Environ. Sci. Technol. 2000, 34, 1747-1750
Combinative Sonolysis and Photocatalysis for Textile Dye Degradation NAOMI L. STOCK,‡ JULIE PELLER, K. VINODGOPAL,# AND PRASHANT V. KAMAT* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556
The merits of combining two advanced oxidation processes, viz., sonolysis and photocatalysis, have been evaluated by investigating the degradation of an azo dye, naphthol blue black (NBB), using a high-frequency ultrasonic generator and UV-photolysis. An additive effect on the degradation rate of the parent compound is observed when the sonolysis and photocatalysis experiments were carried out in a simultaneous or sequential manner. Sonolysis is effective for inducing faster degradation of the parent dye, while TiO2 photocatalysis is effective for promoting mineralization.
Introduction Advanced oxidation processes (AOPs) such as sonolysis, radiolysis, and photocatalysis have emerged as useful methods for mineralizing organic compounds in aqueous media. All these processes which produce hydroxyl radicals as the primary oxidant assist in environmental remediation. Photocatalytic methods involve illumination of large band gap semiconductor particles such as anatase TiO2 either dispersed as a slurry in the contaminated aqueous solutions or immobilized films (see, for example, refs 1-8). Apart from improving mass transfer limitations, sonolysis can also chemically oxidize organic pollutants. Chemical reactions using ultrasound in aqueous medium has been investigated by several researchers (9-13). Higher ultrasound frequencies are especially more favorable for the generation of hydroxide radicals, possibly due to faster production rates (13-16). Combining photocatalysis with another AOP such as sonolysis could be an effective approach to improve the mass transfer of reactants and products to and from the catalytic surface in photocatalytic reactions (17-21). Two possible approaches can be considered for combining photocatalysis and sonolysis techniques. In the first approach the UVirradiation and sonolysis are carried out simultaneously in a single reactor vessel, while in the second approach the two treatments are carried out sequentially by circulating the suspension between the two vessels. We have chosen an azo dye, NBB, as a model contaminant for investigating the merits of these two combinations. Azo dyes are among the largest group of colorants used in a variety of industries ranging from textile to paper. The effluent streams from textile plants are in most cases highly colored, and decolorization is often the responsibility of the publicly * Corresponding author phone: (219)631-5411; fax: (219)631-8068; e-mail:
[email protected] or http://www.nd.edu/∼pkamat. ‡ Co-op Student, University of Waterloo, Canada. # Permanent address: Department of Chemistry, Indiana University Northwest, Gary, IN 46408. 10.1021/es991231c CCC: $19.00 Published on Web 03/16/2000
2000 American Chemical Society
owned water treatment plants (22-24). There is a reasonable urgency for developing a treatment process that not only decolorizes but also mineralizes the dye. We report here the first systematic evaluation of the merits of combining photocatalysis and sonolysis for the degradation of textile azo dye. The beneficial aspect of achieving better mineralization rates using simultaneous photocatalysis and sonolysis is discussed here.
Experimental Section Naphthol blue black (NBB), 80% purity, was obtained from Aldrich Chemical Co. The dye was first recrystallized in ethanol and further purified on a silica column using ethyl acetate, ethanol, and water as eluents. TiO2 (Degussa P-25) was obtained from Degussa and used as supplied. Aqueous solutions were prepared using Milli-Q purified water. The sonolysis experiments were carried out with a 640 kHz sonolysis setup of Ultrasonic Energy Systems (Panama City, FL). Operation of the transducer was in the CW mode with an output power of 240 W. A specially designed glass cell (capacity 600 mL) was attached to the transducer with silicon rubber. The cell was also used for photocatalysis experiment by irradiating it from the top. In addition, the top of the cell had the provision to pass a CuSO4 filter solution during UV photolysis. Simultaneous photocatalysis and sonolysis experiment (Sonolysis ON-Photocatalysis ON) was carried out in a single cell by continuously irradiating the cell during sonolysis. The same cell was also used for sonolysis (Sonolysis ON-Photocatalysis OFF), photocatalysis (Sonolysis OFF-Photocatalysis ON). Sequential combination of sonolysis and photocatalysis (Sonolysis ON-Photocatalysis ON) was carried out in two separate cells having identical geometry (one being irradiated with UV-light and the other being subjected to sonolysis). In the sequential combination experiments a flow rate of 150 mL/min was employed. The volume of the suspension that remained in the tubing was approximately 34 mL. The reaction cell(s) in all experiments was submerged in an ice water bath so that the temperature inside the sonolysis cell was 20 ( 5 °C. (See Supporting Information Figure 1A,B for the cell design and experimental configuration.) Samples of the dye solutions were removed for analysis at appropriate time intervals during all experiments. Solutions were filtered using Whatman 0.2 µm PTFE syringe filters to remove the TiO2 particles. Degradation of the resulting dye solutions (abs. max. 618 nm) was monitored using a Shimadzu UV-vis 3101PC system to record absorption spectra. Total organic carbon (TOC) measurements were carried out using a Shimadzu Total Organic Carbon Analyzer, model TOC5050A equipped with an ASI-500A autosampler. VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Kinetics of the degradation of an aqueous solutions of NBB following 6 h of (a, 4) simultaneous photocatalysis and sonolysis, (b, ×) sequential photocatalysis and sonolysis, (c, 0) sonolysis, and (d, b) photocatalysis. Control experiments using aqueous solutions of NBB under different conditions: (e, 9) photolysis (UV and O2 only) and (f, +) O2 only are also shown. Absorbance values at 618 nm (E ) 2.32 × 104 M-1 cm-1) were measured at various times to determine the concentration of NBB. Inset shows the absorption spectra of an aqueous solution of NBB recorded following simultaneous photocatalysis and sonolysis.
Results and Discussion We conducted both sonolysis and photocatalysis experiments using an aqueous solution (500 mL) containing NBB (50 ( 5 µM) and TiO2 (1 g/L TiO2) using four different configurations: (i) sonolysis only; (ii) photocatalysis only; (iii) simultaneous sonolysis and photocatalysis; and (iv) sequential sonolysis and photocatalysis. Upon photocatalysis, sonolysis, or combinations therein, the concentration of NBB in dye solutions decreases exponentially and follows a pseudo-firstorder kinetic law. The absorption band at 618 nm is quite convenient to monitor the rate of degradation of the parent compound, NBB (Figure 1 inset). The absorption spectra of an aqueous solution of NBB recorded at different time intervals following simultaneous photocatalysis and sonolysis shows disappearance of the absorption band at 618 nm with increasing time. Almost all of the dye is degraded within 200 min by sonolysis + photocatalysis, as is evident from the disappearance of the absorbance band in the visible region. The absorbance decay curves monitored at 618 nm for different sets of experiments are compared in Figure 1. The rate constant for the disappearance of NBB was determined from the pseudo-first-order kinetic analysis of these data. The degradation rate for sonolysis experiment is about two times faster than the photocatalysis experiment. It is also evident that the combination of sonolysis and photocatalysis (simultaneous and sequential) has a beneficial effect in enhancing the rate of NBB degradation. For example, k[-NBB] for the combined method (1.83 × 10-2 min-1) is greater than the one obtained with individual sonolysis (1.04 × 10-2 min-1) or photocatalysis (0.56 × 10-2 min-1) experiment. Upon close examination of these values one can infer that the enhancement observed in the combined methods is an additive effect and that both sequential and simultaneous methods yielded similar degradation rate constants. Thus we can summarize the observed degradation rate constants as
ks+p (ks〈〉p) ≈ ks + kp
(1)
where subscripts s, p, s+p, s〈〉p refer to sonolysis, photocatalysis, simultaneous sonolysis and photocatalysis, and sequential sonolysis and photocatalysis, respectively. 1748
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It has been established in our earlier studies (25-27) that the photocatalytic oxidation of azo dyes proceeds via the hydroxyl radicals generated near the semiconductor surface as well as direct oxidation of the organics by the photogenerated holes. Similarly, hydroxyl radicals and hydrogen peroxide are the major oxygenating species that are responsible for the chemical degradation in sonolytic reactions (912, 28). Although, an estimated 80% of the H• and •OH radicals generated in sonolysis recombine, a significant amount of •OH radicals escape and react with the organic molecules in the bulk of the solution. The rupture of azo group during the initial •OH radical reaction of azo dyes produces several organic intermediates, which need to be further degraded to achieve complete mineralization. The reaction intermediates compete with the parent compound for •OH radicals (or other oxygenating species) and thus slow overall degradation process. The measurement of total organic carbon (TOC) gives us an estimate of the overall mineralization as both the parent dye and the intermediates are totally degraded into inorganic species. We probed the extent of mineralization over a period of 12 h using both individual and combined techniques of sonolysis and photocatalysis. The results are presented in Figure 2. For the individual techniques, we observe an inverse effect as compared to initial NBB degradation rate, i.e., photocatalysis is more effective than sonolysis in achieving mineralization. For example 68% mineralization was achieved following 12 h of photocatalytic treatment of NBB solution. During the same period we achieve only 35% of mineralization in the sonolysis experiment. Combining the two methods had a pronounced effect on the mineralization. Nearly 50% mineralization was achieved in first 4 h and 80% in 12 h when we performed simultaneous photocatalysis and sonolysis experiment. The extent with which we achieved mineralization in this simultaneous combination method is more than an additive effect. Four h of treatment using photocatalysis, sonolysis, or the sequential combination of the two, each resulted in less than 20% mineralization of the textile dye. These results further indicate that intermediates formed during oxidation of NBB are quickly mineralized in the simultaneous sonolysis and
FIGURE 2. Total organic carbon content of an aqueous solution of NBB following 12 h of (4) simultaneous photocatalysis and sonolysis, (×) sequential photocatalysis and sonolysis, (0) sonolysis, and (b) photocatalysis.
FIGURE 3. The dependence on TiO2 catalyst loading, (a) first-order rate constants for the disappearance of NBB(0), and (b) the total organic carbon (b) remaining in the solution following simultaneous sonolytic and photocatalytic treatment of NBB (50 µM). photocatalysis experiments and thus validates its usefulness for environmental remediation. To substantiate the role of photocatalyst in the simultaneous sonolysis and photocatalysis experiment we varied the concentration of TiO2. The dependence of the degradation rate constant and the extent of mineralization achieved in 6 h are shown in Figure 3. The extent of mineralization achieved in these experiments reflects the effectiveness of the photocatalyst in degrading intermediates along with the parent dye, NBB. The rate constant for the disappearance of NBB remains almost the same at TiO2 concentrations greater than 0.1 g/L. As indicated in the previous sections, the rupture of the azo bond is mainly promoted by the sonolysis of the solution. However, we see a maximum mineralization rate at higher TiO2 concentrations. For example, at 2 g/L concentration of TiO2, approximately 90% of NBB is mineralized after 6 h of simultaneous sonolysis + photocatalysis. Again, these results ascertain the role of TiO2 as the photocatalyst favoring the oxidation of reaction intermediates. During the initial stages of NBB degradation in sonolysis experiment, we observe a small increase in the UV absorption (around 275 nm). With increasing time, the transient absorbing at 275 nm builds up, while it is quite efficiently removed in the combined sonolysis and photocatalysis experiment. These results explain why sonolysis alone cannot be effective in achieving complete mineralization of NBB in a short time. The reaction intermediate absorbing at 275 nm
is not observed in the photocatalysis experiment. As shown earlier (28), sonolysis produces reactive species such as •OH, H•, and HO2• radicals in oxygenated aqueous solutions, and their reaction with NBB is expected to yield different intermediates. Preliminary studies carried out by reacting NBB with HO2• in the γ radiolysis experiment indicate the formation of an intermediate with similar absorption characteristics. HPLC analysis is currently underway to identify the intermediates in these experiments. Under the present experimental conditions, the observed rate constant for the disappearance of parent dye in the sonolysis experiment is nearly two times greater than the one observed in the photocatalysis experiment. The faster NBB degradation rate observed in sonolysis experiment and the effectiveness of TiO2 photocatalyst in achieving mineralization is clearly reflected in the experiments carried out with combination of the two. Among the two combinations that we attempted, the simultaneous sonolysis and photocatalysis experiment clearly stands out to be better than sequential combination. There have been suggestions that the sonication prevents the catalyst from aggregation and keeps them as individual nanoparticles (17-19). While this argument is certainly valid in a general sense, it alone cannot explain the difference in two combinative techniques. If dispersion of TiO2 was the only beneficial aspect of sonolysis in promoting photocatalytic activity of TiO2 we would have observed similar mineralization rates for both sequential and simultaneous combinations. The added advantage of sonication on photocatalysis stems from several other sources. For example, during simultaneous sonolysis + photocatalysis experiment we expect the photocatalyst surface to be constantly refreshed. Also, the mass transport of reactants and products to and from the catalyst surface improves as the slurry is constantly agitated. One of the major concerns in any advanced oxidation process is the formation of undesirable chemicals as reaction intermediates. It is not unusual to observe several intermediates with toxicity greater than the parent compound itself. A quick mineralization of an organic contaminant should be the goal to minimize the survival time of toxic intermediates. A combinative AOP approach could serve as a solution to tackle such an issue. As demonstrated in the present study, we have accelerated both the rates of decoloration and mineralization of the azo dye NBB in aqueous solution by combining sonolysis and photocatalysis. Using a combinative AOP approach is certainly a positive step toward achieving quick mineralization and hence deserves careful consideration in future efforts of environmental remediation.
Acknowledgments The work described herein was supported by the Office of the Basic Energy Sciences of the U.S. Department of Energy. This is contribution no. NDRL 4177 from Notre Dame Radiation Laboratory.
Supporting Information Available Figures 1 (cell design and experimental configuration), 2 (absorption spectra of an aqueous solution of NBB), 3 (kinetics of the degradation of aqueous solutions of NBB), and 4 (absorbance difference of an aqueous solution of NBB). This material is available free of charge via the Internet at http:// pubs.acs.org.
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Received for review November 3, 1999. Revised manuscript received February 2, 2000. Accepted February 9, 2000. ES991231C
