SORPTION AND MEMBRANE PROPERTIES OF SOME PURE AND MIXED HYDROUS OXIDES

Abstract

Hydrous oxides behave as ion sorbers due to the presence and pli dependent dissocation of surface hydroxyl groups. These behave as anion or cation sorbers in acidic or basic medium and sometimes lose their exchange characteristics beyond certain pH. To overcome the drawbacks associated with the ion exchange properties of hydrous oxides, metal oxides containing more than one type of cation have been prepared and investigated from both a practical and a fundamental view point. It is suggested that the replacement of metal ion in the oxide matrix by another ion of higher charge may enhance its anion and cation sorption .'ro perties. In this dissertation we have prepared iron-zirconium and cliromium-zirconium mixed oxides and have studied their surf; properties vis a vis pure oxides and also observed the dopin effect of Sn(Il). As the mixed and doped systems exhibit much better surface properties in comparison to pure oxides, these systems have been characterized completely in relation to their utility in ion-exchange adsorption and membrane phenomena. The dissertation also contains the electrochemical performance of the membranes of material under investigation. Fe-Zr and Cr-Zr mixed oxides have been prepared under optimum conditions when the products show maximum stability and sorption capacity. It is observed that mixed oxides prepared in molar ratio of 8:1 show maximum cation as well as anion exchange capacity and therefore chosen for detailed investiga tions. The Fe-Zr mixed and Sn(Il) doped oxide hydrate gels compare well with ferric oxide hydrate gel. The mixed and doped oxide gels were found to consist of ahexagonal close packed arrangements of oxygen sublattices similar to those present in a-FeOOH and cc-Fe^, with iron ion quite randomly distributed in the gels. The gels were found to be composed of hexagonal to rounded and thin rod like particles and their agglomerates. The oarticles in superparamagnetic state have incompletely com pensated antiferromagnetic character. These features support the formation of Zr-substituted ferric oxide hydrate gel as dominant phase along with some Fe-substituted zirconium oxide hydrate gel with Sn(Il) incorporation therein. Cr-zr mixed and Sn(Il) doped oxide hydrate gels compare well with chromium oxide hydrate gel. Eoth the gels were found to have a hexagonal close packed stacking of 0, OH"and H20 ligands with chromium ions distributed in octahedral sites with little degree of order among them. The microstructure of the gels are characterized by the presence of large aggregates of chromium hydroxides, fine granular sheets due to HCr02 phase and Cr(0H)3 microcrystallites. Magnetic susceptibility measurements indicate for an antiferromagnetic behaviour for these gels. These features support th« formation of Zr-substituted chromium oxide hydrate gel as dominant phase along with some Cr-substituted zirconium oxide hydrate gel with Sn(Il) incorporation therein- Cation and anion exchange ca^acit. of !3-Zr mixed oxide is 1.14 and 0.75 meq/g respectively while that of Cr-Zr mixed oxide is 1.30 and 1.03 meq/g respectively. Both the oxides are quite stable in acid and salt solutions. iii The distribution coefficients of various cations and anions on mixed and doped oxides are given in Chapter III. Among the various ions the products exhibit specific affinity for Sr ,Od and Cr cations and Cl", SO2" and PO^" anions. In general it is observed that doping the oxide with Sn(ll) enhances its sorption characteristics. The mechanism of uptake 01 Ci as a function of nitrate ion concentration is found to be non-stoichiometric. The non-stoichiometric nature of uptake of some monovalent cations (Cs+ and Tl+) is also revealed by investigating the variation of distribution coefficient values for these ions with nitric acid and ammonium nitrate concentra tions. The sorption of a number of ions on mixed and doped oxides has been characterized by the sorption isotherms to cover all possible experimental conditions at room tempe: rture. The maximum uptake of individual metal ions has also been determined (Chapter III). Among the monovalent ions studied, the sorption of Cs+ and Rb have been found nearly equivalent to the exchange capa city of the mixed oxides. The sorption sequence of monovalent ions on both the systems follow the order : Cs+ > Rb+ > Tl+ > Ag+. Doping the oxide with Sn(ll) enhances the sorption capacity and the effect is more pronounced for'Ag+. The sorption sequence of these ions on doped oxides follow the order : Ag+ > Tl+ > Rb+ > Cs4". Bivalent ions are tsi m up on the two adsorbents in -che following sequence : Sr2+ >Cd2+ >Ba2+ >Ca2+ >Co2+ >Mn2"1 >Zn2+ >Mg2+ Their adsorption parallels the order of Ionic potentials, with iv the exception of Sr and Cd • The behaviour of Sr "r is diff erent from other ions. In this case the uptake increases steadily with increasing solution concentration and isotherm does not attain constancy even at concentrations higher than 2+ 0.1 M. This observation reflects that sorption of Sr not only involves ion-exchange but physical adsorption as well. Doping the oxides with Sn(ll) improves the cation sorption capacity of the materials. The sorption of trivalent ions follows the order : Cr ' > La3+ > Fe3+ > Al3+. Here with the exception of Cr"rr, this sequence is in increasing order of ionic potentials. The effect 3+ of doping is quite interesting as the uptake of Fe on i?e-zr and Cr°+ on. Cr-Zr increases two times when the mixed oxides are doped with Sn(II). An enhancement of the same magnitude in the uptake of La3+ has also been observed on both the mixed oxide gels. The sorption of monovalent anions (Cl~ > Br > I") inverses the order of their ionic radii. Doping the oxides with Sn(II) enhances its sorption efficiency and the maximum improve ment is observed in the case of chloride ion. Among the poly- 2- valent anions, the uptake is maximum in the case of S04 ions and doping the adsorbent systems does not cause any enhancement in their sorption capacity for polyvalent anions. The availability of a variety of pure and mixed*hydrous oxides as promising .adsorbents-provides a suitable matrix to prepre their membranes and investigate the electrochemical performance. V The investigations include the characterization of zir conium oxide, thorium oxide, iron-zirconium, chromium-zirconium and Sn(II) doped oxide membranes and their use in ion activity measurements. The exchange characteristics of pure zirconium and thorium oxides have been reported by various workers (References 18-21, Chapter IV) while the exchange and sorption characteristics of Fe-Zr, Cr-Zr mixed and Sn(Il) doped hydrous oxides have already been described in preceding paragraphs and in details are given in Chapter III. Homogeneous membranes of these compounds could not be prepared. As such heterogeneous membranes using polystyrene as binder were prepared and relevant functional properties were determined. Hydrous zirconium oxide membrane exhibits a little higher value of water content, porosity, swelling and electrolyte absorption in comparison to hydrous thorium oxide membrane. The conductance in various ionic forms of hydrous zirconium oxide and hydrous thorium oxide membranes (possessing cation exchange characteristics) follow the sequence : Na+ >K+ >Tl+ >Rb+ >Cs+ >Li+ and Mg2+ >Ca2+ >Sr2+ >Ba2+ (for HZO) Na+ > K+ > Tl!" > Rb'1' > Cs+ > Li"1; and Sr2+ > mg2+ > Ca2+ > 5a2+ (for THO) With-few exceptions the conductance decreases with increas ing ionic radii' Results obtained are explained on the basis of the model put forwarded by Albert! and Torracca- (Reference 24, Chapter IV). The conductance of the two membranes in various anionic forms (possessing anion exchange characteristics) follow the sequence : VI CI" >Br" >I" and MoO2" >NO3" >SO2" >WO2" >PO3" >Pe(CU)| (for HZO) CI' >Br" >I" and SO2" >NO3" >WO2" >MoO2" >PC3" >Fe(CN)3" (for THO) Among the mixed oxides, Fe-Zr oxide membrane exhibits a little higher value of water content, porosity, swelling and electrolyte absorption in comparison to Cr-Zr mixed oxide membrane. The conductance of various anions through these two mixed oxide membranes follow the sequence : 3— CI" >Br" >I" and NO3"" >SO2" >MoO2" >WO2" >PO3" >Fe(CN)fi Nevertheless, the swelling and porosity values are quite small in all these membranes suggesting that interstices in membranes are negligible and diffusion through them would occur ainly through exchange sites of the membrane material. _The mbrane conductance increases with an increase in external solution concentration. This may be attributed to the fact that Dorman exclusion becomes less effective at higher solution con centrations thereby leading to a higher value of membrane conductance. It has been reported that /pure zirconium and thorium oxides (References 18-21, Chapter IV) and mixed oxides(Fe-Zr and Cr-Zr, Chapter III) show great selectivity for some ions. ... h,vp usod thP«* oxides for the fabrication of membrane electrodes selective to any desired ionic species. The choice of the ion to be estimated has partly been based on trie selectivity pattern of the oxide but in some cases the selection m me vii has been made by trial. Efforts were made to develop electrodes for molybdate, sodium, sulphate, strontium and chloride ions and the results are as follows : It is observed that 10 ?. polystyrene supported hydrous zirconium oxide (exhibiting anion exchange characteristics) membrane forms a good electrode for the potentiometric estima tion of molybdate ions in xhe concentration range Co to IC"3 M. (Talanta, 30, 285 (1983)). The slope of the linear plot is 30 mV. The response time is r^ 30 sec and remains stable for an hour. Standard deviation of potentials in the working con centration range is 0.6 mV while the membrane can be used for six months without observing any drift in potentials. The electrode is tolerant of pH change in the range 7-11. Alarge number of anions viz. Cl", SCN", CIO", NO " C902~, WO2", 2-3-3 d *+ o ^ 4 4 ' S04 »As04 ,P04~ and Fe(CN)£- do not interfere with the working of the electrode. The electrode system can also be used in non aqueous media and in presence of surfactants. The electrode has also been successfully used as an end point indicator in potentiometric titrations involving molybdate ions with thorium nitrate. Ten percent hydrous zirconium oxide (exhibiting.cation exchange characteristics) membrane electrode is found to be selective to sodium ions in the range 10"1 to 10"5 ;,, inspite of the fact that membrane do >s not show Nernstian behaviour (The Analyst, 000 (1984)). Trimethyl ammonium hydroxide'has been used for maintaining a constant ionic strength. The res ponse time (static) is 20^/ 30 sec. Standard deviation of potentials in the working range is 0.4 mV while the membrane can be used for four months without observing any drift in Vlll potentials. The working pH range is 7-10. Estimation of Na+ ions in the presence of other ions reveals that K+, Tl+, Li+, + - - 2- Ag and anions like Br , NO3 and SO. do not interfere at all. NH. ion would interfere if present in large concentration while H+ ion interferes at all levels of concentration. It is worth mentioning that most of the glass membrane electrodes' available in market exhibit interference by K+ and Ag+ ions. This electrode can be used in acetonic or alcoholic solutions (40-50 '/. non-aqueous content). The electrode assembly can also be used between the temperature range 25° to 45°C without apply ing any correction to it. Besides, it has been used to measure the sodium ion activity in the discharge obtained from the electrolytic cell for the manufacture of sodium metal as well as in discharge given out by a soda ash manufacturing plant by Solvay processo Fifteen percent polystyrene supported hydrous thorium oxide (exhibiting anion exchange characteristics) membrane have been used to measure sulphate ion activity in the concentration —4 range 0.1 to 10 M. Sodium perchlorate along with NI-L,0H has been used to maintain pH as well as ionic strength of the test solution. The working pH range if 6-10. Static respone time is 30 sec and the standard deviation of the data is 0.2 mV« A large number of anions like MoO2", CO2", S0o~f S2~, C0o"", W0?"\ - 2- 3- S^Oo , POT • AsOT etc. do not interfere. Monovalent anions £. O '• 4 like Cl~, B1"", I~, NC~~ can interfere if present in large amounts but the same can be removed if the observations are made after -f- 4* H- 2~f* adjusting the ionic strength. Cations like Na , K , Li , Zn , Cd2+, Mn2+, Mg2+, Al3"1" do not interfere but silver, lead and ix mercury even if present in traces cause serious interference. The electrode system is also tolerant of small amounts Of sur factants and proteins. It has also been used to estimate sul phate ion activity in wastes from tannery and paper and pulp industry. The electrode could also be used for titrations involv ing sulphate ions. Fifteen percent polystyrene supported hydrous thorium oxide (exhibiting cation exchange characteristics) membrane have been used to measure strontium ion activity in the concentration -4 range 0-1 to 10 M« The response of the electrode is Nerstian in nature. The response time is 20-30 sec and a standard devia tion of 0.6 mV is observed in the functional concentration range. The working pH range of the electrode is found to be 2.5-5.5. A large number of mono, bi and trivalent cations do not interfere with the-working of the electrode. This elec trode system however, suffers from a drawback. Even smaller amounts of anionic or cationic surfactants cause appreciable interference. The electrode has also been used as an end point indicator in potentiometric titrations involving strontium ions. Nixed oxides of Fe-Zr, Cr-Zr and the one. doped with Sn(ll) have been used with polystyrene as1'binder material for the esti mation of chloride ions. Potential vs activity plots are linear in the range CI to 10 M'{for mixed oxide membranes) and CI to 5x]0" M (for dope >xide membranes), iuspite of the fact that membranes do no1; Show Nerstian behaviour. The response time of both the electrodes is less than a minute and remains stable for more than 30 minutes. The working pH range for Fe-Zr mixed oxide membrane is 4 to 7.5 while that of Cr-Zr mixed oxide is 4 to 8. A large number of anions viz. Br", I ,no " ciC " ;aa-; CH3C00-, s2o2", c2o2", so2", Wo2", i,0o2"] po3"! aso3"! Fe(CN)3" and Fe(CN)«" do not interfere with the working of the electrodes. Besides the anion, cations like Na+, K+, Rb+, Cs+,' TIJl.+, CCa-2+ , BFtae2+ , Sqvr>2+ , umg2+ , z'n2++ etc., do not interfere even at equivalent concentration while silver, lead and mercury do interfere even if present in traces. Both the electrodes have been used as an end point indicator in potentiometric titrations involving chloride ions. The two electrodes can also be employed in partially non-aqueous media. Fe-Zr mixed oxide membrane electrode can be used in presence of anionic surfactants but Cr-zr mixed oxide membrane cannot be used. Although both the membranes (Fe-Zr and Cr-Zr) show selectivity for chloride ions the second one i.e., Cr-Zr mixed oxide membranes has a wider pH range and exhibits better selec tivity but it has the drawback that it can not be used in the presence of anionic detergents.