Showing papers on "Gibbsite published in 2007"

Charging Behavior at the Alumina–Water Interface and Implications for Ceramic Processing

Abstract: The interaction of water and the alumina surface is comprehensively reviewed. Water can be incorporated in the alumina crystal structure resulting in the formation of aluminum hydroxides such as gibbsite. Alumina dissolves into water to an extent that depends primarily upon the solution pH and temperature. The soluble Al (III)aq species (hydrolysis products) likewise depend upon the solution pH, temperature, aluminum, and other salt concentrations. The development of charge on the surface of alumina is controlled by amphoteric surface ionization reactions. The charging behavior of both alumina powders and single crystal faces is compared. The differences can be explained by the reactivities of different types of surface hydroxyl groups. The substantial difference in surface charging behavior of single crystal sapphire and alumina powders indicates that experiments and modeling conducted on single crystals is of limited use in predicting suspension behavior. The atomic scale structure of the hydroxylated sapphire (0001) basal plane is nearly identical to the gibbsite (001) basal plane. The observed surface structures are consistent with the charging behavior of the surfaces. The role of surface charge on the adsorption of processing additives is briefly discussed. How surface charge and processing additives at the alumina aqueous solution interface influence surface forces between particles is reviewed. The influence of these forces on suspension properties such as rheological behavior is outlined. The importance of controlling these behaviors to improve colloidal ceramic powder processing is stressed.

Molecular Dynamics Modeling of Ion Adsorption to the Basal Surfaces of Kaolinite

Abstract: Molecular dynamics simulation is used to study the mechanisms involved in the adsorption of various ions to the basal surfaces of kaolinite. Analysis of simulation data indicates that cations and anions adsorb preferably on the siloxane and gibbsite surfaces of kaolinite, respectively. Strong inner-sphere adsorption of chlorine at aluminum vacancies on the gibbsite surface and the occurrence of chlorine-driven inner-sphere adsorption of cesium and sodium on the gibbsite surface for high ionic strengths are observed. Cesium ions form strong inner-sphere complexes at ditrigonal cavities on the siloxane surface. Outer-sphere cesium is highly mobile and only weak adsorption may occur. A small amount of sodium adsorbs on the siloxane surface as inner-sphere complexes at less clearly defined sites. Like cesium, sodium only forms very weak outer-sphere complexes on this surface. Inner-sphere complexes of cadmium and lead do not occur on either surface. Relatively strong outer-sphere cadmium and lead complexes are present on the siloxane surface at ditrigonal cavities.

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

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

31P solid-state nuclear magnetic resonance study of the sorption of phosphate onto gibbsite and kaolinite.

Abstract: Sorption of phosphate onto gibbsite (gamma-Al(OH)3) and kaolinite has been studied by both macroscopic and 31P solid-state NMR measurements. Together these measurements indicate that phosphate is sorbed by a combination of surface complexation and surface precipitation with the relative amounts of these phases depending on pH and phosphate concentration. At low pH and high phosphate concentrations sorption is dominated by the presence of both amorphous and crystalline precipitate phases. The similarity between the single-pulse and CP/MAS NMR spectra suggests that the precipitate phases form a thin layer on the surface of the particles in close contact with protons from surface hydroxyl groups or coordinated water molecules. While the crystalline phase is only evident on samples below pH 7, amorphous AlPO4 was found at all pH and phosphate concentrations studied. As pH was increased the fraction of phosphate sorbed as an inner-sphere complex increased, becoming the dominant surface species by pH 8. Comparison of sorption and NMR results suggests that the inner-sphere complexes form by monodentate coordination to singly coordinated Al-OH sites on the edges of the gibbsite and kaolinite crystals. Outer-sphere phosphate complexes, which are readily desorbed, are also present at high pH.

The Behavior of Iron and Aluminum in Acid Mine Drainage: Speciation, Mineralogy, and Environmental Significance

Abstract: Publisher Summary This chapter focuses on the behavior of iron (Fe) and aluminum (Al) in acid mine drainage (AMD) and discusses their speciation, mineralogy, and environmental significance. Ferric iron is known to be mostly dissolved under very acidic conditions (pH<2), and above this pH, Fe(III) is usually hydrolyzed and precipitated as different ocherous minerals such as jarosite, schwertmannite, or ferrihydrite. Aluminum is normally conservative below pH 4.5–5.0 and tends to precipitate above this pH in the form of several oxyhydroxysulfates such as hydrobasaluminite and basaluminite and/or hydroxides such as gibbsite. The precipitation of Fe(III) and Al compounds can retain, by adsorption and/or coprecipitation, many other toxic elements present in the mine effluents such as As, Pb, Cr, Cu, Zn, Mn, Cd, Co, Ni, or U. Therefore, Fe(III) and Al act simultaneously as strong buffering systems of the AMD solutions (at pH 2.5–3.5 and 4.5–5.0 respectively) and natural scavengers of toxic trace elements. The formation of efflorescent, evaporative sulfates from AMD waters is especially abundant during the dry season. The mineralogy of these soluble sulfates is closely associated with their spatial distribution and the pH of the brines from which these salts are precipitated. The dissolution of these salts can also imply a rapid acidification and conductivity increase for the affected streams because they release important amounts of sulfate and free acidity into the water.

Surface area, crystal morphology and characterization of transition alumina powders from a new gibbsite precursor

Abstract: A new procedure was used to prepare a microcrystalline powder constituted by thin euhedral hexagonal gibbsite plates, 0.2 to 0.6 µm in diameter and 32 nm thick. The powder, fired between 200 and 1000 °C, produced chi and kappa transition aluminas. Alpha-alumina is formed from 1000 °C and recrystallized up to 1500 °C. At 1000 °C, kappa- and alpha-alumina coexisted, but kappa-alumina could only be characterized by SAED. The details of the internal organization of the transition alumina pseudomorphs were clearly observable in TEM due to the great thinness of the I-gibbsite plates. The specific surface area varied from pristine I-gibbsite (24.9 m2.g-1) to chi- and kappa transition aluminas (25.4 m2.g-1) at 1000 °C to alpha-alumina (4.0 m2.g-1) at 1500 °C. The maximum value of specific surface area is 347 m2.g-1 in chi-alumina powder at 300 °C, a difference from Bayer gibbsite, in which the chi-alumina highest surface area is 370 m2.g-1 at 400 °C.

Integrated study of the calcination cycle from gibbsite to corundum

Abstract: A comprehensive picture of the gibbsite (Al(OH)3) to corundum (α-Al2O3) calcination process has been developed by multi-technique characterization of an integrated sample set. In 100 °C calcination stages, 27Al nuclear magnetic resonance (NMR), differential thermal analysis (DTA), X-ray diffraction (XRD), and nitrogen sorption surface area measurements were employed to elucidate the structure and chemistry of these calcinates. In addition, positron annihilation lifetime spectroscopy (PALS) has been used to study the defective nature of these aluminas. By exploiting the complementarity of PALS, 27Al NMR, and N2 sorption, new links were established between particle morphology, local atomic coordination, and surface defect chemistry.

Crystallisation processes in aluminium hydroxide gels. IV. Factors influencing the formation of the crystalline trihydroxides

Abstract: The formation of crystalline aluminium trihydroxides from aqueous suspensions of pseudoboehmite, obtained by the hydrolysis of aluminium s-butoxide, has been examined by X-ray diffractometry, nitrogen adsorption and dilatometry. The results obtained have been interpreted in terms of a mechanism involving dissolution of the pseudoboehmite. For those suspensions with which the recrystallisation was followed dilatometrically, first-order kinetics were found, suggesting that random nucleation of trihydroxide crystals is involved. The rate of recrystallisation was decreased by the addition of organic solvents to the aqueous suspension and also, initially, by an increase in temperature. At the same time the proportion of gibbsite produced in admixture with bayerite was reduced and that of nordstrandite increased. It is concluded that the retardation is due, at least in part, to an increase in the crystallinity of pseudoboehmite which has been found to accompany the introduction of the organic solvents. The increased crystallinity must result in a decrease in the solubility and rate of solution of pseudoboehmite; under these conditions nordstrandite formation appears to be promoted. Neutron activation and spectroscopic analysis of hydroxide samples showed that storage of aqueous suspensions in glassware resulted in pick-up of sodium and silica at the ppm level. The changes in these levels as recrystallisation occurred suggested that retention of the products of glass dissolution was occurring in a way which depended on the surface area of the hydroxide. No variation related to the phase composition of the trihydroxide mixtures was observed.

Factors and processes of gibbsite formation in Northern Thailand

Abstract: Gibbsite is usually considered as end product of weathering in tropical environments with potentially high leaching rates. However, there are also hints towards gibbsite formation in initial stages of weathering in different climates. This study reports on a systematic approach based on soil forming factors in order to research the conditions of gibbsite formation in northern Thailand highlands. Therefore, three major study sites were chosen, which differ with respect to parent rock, relief, climate and vegetation. The results show that gibbsite is common in soils of the area. Reasons for its occurrence in soils are manifold. It can be a heritage of the parent rock, a result of initial weathering under free draining conditions or an accumulation under intense chemical weathering caused by high rainfall. Especially the investigation in granite and gneiss areas with a high share of primary minerals indicates that gibbsite can be an early and direct transformation without intermediates from micas and feldspars if free drainage is assured. With progressing soil genesis clay formation reduces drainage and favours kaolinite formation. Only sites with extremely high rainfall and low evaporation (high elevations in northern Thailand) show again a dominance of gibbsite in the clay fraction throughout the whole soil profile.

Zn incorporation in hydroxy-Al- and keggin Al13-intercalated montmorillonite: a powder and polarized EXAFS study.

Abstract: The sorption mechanism of Zn on gibbsite and montmorillonite exchanged with Al3+ (Al-mont) or Keggin Al13 polymer (Al13-mont) was probed by powder and polarized EXAFS spectroscopy as a function of pH (5.85−7), reaction time (1−65 days), and sorbate to sorbent ratio (50−200 μM Zn/2 g montmorillonite). For all Al-mont samples, Zn is octahedrally coordinated to oxygens at ∼2.08(2) A, and surrounded in-plane by six Al atoms at 3.02−3.06(2) A, and another six at 6 A. No out-of plane Si neighbors are detected. These results are interpreted as Zn incorporation in vacant octahedral sites of gibbsite-like layers at the basal and/or interlayer surface of montmorillonite particles. Zinc sorbed on the edges of gibbsite layers would give a split first oxygen shell with bond distances of 2.00(2) and 2.16(3) A, and 2.1(8) nearest Al at 3.02 A with no second-nearest Al, none of which were observed in Al-mont. The binding environment of Zn on Al13-mont after 1 day is similar to that on the edges of gibbsite, and is interp...

Use of reaction path modeling to predict the chemistry of stream water and groundwater: a case study from the Fiume Grande valley (Calabria, Italy)

Abstract: The irreversible water–rock mass exchanges leading to the production of the Fiume Grande valley (Calabria, Italy) stream waters and groundwaters, starting from local rainwaters, were simulated through reaction path modeling in reaction progress (stoichiometric) mode. The simulations assumed bulk dissolution of a phyllitic rock and calcite and precipitation of gibbsite, kaolinite, amorphous silica, illite, a smectite solid mixture, a hydroxide solid mixture, and a trigonal carbonate solid mixture. The analytical contents of major and trace elements in stream waters and groundwaters were satisfactorily reproduced. However, further investigations are necessary to clarify the fate of As in this natural systems.

Sorption of chlorpyrifos to selected minerals and the effect of humic acid.

Abstract: Sorption of chlorpyrifos (CPF) from 2.85 microM (1 mg/L) aqueous solutions in 0.01 M NaCl to montmorillonite, kaolinite, and gibbsite was investigated at 25 degrees C. Uptake of CPF by kaolinite and gibbsite was generally 6 increased negative surface charge results in a lower affinity of CPF for the external surface. In the presence of 50 mg/L humic acid (HA) the amount of CPF sorbed on gibbsite and kaolinite was 3-4 times greater than that in the binary systems. The HA forms an organic coating on the mineral surface, providing a more hydrophobic environment, leading to enhanced CPF uptake. The HA coating on montmorillonite may reduce access of CPF to microporous regions, with CPF tending to accumulate within the HA coating.

Intemperismo subtropical de minerais em saprolitos ácidos do Sul do Brasil

Abstract: Because weathering of minerals releases chemical elements into the biogeochemical cycle, characterization of their weathering products helps to better model groundwater quality, formation of secondary minerals and nutrient flux through the trophic chain. Based on microscopic and elemental analyses, weathering of riodacite from Serra Geral formation was characterized and weathering paths proposed. Three weathering paths of plagioclase phenocrystals were identified: plagioclase to gibbsite (Pg1); plagioclase to gels and gibbsite (Pg2); and plagioclase to gels, gibbsite and kaolinite (Pg3). Pyroxenes weathered to smectite and goethite (Py1), or to goethite and gibbsite (Py2), and magnetite weathered directly into iron oxides. Rock matrix comprises 90% of rock volume, and weathered to kaolinite and gibbsite, which explains why these minerals were the most abundant in the weathering products of these saprolites.

Crystal structures of high-pressure phases in the alumina-water system: I. Single crystal X-ray diffraction and molecular dynamics simulation of η-Al(OH)3

Abstract: The high-pressure phase of gibbsite has been studied by in situ single crystal X-ray diffraction method and molecular dynamics (MD) simulation at 3.0 GPa. The crystal structure of the high-pressure phase, η-Al(OH) 3 , was successfully determined by direct methods based on the intensities of X-ray diffraction. The space group and lattice constants for η-Al(OH) 3 are P2 1 /b11 (#14), a= 8.612(3) A, b = 5.013(2) A, c = 9.194(5) A and a = 90.34(6)°, respectively. The crystal structure of η-Al(OH) 3 consists of an Al octahedral layer, and the layers are connected via H-bonds. The mechanism of the phase transition from gibbsite to η-Al(OH) 3 is also discussed in the context of previously reported powder X-ray diffraction data.

Influence of addition of AlF3 on thermal decomposition of gibbsite and phase transition of the intermediate alumina to α-Al2O3

Abstract: The addition of aluminum fluoride to gibbsite retarded the phase change to boehmite by chemisorption of hydrogen fluoride on gibbsite particles. In the specimen with addition of aluminum fluoride, the spinel type phase transformed directly to α-Al2O3, without passing through κ- and θ-Al2O3 phases. The formed α-Al2O3 consisted of clear, hexagonal plate-like particles about 1 μm, Without taking over a relic of the gibbsite particle. The formation of automorphic crystals suggests that a surface diffusion also contributes to the mechanism of this material transport in addition to the vapor phase transport.

Synthesis of wide‐pore alumina support from gibbsite

Abstract: Gibbsite was treated in a series of primary alcohols at 250°C for 2 h. The surface area of the product increased with the increase in the carbon number of alcohol and reached a maximum when hexanol was used. Pore texture of aluminas, obtained by the calcination of these products, was investigated by means of the nitrogen adsorption technique. The aluminas prepared from the products of the treatment in alcohols other than propanol and butanol had a pore texture identical to the pore texture of the alumina from well-crystallized boehmite prepared by hydrothermal treatment of gibbsite. The pore texture of these aluminas was characterized by the presence of micropores. The aluminas prepared from the treatments in butanol and propanol had quite different pore texture. Micropores were not detected in these aluminas: instead, mesopores of c. 20 nm range were detected. These results are consistent with the reaction mechanisms for the formation of boehmite proposed in a previous paper on the basis of morphology of the products. In lower alcohols boehmite is formed by means of a resolution-crystallization mechanism, whereas in higher alcohols boehmite is formed by means of intraparticle hydrothermal reaction. In butanol and propanol, boehmite is crystallized from the alcoholic media and has a large population of crystal defects; therefore, pore texture of aluminas derived from these products resembles the aluminas from poorly crystallized aluminum hydroxides.

Kinetics of phase transitions of gibbsite during heat treatment in air and in water vapor

Abstract: Results are provided for a study of the kinetics and mechanism of phase transitions (PT) of gibbsite during heat treatment in air and in water vapor (in the range 170–550°C) by iso-and non-isothermal thermogravimetry, x-ray-phase analysis (XPA), transmission electron microscopy (TEM), nuclear magnetic resonance in 27Al nuclei (NMR 27Al), and chemical analysis. It is shown that PT of gibbsite in air there are two interconnected transitions that occur in parallel: gibbsite → boehmite (γ-AlOOH) and gibbsite → x-ray amorphous Al2O3 that are realized by a topochemical mechanism. In water vapor gibbsite only experiences transition to boehmite (PT-3) that occurs by a dissolution — precipitation mechanism. Apparent activation energies for the PT that occur are determined.

Influence of aluminum hydroxide and lime on the hydration of tricalcium silicate

Abstract: Experiments by isothermal calorimetry, indicate that the hydration of 3CaO·SiO2 (C3S) is influenced very little by gibbsite; it is influenced by bayerite to a somewhat larger extent. In the presence of amorphous Al(OH)3 the reaction of C3S with water shows a very complicated course and gives four heat peaks. If CaO is added in addition to this Al(OH)3, the third and the fourth heat peaks are more pronounced. From qualitative d.t.a., infra-red, electron-microscope and X-ray investigations, as well as from quantitative X-ray analysis, a reaction mechanism is proposed. The quantity of C3S reacted, determined by means of quantitative X-ray analysis, is greater during the reaction of 2·00 g C3S with 0·40 g amorphous Al(OH)3, 0·08 g CaO and 2·00 ml water, than during the reaction of 2·00 g C3S with 2·00 ml water.

Role of natural red earth in arsenic removal in drinking water – comparison with synthetic gibbsite and goethite

Abstract: Arsenic contamination in drinking water has been recorded as a problem of global concern. This study aims to investigate As retention on goethite and gibbsite as a function of pH with similar site densities of >FeOH and >AlOH of natural red earth (NRE), to ensure that these two will make out the same adsorption as >Fe and >Al sites of NRE. 0.385 μmol/l of arsenite and arsenate were spiked separately to solid suspensions, with adjusted pH from 4.0 to 9.0, and equilibrated for 24 h. Final pH were recorded and supernatant was analysed for As after centrifugation and filtration. Data were modelled using a generalized diffuse layer model. Arsenite adsorption was low for gibbsite (50%) than goethite ( 90%) for both solids and showed no pH dependency. Model calculations were fitted well and the model validation was successful with a variation of >AlOH and >FeOH (10-fold) site concentration on NRE.

On the aqueous stabilisation of crystalline metastable nanostructures

Abstract: Crystal phase transformations in aqueous solutions generally operate through a dissolution-recrystallisation process to fulfil surface energy minimisation requirements. Indeed, when a solid offers several allotropic crystal phases and polymorphs, it is usually the one with the highest solubility and consequently the lowest stability (i.e., the crystallographic metastable phase), which precipitates first. This is understood by considering the nucleation kinetics of the solids. At a given supersaturation ratio, the germ size is as small (and the nucleation rate as high) as the interfacial tension of the system is low. Thus, given that the solubility is inversely proportional to the interfacial tension, the precipitation of the most soluble phase and consequently the less stable thermodynamically, is therefore kinetically promoted. Due to its solubility and metastability, this particular phase is more sensitive to secondary growth and ageing, which leads to crystallographically more stable phases, essentially by heteronucleation. This is typically the case of metal oxides such as aluminium hydroxide (boehmite to bayerite or gibbsite), titanium dioxide (anatase or brookite to rutile), or iron oxyhydroxides (goethite, akaganeite, or lepidocrocite to hematite). Secondary growth and ageing processes are delicate to control and the phase transformations appear within a few hours to a few days in solution, whence the resulting undesired mixing of allotropic phases and polymorphs. However, by careful consideration of the precipitation conditions such phenomena may be avoided when nanosystems are precipitated at low interfacial tension. Thus, the stabilisation of oxide, hydroxide and oxyhydroxide metastable crystal structures can be achieved at thermodynamically stable conditions. As an attempt to assess such behaviour, experimental studies involving iron (akaganeite, β-FeOOH) and aluminium (boehmite, γ-AlOOH) oxyhydroxides were conducted. The results show that it is possible to significantly slow down and even entirely suppress crystal phase transformation and thus stabilise crystalline metastable phases in aqueous solutions.