Secondary Fe Minerals Research Articles

Biochar mediated uranium immobilization in magnetite rich Cu tailings subject to organic matter amendment and native plant colonization

Organic matter (OM) amendments and plant colonization can accelerate mineral weathering and soil formation in metal mine tailings for ecological rehabilitation. However, the weathering effects may dissolve uranium (U)-bearing minerals (e.g., ianthinite) and increase U dissolution in porewater and seepages. The present study aimed to characterize the U solubility and distribution among different fractions and investigate if biochar (BC) could decrease soluble U levels and facilitate U immobilization in the OM-amended and plant-colonized tailings. A native plant species, Red Flinders grass (Iseilema vaginiflorum) was cultivated in the tailings for four weeks, which were amended with sugarcane residue (SR) with or without BC addition. The results showed that OM amendment and plant colonization increased porewater U concentrations by almost 10 folds from ~ 0.2 mg L−1 to > 2.0 mg L−1. The BC addition decreased porewater U concentrations by 40%. Further micro-spectroscopic analysis revealed that U was immobilized through adsorption onto BC porous surfaces, via possibly complexing with oxygen-rich organic groups. Besides, the BC amendment facilitated U sequestration by secondary Fe minerals in the tailings. These findings provide important information about U biogeochemistry in Cu-tailings mediated by BC, OM and rhizosphere interactions for mitigating potential pollution risks of tailings rehabilitation.

Cysteine induced cascade electron transfer by forming a unique ternary complex with Fe(II) on goethite

The interfacial electron transfer on iron (Fe) minerals with electron shuttles may play a great significance to the fate of pollutants in soils, while the exact surface chemistry is barely understood. This work systematically investigated cysteine induced the reductive transformation of nitrobenzene (NB) on goethite by means of electrochemistry and kinetic modeling. Results show that NB can be efficiently reduced by cysteine on goethite with a rate constant of 0.72 h−1 that is remarkably higher than that of 0.05 h−1 by cysteine. The concomitant production of surface bound Fe(II) has also observed contributing to NB reduction, which cascaded transferring electrons from cysteine to surface Fe(III), and towards NB. Raman spectra and electrochemical results support the formation of a unique ternary FeOOFe(II)-Cyt complex (Emid = − 0.26 V) that is more thermodynamically favorable than surface bound Fe(II) (Emid = − 0.12 V) for electron transfer. Meanwhile, solid state analysis demonstrates that no apparent secondary Fe minerals form, ruling out the effects of the change of crystalline structure on the efficiency of NB reduction. Accordingly, a kinetics model has been developed by combining elementary reactions, finely describing the interfacial reduction of NB on induced by cysteine. Results show that the relative contribution of the ternary complex FeOOFe(II)-Cyt complexes account for more than 60% of NB reductions. Our findings provide new insights into understanding the surface chemistry of electron shuttles on Fe minerals and its significance for the transformation and fate of pollutants in the environment.

The release of arsenic from biologically formed arsenic-containing FeS under aerobic environment: Implications for the stability and species variation of nano-sized iron sulfide in remediation

Biologically formed nano-FeS is a potential and low-cost adsorbent and soil remediation agent. Remediation of arsenic from contaminated groundwater and soil by biological FeS is promising, however, the fate and transport of arsenic associated with nano-FeS in the natural environment are poorly understood. To investigate the fate of adsorbed arsenic on nano-FeS under aerobic conditions, static and dynamic experiments were carried out at different pH values and different oxidants. Batch and column sets showed that arsenic(V)-loaded FeS could release arsenic into solutions at pH 3.0 and 9.0. Especially noteworthy is the release of arsenic sustained at pH 9.0. Introducing Fe3+ and H2O2 at pH 3.0 showed that Fe3+ and a high concentration of H2O2 could reduce the release of arsenic, which might be ascribed to the oxidants promoting secondary Fe mineral phases to form on the arsenic(V)-FeS solid surface. X-ray diffraction patterns indicated that elemental S(0) and arsenic(V)-bearing Fe(II)/Fe(III) minerals formed as the main products. The generated secondary parasymplesite and FeAsO4 were mainly responsible for dissolved arsenic retention. The performance of hydraulic conductivity followed the sequence pH 7.0 > 50 mg/L Fe3+ > 1 mg/L Fe3+ > pH 9.0 > pH 3.0. In addition, H2O2 always kept good hydraulic conductivity. XPS analysis demonstrated that arsenic(III) and arsenic(V) were the main arsenic speciation on the surfaces of the solids, implying that lower valence arsenic was oxidized to arsenic(III/V) during the oxidation processes. The present study facilitates a better understanding of the stability of arsenic-loaded FeS in aerobic environment.

Occurrence, Geochemistry and Speciation of Elevated Arsenic Concentrations in a Fractured Bedrock Aquifer System

The presence of elevated arsenic concentrations (≥ 10 µg L−1) in groundwaters has been widely reported in areas of South-East Asia with recent studies showing its detection in fractured bedrock aquifers is occurring mainly in regions of north-eastern USA. However, data within Europe remain limited; therefore, the objective of this work was to understand the geochemical mobilisation mechanism of arsenic in this geologic setting using a study site in Ireland as a case study. Physicochemical (pH, Eh, d-O2), trace metals, major ion and arsenic speciation samples were collected and analysed using a variety of field and laboratory-based techniques and evaluated using statistical analysis. Groundwaters containing elevated dissolved arsenic concentrations (up to 73.95 µg L−1) were characterised as oxic-alkali groundwaters with the co-occurrence of other oxyanions (including Mo, Se, Sb and U), low dissolved concentrations of Fe and Mn, and low Na/Ca ratios indicated that arsenic was mobilised through alkali desorption of Fe oxyhydroxides. Arsenic speciation using a solid-phase extraction methodology (n = 20) showed that the dominant species of arsenic was arsenate, with pH being a major controlling factor. The expected source of arsenic is sulphide minerals within fractures of the bedrock aquifer with transportation of arsenic and other oxyanion forming elements facilitated by secondary Fe mineral phases. However, the presence of methylarsenical compounds detected in groundwaters illustrates that microbially mediated mobilisation processes may also be (co)-occurring. This study gives insight into the geochemistry of arsenic mobilisation that can be used to further guide research needs in this area for the protection of groundwater resources.

A New Constraint on the Physicochemical Condition of Mars Surface during the Amazonian Epoch Based on Chemical Speciation for Secondary Minerals in Martian Nakhlites

Iddingsite in Martian nakhlites contains various secondary minerals that reflect water–rock interaction on Mars. However, the formation processes of secondary Fe minerals in iddingsite are unclear because they include carbonates precipitated under reductive and alkaline conditions and sulfates that are generally precipitated under oxidative and acidic conditions. Mineral types cannot coexist under equilibrium. Herein, we characterize the carbonate phase of meteorite Yamato 000593 as siderite and Mn-bearing siderite via field-emission electron probe microanalyzer (FE-EPMA). Then, we examined the distribution and speciation of trace Cr and S within the carbonates through synchrotron micro-focused X-ray fluorescence-X-ray absorption fine structure and scanning transmission X-ray microscopy (μ-XRF-XAFS/STXM) analysis to estimate the transition history of Eh-pH conditions during siderite formation to explain the coexistence of carbonate and sulfate phases in the nakhlite vein. Specifically, the distribution and speciation of S in the mesostasis and carbonate phases and the heterogeneous distribution of Mn-FeCO3 incorporating Cr(III) in the carbonate constrain the Eh-pH condition. The conditions and transition of the fluid chemistry determined herein based on speciation of various elements provide a new constraint on the physicochemical condition of the water that altered the nakhlite body during the Amazonian epoch.

Fulvic Acid-Mediated Interfacial Reactions on Exposed Hematite Facets during Dissimilatory Iron Reduction.

Although the dual role of natural organic matter (NOM) as an electron shuttle and an electron donor for dissimilatory iron (Fe) reduction has been extensively investigated, the underlying interfacial interactions between various exposed facets and NOM are poorly understood. In this study, fulvic acid (FA), as typical NOM, was used and its effect on the dissimilatory reduction of hematite {001} and {100} by Shewanella putrefaciens CN-32 was investigated. FA accelerates the bioreduction rates of hematite {001} and {100}, where the rate of hematite {100} is lower than that of hematite {001}. Secondary Fe minerals were not observed, but the HR-TEM images reveal significant defects. The ATR-FTIR results demonstrate that facet-dependent binding mainly occurs via surface complexation between the surface iron atoms and carboxyl groups of NOM. The spectroscopic and mass spectrometry analyses suggest that organic compounds with large molecular weight, highly aromatic and unsaturated structures, and lower H/C ratios are easily adsorbed on Fe oxides or decomposed by bacteria in FA-hematite {001} treatment after iron reduction. Due to the metabolic processes of cells, a significant number of compounds with higher H/C and medium O/C ratios appear. The Tafel curves show that hematite {100} possessed higher resistance (4.1-2.6 Ω) than hematite {001} (3.5-2.2 Ω) at FA concentrations ranging from 0 to 500 mg L-1, indicating that hematite {100} is less conductive during the electron transfer from reduced FA or cells to Fe oxides than hematite {001}. Overall, the discrepancy in the iron bioreduction of two exposed facets is attributed to both the different electrochemical activities of the Fe oxides and the different impacts on the properties and composition of OM. Our findings shed light on the molecular mechanisms of mutual interactions between FA and Fe oxides with various facets.

Extracellular polymeric substances from Shewanella oneidensis MR-1 biofilms mediate the transformation of Ferrihydrite

Extracellular polymeric substances (EPS) of dissimilatory iron-reducing bacteria (DIRB) such as Shewanella oneidensis MR-1 play a crucial role in the biotransformation of iron-containing minerals, but the mechanism has not been fully deciphered. Herein, abiotic and biotic transformation of ferrihydrite (Fh) were compared to clarify the contributions of MR-1, EPS-free MR-1 (MR-1-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS). The results of abiotic Fh transformation indicated that EPS did not block the Fh surfaces and thus has an insignificant effect on the adsorbed Fe(II)-Fh interaction. The complexation of the Fe(III) intermediate (Fe(III)active) with EPS, especially LB-EPS, however, inhibited the nucleation of secondary Fe minerals and changed the crystallization pathway. For biotic Fh transformation, on the other hand, EPS had dual effects that accelerated Fh bioreduction due to the enhanced extracellular electron transfer (EET) and constrained the following Fh mineralization by cutting of the chain reactions leading to mineral crystallization. Our finding also suggested that the effects of EPS on Fh biotransformation largely depend on the chemical properties of EPS, especially the polar functional groups such as carboxyl and phosphate, because of their important abilities for the cell attachment and Fe(II)/Fe(III) binding.

Experimental study of epidote dissolution rates from pH 2 to 11 and temperatures from 25 to 200 °C

The dissolution rates of Green Monster Mine epidote at temperatures of 25, 100 and 200 °C, and over the pH range 2–11, were determined from far-from equilibrium experiments performed in both batch and mixed-flow reactors. Epidote dissolution rates based on silicon release decrease with increasing pH to pH ∼ 8, then increase in response to further pH increases. The apparent relative metal release rates from epidote at chemical steady-state were found to vary with pH and temperature, in part due to secondary phase formation. At 100 and 200 °C, Al is preferentially released relative to Si at alkaline pH, but preferentially retained by the solid at 2 < pH < 8. This behaviour is interpreted to stem from the precipitation and re-dissolution of Al oxy-hydroxides such as boehmite, diaspore, and gibbsite, as observed in solids recovered after the experiments. Calcium tends to be preferentially released compared to Si and Al at acidic and near to neutral pH. Iron was found to be retained in the solid phase due to the prompt formation of secondary Fe minerals. Nevertheless, neither the dissolution of 20 wt% of the initial epidote mass nor the formation of secondary phases on the epidote surface substantially changed epidote dissolution rates. As epidote (i) is almost three times more Ca-rich than a typical basaltic glass, (ii) preferentially releases Ca at pH typical of CO2-charged water injections, and (iii) exhibits increasing dissolution rates as pH increases at alkaline conditions, it seems likely that the presence of epidote in altered basalts may be beneficial for the formation of calcite and other Ca-carbonates during mineral carbonation efforts. In addition, the relatively rapid rates of epidote dissolution at acidic conditions suggest that epidote has a significant role in the weathering of altered mafic rocks.

Natural organic matter inhibits Ni stabilization during Fe(II)-catalyzed ferrihydrite transformation

Trace metals, such as nickel (Ni), are often found associated with ferrihydrite (Fh) in soil and sediment and have been shown to redistribute during Fe(II)-catalyzed transformation of Fh. Fe(II)-catalyzed transformation of Fh, however, is often inhibited when natural organic matter (NOM) is associated with Fh. To explore whether NOM affects the behavior of Ni during Fe(II)-catalyzed transformation of Fh, we tracked Ni distribution, Fe atom exchange, and mineral transformation of Fh and Fh coprecipitated with Suwannee River natural organic matter (SRNOM-Fh). As expected, in the absence of Fe(II), Fh and SRNOM-Fh did not transform to secondary Fe minerals after two weeks. We further observed little difference in Ni adsorption on SRNOM-Fh compared to Fh. In the presence of Fe(II), however, we found that Ni associated with SRNOM-Fh was more susceptible to acid extraction than Fh. Specifically, we found almost double the amount of Ni remaining in the Fh after mild extraction compared to SRNOM-Fh. XRD showed that Fh transformed to goethite and magnetite whereas SRNOM-Fh did not transform despite 57Fe isotope tracer experiments confirmed that SRNOM-Fh underwent extensive atom exchange with aqueous Fe(II). Our findings suggest that Fe atom exchange may not be sufficient for obvious Ni stabilization and that transformation to secondary minerals may be necessary for Ni stabilization to occur.

Mobilization of arsenic during reductive dissolution of As(V)-bearing jarosite by a sulfate reducing bacterium

Microbial sulfidization of arsenic (As)-bearing jarosite involves complex processes and is yet to be fully elucidated. Here, we investigated the behavior of As during reductive dissolution of As(V)-bearing jarosite by a pure sulfate reducing bacterium with or without dissolved SO42− amendment. Changes of aqueous chemistry, mineralogical characteristics, and As speciation were examined in batch experiments. The results indicated that jarosite was mostly replaced by mackinawite in the system with added SO42-. In the medium without additional SO42-, mackinawite, vivianite, pyrite, and magnetite formed as secondary Fe minerals, though 24.55 % of total Fe was in form of an aqueous Fe2+ phase. The produced Fe2+ in turn catalyzed the transformation of jarosite. At the end of the incubation, 41.99 % and 48.10 % of As in the solid phase got released into the aqueous phase in the systems with and without added SO42-, respectively. The addition of dissolved SO42- mitigated the mobilization of As into the aqueous phase. In addition, all As5+ on the solid surface was reduced to As3+ during the microbial sulfidization of As-bearing jarosite. These findings are important for a better understanding of geochemical cycling of elements As, S, and Fe in acid mine drainage and acid sulfate soil environments.

Effects of Cr(VI)-reducing bacteria on the behaviour of Cr(VI) adsorption by goethite and haematite: speciation and distribution

In the soil environment, the existence of Cr(VI)-reducing microorganisms might affect the adsorption, desorption and reduction of Cr(VI) adsorbed on soil minerals (such as Fe oxides). The behaviour of Cr(VI)-reducing microorganisms might affect the adsorption and reduction of Cr(VI) by soil minerals. Goethite and haematite with saturated adsorbed Cr(VI) were incubated with Microbacterium sp. QH-2, a Cr(VI)-reducing bacteria. Scanning electronic microscopy (SEM) and X-ray diffraction (XRD) were used to detect the changes in goethite and haematite. X-ray absorption near-edge structure (XANES) was performed to detect the changes in Cr(III) and Cr(VI) on goethite and haematite. Strain QH-2 adhered to the surfaces of goethite and haematite. No morphological changes in goethite and haematite were detected after incubation. No secondary Fe minerals formed. Cr(III) was the dominant species of Cr on goethite and haematite (78.5% for goethite and 96.7% for haematite) after incubation. Furthermore, the reduction rate of Cr(VI) by strain QH-2 in the liquid phase was faster than that of Cr(VI) adsorbed on goethite and haematite. The existence of strain QH-2 could promote the adsorption of Cr by goethite and haematite. Strain QH-2 could affect the morphological distribution and transformation of Cr in Fe oxides such as goethite and haematite.

Oxidation reactivity of As(III)-containing pyrites: Differences between structurally-incorporated and adsorbed As(III)

Natural pyrites contain high contents of adsorbed and structurally-incorporated arsenic (As), which may in turn affect the oxidation rates of pyrite as well as the transformation and immobilization behaviors of As contaminants. However, the speciation and distribution of As on the oxidation reactivity of pyrites is still not clear yet. In this study, the oxidation reactivity of three different pyrites, namely, pure pyrite (Py-free), As(III)-adsorbed pyrite [Py*-As(III)ad] and As(III)-structurally-incorporated pyrite [Py-As(III)in, including Py-0.02 and Py-0.04] will be compared. In Py-As(III)in, As(III) replaces S(-I) and changes the relative ratio of Fe: S. The XPS results reveal that Fe(III)-S(-II) site densities on the pyrite surface increase with incorporated As, indicating that Py-As(III)in with more As carries more surface defects. The oxidation rates of pyrites are accelerated at pHs 7.0 and 9.0 by both structurally-incorporated and adsorbed As(III), which is more significant in structurally-incorporated As(III). At pH 2.5, structurally-incorporated As(III) promotes the oxidation of pyrite, whereas adsorbed As(III) inhibits its oxidation. The Tafel plots and the electrochemical impedance spectroscopy (EIS) analysis further support that As in Py-As(III)in decreases the charge transfer resistance on the pyrite surface and facilitates electron transfer between dissolved oxygen (DO) and the pyrite surface, thus enhancing the oxidation rates of pyrites. Variations of H2O2 in Py-As(III)in in the full pH range also indicate that As increases the two-electron transfer from pyrite to DO, resulting in the formation of more H2O2. At pH 2.5, the generated OH from H2O2 is the predominant species in pyrite oxidation, and at pHs 7.0 and 9.0, H2O2, Fe(IV) and OH all contribute to pyrite oxidation. Compared with Py-As(III)in, As in Py*-As(III)ad is more readily to be mobilized, with more As(V) being detected at pH 2.5. Subsequent decreases of As(III) and As(V) suggest re-immobilization of the released As via the adsorption or co-precipitation pathways. Solid characterizations further reveal that dissolved or adsorbed As(III) significantly inhibit the development of lepidocrocite, goethite and magnetite, whereas structurally-incorporated As(III) does not, triggering differences in the speciation of secondary Fe minerals. The present study reveals that the speciation and distribution of As(III) significantly affects the chemical and electronic properties of different pyrites, thus affecting the corresponding oxidation reactivity of different pyrites.

Effects of different crystalline iron oxides on immobilization and bioavailability of Cd in contaminated sediment

In situ immobilization remediation has attracted attention due to its effectiveness in treating contaminated sediment. In this study, the effects of three iron oxides with different crystallinities Fe3O4, (α + γ)-Fe2O3, and α-Fe2O3 were investigated as potential in situ amendments for Cd remediation in sediment. The three amendments were mixed with the sediment at dosages of 0%, 2%, and 6% (w/w) for 1–56 d of treatment. The changes of the oxidation-reduction potential (Eh) and pH in the treated sediment samples indicated the related mechanism for Cd immobilization. The experimental results showed that when the sediment sample was treated with synthesized iron oxides for 56 d, the TCLP (toxicity characteristic leaching procedure) leachable Cd in the sediment decreased by 29.43–57.26%, and the PBET (physiologically-based extraction test) extracted Cd decreased by 17.22–39.40%. Additionally, (α + γ)-Fe2O3 exhibited better performances than the other iron oxides. The BCR (Community Bureau of Reference) extraction, results suggested that synthetized iron oxides can effectively reduce the mobile fraction and increase the stable fraction of Cd, which in turn reduces the bioavailability of Cd in the sediment. The analysis of the Fe dissolution risk showed that a high dosage Fe3O4 treatment posed a potential threat to the organisms and suggested that the secondary Fe minerals were involved in the Cd immobilization. Overall, this study provides new insight into the potential applications of iron oxides for Cd immobilization in sediment.

Comparative performance of green rusts generated in Fe0–electrocoagulation for Cd2+ removal from high salinity wastewater: Mechanisms and optimization

The treatment of wastewater containing high concentration of inorganic salts has always been one of the focuses of environmental researchers. In this work, the effect of Cl− and SO42− on the removal of Cd2+ from wastewater using Fe0-electrocoagulation (Fe0-EC) were investigated by evaluating the transformation of Fe mineral. The experimental results indicated that the removal of Cd2+ from wastewater was depended on the property of Fe minerals. The generation of sulfate green rust (GRSO4) produced in the presence of SO42− showed stronger adsorption than the chloride green rust (GRCl) for Cd2+, and GRSO4 was obtained even in the mixture Cl− and SO42− solutions, because Fe(II)–Fe(III) GRs (layered double hydroxides, LDHs) showed stronger affinity for divalent SO42− than monovalent Cl−. High concentration of inorganic anions in wastewater resulted in the negative charged Fe flocs. High concentration of Cl− promoted the oxidation of Fe(II) to Fe(III) by chlorine–containing oxidants, and increased the proportion of Fe(III)/Fe(II) in Fe flocs, secondary Fe mineral magnetite (Fe3O4) was formed because of the increase of pH. Therefore, the presence of GRSO4 intermediate increased the Cd2+ removal by adsorption (coagulation and coprecipitation), and then the generated GRSO4 were gradually transformed into lepidocrocite (γ–FeOOH) by oxygen from air. Finally, the parameter optimization were conducted by adjusting the ratio of Cl− and SO42− (RC:S), current density (j), initial pH (pHi), initial Cd2+ concentration (C0), and temperature (T0). The removal efficiency of Cd2+ reached 99.5% after 10 min Fe0-EC under the optimal parameters: RC:S = 25:50 mmoL/mmol, j = 6 mA/cm2, pHi = 7–9, and T0 = 40 °C.

A novel approach coupling ferrous iron bio-oxidation and ferric iron chemo-reduction to promote biomineralization in simulated acidic mine drainage.

A novel Acidithiobacillus ferrooxidans-mediated approach coupling biological oxidation and chemical reduction for treating acid mine drainage (AMD) was investigated. The results showed that controlled addition of zero valent iron (ZVI) into the coupling system did not exhibit a significant adverse influence on the bacterial activity of Acidithiobacillus ferrooxidans but markedly increased the formation of secondary Fe-minerals. Nutrition did not affect the efficiency of coupling process, except for the bacteria density of A. ferrooxidans. 2 days cyclic treatment performed better than that of 4 and 8 days. After 14 cycles of the coupling process, 89.4% of total iron (2.23 g L−1) was transferred into Fe-minerals finally. In addition, the combined system was highly effective in removing sulfate (63%) from a simulated AMD that contained soluble Cu, Zn, Al, and Mn. Valuable iron-sulfate material e.g. schwertmannite was formed with little co-precipitation of other metals. Therefore, the integration of A. ferrooxidans into the reduction by ZVI may have considerable potential in the enhancement of biomineralization efficiency, which may further decrease soluble TFe and sulfate loads in AMD before lime neutralization.

Geochemical and mineralogical constraints in iron ore tailings limit soil formation for direct phytostabilization

The present study aimed to characterize key physico-chemical and mineralogical attributes of magnetite iron (Fe) ore tailings to identify potential constraints limiting in situ soil formation and direct phytostabilization. Tailings of different age, together with undisturbed local native soils, were sampled from a magnetite mine in Western Australia. Tailings were extremely alkaline (pH > 9.0), with a lack of water stable aggregate and organic matter, and contained abundant primary minerals including mica (e.g., biotite), with low specific surface area (N2-BET around 1.2 m2 g−1). These conditions remained relatively unchanged after four years' aging under field conditions. Chemical extraction and spectroscopic analysis [e.g., X-ray diffraction (XRD) and synchrotron-based Fe K edge X-ray absorption fine structure spectroscopy (XAFS) analysis] revealed that the aging process decreased biotite-like minerals, but increased hematite and magnetite in the tailings. However, the aged tailings lacked goethite, a compound abundant in natural soils. Examination using backscattered-scanning electron microscope - energy dispersive X-ray spectrometry (BSE-SEM-EDS) revealed that aged tailings contained discrete sharp edged Fe-bearing minerals that did not physically integrate with other minerals (e.g., Si/Al bearing minerals). In contrast, Fe minerals in native soils appeared randomly distributed and closely amassed with Si/Al rich phyllosilicates, with highly eroded edges. The lack of labile organic matter and the persistence of alkaline-saline conditions may have significantly hindered the bioweathering of Fe-minerals and the biogenic formation of secondary Fe-minerals in tailings. However, there is signature that a native pioneer plant, Maireana brevifolia can facilitate the bioweathering of Fe-bearing minerals in tailings. We propose that eco-engineering inputs like organic carbon accumulation, together with the introduction of functional microbes and pioneer plants, should be adopted to accelerate bioweathering of Fe-bearing minerals as a priority for initiating in situ soil formation in the Fe ore tailings.

Effects of humic substances on Fe(II) sorption onto aluminum oxide and clay

We studied the effects of humic substances (HS) on the sorption of Fe(II) onto Al-oxide and clay sorbents at pH 7.5 with a combination of batch kinetic experiments and synchrotron Fe K-edge EXAFS analyses. Fe(II) sorption was monitored over the course of 4 months in anoxic clay and Al-oxide suspensions amended with variable HS types (humic acid, HA; or fulvic acid, FA) and levels (0, 1, and 4 wt%), and with differing Fe(II) and HS addition sequences (co-sorption and pre-coated experiments, where Fe(II) sorbate was added alongside and after HS addition, respectively). In the Al-oxide suspensions, the presence of HS slowed down the kinetics of Fe(II) sorption, but had limited, if any, effect on the equilibrium aqueous Fe(II) concentrations. EXAFS analyses revealed precipitation of Fe(II)–Al(III)-layered double hydroxide (LDH) phases as the main mode of Fe(II) sorption in both the HA-containing and HA-free systems. These results demonstrate that HS slow down Fe(II) precipitation in the Al-oxide suspensions, but do not affect the composition or stability of the secondary Fe(II)–Al(III)-LDH phases formed. Interference of HS with the precipitation of Fe(II)–Al(III)-LDH was attributed to the formation organo-Al complexes HS limiting the availability of Al for incorporation into secondary layered Fe(II)-hydroxides. In the clay systems, the presence of HA caused a change in the main Fe(II) sorption product from Fe(II)–Al(III)-LDH to a Fe(II)-phyllosilicate containing little structural Al. This was attributed to complexation of Al by HA, in combination with the presence of dissolved Si in the clay suspension enabling phyllosilicate precipitation. The change in Fe(II) precipitation mechanism did not affect the rate of Fe(II) sorption at the lower HA level, suggesting that the inhibition of Fe(II)–Al(III)-LDH formation in this system was countered by enhanced Fe(II)-phyllosilicate precipitation. Reduced rates of Fe(II) sorption at the higher HA level were attributed to surface masking or poisoning by HA of secondary Fe(II) mineral growth at or near the clay surface. Our results suggest that HS play an important role in controlling the kinetics and products of Fe(II) precipitation in reducing soils, with effects modulated by soil mineralogy, HS content, and HS properties. Further work is needed to assess the importance of layered Fe(II) hydroxides in natural reducing environments.

Arsenic mobilization in spent nZVI waste residue: Effect of Pantoea sp. IMH

Nanoscale zero-valent iron (nZVI) is an effective arsenic (As) scavenger. However, spent nZVI may pose a higher environmental risk than our initial thought in the presence of As-reducing bacteria. Therefore, our motivation was to explore the As redox transformation and release in spent nZVI waste residue in contact with Pantoea sp. IMH, an arsC gene container adopting the As detoxification pathway. Our incubation results showed that IMH preferentially reduce soluble As(V), not solid-bound As(V), and was innocent in elevating total dissolved As concentrations. μ-XRF and As μ-XANES spectra clearly revealed the heterogeneity and complexity of the inoculated and control samples. Nevertheless, the surface As local coordination was not affected by the presence of IMH as evidenced by similar As-Fe atomic distance (3.32–3.36 Å) and coordination number (1.9) in control and inoculated samples. The Fe XANES results suggested that magnetite in nZVI residue was partly transformed to ferrihydrite, and the IMH activity slowed down the nZVI aging process. IMH distorted Fe local coordination without change its As adsorption capacity as suggested by Mössbauer spectroscopy. Arsenic retention is not inevitably enhanced by in situ formed secondary Fe minerals, but depends on the relative As affinity between the primary and secondary iron minerals.

Oxidative dissolution of amorphous FeS and speciation of secondary Fe minerals: Effects of pH and As(III) concentration

Oxidative dissolution of Fe sulfides is an important geochemical process mediating As mobility in aqueous environments. However, little is known regarding the presence of As(III) on the oxidative dissolution rate of FeS and on speciation of secondary Fe minerals under pH-neutral and alkaline conditions. In this study, oxidative dissolution of FeS under different concentrations of As(III) and pHs were carried out. The enhanced dissolution rates of Fe(II) from FeS with increasing As(III) concentrations (≤4.7mM) indicate that strong interactions between As(III) and S(−II) in FeS enhance the dissolution rates of structural Fe(II), whereas such an enhancement cannot be observed in the As(III)-FeS system under anoxic conditions at pHs7.0 and 9.0, suggesting the importance of dissolved oxygen in the enhanced dissolution of FeS by As(III). On the other hand, >98.5% of 6.7 and 4.7mM As(III) can be immobilized by 2gL−1 FeS within 1.5h at pHs7.0 and 9.0, respectively, demonstrating the high efficiency of amorphous FeS in the immobilization of concentrated As(III) under oxic conditions. XPS analysis further reveals that, irrespective of pH and initial As(III) concentrations, >60% As(III) was oxidized to As(V) before it was immobilization in secondary Fe minerals, and no As-S compound could be observed after 48h of oxidation. As(III) also influences speciation of secondary Fe minerals during oxidative dissolution of FeS. Analyses of XRD, FTIR, Raman, SEM and TEM demonstrate that the plate lepidocrocite is the primary oxidation product in the absence of As(III), but shows degenerations with increasing As(III) concentrations, and completely changes to ferric arsenate in the presence of 6.7mM As(III), revealing the inhibition of As(III) and/or As(V) on the development of lepidocrocite at pHs7.0 and 9.0. This also suggests that, apart from adsorption, coprecipitation of ferric arsenate is another important pathway to immobilize concentrated As(III) during oxidative dissolution of FeS.

Reductive dechlorination of carbon tetrachloride by bioreduction of nontronite

Reductive dechlorination of carbon tetrachloride (CT) was investigated during bioreduction of iron-containing clay mineral (i.e., nontronite) by iron-reducing bacteria (Shewanella putrefaciens CN32 (CN32)). In the absence of CT, the production of Fe(II) significantly increased in nontronite suspension with CN32 in 124 d (11.1% of Fe(III) reduction), resulting in formation of new secondary Fe(II) mineral phase (i.e., vivianite (FeII3(PO4)2·8H2O)). In the presence of CT, an acceleration of CT dechlorination was observed after 13 d and it reached almost 68% of removal efficiency at 32 d in nontronite suspension with CN32, which was 1.8 times higher than that by CN32 alone (37%). Significant amounts of formate (30.1%) and CO (2.4%) were measured during the CT dechlorination in the nontronite suspension with CN32. Results obtained from Fe(II) measurement and X-ray diffraction (XRD) showed the acceleration of Fe(II) production after 13 d and the formation of vivianite in the range of 13–25 d, suggesting that the biogenic vivianite enhanced the CT dechlorination in this study. Experimental results from batch kinetic tests, Fe(II) measurements, XRD analysis, and by-product study suggested that the formation of vivianite can play a crucial role for the enhanced reductive dechlorination of CT in phosphorous enriched subsurface environments with iron-containing clay minerals.