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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.23

2002, the Office of Environmental Health Hazard Assessment (OEHHA) published 6 ppb
for adults and 2 ppb for children.
The removal of perchlorate from potable water supplies is somewhat similar to the re-
moval of nitrate. In both cases, ion exchange technology is the best currently known
method. Perchlorate is even more strongly held by ion exchange resins than nitrates are.
Sulfate ions are the only major competitor besides nitrates for exchange sites on strong
base anion resins. When nitrate removal is also required, nitrate capacity becomes the lim-
iting operation and posttreatment for perchlorate removal may be required. However, per-
chlorate is so strongly held by ion exchange resins that it is difficult to regenerate using
normal regeneration methods and traditional regeneration chemicals. Sulfates are less
strongly held by the resin than perchlorates, but they are almost always present at con-
centrations several orders of magnitudes higher and, therefore, significantly reduce the
throughput capacity per cycle.
To control the amount of regenerant waste, a regenerable ion exchange resin must have
high capacity for the contaminant of interest and be efficiently regenerated. The very high
affinity of perchlorate for strong base anion exchange resins goes against this. Type II
strongly basic anion exchange resins and acrylic strong base anion exchange resins can
be regenerated with lower amounts of brine than can type I strong base anion resins. How-
ever, the problem remains of disposal of the brine and/or the destruction of the perchlo-
rate. As of March 2003, much work is being done to address this problem of brine reuse
and perchlorate destruction from the waste brines.
Recent developments in ion exchange technology have resulted in resins that are ex-
tremely deselective of divalent ions. Such resins were first commonly referred to as ni-
trate selective, because they allowed resins to remove nitrates without interference from
sulfates. Although perchlorates are much more strongly held than nitrates, the presence
of sulfates in the water can reduce the operating capacity of type I, type I], and acrylic
strong base anion resins. Further developments in the field of ion exchange technology
have now resulted in resins with even further reduced selectivity for divalent ions such
as sulfate, reducing sulfate interference in perchlorate removal by orders of magnitudes
below the original nitrate selective resins. The result is that today there exist several resin
types that are able to treat over 100,000 bed volumes before perchlorate breakthrough,
based on typical inlet perchlorate concentrations well below 1 ppm.
Typical selectivity coefficients for perchlorate compared to chloride have been reported
in various literature. There are several ways of stating relative affinities between ions and
ion exchange resins. Distribution coefficients, separation factors, and selectivity coeffi-
cients are commonly used. Each defines different mathematical relationships, but they
have in common that they represent ratio relationships for a specified pair of ions. Val-
ues above 1 mean the first ion is preferred, and less than 1 that it isn't. These relation-
ships depend strongly on the solution conditions in which they were measured; therefore
they should be used only for relative comparisons and not as absolute preference of a
resin for one ion compared to another. Table 12.1 shows the selectivity coefficients for
perchlorate versus chloride.
With this development comes the potential use of resins on a nonregenerable basis. Be-
cause of their high affinity for perchlorate compared to sulfate, these resins can be used
without being regenerated on a competitive basis when compared against regenerable, less
expensive resins, including the cost of the regenerant chemicals, the equipment for the re-
generation, the disposal cost, and the treatment cost of the regenerant brines. Because of
their high affinity, these resins pass the leach test for hazardous material so that they can
be disposed of, in most cases, as ordinary landfill. However, specific tests for the resins in-
tended for use must be conducted to ensure compliance with local laws at the time the plant
is being designed to build, as those laws may change from the date of this publication.