Citrate Controls Fe(II)-Catalyzed Transformation of Ferrihydrite by Complexation of the Labile Fe(III) Intermediate

Pb-Bearing Ferrihydrite Bioreduction and Secondary-Mineral Precipitation during Fe Redox Cycling

The significant accumulation of Pb from anthropogenic activities threatens environmental ecosystems. In the environment, iron oxides are one of the main carriers of Pb. Thus, the redox cycling of iron oxides, which is due to biotic and abiotic pathways, and which leads to their dissolution or transformation, controls the fate of Pb. However, a knowledge gap exists on the bioreduction in Pb-bearing ferrihydrites, secondary-mineral precipitation, and Pb partitioning during the bioreduction/oxidation/bioreduction cycle. In this study, Pb-bearing ferrihydrite (Fh_Pb) with various Pb/(Fe+Pb) molar ratios (i.e., 0, 2, and 5%) were incubated with the iron-reducing bacterium Shewanella oneidensis MR-1 for 7 days, oxidized for 7 days (atmospheric O2), and bioreduced a second time for 7 days. Pb doping led to a drop in the rate and the extent of the reduction. Lepidocrocite (23–56%) and goethite (44–77%) formed during the first reduction period. Magnetite (72–84%) formed during the second reduction. The extremely-low-dissolved and bioavailable Pb concentrations were measured during the redox cycles, which indicates that the Pb significantly sorbed onto the minerals that were formed. Overall, this study highlights the influence of Pb and redox cycling on the bioreduction of Pb-bearing iron oxides, as well as on the nature of the secondary minerals that are formed.

Autotrophic Fe-Driven Biological Nitrogen Removal Technologies for Sustainable Wastewater Treatment

Fe-driven biological nitrogen removal (FeBNR) has become one of the main technologies in water pollution remediation due to its economy, safety and mild reaction conditions. This paper systematically summarizes abiotic and biotic reactions in the Fe and N cycles, including nitrate/nitrite-dependent anaerobic Fe(II) oxidation (NDAFO) and anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox). The biodiversity of iron-oxidizing microorganisms for nitrate/nitrite reduction and iron-reducing microorganisms for ammonium oxidation are reviewed. The effects of environmental factors, e.g., pH, redox potential, Fe species, extracellular electron shuttles and natural organic matter, on the FeBNR reaction rate are analyzed. Current application advances in natural and artificial wastewater treatment are introduced with some typical experimental and application cases. Autotrophic FeBNR can treat low-C/N wastewater and greatly benefit the sustainable development of environmentally friendly biotechnologies for advanced nitrogen control.

Understanding the Importance of Labile Fe(III) during Fe(II)-Catalyzed Transformation of Metastable Iron Oxyhydroxides

Transformation of metastable Fe(III) oxyhydroxides is a prominent process in natural environments and can be significantly accelerated by the coexisting aqueous Fe(II) (Fe(II)_aq). Recent evidence points to the solution mass transfer of labile Fe(III) (Fe(III)_labile) as the primary intermediate species of general importance. However, a mechanistic aspect that remains unclear is the dependence of phase outcomes on the identity of the metastable Fe(III) oxyhydroxide precursor. Here, we compared the coupled evolution of Fe(II) species, solid phases, and Fe(III)_labile throughout the Fe(II)-catalyzed transformation of lepidocrocite (Lp) versus ferrihydrite (Fh) at equal Fe(III) mass loadings with 0.2-1.0 mM Fe(II)_aq at pH = 7.0. Similar to Fh, the conversion of Lp to product phases occurs by a dissolution-reprecipitation mechanism mediated by Fe(III)_labile that seeds the nucleation of products. Though for Fh we observed a transformation to goethite (Gt), accompanied by the transient emergence and decline of Lp, for initial Lp we observed magnetite (Mt) as the main product. A linear correlation between the formation rate of Mt and the effective supersaturation in terms of Fe(III)_labile concentration shows that Fe(II)-induced transformation of Lp into Mt is governed by the classical nucleation theory. When Lp is replaced by equimolar Gt, Mt formation is suppressed by opening a lower barrier pathway to Gt by heterogeneous nucleation and growth on the added Gt seeds. The collective findings add to the mechanistic understanding of factors governing phase selections that impact iron bioavailability, system redox potential, and the fate and transport of coupled elements.

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.

Cadmium Isotope Fractionation during Adsorption and Substitution with Iron (Oxyhydr)oxides

Cadmium (Cd) isotopes have great potential for understanding Cd geochemical cycling in soil and aquatic systems. Iron (oxyhydr)oxides can sequester Cd via adsorption and isomorphous substitution, but how these interactions affect Cd isotope fractionation remains unknown. Here, we show that adsorption preferentially enriches lighter Cd isotopes on iron (oxyhydr)oxide surfaces through equilibrium fractionation, with a similar fractionation magnitude (Δ^114/110Cd_{solid-solution}) for goethite (Goe) (-0.51 ± 0.04‰), hematite (Hem) (-0.54 ± 0.10‰), and ferrihydrite (Fh) (-0.55 ± 0.03‰). Neither the initial Cd²⁺ concentration or ionic strength nor the pH influence the fractionation magnitude. The enrichment of the light isotope is attributed to the adsorption of highly distorted [CdO₆] on solids, as indicated by Cd K-edge extended X-ray absorption fine-structure analysis. In contrast, Cd incorporation into Goe by substitution for lattice Fe at a Cd/Fe molar ratio of 0.05 preferentially sequesters heavy Cd isotopes, with a Δ^114/110Cd_{solid-solution} of 0.22 ± 0.01‰. The fractionation probably occurs during the transformation of Fh into Goe via dissolution and reprecipitation. These results improve the understanding of the Cd isotope fractionation behavior being affected by iron (oxyhydr)oxides in Earth's critical zone and demonstrate that interactions with minerals can obscure anthropogenic and natural Cd isotope characteristics, which should be carefully considered when applying Cd isotopes as environmental tracers.

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.

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.

Further insights into the Fe(ii) reduction of 2-line ferrihydrite: a semi in situ and in situ TEM study

The biotic or abiotic reduction of nano-crystalline 2-line ferrihydrite (2-line FH) into more thermodynamically stable phases such as lepidocrocite-LP, goethite-GT, magnetite-MG, and hematite-HT plays an important role in the geochemical cycling of elements and nutrients in aqueous systems. In our study, we employed the use of in situ liquid cell (LC) and semi in situ analysis in an environmental TEM to gain further insights at the micro/nano-scale into the reaction mechanisms by which Fe(ii)_(aq) catalyzes 2-line FH. We visually observed for the first time the following intermediate steps: (1) formation of round and wire-shaped precursor nano-particles arising only from Fe(ii)_(aq), (2) two distinct dissolution mechanisms for 2 line-FH (i.e. reduction of size and density as well as breakage through smaller nano-particles), (3) lack of complete dissolution of 2-line FH (i.e. "induction-period"), (4) an amorphous phase growth ("reactive-FH/labile Fe(iii) phase") on 2 line-FH, (5) deposition of amorphous nano-particles on the surface of 2 line-FH and (6) assemblage of elongated crystalline lamellae to form tabular LP crystals. Furthermore, we observed phenomena consistent with the movement of adsorbate ions from solution onto the surface of a Fe(iii)-oxy/hydroxide crystal. Thus our work here reveals that the catalytic transformation of 2-line FH by Fe(ii)_(aq) at the micro/nano scale doesn't simply occur via dissolution-reprecipitation or surface nucleation-solid state conversion mechanisms. Rather, as we demonstrate here, it is an intricate chemical process that goes through a series of intermediate steps not visible through conventional lab or synchrotron bulk techniques. However, such intermediate steps may affect the environmental fate, bioavailability, and transport of elements of such nano-particles in aqueous environments.

Thermal Stability and Decomposition Products of P-Doped Ferrihydrite

This work aimed to determine the effect of various amounts of P admixtures in synthetic ferrihydrite on its thermal stability, transformation processes, and the properties of the products, at a broad range of temperatures up to 1000 °C. A detailed study was conducted using a series of synthetic ferrihydrites Fe5HO8·4H2O doped with phosphates at P/Fe molar ratios of 0.2, 0.5, and 1.0. Ferrihydrite was synthesized by a reaction of Fe2(SO4)3 with 1 M KOH at room temperature in the presence of K2HPO4 at pH 8.2. The products of the synthesis and the products of heating were characterized at various stages of transformation by using differential thermal analysis accompanied with X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy-energy dispersive X-ray spectroscopy. Coprecipitation of P with ferrihydrite results in the formation of P-doped 2-line ferrihydrite. A high P content reduces crystallinity. Phosphate significantly inhibits the thermal transformation processes. The temperature of thermal transformation increases from below 550 to 710-750 °C. Formation of intermediate maghemite and Fe-phosphates, is observed. The product of heating up to 1000 °C contains hematite associated with rodolicoite FePO4 and grattarolaite Fe3PO7. Higher P content greatly increases the thermal stability and transformation temperature of rodolicoite as well.