Fe Oxy Research Articles

Fe biogeochemistry in reclaimed acid mine drainage precipitates—Implications for phytoremediation

At a 50-year-old coal mine drainage barrens in central Pennsylvania, USA, we evaluated the biogeochemistry of acidic, Fe(III)oxy(hydr)oxide precipitates in reclaimed plots and compared them to untreated precipitates in control areas. Reclaimed plots supported successional vegetation that became established after a one-time compost and lime treatment in 2006, while control plots supported biological crusts. Precipitates were sampled from moist yet unsaturated surface layers in an area with lateral subsurface flow of mine drainage above a fragipan. Fe(II) concentrations were three- to five-fold higher in reclaimed than control precipitates. Organically bound Fe and amorphous iron oxides, as fractions of total Fe, were also higher in reclaimed than control precipitates. Estimates of Fe-reducing and Fe-oxidizing bacteria were four- to tenfold higher in root-adherent than both types of control precipitates. By scaling up measurements from experimental plots, total Fe losses during the 5-yr following reclamation were estimated at 45 t Fe ha−1 yr−1.

Extreme iron isotope fractionation between colloids and particles of boreal and temperate organic-rich waters

Significant and systematic variations of iron isotopic composition in surface water sample fractions obtained by frontal cascade filtration and ultrafiltration have been recorded in (1) subarctic organic-rich boreal river and stream, mire, lake and soil solutions in northern taiga zone (Karelia, NW Russia) and (2) temperate river and lake waters of the southern boreal zone (Central Russia). Water samples were filtered in the field employing progressively decreasing pore size from 100μm to 1kDa followed by iron isotope analysis. In all river samples, there was a gradual increase of δ57Fe relative to IRMM-14 with decreasing pore size, from +0.4±0.1‰ at 100μm up to +4.2±0.1‰ at 10kDa fraction in the subarctic zone and from −0.024±0.2‰ at 100μm up to +1.2±0.2‰ at 10kDa in the temperate zone. In the series of filtrates/ultrafiltrates of subarctic and temperate streams and rivers, the δ57Fe value decreases with increasing molar Fe/Corg ratio. Therefore, small-size, Fe-poor, C-rich colloids (1–10kDa) and Low Molecular Weight (LMW) fractions of oxygenated water exhibit strong enrichment in heavy isotope whereas High Molecular Weight Fe-rich colloids (100kDa–0.22μm) and particles (1–100μm) are isotopically lighter and closer to the continental crust Fe isotope composition.The relative enrichment of 1–10kDa ultrafiltrates in heavy isotopes suggests that low molecular weight ligands bind Fe more strongly (Fe–O–C bonds) than Fe(III)oxy(hydr)oxides (Fe–O–Fe bonds), in accord with quantum mechanics principles. Highly positive δ57Fe of the LMW fraction of labile and potentially bioavailable Fe in small subarctic rivers may turn out to be a very important source of isotopically heavy Fe in the Arctic Ocean. The mechanisms involved in the production of this isotopically heavy Fe may lead this tracer to become a new indicator of environmental changes occurring in the boreal zone.

Rate constants and mechanisms for the crystallization of Al nano-goethite under environmentally relevant conditions

Mobile inorganic and organic nanocolloidal particles originate-from and interact-with bulk solid phases in soil and sediment environments, and as such, they contribute to the dynamic properties of environmental systems. In particular, ferrihydrite and (nano)goethite are the most abundant of nanocolloidal Fe oxy(hydr)oxides in these environments. We therefore investigated the ferrihydrite to goethite phase transformation using experimental reaction conditions that mimicked environmental conditions where the formation of nanocolloidal Fe oxy(hydr)oxides may occur: slow titration of dilute solutions to pH 5 at 25°C with and without 2mol% Al. Subsequently, the rate constants from 54-d nano-goethite aging/crystallization experiments at 50°C were determined using aliquots pulled for vibrational spectroscopy (including multivariate curve resolution, MCR, analyses of infrared spectra) and synchrotron-based X-ray diffraction (XRD). We also present a mechanistic model that accounts for the nano-goethite crystallization observed by the aforementioned techniques, and particle structural characteristics observed by dynamic light scattering (DLS) and transmission electron microscopy (TEM). In contrast to the common assumption that metastable ferrihydrite precipitates first, before it transforms to goethite, the presence of characteristic infrared bands in freshly synthesized nanoparticle suspensions indicate goethite can precipitate directly from solution under environmentally relevant conditions: low Fe concentration, ambient temperature, and pH maintained at 5. However, the presence of 2mol% Al prevented direct goethite precipitation. Rate constants obtained by fitting the contributions from the MCR-derived goethite-like component to the OH-stretching region were (7.4±1.1)×10−7s−1 for 0% Al and (4.2±0.4)×10−7s−1 for 2mol% Al suspensions. Rate constants derived from intensities of OH-bending infrared vibrations (795 and 895cm−1) showed similar values, within error, for both 0 and 2mol% Al nanoparticle suspensions. Thus, the presence of 2mol% Al decreased the rate constants determined from analyses of infrared OH-stretching and OH-bending vibrations by 43–57%. We postulate that dissolution re-precipitation reactions are accelerated in aggregate microenvironments by locally increased supersaturation, yielding the dominant mechanism for transformation of ferrihydrite to goethite and goethite crystal growth when bulk ion concentrations are low. Although we did observe growth of a population of prismatic goethite single crystals by TEM, there was more substantial growth of a population of polycrystalline goethite needles that appeared to retain some defects from a preceding aggregation step that we detected with DLS. Since the presence of Al hinders the dissolution of ferrihydrite, it too reduces the rate of crystallization to goethite and its crystal growth. As exemplified in this nano-particle crystallization study, the combination of advanced spectral-curve-resolution algorithms and sensitive and quantitative infrared sampling techniques opens future opportunities for the quantification of mineral phase dynamics in nanocolloidal suspensions, which is important for many aspects of environmental studies.

Importance of porosity and transfer of matter in the rock weathering processes: two examples in central Spain

Some physical properties (bulk and free porosity, pore size distribution), and the chemical compo- sition and mass balance of two deeply weathered profiles one developed on Hercynian granodiorite and the other on pre-Cambrian slates were studied. Hydric and mercury porosimetry, nitrogen adsorption techniques, chemical analyses and XRD techniques were used. On granodiorite, weathering has created increased porosity with a pore diameter \5 lm, whereas on slates the weathering has produced of ca. 1 lm in diameter. These pore sizes have played an important role in the weathering processes. Assuming that weathering preserves volumes, except in the uppermost part of the profiles, it brought about a loss of matter of more than 12% (*300 kg/m 3 ) on granodiorite and ca. 30% (*800 kg/m 3 ) on slates. These changes are related to shifts in the mineralogical evolution, with the appearance of new 2:1 and 1:1 phyllosilicates and Fe oxy- hydroxides as the main authigenic minerals. The release of matter, at least since the upper Neogene until the present, has led to the lowering of relief in a more or less homo- geneous way, giving rise to gentle hillsides and flat sur- faces below which the current river networks are incised. Porosity studies have the potential to explain several specific landforms as well as affecting landscape devel- opment in general.

Present environment of Dam Lake Sambe, southwestern Japan: a geochemical study of bottom sediments

Geochemical analyses of lakebed and core sediments from Lake Sambe on the outskirts of Oda City in Shimane prefecture in southwestern Japan were carried out in order to assess the water quality and the concentration and distribution patterns of sixteen elements. The lake water showed a stratified condition with respect to dissolved O2, and As, Fe, and Mn concentrations in the bottom layers which increased in the summer. The chemical composition of the sediments, as measured by X-ray fluorescence, included major and trace elements (P, Ca, Sc, Ti, V, Cr, Fe, Ni, Cu, Zn, As, Sr, Zr, Pb, and Th), and total sulfur (TS). Elevated values of As, Zn, V, Fe, P, and TS were present in several layers of the upper cores (from 0 to 5 cm) and other surface sediments. Increases in the abundances of these metals in lake sediments are probably related to the reducing condition of the sediments, fine-grained organic rich sediments, and post-depositional diagenetic remobilization. Moreover, correlations between the concentrations of trace metals and iron in the sediments suggest their adsorption onto Fe (oxy)hydroxides, whereas correlations with sulfur indicate that they were precipitated as Fe-sulfides. The average abundances of As, Pb, Zn, and Cu exceeded the lowest effect level and Interim Sediment Quality Guideline values that the New York State Department of Environmental Conservation and the Canadian Council of Ministers of the Environment determined to have moderate impact on aquatic organisms. In addition, concentrations of As and Zn exceeded the Coastal Ocean Sediment Database threshold value, indicating potentially toxic levels. Therefore, the presence of trace metals in the lake sediments may result in adverse effects on biota health.

Mobility of arsenic and trace element inventories in sediment cores from Masuda City, southwestern Japan

The vertical distribution of arsenic and other trace and major elements has been studied in four sediment cores from Masuda City, Nagashima and Okite in the Shimane Prefecture of southwestern Japan. The sediment cores were also subjected to leaching techniques and 14C dating. The stratigraphic sequences in the cores consist of silt and sandy silt at top, passing downward into gray to black clays. Elevated values of As, Pb, Zn, Cu, Ni, Cr, and V are present in several horizons while abundances of these elements tend to be higher in the black and gray clays, probably due to adsorption onto clay sediments. Higher concentrations of Fe and total sulfur (TS) occur in black clays. The correlations of the trace metals with iron suggest their adsorption onto Fe (oxy)hydroxides, whereas correlations with sulfur in some cores indicate that they were precipitated as Fe-sulfides. Age determinations suggest that clay horizons at ∼5 m depth were deposited at around 5,000 and 6,000 years BP (14C ages) during the transgressive phase of sea level change. The results of the leaching techniques in the core samples show that higher amounts of As were extracted with deionized water. Even at neutral pH, As can be released from sediments to groundwater, and therefore groundwater pollution is a concern in Masuda City and the surrounding area.

Nickel bioavailability in an ultramafic toposequence in the Vosges Mountains (France)

A serpentinised harzburgite outcrop located in the Vosges Mountains hosts a population of the Ni-hyperaccumulator Thlaspi caerulescens J. & C. Presl. A complete study was undertaken to relate the variability of Ni availability along the ultramafic toposequence to pedogenesis, soil mineralogy and functioning with X-Ray Diffraction, Transmission Electron Microscope observations coupled with Isotopic Exchange Kinetics and diethylenetriamine pentaacetic acid extraction of Ni. The soil profiles ranging from Dystric Cambisol to Hypermagnesic Hypereutric Cambisol were distributed unevenly along the toposequence probably due to geochemical variability of the bedrock and also complex quaternary erosion features. The richest soils were characterised by slight mineral weathering leading to Ni, Cr and Fe accumulation in the B horizons whereas the lowest saturated soils had very low-metal contents. Most soil minerals were inherited from the parent materials and there were only few traces of formation of secondary minerals. Primary minerals (e.g. serpentine, chlorite) contained low Ni concentrations (0.2%) whereas neoformed goethite, mainly in the B horizons of the richest soils, contained up to 4.3% Ni. Ni was probably sorbed onto amorphous Fe oxy−hydroxide particles (oxalate extraction) rather than incorporated within the crystal lattice of goethite. Ni availability in the B horizon of Hypereutric Cambisols was extremely high and so was the oxalate extractable Fe. At the toposequence level, there was a high level of Ni availability in the least weathered soils and a very low-availability level in the more intensively weathered soils (strongly acidic pH). Ni availability was unexpectedly positively correlated to pH and was controlled by soil mineralogy and Ni-bearing mineral phases. Ni hyperaccumulation (above 1,000 mg kg−1) by native T. caerulescens was only reached in the Ni-rich soils as a consequence of the local edaphic factors. Ni uptake by T. caerulescens is strongly regulated by Ni availability in soils and therefore related to pedogenesis.

Experimental study of germanium adsorption on goethite and germanium coprecipitation with iron hydroxide: X-ray absorption fine structure and macroscopic characterization

Adsorption of germanium on goethite was studied at 25 °C in batch reactors as a function of pH (1–12), germanium concentration in solution (10 −7 to 0.002 M) and solid/solution ratio (1.8–17 g/L). The maximal surface site density determined via Ge adsorption experiments at pH from 6 to 10 is equal to 2.5 ± 0.1 μmol/m 2. The percentage of adsorbed Ge increases with pH at pH < 9, reaches a maximum at pH ∼ 9 and slightly decreases when pH is further increased to 11. These results allowed generation of a 2-p K Surface Complexation Model (SCM) which implies a constant capacitance of the electric double layer and postulates the presence of two Ge complexes, > FeO – Ge ( OH ) 3 0 and > FeO – GeO ( OH ) 2 − , at the goethite-solution interface. Coprecipitation of Ge with iron oxy(hydr)oxides formed during Fe(II) oxidation by atmospheric oxygen or by Fe(III) hydrolysis in neutral solutions led to high Ge incorporations in solid with maximal Ge/Fe molar ratio close to 0.5. The molar Ge/Fe ratio in precipitated solid is proportional to that in the initial solution according to the equation (Ge/Fe) solid = k × (Ge/Fe) solution with 0.7 ⩽ k ⩽ 1.0. The structure of adsorbed and coprecipitated Ge complexes was further characterized using XAFS spectroscopy. In agreement with previous data on oxyanions adsorption on goethite, bi-dentate bi-nuclear surface complexes composed of tetrahedrally coordinated Ge attached to the corners of two adjacent Fe octahedra represent the dominant contribution to the EXAFS signal. Coprecipitated samples with Ge/Fe molar ratios >0.1, and samples not aged in solution (<1 day) having intermediate Ge/Fe ratios (0.01–0.1) show 4 ± 0.3 oxygen atoms at 1.76 ± 0.01 Å around Ge. Samples less concentrated in Ge (0.001 < Ge/Fe < 0.10) and aged longer times in solution (up to 280 days) exhibit a splitting of the first atomic shell with Ge in both tetrahedral ( R = 1.77 ± 0.02 Å) and octahedral ( R = 1.92 ± 0.03 Å) coordination with oxygen. In these samples, octahedrally coordinated Ge accounts for up to ∼20% of the total Ge. For the least concentrated samples (Ge/Fe < 0.001–0.0001) containing lepidocrocite, 30–50% of total co-precipitated germanium substitutes for Fe in octahedral sites with the next-nearest environment dominated by edge-sharing GeO 6–FeO 6 linkages ( R Ge–Fe ∼ 3.06 Å). It follows from the results of our study that the largest structural change of Ge (from tetrahedral to octahedral environment) occurs during its coprecipitation with Fe hydroxide at Ge/Fe molar ratio ⩽0.0001. These conditions are likely to be met in many superficial aquatic environments at the contact of anoxic groundwaters with surficial oxygenated solutions. Adsorption and coprecipitation of Ge with solid Fe oxy(hydr)oxides and organo-mineral colloids and its consequence for Ge/Si fractionation and Ge geochemical cycle are discussed.

Cu(II) adsorption mechanism on pyrite: an XAFS and XPS study

The surface of a <38 µm ground fraction of pyrite was analysed using x-ray photoelectron spectroscopy (xps), x-ray absorption fine structure spectroscopy (XAFS) and inductively coupled plasma atomic emission spectroscopy (ICPAES) prior to and after Cu(II) adsorption under different pH and Cu(II) concentration conditions. Under all conditions copper is found to be coordinated to sulphur on the surface of pyrite with an average bond length of 2.27±0.02 Å. A Cu–O bond with a length of 2.00 Å due to Cu(OH)2 precipitation was also observed for a sample with copper adsorbed at pH 8.5 from a solution initially containing 2.84 × 10−4 mol m−2 Cu (all concentrations given are normalized against the surface area of pyrite in solution). The Cu 2p XPS signals for the samples with adsorbed copper show the presence of Cu(I) ions on the pyrite surfaces in all cases. For the 2.84 × 10−4 mol m−2 Cu solution at pH 8.5 the pyrite was observed to have both a Cu(I) and a Cu(II) XPS component. The ground pyrite surfaces showed increases in the oxidized S22− (Eb = 163.5 eV) signal on copper adsorption with minor increases in Fe(III) oxy- or hydroxy-species observed on pyrite surfaces exposed to long periods of adsorption. It is proposed that the reduction of Cu(II) to Cu(I) on the surface of the pyrite is accompanied by the oxidation of the S22− species present in the pyrite surface. Solution analysis in conjunction with surface analysis has indicated that a 1 : 1 exchange of Fe(II) ions for Cu(II) ions did not occur during copper adsorption and thus an ion-exchange mechanism is ruled out. Copyright © 2000 John Wiley & Sons, Ltd.

Au/iron oxide catalysts: temperature programmed reduction and X-ray diffraction characterization

Gold on iron oxides catalysts have been characterized by temperature programmed reduction (TPR) and X-ray diffraction spectroscopy (XRD). The influence of preparation method, gold loading and pretreatment conditions on the reducibility of iron oxides have been investigated. On the impregnated Au/iron oxide catalysts as well as on the support alone the partial reduction of Fe(III) oxy(hydroxides) to Fe 3O 4 starts in the 550 and 700 K temperature range. On the coprecipitated samples, the temperature of formation of Fe 3O 4 is strongly dependent on the presence of gold. The reduction temperature is lowered as the gold loading is increased. The reduction of Fe 3O 4 to FeO occurs at about 900 K and is not dependent on the presence of gold and the preparation method. It is suggested that the effect of gold on the reducibility of the iron oxides is related to an increase of the structural defects and/or of the surface hydroxyl groups.

Kinetics of Anodic Processes on Iron in Alkaline Solutions

Anodic transients of iron in 1N solution have been investigated. After the electrode was maintained at a potential until steady state was achieved, either an anodic potential step or a potential sweep were imposed. For a potential step, the current transient followed the relations and where is the time after the imposition of the potential step. For a potential sweep of sweep rate, λ, a peak current was observed where For the current transients for both potential step and potential sweep polarization were interpreted in terms of the logarithmic growth of an oxide. For , a duplex structure of the oxide layer, mixed oxide/solution, was assumed, where the mixed oxide consisted of Fe(II) and Fe(III) oxy compounds. The current transient behavior was attributed to composition changes in the mixed oxide layer due to diffusion of Fe(II). The experimental results can be satisfactorily interpreted by assuming the existence of the complex ions, , in the oxide film.