8 Chemical Methods of Heavy Metal Binding MATTHEW MATLOCK and DAVID ATWOOD University of Kentucky, Lexington, Kentucky, U.S.A. I. INTRODUCTION Toxic heavy metals in air, soil, and water are problems that are constant and escalating threats to the environment in which we live. There are a multitude of sources of heavy metal pollution, most of which the general population cannot avoid [1,2]. In response to this continuing and growing threat, federal and state governments have instituted environmental regulations to protect the quality of surface water and groundwater from heavy metal pollutants [3]. To meet the federal and state guidelines for heavy metal discharge, companies commonly use methods to precipitate or chemically bind the metals before, dur- ing, or after discharge. In order to be competitive economically, many of the commercially utilized reagents are not designed to target specific metals. They are either simple to synthesize or easily obtained as byproducts of other chemical reactions. These readily available reagents offer minimal binding sites for heavy metals. It has been found, in fact, that their mode of action is simply to act as sources of sulfide for the formation of metal sulfides. This is not an acceptable process for metal removal and deactivation. Metal sulfides are known to decompose and release heavy metals back into the environment over varying, but usually short, periods of time [4–6]. Additionally, in forming the metal sulfides, some of the reagents produce toxic byproducts, such as carbon disulfide [4,7]. In an effort to provide an environmentally sound method for chemically re- moving heavy metals from the environment, new, specifically designed chelates for soft heavy metals have been reported. These ligands are not only highly effec- tive for soft metal binding, but are also economical and soon to be produced on a multiton scale. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 128 Matlock and Atwood The present overview will summarize the various chemical methods that are currently being used to chemically bind heavy metals and precipitate them from aqueous sources. Other technologies are available for heavy metal removal, such as micelle-enhanced filtration [8] and ion exchange [9] techniques, but these will not be covered here. II. COMMERCIAL HEAVY METAL REAGENTS A. Thiocarbonates One class of compounds that is commonly used for the precipitation of heavy metals is the thiocarbonates. Thiocarbonates are presumed to bind heavy metals as insoluble metal thiocarbonates (that is, CuCS 3 , HgCS 3 , PbCS 3 , and ZnCS 3 ). One chemical reagent able to potentially effect this precipitation is Thio-Red  ([Na, K] 2 CS 3 ⋅ nH 2 O, where n Ն 0) (Figs. 1a and 1b) [10]. The precipitate, how- ever, with copper, mercury, lead, and cadmium, taken from aqueous solutions, are metal sulfides (that is, CuS, HgS, PbS, and ZnS) [4,7,10]. A byproduct of the metal sulfide precipitation with this reagent is carbon disulfide, a volatile and toxic liquid [4,7]. FIG. 1 (a) Chemical structure of Thio-Red  (a potassium or sodium thiocarbonate). (b) Predicted (and claimed) binding motif between Thio-Red  and a divalent heavy metal (M). TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Chemical Methods of Heavy Metal Binding 129 FIG. 2 (a) Chemical structure of HMP-2000 (a sodium dimehtyldithiocarbamate). (b) Presumed binding motif between HMP-2000 and a divalent heavy metal (M). B. Thiocarbamates A second class of chelating compounds is the thiocarbamates. Typically, thiocar- bamates act as bidentate ligands. One example of a commercially available thio- carbamate is HMP-2000 (Figs. 2a and 2b). Several chemical distribution compa- nies, including Ulrich Chemical Inc., distribute this sodium thiocarbamate. A serious downfall of this compound is its tendency to decompose into toxic sec- ondary products. One major accident associated with the use of HMP-2000 oc- curred in December 1999, when the Guide Corporation (an auto parts manufactur- ing plant in Anderson, Indiana) accidentally released over 1.5 million gallons of contaminated wastewater laced with HMP-2000 into the city’s wastewater sys- tem. Unable to control the chemical contaminate, the HMP-2000-laced wastewa- ter was discharged into local state waters [11]. The compound reportedly decom- posed into toxic compounds (tetramethylthiuram and thiram), which ultimately resulted in the deaths of 117 tons of fish over a 50-mile stretch from Anderson to Indianapolis, Indiana, as reported by the Indiana Department of Environmental Management [11]. C. Potentially Multidenta Ligands A final class of ligands used for heavy metal chelation is potentially multidenta, and bridge to form polymeric compounds. One example of this type of compound TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 130 Matlock and Atwood FIG. 3 (a) Chemical structure of TMT-55 (2,4,6-trimercaptotiazine, trisodium salt non- ahydrate). (b) Binding motif between TMT-55 and a divalent heavy metal (M). is TMT, or 2,4,6-trimercaptotiazine, trisodium salt nonahydrate, Na 3 C 3 S 3 ⋅ 9H 2 O (Figs. 3a and 3b). TMT is a byproduct of chemicals manufactured and distributed by Degussa Corporation USA of Allendale and Ridgefield Park, New Jersey [12]. Despite the widespread use of TMT, only limited information has been available on how the product reacts with heavy metals in aqueous solutions and the chemis- try and stability of the resulting heavy metal–TMT precipitates. Only recently has quantitative and systemic information become available on the chemistry of TMT with metals. This included dissociation constants and the controlled for- mation of metal complexes [5,6,13–15]. While this work demonstrates some limi- tations to the use of TMT as a remediation agent, one of the publications found TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Chemical Methods of Heavy Metal Binding 131 that the reagent could be used as a sacrificial sulfide source for the formation of transition metal sulfide solid-state materials. D. Dosage Formulas Manufacturers of commercial compounds supply dosage formulas that allow us- ers to quickly calculate the amount of compound needed to treat varying volumes of contaminated water. Often these dosage rates are inaccurate and generally lead to an underdosing and consequent failure to meet the permitted discharge limits. As an example, Thio-Red  is distributed as an aqueous solution and utilizes the following formula for calculating the Thio-Red  dosage for contaminated waters (Eq. 1): D ϭ pVY (1) where D ϭ dosage of Thio-Red  in mL p ϭ part per million (ppm) concentration of the metal contaminant V ϭ volume of water to be treated in gallons Y ϭ constant (Cd ϭ 0.0362; Cu ϭ 0.0641; Pb ϭ 0.0196; Zn ϭ 0.0622) Previously, published reports on the Thio-Red  dosage formula have indicated serious problems that have been identified through the use of titration analyses followed by inductively coupled plasma spectroscopy (ICP-OES) or cold vapor atomic fluorescence (CVAF) spectroscopy [4,7]. In the experiments, titration ex- periments and computer modeling with MINTEQA2 indicate that the pH de- creases as Thio-Red  is initially titrated into aqueous divalent heavy metal solu- tions. The pH decline associated with the precipitation of the divalent metals (M 2ϩ ) is explained by the following reaction involving HS Ϫ from the Thio-Red  [4,7]. HS Ϫ ϩ M 2ϩ H 2 O → MS↓ ϩ H 3 O ϩ (2) Once the metals are precipitated, Reaction (2) is no longer dominant and an important source of H 3 O ϩ (acid) is eliminated. Without the presence of significant concentrations of dissolved metals, further addition of pH 12 Thio-Red  results in an increase in pH through the alkalinity of the Thio-Red  and the following sulfide reactions: S 2Ϫ ϩ H 3 O ϩ → HS Ϫ ϩ H 2 O (3) HS Ϫ ϩ H 3 O ϩ → H 2 S 0 ϩ H 2 O (4) The volatilization of some of the H 2 S 0 from the solution would result in a further pH increase by shifting the equilibrium to the right in Reaction (4). According TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 132 Matlock and Atwood to Reactions (2)–(4), then, the pH minimum should coincide with the optimal Thio-Red  dosage for metal removal. E. Effectiveness of the Commercial Compounds on Metal Stock Solutions In an effort to compare the effectiveness of the commercial metal-binding agents, the manufacturers’ dosage formulas were ignored and reactions were carried out using a stoichiometric molar ratio (and also a 10% increase in the stoichiometric molar dosage) between each commercial compound and solutions of mercury, lead, cadmium, copper, and iron(II). Tables 1–3 outline the effectiveness of the commercial compounds under pH conditions of 4.0 and 6.0. Results show that at stoichiometric doses, Thio-Red  , HMP-2000, or TMT were unable to effectively remove the cadmium, lead, copper, or iron from solutions of 50.00 ppm to meet the EPA discharge limits [3]. Even with a 10.00% molar excess, no additional significant removal was observed. HMP-2000 displayed a higher affinity for cad- mium, lead, copper, and iron than the Thio-Red  , but still EPA discharge limits were not achieved. TMT displays similar results as compared with the HMP- 2000, with the highest removal seen for lead and copper. Once again, it is seen that even at a 10% molar increase in dosage, TMT were unable to reduce lead or cadmium concentrations to meet EPA standards [3]. For the mercury analyses, it was found that at stoichiometric and a 10% molar dose increase (in each com- mercially tested compound), mercury concentrations from the 50.00-ppm stock solutions were not reduced to meet the EPA limit of 0.2 ppm [3]. Maximum results for mercury removal with Thio-Red  were seen at 20 hours at a 10% molar dose increase, with a final average value of 3.97 ppm. At one hour the results of HMP-2000, at a 10% molar dose increase, indicated a reasonably high removal of mercury, with a final concentration of 0.69 ppm. Within 20 hours at stoichiometric doses, TMT was able to reduce the 50.00-ppm mercury concentra- tions to an average final concentration of 9.82 ppm. For the metal concentrations that increased over time, the formation of more soluble metal–ligand complexes or the substantial decomposition of the metal– ligand complexes may explain the increase in metal concentration. Additionally, an increase in metal concentration may also contribute to the possible high leach- ing rates of the metal out of the ligand complexes (or, more accurately, leached out of the metal sulfides produced by the ligand complexes) [5,6]. III. RESEARCH ADVANCES ON NOVEL LIGANDS FOR HEAVY METAL CHELATION Immediate concerns with the commercially available ligands focus on the weak potentially multidentate binding abilities for heavy metals. Ligands with alkyl-thio TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Chemical Methods of Heavy Metal Binding 133 TABLE 1 ICP and CVAF Results of Thio-Red  at Stoichiometric Doses and at 10% Molar Dosage Increases EPA Initial metal Final metal discharge Chelating Time Solution concentration concentration limit (ppm) agent Metal Dose (hours) pH (ppm) (ppm) [3] Thio-Red  Pb Stoichiometric 1 6.0 50.00 38.24 5.0 Pb Stoichiometric 6 6.0 50.00 44.83 5.0 Pb Stoichiometric 20 6.0 50.00 48.17 5.0 Pb 10% dose increase 1 5.5 50.00 33.67 5.0 Pb 10% dose increase 6 5.5 50.00 41.55 5.0 Pb 10% dose increase 20 5.5 50.00 47.99 5.0 Cu Stoichiometric 1 5.0 50.00 27.77 Cu Stoichiometric 6 5.0 50.00 28.99 Cu Stoichiometric 20 5.0 50.00 28.86 Cu 10% dose increase 1 4.5 50.00 27.08 Cu 10% dose increase 6 4.5 50.00 25.77 Cu 10% dose increase 20 4.5 50.00 26.79 Cd Stoichiometric 1 5.5 50.00 34.38 1.0 Cd Stoichiometric 6 5.5 50.00 39.53 1.0 Cd Stoichiometric 20 5.5 50.00 47.07 1.0 Cd 10% dose increase 1 5.0 50.00 27.09 1.0 Cd 10% dose increase 6 5.0 50.00 34.87 1.0 Cd 10% dose increase 20 5.0 50.00 41.50 1.0 Fe(II) Stoichiometric 1 6.0 50.00 35.15 2.0 Fe(II) Stoichiometric 6 6.0 50.00 34.38 2.0 Fe(II) Stoichiometric 20 6.0 50.00 32.98 2.0 Fe(II) 10% dose increase 1 5.0 50.00 34.79 2.0 Fe(II) 10% dose increase 6 5.0 50.00 34.56 2.0 Fe(II) 10% dose increase 20 5.0 50.00 33.56 2.0 Hg Stoichiometric 1 6.0 50.00 8.59 0.2 Hg Stoichiometric 6 6.0 50.00 8.07 0.2 Hg Stoichiometric 20 6.0 50.00 6.85 0.2 Hg 10% dose increase 1 6.0 50.00 6.72 0.2 Hg 10% dose increase 6 6.0 50.00 5.20 0.2 Hg 10% dose increase 20 6.0 50.00 3.97 0.2 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 134 Matlock and Atwood TABLE 2 ICP and CVAF Results of HMP-2000 at Stoichiometric Doses and at 10% Molar Dosage Increases EPA Initial metal Final metal discharge Chelating Time Solution concentration concentration limit (ppm) agent Metal Dose (hours) pH (ppm) (ppm) [3] HMP-2000 Pb Stoichiometric 1 3.5 50.00 21.90 5.0 Pb Stoichiometric 6 3.5 50.00 21.89 5.0 Pb Stoichiometric 20 3.5 50.00 23.77 5.0 Pb 10% dose increase 1 4.0 50.00 15.46 5.0 Pb 10% dose increase 6 4.0 50.00 16.21 5.0 Pb 10% dose increase 20 4.0 50.00 16.31 5.0 Cu Stoichiometric 1 3.5 50.00 12.55 Cu Stoichiometric 6 3.5 50.00 12.58 Cu Stoichiometric 20 3.5 50.00 12.46 Cu 10% dose increase 1 4.0 50.00 7.08 Cu 10% dose increase 6 4.0 50.00 7.10 Cu 10% dose increase 20 4.0 50.00 7.19 Cd Stoichiometric 1 3.0 50.00 11.52 1.0 Cd Stoichiometric 6 3.0 50.00 11.51 1.0 Cd Stoichiometric 20 3.0 50.00 12.08 1.0 Cd 10% dose increase 1 4.0 50.00 10.47 1.0 Cd 10% dose increase 6 4.0 50.00 10.54 1.0 Cd 10% dose increase 20 4.0 50.00 10.96 1.0 Fe(II) Stoichiometric 1 4.0 50.00 25.18 2.0 Fe(II) Stoichiometric 6 4.0 50.00 23.07 2.0 Fe(II) Stoichiometric 20 4.0 50.00 23.92 2.0 Fe(II) 10% dose increase 1 4.5 50.00 24.28 2.0 Fe(II) 10% dose increase 6 4.5 50.00 23.21 2.0 Fe(II) 10% dose increase 20 4.5 50.00 23.94 2.0 Hg Stoichiometric 1 4.0 50.00 1.01 0.2 Hg Stoichiometric 6 4.0 50.00 1.50 0.2 Hg Stoichiometric 20 4.0 50.00 2.78 0.2 Hg 10% dose increase 1 4.0 50.00 0.69 0.2 Hg 10% dose increase 6 4.0 50.00 1.24 0.2 Hg 10% dose increase 20 4.0 50.00 1.63 0.2 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Chemical Methods of Heavy Metal Binding 135 TABLE 3 ICP and CVAF Results of TMT at Stoichiometric Doses and at 10% Molar Dosage Increases EPA Initial metal Final metal discharge Chelating Time Solution concentration concentration limit (ppm) agent Metal Dose (hours) pH (ppm) (ppm) [3] TMT Pb Stoichiometric 1 5.0 50.00 18.21 5.0 Pb Stoichiometric 6 5.0 50.00 18.50 5.0 Pb Stoichiometric 20 5.0 50.00 21.05 5.0 Pb 10% dose increase 1 5.5 50.00 16.06 5.0 Pb 10% dose increase 6 5.5 50.00 16.58 5.0 Pb 10% dose increase 20 5.5 50.00 17.31 5.0 Cu Stoichiometric 1 5.0 50.00 16.18 Cu Stoichiometric 6 5.0 50.00 13.30 Cu Stoichiometric 20 5.0 50.00 10.13 Cu 10% dose increase 1 5.5 50.00 16.19 Cu 10% dose increase 6 5.5 50.00 14.21 Cu 10% dose increase 20 5.5 50.00 12.59 Cd Stoichiometric 1 5.0 50.00 37.14 1.0 Cd Stoichiometric 6 5.0 50.00 36.12 1.0 Cd Stoichiometric 20 5.0 50.00 38.22 1.0 Cd 10% dose increase 1 5.5 50.00 21.04 1.0 Cd 10% dose increase 6 5.5 50.00 21.04 1.0 Cd 10% dose increase 20 5.5 50.00 21.62 1.0 Fe(II) Stoichiometric 1 5.0 50.00 25.04 2.0 Fe(II) Stoichiometric 6 5.0 50.00 25.46 2.0 Fe(II) Stoichiometric 20 5.0 50.00 25.27 2.0 Fe(II) 10% dose increase 1 5.5 50.00 23.64 2.0 Fe(II) 10% dose increase 6 5.5 50.00 22.38 2.0 Fe(II) 10% dose increase 20 5.5 50.00 21.77 2.0 Hg Stoichiometric 1 5.5 50.00 18.07 0.2 Hg Stoichiometric 6 5.5 50.00 13.39 0.2 Hg Stoichiometric 20 5.5 50.00 9.82 0.2 Hg 10% dose increase 1 5.5 50.00 15.15 0.2 Hg 10% dose increase 6 5.5 50.00 16.90 0.2 Hg 10% dose increase 20 5.5 50.00 10.50 0.2 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 136 Matlock and Atwood chains that lack either chain length or sufficient bonding sites may produce pre- cipitates that are unstable over time and under certain pH conditions. For this reason our research has been focusing on the design and synthesis of ligands with extended binding sites that not only bind heavy metals, but also produce stable precipitates. In order to create more effective and economical ligands, we have developed a series of new synthetic ligands which utilize biological heavy metal binding motifs. As an example, research has led to the design and patenting of compounds such as 2,6-pyridinediamdoethanethiol (PyDETH 2 ) [16]. This compound utilizes two extended alkyl chains at the 2,6-position to irreversibly bind metals. Computer modeling suggests that the designed ligands have a suffi- cient chain length for interactions between the metal and each terminal sulfur group (Fig. 4). Early results have shown that heavy metal concentrations from aqueous solu- tions can be reduced well below EPA discharge limits, and the resulting precipi- FIG. 4 Chemical structure of Hg-PyDET. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. [...]... contaminated with 10,270 ppm of elemental mercury, the multidentate ligand was capable of immobilizing 99.6% of the mercury [19] The ligands has also proven to be highly effective in the gold heap leachate solutions by selectively reducing mercury levels from an initial concentration of 34.5 ppm to 0.014 ppm within 10 minutes and to 0.0 08 pm within 15 minutes [20] In addition to selectively removing... 141, 261, 2 68. 40, U.S Government Printing Office, Superintendent of Documents, Washington, DC, 1994 4 KR Henke Wat Env Res 70(6):11 78 1 185 , 19 98 5 KR Henke, D Robertson, MK Krepps, DA Atwood Wat Res 34(11):3005–3013, 2000 6 MM Matlock, KR Henke, DA Atwood, JD Robertson Wat Res 35(15):3649–3655, 2001 7 KR Henkee Chemistry and Environmental Implications of Thio-Red and 2,4,6Trimercaptotriazine Compounds... the Hg-ligand compounds are highly stable even in the adverse pH conditions used during the NaCN leaching process IV CONCLUSION There is a definite need for new and more effective reagents to meet growing environmental problems Many reagents on the market today either lack the necessary binding criteria or pose too many environmental risks to be effectively utilized For this reason, ligands utilizing... Strauss Env Sci Technol 32:1007–1012, 19 98; AE Gash, PK Dorhout, SH Strauss Chem Mater 13:2257–2265, 2001 ETUS, Inc Product Information on Thio-Red, Sanford, FL, 1994 State of Indiana’s Data Facts Sheet http:/ /www.state .in. us/idem/macs/factsheets/ whiteriver/ 185 Degussa Corporation Data Sheets on TMT-15 and TMT-55, Ridgefield Park, NJ, 1993 MK Krepps, S Parkin, DA Atwood Crystal Growth Design 1(4):291–297,... of North Dakota, Grand Forks, ND, 1 984 8 UR Dharmawardana, SD Christian, RW Taylor, JF Scamehorn Langmuir 8: 414, 1992; M Tuncay, SD Christian, EE Tucker, RW Taylor, JF Scamehorn Langmuir 10:4 688 , 1994 9 AE Gash, AL Spain, LM Dysleski, CJ Flaschenriem, A Kalaveshi, PK Dorhout, TM Copyright n 2003 by Marcel Dekker, Inc All Rights Reserved 1 38 10 11 12 13 14 15 16 17 18 19 20 TM Matlock and Atwood SH Strauss... utilizing multiple binding sites for heavy metals and mimicking biological systems for metal binding look to be a possible answer to heavy metal remediation and wastewater treatment REFERENCES 1 BJ Alloway, ed Heavy Metals in Soils 2nd ed Chapman and Hall, Glasgow, UK, 1995, Chaps 6, 8, 9, 11 ¨ 2 KR Henke, V Kuhnel, DJ Stephan, RH Fraley, CM Robinson, DS Charlton, HM Gust, NS Bloom Gas Res Inst, Chicago,... Heavy Metal Binding 137 tates have shown no solubility in common organic solvents or aqueous systems over a pH range of 0.0–14.0 [17–19] For example, the synthetic ligands have been able to reduce mercury concentrations from 50.00 ppm to 0.094 ppm and lead concentrations from 50.00 ppm to 0.050 ppm [17, 18] The ligands have also been effective in immobilizing mercury from contaminated soils In recent tests... Hutchison, MK Krepps, DA Atwood Inorg Chem 40(17):4443– 4447, 2001 RJ Bailey, MJ Hatfield, KR Henke, MK Krepps, J Morris, T Otieno, K Simonetti, EA Wall, DA Atwood J Org Met Chem 623: 185 –190, 2001 MM Matlock, BS Howerton, DA Atwood Novel Multidentate Sulfur-Containing Ligands University of Kentucky, 2000 U.S (patent pending) MM Matlock, BS Howerton, KR Henkee, DA Atwood J Haz Mat 82 (1):55–63, 2001 MM Matlock,... Howerton, KR Henkee, DA Atwood J Haz Mat 82 (1):55–63, 2001 MM Matlock, BS Howerton, DA Atwood J Haz Mat B84:73 82 , 2001 MM Matlock, BS Howerton, DA Atwood Adv Env Res (in press) MM Matlock, BS Howerton, MA Van Aelstyn, FL Nordstrom Env Sci Tech 36(7): 1636–1639, 2002 Copyright n 2003 by Marcel Dekker, Inc All Rights Reserved . within 10 minutes and to 0.0 08 pm within 15 minutes [20]. In addition to selectively removing mercury, the Hg-ligand compounds are highly stable even in the adverse pH conditions used during. Regulations (CFR). 40, 141, 261, 2 68. 40, U.S. Government Printing Office, Superintendent of Documents, Washington, DC, 1994. 4. KR Henke. Wat Env Res 70(6):11 78 1 185 , 19 98. 5. KR Henke, D Robertson,. Dekker, Inc. All Rights Reserved. 130 Matlock and Atwood FIG. 3 (a) Chemical structure of TMT-55 (2,4,6-trimercaptotiazine, trisodium salt non- ahydrate). (b) Binding motif between TMT-55 and