The reduction of 4-chloronitrobenzene by Fe(II)-Fe(III) oxide systems - correlations with reduction potential and inhibition by silicate

Mercury Isotope Fractionation during Dark Abiotic Reduction of Hg(II) by Dissolved, Surface-Bound, and Structural Fe(II)

Stable mercury (Hg) isotope ratios are an emerging tracer for biogeochemical transformations in environmental systems, but their application requires knowledge of isotopic enrichment factors for individual processes. We investigated Hg isotope fractionation during dark, abiotic reduction of Hg(II) by dissolved iron(Fe)(II), magnetite, and Fe(II) sorbed to boehmite or goethite by analyzing both the reactants and products of laboratory experiments. For homogeneous reduction of Hg(II) by dissolved Fe(II) in continuously purged reactors, the results followed a Rayleigh distillation model with enrichment factors of -2.20 ± 0.16‰ (ε202Hg) and 0.21 ± 0.02‰ (E199Hg). In closed system experiments, allowing reequilibration, the initial kinetic fractionation was overprinted by isotope exchange and followed a linear equilibrium model with -2.44 ± 0.17‰ (ε202Hg) and 0.34 ± 0.02‰ (E199Hg). Heterogeneous Hg(II) reduction by magnetite caused a smaller isotopic fractionation (-1.38 ± 0.07 and 0.13 ± 0.01‰), whereas the extent of isotopic fractionation of the sorbed Fe(II) experiments was similar to the kinetic homogeneous case. Small mass-independent fractionation of even-mass Hg isotopes with 0.02 ± 0.003‰ (E200Hg) and ≈ -0.02 ± 0.01‰ (E204Hg) was consistent with theoretical predictions for the nuclear volume effect. This study contributes significantly to the database of Hg isotope enrichment factors for specific processes. Our findings show that Hg(II) reduction by dissolved Fe(II) in open systems results in a kinetic MDF with a larger ε compared to other abiotic reduction pathways, and combining MDF with the observed MIF allows the distinction from photochemical or microbial Hg(II) reduction pathways.

Linear Free Energy Relationship for Predicting the Rate Constants of Munition Compound Reduction by the Fe(II)-Hematite and Fe(II)-Goethite Redox Couples

Abiotic reduction by iron minerals is arguably the most important fate process for munition compounds (MCs) in subsurface environments. No model currently exists that can predict the abiotic reduction rates of structurally diverse MCs by iron (oxyhydr)oxides. We performed batch experiments to measure the rate constants for the reduction of three classes of MCs (poly-nitroaromatics, nitramines, and azoles) by hematite or goethite in the presence of aqueous Fe2+. The surface area-normalized reduction rate constant (kSA) depended on the aqueous-phase one-electron reduction potential (EH1) of the MC and the thermodynamic state (i.e., pe and pH) of the iron oxide-Feaq2+ system. A linear free energy relationship (LFER), similar to that reported previously for nitrobenzene, successfully captures all MC reduction rate constants that span 6 orders of magnitude: log ( k S A ) = ( 1.12 ± 0.04 ) [ 0.53 E H 1 59 m V - ( p H + p e ) ] + ( 5.52 ± 0.23 ) . The finding that the rate constants of all the different classes of MCs can be described by a single LFER suggests that these structurally diverse nitro compounds are reduced by iron oxide-Feaq2+ couples through a common mechanism up to the rate-limiting step. Multiple mechanistic implications of the results are discussed. This study expands the applicability of the LFER model for predicting the reduction rates of legacy and emerging MCs and potentially other nitro compounds.

Thermodynamic Two-Site Surface Reaction Model for Predicting Munition Constituent Reduction Kinetics with Iron (Oxyhydr)oxides

Iron (oxyhydr)oxides comprise a significant portion of the redox-active fraction of soils and are key reductants for remediation of sites contaminated with munition constituents (MCs). Previous studies of MC reduction kinetics with iron oxides have focused on the concentration of sorbed Fe(II) as a key parameter. To build a reaction kinetic model, it is necessary to predict the concentration of sorbed Fe(II) as a function of system conditions and the redox state. A thermodynamic framework is formulated that includes a generalized double-layer model that utilizes surface acidity and surface complexation reactions to predict sorbed Fe(II) concentrations that are used for fitting MC reduction kinetics. Monodentate- and bidentate Fe(II)-binding sites are used with individual oxide sorption characteristics determined through data fitting. Results with four oxides (goethite, hematite, lepidocrocite, and ferrihydrite) and four nitro compounds (NB, CN-NB, Cl-NB, and NTO) from six separate studies have shown good agreement when comparing observed and predicted surface area-normalized rate constants. While both site types are required to reproduce the experimental redox titration, only the monodentate site concentration controls the MC reaction kinetics. This model represents a significant step toward predicting the timescales of MC degradation in the subsurface.

The formation of sulfate-green rust through Fe(II) sorption to montmorillonite: Impacts on abiotic nitrate reduction

Green rust (GR) minerals are generally considered to be effective reductants of pollutants and the electron transfer from aqueous Fe(II) to structural Fe(III) in montmorillonite has recently been discovered to be a pathway to GR formation at pH ∼7.8. In this study, we have further delineated the pH conditions and examined the effect of aqueous sulfate concentrations that allow for the formation of sulfate-GR through this unique pathway. Iron(II) sorption experiments demonstrated that the amount of 'sorbed' Fe(II) on montmorillonite semi-quantitatively transformed into sulfate-GR at pH values ≥7.5 in the presence of environmentally-relevant sulfate concentrations (i.e., 10 mM). However, excess sulfate concentrations (100 mM) resulted in comparatively less Fe(II) sorption and sulfate-GR was only observed to form at pH 8. As such, it was concluded that the degree of Fe(II) sorption to montmorillonite is critical to GR formation when aqueous Fe(II) and montmorillonite co-exist. In contrast to sulfate-GR minerals formed through other pathways (e.g., co-precipitation of dissolved Fe(II) and Fe(III) species), this montmorillonite-synthesized GR was significantly less reactive towards nitrate reduction, with <2.5 % of 0.2 mM nitrate being reduced over a 6-day period. This behaviour was correlated to reduction potential and it was, therefore, concluded that the relatively high reduction potential that occurs in the presence of montmorillonite exerts a significant influence on the rate of nitrate reduction by sulfate-GR to the point that it may not be a competitive process to microbiological nitrate denitrification. As such, the environmental relevance of green rust to nitrate reduction cannot be inferred simply by its presence, but rather the reduction potential of the environmental system in which it is found.

Electron Transfer Energy and Hydrogen Atom Transfer Energy-Based Linear Free Energy Relationships for Predicting the Rate Constants of Munition Constituent Reduction by Hydroquinones

No single linear free energy relationship (LFER) exists that can predict reduction rate constants of all munition constituents (MCs). To address this knowledge gap, we measured the reduction rates of MCs and their surrogates including nitroaromatics [NACs; 2,4,6-trinitrotoluene (TNT), 2,4-dinitroanisole (DNAN), 2-amino-4,6-dinitrotoluene (2-A-DNT), 4-amino-2,6-dinitrotoluene (4-A-DNT), and 2,4-dinitrotoluene (DNT)], nitramines [hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and nitroguanidine (NQ)], and azoles [3-nitro-1,2,4-triazol-5-one (NTO) and 3,4-dinitropyrazole (DNP)] by three dithionite-reduced quinones (lawsone, AQDS, and AQS). All MCs/NACs were reduced by the hydroquinones except NQ. Hydroquinone and MC speciations were varied by controlling pH, permitting the application of a speciation model to determine second-order rate constants (k) from observed pseudo-first-order rate constants. The intrinsic reactivity of MCs (oxidants) decreased upon deprotonation, while the opposite was true for hydroquinones (reductants). The rate constants spanned ∼6 orders of magnitude in the order NTO ≈ TNT > DNP > DNT ≈ DNAN ≈ 2-A-DNT > DNP^- > 4-A-DNT > NTO^- > RDX. LFERs developed using density functional theory-calculated electron transfer and hydrogen atom transfer energies and reported one-electron reduction potentials successfully predicted k, suggesting that these structurally diverse MCs/NACs are all reduced by hydroquinones through the same mechanism and rate-limiting step. These results increase the applicability of LFER models for predicting the fate and half-lives of MCs and related nitro compounds in reducing environments.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Reduction of 3-Nitro-1,2,4-Triazol-5-One (NTO) by the Hematite-Aqueous Fe(II) Redox Couple

3-Nitro-1,2,4-triazol-5-one (NTO) is an insensitive munition compound (MC) that has replaced legacy MC. NTO can be highly mobile in soil and groundwater due to its high solubility and anionic nature, yet little is known about the processes that control its environmental fate. We studied NTO reduction by the hematite-Fe²⁺ redox couple to assess the importance of this process for the attenuation and remediation of NTO. Fe²⁺_(aq) was either added (type I) or formed through hematite reduction by dithionite (type II). In the presence of both hematite and Fe²⁺_(aq), NTO was quantitatively reduced to 3-amino-1,2,4-triazol-5-one following first-order kinetics. The surface area-normalized rate constant (k_SA) showed a strong pH dependency between 5.5 and 7.0 and followed a linear free energy relationship (LFER) proposed in a previous study for nitrobenzene reduction by iron oxide-Fe²⁺ couples, i.e., log k_SA = -(pe + pH) + constant. Sulfite, a major dithionite oxidation product, lowered k_SA in type II system by ∼10-fold via at least two mechanisms: by complexing Fe²⁺ and thereby raising pe, and by making hematite more negatively charged and hence impeding NTO adsorption. This study demonstrates the importance of iron oxide-Fe²⁺ in controlling NTO transformation, presents an LFER for predicting NTO reduction rate, and illustrates how solutes can shift the LFER by interacting with either iron species.

Role of Carbonate in Thermodynamic Relationships Describing Pollutant Reduction Kinetics by Iron Oxide-Bound Fe2

The reduction of environmental pollutants by Fe²⁺ bound to iron oxides is an important process that determines pollutant toxicities and mobilities. Recently, we showed that pollutant reduction rates depend on the thermodynamic driving force of the reaction in a linear free energy relationship that was a function of the solution pH value and the reduction potential, E_H, of the interfacial Fe³⁺/Fe²⁺ redox couple. In this work, we studied how carbonate affected the free energy relationship by examining the effect that carbonate has on nitrobenzene reduction rates by Fe²⁺ bound to goethite (α-FeOOH). Carbonate slowed nitrobenzene reduction rates by inducing goethite particle aggregation, as evidenced by surface charge and particle size measurements. We observed no evidence for carbonate affecting Fe³⁺/Fe²⁺ reduction potentials or the mechanism of nitrobenzene reduction. The linear free energy relationship accurately described the data collected in the presence of carbonate when we accounted for the effect it had on the reactive surface area of goethite. The findings from this work provide a framework for determining why common groundwater constituents affect the E_H-dependence of reaction rates involving oxide-bound Fe²⁺ as a reductant.

In-situ generation of multi-homogeneous/heterogeneous Fe-based Fenton catalysts toward rapid degradation of organic pollutants at near neutral pH

In this study, an in-situ generated multi-homogeneous/heterogeneous Fe-based catalytic system was developed, which exhibited a high efficiency for the production of •OH and rapid degradation of various organic pollutants in a near neutral pH range (5-8). The mechanism for the rapid decomposition of H₂O₂ and the generation of •OH were investigated in detail. The results indicated that, besides the introduced Fe²⁺, the in-situ generated various iron species including Fe(OH)⁺, Fe(OH)₂, Fe³⁺, ferrihydrite (Fh), γ-FeOOH and α-FeOOH as well as Fe^II/Fh, Fe^II/γ-FeOOH and Fe^II/α-FeOOH could simultaneously act as homogeneous and heterogeneous Fenton reaction catalysts. The dropwise addition manner of Fe²⁺ greatly improved the catalytic efficiency of Fe²⁺ ions in near neutral pH environment, while the in-situ generated nanosized Fh, γ-FeOOH and α-FeOOH could supply numerous active catalytic sites. After degradation, the ferrous ions could be transformed to various crystalline iron oxides by the catalytic phase transformation. This study presents a method towards the rational design of novel Fenton catalysts for wastewater treatment.

Experimental Validation of Hydrogen Atom Transfer Gibbs Free Energy as a Predictor of Nitroaromatic Reduction Rate Constants

Nitroaromatic compounds (NACs) are a class of prevalent contaminants. Abiotic reduction is an important fate process that initiates NAC degradation in the environment. Many linear free energy relationship (LFER) models have been developed to predict NAC reduction rates. Almost all LFERs to date utilize experimental aqueous-phase one-electron reduction potential ( E_H¹) of NAC as a predictor, and thus, their utility is limited by the availability of E_H¹ data. A promising new approach that utilizes computed hydrogen atom transfer (HAT) Gibbs free energy instead of E_H¹ as a predictor was recently proposed. In this study, we evaluated the feasibility of HAT energy for predicting NAC reduction rate constants. Using dithionite-reduced quinones, we measured the second-order rate constants for the reduction of seven NACs by three hydroquinones of different protonation states. We computed the gas-phase energies for HAT and electron affinity (EA) of NACs and established HAT- and EA-based LFERs for six hydroquinone species. The results suggest that HAT energy is a reliable predictor of NAC reduction rate constants and is superior to EA. This is the first independent, experimental validation of HAT-based LFER, a new approach that enables rate prediction for a broad range of structurally diverse NACs based solely on molecular structures.

Impact of Fe(II) oxidation in the presence of iron-reducing bacteria on subsequent Fe(III) bio-reduction

The interplay of Fe(II) oxidation and Fe(III) bio-reduction occurs widely in both natural and engineered redox-dynamic systems. This study aimed to unravel the impact of Fe(II) oxidation by O₂ in the presence of iron-reducing bacteria on subsequent Fe(III) bio-reduction. Mixed solutions of Fe²⁺ (0.1-0.5 mM) and Shewanella oneidensis strain MR-1 (MR-1, 2.0 × 10⁷ CFU/mL) at neutral pH were first exposed to laboratory air for Fe(II) oxidation and bacterial inactivation, and then the resultant Fe(III) suspensions were switched to anoxic conditions for bio-reduction by the surviving bacteria. In the oxidation step, the coexisting MR-1 was inactivated by 0.8-1.71 orders of magnitude within 60 min. In the subsequent bio-reduction step, the resultant Fe(III) was bio-reduced by the surviving MR-1. Bio-reduction of the resultant Fe(III) by the surviving MR-1 was 1.8-2.5 times faster than that of the Fe(III) that was produced from Fe²⁺ oxidation without MR-1 by fresh MR-1 cells at 2.0 × 10⁷ CFU/mL. Although MR-1 inactivation during Fe(II) oxidation may inhibit Fe(III) bio-reduction, the increase in bio-availability of the resultant Fe(III) and the residual reactivity of dead cells led to net enhancement of bio-reduction under the tested conditions. Lepidocrocite was the sole Fe(III) mineral that was produced from Fe²⁺ oxidation without MR-1, while 19% ferrihydrite was produced from Fe²⁺ oxidation in the presence of MR-1. The formation of low-crystallinity ferrihydrite accounts for the increase in bio-availability of the Fe(III) minerals. The findings of this study highlight an important but overlooked impact underlying the interplay of Fe(II) oxidation and Fe(III) bio-reduction.

Linking Thermodynamics to Pollutant Reduction Kinetics by Fe2+ Bound to Iron Oxides

Numerous studies have reported that pollutant reduction rates by ferrous iron (Fe²⁺) are substantially enhanced in the presence of an iron (oxyhydr)oxide mineral. Developing a thermodynamic framework to explain this phenomenon has been historically difficult due to challenges in quantifying reduction potential ( E_H) values for oxide-bound Fe²⁺ species. Recently, our group demonstrated that E_H values for hematite- and goethite-bound Fe²⁺ can be accurately calculated using Gibbs free energy of formation values. Here, we tested if calculated E_H values for oxide-bound Fe²⁺ could be used to develop a free energy relationship capable of describing variations in reduction rate constants of substituted nitrobenzenes, a class of model pollutants that contain reducible aromatic nitro groups, using data collected here and compiled from the literature. All the data could be described by a single linear relationship between the logarithms of the surface-area-normalized rate constant ( k_SA) values and E_H and pH values [log( k_SA) = - E_H/0.059 V - pH + 3.42]. This framework provides mechanistic insights into how the thermodynamic favorability of electron transfer from oxide-bound Fe²⁺ relates to redox reaction kinetics.

Improvement of zinc substitution in the reactivity of magnetite coupled with aqueous Fe(II) towards nitrobenzene reduction

The reduction of nitrobenzene (NB) by Zn-substituted magnetite coupled with aqueous Fe(II) was studied. A series of Zn-substituted magnetites (Fe_3-xZn_xO₄, x = 0, 0.25, 0.49, 0.74, and 0.99) were synthesized by a coprecipitation method followed by systematic analysis of the variation in structure and physicochemical properties of magnetite using XRD, TEM, TG, BET and XAFS. All of the samples had a spinel structure by Zn substitution. Zn²⁺ primarily occupied the tetrahedral sites, but a portion of them moved to the octahedral sites at higher Zn level. Zn substitution increased the BET specific surface area and surface hydroxyl amount. The electron balance indicated that the NB reduction was primarily through the heterogeneous reaction by Fe_3-xZn_xO₄ and adsorbed Fe(II), where NB in aqueous solution was reduced by structural Fe²⁺ in magnetite recharged by adsorbed Fe(II). Various factors, such as aqueous Fe(II) concentration, magnetite stoichiometry and Zn level, were investigated to illustrate their effects on the reduction processes. Both the rate constant k_obs and electron transfer amount illustrated that Zn substitution generally improved the reduction activity of the Fe_3-xZn_xO₄/Fe(II) system, while overdose of Zn retarded the process. This issue was attributed to the variation in electron conductivity of Fe_3-xZn_xO₄ and Zn²⁺ occupancy.

Fe(II) Interactions with Smectites: Temporal Changes in Redox Reactivity and the Formation of Green Rust

In this study, temporal changes in the redox properties of three 0.5 g/L smectite suspensions were investigated-a montmorillonite (MAu-1) and two nontronites (NAu-1 and NAu-2) in the presence of 1 mM aqueous Fe(II) at pH 7.8. X-ray absorption spectroscopy revealed that the amount of Fe(II) added quantitatively transformed into chloride-green rust (Cl-GR) within 5 min and persisted over 18 days. Over the same time, the reduction potential of all three suspensions increased by 50 to 150 mV to equilibrate at approximately -100 mV vs SHE. The reduction of a model organic contaminant, 4-chloronitrobenzene (4-CINB), also became increasingly slower over time with virtually no 4-CINB reduction being observed after 18 days. There was a strong correlation between reduction potential and the quantity of 4-ClNB reduced by MAu-1, although other factors were likely involved in the decreased redox reactivity observed in the nontronites. It is hypothesized that the temporal increase in reduction potential results from clay mineral dissolution resulting in increased Fe(III) contents in the Cl-GR. These results demonstrate that long-term studies are recommended to accurately predict contaminant management strategies.

Influence of Dissolved Silicate on Rates of Fe(II) Oxidation

Increasing concentrations of dissolved silicate progressively retard Fe(II) oxidation kinetics in the circum-neutral pH range 6.0-7.0. As Si:Fe molar ratios increase from 0 to 2, the primary Fe(III) oxidation product transitions from lepidocrocite to a ferrihydrite/silica-ferrihydrite composite. Empirical results, supported by chemical kinetic modeling, indicated that the decreased heterogeneous oxidation rate was not due to differences in absolute Fe(II) sorption between the two solids types or competition for adsorption sites in the presence of silicate. Rather, competitive desorption experiments suggest Fe(II) was associated with more weakly bound, outer-sphere complexes on silica-ferrihydrite compared to lepidocrocite. A reduction in extent of inner-sphere Fe(II) complexation on silica-ferrihydrite confers a decreased ability for Fe(II) to undergo surface-induced hydrolysis via electronic configuration alterations, thereby inhibiting the heterogeneous Fe(II) oxidation mechanism. Water samples from a legacy radioactive waste site (Little Forest, Australia) were shown to exhibit a similar pattern of Fe(II) oxidation retardation derived from elevated silicate concentrations. These findings have important implications for contaminant migration at this site as well as a variety of other groundwater/high silicate containing natural and engineered sites that might undergo iron redox fluctuations.