Prediction of
Ion Exchange Equilibrium when Several
Cationic Species are Present
STUART E. SMITH, Senior Sanitary Engineer
Industrial Works Section
New York State Department of Health
Albany, New York
INTRODUCTION
In many applications of cation exchange, the system contains more than two
exchanging cations. Examples of such systems are water softening, process water
treatment, plating bath treatment, and advanced waste treatment. In the latter
application, the goal is to produce a potable water from a municipal wastewater
treatment plant discharge which has received at least secondary treatment. Depending upon the municipality's industrialization, industrial variety, and effectiveness of the waste treatment process used, the number of cationic species may
be quite large.
It is readily apparent that in the example applications of ion exchange above,
at least three ions are involved (two entering the exchanger and one leaving). A
vast literature exists of work done with binary systems (one ion entering the exchanger and one ion leaving). There are very few references to systems of three
or more cations (1, 2, 3,4, 5, 6, 7, 8, 9,10,11,12,13,14).
Equilibrium sets the limit for the use of any ion exchange process. When the
process is at equilibirum, the ion exchanger is said to be "exhausted" and must be
regenerated. Besides setting a limit for the amount of material which can be exchanged, equilibirum is important for kinetics. The rate of ion exchange is a
function of the departure from equilibirum.
When an exchanger is being used to remove more than one cation from a
solution flowing through an ion exchange column, it is the emergence of any one
cation being removed from the solution which determines when the entire column must be regenerated. The conditions of equilibirum not only determine the
amount of exchange which will occur but also which ions are preferred by the exchanger and, therefore, which ion determines the need for regeneration, the indicator ion.
In the field of wastewater treatment plant design, it is desirable to use available analytical tools rather than expensive pilot plant operations. If some way
could be found to predict behavior of multicomponent ionic systems from the
available extensive binary data (15), then considerable simplification would be
available for treatment plant design.
The system Barium-Copper-Sodium was selected to investigate the prediction
of multicationic equilibirum behavior. The three binary systems comprising this
ternary system Barium-Copper, Barium-Sodium, and Copper-Sodium had already
been investigated (16,17,18) and afforded a check on equilibirum behavior observed in the ternary system.
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