Ion exchange resins have long been used for nitrate removal. Generally speaking, there are two kinds from which to choose: "standard”and“nitrate-selective”. The major difference between them is the relative affinity of multivalent ions like sulfate and arsenate.
Standard anion resins.
The two types of standard resins commonly used for nitrate removal today are Type 1 and Type 2 strongly basic anion exchange resins. The Type 1 resin derives its ion exchange capabilities from the trimethylamine group. The Type 2 resin derives its functionality from the dimethylethanolamine group. The relative order of affinity for the three most common ions in drinking water compared to nitrates is
Sulfate > Nitrate > Chloride > Bicarbonate
Figure 1a illustrates a breakthrough curve for a type 2 anion resin treating a water containing nitrates.
The term "nitrate selective" refers to resins that retain nitrates more strongly than any other ions including sulfates. A variety of functional groups can and 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 such as 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 in drinking water is
Nitrate > Sulfate > Chloride > Bicarbonate
A fair number of nitrate-selective resins have been synthesized, but only two are available commercially--the tributylamine and triethylamine types. Although both the tributylamine and triethylamine resins are approved by most European countries for potable water applications, they are not listed by FDA.
Generally speaking, nitrate selective resins have from 10 to 100 times higher relative affinities for nitrates against sulfates than the standard resins. Because of this, it is the sulfate ion that would be "dumped." The phenomenon known as "dumping" occurs when nitrate concentration in the treated water exceeds the concentration in the raw water. When a nitrate selective resin is run past the point of exhaustion, the nitrate concentration of the treated water will not rise past the concentration in the raw water.
Each of the two types of nitrate selective resin has its own advantages, depending on the application. The triethlyamine structure, because of its smaller size, yields a resin with a higher operating capacity than the tributylamine type. However, the tributylamine may provide lower chemical operating costs in large systems when regenerant use is minimized through brine reclamation schemes.
Lanlang TA465D ion exchange resin for Nitrate removal