Showing papers on "Gibbsite published in 2005"

Inconsistency in the triple layer model description of ionic strength dependent boron adsorption.

Abstract: Understanding anion adsorption mechanisms is necessary to allow prediction of anion adsorption behavior. This study was conducted to evaluate the ability of the triple layer model, a chemical surface complexation model, to describe the effect of changes in solution ionic strength (0.01–1.0 M NaCl) and solution pH (3–11) on B adsorption by the iron oxide, goethite, the aluminum oxide, gibbsite, the clay minerals, kaolinite and montmorillonite, and two arid zone soils. Ionic strength dependence of adsorption suggests an inner-sphere adsorption mechanism for goethite, kaolinite, montmorillonite, and the two soils and an outer-sphere adsorption mechanism for gibbsite. The triple layer model, containing an inner-sphere adsorption mechanism, was able to describe B adsorption on goethite, kaolinite, montmorillonite, and the two soils. The model was able to describe B adsorption on gibbsite using an outer-sphere adsorption mechanism. A problematic inconsistency exists in the triple layer model description of ionic strength dependent B adsorption between the type of B surface complex defined in the model and the ionic strength dependence of the model result. That is, postulating an inner-sphere adsorption mechanism in the triple layer model resulted in an ionic strength dependence appropriate for the formation of outer-sphere surface complexes and vice versa. Additional tests of the ability of the triple layer model to describe ionic strength dependent adsorption of additional ions are needed to establish whether the inconsistencies are limited to the B system or are of concern in other triple layer model applications. Published by Elsevier Inc.

An investigation on characterization and thermal analysis of the Aughinish red mud

Abstract: Red mud is insoluble, fine-grained waste residue which is generated as a by product during the production of alumina from the Bayer process. In this article, the thermal behavior of Aughinish red mud was investigated using thermal gravimetric analysis (TG) and differential thermal analysis (DTA). For identification of oxide and mineral phases in the red mud sample, XRD method, scanning electron microscopy (SEM), EDAX were used. Iron (30.4%), aluminium (23.6%) and titanium (17.85%) oxides are major oxides in the sample. Two endothermic peaks were shown on DTA curve. The total mass loss in the red mud was found to be 10.1%, which was associated with moisture and water molecules in gibbsite and boehmite phases.

Climatic control on clay mineral formation: Evidence from weathering profiles developed on either side of the Western Ghats

Abstract: Many physico-chemical variables like rock-type, climate, topography and exposure age affect weathering environments. In the present study, an attempt is made to understand how the nature of clay minerals formed due to weathering differs in tropical regions receiving high and low rainfall. Clay mineralogy of weathering profiles in west coast of India, which receives about 3 m rainfall through two monsoons and those from the inland rain-shadow zones (<200 cm rainfall) are studied using X-ray diffraction technique. In the west coast, 1:1 clays (kaolinite) and Fe—Al oxides (gibbsite/goethite) are dominant clay minerals in the weathering profiles while 2:1 clay minerals are absent or found only in trace amounts. Weathering profiles in the rain shadow region have more complex clay mineralogy and are dominated by 2:1 clays and kaolinite. Fe—Al oxides are either less or absent in clay fraction. The kaolinite—smectite interstratified mineral in Banasandra profiles are formed due to transformation of smectites to kaolinite, which is indicative of a humid paleoclimate. In tropical regions receiving high rainfall the clay mineral assemblage remains the same irrespective of the parent rock type. Rainfall and availability of water apart from temperature, are the most important factors that determine kinetics of chemical weathering. Mineral alteration reactions proceed through different pathways in water rich and water poor environments.

Sedimentation and Phase Transitions of Colloidal Gibbsite Platelets

Abstract: We study the competition between sedimentation, gelation, and liquid crystal formation in suspensions of colloidal gibbsite platelets of five different sizes at three ionic strengths. For large particles (with diameters of 350, 420, and 570 nm) sedimentation is initially the most important factor determining the macroscopic behavior. Only after the main part of the sample has sedimented in an amorphous phase, phase separation takes place. For the smallest particles (diameter 210 and 270 nm), it is the other way around: fast (within one week) phase separation or gelation takes place, after which sedimentation determines the final macroscopic appearance. We distinguish six different scenarios within this two-fold scheme and interpret these on the basis of the previously obtained phase diagram of colloidal gibbsite platelets (van der Beek, D.; Lekkerkerker, H. N. W. Langmuir 2004, 20, 8582).

Lateritic soils of Kerala, India : their mineralogy, genesis, and taxonomy

Abstract: In this study, we report the chemical and mineralogical characteristics of 4 benchmark Ultisols of Kerala to elucidate their genesis and taxonomy. The taxonomic rationale of the mineralogy class of Ultisols and other highly weathered soils on the basis of the contemporary pedogenesis is also explained. The Ultisols of Kerala have low pH, low cation exchange capacity, low effective cation exchange capacity and base saturation, with dominant presence of 1 : 1 clays and gibbsite. Presence of gibbsite along with 2 : 1 minerals discounts the hypothesis of anti-gibbsite effect. Since the kaolins are interstratified with hydroxy-interlayered vermiculites (HIV), the formation of gibbsite from kaolinite is not tenable. Thus, gibbsite is formed from primary minerals in an earlier alkaline pedo-environment. Therefore, the presence of gibbsite does not necessarily indicate an advanced stage of weathering. On the basis of a dominant amount of gibbsite, a mineralogy class such as allitic or gibbsitic does not establish a legacy between the contemporary pedogenesis and the mineralogy. The dominance of kaolin–HIV in the fine clays of Ultisols and their persistence, possibly since early Tertiary, suggests that ‘steady state’ may exist in soils developed on long-term weathered saprolite. Since the present acid environment of Ultisols does not allow desilication, the chemical transformation of Ultisols to Oxisols with time is difficult to reconcile as envisaged in the traditional model of tropical soil genesis.

Structural and Electronic Properties of Bulk Gibbsite and Gibbsite Surfaces

Abstract: Structure and electronic structure of gibbsite and gibbsite surfaces are studied using gradient-corrected density functional theory and a density-functional based tight-binding (DFTB) scheme. The electronic density-of-states (DOS) is insignificantly changed when comparing bulk and single-layer (001) surfaces, but changes for other surfaces, as pentacoordinated Al sites appear. Model structures of other chemically relevant surfaces are proposed, including a stripe and an Al6(OH)18 cluster model for the (100) face, which turns out to be equivalent to the (011) face if only covalently bound blocks are considered. In all cases quantitative agreement of GGA-DFT and DFTB for structures and occupied electronic states is observed.

Limitations of potentiometric studies to determine the surface charge of gibbsite γ-Al(OH)3 particles

Abstract: Surface charges of gibbsite particles were probed by potentiometric titration and subsequently analyzed to estimate intrinsic proton affinity constants of OH surface groups. A detailed spectroscopic characterization of the molecular structure of surface OH groups yielded estimates of bond lengths and bond valences of O H surface sites. Based on these results, the effects of the setting parameters of a MUSIC calculation have been shown in comparison with previous predictions yielding higher p K a values ( 2 − p K a 4 ) for the protonation of basal doubly coordinated OH surface groups and lower p K a values ( 7.9 − p K a 9.9 ) for the protonation of lateral singly coordinated OH surface groups. Comparison with experimental data is complicated by reproducible hysteresis between acid and base addition in optimal raw potentiometric titration curves at different ionic strengths. Such effects prevented the determination of a univocal intersection point to provide the global point of zero charge of gibbsite particles, even though the ionic strength dependence of the point of zero net proton charge and the different crossovers between curves indicated that the point of zero charge could be estimated between 8.1 and 9.6, in relative agreement with the lateral affinity constant calculated with the MUSIC model. Still, two main drawbacks remained to differentiate the reactivity of lateral singly and basal doubly coordinated surface groups. First, significant kinetic effects observed in acidic media indicated a dissolution process and/or protonation of basal surface groups. Second, the choice of specific surface areas, especially for a heterogeneous sample, led to several cases for the calculation of the absolute surface charge of particles. Therefore, our results demonstrated the heterogeneous reactivity of gibbsite particles and that the prediction and the experimental determination of respective surface groups are still complex even if some trends emerge.

Transformation of haematite and Al‐poor goethite to Al‐rich goethite and associated yellowing in a ferralitic clay soil profile of the middle Amazon Basin (Manaus, Brazil)

Abstract: Summary The red and yellow colours of ferralitic soils in the tropics have for long intrigued pedologists. We have investigated the upward yellowing in a 10-m thick profile representative of the Ferralsols of the plateaux of the Manaus region of Brazil. We determined changes in the nature and crystal chemistry of their Fe oxides by optical and Mossbauer spectroscopy as well as Rietveld refinement of X-ray diffraction patterns. We attribute the upward yellowing of the soil to a progressive transformation of the Fe oxides at nearly invariant iron contents. Aluminium in contrast is strongly mobilized in the uppermost clay-depleted topsoil where there is preferential dissolution of kaolinite and crystallization of gibbsite. Haematite decreases from 35 to 10% of the Fe oxides from the bottom to the top of the profile and the particles become smaller (75–10 nm). Its Al for Fe-substitution remains almost unchanged (10–15 mol %). The average Al-substitution rate of goethite increases from 25 to 33 mol %, and its mean crystal diameter remains in the range 20–40 nm. The proportion of Al-rich goethite (33 mol %) increases at the expense of less Al-substituted Fe oxides (haematite and goethite). This conversion with restricted transfer of iron means that the amount of Al stored in Fe oxides gradually increases. Kaolinite, haematite and Al-poor goethite are thus witnesses of earlier stages of ferralitization of the soil. In contrast, Al-rich goethite and gibbsite initiate the alitization (or bauxitization) of the soil. They correspond to the last generation of soil minerals, which most likely reflects the present-day weathering conditions. The progressive replacement of kaolinite, haematite and Al-poor goethite by new generations of Al-rich goethite and gibbsite attests to greater activities of water and aluminium and smaller activity of aqueous silica in the topsoil than in the subsoil. We interpret this as a consequence of longer periods of wetting in the topsoil that could result from soil aging, more humid climate or both.

Arsenic Speciation in As(III)- and As(V)-Treated Soil Using XANES Spectroscopy

Abstract: Arsenic (As) is a toxic trace element that occurs naturally in groundwater and soils. Understanding the reactions of arsenite (As(III)) and arsenate (As(V)) with soil and mineral surfaces is critical for predicting the fate and transport of As in the environment and developing better ways to remediate As-contaminated areas. This investigation uses X-ray absorption near edge spectroscopy (XANES) to evaluate the solid phase oxidation state and mineral surface binding sites in three agricultural soil samples from California, USA by fitting linear combinations of XANES spectra derived from several synthetic and well characterized As(III)- and As(V)-treated model compounds (Fe and Al metal hydroxides and aluminosilicate illite clay mineral). The results suggest that As(III) is either partially or completely oxidized to As(V) when reacted with soil in an aqueous, batch reaction. The As(III)-treated Aiken soil was composed of 60% As(III) attached to surfaces similar to lepidocrocite (γ-FeOOH)) and 40% As(V) attached to aluminosilicate (illite). The Fallbrook soil completely oxidized As(III) and the product was As(V) adsorbed on Al hydroxide (gibbsite, γ-Al(OH)3) (62%), illite (16%), and lepidocrocite (γ-FeOOH) (22%). The reaction of As(III) with Wyo soil resulted in 42% As(III) adsorbed on surface similar to goethite and 58% As(V) adsorbed on lepidocrocite. Arsenic(V) adsorption on soil resulted in stable As(V) surface complexes that were well described by XANES spectra from As(V) adsorption complexes on gibbsite, illite, and lepidocrocite.

Seeded growth of monodisperse gibbsite platelets to adjustable sizes.

Abstract: Nanosized hexagonal gibbsite seeds are grown from a mixture of dissolved alumina alkoxides at 85 °C. Centrifugation reduces the polydispersity by 30%. The seeds can be grown further by adding them to a fresh alkoxide mixture and heating it. This procedure was repeated several times to obtain particles of 570 nm ± 11% diameter and a thickness of 47 ± 23%. No indications of a size limit were observed. The thus obtained particles may form easily a columnar phase. Individual gibbsite particles in solution can be seen by confocal microscope.

Comprehensive Model of Synthetic Bayer Liquors. Part 1. Overview

Abstract: A Pitzer model representing the thermodynamic properties of synthetic Bayer liquor solutions, consisting of the components NaOH - NaAl(OH) 4-Na2CO3-Na2SO 4-NaCl-NaF-Na2C2O4 (sodium oxalate)-NaHCOO (sodium formate)-NaCH3COO (sodium acetate)-H 2O, is presented. The model calculates, in a thermodynamically consistent manner, heat capacities, enthalpies, activity and osmotic coefficients, and densities of these solutions as well as the solubilities of gibbsite, Al(OH)3, boehmite, AlOOH, sodium oxalate, sodium sulfate and other relevant solid phases in synthetic Bayer liquors over concentration and temperature ranges of industrial interest.

Copper-glyphosate sorption to microcrystalline gibbsite in the presence of soluble Keggin Al13 polymers.

Abstract: Among the most reactive yet largely neglected adsorbents of toxicant species occurring in acidic aquatic environments are the epsilon-Keggin Al13 polyoxocations [AlO4Al12(OH)24(H2O)12(7+)], known generally as Al13 polymers. Here, we report on the sorption of Cu(II), a common ingredient of pesticides, and glyphosate {N-[phosphonomethyl]glycine (PMG)}, a widely applied herbicide, to microcrystalline gibbsite [gamma-Al(OH)3] in the presence of soluble Al13 polymers over the pH range 4-7. In the presence of gibbsite and soluble Al13 polymers, dissolved Cu(II) decreased gradually with pH, achieving a minimum at pH 5.5. Between pH 5.5 and 6.0, however, soluble Cu increased markedly, with approximately 80% of the added metal remaining in solution at pH 5.86. At pH > 6.0, soluble Cu once again decreased, becoming undetectable at pH 7. The anomalous Cu solubilization was attributed to a concomitant deprotonation of soluble Al13 polymers, yielding surface OH groups possessing high affinity for Cu(II). Removal of Cu from solution at pH > 6.0 is facilitated by flocculation of the Al13 polymers to which Cu had sorbed. The sorption behavior of the zwitterionic PMG in the presence of gibbsite and Al13 polymers was consistent with this interpretation, there being a dramatic increase in sorbed PMG at pH > 6.0 as the Al13 polymers deprotonated and flocculated. Copper and PMG loss from solution with increasing pH when both adsorptives were added to the gibbsite-Al13 polymer system was broadly similar to what was observed in the PMG-free systems, although small differences were detected in response to varying the order of adsorptive addition. The inclusion of soluble Al polymers in our experiments exposes a fundamental limitation of models based on but a single inorganic adsorbent as a means to predict the behavior of trace metals and xenobiotic organic compounds in natural systems.

Precipitation of silica in a bayer process

Abstract: In the Bayer process for the production of alumina, problems are caused by silica dissolving in the caustic liquor. This silica arises from the presence of kaolin in the bauxite. A process for removing this kaolin comprises contacting the bauxite with sodium hydroxide solution to form a mixture, and subjecting the mixture to intense ultrasonic irradiation to cause cavitation; this can be carried out at temperatures below 100°C. This enhances both the dissolution of kaolin and the precipitation of sodium aluminium silicate. Silica remaining in solution in spent Bayer liquor (after digestion and then precipitation of gibbsite) can be removed by a similar ultrasonic irradiation treatment to cause it to precipitate before it forms scale in heat exchangers.

Flame-retardant filler for plastics

Abstract: The invention relates to a flame-retardant filler based on aluminium hydroxide, its use in polymers and a method for its production, in which aluminium hydroxide in the form of a bayerite/gibbsite mixture is modified under pressure of temperatures of at least 170°C in the presence of water and crystal growth regulator, the aluminium hydroxide used as starting material having an average particle size d50 from 01 to 4µm

Washup-flotation process for stacked bauxite

Abstract: The invention discloses a mill run technology of bauxite, especially relates to a ore wash-floatation technology of cumulate bauxite. For cumulate bauxite, changing the single ore wash technology into ore wash and appending dispersant, and floatation to the gangue. The invention can increase the quality of wash-concentrate and decrease the mud ratio of wash-concentrate; by floatation, a number of gibbsite, diasporite, bodhmite can be reclaimed, achieve the intention of using bauxite.