Browsing by Author "Chernev G."
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Item Characterization of Bulgarian Copper Mine Tailing as a Precursor for Obtaining Geopolymers(2024-02-01) Ilieva D.; Angelova L.; Radoykova T.; Surleva A.; Chernev G.; Vizureanu P.; Burduhos-Nergis D.D.; Sandu A.V.Valorization of high-volume mine tailings could be achieved by the development of new geopolymers with a low CO2 footprint. Materials rich in aluminum and silicon with appropriate solubility in an alkaline medium can be used to obtain a geopolymer. This paper presents a study of copper mine tailings from Bulgaria as precursors for geopolymers. Particle size distribution, chemical and mineralogical composition, as well as alkaline reactivity, acidity and electroconductivity of aqueous slurry are studied. The heavy metal content and their mobility are studied by leaching tests. Sequential extraction was applied to determine the geochemical phase distribution of heavy metals. The studied samples were characterized by high alkalinity, which could favor the geopolymerization process. The water-soluble sulphates were less than 4%. The Si/Al ratio in mine tailing was found to be 3. The alkaline reactivity depended more so on the time of extraction than on the concentration of NaOH solution. The main part of the heavy metals was found in the residual fraction; hence, in high alkaline medium during the geopolymerization process, they will stay fixed. Thus, the obtained geopolymers could be expected to exert low environmental impact. The presented results revealed that studied copper mine tailing is a suitable precursor for geopolymerization.Item EFFECT OF GRINDING AIDS ON CEMENT PROPERTIES AND GRINDING PROCESS(2024-01-01) Popov A.; Chernev G.The cement industry is one of the most energy-intensive industries, which is why new energy-reducing and reducing CO2 emissions, additives are being sought. Grinding aid or grinding additives refer to substances that when mixed into the mill contents cause an increase in the rate of size reduction. These terms are very common in cement industries where it increases the throughput of the mill. Grinding aid also affects the cement flowability throughout the circuits. The grinding aid facilitates size reduction so that the mill has to apply less grinding power without hurting any of the properties of the resulting cement. The present research work aimed to investigate the influence of a chemical grinding aid ethylene glycol (EG) on cement properties and mill performance during the grinding of Portland cement beyond 500 ppm reaching up to 1200 ppm. The obtained results show increasing in initial compressive strength and increasing in the productivity of the cement mill.Item EVALUATION OF THE OXIDATIVE THERMAL STABILITY OF FISH OIL WITH THE ADDITION OF PUMPKIN SEED OIL OR ROSEMARY EXTRACT(2024-01-01) Gashi A.; Chernev G.; Symoniuk E.; Jankulovski Z.; De Souza C.K.; Rexhepi F.The present study investigates the thermal stability of the commercial fish oil, rich in unsaturated fatty acids, and compares the sensory properties of pure fish oil with a mixture of fish oil and added rosemary extract (RE), as well as roasted and unroasted pumpkin seed oil, at a concentration of 5 %. All samples were monitored using FTIR spectroscopy to measure the specific absorptivity of conjugated dienes (CDs) and conjugated trienes (CTs), as well as the peroxide value. Additionally, GC/FID was employed to evaluate the oxidative degree of the fish oil and compare the antioxidative effect of roasted pumpkin seed oil, specifically in comparison to rosemary extract. For this purpose, the oil stability was optimized by comparing the oxidation levels of fish oils exposed to range of temperature, including 23°C, 50°C, 70°C, 90°C, and 110°C. This was done in the presence of a low percent of rosemary extract, unroasted pumpkin seed oil (UPSO), and roasted pumpkin seed oil (RPSO). Based on the obtained results, a clear difference is observed in the blended samples, particularly when roasted pumpkin seed oil is used. This difference is evident in the ultraviolet chemical parameters, fatty acid profile, and most notably in the optimized FTIR vibrational bands. The ratios of area peaks such as 3444/2854, 1745/2854 and 3010/2854 are considered important parameters for monitoring the chemical changes and lipid stability. All the chemical parameters confirm the possibility of enhancing the stability of fish oil by blending it with healthy pumpkin seed oil. The composition of pumpkin seed oil increases the stability of fish oil. Consequently, it is evident that pumpkin seed oil, known for its high healthy benefits, can successfully be used to improve the thermal and oxidative stability of fish oil lipids. The principal component analysis (PCA) was used to define clusters, which revealed a wide range of both chemically changed and unchanged samples. The application of FTIR spectroscopy as an alternative method provides excellent parameters for easy operation, affordability, and ecological considerations, making in an efficient tool for controlling the quality of edible oils.Item Examination of Humicola lutea immobilized in sol-gel matrices: Effective source of α-galactosidase(2008-01-01) Spasova D.; Aleksieva P.; Nacheva L.; Kabaivanova L.; Chernev G.; Samuneva B.α-Galactosidase production by the fungus Humicola lutea 120-5 immobilized in a hybrid sol-gel matrix, consisting of tetraethylorthosilicate (TEOS) as a precursor and a mixture of polyethyleneglycol (PEG) and polyvinylalcohol (PVA), was investigated under semicontinuous shake flask cultivation and compared to the enzyme secretion by free cells. The influence of the carrier weight on the α-galactosidase biosynthesis in repeated batch experiments was followed. Best results were obtained with 2 g of the sol-gel particles per culture flask using 144-h runs. The growth behaviour of the immobilized mycelium during both the growth and productive phases was observed by scanning electron microscopy. The presence of abundant mycelial growth of intact hyphae correlated with a 2-fold higher enzyme activity compared to free cells. The obtained biocatalyst retained a high level of enzyme titer exceeding the activity of free cells during four cycles of operation (24 days). This result is confirmed by the micrographs showing the retained viability of the growing vegetative cells due to the protective role of the carrier. © 2008 Verlag der Zeitschrift für Naturforschung.Item Immobilization in nanomatrices of humicola lutea mycelium for alpha-galactosidase biosynthesis in Laboratoryair-lift bioreactor(2010-05-01) Djambaski P.; Aleksieva P.; Spasova D.; Chernev G.; Nacheva L.The sol-gel synthesis of both hybrid nanomatrices containing tetraethylortosilicate (TEOS) as an inorganic precursor and lactic acid, or sepharose as an organic component was made. Crystal as well as surface morphology of the hybrids were investigated using different methods: X-ray diffraction, infrared spectra, BET-analysis and atomic force microscopy (AFM). The obtained nanomatrices were applicated for immobilization of the α-galactosidase producing fungal strain Humicola lutea 120-5. The semicontinuous cultivation was carried out in laboratory air-lift bioreactor. Maximal level of enzyme activity (1050-1130 U/l) that was reached in the third to fourth fermentation cycle using TEOS+40% lactic acid was higher than that obtained in the samples with TEOS+20% sepharose (660-770 U/l). The correlation between enzyme productivity and fungal development in the pore structure of the carriers was examined using scanning electron microscopy observation.Item Incorporation of Synechocystis salina in hybrid matrices. Effect of UV-B radiation on the copper and cadmium biosorption(2010-10-14) Toncheva-Panova T.; Pouneva I.; Chernev G.; Minkova K.The isolated Antarctic cyanobacterium Synechocystis salina was embedded in silica hybrid matrices. The ability of the cyanobacterium to remove copper (about 7.6 mg.g-1 biomass) and cadmium (about 9.0 mg.g-1 biomass) was shown. UV-B treatment of cells prior to their immobilization decreased the percentage of viable cells to 4.5. At the same time it increased the copper sorption to 18.8 mg.g-1 biomass without influencing too much the sorption of cadmium (9.0-8.6 mg.g-1 biomass). The addition of carrageenan to the matrices containing viable UV-B non-irradiated cells increased the accumulation of copper and cadmium by about 2.9 and 3.9 fold, respectively. Results indicate that immobilized in hybrid matrices viable and dead Synechocystis salina cells could be used as an active copper and cadmium removing biocomponent for bioremediation of wastewaters and/or soils.Item Influence of metakaolinite and stone flour on the properties of self-compacting concrete(2013-05-21) Todorova E.; Chernev G.; Markov P.Self-compacting concrete is the new generation composites, characterized with high filling and passing ability, as well as high resistance to segregation. The flexibility and spreading without external vibrations make these materials prefered for producing tense elements. Powder additives and chemical admixtures play an important role for structural behavior and potential abilities of this type of concrete. The aim of the present study was the manifacture and characterization of mixture for self-compacting concrete with participation of powder additives (metakaolinite and stone flour) and super plasticizers (Viscocrete 5370 and Viscocrete 5800). The influence of chemical admixtures and powder additives on concrete properties were made by the different methods: sorption ability; S; FTIR and ζ- potential. Physical and mechanical properties as compressive strength; spreading and fluidity were measured. Using SEM and FTIR analyses were observed structure behavior of prepared samples. FTIR spectra showed characteristic peaks of the used components, without chemical interactions between them. SEM images showed evenly distribution of the fine fraction in concrete mixture. Tendency of decreasing sorption ability with increasing pH of the media is observed. Higher sorption ability showed mixtures with addition of chemical admixture Viscocrete 5370. With increasing amount of chemical admixture decreases ζ- potential of cement paste. Mixtures, containing metakaolinite showed higher values of strength pressure, spreading ability and fluidity.Item New approach for n-hexadecane biodegradation by sol-gel entrapped bacterial cells(2018-06-01) Chernev G.; Christova N.; Kabaivanova L.; Nacheva L.In this study sol-gel hybrid materials in the system SiO2-chitosan (CS) - polyethylene glycol (PEG), as novel structures with potential application in bioremediation were investigated. The organic components - CS and PEG were used as structural modifiers for functionality improvement. The catabolic activity to n-hexadecane of Pseudomonas aeruginosa BN10 free and immobilized cells was estimated. The cell immobilization technique was employed to evaluate its efficiency on biodegradation and protective effect from high levels of hydrocarbons. The characteristics of obtained hybrid materials were investigated via X-ray Diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Atomic-force microscopy (AFM) analyses. The obtained results revealed that the organic part in the synthesized hybrids is important for microstructure and defined properties creation. The rate of n-hexadecane mineralization by the bacterial strain was influenced by variation in cell densities applied in the immobilization procedures. Semi-continuous processes with multiple xenobiotic supplies were carried out. The synthesized by the sol-gel method hybrid matrices proved to be suitable carriers for realizing an effective biodegradation process of n-hexadecane by Pseudomonas aeruginosa BN10. Biodegradation of 50 kg/m3 of n-hexadecane was realized by free cells. Significantly greater quantity (150 kg/m3) was mineralized for 15 active cycles by entrapped bacterial cells. Biodegradation process with gradual increase of xenobiotic concentration reaching 30 kg/m3 for 120 h was also accomplished.Item Silica hybrid biomaterials containing gelatin synthesized by sol-gel method(2010-08-01) Chernev G.; Borisova B.; Kabaivanova L.; Salvado I.This work reports the sol-gel synthesis of silica hybrids. We determined the effect of the type and quantity of silica precursors and organic compounds on the resulting structure, surface area, nanostructure design and size, and potential applications. The structure of the synthesized hybrids was analyzed using FT-IR, XRD, BET-Analysis, SEM, and AFM. We demonstrate the immovilization of whole living thermophilic bacterial cells with cyanocompound degradation activity in the synthesized silica hybrid biomaterials by entrapment, chemical binding, and adsorption. © 2010 Versita Warsaw and Springer-Verlag Wien.Item Silica hybrid nanocomposites(2006-03-01) Chernev G.; Samuneva B.; Djambaski P.; Salvado I.; Fernandes H.In this work we present experimental results about the formation, properties and structure of sol - gel silica based biocomposite containing Calcium alginate as an organic compound. Two different types of silicon precursors have been used in the synthesis: Tetramethylortosilicate (TMOS) and ethyltrimethoxysilane (ETMS). The samples have been prepared at room temperature. The hybrids have been synthesized by replacing different quantitis of the inorganic precursor with alginate. The structure of the obtained hybrid materials has been studied by XRD, IR Spectroscopy, EDS, BET and AFM. The results proved that all samples are amorphous possessing a surface area from 70 to 290 m2/g. It has also been established by FT IR spectra that the hybrids containing TMOS display Van der Walls and Hydrogen bonding or electrostatic interactions between the organic and inorganic components. Strong chemical bonds between the inorganic and organic components in the samples with ETMS are present. A self-organized nanostructure has been observed by AFM. In the obtained hybrids the nanobuilding blocks average in size at about 8-14 nm for the particles. © Central European Science Journals Warsaw and Springer-Verlag Berlin Heidelberg 2006.Item Silica-carrageenan hybrids used for cell immobilization realizing high-temperature degradation of nitrile substrates(2011-04-01) Kabaivanova L.; Chernev G.; Miranda Salvado I.; Fernandes M.In this work the application of hybrid materials, containing TEOS as source of SiO2 and k-carrageenan in different percentage, synthesized by the sol-gel method at room temperature was studied. They were used as matrices for entrapment of whole Bacillus sp. UG-5B cells, producers of thermostable nitrilase. The effect of the surface area and size and quantity of pores in the synthesized materials on the enzyme activity was evaluated. The process of biodegradation of different concentrations of toxic, potentially carcinogenic and mutagenic substrates by the obtained biocatalysts was investigated. The enzyme reaction takes place by the nitrilase pathway, catalysing nitrile hydrolysis directly to the corresponding carboxylic acid, forming ammonia. At batch experiments the influence of the substrate concentration of different nitriles was tested and 20 mM concentration was found most suitable. A two-step biodegradation process in a laboratory-scale column bioreactor of o-, m- and p-tolunitrile as a mixture was followed. After operation of the system for nine hours for the mixture of substrates at a flow rate of 45 mL h-1 and at 60°C, the overall conversion realized was above 90%, showing a good efficiency of the investigated process. © Versita Sp. z o.o.Item Sol - gel silica hybrid materials applicable for external treatment of concrete defects(2015-01-01) Todorova E.; Chernev G.; Djambazov S.; Tsvetkova Y.The sol-gel silica hybrids are materials with application in different fields - pharmacy, medicine, sensors, etc. Concrete self - healing is a new area of applications of this kind of materials. It is connected with filling the cracks and preventing their further destruction as well as providing materials able to form CaCO3 as filler. A combination of hybrid materials containing calcium ions and bacterial cells is the optimal choice for that purpose. The material should exhibit stability, durability and ability to penetrate in the cracks. Furthermore, it should be reactive and biocompatible in order to interact with bacterial cells and promote further formation of CaCO3. Hybrid materials based on silica network and in situ incorporated organic components (chitosan and polyethylene glycol) is synthesized by the sol - gel method. The silica matrix obtained from tetraethylorthosilicate provides the structure with stability and durability, while the organic components account for its flexibility, biocompatibility and reactivity. The results from the structural analysis (XRD, FTIR, SEM and AFM) show that hybrids thus obtained have an amorphous, homogeneous structure. Formation of the silica network and backbone organic units is observed by FTIR spectroscopy. The AFM surface micrographs show the presence of particles of a size from 50 nm to 1 μm. They serve as reactive centers for potential interactions with the bacterial cells. The results from the structural characterization show that the synthesized materials can be used as carriers for bacterial cells' immobilization and applied further as fillers for external treatment of concrete defects.Item Sol-Gel immobilization as a suitable technique for enhancement of A-Amylase activity of aspergillus oryzae PP(2014-10-22) Evstatieva Y.; Yordanova M.; Chernev G.; Ruseva Y.; Nikolova D.Bioencapsulation of microbial cells in silica-based matrices has proved to be a good strategy to enhance the biosynthetic capabilities and viability of bioproducers. In the present study, mycelium and pellet cultures of strain Aspergillus oryzae PP were successfully immobilized in sol-gel hybrid matrices composed of tetraethylorthosilicate as an inorganic precursor, 5% (w/v) starch and 10 or 15% (w/v) polyethylene oxide, or 10% (w/v) calcium alginate as organic compounds. Biosynthetic activity of immobilized cultures was investigated by batch and fed-batch cultivation and the obtained results of 3042.04 IU cm ¡3 were comparable with the enzyme activity of the free cell culture. Immobilized cultures retained their viability and biosynthetic capabilities up to the 744th h during fed-batch fermentation processes. Consequently, sol-gel encapsulation in hybrid matrices could be considered as a promising technique for immobilization of Aspergillus oryzae PP in order to increase the a-amylase production.Item Sol-gel nanomaterials with algal heteropolysaccharide for immobilization of microbial cells, producing a-galactosidase and nitrilase(2009-01-01) Djambaski P.; Aleksieva P.; Emanuilova E.; Chernev G.; Spasova D.; Nacheva L.; Kabaivanova L.; Salvado I.M.M.; Samuneva B.The main purpose of the present work is the sol-gel synthesis and structure of the hybrid nanomaterials as matrices for two types of cells, producing hydrolytic enzymes. The effect of different percent of algal polysaccharide included in them on the hydrolytic activity of fungal and bacterial cells was investigated. The hybrid sol-gel nanomaterials were synthesized from tetraethylortosilicate (TEOS) as a silicon precursor and heteropolysaccharide (AHPS) from the red microalga Dixonella grisea as an organic part. The structure of these matrices was investigated using different methods: FT-IR, XRD, BET-Analysis, EDS, SEM and AFM. The sol-gel hybrids were used for the immobilization of fungal (Humicola lutea) and bacterial (Bacillus sp.) cells, producing α-galactosidase and nitrilase, respectively. It was established the effect of the quantity of the heteropolysaccharide in the matrices on the activity of these hydrolytic enzymes. Using 20% AHPS in the hybrid nanomaterials the α-galactosidase yield exceeded over two-fold the enzyme titre of the free cells in the third cycle of repeated batch shake flask cultivation. These results correlated with a dense growth of immobilized mycelium observed with scanning electron microscopy (SEM). The increase of the percentage of organic part in the sol-gel matrix up to 20% led to an increase in the nitrilase activity. The addition of 40% AHPS did not significantly affect the decrease of the nitrile biodegradation. © 2009 Taylor and Francis Group, LLC.Item Structure and properties of innovative silica hybrid materials synthesized for environmental applications(2015-07-21) Todorova E.; Chernev G.; Okolie N.; Salvado I.M.Today, environmental protection is one of the main goals in the strive to preserve the human existence. Development in this area requires invention of new materials, which can reduce the levels of pollution. Hybrid materials are suitable for this purpose, because they combine different desirable properties existing in separate sources into one unique and accessible structure. Most of the commonly used materials for the degradation of different kind of pollutants are based on titanium dioxide, because of its photocatalytic activity under UV irradiation. Innovative silica hybrid materials, containing an organic component (chitosan) and titanium nanoparticles, were successfully synthesized via the sol gel method and tetraethyl orthosilicate was used as a silica source and network former. Interaction between the chitosan and titanium units, and their influence on the structure of final material, were observed and discussed. A homogeneous structure with an even distribution of titanium and chitosan particles was visible from scanning electron microscopy (SEM) micrographs and the particle size varied between 50 and 150 nm. The formed silica network, characteristic peaks of chitosan and titanium groups and possible interactions between them are observed from Fourier transform infrared (FTIR) spectroscopy spectra and nuclear magnetic resonance (NMR) spectroscopy results. The behaviour of the synthesized silica hybrids after thermal treatment was investigated via differential thermal/thermo-gravimetric analysis (DTA/TG) analysis and the sorption and degradation activities of the obtained hybrid materials were investigated using a solution of methyl orange as model pollutant. The structure and properties of the synthesized silica hybrid materials assert their potential application in environmental remediation due to their photocatalytic degradation and sorption activities against pollutants.Item SYNTHESIS AND CHARACTERIZATION OF NANOPARTICLES FROM COAL FLY ASH(2024-01-01) Markov P.; Chernev G.; Nintianova D.; Kazakova N.; Karakostov H.In this study, amorphous nanoparticles were extracted from fly ash using a sol-gel method. The obtained nanoparticles were characterized using XRF spectroscopy, XRD, FT-IR and TEM. The XRD curves show the presence of both crystalline and amorphous phases. FT-IR analysis indicated the presence of silanol and siloxane groups. Upon analysis, the primary nanoparticles were found to exhibit a roughly spherical shape with an average size of approximately 65 nm. The findings of this study demonstrate the feasibility of applying the sol-gel method to synthesize nanoparticles derived from coal fly ash (CFA), thereby avoiding other expensive and energy-intensive methods of nanoparticle synthesis.Item Synthesis and investigation of the properties of hybrid materials for enzyme immobilization(2018-01-01) Yaneva S.; Semerdzhieva V.; Raykova R.; Marinkova D.; Chernev G.; Iliev I.; Yotova L.The chemical nature of carriers for enzyme immobilization plays an important role for retention of the enzyme activity and stability. Two new materials are synthesized for enzyme immobilization based on trimethoxy silane/cellulose acetate butyrate/poly (amido amine) dendrimers (TMOS/CAB/PAMAM) and methyltriethoxy silane/cellulose acetate propionate/ poly (amido amine) dendrimers (MTES/CAP/PAMAM). The synthesis is carried out via the sol-gel method, which allows the preparation of porous glasses through hydrolysis and poly-condensation at a low temperature using high purity initial materials. The PAMAM dendrimers are mono-dispersive, well defined and have a developed three dimensional structure of a high functional groups concentration. The obtained materials are used to investigate the properties of immobilized enzymes such as lipoxygenase and laccase. These enzymes are widely used in industry as bleaching agents. There is also data on application of laccase and lipoxygenase in preparation of biosensors for toxic pollutants determination. Biosensors with immobilized laccase are used to determine phenolic compounds, whereas immobilized lipoxygenase is applied to biosensors formulation for determining Aflatoxin B1 presence.Item THE EFFECT OF CEMENT KILN DUST ON THE PROPERTIES OF CEMENT(2024-01-01) Popov A.; Chernev G.Due to the nature of the technological process, the cement industry is a significant source of dust. Managing industrial waste is a global problem worldwide; cement kiln dust is an example of such waste. The dust from the cement kiln is a by-product of the cement production process obtained during the grinding and burning of the raw materials inside the cement kiln. Still, due to its high alkaline content, it cannot be returned to the kiln, but its disposal and landfill can cause many environmental problems. It is necessary to find alternative methods for its utilization. Due to its fineness and composition like that of cement, there is a growing interest in the use of this powder as a partial substitute for ordinary Portland cement. The purpose of this paper is to investigate the influence of dust obtained during the production of cement clinker on the properties of cement (specific surface area, standard consistency, setting time, and compressive strength) and its implications in the conditions of its application.