Petros G. Koutsoukos

Ascorbic Acid (AA), an important biomolecule present in relatively high concentrations in blood and other biological fluids, has been rarely investigated with reference to its effect on the biological mineralization–demineralization processes. To our knowledge, the present work is one of an extremely limited few found in the literature in which the effect of the presence of AA in mineralizing or demineralizing electrolyte solutions is addressed in a quantitative way. We have used the constant saturation method for the accurate measurement of the rates of crystal growth of hydroxyapatite (HAP, Ca₅(PO₄)₃OH), the model compound of the inorganic component of the hard tissues of higher mammals. It was found that both crystal growth and dissolution were accelerated significantly. The increase in crystal growth rates showed stronger dependence on the solution supersaturation (120% increase for the highest and 460% for the lowest) in the presence of 0.1 mM of AA, pH 7.40, 37 °C, 0.15 M NaCl. The dissolution rate increase was less dependent (average of ca. 300% increase). It was concluded from the detailed characterization of the solid that the acceleration effect was due to the uptake of AA on the HAP surface.

Ascorbic Acid (AA) a very important biomolecule present in relatively high concentrations in blood and other biological fluids, has been rarely investigated with reference to its effect on the biological mineralization-demineralization processes. To our knowledge the pre-sent work is one of extremely limited found in the literature, in which the effect of the presence of AA in mineralizing or demineralizing electrolyte solutions is addressed in a quantitative way. We have used the constant saturation method for the accurate meas-urement of the rates of crystal growth of hydroxyapatite (HAP, Ca5(PO4)3OH) the model compound of the inorganic component of the hard tissues of higher mammals. It was found that both crystal growth and dissolution were accelerated by 4 times (400% of the rates) in the presence of 0.1mM of AA, pH 7.40, 37C, 0.15M NaCl. It was concluded from the detailed characterization of the solid, that the acceleration effect was due to the uptake of AA on the HAP surface.

The control of calcium carbonate formation is of high importance for a wide range of applications in the pharmaceutical industry and membrane processes as well as in the oil and gas industry. Herein, for the first time, the effect of monoethylene glycol (MEG) on the formation of calcium carbonate (CaCO₃) crystals from supersaturated solutions flowing through microchannels (volume 0.36 mL) of varying wettability was investigated. The use of microdevices enabled the observation of the scaling phenomenon in the early stages. Solutions supersaturated with respect to calcite, containing MEG (10, 20, and 30% v/v), were injected into the microchannel under a constant total flow rate and under laminar flow conditions (Re = 0.052). The growth of calcium carbonate crystals was monitored by video recording. The effect of the wettability on crystal formation was tested using glass and silane-coated microchips. The microchannel walls were wet and neutral-wet by the supersaturated solutions with a low MEG concentration. In the presence of a high MEG concentration in the supersaturated solutions, the walls of both types of microchips were neutral-wet. The results showed that the addition of MEG at a concentration of 10% v/v in the supersaturated solutions decreased the time of observation of the first crystal, favored secondary nucleation, and in general decreased the crystal growth rates. Raman spectroscopy identified the formation of aragonite in most cases, while as SR values increased, the formation of aragonite aggregates was favored. Further increases of the MEG concentration in the supersaturated solutions to 20% v/v, at low supersaturation ratio (SR) values, favored the formation of amorphous calcium carbonate (ACC) while at higher SR values aragonite crystals and aragonite aggregates formed. Further increases in the MEG concentration in the supersaturated solutions up to 30% v/v completely inhibited the nucleation and crystal growth of calcium carbonate.

Phosphorus, P, is a vital element of paramount importance for both humans and for the Environment. Wastewater contains often relatively high concentrations of P which can be recovered as crystalline struvite (MgNH₄PO₄·6H₂O, MAP). This option is quite attractive in assisting sustainable development because struvite can be used as a slow-release fertilizer. Domestic wastewater is usually high in P and nitrogen, N, but relatively poor in magnesium, Mg. It is necessary to develop low-cost solutions for the enrichment of wastewater with Mg. In the present work, sacrificial magnesium anodes were used, which dissolve upon anodic polarization, releasing sufficient magnesium for the selective precipitation of MAP. The application of constant current between two electrodes of which the anode is a low-cost magnesium cylindrical rod (4 cm² exposed surface area) and the other a platinum cathode electrode, both immersed in ammonium phosphate solutions, without adjustment of the solution pH, was investigated. Constant current density over the range 10–100 A·m⁻², between the Mg- Pt electrodes immersed in solutions of ammonium hydrogen phosphate of exactly known initial concentration, was applied using a potentiostat. In the presence of sodium chloride solutions, on the magnesium anode and in the bulk solution, Mg(OH)₂ (brucite) formed because of the passivation of the Mg electrode. In dilute ammonium hydrogen phosphate solutions, the magnesium anode dissolution resulted in struvite precipitation, even at a low applied current (10 mA). Struvite crystals with an average size of 20 μm were precipitated. The behavior of the cell for the electrolyte solutions used was Faradaic as long as the surface coverage of the anode was relatively low. The anodic dissolution of Mg resulted in high pH values (pH 11) eliminating the need for alkali addition.

El objetivo de la investigación fue estudiar la precipitación salina a temperatura constante de 75 °C en un destilador solar experimental. Se utilizó agua subterránea con alto contenido de calcio, bicarbonatos y sulfatos solubles como afluente. Una de las variables más importantes en la operatividad y productividad de agua potable con esta tecnología es la precipitación salina en la base del destilador. Esta ocurre debido a las características fisicoquímicas del agua subterránea típica de las zonas desérticas del norte del estado de Chihuahua. Las variaciones en la concentración de calcio soluble del afluente se midieron cada hora con un espectrofotómetro de absorción atómica y se calculó la cantidad total precipitada de calcio. Con la técnica de difracción de rayos X y microscopía electrónica de barrido se analizó el precipitado. Se determinó que la reacción de la precipitación se estabilizó a la tercera hora. La concentración de calcio aumentó durante la cuarta y la quinta hora, lo que indicó que el calcio soluble remanente no reaccionó. Se concluye que la reacción de precipitación salina no es isotérmica, sino que está impulsada por gradientes de temperatura. En el análisis de la identificación de las fases cristalinas del precipitado en las incrustaciones se encontró CaCO3 en forma de calcita y CaSO4 como anhidrita y basanita. The objective of the research was to study salt precipitation at a constant temperature of 75 °C in an experimental solar still. Groundwater with high content of calcium, bicarbonates and soluble sulfates was used as an input water. One of the most important variables in the operability and productivity of drinking water with this technology is the saline precipitation at the base of the distiller. This occurs due to the physicochemical characteristics of the groundwater typical of the desert areas of the north of the state of Chihuahua. Variations in the soluble calcium concentration of the input water were measured every hour with an atomic absorption spectrophotometer and the precipitated amount was calculated. The precipitate was analyzed using the X-ray diffraction and scanning electron microscopy technique. It was determined that the precipitation reaction occurred between the first and second hours. The calcium concentration increased during the fourth and fifth hours, indicating that the remaining soluble calcium did not react. It concludes that the salt precipitation reaction is not isothermal but is driven by temperature gradients. The analysis of the identification of the crystalline phases of the precipitate in the scales found CaCO3 in the form of calcite and CaSO4 as anhydrite and basanite.

The widespread use of hydrophilic intraocular lenses (IOLs) in eye surgery, fabricated by poly-2-(hydroxyethyl methacrylate) (PHEMA), has highlighted their calcification as a serious problem, implying that their surgical explantation is a rather risky process. The field of biomaterials has been developing rapidly in recent years, with research interest turning to the development of novel materials which involve either copolymers of PHEMA or protective functional coatings. Graphene coatings are particularly attractive because of their respective unique properties. In the present work, we present the results of the investigation of the development of graphene coatings on hydrophilic IOLs and their subsequent performance with respect to calcification opacification. Hydrophilic IOLs with a water content of 18% by weight were coated with graphene oxide (GO) by equilibration with GO suspensions in water. The concentrations of the suspensions ranged from 1 × 10⁻⁴ to 20 × 10⁻⁴% w/v. The GO suspensions were equilibrated with the IOLs for 5 days at a constant temperature, 37 °C, and rotated in 30 mL tubes end over end. This treatment resulted in the formation of a uniform coating of GO on the IOLs verified by scanning electron microscopy (SEM) and other physicochemical methods. The contact angle of the GO-coated IOLs decreased significantly in comparison with the uncoated IOLs. The GO-coated IOLs exhibited a higher tendency to calcify in supersaturated solutions simulating aqueous humor (SAH). The growth rate of hydroxyapatite (Ca₅(PO₄)₃OH, HAP) on GO-coated IOLs was higher in comparison with the respective untreated IOLs. The conversion of the GO coating via a reduction with phenyl hydrazine resulted in the formation of a reduced-graphene (rGO) surface film, as identified by Raman and XPS spectroscopy. The rGO film was hydrophobic (contact angle 100°) and did not calcify in supersaturated calcium phosphate solutions.

The widespread use of hydrophilic intraocular lenses (IOL) in eye surgery, fabricated by poly-2- (hydroxyethylmethacrylic acid) (PHEMA), has highlighted their calcification as a serious problem, which implies their surgical explantation a rather risky process. The field of biomaterials has been developing rapidly in recent years with research interest turning to the development of novel ma-terials which involve either copolymers of PHEMA or protective functional coatings. Graphene coatings are particularly attractive because of the respective unique properties. In the present work we present the results of the investigation of the development of graphene coatings on hydrophilic IOLs and their subsequent performance with respect to calcification opacification. Hydrophilic IOL with water content of 18% by weight, were coated with graphene oxide (GO) by equilibration with GO suspensions in water. The concentrations of the suspensions ranged 1x10-4 to 20x10-4 % w/v. The GO suspensions were equilibrated with the IOLs for 5 days at constant temperature, 37°C, rotated in 30 mL tubes end over end. This treatment, resulted in the formation of a uniform coating of GO on the IOLs verified by scanning electron microscopy (SEM) and other physicochemical methods. The contact angle of the GO coated IOLs decreased significantly in comparison with the uncoated. The GO-coated IOLs exhibited higher tendency to calcify in supersaturated solutions simulating aqueous humor (SAH). The growth rate of hydroxyapatite (Ca5(PO4)3OH, HAP) on GO-coated IOLs was higher, in comparison with the respective untreated IOLs. The conversion of the GO coating by reduction with phenyl hydrazine resulted in the formation of reduced graphene (rGO) surface film, identified by Raman and XPS spectroscopy. of the rGO film was hydrophobic (contact angle 100 o) and did not calcify in supersaturated calcium phosphate solutions.

Mortars, which are very important elements for the integrity of historic monuments, consist mainly of calcium carbonate and silicates in different proportions. Chemical dissolution due to exposure in open air is very important for the degradation of mortars. Inorganic nanoparticles with chemical and crystallographic affinity with mortar components are expected to be effective structure stabilizers and agents offering resistance to chemical dissolution. In the present work, we have developed and applied suspensions of amorphous calcium carbonate (ACC), silicon oxide (am-SiO₂) and composite nanoparticles by the precipitation of ACC on am-SiO₂ and vice versa. The application of suspensions of the synthesized nanoparticles on three different historical mortars of Roman times (1st century AD), retarded their dissolution rate in solutions undersaturated with respect to calcite, in acid pH (6.50, 25 °C). All three test historic mortars, treated with suspensions of the nanoparticles prepared, showed high resistance towards dissolution at pH 6.50. The ability of the nanoparticles’ suspension to consolidate the damaged mortar was the key factor in deciding the corresponding effectiveness in the retardation of the rate of dissolution. The combination of ACC with am-SiO₂ nanoparticles showed high efficiency for protection from the dissolution of calcite rich mortars.