Metal Hydroxide Precipitation

Heavy metals are very common contaminants in industrial waste treatment streams. Due to the fact that many metals are very toxic, they are removed before the waste water is discharged from the facility. The list of most common heavy metal contaminants include lead, copper, chromium, nickel, silver, zinc, mercury, cadmium, and arsenic. The most common way to effectively remove these metals from the waste stream is by precipitating the metal in an insoluble form, such as hydroxides or sulfides.

Precipitation by hydroxide formation is the most common method. Many heavy metals are amphoteric, so their solubility reaches a minimum at a specific pH. Adding caustic materials raises the pH of the waste stream. At certain levels of pH, different heavy metals reaches its solubility minimum and a precipitation reaction occurs, forming a metal hydroxide which precipitates out, thus removing the heavy metals from the waste water and leaving acceptable metal levels in the effluent waste stream.

Heavy metals can also be precipitated by adding sulfide ions creating metal sulfides. Typically, metal sulfides are very insoluble. Metal sulfides have a much lower solubility level than the correlating metal hydroxides. This allows for much lower metal contaminant measurements in effluent waste streams. Sulfide precipitation always takes place under alkaline conditions to promote sulfide ion formation by adding sulfide salts. Sulfide precipitation typically requires the use of a coagulant addition to settle the colloidal particles.