Surface Of Gibbsite Research Articles

Dried hybrid imogolite nanotubes as solids with a changeable surface area: an insight into textural properties based on the correlation between nitrogen gas adsorption, immersion calorimetry into water, and small angle X-ray scattering.

Powdered samples of three imogolite materials with an inner surface covered with Si-CH3 groups (hybrid IMO-CH3) were prepared by ambient drying at 323 K, freeze drying, and spray drying. Reliable estimates of the intra-tube (SA), inter-tube (SB), and inter-bundle (SC) specific surface areas of these samples were inferred from a model-guided correlation between the results of measurements by small angle X-ray scattering (SAXS), nitrogen gas adsorption, and immersion calorimetry into water. Since the SAXS studies indicated no significant deformations of IMO-CH3 nanotubes upon drying, further studies by gaseous N2 adsorption at 77 K indicated the intra-tube and inter-bundle specific surface areas as being only accessible to this adsorbate. The outer curved surfaces of IMO nanotubes, including the inter-tube surface areas, were evaluated based on the calorimetric measurements of the enthalpy of immersion into water, using the enthalpy of immersion per unit surface area for a modelled curved surface of gibbsite as the conversion factor. Given the uncertainty in the determination of surface areas, the IMO-CH3 samples were found to possess the limiting values of specific surface areas ranging between 1150 and 1480 m2 g-1. In contrast to the two other materials, ambient-dried IMO-CH3 was characterized by the highest value of SC parameter due to the much smaller bundles formed by the constituent imogolite nanotubes. The accessibility of these surface areas, together with the hydrophobic and hydrophilic surface domains, was demonstrated to depend on the nature of the surrounding medium and the size of the adsorbing species.

Adsorption of amino acids by montmorillonite and gibbsite: Adsorption isotherms and spectroscopic analysis

The adsorption of glycine, cysteine and glutamic acid by gibbsite and montmorillonite was studied as a function of amino acid concentration and solution pH. Zeta potential measurements, X-ray diffraction (XRD), and attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopic analysis were performed to deduce the mechanism(s) of adsorption and to observe changes in the basic structure of the minerals after interaction with amino acids. The ATR-FTIR analysis revealed that both NH3+ and COO− groups of amino acids were very vital for interactions with the surface of gibbsite and montmorillonite. XRD analysis revealed that the formation of mineral-amino acid composites decreased the d001 value of montmorillonite from 17.95 to 14.2 (glycine), 15.49 (cysteine), and 15.22 Å (glutamic acid). This observation suggests that amino acid adsorption induced structural stress on montmorillonite and expanded its interlayer space. Also, the adsorption of glycine and cysteine to gibbsite and montmorillonite increased their zeta potentials at pH 5.0 while glutamic acid decreased the zeta potentials. Thus, amino acid-induced increase/decrease in the surface positive/negative charge of the minerals indicates that chemical bonds were formed between glycine, cysteine, glutamic acid and the minerals. The findings of this study can serve as a reference for studies related to understanding the bonding interactions and distribution of different amino acids in soils.

Radiolysis of thermally dehydrated gibbsite

Aluminum hydroxide (gibbsite, Al(OH)3) powders and their thermally dehydrated forms were rehydrated and irradiated with gamma rays to examine the effects of the collapse of the crystalline gibbsite structure as it is converted to alumina on the transport of precursors of molecular hydrogen to the surface. The amount of hydrogen gas, H2, production from irradiated gibbsite and its transition phases with adsorbed water increased substantially with increasing temperature for dehydration of the samples, up to a point. The production of H2 for samples without water was considerably lower than the hydrated samples signifying the importance of surface water and the transport of precursors to the surface. EPR spectroscopy showed that the major radiolytic products are trapped electrons and related O− centers. Thermal gravimetric analysis (TGA) measurements of gibbsite dehydration established the temperatures of phase transitions from gibbsite to alumina. Changes to the surface and structure of gibbsite and its transition phases following irradiation were analyzed using nitrogen adsorption, powder X-ray diffraction (pXRD), Raman spectroscopy, electron paramagnetic resonance (EPR), and scanning electron microscopy (SEM). pXRD and Raman spectra showed amorphorization at about 300 °C, coupled to a substantial increase in specific surface area due to increasing porosity.

Radiolytic stability of gibbsite and boehmite with adsorbed water

Aluminum oxyhydroxide (boehmite, AlOOH) and aluminum hydroxide (gibbsite, Al(OH)3) powders with adsorbed water were irradiated with γ-rays and 5 MeV He ions (α-particles) in order to determine overall radiation stability and chemical modification to the surface. No variation in overall phase or crystallinity due to radiolysis was observed with X-ray diffraction (XRD) and Raman spectroscopy for doses up to 2 MGy with γ-rays and 175 MGy with α-particles. Temperature programed desorption (TPD) of the water from the surface to the gas phase indicated that the water was chemisorbed and strongly bound. Water adsorption sites are of similar energy for both gibbsite and boehmite. Observation of the water adsorbed on the surface of gibbsite and boehmite with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed broad peaks at 3100-3600 cm−1 due to OH stretching that slowly decreased on heating to 500 °C, which corresponds well with the water vapor evolution observed with TPD. Both materials were found to be amorphous following heating to 500 °C. X-ray photoelectron spectroscopy (XPS) indicated surface reduction of Al(III) to Al metal on radiolysis with α-particles. Complete loss of chemisorbed water and the formation of bulk O atoms was observed following radiolysis with α-particles.

Competition between bacteria and phosphate for adsorption sites on gibbsite: An in-situ ATR-FTIR spectroscopic and macroscopic study

Sorption and desorption of phosphate (P) on Fe and Al (hydr)oxides may be affected by bacteria in soils because their ubiquitous and strong interactions. The role of Bacillus subtilis and Pseudomonas fluorescens in adsorption of P on gibbsite (γ-AlOOH) was systematically investigated under a wide range of conditions by combining in-situ attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy with batch macroscopic experiments. In-situ ATR-FTIR observations of the ternary systems (bacteria, P, and gibbsite) showed simultaneous desorption of P from, and adhesion of the bacteria to, gibbsite, indicating a competition between the two for surface sites. Batch desorption experiments showed that bacteria could mobilize the P from gibbsite into solution, and macroscopic adsorption data showed that the amount of P adsorbed on the bacteria–gibbsite complex was less than that on gibbsite alone over durations from 0h to 26h, concentrations of P from 0.1mM to 2.0mM, pH from 5 to 8, and ionic strength from 0M to 0.5M, suggesting that bacteria inhibit the adsorption of P on gibbsite. The degree of inhibition increased with the number of bacteria in the system and was significantly but non-linearly correlated with the decline in the positive charge on gibbsite induced by the bacteria. Therefore, competition for suitable sites on the surface of gibbsite between P and the bacteria and reduction in the positive charge on the surface of gibbsite induced by bacteria are proposed as two important mechanisms that inhibit P adsorption. These findings highlight the role of bacteria in regulating the availability of P to plants and its mobility in natural environments.

Changes in the system of chemical bonds in gibbsite under the impact of NH4H2PO4 solutions of different concentrations

The participation of anionic aluminum hydroxo complexes in the binding of phosphate anions on the surface of gibbsite has been shown. The succession of changes in the anionic aluminum phosphate complexes under increasing concentration of phosphate solution has been studied. It has been found that aluminum polyphosphate complexes responsible for the intensive dissolution of gibbsite are formed, along with aluminum orthophosphate complexes, at phosphate solution concentrations of 1 and 2 mol P/L. The decisive role of polyphosphate (P–O–P) groups in the ligand structure of anionic complexes in the transformation of gibbsite to a phosphate mineral (ammonium taranakite) has been revealed. The role of hydrogen bonds with the participation of ligand P(O)OH groups in the formation of ammonium taranakite crystals has been discussed.

NMR spectroscopy of naturally occurring surface-adsorbed fluoride on Georgia kaolinite

AbstractUsing 19F magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, we show that most of the fluoride present in the KGa-lb reference kaolinite from Washington County, Georgia, occurs as a surface-adsorbed species bonded to Al. This surface fluoride can be removed from the <2 µm fraction by acid wash, but is largely retained in the coarse fraction. Correlation of integrated 19F NMR peak intensities with fluoride sorption experiments indicates a bulk F content of ∼144 ppm for KGa-1b, of which ∼30% substitutes for hydroxyl sites in the mineral structure and the remaining 70% occurs adsorbed on particle surfaces, corresponding to an edge surface fluoride density of ∼0.7 F− nm−2. 19F{27Al} TRAPDOR (TRAnsfer of Populations in DOuble Resonance) NMR data for the original kaolinite and for products of F− sorption experiments at pH 4 show that all of the observed 19F signals arise from fluoride bonded to Al atoms. Furthermore, bridging Al-F-Al sites and terminal Al-F give distinctly different TRAPDOR fractions allowing assignment of resolved peaks based on the number of Al in the first coordination sphere. This result was confirmed for fluoride adsorbed to the surface of gibbsite from aqueous suspension. No evidence was found for Si-F-type environments on the kaolinite surfaces.

XPS with fast-frozen samples: A renewed approach to study the real mineral/solution interface

The procedure for XPS study of quick-frozen solutions developed at the end of 1970s [K. Burger, E. Fluck, Inorg. Nucl. Chem. Lett. 10 (1974) 171] was modified and applied to investigate the mineral/solution interface formed at the surface of silica, gibbsite, goethite and manganite in aqueous sodium chloride solution at variable pH and ionic strength. The Na/Cl atomic ratio (frozen samples) allows determination of the pH range where the mineral surface charge is close to zero. This ratio can be used also to prove an ion pair formation at the surface. From the intensity of the O 1s component corresponding to water, an interface thickness of 0.5 nm was estimated. A “built-in” potential at the interface, evincing as matrix elements photoelectron lines shifts after water loss, was found. This potential arises due to the spatial separation of the charged mineral surface and counter-ions in the electrical double layer (EDL). Its value does not exceed 250 mV and the sign depends on the charge of the mineral surface.

An extended X-ray absorption spectroscopy study of copper(II) sorption by oxides

Many important sorption parameters have been determined by isothermal sorption batch experiments; however, only macroscopic and indirect results were obtained. The objective of this study was to derive the species and structural environment of Cu(II) on the surface of gibbsite (Al(OH) 3), pyrolusite (MnO 2), and amorphous silica (SiO 2), using the technique of extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Results show that the equilibrated pH of suspension affects significantly the species of Cu sorbed on the surface of these oxides. Sorbed species (Cu 2+) were present below pH 5.73 and precipitated (Cu(OH) 2) phases above 5.63. On the other hand, there is no EXAFS evidence that sorbent types and concentrations affect the local chemistry environment and species of Cu(II).

The flux of oxygen from the basal surface of gibbsite (α-Al(OH) 3) at equilibrium

Experiments were conducted on gibbsite to determine whether oxygen-isotope exchange rates at hydroxyl bridges (μ 2-OH) on the basal sheet exhibit similar reactivity trends as in large aluminum polyoxocations, for which high-quality kinetic data exist. We followed the exchange of 18O from the mineral surface to solution by using a high-surface-area solid that had been enriched to tens of percent in 18O. To establish this high enrichment, we initially react the solid hydrothermally with highly enriched H 2 18O in order to tag all oxygens near the mineral surface, and then back exchange the most reactive oxygens with isotopically normal water. This enrichment procedure isolates 18O into the least-reactive sites, which are presumably μ 2-OH on the basal surface. By analogy with aqueous aluminum complexes, including large multimers, the η-OH 2 sites exchange within fractions of a second and should be isotopically normal using this procedure. When suspended in isotopically normal electrolyte solutions, we find that the rates of release of 18O from the mineral fall close to the rates of dissolution. The lack of steady isotopic exchange of μ 2-OH on gibbsite surfaces contrasts with the aluminum polyoxocations, where the μ 2-OH exchange many hundreds of times with bulk water molecules before the molecule dissociates. Additional experiments were conducted in solutions at near-neutral pH to determine the flux of oxygens at conditions near thermodynamic equilibrium. As in more acidic solutions, rates are close to values expected from dissolution of the mineral and there is no evidence for steady exchange of hydroxyl bridges with water molecules in the bulk solution.

Dissolution of gibbsite: Direct observations using fluid cell atomic force microscopy

In situ atomic force microscopy (AFM) was used to follow the far-from-equilibrium dissolution of the {001} cleavage surface of natural gibbsite in nitric acid. The main dissolution mechanism was the retreat of straight monolayer steps, the edges of which are parallel to the <110>, <010>, and <100> directions. The stability of these steps can be expressed as <110> > <010> > <100>. The results are explained in terms of the positions of the terminal O atoms and their associated Al atoms at the steps. New steps were formed at etch pits that opened where screw dislocations emerged on the surface. The dissolution rates were calculated from the change in size of pits and islands. The values obtained were 9.5 × 10-9 - 2.3 × 10-8 mol/m2·s, normalized to the total surface area, and 1.8 - 3.6 × 10-7 mol/m2·s, normalized to the surface area of the step fronts. The rates of dissolution calculated using only the surface area of the step fronts are similar to literature values obtained by other methods.

Surface and structural characteristics of gibbsite precepitated from pure, synthetic Bayer liquor

In this study, the ex situ surface and structural characteristics of gibbsite precipitated from pure, synthetic, supersaturated caustic aluminate solutions were examined by various techniques to determine the presence of an amorphous aluminium hydroxide phase which might be linked with the crystal growth process. The results showed that in unseeded, isothermal, batch precipitations from synthetic, supersaturated Bayer liquors at 65, 75 and 80°C, while gibbsite was the main product, a small amount of bayerite was also formed at the early stages. Furthermore, a poorly formed material was observed at the ex situ surfaces of some of these crystals. On further precipitation, the bayerite phase disappeared as did the poorly formed material. The entire crystal, with or without poorly-formed surfaces, was found to be crystalline throughout. No evidence was found to indicate the presence of an additional phase such as amorphous aluminium hydroxide. Hence, the poorly formed material is believed to be associated with the presence of bayerite which may precipitate as poorly formed, globular crystals. In similar precipitation experiments performed with gibbsite seeding, gibbsite possessing well-formed crystal surfaces was the only phase obtained.

The removal of silicate from gibbsite

Desorption of silicate from the surface of gibbsite was studied under typical Bayer process conditions. Desorption of silicate declined with aging and with increasing temperature. Successive extractions increased the desorption of silicate, although desorption was more pronounced in the first extraction than in subsequent extractions.

Surface characterisation, iron removal and enrichment of bauxite ultrafines

The bauxite ultrafines studied consists of < 2 μm particles of gibbsite, boehmite, kaolinite, hematite, goethite, anatase and quartz. Electron microscopy and surface analysis (XPS), have shown that iron is present in this material in three forms: separate hematite and goethite particles: extremely small (1–3nm diameter) particles of iron oxide adhering to the surface of gibbsite, boehmite and kaolinite; and lattice-substituted iron in these three minerals. The first and second forms can be removed by mild reductive leaching, without lattice disruption, reducing the total iron oxide content from 5.1 wt.% to 1.7 wt.%. The lower limit of 1.7% is also obtained by extensive leaching with hydrochloric acid and corresponds to the residual lattice-substituted iron. Further discrimination between the cleaned minerals is possible using dispersion, sedimentation, centrifugation and flocculation techniques. For example, an anatase-enriched product or a gibbsite-rich, iron and boehmite depleted product may be obtained. Relevant separation mechanisms are discussed.

A Study of Silicate Adsorption on Gibbsite (Al(OH)3) by X‐Ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS, ESCA) was utilized to investigate the mechanisms of adsorption of silicates and phosphates on a surface of synthetic gibbsite. Unbuffered solutions of CaSiO3 Ca(H2PO4)2, Si(OH)4, and Ca(NO3)2 at low concentrations, were adsorbed onto powdered Al(OH)3. The samples were dried, leached with large volumes of water, redried, pressed into a pellet, and examined by XPS. The following observations were made: When CaSiO3 was the adsorbate, both Ca and Si were detected on the adsorbent surface by means of the Ca2p, Si2s, and Si2p photoelectron peaks. No Si was detected on the Al(OH)3 surface in the Si(OH)4 treatment which suggests weak adsorption of Si on gibbsite in the absence of Ca. Ca and P were both present on the adsorbent after Ca(H2PO4)2 treatment. Atomic concentration ratio estimates calculated from peak intensities, kinetic energies, and photoelectron cross-sections, showed that Ca/Si exceeds Ca/P on the alumina surface.

KINETICS OF ISOTOPIC EXCHANGE OF PHOSPHATE ADSORBED ON GIBBSITE

SummaryAn Elovich‐type equation has been used to describe the kinetics of isotopic exchange of phosphate adsorbed on the surface of gibbsite. The equation is image where A and B are parameters; θ=bF/[b+a(I‐F]; a and b are the molar concentrations of the phosphate on the crystal surface and in solution respectively and F is the fraction exchange of the radio‐isotope at time t. First‐order rate constants were obtained from the equation. The reference state for the first‐order rate constants, and the distribution of activation energies, for exchange can be related to the Elovich equation parameters. The kinetic results are consistent with SN1 dissociation or SN2 bimolecular solvolysis for the phosphate ligand. The eschange reaction is subject to an acid‐base catalysis the exact nature of which could not be determined from the available data.

ANION ADSORPTION BY GOETHITE AND GIBBSITE

SummaryThe desorption of specifically adsorbed (ligand exchanged) anions, such as phosphate, selenite, and fluoride, from the surface of gibbsite and goethite has been studied by repeatedly washing the adsorption complex with solutions of constant pH and ionic strength but containing no specifically adsorbable anions. The desorption of the anions varies between complete reversibility and almost complete irreversibility. Measurements of surface charge by the uptake of Na+ and Cl‐ revealed that, on washing the adsorption complex, the surface charge returned to its value at the given pH in the absence of specifically adsorbed anions. Thus when the isotherm was irreversible, OH‐ was desorbed (or H+ adsorbed) in preference to the desorption of the specifically adsorbed anion, whereas when the isotherm was reversible the specifically adsorbed anion was desorbed.Irreversibility appears to involve the nature of the adsorption complex at the surface. Where only monodentate ligands form, for example fluoride adsorption, the isotherm is reversible, whereas bridging or multidentate ligands and the formation of ring structures at the surface favour irreversibility.