Physicochemical characterization of the sorption behavior of Cs+ and Sr2+ Ions on natural kaolinite and cliptilolite minerals

Abstract

In this study, aspects of the thermodynamics and kinetic profile of the sorption behavior of Cs+ and Sr2+ ions on natural minerals of kaolinite and clinoptilolite originating from Sındırgı and Manisa regions were investigated.Radioactive wastes that are caused by the increasing usage and production of radioactive substances give rise to big problems rising day by day. Natural substances such as clay minerals and zeolites decrease the dispersion of radioactive isotopes by sorption. Therefore they can be able to retard the radioactive pollution that is caused by radioisotopes, and the reach of these radioisotopes to the underground waters. 137Cs (t1/2 . 30.1 y) and 90Sr (t1/2 . 28.8) are radioisotopes that are important as radioactive wastes due to their long half-lives ,and that produce with high yields at the end of nuclear fission reactions. The experiments were performed using the batch technique at four different initial concentrations, two different temperatures, and time period ranging from 5 minutes up to 48 hours.The experimental findings revealed that the percentage sorption of Cs+ on clinoptilolite ranged 91-94, on kaolinite ranged 28-40 , and that the percentage sorptionof Sr2+ clinoptilolite ranged between 48-93 and 12-47 for the kaolinite case. In order to check the stability of sorption of Sr2+ and Cs+ ions fixed by kaolinite and clinoptilolite,desorption experiments were performed as well with tap water. The results indicated that in the case of Cs+ sorption on kaolinite the percentage desorption amounted to 40, but was smaller than 7 in the case of clinoptilolite. The desorption percentage of Sr2+ from clinoptilolite did not exceed 5, while it is amounted to 12 in kaolinite case, indicating a more stable fixation by clinoptilolite. The results showed that sorption of both Sr2+ and Cs+ on both minerals followed pseudo-second order kinetics, with the rate constants indicating faster sorption on kaolinite. The activation energies were 11.6 and 15.3 kj/mol for Cs+ sorption on kaolinite and clinoptilolite, respectively. Alternatively, the activation energy values for Sr2+ sorption on kaolinite and clinoptilolite were 8.5 and 17.3 kj/mol, respectively. Freundlich and D-R isotherm models were found to adequately describe the adsorption data of Sr2+ and Cs+ on both minerals.Cs+ sorption showed a spontaneous and exothermic behavior on both minerals with .H0 being .6.3 and .11.4 kj/mol for Cs+ uptake by kaolinite and clinoptilolite, respectively. On the other hand, Sr2+ sorption showed a spontaneous and endothermic behavior on both minerals with .H0 being 11.22 and 9.8 kj/mol for Sr2+ sorption on kaolinite and clinoptilolite, respectively. To check the effect of interlayer expansion on the uptake capacity of kaolinite, the clay was intercalated with DMSO, to overcome the tight H-bonding interconnecting the layers of the clay. Expanding the interlayer space of kaolinite from 0.71 nm to 1.12 nm using DMSO intercalation, lead to an increase in the sorption capacity of kaolinite, with the increase being more significant in Sr2+ case. Compared with the percentage sorption on natural kaolinite, the percentage sorption ofCs+ on DMSO-intercalated kaolinite for the initial concentrations of 100 and 500 mg/L of CsCl, increased, respectively, from 35 to 41, and from 27 to 33. On the other hand,the percentage sorption of Sr2+ on DMSO-intercalated kaolinite for the initial concentrations of 100 and 500 mg/L of Sr(NO3)2, increased from 17 to 58, and from 22 to 45,respectively. SEM/EDS characterization was carried out to investigate structural changes accompanying the sorption process. SEM analysis indicated that both minerals has a well-defined crystal structure. The EDS findings revealed some localization in the sorbed Sr2+ and Cs+ on the surfaces of both minerals and demonstrated the higher fixation ability of clinoptilolite.