Formation Of Gibbsite Research Articles

Synthesis and characterization of a dendrimeric polycitrate-alumoxane nano composite

This work illustrates the synthesis of a three-dimensional dendrimeric hyper-branched alumoxane nanostructure, named as polycitrate-Alumoxane (PC-A), through condensation reactions between the hydroxyl groups of boehmite nanoparticles (BNs) and the carboxylic groups of citric acid (CA), producing carboxylate alumoxane linkages. FTIR, SEM, TEM, XRD and TG–DTA characterized the hybrid inorganic–organic nano composite. The dendrimeric and hyper-branched porous network structure, including branches of 5–15 nm in diameter was detected. The mechanism of polybranching and dendrimerization was proposed and discussed. It was coated on the surface of a metal mesh to study its hydrophilicity, through contact angle measurement, using oil and water separately. The material was also proved to be hydrophilic, due to its large number of carboxyl and hydroxyl groups. PC-A nanostructure was thermally stable up to 199 °C. Thermal degradation of the material led to the formation of gibbsite at about 550 °C, proving a reverse change of phase from boehmite to gibbsite. Thus, the PC-A, which was synthesized from BNs, finally produced gibbsite on burning.

Interaction of anoxygenic phototrophic bacteria Rhodopseudomonas sp. with kaolinite

The interaction between freshwater nonsulfur purple bacteria Rhodopseudomonas sp. UZ-25p (Uzon caldera, Kamchatka, Russia) and two kaolinite samples (Zhuravlinyi Log, Chelyabinsk oblast) was investigated. Alterations in the chemical composition of the minerals and solutions, the parameters of bacterial growth, and crystal morphology and mineralogy of the kaolinite samples indicated the interactions between all components of the system (minerals, water, growth medium, and bacteria). Bacteria removed some elements from the medium, used them for growth, and promoted their transition into the mineral exchange pool. In the presence of bacteria, kaolinite cation exchange capacity increased and saturation of kaolinites with bases occured. Partial biodegradation of kaolinites, accompanied by ordering of the crystalline structure of their lamellar phase, was the main factor responsible for the increase in cation exchange capacity. For the first time anoxygenic phototrophic bacteria were found to degrade kaolinite with formation of gibbsite. The theoretical and applied significance of the experimental results is discussed.

Rates of Biotite Weathering, and Clay Mineral Transformation and Neoformation, Determined from Watershed Geochemical Mass-Balance Methods for the Coweeta Hydrologic Laboratory, Southern Blue Ridge Mountains, North Carolina, USA

Biotite is a common constituent of silicate bedrock. Its weathering releases plant nutrients and consumes atmospheric CO2. Because of its stoichiometric relationship with its transformational weathering product and sensitivity to botanical activity, calculating biotite weathering rates using watershed mass-balance methods has proven challenging. At Coweeta Hydrologic Laboratory the coupling of biotite to its transformational weathering product is only valid if the stoichiometric relationship for the two phases is known; this relationship is unlikely layer-for-layer. Rates of biotite weathering and transformation of its secondary weathering product at the Coweeta Hydrological Laboratory are comparable with other Appalachian watersheds. The magnitude and sign of the difference between field- and laboratory-determined biotite weathering rates are similar to those of other silicate minerals. The influence of major-cation proportions in biomass on the rates of biotite weathering and transformational weathering product is greatest for watersheds with high biomass aggradation rates. The watershed with the lowest bedrock reactivity and highest flushing rate yielded the highest gibbsite formation rate of ~500 mol ha−1 year−1 and lowest kaolin-group mineral formation rates of 4–78 mol ha−1 year−1. The kaolin-group mineral formation rate increases as bedrock reactivity increases and flushing rate decreases to a maximum of ~300 mol ha−1 year−1, with a similar minimum gibbsite formation rate. The relative differences in bedrock reactivity and flux of water through Coweeta Hydrological Laboratory watersheds studied appear to be invariant over geologic timescales.

Integrating topography, hydrology and rock structure in weathering rate models of spring watersheds

Weathering rate models designed for watersheds combine chemical data of discharging waters with morphologic and hydrologic parameters of the catchments. At the spring watershed scale, evaluation of morphologic parameters is subjective due to difficulties in conceiving the catchment geometry. Besides, when springs emerge from crystalline massifs, rock structure must be accounted in formulas describing the area of minerals exposed to the percolating fluids, for a realistic evaluation of the rates. These particular features are not included in the available approaches and for that reason a new model was developed, coined THROW model. This is a lumped approach that integrates (T)opography, (H)ydrology, (RO)ck structure and (W)eathering in a single algorithm. The study area comprises several stream watersheds and spring sites of the Vouga River basin (northern Portugal), shaped on granites. Firstly, the THROW model couples a terrain modeling analysis with hydrologic models based on discharge rates, to determine hydraulic conductivities (K), effective porosities (ne) and annual recharges (Vr) at the stream watershed scale. Subsequently, these parameters are used in a water balance model to estimate concomitant groundwater travel times (t). The mean K [(4.7 ± 3.2) × 10−7 m s−1] and ne [(2.0 ± 1.3) × 10−2] values are adopted as proxies for the spring watersheds and a firm regression equation is defined between time and stream watershed area (A). Secondly, two more runs of terrain modeling analysis are executed to extrapolate morphologic parameters for the spring watersheds. The first run hinges on scaling properties of the drainage networks, known as Horton laws, and is used to scale watershed areas across stream orders (i). The scaling function is described by another regression equation. The second run evaluates the order of a spring watershed, defined as equivalent order (ieq) and equated to the mean order of the surrounding stream watersheds. Having calculated the ieq, spring watershed areas and travel times were downscaled using the regression equations (A < 10 km2 and t = 1.4–2.8 year). Standing on the physical and hydrologic parameters of the spring watersheds, the THROW model finally calculates plagioclase weathering rates in the vicinity of the spring sites. The SiB model (Pacheco and Van der Weijden, 1996) was used before to estimate the contribution of plagioclase dissolution to the chemical composition of these springs (Van der Weijden and Pacheco, 2006). The chemical data were now coupled with K, ne and t in a rate equation to estimate chemical weathering rates of plagioclase in the basin. In the THROW model, the rate equation describes the exposed surface area as a function of fracture spacings, openings and porosities (Pacheco and Alencoao, 2006). The calculated rates (WPl = (2.5 ± 1.2) × 10−14 mol m−2 s−1) are consistent with previous reports and with results of experimental kinetic models. The SiB results predict formation of halloysite and gibbsite along the flow path, which were indeed close to equilibrium with the dissolved Al and Si activities. âo Develop a weathering model that incorporates rock structure in the rate equation. âo Conceive a weathering model especially designed for fracture artesian springs. âo Create a model that integrates topography, hydrology, rock structure and weathering

The effect of curvature on weathering rind formation: Evidence from Uranium-series isotopes in basaltic andesite weathering clasts in Guadeloupe

To quantify rates of rind formation on weathering clasts under tropical and humid climate and to determine factors that control weathering reactions, we analyzed Uranium series isotope compositions and trace element concentrations in a basaltic andesite weathering clast collected from Basse-Terre Island in Guadeloupe. U, Th, and Ti elemental profiles reveal that Th and Ti behave conservatively during rind formation, but that U is added from an external source to the rind. In the rind, weathering reactions include dissolution of primary minerals such as pyroxene, plagioclase, and glass matrix, as well as formation of Fe oxyhydroxides, gibbsite and minor kaolinite. Rare earth element (REE) profiles reveal a significant Eu negative anomaly formed during clast weathering, consistent with plagioclase dissolution. Significant porosity forms in the rind mostly due to plagioclase dissolution. The new porosity is inferred to allow influx of soil water carrying externally derived, dissolved U. Due to this influx, U precipitates along with newly formed clay minerals and oxyhydroxides in the rind. The conservative behavior of Th and the continuous addition of U into the rind adequately explain the observed systematic trends of (238U/232Th) and (230Th/232Th) activity ratios in the rind. Rind formation rates, determined from the measured U-series activity ratios with an open system U addition model, increase by a factor of ∼1.3 (0.18–0.24mm/kyr) from a low curvature to a high curvature section (0.018–0.12mm−1) of the core–rind boundary, revealing that curvature affects rates of rind formation as expected for diffusion-limited rind formation. U-series geochronometry thus provides the first direct evidence that the curvature of the interface controls the rate of regolith formation at the clast scale. The weathering rates determined at the clast scale can be reconciled with the weathering rates determined at the watershed or soil profile scale if surface roughness equals values of approximately 1300–2200.

The formation of metastable aluminosilicates in the Al–Si–H 2O system: Results from solution chemistry and solid-state NMR spectroscopy

We present the results of a series of experiments designed to probe the interactions between Al and the amorphous silica surface as a function of thermodynamic driving forces. The results from 27Al single pulse magic angle spinning (SP/MAS) and 27Al{ 1H} rotational echo double resonance (REDOR) allow us to identify the reaction products and constrain their structure. In all cases, despite low Al and Si concentrations we observe the formation of metastable aluminosilicates. Results from low temperature experiments indicate that despite thermodynamic driving forces for the formation of gibbsite we observe the precipitation of separate octahedrally coordinated Al (Al [6]) and tetrahedrally coordinated Al (Al [4]) silicate phases. At higher temperatures the Al [4] silicate phase dominates the speciation. Structural models derived from the NMR data are also proposed, and the results are discussed as they relate to previous work on Al/Si cycling.

Deep weathering and landscape evolution in a tropical plateau

Abstract The thick weathering profiles of humid tropical areas are an important, yet generally neglected, source of information on landscape evolution. Six complete profiles of the weathering mantle were sampled by drilling on the three stepped levels of the Campos do Jordao Plateau, on the NW flank of the Continental Rift of Southeastern Brazil. Mineralogical and micromorphological analyses of drill core samples, complemented by geochemical interpretations and by previous data on the upper saprolite, indicate continuity of a general lateritic trend during the entire process of mantle formation. Lateritization phases of different intensity were defined and considered to reflect adjustment to changes in environmental conditions created by the gradual uplift of the plateau to its present position. Older and more superficial materials related to intense lateritic weathering are characterized by allitization with direct formation of gibbsite from silicates, probably related to tropical climates existing immediately before the formation of the continental rift, during the Paleogene, and also before any significant increase in altitude. Monosialitization phase with general kaolinitization and restricted indirect formation of gibbsite from silicates could be associated to less aggressive climates that followed the Neogene (Miocene?) accentuation of uplift rates along the continental rift. The changes produced by uplift in the tropical climate eventually favored the development of a podzolization trend in soils above 1800 m.

Nanogibbsite: Synthesis and characterization

Nanogibbsite was synthesized using a supersaturated Al(OH)(3) solution which was prepared by titration of AlCl(3) with NaOH at pH 4.6. Excess chloride ions in the system were stripped off by dialyzing the Al(OH)(3) suspension against distilled water. The dialysis step is critical for initiation of gibbsite crystallization or the Al(OH)(3) suspension would remain amorphous. Chloride ions seem to mask the seeding sites and so retard the overall process of gibbsite formation. When subjected to heat treatment, gibbsite→alumina conversion occurred by two mechanisms. Nanogibbsite→α-alumina phase transition occurred forming χ- and κ-alumina polymorphs.

Additive action on boehmite precipitation in sodium aluminate solution

Precipitation of boehmite (Al(2)O(3)·H(2)O) instead of gibbsite (Al(2)O(3)·3H(2)O) from sodium aluminate liquor can be an energy saving option for alumina production. Gibbsite is stable at precipitation temperature lower than 90 °C. Thus when boehmite is precipitated below 90 °C it is always accompanied with a gibbsite phase. However, the addition of certain organic additives favours precipitation of a monophase product i.e. boehmite at a temperature lower than 90 °C. At a temperature as low as 60 °C additives like tartaric acid, xylose and glucose could favour the precipitation of single phase boehmite precipitation. The role of the additive is proposed to be complete inhibition of gibbsite formation and facilitation of boehmite nucleation through different extent of complex formation by multidentate ligands.

Carbonate-Induced Structural Perturbation of Al Hydroxides

AbstractThe chemistry of Al transformation has been well documented, though little is known about the mechanisms of structural perturbation of Al precipitates by carbonates at a molecular level. The purpose of the present study was to investigate the structural perturbation of Al precipitates formed under the influence of carbonates. Initial carbonate/Al molar ratios (MRs) used were 0, 0.1, and 0.5 after aging for 32 days, then the samples were analyzed by X-ray absorption near edge structure spectroscopy (XANES), X-ray diffraction (XRD), Fourier-transform infrared absorption spectroscopy (FTIR), and chemical analysis. The XRD data were in accord with the FTIR results, which revealed that as the carbonate/Al MR was increased from 0 to 0.1, carbonate preferentially retarded the formation of gibbsite and had relatively little effect on the formation of bayerite. As the carbonate/Al MR was increased to 0.5, however, the crystallization of both gibbsite and bayerite was completely inhibited. The impact of carbonate on the nature of Al precipitates was also evident in the increase of adsorbed water and inorganic C contents with increasing carbonate/Al MR. The Al K- and L- edge XANES data provide the first evidence illustrating the change in the coordination number of Al from 6-fold to mixed 6- and 4-fold coordination in the structural network of short-range ordered (SRO) Al precipitates formed under the increasing perturbation of carbonate. The fluorescence yield spectra of the O K-edge show that the intensity of the peak at 534.5 eV assigned to σ* transitions of Al-O and O-H bonding decreased with increasing carbonate/Al MR. The XANES data, along with the evidence from XRD, FTIR, and chemical analysis showed clearly that carbonate caused the alteration of the coordination nature of the Al-O bonding through perturbation of the atomic bonding and structural configuration of Al hydroxides by complexation with Al in the SRO network of Al precipitates. The surface reactivity of an Al-O bond is related to its covalency and coordination geometry. The present findings were, therefore, of fundamental significance in understanding the low-temperature geochemistry of Al and its impacts on the transformation, transport, and fate of nutrients and pollutants in the ecosystem.

Enhancing the P trapping of pasture filter strips: successes and pitfalls in the use of water supply residue and polyacrylamide

SummaryIn intensive pastoral systems the landscape at ground level is clad in dense, filtering vegetation – yet phosphorus losses in overland flow do occur, and pollution of surface waters is a serious consequence. The use of pre‐applied polyacrylamide (PAM) or chitosan to trap particulate phosphorus (PP) and P‐sorbing potable water treatment alum residue (PWTR) to enhance vegetative filtering effects is examined here using field and laboratory overland flow simulation (flows from 0.43 to 0.34 litres s−1 (m width)−1) and analysis.Fitted equations suggest that up to 40% of dissolved reactive P applied (0.75 mg P litre−1) in overland flow could be captured in a flow length of 2.1 m (1 kg PWTR m−2). Unfortunately, drying decreased PWTR effectiveness, though little of the P captured was readily desorbed. This effect did not appear to be the result of gibbsite formation. Compared with the other treatments, there was a strong treatment effect of pre‐applied PAM on the change in PP losses (P &lt; 0.001) over time, though evidence suggests the PAM effect declined during a 44 minute flow period.We showed that the investigated two‐pronged approach to the enhancement of the effectiveness of P trapping by pasture had limitations. Laboratory sheet‐flow simulations suggest that a field‐stable P sorber with sorption characteristics similar to those of the un‐dried PWTR could be an effective retention enhancer for dissolved P. Pre‐applied PAM can have an effect on particulate‐P trapping but was rapidly dissolved and removed by flow.

Deep Regolith Weathering on the Summit Surface of the Southern Mount Lofty Ranges, South Australia: a Contribution to the ‘Laterite’ Debate

AbstractChemical and mineralogical analyses of samples from a 102 m deep borehole and from road cuts on the summit surface of the Mount Lofty Ranges near Willunga Hill, are used to assess critically the view that the regolith here developed as a result of tropical, so‐called ‘lateritic’ weathering processes that developed a standard ‘laterite’ profile of great antiquity. There is no evidence of a former surface crust that comprises the upper part of a ‘standard laterite profile’. Moreover, there is no dramatic variation in chemical composition down the profile, with largely in situ localised weathering transformations explaining mineralogical variability. The presence of 2:1 layer silicates and the absence of gibbsite do not favour the operation of former intense tropical weathering, but the possible presence of chloritised smectite may have inhibited gibbsite formation. An iron‐depleted pallid zone does not occur at depth in the profile, but is present on the plateau margin. Many of the weathering features exposed in the road cuts can be explained in terms of modifications and developments under environmental conditions similar to those of the present, arguing against the view that the summit surface and its associated weathering mantle have been preserved in pristine form since the Mesozoic.

Factors and processes of gibbsite formation in Northern Thailand

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.

Weathering trends and parent material characteristics of polygenetic oxisols from Minas Gerais, Brazil: I. Mineralogy

In geologically stable areas in the tropics, climatic changes and geomorphic cycles give origin to polygenetic soils. Polygenesis involves new soil formation phases taking place on preweathered materials from previous phases, resulting in soils with rather similar chemical and mineralogical properties. Polygenetic soils from Minas Gerais, Brazil, were investigated with the purpose of distinguishing mineralogical aspects in relation to polygenesis. The soil materials were studied by micromorphology and electron microscope and analyzed by XRD. All soils are strongly weathered and even show weathered grains of ilmenite and quartz. The soils show two phases of kaolinite formation (pseudomorphs after mica and precipitation from solution) and of accumulation of iron compounds. Desilication causes gibbsite formation from kaolinite, occasionally even in the deep saprolite. Superposed processes can be due to progressively deeper weathering or to changes of the external environment.

Influence of a Ceramic Substrate on Aqueous Precipitation and Structural Evolution of Alumina Nano-Crystalline Coatings

Either boehmite (γ-AlOOH) or gibbsite (γ-Al(OH)3) nanocrystalline thin films (h≈100 nm) can be precipitated from AlCl3 solution at fixed pH and temperature onto different substrates. It depends on the nature of the substrate (mica flakes, SiO2 flakes, or α-Al2O3 flakes), on their crystallographic properties (crystalline or amorphous), and on some experimental parameters (agitation rate, addition rate). According to the surface charge of the substrates, different alumina species are involved in the precipitation process. When negative charges are present on the substrate, the [Al3O(OH)3(OH2)9]4+ polycation is promoted, leading to the formation of the (Al4) tetramer ([Al4O(OH)10(OH2)5]o) and then to the precipitation of bohemite. When positive charges are present, a ligand bridge containing complex ([Al3O(OH)3(O2H3)3(OH2)9]+) is likely favored, giving rise to hexagonal ring structures or amorphous solids that lead to the formation of gibbsite. Besides the surface effects, crystalline substrates can act as a template during precipitation of aluminum species as shown for the formation of gibbsite on muscovite. Finally, calcination at 850°C of boehmite samples leads to porous γ-Al2O3 layers, while calcination of gibbsite leads to δ-Al2O3 layers.

Mineral weathering in trachydacitic-derived soils and saprolites involving formation of embryonic halloysite and gibbsite at Mt. Amiata, Central Italy

Mount Amiata is a volcano that has been extinct for 200 ka. It is located in southern Tuscany, central Italy, and is composed largely of trachydacitic rocks. The soils of Mt. Amiata show a dark, carbon-rich A horizon that overlies a slightly structured Bw horizon or, directly, a very thick saprolite, which at a depth of about 3 m still maintains some ellipsoidal cores of coherent material. Mineral weathering in two of these soils, one under beech and the other under chestnut, was studied in detail. The mineralogy of the parent rock consists predominantly of volcanic glass with phenocrysts of feldspar (principally K-feldspar), biotite and pyroxene. The main weathering products are an embryonic form of halloysite and gibbsite, while allophane is usually absent or occurs in minor amounts. SEM evidence indicates that the clay minerals form directly from the weathering of volcanic glass in an environment where Si is preferentially depleted in relation to Al. In the saprolite an anomalous situation is observed where optically fresh ferromagnesian minerals are thickly coated by clay particles whereas the surfaces of the glass and feldspar grains are relatively much cleaner. However, the ferromagnesian minerals are also much more heavily colonised by fungal hyphae and it is proposed that this results in the preferential deposition or flocculation of colloidal suspensions which translocate from higher positions in the weathering profile.

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

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.

Biomimetic co-assembly of Gibbsite and mesophases: A novel synthesis route by devitrification of gel derived from hydrothermal aluminosilicate/CTAB solution

AbstractThe effect of an amphiphilic surfactanton the co-assembly of gibbsite and low-dimensional-order, liquid-crystalline mesophases using a hydrothermal-gelafion-devitrification route was examined. Crystal growth by this method occurred either in three dimensions or was limited to only two dimensions. Gibbsite and mesophases were synthesized using a cationic surfactant, cetyltrimethyl ammonium bromide (CTAB), as template and a commercial mineral sample as inorganic precursor. The commercial mineral sample contained pyrophyllite, α-quartz, and minor kaolinite. The syntheses were made at pH 10 under hydrothermal conditions followed by equilibration at room temperature. The hydrothermally soluble portion settled at room temperature to form a translucent hydrous gel. This translucent gel turned white after drying on a glass substrate because of the following events based on chemical analysis, X-ray diffraction, and optical/electron microscopy: (1) gibbsite preferentially nucleated at the gel/air and gel/glass interfaces to form spherulites of tabular gibbsite crystals with entrapped droplets; (2) a ∼26 Å basal-spacing, aluminate-encased, lamellar mesophase formed by 2D growth near the edge of the drying gel; and (3) residual solution in entrapped droplets within the gibbsite phase later devitrified abruptly into an optically isotropic material (an aluminosilicate gel possibly with minor mesophases) with a dendritic morphology. Formation of gibbsite and the lamellar mesophase was initially interface controlled, but later became 2D diffusion-controlled as CTAB concentrations and micelle lengths were increased with crystallization time. A relatively high surfactant/water ratio of the drying gel might account for predominant crystallization of an aluminate-encased lamellar mesophase rather than the hexagonal mesophase known as the Mobil Composition Material MCM-41.

Effect of temperatures on formation and transformation of hydrolytic aluminum in aqueous solutions

The effect of temperature and aging on the formation and transformation of hydrolytic Al species were investigated. Partially neutralized Al solutions with OH/Al ranging from 0.5 to 2.6 were prepared at 25 and 70 °C, and were then aged at 25 °C for up to 180 days. The Al speciation in these solutions was determined using 27Al nuclear magnetic resonance (NMR) spectroscopy, ferron colorimetric and sulfate precipitation methods. Synthesis at high temperature results in a lower degree of hydrolysis. Specifically, the formation of Al13 was lower at OH/Al<1.8 and higher at OH/Al>1.8 when the synthesis temperature was high. Upon aging, a significant difference in the formation of gibbsite was observed for the samples synthesized at different temperatures. After aging for 180 days, gibbsite precipitates were seen in the samples prepared at 25 °C and OH/Al>1.0, while no gibbsite precipitates were collected for the sample prepared at 70 °C except the one of OH/Al=2.6. The polymeric Al fractions decreased in the samples where gibbsite was formed. The comparison of the Al speciation of the samples synthesized at different temperature shows no direct relationship between the Al13 content and the production of gibbsite. The conversion of Al13 into precipitates and the eventual formation of gibbsite may be through the decomposition of Al13 into monomeric and small polymeric an Al, which serves as Al pool for gibbsite formation.

Aqueous high-temperature solubility studies. I. The solubility of boehmite as functions of ionic strength (to 5 molal, NaCl), temperature (100–290°C), and pH as determined by in situ measurements

The solubility of boehmite, γ-AlOOH, was measured in a modified hydrogen-electrode concentration cell, which provided continuous in situ measurements of hydrogen ion molality over the range of ionic strengths from 0.1 to 5.0 mol · kg −1 (NaCl) at temperatures from 100 to 290°C. A series of conventional solubility measurements was also carried out in acidic solutions over the same temperature range (i.e., pH was not monitored, but rather calculated from mass balance). The combined results yielded the molal solubility quotients, Q s0 and Q s4 for the equilibria: AlOOH (cr) + 3H + ⇄ Al 3+ + 2H 2O Q s0 = [Al 3+]/[H +] 3 AlOOH (cr) + 2H 2O ⇄ Al(OH) 4 − + H + Q s4 = [Al(OH) 4 −][H +]In the regression of each isothermal data set, the values for the first hydrolysis quotient, Al 3+ + H 2O ⇄ Al(OH) 2+ + H + Q 1,1 = [Al(OH) 2+][H +]/[Al 3+]were fixed according to a previous potentiometric study (Palmer and Wesolowski, 1993). Moreover, for one series of titrations at 0.1 mol · kg −1 ionic strength at 150°C, the remaining two solubility quotients, Q s2 and Q s3, were determined simultaneously from the regression. However, at all other conditions, the values of Q s2 and Q s3 were also fixed at values consistent with the corresponding 0.03 mol · kg −1 ionic strength results (Bénézeth et al., 2001) by invoking the isocoulombic assumption (i.e., an assumption of minimal ionic strength dependence for reactions with no net change in charge). The stability field with respect to pH of the Al(OH) 2 + and Al(OH) 3 0 aqueous species were found to be very narrow, and hence assumptions concerning their stabilities had little effect on the predicted shape of the solubility profiles at high ionic strength. Global fits were made of the log Q s0 and log Q s4 values as functions of temperature and ionic strength after combining with corresponding values from an analogous study at 0.03 mol · kg −1 ionic strength (Bénézeth et al., 2001), as well as appropriate constants taken from the literature. The fits were further constrained by inclusion of gibbsite solubility data (<100°C) in NaCl media (Wesolowski, 1992; Palmer and Wesolowski, 1992) after adjustment for the relative free energies of formation of gibbsite and boehmite. This treatment ensured that a continuous empirical model exists for Al 3+ and Al(OH) 4 − from ambient conditions to 300°C and infinite dilution to five mol · kg −1 ionic strength.