The negatively charged "Sulfide" ion bonds to positively charged heavy metal ions to create Sulfide molecules.  As Sulfide crystals, toxic substances including Mercury, Lead, Cadmium, Uranium, Copper and Zinc are rendered non-soluble in water dimishing their environmental threat.

 

Aluminum, Iron, Magnesium and other minerals are identified as constituents of Coal Ash and Ordinary Portland Cement.  These elements form specific Hydroxide chains in contact with Calcium Hydroxide.  Absorbtion into Ferrous (Iron) Hydroxide is the preferred method of Environmental Remediation of Arsenic concentrations.  Toxic Hexavalent Chromium (VI) is rendered environmentaly safe by reduction to Chromium (III) Hydroxide.   

 

The US EPA has identified encapsulation as a preferred method of remediation.