What are ionic exchange resins?
The ionic exchange resins are insoluble polymers, which have weak bonds with some ions inside them. Those ions can be exchanged with the ions of the water solutions which come in contact with the resins.
This exchange is reversible and it does not involve any structural change in the resin. One the most important strengths of this method is the resin's capability of being reused. The main application of ionic exchange resins is the water treatment, especially demineralization and softening. If the resin exchanges positive ions it's a cationic exchanger, if it exchanges negative ions it's an anionic exchanger.
The ionic exchange resins are generally prepared as little spheres with a diameters from 0,5 up to 1,0 mm, of solid consistency. They are formed by polymers with a matrix with crossed bonds with an uniform distribution of active ionic sites. This structure is sufficiently open and the solution that crosses the resin passes between the polymers' cross bonds, coming in contact with exchange sites. The cationic exchange resins have sulphonate groups in the active sites, while the anionic ones contain aminic groups.
Characteristics of the ionic exchange resins
Resins' affinity for the ions, i.e. the capability to bond in a stable and strong way, depends on the ions' dimensions and valency. The affinity is usually greater for the ions with big dimensions and high valency.
For dilute solutions the most common cations affinity order is the following:
Hg2+ < Li+ < H+ < Na+ < K+ ≈ NH4+ < Cd2+ < Cs+ < Ag+ < Mn2+ < Mg2+< Zn2+ < Cu2+ < Ni2+ < Co2+ < Ca2+ < Sr2+ < Pb2+ < Al3+ < Fe3+
A similar list for anionic exchange resins is the following:
OH- ≈ F- < HCO3- < Cl-< Br- < NO3- < HSO4- < PO43- < CrO42-< SO42-
Let's suppose that a resin has a better affinity for the ion A than for the ion B. If the resin contains the ion B and it comes in contact with a solution that contains the ion A, it releases ion B to hold ion A.
Resins differ from each others because of different properties:
- Capacity.The total capacity is the total number of the available sites for the ionic exchange. It is analytically determined by measuring the concentration of a ion in the waste solution after the resin regeneration. The operational capacity is the measure of the resin's performance in operative conditions.
- Swelling. The swelling is caused by the hydration of the ionic fixed groups and it increase with the growth of the capacity, up to the limit imposed by the polymers' net. The resins' volume changes with the conversion to ionic forms with different grades of hydration.
- Selectivity. It's the preference for the exchange of an ion compared to another one, it's influenced by the netting grade.
- Kinetics. It's the velocity of an ion exchange, it's determined by the diffusion in the film of the solution, which is in contact with the resins and by the diffusion in the resin between its particles. With low concentrations the flux is determined by the diffusion in the film of the solution, with high concentrations the controlling mechanism is the diffusion between particles. Another important factor is the dimensions of the resin's particles.
- Stability The ion exchange resin are quickly degraded by the strongly oxidizing agents, i.e. nitric acid or cromic acid. Also oxygen and chlorine can cause a damage of the resin. For this reason in the oxidizing solutions some metallic ions, as iron, manganese and copper must be reduced to minimum. In the cation exchanger the polymers' structure are mainly attacked. The cationic resins have a long duration because they contain a lot of sites which can be attacked before their properties are altered. In anionic exchanger the functional groups are attacked, so the resin's chemical properties are immediately altered. The limit of the heat stability in the anionic resins is determined by the carbon – nitrogen bonds' strength. Those bonds are influenced by pH: low pH levels make this bond stronger. Also the cations resins are influenced by pH, and the stability of the carbon - solphur bonds decreases with low pH values, however those resins are more stable and they can work with temperature up to 150°C.
Applications of ionic exchange resins
The most common applications of the ionic exchange resins are:
- ultra pure water production (UPW), that is necessary for semiconductors' production;
- nitrates removal;
- radioactive nuclides removal;
- catalysts of chemical processes, as hydrolysis, inversion, esterification, hydration and dehydration.
The first applications of ionic exchange resins in the industrial sector date back to about 1910, with the use of natural zeolites. The first synthetic materials, used as ions exchanger, based on carbon and phenolic resin, were introduced in the 30s. Some years later polystyrene resins with sulphonate groups for the cation exchange and with amine groups for anionic exchange were introduced. Nowadays those two kinds of resin are the most used.
Water softening takes place by a resin with Na+, with an affinity for Ca2+ and Mg2+ ions. When the water passes across the resin, the solid adsorbs Ca2+ and Mg2+ and releases Na+ ions, softening the water. As the ions are replaced, the total dissolved solids concentration, the anionic contents and pH do not vary. The resins used for water softening is regenerated with regularity, treating it with an high concentration sodium chloride solution. Demineralization is realized using two resins: at the beginning the water passes on a cationic exchange resin with H+ ions, which replace all the cations. The remaining solution is a weak acids' blend. Later the water passes through a second container, full of anionic resin with OH- ions, which replace all the water anions. H+ and OH-ions react to form water molecules. The cationic exchange resin is regenerated with a highly acidic solution (hydrocloric or sulphuric). The anionic exchange resin is regenerated with a sodium hydroxide solution.
Nitrates removal is realized with anionic exchange resins with chlorides.
In the larger plants, the water, leaving the first cationic unit, usually passes through a degassing tower to remove most of carbonic acid and anions. Otherwise carbonic acid would not be captured by the anionic resin after its conversion in carbonate, occupying sites useful to remove other ions. If a complete demineralization is necessary, water must pass into a third container, in which there is hydrogen cationic resin or a cationic and anionic resins' blend. The resin is exhausted when the most of its ions is replaced by the ions removed from the solution. The exhausted resin must be regenerated.
In the demineralization plants this condition is controlled by a conductivity sensor at the device's exit. When the conductivity reaches a specific value a regeneration cycle is activated. Other devices are equipped with a pH sensor. Industrial resins are usually regenerated at intervals varying from 12 to 48 hours, depending on how much they are used.
Benefits and limitations of the ionic exchange resins
The main benefit of using ionic exchanger resins is the low maintenance costs. In effect they need a little energy to work, the chemical regenerants are cheap and if a device is maintained with care, it can last even several years before it must be replaced.
However it's necessary to pay attention to some limitations of those devices, which can be controlled by some precautions. Sulfuric acid is the cheapest regenerant for demineralization plants, so it is used in all the cases where it is possible. However in some waters there is a high calcium concentration that, with this acid, precipitates as calcium sulfate. This precipitate damages the resin and blocks the pipes. In those cases the regenerant that must be used is the hydrochloric acid. The anaerobic water's underground sources often contain iron in Fe2+ form, which can be removed by sodium cationic resins. It's important to pay attention not to let waters to go in contact with air. Iron in contact with oxygen oxidises and passes in its form Fe3+, that precipitates as ferric hydroxide, which can damage the plant.
Organic matter must be removed before water go in contact with the ionic exchange resin. In fact it is adsorbed definitively by the anionic resins and their exchange capability is permanently compromised. Also the same resins can cause an organic matter contamination, however for the most uses it' negligible, but if it must be removed it can be done by membrane filtration.
Resins are not able to remove bacteria or other microorganisms from water. If it's necessary a sterile water other methods, as heat or UV, must be integrated. Resins can be disinfected with formaldehyde, but not with chlorine, that can damage them, as all the oxidising disinfectants. If the inlet water contains chlorine it must be removed, for example using activated carbons.
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