The purpose of the deionization method is to exclude inorganic ions dissolved in water. Like the hard water softener, it is also based on the principle of ion exchange resin. Two resins are used here-a cation exchange resin and an anion exchange resin. Cation exchange resins use hydrogen ions (H +) to exchange cations; while anion exchange resins use hydroxide ions (OH-) to exchange anions. Hydrogen ions and hydroxide ions combine to form neutral water. The reaction equation is as follows:
M + x + xH-Re → MM-Rex + xH + 1
A-z + zOH-Re → A-Rez + zOH-1
In the above formula, M + x represents a cation, x represents a valence number, M + x cations exchange with H-Re hydrogen ions on the cationic resin, Az represents an anion, z represents a valence number, and Az combines with an anion exchange resin OH- ions are released. H + ions combine with OH- ions to form neutral water.
These resins also need to be reduced after their adsorption capacity is exhausted. Cation exchange resins require strong acids to reduce them; in contrast, anions require strong bases to reduce them. The cation exchange resins have different adsorption capacities for various cations, and their strength and relative relationship are as follows:
Ba2 +> Pb2 +> Sr2 +> Ca2 +> Ni2 +> Cd2 +> CU2 +> Co2 +> Zn2 +> Mg2 +> Ag1 +> Cs1 +> K1 +> NH41 +> Na1 +> H1 +
The affinity strength of anion exchange resin with each anion is as follows:
S02-4 +> I-> NO3-> NO2-> Cl-> HCO3-> OH-> F-