ABSTRACT
Precipitation is the addition of a chemical to cause removal of a particular cation or anion 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, nor are the innocuous anions such as chloride and sulfate. The exact nature of silica is not clear. For example, SiO2, SiO, SiOH2 are included in this group by Letterman et al. (1999, p. 69). Others have listed silicic acid, H4SiO4, and its derivatives, H3SiO4, H3SiO4, and H3SiO4. SiO2 is a quartz mineral that occurs widely in geologic formations, including sand and in the dissolved form in ambient waters.
