Fe Oxy Research Articles

Organic amendments temporarily change arsenic speciation and bioaccessibility in a lead and arsenic co-contaminated urban soil.

Organic amendments often reduce the bioaccessibility of soil lead (Pb) but not that of soil arsenic (As). The effect of Pb on As bioaccessibility is rarely studied in co-contaminated soils. In a field study, we assessed the effect of mushroom compost, leaf compost, noncomposted biosolids, and composted biosolids amendments on As speciation in a co-contaminated (As and Pb) soil at 7, 349, and 642 days after amending soils and the change of As speciation during an in vitro bioaccessibility extraction (gastric solution, pH 2.5) using bulk X-ray absorption near-edge structure spectroscopy. Soil was contaminated by coal combustion and other diffuse sources and had low As bioaccessibility (7%-12%). Unamended soil had As(III) sorbed onto pyrite (As(III)-pyrite; ∼60%) and As(V) adsorbed onto Fe oxy(hydr)oxides (As(V)-Fh; ∼40%). In amended soils, except in composted biosolids-amended soils, at 7 days, As(V)-Fh decreased to 15%-26% and redistributed into As(III)-Fh and/or As(III)-pyrite. This transformation was most pronounced in mushroom compost amended soil resulting in a significant (46%) increase of As bioaccessibility compared to the unamended soil. Composted biosolids-amended soils had relatively stable As(V)-Fh. Lead arsenate formed during the in vitro extraction in amended soils, except in composted biosolids-amended soils. Arsenic speciation and bioaccessibility were similar in 349- and 642-day in all the amended and unamended soils. Reduction of As(V)-Fh to As(III) forms in the short term in three of the amended soils showed the potential to increase As bioaccessibility. The formation of stable lead arsenate during the in vitro extraction would counteract the short-term increase of As bioaccessibility in those amended soils.

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

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

Taphonomy of fish, invertebrates and plant remains in the first Tethyan-South Atlantic marine ingression along Cretaceous rift systems in NE-Brazil

The Aptian–Albian Amargosa Bed, Tucano Basin, which was recently considered as a Konservat-Lagerstätten, is a relatively thin, siliciclastic-dominated succession of yellowish and grayish papyraceous shales. We analyzed the taphonomy of fishes, gastric ejecta, freshwater shrimps and conchostracans, euryaline ostracods, and plants. Fish biostratinomic signatures indicate point to changes in water alkalinity/salinity as their causa mortis, occasionally subject to weak currents. The preferential preservation of flexed shrimps and isolated pleons corroborates this interpretation. Conchostracans, recorded in distinct levels, indicate episodes of mass mortality, variable transport and temporal mixing, and at least one faunal change. During the fossilization, primary P and Ca from shrimps and fishes probably generated phosphates, which finally altered to Fe oxy-/hydroxides, yielding a mixed elemental composition, not expressed in other mineralogies than Fe oxy-/hydroxides. The original calcite of ostracod valves likely has been dissolved/replaced by a K-rich aluminosilicate, or by a precursor phase. Plants were preserved by Fe oxy-/hydroxides or by kerogenisation. Previous studies show that marine ingression of Tethyan waters into the central segment of the proto-South Atlantic gulf flooded the continental scale axial river system captured by the Tucano rift basin, affecting base level up to the Araripe Basin. Here it is interpreted that tributary river systems with relatively steeper stream gradients were less affected by marine transgression than the axial system, resulting in local freshwater riverine influx across the elongated epeiric sea developed over the interior rift basins of NE-Brazil, resulting in a mixed biota of distinct water salinities that alternate in the Amargosa Bed type-section.

Dissimilatory nitrate reduction to ammonium (DNRA) potentially facilitates the accumulation of phosphorus in lake water from sediment.

Nitrogen (N) and phosphorus (P) are crucial nutrients for eutrophication in the lacustrine ecosystem and attract the attention worldwide. However, the interaction between them need further clarification. This study aimed to assess the influence of dissimilatory nitrate reduction to ammonia (DNRA) on the cycle of P in lacustrine sediment. Different fractions of N and P in the pore water were measured using high-resolution in-situ measurement techniques, HR-Peeper and DGT, coupling with sequential extraction for solid sediment from a shallow freshwater lake. The results showed that elevated nitrate (NO3-) reduction via DNRA rather than denitrification was verified at deeper sediment layer, suggesting the generation of inorganic ammonia (NH4+) as electron donor under anaerobic episodes. High abundance of DNRA bacteria (nrfA gene) obtained using high-throughput sequencing analysis were detected at upper layer and responsible for the accumulation of NH4+ in the sediment coupling with chemolithoautotrophic metabolism. Additionally, significant desorption of ionic ferrous iron (Fe2+) and dissolved reactive phosphate (DRP) from solid phase and the enrichment in the solution was simultaneously detected. Higher concentration of solid Fe bound P (Fe-P) at deeper layer indicated the potential re-oxidation of Fe2+ as electron donor during DNRA process and sorption of DRP toward the Fe-containing minerals. However, obvious evidence of desorption proved by DGT indicated that higher NH4+ concentrations favored the reduction of Fe(III) oxy(hydr)oxides and the desorption of DRP into the pore water and diffusion toward the overlying water. Finally, noteworthy S2- release from solid sediment was speculated to stimulate the DNRA and facilitated the accumulation of NH4+ in the solution, which further induced the enrichment of DRP in water from the solid phase. Overall, DNRA potentially facilitates the accumulation of P in lake water, and the synchronous control of N and P is important for the eutrophication management and restoration of lake eutrophication.

Achieving simultaneous removal of nitrogen and phosphorus in sediment via combined adsorption and oxygen supplement

Nutrients released from lake sediments contribute to the total nutrient load of an aquatic ecosystem. In this study, we used the oxygen-loaded lanthanum modified zeolites (LOZ) for absorbing internal phosphorus (P) and nitrogen (N) meanwhile with potential to trigger biological transformation of the nutrients. The results showed that the lanthanum modification of the zeolites manifested both the P and N adsorption in comparison to the pristine zeolites (enhanced by 129.13 % for PO43−-P and 27.53% for NH4+−N, respectively). In addition, lanthanum modified zeolites also exhibited the excellent oxygen-loading capability (0.73 mmol/g) than the pristine one. Apart from the blocking of P and N, the LOZ application over surface sediment substantially improved the benthic and aquatic DO level as the DO concentration in overlying water was 3–5 mg/L and diffused to 12 mm depth of sediment. The increased oxygen level in sediment favored the phosphate and Fe oxy(hydr)oxides combination and reduced FeS formation with subsequent decline in PO43−-P release from sediments. The improvement of water DO level also induced the microbial community shifts and promoted the abundance of nitrifiers in the surface sediment while denitrifiers in the deeper layer. Consequently, the release of total phosphorus and total nitrogen from the sediment to overlying water was reduced by 80% and 92%, respectively. Thus, the application of LOZ to sediment was beneficial to not only the anaerobic and aerobic processes that involved in nitrogen removal, but also the inactivation of phosphate. The study delivered positive outcomes regarding the control of internal nutrients release with a quick, efficient and sustainable remediation approach to rehabilitate polluted water systems.

Molecular orbital study of Fe(II) and Fe(III) complexation with salicylate and citrate ligands: Implications for soil biogeochemistry

AbstractThe formation of mineral‐associated organic matter (OM), typically with Fe oxy(hydr)oxide minerals, contributes to the long‐term storage of soil C. However, the changing climate is predicted to alter precipitation frequency and intensity patterns such that soils may experience greater periods of anoxic conditions leading to the reductive dissolution of the Fe oxy(hydr)oxide minerals. This would subject the released OM to microbial decomposition and the Fe cation to react with soil solution constituents. Using salicylate and citrate as model soil ligands, this study uses density functional theory to provide physical insights into the chemical bonds formed between Fe(II) and Fe(III) cations with the organic acids. The ΔG° for the complexation reactions of both salicylate and citrate were negative indicating that the complex formation is energetically favorable. Furthermore, the ΔG0 was more negative for the salicylate ligand as compared with the citrate ligand suggesting that the involvement of carboxyl‐O and phenoxyl‐O to form a complex is more thermodynamically favorable. We show from the molecular electron density data that the bonds involved in the complexation are more electrostatic than covalent in nature. Molecular orbital calculations show that the energies of the highest occupied orbitals of Fe(II)–salicylate and Fe(II)–citrate complexes are less negative than that of the uncomplexed Fe(II)–(H2O)6, suggesting that the complexed Fe(II)–organic species will be preferentially oxidized upon return of the soil to aerobic conditions. Our results provide a more fundamental chemical understanding of the reactions involved in the biogeochemical cycling of soil Fe and C. This knowledge may assist in evaluating the potential role of enhanced soil C sequestration as one approach to mitigate climate change.

On the Role of Poly-Glutamic Acid in the Early Stages of Iron(III) (Oxy)(hydr)oxide Formation

Nucleation of minerals in the presence of additives is critical for achieving control over the formation of solids in biomineralization processes or during syntheses of advanced hybrid materials. Herein, we investigated the early stages of Fe(III) (oxy)(hydr)oxide formation with/without polyglutamic acid (pGlu) at low driving force for phase separation (pH 2.0 to 3.0). We employed an advanced pH-constant titration assay, X-ray diffraction, thermal analysis with mass spectrometry, Fourier Transform infrared spectroscopy, and scanning electron microscopy. Three stages were observed: initial binding, stabilization of Fe(III) pre-nucleation clusters (PNCs), and phase separation, yielding Fe(III) (oxy)(hydr)oxide. The data suggest that organic–inorganic interactions occurred via binding of olation Fe(III) PNC species. Fourier Transform Infrared Spectroscopy (FTIR) analyses revealed a plausible interaction motif and a conformational adaptation of the polypeptide. The stabilization of the aqueous Fe(III) system against nucleation by pGlu contrasts with the previously reported influence of poly-aspartic acid (pAsp). While this is difficult to explain based on classical nucleation theory, alternative notions such as the so-called PNC pathway provide a possible rationale. Developing a nucleation theory that successfully explains and predicts distinct influences for chemically similar additives like pAsp and pGlu is the Holy Grail toward advancing the knowledge of nucleation, early growth, and structure formation.

The Stability of Fe-Isotope Signatures During Low Salinity Mixing in Subarctic Estuaries

We have studied iron (Fe)-isotope signals in particles (> 0.22 µm) and the dissolved phase (< 0.22 µm) in two subarctic, boreal rivers, their estuaries and the adjacent sea in northern Sweden. Both rivers, the Råne and the Kalix, are enriched in Fe and organic carbon (up to 29 µmol/L and up to 730 µmol/L, respectively). Observed changes in the particulate and dissolved phase during spring flood in May suggest different sources of Fe to the rivers during different seasons. While particles show a positive Fe-isotope signal during winter, during spring flood, the values are negative. Increased discharge due to snowmelt in the boreal region is most times accompanied by flushing of the organic-rich sub-surface layers. These upper podzol soil layers have been shown to be a source for Fe-organic carbon aggregates with a negative Fe-isotope signal. During winter, the rivers are mostly fed by deep groundwater, where Fe occurs as Fe(oxy)hydroxides, with a positive Fe-isotope signal. Flocculation during initial estuarine mixing does not change the Fe-isotope compositions of the two phases. Data indicate that the two groups of Fe aggregates flocculate diversely in the estuaries due to differences in their surface structure. Within the open sea, the particulate phase showed heavier δ56Fe values than in the estuaries. Our data indicate the flocculation of the negative Fe-isotope signal in a low salinity environment, due to changes in the ionic strength and further the increase of pH.

High sulfate concentration enhances iron mobilization from organic soil to water

Widespread increases in iron (Fe) concentrations are contributing to ongoing browning of northern freshwaters, but the driver/s behind the trends are not known. Fe mobilization in soils is known to be controlled by redox conditions, pH, and DOC availability for complexation. Moreover, high sulfate concentrations have been suggested to constrain Fe in transition from soil to water, and declining sulfate deposition to have the opposite effect. We studied the effect of these Fe mobilization barriers in a microcosm experiment, applying high (peak S deposition) and low (present day) sulfate treatments and oxic versus anoxic conditions to boreal (O horizon) soil slurries. We hypothesized that anoxic conditions would favor Fe release. On the contrary we expected high sulfate concentrations to suppress Fe mobility, through FeS formation or by lowering pH and thereby DOC concentrations. Anoxia had positive effects on both Fe and DOC concentrations in solution. Contrasting with our hypothesis, Fe concentrations were enhanced at high sulfate concentrations, i.e. increasing acidity in high sulfate treatments appeared to promote Fe mobilization. Establishment of the basidiomycete fungus Jaapia ochroleuca in the oxic treatments 44 days into the experiment had a major impact on Fe mobilization by increasing total Fe concentrations in solution. Thus, anoxia and acidity, along with fungi mediated mobilization, were important in controlling Fe release from soil to the aqueous phase. While Fe is often assumed to precipitate as Fe(oxy)hydroxides in the transition from anoxic to oxic water in the riparian zone, Fe from anoxic treatments remained in solution after introduction of oxygen. Our results do not support reduced atmospheric S deposition as a driver behind increasing Fe concentrations in boreal freshwaters, but confirm the importance of reducing conditions—which may be enhanced by higher soil temperature and moisture—for mobilization of Fe across the terrestrial-aquatic interphase.

Lead Speciation and In Vitro Bioaccessibility of Compost-Amended Urban Garden Soils.

In situ soil amendments can modify the Pb bioavailability by changing soil Pb speciation. Urban soils from three vegetable gardens containing different total Pb concentrations were used. The study evaluated how compost amendment and aging of soil-compost mixture in situ affected the following: (i) soil Pb speciation in the field and (ii) change of soil Pb speciation during an in vitro bioaccessibility extraction mimicking gastric phase dissolution at pH 2.5. X-ray absorption fine structure spectroscopy was used to determine Pb speciation in amended and nonamended soils and residues left after in vitro bioaccessibility extraction of those soils. Compost amendment and aging of compost in the field had a negligible effect on Pb bioaccessibility in the soils. Major Pb species in the soils were Pb sorbed to Fe oxy(hydr)oxide (Pb-Fh) and to soil organic C (Pb-Org). The fraction of Pb-Org was increased as soil-compost mixture aged in the field. During the in vitro extraction, the fraction of Pb-Fh was decreased, the fraction of Pb-Org was increased, and hydroxypyromorphite was formed in both amended and nonamended soils. Freshly incorporated compost enhanced the dissolution of Pb-Fh during the extraction. As soil-compost mixture aged in the field, the dissolution of Pb-Fh was low, demonstrating more stability of the Pb-Fh during the extraction. Compost amendment showed potential to contribute to reduced bioaccessibility of Pb as compost aged in the soil by increasing Pb-Org fraction in the field and stability of Pb-Fh during the in vitro bioaccessibility extraction.

Interaction of Humic Acid with Cu/Pb-Zn Tailings of Different Degrees of Weathering

Aggregation of tailings is instrumental for developing a physical environment permitting water infiltration, aeration, and root penetration. Previous research found that microaggregation of Cu-Pb-Zn tailings was stimulated by amendment of composted sugarcane residues. The present study aimed to identify organic functional groups in humic acids (HA) involved in the aggregation of Cu-Pb-Zn tailings, which were considered to be formed from long-term organic matter decomposition in the engineered tailing-soil. The adsorption of HA by the weathered (WT) and freshly deposited (FT) tailings was evaluated at a range of pH conditions in tailing-HA suspension. Humic acid adsorption decreased with increasing pH from 5.0 to 9.0. The FT was found to contain more oxy-(hydr)oxides, particularly poorly crystalline Fe and Al oxy-(hydr)oxides than the WT, which was one of the factors attributing to FT’s higher HA affinity. The Fourier transform infrared spectroscopy analysis of tailing particles before and after HA adsorption indicated that carboxylic and phenolic groups of HA preferentially interacted with tailing mineral particles, probably through ligand exchange, polyvalent cation bridge and electrostatic attraction. The pH-dependent HA adsorption indicated the potential role of functional groups of organic matters in microaggregation of Cu/Pb-Zn tailings. Moreover, the presence of abundant Al/Fe oxyhydroxides promoted organo-mineral interactions. The present findings will contribute toward developing field application to engineer functional Technosols from the tailings by selecting organic matter rich in functional groups.

Changes in the As solid speciation during weathering of volcanic ashes: A XAS study on Patagonian ashes and Chacopampean loess

X-ray absorption spectroscopy (XAS) was used to determine the oxidation state of As, local chemical coordination and the relative proportion of different As species in recent and ancient Andean volcanic ashes, as well as in Chaco Pampean loess. As K edge XANES analysis indicates that in loess sediments the dominant species is As(V) (i.e., >91%). Conversely, As(III) is dominant in all ash samples. In the Puyehue sample, only As(III) species were determined, while in both, the Chaitén and the ancient tephra samples, As(III) species accounts for 66% of the total As. The remaining 34% corresponds to As(−1) in the Chaitén sample and to As(V) in the weathered tephra. The proposed EXAFS models fit well with the experimental data, suggesting that in ancient and recent volcanic ashes, As(III) is likely related to As atoms present as impurities within the glass structure, forming hydroxide species bound to the Al-Si network. In addition, the identified As(−1) species is related to arsenian pyrite, while in the ancient volcanic ash, As(V) was likely a product of incipient weathering. In loess sediments, the identified As(V) species represents arsenate ions adsorbed onto Fe oxy(hydr)oxides, forming inner-sphere surface complexes, in a bidentate binuclear configuration.

Fe-oxy adducts of heme-Aβ and heme-hIAPP complexes: intermediates in ROS generation.

Iron (Fe) is the most abundant transition metal ion in the human body and its role, in the form of heme, has been implicated in Alzheimer's disease (AD) and type 2 diabetes mellitus (T2Dm). Heme binds both amyloid beta (Aβ) and human islet amyloid polypeptide (hIAPP) to form heme-Aβ and heme-hIAPP complexes, respectively, and form reactive oxygen species (ROS) like H2O2, O2˙-etc., which are known to cause oxidative damage. However the intermediates involved during ROS formation have not yet been isolated. In this study the oxygen bound intermediates of both heme-Aβ(1-16) and heme-hIAPP(1-19) have been isolated and characterized using absorption, EPR and resonance Raman (rR) spectroscopy. Fe-O stretches have been found at 575 cm-1 and 577 cm-1 for heme-Aβ(1-16) and heme-hIAPP(1-19) respectively. The oxy intermediates are stable at low temperatures. The isolation of the intermediates reveals a mechanistic pathway of ROS generation through the two heme complexes.

Evaluación de la movilización del As procedente de los residuos mineros abandonados de una zona semiárida (Rodalquilar, Almería)

Navarro, A., Martin, A. 2015. Evaluation of As mobilization from abandoned mine wastes of a semi-arid area (Rodalquilar, Almeria). Ecosistemas 24(2): 76-89. Doi.: 10.7818/ECOS.2014.24-2.12 Field and laboratory studies and mathematical modeling were used in order to assess the long-term mobilization of As from an uncontrolled tailings deposit, located next to the old mine of Rodalquilar (Almeria). Leaching mine wastes by column experiments indicates the possible movement of As at concentrations ranging between 30.9 and 1.2 mg /L, and pH values between 8.24 and 7.9, in a saline leachate character and showed electrical conductivities of 425-22 100 µS/cm. The mobility of some metalloids such as arsenic may be produced by the weathering of arsenian pyrite, present in the mine wastes in appreciable amounts, and jarosite and Fe oxy-hydroxides, where the arsenic appears in appreciable amounts. In this area groundwater showed high concentrations of Al, As, Cd, Fe, Hg , Mn, Ni, Pb, Se, Sb and Zn, which exceed water quality criteria for human consumption. Highlights the extent reached by As contamination detected throughout the aquifer and with concentrations reaching 0.2 mg /L. The dissolution of Fe oxy- hydroxides in areas of low redox potential could explain the existence of appreciable amounts of As, Fe and other metals dissolved in groundwater. MODFLOW modeling code simulated the transport of As in the aquifer of Rodalquilar and indicated that the main pollutant control mechanism, in addition to the hydrodynamic dispersion is adsorption on Fe oxy- hydroxides. The results suggest a progressive increase in the concentrations of As in groundwater for years.

Evaluation of heavy metal enrichment in Cochin estuary and its adjacent coast: multivariate statistical approach.

Temporal and spatial variations of heavy metals in the Cochin estuary and its adjacent coastline during three seasons were studied to investigate the impact of anthropogenic heavy metal pollution. Total organic carbon, sand, silt, clay and 10 metals (Cd, Pb, Cr, Ni, Co, Cu, Zn, Mn, Mg and Fe) in the surface sediments were analysed. Multivariate statistical analyses like canonical correspondence analysis, principal component analysis and cluster analysis were used for source identification, integration of geochemical data and clustering of stations based on similarities. Enrichment factor, contamination factor and geoaccumulation index were used to assess the contamination level. From the study, it can be understood that estuary and coast are highly polluted especially with Cd, Zn, Pb and Ni. Anthropogenic influence of heavy metals was evidenced from both the principal component analysis and cluster analysis. Finer fractions (mud) of the sediment and the associated Fe oxy hydroxides might be playing major role in the transport of heavy metals in the system. Very high enrichment factor value observed suggested high anthropogenic pressure in the study area. All the stations in the northern part of the estuary showed very high enrichment factors indicating heavy load of Zn and Cd in this area which might have reached from the industrial area lying to the north side of the Cochin estuary. Pollution indices suggested that both the estuary and its adjacent coast were showing low contamination with respect to Cr, Mg, Mn and Fe; all other metals were causing low to extremely high levels of pollution in the study area.

Iron isotope fractionation during Fe(II) and Fe(III) adsorption on cyanobacteria

The present study aimed at testing the hypothesis that Fe adsorption and precipitation on bacterial phytoplankton likely cause significant isotopic fractionation with preferential adsorption of heavy isotopes on the cell surface. We measured the iron isotopic fractionation during the interaction of aqueous Fe with planktonic cyanobacteria (Gloeocapsa sp., Synehococcus sp., and Planthothrix sp.) in six independent experiments using two distinct Fe oxidation states (Fe(III) and Fe(II)) at pH3 and 6. Isotopic analyses demonstrated that the Fe adsorption on bacterial planktonic biomass yields a clear enrichment of heavy isotopes on the cell surfaces, producing isotopically light δ57Fe values in solutions. The adsorption experiments with Fe(II) in the initial solution yielded a Δ57Fecell-solution=2.66±0.14‰, whereas the adsorption experiments with Fe(III) in the initial solution yielded Δ57Fecell-solution=0.97±0.19‰. Because these data approached closed system equilibrium isotopic fractionation lines rather than Rayleigh curves, the most likely mechanism is a steady state isotopic fractionation, linked with short-term reversible Fe adsorption on cells. In agreement with X-ray Absorption Spectroscopy structural data obtained on the same adsorption experiment samples (González et al., 2014), the preferential enrichment of heavy Fe isotopes on the cell surfaces is attributed to the stronger covalent metal-ligand bonding (FeOC/P) of octahedrally coordinated Fe with phosphoryl or carboxyl groups on the cell walls when compared with the Fe aquacomplexes (OFeO) in solution. The larger isotopic fractionation factor in the experiments starting with Fe(II) results from the iron oxidation to Fe(III) which is itself accompanied by a strong isotopic effect. A natural case study of warm hydrothermal spring depositing Fe oxy(hydr)oxide with and without cyanobacterial biomass yielded an isotopic fractionation between the solid phase and aqueous solution (Δ57Fesolid-solution) of 0.62±0.16‰ (biotic) and 0.80±0.06‰ (abiotic). This natural case study therefore confirms experimental results, albeit of smaller magnitude. These findings suggest that Fe adsorption on cyanobacteria cell surfaces in nature should produce cell enrichments in heavier isotopes relative to the coexisting aqueous solution.

Citrate influences microbial Fe hydroxide reduction via a dissolution–disaggregation mechanism

Microbial reduction of ferric iron is partly dependent on Fe hydroxide particle size: nanosized Fe hydroxides greatly exceed the bioavailability of their counterparts larger than 1μm. Citrate as a low molecular weight organic acid can likewise stabilize colloidal suspensions against aggregation by electrostatic repulsion but also increase Fe bioavailability by enhancing Fe hydroxide solubility. The aim of this study was to see whether adsorption of citrate onto surfaces of large ferrihydrite aggregates results in the formation of a stable colloidal suspension by electrostatic repulsion and how this effect influences microbial Fe reduction. Furthermore, we wanted to discriminate between citrate-mediated colloid stabilization out of larger aggregates and ferrihydrite dissolution and their influence on microbial Fe hydroxide reduction. Dissolution kinetics of ferrihydrite aggregates induced by different concentrations of citrate and humic acids were compared to microbial reduction kinetics with Geobacter sulfurreducens. Dynamic light scattering results showed the formation of a stable colloidal suspension and colloids with hydrodynamic diameters of 69 (±37) to 165 (± 65) nm for molar citrate:Fe ratios of 0.1 to 0.5 and partial dissolution of ferrihydrite at citrate:Fe ratios⩾0.1. No dissolution or colloid stabilization was detected in the presence of humic acids. Adsorption of citrate, necessary for dissolution, reversed the surface charge and led to electrostatic repulsion between sub-aggregates of ferrihydrite and colloid stabilization when the citrate:Fe ratio was above a critical value (⩽ 0.1). Lower ratios resulted in stronger ferrihydrite aggregation instead of formation of a stable colloidal suspension, owing to neutralization of the positive surface charge. At the same time, microbial ferrihydrite reduction increased from 0.029 to 0.184mMh−1 indicating that colloids stabilized by citrate addition enhanced microbial Fe reduction.Modelling of abiotic dissolution kinetics revealed that colloid stabilization was most pronounced at citrate:Fe ratios of 0.1 – 0.5, whereas higher ratios led to enhanced dissolution of both colloidal and larger aggregated fractions. Mathematical simulation of the microbial reduction kinetics under consideration of partial dissolution and colloid stabilization showed that the bioaccessibility increases in the order large aggregates<stable colloids<Fe-citrate.These findings indicate that much of the organic acid driven mobilization of Fe oxy(hydr)oxides is most likely due to colloid formation and stabilization rather than solubilisation.

Iron adsorption onto soil and aquatic bacteria: XAS structural study

Although the interaction between Fe and microorganisms has been extensively studied, the main physico-chemical factors controlling the mechanisms of Fe adsorption and precipitation on bacterial cell walls remain poorly understood. In this study, we quantified thermodynamic parameters of the Fe adsorption reaction and characterized the speciation of Fe adsorbed on the surface of cyanobacteria and soil heterotrophic bacteria. For this purpose, the molecular mechanisms of iron interaction with typical aquatic and soil bacteria were investigated by combining batch macroscopic adsorption experiments with atomic-level Fe K-edge X-ray absorption fine structure spectroscopy (XAFS). Three cyanobacteria species (Synechococcus sp., Planktothrix sp. and Gloeocapsa sp.) and aerobic heterotrophic soil rhizobacterium (Pseudomonas aureofaciens) were used for Fe3 + and Fe2 + adsorption experiments. These experiments were carried out for a wide range of initial iron concentration (4.5–57.3 μM) and pH (2.0–6.5). Surface adsorption data were rationalized using a Linear Programming Model (LPM), which allowed quantification of the surface adsorption constants and the number of binding sites. XAS (XANES and EXAFS) analysis of adsorbed iron demonstrated the predominance of O-coordinated Fe3 + species. Moreover, XANES data treatment using a linear combination fit of reference compounds suggested that the atomic environment of iron adsorbed onto soil bacterial surfaces was dominated by phosphoryl moieties with a lesser amount of carboxylates and some contribution of Fe(III)-oxy(hydr)oxide component. Complete oxidation of Fe(II) to Fe(III) was observed in the solid phase as determined by XANES analysis. Binding of Fe(III) to carboxylate groups was only significant for capsular cyanobacteria (Gloeocapsa sp.). The relative proportions of various Fe species at the cell surface determined by thermodynamic analysis of the macroscopic data and by XAS are in a good agreement. Our results suggest that, in the presence of surface organic ligands, the oxidation of divalent iron does occur, but the polymerization of formed Fe(III)oxy(hydr)oxides is partially inhibited and adsorbed iron in the form of both Fe–O–Fe polymers and individual Fe atoms attached to phosphoryl moieties. The presence of EPS reduces metal-cell binding capacity and enhances Fe polymerization at the bacterial surface.

Germanium isotope fractionation during Ge adsorption on goethite and its coprecipitation with Fe oxy(hydr)oxides

Isotopic fractionation of Ge was studied during Ge adsorption on goethite and its coprecipitation with amorphous Fe oxy(hydr)oxides. Regardless of the pH, surface concentration of adsorbed Ge or exposure time, the solution–solid enrichment factor for adsorption (Δ74/70Gesolution–solid) was 1.7±0.1‰. The value of the Δ74Gesolution–solid in Fe–Ge coprecipitates having molar ratio 0.1<(Ge/Fe)solid<0.5 remained constant at 2.0±0.4‰. For (Ge/Fe)solid ratio<0.1, the Δ74Gesolution–solid increased with the decrease of Ge concentration in the solid phase, with the value as high as 4.4±0.2‰ at (Ge/Fe)solid<0.001, corresponding to the majority of natural settings. These results can be interpreted based on available structural data for adsorbed and coprecipitated Ge. It follows that Ge(OH)4° adsorption occurring as bidentate binuclear complexes at the goethite surface is characterised by an enrichment factor of ∼1.7‰, likely related to the distortion of the GeO4 tetrahedron and the formation of Ge–O–Fe bonds at the goethite surface as compared to aqueous solution. In contrast, coprecipitation yields more distorted edge-sharing GeO4 tetrahedra and, in the case of the most diluted samples, part of the Ge is found in coordination 6, replacing Fe(III) in octahedral positions. This produces a greater enrichment of the solid phase in lighter isotopes, mostly due to the increase in Ge–O bond distances and coordination number compared to aqueous solution, which is in line with the basic principles of isotope fractionation. Discharge of hydrothermal fluids, leading to massive Fe(OH)3 precipitation in the vicinity of the springs should, therefore, represent an isotopically-heavy source of dissolved Ge to the ocean. Similarly, groundwater discharge and Fe(OH)3 precipitation at the Earth’s surface, Fe oxy(hydr)oxide formation in soils and riverine organo-ferric colloids coagulation, leading to iron hydroxide precipitation in estuaries, should produce an isotopically heavy Ge aqueous flux to the ocean compared to bedrock sources and particulate fluxes.

Effect of biogeochemical redox processes on the fate and transport of As and U at an abandoned uranium mine site: an X-ray absorption spectroscopy study

Environmental context Uranium and arsenic, two elements of human health concern, are commonly found at sites of uranium mining, but little is known about processes influencing their environmental behaviour. Here we focus on understanding the chemical and physical processes controlling uranium and arsenic transport at an abandoned uranium mine. We find that the use of sedimentation ponds limits the mobility of uranium; however, pond conditions at our site resulted in arsenic mobilisation. Our findings will help optimise restoration strategies for mine tailings. Abstract Although As can occur in U ore at concentrations up to 10wt-%, the fate and transport of both U and As at U mine tailings have not been previously investigated at a watershed scale. The major objective of this study was to determine primary chemical and physical processes contributing to transport of both U and As to a down gradient watershed at an abandoned U mine site in South Dakota. Uranium is primarily transported by erosion at the site, based on decreasing concentrations in sediment with distance from the tailings. Sequential extractions and U X-ray absorption near-edge fine structure (XANES) fitting indicate that U is immobilised in a near-source sedimentation pond both by prevention of sediment transport and by reduction of UVI to UIV. In contrast to U, subsequent release of As to the watershed takes place from the pond partially due to reductive dissolution of Fe oxy(hydr)oxides. However, As is immobilised by adsorption to clays and Fe oxy(hydr)oxides in oxic zones and by formation of As–sulfide mineral phases in anoxic zones down gradient, indicated by sequential extractions and As XANES fitting. This study indicates that As should be considered during restoration of uranium mine sites in order to prevent transport.