionation between aqueous Fe(II) and the outermost layers of Fe(III) on the oxide surface is approximately -3%o and can be explained by equilibrium Fe isotope partitioning between reactive Fe(II) and Fe(III) pools that coexist during DIR. The results indicate that sorption of Fe(II) to Fe(III) substrates cannot account for production of low-delta56Fe values for aqueous Fe(II) during DIR.     

Assessing iron-mediated oxidation of toluene and reduction of nitroaromatic contaminants in anoxic environments using compound-specific isotope analysis

We evaluated compound-specific isotope analysis (CSIA) as a tool to assess the coupling of microbial toluene oxidation by Fe(III)-reducing bacteria and abiotic reduction of nitroaromatic contaminants by biogenic mineral-bound Fe(II) species. Examination of the two processes in isolated systems revealed a reproducible carbon isotope fractionation for toluene oxidation by Geobacter metal-lireducens with a solid Fe(III) phase as terminal electron acceptor. We found a carbon isotope enrichment factor, epsilonC, of -1.0 +/- 0.1 per thousand, which corresponds to an apparent kinetic isotope effect (AKIE(C)) of 1.0073 +/- 0.0009 for the oxidative cleavage of a C-H bond. Nitrogen isotope fractionation of the reduction of nitroaromatic compounds (NAC) by mineral-bound Fe(ll) species yielded a nitrogen isotope enrichment factor, epsilonN, of -39.7 +/- 3.4 per thousand for the reduction of an aromatic NO2-group (AKIE(N) = 1.0413 +/- 0.0037) that was constant for variable experimental conditions. Finally, AKIE values for C and N observed in coupled experiments, where reactive Fe(II) was generated through microbial activity, were identical to those obtained in the isolated experiments. This study provides new evidence on isotope fractionation behavior during contaminant transformation and promotes the use of CSIA for the elucidation of complex contaminant transformation pathways in the environment.     

Effects of carbonate species on the kinetics of dechlorination of 1,1,1-trichloroethane by zero-valent iron

The effect of precipitates on the reactivity of iron metal (Fe0) with 1,1,1-trichloroethane (TCA) was studied in batch systems designed to model groundwaters that contain dissolved carbonate species (i.e., C(IV)). At representative concentrations for high-C(IV) groundwaters (approximately 10(-2) M), the pH in batch reactors containing Fe0 was effectively buffered until most of the aqueous C(IV) precipitated. The precipitate was mainly FeCO3 (siderite) but may also have included some carbonate green rust. Exposure of the Fe0 to dissolved C(IV) accelerated reduction of TCA, and the products formed under these conditions consisted mainly of ethane and ethene, with minor amounts of several butenes. The kinetics of TCA reduction were first-order when C(IV)-enhanced corrosion predominated but showed mixed-order kinetics (zero- and first-order) in experiments performed with passivated Fe0 (i.e., before the onset of pitting corrosion and after repassivation by precipitation of FeCO3). All these data were described by fitting a Michaelis-Menten-type kinetic model and approximating the first-order rate constant as the ratio of the maximum reaction rate (Vm) and the concentration of TCA at half of the maximum rate (K(1/2)). The decrease in Vm/K(1/2) with increasing C(IV) exposure time was fit to a heuristic model assuming proportionality between changes in TCA reduction rate and changes in surface coverage with FeCO3.     

Inhibited Cr(VI) reduction by aqueous Fe(II) under hyperalkaline conditions

This study investigated Cr(VI) reduction by dissolved Fe(II) in hyperalkaline pH conditions as found in fluid wastes associated with the U.S. nuclear weapons program. The results show that Cr(VI) reduction by Fe(II) at alkaline pH solutions proceeds very quickly. The amount of Cr(VI) removed from solution and the amount reduced increases with Fe(II):Cr(VI) ratio. However, the Cr(VI) reduction under alkaline pH condition is nonstoichiometric, probably due to Fe(II) precipitation and mixed iron(III)-chromium-(III) (oxy)hydroxides blocking Fe(II) surface sites, as well as removing Fe(II) from solution through O2 oxidation. After Cr(VI) was reduced to Cr(III), it precipitated out as mixed Fe(x)Cr1-xO3(solids) and various Fe(III) precipitates with an overall Cr:Fe ratio of 1:3; all Cr remaining in the solution phase was unreduced Cr(VI). EXAFS data showed that Cr-O and Cr-Cr distances in the precipitates equal to 1.98 and 3.01 A, respectively, consistent with the spinel-type structure as chromite.