Stanislav Kurajica

Bio: Stanislav Kurajica is an academic researcher from University of Zagreb. The author has contributed to research in topics: Mullite & Crystallization. The author has an hindex of 16, co-authored 92 publications receiving 1305 citations.

Ammoniacal nitrogen removal from water by treatment with clays and zeolites

Abstract: The objective of the present study was to investigate the removal of nitrogen in the form of ammonium ions (N-NH+4) from aqueous solutions using natural clay and zeolite. The Croatian zeolite clinoptilolite from the area of Donje Jesenje and Croatian bentonite clay from the Kutina area were used as natural filtration materials. Alkaline and acid modification of natural clay was performed. The N-NH4+ removal efficiency by clays in a dry state as well as in a liquid colloidal state have been investigated. The highest removal efficiency value for N-NH+4 (61.1 wt%) was achieved with the natural zeolite at the lowest used initial concentration, i.e. at a concentration of 100 mg N-NH+4/l. With the increase of the initial concentration of ammoniacal nitrogen, the removal efficiency quickly decreases. This is expected as zeolites and clays have limited sorption capacities. The practical aspect is important since an ammoniacal nitrogen elimination of more than 60.0 wt% for water containing less than 100 mg N-NH+4/l can be achieved. The ammoniacal nitrogen removal efficiency is higher for clays in a liquid colloidal state compared to the clays in a dry state. It has been found that the acid modification of the clay decreases the efficiency of ammoniacal nitrogen removal.

Thermophysical Comparison of Five Commercial Paraffin Waxes as Latent Heat Storage Materials

Abstract: Thermophysical properties of phase change materials (PCM) are of utmost importance in latent heat thermal energy storage (LHTES) applications. Therefore, an experimental study is conducted in order to determine thermophysical properties of five technical grade paraffin waxes produced by major Croatian oil company, INA d.d. Rijeka. The temperatures and enthalpies of melting and solidification (latent heat capacity) and specific heat capacities of solid and liquid paraffin waxes were measured by differential scanning calorimetry (DSC). The thermal diffusivity of paraffin waxes was determined utilizing transient method. The importance of eliminating phase transformation interferences to thermophysical properties determination is addressed. The densities and the coefficient of thermal expansion were measured using Archimedes methods. A self-adopted simple and inexpensive laboratory procedure for the determination of liquid density as a temperature function is presented. Finally, the thermal conductivities have been calculated from measured densities, heat capacities and diffusivities. Based on results obtained, the investigated paraffin waxes were evaluated in regard to their applicability as PCM for LHTES.

Dehydration of a layered double hydroxide – C2AH8

Abstract: Thermal dehydration of dicalcium aluminate hydrate, C2AH8, has been investigated by simultaneous differential thermal and thermo gravimetric analysis (DTA/TGA), powder X-ray diffraction (XRD), temperature-dependent infrared spectroscopy (FT-IR), and BET method of surface area measurement. The temperature-dependent infrared measurements were studied by two-dimensional infrared (2D-IR) correlation spectroscopy. The structure of aluminum-oxide polyhedron, characterized by 27Al solid state NMR spectrum method and FT-IR, shows tetrahedron and octahedron as the main forms of aluminum-oxide polyhedrons in C2AH8 sample. From the results obtained a variety of structural transformations observed are explained as a consequence of the removal of loosely held interlayer water molecules at lower temperatures, followed by grafting process of the interlayer [Al(OH)4]− anion. Structural model of a grafting process of the interlayer [Al(OH)4]− tetrahedron onto hydroxylated octahedrons of [Ca2Al(OH)6]+ layers has been proposed in order to explain observed loss of one water molecule, shrinkage of interlayer spacing and qualitative changes of FT-IR spectra. At higher temperatures the dehydroxylation of the lattice and decomposition of the interlayer species occurs, yielding amorphous material that crystallizes into C3A and C12A7 at 885 °C. Those findings provide improvement in the interpretation of thermo-analytical results of calcium aluminate cements (CAC) hydration products, and better understanding of CAC conversion process.

Corrosion behavior of alumina ceramics in aqueous HCl and H2SO4 solutions

Abstract: The corrosion behavior of cold isostatically pressed (CIP) high purity alumina ceramics in aqueous HCl and H2SO4 solutions with various concentrations has been studied simultaneously at room temperature (25 °C). Corrosion tests were also performed with 0.65 mol/l HCl and 0.37 mol/l H2SO4 solutions at 40, 55 and 70 °C for 48 h. Chemical stability was monitored by determining the amount of Al3+, Mg2+, Ca2+, Na+ Si4+ and Fe3+ ions eluted in different concentrations of HCl and H2SO4 solutions by means of atomic absorption spectrometry (AAS). By increasing the concentration from 0.37 to 6.5 mol/l, it was notified that the corrosion susceptibility in HCl and H2SO4 solutions for the CIP alumina specimens at room temperature decreases.

The effect of annealing temperature on the structure and optical properties of sol–gel derived nanocrystalline cobalt aluminate spinel

Abstract: The thermal evolution of a sol–gel derived cobalt aluminate (CoAl2O4) of a spinel-type structure was studied. The samples were prepared by the sol–gel technique using aluminum-sec-butoxide [Al(OsBu)3] and cobalt nitrate hexahydrate Co(NO3)2 × 6H2O as starting materials and ethyl-acetoacetate (C6H10O3) as a chelating agent. The powder precursors were annealed at various temperatures ranging from 200 to 900 °C. The properties of products were characterized using differential thermal analysis and thermo-gravimetric analysis (DTA/TGA), UV–vis diffuse reflectance spectroscopy (DRS) and X-ray diffraction (XRD). The UV–vis DRS spectra revealed that in samples annealed at temperatures below 700 °C a part of Co existed in a 3 + oxidation state. The accommodation of Co3+ in octahedral cation sites yields with a green color. Thermal treatment at higher temperatures promote the reduction of Co3+ ions to Co2+ and a change of color from green to blue, later being characteristic of Co2+ ions in tetrahedral coordination. The XRD revealed that annealing of the precursor powder at a lower temperature (500 and 600 °C) resulted in a crystal spinel-type phase Co2AlO4. Annealing at higher temperatures (700–900 °C) induces a reaction of Co2AlO4 with amorphous alumina producing a phase CoAl2O4 having a partially inverse spinel structure, with δ in the range of 0.198–0.260. The spinel powders obtained at all annealing temperatures were in the nanosize range.

Natural zeolites as effective adsorbents in water and wastewater treatment

Abstract: Natural zeolites are abundant and low cost resources, which are crystalline hydrated aluminosilicates with a framework structure containing pores occupied by water, alkali and alkaline earth cations Due to their high cation-exchange ability as well as to the molecular sieve properties, natural zeolites have been widely used as adsorbents in separation and purification processes in the past decades In this paper, we review the recent development of natural zeolites as adsorbents in water and wastewater treatment The properties and modification of natural zeolite are discussed Various natural zeolites around the world have shown varying ion-exchange capacity for cations such as ammonium and heavy metal ions Some zeolites also show adsorption of anions and organics from aqueous solution Modification of natural zeolites can be done in several methods such as acid treatment, ion exchange, and surfactant functionalisation, making the modified zeolites achieving higher adsorption capacity for organics and anions

Silica aerogel: synthesis and applications

Abstract: Silica aerogels have drawn a lot of interest both in science and technology because of their low bulk density (up to 95% of their volume is air), hydrophobicity, low thermal conductivity, high surface area, and optical transparency. Aerogels are synthesized from molecular precursors by sol-gel processing. Special drying techniques must be applied to replace the pore liquid with air while maintaining the solid network. Supercritical drying is most common; however, recently developed methods allow removal of the liquid at atmospheric pressure after chemical modification of the inner surface of the gels, leaving only a porous silica network filled with air. Therefore, by considering the surprising properties of aerogels, the present review addresses synthesis of silica aerogels by the sol-gel method, as well as drying techniques and applications in current industrial development and scientific research.

Ion exchange of Pb2+, Cu2+, Fe3+, and Cr3+ on natural clinoptilolite: selectivity determination and influence of acidity on metal uptake

Abstract: In the present study ion exchange of Pb(2+), Cu(2+), Fe(3+), and Cr(3+) on natural Greek clinoptilolite was examined in terms of selectivity toward the above heavy metals in single- and multicomponent solutions in batch systems. Also examined are the influence of clinoptilolite on solution acidity and the effect of acidity on the ion exchange process. Clinoptilolite increases solution acidity due to the exchange of H(+) cations with the cations initially present in its structure. H(+) cations should be considered as competitive ones in ion exchange processes, and consequently ion exchange of metals is favored at high acidity values. Cu(2+) and Cr(3+) are the most sensitive cations with respect to acidity. Selectivity determination demonstrates that the selectivity at total concentration 0.01 N and acidity 2 in both single- and multicomponent solutions is following the order Pb(2+)>Fe(3+)>Cr(3+) > or =Cu(2+). This order is set since the first days of equilibration. However, Cu(2+) shows remarkable changes in selectivity and generally its uptake and selectivity are increasing with time. On the other hand selectivity in single metal solutions where acidity is not adjusted is following the order Pb(2+)>Cr(3+)>Fe(3+) congruent with Cu(2+).