Browsing by Subject "Bentonite"

Now showing 1 - 6 of 6

Open Access
A characterization study of some aspects of the adsorption of aqueous Co2+ ions on a natural bentonite clay
(Academic Press, 2006-06-13) Shahwan, T.; Erten, H. N.; Unugur, S.
The natural bentonite used in this study contained montmorillonite in addition to low cristobalite. The uptake of aqueous Co2+ ions was investigated as a function of time, concentration, and temperature. In addition, the change in the interlayer space of montmorillonite was analyzed using XRPD, and the distribution of fixed Co2+ ions on the heterogeneous clay surface was recorded using EDS mapping. The sorbed amount of Co2+ appeared to closely follow Freundlich isotherm, with the sorption process showing apparent endothermic behavior. The relevance of the apparent _Ho values is briefly discussed. Analysis of the Co-sorbed bentonite samples using SEM/EDS showed that the montmorillonite fraction in the mineral was more effective in Co2+ fixation than the cristobalite fraction. XRPD analysis demonstrated that the interlayer space of montmorillonite was slightly modified at the end of sorption.
Open Access
Radiochemical and spectroscopic studies of cesium, barium, and cobalt sorption on some natural clays
(2000-08) Shahwan, Talal
The wide growth in the nuclear activities results in an increasing subsequent influx of radioactive wastes into the environment. This problem has manifested a great deal of interest aiming at finding out ways through which those wastes can be harmlessly isolated from the human environment. Geological disposal is considered as one of the most promising solutions that ensures a safe storage of radioactive wastes as long as their activities are above the accepted levels. Clay minerals are proposed as backfill buffering materials in the geological repositories that can delay the migration of the radionuclides through sorption 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. In this study radiochemical, spectroscopic (ToF-SIMS, XPS), and X-ray diffraction techniques were applied to examine different aspects of the sorption behavior of cesium, barium and cobalt on three natural clay minerals containing primarily kaolinite, illite-chlorite, and bentonite. The elements cesium (Z=55), barium (Z=56), and cobalt (Z=27) have the radioactive isotopes superscript 137 Cs (half-life=30.17 years), superscript 140 Ba (half-life=12.79 days), and superscript 60 Co (half-life=5.3 y) which are important in radioactive waste management. The first two radionuclides are produced in high yields in nuclear fission, whereas the third is an activation product. The natural clay samples that were used in this study originated from natural mineralogical beds at Sindırgı, Afyon, and Giresun regions in Turkey. The characterization of these clay samples showed that the primary clay minerals were kaolinite in Sındırgı clay, chlorite and illite in Afyon clay, and montmorillonite in Giresun clay. Each of these clays possesses different structural properties that result in different sorption capabilities. Radiochemical batch experiments were carried out to examine the effects of time, concentration, and temperature on the sorption of cesium, barium and cobalt on clays. Solutions of these cations spiked with several microliters of the radionuclides 137 CS (half-life=30.1 y), 133 Ba (half-life=10.7 y), and 60 Co (half-life=5.3 y) were monitored using gamma-ray spectroscopy prior to and after each sorption experiment. These results showed that equilibrium is achieved within two days in all cases. The sorption data was adequately described by Freundlich and Dubinin-Radushkevich isotherm models. Based on the parameters of those isotherm models, it was found that sorption was nonlinear, and that bentonite showed the highest sorption affinity and sorption capacity towards the sorbed ions. The thermodynamic parameters indicated that while sorption of cesium and barium on the three clays is exothermic that of cobalt is endothermic. The obtained values of Gibbs free energy change, Delta G degrees, were generally in the 8-16 (kJ/mol) energy range that corresponds to ion exchange type sorption mechanism. Since sorption is mainly a surface phenomenon, part of our sorption studies were carried out using the surface sensitive techniques; Time of Flight- Secondary Ion Mass Spectroscopy (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS). In addition, depth profiling up to 70 angstroms was performed using ToF-SIMS to investigate cesium, barium and cobalt concentrations through the clay surface. ToF-SIMS and XPS studies were helpful in figuring out the surface composition of different clays prior to and after sorption. Quantification of the depletion of different alkali and alkaline-earth metals initially contained within the analyzed clay surface showed that ion exchange plays a primary role in the sorption process. In addition, X-Ray Diffraction (XRD) technique was applied to figure out the mineralogical composition of the clay minerals used and examine any structural change a accompanying the sorption process. XRD spectra of the clay samples after sorption showed that -apart from some intensity reductions in some clay features-, no primary changes were detected in the sorption cases of cesium and cobalt. In barium sorption, however, features belonging to barium carbonate were present in the spectra corresponding to sorption on chlorite-illite and bentonite.
Open Access
The sorption behavior of CS + ion on clay minerals and zeolite in radioactive waste management: sorption kinetics and thermodynamics
(Springer, 2011) Yildiz, B.; Erten, H. N.; Kış, M.
In this work, Cs+ ion sorption on some clays and zeolite were investigated. 137Cs was used as a tracer. Activities were measured with a NaI crystal gamma counter. The particle size distribution was determined by a laser sizer. Surface area of the particles were determined by BET (Brunauer, Emmett and Teller method). Structure analysis was made by using X-ray diffraction. The chemical compositions of the solid samples were determined using a ICAP-OE spectrometer. Kinetic and thermodynamic parameters were determined. Due to very high uptake results; clay and zeolite can be proposed as a good sorbents in waste management considerations.
Open Access
Sorption studies of Cs+, Ba2+, and Co2+ ions on bentonite using radiotracer, ToF-SIMS, and XRD techniques
(De Gruyter Oldenbourg, 2001) Shahwan, T.; Erten H. N.
The sorption behaviour of Cs+, Ba2+, and Co2+ ions on bentonite were investigated using the radiotracer method, Time of Flight-Secondary Ion Mass Spectroscopy (ToF-SIMS), and X-Ray Diffraction (XRD). The sorption of Cs+ and Ba2+ were exothermic while sorption of Co2+ was endothermic. The sorption data were well described by Freundlich and Dubinin-Radushkevich isotherms. According to ToF-SIMS results Na+ and Mg2+ were the primary exchanging ions in bentonite. The XRD spectra showed that no structural changes were associated with the sorption of Cs+ and Co2+, and BaCO3 precipitate was formed upon the sorption of Ba2+ on bentonite.
Open Access
Thermodynamic parameters of Cs+ sorption on natural clays
(Akademiai Kiado Rt., 2002) Shahwan, T.; Erten, H. N.
The sorption behavior of Cs+ on kaolinite, chlorite-illite, and bentonite clays as a function of time, cation concentration, and temperature was studied using the radiotracer method. Sorption data were well represented by Freundlich and Dubinin-Radushkevich type isotherms. Bentonite was found to have the highest sorption capacity and the highest exchange affinity to Cs+. In all three cases Cs+ sorption was found to be exothermic with ΔH° (kJ/mol) -13, -8, -19 and ΔS° (J/mol·K) -15, 31, and -3 for kaolinite, chlorite-illite, and bentonite, respectively. Negative ΔG° values were obtained in all cases, indicating the spontaneity of sorption. The magnitudes of ΔG° suggest that ion exchange is the primary sorption mechanism.
Open Access
Uptake of Ba2+ ions by natural bentonite and CaCO3: a radiotracer, EDXRF and PXRD study
(Akademiai Kiado Rt., 2002) Shahwan, T.; Atesin, A. C.; Erten, H. N.; Zararsiz, A.
Ba2+ uptake by natural bentonite, CaCO3 in addition to a number of bentonite-CaCO3 mixtures with variable compositions as a function of pH and Ba2+ concentration was studied. Radiotracer method, EDXRF, and PXRD were used. The results of radiotracer experiments showed that the uptake of Ba2+ by CaCO3 was larger than its uptake by natural bentonite samples, particularly at low initial concentrations of Ba2+ and higher pH values. This finding was supported by the EDXRF results, According to the sorption data, the apparent ΔG° values of sorption were in the range -9±1 to -13±3 kJ/mol. The PXRD studies revealed the formation of BaCO3 upon sorption of Ba2+ on pure CaCO3 and on some of the bentonite-CaCO3 mixtures.