A known thiol salt that makes possible the removal of heavy metal ions from dilute aqueous solutions is the sodium salt of trimercaptotriazine. This compound is usually added to an aqueous solution containing heavy metal ions as a 15% aqueous solution, i.e., in a slight stoichiometric excess, in order to precipitate the heavy metal ions as quantitatively as possible. The precipitate comes down in flakes and can be filtered.
Known precipitating agents based on thiolates always require a slight stoichiometric excess of the thiolate equivalents over the heavy metal equivalents, so that the thiolate excess remains in solution. Thus, when the thiolate is used for the removal of heavy metal ions from waste waters, the thiolate becomes the contaminant.
Swellable silicates with a certain ion exchange capacity, e.g., alkali bentonite, are also used for removal of heavy metal ions from aqueous solutions. (Compare, for example, TIZ Fachberichte, Vol. 106, No. 1 (1982), pages 137-139.) In this case the exchangeable alkali ions are exchanged with the heavy metal ions on the interface sites. Heavy metal ions are also adsorbed.
The binding capacity of the swellable silicates for heavy metal ions, however, leaves something to be desired. Thus, it is necessary, especially with waste waters laden with heavy metals, to use larger quantities (e.g., 5 g/liter) of alkali bentonites in order to arrive at lower heavy metal concentrations (e.g., &lt;1 mg/liter). The amount of slurry precipitating with the filtration is thereby considerable and leads to familiar difficulties, both with the filtration and with the disposal of the metal contaminated sludge.