U(VI) Reduction by Biogenic and Abiotic Hydroxycarbonate Green Rusts: Impacts on U(IV) Speciation and Stability Over Time

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

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

Reduction of Chlorinated Ethenes by Ag- and Cu-Amended Green Rust

Chlorinated ethenes have been used extensively as solvents, degreasers, and dry-cleaning agents in a range of commercial and industrial applications. This has created a legacy of contaminated soils and groundwater, particularly with respect to perchloroethylene (PCE; a.k.a. tetrachloroethene—C2Cl4), and trichloroethylene (TCE; a.k.a. trichloroethene—C2HCl3), prompting the development of a wide array of treatment technologies for remediation of chlorinated ethene-contaminated environments. Green rusts are highly redox-active layered Fe(II)-Fe(III) hydroxides that have been shown to be facile reductants for a wide range of organic and inorganic pollutants. The reduction of chlorinated ethenes [vinyl chloride (VC); 1,1-dichloroethene(11DCE), cis-1,2-dichloroethene (c12DCE), trans-1,2-dichloroethene (t12DCE), TCE, and PCE] was examined in aqueous suspensions of green rust, alone as well as with the addition of Ag(I) (AgGR) or Cu(II) (CuGR). Green rust alone was ineffective as a reductant for the reductive dechlorination for all of the chlorinated ethenes. Near-complete removal of PCE was observed in the presence of AgGR, but all other chlorinated ethenes were essentially non-reactive. Partial removal of chlorinated ethenes was observed in the presence of CuGR, particularly 11DCE (34%), t12DCE (51%), and VC (66%). Significant differences were observed in the product distributions of chlorinated ethene reduction by AgGR and CuGR. The effectiveness of Ag(I)- and Cu(II)-amended green rusts for removal of chlorinated ethenes may be improved under different conditions (e.g., pH and interlayer anion) and warrants further investigation.

Enhancement of the heterogeneous adsorption and incorporation of uraniumVI caused by the intercalation of β-cyclodextrin into the green rust

The influence of intercalated anions on the structure and composition of green rusts supplies a theoretical possibility for the investigation of the structural modification of Fe^II/Fe^III (oxyhydr)oxide materials. β-Cyclodextrin was intercalated into the mixed-valent iron-based hydroxide layers to synthesize new green rust materials (β-CD GRs), pursuing high-capacity uranium^VI (U^VI) sorption. The molar ratios of Fe^II to Fe^III and the molar ratios of β-CD GR to Fe^II + Fe^III had a significant effect on the synthesis of β-CD GRs. The synthesis process was further optimized by the quadric predictor and desirability function in a central composite design in combination. Both strong acidity and alkalinity were harmful to the adsorption of β-CD GRs towards U^VI. The pseudo-first-order kinetic model and Langmuir isotherm model were appropriate in fitting the whole adsorption process. The maximum monolayer adsorption capacity of β-CD GRs was 2548.61 mg/g. The presence of mimic groundwater constituents explicitly deteriorated the interaction between β-CD GR and U^VI species. Nanoscale nodules and particles were formed on the β-CD GR after the adsorption experiments. The peaks at 1159 and 609 cm^-1 vanished with the band at 1103 cm^-1 being left-shifted to 1117 cm^-1 in the FTIR spectra of β-CD GR during the heterogeneous process. The intercalation of β-CD brought obvious enhancement of U^VI species sorption to the GR material, which was combinedly driven by several reaction pathways and different from the unmodified GRs.

Effective adsorption of quadrivalent cerium by synthesized laurylsulfonate green rust in a central composite design

The layered laurylsulfonate intercalated green rust (lauryl-S GR) was synthesized to evaluate the influence of synthesis parameters and aqueous conditions on the adsorption of Ce^IV. The maximum adsorption capacity of 305.58 mg/g by lauryl-S GR was predictably obtained. The pseudo-first-order kinetic model was appropriate in fitting the whole uptake process in a weak acid environment. Three isotherm models including Langmuir, Freundlich, and Tempkin were all reliable in depicting the isotherm adsorption process. The maximum monolayer adsorption capacity of lauryl-S GR towards Ce^IV was 315.46 mg/g. Ce species including CeO and Ce₂O₃ besides CeO₂ were matched in the XPS distribution, directly indicating the reduction reaction brought by Fe^II in the GR occurred to hydrated Ce^IV ions during the adsorption. Nano-sized Ce particles attached to the lauryl-S GRs after the adsorption experiments were observed in the morphological characterization. Flocculated materials were formed on the surface of the lauryl-S GR at a pH of 7, which further reduced the active sites and disrupted the continuous uptake of Ce^IV to the lauryl-S GR. This study expands the application of GRs and supplies an ideal iron-based material for the construction of the affiliated recovery pathway to the traditional separation of Ce.

Reduction of Vanadium(V) by Iron(II)-Bearing Minerals

Fe(II)-bearing minerals (magnetite, siderite, green rust, etc.) are common products of microbial Fe(III) reduction, and they provide a reservoir of reducing capacity in many subsurface environments that may contribute to the reduction of redox active elements such as vanadium; which can exist as V(V), V(IV), and V(III) under conditions typical of near-surface aquatic and terrestrial environments. To better understand the redox behavior of V under ferrugenic/sulfidogenic conditions, we examined the interactions of V(V) (1 mM) in aqueous suspensions containing 50 mM Fe(II) as magnetite, siderite, vivianite, green rust, or mackinawite, using X-ray absorption spectroscopy at the V K-edge to determine the valence state of V. Two additional systems of increased complexity were also examined, containing either 60 mM Fe(II) as biogenic green rust (BioGR) or 40 mM Fe(II) as a mixture of biogenic siderite, mackinawite, and magnetite (BioSMM). Within 48 h, total solution-phase V concentrations decreased to <20 µM in all but the vivianite and the biogenic BiSMM systems; however, >99.5% of V was removed from solution in the BioSMM and vivianite systems within 7 and 20 months, respectively. The most rapid reduction was observed in the mackinawite system, where V(V) was reduced to V(III) within 48 h. Complete reduction of V(V) to V(III) occurred within 4 months in the green rust system, 7 months in the siderite system, and 20 months in the BioGR system. Vanadium(V) was only partially reduced in the magnetite, vivianite, and BioSMM systems, where within 7 months the average V valence state stabilized at 3.7, 3.7, and 3.4, respectively. The reduction of V(V) in soils and sediments has been largely attributed to microbial activity, presumably involving direct enzymatic reduction of V(V); however the reduction of V(V) by Fe(II)-bearing minerals suggests that abiotic or coupled biotic–abiotic processes may also play a critical role in V redox chemistry, and thus need to be considered in modeling the global biogeochemical cycling of V.

Effects of Fe(III) Oxide Mineralogy and Phosphate on Fe(II) Secondary Mineral Formation during Microbial Iron Reduction

The bioreduction of Fe(III) oxides by dissimilatory iron-reducing bacteria may result in the formation of a suite of Fe(II)-bearing secondary minerals, including magnetite (a mixed Fe(II)/Fe(III) oxide), siderite (Fe(II) carbonate), vivianite (Fe(II) phosphate), chukanovite (ferrous hydroxy carbonate), and green rusts (mixed Fe(II)/Fe(III) hydroxides). In an effort to better understand the factors controlling the formation of specific Fe(II)-bearing secondary minerals, we examined the effects of Fe(III) oxide mineralogy, phosphate concentration, and the availability of an electron shuttle (9,10-anthraquinone-2,6-disulfonate, AQDS) on the bioreduction of a series of Fe(III) oxides (akaganeite, feroxyhyte, ferric green rust, ferrihydrite, goethite, hematite, and lepidocrocite) by Shewanella putrefaciens CN32, and the resulting formation of secondary minerals, as determined by X-ray diffraction, Mössbauer spectroscopy, and scanning electron microscopy. The overall extent of Fe(II) production was highly dependent on the type of Fe(III) oxide provided. With the exception of hematite, AQDS enhanced the rate of Fe(II) production; however, the presence of AQDS did not always lead to an increase in the overall extent of Fe(II) production and did not affect the types of Fe(II)-bearing secondary minerals that formed. The effects of the presence of phosphate on the rate and extent of Fe(II) production were variable among the Fe(III) oxides, but in general, the highest loadings of phosphate resulted in decreased rates of Fe(II) production, but ultimately higher levels of Fe(II) than in the absence of phosphate. In addition, phosphate concentration had a pronounced effect on the types of secondary minerals that formed; magnetite and chukanovite formed at phosphate concentrations of ≤1 mM (ferrihydrite), <~100 µM (lepidocrocite), 500 µM (feroxyhyte and ferric green rust), while green rust, or green rust and vivianite, formed at phosphate concentrations of 10 mM (ferrihydrite), ≥100 µM (lepidocrocite), and 5 mM (feroxyhyte and ferric green rust). These results further demonstrate that the bioreduction of Fe(III) oxides, and accompanying Fe(II)-bearing secondary mineral formation, is controlled by a complex interplay of mineralogical, geochemical, and microbiological factors.

Reduction of Hg(II) by Fe(II)-Bearing Smectite Clay Minerals

Aluminosilicate clay minerals are often a major component of soils and sediments and many of these clays contain structural Fe (e.g., smectites and illites). Structural Fe(III) in smectite clays is redox active and can be reduced to Fe(II) by biotic and abiotic processes. Fe(II)-bearing minerals such as magnetite and green rust can reduce Hg(II) to Hg(0); however, the ability of other environmentally relevant Fe(II) phases, such as structural Fe(II) in smectite clays, to reduce Hg(II) is largely undetermined. We conducted experiments examining the potential for reduction of Hg(II) by smectite clay minerals containing 0–25 wt% Fe. Fe(III) in the clays (SYn-1 synthetic mica-montmorillonite, SWy-2 montmorillonite, NAu-1 and NAu-2 nontronite, and a nontronite from Cheney, Washington (CWN)) was reduced to Fe(II) using the citrate-bicarbonate-dithionite method. Experiments were initiated by adding 500 µM Hg(II) to reduced clay suspensions (4 g clay L−1) buffered at pH 7.2 in 20 mM 3-morpholinopropane-1-sulfonic acid (MOPS). The potential for Hg(II) reduction in the presence of chloride (0–10 mM) and at pH 5–9 was examined in the presence of reduced NAu-1. Analysis of the samples by Hg LIII-edge X-ray absorption fine structure (XAFS) spectroscopy indicated little to no reduction of Hg(II) by SYn-1 (0% Fe), while reduction of Hg(II) to Hg(0) was observed in the presence of reduced SWy-2, NAu-1, NAu-2, and CWN (2.8–24.8% Fe). Hg(II) was reduced to Hg(0) by NAu-1 at all pH and chloride concentrations examined. These results suggest that Fe(II)-bearing smectite clays may contribute to Hg(II) reduction in suboxic/anoxic soils and sediments.

Geochemistry and Acid Hydrometallurgy Accessibility of Uraninite from Mianhuakeng Granite-Hosted Uranium Deposit, South China

Systematic study of the surface chemical properties of uranium minerals is necessary to improve the uranium ore extracting process. The presented work aims to argue geochemistry and acid hydrometallurgy accessibility of uraninite from the Mianhuakeng (MHK) granite-hosted uranium deposit, South China, which provides insight on this ore extracting domain. Mineralogy, geochemical composition, U–Th–Pb chemical age, and uranium deportment of the uraninite were systematically analyzed by using scanning electron microscope with energy dispersion spectrum (SEM-EDS), an electron probe microanalyzer (EPMA), and x-ray photoelectron spectroscopy (XPS). The results showed that uraninite was intergrowth with coffinite, probably due to uraninite being partly metasomatized into coffinite along the fissures. The major element content of uraninite such as for UO2, SiO2, and CaO were 79.46 ± 2.03 wt%, 6.19 ± 1.36 wt%, and 5.09 ± 0.80 wt%, respectively. Single-point U–Th–Pb chemical ages for uraninite grains were calculated with the EPMA data, and the results showed ages ranging from a few million to dozens of million years, indicating Pb loss after uraninite formed. Uranium deportment in uraninite generally existed in the forms of UO2, U3O8, and UO3, and mostly showed high valence states suggested by XPS. Uranium on the surface of the uraninite grain was partially oxidized by sulfuric acid leaching, which led to tetravalent uranium converting to hexavalent uranium, suggesting uraninite in the MHK uranium deposit is accessible to be leached by sulfuric acid.

Controls on Iron Reduction and Biomineralization over Broad Environmental Conditions as Suggested by the Firmicutes Orenia metallireducens Strain Z6

Microbial iron reduction is a ubiquitous biogeochemical process driven by diverse microorganisms in a variety of environments. However, it is often difficult to separate the biological from the geochemical controls on bioreduction of Fe(III) oxides. Here, we investigated the primary driving factor(s) that mediate secondary iron mineral formation over a broad range of environmental conditions using a single dissimilatory iron reducer, Orenia metallireducens strain Z6. A total of 17 distinct geochemical conditions were tested with differing pH (6.5-8.5), temperature (22-50 °C), salinity (2-20% NaCl), anions (phosphate and sulfate), electron shuttle (anthraquinone-2,6-disulfonate), and Fe(III) oxide mineralogy (ferrihydrite, lepidocrocite, goethite, hematite, and magnetite). The observed rates and extent of iron reduction differed significantly with k_int between 0.186 and 1.702 mmol L^-1 day^-1 and Fe(II) production ranging from 6.3% to 83.7% of the initial Fe(III). Using X-ray absorption and scattering techniques (EXAFS and XRD), we identified and assessed the relationship between secondary minerals and the specific environmental conditions. It was inferred that the observed bifurcation of the mineralization pathways may be mediated by differing extents of Fe(II) sorption on the remaining Fe(III) minerals. These results expand our understanding of the controls on biomineralization during microbial iron reduction and aid the development of practical applications.

Exclusive microbially driven autotrophic iron-dependent denitrification in a reactor inoculated with activated sludge

Autotrophic iron-dependent denitrification (AIDD) is arising as a promising process for nitrogen removal from wastewater with a low carbon to nitrogen ratio. However, there is still a debate about the existence of such a process in activated sludge systems. This work provides evidence and elucidated the feasibility of autotrophic Fe(II)-oxidizing nitrate-reducing culture for nitrogen removal by long-term reactor operation, batch experimental verification, unstructured kinetic modeling and microbial community analyses. A relatively stable nitrate removal rate was achieved coupled with the oxidation of ferrous ions in 3-month operation of reactor. The kinetic modeling suggests that the iron oxidation was a growth-associated process in AIDD. Utilization of extracellular polymeric substances (and/or soluble microbial products) as electron donor for denitrification by heterotrophic denitrifiers was not mainly responsible for nitrogen removal in the reactor. After long-term operation of the reactor with activated sludge as inoculum, the enrichment culture KS-like consortium, dominated by Fe(II) oxidizer, Gallionellaceae, was successfully acclimated for autotrophic Fe(II)-oxidizing nitrate reduction. This work extents our understanding about the existence of such an autotrophic Fe(II)-oxidizing nitrate-reducing culture in both natural and engineered systems, and opens a door for its potential application in wastewater treatment.

High adsorption performance of synthesized hexametaphosphate green rust towards Cr(VI) removal and its mechanism explorations

Hexametaphosphate intercalated green rust (hexa-P GR) was fabricated by a coprecipitation process in an anaerobic environment to improve the adsorption of hexa-P GR for Cr(VI) and the total Cr under various aqueous conditions. Three kinetic models including the pseudo-first-order, intraparticle, and Elovich were appropriate in describing the adsorption of hexa-P GR towards Cr(VI) and the total Cr. The maximum mono-layer adsorption capacities (mg/g) of hexa-P GR for Cr(VI) at pH of 2 and 7 were 87.64 and 92.25, respectively, with the theoretical maximum capacity (mg/g) of 52.73 being obtained at pH of 7. Some competing cations existing in solutions such as Al³⁺, Ca²⁺, and Mg²⁺ would consume more hexa-P GR to remove Cr species. The neutral and weak alkaline environment was conducive to the hexa-P GR reuse, while the strong alkaline environment was beneficial to the removal of the total Cr. The orthogonal variables including the initial pH, the flow rate, and the Cr(VI) concentration all significantly influenced Cr removal. The sequences of reaction pathways referring to the adsorption of hexa-P GR differently occurred in various pH conditions.

Electron Donor Utilization and Secondary Mineral Formation during the Bioreduction of Lepidocrocite by Shewanella putrefaciens CN32

The bioreduction of Fe(III) oxides by dissimilatory iron reducing bacteria (DIRB) may result in the production of a suite of Fe(II)-bearing secondary minerals, including magnetite, siderite, vivianite, green rusts, and chukanovite; the formation of specific phases controlled by the interaction of various physiological and geochemical factors. In an effort to better understand the effects of individual electron donors on the formation of specific Fe(II)-bearing secondary minerals, we examined the effects of a series of potential electron donors on the bioreduction of lepidocrocite (γ-FeOOH) by Shewanella putrefaciens CN32. Biomineralization products were identified by X-ray diffraction, Mössbauer spectroscopy, and scanning electron microscopy. Acetate, citrate, ethanol, glucose, glutamate, glycerol, malate, and succinate were not effectively utilized for the bioreduction of lepidocrocite by S. putrefaciens CN32; however, substantial Fe(II) production was observed when formate, lactate, H2, pyruvate, serine, or N acetylglucosamine (NAG) was provided as an electron donor. Carbonate or sulfate green rust was the dominant Fe(II)-bearing secondary mineral when formate, H2, lactate, or NAG was provided, however, siderite formed with pyruvate or serine. Geochemical modeling indicated that pH and carbonate concentration are the key factors determining the prevalence of carbonate green rust verses siderite.

Effect of Calcium on the Bioavailability of Dissolved Uranium(VI) in Plant Roots under Circumneutral pH

We integrated field measurements, hydroponic experiments, microscopy, and spectroscopy to investigate the effect of Ca(II) on dissolved U(VI) uptake by plants in 1 mM HCO₃^- solutions at circumneutral pH. The accumulation of U in plants (3.1-21.3 mg kg^-1) from the stream bank of the Rio Paguate, Jackpile Mine, New Mexico served as a motivation for this study. Brassica juncea was the model plant used for the laboratory experiments conducted over a range of U (30-700 μg L^-1) and Ca (0-240 mg L^-1) concentrations. The initial U uptake followed pseudo-second-order kinetics. The initial U uptake rate ( V₀) ranged from 4.4 to 62 μg g^-1 h^-1 in experiments with no added Ca and from 0.73 to 2.07 μg g^-1 h^-1 in experiments with 12 mg L^-1 Ca. No measurable U uptake over time was detected for experiments with 240 mg L^-1 Ca. Ternary Ca-U-CO₃ complexes may affect the decrease in U bioavailability observed in this study. Elemental X-ray mapping using scanning transmission electron microscopy-energy-dispersive spectrometry detected U-P-bearing precipitates within root cell walls in water free of Ca. These results suggest that root interactions with Ca and carbonate in solution affect the bioavailability of U in plants. This study contributes relevant information to applications related to U transport and remediation of contaminated sites.