Ultrasonically Accelerated Photocatalytic Waste Treatment - ACS

Chapter 6 Ultrasonically Accelerated Photocatalytic Waste Treatment

Downloaded by MONASH UNIV on October 26, 2012 | http://pubs.acs.org Publication Date: February 22, 1993 | doi: 10.1021/bk-1993-0518.ch006

A. J. Johnston and P. Hocking Bioengineering Research Laboratory, SRI International, Menlo Park, CA 94025

Semiconductors such as TiO have been widely investigated as materials for the catalytic photodegradation of aqueous environmental contaminants by way of light-induced redox reactions at the semiconductor/liquid interface. Complete mineralization of organics upon UV irradiation of aqueous organics in the presence of TiO has been reported for aliphatics and aromatics such as polychlorinated biphenyls and dioxins. TiO mediated photodegradation may provide a safe and efficient means to destroy a variety of organic pollutants in groundwater and wastewater. Power ultrasound has shown potential for improving this process—concurrent UV and ultrasonic irradiation of the reaction mixture significantly increases degradation rate and efficiency. Potential reasons for the observed reaction rate acceleration are discussed, including cavitational effects, bulk and localized mass transport effects, and sonochemical reactions. 2

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Background Metal chalcogenide semiconductors (T1O2, ZnO, CdS, WO3, Sn02> have been widely investigated as photocatalysis for the degradation of aqueous organics by means of light-induced redox reactions (1-3). The initiating step in this photocatalytic process involves the generation of conduction band electrons (e~ b ) and valence band holes (h b) by illumination of the semiconductor with light of energy higher than the band gap of the semiconductor (10 W/cm ) ultrasonic irradiation in conjunction with a heterogeneous photocatalytic process using photoreactive semiconductors such as titanium dioxide (T1O2). This system has potential to provide increased organic degradation and mineralization rates and efficiencies as compared to the photocatalytic process alone, by • Improved rates and efficiencies (hence throughput) under optimized conditions. • The ability to use impure or cheaper forms or lower levels of the photocatalyst. • Significant increases (up to fivefold in preliminary experiments) in the degradation rate of aqueous organics without poisoning of the catalyst. • Improvements in conventional suspended-catalyst separation methods, such as ultrafiltration. For applications involving treatment of wastewater containing suspended solids, the use of sonication may also assist in the release of hydrophobic organics adsorbed on soil particles, for subsequent reaction with OH-.


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Experimental Initial experiments were designed and conducted to evaluate the scope of this technology for selected pollutants. To this end, the degradation of aqueous solutions of chlorinated phenols and biphenyls in the presence of the heterogeneous photocatalyst T1O2 was investigated. The photocatalysts zinc oxide and cadmium sulfide were also examined to determine the effect of sonication on photoreaction with these catalysts. General Procedure. The aqueous solution containing the substrate (sample size 25-30 ml) was placed in a 40-ml cylindrical jacketed glass cell (~ 3 cm diameter) that allowed continuous water cooling. The temperature of the test solution was controlled at 35± 2°C. For experiments with T1O2, titanium dioxide (Degussa P25, anatase form, surface area = 55± 10 m /g) was added to the cell solution to the desired concentration (normally 0.2% w/w). The cell was irradiated by means of a Blak-Ray B-100A long-wave ultraviolet light source (100-W mercury bulb with a nominal intensity of 2

In Emerging Technologies in Hazardous Waste Management III; Tedder, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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7000 pW/cm at 350 nm) positioned 12.7 cm from the cell outer jacket wall. The solution was sonicated using a 1.27-cm titanium horn immersed 1 cm into the cell solution and powered by a 475-W Heat Systems XL2020 ultrasonic processor. The amplitude of ultrasonic vibration (20 kHz) at the tip of the horn was set to the maximum value (120 μπι); for the particular cell geometry and contents, this required approximately 130 W input power. After sonication or photolysis (or both) for the desired time, the sample was analyzed for chloride ion production using a chloride ion selective electrode. The disappearance of the compound was monitored by UV/visible spectrophotometric analysis after filtration through a 0.20-μιη Teflon filter. Phenol levels were determined colorimetrically at 510 nm using an aminoantipyrine/ferrocyanide assay (18). Ferrioxalate actinometry was used to evaluate light output at 360 nm. Recovery of substrate from T1O2 suspensions that were stirred in the dark for >90 minutes were greater than 95%, indicating that surface adsorption of unreacted substrate was not significant in analyses to determine degradation. Parameters that were varied included time of sonication, concentration of pollutant, and effect of presonication of the catalyst. Control experiments (sonication only, photolysis with high-speed magnetic stirring, photolysis or sonication without catalyst) were also performed. Results. The use of sonication during photolysis had a significant effect on the rate and efficiency of organic destruction as compared with photolysis alone. Significant enhancements in degradation rates were noted with T1O2 and chlorophenols. Rate enhancements effects due to decreases in particle size (increases in total surface area) for 0.2% T1O2 suspensions subjected to sonication were insignificant: photolysis using T1O2 that had been presonicated for 10 minutes showed minor differences in initial rate of disappearance of the substrate (the actual change in total surface area was not established). UV irradiation of catalyst-free solutions with and without sonication did not result in significant degradation of the organic substrate, except for the combined UV irradiation and sonication of 4-chlorophenol solutions, indicating that the UV energy alone was insufficient for significant direct photolysis. Sonication of catalyst/substrate suspensions without simultaneous UV irradiation did not result in significant degradation except, again, for 4-chlorophenol. Pentachlorophenol. The photocatalytic degradation of pentachlorophenol (PCP, 2.4x10" M solution) with and without sonication was investigated. The initial rate of appearance of chloride was about 2.7 times faster with sonication than without (Figure 1). Also, in the absence of sonication, the percent degradation as measured by release of chloride ion reached a value of about 40% after 50 minutes, and continued irradiation up to 200 minutes did not result in any significant increase in degradation. In contrast, combined sonication/photolysis resulted in a rapid initial degradation, with near quantitative release of chloride after 120 minutes. A possible explanation for this observation is the poisoning of the catalyst by some relatively inert intermediate formed during photodegradation; 4


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Accelerated Photocatalytic Waste Treatment 109


χ

100 Time (minutes)

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Figure 1. Degradation of pentachlorophenol as measured by chloride release sonication serves to remove this intermediate from the active sites, allowing continued degradation. Alternatively, decreases in pH during photolysis may lead to a significant decrease in a photochemical degradation process. 3-Chlorobiphenyl. The sparingly soluble PCB isomer 3-chlorobiphenyl was added to water at a level of 75 ppm (4X10" M ) and dispersed using an ultrasonic cleaning bath, resulting in a cloudy solution. T1O2 was added to a level of 0.2% w/w, and 30-ml aliquots were subjected to UV irradiation and to combined UV/ultrasound irradiation. The chloride ion released was monitored over time with results as shown in Figure 2. The rate of appearance of chloride was fairly linear over time for both treatment conditions, but the rate using sonication was approximately three times greater with than without sonication. The highly hydrophobic nature of this solute, evidenced by its low solubility, may be a factor in the enhancement observed. That is, ultrasound may serve to deposit the substrate on the particle surface more effectively than stirring alone, analogous to ultrasonic phase transfer catalysis. 4

2,4-Dichlorophenol. Several reaction parameters were varied in the degradation experiments with 2,4-dichlorophenol (2,4-DCP). In addition to evaluating the rate increase upon sonication, we examined the effects of lower catalyst concentration (0.05% T1O2), of using presonicated T1O2 in UV-only exposures, and of varying the initial concentration of 2,4-DCP. The use of sonication (lxlO" M 2,4-DCP; 0.2% T1O2) in photolysis resulted in enhancing the chloride release rate by a factor of four as compared with UV irradiation only (Figure 3). Experiments with the lower catalyst concentration yielded results similar to degradations using 0.2% T1O2 (Figure 4; data from spectrophotometric measurement on phenol levels). This result suggests that the 3


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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT III

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ι' ι 100 150 200 250 Time (minutes) Figure 2. Degradation of 3-chlorobiphenyl as measured by chloride release

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Time (minutes) Figure 3. Degradation of 2,4-DCP as measured by chloride release


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Accelerated Photocatalytic Waste Treatment 111


1.2

o-o 1 0

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Figure 4. 2,4-DCP degradation with lower catalyst concentration, as measured by UV spectral analysis (510 nm). incident light is completely absorbed by the lower level of T1O2 in this experimental setup. The effect of presonicated T1O2 (0.20%) in UV-only experiments was not significant, consistent with the conclusion that the amount of catalyst present is already sufficient to absorb all of the light. Additional experiments using presonicated T1O2 at lower levels will have to be performed to determine if increases in particle surface area at low levels of T1O2 will be a beneficial effect of using ultrasonic irradiation. Decreasing the initial concentration of 2,4-DCP by one half resulted in little change in the initial rates of phenol disappearance for both sonication/photolysis and photolysis alone (Figure 5). This could indicate that we are near the plateau region of the initial-rate versus initial-concentration curve as described by the Langmuir-Hinshelwood mechanistic model. (3) Further experiments are required in order to more fully characterize the effect of initial substrate concentration on degradation rate and to evaluate the kinetic parameters according to the Langmuir surface-reaction model.


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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT III 1.2 [2,4-DCP] = 5x10-4 M 1.0 0.8 J ο ^0.6 J 0.4 j


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25 50 Time (minutes)

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Figure 5. 2,4-DCP degradation with 1/2 initial concentration of 2,4-DCP as measured by UV spectral analysis (510 nm).

4-Chlorophenol. 4-Chlorophenol (4-CP) degradation has been extensively studied by other researchers as a substrate for T1O2 heterogeneous photodegradation (7,19). For this substrate, combined UV irradiation/sonication of catalyst free solutions resulted in a significant decrease in the substrate concentration over time, as compared with UV irradiation only, which had a minimal effect. Sonication alone resulted in a similar decrease in 4-CP after 30 minutes, suggesting that homogeneous sonochemical reaction of the 4-CP is occurring. The higher vapor pressure and higher initial concentration (7xl0" M) of 4-CP as compared with the other substrates studied may provide a rationale for the sonochemical effects observed. If there is a significant percentage of 4-CP in the vapor phase of the cavitational bubble upon collapse, the likelihood of cavitationrelated thermal decomposition increases. However, sonication of UV-irradiated catalyst solutions resulted in similar initial rates of disappearance of the phenol (Figure 6) and appearance of chloride (Figure 7) as compared with UV irradiation alone. At longer treatment times, the rate of disappearance of substrate was constant for sonication/photolysis but started to level off in the photolysis-only treatment (Figure 6). The relatively high concentration of 4-CP used may have some effect on the catalyst; after short exposure times there was a definite change in the appearance of the T1O2 to a more flocculant yellow solid that settled out rapidly. 3



JOHNSTON AND HOCKING

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Accelerated Photocatalytic Waste Treatment

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Figure 6. Disappearance of phenol in 4-CP as measured by U V spectral analysis (510 nm)

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Ultrasonically Accelerated Photocatalytic Waste Treatment - ACS

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