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12.6 CHAPTER TWELVE
space between the ion exchange groups, which in turn reduces change repulsion and low-
ers the resistance to penetration by the hypochlorite ion into the resin. Therefore, lower-
ing the DVB cross-linker level increases susceptibility to attack by chlorine in two ways.
The life expectancy under oxidizing conditions of a water softening resin is proportional
to the square of the cross-linking level. Reducing the DVB level by 50%, for instance,
will decrease resin life due to oxidation by chlorine by a factor of 4. Most domestic soft-
ener manufacturers use cation resins with 8.0% DVB.
Anion resins are similarly affected by chlorine except that the negatively charged
hypochlorite ion is not excluded from the polymer. Therefore, anion resins are more sus-
ceptible to chlorine attack. Since it is an anion, the hypochlorite ion is exchanged onto
the anion exchange resin. The amine group of the anion resin is the primary point of at-
tack by the hypochlorite ion. Although de-cross-linking does occur, the primary indica-
tor of degradation is loss of functionality.
Chlorine attack on the amine groups of an anion exchange resin can cause either par-
tial oxidation of the amine groups, which lowers the basicity (more weakly basic), or
cleavage of the entire amine group from the resin, which destroys the ion exchange group.
In either case the result is lower water retention values. This tends to counter the effect
of DVB cleavage. Because of the double route that oxidation takes in the anion resins,
the water retention may not immediately be affected by chlorine attack. Therefore, the
best way to measure anion resin degradation from oxidation is to track the changes in
strongly basic, weakly basic, and total capacities.
Chemical Degradation. The ion exchange groups can become impaired by an ion ex-
change reaction that is not easily reversible, or by surface clogging, which occurs when
a resin becomes fouled with oil, rust, or biological slime. When any of this happens, the
resin is referred to as being fouled. Fouling due to coating by water-soluble polymers from
clarification upsets also occurs.
The most common form of cation resin fouling is due to chemical precipitants such as
iron or barium. These can either coat the resins or form clumps of precipitated salts that
cannot be removed by the normal regeneration process.
Anion resins are commonly fouled by naturally occurring organic substances which,
though exchangeable as ions, have such slow diffusion rates that they are not fully re-
moved by the normal regeneration process unless special resins or special regeneration
procedures are used. This often makes their exchange onto the resins an irreversible pro-
cess. When this continues over a long time the resin will become impaired and perfor-
mance will drop off. This condition is commonly referred to as organic fouling.
Proper regeneration techniques and chemical procedures reduce the tendency to foul.
Some types of resins are more resistant to fouling than others, but usually there is a trade-
off in other properties so that selection of the type of anion resin for a given application
depends on several factors, such as water temperature ranges, operating efficiency ca-
pacity requirements, regeneration temperatures, and required quality levels.
THE ION EXCHANGE PROCESS
It is important to keep in mind that the ion exchange process only works with ions. Sub-
stances that do not ionize in water are not removed by ion exchange. Each type of ion
exchange resin exhibits an order of preference for various ions. This can be stated quan-
titatively through selectivity coefficients. These are similar to the equilibrium constants
in ionic equilibria. In a similar manner, the equilibrium concentrations of ions in the
resin phase and in the water phase can be calculated from the selectivity coefficient.