Browsing by Author "Koleva V."

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    Characterization of beryllium selenates by X-ray powder diffraction, DTA and DSC
    (2004-11-01) Koleva V.; Karadjova V.; Georgiev M.
    BeSeO4.2H2O and BeSeO4 have been studied röntgenographically: BeSeO4.2H2O forms orthorhombic / crystals with lattice constants: a = 5.843(2) Å; b = 9,790(3) Å; c = 4.692(1) Å; V = 268.4(1) Å3. The crystals of BeSeO 4 are tetragonal: a = 4.648(1) Å; c = 7.084 (3) Å; V= 153.1(1) Å3, SG I4̄. The thermal dehydration of BeSeO 4.4H2O has been studied by TG, DTA and DSC methods. The dehydration occurs in four steps and intermediate hydrates BeSeO 4.2H2O, BeSeO4.H2O and BeSeO 4.0.5H2O are formed. The enthalpies of dehydration of the observed dehydration processes have been determined. The higher value of ΔHdeh of BeSeO4.4H2O as compared to those of other metal selenate tetrahydrates is discussed in terms of the strong Be-H2O interaction. The enthalpies of formation of BeSeO 4.4H2O and BeSeO4.2H2O have been calculated. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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    Solid liquid extraction of phenolic and flavonoid compounds from Cotinus coggygria and concentration by nanofiltration
    (2014-01-01) Koleva V.; Simeonov E.
    Extraction kinetics of polyphenols and flavonoids from plant material and their separation and concentration by nanofiltration were investigated. The kinetics experiments were carried out with Cotinus coggygria. The influence of the extraction solvent on the extraction rate was defined. Four different extraction solvents or mixtures were applied - methanol, azeotropic ethanol, 50/50 ethanol/water mixture, and pure water. The optimum extraction rate of flavonoids and polyphenols was achieved by using 50 % ethanol as a solvent for extraction. This solvent mixture was used for generating extracts for the nanofiltration experiments. Organic solvent nanofiltration membranes from DuraMem™ series with different pore sizes (200, 300 500 and 900 Da) were tested in dead-end and cross-flow filtration systems. Flux and rejection data were obtained for every membrane type. Very good separation and respectively concentration of the extracted useful compounds was achieved (rejection for polyphenols and flavonoids is 91 and 93 %, respectively). During a feed/extract concentration experiment, the constant polyphenols and flavonoids rejection was measured. The results suggest that the nanofiltration technology could be combined successfully with solid-liquid extraction for natural sourced valuable compounds enrichment.