Ion Exchange resins break down due to physical attrition or chemical attack here we will focus on the mechanisms by which cation resins break down.
Physically, resins shrink and swell during exhaustion and regeneration cycles. For strong acid cation resin commonly used in commercial/industrial softeners the volume change is in the 5-7% range. This “shrinking and swelling” often referred to as “osmotic shock”, varies with the resin type. For example, weak acid gellular resins can shrink / swell up to 100% while strong base anion resins may increase 20% in volume when converted from the exhausted to the regenerated form.
Repeated shrink or swell cycles for these plastic beads eventually leads to bead breakage. Some of those pieces will get removed in the subsequent backwash cycle but if not, those pieces will still perform as functional groups and are distributed throughout the spherical structure. The typical physical attrition rate for SAC resins is 1-3% per year and if those pieces aren’t removed, they will plug the holes between the beads (or void volume) and create pressure drop and / or concerns with flow. Higher pressure drop through the bed causes more beads to break and the cycle will continue until the fines & pieces are removed. Ideally no more than 8% by volume of broken beads should be present in a resin bed to avoid pressure drop and channeling performance issues.
The chemistry of chemical or oxidative attack is well known and the mechanism is the same regardless of the oxidant or its relative oxidative power. Chlorine, chloramine, ozone, peroxide, hypochlorite, chlorine dioxide, nitric and other acids will all cause degradation by attacking the weakest bond in the resin.
That weakest bond is where the functional group attaches to the backbone. Once the nuclear sulfonic acid functional group is cleaved off the S-DVB matrix, a water molecule rushes in to replace it. This increased internal moisture of the resin (also called water retention capacity or WRC) causes the beads to become weaker, kinetic performance is slowed leading to reduced operating capacity and increased leakage. Further, high WRC can make SAC resins more compressible which may in turn lead to channeling, poor distribution of regenerant and short runs. Since DVB is the chemical that gives the S-DVB copolymer strength we studied the effects oxidants have on WRC as it relates to DVB. In the initial lab testing, resins were immersed in a room temperature bath of 3% solution of hydrogen peroxide and stirred to ensure complete contact with the resin. Though it is known that oxidants will attack organic chemicals, the initial 4-hour test using no metals in solution showed very slow attack.
The lab test was then revised where 6% H2O2 was used in conjunction with 1,000 ppm iron. Iron, and other transition metals (such as Cu, Zn, Mn, etc.) catalyze (or make easier) oxidative attack by speeding the decomposition reaction of hydrogen peroxide by iron (aka Fenton reagent) forming very reactive free oxygen radicals in that process. Those radicals then attack the first organic molecule encountered, namely ion exchange resins. Clearly, as DVB level in the resin increases the percent gain in WRC decreases. The 7% SAC resin faired the worse showing nearly a 20% gain in WRC while the highly cross-linked (~16% DVB) macroporous SAC exhibited a modest 5% increase in internal moisture.
In general, all ion exchange manufacturers recommend the influent water have less than 1.0 ppm oxidant entering the ion exchange column. If oxidant levels are kept to this level (or lower), then average resin life will be attainable (see Table IV). There
are exceptions to the rule as chemical reactions depend on temperature as well the presence of metals which act as a catalyst speeding up the rate of oxidation. With the presence of metals present in the feed water and exposure to oxidants it becomes very difficult to predict and or guarantee the useful service life of an ion exchange resin.