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194 results on '"Gochev, Velizar"'

7. Biosorption of Congo Red and Methylene Blue by pretreated waste Streptomyces fradiae biomass - Equilibrium, kinetic and thermodynamic studies

Author

Velkova Zdravka Y., Kirova Gergana K., Stoytcheva Margarita S., and Gochev Velizar K.

Subjects
waste biomass, water treatment, dyes, biosorption, Chemistry, QD1-999
Abstract

Pretreated waste Streptomyces fradiae biomass was utilized as an eco-friendly sorbent for Congo Red (CR) and Methylene Blue (MB) removal from aqueous solutions. The biosorbent was characterized by Fourier transform infrared spectroscopy. Batch experiments were conducted to study the effect of pH, biosorbent dosage, initial concentration of adsorbates, contact time and temperature on the biosorption of the two dyes. The equilibrium adsorption data were analysed using Freundlich and Langmuir models. Both models fitted well the experimental data. The maximum biosorption capacity of the pretreated Streptomyces fradiae biomass was 46.64 mg g-1 for CR and 59.63 mg g-1 for MB, at a pH 6.0, with the contact time of 120 min, the biosorbent dosage of 2 g dm-3 and the temperature of 298 K. Lagergren and Ho kinetic models were used to analyse the kinetic data obtained from different batch experiments. The biosorption of both dyes followed better the pseudo-second order kinetic model. The calculated values for ΔG, ΔS, and ΔH indicated that the biosorption of CR and MB onto the waste pretreated biomass was feasible, spontaneous, and exothermic in the selected temperature range and conditions.

Published
2018
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8. First Order Derivative of Differential Pulse Voltammetry for the Selective and Sensitive 4‐Nitrophenol Determination in the Presence of 2‐Nitrophenol at a Bare Graphite Electrode.

Author

Stoytcheva, Margarita, Velkova, Zdravka, Gochev, Velizar, Toscano‐Palomar, Lydia, Valdez, Benjamin, and Curiel, Mario

Subjects
VOLTAMMETRY, ELECTRODES, SIGNAL processing, PHENOLS
Abstract

Derivative processing of the signal obtained by differential pulse voltammetry was applied in this work as a simple strategy to achieve the sensitive and selective 4‐nitrophenol (4‐NP) determination in the presence of the interfering compound 2‐nitrophenol (2‐NP). The derivative voltammogram displayed a well‐defined and fully separated 4‐nitrophenol anodic peak, which was used for its quantification in two linear concentration ranges extended up to 10 μmol L−1 and up to 100 μmol L−1. A LOD as low as 0.1 μmol L−1 (S/N=3) was achieved using spectroscopic graphite electrode without any modification. The accuracy of the 4‐NP determination in the presence of 2‐NP, assessed by spike‐and‐recovery experiments was found to be satisfactory. 4‐NP determination was not affected by the presence of a range of structurally similar phenolic compounds. [ABSTRACT FROM AUTHOR]

Published
2023
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11. Supplementary Material 1 - Supplemental material for Diterpenoid Constituents of Teucrium scordium L. Subsp. scordioides (Shreb.) Maire Et Petitmengin

Author

Bozov, Petko I., Penchev, Plamen N., Girova, Tania D., and Gochev, Velizar K.

Subjects
FOS: Clinical medicine, 111599 Pharmacology and Pharmaceutical Sciences not elsewhere classified
Abstract

Supplemental material, Supplementary Material 1, for Diterpenoid Constituents of Teucrium scordium L. Subsp. scordioides (Shreb.) Maire Et Petitmengin by Petko I. Bozov, Plamen N. Penchev, Tania D. Girova and Velizar K. Gochev in Natural Product Communications

Published
2020
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21. Supplementary material - Supplemental material for Phytochemical Investigations of Laurel Fruits (Laurus nobilis)

Author

Petkova, Zhana, Stefanova, Galina, Girova, Tania, Antova, Ginka, Stoyanova, Magdalena, Damianova, Stanka, Gochev, Velizar, Stoyanova, Albena, and Valtcho D. Zheljazkov

Subjects
FOS: Clinical medicine, 111599 Pharmacology and Pharmaceutical Sciences not elsewhere classified
Abstract

Supplemental material, Supplementary material, for Phytochemical Investigations of Laurel Fruits (Laurus nobilis) by Zhana Petkova, Galina Stefanova, Tania Girova, Ginka Antova, Magdalena Stoyanova, Stanka Damianova, Velizar Gochev, Albena Stoyanova, and Valtcho D. Zheljazkov in Natural Product Communications

Published
2019
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28. Adsorptive Stripping Voltammetric Determination of 2,4‐Dichlorophenol by Laponite Modified Carbon Paste Electrode.

Author

Stoytcheva, Margarita, Zlatev, Roumen, Velkova, Zdravka, Gochev, Velizar, Ayala, Alan, Montero, Gisela, and Valdez, Benjamín

Subjects
CARBON electrodes, RESORCINOL, CATECHOL, DRINKING water, CATALYTIC activity, PHENOLS
Abstract

A laponite modified carbon paste electrode was prepared, characterized and applied for the 2,4‐dichlorophenol (2,4‐DCP) voltammetric determination. It takes advantage of the ability of laponite to adsorb phenols, as well as of its availability and very low cost. Kinetic and equilibrium data for 2,4‐DCP adsorption by laponite in aqueous dispersions demonstrated that the adsorption process obeyed a pseudo first order kinetic model and was consistent with the formation of adsorbed multilayers on a surface with heterogeneous pore distribution. The composite paste electrode exhibited a heterogeneous surface with 65 % increased surface area and 27 % enhanced catalytic activity compared to the unmodified one. The adsorptive stripping voltammetric determination of 2,4‐DCP at an electrode with an optimized graphite:laponite ratio of 55 : 15 w% using a 3 min accumulation time at pH 5.5 was found to be suitable for its quantification in the linear concentration range extended up to 50 μmol L−1 with a sensitivity of 0.56 μA L μmol−1 and a LOD of 0.2 μmol L−1 (S/N=3).The 2,4‐DCP electrochemical response was not affected by the presence of some structurally similar phenols, like catechol and p‐nitrophenol, while resorcinol, 2‐chlorophenol, and 4‐chlorophenol presented interferences. The results were validated by 2,4‐DCP determination in spiked tap water. [ABSTRACT FROM AUTHOR]

Published
2021
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29. Stripping Voltammetric Determination of Methyl Parathion at Activated Carbon Nanopowder Modified Electrode.

Author

Stoytcheva, Margarita, Zlatev, Roumen, Velkova, Zdravka, Gochev, Velizar, Montero, Gisela, Valdez, Benjamín, and Curiel, Mario

Subjects
METHYL parathion, ACTIVATED carbon, CARBON electrodes, ELECTRODES
Abstract

An activated carbon nanopowder modified glassy carbon electrode (AC‐GCE) was constructed for the sensitive determination of methyl parathion by adsorptive differential pulse anodic stripping voltammetry. The simple and rapid modification procedure included only drop‐coating the electrode surface with a laponite stabilized activated carbon nanopowder suspension and drying. The modifier high adsorption ability, combined with its large electroactive surface area allowed a 30‐fold signal increase to be achieved, compared to bare GCE. Under optimized experimental conditions (activated carbon to laponite ratio, pH and accumulation time), the AC‐GCE exhibited a linear response to methyl parathion in two concentration ranges: from 0.01 μmol L−1 to 1 μmol L−1 and from 1 μmol L−1 to 6 μmol L−1. The LOD of 2.5 nmol L−1 (S/N=3) achieved fitted with regulatory norms. It was demonstrated that the as‐prepared AC‐GCE is suitable for routine real samples analysis. [ABSTRACT FROM AUTHOR]

Published
2021
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30. Biosorption of Pb (II), Cd (II) and Hg (II) Ions from Model Solutions on Pretreated Waste Bacillus thuringiensis Biomass.

Author

Todorova, Kostadinka T., Velkova, Zdravka Y., Kirova, Gergana K., Stoytcheva, Margarita S., Kostadinova, Sonya T., and Gochev, Velizar K.

Subjects
BACILLUS thuringiensis, BIOMASS, PHOSPHORYL group, CARBONYL group, METAL ions, BIOCHAR, SORPTION, MERCURY
Abstract

In the present study waste pretreated Bacillus thuringiensis biomass was applied as an ecofriendly biosorbent for Pb (II), Cd (II) and Hg (II) from model aqueous solutions. Heat inactivated and alkali treated biomass showed ability for removal of metal ions from single solutions in the following order Pb(II) > Cd(II) > Hg(II). It was proved that the major groups involved in biosorption are hydroxyl/amino, alkyl, carbonyl and phosphoryl groups. The influence of different factors as pH, initial sorbate concentration, biosorbent concentration, contact time was evaluated. [ABSTRACT FROM AUTHOR]

Published
2019

40. A nanotechnological approach to biosensors sensitivity improvement: application to organophosphorus pesticides determination.

Author

Stoytcheva, Margarita, Zlatev, Roumen, Montero, Gisela, Velkova, Zdravka, and Gochev, Velizar

Subjects
BIOSENSORS, ORGANOPHOSPHORUS pesticides, NANOTECHNOLOGY, PARAOXON, CARBON nanotubes, HYDROXYAPATITE, CYCLIC voltammetry, CHRONOAMPEROMETRY
Abstract

An electrochemical organophosphorus hydrolase-based sensor for direct organophosphorus pesticides determination was developed, analytically characterized and applied for paraoxon quantification. The biosensing platform was constructed by one-step electrodeposition onto the surface of a glassy carbon electrode of a bionanocomposite of chitosan with carbon nanotube (CNTs), hydroxyapatite (HA) and organophosphorus hydrolase. The sensor, optimized with respect to the CNTs and HA load, was characterized by the application of cyclic voltammetry and chronoamperometry. The limit of detection (LOD) was found to be as low as 0.1 µmol L−1, the linear concentration range was extended up to 80 µmol L−1, and the sensitivity was as high as 5.10 nA L µmol−1. The sensor possesses enhanced sensitivity towards organophosphorus pesticides (OPs), because of the CNTs and the HA nanoparticles synergistic action. [ABSTRACT FROM AUTHOR]

Published
2018
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41. Novel composite biosorbent from Bacillus cereusfor heavy metals removal from aqueous solutions

Author

Todorova, Kostadinka, Velkova, Zdravka, Stoytcheva, Margarita, Kirova, Gergana, Kostadinova, Sonia, and Gochev, Velizar

Abstract

AbstractWaste biomass from Bacillus cereusimmobilized in sodium alginate and co-immobilized with activated carbon or with bentonite into alginate gel was studied for Pb(II), Cd(II) and Hg(II) removal from aqueous solutions. The composite biosorbent consisting of waste B. cereusbiomass co-immobilized with activated carbon into alginate beads was selected as the most prospective for heavy metals removal. Immobilization increased both the removal capacity and the mechanical strength of the biosorbent. Major process parameters were optimized and maximum removal efficiency of 92.13% was reached for Pb(II) ions at pH 5.0, biosorbent dosage 2 g/L, temperature 25 °C, agitation speed 120 rpm for 120 min.

Published
2019
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47. Chemical Composition and Biological Activity of Essential Oils from Wild Growing Aromatic Plant Species of Skimmia laureola and Juniperus macropoda from Western Himalaya

Author

Stappen, Iris, primary, Tabanca, Nurhayat, additional, Ali, Abbas, additional, Wedge, David E., additional, Wanner, Jürgen, additional, Kaul, Vijay K., additional, Lal, Brij, additional, Jaitak, Vikas, additional, Gochev, Velizar K., additional, Schmidt, Erich, additional, and Jirovetz, Leopold, additional

Published
2015
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