Environmental Engineering Reference
In-Depth Information
8
Ion exchange
Discussion is an exchange of knowledge; argument an exchange of ignorance.
- ROBERT QUILLEN
8.1
Objectives
1 Describe the mechanism of ion exchange.
2 List the types of solutions which ion exchange is capable of separating.
3 Differentiate between:
(a) Strong-acid cation exchangers;
(b) Weak-acid cation exchangers;
(c) Strong-base anion exchangers;
(d) Weak-base anion exchangers.
4 Compare ion exchange and adsorption.
5 Design an ion-exchange column via the same kinetic approach used in the design of
adsorption columns.
8.2
Background
Ion exchange is very similar to adsorption; both processes involve mass transfer from a
fluid to a solid phase. Ion exchange can be described as a sorption process, but ions are
sorbed in comparison to electrically neutral species in adsorption. An important difference
between ion exchange and adsorption is that ion exchange requires that the species removed
from the fluid phase is replaced with a species (exchanged) so that electroneutrality is
maintained. Electroneutrality requires that the total charge sorbed and desorbed is the
same. For example, two Na + will exchange with one Ca 2 + .
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