In the Laboratory
Remediation of Water Contaminated with an Azo Dye: An Undergraduate Laboratory Experiment Utilizing an Inexpensive Photocatalytic Reactor
W
John A. Bumpus* Department of Chemistry and The Environmental Science Program, University of Northern Iowa, Cedar Falls, IA 50614;
[email protected] Jennifer Tricker and Ken Andrzejewski Marian High School, Mishawaka, IN 46544 Heather Rhoads Environmental Science Program, University of Northern Iowa, Cedar Falls, IA 50614 Matthew Tatarko Department of Biological Sciences and Center for Bioengineering and Pollution Control, University of Notre Dame, Notre Dame, IN 46556
The enormous contribution of the chemical industry to the world’s economy is due in large measure to the ongoing creative ability of synthetic chemists to discover and develop methods for large-scale production of novel and useful compounds. Recently, the number of chemicals listed by the Chemical Abstracts Service surpassed eighteen million (1). In the past, wastes from chemical manufacturing processes have too often been discharged into the environment with little or no treatment or concern for potential environmental damage. In the United States and several other countries, this has led to legislation aimed at curtailing or eliminating discharge of manufacturing wastes. This regulatory control has obvious beneficial effects for the environment. Interestingly, the requirement or threat of regulatory control has also been the motivation for development of novel processes designed to reduce wastes below levels of regulatory concern. Methods and techniques of organic synthesis are covered in some detail in all undergraduate chemistry programs. However, many times, in instructional settings, little attention is given to what becomes of the wastes generated by such processes. In the present investigation we developed an inexpensive photocatalytic reactor suitable for use in the undergraduate (or secondary school) chemistry laboratory to demonstrate the use of TiO2-mediated photocatalysis for remediation of water contaminated with the azo dye Congo Red. Use of this photocatalytic reactor and associated techniques can be adapted to a number of instructional and student research settings. In our view, the low cost of the photocatalytic reactor, the availability of a wide variety of inexpensive dyes, and the ease of analysis may make expansion of this avenue of student research particularly attractive to undergraduate chemistry departments and high school laboratories where budgetary priorities are a major concern. Background
Titanium Dioxide–Mediated Photocatalysis During the past several years, the possibility of using semiconductors such as titanium dioxide in chemical oxidation processes to remediate contaminated water has received considerable attention. The wide variety of chemicals that are 1680
degraded by this process has been summarized by Venkatadri and Peters (2). Many papers and reviews address the theory and use of this process. References to several of these are provided (2–8). In practice, fine suspensions of TiO2 or another suitable semiconductor are irradiated at wavelengths less than ~380 nm, causing electron excitation from the valence band to the conduction band. This, in turn, generates the formation of “holes” on the surface of the semiconductor, which can react with oxygen, water, and hydroxide ion to form hydroxyl radicals. Furthermore, superoxide and perhydroxyl radicals are formed from the reaction of excited electrons with oxygen molecules. The highly reactive active oxygen species so formed then react with the organic pollutants present resulting in its extensive oxidation. In many cases, carbon dioxide is the final oxidation product of titanium dioxide–mediated photocatalytic oxidation. In addition to the above-described process, TiO 2mediated oxidation of colored pollutants such as dyes may occur by the process known as photosensitization (9–11). In this process a colored compound adsorbed to the TiO2 absorbs radiation in the visible range, becomes photochemically excited, and transfers an electron to the semiconductor particle, which, in turn, reduces molecular oxygen to form a superoxide anion radical. The cation radical generated from the dye may then undergo degradation reactions, possibly with other dye molecules, with superoxide anion, or other active oxygen species that might be generated.
Congo Red The conjugated diazo dye Congo Red was first synthesized in 1884 (12–14) and found commercial success because of its ability to dye cotton by simple immersion. In the textile industry such dyes are know as direct dyes. Congo Red as a textile dye has been replaced by other dyes more resistant to fading and repeated washing (13). However, it is still widely used as a pH indicator and as a histological stain (13, 14 ). Most recently, Congo Red has proved to be important in the study and possibly the treatment of diseases that produce amyloid plaques (e.g., Alzheimer’s disease and spongiform encephalopathies) (15–17). This dye binds the inter-monomer clefts between antiparallel β sheets in amyloid proteins found in such plaques.
Journal of Chemical Education • Vol. 76 No. 12 December 1999 • JChemEd.chem.wisc.edu
In the Laboratory
The ability of Congo Red to serve as a direct dye has been illustrated in at least three organic chemistry laboratory texts (18–20) and its synthesis has been proposed for use as an undergraduate organic chemistry laboratory exercise (21). Here we use aqueous solutions of Congo Red as model wastes from a textile-dyeing facility to demonstrate remediation of such wastewaters by photocatalysis. Experimental Procedures
An Inexpensive Photocatalytic Reactor The photocatalytic reactor used in these studies consisted of a heavy-duty 1-L glass beaker into which was placed a magnetic stirring bar and a 500-mL conical glass vessel that separated the light source from the water.1 A 10-cm ring attached to a ring stand was fitted on top of the conical glass vessel to hold it in place on the beaker, which was placed on a magnetic stirrer. After addition of ~550 mL of dye containing water and TiO2 to the 1-L beaker, the conical glass vessel was sealed with Parafilm where it touched the top of the beaker. Finally, a General Electric 26-W fluorescent lamp2 was placed in the middle of this vessel so that it did not touch the sides and its end was approximately 2 cm from the bottom of this vessel. The fluorescent lamp was held in place by a 3-pronged clamp. Figure 1 is a photograph of a similar although slightly larger photocatalytic reactor whose use we recommend. It should be stressed that at no time was the lamp in direct physical contact with the solution. TiO2-Assisted Photocatalysis of Congo Red Congo Red was purchased from Aldrich Chemical Co. (Milwaukee, WI) and TiO2 was obtained from Degussa Corp. (Dublin, OH). A solution (550 mL) containing 25 mg of Congo Red per liter of water, a stirring bar, and 0.55 g of TiO2 was placed in the photocatalytic reactor as described. The fluorescent lamp was turned on and the solution was stirred for the duration of the experiment. At predetermined times, 1.5-mL aliquots were collected and centrifuged (14,000 rpm, 4 min) in an Eppendorf Model 5415 C centrifuge. The supernatant was carefully removed and centrifuged again. This second centrifugation was necessary to remove fine particles of TiO2. After the second centrifugation the absorbance at 498 nm of the supernatants was determined. Incubations without light and without TiO2 were performed to demonstrate that degradation of the dye was dependent on the presence of light and TiO2.
Figure 1. Photograph of the reactor. For photocatyltic reactions, a stirring bar, ca. 900 mL of dye-contaminated water, and appropriate amounts of titanium dioxide are placed in the 2-L beaker. The 1-L dissolution flask is then placed in the beaker and held in place with the ring, and the light source is placed and secured inside the dissolution flask.
Figure 2. Time course for the TiO2-mediated photocatalytic destruction of Congo Red.
Toxicity Assay The toxicity of Congo Red–contaminated water before and after remediation was assessed using the Daphnia magna assay (22). In these studies, 10 mL of contaminated water or treated water was added to 490 mL of dechlorinated tap water containing 10 D. magna in a 500 mL beaker. The lethality of these solutions was examined visually after 24 and 120 h. Results and Discussion The TiO2-mediated photocatalytic remediation of water contaminated with Congo Red (25 mg/L) is presented in Figure 2, in which remediation was assessed by monitoring decolorization at 498 nm. The UV–vis absorbance spectrum of the Congo Red–contaminated water before and after treatment is presented in Figure 3. In addition to documenting
Figure 3. Absorption spectrum of Congo Red in water (A) before and (B) after photocatalytic degradation.
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that complete decolorization of the dye occurred, Figure 3 also demonstrates that an extensive decrease in absorbance between 240 and 400 nm occurred, thus suggesting that photocatalytic degradation involves destruction of the aromatic rings. Upon centrifugation, visual examination of the pellets showed the dye was adsorbed initially to the TiO2 photocatalyst. Indeed, approximately 43% of the color disappearance observed occurred immediately after addition of TiO2 and was attributable to adsorption to TiO2. Further color disappearance from the aqueous solution only occurred in the presence of light, and eventually the pellets were completely decolorized. Controls also demonstrated that TiO2 was necessary for decolorization to occur. Although extensive destruction of toxic compound(s) is a goal of any wastewater remediation strategy, there is no guarantee that the degradation products are less toxic than the parent compound. Therefore, to determine the effectiveness of a remediation strategy, it is necessary to assess the relative toxicity of the treated and untreated material. In our study, toxicity was assessed using the D. magna assay in contaminated and treated water both of which had been diluted 50:1 with uncontaminated water. In actual wastewater treatment systems TiO2-mediated photocatalytic remediation processes would likely be used as a pretreatment before addition to municipal water treatment systems or further biological treatment (e.g., activated sludge process) in-house prior to discharge to environmental waters. In such systems wastewater treated by TiO2-mediated photocatalysis would undoubtedly be subjected to dilution even greater than the 50:1 dilution selected for these studies. Results of these studies showed that untreated water contaminated with Congo Red (concentration following 50:1 dilution = 1 mg/L) was not toxic to D. magna after a 24-h incubation (100% survival). However, after 120 h, 100% mortality of the Daphnia was observed. In samples of contaminated water after photocatalytic oxidation and dilution, 100% survival of the D. magna was observed after 120 h. Thus, these studies show that photocatalytic destruction of Congo Red also resulted in decreased toxicity of the wastewater. As noted in the background section, there are two general mechanisms by which TiO2-mediated photocatalysis may take place, one in which TiO2 is excited directly by near-UV radiation less than ~380 nm and one (photosensitization) in which less energetic radiation in the visible range excites a colored compound photochemically, which then donates an electron to TiO2 to initiate the photocatalytic process. The emission spectrum of the light source used in these experiments is illustrated in Figure 4. Only minimal amounts of radiation in the range necessary to excite TiO2 directly are emitted by this source. However, contribution by the mechanism in which TiO2 is excited directly by near-UV radiation cannot be ruled out. Indeed, Coon and Freeney (private communication from Shoshanna Coon, University of Northern Iowa) have shown that this is the preferred photocatalytic mechanism by which the triphenylmethane dye crystal violet is photooxidized when adsorbed on solid TiO2. It should also be noted that although Pyrex glass in the photocatalytic reactor absorbs substantial amounts of UV radiation, much of the emitted radiation in the range from 300 to 380 nm is transmitted through the glass and is, therefore, available for direct excitation of TiO2. 1682
Figure 4. Emission spectrum of the radiation source used in these experiments (26-W General Electric Energy Choice electronic compact fluorescent bulb).
The photocatalytic properties of titanium dioxide and experiments concerning it and certain other metal oxides have appeared in several articles in this Journal. The review by Markham (23) provides a nice, brief, overview of some of the early literature. Giglio et al. (24 ) described a photocatalytic reactor that utilizes solar radiation and noted that photocatalytic decomposition of dyes serves as a useful introduction to several important chemical topics (viz., band theory, semiconductors, catalysis, colloids, and waste reduction). Nogueira and Jardin also studied photooxidation of a dye (25). They showed that methylene blue was readily degraded in Petri dishes in the presence of solar radiation. In yet another investigation, Yu et al. described an excellent experiment in which titanium dioxide adsorbed on glass plates mediates photooxidation of organic pollutants in the gas phase (26 ). The photocatalytic reactor described herein is in a number of ways complementary to the solar-driven reactor previously described (24 ). For example, the solar reactor emphasizes nicely the practical idea of using solar energy to purify water, whereas our reactor would be more amenable to methods development and optimization experiments because the radiation source is stable and more easily controlled. All the experiments described (24–26 ) emphasize the need to treat chemical wastes before discharge into the environment. Our photocatalytic reactor is inexpensive, simple to construct, and easy to operate and uses a widely available screw-in fluorescent light. According to the manufacturer, these lamps have an effective lifetime of 10,000 h or greater (27), and we have confirmed this. One of our lamps has been in continuous use for other purposes for more than 26,000 h (although a decrease in intensity has been observed). These light sources also cost less to operate than incandescent lights (27). Of importance from a safety perspective in undergraduate and high school laboratories is the fact that these lights do not produce large amounts of heat and therefore do not require expensive jacketed glassware for water cooling. Although the light source itself is too hot to touch, the stirred liquid in the reactors dissipates heat rapidly. The temperature of the water treated in our experiments was never greater than 44 °C. Conclusions The photocatalytic reactor described herein is inexpensive and suitable for use in a variety of educational settings, and
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it illustrates one way chemists can treat industrial wastes—a topic that is only now starting to be addressed in undergraduate instructional laboratories. In addition to its use in a straightforward laboratory exercise or demonstration, this photocatalytic reactor is also a suitable focus for stimulating student designed research. For example, students could be encouraged to assess the ability of water contaminated by other types of dyes to be remediated by TiO2-mediated photocatalysis, and experiments could be designed to optimize conditions for degradation. The ratio of dye to semiconductor and the effect of pH are two parameters that could be studied. The use of other semiconductors and composite semiconductors (SnO 2/TiO2, for example) are other lines of research that could be pursued. The use of composite semiconductors should be particularly interesting, as SnO2/TiO2 composites for some compounds have already been shown to result in greater rates of degradation than those containing just one of the semiconductors (28). The use of photocatalysis to treat “real-world” aqueous wastes generated in the laboratory is still another avenue of research that could be explored. The use of other light sources can also be studied. However, caution is advised here, as some light sources produce excessive amounts of heat and would not be suitable for use in the reactor described herein. It may be wise to avoid use of some high-intensity light sources in an instructional setting, especially those that are under pressure and thus present a potential explosion hazard when handled. Students can also design experiments to compare the relative efficiency of photocatalytic degradation with other types of wastewater treatment systems. In our experience, biodegradation by white rot fungi (29), ozonolysis, adsorption on chitin, and chemical oxidation using sodium hypochlorite are all effective ways to decolorize water contaminated with Congo Red. Acknowledgments This project was supported in part by a grant from the Howard Hughes Medical Institute to the Department of Biological Sciences and the Department of Chemistry and Biochemistry at the University of Notre Dame. We thank Shoshanna Coon of the Chemistry Department at the University of Northern Iowa for assistance in acquiring emission spectra. We also thank Virginia Berg of the Biology Department at University of Northern Iowa for the loan of the spectroradiometer. The Department of Chemistry and Biochemistry and the Center for Bioengineering and Pollution Control at the University of Notre Dame generously provided laboratory space for this project. Notes W Supplementary materials for this article are available on JCE Online at http://jchemed.chem.wisc.edu/Journal/issues/1999/Dec/ abs1680.html. 1. A variety of glassware can be adapted for use in constructing similar photocatalytic reactors. A critical component from our perspective is the glass vessel used to house the light source. We used the bottom
half of an old 500-mL Virtis lyophilization vessel. Recently we discovered that a 1-L glass dissolution vessel (Fisher Scientific) in a heavy-duty 2-L beaker is very suitable for this use. This reactor is illustrated in Fig.1 and is recommended for use. One criterion to be considered in reactor construction is that the fit of the light source into the glass vessel not be too snug. Too snug a fit might lead to overheating. 2. The light source used in these experiments was a 26-W General Electric Energy Choice Electronic Compact Fluorescent Bulb (product code 21046, GE description number FLC26). This product is no longer manufactured by GE. Its suggested replacement product is a 24-W fluorescent bulb (product order no. FLE24TBXSPX27, GE description no. 12546). In our experience, a number of other fluorescent light sources are suitable, including GE’s Soft White Energy Choice 23-W Electronic Fluorescent Bulb (product code 12546, GE description no. FLE23TBX/SPX27/EC). Lights of America’s Quad Lite, Model 2127 Compact Fluorescent Electronic Light (27 W) and Phillips Earthlight Universal Electronic Energy Saving Bulb (25 W) are also suitable.
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