Showing papers on "Goethite published in 1994"

The Mechanism of Selenate Adsorption on Goethite and Hydrous Ferric Oxide

Abstract: EXAFS spectroscopy has been used to investigate the structure of selenate surface complexes sorbed on dry and wet goethite and hydrous ferric oxide. It is shown that selenate oxyanions always form inner-sphere surface complexes on both sorbents regardless of the hydration state. Selenate, like phosphate and selenite, forms binuclear bridging complexes on (hk0) planes with two singly coordinated A-type OH groups. Furthermore evidence is provided for the attachment of selenate oxyanions on (001) planes by the sharing of edges with Fe octahedra. Selenite and arsenate appear to form the same types of surface complexes as selenate at the surface of goethite and hydrous ferric oxide. It is likely that this sorption mechanism is operative for other anions including phosphate and sulfate. A consequence of this study is the conclusion that the crystallographic planes of Fe oxides (hk0 and 001) do not appear to selectivity bond either cations or anions, at least at high surface coverage.

Effect of solution chemistry on clay colloid release from an iron oxide-coated aquifer sand.

Abstract: This research compared the influence of (1) dissolution of iron oxides and (2) alteration of electrostatic interactions on the mobilization of colloids in a clay- and iron oxide-coated sand obtained from an Atlantic Coastal Plain aquifer in which colloids have been found suspended only in anoxic groundwater. The sediment was flushed with solutions of varying ionic strength, pH, and reductant and surfactant concentrations, and the steady-state rates of clay colloid release and iron oxyhydroxide dissolution were measured. The clay release rates were directly related to the calculated detachment energies and unrelated to rates of iron(III) oxide dissolution, indicating that electrostatic interactions dominated the binding of colloids to grain coatings. Mobilization of colloids by iron(III) oxide dissolution through reductive dissolution was limited by high ionic strength. Flushing of the sediment by a natural groundwater with high dissolved organic carbon concentration released clay without rapidly dissolving iron oxides. 59 refs., 8 figs., 3 tabs.

Oxidation capacity of aquifer sediments.

Abstract: A laboratory extraction method (Ti 3+ -EDTA extraction) for the determination of the oxidation (electron accepting) capacity related to oxides and hydroxides of aquifer sediments was developed. At room temperature, titanous ions (0.008 M Ti 3+ ) in a solution of 0.05 M ethylenediaminetetraacetic acid (EDTA) reduced oxidized aquifer species. This operationally defined oxidation capacity (ORC) was determined as μmoles of electrons accepted per gram of sediment sample. Well-described oxidized iron and manganese minerals were reduced (ferrihydrite 98%, akageneite 100%, goethite and hematite 93%,magnetite 9%, pyrolmite 99%) whereas organic matter in the sediments was not reduced significantly

Uranium retention by weathered schists-The role of iron minerals

Abstract: Uranium sorption experiments were carried out on weathered schist samples which had been treated with citrate/dithionite/bicarbonate (CDB) reagent to remove iron oxides, and the results were compared to those of similar experiments with untreated samples. Uranium sorption was greatly decreased by the CDB extraction, which reduced the surface area of the samples by about 30-40%. Chemical analysis and XRD confirmed that iron oxides were the major mineral phase extracted by the CDB reagent. To further elucidate the role of iron minerals in the natural environment, we carried out transformation (aging) experiments with synthetic ferrihydrite which contained adsorbed natural uranium (predominantly {sup 238}U). In these experiments, the ferrihydrite was partially converted to crystalline forms such as hematite and goethite. The uptake of an artificial uranium isotope ({sup 236}U) and the leaching of {sup 238}U form the samples were then studied in adsorption/desorption experiments. The results showed that the transformation of ferrihydrite to crystalline minerals substantially reduced the ability of the samples to adsorb {sup 236}U from solution. The desorption data for {sup 238}U showed that some of the {sup 238}U which was adsorbed to the samples prior to the transformation step was irreversibly incorporated within the mineral structure during the transformation process. These experiments highlight the potential importance of iron minerals both in the initial sorption of radionuclides and in isolating them from interactions with the aqueous phase. Tranformation of iron minerals from amorphous to crystalline forms provides a possible mechanism for uranium immobilisation. In considering the overall effect on U migration, this must be balanced against the reduced ability of the transformed iron oxide to adsorb U from solution. (orig.)

Molybdenum Adsorption by Aluminum and Iron Oxides and Humic Acid

Abstract: It is hypothesized that soil adsorption properties can be estimated from contents and adsorption properties of individual soil components (additivity principle), if the data are consistent. Therefore, adsorption of molybdenum(VI) by synthetic aluminum oxide, ferrihydrite, goethite, extracted humic acid and a sandy soil sample was determined as a function of Mo concentration (0-1.2 mmol/L) and pH (3.5-8). For each pH and adsorbent, the Langmuir adsorption maximum (adsorption capacity) was calculated. For the oxides, plots of these capacities against pH resulted in adsorption envelopes exhibiting maxima at pH 4-5, i.e., near the pK a1 and pK a2 for molybdenum acid

Phytoavailability of phosphate adsorbed on ferrihydrite, hematite, and goethite

Abstract: Iron (hydr)oxides are important phosphate adsorbents in soils. However, the dependency of phytoavailability of adsorbed phosphate on mineral type and properties has not been established. To examine the influence of mineralogy, one ferrihydrite, two hematites, and two goethites were prepared. They differed widely in morphology (from euhedral crystals to granular aggregates), specific surface area (15–266 m2 g−1) and microporosity (0–87% of total surface area). Their relative affinity for phosphate, which is inversely related to the concentration exponent in the Freundlich equation, b, decreased in the order goethite (b=0.11, 0.09)> hematite (b=0.13, 0.17)> ferrihydrite (b=0.21). Phytoavailability of the adsorbed phosphate was studied by growing sunflower (Helianthus annuus L.) in pots containing suspensions of the phosphated Fe (hydr)oxides in equilibrium with a concentration of 1 mg P L−1. The fraction of the adsorbed phosphate that was phytoavailable, B 1, (1) was lower for goethite (B 1=0.43, 0.38) than for hematite (B 1=0.73, 0.49) and ferrihydrite (B 1=0.79), (2) was not negatively affected by microporosity, and (3) decreased, in general terms, with increasing relative affinity for phosphate. Similar trends were observed for the production of dry matter. These results, together with previous reports showing that goethites usually have higher relative affinity for phosphate than hematites, suggest that phosphate could be more available to plants in hematitic than in similar goethitic soils.

Fe-Speciation in Kaolins: A Diffuse Reflectance Study

Abstract: Diffuse reflectance spectra of kaolins have been recorded in samples from different environ- ments. They show the systematic presence of Fe-oxides, even in bleached kaolins, with no contribution from the Fe 3+ ions substituted in kaolinite. Second derivative spectra of various Fe-phases (hematite, goethite, lepidocrocite, maghemite, akaganeite, ferrihydrite and Fe-polymer) may be differentiated by the position of a diagnostic band corresponding to the 2QA0 ~ 2(4T~(4G)) transition. The systematic com- parison of diffuse reflectance spectra of unbleached and bleached kaolins has demonstrated the differences between the Fe-oxides occurring as coatings and as occluded phases. The features observed in second derivative spectral curves are consistent with assignments of crystal field transitions to goethite, hematite, akaganeite, and aged hydrous ferric oxides. The optical determination of the Fe-phases associated to kaolins assists in the interpretation of the formation conditions of these minerals.

Cobalt, copper, and manganese adsorption by aluminium and iron oxides and humic acid

Abstract: Adsorption of cobalt (Co), copper (Cu), and manganese (Mn) by synthetic aluminium oxide, ferrihydrite, goethite, extracted humic acid, and a sandy soil sample were determined as a function of metals concentration (0–1.2 mM) and pH (3.8–8.2). For each pH and adsorbent, the Langmuir adsorption maximum (adsorption capacity) was calculated. The position of the adsorption curves for Co, Cu, and Mn as a function of pH has been shown to be related to the first hydrolysis constant of the cations in solution (pK1). The sequence of preference of the three metals to the studied adsorbents decreased in order Cu >> Co > Mn. The results obtained from this study showed that the Co, Cu, and Mn adsorption characteristics of soils are probably controlled to large extent by their organic matter and oxides contents.

Geochemical differentiation in laterite and bauxite formation

Abstract: The geochemical differences between laterite and bauxite formation are discussed under the aspect that both weathering products are predominantly formed by residual enrichment of iron and aluminium. Residual enrichment is corroborated by various arguments and clearly proved for laterite formation on ultramafic rocks. Tropical weathering of the most widespread acidic rocks leads, in spite of their high Al/Fe ratio, generally to formation of laterites showing a high enrichment of iron whereas aluminium is not or only slightly enriched. This chemical variation indicates that high amounts of Al are removed additionally to Si in soil solutions when saprolites on acidic rocks are transformed into laterites containing goethite, hematite and kaolinite as main constituents. Dissolved free aluminium exist in equilibrium with kaolinite only in minimal contents, hence the removal of Si and Al probably takes place in form of freshly formed colloidal or subcolloidal, X-ray amorphous particles with kaolinitic or similiar composition. The frequently observed occurrence of two generations of secondary kaolinite in laterites corroborates this conclusion. There is general agreement that bauxites are formed by stronger leaching compared with laterites. Increased leaching lowers the concentration of dissolved silica, thus enabling gibbsite instead of kaolinite stability. The concentration of dissolved free aluminium, however, is increased in equilibrium with gibbsite. Free aluminium shows in contrast to iron an effective solubility atpH= 5, giving rise to downward leaching and re-precipitation at higher pH below a laterite crust which covers the bauxite zone in most of the bauxite deposits. In spite of the stronger leaching, bauxite formation implies a lower mass loss (mainly Si) during weathering than the formation of laterites (loss ofSi + Al) related to their highly increased iron content.

Monosilicate adsorption by ferrihydrite and goethite at pH 3-6

Abstract: The constant capacitance model and the Elovich equation were combined in the following mathematical expression enabling calculation of the amount of silicic acid adsorbed by iron oxides as a function of the Si concentration, pH, soil:solution ratio, and reaction time: Ka1 is the protolytic surface constant and Ksi is the stability constant for the Fe oxide-silicate surface complex; -logKa1 = 6.40–0.54(8-pH) and logKsi = 3.85 for ferrihydrite and goethite. Good agreement was found between calculated Si adsorption and the amount actually found to be adsorbed by synthetic ferrihydrite and goethite at different pH (3–6), Si concentration (1–600 μM), solid:solution ratio (1:300, 1:1200) and reaction time (1–480h).

Desorption and plant-availability of phosphate sorbed by some important minerals

Abstract: Experiments were conducted to study the desorption characteristics and plant-availability of phosphate sorbed by some important variable-charge minerals including kaolinite, goethite and amorphous Al oxide. Phosphate desorption from the complexes of goethite-P, kaolinite-P and Al oxide-P by equilibration with 0.02M KCl, resin or some commonly used chemical extractants was slow compared to desorption from a permanent-charge mineral (montmorillonite). However, rice plants were not observed under P deficiency in a pot trial with a phosphate-mineral complex as the only P source for both the permanent-charge mineral and the variable-charge minerals at either 50% or 100% sorption saturation with the exception of goethite-P at 50% saturation. In the exceptional goethite-P treatment, plant P concentration (1.0 g kg−1) was on the threshold of P deficiency. From 15% to 31% of the applied P was recovered by the plants within a growing period of three months, depending on sorption saturation and mineral type. Both the dry matter yield and P uptake decreased with decreasing sorption saturation for all the tested complexes except for Al oxide-P100 (100% saturation). In the case of Al oxide-P100, Al toxicity may have occurred, for poor root growth and high Al concentration in the plants were observed. The effect of sorption saturation on the yield and P uptake of plant was obvious for kaolinite and goethite but not very significant for montmorillonite. Based on the recovery of applied P, the plant-availability decreased in the following order: kaolinite-P100 > goethite-P100 > Al oxide-P50 > montmorillonite-P100 > montmorillonite-P50 > kaolinite-P50 > goethite-P50. Fractionation of the sorbed P before and after plant uptake showed that most of the P uptake originated from the resin-exchangeable P fraction in montmorillonite-P complex, but came mainly from NaOH-extractable fractions in goethite-P complex, whereas all the resin-P, NaHCO3-P and NaOH-P fractions in kaolinite- and amorphous Al oxide-P complex made a contribution to P uptake.

The use of color to quantify the effects of pH and temperature on the crystallization kinetics of goethite under highly alkaline conditions

Abstract: The crystallization kinetics of goethite were studied colorimetrically under highly alkaline conditions (pH 10. l-12.2) at temperatures from 40 ~ to 85~ Color changes during crystallization from fresh precipitates, plotted on a*-b* colorimetric diagrams, were used to discriminate between pure goethite and mixtures of goethite and hematite. Only the b* value increased as goethite crystallization proceeded, and even a minor increase in the a* value revealed the existence of hematite. The rate of goethite crystallization, estimated from the b* value, could be modeled by a pseudo-first-order rate law. This rate depended both on pH and on temperature. Apparent activation energies for the reactions of 56.1 k.J/mol at pH 11.7 and 48.2 kJ/mol at pH 12.2 were estimated from Arrhenius plots.

Metal transport in groundwater contaminated by acid mine drainage

Abstract: Acid mine drainage from a rock dump at an abandoned mine in Dalarna, Sweden, has contaminated the groundwater and created a leachate plume characterized by low pH and high concentrations of Fe, SO4, and heavy metals. Groundwater samples have been collected in order to explain the observed spatial variations in groundwater chemistry. The mineralogy of soil samples collected at the water table has been identified by X-ray diffraction analysis In order to determine which processes may be controlling the evolution of the leachate plume, the geochemical mass-transfer model PHREEQE is used to evaluate a possible reaction pathway controlling the reaction of the leachate plume with uncontaminated groundwater and the till aquifer. The results suggest that silicate weathering and goethite precipitation are the processes primarily controlling observed pH, pe, and Fe concentrations in groundwater direct down-gradient of the rock dump. Increases in K, Na, and Al concentrations along the flow path can be attributed to the feldspar weathering while Al and SO4 concentrations further down-gradient can be controlled by the precipitation of AlOHSO4. The attenuation of heavy metal concentrations may be the result of adsorption and coprecipitation with goethite and other Fe xides.

The oxidation of Fe(OH)2 in the presence of carbonate ions: Structure of carbonate green rust one

Abstract: Sodium carbonate Na2CO3 is added to a solution containing an Fe(OH)2 precipitate in order to study the influence of CO32− ions on the oxidation of ferrous hydroxide. The first stage of the reaction leads to a ferrous-ferric compound, the carbonate green rust one (GR1), identified by its X-ray diffraction pattern. The Mossbauer spectrum at 78 K of this GR1 displays two ferrous doublets and one ferric doublet in the 3∶1∶2 abundance ratio. The quadrupole splittingsQS are 2.91, 2.58 and 0.42 mm/s, respectively, and the isomer shifts 75 are 1.25, 1.25 and 0.47 mm/s respectively. These values are very close to those of the three doublets of the chloride GR1, 3Fe(OH)2Cl·Fe(OH)2Cl·nH2O. This fact confirms that the crystallographic structures of these two GR1s are similar, formed by the stacking of hydroxide layers and interlayers containing the considered anions (Cl− or CO32−) and water molecules. The chemical formula of carbonate GR1 is Fe4(II)Fe2(III)(OH)12CO3·nH2O, and its standard chemical potential -853 900 cal/mol ifn=0. The second stage of the reaction is the oxidation of GR1, which leads to α-FeOOH goethite.

Effect of the Presence of Aluminum Ions in Iron Solutions on the Formation of Iron Oxyhydroxides (FeOOH) at Room Temperature Under Acidic Environment

Abstract: The hydrolytic behavior of Fe solutions at room temperature under acidic conditions was investigated. In the presence of Al ions, with Cl and NO3 as associated anions, the Fe hydrolysis began almost instantaneously and a crystalline β-FeOOH (akaganeite) was formed in the AlCl3/FeCl3 system within a short period. Initially the particles were small with large surface area. However, with time the particles grew in size and the surface area decreased. After about 42 days of equilibration, the akaganeite particles grew to 60–300 nm long, 10–50 nm wide and with a surface area of 55 m2/g, which is similar to other reports for akaganeite prepared at higher temperatures. In the NO3 system [Al(NO3)3/Fe(NO3)3], lepidocrocite (γ-FeOOH) and goethite (α-FeOOH) were formed. In a mixed anion system (Cl/NO3) solid phases identified were akaganeite (β-FeOOH) and lepidocrocite (γ,-FeOOH). The introduction of poly-nuclear hydroxy-Al along with monomer Al in Cl and NO3 systems of Fe affected the quantity and quality of the solid phase. The crystallinity of β-FeOOH formed in the presence of polynuclear hydroxy-Al ions in a Cl-system was more disordered than when it formed in the presence of monomer Al-ions alone. In NO3 systems, polynuclear hydroxy-Al hindered the formation of goethite (α-FeOOH). Our experiments showed that Fe oxyhydroxides crystallize readily under acidic conditions in the presence of Al ions and the data also indicated that the Cl was essential for the crystallization of akaganeite, whereas goethite was formed in those systems when Cl was absent.

Properties of iron-oxides from red soils derived from volcanic tuff in West Java

Abstract: The properties of iron oxide concentrates of Oxisols and Alfisols on volcanic tuff in West Java, Indonesia, were studied by XRD,TGA, TEM and chemical analysis.Goethite and hematite are the co-dominant iron oxides and are present in all samples. Iron content of the soils (dithionite-soluble Fe) ranges from 6.0 to 15.4%. Aluminium substitution in goethite and hematite ranges between 11 and 26 mol% and 0 and 9 mol% respectively. The Al substitution in hematite increases at about half the rate for goethite within the same samples. Crystal size of goethite and hematite was measured from line broadening of the 110 reflections and ranges between 7 and 18 nm for goethite with a mean value of 10 nm and between 7 and 28 nm for hematite with a mean value of 18 nm. The dehydroxylation temperature of goethite is in the range 290-320 °C and is linearly related to Al substitution by the equation y = 280 + 1.53x, (R(2) = 0.53). The goethite and hematite in these Indonesian soils are very similar to these minerals in tropical and Mediterranean soils from other regions.

Effects of iron-oxygen precursor phase on carbon-carbon bond scission in naphthylbibenzylmethane : Iron-based catalysts for coal liquefaction

Abstract: Eleven iron-oxygen compounds prepared using standard laboratory syntheses were tested as precursors for carbon-carbon bond scission catalysts with the coal model compound naphthyl-bibenzylmethane in the presence of elemental sulfur and a hydrogen-donating solvent. The structure of the iron-oxygen catalyst precursor was found to be the most significant factor determining the reactivity of the catalyst produced. The reactivity of the iron-oxygen compounds showed little apparent correlation with surface area, iron content, or water content. The iron-containing single-phase materials with the best catalytic activity at 400 °C were determined to be ferric oxyhydroxysulfate (Fe 8 O 8 (OH) 8 SO 4 ), six-line ferrihydrite, goethite (α-FeOOH), and akaganeite (β-FeOOH)

Sodium Adsorption onto Goethite as a Function of pH and Ionic Strength

Abstract: The adsorption of sodium ion onto goethite was studied as a function of both pH and ionic strength (NaC104 electrolyte) using Na tracer. The pH ranged from 4.1 to 12.0. The ionic strength was controlled by varying the NaC104 concentration and ranged from 0.005 to 0.016 molar. The results indicate that sodium ion is adsorbed onto goethite as an outer-sphere complex at pH values above the point of zero charge. Below the point of zero charge sodium ions are repelled from the double layer by the positively charged goethite surface. This phenomenon of 'cation exclusion' is similar in principle to the 'anion exclusion effect' [1] observed in soils and clays. The pH dependence for sodium ion adsorption is in good agreement with triple layer surface complexation model. The surface complexation constant log ( Λ ® is 1.2±0.

Modeling of Neptunium(V) Sorption Behavior onto Iron-Containing Minerals

Abstract: Sorption behaviors of neptunium (V) on naturally-occurring magnetite (Fe3O4) and goethite (α-FeOOH) in 0.1M NaN03 electrolyte solution under aerobic conditions were interpreted using the surface complexation model (SCM). The surface properties of these materials were experimentally investigated by C02-free potentiometric titration, and SCM parameters for the constant capacitance model, such as protonation/deprotonation constants of the surface hydroxyl group, were determined. The number of negatively charged sorption sites of goethite rapidly increased with the increase of the bulk solution pH compared with that of magnetite and this tendency was similar to the pH dependence of neptunium sorption. This implies that the neptunyl cation, NpO2+, plays a dominant role in possible sorption reactions. Assuming that the dominant surface complex is XO-NpO2, modeling by means of SCM was carried out, and the results were found to agree with experimental data.

Formation of goethite by oxidative hydrolysis of iron(II) sulphate

Abstract: The kinetics and the mechanism of goethite formation in the process of iron(II)-sulphate oxidative hydrolysis at pH = 4.5 and temperatures of 30 ‡ and 60 ‡C were studied by chemical methods, transmission electron microscopy and Mossbauer spectroscopy. Experimental evidence was found for the role of iron(II) and the hydrolysis intermediates in the formation of a crystalline goethite phase.