In the Laboratory
Removal of Heavy Metals from Water: An Environmentally Significant Atomic Absorption Spectrometry Experiment
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Brian P. Buffin† Department of Chemistry and Biochemistry, University of Detroit Mercy, Detroit, MI 48221;
[email protected] The area of environmental chemistry continues to be of interest to undergraduate students, as evidenced in part by recent reports of the extensive incorporation of environmental projects into the undergraduate curriculum at other institutions (1, 2) and the high number of environmental science programs. As part of ongoing attempts to increase the quality and relevance of the undergraduate laboratories at the University of Detroit Mercy, new real-life experiments have been introduced, many of which have an environmental focus. While a number of excellent laboratory exercises that have an environmental component have appeared in this Journal (3– 11) and other journals (12), few of these experiments involve the analysis of toxic heavy metals (6, 10–12). In addition, although the handling of laboratory waste has received much attention (13), it should be noted that inclusion of the environmentally important topic of industrial waste mediation in undergraduate courses and laboratory exercises has only recently been reported (14–19). An interesting undergraduate laboratory that involves wastewater treatment and analysis of heavy metals by atomic absorption spectrometry (AAS) is presented herein. Background The metal-finishing industry employs various technologies, for example chromium plating, to produce items whose surface is more resistant to corrosion, has higher durability, and has an improved esthetic appearance. Unfortunately, the effluent from the metal-finishing process may contain cyanides and heavy metal contaminants such as cadmium, hexavalent chromium, copper, nickel, iron, silver, and zinc. In addition to the detrimental effects high concentrations of these species may have on organisms in natural waters, many of these contaminants are inimical to municipal sewage treatment processes and must be removed before release to a sewer system or natural waterway. Methods involving chemical precipitation are some of the easiest to perform and most effective for the removal of metallic species from aqueous waste. Most heavy metal cations form sparingly soluble hydroxides or carbonates when subjected to elevated pH conditions by the addition of caustic soda (NaOH) or soda ash (Na2CO3), respectively. Flocculantaided sedimentation followed by filtration can then be performed to remove the precipitated metal-bearing waste from the aqueous effluent. In an industrial wastewater treatment system, a great deal of effort is dedicated to the coagulation of colloidal particles, as well as the engineering of proper flow †
Current address: Department of Chemistry, Western Michigan University, Kalamazoo, MI 49008.
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rates, amounts and points of chemical addition, and settling tank design (20–22). In the laboratory exercise presented herein, students perform a treatment study on a metal-contaminated “wastewater”. In the prelaboratory section of the laboratory, students are encouraged to research wastewater treatment technology, including newer forms of waste recovery. In addition, students are introduced to the complex equilibria involved in metal hydroxide precipitation and soluble complex ion formation. The lab is then performed in two parts. In the first 3-hour section, students perform treatment trials at different pH values. On the second day, students make up standard solutions of the metals of interest and perform AAS analysis, including instrument calibration, and water analysis; they also run QA/QC sample checks. Upon completion of the laboratory, students have enough information to assess the effectiveness of the potential treatment processes. The laboratory has been conducted several times over the past decade at two different institutions, and student feedback is very positive. Although the laboratory has been part of an upper-level course in instrumental analysis, incorporation into a second-year course in quantitative analysis may also be appropriate. One of the unique features of this laboratory is the large number of topics that can be easily introduced. These include, but are not limited to, atomic absorption spectroscopy, environmental chemistry, matrix effects and methods of standard addition, polymeric flocculants, pHdependent solubility and solubility product equilibrium, filtration, and statistics of analysis. Summary of Procedure The laboratory experiment consists of a prelab exercise followed by two laboratory sessions. For the prelab, students perform a literature search and prepare a summary on different aspects of wastewater treatment technology. In addition, a brief discussion of complex equilibria is presented in the first part of the laboratory class to reacquaint the students with this topic. The basics of atomic absorption spectrometry (AAS) are presented in a lecture course that accompanies the laboratory. In the first laboratory session, students perform a treatment study on a simulated wastewater sample. They work in groups of 2–3 throughout this experiment. Slightly acidic “wastewater” samples containing elevated concentrations of hexavalent chromium, copper, nickel, and zinc are made prior to the lab. Other metals, such as silver or cadmium, could easily be substituted or added as additional analytes. First, the students lower the pH of their sample to pH 3 by adding 0.5 M H2SO4. The sample is then treated with sodium metabisulfite to reduce the Cr(VI) to Cr(III). After addition of
Journal of Chemical Education • Vol. 76 No. 12 December 1999 • JChemEd.chem.wisc.edu
In the Laboratory
calcium chloride, which serves as a coprecipitant, the solution is divided into four portions of equal volume. Sample 1 is adjusted to a pH of 6 by the dropwise addition of 10% (w/w) aqueous NaOH. Sample 2 and sample 3 are subsequently raised to pH 8 and pH 10, respectively. Sample 4 is left “untreated”. After pH adjustments, an anionic polyacrylamide flocculant is added to samples 1–3 to promote coagulation and settling of the precipitated metal hydroxides. After letting the samples stand for approximately 30 minutes, solids are removed by decantation through filter paper. The next laboratory session involves the analysis of the treatment filtrates and the untreated sample 4 by atomic absorption spectrometry. Students prepare calibration solutions of the analytes using standard procedures for the analysis of water and wastewater (23) and perform instrument setup and calibration following guidelines given in the instrument manual provided by the manufacturer. The four samples are then sequentially analyzed for each of the metal contaminants. As part of QA/QC protocol, calibration checks are performed and selected samples are spiked with appropriate amounts of analyte to confirm the absence of matrix effects. After completion of the laboratory portion of the experiment, each student is required to prepare a report in standard literature format. The results are typically very interesting, and a dramatic lowering of metal concentrations is readily observed. In addition, careful examination of the data often illustrates the pH-dependent solubility of metal hydroxides in a unique way. Overall, the laboratory gives students hands-on experience in the setup and operation of an atomic absorption spectrometer in an experiment that incorporates important concepts of environmental chemistry and waste treatment. Acknowledgments I am grateful to Jane Schley for helpful comments on this manuscript and to the many students who have performed this experiment in the laboratory and offered suggestions for improvement. I also wish to acknowledge the assistance of Kevin Clemmer, who first introduced me to the treatment of wastewater in the metal-finishing industry.
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Instructor’s notes, reagent and equipment lists, detailed experimental procedures, and an expanded discussion of the experiment are available on JCE Online at http://jchemed.chem.wisc.edu/Journal/issues/ 1999/Dec/abs1678.html.
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