Introduction

Inorganic and organic ion exchangers consist of either a crystalline or a polymeric matrix and functional groups. Depending on the pH value of the liquid phase, these groups can either be protonated or dissociated. By this means exchangers are able to interact with ions from the liquid phase. There is a large variety of such interactions: they may be due to electrostatic and van der Waals forces, heteropolar and covalent binding, or coordination forces. The resulting sorption phenomena take place at the inner pores and/or surface of the exchangers.

The surface charge generates an electric potential normal to the surface. Consideration of the resulting electrostatic interactions has led to several theoretical descriptions, e.g. the Helmholtz, Gouy-Chapman and Stern models. Modern theoretical approaches consider the adsorption of counterions as a result of chemical interactions between the surface groups and dissolved species. Sorption of protons or of any other kind of ion is treated as a local equilibrium reaction. Specific (for protons and hydroxyl ions) and non-specific interactions lead to the formation of ion pairs at the surface that are designated as surface complexes. Different kinds of surface complexes can be discriminated.

Spectroscopic investigations of surfaces have given rise to the assumption that more than one single layer has to be assumed to account for the uptake of counterions. A typical approach used by many authors is the triple-layer model consisting of surface, inner and outer layers.

A further refinement of the triple-layer model is the approach developed by Horst. In this model individual sorption layers are credited to each kind of counter-ions. The respective theoretical approach allows the description of pure cation or anion exchange equilibria as well as of amphoteric equilibria encountered with activated alumina or activated carbon.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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