Editor’s note: One way our readers can connect with water treatment experts to troubleshoot their water challenges and questions is by posting comments to articles found on WaterTechOnline.com. Recently, a reader did just that. A new comment was posted for the article, “Removing nitrates with ion exchange.” Below is the actual question and an answer provided by ResinTech's Frank DeSilva.
Question: “Dear sir, want to know the reaction chemistry held while removing nitrates from drinking water by the resin exchange process.”
Answer: The ion exchange process for the removal of nitrates from drinking water supplies is both simple and effective. It operates in virtually the same manner as a common water softener and can easily remove over 90 percent of nitrates. The ion exchange resin process for nitrate removal uses a strong base anion resin, which is regenerated with common salt (sodium chloride). The anion resin uses the chloride ion as the exchangeable ion.
Type 1 and Type 2 strong base anion resins operating in the chloride cycle are considered non-selective because they have a greater affinity for sulfates over nitrates. For example, if a normal Type 1 or Type 2 resin is run past the end of the normal nitrate removal service cycle, then after the resin has been fully loaded with nitrates, sulfates can continue to load on the resin bed pushing off the nitrates. When this happens, the nitrate concentration in the treated water can exceed the concentration in the raw water. This phenomenon is known as “dumping.”
The dumping phenomenon can be eliminated by using nitrate selective resin. The nitrate selective anion resins are also operated in the chloride form and regenerated in the same manner.
Here’s more information from an article I wrote a few years ago for Water Technology: www.watertechonline.com/articles/being-selective-with-sulfate.
When the Type 1 and Type 2 anion resins are run in the salt form, chlorides initially replace all anions. The effluent sulfates will be near zero throughout the run. Bicarbonates will be exchanged for chlorides in the first part of the run, and then pushed off the resin in the latter part of the run. If nitrates are present, they will be low throughout the run unless the unit is run past its nitrate capacity.
In normal drinking water concentrations, because sulfate has a higher affinity for the resin than nitrate, the sulfate occupies the top portion of the bed and the nitrate, which has the second highest affinity for the resin, takes the next position. On over-exhaustion, the sulfate will displace the nitrate so that the nitrate concentration will rise quite sharply to a level in excess of the nitrate level in the raw water — dumping. pH will be reduced during the first part of the run due to the removal of alkalinity and increased above the influent pH once bicarbonates (alkalinity) begin to leak.
Chlorides will be equal to the sum of SO4, Cl, HCO3 and NO3 during the first part of the run and equal to SO4, Cl and NO3 during the latter part of the run.
Since the anion unit is salt regenerated, materials of construction may be the same as for a water softener. Also, since no caustic is used for regeneration, the influent to the unit does not need to be softened. The resin is typically regenerated with dilute brine, usually at 10 pounds per cubic foot. When operated in the chloride cycle (sodium chloride regeneration), the resin will not exchange for CO2 or silica. Recommended service flow rate is 3 to 5 gpm/cubic foot.
The term nitrate selective refers to resin that retain nitrates more strongly than any other ions, including sulfates. There are a variety of functional groups that have been placed into anion exchange resins that are nitrate selective.
Most of these resins are similar to the Type 1 resins, but they have larger chemical groups on the nitrogen atom of the amine than the methyl groups that comprise a Type 1 resin.
The larger size of the amine groups makes it more difficult for divalent ions, like sulfates, to attach themselves to the resin. This reorders the affinity relationships so that nitrate has a higher affinity for the resin than sulfates — even at drinking water concentrations.
The affinity relationship for nitrate selective resins for the major ions in drinking water is: Nitrate > Chloride > Bicarbonate > Sulfate.
The term nitrate selective resin is actually a misnomer. In fact, all of the commonly referred to nitrate selective resins are non-selective for sulfates and other polyvalent ions. This leaves nitrates as having the greatest affinity for the resin. Nitrate selective resins have larger molecular groups at the exchange sites and a greater level of porosity to enhance kinetics. Because of this, they post a lower total capacity than standard resins.
When the resin is used for nitrate removal on exhaustion, the nitrate concentration climbs gradually until it reaches the same level as the raw water. Nitrates will not exceed their influent levels. Sulfates, however, break before nitrates and will rise to levels higher than the inlet sulfate levels due to dumping. The sulfate levels can achieve a level equal to the sum of the nitrates and sulfates before coming back to the same level as the sulfate concentration in the raw water.
Frank DeSilva is national sales manager with resin manufacturer ResinTech Inc., West Berlin, N.J.