Ion exchangers are resins that are polymers with cross-linking ( connections between long carbon chains in a polymer ). The resin has active groups in the form of electrically charged sites. At these sites, ions of opposite charge are attracted but may be replaced by other ions depending on their relative concentrations and affinities for the sites. Two key factors determine the effectiveness of a given ion exchange resin: favourability of any given ion, and the number of active sites available for this exchange. To maximise the active sites, significant surface areas are generally desirable. The active sites are one of a few types of functional groups that can exchange ions with either plus or minus charge. Frequently, the resins are cast in the form of porous beads.
Cross-linking, usually on the order of 0.5 to 15 percent, comes from adding divinyl benzene to the reaction mixture during production of the resin. The size of the particles also plays a role in the utility of the resin. Smaller particles usually are more effective because of increased surface area but cause large head losses that drive up pump equipment and energy costs. Temperature and pH also affect the effectiveness of ion exchange, since pH is inherently tied to the number of ions available for exchange, and temperature governs the kinetics of the process. The rate-limiting step is not always the same, and temperature’s role is still not thoroughly understood.
Regeneration of the resin is also a feauture of ion exchange. The resin is flushed free of the newly-exchanged ions and contacted with a solution of the ions to replace them. Regeneration is initiated after most of the active sites have been used and the ion exchange is no longer effective. With regeneration, the same resin beads can be used over and over again, and the ions that we are looking to get out of the system can be concentrated in the backwash effluent, which is just a term for the spent fluid used to regenerate the ion exchanger.
Nowadays, the ion exchange substances are used almost exclusively under the name of resins. There are two categories of resins: the resins of the gel type and those of the macroporous or loosely cross-linked type. Their basic structure is identical: the macromolecular structure is obtained in both cases by co-polymerization. The difference between them lies in their porosity.
|Gel type resins have a natural porosity limited to intermolecular distances. It is a microporous type structure||Macroporous type resins have an additional artificial porosity which is obtained by adding a substance designed for this purpose.|
The exchanger is known as monofunctional if there is only one variety of radicals and it is called polyfunctional if the molecule contains various type of radicals.
Nitrate in Drinking water.
What is nitrate?
Nitrates and nitrites are nitrogen-oxygen chemical units which combine with various organic and inorganic compounds. Nitrates in concentrations above 10 ppm expressed as N* (this can be expressed as 35.7 ppm as calcium carbonate or 44.3 ppm as nitrate) are considered unsafe. Nitrates have no detectable color, taste or smell at the concentrations involved in drinking water supplies, and they do not cause discoloration of plumbing fixtures, so they remain undetectable to our senses.
Uses for nitrate.
The greatest use of nitrates is as a fertilizer. Once taken into the body, nitrates are converted to nitrites.
What are nitrate’s health effects?
High nitrate levels in water can cause methemoglobinemia or blue baby syndrome, a condition found especially in infants under six months. The stomach acid of an infant is not as strong as in older children and adults. This causes an increase in bacteria that can readily convert nitrate to nitrite (NO2).
Nitrite is absorbed in the blood, and hemoglobin (the oxygen-carrying component of blood) is converted to methemoglobin. Methemoglobin does not carry oxygen efficiently. This results in a reduced oxygen supply to vital tissues such as the brain. Methemoglobin in infant blood cannot change back to hemoglobin, which normally occurs in adults. Severe methemoglobinemia can result in brain damage and death.
Pregnant women, adults with reduced stomach acidity, and people deficient in the enzyme that changes methemoglobin back to normal hemoglobin are all susceptible to nitrite-induced methemoglobinemia. The most obvious symptom of methemoglobinemia is a bluish color of the skin, particularly around the eyes and mouth. Other symptoms include headache, dizziness, weakness or difficulty in breathing.
Treatment options for nitrate in potable water supply:
- Distillation boils the water, catches the resulting steam, and condenses the steam on a cold surface (a condenser). Nitrates and other minerals remain behind in the boiling tank.
- Reverse osmosis forces water under pressure through a membrane that filters out minerals and nitrate. One-half to two-thirds of the water remains behind the membrane as rejected water. Higher-yield systems use water pressures of 150 psi.
- Ion-exchange using 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
Sulphate > Nitrate > Chloride > Bicarbonate
The term “nitrate selective” refers to resins that retain nitrates more strongly than any other ions including sulphates. 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 sulphates to attach themselves to the resin. This reorders the affinity relationships so that nitrate has a higher affinity for the resin than sulphates even at drinking water concentrations. The affinity relationship for nitrate selective resins in drinking water is
Nitrate > Sulphate > Chloride > Bicarbonate
Activated Carbon is not effective in removing Nitrates from water.
SEPLITE® LSI 106 Plus is a special strong base anion resin developed for nitrate removal, especially from drinking water. Our nitrate selective resins have increased selectivity for nitrate and prefer nitrate over sulphate and other anions, even at very low TDS. This increased preference prevents nitrate dumping.
LSI 106 Plus has good mechanical strength and excellent resistance to osmotic and thermal shock.
SEPLITE® LSI 106 is specially prepared to be taste and odour free and is WQA certified to meet the ANSI/NSF 61 standard for potable water. This resin can be regenerated using 8-10% NaCl, 1-2 BV.