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Magnetic Particle Technology A Simple Preparation of Magnetic Composites for the Adsorption of Water Contaminants Luiz C. A. Oliveira, Rachel V. R. A. Rios, José D. Fabris, and Rochel M. Lago* Departamento Química, ICEx, UFMG, 31270-901 Belo Horizonte, MG, Brazil; *
[email protected] Karim Sapag Laboratorio de Ciencias de Superficies y Medios Porosos, Universidad Nacional de San Luis, Chacabuco 917, 5700 San Luis, Argentina
Recovering contaminated sites, treating industrial effluents, and managing hazardous wastes are challenging topics in environmental science that demand a great deal of attention from chemists, biologists, and engineers. Because of the great variety of factors related to the chemical and physical features of the contaminant and the contaminated site, remediation processes are complex and often require specific solutions where creativity and innovation are key elements. Therefore, laboratory experiments introducing innovative aspects in environmental technology that are capable of stimulating the student’s imagination are of great interest in an environmental chemistry curriculum. An innovative technology that has gained attention is the use of magnetic materials to solve environmental problems, such as accelerating the coagulation of sewage (1), removing radionuclides from milk (2), adsorption of organic dyes (3), and oil spill remediation (4). In this article an undergraduate laboratory experiment to produce magnetic adsorbents is described. These magnetic composites show the following features: (i) they combine the adsorption properties of adsorbents, such as activated carbon or clays, with the magnetic properties of iron oxides, (ii) they can be used to remove different types of contaminants, for example, metals or organics, from water, and (iii) they can be removed from the medium by a simple magnetic separation process. One of the pedagogical benefits of this simple experiment is that the students are exposed to stimulating environmental problems that illustrate several important aspects of basic chemistry, such as the properties, characterization, and application of different materials and composites. Preparation of the Magnetic Composites The preparation of the magnetic composites is simple and can be carried out in a 2-h laboratory by college or even high school students. The chemicals required are readily available and not expensive. The composites can be prepared using the adsorbents, for example, activated carbon or clay, suspended in a 400-mL solution of FeCl3 (7.8 g, 28 mmol) and FeSO4 (3.9 g, 14 mmol) at 70 ⬚C. To this suspension a solution of NaOH (100 mL, 5 mol L᎑1) is added dropwise to precipitate the iron oxides. The quantity of the adsorbent can be varied between 3.3, 6.6, or 9.9 g to obtain adsorbent:iron oxide weight ratios of 1:1, 1.5:1, and 2:1. The composites are washed and dried in an oven at 100 ⬚C for 2 h. A simple test with a magnet (0.3 T) can be carried out to show that the composite material is magnetic. 248
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The characterization of the prepared materials can be of important educational value, especially for more advanced students to get familiar with different techniques, such as: (i) X-ray diffraction—one of the most important techniques in material characterization, it gives the students information on the crystalline structure of the clays and the identification of the iron oxide phases; (ii) Mössbauer spectroscopy—an important tool for the identification of iron compounds; (iii) magnetization measurements—can be the basis for a discussion with the students on the magnetic properties of the different materials and their modern technological applications; (iv) scanning and transmission electron microscopies (SEM and TEM)—can afford textural and structural information of the prepared materials; and (v) surface-area measurement—a fundamental technique in material science, specially in catalysis and adsorption. Detailed characterization of the composite material (available in the Supplemental MaterialW) suggests the formation of the magnetic phase maghemite with small quantities of magnetite and hematite (5, 6). Hazards Concentrated NaOH is caustic and can cause serious damage to the skin. The heavy metal solutions, for example, Cd2+, Zn2+, and Pb2+, are toxic if ingested and must be disposed of in accord with local regulations. Application of the Magnetic Composites The experiments presented have been designed to illustrate applications of the magnetic adsorbents to remove organic and inorganic contaminants from water and the possibility to use them, for example, in an oil spill remediation. Many other applications can be proposed and the students should be encouraged to find new uses for the prepared materials.
Harvesting the Magnetic Composite This experiment illustrates the most important feature of these magnetic materials, namely the facility to remove the adsorbent from the medium by a simple magnetic process. Activated carbon magnetic composite is mixed with a solution containing a colored organic substance (concentration ≈ 1 mg兾L), such as a dye or an acid–base indicator. Upon stirring with a glass rod the solution becomes colorless owing to the adsorption on the activated charcoal. If a magnet
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is put in contact with the flask’s outside wall, the suspended magnetic composite will be attracted to the magnet and the solution is easily separated (Figure 1).
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Figure 2. Ni2+ (䊏), Cu2+ (夹), Cd2+ (䊊), and Zn2+ ( 䉭) adsorption isotherms at room temperature using the clay magnetic composite (Ceq is the metal concentration and qeq is the quantity of metal absorbed at the equilibrium).
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Adsorption of Organic Contaminants onto Activated Carbon Composites For these experiments it will be necessary to simulate organic contaminated water, for example, using colored substances such as a dye or an acid–base indicator. The adsorption experiments can be carried out with 50 mg of the activated carbon composites in 50 mL of 10, 20, 50, and 100 mg兾L solutions of dye. All the adsorption experiments should be carried out at the same pH (usually 7) and stored for a period of 24 h at room temperature. The composites are removed magnetically from the solution and the dye concentration determined by UV–vis measurements. The student can use these data to produce an adsorption isotherm. A typical adsorption isotherm obtained for a textile dye is shown in Figure 3. If no UV–vis spectrophotometer is available, the students can visually evaluate the adsorption for low dye concentrations (ca. 2 mg兾L). This demonstration can also be carried out on an overhead projector using a Petri dish containing a dye solution (5 mg兾L) and adding ca. 15 mg of the activated carbon composite and stirring. If a magnet is put in contact
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Figure 1. Schematic representation of the magnetic composite separation.
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Adsorption of Metal Ion Contaminants onto Clay Composites Removal of toxic heavy metal contaminants by adsorption has been extensively used to treat wastewaters. In this experiment the prepared clay magnetic composites are studied as adsorbent for the metals Cu2+, Ni2+, Cd2+, and Zn2+. The experiments can be carried out with 50 mg of the clay composites in 50 mL of 10, 20, 50, and 100 mg兾L solutions of the metal (e.g., nitrate salts of Cu2+, Ni2+, Cd2+, and Zn2+). The solutions with composites should have their pH adjusted to 5.0 with diluted HCl and stored for a minimum period of 24 h at room temperature. These solutions should be stored in well-sealed flasks to avoid evaporation. The composites are then removed magnetically and the metal concentration in the solution determined by atomic absorption spectrophotometer. The students can use these data to produce an adsorption isotherm. Information on the removal efficiency of the different metals by the adsorbents can be obtained from the isotherms (Figure 2). The isotherms suggest high adsorption capacities for the metals in the following order Cd2+ ≈ Zn2+ > Cu2+ > Ni2+.
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Oil Spill Remediation In this experiment the problem of oil spill accidents, such as those related to petrochemical industries, is introduced and creates the opportunity to discuss the environmental consequences and the remediation strategies with the students. An oil spill simulation can be made by mixing few milliliters of oil, such as a cooking soybean oil, engine oil, or even gasoline in a beaker containing 400 mL of water. To the oil suspension is added 2 g of the magnetic composite. After stirring for 5 min the separation is carried out as described above. The oil-saturated composite material can be washed with an organic solvent such as hexane or even acetone to remove the oil and recover the adsorbent.
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Figure 3. Drimaren red dye adsorption isotherms at room temperature on activated carbon magnetic composites. Ratios are activated carbon:iron oxide, Ceq is the dye concentration and qeq is the quantity of dye absorbed at the equilibrium).
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with the outside wall of the dish the magnetic composite is attracted to the magnet and the projected image will be transparent and colorless as a result of the adsorption. Conclusions The proposed experiments are simple, requiring readily available chemicals and straightforward procedures. The experiments are related to environmental problems and illustrate several important aspects in basic chemistry, such as material sciences, characterization techniques, and adsorption processes (adsorption isotherms, surface area, cation exchange, adsorption of organics and metal ions). The suggested experiments are very flexible allowing the application of the prepared adsorbents to different substances, including real contaminants existing in the student’s neighborhood. Acknowledgments The authors are grateful to CAPES兾SCyT, CNPq, and FAPEMIG for financial support. W
Supplemental Material Materials available in this issue of JCE Online include: • Instructions for the students • Notes for the instructor
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• CAS registry numbers • Time required for each experimental step • General information on magnetic particle technology, clays, and activated carbon • Detailed characterization of the composite materials
Literature Cited 1. Booker, N. A.; Keir, D.; Priestley, A.; Rithchie, C. D.; Sudarmana, D. L.; Woods, M. A. Water Sci. Technol. 1991, 123, 1703–1711. 2. Sing, K. S. Technology Profile, Ground Water Monitor 1994, 21, 60–67. 3. Safarik, I.; Safarikova, M.; Buricova, V. Collection 1995, 60, 1448–1456. 4. Orbell, J. D.; Godhino, L.; Bigger, S. W.; Nguyen, T. M.; Ngeh, L. N. J. Chem. Educ. 1997, 74, 1446–1452. 5. Coey, J. M. D. Magnetic Properties of Iron in Soil Iron Oxides and Clay Minerals; Stucki, J. W., Goodmann, B. A., Schwertmann, U., Eds.; Reidel Publishing: Dordrecht, Holland, 1988; pp 397–466. 6. Oliveira, L. C. A.; Lago, R. M.; Rios, R. V. R. A.; Fabris, J. D.; Soares, W. T.; Sapag, K. Clay–Iron Oxide Magnetic Composites For The Adsorption Of Contaminants In Water; 12th International Clay Conference, Bahia Blanca, Argentina, July, 2001.
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