Chemical Education Today
Letters Note on Photocatalytic Destruction of Organic Wastes: Methyl Red as a Substrate
Concentration Methyl Red / (µmol/L)
The laboratory experiment by Herrera-Melián, et al., “Solar Photocatalytic Destruction of p-Nitrophenol” (1), utilizes colorimetric monitoring of the disappearance of pnitrophenol in a TiO2-catalyzed photochemical reaction. This system is potentially attractive as a freshman chemistry or even high school chemistry experiment because it addresses an environmentally significant problem, gives results that can be observed visually as well as instrumentally, and affords experience using Beer’s law calibration and photometric analysis. However, the hazardous nature of p-nitrophenol, documented in that paper, necessitates the use of protective clothing and respirators by those handling that material. We have adapted this experiment to use methyl red as the substrate for photolysis. Methyl red will be available in virtually any chemistry stockroom, is compatible with aqueous chemistry, and gives excellent results with this experiment. Our students carried out the photolysis in a UV photoreactor, which makes it possible to nearly destroy the methyl red within 30 minutes in the presence of TiO2, starting with a concentration of around 3 × 10–5 M methyl red. We filtered the samples for absorbance measurement using 4.5 µm HPLC filters and monitored the absorbance at 520 nm. A sample data set is shown in Figure 1. 30
The authors reply: The experiment with methyl red is also interesting as it may be with any of the many water-dissolved organic wastes that can be degraded by TiO2 photocatalysis. The advantages of using methyl red instead of p-nitrophenol, such as its availability in any academic lab and excellent results with the experiment, are to be considered. However, one of the most interesting aspects of our article is the use of real wastes, in this case from a previous student experiment (the determination of the Freundlich adsorption isotherm of pnitrophenol on activated carbon). Additionally, aqueous p-nitrophenol solutions are not as hazardous as the solid compound, but of course if no wastes are available (unusual in a lab) the solid compound must be used, prepared by the lab trainer or by the students. We are now repeating a similar experiment with marine science and chemical engineering students, but using aqueous wastes from the HPLC determination of formaldehyde in veterinarian wastes. HPLC wastes are quite common in almost any lab. These contain acetonitrile, low formaldehyde concentrations, and DNPH (dinitrophenylhydrazine). The degradation of a 100 ppm total organic carbon (TOC) solution is monitored by TOC, HPLC, and UV–vis measurements. Also, sunlight and a UV-lamp are used for comparison. Results are also quite interesting since notable degradations are obtained in 30 min for DNPH and longer times for acetonitrile. Additionally, after 1 h of degradation a non-identified HPLC peak corresponding to an intermediate compound appears. This is quite interesting because three aspects of waste treatment are pointed out: 1. The importance of using different analytical techniques
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2. The possible appearance of intermediates that may be even more toxic than the parent compound
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3. The need of combining different treatment methods since quite often the complete elimination of all the contaminants is not achieved
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Time / min Figure 1. Concentration of methyl red solution in UV photoreactor, with and without TiO2 catalyst.
Consequently, other interesting analyses to be added to this kind of experiment, in general to any waste degradation experiment, are toxicity determinations such as those with the marine bacteria Vibrio fisheri, which it is no more complicated than a luminescence measurement. We have observed in other wastes that even after about 80% TOC reductions, toxicity was increased, quite probably due to the formation of more toxic intermediates. José Alberto Herrera-Melián and Javier Araña Mesa
Literature Cited 1. Herrera-Melián, J. A.; Doña-Rodriguez, J. M.; Tello Rendón, E.; A. Soler Vila; Brunet Quetglas, M.; Alvera Azcárate, A.; Pascual Pariente, L. J. Chem. Educ. 2001, 78, 775–777.
Universidad de Las Palmas de Gran Canaria 35017 Gran Canaria, Islas Canarias, Spain
[email protected] L. J. Soltzberg and Virginia Brown Department of Chemistry Simmons College Boston, MA 02115
[email protected] 526
Journal of Chemical Education
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Vol. 82 No. 4 April 2005
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www.JCE.DivCHED.org
