Browsing by Author "Mehandjiev D."
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Item Adsorption of Ag+ ions on hydrolyzed lignocellulosic materials based on willow, paulownia, wheat straw and maize stalks(2016-05-01) Vassileva P.; Radoykova T.; Detcheva A.; Avramova I.; Aleksieva K.; Nenkova S.; Valchev I.; Mehandjiev D.In the present work, the adsorption of Ag+ ions on hydrolyzed plant biomass (willow, paulownia, wheat straw and maize stalks) was investigated. Chemical analyses were performed to establish the composition of the obtained materials. Adsorption mechanism, adsorption sites and specific surface areas of these materials were examined by BET analysis, IR spectroscopy, XPS and EPR. The effects of contact time, acidity of initial solutions and Ag+ ion concentrations were followed. Pseudo-first-order, pseudo-second-order and intra-particle diffusion models were used to analyze kinetic data. In all cases, the adsorption was significantly affected by the pH value. Different types of adsorption isotherms of Ag+ ions (either Langmuir or Freundlich) were registered depending on the adsorbing material. The adsorption mechanism is complex, and the process passes through different stages as clustering of Ag+ ions and formation of elemental Ag. The maximal adsorption capacities varied from 2.05 to 6.07 mg g−1. The obtained results revealed that the examined waste lignocellulosic materials are promising adsorbents for Ag+ ions.Item Calcareous clays: Phase composition and adsorption capacity for heavy metal ions(2019-01-01) Mihailova I.; Vassileva P.; Mehandjiev D.The purpose of this study was to investigate the prospective use of calcareous clay in its natural state and after thermal treatment for removal of heavy metals from aquatic environment. The study included 6 natural products from clay deposits in Bulgaria, five of which were calcareous clays and the sixth one was an iron-rich clay. The structural characterization of the samples was carried out using inductively coupled plasma optic emission spectrometry, X-ray powder diffraction analysis, Fourier-transform infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry. The clay minerals contained in the samples were identified as illite, chlorite, kaolinite and smectite. Quartz, calcite and feldspar were identified in the samples too. The thermal treatment has led to changes in the phase composition of clays as a result of partial dehydration and decomposition of minerals. There was an increase in the adsorption capacity of the calcareous clays relative to the metal ions of Cu(II) and Zn(II) after thermal treatment, regardless of the reduction of their specific surface area, which is determined by the increase of the adsorption centers upon thermal treatment.Item Dependence of the C constant in the brunauer-emmett-teller equation on water pre-adsorbed on activated carbon(1992-01-01) Mehandjiev D.; Nickolov R.The dependence of the C constant in the BET equation on the amount of pre-adsorbed water has been investigated by low temperature nitrogen adsorption on activated carbon containing different amounts of water pre-adsorbed at the same relative water vapour pressure. It is suggested that the rapid decrease in the surface area as determined by the BET method in comparison with the C value in cases of low degrees of sorption volume filling is associated with blocking of part of carbon texture and the parallel formation of extremely narrow micropores.Item Effect of the thermal treatment on the Pb (II) adsorption ability of blast furnace slag(2013-05-21) Mihailova I.; Dimitrova S.; Mehandjiev D.The aim of this study is to evaluate the possibilities for improving the adsorption capacity of blast furnace slag in processes for removal of heavy metal ions from aqueous media. Three samples of slag have been prepared by heating of granulated blast furnace slag. The initial slag and thermally treated slag samples have been characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and BET analysis. It has been found that the main crystalline phase of the analyzed slag samples is melilite. Merwinite and pseudowollastonite have been also identified. The Pb(II) adsorption isotherms have been obtained under bath conditions. All samples demonstrate a good Pb(II) removal ability. The equilibrium adsorption data are described by the Langmuir model. The highest Pb(II) adsorption capacity has been achieved by the blast furnace sample treated at 860 °C. The obtained results suggest that it is possible to improve the adsorption properties of blast furnace slag and slag based materials, by control of the phase composition.Item On the possibility of surface modification of an alumina support with amphiphilic carbonaceous material(1998-01-01) Nickolov R.; Mehandjiev D.A carbon-coated support has been prepared by the impregnation of commercial alumina with an ammoniacal solution of amphiphilic carbonaceous material. It was established that the method used for the deposition of the carbon coating on the support allowed control of both the layer thickness and the degree of filling of the surface in the low carbon content region. Equations proposed for the approximate determination of the thickness of the deposited carbon layer and of the surface occupied by this layer enabled a tentative prediction to be made of the most desirable parameters for such a system.Item Phase composition, structure and catalytic activity in oxidation reactions of fayalite waste from the flotation of copper slag(2017-01-01) Mihailova I.; Radev L.; Mehandjiev D.The study examines the phase composition and structure of fayalite waste (FW), as well as the changes that occur during heat treatment applied in the temperature range of 500 - 1000 °C. Differential thermal analysis, X-Ray diffraction analysis, Fourier transform infrared spectroscopy, Optical microscopy, Scanning electron microscopy and Energy dispersive spectroscopy have been used. The main crystalline phase in waste was fayalite, accompanied by magnetite, amorphous silicate phase and a small amount of copper-iron sulfides. Phase-structural changes were found in all heat treated samples. Their extent was temperature dependent. Fayalite transformed to hematite, magnetite and silica. Magnetite transformed to hematite through oxidation. Glass phase underwent oxidation too. In oxidation at 1000°C FW baseline phases were fully replaced by hematite, magnetite, cristobalite and amorphous silicate phase. At this heat treatment temperature the sample exhibited the highest catalytic activity towards the oxidation of CO.Item Porous texture of CuO prepared from copper oxalate precursor(2006-07-01) Nickolov R.; Donkova B.; Milenova K.; Mehandjiev D.The porous texture of CuO obtained from CuC2O4 • 0.5H2O as a precursor, as well as the interconnection between the texture of the initial oxalate and that of the copper oxide obtained, has been studied by physisorption, XRD, XPS and SEM methods. The dimensions of the particle aggregates and the crystalline particles were altered as a consequence of the thermal decomposition of CuC2O4·0.5H2O to CuO, and this led to an increase of ca. 1.5-times in the specific surface of the CuO obtained relative to that of the initial substance. The comparatively non-uniform sizes of the intra-aggregate mesopores in CuC2O4·0.5H2O were transformed into considerably smaller intra-aggregate mesopores in CuO. A finite increase in the volume of the intra-crystallite pores in CuO was also observed, although this did not change the average size of the intra-crystallite pores in the oxide obtained nor the character of the pore-size distribution with respect to the starting material. In addition, as a result of the thermal decomposition of CuC2O4·0.5H2O, the mesoporosity of the prepared oxide also developed. However, the CuO component retained the textural type characteristic of the initial CuC2O4·0.5H2O.Item Slag-based materials as catalysts in oxidation reactions(2014-01-01) Mihajlova I.; Radoykova T.; Ivanov G.; Stoyanova D.; Mehandjiev D.The ability to create new active catalysts for complete oxidation of carbon monoxide and organic compounds on the basis of the metallurgical slag for waste gas purification was proven. The procedure for the synthesis of the catalysts includes only acid and thermal activation of the metallurgical slag, during which in addition to increasing the catalytic activity, the specific surface area is also increased several times. ICP-OES analysis, Differential thermal analysis (DTA), Thermogravimetric analysis (TGA), X-ray diffraction (XRD) measurements and Fourier transform infrared (FTIR) spec-troscopy have been used for the study of the composition and structure of the resulting materials, and the processes of phase formation. The textural properties and the chemical composition of the surface have been investigated by Scanning electron microscopy (SEM), BET analysis and X-ray photoelectron spectroscopy (XPS). The catalysts obtainedfrom metal-lurgical slag can be successfully used for purification of waste gases in industry and transport.Item Visible light photocatalytic activity of TiO2 deposited on activated carbon(2013-03-01) Eliyas A.; Ljutzkanov L.; Stambolova I.; Blaskov V.; Vassilev S.; Razkazova-Velkova E.; Mehandjiev D.Four photocatalyst samples, prepared from beech sawdust, were synthesized by an original method, combining pyrolysis and impregnation - two of them: TiO2 + activated carbon and other two - only activated carbon. The pyrolysis process has been carried out at two different temperatures - 680 C and 830 C. The prepared samples were characterized by a series of methods - XRD, BET, SEM and DTA/TG. The most important result was achieving visible light photocatalytic activity with an azo dye pollutant for both materials. The TiO2/AC-680 C sample demonstrated higher activity under visible light illumination than the TiO2/AC-830 C sample. The visible light activity was attributed to the active carbon component in the composite materials, which was evidenced by the photocatalytic tests with bare carbon (without any TiO2) manifesting visible light activity. The AC-680 C carbon was superior to the AC-830 C under visible illumination probably due to its higher specific surface area and porous texture. UV-light testing of the photocatalytic activity revealed that the TiO2/AC-680 C sample was higher than that of the TiO2/AC-830 C under polychromatic UV-A illumination (320-400 nm with a maximum at l = 365 nm). The TiO 2/AC-680 C sample was also more efficient with the monochromatic UV-C illumination (l = 254 nm). © 2012 Versita Warsaw and Springer-Verlag Wien.Item Waste Copper Slag/Aluminium Dross-Based Geopolymer(2021-01-01) Mihailova I.; Uzunov I.; Mehandjiev D.One of the important issues of the modern world is the ecological problems, which raise the issues of rational and efficient use of natural resources and the reduction of industrial waste. Materials synthesized from industrial waste are an example of producing new useful materials for various applications as well as efficient utilization of waste. The present study aims to synthesize amorphous aluminosilicates from metallurgical waste - waste copper slag, aluminum dross as well as from agricultural waste (rice husks). A synthesis method was applied by which the necessary silica and alumina were extracted from the waste products by chemical treatment in an alkaline environment. Two types of aluminosilicate materials were obtained depending on the experimental procedure. In the first case during the synthesis, there was a stage of polymerization and gel formation and in the second - of co-precipitation. Accordingly, the materials designated as geopolymer and amorphous aluminosilicate were characterized experimentally. They were found to be X-ray amorphous by XRD. FTIR spectroscopy proved that their structure was characterized by a three-dimensional aluminosilicate network with a high degree of connectivity of silicon and aluminum tetrahedra. The microstructure of the materials was established by SEM-EDS and their homogeneity was confirmed. However, due to the difference in synthesis, some differences in the chemical composition and structure of the geopolymer and the amorphous aluminosilicate were experimentally established. Their composition, the structural and textural features of the obtained geopolymer and amorphous aluminosilicate show that such materials can find application as catalyst carriers.