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Background and aim of the study Calcification remains the major role of failure of implantable biomedical material and in particular of bioprosthetic valves. Various treatments have been proposed to mitigate calcification of glutaraldehyde-fixed bioprosthetic valves but none have succeeded in inhibiting or mitigating efficiently the calcification process of the implantable biological tissues. Since the discovery of calcitonin (CT) and its therapeutic role in treating hypercalcemic patients, CT has never been tried as an anticalcification treatment for biomaterials. It is postulated, that tissue calcification may be efficiently minimized by forming adducts with aldehyde groups thus eliminating the places of the biological tissues onto the calcium cations could be deposited. Material and methods Fresh porcine aortic leaflets were cut radially in three parts. Three groups of tissue were created. Group I (glutaraldehyde only), Group II (glutaraldehyde with 1% CT) and Group III (glutaraldehyde with 10% CT). All tissues were then implanted subdermally in three sets of 8 (Group I) and 9 (Group II and Group III) male Wistar rats of 12 days old. 21 days later the rats were euthanized by inhalation of CO2. The tissues were retrieved and after rinsing with distilled water 3 times, were lyophilized at −40 °C at high vacuum pressure of approximately 100 mmHg for 16 h. The calcium content was then measured with flat atomic absorption technique. Results The preimplantation values of Ca concentration as expressed in mg Ca/g of tissue were 1.79 ± 0.14 in Group I, 4.78 ± 0.0079 in Group II and 2.88 ± 0.17 in Group III (p = ns). 21 days later the values of Ca concentration were 126.95 ± 12.97 for Group I, 24.69 ± 2.71 for Group II (p Conclusion An anticalcification treatment based on calcitonin as an additive to buffered glutaraldehyde, mitigates the calcification process of the implantable biological tissues, as compared to glutaraldehyde treatment only.

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.

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.

The crystal growth of monetite (CaHPO 4 , DCPA), the least stable calcium phosphate has been investigated by the seeded growth method from solutions supersaturated exclusively with respect to this phase alone. The kinetics of crystal growth was measured at constant supersaturation at 37° C, pH 6.0. The measurements of the rates of crystal growth of DCPA showed a second order dependence on the relative supersaturation of the solutions with respect to DCPA, suggesting that the rate determining step is surface diffusion of the growth units to the active growth sites of the crystals. The heterogeneous crystallization of brushite (CaHPO 4 ·2H 2 O, DCPD) on DCPA seed crystals in solutions supersaturated with respect to both DCPD and DCPA showed that DCPD nucleation and growth took place selectively by heterogeneous nucleation on the DCPA seed crystals. The same parabolic dependence of the rates on the relative supersaturation of the solutions with respect to DCPD was found, showing that the mechanism was the same as in the case of growth of DCPA on DCPA seed crystals. Seeding of solutions supersaturated with respect to both DCPA and DCPD at pH 6.0, 37 °C with collagen type II in powdered form, resulted to the formation of an apatitic solid. The morphology of the crystals formed on the collagenous matrix suggested that the identified mineral was probably the result of rapid hydrolysis of metastable calcium phosphatephases. The formation of the mineral phase on collagen took place past long induction time periods inversely proportional to the relative supersaturation with respect to DCPD and showed that the collagen substrate did not stabilize calcium phosphate at the same conditions at which DCPA did.

Purpose To investigate the nature and characteristic features of deposits causing opacification of intraocular lenses (IOLs) based on the examination of clinical findings using scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDX) analysis. Design Retrospective, observational case series. Methods This is a multicenter study of 6 hydrophilic acrylic IOLs (Lentis LS-502-1; Oculentis GmbH, Berlin, Germany) with a hydrophobic surface that were explanted from 5 patients because of opacification. Three patients had an uncomplicated phacoemulsification. One patient underwent combined phacoemulsification and pars plana vitrectomy for retinal detachment and later silicone oil endotamponade owing to redetachment. The last patient had a pars plana vitrectomy and silicone oil instillation combined with phacoemulsification for tractive retinal detachment and diabetic retinopathy. The explanted lenses were submitted to our laboratory and were examined by SEM and EDX in order to identify the morphologic features and the composition of the deposits. Results SEM and EDX analyses confirmed the presence of calcific deposits in the interior of the opacified hydrophilic IOLs, with a pattern showing the formation of lumps on the surface. The lumps were due to subsurface formation of calcium phosphate crystalline deposits. The crystallite clusters seemed to diffuse from the IOL interior to the surface. Conclusions We demonstrated the calcification pattern of the hydrophilic IOL (Lentis LS-502-1) with a hydrophobic surface. Although hydrophilic acrylic lenses have a hydrophobic surface, the development of calcification is a possible threat initiating from the hydrophilic subsurface of the IOLs.

Although landfilling and disposal into the ocean present still the most common methods used in the sewage sludge management, the share of recycling and reuse based on drying and incineration procedures continuously increases during the last decades. In this paper, an energy-efficient thermal treatment of sewage sludge is proposed, aimed at its use in blended cements. A practical application of the designed treatment is performed for the sewage sludge obtained from the municipal wastewater treatment plant in Patras, Greece. The material characterization shows that the sludge thermally treated at 700 °C has significant amounts of silica, alumina, calcium oxide and iron oxide, while the content of amorphous phase is 41%, which results in a good pozzolanic activity. The mechanical properties of hardened cement pastes prepared using blended Portland cement-thermally treated sewage sludge binders are after the 28-days curing period not satisfactory yet, indicating that a substantial part of the sludge acts still as filler only. This discrepancy between the pozzolanic activity and mechanical tests can be explained by too coarse sludge particles which slow down the pozzolanic reaction. The results of chemical tests show that the safe limit for the practical use of sewage sludge as partial Portland cement replacement is 10%, which is mainly due to the relatively high content of chlorides and alkalis. This limit can though be increased by the removal of soluble salts from the raw sludge using water treatment. The life cycle assessment analysis of the blended cements shows that the decrease in energy consumption is nearly 10% for each 10% of sewage sludge in the blend. The emission of greenhouse gases during the thermal treatment of the sludge is found almost twenty times lower than in the Portland cement production. The cost analysis reveals substantial economical benefits related to the application of sewage sludge as partial cement replacement.

The formation of inorganic scale deposits of the alkaline earth metals is a persistent problem. Dispersion of solid particles separating out from the fluids is also very important for fouling due to deposition. Scale formation and stabilization of suspensions are often overcome through the use of water soluble polymers. In the present work, a series of polymeric compounds were tested as inhibitors of calcium carbonate and calcium sulfate dihydrate (gypsum) precipitation and as dispersion agents of hematite (Fe2O3) suspensions in electrolyte solutions. Acrylic acid (AA) and maleic acid (MA) polymers were found to inhibit the precipitation of calcium carbonate and gypsum from supersaturated solutions to extents exceeding 90% at concentrations as low as 2 ppm. The molecular weight (MW) was an important parameter in determining the activity of the tested inhibitors. Lower MW polymers (ca. 2000) proved to be more efficient than higher MW polymers. A similar trend was exhibited for the dispersion ability of the hematite particles. AA and MA copolymers in which functional groups were introduced (pyrrole, sulfono and amide groups) were efficient precipitation inhibitors and dispersion agents, but the efficiency depended strongly on the co-polymer architecture. Copolymers containing sulfono groups improved inhibitory activity and dispersion ability and showed higher calcium ion tolerance.

In the present work, the effect of the presence of strontium ions in supersaturated calcium phosphate solutions on the rates of crystal growth of octacalcium phosphate (Ca 8 H 2 (PO 4 ) 6 ·5H 2 O, OCP) and of hydroxyapatite (Ca 5 (PO 4 ) 3 OH, HAP) was investigated at 37 °C and at constant solution supersaturation. The presence of strontium reduced crystal growth rates of both calcium phosphate phases. The reduction of the OCP crystal growth was ca. 40% for strontium concentration of 1 mM while for HAP was significantly higher, reaching 70% for the same concentration of strontium in the solution. Higher retardation of the rates of HAP crystal growth was achieved at higher supersaturations in the presence of 1 mM strontium concentration. Moreover, strontium ions were shown to stabilize or delay the transformation of the unstable OCP that tends to hydrolyze to the thermodynamically more stable HAP. Strontium ions were incorporated into both OCP and HAP growing on the respective seed crystals.

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.

A new method for the consolidation of loose sand formations has been developed. The method involves in situ precipitation of a composite calcium phosphate–polyelectrolyte salt that binds together with loose sand grains, thus resulting to their consolidation. Three different polyelectrolytes (PE) were tested, i.e., polyacrylic acid (PAA), polyallylamine hydrochloride (PAH), and polyethylenimine (PEI). The effect of PE tested on the thermodynamics and the kinetics of precipitation of calcium phosphate salts was investigated. Three types of experiments were done. Investigation of the adsorption of PE on either hydroxyapatite (Ca5(PO4)3OH, HAP) crystals or on sand grains. Measurement of the kinetics of heterogeneous nucleation of HAP on the solid substrates and the mechanical properties of the obtained crystals in batch experiments of low and high supersaturation solutions, respectively. Evaluation of the consolidation in sand packs in order to investigate the effectiveness of the method. The crystallization rates, Rp, on HAP crystals in the presence of the PE tested were found in the order R p PAA > R p PEI > R p PAH , while nucleation and crystal growth on silicate sand took place only in the absence of adsorbed PE. PAH favored strongly the consolidation process, whereas PEI and PAA resulted in the formation of poorly consolidated grain agglomerates.

The adsorption of, the still widely used, herbicide atrazine on model soil components, such as humic acid and humic acid-silica gel mixtures, was investigated in a series of batch experiments, under different experimental conditions (ionic strength, temperature, and pH). The investigation aimed at obtaining an estimate of the contribution of each of the soil components on the adsorption of atrazine from aqueous solutions. The kinetics of atrazine adsorption on humic acid showed two steps: a fast step, of a few hours duration, and a second slow step, which continued for weeks. The kinetics of adsorption data gave a satisfactory fit to the Elovich equation. Τhe adsorption of atrazine on the test substrates was found to be reversible in all cases. The atrazine uptake data on the test substrates were fitted best with the Freundlich adsorption isotherm. The ionic strength of the atrazine aqueous solutions did affect the amount of the atrazine adsorbed on the test substrates, suggesting that electrostatic forces between atrazine molecules and soil play a significant role in the adsorption process. An increase of temperature resulted in a decrease of atrazine adsorption on humic acid at low atrazine equilibrium concentrations. However, for higher levels of equilibrium concentrations (≥3 mg/L) the amount of atrazine adsorbed onto the test substrate increased as temperature increased. The calculated isosteric enthalpies of adsorption ranged between slightly exothermic at low atrazine uptake and slightly endothermic at high atrazine uptake, all values being in the range of physisorption.

The precipitation of calcium oxalate monohydrate (COM) as a waterproofing material for concrete constructions is a promising approach. The kinetics of spontaneous precipitation of the calcium oxalate from supersaturated solutions in batch reactors was investigated. The precipitation took place in supersaturated solutions of equal concentrated solutions of sodium oxalate and calcium nitrate and was monitored when COM precipitated from the solutions as a function of time. The experiments were done both in unseeded and in solutions seeded with COM crystals and also with concrete grains (Portland cement). COM formed exclusively in all cases. From the analysis of the kinetics data, it was concluded that the prevalent mechanism for COM precipitation was surface diffusion. Satisfactory fit of the data to the polynuclear model yielded values for the surface energy ca. 30 mJ m −2 . The relatively low values obtained suggested mainly heterogeneous nucleation. The rates of COM precipitation increased with increasing amounts of inoculating COM seed crystals suggesting significant contribution of seeded growth in the overall process. Inoculation with concrete grains resulted in lower rates possibly due to the release of inhibiting species in the supersaturated solutions from the partially dissolving substrate.

In contrast to biomineralization phenomena, that are among the most widely studied topics in modern material and earth science and biomedicine, much less is systematized on modern view of demineralization. Biomineralized structures and tissues are composites, containing a biologically produced organic matrix and nano- or microscale amorphous or crystalline minerals. Demineralization is the process of removing the inorganic part, or the biominerals, that takes place in nature via either physiological or pathological pathways in organisms. In vitro demineralization processes, used to obtain mechanistic information, consist in the isolation of the mineral phase of the composite biomaterials from the organic matrix. Physiological and pathological demineralization include, for example, bone resorption mediated by osteoclasts. Bioerosion, a more general term for the process of deterioration of the composite biomaterials represents chemical deterioration of the organic and mineral phase followed by biological attack of the composite by microorganisms and enzymes. Bioerosional organisms are represented by endolithic cyanobacteria, fungi, algae, plants, sponges, phoronids and polychaetes, mollusks, fish and echinoids. In the history of demineralization studies, the driving force was based on problems of human health, mostly dental caries. In this paper we summarize and integrate a number of events, discoveries, milestone papers and books on different aspect of demineralization during the last 400 years. Overall, demineralization is a rapidly growing and challenging aspect of various scientific disciplines such as astrobiology, paleoclimatology, geomedicine, archaeology, geobiology, dentistry, histology, biotechnology, and others to mention just a few.

In situ atomic force microscopy (AFM) has been used to compare the growth of pure calcite and the growth of calcite in the presence of sulfate ions from aqueous solutions at a constant value of supersaturation (S.I. = 0.89) with respect to calcite. The effect of sulfate ions on calcite growth rates is determined and a potential incorporation of sulfate ions is identified in the calcite during growth. Solutions supersaturated with respect to calcite with solution concentration ratio of one and a constant pH of 10.2, were prepared and sulfate was added as Na 2 SO 4 aqueous solution. The solution composition was readjusted in order to keep the supersaturation and pH constant. PHREEQC was used to determine relevant solution concentrations . In situ AFM experiments of calcite growth were performed using a fluid cell and flowing solutions passed over a freshly cleaved calcite surface. Growth rates were determined from the closure of the rhombohedral etch pits induced by initial dissolution with pure water. The spreading rate of 2-dimensional nuclei was also measured. At low concentrations of sulfate (≤ 0.5 mM), no effect on the growth rate of the calcite was observed. At higher concentrations (2 to 3 mM) of sulfate, the growth rate increased, possibly because a higher concentration of calcium and carbonate was necessary to maintain the supersaturation constant. At much higher concentrations of additional sulfate (up to 60 mM) the growth rate of the calcite was substantially decreased, despite the fact that a further increase of calcium and carbonate was required. The morphology of 2-dimensional growth nuclei became increasingly elongated with increasing sulfate content. Measurements of step height showed that newly grown steps were approximately 1 A higher when grown in high sulfate concentrations, compared to steps grown in sulfate-free solutions. At sulfate concentrations above 5 mM the growth mechanism changes from layer growth to surface roughening. These observations suggest that the new growth has incorporated sulfate into the calcite surface.

The conservation of cultural heritage is a demanding and imperative task. Of particular interest is the prevention of damage in monuments in load bearing elements made of reinforced concrete. In this case, the remediative intervention should be done with special caution, especially in places of the structure where earthquakes pose immediate threat for the integrity of historical monuments. The present work focuses on the problems associated with corrosion of steel in reinforced concrete, considered as a composite material consisting of steel and inorganic constituents. The study is concerned with S500s Tempcore ® steel and the associated corrosion phenomena, responsible for the degradation of the mechanical properties of the reinforced concrete. Corrosion of steel bars in reinforced concrete is attenuated due to either of the incomplete coverages of the metal with concrete, leaving it exposed to the aggressive marine environment or from galvanic corrosion. The latter is due to the presence of different types of metallic elements introduced by the welding process employed often in pre-earthquake and post-earthquake repairs in order to strengthen load bearing elements. In a microscopic scale the development of special conditions may lead to major damages of the structures. In the present work, we report on the results of mechanical stress measurements done on S500s Tempcore ® steel bar specimens. Prior to the stress measurements, the specimens were subjected to accelerated corrosion by exposure in salt spray chamber for time periods up to 90 days. Specimens in which welding side-by-side was applied were also exposed at the same conditions and the respective mechanical stresses were measured. The effect of increasing corrosion damage on the strength properties of the steel S500s Tempcore ® was moderate. The effect of salt spray exposure on the tensile ductility however was appreciable. All measurements have shown that corrosion had an adverse effect on the materials strength, while the ductility of the specimens was drastically reduced. The embrittlement of reinforcing steel due to corrosion was confirmed both by the gradual decrease of the specimens’ ductility and the examination of the morphology of the cross-section of the respective specimens by scanning electron microscopy.

The possibility of complete fractionation of olive mill wastewater (OMW) was investigated using membrane technology. Combinations of different membrane processes were used for the fractionation of OMW into by-products that may be further developed to achieve reduction of the expenses involved. Ultrafiltration (UF) in combination with nanofiltration (NF) and/or reverse osmosis (RO) were found to be very efficient for the treatment and fractionation OMW. An extended parametric study for the optimum fractionation yield of OMW involved variation of the operational system parameters including temperature, and the trans-membrane pressure. The final effluent obtained was a clean transparent with very low content of organic compounds and dissolved ionic salts. The chemical composition of the post-treatment effluent showed that it was suitable for disposal in aquatic receptors or for use for irrigation purposes. The ultrafiltration process resulted in the separation of high molecular weight constituents including suspended solid particles. Phenols present in the OMW were removed to an extent exceeding 95% of the initial value following the nanofiltration step. The concentrate obtained at this stage was very rich in phenols. Better efficiency of the OMW treatment was achieved applying RO after UF.

The precipitation of calcium carbonate as a binding salt for the consolidation of loose sand formations is a promising approach. The heterogeneous nucleation and growth of calcite were investigated in supersaturated solutions. The ionic activities in the solutions tested were selected so that they included both supersaturations in which crystal growth took place only following the introduction of seed particles and supersaturations in which precipitation occurred spontaneously past the lapse of induction times. In the latter case the supersaturation conditions were sufficiently low to allow the measurement of induction times preceding the onset of precipitation. The stability domain of the calcium carbonate system was established at pH 8.50, 25 degrees C, measuring the induction times in the range between 30 min and 2 h. The rates of precipitation following the destabilization of the solutions were measured from the pH and/or concentration-time profiles. The induction times were inversely proportional and rates proportional to the solution supersaturation as expected. The high-order dependence of the rates of precipitation on the solution supersaturation suggested a polynuclear growth mechanism. Fitting of the induction time-supersaturation data according to this model yielded a value of 64 mJ/m2 for the surface energy of the calcite nucleus. In the concentration domain corresponding to stable supersaturated solutions, seeded growth experiments at constant supersaturation showed a second-order dependence on the rates of crystal growth of calcite seed crystals. Inoculation of the stable supersaturated solutions with quartz seed crystals failed to induce nucleation. Raising supersaturation to reach the unstable domain showed interesting features: calcite seed crystals yielded crystal growth kinetics compatible with the polynuclear growth model, without any induction time. The presence of quartz seed crystals reduced the induction times and resulted in nucleation in the bulk solution. The kinetic data in the latter case were consistent with the polynuclear growth model and the surface energy for the newly forming embryo was calculated equal to 31.1 mJ/m2, because of the dominantly heterogeneous nature of the process.

We have investigated the presence of foreign ions into calcite bulk structure using periodic ab initio methods. Four cations isovalent to Ca2+ were studied: Mg2+, Sr2+, Ba2+ and Zn2+. Our calculations performed at the Hartree–Fock level show that the incorporation of these ions into calcite strongly depend to their size. Mg2+ and Zn2+ can enter into the host lattice while the incorporation of Sr2+ and Ba2+ into calcite is energetically less favoured at the studied degrees of contamination.

The effect of four different polymers on the precipitation of calcium sulfate was investigated in the present work. The degree of inhibition was estimated from measurements of the calcium ion activity and from specific solution conductivity measurements in the supersaturated solutions during the course of the precipitation process. The effects of polyacrylic acid (PAA, three different polymers with average molecular weight 2000, 50,000, and 240,000, respectively) and of a co-polymer of PAA with polystyrene sulfonic acid (PSA, average molecular weight20,000) were investigated with respect to their effect on the kinetics of spontaneous precipitation of calcium sulfate salts. The results of the kinetics experiments suggested that the spontaneous precipitation from supersaturated calcium sulfate solutions at 25 degrees C yielded exclusively calcium sulfate dihydrate (gypsum) both in the absence and in the presence of the polymeric additives. The induction times, preceding the formation of the solid increased in all cases in the presence of the polymeric additives. Polymer concentrations as low as 2.0 ppm increased induction time from practically zero to 10 min. The rates of precipitation were reduced according to the solutions content in the polymers added and precipitation was completely suppressed in the presence of 6.0 ppm of the polymers tested, depending on their molecular weight. The lower the molecular weight of PAA, the more efficient was the threshold inhibition and the stronger the reduction of the rates of spontaneous precipitation. PSA yielded the poorest inhibition efficiency in comparison with the PAA, possibly because of the relatively lower affinity of the sulfonate groups for the calcium ions of the surface of the solid forming. The kinetics results analysis assuming Langmuir-type adsorption of the polymeric molecules on the growing supercritical gypsum nuclei showed different affinity for the polymers tested in agreement with the respective inhibition efficiency, in the order: PAA1PAA2PSAPAA3. The presence of the polymers in the supersaturated solutions resulted in modification of the precipitated gypsum crystals morphology.

The adsorption of the widely used herbicide atrazine onto three model inorganic soil components (silica gel, gamma-alumina, and calcite (CaCO(3)) was investigated in a series of batch experiments in which the aqueous phase equilibrated with the solid, under different solution conditions. Atrazine did not show discernible adsorption on gamma-alumina (theta=25 degrees C, 3.8

Dissolution processes play an important role in marble weathering. Investigation of the dissolution kinetics of powdered Pentelic marble, a calcitic natural stone, showed that the rates measured at constant undersaturation decreased with time. It was found that the rate of dissolution decreased sharply to about 50% of the initial value before 5% of the initially suspended powder was dissolved. Past a dissolution extent corresponding to 25% with respect to the suspended solid, the rates of dissolution at the same constant undersaturation reached a stable value corresponding to 20-25% of the initial rate measured. The reduction in the rates of dissolution up to 25% of mass loss was in agreement with the reduction in the total surface area exposed to the solution, suggesting a dependence of the rate on the number of active sites on the surface of the dissolving material. The decrease in the dissolution rates did not depend on the undersaturation of the solution or on the physical characteristics of the particles such as specific surface area, porosity, and particle size distribution. Equal mass losses, with respect to the initially suspended solid, of different samples exhibited the same relative decrease in rate regardless of the conditions of the aqueous solutions.

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 cm 2 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 6.070.4, 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. r 2001

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.

The wettability of calcium carbonate, an important parameter in the evaluation of the interaction characteristics of materials used in conservation, was examined by measurements with the Wilhelmy plate technique. Heterogeneous surfaces consisting of glass and calcium carbonate were examined. Stable calcium carbonate layers were deposited on microscope glass slides using supersaturated solutions in which the equimolar concentrations of total calcium and total carbonate used ranged between 0.1 and 0.4 M, over a pH range between 9 and 12 and temperature between 25 and 80°C. In all cases amorphous calcium carbonate was deposited, the extent of surface coverage increasing with increasing supersaturation and temperature. Higher surface coverage of the glass slides with calcium carbonate yielded lower contact angles suggesting increased wettability of the heterogeneous surface.

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 O'Brien theory developed for the electrical conductivity and electro-osmosis in porous plugs consisting of nonconducting particles has been applied for the calculation of zeta-potentials from conductivity and streaming potential measurements at two pH values, 3.3 and 6.5 or 5.8, on plugs composed of titania (undoped and doped with W6+ and Li+ ions) particles. The particles have been prepared both by precipitation (continuous crystallization at constant supersaturation) and by the sol-gel method. It was found that O'Brien's theory is also applicable in n-type semiconductor particles, such as titania (doped and undoped). Specifically, for the correct determination of zeta-potentials measured in solutions of relatively low ionic strength, it is necessary to take into account surface conduction behind the shear plane, for all samples measured at pH 3.3, a value considerably lower than the isoelectric point (i.e.p.) of titania. Under these conditions the surface charge of the suspended titania particles is high, and consequently the contribution of surface conductance to the conductivity of the plug is significant. The effect of surface conduction behind the shear plane on the calculation of zeta-potential is enhanced for the Li+-doped preparations due to the increase of surface conductivity caused by doping with Li+. Measurements of the streaming potential for the samples prepared with the sol-gel method were made at pH 6.5 (5.8 for the undoped titania); i.e., near the pH corresponding to the i.e.p., correct values of zeta-potential may be obtained in suspensions even at low ionic strength without taking into account the polarization of the double layer due to surface conduction, by applying the primitive Smoluchowski theory. This treatment is valid because at pH values near the i.e.p. the surface charge of the particles is very low, and consequently surface conductance is negligible compared with the conductivity of the plug. Copyright 1999 Academic Press.

The formation of oxide layers on aluminum and its alloys has a protective function. Protective oxide layers may be formed by various processes and may be promoted in the presence of chemical compounds present at the metal/water interface. In the present work, the electrochemical behavior of aluminum alloy 7075 in the presence of aqueous oxalate solutions in the concentration range between 10–80 mM was studied by open circuit potential, polarization and ac impedance measurements. It was found that the oxalate ions present in the solutions promoted passivation of the specimens surface. The passivation was attributed to the formation of a resistive oxide layer which was thicker than the corresponding layer formed in distilled water. This finding was confirmed by XPS measurements. Since the interaction between the anionic species, such as the oxalate ions, with the metal and or oxide surfaces seem to play an important role, the influence of the duration of immersion and of the concentration of the aqueous oxalate solutions on the surface oxide film formation was investigated. It was found that in a treatment process in which both the immersion time and the oxalate concentration were increased, a thicker, more resistive film was formed. In the treatment process of the aluminum alloy specimens with oxalate, the effect of the oxalate concentration on the electrical properties of the surface layer formed during the immersion in the oxalate solutions (increase of oxide resistance upon increasing oxalate concentration) was revealed when testing solutions of the same concentration (10 mM) in sodium oxalate were used for conducting electrochemical measurements, regardless of the composition of the treating solutions. The apparently low resistivity observed when the treating and testing solutions had the same, high, oxalate concentration (resulting in solutions of high conductivity) was attributed to the porous structure of the surface oxide layer.

The z-potential and the surface conductivity of pure anatase and anatase doped with Li+, Nb5+ and W6+ prepared with both the precipitation (continuous crystallization at constant supersaturation) and the sol–gel method have been determined at two pH values (3.3. and 6.0 or 6.5). This was carried out by measuring the streaming potential and electrical conductivity of a porous plug containing the anatase particles. The sol–gel preparations exhibited a higher density than those obtained by precipitation. Plots of the plug conductivity as a function of the conductivity of the solution are essentially linear, allowing for the establishment of the surface conductivity, Kσ. The main contribution to Kσ stemmed from the counter charge behind the slip plane. At pH 3.3, i.e. much lower than the i.e.p., doping with Nb5+ and Li+ caused a considerable increase of the surface conductivity in both preparations, whereas at a pH 6.0 or 6.5, i.e. near the i.e.p., doping led to a slight decrease of the relatively high surface conductivity of the sample prepared by precipitation. At pH 6.5 sol–gel preparations showed negligible surface conductivity, independent of the doping, with the exception of the preparation with a high concentration of W6+. Moreover, doping with W6+ did not considerably change the surface conductivity. There is some indirect correlation between the surface and the bulk conductivity of the various samples. Tangential mobilities of Cl− ions in the stagnant layers are of the same order of magnitude as those in the bulk solution.

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.

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.

Manganese oxidic mesoporous particles of nominal size in the micrometer region have been prepared by controlled hydrolysis of the trinuclear complex [Mn3O(CH3COO)6(pyr)3]ClO4(pyr:pyridine). The cation of the complex, which is unstable in water, was hydrolized in various baths containing additives likeM(CH3COO)2(M Mn, Ni),M′(NO3)2–3(M′ Ni, Mn, La, Co, Fe), LiOH, and CH3COOH. The resulting dried products of hydrolysis possess surface areas as high as 200–400 m2g−1atT≤ 300°C, 50–200 m2g−1around 400°C, and 20–100 m2g−1atT= 500–600°C depending on the additive. The particle thus obtained shows no hysteresis loop in the adsorption–desorption of N2at 77 K. The αsplots indicate that the obtained solids are mesoporous. The calculated pores are in the range of 5–10 nm, corresponding most probably to interparticle void space. Particle size analysis showed that the formed particles are in the micrometer region and SEM photographs reveal micrometer scale particles, too. Plots of lnVadsvs ln[ln(P0/P)], whereVadsis the volume of N2adsorbed atT= 77 K and relative pressureP/P0, reveal a surface dimentionalityDof the particles equal to 2.56 ± 0.05. This value approaches the fractal dimentionality of the Koch pyramid 2.58497, and a model is proposed for building up such Koch-like porous particles from Mn4O or MnO4tetrahedra.

A quantitative analytical methodology based on Raman spectroscopy (RS) was developed for the compositional analysis of binary solid mixtures consisting of calcium oxalate monohydrate (COM) and hydroxyapatite (HAP). The ratios of the relative intensities of the Raman bands corresponding to HAP and COM were used for the construction of calibration curves used for the quantitative analysis. The intensities at 960 cm −1 , I RHAP and 1462 cm −1 , I RCOM , for HAP and COM, respectively, in the Raman spectra of each salt were used. A linear dependance was found for the ratio I RHAP / I RCOM on the inverse of the COM concentration (mole fraction) in the solid, binary mixtures. Comparison of this method with quantitative analysis of binary HAP-COM by powder X-ray diffraction (XRD), using the relative intensities of the 002 reflection of HAP and the 020 reflection of COM showed that although the limit of detection was comparable (approx. 1 mol%), RS analysis offered the relative advantage of performing analyses without the need of grinding the samples, thus making it possible for the analysis of layered deposits of the minerals investigated.

Innovative separation techniques like melt crystallization have the potential to fulfil two important demands, namely: a significant reduction of energy consumption by the chemical industry, and the production of high quality products required by industry. Several industrial applications of melt crystallization have been realized but industry is still hesitating to fully commit its resources. One of the most important reasons is that the most promising application of melt crystallization, the suspension-based melt crystallization process, is still under development. For this reason this study had the objective to improve equipment design and to establish optimal process conditions for the separation of crystals from the melt, this being the essential step in crystallization from suspension. The ultimate aim of the study was to test a pilot-plant for the application of melt crystallization to an industrial problem. In this paper, background information on melt crystallization processes and possible solid-liquid separation techniques will be given. The content of the study is briefly discussed and experimental results are given.

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.

The presence of chloride ions accelerates the corrosion of aluminum in aqueous media. The solution pH is also an important parameter which strongly influences the rates of corrosion and the pitting potential of pure aluminum. A large number of studies of the corrosion behavior of aluminum in aqueous media have been done using polarization measurements. Little attention has been given in the variation of the bulk solution pH during the cathodic or the anodic polarization measurements. In the present work, during the course of obtaining the polarization curves of aluminum specimens in aqueous media containing sodium chloride, significant changes in the solution pH occurred which consisted of a shift to alkaline values during cathodic polarization and to acidic values during anodic polarization. The results were obtained at conditions of sustained pH, by using a pH controller which kept the solution pH constant by the addition of alkali or acid, as needed. This gave markedly different results, reflecting the sensitivity of the electrochemical measurements to the bulk solution pH. The pitting potential at constant pH was shifted to more cathodic values at pH 7 while it was minimum in the pH range of 5.00–7.0. The rates of corrosion of aluminum were found to depend strongly on both the solution pH and the solution temperature.

The effect of cartilage proteoglycans on HA seed crystal growth was studied using a system providing constant supersaturation with respect to HA. The monomers were much less effective than the aggregates in reducing the rate of HA growth, which correlates with their affinity for the HA crystals. Hyaluronan, which is a normal constituent of the proteoglycan aggregates, behaved as a strong inhibitor of HA seed crystal growth and had an affinity constant similar to that of proteoglycan aggregates. The results indicate that inhibition of HA seed crystal growth is mediated throught the interaction of hyaluronan with HA crystal surface and that the proteoglycans add to the volume of the adsorbate causing steric hindrance.

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.

The formation of insoluble calcium carbonate scale is often undesirable and the use of water additives which prevent or retard the deposition of this salt is widespread as the alternative solution of water acidification would cause severe corrosion problems. In the present work, the effect of four synthetically prepared compounds, 1,2-dihydroxy-1,2-bis(dihydroxyphosphonyl)ethane (DDPE), 2-dihydroxyphosphonyl-2-hydroxypropionic acid (DHHPA), 1,3-bis[(1-phenyl-1-dihydroxyphosphonyl)methyl]-2-imidazolidinone (BPDMI), and 2,3-bis(dihydroxyphosphonyl)-1,4-butanedioic acid (BDBA), on the spontaneous precipitation of calcium carbonate in aqueous media was investigated. The precipitation process was studied in batch reactors at 25°C and pH 8.50, under conditions of sustained supersaturation. Two of the organophosphorous compounds investigated, DDPE and BDBA, contained PCCP bonds in their molecules. In a third, BPDMI, the two PC bonds were separated by a bulky five-membered ring, and the other, DHHPA, contained only one PC bond. All organophosphorus compounds examined in the present work were synthesized in the laboratory and were found to be resistant to hydrolysis even at elevated temperatures, typical of those existing in geothermal wells. In all cases, the induction periods preceding the onset of the spontaneous precipitation of calcium carbonate were significantly increased depending on the concentration of the additives examined, while the concomitant precipitation rates were markedly slower at concentration levels as low as 2 × 10−8 mol dm−3. It is possible that the inhibiting effect of the organophosphorus compounds tested is due to the blocking of the active growth sites of the crystallites formed after the induction period. The existence of the PCCP bonds in the molecules of the organophosphorus compounds resulted in the most effective inhibitors, while the separation of the two phosphonyl groups by the bulky five-membered ring resulted in the same inhibitory activity as with the compound possessing only one phosphonyl group.

Calcium sulfate scale formation is an important problem in a number of industrially interesting applications. An understanding of the mechanism of its formation in aqueous supersaturated solutions, in which the precipitation takes place spontaneously due to the high supersaturation with respect to this salt, is of key importance for the control of undesirable scale deposits. In the present work, the spontaneous precipitation of equimolar calcium sulfate supersaturated solutions was investigated at conditions of sustained supersaturation over the temperature range 20–60°C. In all cases, the only phase forming was identified as calcium sulfate dihydrate. The kinetic parameters were measured accurately by a potentiometric technique involving probing the conductivity of the supersaturated solutions. From the induction periods, preceding the onset of calcium sulfate precipitation the stability diagrams of the system were constructed. Kinetic analysis of the rates, which depended strongly on the solution supersaturation, suggested a polynuclear mechanism and yielded an activation energy of 44 kcal mole−1, indicative of a surface-controlled mechanism. Finally, the surface energy of the forming phase was calculated at various temperatures from kinetic data.

Polymers such as the commercially available Porapak-N and synthetically prepared polystyrene (1.5% w/w) divinyl benzene copolymer and polystyrene have been converted into substrates appropriate for the deposition of hydroxyapatite (Ca 5 (PO 4 ) 3 OH, HAP) by the introduction of various phosphate groups on their surface. The investigation was carried out at sustained supersaturation at 37°C for the styrene (1.5% w/w) divinyl benzene copolymer and at 25°C for Porapak-N and polystyrene. In the former case the apparent crystal growth order was found to be 1.25 as for the growth on HAP seed crystals. Increasing the content of phosphate in the surface resulted in higher rates of mineralization. At 25°C higher apparent orders of reaction were observed possibly due to the microporous structure of the polymers employed.

Molybdenum oxide (MoO3) supported catalysts are widely used for the catalysis of a number of processes. In the past oxidic carriers and polymers have been used as carriers. In the present work, a temperature resistant copolymer, poly(1,4-phenylene) terephthalamide (PPTA) was used as a substrate for the deposition of MoO3, selectively, from aqueous molybdate solutions, pH 2.25 at 25°C. MoO3 deposition followed the induction period and occurred on specific active sites of the copolymer. The rates of deposition, following the nucleation step, depended not only on the supersaturation of the aqueous solution with respect to MoO3, but on the chemical composition of the copolymer as well.

The stability of calcium carbonate supersaturated solutions was studied between 25−80°C and pH 8.00–9.00. The stability diagrams showed narrow supersaturation ranges, for which very reproducible induction times could be obtained. Investigation of the precipitation process by a plethostatic (i.e., in which a constant number of moles is maintained) experimental approach, enabled us to identify vaterite as the initially forming crystalline phase, throughout the pH and temperature range investigated. The precipitation took place via a polynuclear mechanism. Ethane-1-hydroxyethylideno-1,2-diphosphonic acid (EHDP) caused not only retardation in the rates of spontaneous precipitation but resulted also in stabilization of the initially forming vaterite. An apparent activation energy of 46 kJ mol −1 was obtained for the precipitation reaction of vaterite. Seeded growth experiments on calcite seed crystals over the temperature range 10–37°C, gave second order kinetics suggesting a spiral growth mechanism. In all cases, calcite was found to be the only phase forming. The presence of additives such as orthophosphate and oxalate reversed the surface charge of the calcite particles while metal ions such as Cd 2+ or Zn 2+ resulted in a marked reduction of the rates of precipitation by adsorption on the active growth sites on the surface of the calcite crystals.

The oxides γ-Al 2 O 3 , SiO 2 and TiO 2 , are the most commonly used carriers in the preparation of supported catalysts and are widely used in industry. The deposition of the active phase is the most critical step and is often done by wet impregnation. This phase, which may be anionic or cationic, is deposited on the cationic and anionic sites of the carrier, respectively. In the present work, we present an overview of the methods available for controlling the concentration of the positively or negatively charged sites on the surface of the oxide carriers. Doping with cations (Na+ or Li+) resulted in shifting the p.z.c. at more alkaline values, thus increasing the positively charged groups on the oxides, while doping with F − had the opposite effect. Adsorption of MoO 2− 4 was, therefore, higher in the former samples while the adsorption of metal ions such as Co 2+ and Ni 2+ was higher in the latter samples. Similarly, increasing the suspension temperature shifted the p.z.c. to higher values. The variation of the surface concentrations of the charged groups of the carrier oxides provides a unique methodology for the preparation of the desired catalysts.

The deposition of calcium carbonate in lines carrying hot water or in pumping systems is an undesirable process in most cases. The mechanism of calcium carbonate scale formation is of key importance and has many practical implications. The deposition of calcium carbonate has been investigated in a closed, flow through circuit, in which heated specimens made of 316 stainless steel, aluminum and glass covered internally, 316 stainless steel, were used as points of initiation of the deposition. It was found, at plethostatic conditions, that in the temperature range between 50–80°C, vaterite is the initially forming phase converting to the thermodynamically more stable calcite. The deposition took place within the stationary layers in which the supersaturation gradient was established. The deposition of vaterite was done by a surface diffusion controlled process, as suggested from the apparent activation energy of 36 kJ/mol found. The deposition rates as a function of the solution supersaturation suggested a polynuclear mechanism for the formation of vaterite on the heated specimens. Pretreatment of the specimens with concentrations of EHDP varying from 0.1 μM to 1 mM, increased the induction times significantly and resulted in drastic reduction of the rates of deposition, which were more pronounced for the 316 stainless steel specimens. From the kinetics of precipitation data, the surface energy of the forming vaterite was estimated and it was found to increase with temperature from 77 to 148 mJ m−2.

The adsorption of MoxOyz− 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 MoO42− ions is larger than those for the isopolyanions. (iii) The adsorbed MoxOyz− 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 MoxOyz− ions resulting in the formation of MoxOyz− … MoxOz− 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 MoVI 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.

The adsorption of inorganic orthophosphate on porous glass and SiO 2 surfaces has been investigated. Porous glass exhibited markedly higher affinity as compared to SiO 2 toward phosphate, which was found to strongly depend on ionic strength and temperature. The phosphate uptake was relatively fast and the kinetics followed a simple first order equation. Adsorption data gave a satisfactory fit to a recently developed adsorption isotherm describing the adsorption of ions on heterogeneous surfaces, based on the classical Langmuir isotherm and assuming a Gaussian distribution of the free energies of adsorption.