Browsing by Subject "Magnesite."

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    Sorption behaviour of Ba++, Co++ and Zn++ ions on alumina, kaolinite and magnesite
    (1991) Gökmenoğlu, Zeliha
    The need for ultimate disposal of nuclear wastes has stimulated a renewal of interest in the adsorption behaviour of various nuclides on minerals of the type found in and around the various types of repositories that have been proposed. These adsorption studies are needed in order to estimate rates of transport of the nuclides in the event of water penetration into and through the repository. In this work the sorption-desorption behaviour of Ra·*”*·, and Zn'^'^ on minerals from different regions of Turkey has been investigated by means of a batch technique. The mineral samples used are mainly alumina, kaolinite and magnesite types. and were used as isotopic tracers. The distribution coefficients for the sorption of the three cations studied on different minerals were calculated. Alumina mineral was used for Co·*"*· and Zn'^'^ sorption experiments whereas kaolinite and magnesite were used to study the sorption behaviour of Cc»·*·*· and Zn++. The samples were seperated into different particle size ranges by AndrecLsen Pipette method. The particle size range used throughout the experiments was 20-38//m .The groundwaters used for different sorbing materials were from Beyşehir, Seydişehir and Mihalhçcık. The groundwaters used in the experiments were prepared synthetically in the laboratory. In the solutions prepared with ground- waters, initied concentrations ranged from 1.04x10“® to 1.04x10"® meq/ml for Co·*····, 7.67x10"® to 7.67x10"'* meq/ml for Zn'^'^ and 7.65x10"® to 7.65x10"® meq/ml for The samples were shaken with a shaker at 190 rpm and phase separations were carried out by centrifuging. The measurements were performed by a Nal(Tl) detector. Sorption and desorption kinetics were studied during 16 days except for the adsorption of C0++ on magnesite which was studied during 24 days, and adsorption rates were calculated from first order rate equation. Rapid adsorption was observed at high concentrations. It was observed that, about 2 days of contact was enough for values on kaolinite to reach steady-state whereas for Co^'^ on magnesite at least 25 days were needed. Rd values of on alumina mineral were quite high (2857 ml/g). Rd values for on minerals ranged from 15 (on kaolinite) to 4274 ml/g (on alumina) depending on the type of minéral and the Rd values of Zn'^'^ on minerals ranged from 26 (on magnesite) to 3800 ml/g (on alumina). It was observed that, alumina minerals adsorb ions more than kaolinite and magnesite type of minerals. Adsorption-desorption process was found to be reversible for Ba'^'^ and Co^'^ sorption on alumina and Co^'^ sorption on kaolinite. However, a paxtially reversible mechanism was observed for Zn'^'^ on.alumina and kaolinite and the sorption of (70++ and Zn++ ions on magnesite. Concentration dependent ion sorption isotherms were found to fit to Fre- undlich type of isotherms. The specific sorption concentration or the amount of radionuclide sorbed per gram of soil were calculated. It was seen that the sorbed concentration, Ca decreases with increasing mass M of the adsorbing solid. The change of the system parameter, 7 and specific sorbed concentration, Ca,o with the initial ion concentration is given by appropriate equations. The results obtained from V/M ratios indicate that in batch experiments the influence of the V/M ratio should be taken into account although according to the fundamental distribution law, distribution coefficients should be independent of V/M ratio. Distribution coeiRcients for adsorption of on binary mixtures of various minerals have been determined to see whether Rd of mixtures could be predicted from those of the pure minerals. It is shown that the system corresponds to a diluted experiment and the overall Rd is limited by the high Rd of alumina.
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    Sorption studies of cesium and barium on magnesite using radiotracer and x-ray photoelectron spectroscopy
    (1997-06) Shahwan, Talal
    As the consumption of the radioactive materials is continuously increasing, the problem of disposing the resulting v^^astes safely is becoming more challenging. One way through which these radioactive wastes could be isolated from the biological environment is by disposing them in deep geological formations. Clay minerals are proposed as backfill buffering materials that can delay the migration of the radionuclides and thus decrease the contamination of underground waters. The extent of retardation of the radionuclide migration is dependent on factors like time of contact, pH and Eh of groundwater, concentration, temperature and grain size of the mineral particles. Up to now, several studies were carried out to examine the effect of such parameters on the sorption behavior of different radionuclides on various kinds of minerals. This study was conducted to investigate the effects of time, concentration and temperature on the sorption behavior of cesium and barium ions on magnesite. Cesium 137 140 and barium have the radioactive isotopes Cs (t^^^ = 30.1 y) and Ba (t^^^ = 12.8 d) 1/2 produced in high yields during the fission process which are important in radioactive waste considerations. Magnesite is a mineral composed mainly of magnesium carbonate 2+ together with minor amounts of quartz and has a single exchangeable cation. Mg . The radiotracer method and x-ray photoelectron spectroscopy, which is a powerful surface sensitive tool, were used in this study. The results obtained from both methods complemented each others and were in good agreement. Kinetic studies of the sorption process show that equilibrium was approached within one day of contact for both of cesium and barium ions. The data of the sorption of both cations using different concentrations at various temperatures were most adequately described by the Freundlich type isotherms which correspond to multilayer adsorption on heterogeneous surfaces. The values of the Freundlich constants k and n imply that barium ions have slightly larger adsorption affinity and adsorption intensity than cesium ions. The adsorption data at low concentrations were also observed to obey the Dubinin-Radushkevich type isotherms which describe monolayer adsorption on heterogeneous or homogeneous surfaces. The adsorption data were very poorly described by the Langmuir type isotherms. Thermodynamic parameters such as enthalpy change, AH°, entropy change, AS° and free energy change of adsorption, AG°, were calculated from the sorption data of cesium and barium ions at different temperatures. The values obtained for AH° and AS° were -37 kJ/moL, -0.09 kJ/moL.K and -13 kJ/moL, -0.009 kJ/moL.K for cesium and barium ions respectively. The negative AH° values indicate the exothermic nature of adsorption which means that low temperatures are favored. The decrease in entropy upon adsorption implied by the negative AS° values is indicative of the stability of adsorption for both cations. The values of AG° at different temperatures were all negative indicating the spontaneity of the adsorption process for both cesium and barium ions. The magnitudes of AG° were seen to be within the 8-16 kJ/moL range which is the energy range of ion- exchange type processes.