Slobodan K. Milonjić

Sorption of selenium anionic species on apatites and iron oxides from aqueous solutions.

The sorption of selenite and selenate ions from aqueous solutions was investigated on hydroxyapatite, fluorapatite, goethite and hematite, in order to simulate the behavior of radioactive selenium in natural or artificial sorbing media. Correlation studies with acido-basic properties and solubility of the solids were also performed. The sorption is pH dependant, but these solids are very efficient for retaining selenite at pH values generally encountered in natural waters, with however higher K(d) values for oxides than apatites. Selenate ions are much less sorbed than selenite. Several methods such as electron microscopy and spectroscopic techniques were used to identify the sorption mechanisms. In the case of hydroxyapatite, sorption proceeds by substitution of phosphate groups in the lattice of the apatite crystal in the superficial layers of the solid. In the case of goethite and hematite, sorption can be interpreted and modeled by a surface complexation process, but there is a discrepancy between sorption site densities for selenite and for protons.

Point of Zero Charge and Isoelectric Point of Alumina

The point of zero charge, pHpzc, and the isoelectric point, pHiep, of a commercial alumina sample was investigated in KNO3 solutions. The influence of the concentration and the initial pH value of KNO3 solution used for preparation and equilibration of the alumina suspension, as well as of the alumina/solution ratio on pHpzc and pHiep were investigated. The obtained results indicate that pHiep values are dependent on initial pH values of KNO3 solutions used for equilibration of alumina suspensions. Simultaneously with isoelectric point determination, the particle size of alumina was also measured. It was shown that the particle size of alumina increases with increasing suspension pH due to agglomeration, reaching the maximum value at pHiep.

Influence of synthesis parameters on the structure of boehmite sol particles

Abstract Influence of the type and amount of acid, used for aluminium hydroxide peptization, on the crystallinity and specific surface area of the dispersed phase of boehmite sols as well as the pH of sols, was investigated. The ratio n (HCl)/ n (Al(OH) 3 )=0.1 was found to be optimum for aluminium hydroxide peptization. Higher acid to aluminium hydroxide ratio leads to a decrease in specific surface area and crystallinity of the solid phase as well as in pH of sols. Increase in counter ion (Cl − ) concentration causes a shift in d values of (020) crystallographic plane, from the value corresponding to boehmite to that for pseudoboehmite, and a decrease in specific surface area of the sol solid phase. Using HNO 3 as a peptizing agent, greater crystallinity and higher specific surface area values of the solids were registered.

Determination of isoelectric points of metals and metallic alloys by adhesion of latex particles.

A set-up and a method were developed to determine the isoelectric point of metals and metallic alloys samples (stainless steels, inconel, zircaloy, aluminum and dural) by measuring the adhesion rate of negative latex particles. The concentration of polystyrene spheres with surface carboxylate groups (initially 0.5-1 mg L(-1)) in contact with metallic samples was measured as a function of pH and time by turbidimetry. The simulation of measurements by a model predicting the sticking coefficient based on DLVO theory was used for the determination of the isoelectric point from experimental results. It was found that the isoelectric points of aluminum (8.7) and dural (9.1), treated by boiling water, are close to those of hydrated aluminum oxides powders. For stainless steels, inconel and zircaloy, the values of isoelectric points were found to be between 2.4 and 3.0, far below the isoelectric points measured for metallic oxides constituting the alloy surface layer. This difference was explained by two different charging mechanisms: (1) deprotonation of hydroxyl groups on the surface of the metal oxide in suspension or as a thick layer, (2) adsorption of hydroxide ions on a metal surface covered by a thin oxide layer, as observed on hydrophobic surfaces.

Surface Properties of HAp Particles Obtained by Hydrothermal Decomposition of Urea and Calcium-EDTA Chelates

In this paper surface properties of calcium-hydroxyapatite, synthes ized by modified hydrothermal reaction of urea and Ca(EDTA) 2– in phosphate solutions, as well as the dependence of calcium hydroxyapatite dispersion stability on pH were determined. T he specific surface area of calcium-hydroxyapatite was experimentally found to be 67 m /g. The point of zero charge (PZC) of indifferent KCl electrolyte of 6.8 ± 0.1 was determined by the batch equilibration method. From the dependence of surface charge density on pH and electrolyte concentrati on, the intrinsic equilibrium constant of surface groups p a2 K was determined to be 8.3 ± 0.1, while p int a1 K , calculated from the p int a2 K and point of zero charge values, is 5.3 ± 0.1. Hy droxyapatite suspension stability was investigated, in the pH range from 7 to12, it was r evealed that the most stable dispersion at pH 10. Introduction Hydroxyapatite is one of the main constituent compo unds of bones and teeth. Knowledge of the hydroxyapatite surface chemistry is very important from the point of view not only of bone tissue formation but also of HAP solubility, due to its in teraction with the surrounding fluid. Solubility of apatites is directly connected with various illness es uch as decalcination of bones (osteoporosis) and dental caries[1-2]. Behavior of the aqueous dispersion of hydroxyapatit e depends on the interfacial chemistry of the solid phase and dispersive medium. Surface charge o f hydroxyapatite in aqueous systems plays one of the main roles in determination of its colloidal and sorption properties [3]. The studies of the point of zero charge (pH PZC) and surface charge characteristics of apatites in dicate that H + and OH are potential-determinig ions and that the surface charge development at the apatite-water interface could be described by amphoteric dissociation react ions of surface functional groups [4,5]. In this paper, behavior and surface properties such as specific surface area, presence of particular surface groups, point of zero charge, surface charg e density, and intrinsic equilibrium constants of carbonate HAP obtained by hydrothermal decompositio n of urea and Ca-chelate, were reported. Methods and Materials Synthesis of carbonate hydroxyapatite was performed by a modified method of Fujishiro et al.[6-8], and was described earlier [9]. Specific surface area of HAP was determined by the B.E.T. (Brunauer – Emett – Teller) method, the sample was previously degassed in vacuum for 2 hour s at 150°C. The point of zero charge of HAP was determined by t he batch equilibration method described by one of the authors[10]. The adopted technique is as follows: KCl of different ionic strengths, 0.1 and 0.01 mol/ dm, was used as an electrolyte. First set of measurements included 12 samples with 25 cm 3 KCl of 0.1 mol/dm ionic strength. The initial pH (pHi) values, from 3 to 10, were adjusted by adding sm all volumes of 0.1 mol/dm 3 HCl or KOH to Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 437-440 doi:10.4028/www.scientific.net/KEM.240-242.437

Surface properties of silicon nitride powders

Abstract The surface charge densities of silicon nitride powders in NaNO3 solutions of various concentrations were investigated by the potentiometric acid–base titration method. The points of zero charge, pHpzc, of silicon nitride powders were determined by the batch equilibration method. The site-binding model was applied to calculate the intrinsic acidity constants describing association–dissociation of silicon nitride surface groups.

Deposition of hematite particles on polypropylene walls in dynamic conditions

An experimental circulating water loop has been constructed to study the deposition of hematite particles of average diameter 320 nm on polypropylene pipe walls in the ranges pH 4-11, Re 3300-17,700 at 25 degrees C. Real-time turbidimetric measurements were used to measure the deposited concentrations. Results showed that the deposition rate increased when pH decreased and when the flow rate increased. Adhesion was observed even under repulsive electrostatic conditions (pH>7), where the surfaces of hematite and polypropylene were both negative, indicating that the kinetic energy of at least a part of the particles surpassed the electrostatic repulsive potentials. The experimental curves were fitted by a model assuming simultaneous adhesion and removal of particles, leading to adhesion and removal rate constants, whose values depend on pH and flow rate. Removal is negligible below pH 9.

Influence of AlOOH nanoparticles on the oxidation of iodide by persulphate

Abstract The oxidation of iodide by persulphate has been studied in the presence of the AlOOH nanoparticles. The rate of this reaction is significantly higher in the presence of the AlOOH nanoparticles than when it takes place in an aqueous medium. The dependence of the rate constant on the AlOOH content, temperature, ionic strength and pH has been examined. The results are interpreted in terms of electrostatic interaction between negatively charged reactant species and surface charge of the AlOOH nanoparticles. The sensitivity of the rate constant to the pH provides convenient method for precise detection of the point of zero charge of dispersed material.

Kinetics of 1,8-dihydroxy-2-(pyrazol-5-ylazo)-naphthalene-3,6-disulphonic acid immobilization on anion exchangers

The sorption of 1,8-dihydroxy-2-(pyrazol-5-ylazo)-naphthalene-3,6-disulfonic acid onto the anion exchange resins Dowex 1-X8 and Dowex 2-X8 from aqueous solutions of between pH 1 and 10 was studied spectrophotometrically. Conditions for obtaining modified sorbents with a given capacity for the azo compound were investigated. The absorption and reflectance spectra of the sorbed dye were traced. The immobilized reagent retained chromic characteristics similar to those it had in solution. The mechanism of sorption was investigated under static conditions in the temperature range of 0 to 25°C by following the amount of sorbed dye as a function of contact time. The sorption was a first-order process in all cases. Pore diffusion was found to be the rate-limiting step in the sorption process. Diffusion coefficient values of from 6.0 × 10–7 to 7.4 × 10–7 cm2/s were calculated from the experimental results. The values of the activation energy of PACA sorption were found to be –8.56 and –8.05 kJ/mol for Dowex 1-X8 and Dowex 2-X8, respectively.

Sol-Gel Synthesis of Silicon Carbide and Silicon Nitride Powders and Their Surface Properties

Silicon carbide (SiC) and silicon nitride (Si3N4) are important ceramics for various applications, due to their superior mechanical and electrical properties at higher temperatures. These materials exhibit high strength, good chemical, corrosion and oxidation resistance as well as good thermal shock resistance, and as such can be widely used in future.