Matthew M. Matlock

Chemical precipitation of heavy metals from acid mine drainage.

The 1,3-benzenediamidoethanethiol dianion (BDET, known commercially as MetX) has been developed to selectively and irreversibly bind soft heavy metals from aqueous solution. In the present study BDET was found to remove > 90% of several toxic or problematic metals from AMD samples taken from an abandoned mine in Pikeville, Kentucky. The concentrations of metals such as iron, may be reduced at pH 4.5 from 194 ppm to below 0.009 ppm. The formation of stoichiomietric BDET-metal precipitates in this process was confirmed using X-ray powder diffraction (XRD), proton nuclear magnetic resonance (1H NMR), and infrared spectroscopy (IR).

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.

Effectiveness of commercial reagents for heavy metal removal from water with new insights for future chelate designs

Toxic heavy metals in air, soil, and water are global problems that are a growing threat to the environment. To meet the federal and state guidelines for heavy metal discharge, companies often use chemical precipitation or chelating agents. In order to be competitive economically, many of these chelating ligands are simple, easy to obtain, and, generally offer weak bonding for heavy metals. Laboratory testing of three commercial reagents, trimercaptotriazine (TMT), Thio-Red potassium/sodium thiocarbonate (STC), and HMP-2000 sodium dimethyldithiocarbamate (SDTC) has shown that the compounds were unable to reduce independent solutions containing 50.00 ppm of divalent cadmium, copper, iron, lead, or mercury to meet EPA standards. Additionally, the compounds displayed high leaching rates and in some cases decomposed to produce toxic substances. In contrast, the studies demonstrate that a recently reported sulfur-containing multidentate ligand is both safe and effective for the removal of these metals.

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%).

Low-level mercury removal from groundwater using a synthetic chelating ligand

Mercury is present in many industrial processes at low concentrations and is a cause for concern due to the propensity for mercury to bioaccumulate. As a cumulative toxin, introduction of mercury into the environment at any level has the potential to adversely affect ecologic systems. To date, no commercial precipitants are available that can irreversibly and permanently bind mercury. In the current work, selected commercial reagents were compared alongside the dianionic ligand 1,3-benzenediamidoethanethiolate (BDET(2-)) to test the feasibility of low-level (parts-per-billion, ppb) mercury treatment for groundwater near a chloralkali plant. Of all the reagents examined, only K(2)BDET was capable of reducing mercury concentrations to below instrumental detection limits of 0.05 ppb with the added benefit of producing a stable precipitate.

Covalent coating of coal refuse to inhibit leaching

Acid mine drainage (AMD) is a severe environmental problem that results from the oxidation of pyrite (FeS2) and various other metal sulfides. AMD is often associated with abandoned mines which produce acidic run-off waters that are rich in heavy metals. AMD also occurs in the areas surrounding coal refuse piles that have accumulated from coal cleaning processes. In the present work, the use of the disodium salt of the ligand 1,3-benzenediamidoethanthiol (Na2BDET) is explored as a possible coating to prevent the dissolution of pyrite in coal. Unlike phosphate- and silica-based compounds, Na2BDET is believed to form covalent Fe–BDET linkages along the pyrite lattice. It was found that Fe leaching from BDET-coated coal is reduced by 99.3% when submerged in a pH 6.5 solution, by 97.5% when submerged in a pH 3.0 solution, and 66.4% when subjected to acidic and oxidative conditions. The ligand coating treatment also reduced the leaching of other heavy metals, such as Mn, Cu, Ni, Zn where a reduction of 88.3%, 64.7%, 70.5% and 89.5%, respectively, was observed in leaching after a 14-day simulated acid rain test. Additional leaching reductions were observed for metals such as Zn, Cu, Co, etc. at pH 6.5 and 3.0 when subjected to both oxidative and acidic conditions.

Irreversible binding of mercury from contaminated soil

Abstract The 1,3-benzenediamidoethanethiol dianion (BDET 2− ) has been designed and synthesized to immobilize mercury from contaminated soils. In this study, mercury contaminated soil samples from the Appalachia Region of Eastern Kentucky were collected and tested with the sodium salt of this ligand. Multiple binding sites on the BDET 2− ligand have led to stable mercury–ligand precipitates that are capable of withstanding adverse oxidative and pH conditions. Results indicated that 99.6% of the mercury in the soil samples could be immobilized from an average starting concentration of 10.3 mg of mercury per gram of soil (mg Hg/g soil). The precipitates were tested for stability through leaching and oxidative-digestion tests, and yielded leaching rates less than 5×10 −11 g/ml at times 2, 30 and 60 days at a pH range of 0.0–10.0.

Soft metal preferences of 1,3-benzenediamidoethanethiol.

Recent studies indicate that the sodium salt of 1,3-benzenediamidoethanethiol (BDET) is both economical and effective in precipitating mercury and other heavy metals from water. Because wastewaters and contaminated natural waters may contain a variety of heavy metals, it is important to determine how different heavy metals may interact with BDET, and whether free metals may displace those that are bound. To explore this possibility, Cd-, Cu-, Pb-, Mn-, Hg- and Zn-BDET were leached separately under a nitrogen purge for up to 240 h in pH 3 aqueous solutions containing 0.100 mmol of all five heavy metals. The leaching studies indicate that dissolved Hg has a strong tendency to displace Cd, Cu, Mn, Pb, and Zn from the BDET structure.