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Marble and limestone have been extensively used as building materials in historical monuments. Environmental, physical, chemical and biological factors contribute to stone deterioration. The rehabilitation of stone damage and the delay of further deterioration is of utmost importance. Inorganic nanoparticles having chemical and crystallographic affinity with building materials is very important for the formation of protective coatings or overlayers. In the present work, we have tested the possibility of treating calcitic materials with suspensions of amorphous calcium carbonate (am-CaCO3, ACC) and amorphous silica (AmSiO2). Pentelic marble (PM) was selected as the test material to validate the efficiency of the nanoparticle suspension treatment towards dissolution in undersaturated solutions and slightly acidic pH (6.50). Suspensions of ACC and AnSiO2 nanoparticles were prepared by spontaneous precipitation from supersaturated solutions and by tetraethyl orthosilicate (TEOS) hydrolysis, respectively. The suspensions were quite stable (nine days for ACC and months for AmSiO2). ACC and Am SiO2 particles were deposited on the surface of powdered PM. The rates of dissolution of PM were measured in solutions undersaturated with respect to calcite at a constant pH of 6.50. For specimens treated with ACC and AmSiO2 suspensions, the measured dissolution rates were significantly lower. The extent of the rate of dissolution reduction was higher for AmSiO2 particles on PM. Moreover, application of the nanoparticles on the substrate during their precipitation was most efficient method.

In the present work, the effect of wettability on CaCO3 precipitation was approached by monitoring crystal formation in microchips of different wettability degree. Solutions of calcium chloride and...

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.

BACKGROUND In the present work, a combination of several physicochemical methods has been applied aiming at recovering phosphorus from olive mill wastewater (OMW) in the form of struvite (NH4MgPO4·6H2O, MAP) and at the same time achieving reduction of the chemical oxygen demand (COD) of the wastes. Phosphorus is a valuable raw material used for the production of fertilizers and numerous other products. RESULTS The experiments were conducted in a two-stage process, over a pH range between 5 and 10. The first step consisted of a batch process, in which MgCl2 or MgSO4 were used as coagulants of suspended particles of the OMW. During the second step, a nitrogen source (NH4OH(aq)) was added to the supernatant and solid precipitation took place without delay. The measured COD values of the fluid phase of the OMW was reduced by up to 73% of the initial value, while the final crystalline product consisted of a mixture of struvite and dittmarite (NH4MgPO4·H2O). CONCLUSION Over 90% dissolved phosphorus recovery from OMW samples in the form of struvite was achieved through the application of a two-step process together with a significant reduction of the COD of initial raw wastewaters. © 2017 Society of Chemical Industry

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 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.

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.

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 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 formation of sparingly soluble salts from aqueous solutions and their dissolution has attracted broad research interest. Of particular importance is the formation and transformation of minerals exhibiting polymorphism or encountered in more than one crystalline phase as, for example, in the case of calcium phosphates, formed in biological mineralization and in industrial-scale deposits. Understanding of these processes depends primarily on the equilibrium between the mineral phases considered and the aqueous medium in contact. Precipitation takes place in supersaturated solutions with rates depending on the solution supersaturation. The experimental investigation may reveal mechanistic details if done at sustained supersaturation. The kinetics of crystal growth depends either on surface diffusion or on bulk diffusion, which in turn is controlled by the medium fluid dynamics. In the case of magnesium ammonium phosphate (struvite), the presence of water-soluble organic compounds is responsible for the retardation both of the time needed for the onset of precipitation and for the kinetics of growth of the supercritical nuclei. Dissolution processes are controlled by the same mechanisms. In the case of calcitic marble, the dissolution in alkaline solutions is controlled by surface diffusion. Compounds active at the marble/water interface may in this case be used as protective agents.

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 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.

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...

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.

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

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 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.

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 seeded and unseeded precipitation of calcium carbonate from supersaturated aqueous solutions was investigated in a closed system under conditions of constant supersaturation (supersaturation 1.88−3.39) at 25 °C and at pH 9.0 and 10.0. The analysis of our results based on expressions for the supersaturation dependence of the induction time in seeded and unseeded precipitation according to the classical nucleation theory provides additional information on the mechanism of crystal growth and nucleation of calcium carbonate from supersaturated solutions. High pH values and relatively high supersaturations favor the precipitation of vaterite. The induction times were shorter in the seeded precipitation compared to those in the unseeded precipitation and displayed a marked inverse dependence on the solution supersaturation. Crystal growth proceeds via the spiral growth mechanism. The relatively low surface energy calculated for vaterite from the kinetics data, and the very low nucleation rate constant, indi...

A model system was developed for the in vitro quantitative investigation of the calcification process occurring in heart valves. The process of heart valve calcification consists of the formation of calcium phosphates at the heart valve-biological fluid interface. Calcium phosphate deposits may consist of more than one calcium phosphate mineral phase, differing with respect to their physical and chemical properties. The kinetics of the formation of hydroxyapatite, the model inorganic compound for the calcified deposits, was precisely monitored in a reactor containing supersaturated calcium phosphate solutions in which the heart valves were immersed after being treated with glutaraldehyde and mounted on special racks. The precipitation process, accompanied with proton release in the solution, was monitored by a pair of glass-saturated calomel electrodes. Upon initiation of the formation of calcium phosphate deposits, the supersaturation in the working solution was reestablished through the addition of titrant solutions made with the appropriate concentration to compensate for the ions precipitated. With this methodology, not only the rates were measured very precisely but also the nucleation capability of the various substrates could be evaluated. Moreover, it was possible to identify the formation of intermediate calcium phosphate phases formed during the calcification process. Valves previously treated with glutaraldehyde were shown to nucleate octacalcium phosphate, which at lower supersaturations converted to the thermodynamically more stable hydroxyapatite. The rates measured were found to depend on the solution supersaturation, while the apparent order of the precipitation process was found to be 1.

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.

Objective To develop an in vitro model system for the investigation of calcium oxalate urinary stones under conditions of spontaneous precipitation. Materials and methods Using a calcium-ion selective electrode and automatic titrator, a test solution was kept supersaturated with calcium oxalate and the rate of crystallization measured. Results During the spontaneous precipitation at constant supersaturation at 37°C, the induction times required for the precipitation of calcium oxalate monohydrate were inversely proportional to the supersaturation of the solution. No transient phases were identified and the interfacial energy determined from kinetic analysis was 28.4 mJ/m2 . The rates of precipitation showed a first-order dependence on the degree of supersaturation and were in good agreement with those reported for the in vivo formation of calcium oxalate monohydrate stones. Conclusion This experimental model system allows precise measurements of the kinetics of calcium oxalate monohydrate. From the dependence of the rates of precipitation on supersaturation, a mechanism controlled by surface diffusion is suggested.

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.

Hydroxyapatite (HA) has been widely used as a bone substitute in dental, maxillofacial and orthopaedic surgery and as osteoconductive bone substitute or precoating of pedicle screws and cages in spine surgery. The aim of the present study was to investigate the osteoblastic adhesion strength on HA substrata with different surface topography and biochemistry (pre-adsorption of fibronectin) after blocking of specific integrin subunits with monoclonal antibodies. Stoichiometric HA was prepared by precipitation followed by ageing and characterized by SEM, EDX, powder XRD, Raman spectroscopy, TGA, and specific surface area analysis. Human bone marrow derived osteoblasts were cultured on HA disc-shaped substrata which were sintered and polished resulting in two surface roughness grades. For attachment evaluation, cells were incubated with monoclonal antibodies and seeded for 2 h on the substrata. Cell detachment strength was determined using a rotating disc device. Cell detachment strength was surface roughness, fibronectin preadsorption and intergin subunit sensitive.

The formation of barium sulfate in aqueous media causes problems of insoluble scale formation. On the other hand, the coprecipitation of uranium and transuranium elements with barium sulfate has been suggested for the determination of these elements in environmental monitoring processes. Therefore, the study of barium sulfate crystal growth is required. The investigations done so far suffer from the low sensitivity of the analytical methods used. In the present work we have overcome this problem by using131Ba for the preparation of supersaturated solutions. Thus kinetics parameters such as induction time and precipitation rates were measured. The polynuclear mechanism was found to be operative at high and the spiral growth at low supersaturations.

The formation of barium sulfate scale in oil wells and in several industrial processes in which water supersaturated with respect to this salt is used, is often a serious cause of energy losses. The spontaneous precipitation in aqueous solutions containing relatively high barium and sulfate concentrations was investigated in the present work. In our experiments the supersaturation of the solutions was chosen so that an induction period preceded the onset of precipitation. In all cases the induction time was found to be inversely proportional to the solution supersaturation and the rates of the subsequent precipitation were higher the higher the solution supersaturation. The stability domain of the barium sulfate system was not affected by the barium: sulfate molar ratio which ranged from 0.01to ca. 100. The dependence of the induction time on the initial barium concentration was linear and irrespective of the molar ratio of barium: sulfate the same slope was obtained. The slope may be related to the size of the critical nucleus. The kinetic data obtained suggested a surface-controlled, polynuclear mechanism. On the basis of this model a value of 86 mJ m–2 was obtained for the surface energy of the precipitaing solid phase.

Three different MoO3 samples have been prepared by spontaneous precipitation from (NH4)6Mo7O24 and Na2MoO4 aqueous solutions at pH 2.30 and 1.65. The supersaturated solutions were stable over the entire concentration range examined for pH values over 4.0 and below 1.50. The spontaneous precipitation process showed a high apparent order (p= 3) typical for a surface-controlled mechanism. The solution pH and the nature of the counter-ions influenced the size, texture and symmetry of Mo in the precipitated oxide.Catalytic tests over MoO3 samples prepared by spontaneous precipitation followed by sulfidation, were performed using the hydrosulfurization of thiophene as a probe reaction. The tests showed that the activity increased with the ratio Mo (tetrahedral): Mo (octahedral). The maximum value of this ratio was obtained for the sample prepared by ammonium heptamolybdate at pH 2.30.

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.

Three copolymers containing the functional groups P=O, S=O and C=O were prepared, and upon the introduction in calcium phosphate aqueous solutions at physiological conditions, "in vitro" were induced the precipitation of calcium phosphate crystals. The investigation of the crystal growth process was done at constant supersaturation. It is suggested that the negative end of the above functional groups acts as the active site for nucleation of the inorganic phase. In order to obtain the copolymer further antimicrobial activity, titania (TiO(2)) nanocrystals were incorporated in the polymer matrix after silver coverage by UV radiation. The antimicrobial resistance of the composite material (copolymer-titania/Ag) was tested against Staphylococcus epidermidis (SEM), Staphylococcus aureus (SAM), Candida parapsilosis (CAM) and Pseudomonas aeruginosa (PAM), microorganisms, using cut parts of "pi-plate" that covered with the above mentioned composite. The antimicrobial effect increased as the size of the nanocrystals TiO(2)/Ag decreased, the maximum achieved with the third polymer that contained also quartenary ammonium groups.

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.

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 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 precipitation of calcium carbonate was investigated at pH 8.5 and at temperatures of 25 and 80 °C in stable, supersaturated, synthetic high-salinity solutions (0.62 M NaCl) seeded with calcite, in the presence of fulvic acid (FA), humic acid (HA) and poly(acrylic acid) (PAA), which acted as inhibitors. The rates of crystal growth and the effect of the inhibitors on the mineral formation kinetics were evaluated under conditions of sustained supersaturation. Concentrations of the inhibitors tested in the range 0.02-0.1 ppm for solutions with a supersaturation value (SR) of 8.39 with respect to calcite suppressed the rates of crystal growth by up to 95%. For 0.1 ppm PAA as well as for 0.1 ppm HA the rates of crystal growth showed a linear increase with SRcalcite of the working solution (SRcalcite = 7.31-13.9) at 80 °C. The second-order dependence of the rate on the solution supersaturation suggested surface diffusion control. The inhibitory activity of the compounds tested was ascribed to their interaction with the solid surface. Kinetics analysis of the results obtained for the formation of calcium carbonate in the presence of the compounds tested, based on the assumption of Langmuir-type adsorption on the calcite seed crystals, fitted satisfactorily the data for PAA and FA, while for HA the interaction did not follow this model. In the latter case aggregation phenomena may occur during the growth of crystals, resulting in rate reduction.

The spontaneous precipitation of barium sulphate in aqueous solutions was followed by counting the 131Ba specific activity in the aqueous phase. The main advantages of this method were the increased sensitivity and the possibility of monitoring the process to a larger extent in comparison with other techniques. Kinetics parameters including the size of the critical nucleus and the surface energy of the precipitating solid were estimated according to the model for polynuclear growth.

A new method has been developed to prevent sand reentrainment during oil production from unconsolidated or poorly consolidated reservoir formations. Consolidation of the zformation around the well is achieved through in situ precipitation of a sparingly soluble salt, namely, calcium phosphate. Control of the depth of salt formation is achieved by alternating injection, mixing, and reaction of two aqueous solutions of calcium chloride and potassium phosphate. Calcium phosphate crystals precipitate and grow on the grain surfaces, forming sufficiently uniform coatings. The formation of relatively uniform coatings on the grains causes an acceptably small decrease of the permeability, which is a feature of primary importance for oil production. The grains are gradually "cemented" with bridges of calcium phosphate crystallites and form a consolidated and still porous structure. As a result, the rate of hydrocarbon production for the problematic reservoir can be increased considerably without undesirable reentrainment of sand. The proposed method for consolidation has been successfully tested in sandbeds. Several series of experiments have been carried out under diverse conditions to establish the optimum parameter values for the implementation of this method. A set of optimum conditions at 25 degrees C were determined and these conditions gave satisfactory consolidation with permeability loss of ca. 60% of the initial value. The conditions of precipitation were chosen so that the precipitated phase was octacalcium phosphate [Ca(4)H(PO(4))(3).2.5H(2)O], along with its byproduct hydroxyapatite [Ca(5)(PO(4))(3)OH]. Experiments were also carried out at 70 degrees C and have shown that it is feasible to consolidate loose sandpacks at oil reservoir conditions. Copyright 2000 Academic Press.

Cellulose powder was found to be a substrate favoring the deposition of calcite crystals from stable supersaturated solutions at pH 8.50 and at 25 degrees C. Kinetic analysis of the initial rates showed that they were proportional with the relative supersaturation with respect to calcite. Analysis of the dependence of the induction periods on the initial solution conditions showed that the number of ions forming the critical nucleus was 5. The second-order dependence of the rate of precipitation of calcite on cellulose on the solution supersaturation suggested a surface controlled mechanism. The surface energy of the calcite nuclei growing on cellulose was calculated to be 46 mJ m(-2) from the dependence of the induction time on the solution supersaturation. The overgrowth of calcite on cellulose was done selectively on the macromolecules possibly through active sites formation at ionizable functional groups (-OH). The nucleating capability of cellulose was found to be comparable with that of sulfonated polystyrene and significantly lower in comparison with sulfonated polystyrene divinyl benzene copolymer on which vaterite was formed. This fact together with the selective growth of the most stable calcite suggested that stereochemical factors are very important in determining both the kinetics and the nature of the polymorph formed. Copyright 2000 Academic Press.

Three different MoO3 samples have been prepared by spontaneous precipitation from (NH4)6Mo7O24 and Na2MoO4 aqueous solutions at pH 2.30 and 1.65. The supersaturated solutions were stable over the entire concentration range examined for pH values over 4.0 and below 1.50. The spontaneous precipitation process showed a high apparent order (p= 3) typical for a surface-controlled mechanism. The solution pH and the nature of the counter-ions influenced the size, texture and symmetry of Mo in the precipitated oxide.Catalytic tests over MoO3 samples prepared by spontaneous precipitation followed by sulfidation, were performed using the hydrosulfurization of thiophene as a probe reaction. The tests showed that the activity increased with the ratio Mo (tetrahedral): Mo (octahedral). The maximum value of this ratio was obtained for the sample prepared by ammonium heptamolybdate at pH 2.30.

The formation of barium sulfate scale in oil wells and in several industrial processes in which water supersaturated with respect to this salt is used, is often a serious cause of energy losses. The spontaneous precipitation in aqueous solutions containing relatively high barium and sulfate concentrations was investigated in the present work. In our experiments the supersaturation of the solutions was chosen so that an induction period preceded the onset of precipitation. In all cases the induction time was found to be inversely proportional to the solution supersaturation and the rates of the subsequent precipitation were higher the higher the solution supersaturation. The stability domain of the barium sulfate system was not affected by the barium: sulfate molar ratio which ranged from 0.01to ca. 100. The dependence of the induction time on the initial barium concentration was linear and irrespective of the molar ratio of barium: sulfate the same slope was obtained. The slope may be related to the size of the critical nucleus. The kinetic data obtained suggested a surface-controlled, polynuclear mechanism. On the basis of this model a value of 86 mJ m–2 was obtained for the surface energy of the precipitaing solid phase.

The spontaneous precipitation of cadmium sulphide in aqueous supersaturated solutions has been investigated at 25 °C. In all cases, an induction period, inversely proportional to the solution supersaturation, preceded the onset of precipitation. α-CdS (greenockite) was identified as the only phase being formed. The rates of precipitation were proportional to the solution supersaturation and were affected by the presence of anions arising from the cadmium salts employed in the preparation of the supersaturated solutions. The presence of SO2–4 ions resulted in higher precipitation rates, and the effect of NO–3, Cl–, Br– and I– was also investigated. A fourthorder dependence of the rates of cadmium sulphide precipitation on the solution supersaturation, typical of several sparingly soluble salts, was found.

Four different fly ash samples collected from thermal power plants in Greece burning lignite of coal-lignite mixtures, were characterized using various physicochemical techniques, including specific surface area and particle size distribution determination, X-ray diffraction analysis, potentiometric titration and electrophoretic mobility measurements. The interaction of inorganic orthophosphate at the water/solid interface of these samples was investigated. It was found that the uptake of phosphate is extensive on fly ash surfaces and exceeds by far that corresponding to a monolayer coverage. Pretreating the samples by prewashing with potassium nitrate solutions did not cause any difference in adsorption behavior. Only one sample, containing relatively high amounts of soluble calcium, showed enhanced phosphate uptake. The increased uptake was not shown when the excess of calcium ions was removed from the supernatant by pre-equilibration. Although this suggested that this particular sample would cause precipitation of some calcium phosphate phase, this process could not be identified. Thermodynamic calculations based on the solubility data for calcium, infrared spectra and the shift of the isoelectric point at lower pH values, as phosphate is taken up, suggest that inorganic phosphate was specifically adsorbed on the fly ash particles examined.

The formation of calcium sulfate is important in a number of environmental geological, industrial and energy production processes. At constant calcium activity at 25°C, the kinetics of precipitation were found to be independent of pH, and an apparent order of reaction of n = 4, significantly different from the order of 2 found by seeded growth experiments, suggested a polynucleation mechanism. The electrophoretic mobility of the calcium sulfate dihydrate particles showed a marked dependence on the solution pH, which may affect the crystal growth of the initially formed calcium sulfate dihydrate crystallites by altering the Ca 2+ / SO 2- 4 ratio on their surface.

The crystallization of hydroxyapatite is of importance, since it is considered to be a model compound for tooth and bone formation. In this study, interference by precursot phases has been avoided by studying the kinetics of crystallization at sustained low supersaturations using a highly reproducible constant composition method over a pH range 5.0–8.5. The resultant appreciable extent of crystallization, of two to four times the amount of inoculating crystals, has enabled striking changes in the morphology of the grown crystals to be followed as a function of the extent of growth, pH of the supersaturated solution and the presence of the chloride ion. In general, HAP was found to grow in the form of needle-like crystals whose length increased linearly with the extent of precipitation. The presence of chloride in the crystallizing medium favored the formation of platy HAP crystals, irrespective of pH, at large extents of growth.

Alkaline earth cations present in biological fluids and in natural waters may play an important role in regulating the formation of calcium phosphate solid phases. The kinetics of crystallization of hydroxyapatite (HAP) and fluorapatite (FAP) has been investigated under constant composition conditions in which the supersaturation was maintained constant during the crystallization reactions. Magnesium ions markedly retard the rates of precipitation of both HAP and FAP and the fractional reduction in rate constant can be interpreted in terms of the blocking of active growth sites through adsorption of magnesium ions at the crystal surfaces. During crystallization, the magnesium ions are excluded from the crystal lattices and their adsorption at the surface may be interpreted in terms of a Langmuir isotherm.

A new constant solution composition method is described for the study of calcium phosphate mineralization. It enables large quantities of stoichiometric hydroxyapatite to be formed by direct precipitation from solution at 37°C and pH = 7.40.