Rada Petrović

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.

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

Crystallization Behavior of Alkoxy-Derived Cordierite Gels

Alkoxy-derived cordierite gels were synthesized from tetraethylorthosilicate (TEOS), aluminum isopropoxide (Al(OPri)3), and magnesium ethoxide (Mg(OEt)2). TEOS was partially hydrolyzed at molar ratios H2O/TEOS = 1.2, in the presence of hydrochloric acid as a catalyst, HCl/TEOS = 0.1. Aluminum and magnesium alkoxides were added successively or as a double alkoxide. Phase transformations occurring in the gel were studied by differential thermal analysis, x-ray diffractometry, and Fourier-transform infrared spectroscopy. In all cases, μ-cordierite crystallized at similar temperatures (950–1000°C) with small amounts of spinel, which confirms dominant influence of the optimal conditions for partial hydrolysis of TEOS on the gels homogeneity. The transformation of μ- into α-cordierite began at about 1100°C. Broadening of diffraction peaks and appearance of new bands in the FT IR spectra confirmed the transformation of α- into modulated β-cordierite at temperatures above 1300°C. Differential thermal analysis under nonisothermal conditions also proved homogeneous nucleation with constant rate and three-dimensional crystallite growth during μ-cordierite crystallization. The overall activation energy of the crystallization of μ-cordierite is 580 ± 81 kJ/mol.

Kinetics, thermodynamics, and structural investigations on the removal of Pb2+, Cd2+, and Zn2+ from multicomponent solutions onto natural and Fe(III)-modified zeolites

The adsorption of Pb2+, Cd2+, and Zn2+ from equimolar multicomponent solutions onto natural and Fe(III)-modified zeolites was studied by batch experiments in order to determine the influence of the competitive effect on the efficiency ions removal at different initial concentrations and different temperatures. Kinetic and equilibrium studies showed that affinity of both zeolites was the highest for Pb2+ and much lower for Zn2+ and Cd2+. The competitive effect was negligible at lower concentrations, while Zn2+ and especially Cd2+ ions adsorption was suppressed as the concentrations were increased. Affinity of natural zeolite toward the ions was lower than that of the modified zeolite, as was shown for adsorption from single-component solutions. External film diffusion was the rate-controlling step at lower concentrations and for ions of higher affinity for the adsorbent, while intraparticle diffusion was dominant at higher concentrations and for ions of lower affinity for the adsorbent. The increasing spontaneity of the adsorption process with increasing temperature and the positive values of enthalpy of adsorption indicated the endothermic nature of the adsorption process. Structural investigations of the adsorbents loaded with Pb2+, Cd2+, and Zn2+ ions by X-ray photoelectron spectroscopy (XPS) confirmed the higher content of the ions in the surface and subsurface regions of the Fe(III)-modified than of the natural zeolite. XPS depth profiles of the loaded Fe(III)-modified zeolite showed that surface mechanisms of adsorption were dominant in the case of lead and cadmium, while zinc loading on the zeolite sample was governed mainly by an ion-exchange process.

Sintering Behaviour of Nanosized HAP Powder

The influences of temperature and time on sintering behaviour of nanosized HAP powder were investigate in this paper. The calcium hydroxyapatite powder, with the average crystallite size of 34 ± 1 nm, was uniaxially pressed at a pressure of 500 MPa. Obtained green compacts were sintered at temperature ranging from 1000°C to 1200°C in air atmosphere at various times. According to the results of scanning electron microscopy, X-ray and FTIR analyses, it is shown that HAP compacts with dense microstructure and average grain size below 250 nm is obtained.

The influence of silicon substitution on the properties of spherical- and whisker-like biphasic α-calcium-phosphate/hydroxyapatite particles

In this work, the influence of the morphology of hydroxyapatite particles on silicon substitution through hydrothermal synthesis performed under the same conditions was investigated. Spherical- and whisker-like hydroxyapatite particles were obtained starting from calcium-nitrate, sodium dihydrogen phosphate, disodium-ethylenediaminetetraacetic acid and urea (used only for the synthesis of whisker-like particles) dissolved in aqueous solutions. Silicon was introduced into the solution using tetraethylorthosilicate. X-ray diffraction, infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy and transmission electron microscopy indicate that silicon doping induce different phase compositions and bioactivity of spherical- and whisker-like hydroxyapatite particles obtained under the same hydrothermal conditions. Silicon-substituted, spherical hydroxyapatites particles showed greater phase transformation to silicon-substituted α- calcium-phosphate compared with whiskers-like hydroxyapatite particles synthesized with the same amount of added silicon. Metabolic activity assay performed with SaOs2 osteosarcoma cells showed better biocompatibility of annealed biphasic spherical-like particles compared with annealed whiskerlike particles while dried spherical-like particles induce high cytotoxicity effect.

The effect of grain size on the biocompatibility, cell–materials interface, and mechanical properties of microwave‐sintered bioceramics

The effect of decreasing the grain size on the biocompatibility, cell-material interface, and mechanical properties of microwave-sintered monophase hydroxyapatite bioceramics was investigated in this study. A nanosized stoichiometric hydroxyapatite powder was isostatically pressed at high pressure and sintered in a microwave furnace in order to obtain fine grained dense bioceramics. The samples sintered at 1200°C, with a density near the theoretical one, were composed of micron-sized grains, while the grain size decreased to 130 nm on decreasing the sintering temperature to 900°C. This decrease in the grain size certainly led to increases in the fracture toughness by much as 54%. An in vitro investigation of biocompatibility with L929 and human MRC-5 fibroblast cells showed noncytotoxic effects for both types of bioceramics, while the relative cell proliferation rate, cell attachment and metabolic activity of the fibroblasts were improved with decreasing of grain size. An initial in vivo investigation of biocompatibility by the primary cutaneous irritation test showed that both materials exhibited no irritation properties.

Heavy Metals Fractionation in Agricultural Soils of Pb/Zn Mining Region and Their Transfer to Selected Vegetables

Improved understanding of the relationships between heavy metals fractionation in agricultural soils and biological uptake could be obtained by analysing samples of biota in parallel with sequential extraction of their grown media. The overall goals of this study were to identify the characteristics of metal fractions and their bioavailability to maize and potato plants in the agricultural land of the Ibar River in southern Serbia and northern Kosovo. The concentrations of Pb, Zn, Cd, Ni, Cr and Cu in soil and vegetable samples were determined by the ICP-OES method. Pb/Zn production and industrial waste disposal significantly increased the pseudo-total concentrations of heavy metals in the soil together with their mobile and potentially bioavailable amounts. The Pb concentrations in the vegetable samples were generally above the EU maximum permitted concentrations in foodstuffs. However, the concentrations of Zn, Ni, Cr and Cu in the vegetables were below the critical levels. The results of the present study indicated that the intensive industrial production of Pb/Zn over the years and permanent pollution were responsible for the environmental contamination by heavy metals in the study area, particularly by Pb and Zn. The quantity of the mobile and potentially bioavailable heavy metals in the studied soils threatens the quality of Zea mays L. and Solanum tuberosum L. crops, with a real risk that these elements could enter the food chain.

Nanoscale zerovalent iron (nZVI) supported by natural and acid-activated sepiolites: the effect of the nZVI/support ratio on the composite properties and Cd 2+ adsorption

Natural (SEP) and partially acid-activated (AAS) sepiolites were used to prepare composites with nanoscale zerovalent iron (nZVI) at different (SEP or AAS)/nZVI ratios in order to achieve the best nZVI dispersibility and the highest adsorption capacity for Cd2+. Despite the higher surface area and pore volume of AAS, better nZVI dispersibility was achieved by using SEP as the support. On the other hand, a lower oxidation degree was achieved during the synthesis using AAS. X-ray photoelectron spectroscopy (XPS) analysis of the composite with the best nZVI dispersibility, before and after Cd2+ adsorption, confirmed that the surface of the nZVI was composed of oxidized iron species. Metallic iron was not present on the surface, but it was detected in the subsurface region after sputtering. The content of zerovalent iron decreased after Cd2+ adsorption as a result of iron oxidation during Cd2+ adsorption. The XPS depth profile showed that cadmium was present not only at the surface of the composite but also in the subsurface region. The adsorption isotherms for Cd2+ confirmed that the presence of SEP and AAS decreased the agglomeration of the nZVI particles in comparison to the pure nZVI, which provided a higher adsorption capacity. The results showed that the prevention of both aggregation and oxidation during the synthesis was necessary for obtaining an SEP/AAS–nZVI composite with a high adsorption capacity, but oxidation during adsorption was beneficial for Cd2+ removal. The formation of strong bonds between Cd2+ and the adsorbents sites of different energy until monolayer formation was proposed according to modeling of the adsorption isotherms.

Distribution and mobility of heavy elements in floodplain agricultural soils along the Ibar River (Southern Serbia and Northern Kosovo). Chemometric investigation of pollutant sources and ecological risk assessment.

This work investigates the influence of a high-magnitude flood event on heavy elements (HEs) pollution and mobility in the agricultural soils along Ibar River in Southern Serbia and Northern Kosovo. The study area was one of the most important Pb/Zn industrial regions in Europe. Soil samples (n = 50) collected before and after the floods in May 2014 were subjected to the sequential extraction procedure proposed by the Community Bureau of Reference (BCR). The results indicated that the floods significantly increased not only the pseudo total concentrations of HEs in the soil but also their mobile and potentially bioavailable amounts. Moreover, higher concentrations (both pseudo total and potentially bioavailable) were found in the agricultural soils closer to the industrial hotspots. Principal component analysis and hierarchical cluster analysis successfully grouped the analyzed elements according to their anthropogenic or natural origin. The floods significantly increased the potential ecological risk of HEs associated with Pb/Zn industrial activities in the study area. The potential ecological risk of Cd after the floods was highest and should be of special concern.