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12.52 CHAPTER TWELVE

sulfonates, on anion resins. These ions go on to the resin and do not readily come off.
They permanently occupy a portion of the resin's exchange capacity; this leads to loss of
available capacity.
All strong base anion resins are thermally unstable and undergo a gradual loss of func-
tionality under all conditions. This rate of loss is directly affected by the operating tem-
perature and the internal pH. The weak base resins are the most stable. Of the strong base
resin, the type I styrenic resins are the most resistant, followed by the type II styrenic
resins, and then the strong base acrylic resins which are the least thermally stable. Anion
resins used in the salt form have lower internal pH values and are significantly more sta-
ble than those used in the hydroxide or basic form.
Physical fouling can take one of two directions. Either the foulant can plug up the flow
channels between resin beads and prevent equal distribution of service water or regener-
ant flows, or the foulant can actually coat the resin bead and prevent or retard the ex-
change of ions into and out of the resin. In the former case, the effect of fouling may or
may not be noticed as increased pressure drop because the effect of channeling is uncer-
tain. In cases where the physical foulant coats the surface of the resin beads, the resin ex-
hibits a marked kinetic impairment. At very low flows, the water quality may be some-
what acceptable, but as flow is increased, the water quality deteriorates. Certain types of
chemical fouling also cause this type of flow response. The extent to which a physical
foulant is present in the resin bed can be determined by looking at the resin under a mi-
croscope of 20 to 30 power, or in many cases simply by taking a handful of the resin,
feeling it, and looking at it closely. Chemical foulants are more difficult to identify and
are frequently not visually apparent. Most chemical foulants are ions with very high se-
lectivities for the resin. They are exchanged onto the resin but are not easily removed.
Such is the case with respect to cation resin fouling by barium, lead, or aluminum. Sim-
ilarly, anions such as phosphates can significantly foul anion resins and are quite difficult
to remove during the normal regeneration with NaOH.
Another type of chemical fouling is precipitation. Precipitation can occur on the sur-
face of the resin bead, in which case it resembles other forms of physical fouling, but can
also occur inside the resin beads. Internally precipitated salts are the most difficult to re-
move from ion exchange resins and usually require successive long soaks in fairly high
concentrations of acids or bases.
Silica fouling of anion resins occurs in deionization systems where the ratio of silica
is high and the caustic regeneration dosage and temperature and contact time are not suf-
ficient to fully remove the silica from resin during the regeneration cycle. Silica is only
slightly soluble and polymerizes on the resin during the service cycle. If the regeneration
process fails to remove as much silica as was exchanged, a coating of silica builds up in
a shell around the resin bead. Silica depolymerizes at high pH, and this is accelerated with
high temperatures. So long contact times and elevated temperatures during regeneration
are helpful. Elevated regenerant dosage levels are required to ensure that enough excess
caustic, above and beyond the amount consumed by the regeneration of the resin, is pres-
ent to redissolve and remove the silica from the resin during regeneration.

Resin Leachables
Chemical discharge by ion exchange resins occurs when they begin to oxidize. In the case
of anion resins, this discharge can also be caused by thermal degradation of the ion ex-
change functional groups. In the case of oxidation, the leachables are oligomers of poly-
styrene and may not have any discrete chemical structure, except that they will generally
contain at least one aromatic group and may also contain portions of the functional groups
that happen to be attached to the part of the polymer that breaks away from the bead it-