Ion exchange (IX) resins are polymers matrix on which the functional group are attached. The functional group are capable of exchanging their ions within the polymer (either positively or negatively charge), with ions of different species in solution. These resins are usually designed as granules, beads, or sheets. They appear solid, even under the microscope, but on a molecular scale the structure is quite open. The beads are typically porous, providing a large surface area on and inside them. ‘Catching’ ions occurs along with the accompanying release of other ions: this is the reason why the process is called ion exchange.
The IX resins have been used widely in domestic water to soften the water or to remove other mineral content (demineralization), but there are also use for other applications including removing out some elements like heavy metals, transition of heavy metals and alkaline earth cations.
There are five type of synthetic ion-exchanges resins which are distinguished based on their functional groups:
|Type of Resins||Characteristics|
|Strong-acid cation resins||Strong-acid resins behave in a manner of similar to a strong acid, and are highly ionized in both acid (R-SO3H) and salt (R-SO3Na) form, over the entire pH range|
|Weak- acid cation resins||Weak acid cation exchangers have a weak-acid functional group (-COOH), typically a carboxylic group. These resins behave like weak organic acids that are weakly dissociated|
|Strong-base anion resins||Strong-base resins are highly ionized, having strong-base functional groups such as (OH), and can be used over the entire pH range. This resins are used in the hydroxide (OH-) form for water deionization.|
|Weak-base anion resins||Weak-base resins have weak-base functional groups in which the degree of ionization is dependent on pH|
|Heavy-metals selective Chelating resins||Chelating resins behave like weak-acid cation resins but exhibit a high degree of selectivity for heavy-metal cations. The functional group in most of these resins is R-EDTA-Na|
Water softening is a process in which the water passes through a bed of resin containing Na+ to exchange the hard metal ions (calcium and magnesium) for sodium ions. When there is a need on removing other dissolved mineral contents (TDS), the water then is passed through a bed of resin containing H+ (which replaces all the cations) and then through a second resin containing OH– (which replaces all the anions). The H+ and OH– then react together to give more water.
At regular time interval, the resin reaches its capacity of holding hardness ions, then the regeneration process is performed by passing influent water back up through the resin to remove suspended solids, passing a regenerant solution down through the resin to replace the ions that have bound to the resin and then rinsing again with water to remove the regenerant solution. In water softening the regenerant is a strong solution of sodium chloride.
Purification (Deionization or Demineralization)
In general, most of the dissolved matter in natural water supplies is in the form of charged ions. Deionization (demineralization) of these ions can be achieved by using two resins. The water is first passed through a bed of cation exchange resin contained in a vessel similar to that described for softeners. This is in the hydrogen ion form brought about by the use of a strong acid regenerant (either hydrochloric or sulphuric). During service, cations in the water are taken up by the resin while hydrogen ions are released. Thus the effluent consists of a very weak mixture of acids. After that, the water passes through a second vessel containing anion exchange resin in the hydroxide form for which sodium hydroxide is used as the regenerant. Here the anions are exchanged for hydroxide ions, which react with the hydrogen ions to form water.
Removal/Recovery of Heavy Metals
Chelating resin generally consists of two parts: polymer matrix and chelating group. The resin has capacity to selectively binding multivalent metal ions to form complexes. Hence, this type of resins is suitable for removing heavy metals or recovery of precious metals from solution.
Floc provides high quality of IX resins from Lewatit®. We can help you to identify which suitable type of resins for your water quality solutions.