Hofmeister Effects In Ion Exchange Equilibrium On Zeolite

Zeolites have been widely applied in environmental protection, chemical catalysis, light industry, agriculture, petroleum processing because of its ion-exchange, pore shape selectivity, surface acidityand and hydrophobic properties. Zeolites have a high specific surface area and surface charge density, which lead to strong adsorption and ion exchange properties of zeolites. A large number of studies have found that the zeolites’ adsorption for different ions were different, this selective adsorption of zeolite makes it has a broad application prospects. However, the main reasons of this phenomenon are not clear and the main reasons currently proposed can be summarized as surface complexation, volume effect, ion hydration and dispersion force, classic induction force .etc. These theories cannot explain Hofineister effect of zeolite ion exchange rationally. Li Hang found that the coupling effects of polarization originate from the quantum fluctuation of diverse alkali metal ions and electric field near montmorillonite surface should be the primary reason for specific ion effect in ion exchange on montmorillonite. But what about zeolite? Whether the Hofineister effects also originate from the influence of electric field? And the degree how electric field affect ionic polarization? Which are unknown about this questions. This article selected natural clinoptilolite zeolite and ZSM-5 zeolite as research materials. The Hofineister effects of alkali metal ions and heavy metals ions adsorpted by natural clinoptilolite zeolite and ZSM-5 zeolite were researched in order to explore whether the specific ion effects come from the influence of surface electric field. Some conclusions were summarized as follows:(1) With increase of ionic concentration, exchange ionic adsorption capacity also increase gradually, but the increase gradually trend to be mild, which means the exchange gradually achieve a complete state. Hofineister effects in ion exchange equilibrium of Li, Na, K, Cs four alkali metal ions on Clinoptilolite and ZSM-5 zeolite. The specific ion sequences are both Cs+> K+> Na+> Li+. The maximum adsorption amounts of exchangeable cations in the clinoptilolite in Li-Na, Cs-Na, Li-K and Cs-K ion exchange systems were 23 cmol·kg-1,112 cmol·kg-1，35 cmol·kg-1 and 110 cmol·kg-1. And the maximum adsorption amounts of exchangeable cations in ZSM-5 zeolite were 35 cmol·kg-1，100 cmol·kg-1,32 cmol·kg-1 and 96 cmol·kg-1.(2) Take Pb(NO3)2、Zn(NO3)2、Cd (NO3)2 and Cu(NO3)2 with concentration gradient of 0.001-0.1 mol·L-1 and natural zeolite Na saturated sample to conduct ion exchange experiment. Maximum adsorption capacity of heavy metal ions are higher than Li-Na exchange system, Zn-Na and Pb-Na exchange system have bigger exchange capacity, which all greater than Cs-Na system’s maximum adsorption capacity.(3) The experiment shows that Hofmeister effect of ion exchange equilibrium in zeolite increases with decreasing of electrolytic concentration, classic effect of ion volume, ion hydration, dispersion force, induction force and surface complexation.etc. can’t completely explain this phenomenon. By theoretical calculation and comparison, the analysis shows that Hofmeister effect of alkali metal ion exchange equilibrium in zeolite originate from the coupling effect of surface electric field and ionic quantum fluctuations.(4) Natural clinozeolite and ZSM-5’s strong electric field of molecular sieve surface have extremely strong amplification effect on adion’s polarization. This kind of polarization difference is the basic power for alkali metal ions show selective adsorption in zeolite surface. Analysis shows that the volume effect in classical theory, the dispersion effect has important influence on coupling effect of’electric field-quantum fluctuationin’in natural clinozeolite surface with high charge density, and the complexity of natural clinozeolite’s structure, cause the exchange equilibrium difference between ions in natural clinozeolite surface and surface of montmorillonite and illite.(5) Volume effect (hydration volume) of ion or zeolite’s aperture has important effects on ionic exchange selectivity. Natural clinozeolite has obvious ion specific effects to the ionic exchange adsorption of heavy metals, its selectivity order is:Pb2+ Zn2+>Cd2+>Cu2+. But when four kinds of divalent heavy metal ions exchange adsorption with univalent Na ions, its selectivity coefficient is less than 1. Radius of heavy metal ions and hydrated are bigger, when spread to internal zeolite is impeded, which causes the Na+ exchange in aperture incomplete, result in the selectivity zeolite to Na+ is greater than that of bivalent heavy metal ions.(6) It is the result of comprehensive effect of many factors that zeolite’s adsorption of heavy metal ions has Hofmeister effect, among which zeolite aperture, iron volume, surface complexation effect and coupling effect of’electric field-quantum fluctuationin’ are the main reasons. Ion adsorption of zeolite surface is much more complicate than ordinary nonporous material, the adsorption selectivity can’t be described by existing theory. Ion volume is a primary factor in the research on adsorption effect of porous material surface, followed by the coupling effect of ’electric field-quantum fluctuationin’.