Abiotic transformation of hexahydro-1,3,5-trinitro-1,3,5-triazine by Fe(II) bound to magnetite

RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), a nitramine explosive, is often found as a subsurface contaminant at military installations. Though biological transformations of RDX are often reported, abiotic studies in a defined medium are uncommon. The work reported here was initiated to investigate the transformation of RDX by ferrous iron (Fe(II)) associated with a mineral surface. RDX is transformed by Fe(II) in aqueous suspensions of magnetite (Fe3O4). Negligible transformation of RDX occurred when it was exposed to Fe(II) or magnetite alone. The sequential nitroso reduction products (MNX, DNX, and TNX) were observed as intermediates. NH4+, N2O, and HCHO were stable products of the transformation. Experiments with radiolabeled RDX indicate that 90% of the carbon end products remained in solution and that negligible mineralization occurred. Rates of RDX transformation measured for a range of initial Fe(II) concentrations and solution pH values indicate that greater amounts of adsorbed Fe(II) result in faster transformation rates. As pH increases, more Fe(II) adsorbs and k(obs) increases. The degradation of RDX by Fe(II)-magnetite suspensions indicates a possible remedial option that could be employed in natural and engineered environments where iron oxides are abundant and ferrous iron is present.     

Enhanced dechlorination of chlorinated methanes and ethenes by chloride green rust in the presence of copper(II)

The enhanced removal of carbon tetrachloride (CCl4), tetrachloroethene (C2Cl4), and trichloroethene (C2HCl3) by chloride green rust (GR(Cl)) in the presence of copper ions was investigated. X-ray powder diffraction (XRPD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the crystallization and chemical speciation, respectively, of the secondary mineral phases produced in the GR(Cl)-Cu(II) system. The addition of Cu(II) to GR(Cl) suspensions resulted in enhanced dechlorination of the chlorinated hydrocarbons examined in this study. The degradation reactions followed pseudo-first-order kinetics and the pseudo-first-order rate constant (k(obs)) for CCl4 (20 microM) removal by GR(CI) at pH 7.2 was 0.0808 h(-1). Addition of 0.5 mM Cu(II) completely dechlorinated CCl4 within 35 min, and the k(obs) was 84 times greater than that in the absence of Cu(II). Chloroform (CHCl3), the major chlorinated product in CCl4 dechlorination, accumulated at a concentration up to 13 microM in the GR(Cl) system alone, but was completely dechlorinated within 9 h in the GR(Cl)-Cu(II) suspension. Also, rapid removal of C2Cl4 and C2HCl3 by GR(Cl) was observed when Cu(II) was added. The k(obs) values for the removal of chlorinated ethenes were 4.7-7 times higher than that obtained in the absence of Cu(II). In addition, the k(obs) for PCE removal increased linearly with respect to Cu(II) concentrations in the range from 0.1 to 1.0 mM. Addition of Cu(II) at a concentration higher than 1.0 mM decreased the k(obs) for the removal of both C2Cl4 and C2HCl3 due to the decrease in structural Fe(II) concentration in GR(Cl) and the changes in redox potentials and pH values. Moreover, the highest removal efficiency and rate of C2Cl4 was obtained at near-neutral pH when Cu(II) was added into the GR(Cl) suspension. XPS and XRPD results showed that the Fe(II) in the GR(Cl) suspension could reduce Cu(II) to both Cu(I) and metallic Cu. These findings are relevant to the better understanding of the role of abiotic removal of chlorinated hydrocarbons during remediation and/or natural attenuation in iron-reducing environments.     

Iron oxide surface-catalyzed oxidation of ferrous iron by monochloramine: implications of oxide type and carbonate on reactivity

The maintenance of monochloramine residuals in drinking water distribution systems is one technique often used to minimize microbial outbreaks and thereby maintain the safety of the water. Reactions between oxidizable species and monochloramine can however lead to undesirable losses in the disinfectant residual. Previous work has illustrated that the Fe(II) present within distribution systems is one type of oxidizable species that can exert a monochloramine demand. This paper extends this prior work by examining the kinetics of the reactions between Fe(II) and monochloramine in the presence of a variety of iron oxide surfaces. The identity of the iron oxide plays a significant role in the rate of these reactions. Surface area-normalized initial rate coefficients (k(init)) obtained in the presence of each oxide at pH approximately 6.9 exhibit the following trend in catalytic activity: magnetite > goethite > hematite approximately = lepidocrocite > ferrihydrite. The differences in the activity of these oxides are hypothesized to result from variations in the amount of Fe(II) sorbed to each of the oxides and to dissimilarities in the surface site densities of the oxides. The implications of carbonate on Fe(II) sorption to iron oxides are also examined. Comparing Fe(II) sorption isotherms for goethite obtained under differential carbonate concentrations, it is apparent that as the carbonate concentration (C(T,CO3)) increased from 0 to 11.7 mM that the Fe(II) sorption edge (50% sorption) shifts from a pH of approximately 5.8 to a pH of 7.8. This shift is hypothesized to be the result of the formation of aqueous and surface carbonate-Fe(II) complexes and to competition between carbonate and Fe(II) for surface sites. The implications of these changes are then discussed in light of the variable oxide studies.     

Reductive dechlorination of carbon tetrachloride in aqueous solutions containing ferrous and copper ions

Fe(II) associated with iron-containing minerals has been shown to be a potential reductant in natural subsurface environments. While it is known that the surface-bound iron species has the capacity to dechlorinate various chlorinated compounds, the role of transition metals to act as catalysts with these iron species is of importance. We previously observed that the reduction of Cu(II) by Fe(II) associated with goethite enhanced the dechlorination efficiency of chlorinated compound. In this study, the reductive dechlorination of carbon tetrachloride (CCl4) by dissolved Fe(II) in the presence of Cu(II) ions was investigated to understand the synergistic effect of Fe(II) and Cu(II) on the dechlorination processes in homogeneous aqueous solutions. The dechlorination efficiency of CCl4 by Fe(II) increased with increasing Cu(II) concentrations over the range of 0.2-0.5 mM and then decreased at high Cu(II) concentrations. The efficiency and rate of CCl4 dechlorination also increased with increasing dissolved Fe(II) concentration in the presence of 0.5 mM Cu(II) at neutral pH. When the Fe(II)/Cu(II) ratio varied between 1 and 10, the pseudo-first-order rate constant (k(obs)) increased 250-fold from 0.007 h(-1) at 0.5 mM Fe(II) to 1.754 h(-1) at 5 mM Fe(II). X-ray powder diffraction and scanning electron microscopy analyses showed that Cu(II) can react with Fe(II) to produce different morphologies of ferric oxides and subsequently accelerate the dechlorination rate of CCl4 at a high Fe(II) concentration. Amorphous ferrihydrite was observed when the stoichiometric Fe(II)/Cu(II) ratio was 1, while green rust, goethite, and magnetite were formed when the molar ratios of Fe(II)/Cu(II) reached 4-6. In addition, the dechlorination of CCl4 by dissolved Fe(II) is pH dependent. CCl4 can be dechlorinated by Fe(II) over a wide range of pH values in the Cu(II)-amended solutions, and the k(obs) increased from 0.0057 h(-1) at pH 4.3 to 0.856 h(-1) at pH 8.5, which was 9-25 times greater than that in the absence of Cu(II) at pH 7-8.5. The high reactivity of dissolved Fe(II) on the dechlorination of CCl4 in the presence of Cu(II) under anoxic conditions may enhance our understanding of the role of Fe(II) and the long-term reactivity of the zerovalent iron system in the dechlorination processes for chlorinated organic contaminants.     

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.     

Mechanisms and products of surface-mediated reductive dehalogenation of carbon tetrachloride by Fe(II) on goethite

Natural attenuation processes of chlorinated solvents in soils and groundwaters are increasingly considered as options to manage contaminated sites. Under anoxic conditions, reactions with ferrous iron sorbed at iron(hyro)xides may dominate the overall transformation of carbon tetrachloride (CCl4) and other chlorinated aliphatic hydrocarbons. We investigated mechanisms and product formation of CCl4 reduction by Fe(II) sorbed to goethite, which may lead to completely dehalogenated products or to chloroform (CHCl3), a toxic product which is fairly persistent under anoxic conditions. A simultaneous transfer of two electrons and cleavage of two C-Cl bonds of CCl4 would completely circumvent chloroform production. To distinguish between initial one- or two-bond cleavage, 13C-isotope fractionation of CCl4 was studied for reactions with Fe(II)/ goethite (isotopic enrichment factor epsilon = -26.5% percent per thousand) and with model systems for one C-Cl bond cleavage and either single-electron transfer (Fe(II) porphyrin, epsilon = -26.1 percent per thousand) or partial two-electron transfer (polysulfide, epsilon = -22.2 percent per thousand). These epsilon values differ significantlyfrom calculations for simultaneous cleavage of two C-Cl bonds (epsilon approximately equal to -50 percent per thousand), indicating that only one C-Cl bond is broken in the critical first step of the reaction. At pH 7, reduction of CCl4 by Fe(II)/ goethite produced approximately 33% CHCl3, 20% carbon monoxide (CO), and up to 40% formate (HCOO-). Addition of 2-propanol-d8 resulted in 33% CDCl3 and only 4% CO, indicating that both products were generated from trichloromethyl radicals (*CCl3), chloroform by reaction with hydrogen radical donors and CO by an alternative pathway likely to involve surface-bound intermediates. Hydrolysis of CO to HCOO-was surface-catalyzed by goethite butwastoo slow to account for the measured formate concentrations. Chloroform yields slightly increased with pH at constant Fe(II) sorption density, suggesting that pH-dependent surface processes direct product branching ratios. Surface-stabilized intermediates may thus facilitate abiotic mineralization of CCl4, whereas the presence of H radical donors, such as natural organic matter, enhances formation of toxic CHCl3.     

Synergistic effect of copper ion on the reductive dechlorination of carbon tetrachloride by surface-bound Fe(II) associated with goethite

The dechlorination of carbon tetrachloride (CT) by Fe(II) associated with goethite in the presence of transition metal ions was investigated. X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRPD) were used to characterize the chemical states and crystal phases of transition metals on solid phases, respectively. CT was dechlorinated to chloroform (CF) by 3 mM Fe(II) in 10 mM goethite (25.6 m2 L(-1)) suspensions. The dechlorination followed pseudo-first-order kinetics, and a rate constant (k(obs)) of 0.036 h(-1) was observed. Transition metal ions have different effects on CT dechlorination. The addition of Ni(II), Co(II), and Zn(II) lowered the k(obs) for CT dechlorination, whereas the amendment of 0.5 mM Cu(II) into the Fe(II)-Fe(III) system significantly enhanced the efficiency and the rate of CT dechlorination. The k(obs) for CT dechlorination with 0.5 mM Cu(II) was 1.175 h(-1), which was 33 times greater than that without Cu(II). Also, the dechlorination of CT by surface-bound iron species is pH-dependent, and the rate constants increased from 0.008 h(-1) at pH 4.0 to 1.175 h(-1) at pH 7.0. When the solution contained Cu(II) and Fe(II) without goethite, a reddish-yellow precipitate was formed, and the concentration of Fe(ll) decreased with the increase in Cu(II) concentration. XPS and XRPD analyses suggested the possible presence of Cu2O and ferrihydrite in the precipitate. Small amounts of aqueous Cu(I) were also detected, reflecting the fact that Cu(II) was reduced to Cu(I) by Fe(II). A linear relationship between k(obs) for CT dechlorination and the concentration of Cu(II) was observed when the amended Cu(II) concentration was lower than 0.5 mM. Moreover, the k(obs) for CT dechlorination was dependent on the Fe(II) concentration in the 0.5 mM Cu(II)-amended goethite system and followed a Langmuir-Hinshelwood relationship. These results clearly indicate that Fe(II) serves as the bulk reductant to reduce both CT and Cu(II). The resulting Cull) can further act as a catalyst to enhance the dechlorination rate of chlorinated hydrocarbons in iron-reducing environments.     

Role of organically complexed iron(II) species in the reductive transformation of RDX in anoxic environments

Organically complexed iron species can play a significant role in many subsurface redox processes, including reactions that contribute to the transformation and degradation of soil and aquatic contaminants. Experimental results demonstrate that complexation of Fe(II) by catechol- and thiol-containing organic ligands leads to formation of highly reactive species that reduce RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) and related N-heterocyclic nitramine explosive compounds to formaldehyde and inorganic nitrogen byproducts. Under comparable conditions, relative reaction rates follow HMX < RDX < MNX < DNX < TNX. Observed rates of RDX reduction are heavily dependent on the identity of the Fe(II)-complexing ligands and the prevailing solution conditions (e.g., pH, Fe(II) and ligand concentrations). In general, reaction rates increase with increasing pH and organic ligand concentration when the concentration of Fe(II) is fixed. In solutions containing Fe(II) and tiron, a model catechol, observed pseudo-first-order rate constants (k(obs)) for RDX reduction are linearly correlated with the concentration of the 1:2 Fe(II)-tiron complex (FeL2(6-)), and kinetic trends are well described by -d[RDX]/dt= k(FeL2)6-[FeL2(6-)][RDX], where k(FeL2)6- = 7.31(+/-2.52) x 10(2) M(-1) s(-1). The reaction products and net stoichiometry (1 mol of RDX reduced for every 2 mol of Fe(II) oxidized) support a mechanism where RDX ring cleavage and decomposition is initiated by sequential 1-electron transfers from two Fe(II)-organic complexes.     

Degradation of drinking water disinfection byproducts by synthetic goethite and magnetite

Corrosion of iron pipes leads to the release of ferrous iron, Fe(II), and the formation of iron oxides, such as goethite and magnetite, on the pipe surface. Fe(II), a potent reductant when associated with iron oxide surfaces, can mediate the reduction of halogenated organic compounds. Batch experiments were performed to investigate the kinetics and pathways of the degradation of selected chlorinated disinfection byproducts (OBPs) by Fe(II) in the presence of synthetic goethite and magnetite. Trichloronitromethane was degraded via reduction, while trichloroacetonitrile, 1,1,1-trichloropropanone, and trichloroacetaldyde hydrate were transformed via both hydrolysis and reduction. Chloroform and trichloroacetic acid were unreactive. Observed pseudo-first-order reductive dehalogenation rates were influenced by DBP chemical structure and identity of the reductant. Fe(II) bound to iron minerals had greater reactivity than either aqueous Fe(II) or structural Fe(II) present in magnetite. For DBPs of structure Cl3C-R, reductive dehalogenation rate constants normalized by the surface density of Fe(II) on both goethite and magnetite correlated with the electronegativity of the -R group and with one electron reduction potential. In addition to chemical transformation, sorption onto the iron oxide minerals was also an important loss process for 1,1,1-trichloropropanone.     

Effect of aqueous Fe(II) on arsenate sorption on goethite and hematite

Biogeochemical iron cycling often generates systems where aqueous Fe(II) and solid Fe(III) oxides coexist. Reactions between these species result in iron oxide surface and phase transformations, iron isotope fractionation, and redox transformations of many contaminant species. Fe(II)-induced recrystallization of goethite and hematite has recently been shown to cause the repartitioning of Ni(II) at the mineral-water interface, with adsorbed Ni incorporating into the iron oxide structure and preincorporated Ni released back into aqueous solution. However, the effect of Fe(II) on the fate and speciation of redox inactive species incompatible with iron oxide structures is unclear. Arsenate sorption to hematite and goethite in the presence of aqueous Fe(II) was studied to determine whether Fe(II) causes substantial changes in the sorption mechanisms of such incompatible species. Sorption isotherms reveal that Fe(II) minimally alters macroscopic arsenate sorption behavior except at circumneutral pH in the presence of elevated concentrations (10?3 M) of Fe(II) and at high arsenate loadings, where a clear signature of precipitation is observed. Powder X-ray diffraction demonstrates that the ferrous arsenate mineral symplesite precipitates under such conditions. Extended X-ray absorption fine structure spectroscopy shows that outside this precipitation regime arsenate surface complexation mechanisms are unaffected by Fe(II). In addition, arsenate was found to suppress Fe(II) sorption through competitive adsorption processes before the onset of symplesite precipitation. This study demonstrates that the sorption of species incompatible with iron oxide structure is not substantially affected by Fe(II) but that such species may potentially interfere with Fe(II)-iron oxide reactions via competitive adsorption.     

Kinetics and mechanisms for reactions of Fe(II) with iron(III) oxides

Uptake of Fe(II) onto hematite (alpha-Fe2O3), corundum (alpha-Al2O3), amorphous ferric oxide (AFO), and a mixture of hematite and AFO was measured. Uptake was operationally divided into adsorption (extractable by 0.5 N HCl within 20 h) and fixation (extractable by 3.0 N HCl within 7 d). For 0.25 mM Fe(II) onto 25 mM iron(III) hematite at pH 6.8: (i) 10% of Fe(II) was adsorbed within 1 min; (ii) 20% of Fe(II) was adsorbed within 1 d; (iii) uptake slowly increased to 24% of Fe(II) during the next 24 d, almost all adsorbed; (iv) at 30 d, the uptake increased to 28% of Fe(II) with 6% of total Fe(II) fixed; and (v) uptake slowly increased to 30% of Fe(II) by 45 d with 10% of total Fe(II) fixed. Similar results were observed for 0.125 mM Fe(II) onto 25 mM iron(III) hematite, except that percent of adsorption and fixation were increased. There was adsorption but no fixation for 0.25 mM Fe(II) onto corundum [196.2 mM Al(III)] at pH 6.8, for 0.125 mM Fe(II) onto 25 mM iron(III) hematite at pH 4.5, and for 0.25 mM Zn(II) onto 25 mM iron(III) hematite at pH 6.8. A small addition of AFO to the hematite suspension increased Fe(II) fixation when 0.25 mM Fe(II) was reacted with 25 mM iron(III) hematite and 0.025 mM Fe(III) AFO at pH 6.8. Reaction of 0.125 mM Fe(II) with 2.5 mM Fe(III) AFO resulted in rapid adsorption of 30% of added Fe(II), followed by conversion of AFO to goethite and a decrease in adsorption without Fe(II) fixation. The fixation of Fe(II) by hematite at pH 6.8 is consistent with interfacial electron transfer and the formation of new mineral phases. We propose that electron transfer from adsorbed Fe(II) to structural Fe(III) in hematite results in oxidation of Fe(II) to AFO on the surface of hematite and that solid-phase contact among hematite, AFO, and structural Fe(II) produces magnetite (Fe3O4). The unique interactions of Fe(II) with iron(III) oxides would be environmentally important to understand the fate of redox-sensitive chemicals.     

Effect of oxide formation mechanisms on lead adsorption by biogenic manganese (hydr)oxides, iron (hydr)oxides, and their mixtures

The effects of iron and manganese (hydr)oxide formation processes on the trace metal adsorption properties of these metal (hydr)oxides and their mixtures was investigated by measuring lead adsorption by iron and manganese (hydr)oxides prepared by a variety of methods. Amorphous iron (hydr)oxide formed by fast precipitation at pH 7.5 exhibited greater Pb adsorption (gamma(max) = 50 mmol of Pb/mol of Fe at pH 6.0) than iron (hydr)oxide formed by slow, diffusion-controlled oxidation of Fe(II) at pH 4.5-7.0 or goethite. Biogenic manganese(III/IV) (hydr)oxide prepared by enzymatic oxidation of Mn(II) by the bacterium Leptothrix discophora SS-1 adsorbed five times more Pb (per mole of Mn) than an abiotic manganese (hydr)oxide prepared by oxidation of Mn(II) with permanganate, and 500-5000 times more Pb than pyrolusite oxides (betaMnO2). X-ray crystallography indicated that biogenic manganese (hydr)oxide and iron (hydr)oxide were predominantly amorphous or poorly crystalline and their X-ray diffraction patterns were not significantly affected by the presence of the other (hydr)oxide during formation. When iron and manganese (hydr)oxides were mixed after formation, or for Mn biologically oxidized with iron(III) (hydr)oxide present, observed Pb adsorption was similar to that expected for the mixture based on Langmuir parameters for the individual (hydr)oxides. These results indicate that interactions in iron/manganese (hydr)oxide mixtures related to the formation process and sequence of formation such as site masking, alterations in specific surface area, or changes in crystalline structure either did not occur or had a negligible effect on Pb adsorption by the mixtures.     

Effect of pH on aqueous Se(IV) reduction by pyrite

Interaction of aqueous Se(IV) with pyrite was investigated using persistently stirred batch reactors under O2-free (<1 ppm) conditions at pH ranging from 4.5 to 6.6. Thermodynamic calculations, an increase in pH during the experiments, and spectroscopic observation indicate that the reduction of aqueous Se(IV) by pyrite is dominated by the following reaction: FeS2+3.5HSeO3?+1.5H+=2SO4(2?)+Fe2++3.5Se(0)+2.5H2O. The released Fe(II) was partitioned between the bulk solution and pyrite surface at pH≈4.5 and 4.8, with the Fe2+ density at pyrite-solution interface about 4 orders of magnitude higher than that in the bulk solution, while iron oxyhydroxide precipitated at pH≈6.6, resulting in the decrease of dissolved iron. In the Se(IV) concentration range of the experiments, aqueous Se(IV) reduction rate follows the pseudofirst order which is in the form of ln mSe(IV)=?k′t+ln mSe(IV)0, where k′ is apparent rate constant combining the rate constant k and pyrite surface area to mass of solution ratio (A/M). And the aqueous Se(IV) reduction rate constant for a standard system (k) with 1 m2 pyrite surface area per 1 kg solution was obtained to be 1.65×10(?4) h(?1), 3.28×10(?4) h(?1), and 4.76×10(?4) h(?1) at pH around 4.5, 4.8, and 5.1, respectively. The positive correlation between reaction rate and pH disagrees with the theories that protons are consumed when HSeO3? is reduced to Se0, and negative charge density on pyrite surface increases as pH increases. Thus, a ferrous iron mediated electron transfer mechanism is proposed to operate during the reduction of aqueous Se(IV) by pyrite. pH and iron concentration affect significantly on Se(IV) reaction rate and reaction product.     

Kinetics of nitrate, nitrite, and Cr(VI) reduction by iron metal

The kinetics of nitrate, nitrite, and Cr(VI) reduction by three types of iron metal (Fe0) were studied in batch reactors for a range of Fe0 surface area concentrations and solution pH values (5.5-9.0). At pH 7.0, there was only a modest difference (2-4x) in first-order rate coefficients (k(obs)) for each contaminant among the three Fe0 types investigated (Fisher, Peerless, and Connelly). The k(obs) values at pH 7.0 for both nitrite and Cr(VI) reduction were first-order with respect to Fe0 surface area concentration, and average surface area normalized rate coefficients (kSA) of 9.0 x 10(-3) and 2.2 x 10(-1) L m(-2) h(-1) were determined for nitrite and Cr(VI), respectively. Unlike nitrite and Cr(VI), Fe0 surface area concentration had little effect on rates of nitrate reduction (with the exception of Connelly Fe0, which reduced nitrate at slower rates at higher Fe0 surface areas). The rates of nitrate, nitrite, and Cr(VI) reduction by Fisher Fe0 decreased with increasing pH with apparent reaction orders of 0.49 +/- 0.04 for nitrate, 0.61 +/- 0.02 for nitrite, and 0.72 +/- 0.07 for Cr(VI). Buffer type had minimal effects on reduction rates, indicating that pH was primarily responsible for the differences in rate. At high pH values, Cr(VI) reduction ceased after a short time period, and negligible nitrite reduction was observed over 48 h.     

Kinetics of reductive dissolution of hematite by bioreduced anthraquinone-2,6-disulfonate

The reductive dissolution of hematite (alpha-Fe2O3) was investigated in a flow-through system using AH2DS, a reduced form of anthraquinone-2,6-disulfonate (AQDS), which is often used as a model electron shuttling compound in studies of dissimilatory microbial reduction of iron oxides. Influent flow rate, pH, and Fe(II) and phosphate concentrations were varied to investigate the redox kinetics in a flow-through reactor. The hematite reduction rates decreased with increasing pH from 4.5 to 7.6 and decreased with decreasing flow rate. The rates also decreased with increasing influent concentration of Fe(II) or phosphate that formed surface complexes at the experimental pH. Mineral surface properties, Fe(II) complexation reactions, and ADDS sorption on hematite surfaces were independently investigated for interpreting hematite reduction kinetics. AH2DS sorption to hematite was inferred from the parallel measurements of AQDS and AH2DS sorption to alpha-Al2O3, a redox stable analog of alpha-Fe2O3. Decreasing Fe(ll) and increasing AH2DS sorption by controlling flow rate, influent pH, and Fe(II) and phosphate concentrations increased the rates of reductive dissolution. The rates were also affected by the redox reaction free energy when reductive dissolution approached equilibrium. This study demonstrated the importance of the geochemical variables for the reductive dissolution kinetics of iron oxides.     

Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen

The corrosion of zero-valent iron (Fe0(s)) by oxygen (O2) can lead to the oxidation of organic compounds. To gain insight into the reaction mechanism and to assess the nature of the oxidant, the oxidation of methanol, ethanol, 2-propanol, and benzoic acid by the reaction of nanoparticulate zero-valent iron (nZVI) or ferrous iron (Fe[II]) with O2 in the absence of ligands was studied. At pH values below 5, Fe0(s) nanoparticles were oxidized by O2 within 30 min with a stoichiometry of approximately two Fe0(s) oxidized per O2 consumed. The yield of methanol and ethanol oxidation products increased from 1% at acidic pH to 6% at pH 7, relative to nZVI added. Product yields from 2-propanol and benzoic acid were highest under acidic conditions, with little oxidation observed at neutral pH. At pH values below 5, product formation was attributable to hydroxyl radical (OH.) production through the Fenton reaction, involving hydrogen peroxide and Fe(II) produced during nZVI oxidation. At higher pH values, the oxidation of Fe(II), the initial product of nZVI oxidation, by oxygen is responsible for most of the oxidant production. Product yields at circumneutral pH values were consistent with a different oxidant, such as the ferryl ion (Fe[IV]).     

Fe(II) sorption on hematite: new insights based on spectroscopic measurements

We collected M?ssbauer spectra of 57Fe(II) interacting with 56hematite (alpha-Fe2O3) over a range of Fe(II) concentrations and pH values to explore whether a sorbed Fe(II) species would form. Several models of Fe(II) sorption (e.g., surface complexation models) assume that stable, sorbed Fe(II) species form on ligand binding sites of Fe(III) oxides and other minerals. Model predictions of changes in both speciation and concentration of sorbed Fe(II) species are often invoked to explain Fe(II) sorption patterns, as well as rates of contaminant reduction and microbial respiration of Fe(III) oxides. Here we demonstrate that, at low Fe(II) concentrations, sorbed Fe(II) species are transient and quickly undergo interfacial electron transfer with structural Fe(III) in hematite. At higher Fe(II) concentrations, however, we observe the formation of a stable, sorbed Fe(II) phase on hematite that we believe to be the first spectroscopic confirmation for a sorbed Fe(II) phase forming on an iron oxide. Low-temperature M?ssbauer spectra suggest that the sorbed Fe(II) phase contains varying degrees of Fe(II)-Fe(II) interaction and likely contains a mixture of adsorbed Fe(II) species and surface precipitated Fe(OH)2(s). The transition from Fe(II)-Fe(III) interfacial electron transfer to formation of a stable, sorbed Fe(II) phase coincides with the macroscopically observed change in isotherm slope, as well as the estimated surface site saturation suggesting that the finite capacity for interfacial electron transfer is influenced by surface properties. The spectroscopic demonstration of two distinctly different sorption endpoints, that is an Fe(III) coating formed from electron transfer or a stable, sorbed Fe(II) phase, challenges us to reconsider our traditional interpretations and modeling of Fe(II) sorption behavior (as well as, we would argue, of any other redox active sorbate-sorbent couple).     

Kinetics and structural constraints of chromate reduction by green rusts

Green rusts, ferrous-ferric iron oxides, occur in many anaerobic soils and sediments and are highly reactive, making them important phases impacting the fate and transport of environmental contaminants. Despite their potential importance in environmental settings, reactions involving green rusts remain rather poorly described. Chromate is a widespread contaminant having deleterious impacts on plant and animal health; its fate may in part be controlled by green rust. Here we examine chromate reduction by a series of green rust phases and resolve the reaction kinetics at pH 7. The overall kinetics of the reactions are well described by the expression d[Cr(VI)]/dt = -k[Cr(VI)][GR], and this model was successfully used to predict rates of reaction at varying chromium concentrations. The rates of reduction are controlled by the concentration of ferrous iron, surface area, and chemical structure of the green rust including layer spacing. On a mass basis, green rust (GR) chloride is the most rapid reductant of Cr(VI) followed by GRCO3 and GRSO4, with pseudo-first-order rate coefficients (k(obs)) (with respect to Cr(VI) concentration) ranging from 1.22 x 10(-3) to 3.7 x 10(-2) s(-1). Chromium(III)-substituted magnetite and lepidocrocite were identified as the major oxidation products. The nature of the oxidation products appears to be independent of the anionic class of green rust, but their respective concentrations display a dependence on the initial GR. The mole fraction of Fe(III) in the Cr(x),Fe(1-x)(OH)3 x nH2O reaction product ranged from 17% to 68%, leading to a highly stabilized (low solubility) phase.     

Nitrate reduction by zerovalent iron: effects of formate, oxalate, citrate, chloride, sulfate, borate, and phosphate

Recent studies have shown that zerovalent iron (Fe0) may potentially be used as a chemical medium in permeable reactive barriers (PRBs) for groundwater nitrate remediation; however, the effects of commonly found organic and inorganic ligands in soil and sediments on nitrate reduction by Fe0 have not been well understood. A 25.0 mL nitrate solution of 20.0 mg of N L(-1) (1.43 mM nitrate) was reacted with 1.00 g of Peerless Fe0 at 200 rpm on a rotational shaker at 23 degrees C for up to 120 h in the presence of each of the organic acids (3.0 mM formic, 1.5 mM oxalic, and 1.0 mM citric acids) and inorganic acids (3.0 mM HCl, 1.5 mM H2SO4, 3.0 mM H3BO3, and 1.5 mM H3PO4). These acids provided an initial dissociable H+ concentration of 3.0 mM available for nitrate reduction reactions under conditions of final pH < 9.3. Nitrate reduction rates (pseudo-first-order) increased in the order: H3PO4 < citric acid < H3BO3 < oxalic acid < H2SO4 < formic acid < HCl, ranging from 0.00278 to 0.0913 h(-1), corresponding to surface area normalized rates ranging from 0.126 to 4.15 h(-1) m(-2) mL. Correlation analysis showed a negative linear relationship between the nitrate reduction rates for the ligands and the conditional stability constants for the soluble complexes of the ligands with Fe2+ (R2 = 0.701) or Fe3+ (R2 = 0.918) ions. This sequence of reactivity corresponds also to surface adsorption and complexation of the three organic ligands to iron oxides, which increase in the order formate < oxalate < citrate. The results are also consistent with the sequence of strength of surface complexation of the inorganic ligands to iron oxides, which increases in the order: chloride < sulfate < borate < phosphate. The blockage of reactive sites on the surface of Fe0 and its corrosion products by specific adsorption of the inner-sphere complex forming ligands (oxalate, citrate, sulfate, borate, and phosphate) may be responsible for the decreased nitrate reduction by Fe0 relative to the chloride system.