Ind. Eng. Chem. Res. 2002, 41, 1579-1582
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SEPARATIONS Chemical Precipitation of Lead from Lead Battery Recycling Plant Wastewater Matthew M. Matlock, Brock S. Howerton, and David A. Atwood* Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055
The 1,3-benzenediamidoethanethiol dianion (BDET2-) binds soft heavy metals from aqueous stock solutions as highly stable metal-ligand compounds. In the present study, the potassium salt of this ligand was applied to field samples collected from an operating lead battery recycling site (LBRS) which generates wastewaters containing concentrations of lead from 2 to 300 ppm at an average pH of 1.5. Goals of this study were to reduce lead concentrations to below 0.5 ppm and produce stable Pb-BDET precipitates. It was found that more than 99.4% removal of lead from an average initial concentration of 3.61 ppm could be accomplished within 15 min using an equimolar dose of the BDETK2 ligand. Moreover, the precipitates remained stable during 30-day leaching tests. Introduction Heavy-metal pollution is a serious problem that adversely affects public health. Metals such as mercury, cadmium, and lead pose a direct and serious health hazard to all forms of life. Of these metals, the U.S. Agency for Toxic Substances and Disease Registry (ASTDR) and the Center for Disease Control (CDC) ranked lead as the most hazardous metal in America.1,2 It has been estimated that 1 out of 11 American preschoolers have dangerously high levels of lead in their blood. In terms of toxicity, lead has various detrimental biological effects including inhibition of the synthesis of hemoglobin and dysfunction in the kidneys, reproductive system, liver, and the central and peripheral nervous systems.3-5 Worldwide, the largest current use of lead (≈80%) is in the production of automotive and industrial leadacid batteries.6 About 95% of these batteries undergo recycling. In a typical lead battery-recycling site (LBRS), nearly 50 000 batteries are recycled per week.7 Typically, the battery casings are crushed and the sulfuric acid extracted. The lead is then extracted as Pb0, PbSO4, PbO, and PbO2 from the remains. 8 These products are subsequently smelted to produce Pb0. Other metals found in lower concentrations include Sb, As, Cd, Cu, Se, and Sn, which are used as additives and as grid hardening agents in the battery. During the extraction and smelting processes, large volumes of heavy-metalcontaining wastewater are produced. This water, if not handled appropriately, can (and often does) ultimately contaminate surrounding soils and waters. Federal and state governments have instituted environmental regulations to protect the quality of surface and groundwater from heavy-metal pollutants.9 In response to the regulatory requirements, numerous companies have marketed common and easily available chemicals as supposed heavy-metal treatment agents. Some examples of these reagents include sodium di-
methyldithiocarbamate (SDTC) and sodium thiocarbonate (STC). In addition to requiring elevated doses (above stoichiometric values), many of the thiocarbonate and thiocarbamate ligands decompose to hazardous materials.10,11 There are other problems with these simple ligands since they bond indiscriminately and, often, weakly.12 This leads to unstable metal-ligand complexes that tend to decompose and release the heavy metals back into the environment over varying, but usually short, periods of time. This is especially true under adverse pH conditions such as those present in a LBRS. To address these problems, we have begun to synthesize ligands that are specifically designed to bind soft heavy metals and that result in stable precipitates. One such ligand is 1,3-benzendiamidoethanethiol (BDET), which can be used as a water-soluble alkali salt (Figure 1). Bonding through the terminal sulfurs with potential secondary interactions between the amide groups in the ligand backbone should enhance the strength of the metal-ligand bonding. It has been demonstrated that addition of the BDET2- to stock solutions of Pb2+, Hg2+, and other metals produces stable and insoluble metalligand precipitates over pH ranges of 0.0-14.013-15 (eq 1). Additionally, the concentrations of these metal solutions can be reduced to meet EPA discharge limits. The present work will explore the effectiveness of the ligand in treating actual samples from a LBRS.
K2C12H14N2O2S2(aq) + M2+(aq) 9 8 H O 2
C12H14N2O2S2M(s) + 2K+(aq) (1) where M2+ ) Cd, Pb, Hg, Fe, Zn, etc. Experimental Section Sampling Sites. The LBRS being examined in this study generates discharge wastewaters derived from the
10.1021/ie010800y CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002
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Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 Table 1. Initial Metal Concentrations from Raw Field Collected Wastewater Solutions at pH 1.5
b
metal
average initial concentration (ppm)
Sb Cd Cu Fe Pb
0.067
