Browsing by Author "Georgieva R."
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Item Adsorption properties of a nanostructured hybrid material containing aluminium towards some metal ions(2011-10-01) Detcheva A.; Vassileva P.; Georgieva R.; Voykova D.; Gerganova T.; Ivanova Y.In the present work the adsorption of some transition metal ions from aqueous solutions on a silica-based nanostructured hybrid material modified by aluminium was investigated. The novel organic-inorganic material was synthesized via a sol-gel method through hydrolysis and co-condensation reactions. Its structure was characterized by means of SEM, XRD and FTIR. Based on the data obtained the most probable cross-linking mechanism for the derived xerogel was proposed. The characterization of its texture parameters was carried out by low-temperature adsorption of nitrogen. The adsorption properties of this material with respect to Cu(II), Cr(III) and Pb(II) ions from single-component aqueous solutions and multi-component aqueous solutions containing also Cd(II) and Fe(III) were evaluated. The effect of contact time, acidity of initial solutions and metal ion concentrations was investigated using the batch method. Pseudo-first order, pseudo-second order and intraparticle diffusion models were used to analyze kinetic data. In all cases the adsorption was significantly affected by the pH value. Equilibrium modelling data were fitted to linear Langmuir, Freundlich and Dubinin-Radushkevich models. Best fit was observed for Langmuir model, which showed determination coefficients greater than 0.992 for all ions studied. The maximum adsorption capacities for single- and multi-component adsorption were calculated. © Versita Sp. z o.o.Item Antimicrobial activity and antibiotic susceptibility of Lactobacillus and Bifidobacterium spp. intended for use as starter and probiotic cultures(2015-01-01) Georgieva R.; Yocheva L.; Tserovska L.; Zhelezova G.; Stefanova N.; Atanasova A.; Danguleva A.; Ivanova G.; Karapetkov N.; Rumyan N.; Karaivanova E.Antimicrobial activity and antibiotic susceptibility were tested for 23 Lactobacillus and three Bifidobacterium strains isolated from different ecological niches. Agar-well diffusion method was used to test the antagonistic effect (against Staphylococcus aureus, Escherichia coli, Bacillus cereus and Candida albicans) of acid and neutralized (pH 5.5) lyophilized concentrated supernatants (cell-free supernatant; CFS) and whey (cell-free whey fractions; CFW) from de Man Rogosa Sharpe/trypticase-phytone-yeast broth and skim milk. Acid CFS and CFW showed high acidification rate- dependent bacterial inhibition; five strains were active against C. albicans. Neutralized CFS/CFW assays showed six strains active against S. aureus (L. acidophilus L-1, L. brevis 1, L. fermentum 1, B. animalis subsp. lactis L-3), E. coli (L. bulgaricus 6) or B. cereus (L. plantarum 24-4%). Inhibition of two pathogens with neutralized CFS (L. bulgaricus 6, L. helveticus 3, L. plantarum 24-2L, L. fermentum 1)/CFW (L. plantarum 24-5D, L. plantarum 24-4%) was detected. Some strains maintained activity after pH neutralization, indicating presence of active substances. The antibiotics minimum inhibitory concentrations (MICs) were determined by the Epsilometer test method. All strains were susceptible to ampicillin, gentamicin, erythromycin and tetracycline. Four lactobacilli were resistant to one antibiotic (L. rhamnosus Lio 1 to streptomycin) or two antibiotics (L. acidophilus L-1 and L. brevis 1 to kanamycin and clindamycin; L. casei L-4 to clindamycin and chloramphenicol). Vancomycin MICs > 256 μg/mL indicated intrinsic resistance for all heterofermentative lactobacilli. The antimicrobially active strains do not cause concerns about antibiotic resistance transfer and could be used as natural biopreservatives in food and therapeutic formulations.Item Chemical and technological characterization of medieval glass bracelets from South-East Bulgaria(2014-01-01) Georgieva R.; Detcheva A.; Dimitriev Y.The present paper deals with chemical and physicochemical characterization of seven glass fragments of medieval glass bracelets from South-East Bulgaria. Samples were investigated by Fourier-transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and differential thermal analysis (DTA). Gravimetric chemical analysis was used for silica assessment. Flame photometry was applied to determine sodium and potassium content while aluminium, calcium and magnesium were determined by flame atomic absorption spectrometry after dissolution using a mixture of acids. All investigated artefacts are soda-lime-silica glasses and when produced two different recipe norms were used. Important technological parameters of glass manufacturing were determined and discussed. This investigation throws light on the technological development and production of glass during medieval times. ©Versita Sp. z o.o.Item Chemical and technological characterization of medieval glass bracelets from South-East Bulgaria(2014-01-01) Georgieva R.; Detcheva A.; Dimitriev Y.The present paper deals with chemical and physicochemical characterization of seven glass fragments of medieval glass bracelets from South-East Bulgaria. Samples were investigated by Fourier-transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and differential thermal analysis (DTA). Gravimetric chemical analysis was used for silica assessment. Flame photometry was applied to determine sodium and potassium content while aluminium, calcium and magnesium were determined by flame atomic absorption spectrometry after dissolution using a mixture of acids. All investigated artefacts are soda-lime-silica glasses and when produced two different recipe norms were used. Important technological parameters of glass manufacturing were determined and discussed. This investigation throws light on the technological development and production of glass during medieval times. ©Versita Sp. z o.o.Item Physicochemical parameters and rheological properties of yoghurts during the storage(2013-11-05) Karsheva M.; Paskov V.; Tropcheva R.; Georgieva R.; Danova S.The aim of this study is to investigate the changes in different physicochemical, texture and sensory properties of milk fermented with different strains during the storage. An experimental study of the effect of the storage time on the physicochemical and rheological properties of milk fermented with commercial starter (StLb) and the same starter with a probiotic Lactobacillus salivarius LS adjunct was carried out. The effect of the storage on the pH, titrable acidity, lactic acid concentration, microorganisms count, gel firmness and rheological parameters is investigated. During the fermentation a stable decrease of the pH values and increase in the titrable acidity was observed. Through the whole storage period at 4°C (till 28th day) the both fermented milks kept stable rheological and organoleptic properties. This determines the fermented milk as a suitable form for oral administration of tested probiotic strain LS. The rheological tests showed that the products studied demonstrated non-Newtonian behaviour, which could be described with Herschell-Bulkley rheological model. A correlation between the model parameters and the results of the penetration test was found. The results of this study can be used in production of new fermented milk products with functional properties.Item Silica-based organic-inorganic hybrid materials as potential adsorbents for Hg(II) ions(2013-12-17) Velikova N.; Vueva Y.; Ivanova Y.; Salvado I.; Fernandes M.; Vassileva P.; Georgieva R.; Detcheva A.In the present study four functionalized mesoporous silica materials denoted as BIS-1, BIS-2, BIS-3 and BIS-4 have been synthesized through a simple co-condensation of bis-[3-(trimethoxyosilyl)propyl] amine (BTPA) and tetraethyl orthosilicate (TEOS). The influence of the BTPA amount on the structural and adsorption properties of the resultant materials was investigated. The novel hybrid materials have been characterized by FT-IR, BET, 13C CP MAS NMR and elemental analysis. The adsorption properties of mesoporous organic-inorganic materials containing amine functional groups, regarding Hg(II) ions were studied by the batch method. Experiments were carried out in acidic media. The material BIS-3 exhibits highest adsorption capacity for mercury ions. Kinetics of Hg (II) adsorption on BIS-3 was investigated. Equilibrium experimental data for BIS-3 were fitted to linear Langmuir and Freundlich models. All studied hybrid materials could be successfully used for effective removal of mercury ions from acidic aqueous solutions.Item Synthesis, characterization and adsorption properties of nanostructured hybrid materials modified by boron and zirconium(2012-10-01) Georgieva R.; Vassileva P.; Detcheva A.; Voykova D.; Gerganova T.; Ivanova Y.The adsorption properties of two new nanostructured hybrid materials containing B 2O 3 and ZrO 2 were studied. The new organic- inorganic materials were synthesized via a sol-gel method. As a modifying agent, a quantity of 10 wt.% Zr(OPr) 4 or B(OCH 3) 3 was added. The structure of the hybrid materials was investigated by means of (Fourier transform infrared spectroscopy (FTIR), x-ray diffractometry (XRD), scanning electron microscopy (SEM), (atomic force microscopy (AFM) and nuclear magnetic resonance spectroscopy (NMR). Based on the obtained data, the most probable cross-linking mechanism for the derived gels was proposed. The characterization of texture parameters of both materials was carried out with the use of low-temperature adsorption of nitrogen. Adsorption of Cu(II), Fe(III), Cr(III), Cd(II) and Pb(II) ions on both materials was investigated using multi-component solutions with different concentrations and acidity by means of the batch method. Kinetics of adsorption was also investigated. Pseudo-first order, pseudo-second order and intraparticle diffusion models were used to analyze kinetic data. The adsorption was significantly affected by the pH value. Equilibrium data were fitted to linear Langmuir, Freundlich and Dubinin-Radushkevich models and maximum adsorption capacities were calculated. © Versita Sp. z o.o.