SEPARATION OF SOME SELECTED ALKALI AND ALKALINE EARTH METAL IONS BY INORGANIC ION EXCHANGERS

Abstract

The work described in this thesis consists of the synthesis, characterization and application of the supported ion exchange materials for use in connection with the separation of lithium, rubidium, cesium and strontium ions from the nitric acid solutions. Some of these metals, especially the Li, are of immense importance due to their use in industrial and defense sectors being vital for a special type of atomic power plants, alloy making, hydrogen storage, heat-resistant ceramic technologies, pharmaceutical industry and power sources including the storage batteries. Such uses, naturally, are expected to lead to environmental issues with the envisaged environmental induction leading to deterioration of the latter. Increasing energy demands and hence more nuclear power production seems to be inevitable and feasible alternate. However such steps may result in the production of more than forty radioactive by-products including Rb86, 87, Cs134, 137 and Sr90. These radio nuclides with their high transport abilities are expected to find their way to water bodies and soils and thus incorporated into animals and human beings through the food chain. The present work describes separation of such hazardous metal ions from the effluents and thus suggests environmental remedial mitigation and control by using new types of materials and appropriate separation methodologies. For this purpose supported inorganic ion exchange materials based upon hexacyanoferrates of different metal ions were prepared and used for the separation of the lithium, rubidium, cesium and strontium ions from aqueous HNO3 solutions. Potassium iron(III) hexacyanoferrate(II) and potassium iron(II) hexacyanoferrate(III) supported on polymethylmethacrylate (PMMA) were prepared by impregnation-precipitation method. However silica gel supported potassium iron(III) hexacyanoferrate(II), potassium iron(II) hexacyanoferrate(III), potassium nickel(II) hexacyanoferrate(III) and potassium copper(II) hexacyanoferrate(III) were synthesized, by in-situ impregnation–precipitation of the silica gel, prepared from trans silicate a commercial sodium silicate product cheaply available in the local market. All the synthesized materials were characterized by FTIR, XRPD, SEM/EDX, CHNS, TGA and surface area analysis in addition to the solubility and stability studies using different solutions of the mineral acids. Using the synthesized ion exchange materials separation studies for each of the lithium, rubidium, cesium and strontium ions, in HNO3 solution, were conducted independently, by carrying out optimizations procedures for establishment of various factors on the separation processes, to elucidate effects of the changing concentration of the metal ion, extent of the ion exchange process, effect of temperature variations on sorption, xii sorption capacity and distribution coefficient determination under respective optimized concentration for each of the above mentioned ions. All procedures, as designed, were tested before undertaking actual separations using authenticated standard solutions. Results on these optimizations are presented and discussed. Distribution coefficient (Kd) values determined for each of the Li+, Rb+, Cs+ and Sr2+ sorption on each of the synthesized material suggested their metal ion removal efficiency. Distribution coefficient values of these metal ions on PMMA supported potassium iron(III) hexacyanoferrate(II) represented a decreasing order of the Kd values as Li+ < Rb+< Cs+< Sr2+ showing the maximum capability for the Sr2+ removal. Deviation in the above cited trend was observed in the case of PMMA supported potassium iron(II) hexacyanoferrate(III) as Kd values followed the trend Sr2+ < Li+ < Rb+< Cs+ showing the lowest distribution of the Sr2+. Distribution of the metal ions on silica gel supported potassium iron(III) hexacyanoferrate(II) indicated the lowest efficiency of the sorbent for Sr2+ removal, while alkali metals have shown minimum values of Kd for Rb+, intermediate for Li+ and highest for Cs+. Potassium iron(III) hexacyanoferrate(II) supported on silica gel has followed the order Li+<Rb+<Sr2+<Cs+, while silica gel supported potassium nickel(II) hexacyanoferrate(III) has depicted the decreasing order of the distribution coefficient values as Rb+< Li+≤ Sr2+< Cs+ showing the least capability for the Rb+ extraction, intermediate for Li+ and Sr2+, while maximum for Cs+. Efficacy of the silica gel supported potassium copper(II) hexacyanoferrate(III) was minimum for Li+ and maximum for the Cs+ as indicated by the Kd values which have shown the decreasing order Li+< Sr2+<Rb+<Cs+. Distribution coefficient values stated that the material PMMA supported potassium iron(III) hexacyanoferrate(II) appeared to be the best for the Sr2+ removal under respective optimized conditions. Maximum efficiency for the Li+ extraction was observed in silica gel supported potassium iron(II) hexacyanoferrate(III), while silica gel supported potassium copper(II) hexacyanoferrate(III) appeared to be the best for Rb+ removal. All the materials were found to be excellent Cs+ scavenger under respective optimized conditions. The results obtained have been discussed keeping in view the available information and stipulated use of some of these materials have been suggested for remedial mitigation of the environmental problems.