THEORY OF ION EXCHANGE
Introduction:
Ion exchange technology encompasses the sciences of thermodynamics,
kinetics, ion chemistry, fluid mechanics, and economics. Understanding the finer points
of ion exchange helps to determine whether or not ion exchange will be useful for
a particular application. Furthermore,
new computer modeling capabilities are used to find new ion exchange substances 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 effectveness.
Basic Theory:
Ion exchangers are resins that are polymers with cross-linking
( connections between long carbon chains in a polymer ). The
resin has active groups in the form of electrically charged sites. At these sites, ions of opposite
charge are attracted but may be replaced by other ions depending on their relative
concentrations and affinities for the sites.
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. To maximize
the active sites, significant surface areas are generally desirable. The active sites are
one of a few types of functional groups that can exchange ions with either plus or minus charge.
Frequently, the resins are cast in the form of porous beads.
Cross-linking, usually on the order of 0.5 to 15 percent, comes from
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
because of increased surface area but cause large head losses that drive up pump equipment and energy
costs. Temperature and pH also
affect the effectiveness of ion exchange, 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 temperature's role is still not thoroughly understood.
Regeneration of the resin is also a feauture of ion exchange. The resin is
flushed free of the newly-exchanged ions and contacted with a solution of the ions to replace them. Regeneration is initiated after 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 spent fluid used to regenerate the ion exchanger.
Summary:
Ion exchange chemistry covers both production of the ion exchange substance, a resin,
and its operation, which depends upon the conditions ofthe resin 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