In the Laboratory
Chemical Remediation of Nickel(II) Waste: A Laboratory Experiment for General Chemistry Students K. Blake Corcoran, Brian E. Rood, and Bridget G. Trogden* Department of Chemistry, Mercer University, Macon, Georgia 31207, United States *
[email protected] Well-established Beer's law experiments (1-4) in the general chemistry laboratory curriculum involve making solutions of varying concentrations. Our procedure utilizes the nickel(II) cation, generated by dissolving nickel(II) sulfate hexahydrate in water (5). The distinct absorption line of nickel, which is proportional to its concentration, makes it ideal for a Beer's law experiment, but by the end of the experiment, a single student accumulates 260 mL of solution containing 4.0 g of soluble nickel(II) sulfate hexahydrate. Similar to many universities, this university experiences large enrollments in the general chemistry laboratory and these volumes quickly add up to an impressive 88 L of waste. Current methods of waste stream alleviation require the stockroom workers to carry the solution to the roof of the chemistry building where it is evaporated and decanted. The nature of the waste is potentially problematic in that nickel(II) compounds are widely considered to be carcinogenic at high doses via inhalation (6-8). Whereas the limits are fine for each student conducting the lab, the total waste accumulated at the end of the experiment must be removed by chemical contractors according to EPA regulations. Not only is this an economically expensive process, but the transportation of waste relies upon fossil fuels, adding to the environmental costs. Precipitation of the waste in the laboratory environment allows for an immediate method of waste reduction and provides valuable learning outcomes for the students. It is expected that the precipitation methods described herein can be adapted to Beer's law experiments using nonnickel metals or even to other experiments where aqueous metal solutions are a byproduct. The goal of this project was thus not only to provide a method for decreasing the sheer volume of the nickel(II) waste, but to develop a new pedagogy for the general chemistry laboratory curriculum. Included in the tenets of green chemistry are the importance of pollution prevention and the design of processes to be environmentally benign (9). Yet many students receive no exposure to these practices and others only receive such knowledge later in their chemistry careers. Reflection by many scientists shows a desire for significant exposure to the field at an earlier date in their chemical careers (10). The large and diverse enrollments in the general chemistry laboratory provide an ideal audience for beginning a discussion on the role of the chemist in environmental protection. Experiments of pedagogical importance produce waste, but the remediation of that waste can also be pedagogical. The implementation of this experiment involving waste remediation thus provides an important juncture point in the education of the general university student and those who continue on with careers in science. 192
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In this experiment, waste nickel solutions were combined with students' standardized sodium hydroxide solutions from an earlier experiment; thus, two potential waste streams were alleviated at the same time. Whereas the original nickel(II) sulfate is water-soluble (eq 1), treating the solution with sodium hydroxide caused the nickel to precipitate out of solution as an insoluble nickel(II) hydroxide hydrate. The benign sodium and sulfate ions remain in solution (eq 2). NiSO4 ðsÞ f Ni2þ ðaqÞ þ SO4 2 - ðaqÞ
ð1Þ
Ni2þ ðaqÞ þ SO4 2 - ðaqÞ þ 2Naþ ðaqÞ þ 2OH - ðaqÞ f NiðOHÞ2 3 H2 OðsÞ þ 2Naþ ðaqÞ þ SO4 2 - ðaqÞ
ð2Þ
By adding a stoichiometric 31.0 mL of 1 M sodium hydroxide solution, each student can decrease his or her waste stream from 260 mL of nickel(II) solution to 1.68 g of solid nickel(II) hydroxide hydrate. Students can verify mathematically that the filtrate retains minimal nickel(II) and the resulting filtrate can then be washed down the sink. Method Development and Discussion In developing the method for the waste remediation, we were intent upon using a stoichiometric amount of sodium hydroxide to react with the nickel(II) ions. Lab students can easily perform this calculation based upon their original nickel(II) measurements to ensure that the appropriate molar ratios of hydroxide and nickel(II) are present. To keep the waste stream small, we performed all trials using 1 g of nickel sulfate hexahydrate rather than the 4 g that are necessary for the Beer's law experiment (5). Once the hydroxide was added, the solution was either filtered immediately or allowed to set for a week to determine the best stopping point for general chemistry laboratory students. A flow chart for the overall method development is shown in Figure 1. It was found that solutions that were filtered immediately (samples 1-4 in Table 1) gave better results: less nickel(II) remaining in solution and more precipitate recovered. In letting the solution set over a week, some of the nickel would leach back into solution, as verified by inductively coupled plasma optical emission spectrometry (ICP-OES). The solutions that were not filtered immediately (samples 5-8) also gave a lower overall yield of precipitate, as would be expected based upon the ICP-OES data. The solutions containing higher concentrations of nickel(II) ion required further treatment to remove the waste and would be an undesired result in this experiment.
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In the Laboratory
Figure 1. Flow chart of method development. Table 1. Analysis of Time Delay and Filtration and Drying Methods Filtration Delay None
One Week
Filtration Method
Sample Numbera
Ni(II) Remaining in Solution (ppm)b
Drying Method for Precipitate
Mass Ni(OH)2xH2O Recovered/g
Percent Yield (%)c
Gravity
1
0.1 ( 0
2
6.1 ( 0.1
Vacuum
3 4
168 ( 3 57.7 ( 0.5
Air Oven Air Oven Vacuum desiccator Vacuum desiccator
0.4246 0.3627 0.4526 0.3689 0.4391 0.4567
100.9 86.15 107.5 87.62 104.3 108.5
Gravity
5
44.4 ( 0.2
6
68.1 ( 0.4
7 8
73 ( 8 78 ( 6
Air Oven Air Oven Vacuum desiccator Vacuum desiccator
0.3951 0.2774 0.5070 0.4397 0.3911 0.3787
93.95 65.89 120.4 104.4 92.90 89.90
Vacuum
a Air-dried samples were analyzed and then subjected to oven drying for further analysis. b As determined by inductively coupled plasma optical emission spectroscopy. c Percent yield is based upon the monohydrate.
Further examination of the solutions by ICP-OES took into consideration any factors that the filtration method might have upon nickel(II) concentration. Although gravity filtration took around 1 h for fresh solutions, vacuum filtration via water aspirator was completed in 15 min. Both methods were easy and straightforward and students with a good lab technique could obtain good volumes of filtrate from either method. However, in the samples that were filtered immediately via gravity (samples 1 and 2), much less nickel remained in the solution, so gravity was preferred. This difference was less pronounced in the samples with a week delay, but gravity filtration seemed preferable here as well. The nickel(II) hydroxide hydrate formed immediately upon addition of the 1 M sodium hydroxide, but the solid was very hygroscopic and formed a gel-like layer in the water. Calculations here were based upon the monohydrate as the final product (11), although some samples were difficult to dry and resulted in a yield slightly over 100%. Various drying methods were used to remove most of the water so that the general chemistry students could better understand the stoichiometry of the process. Drying methods included air drying in a lab drawer versus vacuum drying in a desiccator, both for a week. The air-dried samples were then dried in an oven set at 100 °C to remove more water.
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As seen in Table 1, certain drying methods were more effective than others. The air-dried samples saw a considerable weight decrease after they were placed in the oven. The vacuum desiccator-dried samples were on average heavier than the ovendried samples, showing that this method was not the best for removal of water. It was determined that, in the interest of time and feasibility, samples would be allowed to dry in students' lab drawers for 1 week before final mass and percent recovery determination. Working with this substance allowed the students a chance to relate principle to practice in looking at the hygroscopic nature of a solid. Experimental Rationale and Troubleshooting This experiment would be appropriate for college-level introductory general chemistry courses, both for science majors and general university students. It would also be appropriate for general high school chemistry laboratories. The simple experiment allows students to explore reactivity and stoichiometry of a metal hydrate to reinforce these important chemical concepts. The main experimental techniques involve filtration and drying. As a straightforward and simple experiment, there is no special apparatus or instrumentation needed. An inductively
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coupled plasma optical emission spectrometer (ICP-OES) was utilized during the method development, but this is not necessary to conduct the experiment. The only troubleshooting necessary was that, if vacuum filtration is used, a room full of water aspirators going at the same time caused a large water-pressure drop in the laboratory. If labs are equipped with house vacuum, this problem is alleviated. Alternatively, the gravity filtration method could be used and the experiment could still be completed within one lab period. Hazards Sodium hydroxide is caustic; it causes burns to any area of contact. Nickel compounds are considered suspected cancer agents via inhalation, but exposure is not expected in this experiment. A 2005 letter to this Journal addresses these concerns and points out appropriate literature (12). Conclusion This experiment serves a two-fold purpose in allowing students the opportunity to remove a metal from a large waste stream while exploring stoichiometry and measurement applications. The implementation of this experiment allows for the presentation of the principles of process design and gives students a chance for handson experience with waste remediation. With the addition of one lab period and minimal materials, this procedure gives the students a valuable look into real-world chemistry problem solving. Acknowledgment The inductively coupled plasma optical emission spectrometer (ICP-OES) was purchased through an NSF-CCLI Grant No. 0410461 (2004).
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Literature Cited 1. Ricci, R. W.; Ditzler, M. A.; Nestor, L. P. J. Chem. Educ. 1994 71, 983–985. 2. Stewart, S. A.; Sommer, A. J. J. Chem. Educ. 1999, 76, 399–400. 3. Martinez-Dawson, R.J. Stat. Educ. [Online] 2003, 11 (No. 1). http://www.amstat.org/publications/jse/v11n1/martinez-dawson. html (accessed Oct 2010). 4. Pfeiffer, H. G.; Liebhafsky, H. A. J. Chem. Educ. 1951, 28, 123–125. 5. Mercer University Department of Chemistry Laboratory. http:// chemistry.mercer.edu/genchem/chm111.htm (accessed Sep 2010). 6. Ollera, A. R.; Costab, M.; Oberdorster, G. Toxicol. Appl. Pharmacol. 1997, 143, 152–166. 7. (a) Costa, M.; Daoji, Y. Y.; Salnikow, K. J. Environ. Monit. 2003, 5, 222–223. (b) National Toxicology Program: 11th Report on Carcinogens. The Hazards of Nickel Compounds. http://ntp.niehs.nih.gov/ ntp/roc/eleventh/profiles/s118nick.pdf (accessed September 2009). 8. The Hazards of Nickel Compounds. http://ntp.niehs.nih.gov/ntp/ roc/eleventh/profiles/s118nick.pdf (accessed Sep 2010). 9. Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998; p 30. 10. Braun, B.; Charney, R.; Clarens, A.; Farrugia, J.; Kitchens, C.; Lisowski, C.; Naistat, D.; O'Neil, A. J. Chem. Educ. 2006, 83, 1126–1169. 11. CRC Handbook of Chemistry and Physics, 82nd ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2001; p 4-71. 12. Bentley, A. K.; Farhoud, M.; Ellis, A. B.; Lisensky, G. C.; Nickel, A.-M. L.; Crone, W. C. J. Chem. Educ. 2005, 82, 1775.
Supporting Information Available Notes for the instructor; student instructions. This material is available via the Internet at http://pubs.acs.org.
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