N. Spanos

The transformation of vaterite to calcite: effect of the conditions of the solutions in contact with the mineral phase

Abstract The transformation of the thermodynamically unstable vaterite into the stable calcite was investigated during the precipitation of calcium carbonate, under conditions of constant supersaturation at 25, 35 and 45°C and pHs 8.5, 9.0 and 10.0. The calcite content in the solid precipitates was determined quantitatively by the X-ray diffraction technique (XRD) using the appropriate mixed standards of known composition. Scanning electron microscopy (SEM) and particle number and size measurements were also used for the characterisation of the solid precipitates. It was found that the transformation of vaterite into calcite does not depend on pH and temperature in the range 25–45°C. It depends only on the supersaturation, and this, only at relatively high supersaturation ratios (1.5–1.9). Specifically, the rate of transformation decreases as supersaturation increases. It was suggested on the basis of the kinetics results, that the transformation takes place through dissolution of vaterite, preferably of the small crystals followed by the crystallisation of calcite. At relatively high supersaturation ratios (1.5–1.9) the transformation is controlled by the dissolution of vaterite, whereas at lower supersaturation ratios (1.2–1.5) the rate of dissolution of vaterite is similar with that of crystallisation of calcite. Finally, a model equation which predicts a linear dependence of the rate of transformation on supersaturation ratio has been derived at relatively high supersaturation ratios.

Molybdenum-oxo species deposited on alumina by adsorption: I. Mechanism of the Adsorption

Abstract The mechanism of adsorption of molybdates on the surface of γ-alumina was investigated by use of adsorption equilibrium experiments, potentiometric titrations, microelectrophoresis and UV/VIS spectroscopy. From the experimental results and the theoretical analysis of the isotherms it was concluded that Mo VI deposition was done by adsorption of the Mo x O y z − ions on energetically equivalent and distinct sites of the inner Helmholtz plane (IHP) of the double layer on the surface of the γ-alumina particles suspended in the aqueous medium. The creation of these sites was mainly attributed to the presence of the protonated surface hydroxyls of γ-alumina and not to the neutral hydroxyls. Analysis of the adsorption isotherms suggested that considerable lateral interactions are exerted between the adsorbed Mo x O y z − ions, resulting in the formation of Mo x O y z − … Mo x O y z − … oligomers in the IHP. It is proposed that these polymeric ions are deposited on the surface in the drying step.

Effect of inorganic phosphate ions on the spontaneous precipitation of vaterite and on the transformation of vaterite to calcite

Abstract The effect of inorganic orthophosphate ions on both the precipitation of vaterite and the transformation of vaterite to the thermodynamically stable calcite was investigated during the precipitation process of calcium carbonate, under conditions of constant supersaturation at 25°C and at pH 9.0 and 10.0. The calcite content of the solid precipitates was determined quantitatively by powder X-ray diffraction (XRD) using the appropriate standards mixed at known amounts. The presence of very small amounts of inorganic phosphate ions was found to reduce the rate of vaterite crystallization. Application of the kinetic data to a Langmuir-type model suggested that adsorption of phosphate at the active growth sites was responsible for the reduction in the crystal growth rates. At relatively high supersaturation ratios (1.5–2.1), where the transformation rate decreases with increasing supersaturation ratio, it was found that the transformation of the initially formed vaterite to calcite depends only on the supersaturation ratio and not on the pH of the supersaturated solutions in which precipitation takes place. It was suggested on the basis of the kinetic results, that the transformation takes place through dissolution of vaterite followed by crystallization of calcite. At relatively high supersaturation ratios (1.5–2.1) the transformation is controlled by the dissolution of vaterite, whereas at lower supersaturation ratios (1.1–1.5) the rate of vaterite dissolution is comparable with the respective rate of calcite crystallization. Finally, it was found that the presence of phosphate ions in the precipitating solution stabilized the initially formed vaterite, by decreasing markedly the rate of its transformation to calcite. The stability of vaterite in the presence of the phosphate ions was ascribed to the retardation of both the dissolution of vaterite and of the crystallization of calcite, caused by blocking of the active sites for the dissolution of vaterite and for the crystallization of calcite, by the adsorbed phosphates.

Molybdenum-oxo species deposited on titania by adsorption: Mechanism of the adsorption and characterization of the calcined samples

The mechanism of the adsorption of molybdates on the titania surface has been investigated using adsorption equilibrium experiments, potentiometric titrations, and microelectrophoretic mobility measurements. Comparison of adsorption data with the surface charge of titania, regulated by changing the pH of the impregnating solution, demonstrated that the groups responsible for the creation of the adsorption sites are mainly the protonated surface hydroxyls of titania in addition to the neutral sites. Moreover, the results obtained by the combination of potentiometric titrations and microelectrophoretic mobility measurements, and the variation of pH before and after adsorption with the Mo(vI) concentration suggested that the MoO22− ions are adsorbed on the Inner Helmholtz Plane (IHP) of the electrical double layer, which is developed between the surface of the titania particles and the impregnating solution. Finally, from the analysis of the isotherms obtained it was concluded that the adsorbed MoO2− ions are located on energetically equivalent sites of the IHP and that relatively weak lateral interactions are exerted. On the basis of the abovementioned menhanism, an explanation of the dependence of the sorptive capacity of titania on the pH of the impregnating solution is provided. The combined use of NO and CO2 adsorption, as well as XPS, RAMAN spectroscopy, and temperature programmed reduction measurements, showed that at pH 7.3, corresponding to about 2 wt% MoO3, the titania surfaceis completely covered and the active surface reaches its maximum value. At pH values lower than 7.3, a second molybdena layer starts to form until the Mo03 content reaches the 5.4 wt% MoO3, a point at which Mo03 crystallites are formed.

Molybdenum-oxo species deposited on alumina by adsorption: II. Regulation of the Surface MoVI Concentration by Control of the protonated Surface Hydroxyls

Abstract The adsorption of Mo x O y z − species from aqueous solutions on the surface of pure and Na + or Li + -doped γ-aluminas was studied over a pH range between 3.0 and 8.5 and temperatures ranging from 10 to 55°C. The variations in pH with the concentrations of the impregnating solutions observed and analysis of the isotherms demonstrated that the following findings reported for adsorption on pure γ-alumina at T = 25°C and pH 5 are also valid for adsorption on pure and sodium-doped aluminas performed at various pH values and temperatures: (i) The contribution of precipitation to the deposition is negligible. (ii) The adsorption constant for the MoO 4 2− ions is larger than those for the isopolyanions. (iii) The adsorbed Mo x O y z − ions are located on energetically equivalent but distinct sites of the inner Helmholtz plane, created mainly by the protonated surface hydroxyls. (iv) Lateral interactions are exerted between the adsorbed Mo x O y z − ions resulting in the formation of Mo x O y z − … Mo x O z − oligomers. Moreover, it was demonstrated that the regulation of the concentration of the protonated surface hydroxyls makes it possible to deposit by adsorption very large amounts of Mo VI on the γ-alumina surface. Although the sodium doping was found to be the most attractive from the view point of maximization of the extent of adsorption, it may be suggested that it promotes the formation of the catalytically inert sodium molybdate. Therefore the second best method of regulation, namely the change in the impregnating temperature, may prove to be the most convenient in practice.

Preparation and characterization of anatase powders

Anatase powders have been prepared by precipitation and by sol–gel methods. In the former, titania was continuously precipitated at 25°C, pH 1.97 in a stirred reactor by mixing TiOSO4 and potassium hydroxide so as to keep solution supersaturation constant throughout the precipitation process. The preparations were performed in the absence and in the presence of Li+, Nb5+ and W6+ ions. In all cases anatase was the sole phase forming. The presence of these metal ions (1–7×10-5M) did not influence the precipitation kinetics, which was controlled by surface integration. XPS analysis showed that the dopant ions were incorporated into the anatase lattice for the preparations in supersaturated solutions. Microelectrophoresis experiments did not show any differentiation of the electric charge of the preparations in the presence and in the absence of these ions. The relatively high specific surface area (SSA) of the anatase obtained (135 g-1) increased (up to 250 g-1) by the incorporation of the dopant ions. In the sol–gel preparations the process was found to depend on the supersaturation of the sol with respect to the solid phase forming. A threshold sol composition corresponding to a total titanium concentration of CTi=0.06 M (25°C, pH=2.5–3.0) was found to be critical for the formation of the gel. Anatase was exclusively formed both in the absence and in the presence of dopant ions in the gel. The SSA obtained was low although it increased in the presence of dopant metal ions. Electrokinetic measurements of the solids formed by the sol–gel method, suggested that the dopant ions tend to accumulate on the surface of the anatase particles.

Functionalization of synthetic polymers for potential use as biomaterials: selective growth of hydroxyapatite on sulphonated polysulphone

A novel composite made of biocompatible synthetic polymer (Sulphonated Polysulphone, SPSPH) which may be easily fabricated in various shapes and synthetic hydroxyapatite (HAP) was prepared. The preparation was done by the spontaneous precipitation of HAP in aqueous suspensions of the polymer particles. The time the precipitation process was allowed to proceed was used to regulate the inorganic content of the composite. The preparation thus obtained, in addition to its effectiveness in inducing HAP formation, could be easily fabricated in various shapes, including films. The SPSPH-HAP composite films, surface area totaling ca. 30 cm2 induced the exclusive formation of HAP with rates proportional to the solution supersaturation. No induction times preceded the formation of HAP. Kinetics analysis with respect to HAP yielded an apparent order of precipitation of 6.0+/-0.4, suggesting polynuclear growth with the formation of nuclei above nuclei. The surface energy calculated from the rates of crystal growth on the polymeric substrate gave for HAP the value of 185 mJ m(-2) of order of magnitude typical for crystalline solids.

Preparation and characterization of various titanias (anatase) used as supports for vanadia-supported catalysts

Abstract Samples of anatase, prepared using different methodologies, and an industrial sample were characterized using X-ray photoelectron spectroscopy, X-ray diffraction analysis, electron microscopy, diffuse reflectance spectroscopy, temperature-programmed desorption of ammonia, nitrogen adsorption, microelectrophoresis and potentiometric titrations. It was found that anatase prepared by hydrolysis of the titanium isopropoxide exhibited the largest specific surface area, the highest Lewis acidity and the highest concentration of the protonated plus neutral surface hydroxyl groups. These hydroxyl groups are considered to be the deposition sites for the VO 3 − ions in the preparation of the V 2 O 5 /anatase catalysts by equilibrium deposition-filtration. The above findings as well as the absence of foreign ions on the surface of the anatase prepared by the hydrolysis of titanium isopropoxide renders this material the most suitable for preparing the above catalysts used in NO reduction.

Hydroxyapatite precipitation on a carboxylated vinyl chloride-vinyl acetate copolymer

The kinetics of crystal growth of hydroxyapatite [Ca5(PO4)3OH, HAP], the thermodynamically most stable calcium phosphate phase, on a commercially available carboxylated (1%) vinylchloride (83%) vinyl acetate (13%) copolymer was investigated at relatively low ionic strength, at 37 °C and pH 7.4. Stable supersaturated solutions were seeded with HAP seed crystals as reference and with the polymer in powder form. Well defined induction times preceded the growth of HAP on the polymeric substrate. The induction times measured were inversely proportional to the solution supersaturation. The rates of crystallization both on HAP and on the polymer were measured at conditions of constant supersaturation. HAP growth took place on both substrates and the parabolic dependence of the rates of crystal growth on the relative solution supersaturation suggested a surface diffusion controlled mechanism. The rates of HAP crystal growth on the polymeric substrate were found to depend on the amount of the seed material suggesting selective overgrowth of HAP. A possible explanation for this selectivity is the development of active growth sites through the formation of surface complexes of the calcium ions with the dissociated carboxyl groups.

Mechanistic aspects of the deposition of the Cr(VI) species on the surface of TiO2 and SiO2

Abstract The deposition of the negatively charged Cr(VI) species on TiO2 and SiO2 surfaces suspended in aqueous electrolyte media has been studied in the pH range 3.0–8.0. The investigation included deposition isotherm and microelectro-phoretic mobility measurements. From the experimental results and a theoretical analysis of the deposition isotherms, it was concluded that the CrxOyz− ions taken up were located in energetically equivalent sites at the inner Helmholtz plane (IHP) of the electric double layer that had developed between the support particles and the impregnating solution. The deposition sites were mainly created by protonated surface hydroxyl groups in the case of SiO2 and by protonated and undissociated surface hydroxyl groups in the case of TiO2. The lateral interactions operative among the deposited Cr(VI) species were, in general, strong, being stronger in SiO2 supports in comparison with those of TiO2. Moreover, it was found that upon a pH decrease from 6.0 to 3.0, the extent of deposition of the Cr(VI) species on silica increased from 0.01 to 0.13 μmol Cr(VI) m−2. The extent of deposition of the Cr(VI) species on titania increased also from 0.23 to 1.68 μmol Cr(VI) m−2 with a pH decrease from 8.0 to 4.8. Further decrease in the pH in the case of titania resulted in a considerable decrease in the extent of deposition.