In the Laboratory
Sedimentation Time Measurements of Soil Particles by Light Scattering and Determination of Chromium, Lead, and Iron in Soil Samples via ICP
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Patricia Metthe Todebush† and Franz M. Geiger* Department of Chemistry, Northwestern University, Evanston, IL 60208; *
[email protected] Environmental science is increasingly important in the life of the general public (1). A search on the New York Times Web site on “pollution” returns 956 full stories for the period of Jan 1, 1996–July 15, 2003, among which 64 were published as headline stories within the last 18 months. One of the central issues in environmental chemistry is how toxic metals such as chromium (2), arsenic (3), or lead (4) interact with geosorbents such as soils, clays, and rocks as well as with solid matter suspended in the aqueous phase, that is, colloids. Understanding sorption processes is important as geosorbent and colloid surfaces can bind toxic species and promote reactions that change the toxicity of the surfacebound species. This six-hour, two-day chemistry laboratory activity introduces general chemistry students to soil chemistry and physics in the context of chromium contamination in the environment. The activity demonstrates that chemical binding and pollutant transport in the environment are intricately linked. A key consequence is that chemistry occurring on the molecular scale can have consequences on the regional scale. The activity can be expanded to lead (4, 5) and arsenic (5, 6), two other major pollutants in soils and addresses the need for developing soil environmental chemistry laboratory experiments in the undergraduate curriculum (7, 8). Chromium in the Environment Chromium contamination in populated areas is widespread (2, 9) and chromium traces are found even in the remote atmosphere (10). Chromium is the second-most abundant toxic metal in the environment after lead (11). It originates mainly from fossil-fuel combustion; metal, alloy and wood industries; and cooling towers (9). Cr(VI) in the form of chromate, CrO42− and Cr(III) are the two stable oxidation states in the environment, with Cr(VI) dominating under oxidizing conditions (12). Cr(VI) contamination is mainly anthropogenic (12). Its carcinogenicity, toxicity, and high mobility in most soils causes great environmental concern (12). In contrast, Cr(III) occurs naturally, is less mobile than Cr(VI), and is an essential trace element for humans and animals (9, 12). The apparent dependence of chromium toxicity on its speciation is known as the “chromium paradox” (12). Chromium(VI) can efficiently bind to iron-containing solid matter in soils (13), and certain oxides and organic compounds can convert Cr(III) back to Cr(VI). The recognition of these redox processes led to the assessment that all chromium compounds are potentially carcinogenic (12) † Current address: Department of Natural Sciences, Clayton State University, Morrow, GA 30260;
[email protected].
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and can serve as a motivating tool for introducing redox chemistry in the general chemistry curriculum. Soils and Colloids Soil chemistry is of great importance for our society and human survival on earth (14–17). While soil is necessary for the production of food crops, it can also act as a medium for waste disposal and pollutant transport (15–17). Colloids (18) are ubiquitous in soil environments and underground water and can bind toxic species and promote reactions that chemically transform toxic species into more or less toxic species (17). The physical behavior of colloids and their chemical reactivity in soils are thus important factors in determining soil productivity and pollutant transport. Since they can transport toxic metals over long distances in ground water, colloids play a chief role in pollutant migration from contaminant sites to populated areas or agricultural centers Laboratory Overview In this activity for the general chemistry teaching laboratory, experiments are carried out that center on determining (i) how long colloid particles obtained from soil samples remain suspended in water and (ii) the quantity of iron, lead, arsenic, and chromium contained in the solid phase of the soil samples. Students collect soil samples from areas around the campus dorms and also work on samples sent to them from home. This motivates them and inspires curiosity and care for the experiment. The colloid lifetime in the aqueous soil phase is mainly limited by sedimentation, that is, downward motion following gravity. For colloids, the sedimentation rate is slow enough to be measured using a light scattering setup over a series of multiple filtrations. In addition to determining colloid sedimentation rates, students also characterize the quantity and type of toxic and nontoxic metal ions in the soil samples using an inductively coupled plasma (ICP) spectrometer. While the photometric direct current plasma optical emission spectrometric (DPC) method has been applied successfully in a chromium laboratory for freshmen engineers (19), ICP has the advantage that multiple metals can be measured simultaneously with comparable sensitivity. Using their data, students then test the hypothesis that toxic metals are often associated with iron-bearing soils. The student database is accessed via a Web-based interactive map containing the toxic metal concentrations found in the soil samples. These two experiments allow students to draw conclusions about chemical binding and transport of pollutants in the environment.
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In the Laboratory
Hazards This experiment involves relatively few hazards. Concentrated hydrochloric acid, 6 M, is used to extract metals from the soil. Since HCl is toxic, it should only be used after reading and understanding the appropriate MSDS safety sheets. Also, unfiltered samples should never be measured using the ICP spectrometer, as the inlet will become clogged by larger, unfiltered soil particles. Experimental
Sample Collection At the beginning of the course, students are asked to contact their parents for a 20-gram soil sample collected from home in a double-sealed plastic bag. The students are also asked to collect their own soil samples from areas around the campus dorms or apartments they live in. After each sample is labeled properly with the student’s name, lab section, the TA’s name, and the location and date where the sample was collected, the samples are stored in the general chemistry laboratory. Students put general observations about the conditions on the day of sampling and the sampling location in their laboratory notebook. Prior to storage, a digital photo of each soil sample is taken and added to the interactive database. Sedimentation Rates Ten grams of each sample are thoroughly mixed with 10 mL of deionized water and the solution pH is measured. The sample is filtered four times using filtration paper with decreasing pore size, generating four colloid fractions. The quantity of light scattered by each fraction is measured us-
ing a previously calibrated Labworks colorimeter. A basic introduction to light scattering (20) and colloid sedimentation is provided during the prelab. As the pore size on the filter paper is decreased, the students isolate smaller and smaller particles in each fraction, allowing them to predict that the colloid sedimentation rate will decrease with decreasing filter pore size. To test this hypothesis, the students record the quantity of light scattered as a function of time for the third and fourth fraction. Plotting the logarithmic signal of the scattered light intensity as a function time yields a straight line for a given group of colloid particle sizes. The slope of the line yields the sedimentation rate for this given group of colloid particle sizes. Since multiple groups of colloid particle sizes are generally present in each fraction, and since smaller colloid particles will settle at a slower rate than bigger colloid particles, more than one slope in the time plots are often found (Figure 1). The number of slopes will depend on the number of distinct colloid particle sizes that are spaced apart sufficiently so they can be detected by the colorimeter in the solution fraction of the soil sample. Students will be asked how many distinct straight lines were obtained in the data, how they ensure that the slopes are in fact distinct, and how the sedimentation rate of colloids is related to their capacity for transporting pollutants over long distances.
ICP Spectroscopy To determine the concentration of chromium, lead, and iron in the soil samples, the metal ions are extracted from the samples using water-based and acid-based methods. Background information (20) on ICP is given, referring specifically to an earlier experiment in the course based on flame emission spectroscopy. It is emphasized that ICP is highly sensitive and operates on the same principle as the flame emis-
0
-1
Ln
I I0
-2
-3
-4
-5 0
1000
2000
3000
Time / s Figure 1. Typical light scattering versus time trace obtained from colloids in a soil sample filtered with the smallest pore size. The straight line is a linear least squares fit to the data and yields the sedimentation rate.
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Figure 2. ICP spectra from one soil sample washed with distilled water (top line) and 6 M HCl (bottom line).
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In the Laboratory
sion technique. In this part of the laboratory activity, the students use an ICP spectrometer that is located in the analytical services laboratory (ASL)—a lab mainly associated with research activities. Students measure their samples with the help of a TA who had been trained by an ASL laboratory technician. Students work on the ICP in groups of five and are asked to report on color changes in the plasma depending on what kind of ions are present in the samples. Further, they are asked to draw a picture of the instrument in their lab notebook and to describe what occurs as the sample flows through the plasma coil. Finally, they are asked to compare the chromium, lead, and iron concentrations obtained using the water-extraction method and the acid-extraction method (Figure 2). If an ICP is not readily available, atomic absorption-based experiments provide an alternative.
Web-Based Results After all of the samples have been analyzed by ICP the results are presented in an interactive Web format: using a clickable map of the United States, students can select a state and examine results from the ICP measurements and photos of the corresponding soil samples (21). In the lab writeup, the students are asked to explain why soils from different regions have different colors, and a connection is made between high iron concentrations and the red color in soils. Finally, by plotting the measured chromium and lead concentrations versus iron content in the soil samples, students can test the hypothesis that iron-bearing soils bind toxic metals such as chromium better than soils that do not contain significant quantities of iron (Figure 3). Special attention is given to the issue on how to deal with scattered data in the context of an early introduction to error analysis. Contaminant Transport Modeling To determine the impact of the sedimentation rate on contaminant transport in ground water, students use the simple but widely used contaminant transport Kd model (17). The model assumes that contaminant interaction with the mobile solid phase in ground water is completely reversible and that the fluid flow in the porous soil medium is isotropic. Based on this model, and based on sedimentation rates measured in the laboratory, students can assess the extent of pollutant transport in soils. Further, for soil-contaminant systems with high Kd values, students can assess the importance of colloids in transporting contaminants. This information is crucial for predicting how far a colloid-bound contaminant such as chromium(VI) can travel with respect to uncontaminated ground water. The rate at which the colloid-bound contaminant travels through the aqueous phase is given by vc. The ground water flow rate, which is measured by EPA stations across the country, is vg. The ratio of vg/vc yields the retardation factor R, which is given by R = 1+
ρ Kd n
(1)
where ρ is the average soil density (a representative value of 2.65 g兾cm3 is used here,), n is a parameter that describes the
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1.0
X = Pb X = Cr
[X]/[X]max
0.8
0.6
0.4
0.2
0.0 0.0
0.2
0.4
0.6
0.8
1.0
[Fe]/[Fe]max Figure 3. Normalized chromium (solid circles) and lead (open circles) concentrations versus normalized iron concentration measured in 170 soil samples across the United States. For reference, the solid gray diagonal line shows a 1:1 correlation.
Table 1. Rounded Kd Valuesa for Cr(VI) Adsorption onto Four Soils at Various Values of pH Ocala Soil
pH
Kenoma Soil
Cecil/Pacolet Soil
Holton/Cloudland Soilb
5
30
400
1000
1
6
20
200
400
1
7
8
70
60
1
1
20
5
1
8 a
Data from ref 24.
b
Holton/Cloudland soil has the highest Fe content.
soil porosity (a typical value of 0.2 is used here), and Kd is an partition parameter for characterizing the interaction of a given pollutant with a geosorbent. A typical Kd value for Cr(VI) interaction with iron-rich soils is 1000 cm3兾g (Table 1) and leads to a situation in which Cr(VI) moves about ten thousand times more slowly than ground water. Thus in nonmoble soil, that is, in the absence of colloids, Cr(VI) would require about 1300 years to travel 30 m, provided a typical ground water flow velocity of 0.3 m兾day (22). In the light of this result, colloid-mediated transport mechanisms become clearly important, since Cr(VI) bound to colloidal surfaces could travel as far as the colloid sedimentation rate allows. Based on the measured colloid sedimentation rates, the students can then calculate how far colloid-bound Cr(VI) will travel, by how much the size of a contaminated area would change following a Cr(VI) spill, and how the aqueous Cr(VI) concentration will change over time following cleanup procedures.
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Summary In this two-part general chemistry laboratory activity, students study soil samples from home and from campus using light scattering and inductively coupled plasma spectrometry to determine (i) colloid sedimentation rates and (ii) the quantity of chromium, lead, and iron in the samples. The experiments can be expanded to other important toxic metals such as arsenic. Through these experiments students can draw conclusions about the physical and chemical behavior of solid components in soil, paying particular attention to their propensity for transporting and chemically transforming pollutants in the environment. The activity points out how molecular-scale phenomena such as pollutant binding to colloid surfaces can have direct effects on a local and even global scale. Use of a complex instrument such as an ICP spectrometer allows the students to internalize the importance of their work. Working in a multi-user scientific facility within the chemistry department provides a means for integrating general chemistry students into the scientific culture of the department. Studying soil samples from areas where they live motivates students and inspires curiosity and care for the experiment. Finally, students are introduced early on to the important concept of data scattering (23) inherent to most real-life samples. Acknowledgments We gratefully acknowledge Saman Shafaie from the Northwestern University Analytical Services Laboratory for training and overseeing the use of the ICP. We also acknowledge Janet Maher for all of her help with the experiment including developing and coding the Web site. Funding from the National Science Foundation through its CAREER program is gratefully acknowledged. W
Supplemental Material
Instructions for the students are available in this issue of JCE Online. Literature Cited 1. Manahan, S. E. Fundamentals of Environmental Chemistry; Lewis Publishers: New York, 1993. 2. Ellis, A. S.; Johnson, T. M.; Bullen, T. D. Science 2002, 295, 2060.
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3. Gebel, T. W. Science 1999, 283, 1455. 4. Goldman, L. R.; Lanphear, B. P. Science 1998, 282, 1823. 5. Brown, G. E.; Foster, A. L.; Ostergren, J. D. Proceedings of the National Academy of Sciences of the United States of America 1999, 96, 3388. 6. Chen, C. Y.; Folt, C. L. Environ. Sci. Technol. 2000, 34, 3878. 7. Tran, T. H.; Bigger, S. W.; Kruger, T.; Orbell, J. D.; Buddhadasa, S.; Barone, S. J. Chem. Educ. 2001, 78, 1693. 8. Willey, J. D.; Avery, G. B., Jr.; Manock, J. J.; Skrabal, S. A. J. Chem. Educ. 1999, 76, 1693. 9. Nriagu, J. O.; Nieboer, E. Chromium in the Natural and Human Environments; John Wiley & Sons: New York, 1988. 10. Murphy, D. M.; Thomson, D. S.; Mahoney, M. J. Science 1998, 282, 1664. 11. Kavanaugh, M. C. Alternatives for Groundwater Cleanup; Academic Press: Washington, DC, 1994. 12. Katz, S. A.; Salem, H. The Biological and Environmental Chemistry of Chromium; VCH: New York, 1994. 13. Grossl, P. R.; Eick, M.; Sparks, D. L.; Goldberg, S.; Ainsworth, C. C. Environ. Sci. Technol. 1997, 31, 32. 14. Sparks, D. L. Soil Physical Chemistry, 2nd ed.; CRC Press: Boca Raton, FL, 1999. 15. Bolt, G. H.; van Riemsdijk, W. H. Surface chemical processes in soil. In Aquatic Surface Chemistry; Stumm, W., Ed.; John Wiley & Sons: New York, 1987. 16. Evangelou, V. P. Environmental Soil and Water Chemistry; John Wiley & Sons: New York, 1998. 17. Langmuir, D. Aqueous Environmental Geochemistry; PrenticeHall, Inc: Englewood Cliffs, NJ, 1997. 18. Atkins, P. W. Physical Chemistry, 6th ed.; Oxford University Press: Oxford, 1998. 19. Seymour, P. J. Chem. Educ. 1999, 76, 927. 20. Skoog, D. A.; Leary, J. J. Principles of Instrumental Analysis; Saunders College Publishing: New York, 1992. 21. Chem 101 Soil Lab Home Page. http:// www.chem.northwestern.edu/~geigerf/genchem/soilscience50.htm (accessed Jun 2005). 22. Freeze, R. A.; Cherry, J. A. Groundwater; Prentice-Hall: Englewood Cliffs, NJ, 1979. 23. Dunnivant, F. M. J. Chem. Educ. 2002, 79, 718. 24. Rai, D.; Zachara, J. M.; Eary, L. E.; Ainsworth, C. C.; Amonette, J. E.; Cowan, C. E.; Szelmeczka, R. W.; Resch, C. T.; Schmidt, R. L.; Girvin, D. C.; Smith, S. C. Chromium Reactions in Geoloical Materials; Electric Power Research Institute: Palo Alto, CA, 1988.
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