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12.12 CHAPTER TWELVE

Hydrogen form weak acid resins combine readily with the hydrogen ion and are very
efficiently converted back to the hydrogen form. Only about 20% excess regenerant above
the theoretical dose is needed. Hydrochloric acid as a regenerant has the advantage that
it can be used at high concentrations, which means lower waste volume. Hydrochloric
acid regenerant concentrations of 6% to 10% are typical. Concentrated hydrochloric acid
is hazardous and corrosive and gives off toxic fumes. Proper precautions must be taken
when using this chemical. Sulfuric acid cannot be used except at low concentrations,
< 0.7%, because it would form insoluble calcium (and barium) sulfate, which would foul
the resin bed.
A weak acid resin can be used in the sodium form to remove barium and hardness. This
involves a two-stage regeneration: first acid, to remove hardness, then an alkali, to neu-
tralize the bed using sodium carbonate or sodium hydroxide. Sodium bicarbonate can be
used but is less effective.
The operation of a weak acid resin whether in the hydrogen or sodium cycle is more
complex and usually more expensive than the use of salt regenerated strong acid cation
resins because of higher chemical costs, the need for acid-resistant construction, materi-
als, wastewater neutralization, and the need to deal with or strip CO2 from the product
water.

Radium
Radium 226 and radium 228 are natural groundwater contaminants that usually occur at
trace levels. A strong acid cation exchange resin operated in the sodium cycle is a very
effective method of radium removal. Like barium, radium has a higher selectivity for
cation exchange resins than hardness and will be removed during the normal water soft-
ening cycle. In a manner similar to barium, on the first cycle of exhaustion, radium will
continue to load on the resin bed until well after hardness breakthrough by displacing all
the other ions previously loaded, including calcium and magnesium. This effect is valid
only for the first cycle and can be misleading.
Point-of-use cartridges, which use ordinary softener resins on a one-time basis for ra-
dium removal, will provide decent radium removal for over 5 to 15 times (depending on
the particular resin) as long as they will produce softened water. In regenerable systems,
however, radium like barium is much harder to regenerate off the resin, and more of it
will remain in the resin after regeneration. The radium will be pushed toward the exit
(bottom) of the resin bed during the regeneration cycle. As the softener becomes ex-
hausted, hardness leakage reaches the radium-rich end of the resin bed. The hardness,
which is less preferred by the resin, will displace only a small, but nevertheless, signifi-
cant amount of radium from the resin, causing radium levels to increase to unacceptable
levels. Therefore, in systems that are regenerated it is necessary to limit the service cycle
to the softener capacity for hardness. Since the amount of radium is insignificant com-
pared to hardness, the softener design calculations are made for an ordinary softener.

ANION EXCHANGE PROCESSES

Dealkalization, sulfate, nitrate, selenium, arsenic, and uranium can all be removed by an-
ion exchange. These substances are all present as anions. The most common process is
to use salt (NaC1) regenerated strongly basic anion exchange resins operating in the chlo-
ride cycle. The design of the equipment is essentially the same for any of these substances.
Only the throughput capacities and the resin volumes are different because of the differ-