Paunka Vassileva

• PhD
• Professor (Associate) at Bulgarian Academy of Sciences

This study explores the structural and chemical interactions between glacial water, ice Ih, and hydration clusters of divalent cations (Ca²⁺ and Mg²⁺). Ice Ih, with its hexagonal lattice and tetrahedral bonding network, is incompatible with the hydration shells of Ca²⁺ and Mg²⁺, which adopt octahedral geometries in aqueous solutions. During freezin...

Featured Application This study highlights the potential of biochars for industrial and domestic wastewater treatment by leveraging their adsorption properties. Converting renewable agricultural waste into value-added products enhances environmental sustainability. Our findings demonstrate the dual functionality of activated rice husk biochar, comb...

Graphene materials and their derivatives have shown promising capabilities in removing anionic and cationic dyes from wastewater. The present study aims at the synthesis of graphene-based material with a high specific surface area and evaluates its use as an adsorbent for removing toluidine blue and methyl violet from aqueous solutions. The physico...

The present study deals with the removal efficiency regarding Reactive Blue 13 (RB) of a cationized cellulosic adsorbent prepared from agricultural waste. The rice husks were used as a raw material to extract and separate the cellulose by alkali and bleaching treatment. The cellulosic material was modified with N, N-dimethyl-1-octadecylamine to pre...

Various methods are applied to improve the quality of cement and concrete. Many efforts are made to enhance their quality by introducing additional compounds that can stabilize the chemical bonds between their components. Significant effects on hydrogen bonds under the influence of magnetic and electromagnetic waves have been observed. Results have...

Biosorption is process for the removal of toxic metals using a passive binding process with nonliving organisms. Medicinal plants are of great interest as biosorbents, because they contain numerous active phytochemicals which are prone to metal binding. In the present study the feasibility of a material based on the widely used herb yarrow (Achille...

In the present work an attempt is made to elucidate the mechanism of Cu(II) adsorption on a plant material based on Anethum graveolens L. Batch experiments were carried out in order to study the effects of acidity, contact time and initial metal concentration on Cu(II) removal. Langmuir, Freundlich and Dubinin-Radushkevich isotherm models are used...

The survey aimed to investigate the adsorption properties of cellulose-based materials derived from cereal by-products towards Ag+ ions in water solutions, and to shed light on the mechanism of adsorption. Cellulose was isolated from rice and einkorn husks using alkali and bleaching treatments. Untreated einkorn husks and commercial cellulose serve...

There are partial or total solar eclipses every year on our planet. They are observed from relatively small areas. From 1950 to 2100, three total solar eclipses fell within the territory of Bulgaria. The two solar eclipses from the 20th century were observed on 15.02.1961 and 11.08.1999. The next total solar eclipse will happen on 3.09.2081. The pa...

The present study deals with the application of an experimental design approach in order to clarify the optimal experimental conditions for biosorption output signals (A% and Qe) depending on three input factors (pH, mass and concentration C0). The major goal is to create adequate regression models able to correctly describe the area determined by...

In the current investigation, the removal efficiency regarding a cationic dye, methylene blue (MB), from three graphene-based materials was investigated. The materials’ characterization process involved instrumental methods such as XRD, XPS, SEM, TEM, FTIR, and nitrogen adsorption at 77 K. The survey examined how various process factors influenced...

In the present work an attempt is made to elucidate the mechanism of Cu(II) adsorption on a plant material based on lemon balm. Batch experiments were carried out in order to study the effects of acidity, contact time and initial metal concentration on Cu(II) removal. Langmuir, Freundlich and Dubinin-Radushkevich isotherm models were used to explai...

An investigation of simultaneous osmosis and diffusion was conducted in a setup consisting of a cylindrical ceramic element initially filled with 0.9 % sodium chloride (NaCl) aqueous solution and immersed a beaker initially filled with deionized water. The cylindrical element was made of kaolin with chemical composition Al 2 O 3 (52 %) SiO 2 (47 %)...

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 wa...

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-1-octadecylamine to pre...

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 inductiv...

The efficiency of a material based on the plant Thymus vulgaris L. for Cu(II) removal from aqueous solutions is investigated. The estimation of optimal parameters influencing the Cu(II) adsorption was studied by the batch method. Optimum pH value was found to be about 4. Kinetic of Cu(II) adsorption was very fast. Equilibrium experimental data were...

In the present work an attempt is made to elucidate the mechanism of Cu(II) adsorption on a plant material based on Anethum graveolens L. Batch experiments were carried out in order to study the effects of acidity, contact time and initial metal concentration on Cu(II) removal. Langmuir, Freundlich and Dubinin-Radushkevich isotherm models are used...

The publication examines the properties of the electrochemically activated waters catholyte and anolyte with Electrolyzer device. It is accepted that the acid-alkaline nature of water in the human body has a pH of 7.0. When the pH is above 7.0, the water is alkaline. When the pH is less than 7.0, the water is acidic. The water Catholyte has using w...

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 Cu2+ ions from aqueous solutions. XRD, TGA, FTIR, SEM and low-temperature nitrogen adsorption were used for the analytical characterization of the material. Particle s...

The present study aims at removal of hexavalent chromium from aqueous solution using manganese ferrite (MnFe 2 O 4) nanoparticles. The investigated material was obtained by solution combustion method using starting reagents Mn(NO 3) 2 .4H 2 O and Fe(NO 3) 3 .9H 2 O (as oxidizers) and citric acid (as a re-ducer). Nanoparticles of MnFe 2 O 4 were cha...

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...

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 a...

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 c...

The present study investigated the potential use of Triticum monococcum L. (einkorn wheat) husks for the removal of methylene blue (MB) from aqueous solutions. The studied materials were obtained by pyrolysis and incineration of Triticum monococcum L. husks. The porous structure of both biomaterials was investigated by low-temperature nitrogen adso...

The feasibility of using Mentha spicata L. (MS) and Ruta graveolens L. (RG) for the removal of Cd²⁺ions from aqueous solution was investigated. Batch experiments were performed to evaluate the effect of contact time, acidity and initial metal concentration on Cd²⁺removal from aqueous solutions. Equilibrium experimental data were fitted to linear La...

In the present study raw coal bottom ashes from four Bulgarian coal-fired power plants – “Bobov dol”, “Maritsa Iztok-3”, “Republika” and “Sliven” were investigated by a combination of different analytical methods. They were characterized in terms of mineralogical and chemical composition, grain size distribution, conductivity, moisture, slurry pH,...

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, I...

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)...

Several characteristics of natural diatomites from Dragovishtitsa and Ig- natievo deposits (Bulgaria) such as chemical composition, surface area, pore volumes, XRD and IR spectra were studied. The sorption properties of the di- atomites towards Fe3+, Pb2+, Cu2+, Cd2+, Tl+, Mn2+, Ni2+ and Co2+ were in- vestigated and the sorption capacities towards...

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 TM X-114. The experimental parameters, such as grain size, concentrations of fly ash slurry and excitation time were optimised. For valid...

The effect of autoclaving of the granulated blast furnace slag on its catalytic activity with regard to the oxidation of CO was investigated. The results show that autoclaving after chemical treatment of slag with nitric acid or nitric acid and ammonia, leads to the increase in the specific surface area, but reduces the concentration of catalytical...

In the present study the adsorption properties of coal fly ashes from Bulgarian power plants were investigated. Batch experiments were performed to evaluate the effect of contact time, acidity, initial metal concentration on Cu(II) removal from aqueous solutions. Equilibrium experimental data were fitted to linear Langmuir and Freundlich models. It...

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. Equil...

In the present study the adsorption properties of coal fly ashes from Bul- garian power plants were investigated. Batch experiments were performed to evaluate the effect of contact time, acidity, initial metal concentration on Cu(II) removal from aqueous solutions. Equilibrium experimental data were fitted to linear Langmuir and Freundlich models....

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. Equil...

Four carbon/silica-containing materials obtained by pyrolysis of rice husks were characterized and their adsorption properties towards some metal ions were evaluated. GC/MS analysis and FT-IR spectroscopy were applied for identification of functional groups finely dispersed on the surface. Their amounts were gravimetrically determined after extract...

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 mater...

The influence of the crystallization and the mineral composition on the adsorption properties of slag-based glass ceramics has been studied with respect to lead ions. Four slag-ceramic samples have been prepared by sintercrystallization process of pressed glass powders. All samples have been characterized by XRD, FTIR and SEM analyses and lead batc...

The adsorption properties of two new nanostructured hybrid materials containing B2O3 and ZrO2 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(OCH3)3 was added. The structure of the hybrid materials was investigated by means of (Fourier transform infra...

The main purpose of the present work was to study the adsorption of Mn(II), Fe(III) and Cr(III) from aqueous solutions onto natural and pretreated Bulgarian clinoptilolite from Beli plast deposit. Batch adsorption studies were carried out to evaluate the effect of contact time, temperature, solution pH and initial concentration of investigated ions...

Natural Bulgarian diatomite modified by oxidation with sulfuric acid and H2O2 or by coating with manganese oxide was characterized considering its chemical composition, surface area, pore volume, and structure. Modified diatomites displayed larger surface area and pore volumes in comparison with untreated natural diatomite, which favored their sorp...

The possibilities for removal of toxic ions from aqueous solutions by novel porous material obtained from Bulgarian lignite and its oxidized modifications were studied. Adsorption of Hg(II), Cd(II), Pb(II), As(III), and As(V) ions was investigated using aqueous solutions with different concentrations and acidity. Equilibrium modeling data were fitt...

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,...

Several characteristics of natural diatomites from Dragovishtitsa and Ig-natievo deposits (Bulgaria) such as chemical composition, surface area, pore volumes, XRD and IR spectra were studied. The sorption properties of the di-atomites towards Fe 3+ , Pb 2+ , Cu 2+ , Cd 2+ , Tl + , Mn 2+ , Ni 2+ and Co 2+ were investigated and the sorption capacitie...

Several characteristics of natural diatomites from Dragovishtitsa and Ig-natievo deposits (Bulgaria) such as chemical composition, surface area, pore volumes, XRD and IR spectra were studied. The sorption properties of the di-atomites towards Fe 3+ , Pb 2+ , Cu 2+ , Cd 2+ , Tl + , Mn 2+ , Ni 2+ and Co 2+ were investigated and the sorption capacitie...

In the present work the adsorption of lead from aqueous solutions on a novel nanostructured di-urethanesil hybrid material modified by aluminium was investigated. The adsorption properties of this material with respect to Pb(II) ions from single-component aqueous solutions and multi-component aqueous solutions containing also Cu(II), Cr(III), Cd(II...

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...

A two sorbents was to synthesize for heavy metals removal: silica gel modified with 5-amino-1,3,4-thiadiazole-2-thiol (S5A) and pure silica gel. Modification of silica gel is achieved by tetraethyl orthosilicate (TEOS) preliminary hydrolyzed at room temperature. 5-Amino-1,3,4-thiadiazole-2-thiol is added, with continuous stirring with a magnetic st...

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...

The possibilities for ammonium ions removal from aqueous solutions by natural and NaCl pretreated Bulgarian clinoptilolite from Beli plast deposit were studied. Experiments were carried out using batch method. The adsorption of NH4(+) was investigated as a function of the solution pH, dosage of adsorbent, initial ammonium concentration and temperat...

The main purpose of this work is to study the possibilities for removal of ammonium ions from aqueous solutions by the two coal-based activated carbons (one obtained from Bulgarian lignite from the Chukurovo deposit, and the second, available as commercial product) and their oxidized modifications. The porous texture and surface chemistry of the ad...

An activated carbon sorbent containing thioetheric sites (ACTS) was prepared by modification of the activated carbon with 2,2′-thiodiethanol. The specific surface area, pore volume, concentration of oxygen-containing groups and sulfur content of the sorbent were determined. The sorption behavior towards ions of some precious metals—Au(III), Pt(IV),...

A new sorbent is prepared by modifying activated carbon obtained from apricot stones with 2-hydroxy-2-mercaptopyrimidine (2-thiouracil). The products have been characterized as to their surface area, pore volumes, content of the functional groups with basic and acidic properties, IR spectra, sulfur content. Their sorption properties with respect to...

The sorption behavior of a newly synthesized silica gel sorbent with thioetheric sites (STS) towards microgram levels of Au(III), Pt(IV) and Pd(II) was studied. Au(III) is quantitatively (>95%) sorbed in the pH region of 1–9. The sorption of Pt(IV) starts at pH 1 and does not exceed 25% in the entire pH region examined. The sorption of Pd(II) start...

The binding energies of nitrogen and phosphorus in some cyclic phosphazene molecules in the cases of identical or different substituents, and in linear polymers having electron-donating or electron-accepting ligands were analyzed using x ray photoelectron spectroscopy (XPS). All XPS spectra were obtained using a Perkin Elmer Physical Electronics Mo...

The synthesis of adsorbents based on tetraethyl orthosilicate (TEOS) and 2,2′-thiodiethanol (TDE) was discussed. The sorption properties of the adsorbents with respect to cadmium , lead, and mercury were investigated using nuclear magnetic resonance spectroscopy. A change in favor of TDE reduced the porosity and increased the elasticity of the prod...

Activated carbons were prepared from apricot and cherry stones and modified with 3-methyl-1-phenyl-pyrazolone-5 (HMPP). Synthesis of the sorbents occurred in three consecutive steps: pyrolysis of the raw material in a stream of water vapor; preparation of cyanuric chloride derivatives of the activated carbons thus obtained; and a treatment anchorin...

The sorption capacity of 3-methyl-1-phenyl-pyrazolone-5 (HMPP)- modified inorganic sorbents has been investigated. The new sorbents are suitable for simultaneous concentration of the 3d elements being investigated over a wide pH range. Choosing appropriate pH values for the corresponding elements, it is possible to achieve their separate extraction...

New inorganic sorbents are obtained by modifying four different kieselguhrs and silica gel with 3-methyl-1-phenyl-pyrazolone-5 (HMPP). The products are characterized by elemental analysis and IR spectroscopy. The complexation ability of the sorbents with respect to some 3d elements such as Cu(II), Mn(II), Cd(II) and Pb(II) is investigated. The new...

Hexachlorocyclotriphosphazene (T) and poly-bis-(trifluoroethoxy)phosphazene (A) deposited separately on wide-pore kieselguhr supports result in composite adsorbents of an increased adsorption capacity for water vapour per unit surface area. Deposition alters the adsorption behaviour of T and A themselves due to high dispersion and conformation chan...

We report the adsorption isotherms of water vapour at 22°C on six representatives of the phosphazene family: hexachlorocyclotriphosphazene (I), bis-(dimethylamino)-tetrachlorocyclotriphosphazene (II), bis-(dimethylamino)-bis-(trifluoroethoxy)-dichlorocyclotriphosphazene (III), octabutylaminocyclotetraphosphazene (IV), phospham (V) and poly-bis-(tri...