DEVELOPMENT OF MULTIDENTATE LIGANDS TO INCREASE THE EFFECTIVENESS OF HEAVY METAL PRECIPITATION FROM WATER

Numerous companies have developed and marketed chemical products to precipitate heavy metal contaminants (such as: mercury, lead, and cadmium) from wastewaters and natural waters. Our studies of Thio-Red(R) (an aqueous solution of alkali thiocarbonates [(Na,K)2CS3·nH2O] manufactured and distributed by ETUS, Inc. of Sanford, Florida, USA) and TMT-55 (a sodium salt of 2,4,6-trimercapto-triazine [Na3C3N3S3·9H2O], a product of Degussa-Hüls Corporation USA of Allendale, New Jersey) have demonstrated that the chemistry of these products is much more complex than previously realized. Specifically, Thio-Redâ precipitates divalent heavy metals as sulfides rather than thiocarbonates. Reactions between thiocarbonates and heavy metal cations also generate toxic carbon disulfide (CS2), which precludes the use of Thio-Redâ for the in-situ remediation of soils, sediments and groundwater. Depending upon pH conditions, TMT-55 removes divalent heavy metals from water through the formation of various precipitates (M3(C3N3S3)2·nH2O, MHC3N3S3·nH2O and M2(HC3N3S3)2·nH2O, where M=Hg2+, Cd2+, Pb2+, etc.). Some of these compounds are unstable at pH conditions below 5. Furthermore, in the presence of oxygen and oxygenated water, many heavy metal TMT compounds decompose to metal sulfides and cyanuric acid over weeks to months. Increasing the coordination of the bonds between the metals and sulfur-based ligands may enhance the stability of heavy metal precipitates. Recently, the Atwood Group developed two relatively inexpensive and non-toxic multidentate ligands, which contain either pyridine-thiol (DTPY2-) or benzene-thiol (1,3-benzenediamidoethanethiol, BDET2-). As expected, the relatively high coordination values between the metals and the ligands yielded heavy metal precipitates, which were very stable under oxidizing and diverse pH conditions, including pH values lower than 2.0. As an example, BDET was added to pH 1.5 wastewater samples from a lead battery recycling plant. Within 15 minutes, the ligand reduced the initial 3.61 mg/L lead concentration of the samples by 99.4%. Over weeks to months, heavy metal DTPY and BDET compounds were generally more stable in aqueous solutions than their TMT analogues. The ligands may also have applications in the processing of gold and silver ores.