Chemistry of heavy metal precipitates resulting from reactions with Thio-Red®

Thio-Red(R) is a commercial product that is widely used to precipitate divalent heavy metal contaminants from water. To gain a better understanding of how Thio-Red(R) reacts with dissolved divalent metals, the product was added to laboratory aqueous solutions, each of which contained one of several dissolved copper, lead, mercury, and zinc compounds. In all of the solutions, precipitates rapidly formed after adding Thio-Red(R). X-ray diffraction analyses of the precipitates detected ordinary metal sulfides rather than metal thiocarbonates. Further investigations identified reactions that may be responsible for the formation of the precipitates.