Showing papers on "Goethite published in 1997"

Arsenate and Chromate Retention Mechanisms on Goethite. 1. Surface Structure

Abstract: The molecular structure of ions retained on mineral surfaces is needed to accurately model their sorption process and to determine their stability. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used in this study to deduce the local coordination environment of two environmental contaminants, arsenate and chromate, on the mineral goethite (α-FeOOH). Based on the oxyanion−Fe distances, it was concluded that three different surface complexes exist on goethite for both oxyanions: a monodentate complex, a bidentate-binuclear complex, and a bidentate-mononuclear complex. At low surface coverages, the monodentate complex was favored while at higher coverages the bidentate complexes were more prevalentthe bidentate-binuclear complex appears to be in the greatest proportion at these highest surface coverages. Therefore, modeling efforts for chromate or arsenate retention on goethite need to consider a monodentate complex at very low coverages, both the monodentate and bidentate complexes at in...

Arsenate and Chromate Retention Mechanisms on Goethite. 2. Kinetic Evaluation Using a Pressure-Jump Relaxation Technique

Abstract: The kinetics of arsenate and chromate adsorption/desorption on goethite (α-FeOOH) were investigated using a pressure-jump (p-jump) relaxation technique. Information provided by this technique was used to elucidate the fate of arsenate and chromate in natural environments. Chemical relaxations resulting from rapidly induced pressure changes were monitored via conductivity detection. The adsorption/desorption of these oxyanions on goethite involved a double relaxation event. The proposed mechanism for the adsorption of arsenate and chromate on goethite is a two-step process resulting in the formation of an inner-sphere bidentate surface complex. The first step, associated with the fast τ values, involved an initial ligand exchange reaction of aqueous oxyanion species H2AsO4- or HCrO4- with OH ligands at the goethite surface forming an inner-sphere monodentate surface complex. The subsequent step, associated with the slow τ values, involved a second ligand exchange reaction, resulting in the formation of an ...

Dissolved organic matter sorption on sub soils and minerals studied by 13C‐NMR and DRIFT spectroscopy

Abstract: Summary Sorption on the mineral matrix is an important process restricting the movement of dissolved organic matter (DOM) in soils. In this study, we aimed to identify the chemical structures responsible for the retention of DOM by sorption experiments with total DOM and acidic humic substances (AHS), containing humic and fulvic acids, on soil samples and minerals (goethite, ferrihydrite, and amorphous Al(OH)3). The AHS remaining in solution after sorption were studied by 13C nuclear magnetic resonance (NMR) analysis, and total DOM and AHS for bed on the surfaces of minerals by diffuse reflectance Fourier-transform infrared (DRIFT) spectroscopy. The soil samples were taken from strongly sorbing Bw horizons of two Inceptisols rich in pedogenetic Fe (29 and 35 g kg −1) and containing little C (7 and 22 g kg−1). The 13C-NMR spectra showed that sorption causes a preferential removal of aromatic and carboxyl C from the solution, whereas alkyl-C accumulates in the solution. No change was observed for O-alkyl C. The DRIFT spectra of sorbed total DOM and AHS showed a relative increase of the band intensity of carboxyl groups compared to DOM in the initial solution, confirming the importance of those groups for the sorption to mineral surfaces. The spectra also indicated reactions of carboxyl groups with metals at the mineral surfaces. The extent to which the carboxyl groups are bound depended on the surface coverage with DOM and the type of mineral.

Effects of crystallinity of goethite: II. Rates of sorption and desorption of phosphate

Abstract: Summary Eight samples of goethite ranging in surface area from 18 to 132 m2 g-1 were mixed with phosphate at a range of pH values for periods which ranged from 0·5 h to 6 weeks. The sample with a surface area of 18 m2 g-1 had been hydrothermally treated to improve its crystallinity. Its rate of reaction with phosphate depended on pH but was complete within a day. Its maximum observed reaction was close to the theoretical maximum for surface adsorption of 2·5 μmol m−2. For the other samples, phosphate continued to react for up to 3 weeks and exceeded the value of 2·5 μmole m−2. The duration and extent of the reaction depended on the crystallinity of the goethite. The results were closely described by a model in which the phosphate ions were initially adsorbed on to charged external surfaces. The phosphate ions then diffused into the particles. This was closely described using equations for diffusion into a cylinder. Samples of goethite which had been loaded with phosphate dissolved more slowly in HCl, and had a longer lag phase, than phosphate-free goethite. For the hydrothermally treated goethite, HCl removed much of the phosphate when only a small proportion of the iron had been dissolved. For a poorly crystallized goethite, it was necessary to dissolve much more of the iron to obtain a similar removal of phosphate. Brief treatment with NaOH removed most of the phosphate from the hydrothermally treated goethite but only half the phosphate from a poorly crystallized goethite. These results are consistent with the idea that phosphate ions were not only bound on external surface sites but had also penetrated into meso- and micro-pores between the domains of the goethite crystals and were then adsorbed on internal surface sites. This penetration tied the domains together more firmly thus increasing the lag phase for dissolution. Differences between sites for phosphate adsorption are therefore caused mainly by their location on either external or internal sites. Models that ignore this are incomplete.

Products of Chromate Reduction on Proposed Subsurface Remediation Material

Abstract: Recent studies have shown promising results for subsurface remediation of dissolved chromate using permeable-reactive redox walls. Chromate reduction in the presence of iron filings and quartz grains was studied to determine the fate of reduced chromium in proposed wall material. Using a flow-through column apparatus, iron filings mixed with quartz grains were reacted with solutions that contained about 20 mg/L dissolved Cr(VI). Reacted iron filings developed coatings comprised of goethite with chromium concentrated in the outermost edges. Surface analysis showed all detectable chromium occurred as Cr(III) species. In addition, in regions of increased chromium concentration, goethite acquired chemical and structural characteristics similar to Fe2O3 and Cr2O3. Results of the study show that complete reduction of Cr(VI) to Cr(III) occurred and that Cr(III) was incorporated into sparingly soluble solid species.

Visible spectrometric indices of hematite (Hm) and goethite (Gt) content in lateritic soils: the application of a Thematic Mapper (TM) image for soil-mapping in Brasilia, Brazil

Abstract: Data on mineralogy and diffuse reflectance spectra, obtained from 56 samples of Brazilian lateritic soils, were used to establish quantitative relationships between spectral parameters and iron oxides contents (hematite and goethite). A redness index RI H calculated from Helmholtz chromatic co-ordinates ( lambda d, P e, Y) and correlated to hematite content is found. A good estimate of the ratio Hematite/ (Hematite + Goethite) is given by the dominant wavelength lambda which takes into account a large proportion of the tint detected on the soil sample. Based on these laboratory results, radiometric indices were elaborated by combining only those parts of the visible spectrum that correspond to Thematic Mapper (TM) channels: (i) a hematite index I Hm to estimate hematite concentration; and (ii) a ferric index I Fe to estimate the ratio hematite/(hematite goethite). The hematite index, applied to a TM image of the area around Brasilia, allowed hematite contents in the surface horizons of lateritic ...

Pyromorphite Formation from Goethite Adsorbed Lead

Abstract: Conversion of labile soil Pb species into stable Pb minerals is a crucial process in the development of an in situ soil Pb remediation strategy. A complete understanding of the reaction with specific soil Pb species is therefore of fundamental importance. A synthetic goethite, α-Fe(OH)3, with surfaces saturated with Pb2+ was used to represent adsorbed Pb, one of the primary soil Pb species. Hydroxyapatite (Ca5(PO4)3OH) and NaH2PO4 were the source of solid and soluble phosphate, respectively. When stoichiometric concentrations of soluble phosphate were added directly to the suspension of Pb-adsorbed goethite, the thermodynamically stable lead phosphate mineral chloropyromorphite (Pb5(PO4)3Cl) was rapidly precipitated. By contrast, when the same goethite suspension was reacted in a dialysis system in the presence of hydroxyapatite, the formation of chloropyromorphite is slow and appears to be controlled by the rate of dissolution of the hydroxyapatite. Chloropyromorphite minerals produced in these experimen...

Interaction of Cadmium with Phosphate on Goethite

Abstract: Interactions between different ions are of importance in understanding chemical processes in natural systems. In this study simultaneous adsorption of phosphate and cadmium on goethite is studied in detail. The charge distribution (CD)-multisite complexation (MUSIC) model has been successful in describing extended data sets of cadmium adsorption and phosphate adsorption on goethite. In this study, the parameters of this model for these two data sets were combined to describe a new data set of simultaneous adsorption of cadmium and phosphate on goethite. Attention is focused on the surface speciation of cadmium. With the extra information that can be obtained from the interaction experiments, the cadmium adsorption model is refined. For a perfect description of the data, the singly coordinated surface groups at the 110 face of goethite were assumed to form both monodentate and bidentate surface species with cadmium. The CD-MUSIC model is able to describe data sets of both simultaneous and single adsorption of cadmium and phosphate with the same parameters. The model calculations confirmed the idea that only singly coordinated surface groups are reactive for specific ion binding.

Measurement of the hematite:goethite ratio using field visible and near‐infrared reflectance spectrometry in channel iron deposits, Western Australia

Abstract: Empirical relationships and a field method have been developed for the measurement of the hematite:goethite ratio in Tertiary ooidal ironstones, locally named channel iron deposits, from the Hamersley region of Western Australia using visible to near‐infrared (400 to 1000 nm) refiectance spectrometry. The hematite:goethite ratio is important in the characterisation of these iron deposits as Al, P, water and Si are deleterious components commonly associated with goethite. The channel iron deposits typically comprise iron oxy‐hydroxides with less than 1% Fe2+ (present in maghemite or kenomagnetite), less than 8% Al3+‐substitution and with a mean crystal dimension of approximately 20 nm. The natural variations in the hematite:goethite ratio of the channel iron deposits were modelled using laboratory mixtures of pure hematite and goethite. The resultant spectral mixing trends produced consistent relationships with the hematite:goethite ratio, especially for the wavelength of the 6A1?4T1 crystal field absorpti...

Complexation of Humic Substances with Oxides of Iron and Aluminum

Abstract: Complexation of HA (and FA) by goethite, hematite, gibbsite, and boehmite was studied in pre-dried systems. Hematite showed the highest fixation of HA at various oxide: HA ratios and at all pH ≥ 7.0. A gradual reduction in HA/FA fixation from 2.0 to 10.0 was observed for all minerals except gibbsite, which showed a very sharp decrease at pH > 7.0 and a maximum at pH 5.0. Exchangeable cations have a remarkably dissimilar influence on HA complexation by the four minerals. Thus, various cationic forms of gibbsite showed a drastic loss of HA fixation capacity compared with the original (pronated) surface, whereas in boehmite, the reverse behavior was observed. Most of the metal ion-substituted hematites showed excellent HA fixation, but goethites revealed a mixed trend. It is inferred that two major modes of HA bonding are operative in hematite and goethite, viz., cation bridges forming oxide-M-HA links and direct bonding of HA to coordination centers at the oxide surface; forces of such bonding are strongest in hematite. In boehmite, cation bridging is the major interactive mode, whereas in gibbsite, HA fixation occurs primarily to coordination centers at the surface.

Effects of crystallinity of goethite: I. Preparation and properties of goethites of differing crystallinity

Abstract: Summary We prepared five samples of pure goethite and three samples of aluminous goethite with mole fractions of aluminium ranging from 0·05 to 0·20. One of the samples of pure goethite was repeatedly heated in an autoclave at 150°C. We studied the samples using transmission electron microscopy and XRD and we measured the surface area (BET), rate of dissolution of the samples in acid, the surface charge in four concentrations of sodium nitrate from pH 4–10, and the sorption of calcium and chloride from a dilute solution of calcium chloride. Surface areas ranged from 18 to 132 m2 g−1. The BET adsorption-desorption isotherms had hysteresis loops which increased as the surface area increased. These showed that there were pores present with diameters smaller than 2 nm and that the number of pores increased as the surface area increased. All samples of goethite showed typical X-ray diffraction patterns but with peak heights decreasing, and width increasing, as surface area increased. Mean coherence lengths, as determined by XRD line broadening, were smaller than those determined by transmission electron microscopy. This was because each crystal was composed of several domains, and the XRD data reflect the size of the domains. Heating one of the goethite samples in an autoclave caused most of the domains to coalesce and slightly decreased the surface area. As the surface area of the eight goethite samples increased, the mean coherence length in the direction of the a, b and c axes decreased. The better crystallized samples dissolved more slowly in acid and the dissolution curves were sigmoid because the acid was able to penetrate between the domains allowing access to a larger surface area and consequently accelerated dissolution. The points of zero charge increased with increasing surface area of the goethite. The charge carried by the goethite at pH values distant from the point of zero charge increased as the surface area increased. This may indicate movement of protons or hydroxyl ions into pores or other defects in the crystal. The effects of pH and of salt concentration on surface charge were closely described by the variable-charge/variable-potential model. This model also closely described adsorption of Ca2+ and Cl− from calcium chloride solutions requiring only the allocation of constants for these ions. The substitution of aluminium for iron did not affect the charge properties of goethite.

Hydroxamate Complexes in Solution and at the Goethite−Water Interface: A Cylindrical Internal Reflection Fourier Transform Infrared Spectroscopy Study

Abstract: The infrared spectra of aqueous solutions of acetohydroxamic acid (aHA), an analogue for important iron(III) chelating ligands in soils and groundwaters, and of suspensions of goethite with adsorbed aHA were measured using a cylindrical internal reflectance cell. Using molecular orbital theory and data from the literature on the infrared spectra of solid hydroxamates and metal−hydroxamate complexes, we identify the spectral changes occurring upon complexation with both aqueous Fe(III) and Fe atoms on the surface of goethite. The effect of pH upon the solution and surface spectra and the possibility of aHA hydrolysis at the goethite surface at pH = 3 were also examined. There was no significant difference in adsorbed aHA spectra between pH = 3 and pH = 6, indicating identical surface complexes form over the pH range where the solution phase complex changes from the bis- to the tris(hydroxamato)iron(III) complex. The infrared technique was not capable of detecting the low concentrations of hydrolysis produc...

Preparation and properties of glass-ceramic materials obtained by recycling goethite industrial waste

Abstract: The recycling of toxic goethite waste, originated in the hydrometallurgy of zinc ores, in glass-ceramic matrices has been studied. Oxide compositions suitable to form glasses were prepared by mixing the goethite waste with granite scraps and glass cullet, yielding the following oxide composition (wt%): SiO2, 44.6; Al2O3, 3.3; Fe2O3, 25.5; MgO, 1.6; CaO, 4.5; Na2O, 5.9; PbO, 3.1; ZnO, 6.5; K2O, 1.0; TiO2, 2.0; other 2.0. By proper addition of carbon powder, the initial Fe3+/Fe2+ ratio (12) of glasses melted in air at 1450 °C was approximated to the stoichiometric value of magnetite (2) to obtain high nucleation and crystallization rates. The heat treatment of iron supersaturated goethite glasses above 630 °C led to the formation of magnetite nuclei with a high tendency to grow and coalesce with time. The crystallization of pyroxene, occurring on the magnetite crystals above 800 °C, was found to be influenced by the nucleation period, so that the highest crystalline volume fraction, V f (0.80–0.85), was obtained for 90–120 min nucleation time at 670 °C and 120 min crystallization at 860 °C.

Characterization of Colloids Mobilized from Southeastern Coastal Plain Sediments

Abstract: Two subsurface samples representing highly weathered sediments of the Upper Coastal Plain (Aiken, SC) that differed in Fe-oxide content (7.3 vs 35.9 g Fe kg -1 ) were evaluated in terms of mobile colloid generation in response to changes in solution composition. In repacked columns, the two sediments were leached (∼0.72 m d -1 Darcy velocity) with either 0.001 N NaCI or CaCl 2 solutions for 10 pore volumes followed by 10 pore volumes of deionized water. Bulk clay mineralogy from the two samples and suspensions generated in the column studies was characterized by energy dispersive X-ray analysis (EDXA) using a field-emission scanning electron microscope (SEM). SEM results were confirmed by selected-area electron diffraction (SAED) and EDXA in a transmission electron microscope. In column experiments, the sample lower in Fe oxide displayed increased effluent turbidity that coincided with a decrease in effluent pH during injection of the CaCl 2 solutions. In contrast, no effluentturbiditywas observed during the leaching of NaCl solutions. The sample higher in iron oxide displayed greater effluent turbidity from the onset of leaching that decreased over the duration of injection for both the CaCl 2 and NaCl solutions. Colloids generated from both materials displayed positive electrophoretic mobilities, confirming the importance of iron oxides in controlling dispersion and surface charge properties of the mobile colloids. Minor differences in the clay mineralogy (i.e., iron oxide/kaolinite content) between the samples were reflected in both their sensitivity to pore-solution composition and the relative degree of dispersion observed for each sample. Electron microscopy, EDXA, and thermal gravimetric analysis (TGA) confirmed that the mobile colloids consisted mainly of submicron sized Al-rich goethite, with lesser amounts of kaolinite,the predominate layer silicate of the bulk sediments, and varying amounts of crandallite.

The effect of phosphate on the transformation of ferrihydrite into crystalline products in alkaline media

Abstract: The presence of phosphate retards the transformation of ferrihydrite into crystalline products. Increasing phosphate from 0 to 1 mole % results in an order of magnitude decrease in the rate of transformation of ferrihydrite at pH 12. Levels of phosphate of ∼1 mol % suppress the formation of goethite (α-FeO(OH)) and result in the formation of a product consisting ofη-Fe2O3. Higher levels of phosphate result in the ferrihydrite remaining amorphous, even after several hundred hours. Phosphate prevents formation of goethite by hindering the dissolution of ferrihydrite rather than by interfering with nucleation and growth of goethite in solution. The transformation rate of pure ferrihydrite is also strongly inhibited in the presence of dissolved phosphate. This is due to surface complexation. The transformation rate was measured at temperatures of 60 °C and 70 °C. The rate of transformation was found to be described by either (i) a solid-state reaction equation for powdered compacts or (ii) a zero-order reaction controlled by desorption. The transformation of the ferrihydrite matrix was accompanied by the loss of the phosphate trace component. X-ray diffraction indicates that no solid solution involving phosphate substitution intoη-Fe2O3 is formed. Transmission electron microphotographs of the original precipitates containing phosphate confirm the presence of the phosphate and demonstrate its involvement in linking together extremely small particles of ferrihydrite.

Properties of goethite and jarosite precipitated from acidic groundwater, Dalarna, Sweden

Abstract: This study characterizes various chemical and mineralogical properties of goethite and jarosite from a mine drainage environment using chemical extraction techniques, X-ray diffractometry (XRD), Fe-57 Mossbauer spectroscopy and scanning electron microscop

Epitaxial overgrowth of goethite on hematite synthesized in phosphate media: A scanning force and transmission electron microscopy study

Abstract: We used X-ray diffraction (XRD), scanning force microscopy (SFM), transmission electron microscopy (TEM), and color to investigate the effect of phosphate on the crystallization rate, nature, and morphology of iron oxides prepared from ferrihydrite in the laboratory. Synthesis was performed at two temperatures (323 and 373 K) and two pH values (9 and 12) from ferric nitrate, for P/Fe atomic ratios ranging from 0 to 2.5%. The presence of phosphate retarded crystallization, tended to favor hematite over goethite, and markedly influenced the morphology of the goethite crystals formed at high pH. Application of SFM in the deflection mode was useful to investigate the morphology of the small goethite crystals, with careful attention paid to operating conditions; in particular, sharp silicon probes were found to produce fewer artifacts than coarser silicon nitride ones. At low P/Fe ratios (,0.2%), the goethite crystals were thin, elongated, multidomain laths; at high P/Fe ratios (.1.5%), star-shaped, twinned crystals were produced. All the theoretical shapes, derived from the assumption that star-shaped crystals result from the epitaxial growth of goethite on a hematite core, were observed by SFM and TEM. The presence of such hematite nuclei was supported by XRD, selected-area electron diffraction, color, and preferential dissolution of the samples in HCl, because it is known that hematite dissolves faster than goethite in acid. With increasing P/Fe ratio, the arms of the star-shaped crystals became shorter. This was likely due to the higher density of P-adsorbing pairs of singly coodinated OH groups on terminal {021} faces relative to prismatic {110} arm faces.

Factors affecting phosphate sorption along a Mediterranean, dolomitic soil and vegetation chronosequence

Abstract: Phosphate sorption by calcareous soils has been studied mainly on heavily fertilized agricultural soils and soils with calcite as the main carbonate mineral. We examined factors affecting phosphate adsorption in the soils of a semi-arid, mediterranean, dolomitic, soil and vegetation chronosequence in southeastern Spain. The youngest soils are highly eroded, Sandy Regosols (Typic Xerorthents) under gorse-scrubland vegetation. These have small P sorption capacities, large MgCa carbonate contents but small amounts of Fe and Al oxides. Small total P (HNO 3 /HClO 4 digestion) concentrations (30-130 μ g P g -1 ), of which up to 90% is Ca-bound (HCl-extractable), are typical of these young soils. P sorption markedly increased when Ca 2+ was added to the solution. The fractionation of previously sorbed P indicates that the fate of most of this extrasorbed P is the labile-P fraction sorbed on to (carbonate) surfaces and the apatite-like fraction (NaHCO 3 -extractable and HCl-extractable fractions). At the other extreme, older more-intensively weathered, sandy-clay-loam rendzinas (Entic Haploxerolls), supporting dense mature garrigue, have a much greater P adsorption capacity and larger clay and Fe and Al oxide concentrations. They have more total P (ca 400 μg P g -1 ), much of it in occluded form (residual fraction). These soils show no significant differences in P sorption whether or not CaC12 was used as a background electrolyte. Considering the overall variations within the chronosequence, dithionite extractable Fe and Al are the properties best correlated with P sorption. This support the general finding that crystalline Fe-oxides (e.g. goethite and haematite) appear to be the most important P-sorbing component for soils in the Mediterranean region, rather than amorphous Fe-oxides (e.g. ferrihydrite) as is reported for more mesic areas. Stepwise multiple regression and fractionation data, however, suggest that, provided the soil solution is rich in Ca 2+ , carbonate may also be a significant contributing factor to P sorption, especially in the youngest of these dolomitic soils.

An Fe-Berthierine from a Cretaceous Laterite: Part II. Estimation of Eh, pH and pCO2 Conditions of Formation

Abstract: Transgression by the Western Interior Sea during the Late Cretaceous in southwestern Min- nesota caused swampy conditions to be imposed upon a laterite consisting of gibbsite, goethite and kaolinite. Reducing conditions overprinted upon the laterite reduced ferric Fe in goethite for incorporation of ferrous Fe into Fe-berthierine. Attendant oxidation of organic matter provided CO2 for siderite's for- mation. Thermodynamic calculations indicate that berthierine, gibbsite, goethite, kaolinite and siderite were in equilibrium with a solution whose pH was 5.2 and whose pCO 2 was on the order of 0.3 atm. Formation of Fe-berthierine is favored by solutions having: 1) low silica concentration; 2) low (Mg2+)/ (Fe 2§ ratio; 3) high pCO2; 4) extremely low sulfate content before reduction takes place; and 5) moderate reducing conditions (Eh around -0.05 V).

Surface Structure and Site Density of the Oxide-Solution Interface

Abstract: The site densities of the surface of different oxides and hydroxides have been calculated from crystallographic data. The following values for the common habits are recommended: 2.7 nm-2 (Corundum), 2.3 nm-2 (Boehmite and Gibbsite), 2.2 nm-2 (Hematite and Rutile), 2.1 nm-2 (Lepidocrokite), 1.9 nm-2 (Diaspore), and 1.7 nm-2 (Goethite). The probable deviations due to the variations in habits are about +/-0.2 nm-2, except Rutile (+/-0.4 nm-2). For the concrete faces the values of site density are adduced. Copyright 1997 Academic Press.