Iron Atom Exchange between Hematite and Aqueous Fe(II)

Divergent redistribution behavior of divalent metal cations associated with Fe(II)-mediated jarosite phase transformation

The Fe(II)/Fe(III) cycle is an important driving force for dissolution and transformation of jarosite. Divalent heavy metals usually coexist with jarosite; however, their effects on Fe(II)-induced jarosite transformation and different repartitioning behavior during mineral dissolution-recrystallization are still unclear. Here, we investigated Fe(II)-induced (1 mM Fe(II)) jarosite conversion in the presence of Cd(II), Mn(II), Co(II), Ni(II) and Pb(II) (denoted as Me(II), 1 mM), respectively, under anaerobic condition at neutral pH. The results showed that all co-existing Me(II) retarded Fe(II)-induced jarosite dissolution. In the Fe(II)-only system, jarosite first rapidly transformed to lepidocrocite (an intermediate product) and then slowly to goethite; lepidocrocite was the main product. In Fe(II)-Cd(II), -Mn(II), and -Pb(II) systems, coexisting Cd(II), Mn(II) and Pb(II) retarded the above process and lepidocrocite was still the dominant conversion product. In Fe(II)-Co(II) system, coexisting Co(II) promoted lepidocrocite transformation into goethite. In Fe(II)-Ni(II) system, jarosite appeared to be directly converted into goethite, although small amounts of lepidocrocite were detected in the final product. In all treatments, the appearance or accumulation of lepidocrocite may be also related to the re-adsorption of released sulfate. By the end of reaction, 6.0 %, 4.0 %, 76.0 % 11.3 % and 19.2 % of total Cd(II), Mn(II), Pb(II) Co(II) and Ni(II) were adsorbed on the surface of solid products. Up to 49.6 %, 44.3 %, and 21.6 % of Co(II), Ni(II), and Pb(II) incorporated into solid product, with the reaction indicating that the dynamic process of Fe(II) interaction with goethite may promote the continuous incorporation of Co(II), Ni(II), and Pb(II).

Precise measurement of Fe isotopes in marine and biological samples by pseudo-high-resolution multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS)

This paper introduces an enhanced technique for analyzing iron isotopes in complex marine and biological samples. A dedicated iron purification method for biological marine matrices, utilizing three ion exchange columns, is validated. The MC-ICPMS in pseudo-high-resolution mode determines precise iron isotopic ratios, with sensitivity improved through the DSN-100 desolvating nebulizer system and Apex-IR. Only 2 µg of iron on DSN versus 1 µg on Apex is needed for six replicates (30-60 times improvement) while 10 to 20 µg is required for a single measurement on a wet system considering the resolution power (Rp) is maintained at 11,000-13,000. The Ni-doping method with a Fe/Ni ratio of 1 yields more accurate isotopic ratios than standard-sample bracketing alone. Measurement reproducibility of triplicate samples from marine biological experiments on MC-ICPMS is ± 0.03‰ (2SD) for δ56Fe and ± 0.07‰ for δ57Fe (2SD). This study introduces a novel iron purification process specifically designed for marine and biological samples, enhancing sensitivity and enabling more reliable measurements with smaller sample sizes and reduced uncertainties. It proposes iron isotopic compositions for biological reference materials, offering a valuable reference dataset in diverse scientific disciplines.

Ferrihydrite transformation impacted by coprecipitation of lignin: Inhibition or facilitation?

Lignin is a common soil organic matter that is present in soils, but its effect on the transformation of ferrihydrite (Fh) remains unclear. Organic matter is generally assumed to inhibit Fh transformation. However, lignin can reduce Fh to Fe(II), in which Fe(II)-catalyzed Fh transformation occurs. Herein, the effects of lignin on Fh transformation were investigated at 75°C as a function of the lignin/Fh mass ratio (0-0.2), pH (4-8) and aging time (0-96 hr). The results of Fh-lignin samples (mass ratios = 0.1) aged at different pH values showed that for Fh-lignin the time of Fh transformation into secondary crystalline minerals was significantly shortened at pH 6 when compared with pure Fh, and the Fe(II)-accelerated transformation of Fh was strongly dependent on pH. Under pH 6, at low lignin/Fh mass ratios (0.05-0.1), the time of secondary mineral formation decreased with increasing lignin content. For high lignosulfonate-content material (lignin:Fh = 0.2), Fh did not transform into secondary minerals, indicating that lignin content plays a major role in Fh transformation. In addition, lignin affected the pathway of Fh transformation by inhibiting goethite formation and facilitating hematite formation. The effect of coprecipitation of lignin on Fh transformation should be useful in understanding the complex iron and carbon cycles in a soil environment.

Decoding the divalent cation effect on sulfidation of zero-valent iron: Phase evolution and FeSx assembly

The decontamination ability of sulfidated zero-valent iron (S-ZVI) can be enhanced by the effective assembly of iron sulfides (FeSx) on neglected heterogeneous surfaces by liquid-phase precipitation. However, S-ZVI preparation with the usual pickling is detrimental to orderly interfacial assembly and leads to an imbalance between electron transfer optimization and electron storage. In this work, S-ZVI was prepared in solutions containing trace divalent cation, and it removed Cr(VI) up to 323.25 times higher than ZVI. This result is achieved by surface sites protonation of divalent cations regulating the phase evolution on the ZVI surface and inducing FeSx chemical assembly. Regulation of divalent cation and S(-II) content further promotes FeSx targeted assembly and reduces electron storage consumption as much as possible. The barrier for FeSx assembly is found to lie at the ZVI interface rather than in the deposition between FeSx. Chemical assembly at heterogeneous interfaces is a prerequisite for the ordered assembly of FeSx. In addition, S-ZVI prepared in simulated groundwater showed extensive preparation pH and universality for remediation scenarios. These findings provide new insights into the development of in-situ sulfidation mechanisms with particular implications for S-ZVI applied to soil and groundwater remediation by the regulation of heterogeneous interfacial assembly.

Nalidixic Acid and Fe(II)/Cu(II) Coadsorption at Goethite and Akaganéite Surfaces

Interactions between aqueous Fe(II) and solid Fe(III) oxy(hydr)oxide surfaces play determining roles in the fate of organic contaminants in nature. In this study, the adsorption of nalidixic acid (NA), a representative redox-inactive quinolone antibiotic, on synthetic goethite (α-FeOOH) and akaganéite (β-FeOOH) was examined under varying conditions of pH and cation type and concentration, by means of adsorption experiments, attenuated total reflectance-Fourier transform infrared spectroscopy, surface complexation modeling (SCM), and powder X-ray diffraction. Batch adsorption experiments showed that Fe(II) had marginal effects on NA adsorption onto akaganéite but enhanced NA adsorption on goethite. This enhancement is attributed to the formation of goethite-Fe(II)-NA ternary complexes, without the need for heterogeneous Fe(II)-Fe(III) electron transfer at low Fe(II) loadings (2 Fe/nm2), as confirmed by SCM. However, higher Fe(II) loadings required a goethite-magnetite composite in the SCM to explain Fe(II)-driven recrystallization and its impact on NA binding. The use of a surface ternary complex by SCM was supported further in experiments involving Cu(II), a prevalent environmental metal incapable of transforming Fe(III) oxy(hydr)oxides, which was observed to enhance NA loadings on goethite. However, Cu(II)-NA aqueous complexation and potential Cu(OH)2 precipitates counteracted the formation of ternary surface complexes, leading to decreased NA loadings on akaganéite. These results have direct implications for the fate of organic contaminants, especially those at oxic-anoxic boundaries.

Rape straw biochar enhanced Cd immobilization in flooded paddy soil by promoting Fe and sulfur transformation

Cd is normally associated with sulfide and Fe oxides in flooded paddy soil. The mechanisms of biochar enhanced Cd immobilization by promoting Fe transformation and sulfide formation are unclear. Rape straw biochar (RSB) pyrolyzed at 450 °C (LB) and 800 °C (HB) was added to Cd-contaminated paddy soil at 1% (LB1, HB1) and 2% (LB2, HB2) doses. The results showed that Fe/Mn oxide-Cd (Fe/Mn-Cd) and free Fe oxide (Fed) concentrations decreased in the first 12 days and then rose, while Fe2+ in pore water (W-Fe2+) tended to rise first and then fall. The electron transfer rate of soil in the HB2 treatment was 4.9-fold higher than that in the treatment without biochar (CK). Fe oxide reduction was enhanced by RSB, with a maximum increase in W-Fe2+ by 62.1% in HB2 on Day 12. The negative correlation between W-Fe2+ and Fed showed that Fe2+ promoted the reformatted of seconded Fe minerals after Day 12, and the Fed in the HB2 treatments increased by 31.5% in this period. RSB addition also promoted the reformation of poorly crystallized Fe oxide (Feo) by increasing soil pH, which increased by 17.2% and 15.1% on average in the LB2 and HB2 treatments, respectively, compared to CK. Compared to Day 7, the increased rate of Fe/Mn-Cd on Day 30 in RSB was approximately twice that of CK. Compared to the molybdate group, the maximum decrease in CaCl2-Cd was 29.1% in LB2 on Day 12. LB2 increased SO42- and acid-volatile sulfide concentrations by 6.9- and 4.1-fold, respectively, compared to CK. These results suggested that RSB, particularly HB, promoted more Cd adsorption in Fe minerals by increasing Fe hydroxylation and recrystallization processes. LB increased the contribution of sulfide to Cd immobility.

Electron Transfer, Atom Exchange, and Transformation of Iron Minerals in Soils: The Influence of Soil Organic Matter

Despite substantial experimental evidence of electron transfer, atom exchange, and mineralogical transformation during the reaction of Fe(II)_aq with synthetic Fe(III) minerals, these processes are rarely investigated in natural soils. Here, we used an enriched Fe isotope approach and Mössbauer spectroscopy to evaluate how soil organic matter (OM) influences Fe(II)/Fe(III) electron transfer and atom exchange in surface soils collected from Luquillo and Calhoun Experimental Forests and how this reaction might affect Fe mineral composition. Following the reaction of ⁵⁷Fe-enriched Fe(II)_aq with soils for 33 days, Mössbauer spectra demonstrated marked electron transfer between sorbed Fe(II) and the underlying Fe(III) oxides in soils. Comparing the untreated and OM-removed soils indicates that soil OM largely attenuated Fe(II)/Fe(III) electron transfer in goethite, whereas electron transfer to ferrihydrite was unaffected. Soil OM also reduced the extent of Fe atom exchange. Following reaction with Fe(II)_aq for 33 days, no measurable mineralogical changes were found for the Calhoun soils enriched with high-crystallinity goethite, while Fe(II) did drive an increase in Fe oxide crystallinity in OM-removed LCZO soils having low-crystallinity ferrihydrite and goethite. However, the presence of soil OM largely inhibited Fe(II)-catalyzed increases in Fe mineral crystallinity in the LCZO soil. Fe atom exchange appears to be commonplace in soils exposed to anoxic conditions, but its resulting Fe(II)-induced recrystallization and mineral transformation depend strongly on soil OM content and the existing soil Fe phases.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Magnetic anaerobic granular sludge for sequestration and immobilization of Pb

The development of magnetic adsorbents with high capacity to capture heavy metals has been the subject of intense research, but the process usually involves costive synthesis steps. Here, we propose a green approach to obtaining a magnetic biohybrid through in situ grown anaerobic granular sludge (AGS) with the help of magnetite, constituting a promising adsorbent for sequestration and immobilization of Pb in aqueous solutions and soils. The resultant magnetite-embedded AGS (M-AGS) was not only capable of promoting methane production but also conducive to Pb adsorption because of the large surface area and abundant function groups. The uptake of Pb on M-AGS followed the pseudo-second order, having a maximum adsorption capacity of 197.8 mg gDS^-1 at pH 5.0, larger than 159.7, 170.3, and 178.1 mg gDS^-1 in relation to AGS, F-AGS (ferrihydrite-mediated), and H-AGS (hematite-mediated), respectively. Mechanistic investigations showed that Pb binding to M-AGS proceeds via surface complexation, mineral precipitation, and lattice replacement, which promotes heavy metal capture and stabilization. This was evident from the increased proportion of structural Pb sequestrated from the aqueous solution and the enhanced percentage of the residual fraction of Pb extracted from the contaminated soils.

Organic buffers act as reductants of abiotic and biogenic manganese oxides

Proton activity is the master variable in many biogeochemical reactions. To control pH, laboratory studies involving redox-sensitive minerals like manganese (Mn) oxides frequently use organic buffers (typically Good's buffers); however, two Good's buffers, HEPES and MES, have been shown to reduce Mn(IV) to Mn(III). Because Mn(III) strongly controls mineral reactivity, avoiding experimental artefacts that increase Mn(III) content is critical to avoid confounding results. Here, we quantified the extent of Mn reduction upon reaction between Mn oxides and several Good's buffers (MES, pK_a = 6.10; PIPES, pK_a = 6.76; MOPS, pK_a = 7.28; HEPES, pK_a = 7.48) and TRIS (pK_a = 8.1) buffer. For δ-MnO₂, Mn reduction was rapid, with up to 35% solid-phase Mn(III) generated within 1 h of reaction with Good's buffers; aqueous Mn was minimal in all Good's buffers experiments except those where pH was one unit below the buffer pK_a and the reaction proceeded for 24 h. Additionally, the extent of Mn reduction after 24 h increased in the order MES < MOPS < PIPES < HEPES << TRIS. Of the variables tested, the initial Mn(II,III) content had the greatest effect on susceptibility to reduction, such that Mn reduction scaled inversely with the initial average oxidation number (AMON) of the oxide. For biogenic Mn oxides, which consist of a mixture of Mn oxides, bacterial cells and extracelluar polymeric substances, the extent of Mn reduction was lower than predicted from experiments using abiotic analogs and may result from biotic re-oxidation of reduced Mn or a difference in the reducibility of abiotic versus biogenic oxides. The results from this study show that organic buffers, including morpholinic and piperazinic Good's buffers and TRIS, should be avoided for pH control in Mn oxide systems due to their ability to transfer electrons to Mn, which modifies the composition and reactivity of these redox-active minerals.

Iron isotopic fractionation driven by low-temperature biogeochemical processes

Iron is geologically important and biochemically crucial for all microorganisms, plants and animals due to its redox exchange, the involvement in electron transport and metabolic processes. Despite the abundance of iron in the earth crust, its bioavailability is very limited in nature due to its occurrence as ferrihydrite, goethite, and hematite where they are thermodynamically stable with low dissolution kinetics in neutral or alkaline environments. Organisms such as bacteria, fungi, and plants have evolved iron acquisition mechanisms to increase its bioavailability in such environments, thereby, contributing largely to the iron cycle in the environment. Biogeochemical cycling of metals including Fe in natural systems usually results in stable isotope fractionation; the extent of fractionation depends on processes involved. Our review suggests that significant fractionation of iron isotopes occurs in low-temperature environments, where the extent of fractionation is greatly governed by several biogeochemical processes such as redox reaction, alteration, complexation, adsorption, oxidation and reduction, with or without the influence of microorganisms. This paper includes relevant data sets on the theoretical calculations, experimental prediction, as well as laboratory studies on stable iron isotopes fractionation induced by different biogeochemical processes.

Iron Vacancy Accelerates Fe(II)-Induced Anoxic As(III) Oxidation Coupled to Iron Reduction

Chemical oxidation of As(III) by iron (Fe) oxyhydroxides has been proposed to occur under anoxic conditions and may play an important role in stabilization and detoxification of As in subsurface environments. However, this reaction remains controversial due to lack of direct evidence and poorly understood mechanisms. In this study, we show that As(III) oxidation can be facilitated by Fe oxyhydroxides (i.e., goethite) under anoxic conditions coupled with the reduction of structural Fe(III). An excellent electron balance between As(V) production and Fe(III) reduction is obtained. The formation of an active metastable Fe(III) phase at the defective surface of goethite due to atom exchange is responsible for the oxidation of As(III). Furthermore, the presence of defects (i.e., Fe vacancies) in goethite can noticeably enhance the electron transfer (ET) and atom exchange between the surface-bound Fe(II) and the structural Fe(III) resulting in a two time increase in As(III) oxidation. Atom exchange-induced regeneration of active goethite sites is likely to facilitate As(III) coordination and ET with structural Fe(III) based on electrochemical analysis and theoretical calculations showing that this reaction pathway is thermodynamically and kinetically favorable. Our findings highlight the synergetic effects of defects in the Fe crystal structure and Fe(II)-induced catalytic processes on anoxic As(III) oxidation, shedding a new light on As risk management in soils and subsurface environments.

The nature of metal atoms incorporated in hematite determines oxygen activation by surface-bound Fe(II) for As(III) oxidation

The incorporation of secondary metal atoms into iron oxyhydroxides may regulate the surface chemistry of mediating electron transfer (ET) and, therefore, the biogeochemical pollutant processes such as arsenic (As) in the subsurface and soils. The influence of incorporating two typical metals (Cu and Zn) into a specific {001} hematite facet on O₂ activation by surface-bound Fe(II) was addressed. The results showed that Cu-incorporated hematite enhances As(III) oxidation in the presence of Fe(II) under oxic conditions and increases with increasing Cu content. Conversely, Zn incorporation leads to the opposite trend. The As(III) oxidation induced by surface-bound Fe(II) is positively related to the Fe(II) content and is favorable under acidic conditions. Reactive oxygen species (ROS), such as superoxide (·O^2-) and H₂O₂, predominantly contribute to As(III) oxidation as a result of 1-electron transfer from bound Fe(II) to surface O₂ on hematite and radical propagation. Electrochemical analysis demonstrates that Cu incorporation significantly lower the oxidation potential of Fe(II) on hematite, whereas Zn led to a higher reaction potential for Fe(II) oxidation. Subsequently, distinct surface reactivities of hematite for the activation of O₂ to form ROS by surface-bound Fe(II) are evidenced by metal incorporation. Our study provides a new understanding of the changes in the surface chemistry of iron oxyhydroxides because of incorporating metals (Zn and Cu), and therefore impact the biogeochemical processes of pollutants in soils and subsurface environments.

Iron(II)-activated phase transformation of Cd-bearing ferrihydrite: Implications for cadmium mobility and fate under anaerobic conditions

The factors and mechanisms affecting the fate of the associated Cd during the Fe(II)-activated Cd-bearing ferrihydrite transformation remain poorly understood. Herein we have conducted a series of batch reactions containing ferrihydrite with diverse pH values and initial Fe(II) and Cd concentrations coupled with chemical analyses and spectroscopic examination on the transformation products to probe the mechanisms of the Cd partitioning and the processes of Fe(II)-activated Cd-bearing ferrihydrite transformation under anaerobic conditions. Chemical analyses, Fourier transform infrared spectroscopy (FTIR), and powder X-ray diffraction (PXRD) results show that the initial Fe(II) and Cd concentrations as well as pH values all have significant effects on the rates and pathways of ferrihydrite transformation. Increasing Cd loading enhances the inhibition of the Fe(II)-activated ferrihydrite transformation rates. High Cd loading alters the Fe(II)-activated ferrihydrite transformation pathways by hindering the recrystallization of both ferrihydrite to more stable iron minerals and the newly formed lepidocrocite to goethite. Chemical analyses show that the release of Cd to solutions during ferrihydrite transformation is accompanied by a reduction in the 0.4 M HCl extractable Cd fraction and that a significant amount of the released Cd is transformed to a 0.4 M HCl unextractable form. Moreover, enhanced Cd release during the Fe(II)-activated ferrihydrite transformation is observed by reducing the pH value or increasing the initial Cd concentration. Results from synchrotron X-ray absorption spectroscopy (XAS) confirm that the majority of the 0.4 M HCl unextractable Cd form is associated with structural incorporation into the recrystallized iron (hydr)oxides via isomorphous substitution for Fe(III). These findings not only provide molecular-level understanding on the behavior of Cd under natural anoxic environments, but also are useful in predicting the geochemical cycling of Cd and developing long-term Cd contaminant management strategies.

Mobilization of arsenic from As-containing iron minerals under irrigation: Effects of exogenous substances, redox condition, and intermittent flow

Irrigation activities can cause strong geochemical and hydrological fluctuations in the unsaturated zone, and affect arsenic (As) migration and transformation. The As geochemical cycle in the unsaturated zone is coupled with that of iron minerals through sorption-desorption, coprecipitation and redox processes. Dynamic batch experiments and wetting-drying cycling column experiments were conducted to evaluate As mobilization behaviors under the effects of exogenous substances, redox condition and intermittent flow. Our results show that As release under exogenous substances carried by irrigation (e.g., phosphate, carbonate, fulvic acid, humic acid, etc.) followed three trends with the types of exogenous inputs. Inorganic anions and organic matter resulted in opposite trends of arsenate release in different redox conditions. In anoxic environments, As(V) release was favored by the addition of phosphate and carbonate, while in oxic environments, the mobilization of As(V) was promoted by the addition of fulvic acid (FA). Further, intermittent irrigation promoted the reductive dissolution of Fe oxides and the mobilization of As. The addition of humic acid (HA) resulted in the mobilization of arsenate as As-Fe-HA ternary complexes. The mechanism of arsenic mobilization under irrigation has importance for prevention of arsenic exposure through soil to food chain transfer in typical high arsenic farmland.

Exogenous iron alters uptake and translocation of CuO nanoparticles in soil-rice system: A life cycle study

The abundant iron in farmland soil may affect the environmental fate of metal-based nanoparticles (MNPs). In this study, the effect of FeSO4 and nano-zero-valent iron (nZVI) as exogenous iron on the uptake and translocation of CuO nanoparticles (NPs) in soil-rice system was performed in a life cycle study. The results show that exogenous iron basically elevated the soil pH and electrical conductivity but lowered the redox potential. Moreover, the Cu bioavailability in soil was significantly increased by 86-269% with exogenous iron at the tillering stage, while was reduced by 15-45% with medium and high concentrations of Fe(II) at the maturation stage. Meanwhile, the addition of exogenous iron resolved the unfilling of grains caused by CuO NPs. Notably, except for highest Fe(II) treatment, both Fe(II) and nZVI reduced Cu accumulation from 31% to 84% in roots and leaves due to more iron plaque. Especially, medium Fe(II) level markedly decreased the Cu content in the brown rice. μ-XRF analysis suggests that high intensity of Cu was primarily located in the rice hull and embryo under Fe(II) treatment. The reduction of CuO NPs to Cu2O caused by Fe(II) can explain the positive effect of exogenous iron on controlling the environmental risk of MNPs.

Exchange of Adsorbed Pb(II) at the Rutile Surface: Rates and Mechanisms

The dynamics of Pb(II) at mineral surfaces affect its mobility in the environment. Pb(II) forms inner- and outer-sphere complexes on mineral surfaces, and this adsorbed pool often represents a large portion of the bioaccessible Pb in contaminated soils. To assess the lability of this potentially reactive adsorbed Pb(II) pool at metal oxide surfaces, we performed Pb(II) isotope exchange measurements between dissolved Pb(II) enriched in ²⁰⁷Pb and natural isotopic abundance Pb(II) adsorbed to rutile at pH 5, 6, and 7. We find that ∼95% of the adsorbed lead is exchangeable. An initially fast exchange (<1 h) is followed by a slower exchange that occurs on a time scale of hours to days. Pb L_III-edge extended X-ray absorption fine structure spectra indicate that similar binding mechanisms are present at all pH values and Pb(II) loadings, implying that differences in exchange rates across the pH range examined are not attributable to changes in the coordination environment. The slower exchange at pH 5 may be associated with interparticle and intraparticle diffusion resulting from particle aggregation. These findings demonstrate that the dissolved Pb(II) pool can be rapidly replenished by adsorbed Pb(II) if this pool is drawn down incrementally by biological uptake or a shift in chemical conditions.

Vanadate Retention by Iron and Manganese Oxides

Anthropogenic emissions of vanadium (V) into terrestrial and aquatic surface systems now match those of geogenic processes, and yet, the geochemistry of vanadium is poorly described in comparison to other comparable contaminants like arsenic. In oxic systems, V is present as an oxyanion with a +5 formal charge on the V center, typically described as H _x VO₄ ^(3-x)-, but also here as V(V). Iron (Fe) and manganese (Mn) (oxy)hydroxides represent key mineral phases in the cycling of V(V) at the solid-solution interface, and yet, fundamental descriptions of these surface-processes are not available. Here, we utilize extended X-ray absorption fine structure (EXAFS) and thermodynamic calculations to compare the surface complexation of V(V) by the common Fe and Mn mineral phases ferrihydrite, hematite, goethite, birnessite, and pyrolusite at pH 7. Inner-sphere V(V) complexes were detected on all phases, with mononuclear V(V) species dominating the adsorbed species distribution. Our results demonstrate that V(V) adsorption is exergonic for a variety of surfaces with differing amounts of terminal -OH groups and metal-O bond saturations, implicating the conjunctive role of varied mineral surfaces in controlling the mobility and fate of V(V) in terrestrial and aquatic systems.

Hydroxyl radical formation during oxygen-mediated oxidation of ferrous iron on mineral surface: Dependence on mineral identity

Many studies have examined the redox behavior of ferrous ions (Fe(II)) sorbed to mineral surfaces. However, the associated hydroxyl radical (^•OH) formation during Fe(II) oxidation by O₂ was rarely investigated at circumneutral pH. Therefore, we examined ^•OH formation during oxygenation of adsorbed Fe(II) (Fe(II)_sorbed) on common minerals. Results showed that 16.7 ± 0.4-25.6 ± 0.3 μM of ^•OH was produced in Fe(II) and α/γ-Al₂O₃ systems after oxidation of 24 h, much more than in systems with dissolved Fe(II) (Fe²⁺_aq) alone (10.3 ± 0.1 μM). However, ^•OH production in Fe(II) and α-FeOOH/α-Fe₂O₃ systems (6.9 ± 0.1-8.3 ± 0.1 μM) slightly decreased compared to Fe²⁺_aq only. Further analyses showed that enhanced oxidation of Fe(II)_sorbed was responsible for the increased ^•OH production in the Fe(II)/Al₂O₃ systems. In comparison, less Fe(II) was oxidized in the α-FeOOH/α-Fe₂O₃ systems, which was probably ascribed to the quick electron-transfer between Fe(II)_sorbed and Fe(III) lattice due to their semiconductor properties and induced formation of high-crystalline Fe(II) phases that hindered Fe(II) oxidation and ^•OH formation. The types of minerals and solution pH strongly affected Fe(II) oxidation and ^•OH production, which consequently impacted phenol degradation. This study highlights that the properties of minerals exert great impacts on surface-Fe(II) oxidation and ^•OH production during water/soil redox fluctuations.

Facet-dependent Fe(II) redox chemistry on iron oxide for organic pollutant transformation and mechanisms

Fe(II) redox chemistry is a pivotal process of biogeochemical Fe cycle and the transformation of organic pollutants in subsurface aquifers, while its interfacial reactivity on iron oxides with varying surface chemistries remains largely unexplored. In this study, the redox processes of Fe(II) on two hematite with highly exposed {001} and {110} facets and their impacts on the transformation of nitrobenzene were investigated. Results suggest that Fe(II) adsorption is the rate-limiting step of the redox chain reactions, controlling the reduction potential (E_H). Nitrobenzene activates the facet electron transfer on hematite, leading to nitrobenzene reduction and Fe(II) oxidation. Moreover, {001} facet exhibits a higher reactivity and electron transport efficiency than {110} facet, which is attributed to a lower site density (0.809 #Fe/nm²) and a lower E_H of hematite {001} facet than that of {110} facet. It is worth noting that the facet-dependent reduction activity is more intense at low pH or high Fe(II) activity. A slight dissolution of {110} facet was observed, indicating hematite {001} facet exhibits higher thermodynamic stability than {110} facet. This study confirms the facet-dependent reducing activity of surface bound Fe(II) on hematite, providing a new perspective for in-depth understanding of the interfacial reactions on hematite. The findings of this work broaden the biogeochemical process of Fe cycle in subsurface environments and its impact on the fate of organic pollutants in groundwater.

Redox-Driven Recrystallization of PbO2

Lead(IV) oxide (PbO₂) is one of the lead corrosion products that forms on the inner surface of lead pipes used for drinking water supply. It can maintain low dissolved Pb(II) concentrations when free chlorine is present. When free chlorine is depleted, PbO₂ and soluble Pb(II) will co-occur in these systems. This study used a stable lead isotope (²⁰⁷Pb) as a tracer to examine the interaction between aqueous Pb(II) and solid PbO₂ at conditions with no net change in dissolved Pb concentration. While the dissolved Pb(II) concentration remained unchanged, significant isotope exchange occurred that indicated that substantial amounts (24.3-35.0% based on the homogeneous recrystallization model) of the Pb atoms in the PbO₂ solids had been exchanged with those in solution over 264 h. Neither α-PbO₂ nor β-PbO₂ displayed a change in mineralogy, particle size, or oxidation state after reaction with aqueous Pb(II). The combined isotope exchange and solid characterization results indicate that redox-driven recrystallization of PbO₂ had occurred. Such redox-driven recrystallization is likely to occur in water that stagnates in lead pipes that contain PbO₂, and this recrystallization may alter the reactivity of PbO₂ with respect to its stability and susceptibility to reductive dissolution.

Semiconducting hematite facilitates microbial and abiotic reduction of chromium

Semi-conducting Fe oxide minerals, such as hematite, are well known to influence the fate of contaminants and nutrients in many environmental settings through sorption and release of Fe(II) resulting from microbial or abiotic reduction. Studies of Fe oxide reduction by adsorbed Fe(II) have demonstrated that reduction of Fe(III) at one mineral surface can result in the release of Fe(II) on a different one. This process is termed “Fe(II) catalyzed recrystallization” and is believed to be the result of electron transfer through semi-conducting Fe (hydr)oxides. While it is well understood that Fe(II) plays a central role in redox cycling of elements, the environmental implications of Fe(II) catalyzed recrystallization require further exploration. Here, we demonstrate that hematite links physically separated redox reactions by conducting the electrons involved in those reactions. This is shown using an electrochemical setup where Cr reduction is coupled with a potentiostat or Shewanella putrefaciens, a metal reducing microbe, where electrons donated to hematite produce Fe(II) that ultimately reduces Cr. This work demonstrates that mineral semi-conductivity may provide an additional avenue for redox chemistry to occur in natural soils and sediments, because these minerals can link redox active reactants that could not otherwise react due to physical separation.

Stabilization of Ferrihydrite and Lepidocrocite by Silicate during Fe(II)-Catalyzed Mineral Transformation: Impact on Particle Morphology and Silicate Distribution

Interactions between aqueous ferrous iron (Fe(II)) and secondary Fe oxyhydroxides catalyze mineral recrystallization and/or transformation processes in anoxic soils and sediments, where oxyanions, such as silicate, are abundant. However, the effect and the fate of silicate during Fe mineral recrystallization and transformation are not entirely understood and especially remain unclear for lepidocrocite. In this study, we reacted (Si-)ferrihydrite (Si/Fe = 0, 0.05, and 0.18) and (Si-)lepidocrocite (Si/Fe = 0 and 0.08) with isotopically labeled ⁵⁷Fe(II) (Fe(II)/Fe(III) = 0.02 and 0.2) at pH 7 for up to 4 weeks. We followed Fe mineral transformations with X-ray diffraction and tracked Fe atom exchange by measuring aqueous and solid phase Fe isotope fractions. Our results show that the extent of ferrihydrite transformation in the presence of Fe(II) was strongly influenced by the solid phase Si/Fe ratio, while increasing the Fe(II)/Fe(III) ratio (from 0.02 to 0.2) had only a minor effect. The presence of silicate increased the thickness of newly formed lepidocrocite crystallites, and elemental distribution maps of Fe(II)-reacted Si-ferrihydrites revealed that much more Si was associated with the remaining ferrihydrite than with the newly formed lepidocrocite. Pure lepidocrocite underwent recrystallization in the low Fe(II) treatment and transformed to magnetite at the high Fe(II)/Fe(III) ratio. Adsorbed silicate inactivated the lepidocrocite surfaces, which strongly reduced Fe atom exchange and inhibited mineral transformation. Collectively, the results of this study demonstrate that Fe(II)-catalyzed Si-ferrihydrite transformation leads to the redistribution of silicate in the solid phase and the formation of thicker lepidocrocite platelets, while lepidocrocite transformation can be completely inhibited by adsorbed silicate. Therefore, silicate is an important factor to include when considering Fe mineral dynamics in soils under reducing conditions.

Sulfidation of ferric (hydr)oxides and its implication on contaminants transformation: a review

Rapid industrialization and urbanization have resulted in elevated concentrations of contaminants in the groundwaters and subsurface soils, posing a growing hazard to humans and ecosystems. The transformation of most contaminants is closely linked to the mineralogy of ferric (hydr)oxides. Sulfidation of ferric (hydr)oxides is one of the most significant biogeochemical reactions in the anoxic environments, causing reductive dissolution and recrystallization of ferric (hydr)oxides and further affecting the transformation of iron-associated contaminants. This paper provides a comprehensive review on the sulfidation process of ferric (hydr)oxides and the transformation of relevant contaminants. This review presents detailed reaction mechanisms between ferric (hydr)oxides and dissolved sulfide, and elucidates the factors (e.g. crystallinity of ferric (hydr)oxides, the ratio of sulfide concentration to the surface area concentration of ferric (hydr)oxides) that control the formation of surface associated Fe(II), iron sulfide minerals, as well as transformation of secondary minerals. Then, we summarized the transformation mechanisms of a variety of typical environmentally relevant contaminants existing in groundwater and subsurface soils, including heavy metals, metal(loid) oxyanions (arsenic, antimony, chromium), radionuclides (uranium, technetium), organic contaminants and phosphate/nitrate species. The general mechanisms of contaminant transformation involve a combination of release, reduction and re-adsorption/incorporation processes, the specific pathway of which is highly dependent on the properties of the contaminant itself and the extent of sulfidation. Moreover, the challenge of extending our knowledge towards in situ remediation, as well as further research needs are identified.

Persistence of the Isotopic Signature of Pentavalent Uranium in Magnetite

Uranium isotopic signatures can be harnessed to monitor the reductive remediation of subsurface contamination or to reconstruct paleo-redox environments. However, the mechanistic underpinnings of the isotope fractionation associated with U reduction remain poorly understood. Here, we present a coprecipitation study, in which hexavalent U (U(VI)) was reduced during the synthesis of magnetite and pentavalent U (U(V)) was the dominant species. The measured δ²³⁸U values for unreduced U(VI) (∼-1.0‰), incorporated U (96 ± 2% U(V), ∼-0.1‰), and extracted surface U (mostly U(IV), ∼0.3‰) suggested the preferential accumulation of the heavy isotope in reduced species. Upon exposure of the U-magnetite coprecipitate to air, U(V) was partially reoxidized to U(VI) with no significant change in the δ²³⁸U value. In contrast, anoxic amendment of a heavy isotope-doped U(VI) solution resulted in an increase in the δ²³⁸U of the incorporated U species over time, suggesting an exchange between incorporated and surface/aqueous U. Overall, the results support the presence of persistent U(V) with a light isotope signature and suggest that the mineral dynamics of iron oxides may allow overprinting of the isotopic signature of incorporated U species. This work furthers the understanding of the isotope fractionation of U associated with iron oxides in both modern and paleo-environments.

Anoxic oxidation of As(III) during Fe(II)-induced goethite recrystallization: Evidence and importance of Fe(IV) intermediate

Under anoxic conditions, aqueous Fe(II) (Fe(II)_aq)-induced recrystallization of iron (oxyhydr)oxides changes the speciation and geochemical cycle of trace elements in environments. Oxidation of trace element, i.e., As(III), driven by Fe(II)_aq-iron (oxyhydr)oxides interactions under anoxic condition was observed previously, but the oxidative species and involved mechanisms are remained unknown. In the present study, we explored the formed oxidative intermediates during Fe(II)_aq-induced recrystallization of goethite under anoxic conditions. The methyl phenyl sulfoxide-based probe experiment suggested the featured oxidation by Fe(IV) species in Fe(II)_aq-goethite system. Both the Mössbauer spectra and X-ray absorption near edge structure spectroscopic evidenced the generation and quenching of Fe(IV) intermediate. It was proved that the interfacial electron exchange between Fe(II)_aq and Fe(III) of goethite initiated the generation of Fe(IV). After transferring electrons to goethite, Fe(II)_aq was transformed to labile Fe(III), which was then transformed to Fe(IV) via a proton-coupled electron transfer process. This highly reactive transient Fe(IV) could quickly react with reductive species, i.e. Fe(II) or As(III). Considering the ubiquitous occurrence of Fe(II)-iron (oxyhydr)oxides reactions under anoxic conditions, our findings are expected to provide new insight into the anoxic oxidative transformation processes of matters in non-surface environments on earth.

Fe(II) Induced Reduction of Incorporated U(VI) to U(V) in Goethite

Over 60 years of nuclear activities have resulted in a global legacy of radioactive wastes, with uranium considered a key radionuclide in both disposal and contaminated land scenarios. With the understanding that U has been incorporated into a range of iron (oxyhydr)oxides, these minerals may be considered a secondary barrier to the migration of radionuclides in the environment. However, the long-term stability of U-incorporated iron (oxyhydr)oxides is largely unknown, with the end-fate of incorporated species potentially impacted by biogeochemical processes. In particular, studies show that significant electron transfer may occur between stable iron (oxyhydr)oxides such as goethite and adsorbed Fe(II). These interactions can also induce varying degrees of iron (oxyhydr)oxide recrystallization (<4% to >90%). Here, the fate of U(VI)-incorporated goethite during exposure to Fe(II) was investigated using geochemical analysis and X-ray absorption spectroscopy (XAS). Analysis of XAS spectra revealed that incorporated U(VI) was reduced to U(V) as the reaction with Fe(II) progressed, with minimal recrystallization (approximately 2%) of the goethite phase. These results therefore indicate that U may remain incorporated within goethite as U(V) even under iron-reducing conditions. This develops the concept of iron (oxyhydr)oxides acting as a secondary barrier to radionuclide migration in the environment.

Rapid oxygen exchange between hematite and water vapor

Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surprisingly reactive. Specifically, we show that all the 3-fold coordinated lattice oxygen atoms on a defect-free single-crystalline "r-cut" ([Formula: see text]) surface of hematite (α-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70 °C, while the atomic-scale surface structure is unperturbed by the process. A similar behavior is observed after liquid-water exposure, but the experimental data clearly show most of the exchange happens during desorption of the final monolayer, not during immersion. Density functional theory computations show that the exchange can happen during on-surface diffusion, where the cost of the lattice oxygen extraction is compensated by the stability of an HO-HOH-OH complex. Such insights into lattice oxygen stability are highly relevant for many research fields ranging from catalysis and hydrogen production to geochemistry and paleoclimatology.

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.

Ferrous iron enhances arsenic sorption and oxidation by non-stoichiometric magnetite and maghemite

Arsenic-contaminated waters affect millions of people on a daily basis. Because the toxicity of As is dependent on the redox state, understanding As biogeochemistry, particularly in reducing environments, is critical to addressing the environmental risk that As poses. Sorption of As to Fe(III)-(oxyhydr)oxides is an important mechanism for As removal from solution under anoxic conditions. However, dissolved ferrous Fe (Fe(II)) also occurs under anoxic conditions, and the impact that Fe(II)-catalyzed recrystallization of crystalline Fe minerals has on As sorption mechanisms is not clear. Our research investigates the potential for non-stoichiometric magnetite, a commonly occurring mixed-valence Fe oxide in anoxic aquifers, to adsorb and/or incorporate inorganic As species during Fe(II)-catalyzed recrystallization at neutral pH, with particular focus on the impact of mineral stoichiometry (Fe(II):Fe(III) = 0.23 and 0.0) and varying Fe(II) concentrations. By following aqueous As concentrations and speciation over time coupled with As K-edge X-ray absorption spectroscopy, our results demonstrate that the presence of Fe(II) substantially enhanced As removal from solution. In addition, we highlight a Fe(II)-induced mechanism through which highly mobile, toxic As(III) species are oxidized on the mineral surface to form As(V). Furthermore, the presence of Fe(II) promotes the structural incorporation of As(V) into the non-stoichiometric magnetite and maghemite structures. These results highlight the potential of Fe(II)-reacted non-stoichiometric magnetite or maghemite as pathways for long-term As sequestration in anoxic environments.

Evidence for a Diagenetic Origin of Vera Rubin Ridge, Gale Crater, Mars: Summary and Synthesis of Curiosity's Exploration Campaign

This paper provides an overview of the Curiosity rover's exploration at Vera Rubin ridge (VRR) and summarizes the science results. VRR is a distinct geomorphic feature on lower Aeolis Mons (informally known as Mount Sharp) that was identified in orbital data based on its distinct texture, topographic expression, and association with a hematite spectral signature. Curiosity conducted extensive remote sensing observations, acquired data on dozens of contact science targets, and drilled three outcrop samples from the ridge, as well as one outcrop sample immediately below the ridge. Our observations indicate that strata composing VRR were deposited in a predominantly lacustrine setting and are part of the Murray formation. The rocks within the ridge are chemically in family with underlying Murray formation strata. Red hematite is dispersed throughout much of the VRR bedrock, and this is the source of the orbital spectral detection. Gray hematite is also present in isolated, gray-colored patches concentrated toward the upper elevations of VRR, and these gray patches also contain small, dark Fe-rich nodules. We propose that VRR formed when diagenetic event(s) preferentially hardened rocks, which were subsequently eroded into a ridge by wind. Diagenesis also led to enhanced crystallization and/or cementation that deepened the ferric-related spectral absorptions on the ridge, which helped make them readily distinguishable from orbit. Results add to existing evidence of protracted aqueous environments at Gale crater and give new insight into how diagenesis shaped Mars' rock record.

Multifaceted aspects of charge transfer

Charge transfer and charge transport are by far among the most important processes for sustaining life on Earth and for making our modern ways of living possible. Involving multiple electron-transfer steps, photosynthesis and cellular respiration have been principally responsible for managing the energy flow in the biosphere of our planet since the Great Oxygen Event. It is impossible to imagine living organisms without charge transport mediated by ion channels, or electron and proton transfer mediated by redox enzymes. Concurrently, transfer and transport of electrons and holes drive the functionalities of electronic and photonic devices that are intricate for our lives. While fueling advances in engineering, charge-transfer science has established itself as an important independent field, originating from physical chemistry and chemical physics, focusing on paradigms from biology, and gaining momentum from solar-energy research. Here, we review the fundamental concepts of charge transfer, and outline its core role in a broad range of unrelated fields, such as medicine, environmental science, catalysis, electronics and photonics. The ubiquitous nature of dipoles, for example, sets demands on deepening the understanding of how localized electric fields affect charge transfer. Charge-transfer electrets, thus, prove important for advancing the field and for interfacing fundamental science with engineering. Synergy between the vastly different aspects of charge-transfer science sets the stage for the broad global impacts that the advances in this field have.

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.

Nanostructure-assisted charge transfer in α-Fe2O3/g-C3N4 heterojunctions for efficient and highly stable photoelectrochemical water splitting

The development of semiconductor heterojunctions is a promising and yet challenging strategy to boost the performance in photoelectrochemical (PEC) water splitting. This paper describes the fabrication of a heterojunction photoanode by coupling α-Fe₂O₃ and g-C₃N₄via aerosol-assisted chemical vapour deposition (AACVD) followed by spin coating and air annealing. Enhanced PEC performance and stability are observed for the α-Fe₂O₃/g-C₃N₄ heterojunction photoanode in comparison to pristine α-Fe₂O₃ and the reason is systematically discussed in this paper. Most importantly, the fabricated α-Fe₂O₃/g-C₃N₄ film shows impressive stability, retaining more than 90% of the initial current over 12 h operating time. The excellent stability of the heterojunction photoanode is achieved due to the unique nanoflake structure of α-Fe₂O₃ induced by AACVD. This nanostructure promotes good adhesion with the g-C₃N₄ particles, as the particles tend to be trapped within the α-Fe₂O₃ valleys and eventually create strong and large interfacial contacts. This leads to improved separation of charge carriers at the α-Fe₂O₃/g-C₃N₄ interface and suppression of charge recombination in the photoanode, which are confirmed by the transient decay time, charge transfer efficiency and electrochemical impedance analysis. Our findings demonstrate the importance of nanostructure engineering for developing heterojunction structures with efficient charge transfer dynamics.

Adsorption of Cr(VI) on Al-substituted hematites and its reduction and retention in the presence of Fe2+ under conditions similar to subsurface soil environments

Aluminum substitution is common in iron (hydr)oxides in subsurface environments, and can significantly modify mineral interactions with contaminants. However, few studies investigate Cr(VI) adsorption and its subsequent mobility on Al-substituted iron (hydr)oxide surfaces. Here shows that Al substitution gradually modifies hematite crystals from {101}, {112}, {110} and {104} faceted rhombohedra to {001} faceted plates, resulting in a general decrease in Cr(VI) adsorption density and favoring of monodentate mononuclear over bidentate binuclear Cr(VI) adsorption complexes. Consequently, the mobility of Cr(VI) might be increased in environments with an abundance of Al-containing iron (hydr)oxides. However, pre-adsorption of Fe²⁺ on hematite promotes Cr(VI) adsorption, reduction and fixation, and Al-substituted hematite removes more Cr(VI) than pure hematite. Similarly, although addition of Fe²⁺ to Cr(VI)-adsorbed hematite remobilizes a small proportion of Cr, it greatly increases the proportion of Cr fixed. As the coexistence of Fe²⁺ and iron (hydr)oxides is common in subsurface environments, Al-containing iron (hydr)oxides will promote Cr(VI) uptake and retention, with a significant proportion fixed as Cr(III), limiting Cr mobility and toxicity. These results offer new insights into how iron (hydr)oxides might control the behaviors of other high-valence redox-sensitive contaminants, and provide a platform for modeling such processes in complex soil and sediment systems.

Contaminant Degradation by •OH during Sediment Oxygenation: Dependence on Fe(II) Species

It has been documented that contaminants could be degraded by hydroxyl radicals (•OH) produced upon oxygenation of Fe(II)-bearing sediments. However, the dependence of contaminant degradation on sediment characteristics, particularly Fe(II) species, remains elusive. Here we assessed the impact of the abundance of Fe(II) species in sediments on contaminant degradation by •OH during oxygenation. Three natural sediments with different Fe(II) contents and species were oxygenated. During 10 h oxygenation of 200 g/L sediment suspension, 2 mg/L phenol was negligibly degraded for sandbeach sediment (Fe(II): 9.11 mg/g), but was degraded by 41% and 52% for lakeshore (Fe(II): 9.81 mg/g) and farmland (Fe(II): 19.05 mg/g) sediments, respectively. •OH produced from Fe(II) oxygenation was the key reactive oxidant. Sequential extractions, X-ray diffraction, Mössbauer, and X-ray absorption spectroscopy suggest that surface-adsorbed Fe(II) and mineral structural Fe(II) contributed predominantly to •OH production and phenol degradation. Control experiments with specific Fe(II) species and coordination structure analysis collectively suggest the likely rule that Fe(II) oxidation rate and its competition for •OH increase with the increase in electron-donating ability of the atoms (i.e., O) complexed to Fe(II), while the •OH yield decreases accordingly. The Fe(II) species with a moderate oxidation rate and •OH yield is most favorable for contaminant degradation.

The geologic history of seawater oxygen isotopes from marine iron oxides

The oxygen isotope composition (δ¹⁸O) of marine sedimentary rocks has increased by 10 to 15 per mil since Archean time. Interpretation of this trend is hindered by the dual control of temperature and fluid δ¹⁸O on the rocks' isotopic composition. A new δ¹⁸O record in marine iron oxides covering the past ~2000 million years shows a similar secular rise. Iron oxide precipitation experiments reveal a weakly temperature-dependent iron oxide-water oxygen isotope fractionation, suggesting that increasing seawater δ¹⁸O over time was the primary cause of the long-term rise in δ¹⁸O values of marine precipitates. The ¹⁸O enrichment may have been driven by an increase in terrestrial sediment cover, a change in the proportion of high- and low-temperature crustal alteration, or a combination of these and other factors.

Linking Isotope Exchange with Fe(II)-Catalyzed Dissolution of Iron(hydr)oxides in the Presence of the Bacterial Siderophore Desferrioxamine-B

Dissolution of Fe(III) phases is a key process in making iron available to biota and in the mobilization of associated trace elements. Recently, we have demonstrated that submicromolar concentrations of Fe(II) significantly accelerate rates of ligand-controlled dissolution of Fe(III) (hydr)oxides at circumneutral pH. Here, we extend this work by studying isotope exchange and dissolution with lepidocrocite (Lp) and goethite (Gt) in the presence of 20 or 50 μM desferrioxamine-B (DFOB). Experiments with Lp at pH 7.0 were conducted in carbonate-buffered suspensions to mimic environmental conditions. We applied a simple empirical model to determine dissolution rates and a more complex kinetic model that accounts for the observed isotope exchange and catalytic effect of Fe(II). The fate of added tracer ⁵⁷Fe(II) was strongly dependent on the order of addition of ⁵⁷Fe(II) and ligand. When DFOB was added first, tracer ⁵⁷Fe remained in solution. When ⁵⁷Fe(II) was added first, isotope exchange between surface and solution could be observed at pH 6.0 but not at pH 7.0 and 8.5 where ⁵⁷Fe(II) was almost completely adsorbed. During dissolution of Lp with DFOB, ratios of released ⁵⁶Fe and ⁵⁷Fe were largely independent of DFOB concentrations. In the absence of DFOB, addition of phenanthroline 30 min after tracer ⁵⁷Fe desorbed predominantly ⁵⁶Fe(II), indicating that electron transfer from adsorbed ⁵⁷Fe to ⁵⁶Fe of the Lp surface occurs on a time scale of minutes to hours. In contrast, comparable experiments with Gt desorbed predominantly ⁵⁷Fe(II), suggesting a longer time scale for electron transfer on the Gt surface. Our results show that addition of 1-5 μM Fe(II) leads to dynamic charge transfer between dissolved and adsorbed species and to isotope exchange at the surface, with the dissolution of Lp by ligands accelerated by up to 60-fold.

Ferrihydrite Growth and Transformation in the Presence of Ferrous Iron and Model Organic Ligands

Ferrihydrite (Fh) is a poorly crystalline Fe(III)-oxyhydroxide found in abundance in soils and sediments. With a high specific surface area and sorption capacity at circumneutral pH, ferrihydrite is an important player in the biogeochemical cycling of nutrients and trace elements in redox-dynamic environments. Under reducing conditions, exposure to Fe(II) induces mineral transformations in ferrihydrite; the extent and trajectory of which may be greatly influenced by organic matter (OM). However, natural OM is heterogeneous and comprises a range of molecular weights (MWs) and varied functional group compositions. To date, the impact that the chemical composition of the associated OM has on Fe(II)-catalyzed mineral transformations is not clear. To address this knowledge gap, we coprecipitated ferrihydrite with model organic ligands selected to cover a range of MWs (25 000-50 000 vs <200 Da) as well as carboxyl content (polygalacturonic acid (PGA) > citric acid (CA) > galacturonic acid (GA)). Coprecipitates (C:Fe ≈ 0.6) were reacted with 1 mM ⁵⁷Fe(II) for 1 week at pH 7, with time-resolved solid-phase analysis (via X-ray diffraction, X-ray absorption spectroscopy, and electron microscopy) revealing that all ligands inhibited Fe(II)-catalyzed ferrihydrite mineral transformations and the formation of crystalline secondary mineral phases compared to a pure ferrihydrite. For carboxyl-rich coprecipitates (Fh-PGA and Fh-CA), mineral transformations were less inhibited than in the carboxyl poor Fh-GA, and a crystalline lepidocrocite "shell" was formed surrounding the residual ferrihydrite core. However, Fe isotope analysis revealed that all coprecipitates underwent near complete atom exchange. Collectively, our results highlight that ferrihydrite is indeed an active mineral phase in redox-dynamic environments, but that its stability under reducing conditions, and thus capacity for nutrient and trace element retention, depends on the chemical characteristic of the associated OM, specifically OM-induced changes in the particle surface charge and the distribution of organic functional groups.

Water Flow Variability Affects Adsorption and Oxidation of Ciprofloxacin onto Hematite

The mobility of pharmaceuticals in environmental systems is under great scrutiny in the scientific literature and in the press. Still, very few reports have focused on redox-driven transformations when these compounds are bound to mineral surfaces, and how their transport is affected under flow-through conditions. In this study, we examined the adsorption and electron transfer reactions of ciprofloxacin (CIP) in a dynamic column containing nanosized hematite (α-Fe₂O₃). CIP binding and the subsequent redox transformation were strongly dependent on inflow pH and residence time. These reactions could be predicted using transport models that account for adsorption and transformation kinetics. Our results show that flow interruption over a 16 h period triggers oxidation of hematite-bound CIP into byproducts. These reactions are likely facilitated by inner-sphere iron-CIP complexes formed via the sluggish conversion from outer-sphere complexes during interrupted flow. When intermittent flow/no-flow conditions were applied sequentially, a second byproduct was detected in the column effluent. This work sheds light on a much overseen aspect of redox transformations of antibiotics under flow-through conditions. It has important implications in adequately predicting transport, and in developing risk assessments of these emerging compounds in the environment.

Redistribution of Electron Equivalents between Magnetite and Aqueous Fe2+ Induced by a Model Quinone Compound AQDS

The complex interactions between magnetite and aqueous Fe²⁺ (Fe²⁺_(aq)) pertain to many biogeochemical redox processes in anoxic subsurface environments. The effect of natural organic matter, abundant in these same environments, on Fe²⁺_(aq)-magnetite interactions is an additional complex that remains poorly understood. We investigated the influence of a model quinone molecule anthraquinone-2,6-disulfonate (AQDS) on Fe²⁺_(aq)-magnetite interactions by systematically studying equilibrium Fe²⁺_(aq) concentrations, rates and extents of AQDS reduction, and structural versus surface-localized Fe(II)/Fe(III) ratios in magnetite under different controlled experimental conditions. The equilibrium concentration of Fe²⁺_(aq) in Fe²⁺-amended magnetite suspensions with AQDS proportionally changes with solution pH or initial AQDS concentration, but independent of magnetite loadings through the solid concentrations that were studied here. The rates and extents of AQDS reduction by Fe²⁺-amended magnetite proportionally increased with solution pH, magnetite loading, and initial Fe²⁺_(aq) concentration, which correlates with the corresponding change of reduction potentials for the Fe²⁺-magnetite system. AQDS reduction by surface-associated Fe(II) in the Fe²⁺-magnetite suspensions induces solid-state migration of electron equivalents from particle interiors to the near-surface region and the production of nonmagnetic Fe(II)-containing species, which inhibits Fe²⁺_(aq) incorporation or electron injection into the magnetite structure. This study demonstrates the significant influence of quinones on reductive activity of the Fe²⁺-magnetite system.

Visualizing the iron atom exchange front in the Fe(II)-catalyzed recrystallization of goethite by atom probe tomography

The autocatalytic redox interaction between aqueous Fe(II) and Fe(III)-(oxyhydr)oxide minerals such as goethite and hematite leads to rapid recrystallization marked, in principle, by an atom exchange (AE) front, according to bulk iron isotopic tracer studies. However, direct evidence for this AE front has been elusive given the analytical challenges of mass-resolved imaging at the nanoscale on individual crystallites. We report successful isolation and characterization of the AE front in goethite microrods by 3D atom probe tomography (APT). The microrods were reacted with Fe(II) enriched in tracer ⁵⁷Fe at conditions consistent with prior bulk studies. APT analyses and 3D reconstructions on cross-sections of the microrods reveal an AE front that is spatially heterogeneous, at times penetrating several nanometers into the lattice, in a manner consistent with defect-accelerated exchange. Evidence for exchange along microstructural domain boundaries was also found, suggesting another important link between exchange extent and initial defect content. The findings provide an unprecedented view into the spatial and temporal characteristics of Fe(II)-catalyzed recrystallization at the atomic scale, and substantiate speculation regarding the role of defects controlling the dynamics of electron transfer and AE interaction at this important redox interface.

Low Fe(II) Concentrations Catalyze the Dissolution of Various Fe(III) (hydr)oxide Minerals in the Presence of Diverse Ligands and over a Broad pH Range

Dissolution of Fe(III) (hydr)oxide minerals by siderophores (i.e., Fe-specific, biogenic ligands) is an important step in Fe acquisition in environments where Fe availability is low. The observed coexudation of reductants and ligands has raised the question of how redox reactions might affect ligand-controlled (hydr)oxide dissolution and Fe acquisition. We examined this effect in batch dissolution experiments using two structurally distinct ligands (desferrioxamine B (DFOB) and  N, N'-di(2-hydroxybenzyl)ethylene-diamine- N, N'-diacetic acid (HBED)) and four Fe(III) (hydr)oxide minerals (lepidocrocite, 2-line ferrihydrite, goethite and hematite) over an environmentally relevant pH range (4-8.5). The experiments were conducted under anaerobic conditions with varying concentrations of (adsorbed) Fe(II) as the reductant. We observed a catalytic effect of Fe(II) on ligand-controlled dissolution even at submicromolar Fe(II) concentrations with up to a 13-fold increase in dissolution rate. The effect was larger for HBED than for DFOB. It was observed for all four Fe(III) (hydr)oxide minerals, but it was most pronounced for goethite in the presence of HBED. It was observed over the entire pH range with the largest effect at pH 7 and 8.5, where Fe deficiency typically occurs. The occurrence of this catalytic effect over a range of environmentally relevant conditions and at very low Fe(II) concentrations suggests that redox-catalyzed, ligand-controlled dissolution may be significant in biological Fe acquisition and in redox transition zones.

Impact of Organic Matter on Iron(II)-Catalyzed Mineral Transformations in Ferrihydrite-Organic Matter Coprecipitates

Poorly crystalline Fe(III) (oxyhydr)oxides like ferrihydrite are abundant in soils and sediments and are often associated with organic matter (OM) in the form of mineral-organic aggregates. Under anoxic conditions, interactions between aqueous Fe(II) and ferrihydrite lead to the formation of crystalline secondary minerals, like lepidocrocite, goethite, or magnetite. However, the extent to which Fe(II)-catalyzed mineral transformations are influenced by ferrihydrite-associated OM is not well understood. We therefore reacted ferrihydrite-PGA coprecipitates (PGA = polygalacturonic acid, C:Fe molar ratios = 0-2.5) and natural Fe-rich organic flocs (C:Fe molar ratio = 2.2) with 0.5-5.0 mM isotopically labeled ⁵⁷Fe(II) at pH 7 for 5 weeks. Relying on the combination of stable Fe isotope tracers, a novel application of the PONKCS method to Rietveld fitting of X-ray diffraction (XRD) patterns, and ⁵⁷Fe Mössbauer spectroscopy, we sought to follow the temporal evolution in Fe mineralogy and elucidate the fate of adsorbed ⁵⁷Fe(II). At low C:Fe molar ratios (0-0.05), rapid oxidation of surface-adsorbed ⁵⁷Fe(II) resulted in ⁵⁷Fe-enriched crystalline minerals and nearly complete mineral transformation within days. With increasing OM content, the atom exchange between the added aqueous ⁵⁷Fe(II) and Fe in the organic-rich solids still occurred; however, XRD analysis showed that crystalline mineral precipitation was strongly inhibited. For high OM-content materials (C:Fe ≥ 1.2), Mössbauer spectroscopy revealed up to 39% lepidocrocite in the final Fe(II)-reacted samples. Because lepidocrocite was not detectable by XRD, we suggest that the Mössbauer-detected lepidocrocite consisted of nanosized clusters with lepidocrocite-like local structure, similar to the lepidocrocite found in natural flocs. Collectively, our results demonstrate that the C content of ferrihydrite-OM coprecipitates strongly impacts the degree and pathways of Fe mineral transformations and iron atom exchange during reactions with aqueous Fe(II).

Fe(II)-Catalyzed Transformation of Organic Matter-Ferrihydrite Coprecipitates: A Closer Look Using Fe Isotopes

Ferrihydrite is a common Fe mineral in soils and sediments that rapidly transforms to secondary minerals in the presence of Fe(II). Both the rate and products of Fe(II)-catalyzed ferrihydrite transformation have been shown to be significantly influenced by natural organic matter (NOM). Here, we used enriched Fe isotope experiments and ⁵⁷Fe Mössbauer spectroscopy to track the formation of secondary minerals, as well as electron transfer and Fe mixing between aqueous Fe(II) and ferrihydrite coprecipitated with several types of NOM. Ferrihydrite coprecipitated with humic acids transformed primarily to goethite after reaction with Fe(II). In contrast, ferrihydrite coprecipitated with fulvic acids and Suwannee River NOM (SRNOM) resulted in no measurable formation of secondary minerals. Despite no secondary mineral transformation, Mössbauer spectra indicated electron transfer still occurred between Fe(II) and ferrihydrite coprecipitated with fulvic acid and SRNOM. In addition, isotope tracer experiments revealed that a significant fraction of structural Fe in the ferrihydrite mixed with the aqueous phase Fe(II) (∼85%). After reaction with Fe(II), Mössbauer spectroscopy indicated some subtle changes in the crystallinity, particle size, or particle interactions in the coprecipitate. Our observations suggest that ferrihydrite coprecipitated with fulvic acid and SRNOM remains a highly dynamic phase even without ferrihydrite transformation.

Cr Release from Cr-Substituted Goethite during Aqueous Fe(II)-Induced Recrystallization

The interaction between aqueous Fe(II) (Fe(II)aq) and iron minerals is an important reaction of the iron cycle, and it plays a critical role in impacting the environmental behavior of heavy metals in soils. Metal substitution into iron (hydr)oxides has been reported to reduce Fe atom exchange rates between Fe(II)aq and metal-substituted iron (hydr)oxides and inhibit the recrystallization of iron (hydr)oxides. However, the environmental behaviors of the substituted metal during these processes remain unclear. In this study, Fe(II)aq-induced recrystallization of Cr-substituted goethite (Cr-goethite) was investigated, along with the sequential release behavior of substituted Cr(III). Results from a stable Fe isotopic tracer and Mössbauer characterization studies show that Fe atom exchange occurred between Fe(II)aq and structural Fe(III) (Fe(III)oxide) in Cr-goethites, during which the Cr-goethites were recrystallized. The Cr substitution inhibited the rates of Fe atom exchange and Cr-goethite recrystallization. During the recrystallization of Cr-goethites induced by Fe(II)aq, Cr(III) was released from Cr-goethite. In addition, Cr-goethites with a higher level of Cr-substituted content released more Cr(III). The highest Fe atom exchange rate and the highest amount of released Cr(III) were observed at a pH of 7.5. Under reaction conditions involving a lower pH of 5.5 or a higher pH of 8.5, there were substantially lower rates of Fe atom exchange and Cr(III) release. This trend of Cr(III) release was similar with changes in Fe atom exchange, suggesting that Cr(III) release is driven by Fe atom exchange. The release and reincorporation of Cr(III) occurred simultaneously during the Fe(II)aq-induced recrystallization of Cr-goethites, especially during the late stage of the observed reactions. Our findings emphasize an important role for Fe(II)aq-induced recrystallization of iron minerals in changing soil metal characteristics, which is critical for the evaluation of soil metal activities, especially those in Fe-rich soils.

Aqueous Fe(II)-Induced Phase Transformation of Ferrihydrite Coupled Adsorption/Immobilization of Rare Earth Elements

The phase transformation of iron minerals induced by aqueous Fe(II) (Fe(II)aq) is a critical geochemical reaction which greatly affects the geochemical behavior of soil elements. How the geochemical behavior of rare earth elements (REEs) is affected by the Fe(II)aq-induced phase transformation of iron minerals, however, is still unknown. The present study investigated the adsorption and immobilization of REEs during the Fe(II)aq-induced phase transformation of ferrihydrite. The results show that the heavy REEs of Ho(III) were more efficiently adsorbed and stabilized compared with the light REEs of La(III) by ferrihydrite and its transformation products, which was due to the higher adsorptive affinity and smaller atomic radius of Ho(III). Both La(III) and Ho(III) inhibited the Fe atom exchange between Fe(II)aq and ferrihydrite, and sequentially, the Fe(II)aq-induced phase transformation rates of ferrihydrite, because of the competitive adsorption with Fe(II)aq on the surface of iron (hydr)oxides. Owing to the larger amounts of adsorbed and stabilized Ho(III), the inhibition of the Fe(II)aq-induced phase transformation of ferrihydrite affected by Ho(III) was higher than that by La(III). Our findings suggest an important role for the Fe(II)aq-induced phase transformation of iron (hydr)oxides in assessing the mobility and transfer behavior of REEs, as well as for their occurrence in earth surface environments.

The Role of Defects in Fe(II)-Goethite Electron Transfer

Despite substantial experimental evidence for Fe(II)-Fe(III) oxide electron transfer, computational chemistry calculations suggest that oxidation of sorbed Fe(II) by goethite is kinetically inhibited on structurally perfect surfaces. We used a combination of ⁵⁷Fe Mössbauer spectroscopy, synchrotron X-ray absorption and magnetic circular dichroism (XAS/XMCD) spectroscopies to investigate whether Fe(II)-goethite electron transfer is influenced by defects. Specifically, Fe L-edge and O K-edge XAS indicates that the outermost few Angstroms of goethite synthesized by low temperature Fe(III) hydrolysis is iron deficient relative to oxygen, suggesting the presence of defects from Fe vacancies. This nonstoichiometric goethite undergoes facile Fe(II)-Fe(III) oxide electron transfer, depositing additional goethite consistent with experimental precedent. Hydrothermal treatment of this goethite, however, appears to remove defects, decrease the amount of Fe(II) oxidation, and change the composition of the oxidation product. When hydrothermally treated goethite was ground, surface defect characteristics as well as the extent of electron transfer were largely restored. Our findings suggest that surface defects play a commanding role in Fe(II)-goethite redox interaction, as predicted by computational chemistry. Moreover, it suggests that, in the environment, the extent of this interaction will vary depending on diagenetic history, local redox conditions, as well as being subject to regeneration via seasonal fluctuations.

Magnetite and Green Rust: Synthesis, Properties, and Environmental Applications of Mixed-Valent Iron Minerals

Mixed-valent iron [Fe(II)-Fe(III)] minerals such as magnetite and green rust have received a significant amount of attention over recent decades, especially in the environmental sciences. These mineral phases are intrinsic and essential parts of biogeochemical cycling of metals and organic carbon and play an important role regarding the mobility, toxicity, and redox transformation of organic and inorganic pollutants. The formation pathways, mineral properties, and applications of magnetite and green rust are currently active areas of research in geochemistry, environmental mineralogy, geomicrobiology, material sciences, environmental engineering, and environmental remediation. These aspects ultimately dictate the reactivity of magnetite and green rust in the environment, which has important consequences for the application of these mineral phases, for example in remediation strategies. In this review we discuss the properties, occurrence, formation by biotic as well as abiotic pathways, characterization techniques, and environmental applications of magnetite and green rust in the environment. The aim is to present a detailed overview of the key aspects related to these mineral phases which can be used as an important resource for researchers working in a diverse range of fields dealing with mixed-valent iron minerals.

Recrystallization of Manganite (γ-MnOOH) and Implications for Trace Element Cycling

The recrystallization of Mn(III,IV) oxides is catalyzed by aqueous Mn(II) (Mn(II)_aq) during (bio)geochemical Mn redox cycling. It is poorly understood how trace metals associated with Mn oxides (e.g., Ni) are cycled during such recrystallization. Here, we use X-ray absorption spectroscopy (XAS) to examine the speciation of Ni associated with Manganite (γ-Mn(III)OOH) suspensions in the presence or absence of Mn(II)_aq under variable pH conditions (pH 5.5 and 7.5). In a second set of experiments, we used a ⁶²Ni isotope tracer to quantify the amount of dissolved Ni that exchanges with Ni incorporated in the Manganite crystal structure during reactions in 1 mM Mn(II)_aq and in Mn(II)-free solutions. XAS spectra show that Ni is initially sorbed on the Manganite mineral surface and is progressively incorporated into the mineral structure over time (13% after 51 days) even in the absence of dissolved Mn(II). The amount of Ni incorporation significantly increases to about 40% over a period of 51 days when Mn(II)_aq is present in solution. Similarly, Mn(II)_aq promotes Ni exchange between Ni-substituted Manganite and dissolved Ni(II), with around 30% of Ni exchanged at pH 7.5 over the duration of the experiment. No new mineral phases are detected following recrystallization as determined by X-ray diffraction and XAS. Our results reveal that Mn(II)-catalyzed mineral recrystallization partitions Ni between Mn oxides and aqueous fluids and can therefore affect Ni speciation and mobility in the environment.