Synthesis Characterisation and Applications of some Inorganic Ion-Exchangers

Abstract

Recently, the interest in inorganic ion-exchangers has been revived with the need for the separation of ionic components in radioactive wastes at high temperatures. Inorganic ion-exchangers are the only materials that can be used for such applications due to their high selectivity and remarkable stability against high radiation levels. Inorganic ion-exchangers based on tin(lV) and zirconium have been synthesized, characterised and studied for their analytical applications. Due to better ion-exchange characteristics, double salts of zirconium Le., zirconium antimonophosphate and zirconium phosphotungstate are being reported as novel materials with good ion exchange characteristics. Synthetic procedures have been optimized to prepare good quality lon-exchangers possessing workable particle geometry, chemical and thermal stabHities. These ion-exchangers have been characterised as weakly acidic ion exchangers Studies like ion-exchange capacity for alkali and alkaline earth metal ions, distribution coefficients for a number of metal ions, effect of heat treatment on ion-exchange capacity, chemical composition, thermal and chemical stabilities have been done on the newly synthesized ion-exchangers. Structural studies of these materials and some of those prepared earlier like tin (IV) antimonoarsenate have been carried out with the help of X-ray powder patterns, Infrared analysis and response to thermal treatment upto 900" C. Analysis of the X - ray diffractograms of tin(IV) antimonoarsenate reveals that crystaUinityof the ion-exchanger is due to weak Vanderwaals forces holding the structure in shape, because of the reason that the structures lose crystallinity when subjected to a temperatute as low as 60" C. A decrease in the crystalline character continues upto 300" C and at 600" C, reorientation of the structure is observed. Exchange kinetic studies have been done on one of these ion. exchangers to understand the exchange reaction mechanism. Experiments have been conducted by using limited bath technique with alkaline earth metal ions as exchanging electrolytes. Thermodynamic parameters like activation energy. entropy of activation and self. diffusion coefficients have been deternined. The exchange process seems to be comprising of a sequence of steps like change of state of hydration of the incoming ion, diffusion through reticular structure of the polymer and stoichiometric exchange of the incoming counter ions with the mobile protons bound electrostatically to the fixed anionic sites of the Jon-exchanger Electrochemical studies have been done on zirconium phosphotungstate by preparing an inorganic ion-exchange membrane us ng PVC as a support material. Diffusion of the counter Ions across the membrane fixed in a concentration cell has been used to know the electrochemical characteristics like transport numbers, permselectivity and fixed charge density of the ion-exchange material. The membrane system shows Nernst breakdown for Na" ions as these ions are not thermodynamically favoured 10 the exchanger matrix. Due to high chemical stability, zirconium boratophosphate has been used to prepare an heterogeneous solid membrane electrode. The electrode shows linear response towards thorium(IV) ions in the concentration range of 10-4 to 10" M with an overNernstian slope of 50 mV/decade. Selectivity coefficients for a number of mterfenng ions like rare earth, alkali metal and alkaline earth metal cations have been studied, The electrode responds selectively to the primary ions Le., Th<4> ions in the presence of these interfering cations. The interference from these ions increases with increase in their concentration. Effect of the concentration of interfenng ions on the slope of calibration curve has also been studied. The electrode system responds reasonably well as an Indicator electrode in the potel1tiometric titrations of thorium(IV) ions against complexing agents.