Removal of Aromatic Pollutant Surrogate from Water by Recyclable

Aug 28, 2014 - A general chemistry laboratory experiment using readily available chemicals is described to introduce college students to an exciting c...
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Removal of Aromatic Pollutant Surrogate from Water by Recyclable Magnetite-Activated Carbon Nanocomposite: An Experiment for General Chemistry Ping Y. Furlan* and Michael E. Melcer Math and Science Department, U.S. Merchant Marine Academy, Kings Point, New York 11024, United States S Supporting Information *

ABSTRACT: A general chemistry laboratory experiment using readily available chemicals is described to introduce college students to an exciting class of nanocomposite materials. In a one-step room temperature synthetic process, magnetite nanoparticles are embedded onto activated carbon matrix. The resultant nanocomposite has been shown to combine the adsorption ability of the activated carbon and the magnetic properties of the magnetite nanoparticles, enabling its application as a fast, effective, low-cost, and recyclable aromatic water pollutant adsorbent. This quality is illustrated by its rapid removal of the surrogate “pollutants”, made of several dyes in the Fisher universal indicator, within 2−3 min. A successful “pollutant” removal is indicated by the absence of the rainbow colors because of the presence of the “pollutants” in the “polluted” water when different quantities of an acid or a base are added. The nanocomposite’s reusability as the “pollutant” adsorbent is demonstrated after its used surface is regenerated using ethanol as the extracting solvent. The exercise allows students to (i) gain awareness of timely environmental issues; (ii) be exposed to the modern field of nanoscience; and (iii) appreciate the roles new and advanced materials play in keeping our water clean. Students have fun working in the lab and find the experience interesting and motivating. The experiment is also suitable for advanced high school students. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Environmental Chemistry, Interdisciplinary/Multidisciplinary, Hands-On Learning/Manipulatives, Acids/Bases, Laboratory Instruction, Magnetic Properties, Materials Science, Nanotechnology

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INTRODUCTION

stress dissipating organic polymers. Nanocomposites offer unique properties that include valuable technological applications and that are not achieved through use of conventional materials. An early exposure to this important class of materials may peak students’ interest in science.

Nanomaterials and Nanocomposites

In recent years, cutting-edge “nano”-based topics and laboratory exercises have been introduced into early college curricula as a successful strategy to raise students’ enthusiasm toward science, technology, engineering and mathematics to enhance their academic and occupational advancement opportunities.1,2 Since the 1990s, the Journal has published a series of colorful (literally and figuratively), attention-grabbing and easily adaptable laboratory exercises connecting nanoscale science and technology to our everyday lives. Classic examples include the preparations and properties of ferrofluids, gold nanoparticles, nickel nanowires, and quantum dots.3−6 However, hands-on exercises illustrating the concept, synthesis and applications of nanocomposites at the introductory level are limited. A nanocomposite is a multiphase solid material where one of the phases has one, two, or three dimensions of less than 100 nm. Nanocomposites represent an exciting class of advanced materials due to their synergistic and/or hybrid properties derived from their components. Natural nanocomposites are widespread. Examples are hard and tough bones, teeth, molluscan shells and sea urchin spine that are built of alternating nanolayers of hard inorganic minerals and soft This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society

Magnetite-Activated Carbon (MAC) Nanocomposite as a Recyclable Aromatic Pollutant Adsorbent

The dramatic demonstration of attraction of ferrofluid to magnets in science museums and classrooms and their easy preparation in laboratories3 have allowed magnetite (Fe3O4) nanoparticles to gain popularity among college and high school students. With few exceptions,7 direct and personal experience with these captivating nanoparticles’ real-life applications, including those that solve environmental problems, is lacking. Our motivation here is to fill this gap via a new laboratory exercise, which we believe would add tremendously to students’ enthusiasm for science.1,2,8−11 Much of today’s research has been focused on magnetite nanoparticles for their potential for removing various contaminants, due to (1) their easy separation by applying a magnetic field; (2) their reactive surfaces; (3) their large surface

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to volume ratio; and (4) their relatively low cost.12 This contaminant removal is based on the simple and effective adsorption mechanism. Our recent work, however, showed the nanoparticles’ ability for adsorbing certain organic molecules, including colored dyes, was limited when the water was around neutral pH or under basic conditions. For instance, we found that their adsorption of bromocresol green dye molecules did not occur rapidly and/or significantly until the water pH was lowered to 3.4. Under such a pH condition, however, the magnetite nanoparticles appeared less magnetic. Activated carbon (AC) is renowned for being an effective absorbent for the removal of a wide variety of organic matter, including organic dyes and polycyclic aromatic hydrocarbons (PAHs). AC’s adsorbent qualities are due to its extended surface area, complex porous structure, high adsorption capacity and high degree of surface reactivity.13−16 Because of its fine particle sizes, AC passes through filters and is therefore difficult to separate from solutions. In large scale water treatment processes, this and related problems may result in high operational cost. In this manuscript, we describe a simple 2-h laboratory exercise that allows college or high school students to prepare a nanocomposite by incorporating magnetite nanoparticles onto an AC matrix in a one-step room-temperature synthetic process. The synthesis is based on the procedure for making aqueous magnetite nanoparticles by simply adding AC to the reaction mixture.3,17,18 The resultant magnetite-activated carbon (MAC) nanocomposite has shown the desired properties of both components: the adsorption ability of the AC and the magnetic properties of the magnetite nanoparticles. Its environmental application is exemplified through its rapid and effective removal of selected pollutant surrogates from water. The described exercise also demonstrates the recyclability of the MAC nanocomposite for repeated use as a pollutant adsorbent.

Figure 1. Structures of dye molecules in a Fisher universal indicator solution.

of them are identified as carcinogenic, mutagenic, and teratogenic. The United States Environmental Protection Agency (EPA) has included PAHs in its list of priority pollutants to be monitored.16 Finally, the FUI is selected to allow students to observe the spectacular rainbow colors the indicator generates over the pH range from 4 to 10 and to make a visual estimate of the pH of a water solution (Figure 2).

Fisher Universal Indicator as the Water Pollutant Surrogate

The Fisher universal indicator (FUI) is composed of a solution of several dye molecules that show smooth color changes from red to violet over a pH value range from 4 to 10. It is composed of the following colored molecules: methyl red, thymol blue, bromothymol blue, and phenolphthalein. The structures of these dye molecules are shown in Figure 1. The FUI is used to simulate aromatic pollutants or pollutants containing one or more benzene rings, such as dyes and PAHs. Due to the benzene ring stability, aromatic pollutants are among the most abundant environmental pollutants. The use of different types of dyes in textile, paper, leather, cosmetics, and other industries is well-known. The effluents of these industries dispose a large quantity of dye contents into streams and rivers which can cause environmental problems by absorbing light and interfering with fundamental aquatic biological process.19 In addition, they are relatively stable and potentially carcinogenic and toxic.19 In our students’ case who are studying for maritime careers, PAHs are a ubiquitous class of organic compounds which have been identified in high concentrations in a variety of water bodies.15 PAHs occur in oil, coal and tar deposits and are produced as byproducts of oil heating or fuel burning. Most PAHs enter the marine environment in urban runoff, municipal and industrial waste discharges, bilge and fuel oil leaks associated with day-to-day shipping operations, and oil spills from maritime accidents.20 They are of concern because some

Figure 2. 2% (w/w) Fisher universal indicator solution. The solution contains a mixiture of colored organic dye molecules that simulate water “pollutants”.

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PROCEDURE

Materials and Equipment

• Neodymium magnets • Ring stand, 25 mL or 50 mL buret, buret clamp, 50 mL beakers, test tubes, a glass stirrer, and disposable droppers • Two 5 mL or 10 mL graduated pipettes with pipet controllers, two 10 mL graduated cylinders • 2.0 M FeCl2 in 2 M HCl, 1.0 M FeCl3 in 2 M HCl, 1.4 M NH3, 0.10 M HCl, 0.10 M NaOH, 2% (w/w) FUI aqueous solutions, ethanol • Activated carbon powder treated with 4 M HNO3 • pH = 6 phosphate buffer • “Polluted” water [2% (w/w) FUI solution and phosphate buffer in 10:1 (w/w) ratio] Preparation of MAC Nanocomposite

The MAC nanocomposite was prepared by mixing 4.0 mL of FeCl3 (1.0 M), 1.0 mL of FeCl2 (2.0 M), and 0.10 g of activated B

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carbon powder with a glass stirring rod in a 50 mL beaker for 2−3 min. Into the continuously stirred mixture, a 25 mL of 1.4 M ammonia aqueous solution was added dropwise through a buret over a period of 5−10 min. The beaker was then placed on a magnet to allow the MAC nanocomposite particles to settle. The solution was decanted with the magnet in contact to the bottom of the beaker. The particles were washed with deionized water 1−2 additional times. Removal of Organic Water Surrogate “Pollutants”

A 10 mL portion of the “polluted” water was added to the damp MAC nanocomposite particles in the 50 mL beaker, and the mixture was stirred for 2−3 min. The beaker was placed on a magnet to allow the upper water to turn clear or be “cleaned” (this may take 5−10 min). A 5 mL portion of the “cleaned” water and a 5 mL portion of the “polluted” water were placed into two test tubes. A 0.10 M HCl solution was added dropwise to these water samples, followed by dropwise addition of the 0.10 M NaOH solution. “Pollutant” removal was examined qualitatively based on a visual inspection of the relative intensities of the colors induced in these samples by the added acid or base.

Figure 3. Atomic force microscope (AFM Workshop, dynamic mode, 0.1 Hz, 1024 pixels/line, tip radius = 8 nm) image of magnetite nanoparticles. Scale bar = 500 nm. The magnetite nanoparticles were prepared by following the procedure for making ferrofluid. The image shows uniform particles with sizes smaller than 20 nm (a particle with diameter of 16 nm is shown).

Recovery and Recyclability of MAC Nanocomposites

The MAC nanocomposite with adsorbed “pollutant” was collected by decanting off the “cleaned” water (the upper clear water in the 50 mL beaker in the previous step) with the aid of a magnet. A 10 mL portion of ethanol was added to the used MAC nanocomposite. After stirring for 2−3 min, the regenerated MAC nanocomposite was collected with the aid of a magnet. The quantity of extracted “pollutant” surrogate in ethanol was visually compared to the quantity in a 50% “polluted” water (for comparison, two equal volume samples were adjusted to a similar color by adding the 0.10 M HCl or NaOH solution). The recovered MAC nanocomposite was reused and the cycle of adsorption−desorption was repeated. Further procedural details are provided in the Supporting Information.

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HAZARDS Gloves and goggles must be worn at all times. Aqueous ammonia, FeCl2, FeCl3, HCl and NaOH are corrosive. FeCl2 is also a mutagen. Ethanol is flammable. Handle all of these materials with care and wash immediately with water in case of skin contact. The magnets strongly attract each other. Avoid placing two magnets in close proximity to avoid pinching.

Figure 4. Atomic force microscope (AFM Workshop, dynamic mode, 0.1 Hz, 1024 pixels/line, tip radius = 8 nm) image of activated carbon. Scale bar = 500 nm. The image reveals a large number of nanopores that are attributable to the large surface area of activated carbon for adsorption (a pore with diameter of 25 nm is shown).

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RESULTS AND DISCUSSION The synthetic method typically produced 2−3 g of damp MAC nanocomposite, equivalent to 0.2−0.4 g of dry product. The approach described here is to embed nano-Fe3O4 particles onto the activated carbon matrix. The ferrofluid, prepared based on ref 3, produced small and uniform magnetite particles with sizes smaller than 20 nm as shown in the atomic force microscope (AFM) image in Figure 3. The magnetite particle size appeared to depend on the duration over which the ammonia solution was added to the ferric and ferrous salt mixture: shorter duration (about 5 min) produced smaller particles that “spiked” readily when placed near a magnet. Small magnetite particles are desirable, since they have large surface to volume ratio, large reactive surfaces and strong magnetism.18 Figure 4 is the AFM image of the AC, which clearly revealed a large number of nanopores that are attributable to the large surface area of AC for adsorption. Figure 5 shows the agglomerates of the

magnetite nanoparticles embedded onto the surfaces of the microsized activated carbon particles. The embedded nanoparticles show a similar size as that of the isolated ones. The procedure involved mixing or wetting the porous AC with a magnetic precursor in solution. This allowed the small magnetic particles, when formed, to be intimately mixed with and well integrated into the AC matrix, a desirable structure for the nanocomposite’s magnetic separation. All of the AFM images were scanned by the faculty. The student procedure effectively utilizes the student lab time and minimizes glassware transferring. We chose to let students prepare the product at room temperature using 0.1 g of the AC. Student titrated with 25 mL of 1.4 M NH3 instead of C

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Since the MAC nanocomposite has a finite number of adsorption sites, its adsorption potential eventually ceases when all the sites are occupied. To illustrate that the occupied surface can be regenerated, we let students use ethanol, the common and widely available solvent, to extract the “pollutants” from the used nanocomposite. Visual inspection showed a much higher than 50% recovery based on the color intensities between 50% “polluted” water and the ethanol containing the extracted “pollutants”. The absorption−desorption process was successfully repeated that demonstrated the reusability of the MAC nanocomposite. Our results showed that, even after 5−6 cycles, no noticeable loss in the MAC nanocomposite’s cleaning ability was observed. The process can also be used to greater advantage by recovering the desorbed “pollutants” via the removal or recovery of the ethanol. The exercise was performed along with the classic ferrofluid experiment3 by forty-six freshmen (plebes) and one senior student during a 2 h laboratory period at the United States Merchant Marine Academy (USMMA). The participating students majored in marine engineering. Groups of 3−5 students were formed that were further divided into A and B subgroups. Those in subgroup A followed the procedure for the ferrofluid experiment; those in subgroups B followed the MAC nanocomposite experiment. Subgroups A and B then shared and combined their results. Another approach utilized the ferrofluid prepared in advance as a demonstration and had the students carry out the MAC nanocomposite procedure. The student survey results showed a very positive attitude toward this laboratory experience. 100% (N = 47 students) agreed the lab exercise helped them learn new things, see the use of chemistry, experience the fun of science, and improve their interest in and attitude toward science. These sentiments were further illustrated by students’ comments. The comments on the concepts they learned included the following: • It is possible to clean polluted water through the use of nanoparticle-based material. • Fluid can be magnetic and can spike when magnetized; magnetite nanoparticles can help clean water. • Nanoparticles are really small particles with sizes below 100 nm. They are special and can be very useful. • Magnetic properties came from unpaired, parallel spinning electrons. • Magnetite nanoparticles combined with activated carbon can easily remove water pollutants. • Magnetite nanoparticles react very strongly to magnet; pure carbon makes things very dark and difficult to clean; polluted water can easily be decontaminated using carbons decorated with magnetite nanoparticles! • The exercise allowed me to understand what makes compounds/elements magnetic and how we could use them to make a cleaner planet. • Magnetite nanoparticles + magnets → FUN!!! Other comments relating to attitudes are as follows: • This is interesting because it relates to environmental concerns that we have been discussing in Marine Engineering. • This lab showed me how interesting science can be, especially when I could see the first-hand results of its applications. • I thought it was a very interesting lab. The results we got were way cool! I also liked all the colors the “polluted” water displayed.

Figure 5. Atomic force microscope (AFM Workshop, dynamic mode, 0.1 Hz, 1024 pixels/line, tip radius = 8 nm) image of magnetiteactivated carbon nanocomposite. Scale bar = 500 nm. The image shows the agglomerates of the magnetite nanoparticles embedded onto the surfaces of the microsized activated carbon particles. The embedded nanoparticles show a similar size as that of the isolated ones (a particle with diameter of 18 nm is shown).

50 mL of 0.7 M NH3.3 The total product was then applied to clean 10 mL of 2% (w/w) “polluted” water (pH = 6). After stirring the mixture and allowing the particles to settle, the yellow colored “polluted” water visibly turned clear (see Figure 6)! When dropwise addition of, first a 0.10 M HCl solution,

Figure 6. USMMA students enjoy the “nano” time during their 2 h lab period. The yellow solution, buffered at pH = 6, contains a mixture of dye molecules. It simulates the “polluted” water, which becomes clear after the dye molecules are adsorbed onto the magnetite-activated carbon nanocomposite and removed, along with the magnetic nanocomposite, from the water.

and then a 0.10 M NaOH solution, was applied to a 5 mL portion of the “polluted“ water, the water exhibited reversible spectacular rainbow colors (Figure 1). On the other hand, the “cleaned” water stayed clear when the same addition of the acid and base solutions was performed, suggesting no noticeable presence of the pollutant surrogates in the water. D

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• This experiment was very interesting and hands-on. It gave us a chance to make something cool and useful. It was really interesting learning about nanoparticles and how they function to protect our environment. • It was interesting to see the physical property changes as we conducted the experiment. • I once saw a video on ferrofluids and how they reacted to magnetic fields. I was intrigued by the video and now I finally get to learn the science behind it, as well as making ferrofluid myself and actually seeing how they can help clean water!

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CONCLUSION We have developed a simple experiment that allows college students to prepare a MAC nanocomposite by incorporating magnetite nanoparticles onto an AC matrix in a one-step room temperature synthetic process. The MAC nanocomposite has been shown to combine the adsorption ability of the AC and the magnetic properties of the magnetite nanoparticles, enabling its applicability as a fast, efficient, low-cost, and recyclable pollutant adsorbent. This principle is illustrated by its removal of the pollutant surrogates made of a mixture of several dyes in the Fisher Universal Indicator. The exercise has allowed students to gain exposure to the modern field of nanoscience and technology and to gain direct working experience with functional nanocomposite materials. Students enjoy and appreciate the experience and find it interesting and motivating. The experiment can be done within 2 h and is also suitable for advanced high school students.

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ASSOCIATED CONTENT

* Supporting Information S

Handouts for students and notes for instructors. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare the views expressed in this article are the authors’ own and not those of the U.S. Merchant Marine Academy, the Maritime Administration, the Department of Transportation, or the United States government. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Jerry Doumas, James Schlauch, the students taking chemistry classes, and the Math and Science faculty and staff for their enthusiastic support of this project.

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REFERENCES

(1) Furlan, P. Y. Integrating nanoscience and technology in natural science associate degree and relocation programs serving underrepresented students. J. Nano. Educ. 2014, 6, 12−24. (2) Furlan, P. Y. Engaging students in early exploration of nanoscience topics using hands-on activities and scanning tunneling microscopy. J. Chem. Educ. 2009, 86 (6), 705−711. (3) Berger, P.; Adelman, N. B.; Beckman, K. J.; Campbell, D. J.; Eillis, A. B.; Lisensky, G. C. Preparation and properties of an aqueous ferrofluid. J. Chem. Educ. 1999, 76 (7), 943−948. Breitzer, J.: Lisensky, G. Synthesis of aqueous ferrofluid. http://education.mrsec.wisc.edu/ nanolab/ffexp/index.html (accessed Aug 2014). E

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