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.
Perchlorate in Drinking water.
Perchlorate is both a naturally occurring and man-made chemical that is used to produce rocket fuel, fireworks, flares and explosives. Perchlorate can also be present in bleach and in some fertilizers. Perchlorate may have adverse health effects because scientific research indicates that this contaminant can disrupt the thyroid’s ability to produce hormones needed for normal growth and development.
What is Chlorate & Why is it an Indicator to Monitor?
- Sodium hypochlorite (NaOCl) solutions decompose slowly to produce chlorate and chlorite ions.
- The majority of water service providers use chlorine gas for disinfection but use of sodium hypochlorite (NaOCl) as a primary disinfectant is widespread.
- Regardless of the primary disinfection method used, sodium hypochlorite is often used at booster stations and also in pre-chlorination for iron and manganese removal.
- International research indicates that chlorate presents a potential health risk to consumers in that it causes damage to red blood cells. A recent study has also demonstrated perturbation of thyroid cells.
- The World Health Organisation has established a provisional guideline of 0.7 mg/L for chlorate in drinking water supplies.
- A new Australian Drinking Water Guideline (ADWG) is expected to be released in 2011. The draft of the proposed guideline released in 2009 for the first time included a chlorate health limit of 0.3 mg/L. The inclusion of chlorate into the ADWG is undergoing robust debate and may or may not be included in the 2011 revision. A limit of 0.3 mg/L for chlorite has been previously established.
SEPLITE® LSI 106 is a special strong base anion resin developed for perchlorate removal, especially from drinking water.
Its’ special matrix and tributylamine function group makes it high selective for perchlorate.
LSI 106 has good mechanical strength and excellent resistance to osmotic and thermal shock.
SEPLITE® LSI 106 selective resin has the highest possible capacity for perchlorate and will load with perchlorate even when the concentration of nitrate and other anions is quite high. The combination of high capacity and high selectivity makes SEPLITE® LSI 106 selective anion resin the most cost effective choice for perchlorate removal systems.
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.
As it is not easy handling the regeneration effluent, this resin is not suggested to be regenerated forrecycling use.