Sorption behaviour of Ba++, Co++ and Zn++ ions on alumina, kaolinite and magnesite

Abstract

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.