Drinking water
Ion exchange processes
Introduction
Ion exchange is not a general method to treat drinking water. The production of potable water is usually done with other techniques: coagulation, filtration, disinfection with chlorine, hypochlorite, or ozone, activated carbon, ultrafiltration, and, when desalting sea water, reverse osmosis. The main purpose of drinking water treatment is to ensure a good bacteriological quality of the water. Nevertheless, ion exchange can be very helpful to remove selectively targeted contaminants, mostly inorganic.
Contaminants
The table below shows the contaminants that can be removed with ion exchange, and some that must be removed with other processes.
| Contaminant | Comments | Ion exchange removal |
|---|---|---|
| Hardness | There is no prescribed limit for hardness in drinking water. Softening is more a question of comfort. | The process is a normal softening, using special resins agreed for the treatment of drinking water. |
| Nitrate | Nitrate is not a problem for adults, but it is harmful for infants. | Nitrate can be removed with special, selective resins. |
| Perchlorate | Perchlorate contaminates some water wells close to production sites of rocket fuels. The recommended level is less than 6 µg/L in California. | Perchlorate can be removed with special, selective anion exchange resins. Resins used for nitrate removal are also effective for perchlorate. |
| Boron | Boric acid is present in sea water RO permeate. The recommended level is 0.5 mg/L. | Boron can be removed with special, selective resins. See Boron removal. |
| Lead | Excessive Pb levels may be due to old lead pipes. The recommended level is less than 10 µg/L. | Lead can be removed with carboxylic (WAC) resins. Softeners also remove Pb. |
| Barium | Barium is a component of hardness, and forms insoluble salts. The recommended WHO limit is 0.7 mg/L. | Sulphonic (SAC) resins have a high selectivity for barium. |
| Chromate | The presence of chromate in underground water is mostly due to industrial pollution. The recommended limit for Cr(VI) is 50 µg/L. | Special WBA resins can be used to remove chromate from water. |
| Radium | Radium is a component of hardness, but it has natural radioactivity. The American EPA has set a limit for a maximum combined value of 5 pCi/L (0.185 Bq/L). Ra is found in trace amounts in uranium ores. | Radium can be removed with sulphonic (SAC) resins. |
| Uranium | The toxicity of uranium is not due to its radioactivity, it is purely chemical. The WHO guideline is 30 µg/L. | Uranium(VI) is present in nature as the uranyl ion UO2++ and can be removed with WBA or SBA resins. |
| Other heavy metals | Heavy metals may be present in underground water, either naturally or due to pollution. | Many metals can be removed with chelating resins, but those are not authorised in all countries. |
| Arsenic | As is found in some natural water sources. The recommended limit is 10 µg/L. | There is no proven ion exchange process to remove arsenic selectively. Other media, such as granulated iron hydroxide or titanium oxide, can be used. |
| Fluoride | Ground water is sometimes contaminated with fluoride originating from industrial waste, e.g. aluminium mills. The recommended WHO limit for the F— anion is 1.5 mg/L. | Anion resins have a low selectivity for fluoride, so that selective removal is not practical. The usual process involves activated alumina. |
| NOM | Certain natural organic matters (NOM), notalby those giving an undesirable colour to the water, can be removed by ion exchange. | The resins used in this case are strongly basic anion exchangers (SBA) in the Cl– form. They are regenerated with sodium chloride. |
Other information
- See the periodic system of the elements (Mendeleev table) with some data about selective ion exchange removal.
- See also the page in another section of my web site about drinking water and bottled water.
- Drinking water quality standards on Wikipedia.
