• Ion exchange resins form much of the basic methods of waste water clean-up.WATER TREATMENT image by BILLY WELLBORN from Fotolia.com
  
  Ion exchange technology is widely used by industrial companies and governmental agencies to clean up waste water. The same technology and techniques are used by consumers in their homes to soften the water consumed on a daily basis. An integral piece of equipment for these practices is ion exchange resins. There are many forms of resin and types of ion exchangers available, depending on the intended applications. The specific type of exchanger selected will define the length of time required to reach equilibrium with the water treated and determine the ion content of the resulting water.
  
  

Substrates for Ion Exchange Resins

• Historically, scientists developed ion exchange resins on the surface of glass or silica beads. Chemical modification of the surface of the bead allowed the bonding of various functional groups to the surface. Each functional group on the outer surface of the beads was capable of forming an ionic compound with either positive or negative ions.
  
  As polymer science developed, polymeric substrates displaced the glass and silica beads. Polystyrene and polydivinylbenzene (PDVB) are the most common substrates currently used. The size of the beads may vary depending on the conditions imposed during the polymerization process. Modification of the backbone of the polymeric beads occurs---following their formation---to include the required functional groups for the ion exchange process. Depending on the functional groups bound to the backbone, both anionic and cationic resins are possible.
  
  

Acid Functional Groups

• When ion exchange beads contain sulfonic acid functional groups, the resin behaves like a strong acid. It ionizes readily in both the acid and salt form. This ability provides the ability to convert a solution of metal salts to the corresponding acid by exchanging the hydrogen ion on the functional group, when in the hydrogen form, with the metal ion in solution independent of the pH of the solution. When the functional group is in the sodium form the ion exchange resin softens the water in areas where the water supply is hard. Regeneration of the ion exchange resin after use results by passing a concentrated solution of either acid, for the hydrogen form, or sodium chloride, for the sodium form, through the bed of beads. A quick rinse of the bed with deionized water and it is ready for use again. Strong acid resins exchange extremely quickly with the liquid they are in contact with and will allow a higher flow rate during the operation.
  
  Weak acid exchangers also are available, but require longer equilibration times with the liquids and thus, a slower flow rate. A typical functional group for a weak acid resin is the carboxyl group. The dissociation of the salt from the carboxyl group is slower and is dependent on the pH of the solution. The capacity of the resin is dependent on the pH of the solution and has limited capacity below approximately a pH of 6. This limitation generally makes weak acid resins unsuitable for industrial waste water applications.
  
  

Basic Functional Group

• Much like the strong acid resins discussed, strong base resins dissociate to a high degree. This strong prevalence to dissociate makes these resins usable over the entire pH range. The primary application of strong base resins is for the treatment of industrial waste water. Acid effluents from factories and other industrial sources---when treated with ion exchange resins---resulted in pure water. The regeneration of these strong base resins occurs by washing the resin with a strong sodium hydroxide solution. This washing returns the resin to the hydroxide form.
  
  The pH strongly effects the degree of dissociation of weak base resins. As a result, resins of weak bases show a minimum of exchange capacity above the pH of 7. Unlike the strong base resins, those of weak bases do not have a hydroxide form but serve to simply absorb strong acids. The regeneration of these resins requires inexpensive re-agents, such as ammonia or sodium carbonate. These re-agents neutralize the absorbed strong acid and regenerate the activity of the functional group.