STUDIES ON SOME INORGANIC ION EXCHANGERS AND THEIR APPLICATIONS

Abstract

The wide application of ion-exchange phenomena, in so many divergent fields, have remarkably contributed to the social, scientific and technological advancement of mankind. This dissertation includes the preparation of some inorganic gels, having ion-exchange characteristics and using these materials in column operations for the separation of metal Ions. These substances have also been tried as suitable matrix for the fabrication of some solid membrane ion sensors. In general, inorganic 1on exchangers exhibit better selectivity and higher stability towards temperature and ionizing radiations, as compaired to organic resins. Insoluble ferrocyanides of Ag, Zn, Cd, Cu, Ni, Co, Sn, Pb, Mn, Zr, Cr etc. have been studied for their ion-exchange characteristics. Most of these exhibit a great affinity for heavy alkali metal ions, specially for Cs+. These are prepared by mixing salt solutions with H^Fe(CN)6, Na^FeCCN^ or K Fe(CN), when precipitates of various compositions are formed depending upon experimental conditions, i.e. Initial ratio of reactants, acidity of reaction mixtures and order of mixing. Although metal ferricyanides have been reported to be more stable than ferrocyanides, little effort has been made to prepare insoluble metal ferricyanides and exploit their ion-exchange pro perties. The sole reference of such compounds available till recent past was that of Ganzerli et al who studied zinc ferro and ferricyanide vis avis their exchange properties. Chromium ferro ii and ferricyanide were prepared in these laboratories a few years back and their exchange characteristics were explored. Since the ferrocyanides of titanium have already been investigated, the ferricyanide salt of this metal ion has been prepared and explored under varying physical conditions and in this dissertation the exchange capacity, selectivity and separation potential of the product is being reported. A number of samples of titanium ferricyanide were prepared under different experimental conditions (Page-3i) and the sample (4A, Table-i, chapter-II) having maximum exchange capacity (0.18 meq g-1) has been employed for all subsequent investigations. The chemical analysis of this sample in H+ form gives a Fe/Ti ratio of 0.387. Efforts were made to establish a chemical formulae of this product on the basis of analysis data and the one approxi mately close to the reported analytical values is; K(T10)2(0H) [TlFe(CH)J 2 X-ray diffraction studies reveal the product to be amorphous in nature. The exchange capacity of the product decreases with ftdii hydrated lonic^of the exchanging ion and the values are; L1+ = 0.09; Na+ = 0o15; K+ = 0.18 meq g"1 The selectivity of the compound as obtained in terms of distribution coefficient values (Krf) follows the following sequence: Rb+> Cs+> Ag+; Pb2+> Mg2+> N12V Mn2+> Zn2+> Hg2V Ca2+> Ba2+> Sr2+>Co2+>Cd2+; Fe3Vcr3V Al3Vl_a3+>Bl3+; ThA+ (Page39, Table-5) iii Some binary separations viz., Cd2+ from Rb+; Mn2' from Rb+; Cr3+ from Rb+ etc. have been achieved on the columns of this compound. All the separations are clean and quantitative. It 1s possible to use the material for six to eight cycles without any loss in its adsorption capacity or separation efficiency. Hydrous oxides behave as 1on sorbers due to the presence and pH dependent dissociation of surface hydroxyl groups. These behave as anion or cation sorbers 1n acidic or basic meditJ and sometimes lose their exchange characteristics beyond certain pH. To minimise the drawbacks associated with the ion-exchange properties of hydrous oxides, mixed oxides containing more than one type of cation have been prepared and investigated. The induced sorption property depends on the extent of the formation of mixed oxide. Mixed oxides of Ti(IV)-Fe(III) have been prepared and their structural and adsorption properties have also been investigated. The structure of mixed oxides of the two metal Ions have some resemblance with that of its constituents. Titanium oxyhydroxide have a general formula TiC^'n H20 . The hydrous oxides, Tl02*nH20, are obtained by addition of abase to Ti(IV) solutions. Titanium dioxide exists in three crystalline modifications i.e., rutile, anatase and brookite. All these occur in nature. Naturally occurring iron oxyhydroxides show great varience in their structure compositions and the degree of crystal 11nity depen ding on the conditions of formation. Four important modifications of iron oxyhydroxides designated as of-FeOOH(goethite)y^ -FeOOH iv (akagenite),Y-FeO0H (lepidocrocite) and^f-FeOOH are known either as naturally occurring minerals or as synthetic preparations. Since the structural character and hence the exchange behaviour of mixed oxyhydroxide gels not only depend on the nature of its constituents but also on their atomic ratio and conditions of pre paration, efforts have been made to prepare the mixed oxide gels having optimum sorption properties. Many samples of mixed oxides containing different titanium to iron ratio were prepared and those having Ti-Fe ratio of 3:1, 1:8 and 5:5 respectively were selected, in view of their good sorption and exchange characteris tics, for further investigations. Various types of techniques were used for the characterization of mixed hydroxides. X-ray diffraction studies revealed the material to be largely amorphous 1n nature. Based on the experimental results the following conclusions have been drawn: 1. The bulk composition of pure titanium gel is given by T102#1.68H20 2. The bulk composition of mixed titanium-iron hydrate gel can be represented by Fe3-nTin°i, 3'58H2° 3. Both the pure as well as mixed titanium-iron hydrate gel are antiferromagnetic at room temperature. Pure gal possesses an Inhomogeneous structure characterized by the simultaneous presence of TiO(OH)2*nH2° and T10 *1.68H2<) with brooklte like characteristics. Mixed hydrate gel consists of TiO(OH) and Fe2Ti0^3.58H20 k. The structure of both gels may be depicted by the distorted octahedral closepacked stackings of 0, OH and HO 1igands. l 5. The general formula of mixed titanium-iron hydrate gel can be given as T10(OH)2Fe>nTinO/j-3.58 H20 (1>n>0) Besides the characterization of pure and mixed hydrous oxides we have studied their surface properties and also observed the doping effect of Sn(IV), Ce(IV) 6- Ce(III). Ti-Fe mixed and Sn(IV), Ce(IV) 6- Ce(III) doped oxides have been prepared under optimum conditions to get the product of maximum stability and sorption capacity. It is observed that mixed oxide prepared in molar ratio of 8:1 and 9:1 show maximum cation exchange capacity and therefore chosen for detailed Investigations. For the gels having Ti-Fe ratios 8:1 and 9: 1 a constant value of cation exchange capacity (0.22 meq/g) is obtained, while the one doped with Sn(IV), Ce(lv) and Ce(III) have exchange capacity equivalent to 0.65, 0.57 and 0o1j5 meq/g respectively. A comparative study of distribution co efficient and maximum sorption values of various cations on mixed and doped samples heated to 80° and 800°C are given in chapter (IV). The surface characteristics of doped and mixed oxides decrease sharply when the samples are heated to 800 C. Besides the structural and surface properties these compounds provide an appropriate matrix for the preparation of membranes to be used for ion activity measurements. As such it was considered worth while to prepare the membranes of titanium ferricyanide, titanium-iron mixed as well as doped oxides to observe their electrovi chemical performance. Homogeneous membranes of these compounds could not be prepared. As such heterogeneous membranes using araldite as binder were prepared and relevant functional properties were determined. Water content, porosity, swelling, electrolyte absorption and conductance data of titanium ferricyanide membrane are given in chapter-V. With the exception of potassium, the water content decreases with increasing radii of the ions present in membrane. The lower value of porosity and the extent of swelling suggests that diffusion through this membrane would be dependent more on the exchange sites rather than porosity. The membrane, under investi gation, may therefore, be expected to show better selectivity. Specific conductivity data for titanium ferricyanide membrane exhibits the following sequence; Na+>K+> Li+>Tl+>Rb+>Cs+ and Ba2+>Sr2+>Ca2+>Mg2+. Investigations with titanium ferricyanide membrane reveal that the same can be used for the estimation of K ions, 1n the concentration range 10~1 to 10 M. This electrode assembly can also be used in 30% non aqueous (alcohol, acetone) media also. The performance of the electrode system has been observed in the presence of various interfering ions and its selectivity for potassium ions over other cations has been determined by separate solution method. Sodium ion causes a serious Interference while other monovalent cations would interfere if present 1n larger amounts. Bivalent and trivalent ions would cause absolutely no interference. vii T1-Fe mixed hydrous oxide membrane exhibits a little higher value of water content, porosity, swelling and electrolyte absorp tion 1n comparison to Sn(IV) and Ce(IV) doped membranes. The conductance in various ionic forms of Ti-Fe mixed, Sn(IV) and Ce(IV) doped membranes are given 1n chapter-V, With few exceptions the conductance decreases with increasing ionic radii and also increases with Increasing external solution concentration. This may be attributed to the fact that Donnan exclusion becomes less effective at higher solution concentrations thereby leading to a higher value of membrane conductance. In view of the fact that Ti-Fe mixed, Sn(IV) and Ce(IV) doped gels give very high values of Kd and Q for Ba2+ ion, their membranes have been tried for measuring barium 1on activity. It is found that a A0%araldite supported membrane of Ti-Fe mixed and Sn(IV) & Ce(IV) doped oxide forms a good electrode for the potentlometric estimation of Ba2+ in the concentration range 10"1 to lO'Sl. The response of the electrode has also been Inves tigated 1n partially 30% non-aqueous media and a linear relation ship is observed 1n the concentration range 10~ to 10" M. The performance of the electrode system is also observed in the presence of various Interfering ions. In case of mixed and doped hydrous oxides membrane all the monovalent ions cause serious interference. Bivalent and trivalent ions have very low values of selectivity coefficients. It is further observed that the Sn(IV) doped membrane functions 1n a much better way, out of the three, as far as the selectivity of barium ion is concerned.