During partial desalination (= selective ion exchange), the ion exchanger is loaded only with H+ ions or OH- ions. This means that the ion exchanger can exchange only positively or negatively charged ions.
Partial desalination usually is used for the removal of hydrogen carbonate salts. This means only the positively charged cations CA++ and Mg++ from the carbonate hardness elements calcium carbonate and magnesium carbonate are replaced with hydrogen ions H+.
The CaCO3 carbonate hardness thus results in carbon dioxide.
(H+) + (HCO3) = H2CO3
In contrast to fully demineralized water, all other salts in the water remain in place. A partial desalination is therefore only useful if simple water softening is sufficient for the application, for example (as described in the process above).
It is also possible to selectively load the ion exchanger to use adsorption to remove only specific heavy metals (e.g. lead, cadmium or mercury) but also anions such as nitrate and sulfate.
The choice of desalination process, as well as the degree of desalination arise from the desired purity of water as well as the procedures and the operational and economic conditions.