chemical analysis of soils: an environmental chemistry laboratory for undergraduate science majors
TRANSCRIPT
In the Laboratory
JChemEd.chem.wisc.edu • Vol. 76 No. 12 December 1999 • Journal of Chemical Education 1693
Environmental chemistry laboratory experiments arenow an important part of many undergraduate chemistrycurricula. This reflects increasing interest among undergraduatestudents as well as by the American Chemical Society, whichendorses an environmental chemistry option for undergraduatechemistry majors.
Soil chemistry is an area of environmental chemistryoften neglected by chemists. However, it has great importancein environmental science. For example, soils are used forfood production as well as for disposal of hazardous wastes.Successful management of these conflicting uses requiresunderstanding of how soil characteristics affect the mobilityand fate of soil contaminants and soil nutrients.
We have developed a laboratory exercise for undergraduatescience students in which they evaluate soil samples for variousparameters related to suitability for crop production andcapability for retention of contaminants (1). Soil is a variablemixture of minerals, organic matter, water, and air. It is producedby physical, chemical, and biological processes collectivelycalled weathering (2). This lab emphasizes the heterogeneousnature of soil, and the difficulty of obtaining representativesamples for analysis. One of the first steps in any chemicalanalysis is to obtain a representative analytical sample from abulk or larger sample (3), yet undergraduate students rarelydo this in practice (4, 5). In this lab, students mix a largesoil sample and attempt to obtain a smaller representativesample for analysis. They assess the success of their work bycomparing their individual data with the mean and standarddeviation for samples compiled from the whole class.
Explanations and Procedures
Physical DescriptionSoil sampling strategies are discussed prior to this lab
(6, 7 ), and students are encouraged to bring in their ownsoil samples. They are also provided with (or collect them-selves as a group) two soil samples, beach sand and a darkwoods soil for contrast, in order to generate class data forthe same samples. Students describe their soil samples withrespect to color, texture (pebbles, sand, and mud), moisture,and organic content. They mix a soil sample well, form acone, and divide the cone into quarters. Opposing quartersare removed, a new, smaller cone is formed, and the processis repeated until students obtain what they consider to be arepresentative sample of approximately 20 g (cone and quartermethod [8]). They do this twice for each sample.
Water, CaCO3, and Organic ContentWater content is highly variable in surface soils because it
is dependent upon recent rainfall. Water retention is a functionof mineralogy (clays hold more water in their crystal latticesthan larger particles), organic content (organic material retainswater), and texture, which controls porosity. On a global scale,the capacity of soil for retention of water is similar in volume toall the world’s lakes (9). Calcium and magnesium carbonateminerals buffer against soil and lake acidification, both naturaland anthropogenic (1). Soil organic material (humus) retainspollutant trace metals and many organic pollutants as well,and hence slows their rate of flow through the environment.Soil humus contains approximately twice as much carbon asis present in the atmosphere as carbon dioxide (10).
Water content is determined by change in weight beforeand after oven-drying in aluminum pans (approximately 20 gof soil, 1 week at 110 °C, 20 minutes in a desiccator at roomtemperature). Some volatile organic compounds might alsobe lost in this step. The second sample is used to determineCaCO3 plus water, and then organic matter content. Thissample is treated with 10% HCl to remove carbonate minerals(primarily calcium carbonate), and after no more bubblingis observed, it is rinsed with deionized water. It is then driedand weighed as above to determine weight loss, which is equalto calcium carbonate plus water content (7 ). Carbonatecontent is determined by subtracting the percent water(determined on the other aliquot) from the sum of percentwater plus carbonates. Organic content is determined on thissample after acidification and drying as loss on ignition (LOI),by measuring the change in weight after combustion in amuffle furnace (550 °C overnight) or crucible over a high-temperature burner to red heat (8). Removal of carbonatesbefore ignition is necessary because calcium carbonate de-composes at temperatures required for complete combustionof soil organic material (550–800 °C) (6, 8).
Determination of Representative SampleDuring the second lab, students write their individual
data for the two class soil samples for %water, %CaCO3, and%LOI on the board. When data are complete, a mean andstandard deviation are computed. Students assess the repre-sentativeness of their samples by whether they are within onestandard deviation of the mean for the class.
Grain SizeSoil texture controls the surface area available for reaction.
Because of their small size and large surface area, clay-sized
Chemical Analysis of Soils W
An Environmental Chemistry Laboratory for UndergraduateScience Majors
Joan D. Willey,* G. Brooks Avery Jr., John J. Manock, and Stephen A. SkrabalDepartment of Chemistry, University of North Carolina at Wilmington, Wilmington, NC 28403-3201;*[email protected]
Charles F. StehmanNorth Carolina Department of Environment and Natural Resources, Wilmington, NC 28405
In the Laboratory
1694 Journal of Chemical Education • Vol. 76 No. 12 December 1999 • JChemEd.chem.wisc.edu
minerals tend to retain organic compounds and trace metalsmore than larger particles. Soil samples are rinsed through a63-µm sieve into a beaker; the sand fraction (63–200 µm)and larger particles are retained on the screen. The suspen-sion in the beaker is transferred to a glass graduated cylin-der. Failure to settle within a few minutes indicates the pres-ence of clay (particle diameter < 4 µm). Percentages of sand (63–200 µm) and mud (silt plus clay, < 63 µm) are estimated.
Soil pH and SalinitypH and salinity are usually considered to be properties
of aqueous solutions, not solid phases. However, the pH andsalinity of the water contained in soils determine in partwhich crops will grow in both inland and coastal locations.Soil-moisture pH is set by ion exchange reactions betweenthe surfaces of soil minerals, organic material, and water, aswell as by hydrolysis reactions of Al3+ and Fe3+ (1).
Soil and deionized water are mixed in equal proportionsto form a slurry. A pH electrode or multi-indicator pH papercan be used to measure pH. The water and soil slurry can becentrifuged or filtered (pore size is not critical) to generate a clearsolution for salinity analysis by refractometer (for example, aLeica Model 10419 temperature-compensated hand-heldrefractometer), which requires only a few drops of solution.
Soil Partition Coefficient DemonstrationThe tendency of contaminants to adsorb to organic
components (lipophilicity) versus remaining more mobile inthe aqueous phase (hydrophilicity) determines in part therate of migration of a specific compound through soil. Thepartition coefficient Kow = [x(oct)]/[x(aq)] is used to quantifythe tendency of a compound to accumulate in 1-octanol,which is lipophilic and imitates biota lipids, versus water.Many pesticides and PCBs have very large values for Kow(>105) because they are fat-soluble. Compounds with Kow >100 have low mobility through soils (11) and so will remainnear the source of application. High Kow compounds tend toaccumulate in soil organic material (12) and to bioaccumulateand bioamplify in the food chain. Compounds with lowvalues for Kow (<100) have high water solubility and highmobility within soils (11). Examples include acetone, methyl-ene chloride, and methyl tert-butyl ether (MTBE).
CAUTION: The material safety data sheet (MSDS)for 1-octanol, available at several sites on the Web(e.g., http://hazard.com or http://siri.org/msds/ ) indi-cates that students must wear plastic gloves andwork in a hood during this part of the exercise.
Approximately 2 mL of deionized water is placed in onetest tube, and 2 mL of 1-octanol in another test tube. A smallquantity of sample is added to each and mixed with a spatulaor stirring rod. Supernatant from each test tube is carefullypoured into a third test tube (leaving behind the solids) toillustrate the two layers of different colors. Students estimatewhether the value for Kow is greater or less than 1 by comparingthe intensity of colors in the octanol top layer versus thewater bottom layer for several substances used as examples.Two separate test tubes are used to ensure that the samplecomes into contact with both layers. Examples can includegreen plant material, β-carotene, humic acid sodium salt, anda colored inorganic salt such as CuSO4 (which has been usedas an aquatic herbicide). These visually illustrate the prin-
ciple of partitioning; β-carotene is found in the octanol layer,humic compounds and copper sulfate in the aqueous phase,and chlorophyll in both phases, with greater intensity in theaqueous layer.
Summary
Most students taking this lab at our university are under-graduate environmental studies majors. This exercise givesthese students exposure to real, environmentally importantsamples. Students test the hypothesis that their sample is rep-resentative of the whole. The exercise provides an overviewof soil composition in two 3-hour lab periods using readilyavailable equipment and inexpensive reagents. With somemodifications, this lab could be revised for high school students(13). If more time is allocated, additional concepts could beintroduced, for example soil ion mobility (14 ) or analyses ofspecific soil components (15–18), depending on the interestsand expertise of the instructor. The lab can also be modifiedto address issues of local importance. For example, wecompare soils (taken at three different depths) from forestconservation areas on our campus that undergo controlledburns with similar areas that are not burned. Students enjoythis lab and often think beyond the actual assignments, asindicated by their questions.
NoteWExperimental procedures for students and additional references
for instructors are available on JCE Online at http://jchemed.chem.wisc.edu/Journal/issues/1999/Dec/abs1693.html.
Literature Cited
1. McBride, M. B. Environmental Chemistry of Soils; Oxford Uni-versity Press: New York, 1994.
2. Manahan, S. E. Environmental Chemistry, 6th ed.; Lewis Publish-ers, CRC: Boca Raton, FL, 1994.
3. Harris, D. C. Quantitative Chemical Analysis, 4th ed.; Freeman:New York, 1995.
4. Clement, R. E. Anal. Chem. 1992, 64, 1076A–1081A.5. Guy, R. D.; Ramaley, L.; Wentzell, P. D. J. Chem. Educ. 1998,
75, 1028–1033.6. Soil Sampling and Methods of Analysis; Carter, M. R., Ed.; Lewis
Publishers, CRC: Boca Raton, FL, 1993.7. Rowell, D. L. Soil Science: Methods and Applications; Longman
Scientific & Technical: Essex, England, 1994.8. Head, K. H. Manual of Soil Testing, Vol. 1, Soil Classification and
Compaction Tests; Pentech Press, London: Plymouth, England, 1980.9. Kutilek, M.; Nielsen, D. R. Soil Hydrology; Catena: Cremlingen-
Destedt, Germany, 1994.10. Schimel, D.; Alves, D.; Enting, I.; Heimann, M.; Joos, F.;
Raynaud, D.; Wigley, T. In Climate Change 1995: The Science ofClimate Change; Houghton, J. T.; Meiro Filho, L. G.; Callander,B. A.; Harris, N.; Kattenberg, A.; Maskell, K., Eds.; CambridgeUniv. Press: New York, 1996; pp 76–86.
11. Fetter, C. W. Applied Hydrogeology; Macmillan: New York, 1988.12. Connell, D. W. Basic Concepts of Environmental Chemistry; Lewis
Publishers, CRC: Boca Raton, FL, 1997; pp 35–40 and Chapter 17.13. Eisenmann, M. A. J. Chem. Educ. 1980, 57, 897–899.14. Brown, M.; Sutherland, M.; Lehame, S. J. Chem. Educ. 1987,
64, 448–449.15. VanDoren, J. B. J. Chem. Educ. 1987, 64, 447.16. Baker, R. C. Jr. J. Chem. Educ. 1995, 72, 57–59.17. Butala, S. J.; Zarrabi, K.; Emerson, D. W. J. Chem. Educ. 1995,
72, 441–444.18. Kegley, S. E.; Hansen, K. J.; Cunningham, K. L. J. Chem. Educ.
1996, 73, 558–563.