In the Laboratory edited by
Safety Tips
Timothy D. Champion
Solar Photocatalytic Destruction of p-Nitrophenol: A Pedagogical Use of Lab Wastes
Johnson C. Smith University Charlotte, NC 28216
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J. A. Herrera-Melián,* J. M. Doña-Rodríguez, E. Tello Rendón, A. Soler Vila, M. Brunet Quetglas, A. Alvera Azcárate, and L. Pascual Pariente Departamento de Química, Universidad de Las Palmas de Gran Canaria, 35017 Gran Canaria, Islas Canarias, Spain; *
[email protected] Rationale
Experimental Procedure
In this article we propose the use of lab wastes to develop new experiments for students. Besides increasing students’ awareness of the wastes they generate, this has other advantages such as low cost, reduction in waste production and management, use of a non-contaminating renewable energy source, and an opportunity to introduce TiO2-photocatalysis for hazardous waste destruction. In addition, the experiment demonstrates the need to use blanks and controls and shows the kind of information that may be obtained from them. TiO2 is a semiconductor with a band-gap energy of 3.2 eV. When colloidal anatase-TiO2 is photoexcited (λ < 380 nm) an electron and a positive hole, h+ are produced:
Equipment and Chemicals
TiO2 + hν → e᎑ + h+ The hole and electron may give rise to oxidative and reductive reactions, respectively. In aqueous solution, dissolved oxygen acts as an electron sink and water may combine with the positive hole. In this reaction a hydroxyl radical may be obtained: H2O + h+ → ⭈OH + H+ The very high oxidizing capacity of the hydroxyl radical (redox potential = 2.8 V) (1) enables it to react rapidly with most organic and inorganic compounds in water (2). We used p-nitrophenol wastes in this experiment. Since p-nitrophenol and its derivatives are used as insecticides and herbicides and in the production of pesticides (3) and many synthetic dyes (4 ), they are common water pollutants (5).
Cylindrical 800-W UV lamp (Hg high-pressure, HPA 8068, ENCO, Spain) UV–vis spectrophotometer (Shimadzu 1203) TOC analyzer (Shimadzu 5000-A) TiO2 (Degussa P-25) p-Nitrophenol
p-Nitrophenol Waste Water Origin Aqueous wastes were generated by students following the adsorption of p-nitrophenol on activated carbon by UV–vis spectrophotometry, as described by Jenkins et al. (6 ). Each group of four students made its own calibration curve, obtaining generally good correlation coefficients (Fig. 1). We used p-nitrophenol because these wastes had been generated in previous lab experimentation. However, TiO2photocatalysis can be employed to degrade many different organic pollutants dissolved in water, such as chlorinated aromatics, chlorinated aliphatic and olefinic compounds, hydrocarbons, carboxylic acids, alcohols, halocarbons, nitrogenous compounds, and heteroatom compounds (7). Regardless of the origin of pollutants treated, it is worthwhile to use lab wastes generated by the students in their own experiments. We recommend adopting this experiment as a second part of the adsorption experiment.
JChemEd.chem.wisc.edu • Vol. 78 No. 6 June 2001 • Journal of Chemical Education
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In the Laboratory 3
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Figure 1. Calibration plot of absorbance for determination of p -nitrophenol.
Figure 2. Effect of experimental conditions on disappearance of p-nitrophenol.
The Photocatalytic Experiment Each student group followed the p-nitrophenol disappearance in a sample and two blanks by absorbance measurements. The sample contained 1 g/L of TiO2 and was exposed to sunlight. One control contained the same TiO2 concentration but was covered with aluminum foil to keep it in the dark. The other control was exposed to sunlight, but no TiO2 was added. The pH of the sample and blanks was adjusted to about 4.5 to obtain the highest degradation rate (8). Total organic carbon (TOC) measurements were also carried out to show the total destruction of p-nitrophenol and its intermediates. To obtain reliable TOC measurements activated carbon particles must be totally removed from the samples and inorganic carbon must be reduced at a minimum. Magnetic stirrers were used to keep TiO2 in suspension and provide O2 to the solution. This simplifies the required equipment, since the reactor apparatus is reduced to magnetic stirrers and bars, aluminum foil, and Erlenmeyer flasks. The Effect of Light Source Some groups of students were encouraged to test different experimental variables. Two groups compared their results when using a 800-W UV lamp with those obtained by other groups who employed sunlight on sunny days. Another group of students carried out their experiment on a dark, rainy afternoon. The Effect of H2O2 The addition of an oxidizer enhances the TiO2-photocatalytic degradation of many organic pollutants. This effect may be attributable to the oxidizing action itself or to the ability of the oxidizer to accept electrons from the excited TiO2 surface and reduce the electron–hole recombination. To test the effect of adding H2O2, another group followed the evolution of three p-nitrophenol samples: (i) TiO2 plus H2O2, (ii) only H2O2, and (iii) only TiO2.
Minimum protective clothing: If Tyvek-type disposable protective clothing is not worn during handling of this chemical, wear disposable Tyvek-type sleeves taped to your gloves. Recommended respirator: Where p-nitrophenol or TiO2 is weighed and diluted, students must wear a NIOSH-approved half-face respirator equipped with an organic vapor/acid gas cartridge (specific for organic vapors, HCl, acid gas, and SO2) with a dust/mist filter. Spills and leakage: Should a spill occur, first remove all sources of ignition, then dampen the solid spill material with 60–70% ethanol and transfer the dampened material to a suitable container.
Hazards Acute or chronic hazards: p-Nitrophenol is highly toxic by ingestion, inhalation, or absorption through the skin.
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Results and Discussion From the plot of the p-nitrophenol concentration versus time for the sample and controls it is apparent that both TiO2 and light are necessary for efficient degradation of p-nitrophenol (Fig. 2). Depending on the initial p-nitrophenol concentration and sunlight, the experiment may be performed in 1–2 hours. On a sunny afternoon the p-nitrophenol concentration may be reduced to about 20 ppb, which is the maximum concentration allowed. From the experiments with different light sources (Fig. 3) we may conclude that the reaction rates were not especially lower under the weak sunlight of a dark rainy day. This shows that the experiment performed under bad weather conditions will yield similar qualitative results. The effect of adding H2O2 or any other oxidant gives new alternatives to the experiment (Fig. 4). If only H2O2 and no TiO2 is present in the sample, a slow reduction in pnitrophenol concentration is observed. If the sample contains only TiO2 the reaction rate is significantly higher; but the addition of both H2O2 and TiO2 causes an even more rapid disappearance of p-nitrophenol: in the sample with TiO2 and H2O2, the p-nitrophenol concentration decreases to 20 ppb half an hour before it reaches this level in the sample without H2O2.
Journal of Chemical Education • Vol. 78 No. 6 June 2001 • JChemEd.chem.wisc.edu
In the Laboratory 8
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Time / min Figure 3. TiO2 -photocatalytic destruction of p-nitrophenol under different light conditions.
Figure 4. Effect of H2O2 on the photocatalytic degradation of p-nitrophenol.
TOC measurements give an insight into the efficiency of mineralization. When wastes are adequately pretreated, typical TOC reductions (from 10.5 ppm to 2.2 ppm) may be obtained. These final TOC values fall in the range of our tap water TOC measurements (1.5–2.93 ppm). The experiments described here have been conducted by fourth-year marine science students. Their lab skills may be considered medium or high. These experiments do not require sophisticated equipment or special lab training for the students. We recommend these experiments for students of chemistry, marine or environmental sciences, and chemical engineering.
Literature Cited
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Supplemental Material
A more detailed version of this article is available in this issue of JCE Online. Notes and questions for students are also included.
1. Masten, S. J.; Davies S. H. R. Environ. Sci. Technol. 1994, 28, 180A–185A. 2. Buxton, G. W.; Greenstock C. L.; Helman, W. P.; Ross A. B. J. Phys. Chem. Ref. Data 1988, 17, 513–886. 3. Alif, A.; Bolue P. J. Photochem. Photobiol., A 1991, 59, 357– 367. 4. Takahashi, N.; Nakai, T.; Satoh, Y.; Katoh, Y. Water Res. 1994, 28, 1563–1570. 5. Dieckmann, M. S.; Gray, K. A. Water Res. 1996, 30, 1169– 1183. 6. Jenkins, D.; Snoeyink, V. L.; Ferguson, J. F.; Leckie, J. O. Water Chemistry, Laboratory Manual, 3rd ed.; Wiley: New York, 1980. 7. Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69–96. 8. Chen, D.; Ray, A. K. Water Res. 1998, 32, 3223–3234.
JChemEd.chem.wisc.edu • Vol. 78 No. 6 June 2001 • Journal of Chemical Education
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