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Removing Nitrates With Ion Exchange Dec 22, 2014

Nitrates in concentrations above 10 parts per million (ppm) expressed as N (this can be expressed as 35.7 ppm as calcium carbonate or 44.3 ppm as nitrate) are considered unsafe in drinking water. Nitrates have no detectable taste or smell compared with ordinary substances in water, so nitrate removal processes must either be foolproof or include extensive monitoring of the treated water.

At levels which would not cause harm to adults, nitrates can cause methemoglobinemia, also known as “blue baby syndrome,” in infants. The high nitrate levels interfere with the ability of the infant’s blood to carry oxygen.

Nitrate-selective resin vs. standard resins

Ion exchange resins have long been used for nitrate removal. Generally speaking, there are two kinds from which to choose: “nitrate-selective” and “standard.” The major difference between them is the relative affinity of multivalent ions like sulfate and arsenate.

The relative affinity for some common ions found in drinking water for standard Type 1 and 2 resins is (from greater to lesser affinity): Perchlorate >> Sulfate > Arsenate > Nitrate > Chloride > Bicarbonate.

In contrast, nitrate-selective resins actually are de-selective for multivalent ions, like sulfate and arsenate. These resins have higher affinities for nitrates than all other common ions found in drinking water, including sulfates.

The relative order of affinity for some common ions found in drinking water for nitrate-selective resins is: Perchlorate >>> Nitrate > Sulfate > Arsenate > Chloride > Bicarbonate.

A fair number of nitrate-selective resins have been synthesized. The two types most widely used for potable water are tributylamine and triethylamine. Both are available with NSF certification for potable water.