Ion exchange phenomena

O.A.G.J. van der Houwen, R.H.A. Sorel, A. Hulshoff, J. Teeuwsen, and A.W.M. Indemans, Ion-exchange phenomena and concomitant pH shifts on the equilibration of reversed-phase stationary phase materials with ion-pairing reagents , J. Chromatogr., 1981,209, 393. 

Numerous experimental and theoretical studies of problems in this field have appeared and these have been reviewed under the title Metal Com-plexing by Polyphosphates by van Wazer (342). The phenomena which have been mentioned may be considered in general as due to complex formation. However, a simple qualitative and quantitative description of the facts may also be obtained if solutions of high-molecular polyphosphates arc considered as micro-heterogeneous systems and if the bonding of polyvalent ions is interpreted as involving ion exchange phenomena on the polyphosphate chains (77, 313, 319, 324). 

For each nuclide studied, the sorption distribution coefficients appeared to result from a minimum of two separate mechanisms. In all cases, one mechanism appears to be an ion-exchange phenomena associated with the silicate phases and appears to have a relatively much larger sorption capacity than the other mechanism. In the case of cesium (and probably rubidium) the second mechanism appears to also be related to the silicate phases and may or may not be an ion-exchange phenomena. However, for the other elements studied, the second mechanism appears to be related to the hydrous iron and manganese oxides and again may or may not be an ion-exchange process. 

Also, in the late 1950s and 1960s some particularly seminal papers on ion exchange kinetics appeared by Helfferich (1962b, 1963, 1965) that are classics in the field. In this research it was definitively shown that the rate-limiting steps in ion exchange phenomena were film diffusion (FD) and/ or particle diffusion (PD). Additionally, the Nernst-Planck theories were explored and applied to an array of adsorbents (Chapter 5). 

There are some cases where a reaction, that is, the formation or dissolution of a chemical bond, is involved along with ion exchange phenomena (Helfferich, 1983). Examples of this are acid-base neutralization, dissociation of weak electrolytes in solution or weak ionogenic groups in ion exchangers, complex formation, or combinations of these (Table 5.2). With some of these, very low apparent D in ion exchangers have been noted. 

In ion exchange phenomena accompanied by reaction the increase or decrease in concentration of a species results from both mass transfer and reaction,... 

Birrell, K.S. and Gradwcll, M., 1956. Ion-exchange phenomena in some soils containing amorphous mineral constituents. / Soil. Sci, 7 130-147. 

Contrary to empirical approaches, a fundamental approach has value in that the results demonstrate the validity or otherwise of a particular mechanism or model chosen for the system. For example, the application of thermodynamics to an ion exchange system does not necessarily require the setting up of a physicochemical model, but eventually the results must still be interpreted in terms of the molecular forces acting within the system. Selected molecular models enable the mechanisms of ion exchange phenomena to be better interpreted, but their success must be measured in terms of predicted accuracy which in turn depends upon the validity of the model and the accessibility of the various molecular parameters. Ideally, the mathematical equations describing the perfect model would contain quantities which were derived from the known fundamental data for the components of the system. 

That the development of a perfect understanding of ion exchange phenomena at a molecular level should prove so challenging and elusive is not surprising when one considers the complex mechanistic steps involved, namely ... 

Twenty-two experiments using Inexpensive equipment are detailed, which not only complement a chapter dedicated to the characterization of organic exchangers, but ateo serve to illustrate several other pure and applied principles related to ion exchange phenomena. It is anticipated that the unique inclusionpf experiments and the broad coverage of the whole tew should appeal to a wide readership and offer particular relevance to practitioners in schools, colleges, and industry. 

A review of some leading semiempirical models precedes examination of physicochemical modeling of ion exchange. Such models will likely be used for the foreseeable future to describe ion-exchange phenomena in complex svstems. Thus, they represent the reference point for development of improved models. Methods of incorporating the semiempirical ion-exchange equations in general chemical equilibrium models are also described. 

The present review, however, is confined to kinetics of local, momentary ion-exchange phenomena. After a brief overview of kinetic models commonly used to interpret ionic reactions on soil constituents (essentially based on kinetic theory of homogeneous systems), current kinetic models of heterogeneous systems will be examined in order to identify the limitations of their application to inorganic soil constituents. 

LIMITATIONS OF THE NERNST-PLANCK APPROACH AS APPLIED TO ION-EXCHANGE PHENOMENA IN SOILS... 

Physicochemical reactions within the sea-sediment sphere tend to reach equilibrium. Those reactions that are so rapid that they occur prior to burial in the bottom sediments are referred to as "halmyrolysis" (e.g. formation of clay aggregates), while those that take place in the upper part of the sediment are termed "early diagenesis". The diagenetic processes include cementation, compaction, diffusion, redox reactions, transformation of organic and inorganic material, and ion exchange phenomena. A short... 

An important observation that derives from the solvent selectivity data is that water is preferred highly over alcohols by the zeolite phase. As a consequence, the zeolite solvent is essentially aquatic even when in contact with alcohol-rich solutions. Since this is the case, the osmotic model should be applicable as well to the examination of ion-exchange phenomena in mixed media which contain a high proportion of the non-aqueous material. 

Ion interaction. This suggestion is based neither on ion pair or ion exchange phenomena, though the lipophilic ions are adsorbed onto the surface but are associated with a primary ion giving an electrical double layer. The analyte then interact dynamically with this double layer by both electrostatic and van der Waal s type forces. 

Ion exchange phenomena are stoichiometric, i.e. 37 mg of potassium are exchanged by 23 mg of sodium and 40 mg of calcium by 46 mg of sodium. The ion exchange rate depends on the type of exchanger grain size, porosity and distensibility. 

---