Environmental problems provide an excellent vehicle for teaching concepts and techniques in the first-year laboratory. We report here a project based upon the remediation of heavy metals in soil, which has a firm grounding in solubility and precipitation reactions based upon relative solubilities. The project provides experience in determining solubilities and exploring precipitation reactions that can be used to precipitate a heavy metal ion. It provides experience in both qualitative and quantitative analyses that are also based upon relative solubilities. Importantly, it provides experience dealing with an environmental problem.
Lead in soil can be stabilized by treatment with phosphate to form lead phosphate or by treatment with a mixture of phosphate and chloride to form the less soluble double salt, pyromorphite (Pb5(PO4)3Cl) (1,2). In order to simulate contamination by lead but avoid its toxicity, this experiment relies on the barium ion. The solubilites of salts of the two ions are sufficiently similar to allow easy extrapolation from one to the other. For example, the sulfates of barium and lead are both insoluble, as are their carbonates and phosphates. The relative toxicities of the two can be seen in the LD50 values (intraperitoneal, mouse) of their nitrates: 74 mg/kg for lead, 293 mg/kg for barium (3,4).
The hazards associated with this experiment are due to the relatively low toxicity of barium salts (LD50 = 293 mg/kg (1,2)) and the corrosive nature of acids and ammonia. All work should be done in a hood and appropriate eye protection should be used. We also suggest the use of gloves and lab coats/aprons and the handling of all solids in beakers, flasks, or vials, not on watch glasses.
The project begins with qualitative observations on the relative solubilities of barium salts. The student adds a variety of anions, including sulfate, phosphate, oxalate, acetate, carbonate, and chloride to samples of Ba2+ to determine which salts are insoluble. After the student determines which of these compounds are insoluble, the instructor should provide a discussion of the factors that affect solubility. These include temperature, the common ion effect, and the effect of pH. The effect of pH is especially important in this project because of the importance of the anions of weak acids. These anions, such as carbonate, oxalate, acetate, and phosphate, will react with acid and the solubilities of these salts will therefore be pH dependent.
Quantitative Determination of Solubilities
Samples of the least soluble barium salts are then available for the student to determine the solubility in grams per liter. This is accomplished by suction extraction of the salt in a filter crucible with one liter of distilled water as reported in a previous paper in this journal (5). The weight of the sample of salt before and after extraction yields the solubility in grams per liter. This solubility is then used to calculate the molar solubility and approximate value of the Ksp, if desired. Because the solubilities of the compounds that contain an anion of a weak acid are dependent upon pH, the distilled water should have a pH in the range 5.5-7.0. In this range, the solubilities are reproducible and in good agreement with literature values (Table 1).
|Compound||Exp Sol/g L-1||Lit Sol/g L-1|
Selection of Remediating Anion
The student is now in a position to select an anion that will produce the lowest concentration of barium in solution. Before the final selection of an anion, the students are told that a double salt of barium may be even more insoluble than one of the simple salts. Using the analogy of the insolubility of chloroapatite (Ca5(PO4)3Cl), a double salt of calcium phosphate and calcium chloride, the instructor suggests the preparation of Ba5(PO4)3Cl, the naturally occurring substance alforsite.
Preparation and Analysis of Ba5(PO4)3Cl
This double salt is prepared through combination of BaCl2 with sodium dihydrogen phosphate in basic solution. The dry compound is dissolved in nitric acid and then analyzed qualitatively for the presence of barium, phosphate, and chloride ions by the addition of sulfuric acid (precipitation of BaSO4), ammonium molybdate (precipitation of ammonium molybdophosphate), and silver nitrate (precipitation of AgCl). Finally, the compound is analyzed quantitatively for barium ion and chloride ion using standard gravimetric procedures (precipitation of BaSO4 and AgCl, respectively). Table 2 shows the barium and chloride composition for some possible products and starting materials as well as a typical student experimental result. The high atomic weight of barium results in composition values that are very similar in the percent barium. However, the percent chloride in those compounds that contain chloride varies sufficiently to make a determination of empirical formula.
|Compound||% Ba||% Cl|
The solubility of the double salt is then determined by the suction extraction procedure and the students are asked to reevaluate their choice of an anion. The experimental water solubility of a student preparation of the double salt is 0.015 g/L, which corresponds to a molar solubility of 1.6 x 10-5 M and a Ksp of 5 x 10-39.
Preparation of Contaminated Soil
Barium contaminated soil is prepared using soil obtained from a nursery or gardening store sieved to insure a particle size no larger than 1.7 mm. A sample of the soil is washed with water, oven-dried, and then treated with barium acetate (the acetate is more soluble than the nitrate and therefore provides a greater concentration of barium ion) to give a contamination of about 3% Ba by weight.
A portion of the contaminated soil is then treated with a mixture of ammonium dihydrogen phosphate and NaCl and is brought to a pH of 9. The pH of the soil is maintained at a pH of 9 by the addition of ammonia during the ensuing two-week period. After two weeks the dry soil is thoroughly mixed in order to improve its homogeneity.
Determination of the Effectiveness of the Remediation
A portion of the dried, remediated soil is then extracted with water using the suction extraction method. The amount of barium released is determined by gravimetric BaSO4 analysis and this amount is compared to the amount of barium released by a control sample of the unremediated soil. Typical results show that approximately 37% of the barium used to treat the soil is released during washing of the control sample, whereas only 2% is released after remediation. [The retention of considerable barium in the unremediated soil is probably a result of of reaction with sulfates, carbonates, phosphates, zeolites, and clay in the soil sample.]
Discussion of Results
The following topics can be discussed as desired:
- The equilibria involved in the precipitation reactions and the driving force of ion-combination reactions.
- The dependence of the solubilities of the barium salts on the size, charge, and basicity of the anion.
- The factors that affect the accuracy and precision of gravimetric analysis.
- The conditions required for the synthesis of the double salt, especially the effect of high pH.
- The relative toxicities of lead and barium compounds and the methods used to express these toxicities.
- The principles and advantages of the various methods used for in situ remediation of metal-contaminated soils. An excellent reference summarizing the four principal methods of in situ remediation is available (1).
The authors are indebted to the Camille and Henry Dreyfus Foundation for a Special Grant in the Chemical Sciences and to Neal Langerman of Advanced Chemical Safety (San Diego, CA 92111) for helpful suggestions.
- "Recent Developments for In Situ Treatment of Metal Contaminated Soils," EPA (68-W5-0055), Washington D.C., 1997.
- Chen, X.; Wright, J. V.; Conca, J. L.; Peurrung, L. M. Water, Air, Soil Poll. 1997, 98, 59.
- Bulletin of Environmental Contamination and Toxicology. (Springer-Verlag New York, Inc., Service Center, 44 Hartz Way, Secaucus, NJ 07094) 1973, 9, 80.
- Gigiena Truda i Professional 'nye Zabolevaniya. Labor, Hygiene and Occupational Diseases. (V/O Mezhdrunarodnaya Kniga, 113095 Moscow, USSR) 1984, 28, 45.
- Harle, H.D.; Ingram, J. A.; Leber, P.A.; Hess, K. R.; Yoder, C. H. J. Chem. Educ., in press.
- Lange's Handbook of Chemistry, 13th Ed, McGraw-Hill, NY, 1985
- Gmelin's Handbuch der Anorganischen Chemie, Barium, Verlag chemie, Weinheim, 1960.