In the Classroom
Issues-Directed Chemistry: Teaching Chemical Reactions Using Waste Treatment David L. Adams Department of Natural Sciences, Babson College, Babson Park, MA 02457;
[email protected] Effective science education plays an important role in establishing a citizenry literate in science, maintaining a wellprepared work force, and recruiting students into scientific careers. Because of this, alternative approaches to the instruction of science courses for collegiate non-science majors have recently been discussed (1). One such approach is the issuesdirected or in-context approach (2–6 ). Several introductory chemistry courses using this approach have been described (7–10). In general, these courses introduce the principles of chemistry on an “as-needed” basis within the framework of an issue. Useful issues are timely, interesting, important to students’ future careers, and interrelated with other course issues, and they allow development of chemical principles (2). At Babson College, a business specialty school in Wellesley, MA, we offer a one-semester introductory chemistry course. This issues-directed chemistry course, including the issues and chemical concepts covered, was previously described (7, 8). Since the earlier description the chemical industry issue has been deleted and the material covered integrated into the remaining three issues of materials, energy, and waste. Collectively, these three issues parallel the manufacturing process. Raw materials are needed to make a product, energy is used to transform the raw materials into the desired product, and waste is invariably produced during this transformation. Waste treatment constitutes about one-third of the course, and this is the portion described in this paper. It has proven to be an effective issue. Students view it as both interesting and important, and it enables us to explore chemical reactions. For the past several years we have used Chemistry—An Environmental Perspective by P. Buell and J. Girard (11) as a textbook. This paper examines (i) how the course organizes and approaches the issue of waste treatment to allow for optimal coverage of fundamental chemical reactions and (ii) the content and scope of coverage of those chemical reactions. The course described here also requires a laboratory in which standard experiments in acid–base and redox chemistry, kinetics, and equilibrium are used to reinforce lecture concepts. Waste Treatment Waste treatment is defined as a process that reduces the volume, mobility, or toxicity of waste. Waste treatment processes are subdivided into physical and chemical methods, reflecting the definitions of physical and chemical changes and reinforcing these concepts discussed earlier in the course.
Physical Methods Physical methods do not change the chemical composition of the waste. They involve physical changes and include (i) separation (filtration and distillation), (ii) sedimentation, and (iii) stabilization. Separation and sedimentation techniques reduce waste volume, whereas stabilization isolates waste, thereby reducing its mobility. The inherent toxicity of the 1088
waste remains unchanged because its chemical composition remains unchanged. Separation and sedimentation are both employed in municipal drinking water purification. Separation by filtration is used in the screening process in the primary system. Sedimentation is used in the secondary treatment. We examine both in detail. Separation by distillation is illustrated by the separation of solvents from petroleum samples generated during oil exploration. This reinforces discussions of fractional distillation and petroleum refining covered earlier during the materials issue. Stabilization is illustrated by vitrification, also called glassification. One form of vitrification is in situ vitrification (ISV), a technology that employs heat from an electrical current to melt soil contaminated with wastes (12). Upon cooling, the soil solidifies into a solid mass, encapsulating the wastes and preventing them from migrating in the soil or elsewhere. A vitrification process is currently used at the Savannah River Site (SRS) in South Carolina to immobilize high-level radioactive waste in glass (13). This discussion provides continuity with previous examinations of fission and radioactive waste disposal, both discussed earlier in the energy issue.
Chemical Methods Chemical methods transform waste substances (reactants) into new substances (products) that are nontoxic or less toxic than the original waste material. Chemical methods may be categorized by the agent causing the chemical change. These include added reagent, thermal, and biological. Added reagents are chemical substances selected to react with waste materials and lead to products of lower toxicity. Wastes may undergo oxidation–reduction, neutralization, or precipitation reactions depending on the chemical nature of the added reagent, thus offering many opportunities to discuss reaction fundamentals. Thermal and biological methods also involve chemical changes and provide additional opportunities to reinforce concepts such as the kinetic–molecular theory, bond energies, and thermochemistry. Added Reagent Methods O XIDATION –R EDUCTION OR E LECTRON T RANSFER REACTIONS. Chlorination of public water supplies provides an example of an oxidation–reduction reaction. We begin the discussion by considering the chemical reaction that occurs when chlorine gas dissolves in water (eqs 1 and 2) (14 ). Cl2(g) + H2O(,) HClO(aq)
H+(aq) + Cl {(aq) + HClO(aq) (1) H+(aq) + ClO {(aq)
(2)
The most active disinfectant is hypochlorous acid (HClO), which is a good electron acceptor or oxidizing agent. Since electron acceptors react with electron donors, we consider electron sources. Certain critical molecules, usually
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In the Classroom
enzymes containing mercapto (–SH) groups, in bacterial cells are willing electron donors. When an electron transfer occurs from the enzyme to HClO, the enzyme is deactivated, leading to cell death. In this way chlorine, added to water in concentrations of less than 2 ppm, destroys pathogenic microorganisms that cause cholera, dysentery, and typhoid fever (15). In this example we also discuss some descriptive chemistry of chlorine gas (Cl2), such as its color, physical state, and health effects. We also talk about the hypochlorite ion (ClO{) found in household bleach. We relate its ability to decolorize with its activity as an electron acceptor, and the role of electronic excitation–relaxation in color production. The concepts of electron transfer, oxidation, reduction, oxidizing agent, and reducing agent flow logically from this discussion. At this point students often ask what other electron acceptors (oxidizing agents) might be effective in water purification. This line of questioning prompts discussion of other chemical oxidizers, such as the hydroxyl radical (?OH), ozone (O3), and oxygen (O2). Consideration of the reaction of sodium metal with chlorine gas to form sodium chloride leads to generalization of the electron transfer process. We revisit the terms oxidizing agent and reducing agent and introduce half reactions. From a waste treatment point of view, we emphasize the principle that the chemical nature of the waste material must first be known to determine how best to treat it with added reagents. For example, wastes containing potentially good electron donors such as polychlorobiphenyls (PCBs) will likely be detoxified by an oxidizing agent such as oxygen gas (O2) at high temperatures or under the influence of a catalyst (16 ). Integration of current applications with chemical concepts is usually motivational. During the presentation of oxidation– reduction reactions we describe solar photocatalytic detoxification. This technology, developed at the Lawrence Livermore and Sandia National Laboratories, uses sunlight to produce powerful oxidizing agents directly in wastewater (17). In this process, sunlight is focused on a pipe carrying contaminated water. Inside the pipe, titanium dioxide (TiO2) catalyst is in contact with the water. Ultraviolet radiation in the sunlight causes the catalyst to accept electrons from hydroxide ions in the water, producing hydroxyl radicals (?OH) (eq 3). OH{ → ?OH + e{ (to catalyst) (3) These hydroxyl radicals, powerful electron acceptors, react with reducing agents in the water such as PCBs, chlorinated solvents, dioxins, and pesticides. This also provides the opportunity to reinforce the notion introduced earlier of the high chemical reactivity of electron deficient species. We then cover the rusting of iron metal in detail as another example of an economically important oxidation– reduction reaction. This section concludes with a consideration of the spontaneous electron transfer reaction between zinc metal and cupric ion. Discussion and demonstrations explore the operation of this voltaic cell in the production of electrical current and also illustrate the requirements for producing a useful battery. NEUTRALIZATION OR PROTON TRANSFER REACTIONS. There is a brief overview of the classical properties of acids and bases and Arrhenius’ definitions introduces acid–base reactions. Specific examples of acids and bases and the general Arrhenius neutralization reaction follow. We then develop neutralization as a waste treatment technology, using leather manufacturing
as an example. The leather production process generates both acidic and basic wastewater. By mixing them, both streams may be detoxified via the resulting neutralization reaction. Specifically, calcium hydroxide, or slaked lime, is used to aid in hair removal from animal hides. Later, in the tanning operation, sulfuric acid is used for cleaning (18). If these two waste streams are mixed in appropriate amounts, both the acid and the base are neutralized (eq 4). In this reaction, it is further noted that calcium sulfate (CaSO 4), or gypsum, is produced. H2SO4(aq) + Ca(OH) 2(aq) → CaSO4(s) + 2H2O(,) (4) Using stoichiometry concepts developed during the energy issue, students determine mole and mass ratios and perform mass–mass calculations for this reaction. This is followed by exploration of the general Arrhenius neutralization reaction, acid + base = salt + water, and the concept of a metathesis reaction is explored. The Brönsted–Lowry (B/L) approach to acids and bases is also developed. The concept of proton donor and proton acceptor flows naturally from the earlier notions of electron donor and acceptor. This parallel is highlighted to demonstrate the conceptual similarity of the two reaction types. We also delve into B/L acid–base reactions because they provide examples of reversible reactions and equilibrium. We examine acid strength tables and explore the notion that when using acid–base reactions in a waste treatment operation, the equilibrium should lie heavily in the direction of the less toxic substance. The added reagent, therefore, must be chosen carefully to ensure this. A typical exam question illustrating this point might be: “A tanker full of vinegar (5% HC2H3O2 in water) has overturned and spilled its contents on a street in your neighborhood. You have been asked to advise on how best to remove the acid as cheaply and completely as possible. How do you respond?” An acceptable answer proposes a neutralization reaction with a base, which establishes an equilibrium favoring the acetate ion, and considers the cost of that base. Household ammonia and lye are two frequent responses. Lecture and demonstrations show that aqueous solutions of acids and bases conduct electrical current and are therefore electrolytes. Discussion ensues on the definition of electrolyte and the categories of compounds that are electrolytes. This provides an opportunity to further consider the nature of ionic and covalent bonding and ionic and molecular compounds. The processes of ionization, dissolution of a molecular compound in water to yield ions, and dissociation, dissolution of an ionic compound in water to yield ions, are explained in detail in the context of electrolytes. We also make the connection between acid and base concentration and electrolytic strength. Class discussion includes the analysis of an electrolytic cell using sodium chloride and presents electroplating as an application of an electrolytic cell. In the course of this presentation, we also briefly discuss the topics of concentration, molarity, pH, and the autoionization of water, Kw. PRECIPITATION R EACTIONS. We approach precipitation reactions by suggesting that soluble toxic heavy metal ions in water solution may be removed by converting them to insoluble precipitates, which may be removed by filtration. Examples of precipitation of toxic metal ions from the leather (Cr3+) and electronics (Pb2+) industries are provided. The principles of water solubility and the water solubility rules are illustrated with demonstrations. We point out in these
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examples that many of the heavy metal ions are precipitated as hydroxides and sulfides. Students learn to use the solubility rules to predict the products obtained by mixing two solutions of soluble ionic compounds. Students are asked to apply their knowledge of solubility in the context of waste treatment. For example, one examination question stated: “A wastewater stream contains Cr3+ ion and is slightly basic (pH = 9). Explain how you would remediate this wastewater and why you chose this treatment.” We present the role of precipitation in the sedimentation of municipal water supplies. In secondary treatment, dirt and debris are swept from the water by allowing a sticky, gelatinous precipitate to settle from the top to the bottom of the water tank. The gelatinous precipitate (coagulating agent) is produced at the top of the tank by mixing two water-soluble salts, calcium hydroxide (sometimes sodium bicarbonate is used) and hydrated aluminum sulfate. The reactions yield the sticky precipitate aluminum hydroxide (eq 5). 3Ca(OH)2(aq) + Al2(SO4)3(aq) → 2Al(OH)3(s) + 3CaSO4(s) (5)
This discussion, when coupled with the filtration and chlorination topics discussed earlier in this unit, provides a good overview of the physical and chemical methods used in the purification of public drinking water supplies. Thermal Methods Thermal treatment methods use heat to degrade waste materials. They depend on providing sufficient kinetic energy to rupture chemical bonds. This topic provides an opportunity to reinforce the kinetic-molecular theory. We define both aerobic incineration, descriptively called thermal oxidation, and anaerobic pyrolysis. We restrict the remainder of the discussion to aerobic incineration. We discuss the differences in the response of organic and inorganic materials to incineration, and the products of the incineration process. In general, organic materials such as paper, plastics, and food end up as gases exiting from incinerator stacks, while noncombustible inorganic materials end up as ash. Incinerated organic materials contain several chemical elements, the most abundant of which are C, H, O, and Cl. These elements are converted to CO2 , H2O, and HCl. Students are asked to propose a method to rid the stack gas of HCl prior to atmospheric release. This relates to the earlier discussion of acids and bases. Many suggest use of a base such as lime (CaO), and this is exactly how HCl is removed in practice. Biological Methods Biological treatment methods, collectively called bioremediation, involve the use of living organisms to chemically change waste molecules into less toxic ones. Living organisms used for remediation may be either plant or animal. In the examples described in this section, we examine the reactants, products, and overall changes, but not the complex details of the changes. The applications of the technologies are stressed. PLANT. Plant remediation, or phytoremediation, is exemplified by the recent use of sunflowers to absorb radioactive isotopes from water (19). We discuss in greater detail, however, the first genetically engineered plant to be used in remediation, a mustard weed, produced in 1996 (20). The mustard weed was altered by the introduction of a bacterial gene encoding a protein that helps reduce toxic ionic mercury 1090
to the less toxic metallic mercury (Hg2+ → Hg0). The metallic mercury is then absorbed by the plant and released into the environment. This emphasizes the potential of genetic engineering in extending bioremediation. BACTERIA. By far the most common organisms used in bioremediation are bacteria, both aerobic and anaerobic. Either type can be intrinsic (native) or extrinsic (nonnative) (21). In aerobic bacteria, waste molecules act as electron donors (reducing agents) and oxygen acts as the electron acceptor in energy-producing metabolic reactions. In anaerobic organisms waste molecules function as electron acceptors. Anaerobic bacteria use various electron donors. Molecular hydrogen is used by a recently discovered and unusual species of bacterium (22). The naturally occurring anaerobic bacterium Dehalococcoides ethenogenes strain 195 has the ability to remove all the chlorine from chlorinated solvents such as tetrachloroethylene (TCE or perc) or trichloroethylene. Other bacteria remove only the first few chlorine atoms, leaving either dichloroethylene or vinyl chloride. The strain 195 bacterium reductively dechlorinates TCE to ethylene and hydrochloric acid (eq 6). in living bacterial cell
4H2 + C2Cl4 → C 2H 4 + 4HCl
(6)
Intrinsic microbes may be fertilized to encourage their growth as illustrated by the use of this on-site technique at the Exxon Valdez oil spill in Prince William Sound in 1989 (23). Rocks containing aerobic bacteria known to have an appetite for crude oil were sprayed with specially prepared nitrogencontaining growth fertilizer. As a result the bacteria remained active well above normal levels for 5 months. Bragg and coworkers (24 ) proved that intrinsic bacteria degraded the oil at a rate proportional to the nitrogen content of the fertilizer. The use of extrinsic aerobic microbes may include naturally occurring bacteria from another location or genetically engineered bacteria. The experience of the Rolscreen Corporation (now Pella Corporation) is illustrative here (25). Naturally occurring bacteria of the Flavobacterium genus were introduced into soil containing pentachlorophenol (PENTA or PCP), a wood preservative. Within 14 months PCP contamination was reduced by two-thirds. The aerobic bacteria used oxygen as the oxidizing agent to react with the reducing agent PCP in the production of energy for the bacteria. The bacteria convert the PCP (C6Cl5OH) into CO2, H 2O, and Cl{. This process nicely lends itself to a parallel discussion of animal cell reduction of glucose to CO2 and H 2O. The use of anaerobic microbes is especially helpful when dealing with remediation of bottom sediments in rivers and deep water areas where there is little or no oxygen. In these cases, the waste materials function as oxidizing agents for the bacteria. An in-class case study details the research conducted by General Electric in the Hudson River in New York using anaerobic bacteria to degrade PCBs (26 ). FUNGI. In 1985 it was observed that the common white rot fungus Phanerochaete chrysosporium degrades a variety of common hazardous wastes such as DDT, TNT, PCP, creosote, coal tars, and heavy fuels (27). Studies showed the DDT to be about 32% degraded after 32 days. The white rot fungus occurs naturally and is responsible for breaking down lignin in wood, which causes it to rot. Further, it is believed that the fungus produces several enzymes that catalyze oxidation reactions, which degrade the hazardous molecules in a manner similar to lignin degradation (27 ). Several commercial ventures
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exploiting this technology have been formed. For example, Groundwater Technology Inc. of Norwood, MA, has used fungal bioremediation on petroleum hydrocarbons. Summary Many students approach learning chemistry with increased interest when it is presented in the context of a meaningful issue. Waste treatment is one such issue. Waste treatment technologies can be categorized as physical or chemical. The chemical methods include oxidation–reduction, acid–base, and precipitation reactions. In addition, while discussing these reaction types, many previously discussed topics may be reinforced. Further, waste treatment processes using thermal and biological methods extend the scope of topics while providing additional opportunities to both reinforce previously discussed concepts and introduce new ones. Current and returning students have emphasized their interest in chemistry and their enhanced ability to recall it when it is presented in this fashion. Clearly there are many issues and contexts within which chemistry can be taught, but waste treatment works well with business students at Babson College. Approaches and content used in this course might work well in other introductory chemistry courses. Literature Cited 1. Committee on Undergraduate Science Education. Science Teaching Reconsidered; National Academy of Sciences: Washington, DC, 1997; pp 1–7. 2. Adams, D. L.; Philips, J. M. Bull. Sci. Technol. Soc. 1991, 11(3), 155–160.
3. Bunce, D. M. J. Coll. Sci. Teach. 1995, 25(3), 169–171. 4. Leamnson, R. N. J. Coll. Sci. Teach. 1996, 25(5), 334–336. 5. Moore, C. B. Sweeping Change in Manageable Units: A Modular Approach to Chemistry Curriculum Reform; The ModularCHEM Consortium; http://mc2.cchem.berkeley.edu (accessed April 1999). 6. Spencer, B. ChemLinks Coalition: Curricular Reform Using Thematic Modules to Change How Undergraduates Learn Chemistry; http:// chemlinks.beloit.edu (accessed April 1999). 7. Adams, D. L. J. Chem. Educ. 1991, 68, 483–485. 8. Adams, D. L. J. Chem. Educ. 1993, 70, 574. 9. Nakhleh, M. B.; Bunce, D. M.; Schwartz, A. T. J. Coll. Sci. Teach. 1995, 25(3), 174–180. 10. Schwartz, A. T.; Bunce, D. M.; Silberman, R. G.; Stanitski, C. L.; Stratton, W. J.; Zipp, A. P. J. Chem. Educ. 1994, 71, 1041–1044. 11. Buell, P.; Girard, J. Chemistry—An Environmental Perspective; Prentice-Hall: Englewood Cliffs, NJ, 1994. 12. Shelly, S. Chem. Eng. 1990, 97(6), 47–54. 13. Hileman, B. Chem. Eng. News 1996, 74(30), 14–20. 14. White, G. C. The Handbook of Chlorination; Van Nostrand Reinhold: New York, 1986; pp 194–197. 15. Meltzer, T. H. High-Purity Water Preparation; Tall Oaks: Littleton, CO, 1993; p 308. 16. Williams, W. Environ. Sci. Technol. 1994, 28, 630–635. 17. Rajeshwar, K.; Ibanez, J. G. J. Chem. Educ. 1995, 72, 1044–1049. 18. Traubel, H. Chemtech 1992, 22, 340–345. 19. Anonymous. Chem. Ind.. 1996, No. 6, 193. 20. Meagher et al. Proc. Natl. Acad. Sci. USA 1996, 93, 3182-3187. 21. Ellis, B.; Gorder, K. Chem. Ind. 1997, No. 3, 95–99. 22. Maymó-Gatell, X.; Chien, Y.-t.; Gossett, J. M.; Zinder, S. H. Science 1997, 276, 1568–1571. 23. Crawford, M. Science 1990, 247, 1537. 24. Bragg, J. R.; Prince, R. C.; Harner, E. J.; Atlas, R. M. Nature 1994, 368, 413–418. 25. Chollar, S. Discover 1990, 11(4), 76–78. 26. Stover, D. Pop. Sci. 1992, 241(1), 70–74,93. 27. Illman, D. L. Chem. Eng. News 1993, 71(28), 26–29.
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