PHYSICO-CHEMICAL STUDIES ON INORGANIC COLLOIDAL PRECIPITATES

Abstract

Colloidal and surface properties of some inorganic com pounds, possessing some specific morphology, have assumed importance due to their use as ion exchangers and these also provide a suitable material for the formation of membranes to be used as electrodes, or in desalination of water or in the treatment of industrial effluents. Metal ferrocyanogen compounds are the first amongst the metal complexes which were comprehensively studied from the colloid-chemical point of view Studies on their composition, physical characteristics and structure were initiated by Malik and coworkers ( References 9-11 Chapter I ). Ion exchange and membrane properties, of some of these compounds and their use in waste treatment has also been recently explored ( References 30-33 Chapter I ). Compounds like metal arsenates, molybdates, tungstoarsenates etc. have also been investigated by Malik and Srivastava (References 34-39 Chapter i) for their colloid-chemical, ionexchange and membrane properties. Zirconium compounds with a variety of polyanions have attracted the attention of workers in this field ( References 16,19,20 Chapter I ). A survey of structure reveals that metal molybdates or phosphomolybdates etc. have not been comprehen sively studied and no consistent data is available on these compounds. -ii- Zirconium molybdate has been prepared by various workers and is found to exhibit cation sorption properties, zirconium molybdophosphate too has been investigated but the stability as well as exchange characteristicsfreported by various work ers, are not promising at all. Exhaustive investigations on the colloidal properties of zirconium molybdate and octacyanomolybdate and ion-exchange as well as membrane properties of zirconium molybdate and molybdophosphate have been undertaken and the results are reported in this dissertation. Colloid-chemical behaviour of the two zirconium compounds viz. zirconium molybdate and zirconium octacyanomolybdate have been investigated to know the nature of the double layer as well as to evaluate various viscometric constants which affect the gel formation ( Chapter II). Preliminary experiments were carried out to prepare stable zirconium molybdate and zirconium octacyanomolybdate colloidal solutions by adding zirconium oxychloride to sodium molybdate and potassium octacyanomolybdate respectively, in different stoichiometric ratios. It is observed that different products ranging from soluble complex to colloidal precipitates and gels are formed and in both the cases with a little excess of zirconium oxychloride, a positively charged colloidal solution is obtained. Zeta- potential values are determined by measuring the cataphoretic velocity of the colloidal particles by a modified Burton's apparatus. The values of the same are found -lllto be 25.5 mV and 40.9 mV for zirconium molybdate and zirconium octacyanomolybdate respectively• Since the sols are positively charged the effect of potassium salts of various anions ( viz. Cl~, SO - and Fe(CN),") on the electrokinetic potential and flow behaviour of the sus pension is investigated. Particle- particle interaction forces and mode of particle association existing in suspensions is reported in terms of viscumetric constants viz. specific vis cosity ( Q__), intrinsic viscosity^ 1^3 •> axial ratio (j), sp interaction index (k ) etc. In both the sols an increase in intrinsic viscosity and axial ratio values with the concentra tion of the electrolyte added show that at higher concentrations larger units of higher disymmetry are formed. On the other hand decreasing values of (k ) indicate a reduction in the hydrodynamic and electrical interactions existing between the suspended units. The effect of the three electrolytes on zeta-potential and viscosity of both the sols follow the sequence FeCcN)^ " > SO ~>C1~. Gel formation of zirconium molybdate and zirconium octacy anomolybdate has been investigated by observing the time of ge- 2+ 2- 1 ation against the various stoichiometric ratios of ZrO /MoO. and ZrO j Mo(CN^~. It is observed that zirconium molybdate gel is formed when ZrO2 +/MoO2. ratio is 1.*1, while zirconium 2 ♦ octacyanomolybdate gel is formed at 2 ." 1 ratio of ZrO / Mo(CN)4"" . 8 -iv- It is observed that zirconium octacyanomolybdate does not possess desired stability and hence its ion-exchange and sor ption properties could not be investigated Zirconium molybdate has so far been used as cation exchanger ( References 32-^6 Chapter III). Earlier workers have prepared this compound in basic medium ( pH > 6.0) by adding zirconium oxychloride in higher concentration than molybdate and found it to possess cation exchange properties. However, when we prepared the com pound by adding equimolar concentrations of the two reagents (pH 6»0) it is found to possess anion exchange characteristics. It is both chemically and thermally stable. The chemical and thermogravimetric analysis of the product » X-ray and IR spectra are also recorded* The product is amorphous in nature and the emperical formula is found to be ZrO(MoO. ). 2.5 HpO . •7.C. The anion ( CI ) uptake capacity is found to be 2.0 meq/g , which decreases with rise in temperature. The distribution coefficients (K.)of various anions as d determined by spectrophotometry, titrimetric and radiometric methods are given in Table III (chapter III). Among the various monovalent and polyvalent anions the product exhibits maximum 2- 2- affinity for SCN , SO. and MoO. . The mechanism of uptake of CI as a function of nitrate ion concentration is found to be non-stoichiometric On the basis of distribution coefficient measurements, various separations of analytical and radiochemical interest have been performed on the columns of this exchanger material. For example MoO. ions have been seperated with other _vanions viz., Cl~, Br", SCN~, P0^~ etc. with 100%" recovery. Many other binary separations of various anion pairs are also po ssible with the help of this exchanger material. Another zirconium compound belonging to heteropolyacid salts viz. zirconium molybdophosphate has also been reported as a cation exchanger by Vinter et al and others ( References 44 and 45 Chapter III ). However the product has not been fully investigated for the sorption of a wide variety of cations, their selectivity order and separations etc. They have pre pared this compound at pH > 8 in presence of NHjNOj. We have taken a higher concentration of 12-molybdophosphoric acid and zirconium salt and prepared this compound at a low pH < 1. The compound is found to possess better stability and cation exchange characteristics in comparison to what has been report ed earlier. X-ray analysis of the compound reveals its poorly crys talline nature. The emperical formula of the product, as obtain ed by chemical analysis, TGA etc. is ( ZrO ) (MO-^PO.q^ ' 2^H20, The exchange capacity in the H form of the compound is 0.42 meq/g . The K, values for various cations ( Table VII, Chapter a >. . + III ; show that the product exhibits a great affinity for Ag , Tl+, UOp and Th , For other mono-, bi** and trivalent cations the K, values are quite low. The non-stoichiometric nature of Q the uptake of some monovalent ions (Cs , Tl , Ag ) on this exchanger is revealed by investigating the variation of -vidistribution coefficient values with nitric acid and ammonium nitrate concentrations. The high selectivity of the exchanger for above mentioned cations prompted us to carryout their separations with other cations on the columns of this exchanger material. It is ob served that Tl can be selectivity separated from all other cations with the help of this exchanger material and the re covery is hundred percent. Besides this, some separations bet ween cation pairs, having low separation factor, could also be carried out fruitfully ( Chapter III ). With the development of ion*-selective electrodes and importance of ion-exchange membranes in various fields like water desalination, reverse osmosis etc. it was felt necessary to prepare and study the membranes of these two exchangers viz* zirconium molybdate and zirconium molybdo phosphate . Attempts to prepare homogeneous membranes of these two products with desired stability failed. As such heterogeneous membranes using polystyrene and araldite as binders were pre pared and different functional properties of the same were de termined. Zirconium molybdate membrane exhibits little higher values of water content, porosity, swelling and electrolyte absorption in comparison to zirconium molybdophosphate membrane. Nevertheless, the swelling and porosity values are quite small in both the membranes suggesting that interstices in membranes are negligible and diffusion through them would occur mainly through exchange sites of the membrane material. The conductance -viiof various anions through zirconium molybdate membrane( possess ing anion exchange characteristics) and cations through zirconium molybdophosphate membrane ( possessing cation exchange charac teristics ) follow the sequence ." NO" >Cr042~ >S0^~> P0^~ >Fe(CN)^" and CI" >Br" .... for zirconium molybdate ♦ •' ' ♦ + + + . „ 2+n 2 + and Li > K > Rb > Cs > Na and Ca > Ba .... for zirconium molybdophosphate With few exceptions the conductance decreases with in creasing ionic radii. Results obtained are explained on the basis of the model put forwarded by Alberti and Torracca. These results have also been compared with the membranes of the com pounds of similar type reported by other workers (Chapter IV, Section a). Further investigations were carried out by observing the permeability of ions and membrane potential of these mem branes ( Chapter IV, Section b). The permeability experiments could be performed only with zirconium molybdate membrane. The less porous and non-swelling zirconium molybdophosphate mem brane does not permit sufficient diffusion of ions to get meaningful permeability data. The permeability values obtained for various anions with zirconium molybdate membrane at different temperatures show the same order as obtained in the conductance measurements -viiiof this membrane in various anionic forms. The thermodynamic parameters ( viz. energy of activation E , &F, £H and -AS ) of the process of diffusion as evaluated by applying the theory of absolute reaction rates have been used to explain the di ffusion of anions across this membrane, A positive value of AS, the entropy of activation, for all the anions, reflects that a greater region of disorder arises when a molecule is diffusing throuyh the membrane phase. Membrane potentials generated when the membranes are in contact with 1 .* 1 electrolytes, follow the sequence Cl"> NO" 3 in the case of zirconium molybdate and Tl+ > Cs+ > K* > Li+ in the case of zirconium molybdophosphate membrane. In both the membranes the order remains practically constant over the con centration range studied. The transport number of the permeating ion in membrane phase and the permselectivity of the two mem branes have also been evaluated. The transport number of anions as well as the permselectivity of zirconium molybdate membrane decreases with increase in external electrolyte concentrations while cation transport and the permselectivity of zirconium molybdophosphate membrane increases with increase in external electrolyte concentration. In the latter membrane the two parameters reach a maximum value and then decrease again. This indicates that the equilibrations of the zirconium molybdophos phate membrane takes place only at certain optimum concentra tion and beyond this the values of the two parameters decrease again due to co-ion transfer. The fixed charge density of the -ixtwo membranes has also been evaluated. These are found to depend upon the concentrations of solutions separating the two membranes (chapter IV, Section b). It has already been discussed ( Chapter III) that zir conium molybdate ana zirconium molybdophosphate show great selectivity for some ions. As such it was considered necessary to investigate their use as membrane electrodes (Chapter v) for ions for which these exchangers show high exchangeability. It is observed that *\5% polystyrene supported zirconium molybdate membrane forms a good electrode for the potentiometric estimation of molybdate ions in the concentration range 10"^ to 5 x 10"^ M,inspite of the fact that membrane does not show Nernstian behaviour ( Anal. Chem.,J54_, 1399(1982)). The working pH range of the electrode is found to be 6.5 to 9.0. The response time is less than a minute and remains stable for mo re' than 15 min. A large number of anions viz. Cl~, Br , I , S027—~, WO 2 ~, CrO^?r - do not interfere with the working of the electrode even if present in large amounts. The electrode has also been successfully used as end point indicator in poteniometric titrations involving molybdate ions ( MoO.~) with titanium (iv) tetrachloride and cerium (iv) ammonium nitrate. 30$ araldite supported zirconium molybdophosphate mem brane electrode is found to be selective to thallium ions. Although the potentials generated across the membrane are less than the Nernstain values it could still be used to determine -Xthe activity of Tl ions over a fairly wide concentration range ( 10" to 2 x 10 m). The working pH range of the electrode system is 4 to 6- A large number of mono-, bi, and trivalent cations do not interfere even if present in equi valent amounts. The electrode could also be used for titrations involving thallium ions. The two electrodes can also be employed in partially non-aqueous medium.