By Peter Meyers
Summary: Significant discussion has taken place recently regarding leachables and materials safety of water treatment equipment. As it relates to softeners, solvent resins—which can impart contaminants into water in the short term if not properly preconditioned—have been the key topic, but there are other long term leachable issues that bear watching.
There have been several articles published over the last few years concerning the potential for traces of low molecular weight solvents to leach out of new cation resin. The absence of solvents appears to be very important to the manufacturers of solventless resins. In the near future there will most likely be a standard set for maximum solvent concentrations in new cation resins. This will certainly benefit the resin manufacturers that sell solventless cation resins. Will it also benefit or protect a homeowner, who purchases a water softener? To answer this question, we need a fresh look at leachables, both from new and from oxidatively-stressed resins.
In the quest for low solvent concentrations, the focus has been on one type of leachable that’s rapidly purged from new resin. What about the leachables released by oxidatively stressed resins? Are we also ignoring the higher molecular weight leachables that cause objectionable taste and odor in “new” resins that aren’t recently manufactured? The purpose of this article is to help the industry refocus on all the leachables that may be present in a softener effluent.
Regeneration and residue
Of the total mass of organic leachables released over the life of a cation exchange resin, the contribution of low molecular weight solvents and other organic chemicals used in manufacturing represents a very small fraction of the total. Solvents are generally purged during the initial regeneration and conditioning of the resin prior to use. As a rule of thumb, initial solvent concentrations are reduced by at least 80 percent in the typical regeneration cycle that most softeners receive as part of the installation protocol and will continue to be reduced in each backwash and rinse cycle. The regeneration cycle also purges at least half of the other higher molecular weight organics that are always present in unused resin.
Cation resin, no matter how carefully manufactured and cleaned, contains some organic residue. The concentrations of the various kinds of organic chemicals (including solvents) are very low in clean and recently manufactured resin. Cation resin deteriorates slowly, even when properly stored. Organic residue builds up inside unused cation resin, and causes the brown shoe polish color that’s sometimes seen when a softener is first put into service. The cation resin inside the softener gradually decomposes over the life of the resin. The rate of decomposition increases with increasing temperature and with decreasing crosslinking agent added to the polymer. These leachables are usually present at such low concentrations that they can’t be measured, except by special laboratory extractions and concentration prior to analysis. However, if a resin is improperly manufactured, poorly cleaned or old, the leachable concentrations can be high enough to be objectionable, due to taste and odor.
The question of cation resin stability with respect to leachables was first studied by the nuclear power industry. Although it may seem a nuclear power plant has little in common with a domestic water softener, both use cation resin with the same chemical formula (but regenerated with different chemicals). At some nuclear plants, cation resins were found to release a large fraction of organic leachables. These were responsible for a significantly shorter than expected steam generator life at many nuclear plants.
The problem led to careful examination of cation resin leachables for both unstressed and oxidatively-stressed conditions. The method of manufacturing the polymer and degree of crosslinkage significantly affect molecular weight and concentration of leachables that form. Subsequent work demonstrated lower crosslinked resins are more susceptible to producing oxidatively-
stressed leachables than more highly crosslinked cation resins.
The work done by the nuclear power industry focused on leachables caused by oxidation with oxygen at elevated temperatures. It didn’t address oxidation of softening resins by chlorine. Although we know a lot about what happens to cation resin when it’s exposed to chlorine, little has been published about the leachables created when chlorine reacts with cation resin. To see what the leachables might be, a few simple tests were performed. Before presenting the data, it is instructive to know something about how a cation resin would be expected to decompose. Perhaps the best place to start is with a brief theoretical discussion of cation resin chemistry.
Styrene and divinylbenzene are mixed and a catalyst is added to cause the two monomers to react and form an “insoluble” copolymer. The copolymer is then functionalized by a second reaction with either fuming sulfuric acid or with sulfur trioxide. For softening purposes, the hydrogen form of cation resin is neutralized with caustic or other base. A solvent may or may not be used to swell the copolymer prior to the sulfonation reaction.
The copolymer backbone is susceptible to certain kinds of oxidation reactions, particularly cleavage of the carbon-to-carbon bonds that connect the monomers together. This leads to a variety of leachables, from very low molecular weight chlorinated hydrocarbons, to very high molecular weight “pieces” of the polystyrene backbone.
One of the simplest leachables, chloroform, always forms and may be the dominant leachable from chlorinated cation resin. Other higher molecular weight chlorinated organic compounds also form. The exact species depends on the polymer structure. Oxidation of cation resin is generally slow at room temperature in the absence of chlorine or other strong oxidants. Oxidation accelerates in the presence of iron and other catalysts.
What does this all mean to a water softener? It means that all cation resins continuously release a certain amount of leachable organic material. The cleanest of resins, if stored for more than a few months, will release organic leachables when first placed into service. These leachables can reach sufficiently high levels that they’re likely to be found objectionable by a homeowner, since they’re often colored and sometimes malodorous.
Almost all municipal drinking water supplies are chlorinated; thus, many residential softeners see at least some oxidative stress. Oxidatively stressed cation resin can contribute significant concentrations of organic leachables into product water. These leachables vary in molecular weight depending on how the resin was polymerized and its degree of crosslinkage (polymerization is performed prior to the steps that may or may not use a solvent).
However, some fraction of leachables will always be low molecular weight volatile hydrocarbons, similar to solvents that may or may not have been used during a resin’s manufacture. Of this fraction, some chloroform will always be present. Thus, a softener resin that’s exposed to chlorine will always be a source of tri-halogenated methane and other low molecular weight chlorinated hydrocarbons—by-products known as THMs. This shouldn’t come as a surprise. If chlorine reacts with natural organic acids to form THM, then what would you expect from a man-made organic acid such as cation resin?
To illustrate the point, a brand new, very clean, solventless sulfonated cation resin, was soaked in approximately 10 parts per million (ppm) of bleach overnight and the extract sent out for analysis of volatile organics. As should be expected, there was a significant concentration of chloroform that formed. We did not look for higher molecular weight organics, although they were undoubtedly also present (albeit at low concentrations).
Few drinking water supplies contain as much as 10 ppm of chlorine. The test was performed with a high level of chlorine in order to make certain that chloroform concentrations would be high enough to measure and to be able to distinguish the difference between the chloroform in the bleach from that in the resin after exposure. The concentration of chloroform expected at a typical chlorine dose of about 1 ppm would probably be much lower. However, we performed a second extraction of the same resin sample and this analysis showed a higher chloroform concentration than the first. This suggests the rate of chloroform formation may increase with long-term exposure. Certainly, other studies have shown that the general rate of decomposition accelerates as a resin becomes progressively weakened by oxidation.
Not necessarily safe
None of this means water softeners are unsafe or the low levels of organics that generally come from cation resins are dangerous. Powdered cation resins are sometimes used in medicine as adsorbents or as carriers of drugs and can be safely ingested. It does mean the concern about solvent concentrations in new resins is probably excessive, in light of the bigger picture. It also means that if we plan to use softeners in areas with high chlorine concentrations, we should be taking steps to minimize oxidative leachables. Here are some suggestions.
First, if we’re concerned about minimizing exposure to chlorinated organics and other leachables, we should consider use of more highly crosslinked cation resins. Many years ago, Robert Kunin—practically the father of ion exchange—reported relative rates of oxidation for various percentages of crosslinkage noted in Table 3.
Keep in mind that many so-called 8 percent DVB cation resins available today are really around 7 percent and that some domestic softening-grade resins are as low as 6 percent DVB. The table above can be used to determine the approximate DVB content of gel-type cation resins. If in doubt, have the resin analyzed (some resin manufacturers charge for resin analysis and some don’t). Be prepared to pay extra for higher crosslinked cation resin, as they’re somewhat more expensive to make (the premium for a 10 percent DVB resin in a typical domestic softener is probably around $50-to-$100).
A second way to reduce leachables (and extend the life of the resin) is to consider dechlorination ahead of the water softener. There is an obvious drawback to dechlorination—the potential for biogrowth—but for areas with high chlorine concentrations, carbon or other methods of chlorine removal will significantly reduce chloroform and other organic leachables.
We should work on a standardized procedure for purging leachables from resin, immediately prior to first use and after any period of non-use longer than a few hours. It should not be too difficult to design a purge feature into the automatic softener control. The addition of a purge step virtually eliminates any concern over solvents used in manufacturing and helps minimize exposure to higher molecular weight leachables.
If the water treatment industry is concerned enough to test for volatile organics from ion exchange resins that may contain solvents, a test should be devised that takes the potential for oxidatively-stressed leachables into account. Granted, a portion of the leachables are flushed during each regeneration cycle and not consumed, but the total potential generation of volatile chlorinated organics over the life of the resin is at least as important as the initial leachables from new resin.
- Kunin, Robert, Ion Exchange Resins, John Wiley and Sons Inc., New York, 1958.
- Stahlbush, J.R, R.M. Strom, R.G. Byers, J.B. Henry and N.E. Skelly, “Prediction and Identification of Leachables from Cation Exchange Resins,” Proceedings of the 48th International Water Conference, IWC-87-10, November 1987.
- Fisher, S.A., and G. Otten, “Extractables in New Resins: A Critical Look at the How, What and Why of their Measurements,” Proceedings of the 45th International Water Conference, IWC-84-70, October 1984.
- “Purgeable Volatiles,” a test report by Lancaster Labs performed for ResinTech, December 1999.
- Byers, R.G., and Y.R. Dhingra, “Sources of Sulfates, Sulfonates, and Other Leachables from New Production Cation and Anion Resins,” Dow Chemical Company.
About the author
Peter Meyers is technical manager for ResinTech Inc., a resin manufacturer and distributor in Cherry Hill, N.J. He has nearly 30 years industry experience covering a wide range of applications from de-mineralizers, polishers and softeners to industrial process design and hardware operation. A member of the WC&P Technical Review Committee, he attended the University of California-Santa Barbara and holds an associate’s degree from Mt. San Antonio College in Walnut, Calif. Meyers can be reached at (856) 354-1152, (856) 354-6165 (fax) or email: firstname.lastname@example.org