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The Process of Ion-exchange
In the context of water purification, ion-exchange is a rapid and reversible process in which impurity ions present in the water are replaced by ions released by an ion-exchange resin. The impurity ions are taken up by the resin, which must be periodically regenerated to restore it to the original ionic form. (An ion is an atom or group of atoms with an electric charge. Positively-charged ions are called cations and are usually metals; negatively-charged ions are called anions and are usually non-metals).
The following ions are widely found in raw waters:
Ion Exchange Resins
There are two basic types of resin - cation-exchange and anion-exchange resins. Cation exchange resins will release Hydrogen (H+) ions or other positively charged ions in exchange for impurity cations present in the water. Anion exchange resins will release hydroxyl (OH-) ions or other negatively charged ions in exchange for impurity anions present in the water.
Today’s modern ion-exchange resins are prepared from synthetic polymers such as styrenedivinylbenzene copolymers which have either been sulphonated to form strongly acidic cation-exchangers or aminated to form strongly basic or weakly basic anion-exchangers.
The application of ion-exchange to water treatment and purification
These are three ways in which ion-exchange technology can be used in water treatment and purification: first, cation-exchange resins alone can be employed to soften water by base exchange; secondly, anion-exchange resins alone can be used for organic scavenging or nitrate removal; and thirdly, combinations of cation-exchange and anion-exchange resins can be used to remove virtually all the ionic impurities present in the feedwater, a process known as deionization.
The first two technologies are forms of water treatment in which either the chemical nature of the impurities is changed (as in base-exchange softening) or certain impurities are selectively removed (as in organic scavenging or nitrate removal). By contrast, deionization is a purification process which can produce water of exceptionally high quality.
Softening was the first industrial application involving ion exchange. The process was first proposed by Gans in 1905. Except for certain improvements in the type of ion exchange material and the equipment, Gans’ process is still one of the simplest methods for softening water.
The process involves passing water containing hardness ions, namely calcium (Ca2+) and magnesium (Mg2+) through a column containing a strongly acidic cation exchange resin in the sodium (Na+) form (i.e. the exchangeable cations are sodium). The calcium and magnesium ions are exchanged for an equivalent number of sodium ions. The resin, once exhausted, (i.e. all the available sodium ions have been exchanged) must be re-charged. This entails passing a solution containing a high concentration of sodium salts such as brine (sodium chloride) through the ion exchange resin - a process known as regeneration.
Main Usages of Softened Water
- To prevent scale formation in boilers, water heaters, steam irons and dish-washing machines etc.
- To eliminate the production of insoluble ‘scums’ formed as a result of the reaction between calcium and magnesium ions with fatty acids found in soaps - in the textile industry, washing machines etc.
- To prevent unsightly stains on glassware, mirrors, etc.
- To pre-treat reverse osmosis feed water to prevent fouling of reverse osmosis membranes.
Organic scavengers are fully automatic plants designed primarily to remove naturally-occurring organic contaminants - mainly humic and fulvic acids - from water supplies. These are weakly-ionised compounds which can irreversibly foul normal anion resins and reverse osmosis membranes, but which can readily be removed from water by a combination of adsorption and ion-exchange.
Organic scavengers contain special macroporous anion-exchange resins operated in the chloride form. They have an open structure with large pores that allow the bulky organic anions to be removed from the feedwater and then eluted out again during regeneration.
Regeneration is initiated automatically by a clock cycle timer. The regenerant is sodium chloride in the form of a 10% brine solution which is drawn into the scavenger from a brine tank.
Nitrates are a particular hazard to infants under six months old. The nitrates are reduced to nitrites in the child’s gastro-intestinal system, reducing the capacity of the blood to carry oxygen (‘blue baby syndrome’). The simplest and most cost-effective method of removing nitrates from water is by anion-exchange, using resins operated in the chloride form and regenerated with brine. Special resins are available to treat sulphate-rich waters. (Conventional resins have a stronger affinity for sulphate than nitrate, reducing their capacity for nitrate removal).
For many laboratory and industrial applications, high-purity water which is essentially free from ionic contaminants is required. Water of this quality can be produced by deionization.
The two most common types of deionization are:
- Two-bed deionization
- Mixed-bed deionization
The two-bed deionizer consists of two vessels - one containing a cation-exchange resin in the hydrogen (H+) form and the other containing an anion resin in the hydroxyl (OH-) form. Water flows through the cation column, whereupon all the cations are exchanged for hydrogen ions.
To keep the water electrically balanced, for every monovalent cation, e.g. Na+, one hydrogen ion is exchanged and for every divalent cation, e.g. Ca2+, or Mg2+, two hydrogen ions are exchanged. The same principle applies when considering anion-exchange.
The decationised water then flows through the anion column. This time, all the negatively charged ions are exchanged for hydroxide ions which then combine with the hydrogen ions to form water (H2O).
In mixed-bed deionizers the cation-exchange and anion-exchange resins are intimately mixed and contained in a single pressure vessel. The thorough mixture of cation-exchangers and anion-exchangers in a single column makes a mixed-bed deionizer equivalent to a lengthy series of two-bed plants. As a result, the water quality obtained from a mixed-bed deionizer is appreciably higher than that produced by a two-bed plant.
The vessel can be in the form of a large stainless steel or reinforced fibreglass column containing many hundreds of litres of resin, or a small disposable/regenerable cartridge which, when exhausted, can either be thrown away or sent back to the original supplier for regeneration. The large deionizers - whether two-bed or mixed-bed - regenerate themselves automatically, in situ, when the water quality drops to a pre-set level.
Although more efficient in purifying the incoming feedwater, mixed-bed plants are more sensitive to impurities in the water supply and involve a more complicated regeneration process. Mixed-bed deionizers are normally used to ‘polish’ the water to higher levels of purity after it has been initially treated by either a two-bed deionizer or a reverse osmosis unit.
The deionizers used in laboratory applications are almost invariably small mixed-bed units containing exchangeable or disposable cartridges of resin. Large, self-generating deionizers are sometimes used in water purification systems supplying substantial volumes of water to suites of laboratories, or providing large quantities of industrial process water.