In water treatment, precipitation is the addition of a chemical to cause an ion, usually a cation, to be removed as a solid. The ‘‘target’’ ions are those that may impair the use of water. Cations that are most likely to be an issue include calcium, magnesium, iron, manganese, and various heavy metals. Other cations may cause problems, depending on the situation and the concentration. Fewer anions are amenable to precipitation. Those that are amenable include phosphates, silica, sulﬁdes, and cyanide. Nitrate, a ‘‘contaminant’’ in drinking water, is not amenable to removal by precipitation; neither are the innocuous anions such as chloride and sulfate. The exact nature of silica is complex. For example, SiO2, SiO
, SiOH2 þ are
included in the silica group by Letterman et al. (1999, p. 6.9). Stumm (1992, p. 175) gives a dissolution reaction as, SiO2 (s)þ H2O! H4SiO4(ag). The latter is orthosilicic acid, which may dissociate to H3SiO4
(and there are other derivatives).