THEORY OF ION EXCHANGE
Introduction:
Ion exchange is a technology that encompasses the sciences of thermodynamics,
kinetics, ion chemistry, fluid mechanics, and economics. In understanding the finer points
of ion exchange, it can be determined whether or not ion exchange will be useful toward
whatever application may be required. Furthermore,
together with new computer modeling capabilities, new ion exchange substances can be
created with more specificity. The word specificity is a key word in ion exchange, as we
generally want to create exchangers with one unique (specific) ion in mind, which will
maximize efficiency, quality, and cost.
Basic Theory:
Ion exchangers are resins. These resins are polymers that have cross-linking, which
is basically the amount of connections between long carbon chains in a polymer. The
resin has active groups in the form of electrically charged sites. At these sites, ions, being
electrically charged one way or another, may be replaced by other ions if it is favorable.
Two key factors determine the effectiveness of a given ion exchange resin: favorability of
any given ion, and the number of active sites available for this exchange. In order to maximize
the active sites, significant surface areas are generally desirable. The active sites are generally
one of a few types of functional groups that can exchange hydrogen or hydroxyl ions for whatver
ion is desired to be exchanged. Frequently, the resins come in the form of porous beads.
Cross-linking is usually on the order of .5 to .15 percent, and cross-linking is usually accomplished
adding divinyl benzene to the reaction mixture during production of the resin. The size of
the particles also plays a role in the utility of the resin. Smaller particles usually are more effective, due
to the increased surface area, but cause large head losses that drive up pump equipment and energy
costs. Large particles aren't as effective, but are cheaper to operate. Temperature and pH also
affect the effectiveness of the ion exchange process, since pH is inherently tied to the number of
ions available for exchange, and temperature governs the kinetics of the process. The rate-limiting
step is not always the same, and how temperature's role in ion exchange for maximizing the process is still not thoroughly
understood.
Regeneration of the resin is also a feauture of ion exchange, in which the resin can be
flushed free of the newly-exchanged ions in favor of it's original structure. The regeneration is
usually desirable once a breakthrough point has been reached, which is the point at which most of
the active sites have been used and the ion exchange is no longer effective. With regeneration,
the same resin beads can be used over and over again, and the ions that we are looking to
get out of the system can be concentrated in the backwash effluent, which is just a term for
the fluid used to regenerate the ion exchanger.
Summary:
Ion exchange is a chemical process in both production of the ion exchange substance, a resin,
and in it's operation, which depends upon the conditions the resin is subjected to during use.
The mechanisms behind a given process need to be understood for an intelligent choice
to be made about whether or not ion exchange can and should be used.
There are a series of BASIC programs compiled by Dr. Henry Bungay
that are very helpful in learning and applying ion exchange.
Last update: 12/11/94