Showing papers on "Goethite published in 2007"

Adsorption of sulfur dioxide on hematite and goethite particle surfaces

Abstract: The adsorption of sulfur dioxide (SO2) on iron oxide particle surfaces at 296 K has been investigated using X-ray photoelectron spectroscopy (XPS). A custom-designed XPS ultra-high vacuum chamber was coupled to an environmental reaction chamber so that the effects of adsorbed water and molecular oxygen on the reaction of SO2 with iron oxide surfaces could be followed at atmospherically relevant pressures. In the absence of H2O and O2, exposure of hematite (α-Fe2O3) and goethite (α-FeOOH) to SO2 resulted predominantly in the formation of adsorbed sulfite (SO32−), although evidence for adsorbed sulfate (SO42−) was also found. At saturation, the coverage of adsorbed sulfur species was the same on both α-Fe2O3 and α-FeOOH as determined from the S2p : Fe2p ratio. Equivalent saturation coverages and product ratios of sulfite to sulfate were observed on these oxide surfaces in the presence of water vapor at pressures between 6 and 18 Torr, corresponding to 28 to 85% relative humidity (RH), suggesting that water had no effect on the adsorption of SO2. In contrast, molecular oxygen substantially influenced the interactions of SO2 with iron oxide surfaces, albeit to a much larger extent on α-Fe2O3 relative to α-FeOOH. For α-Fe2O3, adsorption of SO2 in the presence of molecular oxygen resulted in the quantitative formation of SO42− with no detectable SO32−. Furthermore, molecular oxygen significantly enhanced the extent of SO2 uptake on α-Fe2O3, as indicated by the greater than two-fold increase in the S2p : Fe2p ratio. Although SO2 uptake is still enhanced on α-Fe2O3 in the presence of molecular oxygen and water, the enhancement factor decreases with increasing RH. In the case of α-FeOOH, there is an increase in the amount of SO42− in the presence of molecular oxygen, however, the predominant surface species remained SO32− and there is no enhancement in SO2 uptake as measured by the S2p : Fe2p ratio. A mechanism involving molecular oxygen activation on oxygen vacancy sites is proposed as a possible explanation for the non-photochemical oxidation of sulfur dioxide on iron oxide surfaces. The concentration of these sites depends on the exact environmental conditions of RH.

Sorptive stabilization of organic matter by microporous goethite: sorption into small pores vs. surface complexation

Abstract: Summary Stabilization of organic matter (OM) by sorption to minerals is thought to be due to (i) sorption into small pores (O < 50 nm) that prevents hydrolytic enzymes approaching and decomposing the organic substrate or (ii) reduced availability of organic molecules after formation of strong multiple bonds by complexation of organic ligands at mineral surfaces. We tested these two potential mechanisms by studying the binding of dissolved OM to microporous goethite (α-FeOOH). The size of organic molecules dissolved prior to and after equilibration with goethite was determined using atomic force microscopy (AFM). The goethite–OM complexes were analysed for bulk and surface elemental composition (by X-ray photoelectron spectroscopy, XPS), specific surface area (SSA) and mesopore and micropore volumes (by N2 adsorption/desorption), by scanning electron microscopy (SEM), and by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The absolute density of goethite–OM complexes was determined by gas pycnometry and the sorbed OM’s apparent density was calculated by assuming no major changes in the volumes of the goethite upon sorption of OM. The stability of the OM–mineral interactions was tested in desorption experiments and by treatment with NaOCl. Surface accumulation of OM by sorption decreased the N2-accessible SSA of the goethite, mostly because micropores (O < 2 nm) were rendered inaccessible to N2. The decrease in accessibility of micropores was most pronounced at small surface OM concentrations. The majority of dissolved organic molecules detected with AFM prior to interaction with goethite were globular with a diameter of 4–10 nm, the rest were mainly linear, 20–100 nm long and 4–8 nm thick. After contact with goethite, the latter type of molecules dominated, which suggests preferential sorption of globular molecules. Their size exceeded or equalled the size of micropores and small mesopores (O < 10 nm) and so sorption therein is unlikely. Also, the changes in volumes of pores with a size of 2–50 nm were smaller than the estimated volume of the OM sorbed. The apparent density of sorbed OM always exceeded that of the freeze-dried OM and was largest at small surface concentrations. DRIFT spectroscopy showed that most carboxyl groups at the goethite surface were in their complexed form. The proportion of complexed carboxyl groups dropped at larger surface concentrations, parallel to the decrease in micropore volume. Thus, micropores seem to favour the formation of multiple complex bonds per molecule. Scanning electron microscopy showed that at small surface concentrations, OM coated the goethite crystals and crystallites tightly, while at larger surface concentrations bulky accumulations of OM were more abundant. Even strongly desorbing reagents such as NaOH and Na pyrophosphate released only part of the sorbed OM. Treatment with NaOCl removed mainly bulky accumulations of OM; the OM tightly bound to goethite crystals was hardly affected by NaOCl. We conclude that molecules tightly bound via multiple complex bonds, probably at the mouths of small pores, are barely desorbable and resist the attack of chemical reagents and probably also of enzymes.

Phosphate imposed limitations on biological reduction and alteration of ferrihydrite.

Abstract: Biogeochemical transformation (inclusive of dissolution) of iron (hydr)oxides resulting from dissimilatory reduction has a pronounced impact on the fate and transport of nutrients and contaminants in subsurface environments. Despite the reactivity noted for pristine (unreacted) minerals, iron (hydr)oxides within native environments will likely have a different reactivity owing in part to changes in surface composition. Accordingly, here we explore the impact of surface modifications induced by phosphate adsorption on ferrihydrite reduction by Shewanella putrefaciens under static and advective flow conditions. Alterations in surface reactivity induced by phosphate changes the extent, decreasing Fe(Ill) reduction nearly linearly with increasing P surface coverage, and pathway of iron biomineralization. Magnetite is the most appreciable mineralization product while minor amounts of vivianite and green rust-like phases are formed in systems having high aqueous concentrations of phosphate, ferrous iron, and bicarbonate. Goethite and lepidocrocite, characteristic biomineralization products at low ferrous-iron concentrations, are inhibited in the presence of adsorbed phosphate. Thus, deviations in iron (hydr)oxide reactivity with changes in surface composition, such as those noted here for phosphate, need to be considered within natural environments.

Spectroscopic investigation of ciprofloxacin speciation at the goethite-water interface.

Abstract: We investigated ciprofloxacin (a fluoroquinolone antibiotic) speciation as a function of pH in aqueous solution and in the presence of dissolved ferric ions and goethite using ATR-FTIR and UV−vis spectroscopy. The presence of dissolved and surface bound ferric species induced the deprotonation of the ciprofloxacin carboxylic acid group at pH < pKa1. The resultant ciprofloxacin zwitterions appeared to interact via both carboxylate oxygens to form bidentate chelate and bridging bidentate complexes within colloidal iron oxide−ciprofloxacin precipitates and bidentate chelates on the goethite surface. However, the structure of the aqueous ferric−ciprofloxacin complexes remains unclear. Our evidence for bidentate chelates (involving only the carboxylate oxygens) on the goethite surface was distinct from previous IR studies of fluoroquinolone sorption to metal oxides that have proposed surface complexes involving both the keto and the carboxylate groups. We find that the distinct ciprofloxacin surface complex pr...

A laboratory study to determine the effect of iron oxides on proton NMR measurements

Abstract: Using laboratory methods, we investigate the effect of the presence and mineralogic form of iron on measured proton nuclear magnetic resonance (NMR) relaxation rates. Five samples of quartz sand were coated with ferrihydrite, goethite, hematite, lepidocrocite, and magnetite. The relaxation rates for these iron-oxide-coated sands saturated with water were measured and compared to the relaxation rate of quartz sand saturated with water. We found that the presence of the iron oxides led to increases in the relaxation rates by increasing the surface relaxation rate. The magnitude of the surface relaxation rate was different for the various iron-oxide minerals because of changes in both the surface-area-to-volume ratio of the pore space, and the surface relaxivity. The relaxation rate of the magnetite-coated sand was further increased because of internal magnetic field gradients caused by the presence of magnetite. We conclude that both the concentration and mineralogical form of iron can have a significant impact on NMR relaxation behavior.

Increased Stability of Organic Matter Sorbed to Ferrihydrite and Goethite on Aging

Abstract: Sorption to micro- and mesoporous mineral phases can stabilize organic matter (OM) against microbial decay in soil Formation of strong bonds that reduce desorbability is one plausible explanation for that effect With time after sorption, sorbed OM may undergo changes in configuration or may migrate into intraparticle spaces We tested the possible effects of residence time of OM sorbed to ferrihydrite and goethite The minerals were loaded with different amounts of water-soluble OM from an Oa horizon, then stored moist (10% w/w water) for up to 1080 d at 4°C We monitored the content of organic C, the desorbability and chemical stability (by extraction with 01 MNaOH-04 MNaF and treatment with 1 M NaOCl), and, after freeze-drying, the micro- and mesopore volume (by N 2 and CO 2 adsorption-desorption) Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopywas used to characterize the OM on the mineral surfaces at the beginning and end of the experiment There was no detectable decrease in sorbed organic C during the experiment; also, the micro- and mesoporosity of the samples remained unchanged The proportion of desorbable organic C, however, decreased by up to 16% This was paralleled by more pronounced bands indicative of complexed organic functional groups in the DRIFT spectra We conclude that with increasing residence time, OM sorbed to porous minerals becomes decreasingly desorbable by the formation of additional chemical bonds to the surface via ligand exchange but not by diffusion into small pores The decrease in desorbability was accompanied by a decrease in chemical destructibility with NaOCl The stability of sorbed OM against biological degradation may similarly increase with residence time

Timing of acid weathering on Mars: A kinetic-thermodynamic assessment

Abstract: [1] Weathering of olivine basalt by H2SO4-HCl aqueous solutions at the conditions of early Mars was investigated through numerical modeling in a system open with respect to CO2 and O2 only. The model includes dissolution rates of primary and secondary minerals and oxidation rate of aqueous Fe2+, as well as chemical equilibration among solutes, dissolved gases, and precipitates. The results reveal fast dissolution of Fe-Mg minerals at low pH, followed by preferential dissolution of plagioclase at higher pH. Correspondingly, solutions evolve from acidic, Mg-Ca-Fe2+-Fe3+-Al3+ compositions toward Na-rich alkaline fluids. The period over which neutralization and mineral precipitation may occur is shorter at higher initial pH, lower water to rock ratios, and larger mineral surface areas. Early stages of weathering are characterized by the formation of amorphous silica, goethite, and kaolinite, while zeolites and carbonates form considerably later at higher pH, where silica dissolves. Slow oxidation of Fe2+ causes precipitation of ferrous phyllosilicates. Comparison with Martian observations indicate that amorphous silica, Fe3+ oxyhydroxides, and Mg-, Ca-, and Fe-sulfates could have formed during multiple short-term episodes of acid weathering that were terminated by freezing and/or evaporation. Throughout history, impact generation of oxidants (e.g., O2, SO3, NO2) caused formation of strong acids and incremental Fe2+ oxidation, the processes that are not efficient during O2-deficient periods of volcanic degassing. Although impact-generated acid rainfalls could have caused intense weathering and erosion in Noachian time, dilution of acids and a prolonged existence of surface solutions favored local neutralization of solution, and formation, transport and deposition of clays.

Iron speciation in soils and soil aggregates by synchrotron-based X-ray microspectroscopy (XANES,μ-XANES)

Abstract: Summary Iron speciation in soils is still poorly understood. We have investigated inorganic and organic standard substances, diluted mixtures of common Fe minerals in soils (pyrite, ferrihydrite, goethite), soils in a forested watershed which constitute a toposequence with a hydrological gradient (Dystric Cambisol, Dystric Planosol, Rheic Histosol), and microsites of a dissected soil aggregate by X-ray Absorption Near Edge Spectroscopy (XANES) at the iron K-edge (7112 eV) to identify different Fe(II) and Fe(III) components. We calculated the pre-edge peak centroid energy of all spectra and quantified the contribution of different organic and inorganic Fe-bearing compounds by Linear Combination Fitting (LCF) conducted on the entire spectrum (E = 7085–7240 eV) and on the pre-edge peak. Fe-XANES conducted on organic and inorganic standards and on synthetic mixtures of pyrite, ferrihydrite and goethite showed that by calculating the pre-edge peak centroid energy, the Fe(II)/Fe(III) ratio of different Fe-bearing minerals (Fe sulphides, Fe oxyhydroxides) in mineral mixtures and soils can be quantified with reasonable accuracy. A more accurate quantification of the Fe(II)/Fe(III) ratio was possible with LCF conducted on the entire XANES spectrum. For the soil toposequence, an increased groundwater influence from the Cambisol to the Histosol was reflected in a larger contribution of Fe(II) compounds (Fe(II) silicate, Fe monosulphide, pyrite) and a smaller contribution of Fe(III) oxyhydroxides (ferrihydrite, goethite) to total iron both in the topsoil and the subsoil. In the organic topsoils, organically bonded Fe (33–45% of total Fe) was 100% Fe(III). For different microsites in the dissected aggregate, spatial resolution ofμ-XANES revealed different proportions of Fe(II) and Fe(III) compounds. Fe K-edge XANES andμ-XANES allows an approximate quantification of Fe(II) and Fe(III) and different Fe compounds in soils and (sub)micron regions of soil sections, such as mottles, concretions, and rhizosphere regions, thus opening new perspectives in soil research.

Inhibition of phosphorus sorption to goethite, gibbsite, and kaolin by fresh and decomposed organic matter

Abstract: The direct effects of dissolved organic matter (DOM) on the sorption of orthophosphate onto gibbsite, goethite, and kaolin were examined using a one-point phosphorus sorption index and the linear Tempkin isotherm model. DOM extracted from fresh and decomposed agricultural residues, as well as model organic and humic acids, were used. Changes in the chemical and sorptive characteristics of the DOM in the absence and presence of added orthophosphate (50 mg l−1) were also determined. For residue-derived materials, DOM sorption to all minerals correlated well with percent hydrophobicity, apparent molecular weight, and phenolic acidity in the absence of added orthophosphate. Sorption of DOM to goethite and gibbsite was significantly decreased in the presence of added P. The correlation coefficient values of percent hydrophobicity, apparent molecular weight, and phenolic acidity to sorption also declined in the presence of added P. Thus, the addition of P substantially lowered fractionation of DOM after sorption to goethite and gibbsite. In contrast, few significant P sorption-induced differences were observed in the kaolin system. According to one-point P sorption results, DOM in the form of Aldrich humic acid, oxalate, and decomposed clover and corn residue, significantly inhibited P sorption to goethite at concentrations of 50 and 200 mg total soluble carbon (CTS l−1). Phosphorus sorption to gibbsite was significantly inhibited by 50 mg CTS l−1 derived from decomposed corn residue, fresh dairy manure residue, and oxalate solution. At 200 mg CTS l−1, all DOM solutions were found to inhibit P sorption to gibbsite. This study suggests that DOM inhibition of P sorption depends on the chemical properties of both the sorbent and the DOM itself. In general, DOM from decomposed organic materials inhibited P sorption to a greater extent than did DOM derived from fresh materials. This stronger inhibition highlights the importance of microbial processes in the release of soluble soil P, a key determinant of P availability to plants.

Arsenic-bicarbonate interaction on goethite particles.

Abstract: The As(V) and As(III) interaction with HCO3 has been studied for goethite systems using a pH and As concentration range that is relevant for field situations. Our study shows that dissolved bicarbonate may act as a competitor for both As(V) and As(III). In our closed systems, the largest effect of bicarbonate occurs at the lowest experimental pH values (pH approximately 6.5), which is related to the pH dependency of the carbonate adsorption process. The experimental data have been modeled with the charge distribution (CD) model. The CD model was separately parametrized for goethite with "single ion" adsorption data of HCO3, As(III), and As(V). The competitive effect of HCO3 on the As(III) and As(V) release could be predicted well. Application of the model shows that the natural As loading of aquifer materials (approximately 1-2 orders of magnitude smaller than the As loading based on the competition of As-HCO3 alone. It indicates that another, very prominent competitor, like phosphate and natural organic matter, will strongly contribute to the control of As in natural systems.