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Öğe A comparative study on heavy metal biosorption characteristics of some algae(ELSEVIER SCI LTD, 1999) Donmez, GC; Aksu, Z; Ozturk, A; Kutsal, TThe biosorption of copper(II), nickel(II) and chromium(VI) from aqueous solutions on dried (Chlorella vulgaris, Scenedesmus obliquus and Synechocystis sp.) algae were tested under laboratory conditions as a function of pH, initial metal ion and biomass concentrations. Optimum adsorption pH values of copper(II), nickel(II) and chromium(VI) were determined as 5.0, 4.5 and 2.0. respectively, for all three algae. At the optimal conditions, metal ion uptake increased with initial metal ion concentration up to 250 mg l(-1). Experimental results also showed the influence of the alga concentration on the metal uptake for all the species. Both the Freundlich and Langmuir adsorption models were suitable for describing the short-term biosorption of copper(IT), nickel(II) and chromium(VI) by all the algal species. (C) 1999 Elsevier Science Ltd. All rights reserved.Öğe AC losses and determination of some superconducting parameters of Ag2O added Bi2223 superconductors(ELSEVIER SCIENCE BV, 2006) Duzgun, I; Karaca, I; Ozturk, A; Celebi, SThe effects of Ag2O addition on electromagnetic properties in a Bi2223 superconductor have been investigated by means of AC susceptibility measurements and theoretical analysis. Intergranular critical current density was found to be decreased with increased amount of Ag2O addition (0, 15, 30 wt.%). The effective volume fractions of the grains were estimated as a function of Ag2O amount. The field and temperature dependence of the intergranular critical current density for each sample has been accounted by the function J(cm) (T) = (J(0)/B-n) (1 - T/T-cm)(P), where alpha(0) denotes the pinning strength parameter at T = 0. The field exponent n, temperature exponent p, and maximum intergranular loss peaks, which are determined from the comparison of the experimental and calculated curves, increased with increasing amount of Ag2O addition. Experimental isothermal hysteresis losses as a function of field amplitude are also seen to adequately be reproduced by model calculation using the same fitting parameters as for AC susceptibility chi(n)(T) data. (c) 2006 Elsevier B.V. All rights reserved.Öğe Biosorption of nickel(II) and copper(II) ions from aqueous solution by Streptomyces coelicolor A3(2)(ELSEVIER SCIENCE BV, 2004) Ozturk, A; Artan, T; Ayar, AThe biosorption of nickel(II) and copper(II) ions from aqueous solution by dried Streptomyces coelicolor A3(2) was studied as a function of concentration, pH and temperature. The optimum pH range for nickel and copper uptake was 8.0 and 5.0, respectively. At the optimal conditions, metal ion uptake was increased as the initial metal ion concentration increased up to 250 mg l(-1). At 250 mg l(-1) copper(II) ion uptake was 21.8% whereas nickel(II) ion uptake was found to be as high as 7.3% compared to those reported earlier in the literature. Metal ion uptake experiments were carried out at different temperatures where the best ion uptake was found to be at 25 degreesC. The characteristics of the adsorption process were investigated using Scatchard analysis at 25 degreesC. Scatchard analysis of the equilibrium binding data for metal ions on S. coelicolor A3(2) gave rise to a linear plot, indicating that the Langmuir model could be applied. However, for nickel(II) ion, divergence from the Scatchard plot was evident, consistent with the participation of secondary equilibrium effects in the adsorption process. Adsorption behaviour of nickel(II) and copper(II) ions on the S. coelicolor A3(2) can be expressed by both the Langmuir and Freundlich isotherms. The adsorption data with respect to both metals provide an excellent fit to the Freundlich isotherm. However, when the Langmuir isotherm model was applied to these data, a good fit was obtained for the copper adsorption only and not for nickel(lI) ion. (C) 2004 Elsevier B.V. All rights reserved.Öğe Fabrication and characterization of Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10 superconductors prepared by a wet technique(ELSEVIER SCIENCE SA, 1998) Celebi, S; Karaca, I; Ozturk, A; Nezir, SWe have carried out measurements of resistivity on Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10 granular samples prepared by the liquid ammonium nitrate method. The standard four-probe technique was used to measure the resistivity and the critical current of the specimen. We estimated the irreversibility line from the broadening of the resistive-superconducting transition in external DC magnetic fields up to 447 mT for the sample fabricated. A non-linear Arrhenius plot of resistive tail has been found. The power law relation B-R(irr)=B-0(1-T/T-c)(3/2) gives a good fit to OUT data Obtained from zero-resistivity temperature in selected magnetic fields. We have also characterized the prepared samples in terms of XRD, SEM and critical currents. (C) 1998 Elsevier Science S.A.
