Laboratory Experiment pubs.acs.org/jchemeduc
Environmental Indicators of Metal Pollution and Emission: An Experiment for the Instrumental Analysis Laboratory John A. Bowden,† Brian A. Nocito,† Russell H. Lowers,‡ Louis J. Guillette, Jr.,§ Kathryn R. Williams,† and Vaneica Y. Young*,† Departments of †Chemistry and §Zoology, University of Florida, Gainesville, Florida 32611, United States ‡ Innovative Health Applications (IHA), Ecological Programs, Kennedy Space Center, Florida 32899, United States S Supporting Information *
ABSTRACT: This experiment enlightens students on the use of environmental indicators and inductively coupled plasma−atomic emission spectroscopy (ICP−AES) and demonstrates the ability of these monitoring tools to measure metal deposition in environmental samples (both as a result of lab-simulated and real events). In this twopart study, the initial experiment is designed to simulate the metal pollutants that originate from sparklers (small-scale fireworks). Two environmental indicators, surface water and Spanish moss (Tillandsia usneoides), are exposed to a lit sparkler in a closed container. The Spanish moss is subsequently microwave digested, and both samples are analyzed for 16 elements by ICP−AES. In part two, the metal content is analyzed in real samples taken from the environment (topsoil and surface water) surrounding the Kennedy Space Center (KSC) launch pads before and after a shuttle launch. Students learn several fundamental concepts including atomic spectroscopy, biomonitoring, microwave digestion, matrix interferences, matrix-matching, detection limits, and the standard additions method. KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Environmental Chemistry, Hands-On Learning/Manipulatives, Atomic Spectroscopy, Instrumental Methods, Metals
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bioindicators have been employed. Epiphytes, or plants without a root system (e.g., Spanish moss, Tillandsia usneoides), acquire their metals from the atmosphere and have been used to measure atmospheric pollution.2 Spanish moss was specifically chosen for this experiment due to the local availability (it drapes beautifully from the trees throughout northern Florida and the Southeastern United States). Owing to its many branches and scale-like stems, the plant has a very large surface area. Furthermore, it is classified as superhydrophilic,3 so the surface will strongly adsorb metal salts present in a mist of water vapor carried by air. A scanning electron microscope (SEM) study of mercury localization in the plant has shown that mercury is not detected in the inner parts of the plant.4 Presumably, other metals are also confined to the surfaces of the plant. Faculty in areas without native sources of Spanish moss may readily obtain it at very little cost on the Web.5 For this experiment, a fireworks display is simulated by lit sparklers, and the plume emissions are confined to plastic boxes containing Spanish moss as the environmental indicator. After the mock pyrotechnic event, the moss samples are dried, pulverized, dissolved in concentrated nitric acid using a microwave acid digestion bomb, and analyzed by inductively coupled plasma−atomic emission spectroscopy (ICP−AES). As a follow-up experiment or special project, surface water and topsoil samples collected pre- and postlaunch from the
hen students learn about the various forms of pollution originating from anthropogenic activities, an often overlooked and underrepresented factor is metal pollution or emission. Environmental indicators are naturally occurring entitiesplant or animal organisms as well as nonliving componentsthat can be used to monitor environmental changes, including the state of dispersal and accumulation of metals. The objective of this instrumental analysis laboratory experiment is to demonstrate to students the use of an environmental indicator and atomic spectroscopy for the monitoring of metals derived from a pyrotechnic (fireworks) event and a space shuttle launch.
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EXPERIMENTAL BACKGROUND Metals are added to fireworks, in varying amounts, because of their ability to produce color and provide propellant, stabilizing, oxidizing, and fuel properties.1 After a fireworks event, the dispersal of the resultant aerosol clouds can allow highly concentrated metal pollutants to enter and integrate into several tiers of the biosphere. Soil has been useful as a “matrix” (a medium that contains pollutants) to investigate contaminant deposition due to its ability to accumulate and retain metals over time, whereas aquatic pollution has been routinely monitored by analyzing surface and groundwater. Atmospheric pollution has been measured predominantly using traditional filters or extraction protocols (e.g., grade 40 or grade 41 Whatman filter paper or solid-phase extraction), although more recently unconventional © 2012 American Chemical Society and Division of Chemical Education, Inc.
Published: May 15, 2012 1057
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deionized water. The water used to extinguish the sparklers is acidified with 2% HNO3. These general procedures can also be implemented for the soil samples collected from KSC.7 All standards and samples are analyzed using a Varian Vista CCD Simultaneous ICP−AES. The metals (Al, Ba, Ca, Cr, Cu, Er, Fe, Ga, K, La, Mg, Mn, Na, Sr, V, and Zn) are quantified using both the external standardization and standard additions methods.
environment surrounding the launch pads at the Kennedy Space Center (KSC) are analyzed by the same procedures. Because the KSC has been the principal launch and construction site for space vehicles since 1960,6 these samples provide a novel platform to monitor real-world metal pollution. Alternatively, because shuttle launch samples are difficult to acquire, samples from other real anthropogenic activities can be substituted (e.g., samples near a busy roadway or factory, especially those areas where a wetland and stormwater runoff is captured over time). For interested faculty and students, previously recorded KSC data are included in the Supporting Information.
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HAZARDS Adequate eye protection is needed to block any possible debris emitted from the lit sparkler. The use of sparklers should be done outside or in a well-ventilated area. Highly concentrated nitric acid is needed to ensure complete digestion,8 thus, emphasis should be placed on informing the student to proceed with caution when handling this solution. Highly concentrated nitric acid is corrosive and causes burns to all body tissue. The bomb should be properly depressurized (by cooling) before opening. Students should be careful for potential oxides of nitrogen being released from the cup while opening the box.
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PROCEDURE The experiment is designed to be completed in one 5 h laboratory period by a group of 2−3 students. The apparatus for the sparkler simulation procedure is shown in Figure 1.
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RESULTS The strontium calibration plots for six student groups are shown in Figure 2. Plots for other detected elements show
Figure 1. Fireworks simulation: (A) the arrangement of the moss bag, dish of water, and sparkler in the plastic box; (B) lit sparkler depositing smoke into the box; (C) lit, smoking sparkler extinguished in the water dish; and (D) sealed box containing smoke, moss, and water dish.
Students weigh four 0.05 g samples of dried Spanish moss and place them into clean sandwich-sized plastic bags, which are arranged in a plastic snap-top food container with a water dish (containing 25 mL of deionized water) and an unlit sparkler. Note that in Figure 1A, the lid of the container is slightly ajar, with the handle of the sparkler protruding from the box. With teaching assistant supervision, students light a green-colored sparkler and allow it to burn for 3 s, while exposing the contents of the box to the smoke (Figure 1B). The sparkler is then extinguished in the dish of water and the lid is positioned for closing after removal of the extinguished sparkler (Figure 1C). Finally, the extinguished sparkler is removed from the box, and the container is sealed (Figure 1D). The sealed container is allowed to set for 1 h while the smoke disperses inside. The same procedure is repeated for a red sparkler and a blue sparkler, and the fourth moss sample is used as a control (no sparkler added). During the 1-h waiting period, students prepare multielement calibration standards for ICP analysis (Al, Ba, Ca, Cr, Cu, Er, Fe, Ga, K, La, Mg, Mn, Na, Sr, V, and Zn in the range of 0.01−10 ppm). Several still shot images of this procedure are provided in the Supporting Information. After 1 h, students pulverize the moss samples and digest them in a Teflon bomb using 2.5 mL of high-purity concentrated nitric acid (HNO3) in a microwave oven (see the Supporting Information for details). The digested samples are transferred to 100 mL volumetric flasks and diluted with deionized water. An instrument blank is prepared containing 2.5 mL of the high-purity nitric acid diluted to 100 mL with
Figure 2. Linear dynamic range (LDR) strontium calibration plots for six student groups.
similar scatter. Students calculate both the limit of detection (LOD) and the limit of quantitation (LOQ) for each element. This is done by collecting data for a 10 run sequence of measurements on the blank solution and determining the mean and standard deviation of the measured intensities. LOD and LOQ (both represented by c) are calculated using the following equation, ks bl (1) m where sbl is the standard deviation of the blank, m is the slope of the calibration plot, and k = 3 for LOD and k = 10 for LOQ.9 The LOD for strontium varied from 0.0096 to 0.095 μg/mL, all significantly higher than the manufacturer’s specified value of 1 × 10−5 μg/mL. It was observed that when a sequence of 10 runs is performed on the blank, and when the average values of the 3 replicate measured intensities for each run are plotted versus run number, the time series plot shows that the intensities oscillate about a line with a slightly negative slope. Oscillation about a line of zero slope would be expected. Mathematical correction for the monotonically decreasing signal gives a factor of 10 decrease for the calculated LOD. c=
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Space Center are provided as a supplementary exercise to allow students the opportunity to monitor potential metal pollution from a real anthropogenic practice. Surface water and topsoil samples were collected both immediately prior to and after space shuttle launch STS127. As the KSC samples are difficult to acquire, a sample set of previously recorded KSC data are provided in the Supporting Information. As shown in the sample data for Ca, Na, Mg, Al, and Fe, using similar acquisition parameters, there were significant increases in all of these elements in the surface water samples post shuttle launch (2−4-fold in comparison to the prelaunch values). However, the metal increases were not as prevalent in the topsoil samples. The difference in metal deposition on each matrix (either surface water or topsoil) following a shuttle launch could be the result of numerous factors, such as weather, sample collection specifics, water levels, among other things. The variance associated with this experiment, in coordination with the variance inherent in all real-world analyses, could provide a profitable topic for further discussion with students. In addition, it is also worth noting that the increased concentrations of Al and Fe in the surface water samples are likely due to the presence of these two elements in solid rocket fuel.
To minimize this systematic error, the experimental design must be changed to collect the blank run sequences first. The moss results for Al, Ba, and Sr resulting from the plume of the green sparkler are shown in Table 1. There is a wide Table 1. Green Sparkler Student Results for Al, Ba, and Sr in Spanish Moss by External Standardization Group
Al/(μg metal/g moss)a
Ba/(μg metal/g moss)a
Sr/(μg metal/g moss)a
1 2 3 4 5 6
nd 72 1170 nd 725 17.6
nd 329 1120 nd 936 23.4
nd nd nd nd nd nd
a
Results corrected for controls; nd is below the LOD.
variation here as well. Contributions to the total variance of a single moss specimen were analyzed by groups of students in the spring of 2008. The results of this study are included in the Supporting Information. Given the inexperience of the students and the complexity of the analysis, it is not realistic to expect a small variation in the results for the mosses. The results for the water analysis, where no microwave digestion is performed, are shown in Table 2. There is variation in this analysis also, but it is clearly much less than that for the moss.
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CONCLUSION This laboratory experiment provides students with a tangible example of environmental monitoring and demonstrates the potential of environmental indicators for the monitoring of metal pollution. For obvious reasons, the analysis of simulated sparklers and real samples typically generates positive student interest, whereas the experimental setup allows instructors to adapt the design and concepts to fit their personal laboratory objectives. Students look forward to doing this lab well before the scheduled date. During the lab, lit sparklers outside the building quickly draw outside attention from the students in the general chemistry laboratories. When compared to other laboratory experiments, there is less idle time spent waiting for the collection of data and more hands-on involvement with microwave acid digestion, sparkler preparation, or general ICP understanding. Furthermore, this laboratory gives the students a “hands-on” experience with innovative analytical techniques and provides the instructor a medium to emphasize the parallels between academic and professional lab work. Moreover, the analysis of real samples helps the students become aware of several environmental monitoring problems and concerns associated with the analysis of real samples. The students learn several fundamental concepts including atomic spectroscopy, biomonitoring, microwave digestion, matrix interferences, matrix-matching, and detection limits.
Table 2. Green Sparkler Student Results for Al, Ba, and Sr in Milli-Q Water by External Standardization
a
Group
Al (μg/mL)a
Ba (μg/mL)a
Sr (μg/mL)a
1 2 3 4 5 6
108 259 8.50 42.0 28.5 42.2
379 368 41.3 131 155 178
6.5 2.0 0.06 0.56 0.63 0.62
Results corrected for controls.
A typical set of pie charts for the sparkler water samples are shown in Figure 3. These results should correlate with those of the active sparker material. Students recognize that the green color is due to the barium flame emission. The red sparkler shows the combined flame colors of potassium (purple) and strontium (red). The combination of Ba, K, and Cu gives the blue sparkler its color. In addition to the sparkler experiment, samples taken from the environment surrounding the launch pads at the Kennedy
Figure 3. Sparkler metal compositions as determined from ICP measurements on water samples. 1059
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Laboratory Experiment
ASSOCIATED CONTENT
S Supporting Information *
Information for the students including prelab questions; notes for the instructor; data from the Kennedy Space Center. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: vanyoung@ufl.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The pie chart was taken from the report of Maria Redkozubova. REFERENCES
(1) Vecchi, R.; Bernardoni, V.; Cricchio, D; D’Allessandro, A; Fermo, P.; Lucarelli, F.; Nava, S.; Piazzalunga, A.; Valli, G. Atmos. Environ. 2008, 42, 1121−1132. (2) Szczepaniak, K.; Bizuik, M.. Environ. Res. 2003, 93, 221−230. (3) Koch, K.; Barthlott, B. Philos. Trans. R. Soc., A 2009, 367, 1487− 1509. (4) Filho, G. M. A.; Andrade, L. R.; Farina, M.; Malm, O. Atmos. Environ. 2002, 36, 881−887. (5) Native sources of Spanish moss may be readily obtained at very little cost on the Web: http://www.spanishmossdirect.com or http:// www.amazon.com. (accessed Apr 2012). (6) Schmalzer, P. A.; Hensley, M. A.; Mota, M.; Hall, C. R.; Dunlevy, C. A. NASA Technical Memorandum # 2000-208583. Dynamac Corporation, NASA Environmental Program Office, 2000. (7) The students do not actually do this analysis; the KSC data is given to them. However, the analysis could be done using the same procedure. In such a case, the soil analysis would be done instead of the sparkler experiment. It would also be a 5 h lab. (8) Operating Instructions for Parr Microwave Acid Digestion Bombs; Parr Instrument Co.: Moline, IL, 1997. (9) Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis, 6th ed.; Thomson Brooks/Cole: Belmont, CA, 2007; pp 20− 21.
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