Paunka Vassileva - Academia.edu (original) (raw)
Papers by Paunka Vassileva
Open Chemistry, Oct 1, 2012
Separation Science and Technology, Mar 2, 2016
ABSTRACT In the present study agricultural waste (rice husks) was used for the production of biog... more ABSTRACT In the present study agricultural waste (rice husks) was used for the production of biogenic silica. The resulting material was obtained by incineration of rice husks at a temperature of 800°С and was characterized by means of XRD, FTIR, SEM-EDS, Hg-porosimetry and N2-adsorption. The adsorption ability of the rice husks ash towards Fe(III), Pb(II), Cr(III) and Cu(II) ions in single- and multi-component aqueous solutions was studied in a batch system. The effects of contact time, acidity of initial solutions and metal ion concentrations were followed. Different models were used to determine the adsorption mechanism. Equilibrium experimental data were fitted to linear Langmuir and Freundlich models. The maximum adsorption capacities for single- and multi-component adsorption were calculated.
Journal of Porous Materials, Jul 7, 2009
... Chem. 355, 938 (1989). doi:10.1007/BF00466386 6. A. Howard, M. Volkan, O.Ataman, Analyst (Lon... more ... Chem. 355, 938 (1989). doi:10.1007/BF00466386 6. A. Howard, M. Volkan, O.Ataman, Analyst (Lond) 112, 159 (1987). doi:10.1039/an9871200159 7. M. Volkan, O. Ataman, A. Howard, Analyst (Lond) 112, 1409 (1987). doi ...
Cellulose Chemistry and Technology, Nov 29, 2022
The present study deals with the removal efficiency towards Congo red dye of two cationized cellu... more The present study deals with the removal efficiency towards Congo red dye of two cationized cellulose adsorbents prepared from cereal by-products. Rice and einkorn husks were used as raw materials to extract and separate the cellulose by alkali and bleaching treatments. The cellulose materials were modified with N,N-dimethyl-1octadecylamine to prepare cationized adsorbents. Instrumental methods, such as XRD, DTA, FTIR and SEM, as well as low-temperature nitrogen adsorption, were used for their characterization. The results showed that the quaternary ammonium group was successfully grafted onto the cellulose structures. The survey mainly focused on the effect of process parameters on the adsorption capacity of the investigated materials, including contact time, initial Congo red concentrations, solution pH and temperature. The adsorption process was well described by the pseudo-second-order and Langmuir models. The values of entropy, enthalpy and Gibbs free energy for the Congo red removal were determined.
Fresenius Journal of Analytical Chemistry, 1993
Silicon oxycarbide glasses have been of interest because of the potential range of properties the... more Silicon oxycarbide glasses have been of interest because of the potential range of properties they might exhibit through a change in carbon-to-oxygen ratio. They are metastable materials and, as such, their structures and properties are very dependent upon the synthesis method. Silicon oxycarbide bonding has been seen in materials made by melting, oxidation, polycarbosilane or sol/gel pyrolysis, and chemical vapor deposition. In this work, the radio-frequency reactive sputtering of silicon carbide targets was explored for synthesis of amorphous silicon oxycarbide thin films. SiO ͑2−2x͒ C x films, with a continuous range of compositions where 0 ഛ x ഛ 1, were deposited by controlling the amount of oxygen present in the plasma with a SiC target. This resulted in a density range from 1.9 to 2.8 g / cm 3 and a range of refractive indexes from 1.35 to 2.85. Analysis of the film compositions, structures, and properties were performed using x-ray photoelectron spectroscopy, infrared spectroscopy, nuclear magnetic resonance, profilometry, electron microscopy, grazing incidence x-ray reflectivity, and UV-visible transmission and reflection. The compositional range obtainable by this rf sputtering method is much wider than that of other synthesis methods. It is shown here that for oxygen-to-carbon ratios between ϳ0.10 and 10.0, silicon oxycarbide bonding comprises 55%-95% of the material structure. These sputter-deposited materials were also found to have significantly less free carbon as compared to those produced by other methods. Thus, the unique properties for these novel oxycarbide materials can now be established.
Chemical Papers, 2015
The contents of Cl, Ca, K, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Rb, Sr, Ba and Pb in raw coal f... more The contents of Cl, Ca, K, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Rb, Sr, Ba and Pb in raw coal fly ash from five Bulgarian power plants were determined by total reflection X-ray fluorescence (TXRF), using gallium as the internal standard. The samples were analysed as in slurry form in Triton
Plant Science Today
The effectiveness of the application of EVOagri technology for purification of water and its use ... more The effectiveness of the application of EVOagri technology for purification of water and its use in agriculture in some African countries (Zimbabwe, Burundi, Egypt and Uganda), Nepal (Asia) and in Europe (Italy and Bulgaria) was evaluated. The performance of EVOdrop turbine for water filtration was analysed and the physicochemical composition of water was tested. This was determined in compliance with Ordinance No. 9/2001, published in the Official State Gazette, issue 30, and decree No. 178/23.07.2004, Bulgaria, European Union on the quality of water intended for drinking and household purposes. The results showed a significant increase in production yield of the tested Solanum spp. and lettuces in Zimbabwe, as well as of dill, spinach and onions in Egypt. Treatment of high-salt irrigation water in Burundi with EVOagri reduced its toxic effect upon vegetable plants. In Uganda, plants absorbed 23% less water, when filtered with EVOagri technology. In Italy, 21% water conservation wa...
Cellulose Chemistry and Technology, 2019
The antibacterial activity of samples prepared from waste lignocellulosic materials, namely wheat... more The antibacterial activity of samples prepared from waste lignocellulosic materials, namely wheat straw and maize stalks, doped with silver on their surface, was investigated. In order to use the various functional groups of these materials, Ag + was applied to the carrier by the adsorption method. EPR and SEM investigations demonstrated that Ag + ions were agglomerated into clusters of elemental silver on the carrier. The samples thus produced exhibited high antibacterial activity against gram-positive Bacillus subtilis strain 3563 and gram-negative Escherichia coli K12.
Analytical Letters, 2019
Two biomaterials based on the abundant Bulgarian medicinal plants Mentha spicata L. (denoted as M... more Two biomaterials based on the abundant Bulgarian medicinal plants Mentha spicata L. (denoted as MS) and Ruta graveolens L. (denoted as RG) were investigated as environmentally friendly biosorbents for Cu(II) removal from aqueous solutions. Grain size distribution, slurry pH, texture parameters, thermal behavior and mineralogical composition of MS and RG were determined. Instrumental methods such as scanning electron microscopy (SEM), X-ray-diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET) analysis, differential thermal analysis (DTA), and thermogravimetric analysis (TGA) were used for their characterization. It was demonstrated that the surface morphology of both materials is rough and contains pores that could entrap different pollutants, as well as functional groups that can attach metal ions. In order to examine the feasibility of these materials for use as biosorbents for heavy metals, adsorption experiments were performed. The results for the removal of Cu(II) ions from aqueous solution reveal capabilities suggesting that both materials have potential to be used as promising, efficient and low-cost biosorbents.
The main purpose of the present work was to study the adsorption of some transition metal ions fr... more The main purpose of the present work was to study the adsorption of some transition metal ions from aqueous solution via a novel porous material obtained from Bulgarian lignite (Chukurovo deposit) and its oxidized modifications. The adsorption of Cu(II), Fe(III), Cr(III) and Au(III) ions was investigated using batch methods to study solutions with different concentrations and acidities. It was found that the adsorption process was affected significantly by the pH value of the aqueous solution. Treatment of the equilibrium data using the linear Langmuir, Freundlich and Dubinin–Radushkevich models allowed the maximum adsorption capacities to be calculated. The uptake of Au(III) ions was almost 100% for the three adsorbents investigated, being greater than 300 mg/ and independent of the pH over the pH range studied. The initial activated carbon proved to be the most suitable for the selective adsorption of Au(III) ions from aqueous solutions in the presence of other transition metal io...
Comptes rendus de l'Académie bulgare des sciences: sciences mathématiques et naturelles
Raw coal fly ashes from five Bulgarian power plants were used in the present study. They were cha... more Raw coal fly ashes from five Bulgarian power plants were used in the present study. They were characterized in terms of mineralogical and chemical composition, grain size distribution, specific surface area and pore size, conductivity, moisture, slurry pH, ignition losses and adsorption properties by the combined use of analytical techniques. Equilibrium experimental data for Pb(II) adsorption onto the fly ash samples were fitted to linear Langmuir and Freundlich models. All investigated materials could be successfully applied for effective removal of lead ions from aqueous solutions
Journal of Porous Materials, 2009
Cellulose Chemistry and Technology, Nov 11, 2020
The objective of the present study is the characterization of an adsorbent plant material based o... more The objective of the present study is the characterization of an adsorbent plant material based on Hypericum perforatum L. and the investigation of its feasibility for the removal of Cu 2+ ions from aqueous solutions. XRD, TGA, FTIR, SEM and low-temperature nitrogen adsorption were used for the analytical characterization of the material. Particle size distribution and slurry pH of the investigated material were also determined. The effect of contact time, solution acidity and initial metal concentration on the adsorption of Cu(II) ions was studied by means of the batch method. Desorption studies have also been performed. Equilibrium experimental data were fitted to the linear Langmuir, Freundlich and Dubinin-Radushkevich isotherm models. The maximum adsorption capacity was calculated and it was concluded that the material could be used as an effective biosorbent for the removal of copper ions from aqueous media.
International Journal of Environmental Science and Technology, 2016
In the present work, the adsorption of Ag ? ions on hydrolyzed plant biomass (willow, paulownia, ... more 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-firstorder, 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.
Proceedings of the Bulgarian Academy of Sciences
The aim of the present study is the effective application of chemometric approach to the elementa... more The aim of the present study is the effective application of chemometric approach to the elemental composition of a data set of ten ethereal oil plant materials. The contents of 28 essential and toxic mineral elements at main, minor and trace level are determined in each sample by means of direct solid sampling electrothermal vapourization inductively coupled plasma optical emission spectrometry (ETV-ICP-OES) and are subjected to hierarchical and non-hierarchical cluster analysis and principal component analysis. Z-transformation of the raw data, Ward’s method of linkage and squared Euclidean distance as similarity measure are used in order to discover appropriate linkage between the plant materials and between the chemical parameters characterizing them. Thus, the investigated plant samples are classified, depending on their chemical composition. On the other hand, the mineral elements are categorized into several patterns which are related to the specific soil characteristics.
Materials Today: Proceedings, 2022
International Research Journal of Pure and Applied Chemistry, 2020
Chemical processes that could occur in the anode chamber of an electrolytic cell divided by a sem... more Chemical processes that could occur in the anode chamber of an electrolytic cell divided by a semi-permeable membrane when direct current passes through a water solution of weak mineralization are described. The interest concerns the possibility of a formation of chemical substances with strong oxidative action as O2 and H2O2, due to nascent oxygen. These substances can destroy the structure of pathogenic bacteria and viruses. The preservation of these properties for a long period of time without specific storage requirements makes the anolyte cheap and effective disinfection and therapeutic means.
Open Chemistry, Oct 1, 2012
Separation Science and Technology, Mar 2, 2016
ABSTRACT In the present study agricultural waste (rice husks) was used for the production of biog... more ABSTRACT In the present study agricultural waste (rice husks) was used for the production of biogenic silica. The resulting material was obtained by incineration of rice husks at a temperature of 800°С and was characterized by means of XRD, FTIR, SEM-EDS, Hg-porosimetry and N2-adsorption. The adsorption ability of the rice husks ash towards Fe(III), Pb(II), Cr(III) and Cu(II) ions in single- and multi-component aqueous solutions was studied in a batch system. The effects of contact time, acidity of initial solutions and metal ion concentrations were followed. Different models were used to determine the adsorption mechanism. Equilibrium experimental data were fitted to linear Langmuir and Freundlich models. The maximum adsorption capacities for single- and multi-component adsorption were calculated.
Journal of Porous Materials, Jul 7, 2009
... Chem. 355, 938 (1989). doi:10.1007/BF00466386 6. A. Howard, M. Volkan, O.Ataman, Analyst (Lon... more ... Chem. 355, 938 (1989). doi:10.1007/BF00466386 6. A. Howard, M. Volkan, O.Ataman, Analyst (Lond) 112, 159 (1987). doi:10.1039/an9871200159 7. M. Volkan, O. Ataman, A. Howard, Analyst (Lond) 112, 1409 (1987). doi ...
Cellulose Chemistry and Technology, Nov 29, 2022
The present study deals with the removal efficiency towards Congo red dye of two cationized cellu... more The present study deals with the removal efficiency towards Congo red dye of two cationized cellulose adsorbents prepared from cereal by-products. Rice and einkorn husks were used as raw materials to extract and separate the cellulose by alkali and bleaching treatments. The cellulose materials were modified with N,N-dimethyl-1octadecylamine to prepare cationized adsorbents. Instrumental methods, such as XRD, DTA, FTIR and SEM, as well as low-temperature nitrogen adsorption, were used for their characterization. The results showed that the quaternary ammonium group was successfully grafted onto the cellulose structures. The survey mainly focused on the effect of process parameters on the adsorption capacity of the investigated materials, including contact time, initial Congo red concentrations, solution pH and temperature. The adsorption process was well described by the pseudo-second-order and Langmuir models. The values of entropy, enthalpy and Gibbs free energy for the Congo red removal were determined.
Fresenius Journal of Analytical Chemistry, 1993
Silicon oxycarbide glasses have been of interest because of the potential range of properties the... more Silicon oxycarbide glasses have been of interest because of the potential range of properties they might exhibit through a change in carbon-to-oxygen ratio. They are metastable materials and, as such, their structures and properties are very dependent upon the synthesis method. Silicon oxycarbide bonding has been seen in materials made by melting, oxidation, polycarbosilane or sol/gel pyrolysis, and chemical vapor deposition. In this work, the radio-frequency reactive sputtering of silicon carbide targets was explored for synthesis of amorphous silicon oxycarbide thin films. SiO ͑2−2x͒ C x films, with a continuous range of compositions where 0 ഛ x ഛ 1, were deposited by controlling the amount of oxygen present in the plasma with a SiC target. This resulted in a density range from 1.9 to 2.8 g / cm 3 and a range of refractive indexes from 1.35 to 2.85. Analysis of the film compositions, structures, and properties were performed using x-ray photoelectron spectroscopy, infrared spectroscopy, nuclear magnetic resonance, profilometry, electron microscopy, grazing incidence x-ray reflectivity, and UV-visible transmission and reflection. The compositional range obtainable by this rf sputtering method is much wider than that of other synthesis methods. It is shown here that for oxygen-to-carbon ratios between ϳ0.10 and 10.0, silicon oxycarbide bonding comprises 55%-95% of the material structure. These sputter-deposited materials were also found to have significantly less free carbon as compared to those produced by other methods. Thus, the unique properties for these novel oxycarbide materials can now be established.
Chemical Papers, 2015
The contents of Cl, Ca, K, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Rb, Sr, Ba and Pb in raw coal f... more The contents of Cl, Ca, K, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Rb, Sr, Ba and Pb in raw coal fly ash from five Bulgarian power plants were determined by total reflection X-ray fluorescence (TXRF), using gallium as the internal standard. The samples were analysed as in slurry form in Triton
Plant Science Today
The effectiveness of the application of EVOagri technology for purification of water and its use ... more The effectiveness of the application of EVOagri technology for purification of water and its use in agriculture in some African countries (Zimbabwe, Burundi, Egypt and Uganda), Nepal (Asia) and in Europe (Italy and Bulgaria) was evaluated. The performance of EVOdrop turbine for water filtration was analysed and the physicochemical composition of water was tested. This was determined in compliance with Ordinance No. 9/2001, published in the Official State Gazette, issue 30, and decree No. 178/23.07.2004, Bulgaria, European Union on the quality of water intended for drinking and household purposes. The results showed a significant increase in production yield of the tested Solanum spp. and lettuces in Zimbabwe, as well as of dill, spinach and onions in Egypt. Treatment of high-salt irrigation water in Burundi with EVOagri reduced its toxic effect upon vegetable plants. In Uganda, plants absorbed 23% less water, when filtered with EVOagri technology. In Italy, 21% water conservation wa...
Cellulose Chemistry and Technology, 2019
The antibacterial activity of samples prepared from waste lignocellulosic materials, namely wheat... more The antibacterial activity of samples prepared from waste lignocellulosic materials, namely wheat straw and maize stalks, doped with silver on their surface, was investigated. In order to use the various functional groups of these materials, Ag + was applied to the carrier by the adsorption method. EPR and SEM investigations demonstrated that Ag + ions were agglomerated into clusters of elemental silver on the carrier. The samples thus produced exhibited high antibacterial activity against gram-positive Bacillus subtilis strain 3563 and gram-negative Escherichia coli K12.
Analytical Letters, 2019
Two biomaterials based on the abundant Bulgarian medicinal plants Mentha spicata L. (denoted as M... more Two biomaterials based on the abundant Bulgarian medicinal plants Mentha spicata L. (denoted as MS) and Ruta graveolens L. (denoted as RG) were investigated as environmentally friendly biosorbents for Cu(II) removal from aqueous solutions. Grain size distribution, slurry pH, texture parameters, thermal behavior and mineralogical composition of MS and RG were determined. Instrumental methods such as scanning electron microscopy (SEM), X-ray-diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET) analysis, differential thermal analysis (DTA), and thermogravimetric analysis (TGA) were used for their characterization. It was demonstrated that the surface morphology of both materials is rough and contains pores that could entrap different pollutants, as well as functional groups that can attach metal ions. In order to examine the feasibility of these materials for use as biosorbents for heavy metals, adsorption experiments were performed. The results for the removal of Cu(II) ions from aqueous solution reveal capabilities suggesting that both materials have potential to be used as promising, efficient and low-cost biosorbents.
The main purpose of the present work was to study the adsorption of some transition metal ions fr... more The main purpose of the present work was to study the adsorption of some transition metal ions from aqueous solution via a novel porous material obtained from Bulgarian lignite (Chukurovo deposit) and its oxidized modifications. The adsorption of Cu(II), Fe(III), Cr(III) and Au(III) ions was investigated using batch methods to study solutions with different concentrations and acidities. It was found that the adsorption process was affected significantly by the pH value of the aqueous solution. Treatment of the equilibrium data using the linear Langmuir, Freundlich and Dubinin–Radushkevich models allowed the maximum adsorption capacities to be calculated. The uptake of Au(III) ions was almost 100% for the three adsorbents investigated, being greater than 300 mg/ and independent of the pH over the pH range studied. The initial activated carbon proved to be the most suitable for the selective adsorption of Au(III) ions from aqueous solutions in the presence of other transition metal io...
Comptes rendus de l'Académie bulgare des sciences: sciences mathématiques et naturelles
Raw coal fly ashes from five Bulgarian power plants were used in the present study. They were cha... more Raw coal fly ashes from five Bulgarian power plants were used in the present study. They were characterized in terms of mineralogical and chemical composition, grain size distribution, specific surface area and pore size, conductivity, moisture, slurry pH, ignition losses and adsorption properties by the combined use of analytical techniques. Equilibrium experimental data for Pb(II) adsorption onto the fly ash samples were fitted to linear Langmuir and Freundlich models. All investigated materials could be successfully applied for effective removal of lead ions from aqueous solutions
Journal of Porous Materials, 2009
Cellulose Chemistry and Technology, Nov 11, 2020
The objective of the present study is the characterization of an adsorbent plant material based o... more The objective of the present study is the characterization of an adsorbent plant material based on Hypericum perforatum L. and the investigation of its feasibility for the removal of Cu 2+ ions from aqueous solutions. XRD, TGA, FTIR, SEM and low-temperature nitrogen adsorption were used for the analytical characterization of the material. Particle size distribution and slurry pH of the investigated material were also determined. The effect of contact time, solution acidity and initial metal concentration on the adsorption of Cu(II) ions was studied by means of the batch method. Desorption studies have also been performed. Equilibrium experimental data were fitted to the linear Langmuir, Freundlich and Dubinin-Radushkevich isotherm models. The maximum adsorption capacity was calculated and it was concluded that the material could be used as an effective biosorbent for the removal of copper ions from aqueous media.
International Journal of Environmental Science and Technology, 2016
In the present work, the adsorption of Ag ? ions on hydrolyzed plant biomass (willow, paulownia, ... more 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-firstorder, 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.
Proceedings of the Bulgarian Academy of Sciences
The aim of the present study is the effective application of chemometric approach to the elementa... more The aim of the present study is the effective application of chemometric approach to the elemental composition of a data set of ten ethereal oil plant materials. The contents of 28 essential and toxic mineral elements at main, minor and trace level are determined in each sample by means of direct solid sampling electrothermal vapourization inductively coupled plasma optical emission spectrometry (ETV-ICP-OES) and are subjected to hierarchical and non-hierarchical cluster analysis and principal component analysis. Z-transformation of the raw data, Ward’s method of linkage and squared Euclidean distance as similarity measure are used in order to discover appropriate linkage between the plant materials and between the chemical parameters characterizing them. Thus, the investigated plant samples are classified, depending on their chemical composition. On the other hand, the mineral elements are categorized into several patterns which are related to the specific soil characteristics.
Materials Today: Proceedings, 2022
International Research Journal of Pure and Applied Chemistry, 2020
Chemical processes that could occur in the anode chamber of an electrolytic cell divided by a sem... more Chemical processes that could occur in the anode chamber of an electrolytic cell divided by a semi-permeable membrane when direct current passes through a water solution of weak mineralization are described. The interest concerns the possibility of a formation of chemical substances with strong oxidative action as O2 and H2O2, due to nascent oxygen. These substances can destroy the structure of pathogenic bacteria and viruses. The preservation of these properties for a long period of time without specific storage requirements makes the anolyte cheap and effective disinfection and therapeutic means.