Formation Of Ferrihydrite Research Articles

Seasonal Variations in Ochreous Precipitates and Drainage Waters in the Grantcharitsa Tungsten Deposit, Western Rhodopes, Bulgaria

Seasonal variations of drainage waters and ochreous products of their discharge from the closed abandoned old gallery at the Grantcharitsa scheelite deposit (Bulgaria) were studied by field and laboratory methods for the period 2019–2023. The drainage is generated under anoxic conditions and is inherently diluted (EC = 100–202 µS/cm) with S (6–12 mg/L), Si (6–22 mg/L), Na (6–10 mg/L), Fe (0.2–3.3 mg/L), and W (0.19–3.5 µg/L), at a pH 4.4–6.5 and temperature 7–11.5 °C, with dissolved oxygen DO (2.1–7.7 mg/L). The concentrations of Fe and W and the pH of the water are variable and reach their maximum values during the dry (autumn) season. It was found that such parameters as pH, Eh, DO, Fe and W content change dramatically at a distance of up to 3 m from the water outlet; the values of pH, DO and Eh are sharply increased with a simultaneous nearly 5–6-times reduction in iron and tungsten content. The decrease in the contents of these elements is associated with the precipitation of ochreous material consisting of nanoscale ferrihydrite with an intermediate structural ordering between 2-line and 6-line ferrihydrite (major phase), hematite, goethite, quartz, montmorillonite and magnetite. The formation of ferrihydrite occurs as a result of abiotic and biotic processes with the participation of iron-oxidizing bacteria. Besides Fe2O3 (55.5–64.0 wt.%), the ochreous sediment contains SiO2 (12.0–16.4 wt.%), SO3 (1.3–2.4 wt.%), Al2O3 (3.1–6.8 wt.%) and WO3 (0.07–0.11 wt.%). It has been shown that drainage waters and ochreous sediments do not inherently have a negative impact on the environment. The environmental problem arises with intense snowmelt and heavy rainfall, as a result of which the accumulated sediments are washed away and carried in the form of suspensions into the water systems. It is suggested that by providing atmospheric oxygen access to the closed gallery (via local boreholes), it is possible to stop the generation of iron-enriched drainage.

Biogenic amorphous FeOOH activated additional intracellular electron flow pathways for accelerating reductive dechlorination of tetrachloroethylene

Dissimilatory iron-reducing bacteria (DIRB) with extracellular electron transfer (EET) capabilities have shown significant potential for bioremediating halogenated hydrocarbon contaminated sites rich in iron and humic substances. However, the role and microbial molecular mechanisms of iron-humic acid (Fe-HA) complexes in the reductive dehalogenation process of DIRB remains inadequately elucidated. In this study, we developed a sustainable carbon cycling approach using Fe-HA complexes to modulate the electron flux from sawdust (SD), enabling almost complete reductive dechlorination by most DIRB (e.g., Shewanella oneidensis MR-1) that lack complex iron-sulfur molybdo enzymes. The SD-Fe-HA/MR-1 system achieved a 96.52% removal efficiency of tetrachloroethylene (PCE) at concentrations up to 250 μmol/L within 60 days. Material characterization revealed that DIRB facilitated the hydrolysis of macromolecular carbon sources by inducing the formation of amorphous ferrihydrite (FeOOH) in Fe-HA complexes. More importantly, the bioavailable FeOOH activated additional intracellular electron flow pathways, increasing the activity of potential dehalogenases. Transcriptome further highlight the innovative role of biogenic amorphous FeOOH in integrating intracellular redox metabolism with extracellular charge exchange to facilitate reductive dechlorination in DIRB. These findings provide novel insights into accelerating reductive dechlorination in-situ contaminated sites lacking obligate dehalogenating bacteria.

Sequestration of Labile Organic Matter by Secondary Fe Minerals from Chemodenitrification: Insight into Mineral Protection Mechanisms.

Labile organic matter (OM) immobilized by secondary iron (Fe) minerals from chemodenitrification may be an effective way to immobilize organic carbon (OC). However, the underlying mechanisms of coupled chemodenitrification and OC sequestration are poorly understood. Here, OM immobilization by secondary Fe minerals from chemodenitrification was investigated at different C/Fe ratios. Kinetics of Fe(II) oxidation and nitrite reduction rates decreased with increasing C/Fe ratios. Despite efficient sequestration, the immobilization efficiency of OM by secondary minerals varied with the C/Fe ratios. Higher C/Fe ratios were conducive to the formation of ferrihydrite and lepidocrocite, with defects and nanopores. Three contributions, including inner-core Fe-O and edge- and corner-shared Fe-Fe interactions, constituted the local coordination environment of mineral-organic composites. Microscopic analysis at the molecular scale uncovered that labile OM was more likely to combine with secondary minerals with poor crystallinity to enhance its stability, and OM distributed within nanopores and defects had a higher oxidation state. After chemodenitrification, high molecular weight substances and substances high in unsaturation or O/C ratios including phenols, polycyclic aromatics, and carboxylic compounds exhibited a stronger affinity to Fe minerals in the treatments with lower C/Fe ratios. Collectively, labile OM immobilization can occur during chemodenitrification. The findings on OM sequestration coupled with chemodenitrification have significant implications for understanding the long-term cycling of Fe, C, and N, providing a potential strategy for OM immobilization in anoxic soils and sediments.

Extracellular polymeric substances altered ferrihydrite (trans)formation and induced arsenic mobilization

The behavior of As is closely related to trans(formation) of ferrihydrite, which often coprecipitates with extracellular polymeric substances (EPS), forming EPS-mineral aggregates in natural environments. While the effect of EPS on ferrihydrite properity, mineralogy reductive transformation, and associated As fate in sulfate-reducing bacteria (SRB)-rich environments remains unclear. In this research, ferrihydrite-EPS aggregates were synthesized and batch experiments combined with spectroscopic, microscopic, and geochemical analyses were conducted to address these knowledge gaps. Results indicated that EPS blocked micropores in ferrihydrite, and altered mineral surface area and susceptibility. Although EPS enhanced Fe(III) reduction, it retarded ferrihydrite transformation to magnetite by inhibiting Fe atom exchange in systems with low SO42-. As a result, 16% of the ferrihydrite was converted into magnetite in the Fh-0.3 treatment, and no ferrihydrite transformation occurred in the Fh-EPS-0.3 treatment. In systems with high SO42-, however, EPS promoted mackinawite formation and increased As mobilization into the solution. Additionally, the coprecipitated EPS facilitated As(V) reduction to more mobilized As(III) and decreased conversion of As into the residual phase, enhancing the potential risk of As contamination. These findings advance our understanding on biogeochemistry of elements Fe, S, and As and are helpful for accurate prediction of As behavior.

Computational study of the interactions of tetravalent actinides (An = Th-Pu) with the α-Fe13 Keggin cluster.

In recent years, evidence has emerged that actinide (An) uptake at the enhanced actinide removal plant (EARP) at the UK's Sellafield nuclear site occurs via An interactions with an α-Fe13 Keggin molecular cluster during ferrihydrite formation. We here study theoretically the substitution of aquo complexes of the actinides Th-Pu onto a Na-decorated α-Fe13 Keggin cluster using DFT at the PBE0 level. The optimised Pu-O and Pu-Fe distances are in good agreement with experiment and Na/An substitutions are significantly favourable energetically, becoming more so across the early 5f series, with the smallest and largest ΔrG° being for Th and Pu at -335.7 kJ mol-1 and -396.0 kJ mol-1 respectively. There is strong correlation between the substitution reaction energy and the ionic radii of the actinides (Δrε0R2 = 0.97 and ΔrG° R2 = 0.91), suggesting that the principal An-Keggin binding mode is ionic. Notwithstanding this result, Mulliken and natural population analyses reveal that covalency increases from Th-Pu in these systems, supported by analysis of the occupied Kohn-Sham molecular orbitals where enhanced An(5f)-O(2p) overlap is observed in the Np and Pu systems. By contrast, quantum theory of atoms in molecules analysis shows that U-Keggin binding is the most covalent among the five actinides, in keeping with previous studies.

Formation and transformation of iron oxy-hydroxide precursor clusters to ferrihydrite

Rapid precipitation of iron oxy-hydroxide clusters at pH > 4.5 during hydrolysis can lead to the formation of metastable ferrihydrite while their aging at acidic pH (<2.5) forms stable products such as goethite and lepidocrocite.

Effects of in situ Fe oxide precipitation on As stabilization and soil ecological resilience under salt stress

Iron (Fe) oxide precipitation is a promising method for stabilizing arsenic (As) in contaminated soils; however, the addition of salts during the process can negatively affect soil functions. This study investigated the effects of in situ Fe oxide precipitation on As stabilization and the impact of salt stress on soil functions and microbial communities. Fe oxide precipitation reduced the concentration of bioaccessible As by 84% in the stabilized soil, resulting in the formation of ferrihydrite and lepidocrocite, as confirmed by XANES. Nevertheless, an increase in salt stress reduced barley development, microbial enzyme activities, and microbial diversity compared to those in the original soil. Despite this, the stabilized soil exhibited natural resilience and potential for enhanced microbial adaptations, with increased retention of salt-tolerant bacteria. Washing the stabilized soil with water restored EC1:5 to the level of the original soil, resulting in increased barley growth rates and enzyme activities after 5-d and 20-week incubation periods, suggesting soil function recovery. 16 S rRNA sequencing revealed the retention of salt-tolerant bacteria in the stabilized soil, while salt-removed soil exhibited an increase in Proteobacteria, which could facilitate ecological functions. Overall, Fe oxide precipitation effectively stabilized soil As and exhibited potential for restoring the natural resilience and ecological functions of soils through microbial adaptations and salt removal.

Understanding the growth mechanism of Al-bearing hematite nanoparticles via two-dimensional oriented attachment

Hematite occurs ubiquitously in nature and is important for controlling the migration and conversion of various nutrients and pollutants. The morphology of hematite is a key factor affecting its reactivity. Naturally occurring hematite usually contains an amount of aluminum (Al) impurities. Al has a significant effect on the morphology of hematite. However, the details of Al-bearing hematite formation and the role of Al in the morphological evolution remain poorly understood. In this work, a series of Al-bearing hematite were synthesized using hydrothermal methods. Their growth process was followed by examining the intermediate products harvested at different intervals of reaction time using the XRD and electron microscopy. DFT calculations were used to investigate the effect of Al on the stability of hematite {001} and {110} crystal facets. The results indicate that the formation of Al-bearing hematite follows two stages: the initial nucleation of hematite nuclei and the subsequent ripening of nuclei into nanoplates. The nucleation process involves the formation of ferrihydrite, followed by the amorphous nanoparticles undergoing phase transition into more thermodynamically stable hematite nuclei. In the ripening stage, hematite nuclei aggregate into single-crystalline hematite particles through oriented attachment and then crystallize into nanoplates. The surface energy of the {110} crystal facet is higher than the {001} crystal facet at high concentrations of Al, which promotes oriented attachment of {110} facets between hematite nuclei. The proposed mechanism in this article not only provides deep insight into the formation of naturally occurring hematite but also a path for the efficient fabrication of environmentally friendly iron oxide materials.

Formation of ferrihydrite induced by low pe+pH in paddy soil reduces Cd uptake by rice: Evidence from Cd isotope fractionation

Ferrihydrite (Fh) is an important iron mineral in paddy soil and is prone to phase transition during dynamic redox condition, which affects Cd distribution and induces Cd isotope fractionation across soil to rice. Here, we conducted rice culture experiments with or not Fh application under different irrigation regimes to study the relationship between Fe species and Cd availability, as well as the isotope ratio of Cd in different Cd pools in paddy soil-rice system. Fh addition under continuous flooding (FL) with the decrease of pe + pH from 9.36 to 3.44 promoted the formation of amorphous Fe oxides as increased by 120.1% and facilitated Cd immobilization along with the increase of Fe/Mn oxides bound Cd by 25.3%, compared with continuous drying (DY) treatment. The isotopically heavy Cd were preferentially enriched from soil to extractable Cd (Δ114/110Cdextractable Cd-soil = 0.39–0.62‰) and from soil to grain (Δ114/110Cdgrain-soil = 0.40–0.66‰) particularly at low pe + pH and with Fh addition, while light Cd were enriched in Fe/Mn oxides (Δ114/110CdFe/Mn oxides bound Cd-extractable Cd = −0.65 ∼ −0.14‰). Besides, the expression of transporters involved in Cd transport in rice like OsNRAMP1, OsNRAMP1, OsHMA3, OsHMA2 and OsLCT1 were suppressed under low pe + pH condition. These findings indicated that low pe + pH facilitated Cd stabilization by the existence of more amorphous iron oxides, which induced the enrichment of heavy Cd isotope in liquid phase and light in Fe/Mn (oxy)hydroxides, respectively.

Ferrihydrite synthesis in the presence of amino acids and artificial seawater.

Ferrihydrite is widespread in clays, soils, and living organisms and was found on Mars. This iron-mineral could be found on the prebiotic Earth, which also contained simple monomeric amino acids. For prebiotic chemistry, it is important to understand how amino acids have an effect on the process of iron oxide formations. There are three important results in this work: (a) preconcentration of cysteine and aspartic acid, (b) formation of cystine and probably the cysteine peptide occurred during ferrihydrite syntheses, and (c) amino acids have an effect on iron oxide synthesis. For samples containing aspartic acid and cysteine, their presence on the surface or mineral structure can be confirmed by FT-IR spectra. Surface charge analysis showed a relatively high decrease for samples synthesized with cysteine. Scanning electron microscopy did not show marked morphological differences among the samples, except for the seawater sample containing cysteine, which had a lamina-shaped morphology surrounded by circular iron particles, indicating the possible formation of a cysteine structure involving iron oxide particles. The thermogravimetric analysis of the samples indicates that the presence of salts and amino acids in the synthesis of ferrihydrite has an effect on the thermal behavior of the iron oxide/amino acids and modifying the water-loss temperature. The heating of the cysteine samples, synthesized in distilled water and artificial seawater, showed several peaks of degradation of cysteine. In addition, heating of the aspartic acid samples produced the polymerization of this amino acid and peaks of degradation of it. FTIR spectra and XRD patterns did not indicate the precipitation of methionine, 2-aminoisobutyric acid, lysine, or glycine with the iron oxide formations. However, the heating of the glycine, methionine and lysine samples, synthesized in artificial seawater, showed peaks that could be attributed to the degradation of them. Then this could be an indication that these amino acids precipitate with the minerals during the syntheses. Also, the dissolution of these amino acids in artificial seawater prevents the formation of ferrihydrite.

Atomic insights into the mechanisms of Al3+ or Cr3+ affecting ferrihydrite nucleation.

Various exotic metal cations commonly coexist with ferrihydrite nanoparticles in natural environments. Lack of knowledge about the metal cations effects on ferrihydrite nucleation and growth greatly blights the deep understanding of ferrihydrite mineralogical properties and reactivities, and thus the fates of associated nutrients, heavy metals/metalloids, and organic pollutants. Here, the nucleation processes and mechanisms of ferrihydrite nanoparticles in the presence of Al3+ or Cr3+ were studied by combining visible spectroscopy, in situ quick X-ray absorption fine structure spectroscopy and quantum chemical calculations. The formation of ferrihydrite can be divided into three stages. In stage 1, Fe(H2O)63+ forms μ-oxo Fe dimers, with the gradual increase of Fe-O bond length (dFe-O) and disappearance of Fe-O multiple scattering. In stage 2, ferrihydrite particles begin to form and grow slowly, during which dFe-O continues to increase and edge- and corner-sharing Fe-Fe bonds appear. In stage 3, ferrihydrite growth rate increases significantly and continues to the end of the reaction, with the decreases of dFe-O. The presence of metal cations at a molar ratio of 0.1 to Fe hinders the formation of μ-oxo dimers by affecting the Fe3+ hydrolysis and polymerization at stage 1 and stage 2, but promotes the conversion of the μ-oxo dimer to the dihydroxo dimer with lower energy barrier and the creation of crystal growth sites and thus enhances the ferrihydrite nucleation and growth at stage 3.

Formation of iron oxyhydroxides as a result of glauconite weathering in soils of temperate climate

The aim of this study was to investigate glauconite weathering in soils of temperate climate, with special attention paid to formation of iron oxyhydroxides at the expense of the glauconite. Five soil profiles developed on parent material in which glauconite was the main (if not the only) phyllosilicate and source of Fe were selected for the study. Basic physical and chemical properties of bulk (<2 mm) soil samples (color, texture, pH, and carbonate and soil organic carbon content) were determined. Clay fractions (<2 µm and <0.2 µm) and green pellets were separated from the studied soils and analyzed using X-ray diffractometry, Fourier-transform infrared spectroscopy, Mössbauer spectroscopy, and inductively coupled plasma - optical emission spectrometry. Thin sections prepared from selected undisturbed soil materials were observed under optical microscope. Three of the soils were acidic with pH ranging from 4.3 to 5.5 and the remaining two were alkaline with pH from 6.8 to 8.6. Parent glauconites from each profile showed quite similar chemical composition, except that in one case Fe(II) was much more abundant than in the others. In both acidic and alkaline soils, glauconite weathering products included Al-smectite-rich glauconite-smectite mixed-layered minerals (G-S), kaolinite, and Fe oxyhydroxides. G-S were likely formed by transformation of glauconite, whereas kaolinite and iron oxyhydroxides (i.e. goethite, lepidocrocite, and perhaps ferrihydrite) crystallized in the studied soils from substances dissolved from parent glauconites. Leaching of Fe and Mg from the glauconite structure and loss of K were indicated. In acidic soils the weathering products concentrated in the clay fractions, whereas in alkaline soils they occurred both in clays and in the glaucony pellets. Mössbauer spectroscopy indicated that fine crystalline and especially nanocrystalline iron oxyhydroxides are substantial products of glauconite weathering in all the studied clays. Formation of the Fe oxyhydroxides was controlled by hydrological conditions (and related redox activity) and by soil pH. Alkaline conditions seemed to favor goethite crystallization, whereas redox activity led to formation of lepidocrocite and likely ferrihydrite. Soil color was mostly controlled by the presence of glauconite. Only the relatively significant lepidocrocite content in a few of the studied soil horizons was marked by brown or rusty colors indicating high redox activity.

Replacement of magnetite by hematite in hydrothermal systems: A refined redox-independent model

The replacement of magnetite by hematite is commonly observed in various geologic systems. In contrast to the formation of hematite by a solid-state oxidation, numerous experimental results have demonstrated that it can also occur by a redox-independent dissolution-reprecipitation reaction. However, the orientation relationship and the intermediate products between magnetite and hematite remain unknown during the redox-independent replacement. In this work, high-resolution transmission electron microscopy (TEM) was used to investigate porous hematite from the Baishiya skarn deposit, East Kunlun orogenic belt, aiming to build a refined growth model applicable to the replacement of magnetite by hematite in natural hydrothermal systems. Initially, hydrothermal leaching of Fe2+ from magnetite led to the formation of ferrihydrite that transformed to goethite and hematite nanocrystals successively. Oriented attachment of pseudocubic hematite nanoparticles induced by Cl along specific crystallographic directions formed hematite mesocrystals on the exposed dodecahedral facets of magnetite, leading to an orientation relationship between magnetite and adjacent hematite (i.e., (110)magnetite || (012)hematite), which is different from that of oxidation (i.e., (110)magnetite || (110)hematite). However, oriented attachment can be imperfect in some instances, and dislocations of adjacent nanoparticles result. The dislocations in the hematite mesocrystals have acted as a closed space to capture the remaining ferrihydrite. When the Si concentration in ferrihydrite was sufficient, the solid-state conversion of the remaining ferrihydrite to hematite was blocked. We propose that repeated dissolution and reprecipitation of hematite mesocrystals (i.e., defective crystals) are required to remove Si, and thereby form defect-free hematite crystals, providing a genetic link between the widespread hematitisation and related multistage fluid infiltration in some of the world's richest deposits (e.g., Olympic Dam and Bayan Obo deposits). This is the first time that Si and Cl have been verified to act as key factors to shape the hematite growth pathway during the ore-forming processes, challenging the ‘ion-by-ion’ growth model that has underpinned our knowledge of mineral solubility, nucleation, and mass transfer from nano- to micron-scales in natural hydrothermal systems.

Insights from modern diffuse-flow hydrothermal systems into the origin of post-GOE deep-water Fe-Si precipitates

Post-GOE deep-water Fe-Si precipitates associated with volcanogenic massive sulfide deposits are an important feature of the Proterozoic rock record. Although it is clear that these enigmatic deposits formed in oxygen-deficient and hydrothermally influenced deep-water settings, the oxidation mechanism(s) resulting in their precipitation remain(s) unclear. Whilst existing genetic models typically couple direct and/or bacterially-mediated iron oxidation with abiotic silica precipitation, low temperature diffuse hydrothermal fluids offer a potential mechanistic alternative to explain the observed layering. Herein, via combination of petrographic observations with elemental and isotopic data (Fe, Si, and O), we explore the genesis of the primary mineral phases present within recent Fe-Si precipitates obtained from the Southwest Indian Ridge. Formation of ferrihydrite and opal-A in these precipitates provides insight into the genesis of widely invoked precursor minerals to post-GOE deep-water Fe-Si precipitates. Specifically, we find that the mineralogical layers that typify these Fe-Si precipitates may have originated via biologically mediated ferrihydrite precipitation and abiotic precipitation of opal-A from oversaturated fluids during diffuse flow. By analogy, we propose that diffuse hydrothermal fluid flow played an important role in the formation of post-GOE deep-water Fe-Si precipitates.

Iron incorporation in synthetic precipitated calcium silicate hydrates

Effect of Fe3+ on the precipitation of synthetic calcium silicate hydrates (C-S-H) under controlled conditions has been evaluated. Using extremely basic either initial calcium nitrate or sodium silicate solutions (pH > 11), incipient formation of ferrihydrite in both solutions was observed. In this case both C-S-H and iron-containing siliceous hydrogarnet are formed. For pH < 11, ferrihydrite was absent. Only C-S-H is formed. The presence of magnetically dilute Fe3+ in the C-S-H was shown by EPR and the atomic Fe/Si ratio of the C-S-H phase was shown to be between 0.01% and 0.1% by measuring and modelling the 29Si spin-lattice relaxation in NMR experiments. A uniform distribution of Fe3+ when C-S-H precipitates in the absence of initial ferrihydrite is obtained. When initial ferrihydrite is present, a non-uniform distribution of Fe3+ is found and the concentration of Fe3+ in the C-S-H is merely 3% of the overall iron concentration, suggesting the initial formation of ferrihydrite restricts the incorporation of iron into C-S-H.

Study of the formation of ferrihydrite under prebiotic chemistry conditions: artificial seawater 4.0 Gy and ammonium thiocyanate

AbstractAmong the several steps involved in molecular evolution, molecular preconcentration is the first and most important. If the molecules are not preconcentrated the other steps of molecular evolution cannot occur. There are several ways to preconcentrate molecules: sorption, wetting/drying cycles, freezing/sublimation and sorption/precipitation with minerals. In the present work, the effect of NH4SCN and artificial seawater 4.0 Gy on the synthesis of ferrihydrite was studied. It should be noted that thiocyanate could play the same role as that of CN− in the Strecker reaction. Unlike today's seawater that has high Na+ and Cl− concentrations, the seawater used in this work has high Mg2+, Ca2+ and SO42− concentrations. Two results stand out, first SCN− and NH4+ were preconcentrated by sorption/precipitation in some syntheses and second, in some experiments, a mixture of goethite, hematite and magnetite was obtained. The sorption/precipitation of SCN− is always associated with the synthesis of goethite. This could be an indication that SCN− interacts with Fe3+ through the sulphur group of SCN−. In addition, the synthesis of magnetite could be an indication that the SCN− ion oxidized, forming thiocyanogen-(SCN)2 or trithiocyanate ion-(SCN)−3 and that Fe3+ reduced to Fe2+. Besides the sorption/precipitation of SCN− and NH4+, Fourier-transform infrared spectroscopy also showed that sorption/precipitation of SO42− and CO32− occurred. Ferrihydrite synthesized with artificial seawater presented the highest surface area and pore size. The pHpzc values of the samples were in the range of pHpzc described in the literature. The X-ray photoelectron spectroscopy (XPS) measurements performed show proportions of iron present in different oxidation states, however, the electronic similarities observed in the mixtures of iron oxides and oxy-hydroxides make it difficult to quantify them. Direct comparison between XPS spectra of the Fe2p and O 1s core-levels reveal no significant differences from the effect of artificial seawater 4.0 Gy on the synthesis of ferrihydrite.

Stability of magnetic LDH composites used for phosphate recovery

Layered double hydroxides (LDH) and their magnetic composites have been intensively investigated as recyclable high-capacity phosphate sorbents but with little attention to their stability as function of pH and phosphate concentration. The stability of a Fe3O4@SiO2-Mg3Fe LDH P sorbent as function of pH (5–11) and orthophosphate (Pi) concentration (1–300 mg P/L) was investigated. The composite has high adsorption capacity (approx. 80 mg P/g) at pH 5 but with fast dissolution of the LDH component resulting in formation of ferrihydrite as evidenced by Mössbauer spectroscopy. At pH 7 more than 60% of the LDH dissolves within 60 min, while at alkaline pH, the LDH is more stable but with less than 40% adsorption capacity as compared to pH 5. The high Pi sorption at acid to neutral pH is attributed to Pi bonding to the residual ferrihydrite. Under alkaline conditions Pi is sorbed to LDH at low Pi concentration while magnesium phosphates form at higher Pi concentration evidenced by solid-state 31P MAS NMR, powder X-ray diffraction and chemical analyses. Sorption as function of pH and Pi concentration has been fitted by a Rational 2D function allowing for estimation of Pi sorption and precipitation. In conclusion, the instability of the LDH component limits its application in wastewater treatment from acid to alkaline pH. Future use of magnetic LDH composites requires substantial stabilisation of the LDH component.

Performance improvement of aqueous zinc-iron flow batteries through organic ligand complexation of Fe(II)/Fe(III)

In aqueous iron-based redox flow batteries (RFBs), there occurs a fatal performance degradation due to the formation of ferrihydrite via Fe(III) hydrolysis during long-term cycling. This paper describes an organic-ligand complexation of Fe(II)/Fe(III), which allows a high stability and reversibility of the Fe(II)/Fe(III) redox couple in aqueous zinc-iron RFBs. The remarkable effectiveness of pyridine chosen as a best complexing ligand is compared to seven organic ligands through various experiments including cell-cycling tests. It is experimentally confirmed that the pyridine initially coordinated to a Fe(II) cation forms a much strong bonding despite pH < 2, which leads to best electrochemical performances containing improved reaction kinetics and electron transfer. It also exhibits a slight discharge-capacity loss of only 2.9 % vs. 1st cycle at 100th cycle as compared to the pristine with an enormous loss of 78.7 % due to ferrihydrite precipitation, resulting in dynamically improved current efficiency with 73.34 % higher than the pristine one. Consequently, these results indicate that the pyridine as a complexing ligand agent can not only inhibit ferrihydrite during long-term cycling, but also enhance electrochemical stability and reversibility of the Fe(II)/Fe(III) redox couple.

Iron(III) fate after complexation with soil organic matter in fine silt and clay fractions: An EXAFS spectroscopic approach

Iron (Fe) speciation in soils is highly dependent on environmental conditions, mineralogy, and chemical interactions with soil organic matter (SOM). The fine silt and clay (FSi + Cl) particle size fraction of soils constitutes a primary organo-mineral fraction and contains SOM with long turnover time. In this study, the FSi + Cl particle size fractions isolated from a coniferous forest, a grassland, a technosol, and an agricultural soil were reacted with Fe(III) at pH 7. Unreacted and reacted samples were then investigated by means of extended X-ray absorption fine structure (EXAFS) spectroscopy. Statistical methods were used to determine goodness-of-fit parameters for linear combination fitting (LCF) and wavelet transformation (WT) of the Fe K-edge EXAFS data. WT separated spectral contributions from different backscattering atoms in higher coordination shells located at similar interatomic distances from the central absorbing Fe atom. LCF results paired with WT showed that the FSi + Cl particle size fractions consisted of a mixture of Fe phyllosilicates, Fe (hydr)oxides, and organically complexed Fe in different proportions. Our research revealed that after sorption experiments, in which Fe(III) was added to the system, increasing amounts of Fe(III)-SOM complexes were found in the solid phase of grassland and agricultural soils, whereas the precipitation of Fe(III) led to the preferential formation of ferrihydrite in the coniferous forest soil and in the technosol. Although the quantitative Fe-mediated organic carbon stabilization effect after Fe(III) addition is shown in this work, Fe speciation is not clearly related to SOM amount or quality (i.e., carbon-to-nitrogen ratio). The variation of Fe chemical speciation among the soil fractions likely translates into differences in their environmental fate.

Enhanced adsorption and photocatalytic degradation of perfluorooctanoic acid in water using iron (hydr)oxides/carbon sphere composite

Perfluorooctanoic acid (PFOA) has been widely detected in aquatic systems. Yet, cost-effective technologies for degrading PFAS have been lacking. We prepared and tested an adsorptive photocatalyst consisting of iron (hydr)oxides and carbon spheres (FeO/CS) through a one-step hydrothermal process. Characterization results revealed that the presence of carbon spheres affected the crystal formation of iron (hydr)oxides, and resulted in mutually modified mixed phases ferrihydrite and carbon spheres. FeO/CS was able to effectively adsorb and then degrade the pre-sorbed PFOA under simulated solar light. FeO/CS(1:1), prepared at an Fe:Glucose molar ratio of 1:1, showed the highest PFOA adsorption capacity and photoactivity. At a dosage of 1.0 g/L, FeO/CS(1:1) adsorbed nearly all 200 µg/L of PFOA within 1 h, and when the PFOA-laden FeO/CS(1:1) was subjected to simulated solar light at neutral pH, 95.2% of pre-concentrated PFOA was photodegraded and 57.2% defluorinated in 4 h. The efficient degradation also regenerated the material, allowing for repeated uses of the material without chemical regeneration. The much enhanced adsorption and photocatalytic activity of FeO/CS was attributed to: 1) CS facilitated formation of ferrihydrite, leading to adsorption of PFOA via binuclear and bidentate complexation, and 2) a hybrid ferrihydrite-CS structure, enabling multi-point, corporative adsorption of PFOA, and increasing direct electron extraction from PFOA under solar light irradiation. Moreover, OH radicals played an important role in PFOA degradation. Lastly, a photodegradation mechanism is proposed based on experimental findings and density functional theory calculations.