Adsorption of Arsenic by Iron Oxide Nanoparticles: A Versatile, Inquiry

LABORATORY EXPERIMENT pubs.acs.org/jchemeduc

Adsorption of Arsenic by Iron Oxide Nanoparticles: A Versatile, Inquiry-Based Laboratory for a High School or College Science Course Daniel VanDorn,† Matthew T. Ravalli,† Mary Margaret Small,‡ Barbara Hillery,§ and Silvana Andreescu*,† †

Department of Chemistry and Biomolecular Science, Clarkson University, Potsdam, New York 13699-5810, United States Office of Educational Partnerships, Clarkson University, Potsdam, New York 13699-5505, United States § Department of Chemistry, SUNY Old Westbury, Old Westbury, New York 11568, United States ‡

bS Supporting Information ABSTRACT:

There has been much interest in magnetite (Fe3O4) due to its utility in adsorbing high concentrations of arsenic in contaminated water. The magnetic properties of the material allow for simple dispersion and removal from an aqueous system. An inquiry-based laboratory has been developed that illustrates these unique properties of magnetite nanoparticles while developing cross-disciplinary and critical-thinking skills. The versatility of the pedagogic approach makes it suitable for both high school (grades 11 and 12) and college-level STEM (science, technology, engineering, and math) courses. The lab experience exposes students to experimental design, environmental remediation, adsorption, and the surface area-to-volume ratio concept of nanotechnology. KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Analytical Chemistry, Environmental Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Applications of Chemistry, Nanotechnology, Water/Water Chemistry

’ EQUIPMENT AND REAGENTS Optimally, flame atomic absorption (AA) should be used to introduce students to spectroscopy. However, this was not available at the institutions visited. Thus, an arsenic test kit was used (HACH, Cat. 28228-00). The kit has a limited sensitivity, necessitating the high arsenate concentration described below. Iron oxide nanoparticles of 10 12 ((5) nm, were prepared using established protocols.13,14

T

here has been much interest in magnetite (Fe3O4) due to its high efficiency adsorption of arsenic(III) and arsenic(V) in contaminated water. Arsenate forms a strong complex with the iron material, which can then be removed from the aqueous system.1 3 The greater surface area per unit mass of magnetite nanoparticles leads to a higher rate of adsorption than bulk iron. Also, the magnetic properties of the magnetite material allow for simple dispersion and removal from an aqueous system.4,5 Thus, magnetite nanoparticles are promising materials for remediation of arsenic contaminated source water. An inquiry-based laboratory has been developed to demonstrate these unique properties of magnetite nanoparticles while also developing cross-disciplinary and critical-thinking skills. Inquiry-based pedagogy has been described recently in this Journal and elsewhere.6 8 This inductive approach has been assessed at a range of institutions and has gained popularity due to its clear efficacy in increasing student engagement.9 12 In cooperation with K 12 Project-Based Learning Partnership Program, this activity was conducted with 55 high school students enrolled in advanced-placement chemistry and physics for two consecutive school years at Clarkson University. Additionally, this activity was tested in a general chemistry course with 22 students at SUNY Old Westbury during the summer of 2009. Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.

’ HAZARDS Arsenic is a highly toxic substance. The instructor should prepare the vials beforehand to reduce student exposure. All lab work should be performed on surfaces protected with absorbent covering. Students must be required to wear goggles, gloves, and lab coats/aprons to reduce risk of exposure. The HACH EZ Arsenic Detection Kit releases arsine and hydrogen gas during operation. Accordingly, all work must be performed under a fume hood if this apparatus is used in place of spectroscopic analysis. The test strips used contain mercury. The neodymium Published: May 25, 2011 1119

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Journal of Chemical Education

Figure 1. Treatment systems for arsenic removal: all vials contain 10 mL of 500 ppb As. From left to right: control sample (no treatment), nanoparticle-treated system with 50 mg of magnetite, bulk iron-treated system with 50 mg of iron shavings.

LABORATORY EXPERIMENT

Figure 3. HACH arsenic assay results from a high school chemistry class: the colorimetric assay allows for comparative analysis of arsenic concentration.

students who have not been previously exposed to nanotechnology.

’ LAB ACTIVITY Sample treatment (Figure 1) consisted of placing the iron material (50 mg) into a vial with 10 mL of 500 ppb arsenic (CAS 7440-38-2; a much lower concentration can be used if spectroscopic analysis is available). Students recapped the vials and agitated the solution for an assigned period of time. At this point, the students were asked to propose a method to remove the iron material from the sample. After filtration was discouraged by the instructor as time-consuming and costly (for the water treatment facility), students mentioned magnets after reviewing the physicochemical and magnetic properties of these materials. Neodymium magnets were supplied to each group for iron removal (Figure 2). After iron separation, students poured the supernatant into the HACH EZ test vessel for arsenic quantification. Figure 2. Separating the iron from the system. Students place the nanoparticle-treated system on the neodymium magnet to separate the iron material. After 5 min, the supernatant may be tested for arsenic content. The control sample is also shown.

magnets used for iron removal are powerful and should be handled with care to avoid pinching.

’ PRELAB ACTIVITY Students were presented with a scenario in which a water treatment facility was concerned with arsenic in the source groundwater. Students were divided into groups of three and given a set of focus questions (see the Supporting Information) to discuss. The groups were asked to generate a hypothesis regarding the effect of both nanosized and bulk iron materials on a solution containing high arsenic concentration. Additionally, students were asked to predict this effect over time (time of treatment). The instructor provided a guided-introductory discussion to introduce nanoparticles and basic nanotechnology concepts such as surface area-to-volume ratio (SVR). An example of such guided discussion is provided in the teaching guide in the Supporting Information. This discussion is particularly useful for

’ POSTLAB ACTIVITY Results from a high school class are shown in Figure 3. Students discovered that the systems treated with magnetite significantly reduced the arsenic. The colorimetric assay made it easy for students to see that the iron shavings removed less arsenic compared to the nanoparticles. Students quickly grasped the idea that the size of the iron material has a strong influence on its ability to remove the arsenic, but the question remains as to why. Questions that are helpful in guiding the students’ understanding of adsorption and the surface area-to-volume ratio (SVR) concept of nanotechnology15 are available in the Supporting Information. The high school students, as a culminating experience, were further challenged to design an arsenic treatment system as consultants to a water treatment facility. Each group of students prepared unique designs including one that consisted of a separate holding tank with series of magnets to remove the residual particles as well as a design similar to that described in recent literature.4 ’ EVALUATION Mastery of content knowledge was assessed by having the students take a pre- and postlab test. The tests consisted of 10 1120

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Figure 4. Lab knowledge gains for college students at SUNY Old Westbury (n = 22 students) represented as content-knowledge test scores before and after the lab experiment. The test consisted of 10 nanotechnology-themed questions indicated by the key on the right.

Figure 5. Lab knowledge gains for upper-level advanced placement chemistry and physics high-school students (n = 27 students) represented as contentknowledge test scores before and after the lab experiment. The test consisted of 10 nanotechnology-themed questions indicated by the key on the right.

Table 1. General Chemistry Student SALG Responses

a

How much did the following aspects help class learning?

SALGaa,,b

Instructional approach

6.1 ( 1.0

Class topics, activities, and assignments fit together

5.8 ( 1.3

Pace of class

5.4 ( 1.5

Had definable objective for completion

6.1 ( 0.9

Achievable goals for period

6.1 ( 1.2

How much did the following aspects help gains in skills?

SALGaa,,b

Making predictions about chemical systems

5.7 ( 1.16

Making observations about chemical systems

5.7 ( 1.0

Explaining observations

5.7 ( 1.1

Developing a hypothesis

5.6 ( 0.94

n = 22 responding. strongly agree.

b

1 = strongly disagree; 4 = neither; 7 =

questions about various nanotechnology topics. Both college and high school students16 showed a significant improvement in the

postlab test with ∼34% increase of the class average (Figures 4 and 5). The high school and college students completed SALG-type surveys (Student Assessment of Learning Gains). Some representative SALG responses from the college students are shown in Table 1. The mean score on all items was above 5.4, indicating a positive attitude toward the lab. This attitude is further illustrated by student comments: • “I was able to see demonstrations of the experiment and it helped me think about the hypothesis of the question; they helped me understand the point of the lab.” • “(It was) well instructed/interesting; it helped see how to test for Arsenic and how the nanoparticles can be used for cleaning water.” • “Class discussion allow(ed) for better thinking process rather than alone. (It) made the lab less overwhelming and the collaborative effect helped as well.” • “I am not majoring in science, but these lab experiments were actually interesting and fun to me. If more labs were done this way, I believe it would be more interesting” 1121

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’ CONCLUSION This lab activity has been used in physics and advanced placement chemistry courses in high schools, as well as a general chemistry college course with little modification. Students were challenged to think critically on a subject about which they knew little. Rather than becoming discouraged, students were enthusiastic and motivated to learn the principles driving the observed phenomenon. A core concept of nanotechnology is demonstrated and used to connect course content from several fields of science by tapping the students’ creativity via experimental design and a challenging problem solving scenario directly related to environmental remediation. Students indicated that making these interdisciplinary connections was a rewarding experience. ’ ASSOCIATED CONTENT

bS

Supporting Information Laboratory student handout; remediation problem scenario; teaching guide; teacher’s copy of key questions with answers. This material is available via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by NSF grants: DUE-0737395, DMR0804506, and DMR-CAREER-0954919 to S.A., DUE-0737202 to B.H., and DGE-0338216 to M.M.S. D.V. and M.T.R. acknowledge K12 fellowships supported by DGE-0338216. The authors would like to thank Earl Peace for helping with the design and implementation of this experiment in an inquiry-based format. ’ REFERENCES (1) O’Reilly, S; Strawn, D.; Sparks, D. Soil Sci. Soc. Am. J. 2001, 65, 67–77. (2) Raven, K.; Jain, A.; Loeppert, R. Environ. Sci. Technol. 1998, 32 (3), 344–349. (3) Manning, B.; Fendorf, S.; Goldburg, S. Environ. Sci. Technol. 1998, 32 (16), 2383–2388. (4) Mayo, J. T.; Yavuz, C.; Yean, S.; Cong, L.; Shipley, H.; Yu, W.; Falkner, J.; Kan, A.; Tomson, M.; Colvin, V. L. Sci. Technol. Adv. Mater. 2007, 8, 71–75. (5) Dixit, S.; Hering, J. G. Environ. Sci. Technol. 2003, 37 (18), 4182–4189. (6) Hmelo-Silver, C. E.; Duncan, R. G.; Chinn, C. A. Educ. Psychologist 2007, 42 (2), 99–107. (7) Kalivas, J. H. J. Chem. Educ. 2008, 85 (10), 1410–1415. (8) Green, W.; Elliott, C.; Cummins, R. J. Chem. Educ. 2004, 81 (2), 239–241. (9) Hanson, D.; Wolfskill, T. J. Chem. Educ. 2000, 77, 120–130. (10) Hinde, R. J.; Kovac, J. J. Chem. Educ. 2001, 78, 93–99. (11) Lewis, J. E.; Lewis, S. E. J. Chem. Educ. 2005, 82 (1), 135–139. (12) Gaddis, B. A.; Schoffstall, A. M. J. Chem. Educ. 2007, 84, 848–851. (13) Huang, S.-H.; Liao, M.-H.; Chen, D.-H. Biotechnol. Prog. 2003, 19, 1095–1100. (14) Mehta, R. V.; Upadhyay, R. V.; Charles, S. W.; Ramchand, C. N. Biotechnol. Techniques 1997, 11, 493–496. (15) Sohlberg, K. J. Chem. Educ. 2006, 83 (10), 1516–1520. (16) Knowledge gain was not assessed for high school students from the first year, so these data are not available. 1122

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