Aqueous leaching properties and environmental implications of cadmium, lead and zinc trimercaptotriazine (TMT) compounds.

2,4,6-Trimercaptotriazine, trisodium salt nonahydrate (TMT-55) is a commercial product that is widely used to chemically precipitate cadmium, lead, zinc and other heavy metals from wastewaters and contaminated natural waters. When mixed with aqueous solutions of TMT-55, aqueous solutions of either reagent-grade zinc, cadmium, or lead salts precipitate crystalline "Zn-TMT", amorphous or crystalline "Cd-TMT" or amorphous "Pb-TMT" (M3[S3C3N3]2.nH2O, where M=Cd2+, Pb2+, and Zn2+ and n> or = 0) that may eventually crystallize if stored in air. Laboratory aqueous leaching studies over 78-106 days using pH 3 HCl, distilled water (pH 6) and pH 9-10 NaOH evaluated the stability of the Cd-, Pb-, and Zn-TMT precipitates. Under pH 3 conditions, the amorphous Cd- and Pb-TMT compounds converted to their crystalline forms and amorphous Cd-TMT also crystallized in distilled water. Otherwise, no decomposition products were detected in the leached solid residues. When compared with the aqueous solubilities of corresponding sulfides and most hydroxides, the TMT compounds were significantly more soluble in distilled water and pH 3 HCl.

Irreversible precipitation of mercury and lead.

There are immediate concerns with current commercial reagents that are used for heavy metal precipitation; in particular the fact that the reagents are not specifically designed to bind the targeted metals. The current literature reveals that not only do commercial reagents lack sufficient ability to strongly bind the metals, but they also fail to provide long-term stability as ligand-metal complexes under a variety of moderate conditions. For this reason a new ligand was designed and synthesized: 1,3-benzenediamidoethanethiol (BDETH2). It offers multiple, concerted, bonding sites for heavy metals and forms a stable metal-ligand precipitate. In this study, the formation of compounds comprised of this ligand with the divalent metals, lead and mercury, was explored and the pH stability of the water insoluble precipitates was determined. The leaching properties of the metal-ligand precipitates were determined using inductively coupled plasma (ICP) spectroscopy and cold vapor atomic fluorescence spectroscopy (CVAF). The results indicate that a 50.00 ppm lead solution at a pH of 4.0 may be reduced to a concentration of 0.05 ppm (99.9% lead removal) and to 0.13 ppm (99.7% lead removal) at a pH 6.0. A 50.00 ppm mercury solution at pH 4.0 may be reduced to a concentration of 0.02 ppm (99.97% mercury removal) and to 0.02 ppm (99.97% mercury removal) at a pH of 6.0.

A pyridine-thiol ligand with multiple bonding sites for heavy metal precipitation.

There are immediate concerns with current commercial ligands that are used for heavy metal precipitation, especially the limited arrays of bonding sites. Previous research has indicated that not only do commercial reagents lack sufficient bonding criteria, but they also fail to provide long-term stability as ligand-metal complexes. For this reason, we have developed a pyridine-based thiol ligand (DTPY) which not only offers multiple bonding sites for heavy metals but also should form stable metal-ligand precipitates. In this study, we used the divalent metals cadmium and copper to model the reactivity and pH stability of divalent metal complexes with the DTPY ligand. Using inductively-coupled plasma spectrometry (ICP), results indicate that a 50.00ppm (parts per million) copper solution, pH of 4.5, can be reduced to below the ICP detection limits of 0.00093ppm (>99.99% removal), and a 50.00ppm cadmium solution, pH of 6.0, can be reduced to 0.06ppm (99.88%).