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Crystallization of struvite (MgNH4PO4·6H2O, MAPH) for both N and P recovery from municipal wastewater is an attractive alternative. The low magnesium concentration in municipal wastewater calls for low-cost magnesium sources. In the present work, the precipitation of struvite from supersaturated solutions was investigated. The supersaturated solutions were prepared by the addition of either slurry of Mg(OH)2 (brucite) obtained from the hydrothermal treatment of magnesium oxide (MgO) or using the aqueous solution equilibrated with brucite after separation from the solid. In both cases, in the supersaturated solutions prepared by the addition of ammonium dihydrogen phosphate, the Mg:NH4:P ratio was 1:1:1, pH 7.0–11.0. The apparent order for the precipitation suggested that the dominant mechanism was surface diffusion according to the polynuclear model. The apparent rate constant for struvite precipitation was two orders of magnitude smaller for the slurry prepared supersaturated solution in comparison with the respective prepared from the brucite-equilibrated solution. Simulated wastewater (SWW), was supersaturated with respect to struvite, by inoculation with brucite. The apparent rate constant was two orders of magnitude higher than that corresponding to the supersaturated solutions prepared by the addition of solution equilibrated with brucite. The P recovery from SWW reached 70%.

Microalgae and other microorganisms often play a significant role in the transportation of heavy metal ions in the environment, while at the same time they are closely related to the formation of minerals in aquatic systems, especially with the formation of calcium carbonate (CaCO3). The biomineralization of calcite was investigated in aqueous solutions, supersaturated with respect to calcium carbonate (7.94 < SRcalcite < 31.36) in the presence of heavy metals (Zn, Cd, Ni, Cu) and of colonies of Acutodesmus obliquus (A. obliquus). The presence of metals tested in the supersaturated solutions, at concentration levels below the threshold of precipitation of the respective hydroxides, reduced the rate of calcium carbonate precipitation by 40 to 90% depending on the solution supersaturation. The presence of A. obliquus culture increased the rates of calcium carbonate precipitation by 80%. The presence of the test metals inhibited the growth of A. obliquus, especially the presence of Cd. The uptake of the test metals on calcite fitted Langmuir adsorption isotherm. Cadmium uptake onto calcite reached 85% of the total amount in the solutions. Charged ion pairs of test metals play an important role in their activity with respect to calcium carbonate precipitation and algal growth.

Biomaterials calcify upon implantation in contact with biological fluids, which are supersaturated with respect to more than one crystalline phase of calcium phosphate. The implantation of intraocular lenses (IOLs) for cataract treatment has been hailed as a major advance. Hydrophilic acrylic IOLs, made of Poly(2-hydroxyethyl methacrylate) (PHEMA), upon contact with aqueous humor, exhibit significant incidence of opacification, due to the formation of calcium phosphate crystals, mainly hydroxyapatite (Ca5(PO4)3OH, HAP) on the surface or in their interior. The aqueous humor is supersaturated with respect to HAP. Clinical findings were duplicated by laboratory experiments through the development of appropriate experimental models which included batch reactors, well stirred operating at constant supersaturation (CCR) and reactors simulating anterior eye chamber (ECSR). In both CCR and ECSR, simulated aqueous humor was used. In ECSR the flow rate was the same as in the eye chamber (2.5 mL per 24 h). HAP formed both on the surface and inside the IOLs tested. Induction times preceding the crystallization of HAP on the surface of the IOLs and crystal growth rates were measured. Surface hydroxyl ionized groups favored the development of locally high supersaturation by surface complexation. In the interior of the IOLs, HAP formed by the diffusion of the calcium and phosphate ions inside the polymeric matrix.

Calcification is among the principal causes of biological heart valve substitute failure. Glycosaminoglcans (GAGs) are negatively charged molecules, possessing anticoagulation and anti-inflammatory activity that make them a potential solution against calcification. In the present work, the anticalcification potential of heparin was investigated under constant supersaturation conditions with respect to hydroxyapatite (Ca 5 (PO 4 ) 3 OH; HAP). Heparin concentration in the supersaturated solutions was in the range between 0.25 and 3 ppm, at pH 7.40 and 37 °C. Heparin showed inhibitory activity, which was attributed to adsorption at the active crystal growth centres. Heparin concentration as low as ca. 50 μM, reduced the rate of HAP crystal growth by more than 80%, while further increase (up to 200 μM) failed to completely inhibit the process beyond 90%. Heparin uptake studies at equilibrium conditions and analysis of the kinetics data in the presence of heparin, strongly suggest that the inhibition is due to the adsorption of heparin onto the HAP crystals, which resulted in the poisoning of the active growth sites.

Recalcitrant barium sulfate (BaSO4) scale deposits are commonly encountered in industrial applications. They are hard to remove because of the pH-independence of their solubility. The formation of BaSO4 from supersaturated solutions was studied in two series of experiments: (a) the first series was performed under conditions of high supersaturation to determine the domain of stability of the system and (b) the second at low supersaturations, under conditions in which the supersaturated solutions were stable (seeded growth experiments). From the first series, the stability diagram of the system was determined. Through the second series of precipitation experiments, the effect of two types of triphosphonic acid additives on BaSO4 formation was assessed: the rigid aromatic 1,3,5-benzenetriphosphonic acid (BTP) and the flexible aliphatic amino-tris(methylenephosphonic) acid (AMP). Initially, the pH of the solutions was adjusted at 9.5 so that all phosphonate groups were deprotonated. For both additives, the rate of crystal growth of BaSO4 was inhibited. At low pH (3.60) a decrease of the rate of crystal growth with increasing additive concentrations was found. At pH 9.50 however, an irregular trend was observed showing a relative increase of the rates of crystal growth with increasing inhibitor concentration. These inconsistencies were attributed to changes of the solubility of BaSO4 in the presence of additives. The solubility changes in the presence of each test additive were quantified by measuring the apparent solubility of BaSO4 seed crystals. The measurements showed that the changes were negligible at pH 3.60. Kinetics measurements in the presence of the test additives suggested that their adsorption on the BaSO4 seed crystals was responsible for the inhibition. Modelling the adsorption according to the Freundlich isotherm gave a satisfactory fit of the kinetics data in the presence of BTP and AMP at pH 9.50. The presence of the test additives at both pH values did not affect the morphology of the growing BaSO4 crystals.

International audience; The crystal growth of aragonite was investigated at pH 7.8, 37 °C and constant solution supersaturation from aragonite-seeded supersaturated solutions. The effect of the presence of orthophosphate ions in the supersaturated solution on the kinetics of crystallization of aragonite was investigated over the range of orthophosphate concentrations of 0.25 μM–1 mM. In the presence of orthophosphate in the range of 0.25 μM–8 μM, the crystal growth rate of aragonite decreased with increasing phosphate concentration. At orthophosphate concentration levels exceeding 2 μM, induction times were measured and were found to increase with orthophosphate concentration. At orthophosphate concentration levels > 8 μM, the crystal growth of aragonite was inhibited, suggesting the blockage of the active growth sites by the adsorption of orthophosphate ions. Adsorption was confirmed by the investigation of orthophosphate uptake on aragonite, which was: i) found to depend on the equilibrium concentration of orthophosphate in aqueous solutions saturated with respect to aragonite; ii) not influenced by the ionic strength of the electrolyte up to 0.15 M NaCl, showing that electrostatic interactions between orthophosphate and CaCO3 did not play a significant role in this concentration range. Adsorption data of orthophosphate on the aragonite crystals gave satisfactory fit to the Langmuir adsorption model and was confirmed by XPS analysis.

Sparingly soluble salts precipitation, e.g. calcium carbonate or calcium sulfate, results in pore clogging in rock formations and in the concomitant reduction of the local permeability of oil wells during the oil extraction processes. On the other hand, in situ controlled salt precipitation is desirable in various applications e.g. waterproofing of concrete constructions suffering from leakages, etc. In the present study, calcium carbonate (CaCO3) precipitation in sandbeds was investigated, in the presence of organic solvents simulating the conditions prevalent in oil-well zones. CaCO3 precipitation was investigated from supersaturated solutions prepared by in-situ mixing of NaHCO3 and CaCl2.2H2O solutions before the inlet of sandbeds. The solution resulting from the mixing of the two solutions was supersaturated with respect to all calcium carbonate polymorphs. Three types of experiments were performed depending on the supersaturated solutions: a) aqueous solutions b) aqueous supersaturated solutions in contact with sandbeds pre-saturated with n-dodecane c) aqueous solutions containing monoethylene glycol (MEG). Results showed that oil–water interfaces enhanced the heterogeneity of the supersaturated solutions and accelerated crystal growth of calcium carbonate at the inlet of the sandbed, resulting in early pore clogging and limitation of local permeability. Maximum sandbed consolidation was obtained with the solutions containing MEG.

Visualization experiments using one-dimensional (1-D) porous media made of Plexiglas and two-dimensional glass porous networks were conducted to obtain qualitative and quantitative information concerning the precipitation and crystal growth of CaCO3 under varying flow and concentration conditions. Supersaturated solutions were prepared by mixing sodium bicarbonate and calcium chloride solutions before the pore networks. Nucleation and crystal growth were assumed to occur within the porous media. Changes in the initial and final solution composition were monitored. At low initial supersaturation values (SRinitial), a few crystals were observed within the flow channels and crystal growth took place exclusively on the newly formed crystals. As the SRinitial increased, more crystals were formed along the flow channels and new crystallites were continuously formed during the course of the experiments. Nucleation and crystal growth were not uniform. The crystal growth rates depended on the initial value of SR a...

The crystallization of sodium sulfate decahydrate (Na 2 SO 4 ·10H 2 O, mirabilite) from supersaturated solutions was investigated using stable supersaturated solutions seeded with mirabilite seed crystals. The experiments were done in batch, stirred reactors in which the supersaturated solutions were prepared either by dissolution of sodium sulfate anhydrous at 32 °C followed by cooling to 18 or 20 °C or by mixing equal volumes of equimolar ammonium sulfate and sodium hydroxide solutions at 20 °C. Inoculation of the solutions supersaturated only with respect to mirabilite with seed crystals was accompanied with temperature increase of the thermostated solution. Despite the fact that crystal growth was initiated with seed crystals, the process started past the lapse of induction times inversely proportional to the solution supersaturation. The rates of crystal growth were measured both from the temperature rise and from the concentration–time profiles, which were linearly correlated. The measured crystal growth rates showed a parabolic dependence on supersaturation at low supersaturations. For higher values this dependence changed to linear, a behavior consistent with the BCF spiral crystal growth model. The morphology of the crystals growing at 20 °C showed typical prismatic habit, while at 18 °C when crystallized from cooled sodium sulfate solutions changes in the crystal habit to a leaf like morphology were observed.

The hierarchically structured glass sponge Caulophacus species uses the first known example of a silica and calcite biocomposite to join the spicules of its skeleton together. In the stalk and body skeleton of this poorly known deep-sea glass sponge siliceous spicules are modified by the addition of conical calcite seeds, which then form the basis for further silica secretion to form a spinose region. Spinose regions on adjacent spicules are then joined by siliceous crosslinks, leading to unusually strong cross-spicule linkages. In addition to the biomaterials implications it is now clear, from this first record of a biomineral other than silica, that the hexactinellid sponges are capable of synthesizing calcite, the ancestral skeletal material. We propose that, while the low concentrations of calcium in deep sea waters drove the evolution of silica skeletons, the brittleness of silica has led to retention of the more resilient calcite in very low concentrations at the skeletal joints.

The adhesion of human erythrocytes settled for different times (15, 30 and 45 min) onto opaque zirconia surfaces of different surface roughness (Ra 0.08, 0.8 and 1.0 µm) was investigated with detachment experiments. The parallel plate/laminar flow experimental set up was employed, whereby the cells were subjected to increasing shear stress, applied via controlled flow of phosphate buffer solution (PBS), and gradually detached. The experimental results showed that the critical shear stress, τs,c, corresponding to 50% detachment, was about 15 dyn/cm2. The distribution curves of attachment suggested that smoother surfaces and longer settling times favoured strengthening of cells adherence, but the influence of settling time was sounder in smoother than in rougher surfaces. Assuming that the cells’ shape relaxes over prolonged settling time, a geometric model, providing satisfactory interpretation of the experimental results, is consistent with the prediction that the surface morphology precisely determines the maximum extent of erythrocyte/surface contacting area and consequently the adhesion strength.

Growth and dissolution of calcite {1014} surfaces in aqueous solutions in the presence of fluoride ions have been studied by in situ atomic force microscopy (AFM). Supersaturated and undersaturated solutions with respect to calcite were prepared for the growth and dissolution experiments, respectively. The concentration range of solutions containing fluoride ions varied between 0.025 and 50 mM. The crystal growth rates were measured from the step growth of closing of etch pits along the [010] direction as well as the spreading of two-dimensional growth nuclei in the same direction. Dissolution rates were measured from the time-dependent development of the rhombohedral etch pits along both [010] and [421] directions. Low fluoride concentrations (≤0.33 mM) in the supersaturated solutions did not significantly affect the crystal growth rates of calcite. At higher concentrations (up to 5 mM), the growth rate decreased substantially, reaching 50% of the respective rate in the absence of fluoride for the same...

A novel methodology for the synthesis of magnesium ammonium phosphate (MgNH4PO4·6H2O, MAP or struvite) through precipitation from supersaturated solutions was developed. The development of supersat...

The formation of phosphorus containing sparingly soluble salts from supersaturated solutions which may be formed at locations such as subsoil or in subsediment formations consisting of loose sand is a promising alternative solution for attaining stabilization of loose particulate systems. In the present work, we present a novel methodology for the development of supersaturation by the release of phosphate through the hydrolysis of polyphosphate substrates by phosphatase enzymes (acid and alkaline). The phosphate released through the enzymic activity in the presence of excess of alkaline earth metals concentrations are critical in developing the supersaturation needed for the precipitation of the respective phosphate salt. In the present work, the methodology was demonstrated by the precipitation of calcium phosphate and magnesium ammonium phosphate in silicic sand and topsoil particulate samples. The rate of the enrichment of the solutions with phosphates and the extent of decomposition of the substrates ...

The effect of citrate and phosphocitrate ions on the spontaneous precipitation of magnesium ammonium phosphate hexahydrate (MgNH4PO4·6H2O, struvite) at pH 8.50 and 25 °C was investigated at conditions of constant solution supersaturation. The supersaturated solutions were prepared in synthetic wastewater. The presence of citrate and phosphocitrate in the working solutions showed reduction of the initial rates of struvite precipitation when compared with solutions in their absence. The parabolic dependence of the rates of precipitation on the relative solution supersaturation suggested a surface-diffusion-controlled mechanism. Presence of citrate and phosphocitrate in the supersaturated solutions at concentration levels from 1 to 10 µM resulted in a drastic increase in induction times. The presence of citrate and phosphocitrate inhibited struvite precipitation up to 75–80%. The reduction of precipitation rates was related to additive concentration following an expression suggested by the kinetic Langmuir-t...

The present work deals with the influence of pH and temperature on the spontaneous precipitation of calcium phosphate from simulated milk ultrafiltrate (SMUF) solutions. The pH range investigated is 5.7−7.0, and the temperature varied from 55 to 75 °C. It was found that the precipitates were prisms of hydroxyapatite (HAP) with low crystallinity preceded by amorphous calcium phosphate (ACP). Crystallinity improved with solution aging. Microscopic particles in the range of 200 nm were obtained at relatively high supersaturation (pH = 6.8), whereas at relatively low supersaturation (pH = 6.0), aggregates in the range of 1 μm were formed. Moreover, it was found that although the SMUF solution is also supersaturated with respect to magnesium phosphate, no magnesium salt was identified in the precipitates, thus precluding the coprecipitation of magnesium phosphate. At the same temperature, the supersaturation was regulated by adjusting the solution pH. Kinetics study of precipitation showed a parabolic dependen...

The rates of dissolution of calcitic Carrara marble have been reported to be significantly reduced in alkaline pH (pH 8.25) at 25 degrees C in the presence of (1-hydroxyethylidene)-1,1 diphosphonic acid (HEDP). The adsorption takes place at the calcite/water interface at the double layer through the interaction of charged surface species with the charged solution species of the adsorbate. The present work focused on obtaining a better understanding of the interaction of the calcite surface with HEDP. Calculations were performed according to the triple layer model, assuming the formation of surface complexes between the charged surface species of calcite and the species of HEDP dominant at pH 8.25. According to the model, the adsorbed species are located at the inner Helmholtz plane of the electrical double layer. Strong lateral interactions between the adsorbed species were suggested and were corroborated from the calculation of the respective energy, which was equal to 69 kJ mol(-1). The adsorption isotherm was consistent with the proposed model at low surface coverage values, while discrepancies between the values experimentally measured and the predicted were found at higher adsorbate concentrations. The deviations from the predicted values were attributed to the fact that HEDP adsorption on calcite resulted in the formation of multiple layers. The model explained adequately the changes in the zeta-potential values of calcite in the presence of HEDP in the solution which resulted in charge reversal upon adsorption.

Phosphorus recovery from wastewater by precipitation in the form of crystalline struvite is an attractive option contributing toward sustainable development. Struvite or magnesium ammonium phosphate hexahydrate (MgNH4PO4·6H2O) is a crystalline salt, which may be used either as a fertilizer or as raw material for the production of phosphorus. The supersaturation of wastewater with respect to the target mineral is the key parameter determining the extent and the rate of recovery of struvite. In the present work, we investigated the stability domain of struvite in synthetic wastewater (SWW) and report on the kinetics of precipitation of the respective salt forming spontaneously. The investigation was carried out in aqueous solutions with compositions typically encountered in municipal wastewaters. The supersaturation with respect to struvite was varied through the appropriate variation of the concentrations of the Mg2+, NH4+, and PO43- ions, for which the stoichiometric molar ratio of 1:1:1 was observed. All...

The dissolution of powdered Carrara marble was investigated in the absence and in the presence of (1-hydroxyethylidene)-1,1-diphosphonic acid (HEDP) at conditions of constant undersaturation at pH 8.25. The dependence of the rates of dissolution on the relative undersaturation suggested a surface diffusion controlled mechanism. According to this mechanistic information, the interaction of HEDP with calcitic Carrara marble was investigated. Two types of interaction of the HEDP with the marble suface were studied: preadsorption and addition of HEDP in the undersaturated solutions (1 μM to 1 mM). In both cases the rates of dissolution were drastically reduced depending on the concentration of the organophosphorus compound on the marble surface or in the undersaturated solutions. The rate reduction was attributed to the poisoning of the active dissolution sites through the uptake of HEDP on the calcite crystallites of the Carrara marble. The HEDP uptake by marble was measured quantitatively. The zeta potenti...

The effect of phospho-l-serine in supersaturated solutions of calcium phosphate was investigated at plethostatic conditions. Phospho-l-serine inhibited the crystal growth of hydroxyapatite {Ca5OH(P...

The kinetics of dissolution of powdered Pentelic marble from two different sites located on Mount Pentelicon of Attica, Dionysos and Cave, were investigated. The results were compared with those of synthetically precipitated calcite, the main inorganic constituent (>98%) of the tested marbles. The dissolution process at solution undersaturation near equilibrium in all cases showed a linear dependence of the rates of dissolution on the relative solution undersaturation. The dissolution was found to be polynuclear, surface diffusion controlled. Morphological examination of the specimens showed that dissolution proceeds through formation of steps, cracks, pores, and layerwise disintegration, corroborating the conclusion from the kinetics results. Powdered marble is suggested to be an excellent model system for the investigation of marble dissolution as it exhibited remarkable similarities with the dissolution of whole marble blocks.

The kinetics of the spontaneous precipitation of struvite was investigated in aqueous supersaturated solutions containing stoichiometric concentrations of Mg 2+ , NH 4 + and PO 4 3− ions, ionic strenght 0.15 M NaCl and at 25°C in a batch, stirred reactor at constant supersaturation. The induction times preceding the onset of struvite precipitation and the initial rates of precipitation were measured directly from the traces of titrants added in order to maintain the solution supersaturation. From the measurement of the induction times as a function of the solution supersaturation, the stability diagram of the system was constructed. In all cases the only solid-phase forming was identified as struvite. Kinetic analysis of the rates, which depended strongly on the solution supersaturation yielding a second-order dependence, suggested a surface diffusion mechanism. The precipitated struvite crystals showed a high negative charge which increased as a function of the solution pH while the presence of magnesium ions affected the microelectrophoretic mobility of struvite dispersions yielding an isoelectric point at pMg of 1.75.

Among semiconductors, metal sulfides are very interesting materials that may be used for solar energy conversion arrangements. The authors prepared a series of Cu{sub x}Ni{sub 1{minus}x}S powders, where x = 0.05, 0.20, 0.50, and 0.80, by spontaneous precipitation. Various stoichiometries of Cu(NO{sub 3}){sub 2}, Ni(NO{sub 3}){sub 2}, and (NH{sub 4}){sub 2}S solutions at 25 C and pH 2.50, at conditions in which the aqueous solutions were supersaturated with respect to CuS and NiS, yielded precipitates corresponding to intermediate mixed sulfides. The appearance of new peaks in the powder X-ray diffraction spectra suggested the formation of new phases. Measurements of the resistance of the bulk solids as a function of temperature for the preparations with x = 0.05, 0.20, and 0.50 showed a decrease with increasing temperature. A hysteresis was found upon thermal cycling of the sulfide preparations, possibly due to changes in the solid state that affect the energy bands of the semiconductors. The Cu and Ni sulfide preparations with x = 0.80 showed metallic behavior. All preparations were highly charged with suspended in aqueous media, and the electrical charge was found to depend strongly on the activity of copper ion in solutions equilibrated with the solid phase. Finally, currentmore » potential curves of electrodes prepared from the precipitated sulfides under constant illumination gave values for the open circuit potential as high as 0.868 V, with 0.119 mA for the photocurrent and 0.58 for the field factor. These values are encouraging for the potential application of these materials in cells for the conversion of solar into electric energy.« less

Poly(vinyl chloride-co-vinyl acetate-co-maleic acid) (vinyl chloride 83%, vinyl acetate 13%, carboxylated 1%) was found to be a substrate favoring the deposition of vaterite crystals from stable supersaturated solutions at pH 8.50 and 25 °C. Induction times preceding calcium carbonate precipitation were inversely proportional to the solution supersaturation, and a surface energy of 23 mJm-2 was calculated according to classical nucleation theory. The relatively low value may be attributed to the heterogeneous character of vaterite nucleation. The linear dependence of the rates of vaterite formation on the solution supersaturation, in which the crystallization took place, in combination with the independence of the measured rates on the fluid dynamics, suggested that vaterite overgrowth was controlled by surface diffusion. This finding was in agreement with the results obtained for the crystallization of vaterite on cholesterol. Our results suggest that the kinetics of overgrowth may be very important for ...

A series of Cu−Cd sulfides were prepared by coprecipitation in aqueous supersaturated solutions at 25 °C, pH 2.50. The solids obtained were characterized by powder X-ray diffraction and their elemental composition was determined by chemical analysis. The results showed that in all cases mixed CuS−CdS precipitates were prepared, the ratio of each phase depending on the composition of the supersaturated solutions. A series of precipitates, of (CuS)x (CdS)1-x, were prepared with x = 0.0, 0.20, 0.25, 0.30, 0.35, 0.50, and 1.0. All polycrystalline preparations with x = 0.30 showed metallic behavior typical for CuS, while the mixed sulfides with x = 0.20 and 0.25 showed semiconducting behavior, which however could not be described by any of the models reported in the literature. The band gap energies of 2.40 and 2.39 eV calculated for the two preparations showed that at this composition the material was dominated by the behavior of CdS. The activation energies estimated from the dependence of the solids conduct...

While calcification of bioprosthetic valves presents a clinical problem, the mechanism of formation of calcific deposits in the leaflets remains unclear. A new method for in vitro calcification is employed, in parallel with an in vivo (subcutaneous animal) model. The nature of crystal phases grown by both methods on porcine bioprosthetic heart valves was investigated. Light microscopy, scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (SEM-EDS) and stoichiometric chemical analysis were used as tools for the qualitative and quantitative assessment of the calcification process. Photomicrographs from leaflets calcified early in vitro (24–48 h) and in vivo (5–10 days) confirmed that calcification initiated in the central region of the tissue, mainly in fibrosa, and extended with time (after 28 and 56 days) in vivo towards the outer surface of the tissue. SEM micrographs from tissue sections of early in vivo and in vitro calcified leaflets showed coexistence of different sizes (1–30 μm) of crystal phases in contact with tissue fibers. EDS analysis confirmed that the deposits were calcium phosphate salts. Chemical analysis of samples calcified in vivo for longer time periods, showed that the content of the salts was mainly calcium and phosphate with a Ca/P molar ratio 1.78 and 1.94 after 28 and 56 days, respectively. These results suggested that the calcification process is followed by a sequential hydrolytic transformation of CaHPO 4 ·2H 2 O (DCPD) to Ca 4 H(PO 4 ) 3 ·2.5H 2 O (OCP) and to Ca 5 (PO 4 ) 3 OH (HAP). Calcium phosphate crystal phases grown in vitro under controlled conditions and at constant supersaturation provide a successful simulation of the calcification process in the animal model and may be applied in screening new biomaterials with respect to their potential for calcification.

The role of carbonate on the kinetics and physicochemical characteristics of the crystal growth of stoichiometric hydroxyapatite was investigated at 37 °C, 0.15 M NaCl ionic strength, and pH 7.40 at constant supersaturation. It was found that at low supersaturations the presence of carbonate ions at concentration as high as 0.1 mM had no effect on the crystal growth kinetics while at higher supersaturation (>3.8) the presence of carbonate ions resulted in reduction of the crystal growth rates and changed the dependence of the rates of crystal growth on the solution supersaturation from parabolic to linear. Carbonate was incorporated to the apatite growing on the seed crystals and caused changes in their morphology, favoring platelike formations.

The spontaneous precipitation of calcium sulfate in supersaturated solutions over the temperature range between 25.0 and 80.0 °C was investigated by monitoring the solution specific conductivity during desupersaturation. From measurements of the induction times preceding the onset of precipitation the surface energy of the forming solid, identifield as gypsum, was found between ca. 12 and 25 mJ m-2 for the temperature range between 80.0 and 25.0 °C, respectively. Kinetics analysis showed that over 50 °C it is possible that anhydrous calcium sulfate is forming as a transient phase converting into the more stable calcium sulfate dihydrate. The linear dependence of the rates of precipitation on the relative solution supersaturation suggested a mechanism according to which the growth units are integrated into the active sites of the supercritical nuclei by surface diffusion. According to the morphological examination of the crystals it is possible that crystal growth occurs by the advancement of steps.

The kinetics of spontaneous precipitation of calcium sulfate dihydrate at pH 7.0, 30°C were investigated in the presence of organophosphorus compounds. The measurement of the kinetics parameters was done at conditions of constant supersaturation probing the conductivity of the supersaturated solutions. Aqueous solutions of 1-hydroxy-ethylidene-1,1-diphosphonic acid (EHDP), nitrilotrimethylene phosphonic acid (NTMP) and N,N,N′,N′-ethylene diaminotetramethylene phosphonic acid (ENTMP) were included in unstable supersaturated solutions of calcium sulfate. NTMP and ENTMP at concentration levels between 0.1 and 1.0 μM increased drastically the induction times preceding precipitation while the rates of precipitation were reduced by 90% at concentrations below 1μM. EHDP was effective in reducing the precipitation rates but at considerably higher concentrations (70% reduction at 0.1 μM). Examination of the morphology of the precipitated crystals suggested that the kinetic results obtained could be attributed to the adsorption of the organophosphorus compounds onto different gypsum crystal faces. EHDP did not inhibit the [0 2 0] face resulting in elongated prismatic crystals, while NTMP and ENTMP favored the development of [0 2 0] resulting in shorter and broader crystallites.

Transition metal sulfides, because of their electronic and spectroscopic properties, are a very interesting class of materials that may be used efficiently for the construction of photoelectric chemical solar cells. In the present work we have prepared a series of mixed Cu and Zn sulfides with the general stoichiometry Cu x Zn 1-x S by coprecipitation in aqueous solutions. Precipitates with x = 0.10, 0.15, 0.20, 0.22, 0.25, and 0.50 were synthesized by mixing metal nitrates with ammonium sulfide at pH 2.50. The precipitates behaved as n-type semiconductors. The dependence of the conductivity of the materials on temperature and calculations of the thermoelectric power coefficient showed the presence of metallic conducting islands separated by nonconducting parts, for copper content corresponding to x = 0.15, 0.20, and 0.22. For x = 0.10, the precipitates behaved as insulators, whereas forx = 0.50, the precipitates showed metallic character. The morphology and the mean particle size of the precipitates varied with increasing Zn content, yielding for pure CuS, prismatic crystals with a mean size of ∼5 μm. Upon increasing Zn content, polycrystalline materials were obtained, forming aggregates of submicron, spherullitic particles. The measurements of the electrical properties of the materials showed that the conditions of precipitation play an important role for the design of their properties.

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.

The crystal growth of pyrite in aqueous supersaturated solutions at pH 6.50, 25 °C was investigated using the seeded growth, pH-stat technique. Crystal growth started immediately after inoculation of iron(II) sulfide supersaturated solutions with pyrite seed crystals. The crystal growth rates measured from the desupersaturation curves were found to depend strongly on the relative solution supersaturation with respect to pyrite. Kinetics analysis yielded an apparent order of 3.5 ± 0.5 suggesting a surface diffusion controlled mechanism. The presence of organophosphorus compounds such as 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), N,N,N',N'-ethylenediaminetetrakis(methylenephosphonic acid) (ENTMP) in the supersaturated solutions at concentration levels between 0.1 and 1000 nM inhibited the crystal growth of pyrite by 70-100% with respect to the blank experiments. Moreover, in the presence of the compounds tested the crystal growth of pyrite was preceded by well-defined induction times which increased sharply with inhibitor concentration. At concentration levels as low as 1 mM, crystal growth was completely suppressed. The presence of nitrilotris(methylenephosphonic acid) (NTMP) had no inhibition effect on pyrite crystal growth. The influence ofthe inhibitors tested on the kinetics of crystal growth was attributed to the blocking of the active growth sites due to adsorption.

The nucleation of e-caprolactam in melts containing water impurities between 3% and 7% w/w was investigated in a batch reactor monitoring melt temperature. The induction times preceding the formation of e-caprolactam nuclei were inversely proportional to the melt supersaturation and a threshold in the supersaturation of about 6% was found for homogenous nucleation. The interfacial tensions calculated using the classical nucleation theory ranged between 0.8 and 1.5 mJ m −2 for water content of 3%–7%, respectively. The concomitant increase of the size of the critical nucleus suggested that water inhibits the process of nucleation of e-caprolactam in melts containing water impurity. This conclusion was further confirmed for the rates of crystal growth of the e-caprolactam in the melts measured from the rates of temperature increase with time.

The kinetics of the spontaneous formation of iron sulfides were studied in low ionic strength deaerated aqueous solutions at 25°C at both nearly neutral and moderately acidic pH. The constant supersaturation approach was adopted for this investigation. In nearly neutral pH (7.1–7.4) mackinawite precipitated directly, while at pH 5.4–5.6 amorphous FeS formed, showing marked sensitivity to oxidation by atmospheric oxygen. The rates of precipitation were found to be proportional to the relative solution supersaturation and the value m = 2 was found for the apparent rate order of precipitation. The investigation was extended at 80°C at comparable supersaturations with the 25°C experiments using the free drift method. At pH > 7, only iron oxides were produced while at pH 5.5 amorphous FeS precipitated. The rates of precipitation measured, showed a first order dependence on the relative solution supersaturation. The apparent orders of 2 and 1 for the precipitation of amorphous iron sulfide at 25°C and 80°C, respectively, in combination with an estimated activation energy of 40 kJ mol −1 and with the insensitivity of the kinetics measured on the fluid dynamics suggested a surface controlled process.

The dissolution of calcium carbonate polymorphs was investigated at conditions of constant undersaturation at neutral pH, 25°C. The kinetics data suggested that the order of dissolution velocity is: aragonite calcite ≈ vaterite. The calcium carbonates with phosphate uptake gave reduced dissolution rates but a stronger dependence on the solution undersaturation was exhibited in comparison with the untreated crystals.

The spontaneous precipitation of strontium sulfate from equimolar supersaturated aqueous solutions was investigated under conditions of constant supersaturation. The stability diagram of the system was drawn and it showed a marked inverse dependence of the induction times on the solution supersaturation. At supersaturation ratios Ω ≥ 8, the precipitation was homogeneous and a value of 88 mJ m−2 was calculated for the precipitating strontium sulfate. The rates of precipitation showed a first-order dependence on the relative solution supersaturation. This order is consistent with a surface-controlled mechanism suggested in the literature. At higher supersaturations however (Ω ≥ 8), the order increased to 4 suggesting prevalence of the nucleation process. The habit of the crystals formed included rhombohedral and hexagonal shapes irrespective of the solution supersaturation, within the range examined in the present work.

Microemulsion gels and coatings have been obtained by the sol-gel method using titanium(IV) isopropoxide. Three types of fine water dispersions have been used as the basis sol: reverse Triton X-100 micelles in cyclohexane; quaternary water-in-oil microemulsions containing cyclohexane, 1-pentanol, sodium dodecyl sulfate, and water; and dispersions of water in pentanol in the presence of sodium dodecyl sulfate. Titanium(IV) isopropoxide was dissolved in the continuous phase and hydrolyzed by the dispersed water. Gelation occurred in a few hours in the case of cyclohexane, as the solvent of the continuous phase, and in few days in the case of pentanol. The gelation processes and the obtained microenvironment were probed with fluorescent probes. Films of thickness of the order of a hundred nanometers can be obtained by dipping glass, plastic, or aluminum plates into the solutions at their early stage of gelation. The absorption spectra of the films were characteristic of TiO[sub 2] absorption. X-ray analysis of films heated to 400-500[degree]C revealed a rutile structure. 44 refs., 8 figs., 2 tabs.

The kinetics of nucleation and crystal growth of octacalcium phosphate [Ca8(PO4)6H2·5H2O, OCP] from calcium phosphate supersaturated solutions inoculated with seed crystals was investigated at 37 °C, pH 7.40, at conditions of constant solution supersaturation. Stable calcium phosphate solutions, supersaturated with respect to OCP and hydroxyapatite [Ca5(PO4)3OH, HAP] were inoculated with calcium carbonate-based bone cement powder consisting of mixed aragonite and calcite crystals and with well-characterized calcite seed crystals. On all substrates tested, OCP nucleated followed by crystal growth of the nuclei formed past the lapse of induction times, inversely proportional to the solution supersaturation. From the dependence of the induction time on the solution supersaturation with respect to OCP, a value of 10 mJ m−2 was calculated for the nucleating phase. The rates of OCP crystal growth on the carbonate substrates showed linear dependence on the solution supersaturation that in combination with the independence from the fluid dynamics in the reactor suggested a surface diffusion-controlled mechanism. Moreover, the independence of the crystallization rates on the amount of the inoculating seed crystals suggested that nucleation and growth took place exclusively on the crystalline substrates. The transient calcium phosphate phase, OCP, was stabilized in our experiments, and it was the only phase growing at constant driving force, despite the fact the solutions were supersaturated with respect to HAP as well.

The formation of calcium carbonate in artificial seawater was investigated at conditions of constant solution supersaturation at pH 8.50 and 25°C. It was found that the only phase growing was aragonite irrespective of the substrate. The key factor is the magnesium concentration in solution. The nucleating capability was found to be aragonite > vaterite > calcite, while the apparent reaction orders were rather high for the heterogeneous nucleation of aragonite. The polynuclear model gave a satisfactory fit for the overgrowth on calcite and vaterite.

Conducting polypyrrole polymers were doped during polymerization with {alpha}-CdS crystals. Doping made the polymers effective nucleators of {alpha}-CdS. Once the doped polypyrrole specimens were introduced into supersaturated cadmium sulfide solutions, {alpha}-CdS precipitated immediately without any precursor phase or induction period. The rates of precipitation were found to depend on the solution supersaturation and were maximum at an optimum dopant concentration of 6.4% w/w. Increasing dopant concentrations decreased the polymer conductivity and increased its p-semiconducting character, possibly through structural changes induced by doping.

The growth of one crystalline phase on the surface of another that offers a good crystal lattice match, may be important in environmental, physiological and pathological mineralization processes. The epitaxial relationships and kinetics of growth of hydroxyapatite on crystals of dicalcium phosphate dihydrate, calcium fluoride and calcite have been studied at sustained low supersaturation with respect to hydroxyapatite. At the very low supersaturations, the crystallization of hydroxyapatite takes place without the formation of precursor phases. The experimental results are in agreement with theoretical predictions for epitaxial growth, while the kinetics of hydroxyapatite crystallization on the foreign substrates is the same as that for the growth of hydroxyapatite on synthetic hydroxyapatite crystals.

The kinetics of growth of dicalcium phosphate dihydrate seed crystals has been investigated by a method in which the activities of the lattice ions were maintained constant during the reaction. The constant composition procedure has also been used to study the crystallization of hydroxyapatite at very low supersaturation. Changes in specific area during the resulting appreciable extents of growth have been used to predict the developing morphologies of the crystals. For dicalcium phosphate dihydrate and hydroxyapatite, the predominant crystal growth takes place in two dimensions and one dimension, respectively. In both cases the growth rates are pH dependent. Dimensionless representation of the crystallization rates and driving forces enable comparisons to be made between data for electrolytes of different charge types.