Softening with Weak Acid Cation Resin
Weak acid cation (WAC) resin is an acrylic divinylbenzene copolymer with carboxylic functional groups. Carboxylic acid is much weaker than sulfuric acid, hence the name “weak acid” cation resin. Weak acid resins are very different than strong acid cation (SAC) resins normally used for water softening. Where SAC resin selectivity coefficients (K) for calcium over sodium is around 2.5, WAC resin K for calcium over sodium is more than 20. SAC resin preference for hydrogen is at the bottom of the selectivity chart but WAC resin prefers hydrogen to every other cation.
When WAC resin is completely in the hydrogen form, it does not exchange ions, and can only be used to neutralize alkalinity and will therefore only remove cations associated with alkalinity. On the other hand, when a WAC resin is in the sodium form it is a true ion exchanger and can remove hardness more efficiently than SAC resin. In fact, sodium form WAC resins preference for all divalent ions is much higher than a SAC resin, making WAC resins a better choice for removal of heavy metals. The type of media used in most pour through pitchers that claim to remove lead from water use WAC resin (mixed with granular activated carbon).
WAC resins cannot be regenerated with salt. They must be regenerated with acid. If they are going to be used as softeners, they must then be neutralized with caustic soda or some other base to be converted to the sodium form. During the neutralization step, the WAC resin doubles in size and becomes quite hot. The complications of regeneration prevent the widespread use of WAC resins in household softeners, although they are ideal for use in single use cartridges and have potential advantages for service exchange softeners (that are regenerated at a central facility).
The advantages of a WAC softener include higher operating capacity (usually above 30 kilograins per cubic foot based on the sodium form volume), much lower hardness leakage (usually less than 0.1 ppm leakage regardless of TDS or inlet hardness), and improved removal of other divalent contaminants such as iron and lead.
The disadvantages of WAC resins include higher resin cost (at least twice regular cation resin), much higher regeneration cost, and the complications of regenerating with both acid and caustic.