The removal of any trace contaminant by ion exchange depends as much or more on the other ions that surround the trace than on the concentration of the trace itself. This fundamental principal is true, no matter if the trace contaminant is highly preferred, such as uranium or perchlorate, or not preferred at all, such as fluoride or nitrite. The fundamental reason why this is true is the concentration difference between the ion exchange resin — cation resins are around 100,000 ppm TDS, while anion resins are a bit less — and the water being treated (typically 500 ppm or less). This article focuses on the basic theory of trace ion removal.
Examining ions
Assume for the moment that an ion exchange resin has equal preference for every different type of ion. They don’t of course, some ions are much more preferred than others, but what causes selectivity is a different subject not necessary for this discussion.
In order to make some simple calculations we need a few simplifying assumptions.
1. Resin concentration is something like 100,000 ppm (as CaCO3). It’s not quite that simple, anion resins tend to be close to 70,000 ppm, cation resins (especially WAC type) can be higher.2. Resin starts out being close to 100 percent in one particular ionic form, generally sodium for cation resin and chloride for anion resin.
3. The water to be treated is 100 ppm TDS (as CaCO3), including the trace contaminant. TDS can vary of course, 100 ppm was chosen simply because 100 is an easy number to multiply and divide by.4. Water contains 1 percent (or less) of some trace contaminant that we don’t like and want to remove. The mathematics of ion exchange becomes increasingly complex when a contaminant concentration is significantly more than a trace.
Assuming the resin’s preference for sodium and the trace contaminant are equal, how much of the trace contaminant will concentrate on the resin? The answer is fairly simple — the trace contaminant will concentrate to 1 percent of the resin’s exchange sites, the same relative composition as the water. Yes, this assumption glosses over a number of issues including equivalencies, later we’ll include them but for now assume everything is already on an equivalent basis.
It’s a numbers game
One percent utilization of a resin’s capacity might not sound like such a good deal but keep in mind the resin concentration is 100,000 ppm and the water is only 100 ppm (99 ppm TDS plus 1 ppm of the trace). The resin will therefore process 1,000 volumes of water per volume of resin (100,000/100 = 1,000) before it is saturated with the trace contaminant. One thousand bed volumes capacity (7,480 gallons per cu. ft.) isn’t too shabby for a contaminant that isn’t preferred over the bulk ions in the solution. The first principal of trace contaminant removal is that it isn’t absolutely necessary that the trace contaminant be preferred by the resin. Trace contaminants will be concentrated by the resin, provided they are not present in the resin to start with and that the TDS of the water is significantly lower than the TDS of the resin.
OK, time for a bit of math to help illustrate the relationship between the resin and the trace. What happens if the concentration of the trace contaminant is half as much (0.5 ppm)?
The resin concentration stays at 100,000, but the solution concentration drops to 99.5 ppm. Thruput changes from 1000 bed volumes to 1005 bed volumes (hardly at all). This is one of the most important things to consider when thinking about thruput and trace contaminants. So long as the contaminant remains a trace, thruput is rather insensitive to the concentration of the trace.
Point one: Thruput is insensitive to the concentration of the trace.
What happens if the solution concentration doubles from 100 to 200 ppm? Again the resin concentration remains unchanged but the total solution concentration changes from 100 ppm to 200 ppm. Thruput changes from 1000 bed volumes to 500 bed volumes.
Point number two: Thruput is extremely sensitive to TDS.
This little exercise illustrates the reason why TDS is at least as important as the concentration of the trace contaminant itself.
Complications that stir the plot
Before going any further it is necessary to introduce a couple of plot complications.
The first complication is the simplifying equation doesn’t always apply. The equations that govern the way that ions exchange into and out of a resin are more complicated than the simple one used to illustrate the principle. When the trace contaminant is 1 percent or less of the total TDS, the error is generally small; but, as the trace contaminant builds up to a higher and higher concentration in the resin, the error gets bigger and bigger. Beyond about 10 percent, the error becomes too large to use the simplifying assumption safely. If the simple approach is used to estimate the capacity for ions that are more than a trace, huge mistakes can easily be made.
A second complication is TDS often changes apparent selectivity. The simplifying assumption completely overlooks the relationship between divalent and monovalent ions. Ion exchange resin preference for divalent ions is concentration dependent, higher concentrations favor the monovalent ion while lower concentrations favor the divalent ion.
The importance of TDS on apparent selectivity is very significant in the case of uranium removal. Uranium forms a tetravalent anion that strong base anion resins have extremely high preference for at low to moderate TDS. But when the TDS gets up over about 500 ppm, the preference drops rapidly. At TDS above about 2000 ppm the removal of uranium by strong base anion resins becomes problematic.
The third complication is that the trace contaminant is not always the most preferred. What happens when the trace contaminant is not the most preferred? Here it is probably necessary to remove the most preferred contaminant along with the trace contaminant. Thruput might be drastically reduced if the more preferred contaminant is present at a meaningfully concentration. How well the trace contaminant continues to be removed once the resin is saturated by the preferred contaminant depends on relative concentrations and preference. In some cases the resin will continue removing the trace contaminant, in others it might dump the trace in favor of the one more preferred. Chromatographic effects such as dumping are probably best left to simulator programs that use mathematical models to calculate resin and liquid compositions across a resin exhaustion profile.
That’s as far as we can go in an introductory article. The main point that readers should hope to understand is that TDS, the mix of ordinary ions and the concentrations of those ions present in water are at least as important as the concentration of the trace contaminant itself. Consequently, the sizing advice a water treatment professional is able to provide is only as accurate as the water analysis provided to them.
A brief introduction to long life resins
A few words need to be used to introduce a couple of basic caveats regarding long life resins. Long life resins are those that have thruputs in the tens of thousands of bed volumes or more and that stay in service for months or years before they become saturated with a trace contaminant. Long life resins generally share a few common characteristics and are susceptible to certain types of problems associated with the huge volume of water compared to resin.
First of all, long life resins are slow kinetically, meaning they do not exchange ions as rapidly as other resins we may be more familiar with. The processes that make them selective for a particular trace contaminant generally make them slow to exchange ions of all kinds. This means that long life resins will exhibit premature leakage if they are operated at too high a flow rate. Slowing the flow rate down after the trace contaminant has started to leak probably won’t help much because it will have been pushed down toward the bottom of the resin bed where it is close to the outlet and more readily displaced by other ions in the water.
If a long life resin is part way through its exhaustion cycle and the bed is disturbed, perhaps by backwashing to remove suspended solids, the resin at the top that is exhausted gets mixed up with the resin at the bottom that is still fresh. The entire bed gets mixed up together and the leakage goes up. Since most long life resins are operated to a leakage endpoint this could mean the resin has to be replaced prematurely. The problem can be partly overcome by backwashing the resin before first use to classify the bed. Classification places the largest beads at the bottom and the smallest beads at the top. Later backwashing maintains the original classification and partly avoids the issue.
However, there’s no getting around it. If a long life bed needs to be backwashed there is a good chance it will have much higher leakage of the trace contaminant afterward and will need to be replaced sooner than if it were never backwashed. Long life resins are necessarily more sensitive to suspended solids than resins that are backwashed and then regenerated because there is no mechanism to remove suspended solids. Since most long life resins are kinetically sensitive it would make sense from a kinetic standpoint to make the beads smaller (smaller beads mean more surface area and a shorter pathway to the ion exchange sites). However, fine mesh resins are especially sensitive to plugging with suspended solids and therefore require very clean feedwater. Waters with almost any level of suspended solids will require prefiltration ahead of a long life resin. Waters with high levels of suspended solids may require several stages of filtration in order to adequately protect the resin that follows.
A long life resin that lasts 100,000 bed volumes stays in service for months or years and might be exposed to as much as a million pounds of water per pound of resin. This means that other trace contaminants might also concentrate on the resin, even if present in the feedwater at concentrations too low to measure. Long life resins are therefore susceptible to a number of fouling issues, some of which might not be easily identified. Promises of thruputs in the millions of gallons per cubic foot are rather unrealistic; the sheer volume of liquid passing through the resin makes it practically certain that something bad will happen to the resin before the theoretical thruput is reached.
But that doesn’t mean that thruput predictions for long life resins are not accurate. It simply means that the more we know about a water’s general characteristics, the better off we’re going to be.
This is probably as good a place as any to stop. In future articles, I hope to discuss the cases of specific trace contaminants and long life resins. In the meanwhile, please insist on complete water analysis before asking for sizing guidance for any ion exchange system. And don’t forget those ordinary ions such as sodium and chloride as they are just as important, if not more so, as the trace contaminant that needs to be removed.
Peter Meyers is technical director for ResinTech Inc. of West Berlin, N.J. ResinTech is an ion exchange resin manufacturer and, through its two divisions Aries and ACM Company, also offers activated carbon, inorganic selective exchangers, cartridge filters and PEDI regeneration services. Mr. Meyers has more than 35 years of experience covering a wide range of ion exchange applications from demineralizers, polishers and softeners to industrial process design and operation. He can be reached at 856-768-9600 or by email at [email protected].
