Which method of water treatment involves the addition of sodium ion?

The ion exchange method for water treatment specifically involves the process of exchanging undesirable ions in the water, such as calcium or magnesium, with more desirable ions, such as sodium. This method is particularly effective in softening water, as the sodium ions replace hardness-causing ions.

During this process, water passes through a resin that has sodium ions attached to it. When the hard water contacts the resin, the calcium and magnesium ions in the water are attracted to the resin and are exchanged for sodium ions. As a result, the water becomes softer and more suitable for use in various applications, such as washing or in household plumbing, where the presence of hard minerals can cause scaling and other issues.

The other methods mentioned, such as reverse osmosis, filtration, and chlorination, do not specifically incorporate sodium ions in their treatment processes. For example, reverse osmosis relies on a membrane to remove contaminants and does not involve ion replacement, and chlorination is primarily used for disinfection. Thus, the ion exchange method is the correct choice regarding the addition of sodium ions in water treatment.