Showing papers on "Goethite published in 1999"

In situ study of the goethite-hematite phase transformation by real time synchrotron powder diffraction

Abstract: The temperature induced goethite-hematite phase transformation that occurs at about 250 °C was studied using in situ synchrotron X-ray powder diffraction with a capillary Debye-Scherrer geometry and a translating image plate system (TIPS). To our knowledge, this is the first time the goethitehematite transformation has been investigated in real time. The sample was a pure, synthetic, stoichiometric goethite with 1 μm long needle-shaped crystals. The microstructural characterization showed that the sample was well crystallized. The Rietveld refinement of 30 powder patterns extracted from the image in the range 25‐800 °C demonstrates that an intermediate phase with nonstoichiometric composition (“protohematite” ) forms after the decomposition of goethite. The cell parameter b of goethite dramatically decreased during the phase transformation while a and c instead continued to increase. Protohematite is iron-deficient and retains residual hydroxyls for charge balance. With temperature protohematite progressively transforms into hematite. Empty layers (pores) are consequently formed about the hematite clusters. The distribution of iron vacancies was modeled in the powder patterns with stacking faults that were simulated using anisotropic broadening coefficients of the pseudo-Voigt profile function. Its disappearance with temperature was effectively followed with a decrease of the density of stacking faults.

Interaction Of Inositol Hexaphosphate On Clays: Adsorption And Charging Phenomena

Abstract: The interaction of myoinositol hexaphosphate (IHP) with goethite, and with phyllosilicates such as illite and kaolinite, was studied by assessing the adsorption mechanisms and the electrochemical modifications induced by adsorption of a molecule at such a high-charge density. In addition to quantitative studies, Fourier-Transform Infrared (FT-IR) spectroscopy was used to establish the mechanisms of interaction. Laser Doppler Velocimetry-Photon Correlation Spectroscopy (LDV-PCS) was employed to determine the electrophoretic mobility and the size of the particles. The experiments were also run with orthophosphate (Pi) for comparison. The quantity of adsorbed IHP reached 0.64 μmol m -2 on goethite, 0.38 μmol m -2 on illite, and 0.27 μmol m -2 on kaolinite. The mechanism of adsorption of IHP involved the phosphate groups, whereas the organic moiety affected the process only in terms of conformational hindrance. Thus, on goethite surfaces, because IHP occupied an area equivalent to four sites for Pi, it was supposed to be bound to the oxide by four of its six phosphate groups, whereas the other two were free. On the illite and kaolinite surfaces, the area occupied by IHP was equivalent to about two sites for Pi, suggesting that a lower number of phosphate groups are bound to phyllosilicates. The phosphate groups that did not react with the minerals caused a modification of the electrochemical properties. In particular, IHP adsorption caused dispersion of the particles and a net increase of the negative charge of the surface.

Kinetics of lead adsorption/desorption on goethite : Residence time effect

Abstract: Sorption/desorption reactions at the mineral/water interface will control solution concentrations of trace elements and their bioavailability in natural environments. While there has been a great deal of research examining trace element sorption reactions on soil and soil constituent surfaces, there is relatively little research examining desorption reactions and the effect of residence time. In this study, the authors examined the influence of sorption density and residence time on the sorption/desorption kinetics of Pb on goethite. Lead sorption was rapid and nearly complete in less than 1 h, with essentially no change in the quantity of Pb sorbed over the 12-week sorption period. Desorption of Pb was slow and was modeled best by the parabolic diffusion equation. At all sorption densities investigated, desorption rate coefficients and the quantity of Pb desorbed were greater for the short-term experiments. However, statistical analyses indicated that these differences were not statistically significant. It is suggested that an insufficient desorption period caused by the high affinity of Pb for the goethite surface or the slow diffusion of Pb(OH)[sup +] into crystal defects may be responsible for the absence of a residence time effect. Further studies are necessary to determine if the observed trends are real ormore » are artifacts of the experiment.« less

Influence of Aqueous and Solid-Phase Fe(II) Complexants on Microbial Reduction of Crystalline Iron(III) Oxides†

Abstract: The influence of aqueous (NTA and EDTA) and solid-phase (aluminum oxide, layer silicates) Fe(II) complexants on the long-term microbial reduction of synthetic goethite by Shewanella alga strain BrY was studied. NTA enhanced goethite reduction by promoting aqueous Fe(II) accumulation, in direct proportion to its concentration in culture medium (0.01−5 mM). In contrast, EDTA failed to stimulate goethite reduction at concentrations ≤1 mM, and 5 mM EDTA enhanced the final extent of reduction by only 25% in relation to nonchelator controls. The minor effect of EDTA compared to NTA, despite the greater stability of the Fe(II)−EDTA complex, likely resulted from sorption of Fe(II)−EDTA complexes to goethite. Equilibrium Fe(II) speciation calculations showed that Fe(II)aq should increase with NTA at the expense of the solid-phase Fe(II) species, whereas the opposite trend was true for EDTA due to Fe(II)EDTA adsorption. The presence of aluminum oxide and layer silicates led to a variable but significant (1.5 to > 3...

Release of natural organic matter sorbed to oxides and a subsoil

Abstract: Sorption to mineral surfaces is an important process controlling the mobility and stability of natural organic matter (NOM) in soil, yet only knowledge of the reversibility of this process enables the prediction of element cycling and NOM-induced transport in soils. We have elucidated the desorption of mineral-bound NOM in batch experiments with amorphous Al(OH) 3 , goethite, and a subsoil low in organic C. These sorbents were equilibrated with increasing amounts of water-extractable NOM from the Oa horizon of a mor forest-floor layer and then extracted with solutions of different ionic strengths, pH, and concentrations of inorganic anions (Cl - , SO 4 2- , H 2 PO 4 - ). Sorbed NOM was extracted after 24, 48, 72, and 120 h. We investigated structural and functional characteristics of the desorbed NOM by XAD-8 (macroporous resin) fractionation and by 13 C-NMR spectroscopy. Desorption of NOM from minerals and soils was negligible (<3%) under solution conditions similar to those during the sorption (hysteresis). It was not influenced by increasing concentrations of noncompeting inorganic anions such as Cl - . Increased concentrations (≤0.1 M) of competing anions like SO 4 2- or H 2 PO 4 - increased the NOM desorption. Though H 2 PO 4 - was most efficient in desorbing NOM, the extractability was only ≤60% at the highest H 2 PO 4 - concentration. The most significant desorption occurred when solution pH was raised. For goethite, NOM desorption reached a maximum at a pH above the point of zero charge (PZC) of the mineral. With increasing surface coverage of the sorbent by NOM, the proportion of desorbable NOM decreased for all extractants. Increased sorption hysteresis was also observed with an increasing time period between sorption and desorption. The desorption was more pronounced for NOM compounds that exhibit hydrophilic properties and have low contents of aromatic structures and carboxyl groups. The irreversible binding of NOM, especially of the lignin-derived portion, to soil minerals seems to result from its polyelectrolytic nature. This may favor the formation of multi-site coordinative bonds and effective shielding of the binding ligands by other parts of the sorbed molecule.

Effect of phosphate on the crystallization of hematite, goethite, and lepidocrocite from ferrihydrite

Abstract: We investigated the crystallization of ferrihydrite prepared by hydrolysis of Fe(NO3)3 solutions containing phosphate. Crystallization was studied at different pH (3-9), temperatures (298, 323, and 373 K), and initial P/Fe atomic ratios for periods to 730 d. Generally, crystallization was inhibited or only poorly crystallized lepidocrocite was formed at P/Fe > 2.5%. Phosphate favored the formation of hematite over goethite at all temperatures for most of the pH and P/Fe ranges investigated. This result is consistent with a model in which phosphate acts as a template for hematite formation, in analogy with other anions, such as oxalate. However, goethite was preferentially formed at alkaline pH and P/Fe > 1%, probably because high phosphate concentration resulted in a large increase in the negative charge of the ferrihydrite particles. This resulted in turn in less aggregation, a process that is known to precede dehydration to hematite. Phosphate greatly influenced the morphology of hematite and goethite. Hematite was often ellipsoidal or spindle-shaped. Twinned goethite crystals with a hematite core were formed at alkaline pH at P/Fe > 1%. Both hematite and goethite particles incorporated phosphate in an occluded form not desorbable by repeated alkali treatments.

Reactions of copper and cadmium ions in aqueous solution with goethite, lepidocrocite, mackinawite, and pyrite

Abstract: The uptake of Cu and Cd in aqueous solution by interaction with the surfaces of carefully synthesized finely sized particles of goethite, lepidocrocite, mackinawite, and pyrite was measured as a function of initial metal concentration in solution. The results show how reactions between metal ions in solution and mineral surfaces depend in a subtle way on the nature of the surfaces and, in certain cases, on the initial concentration of metal in solution. The uptake curves fall into two groups; type I in which the efficiency of uptake decreases with increasing concentration (Cu and Cd on goethite and lepidocrocite, Cd on pyrite), and type II in which it remains constant (Cu on mackinawite and pyrite, Cd on mackinawite). The total uptake is an order of magnitude greater for the latter group. X-ray absorption spectroscopies (XANES and EXAFS) were used to define the local environments of the metals taken up at the mineral surfaces. All examples showing the type I behavior yield information on local environments consistent with their being bound to the surfaces by an inner sphere complex formation mechanism. Thus, Cu on goethite appeared to form a Jahn-Teller distorted octahedral complex (four O atoms at 1.94 Aa; two O atoms at 2.41 Aa) but with evidence for interaction with two further Fe (or Cu) at 2.92 Aa. On lepidocrocite, the first coordination sphere was essentially identical to that on goethite but with a second Cu or Fe shell at 3.04 Aa and, for the highest Cu loadings, a third shell (2 Cu or Fe) at 3.67 Aa. The Cd on goethite showed best fits for sixfold coordination to O (at 2.26 Aa) but with evidence for a second shell of Fe atoms at 3.75 Aa. On lepidocrocite, the first shell was essentially the same as for goethite, but a second shell of Fe atoms appeared to occur at the shorter distance of 3.31 Aa. For Cd-loaded pyrite, best fits were given by a single shell of six O atoms at 2.27 to 2.28 Aa and with no evidence for a second shell of metal atoms. The systems exhibiting the type II behavior yielded spectra consistent with the formation of new phases on the surfaces, either by precipitation or replacement reactions. In the case of Cu interaction with mackinawite, a chalcopyrite phase appeared to form, whereas interaction with pyrite seemed to produce binary Cu sulfides--covellite at lower loadings and chalcocite at higher loadings. Cd interaction with mackinawite seemed to produce a CdS phase.

Red palaeolithic pigments: natural hematite or heated goethite?*

Abstract: Yellow goethite (α-FeOOH) and red hematite (α-Fe 2 O 3 ) were used as pigments by Palaeolithic painters. When heated, goethite dehydrates and transforms to hematite. Archaeologists wonder whether prehistoric people knew of this property and used it. We present a way to distinguish heated goethite from natural hematite. The methods used are X-ray diffraction and transmission electron microscopy. A double 'signature', namely selective broadening of diffraction reflections and porous nanostructure, is evidenced in heated goethite. Coupled with mineralogical associations, this allows us to establish a procedure to investigate the natural or artificial origin of hematite samples. Applied to the Palaeolithic site of Troubat, it reveals no difference between the Magdalenian and Azilian cultures concerning the heat-treatment of pigments; an area specializing in heat-treating pigments may, nevertheless, have existed inside the cave, which would indicate that heating was deliberate.

Bacterial and Chemical Reductive Dissolution of Mn-, Co-, Cr-, and Al-Substituted Goethites

Abstract: Goethite samples in which 5% of the iron atoms were replaced with different noniron metal (NIM) atoms (Mn, Co, Al, Cr) were synthesized and characterized by X-ray diffraction. Their dissolution (a) by bacterial (enzymatic) reduction and (b) by chemical dissolution through reduction and complexing with a citrate-bicarbonate-dithionite reagent was investigated and compared with the response of similarly treated nonsubstituted goethite. Chemical dissolution differed in rate of iron solubilization depending on NIM substituents. Initial rates of iron dissolution were faster with goethite-lacking metal substituents than with Al-goethite. Bacterial fermentation and reduction processes were not significantly modified by the presence of Mn, Co, Al, or Cr, but the dissolution rates of iron decreased with substitution. During bacterial iron reduction, Al-goethite was more resistant than Mn-goethite, Co-goethite, Cr-goethite, and pure goethite. Both chemical and bacterial reduction of iron resulted in simultaneous so...

Dynamics of phosphorus in the rhizosphere of maize and rape grown on synthetic, phosphated calcite and goethite

Abstract: In calcareous soils the dynamics of phosphorus is controlled by calcite and iron oxides such as goethite which strongly retain P and consequently maintain low P concentrations in soil solution. Plants can drastically change chemical conditions in the rhizosphere, in particular by releasing H+ or OH− or by excreting organic anions. By modifying the dissolution/precipitation and desorption/adsorption equilibria, roots can influence the mobility of soil P. The aim of this work was to test whether H+ or OH− release can induce the mobilization of P in the rhizosphere of maize and rape supplied with NO3-N or NH4-N and grown on synthetic phosphated calcite or goethite as sole source of P. With P-calcite, the mobilization of P was generally related to the acidification of the rhizosphere. With P-goethite, rhizosphere acidification induced some increase of DTPA-extractable Fe and hence dissolution of goethite. Rhizosphere P was concomitantly depleted but the mechanisms involved are less clear. The difference in behavior of the two species is discussed.

The role of goethite in the formation of the protective corrosion layer on steels

Abstract: The corrosion products formed on carbon and weathering steels exposed in marine, industrial and rural environments in the United States for 16 years have been investigated using Mossbauer spectroscopy, Raman spectrometry and chemical analysis. Mossbauer spectroscopy was used to measure the fraction of each oxide in the corrosion coatings and micro-Raman spectrometry was used to locate and map the oxides to 2 µm spatial resolution. Mossbauer spectroscopy identified the corrosion products in the weathering steels as 75% goethite, 20% lepidocrocite and 5% maghemite. Raman analysis showed that the corrosion products generally formed as alternating layers containing different oxides. For the weathering steels the protective inner-layer closest to the steel substrate consisted of nano-sized goethite ranging in size from 5–30 nm and having a mean particle size of about 12 nm. The outer-layer close to the coating surface, consisted of lepidocrocite and goethite with the former oxide being most abundant. Electron probe micro-analysis measured significant chromium in the goethite close to the steel substrate. Comparison of the goethite in the corrosion products was made with synthetic chromium substituted goethite with nearly identical microstructural characteristics being recorded. It is concluded that chromium inclusions in the goethite are important for formation of a nano-phase oxide layer which may help protect the weathering steel from further corrosion.

Fenton-like soil remediation catalyzed by naturally occurring iron minerals

Abstract: The ability of the iron minerals hematite and magnetite to catalyze the decomposition of hydrogen peroxide (H2O2) and initiate the Fenton-like oxidation of pentachlorophenol (PCP) was investigated in batch, bench-scale systems in which PCP was spiked onto silica sand. Pentachlorophenol degradation was documented in silica sand–mineral–H2O2 systems by the release of chloride and the loss of total organic carbon. The most efficient oxidation stoichiometry was the magnetite-catalyzed reaction over the first 8 h with 490 mol H2O2 consumed/mol PCP degraded. After 8 h, the peroxide efficiency decreased significantly; amorphous iron hydroxide formation on the magnetite surface may have catalyzed the decomposition of H2O2 to oxygen species other than hydroxyl radicals. Mineral-catalyzed Fenton-like treatment in two natural soils was demonstrated after spiking the soils with PCP; the contaminant was degraded with no iron addition. The oxidation stoichiometry in the two soils was 1,100 and 2,930 mol H2O2 c...

The Influence of Metal Ions on the Adsorption of Phosphonates onto Goethite

Abstract: Mono- and polyphosphonates, which contain R3CP(O)(OH)2 functional groups, are used in an increasing variety of industrial and household applications including cooling water systems, oil production, textile production, and detergents. The main pathway of phosphonate removal in the environment is via adsorption onto surfaces. This study examines the effect of Ca, Cu, Zn, and FeIII on the adsorption of six phosphonates onto the iron (hydr)oxide goethite. HEDP, NTMP, EDTMP, and DTPMP are commonly used, while AMP and IDMP have been identified as breakdown products of NTMP. When the molar concentration of Ca, Cu, Zn, and FeIII is equal to the concentration of the phosphonate, the effects on phosphonate adsorption are either negligible or slight. This can be explained by dissociation of the metal−phosphonate complex and separate adsorption of the metal ion and phosphonate onto different surface sites. When Ca and Zn concentrations are added in excess of the phosphonate concentration, considerable increases in ad...

Quantification and Characterization of Maghemite in Soils Derived from Volcanic Rocks in Southern Brazil

Abstract: Many soils developed from volcanic rocks in southern Brazil exhibit spontaneous magneti- zation caused by the presence of fine-grained maghemite (3J-Fe203), but few attempts were made to quantify or characterize this important soil component. To that end, clays were separated from freely drained soils derived from acid (>~63% Sit2), intermediate (54-62% SiO~), and basic (--<53% Sit;) igneous rocks produced by the Paranfi flood volcanism. The sample set included soils with a wide range of pedogenic development on different landscape positions. The Fe oxide mineralogy of these samples was examined by using a combination of selective dissolution, magnetic susceptibility, and X-ray dif- fraction (XRD) techniques. Hematite and maghemite were the primary Fe oxides in mature soils (Oxisols, Ultisols, and Alfisols) developed from basic rocks; whereas goethite was dominant in all other soils, especially those formed from acid-intermediate rocks. The association of maghemite with basic rock materials suggests that it was primarily formed by oxidation of lithogenic magnetite. A strong, positive correlation (R 2 = 0.89) was obtained between mass specific magnetic susceptibility (X) of the clay frac- tions and maghemite contents estimated by XRD. Either method could be used for quantitative analyses, but X was more sensitive than XRD at low maghemite concentrations (<2 wt. %). The clay-sized maghem- ites were superparamagnetic with an estimated value for the mass specific magnetic susceptibility (X~f) value of 91,000  10 -8 m 3 kg ~ and frequency dependent variations of 10-15%. The maghemites also had low unit cell constants, which, if attributed entirely to replacement of Fe by AI, would correlate with AI substitutions in the range of 5-16 mole %. Selective dissolution of the soil maghemites was achieved by treatment of Fe oxide concentrates with 1.8 M I-I2SO 4 at 75~ for 2 h.

Complexation of Methylphosphonic Acid with the Surface of Goethite Particles in Aqueous Solution

Abstract: Adsorption of methylphosphonic acid (MPA) onto goethite (α- FeOOH) was studied as a function of pH and the concentration of MPA in solution. In situ attenuated total reflectance-FTIR (ATR-FTIR) techniques were used to identify the type of surface complexes formed under each of the solution conditions used in the adsorption studies. Additionally, electrophoretic mobility measurements were performed in each of these systems. At low pH and high Γ, MPA was bound to the surface of goethite predominantly as a monodentate protonated species, while at high pH and low Γ the prevailing structure was that of a bridging bidentate complex. The change in denticity of the surface MPA under different solution conditions may explain the charge reversals shown in the electrophoretic mobility curves.

The Effect of Oxyanions on the Oxalate-Promoted Dissolution of Goethite

Abstract: Organic ligand-promoted dissolution of oxide minerals can be enhanced or inhibited in the presence of specifically adsorbed oxyanions. It has been proposed that an oxyanion inhibits or enhances dissolution depending on the type of surface complex formed and the strength of the bond. Mononuclear complexes (especially if they are bidentate) accelerate dissolution, while binuclear complexes inhibit dissolution. Recent spectroscopic evidence indicates that chromate and arsenate form different surface complexes depending on surface coverages. This study examined the influence of chromate and arsenate on the oxalate promoted dissolution of goethite. Based on a previous spectroscopic study, oxyanion surface coverages were varied to generate both mononudear and binuclear surface complexes. Chromate and arsenate inhibited the oxalate promoted dissolution of goethite at all surface coverages investigated except at pH 6 (Γ arsenate = 1.70 × 10 -6 and Γ chromate = 1.66 × 10 -6 mol m -2 ). It is proposed that chromate and arsenate inhibit goethite dissolution by decreasing oxalate adsorption. This is accomplished because arsenate and chromate are more effective competitors for goethite surface sites than oxalate and upon adsorption increase the negative charge of the goethite surface. At pH 6 the adsorption of chromate and arsenate increases the negative charge of the goethite surface which in turn increases proton adsorption. Since proton adsorption is a necessary step for oxalate-promoted dissolution of goethite, and since proton activity at pH 6 is low, an increase in the negative charge of goethite upon adsorption of the oxyanions accelerates dissolution.

Competitive adsorption of sulfate and oxalate on goethite in the absence or presence of phosphate

Abstract: A study was carried out to examine the competition in adsorption of sulfate (SO4) and oxalate (OX) on goethite as affected by pH, concentration of the ligands, order of anion addition, and the presence of phosphate (PO4). The purpose was to obtain useful information about the factors that influence