Environ. Sci. Technol. 2005, 39, 8557-8570
Sonochemistry in Environmental Remediation. 2. Heterogeneous Sonophotocatalytic Oxidation Processes for the Treatment of Pollutants in Water
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YUSUF G. ADEWUYI* Department of Chemical Engineering, North Carolina A&T State University, Greensboro, North Carolina 27411
Recent advances in advanced oxidation technologies for applications in environmental remediation involve the use of acoustic cavitation. Cavitation is the formation, growth, and implosive collapse of gas- or vapor-filled microbubbles formed from acoustical wave-induced compression/ rarefaction in a body of liquid. Cavitation is effective in treating most liquid-phase pollutants but it is highly energy intensive and not economical or practically feasible when used alone. One of the most interesting topics in the recent advances in environmental sonochemistry is the intensification of the ultrasonic degradation process by coupling ultrasound with other types of energy, chemical oxidants, or photocataysts. In Part II of this series, a critical review of the applications of ultrasound in environmental remediation focusing on the simultaneous or hybrid use of ultrasonic irradiation and photocatalysis in aqueous solutions, namely, sonophotocatalytic oxidation processes, is presented.
Introduction Contamination of soil and groundwater from industrial wastestreams is a serious health and environmental problem. Current developments in environmental technologies to address the problem include the oxygen-based chemical oxidation technologies termed Advanced Oxidation Processes (AOPs) (1-6). Recent advances in AOPs include environmental sonochemistry, which involves the application of ultrasound to induce in situ cavitation to destroy or accelerate the destruction of liquid-phase contaminants (7-12). Cavitation is the nucleation, growth, and sudden collapse of gasor vapor-filled microbubbles formed from acoustical waveinduced compression/rarefaction in a body of liquid (7). The lifetime and properties of a bubble are directed by the coupling between its size and parameters of the ultrasonic wave (amplitude, wavelength). Close to or on a boundary the bubble may pulse during several cycles (stable cavitation), the diameter increases by rectified diffusion, and for specific values the cavity collapses (transient cavitation). The implosion of the microscopic bubbles in the liquid generates energy, which induces chemical and mechanical or physical effects. It is well-known that the sudden collapse leads to localized, transient high temperatures (g5000 K) and pressures (g1000 atm), resulting in an oxidative environment due to the generation of highly reactive species including hydroxyl (•OH), hydrogen (H•), and hydroperoxyl (HO2•) radicals, and hydrogen peroxide (13-14). These and other * Corresponding author phone: (336) 334-7564; fax: (336) 3347904; email:
[email protected]. 10.1021/es0509127 CCC: $30.25 Published on Web 10/11/2005
2005 American Chemical Society
reactive species that can be formed by ultrasonic irradiation (denoted by [)))]) in the presence of oxygen (O2), ozone (O3), hydrogen peroxide (H2O2), and persulfate (S2O82-) are summarized in Table 1 [eqs R1-R27] (15, 16). Reactions involving these free radicals can occur within the collapsing bubble, at the interface of the bubble, and in the surrounding liquid. When solid particles are present, the cavitation bubble can implode symmetrically or asymmetrically. If solid particles are present in the proximity of the bubble, the cavity implodes asymmetrically and a microjet is formed (17-20). This high-speed microjet can break up particles resulting in a larger contact area. Symmetric cavitations create shock waves that propagate in the surrounding liquid, causing microscopic turbulences. This so-called microstreaming, results for instance in smaller liquid droplets and hence higher mass transfer rates, increases the liquid renewal rate at the surface interface and causes surface cleaning. That is, in heterogeneous catalytic systems, ultrasound can increase the overall surface area of the solid particles used as catalyst and prevent deactivation by continuously cleaning the catalyst surface. In Part I of this series, an overview of the fundamentals of ultrasound and a critical review of the environmental remediation applications of combinative and hybrid sonophotochemical oxidation processes involving the coupling of ultrasonic irradiations with chemical oxidation, UV photolysis, and hydrothermal oxidation techniques in aqueous homogeneous solutions, were presented (15). In Part II of this series, we present here the fundamentals of photocatalysis and sonophotocatalysis, and a critical review of the remediation of organic pollutants by sonophotocatalytic oxidation processes (i.e., the simultaneous or sequential irradiations of ultrasound and light with a photocatalyst).
Aqueous-Phase Photocatalysis The use of aqueous-phase photocatalysis for the remediation of organic and inorganic contaminants has been the subject of a number of recent studies and reviews (6, 21-41) and will be discussed only briefly here. Photocatalysis exploits the unique electronic structure of semiconductor particles such as TiO2 or ZnO to catalyze redox reactions. For a semiconductor catalyst (i.e., TiO2) the threshold or ideal wavelength corresponding to the band-gap energy of 3.02 eV is 300 nm (i.e., near-UV radiation). When exposed to ultraviolet, near UV-light, or sunlight, an electron (e-) from the valence band (VB) is promoted to the conduction band (CB) resulting in the simultaneous generation of a positive oxidant hole (h+) in the VB. The step of electron-hole formation is a very fast one (time constant ≈ 1015 s-1) in a well-illuminated reactor (37). The photogeneration and interaction of radical species can be represented by the reactions in Table 1 (eqs R28R68). The e--h+ pair of carriers could recombine at the surface of the semiconductor particle or in the bulk to produce VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Chemistry of Coupled Sonophotocatalytic Oxidation Processes Section A: Sonolysis in the Absence and Presence of O2, H2O2, O3, or S2O82(i) Ultrasound Only H2O + ))) f H• + •OH •OH + •OH f H O + O• 2 •OH + H O f H O + H• 2 2 2 • • H + OH f H2O H• + H• f H2 O• + O• f O2 O• + H2O f 2 •OH •OH + •OH f H + O 2 2 •OH • (aq) + OH(aq) f H2O2(aq) • • H + O2 f HO2 HO2• + H• f H2O2 HO2• + HO2• f H2O2 + O2 •OH + HO • f H O + O 2 2 2
(R1) (R2) (R3) (R4) (R5) (R6) (R7) (R8) (R9) (R10) (R11) (R12) (R13)
(ii) In the Presence of O2 and H2O2 O2 + ))) f 2 O• O• + HO2• f •OH + O2 O2 + O• f O3 H2O2 + ))) f 2 •OH
(R14) (R15) (R16) (R17)
(iii) In the Presence of O3 )))
O3(g) 98 O2(g) + O(3P)(g) )))
9
(R19)
O(3P)(g) + H2O(g) 98 2 •OH(g) • (aq) + O3(aq) f HO2(aq) + O2(aq) • •OH (aq) + H2O2(aq) f HO2(aq) + H2O HO2(aq)• h O2(aq)•- + H + •- f OH - + O •OH + O 2(aq) 2 •OH + O •- + H + 2(aq) (aq) f H2O + O2(aq)
(R20) (R21) (R22) (R23) (R24)
(iv) In the Presence of S2O82S2O82- +))) f 2SO4-• SO4-• + H2O f HSO4- + •OH SO4-• + SO4-• f S2O82-
(R25) (R26) (R27)
Section B: TiO2-UV-O2-H2O2-O3-S2O82--HSO5- Reaction Systems (i) TiO2-O2 System TiO2 + hν(g3.2 eV, BrO3- > ClO3- (54). However, in general, the photocatalytic degradation rates are considerably reduced by impaired adsorption of pollution on the TiO2 surface. The isoelectric point (pI) of TiO2 is around 6.3, so that the TiO2 particles carry positive charges when the solution pH < 6.3 (56-57). Therefore, for example, chloride ions affect the adsorption step strongly by adsorbing onto the positively charged TiO2 particle surface and also partly absorb UV light, negatively impacting photocatalytic degradation rate especially at low pH (51, 56, 64). Carbonate and bicarbonate act as radical scavengers and also affect the adsorption process. Yawalkar et al. (59) reported the detrimental effect of anions on the photocatalytic process in the order SO42- < CO32- < Cl- < HCO3-. However, Abdullah et al. (50) found that the anions perchlorate (ClO4-) and nitrate made little difference to the photocatalytic oxidation rate despite the fact that their ionic strength contribution at equivalent concentrations is no different from that of Clwhere a pronounced effect was observed, supporting the view that the effect of Cl- involves more than mere blocking of active sites. For example, the reaction of surface holes with the undesirable chloride ions can decrease the formation of •OH radical (eq R85) resulting in low photocatalytic efficiencies. Also, both carbonate and bicarbonate ions show scavenging effects on •OH radicals (eqs R76-R77) (57). It is well-known that the photocatalytic degradation rates of organic compounds follow the Langmuir-Hinshelwood (L-H) type kinetic model assuming the rate is controlled by Langmuir-type adsorption of substrate, which is the case usually at low substrate adsorption on the photocatalyst (2628, 36):
r)-
KaC ∂C ) kr ∂t 1 + KaC
(1)
where Ka is the equilibrium adsorption coefficient of substrate/reactant, C is the concentration of reactant at any time t, kr is the reaction rate constant of substrate reaction on the catalyst, and a linear increase of the reaction rate, r (rate per gram catalyst), is considered proportional to the fraction of surface sites occupied by the substrate. For low concentration of pollutant in water, the term KaC could be considered insignificant and the rate is essentially first order (assuming no other reaction paths appear at the lower concentrations)
r ) krKaC
(2)
and the apparent reactivity of a molecule is the product of two characteristics: its tendency to be converted when adsorbed, kr, and its apparent adsorption constant, Ka. Also, eq 1 can be rewritten as
∂C C ) kr ∂t 1 +C Ka
-
(3)
Thus, from eq 3, it can be noted that the photodegradation rate of organic substrate depends on both values of chemical reaction constant (kr) and adsorption coefficient (Ka). When both kr and Ka increase, the photodegradation rate will increase accordingly. Using different initial concentrations of the pollutant, the reaction rate of the initial stage is determined by each slope of the pollutant vs time plot. When the initial degradation rate r0 is measured for an arbitrary 8560
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initial concentration C0, eq 1 can be written in a linearized form as
C0 1 1 ) C + r0 kr 0 krKa
(4)
where the values of kr and Ka are obtained from the intercept and slope of the straight line, obtained from a least-squares regression. Also, for a batch reactor, eq 1 is written in the form (26)
∂C mAkrKaC ) ∂t 1 + KaC
-V
(5)
which can be integrated to yield C(t)
()
( )
1 C mA ln + (C - C0) ) kt Ka C0 V r
(6)
where V is the liquid volume, A denotes moles of adsorption sites per gram of catalyst, and m is mass of the catalyst. Hence, the apparent half-life or time required for the decomposition of C0 to C0/2 is given by
t1/2 )
(0.693/krKa + C0/2kr) mA/V
(7)
where for C0 , 1/Ka, the reaction is always first order and a reaction half-life is determined which is independent of reactant concentration but is dependent on catalyst concentration (m/V) and apparent reactivity, krKa (in turn dependent on intensity) and hence, t1/2 and krKa provide equivalent sequence of apparent reactivities (26).The solvent may interact competitively with the illuminated surface. The reaction rate for a surface reaction, where the reactant is significantly more strongly adsorbed than the product follows (26, 36)
ra )
krKaC 1 + KaC + KsCs
(8)
or linearized form for estimating parameters
1 1 + K s Cs 1 1 ) + r0 krKa C0 kr
(9)
where Ks is the solvent adsorption constant and Cs is the concentration of the solvent (in water, Cs ≈ 55.5 M), and the slope is now dependent also on solvent properties. As Cs . C and Cs remains practically constant, the part of the catalyst covered by water is unaltered over the whole range of concentration C and eq 8 can be integrated as follows:
C0 krKa Ka ln + (C - C) ) t C 1 + K s Cs 0 1 + KsCs
(10)
C0 ln + Kapp(C0 - C) ) krKappt C
(11)
where Ka/(1 + KsCs) is usually denoted by Kapp, and eq 11 represents the sum of zero-order and first-order rate equations and their contribution depends on C0. Thus, the effect of competitive adsorption by solvent present at constant concentration is to reduce the true binding constant Ka to an apparent value Kapp ) Ka/(1 + KsCs). When KaC . (1 + KsCs), eq 8 reduces to zero order asymptote
(C0 - C) ) krt
(12)
The efficiency of the photocatalytic process itself is measured as a quantum yield (Φ), which is defined as the number of events occurring per photo absorbed or moles of pollutant degraded or yield of a particular product per Einstein incident (i.e., intensity, I, in Einstein s-1cm-2) upon the sample. It has been demonstrated that Φ varies with I as follows (6). (i) Low I: rate varies as I and Φ is constant; (ii) Intermediate I: rate varies as I0.5 and Φ varies as I-0.5; and (iii) High I (mass transfer limit): rate varies as I0.0 (constant) and Φ varies as I-1.0. Hence, increase in intensity always results in an increase in volumetric reaction rate until the mass transfer limit is encountered. At high-intensity levels Φ decreases as I-0.5, indicating an efficiency penalty for sufficiently intense lamps or concentrated solar sources. When reactor cost is the most expensive part of the process, increase in I below the mass transfer limit is suggested to increase the rate per volume, whereas, lower intensity provides cheaper treatment cost if the photo collection or generation is the major cost of the process (6). Crittenden et al. (65) also reported that the apparent photocatalytic degradation rate constant for trichloroethylene (TCE) increased as the light intensity (I) increased for all catalyst dosages. However, the rate increase with I depends on catalyst dosage. The dependence of the rate constant KI on I was KI ) I/[a + I0] where a is constant. The photocatalytic activity of suspended TiO2 in solution strongly depends on the physical properties of TiO2 (e.g., crystal structure, surface area, surface hydroxyls, and particle size) and operating conditions (e.g., light intensity, oxygen, initial concentration of chemicals, amount of TiO2, pH, and ionic strength of solution) (66-67). Also, for photocatalytic reactors, the optimal concentration of TiO2 loading has to be determined since it is strongly dependent on the geometry of the photoreactor and the incident flux as well as on the mean optical pathway within the suspension (25, 37). In general, the most common problem is the reduced efficiency of the photocatalyst with continuous operation possibly due in part to the adsorption of contaminants on the catalyst surface and blocking of active sites. Practical applications of photocatalytic oxidation require improvements in reaction efficiency.
Sonophotocatalytic Oxidation Processes The coupling of photocatalysis (PC) with ultrasound, i.e., sonophotocatalysis (SPC), represents an example of recent advances targeted at improving photocatalytic processes. Sonophotocatalytic reaction here implies a sequential photocatalytic reaction and ultrasonic irradiation or the simultaneous irradiation of ultrasound and UV light with a photocatalyst in the presence of oxygen or other chemical oxidants such as O3 or H2O2. In addition to activation of the photocatalyst surface, enhancement of mass transport of organic compounds to the surface of the TiO2 from bulk solution and aggregate breakage, ultrasound also provides an extra source of •OH radicals from cavitation events or by promoting the scission of photocatalytically and sonolytically produced H2O2, which, together with the photogenerated holes in the semiconductor valence band, are the major oxidizing species in sonophotocatalytic process. Hence, when the two irradiations are operated simultaneously, more free radicals are likely to be available for the reaction with the pollutants and the synergistic effect is to increase the rates of reaction. Ultrasonic irradiation is also expected to alleviate the detrimental effect of blockage of active sites by continuous cleaning of the catalyst surface (in case of simultaneous irradiation of US and UV light rather than sequential operation) and providing additional surface area due to fragmentation, deagglomeration, or deaggregation of the catalyst particles. As a result of these effects, in photocatalytic oxidation processes where the adsorption of the pollutants at the specific sites is the rate-limiting step, ultrasound plays
a profound role in the global rates of the combinative sonophotocatalytic process. In other words, sonophotocatalytic oxidation results in the elimination of the main disadvantages of photocatalytic operation: fouling of the catalysts and mass transfer resistance due to the turbulence generated by the ultrasonic action (68). The mechanism of sonophotocatalytic oxidation is a hybrid of those of sonolysis and photocatalysis. The modes of reactivity in sonolysis include pyrolytic decomposition and hydroxyl radical oxidation and have been discussed in detail elsewhere (7, 15). It should be noted that •OH radical is the primary oxidizing species produced by both sonolytic and photocatalytic degradations (Table 1). Serpone et al. (69) studied the kinetics of 2-, 3-, and 4-chlorophenol decomposition in air-equilibrated media by low-frequency ultrasound irradiation. They reported that reaction products and kinetics were parallel to those observed in heterocatalytic oxidation of these compounds with semiconductor particles. Vinodgobal and Kamat (70) reported the results of three hydroxyl mediated oxidation reactions (photocatalysis, γ-radiolysis, and sonolysis) for the degradation of reactive dye, Acid Orange-7, under the effect of saturation with O2. They also noted the similarity of reaction pathways in all three processes as made evident from the single identifiable intermediate produced in all experiments, and concluded that textile azo dyes are effectively destroyed by advanced oxidation, or any hydroxyl radical mediated reaction pathways. However, sonolysis adds another unique dimension to sonophotocatalytic systems, with the ability of ultrasound waves to be transmitted perfectly through opaque systems, unlike that of ultraviolet light. Therefore, the sonochemical approach also has the advantage of being adaptable to mixed solid-liquid wastes. Generally, it has been shown that ultrasound enhances the effectiveness of photocatalytic oxidation resulting in the complete mineralization of the pollutants in some cases. For a given pollutant, its extent of adsorption on TiO2 and its hydrophobicity, which governs the probability of presence of the organic in the cavitation bubbles and their surrounding, are two of the most important parameters that would dictate the effectiveness of degradation. For hydrophobic compounds, with a long hydrocarbon chain, which interact poorly with TiO2 in water, the use of ultrasound initially to ensure faster COD decrease and mineralization seems more appropriate. On the other hand, photocatalysis is more suitable for hydrophilic compounds, which are repelled from the cavitation bubbles, and is more adapted to achieve the mineralization, especially to remove the carboxylic acids formed intermediately. Simultaneous use of both photocatalysis and ultrasound are therefore of interest to treat waters polluted with numerous compounds having various hydrophobic properties. In terms of convenience and simplicity of operation, sonolysis could prove to be economically competitive and far superior to many alternative approaches. These include high-temperature catalytic combustion or incineration, activated carbon or zeolite adsorption, supercritical fluid extraction or oxidation, substrate-specific biodegradation, membrane separation, electron-beam irradiation, UV-photolytic, and other chemical degradation methods. Ultrasonic degradation is several times (about 10 000-fold) faster than natural aerobic oxidation, for example (71). In a recent economic analysis of a dilute p-nitrophenol aqueous waste treatment, the cost of sonochemical oxidation was found to be comparable to incineration (72). The relative efficiency of ultrasound in terms of p-nitrophenol degraded per liter of water was also shown to be far superior to conventional UV-photolytic degradation (8). Calculated G-value efficiencies from the literature also indicate sonochemical systems are competitive with other AOPs such as UV photocatalysis and VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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supercritical water oxidation (7, 73). Also, sonolysis does not require the addition of chemical additives to achieve viable degradation rates. However, some chemicals may be utilized as an effective sonolytic catalyst for reactions involving •OH radical, for example. Sonochemical degradation also occurs over wide concentrations varying by order of magnitude (8). It is expected that sonophotocatalytic oxidation processes when fully developed should have potential economic advantages over other conventional processes for the remediation of recalcitrant pollutants.
Applications of Sonophotocatalytic Oxidation in Wastewater Remediation Ultrasound-assisted photocatalytic destructions of several organic pollutants have been reported recently (64, 74-94). The studies involving sonophotocatalytic oxidation (i.e., coupling of ultrasonic irradiation with UV photocatalysis in the presence of O2, and/or H2O2, O3, Fe (II), or Fe (III) ions) are reviewed here. Aromatic Compounds. Chen and Smirniotis (64) studied the synergistic effect of ultrasound and photocatalysis on the degradation of phenols and chlorophenols (4-CP; 2,4DCP; 2,4,6-TCP) and the influence of intensity of US energy, the ionic strength of the solution (using NaCl and Na2SO4), and chemical properties of chlorophenols (i.e., number of chlorine atoms per molecule) by conducting experiments involving US (70 W, 20 kHz), PC (0.25 g/L Hombikat UV 100 photocatalyst; 450 W, 320-nm Hg lamp), or the combination (sonophotocatalysis) at 30(2 °C). Hombikat UV 100 was chosen for the study because it was determined to exhibit the highest enhancement with US when tested against Degussa P25 and Ishihara ST-21. Significant enhancement was reported for the reaction rate of phenol (1.0 mM initial concentration) for the sonophotocatalytic decomposition (k ) 7.0 µM/min) compared to that with US alone (4.3 µM/ min), an increase of about 63%. The enhancement was also intensified by reducing reaction volume or increasing US power density within a reasonable range with an optimal usage of 0.7 W/mL. It was reported that for a 50-ml reaction volume (with k ) 0.5 µM/min for US and 5.8 µM/min for UV), the synergistic effect from the combination of UV and US achieved 9 times greater effectiveness (k ) 4.4 µM/min) than US alone in terms of the usage of US energy, i.e., average US power density or applied US power per reaction volume. Since results of experiments in the presence of a mixer or use of high flowrates of O2 (1000 sccm) without sonication indicated no enhancement, the enhancement observed in the presence of US was attributed mainly to the supported functions of US in the photocatalysis, i.e., deagglomeration and surface cleaning and not the better mixing provided by US. The sonophotocatalytic degradation of phenol after 140 min was reduced from 96% (without the salt) to 86% in the presence of 0.25 M Na2SO4, and drastically to 39% in the presence of 0.5 M NaCl. Johnston and Hocking (74) investigated the UV photolytic and photocatalytic degradation of 2,4-dichlorophenol (2,4DCP) at 350 nm with and without sonication. They found that the use of sonication (1× 10-3 M 2,4-DCP; 0.2% or 0.05% TiO2) in photolysis resulted in the enhancement of chloride release rate by a factor of 4 compared with UV irradiation only. They also studied the photocatalytic and photolytic degradation of a 2.4 × 10-4 M solution of pentachlorophenol (PCP) with and without sonication. They demonstrated that the initial rate of chloride formation due to photocatalysis using TiO2 was about 2.7 times faster with sonication than without. The photocatalytic/photolytic degradation of PCB isomer 3-chlorobiphenyl at 75 ppm level (4 × 10-4 M) was also investigated by monitoring the chloride formation. TiO2 (0.2% w/w) and 30-mL aliquots were subjected to UV 8562
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irradiation and to combined UV/ultrasonic irradiation. They found a linear rate of appearance of chloride with time for both conditions but rates with sonication were approximately three times greater than that without sonication, and they attributed the enhanced rates partially to the highly hydrophobic nature (i.e., low solubility) of substrate. They also attributed the significant increase in degradation rates and efficiency of the concurrent UV/ultrasonic irradiation to cavitational effects, bulk and localized mass transport effects, and sonochemical reactions. Ragaini et al. (75) studied the kinetics of the degradation of 2-chlorophenol using sonication (at 20 kH and 7.5 W) in different dissolved gas media (Ar, O2, O2/O3), and photocatalysis (using 0.1 g/L Degussa P25 TiO2 (80% anatase/20% rutile, BET area ) 50 m2/g; 315-400 nm/250 W UV light) individually and simultaneously. In general, the following reactivity scale emerged from the results of the study: O3UV-TiO2 g O3-US-UV-TiO2 > O3-US > O3-UV > O3 > US-UV-TiO2 > UV-TiO2 . US. Typical first-order rate constants for US, UV-TiO2, US-UV-TiO2 in using air as dissolved gas were 0.61 ( 0.05 × 10-5 s-1, 5.89 ( 0.08 × 10-5 s-1, and 4.92 ( 0.11 × 10-5 s-1 respectively, and 0.69 ( 0.05 × 10-5 s-1, 13.4 ( 0.8 × 10-5 s-1, and 9.14 ( 0.13 × 10-5 s-1 respectively in O2/Ar (80%/20%). The higher rate in the O2/ Ar mixture was attributed to increased surface concentration of active species on the photocatalyst in the presence of higher O2 content of the gas mixture in equilibrium with the treated suspensions. The effect of stirring was also investigated by using a gas mixture of O2 and Ar as the bubbling gas. The lower decrease in reaction rate observed without stirring under sonophotocatalysis compared with photocatalysis only was ascribed to the beneficial stirring effect of US, which inhibited the deposition of TiO2 particles and maintained them in suspension, where they could be efficiently illuminated. Shirgaokar and Pandit (76) observed that the sonophotocatalytic degradation of 100 ppm aqueous solution of 2,4,6-trichlorophenol (TCP) using combined ultrasound (22 kHz, 40 Wcm-2) and photocatalysis (using anatase-grade TiO2, 0.1 gL-1 and 15 W UV, 254 nm light) at 30(2 °C after 5 h. of sonication was substantially higher (increase of 50 to 100%) compared to that obtained in the absence of the catalyst. They ascribed the increase in percent degradation to the interfacial cavitation at the surface of the catalyst resulting in the pitting and cleaning of the catalyst surface and hence more availability of fresh catalyst surface for reaction. Nakajima et al. (77) studied the degradation of 1,4-dioxane in water (C0 ) 50 ppm) using sonolysis (20 kHz, 50 W), photocatalysis (365 nm, 144 mW/cm2 UV with TiO2 and HFtreated TiO2) and the combined sonophotocatalytic systems at 5 °C. It was shown that HF treatment of TiO2 surface enhanced its absorption capabilities for both 1,4-dioxane and improved the overall decomposition rate of 1,4-dioxane by the sonophotocatalytic treatment. The pseudo-first-order rate constants (min-1) obtained for the decomposition of 1,4-dioxane were 2.8 × 10-5, 1.4 × 10-4, 1.6 × 10-4, 4.7 × 10-4, and 5.4 × 10-4, respectively, for US, UV-TiO2, UV-HFTiO2, US-UV-TiO2, and US-UV-HF-TiO2 systems. The results suggest that the rate constant for the US-UV-HFTiO2 system is slightly higher than that of US-UV-TiO2 despite a low specific surface area (66 m2/g) of the HF-TiO2 powder compared that of the TiO2 without HF treatment (84 m2/g). Matsuzawa et al. (78) investigated the photocatalysis of 5 × 10-4 M solutions of dibenzothiophene (DBT) and 4,6dimethyldibenzothiophene (4,6-DMDBT) using 0.2 g of different TiO2 photocatalysts (Degussa P25, 50 m2/g; PC-1, 300 m2/g; PC-2, 250 m2/g) and UV irradiation (Hg-Xe lamp, >290 nm, 19 mW/cm2), and the effect of H2O2 (3%) and/or ultrasound (45 kHz, 50 W) in the presence of air. The
photocatalytic conversion of the DBTs using the P25 photocatalyst showed the highest rate of photooxidation but was less than 40% after 10 h of irradiation. The pseudo-firstorder rate constants (s-1) for the P25, P25-H2O2, P25-H2O2US, H2O2, and P25-US systems are 9.74 × 10-6, 3.1 × 10-5, 5.3 × 10-5, 4.4 × 10-5, and 2.9 × 10-5, respectively for DBT, and 1.21 × 10-5, 5.7 × 10-5, 9.7 × 10-5, 1.0 × 10-4, and 4.1 × 10-5 for 4,6-DMDBT. The data demonstrated that the addition of H2O2 to the TiO2 (P25)-containing system or irradiating the system with US accelerated the photooxidation but these methods were not much superior to photooxidation using H2O2 alone in the solution. Organic Dyes. Mrowetz et al. (79) studied the kinetics of the degradation of 2-chlorophenol (2-CP) and two azo dyes, acid orange 8 (AO8) and acid red 1 (AR1), at 35 ( 1 °C under sonolysis (20 kHz, 15 W, 37 WL-1) and photocatalysis in the presence of 0.1 or 0.2 g/L Degussa TiO2 particles (35 m2/g) and UV light (250 W, 315-400 nm, and intensities of 4.5 × 10-8 and 5.8 × 10-7 Einstein s-1cm-2) and the combined SPC in the absence and presence of FeCl3 (2 × 10-5 M). They demonstrated that the two azo dyes and 2-CP were stable and fairly stable, respectively, in the absence of TiO2 either under UV or US irradiation but the degradation was enhanced under TiO2 sonophotocatalysis in each case. The degradation of AO8 was found to be slow under US-TiO2 and higher under UV-TiO2 but under UV-US-TiO2, further significant increase of reaction rate, k, and a synergistic effect were observed. The synergy was quantified as the normalized difference of SPC and the sum of the separate PC and US in the presence of TiO2
Synergy )
kUS+UV+TiO2 - (kUS+TiO2 + kUV+TiO2) kUS+UV+TiO2
(13)
Equation 13 suggests that a synergistic effect is evidence when the combined effect of sonolysis and photocatalysis leads to a degradation rate constant kUS+UV+TiO2, which is greater than the sum of the degradation rate constants measured under photocatalysis, kUV+TiO2 and sonolysis kUS+TiO2; that is, kUS+UV+TiO2 > kUV+TiO2 + kUS+TiO2. Typical pseudo-first-order rate constants (s-1) obtained for the kUS+TiO2, kUV+TiO2, kUS+UV+TiO2 and synergy for the degradation of AO8 (C0 ) 4 × 10-5 M) at irradiation intensity of 4.5 × 10-8 Einstein s-1cm-2 were 0.105 ( 0.002 × 10-4, 1.24 ( 0.02 × 10-4, 2.61 ( 0.04 × 10-4, and 0.49 ( 0.04, respectively, at a catalyst loading of 0.1 g/L and 0.089 ( 0.002 × 10-4, 1.93 ( 0.09 × 10-4, 3.57 ( 0.12 × 10-4, and 0.43 ( 0.10 at a catalyst loading of 0.2 g/L. Also, the pseudo-first-order rate constants (s-1), kUS+TiO2, kUV+TiO2, kUS+UV+TiO2 and synergy at irradiation intensity of 4.5 × 10-8 Einstein s-1cm-2 and catalyst loading of 0.1 g were 0.038 ( 0.002 × 10-4, 1.22 ( 0.03 × 10-4, 2.89 ( 0.13 × 10-4, and 0.56 ( 0.09 respectively for ARI (C0 ) 2.5 × 10-5 M) and 0.59 ( 0.02 × 10-4, 0.88 ( 0.04 × 10-4, 1.58 ( 0.06 × 10-4, and < 0.1 respectively for 2-CP (C0 ) 4 × 10-4M). For ARI (C0 ) 1.5 to 4 × 10-4 M), it was demonstrated that a decrease in reaction rate with increasing substrate concentration was due to the progressive decrease of the percent of substrate adsorbed on the semiconductor. Also, the reaction rate constants increase with catalyst loading to maximum value around 0.1 g/L and remained practically constant for higher TiO2 content. On the basis of these results it was suggested that the synergy between photocatalysis and sonolysis could not be attributed to water-semiconductor interface phenomena, but rather involvement of active species in the liquid phase. Also, the use of pre-sonicated TiO2 resulted in only slightly higher rate of degradation and mineralization in line with the fact that sequential application of sonolysis and photocatalysis results in less than additive reaction rates. Results from ARI (C0 ) 2 × 10-5 M) experiments with 0.1 g/L
TiO2 at irradiation intensity of 5.8 × 10-7 Einstein s-1cm-2 indicated that small amounts of FeCl3 (2 × 10-5 M) affected both the adsorption equilibrium on the semiconductor (resulting from the adsorption of about 30% of the Fe(III) on the photocatalyst surface) and the degradation paths [possibly due to reactive radicals produced through photolysis of FeX2+ (X ) Cl, OH) complex in solution], resulting in slightly higher degradation rate. It was also shown in the oxidation of 2-CP that H2O2 evolution was lower under US-UV-TiO2 than under UV-TiO2 suggesting that US contributes to the decreasing of the H2O2 amount, while it increases the degradation rate in a synergistic way. They concluded that the main effect of US was to promote deaggregation of the photocatalyst, induce desorption of organic substrates and degradation intermediates from the photocatalyst surface, and to contribute through cavitation to the scission of H2O2 produced by both photocatalysis and sonolysis with consequent increase of aqueous-phase oxidizing species responsible for the observed synergy. Stock et al. (80) investigated the degradation of an azo dye, naphthol blue black (NBB), using sequential combination of high-frequency sonolysis and photocatalysis using medium-pressure Hg lamp (>315 nm) in four different configurations: (1) US only; (2) PC only; (3) simultaneous US and PC; and (4) sequential US and PC. They observed that for individual techniques, sonolysis was effective for inducing faster degradation of the parent dye, while TiO2 photocatalysis was effective for promoting minerilization. For example, photocatalysis was responsible for 68% minerilization compared to 35% for sonolysis after 12 h. It was also shown that the first-order rate was enhanced by the combined simultaneous and sequential systems. For example, pseudo-first-order rate constant was 1.83 × 10-2 min-1 for the combined case compared with 1.04 × 10-2 min-1 for sonolysis or 0.56 × 10-2 min-1 for photocatalysis experiments. TOC analysis indicated both the parent dye and the intermediates formed were totally mineralized into inorganic species using the combined AOP approach. Selli (81) also studied the degradation of azo dye Acid Orange (A08) in aqueous suspension (C0 ) 4 × 10-5 M) at 35 °C under sonolysis (20 kHz, 20 W), photocatalysis (315-400 nm, 250 W), and sonophotocatalysis, and evaluated the effects of dye concentration, type, and amount of catalyst (TiO2 or ZnO). They found that the use of ZnO (0.05 gL-1) as a photocatalyst resulted in greater amount of H2O2 accumulated during the runs and correspondingly higher synergistic effects compared with TiO2 (0.59 vs 0.49 based on eq 13) even with higher concentration of TiO2 (0.1 g L-1). Typical first-order rate constants for the US-TiO2, UV-TiO2, and US-UV-TiO2 were 0.105 ( 0.002 × 10-4, 1.24 ( 0.02 × 10-4, and 2.61 ( 0.04 × 10-4, respectively, and for US-ZnO, UV-ZnO, and US-UV-ZnO were 0.01 × 10-4, 4.33 ( 0.21 × 10-4, and 10.7 ( 0.5 × 10-4, respectively. It is well-known that the amount of H2O2 accumulated under irradiation in aqueous suspensions containing ZnO is greater than in those containing TiO2 (33, 40). They concluded that the main effect of ultrasound is to contribute, together with photocatalysis, to the scission of H2O2, produced by both photocatalysis and sonolysis, with an increase of the reactive radical species inducing the degradation of the substrate. Also, it was demonstrated that the synergistic effect was a direct consequence of the simultaneous use of ultrasound and photocatalysis as pre-sonication of aqueous suspensions up to 2 h did not lead to any significant increase of AO8 degradation rate under photocatalysis. An et al. (82) evaluated the decolorization and COD removal kinetics of Reactive Brilliant Orange K-R (RBOKR) dye contained in a synthetic wastewater (C0 ) 0.5 mmol dm-3, pH ) 5.8) using sonophotocatalysis and a hybrid process of ultrasound (47 kHz, 30 W)) and photocatalytic oxidation at VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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315 nm. Using TiO2 concentration ranging from 0.01 to 3.0 g L-1 at 315 nm it was shown that the decolorization and COD removal both increased greatly with initial TiO2 concentration, and then leveled off when the dosage of TiO2 exceeded 0.75 g dm-1. With the amount of TiO2 increased, more active sites are presented and the photocatalytic rate increased as expected. The lack of continuous rate improvement was attributed to the increase of the scattering of UV light with increasing TiO2 concentration. At the condition of 0.1 m3 h-1 airflow, 0.75 g dm-3 TiO2, and 0.5 mmol dm-3 RBOKR solution, the first-order rate constants of decolorization and COD removal were 0.0750 and 0.0143 min-1, respectively, for the US-UV-TiO2 process, 0.0197 and 0.0046 min-1, respectively, for UV-TiO2 process, and 0.0005 and 0.001 min-1, respectively, for the sonochemical process. The rate constants for the sonophotocatalytic process were greater than that of both photocatalytic and sonchemical processes either in isolation or as a sum of individual processes indicating an apparent synergistic effect between photo- and sonoprocesses ((i.e., synergistic factor, SF ) Rspc - (Rpc + RUS) > 1, R ) kinetic rate constant)). Also, sparged oxygen was found to have a more pronounced effect than sparged air while nitrogen had no significant impact compared with base experiment without any sparged gas, indicating the increased decoloring rate is due to photoelectron capture role of oxygen leading to increased generation of active species (eqs R33-R38) rather than by increasing agitation. Herbicides and Pesticides. Peller et al. (83) investigated the sonochemical, photocatalytic, and sonophotocatalytic oxidation of two herbicides, 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-(2,4-dichlorophenoxy) propionic acid (2, 4-DP), and chlorophenols, 2,4-dichlorophenol (2,4-DCP) and 2,4,6trichlorophenol (2,4,6-TCP), at 308 K in the concentration range of 1.0 × 10-4 M to 7.0 × 10-4 M in Ar- or O2-saturated solutions using high-frequency sonolysis (660 kHz, 50 W) and 10 to 500 mg/L TiO2 (Degussa-Huls P25) with mediumpressure UV Hg lamp. They reported faster and complete mineralization with no build-up of toxic intermediates even at low catalyst loading when sonolysis and photocatalysis were carried out simultaneously. The lifetimes (( 1min) for the sonophotocatalytic degradation of 2,4-D, 2,4-DCP, 2,4DP, and 2,4,6-TCP were 6.6, 7.7, 5.9, and 8.2, respectively, compared to 8, 20, 6.2, and 18 for PC only and 12, 11, 11, and 12 for sonolysis only. It was shown that catalyst loading as low as 10 mg/L was effective to maintain an effective degradation rate and mineralization of 0.2 M 2,4-DCP in the combination process (lifetime of 8.8 ( 1 min compared with > 60 min for PC only) and the extent of mineralization remained insensitive to catalytic loading. It was also shown that the observed degradation rate followed approximately additive rate enhancement of the individual rates according to
kobsd ) ksono + kphotocat
(14)
where k values are pseudo-first-order rate constant determined from reciprocal of the lifetimes. However, the presence of high-frequency sonication was deemed to offer more than just an additive process through a cavitation-induced degradation since the combination approach displayed a degree of synergy at levels of TiO2 that are inefficient in the individual photocatalytic process (i.e., at e65 mg/L loading). Results of controlled experiments with US-UV in the absence of photocatalyst showed no difference from US only, confirming that the additive effect observed in the combination experiment arose from the photocatalytic activity of the TiO2 nanoparticles and not the UV light. Also, results from photocatalytic experiments using sonolytic pretreated photocatalyst indicated limited beneficial effects, which could not fully account for enhancement observed in the simultaneous sonophotocatalytic experiments. 8564
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Yano et al. (84) studied the decomposition of the popular herbicide, propyzamide [3,5-dichloro-N-(3-methyl-1-butyn3-yl) benzamide] using photocatalytic oxidation and coupled TiO2-UV-H2O2 and TiO2-UV-H2O2-US oxidation systems. In the photocatalytic system containing 200 mg of TiO2 (0.133 wt %), propyzamide (initially 29 µmol/dm3) completely vanished in 40 min but beyond 200 mg propyzamide increased due probably to the fact that the light-shielding of the TiO2 itself affected the decomposition. While the photocatalytic oxidation of propyzamide was further enhanced by the addition of H2O2 (TiO2-UV-H2O2 system with 200 mg TiO2, 0.588 mmol dm-3 H2O2 at 30 °C, pH ) 5.88) leading to complete decomposition in 20 min, intermediates remained and no decrease of propyzamide was observed without light irradiation (i.e., in the dark), suggesting that H2O2 acted as an effective electron-trapping agent and generated •OH to decompose propyzamide (R45). However, the hybrid effect of H2O2 and US (200 kHz, 200 W) on TiO2/ UV (100 W) at the same experimental conditions resulted in a complete mineralization of the propyzamide. Theron et al. (85) determined the removal rates of phenyltrifluoromethyl ketone (160 µmol L-1 PTMK or C6H5COCF3) by sonolysis at 30 and 515 kHz, by UV-irradiated TiO2 (Degussa P25 and Rhodia), and by simultaneous photocatalysis and ultrasonic irradiations. The PTMK firstorder removal rate constant was reported to be 14 times greater at the higher frequency compared with the lower frequency at the same energy and 2.5 times higher when synthesized Rhodia TiO2 was used instead of Degussa P25. However, a synergy between photocatalysis with both TiO2 samples and the 30 kHz ultrasound was observed resulting in the rate constant values about twice the sum of the individual photocatalytic and ultrasonic rate constants but this was not the case with 515 kHz ultrasound regardless of the TiO2 sample. Theron et al. (86) also investigated the degradation rate of PTMK and octano-1-ol either alone or mixed using photocatalysis (degussa P25, 30 nm particles, 50 m2/g, 3.5 g/L; 125 W Hg lamp with radiant flux of 55 MW/ cm2) and ultrasound (515 kHz, 16.0 ( 0.3) separately and simultaneously at 293 K and pH ) 6-6.5. When both PC and US were employed concurrently, the presence of octanol1-ol did not affect the first-order rate constant, k, of PTMK, which was approximately the sum of the k values (in min-1) found when each method was used separately (e.g., k ) 3.2 × 10-2 min-1 for PC, 1.1 × 10-2 min-1 for US, 4.3 × 10-2 min-1 for combined PC + US for degradation of PTMK only; k ) 2.5 × 10-2 min-1 for PC, 4.0 × 10-3 min-1 for US, 2.9 × 10-2 min-1 for PC + US for mixed PTMK and Octan-1-ol degradation). These results are consistent with the interpretation that PTMK is nearly excluded from the cavitation bubbles and their surrounding when the octan-1-ol is present, whereas the octan-1-ol, which tends to form emulsions in water, partitions itself predominantly in cavitation bubbles compared with the TiO2 surface. As a result, PTMK was principally removed by photocatalysis, especially in the presence of octan-1-ol and the micromovements produced by US did not affect the reaction rate constant of PTMK with the photocatalytically generated active species. Chemical Warfare Agents. Chen et al. (87) studied the mechanistic aspects of ultrasound (20 kHz, 70 W) in the photocatalytic oxidation (320 nm UV, 450 W) of dimethyl methylphosphonate (DMMP), a nerve chemical warfare agent, in aqueous solution with O2 sparging (500 mL min-1) at 25 ( 2 °C. It was demonstrated that low-frequency sonication alone did not produce a significant concentration of hydroxyl radicals or sonochemical pyrolysis for the ultrasonic degradation of DMMP to be significant. However, the results of sonophotocatalytic experiments conducted with 100 mL of aqueous slurry containing 0.25 g L-1 TiO2 (microporous Hombikat UV 100 with primary particle size
< 10 nm) and 0.25 g L-1 indicated ultrasound increased the rate of DMMP mineralization. It was shown that the sonophotocatalytic oxidation of DMMP generated the same set of nonvolatile intermediates as photocatalytic oxidation. Despite the absence of degradation of DMMP under ultrasonic irradiation only, the addition of sonication during DMMP photocatalysis increased the mean rate of TOC removal from 0.15 to 0.34 mg L-1. Also, the results of photocatalytic oxidation performed under stirring indicated that agitation of the suspension increased the reaction rate. However, the rate increase was smaller than that induced by ultrasound, probably due to the better acceleration of mass transport by ultrasound compared with mechanical agitation. They also conducted photocatalytic experiments interrupted periodically by turning off the UV light and applying ultrasound in the dark to break up the particles. The rates of photocatalytic mineralization under these conditions were lower than those for pure photocatalytic reactions. The absence of reaction acceleration due to periodic deagglomeration suggests that it does not influence the reaction. Since mass transport in micropores is very slow and can be accelerated by ultrasonic waves and microstreaming, they concluded that the increase in the rate of photocatalytic mineralization of DMMP in the presence of ultrasound was not due to deagglomeration of TiO2 nor positive influence of US via hydroxyl radicals but was associated with enhanced mass transport of reagents into the micropores of the catalyst. It was shown through kinetic modeling that sonication increased the apparent rate constants of all stages of DMMP photocatalytic oxidation and introduced an additional channel for DMMP sonophotocatalytic mineralization without releasing intermediate products, presumably via enhanced mass transport to and from the micropores of the TiO2. Aliphatic Carboxylic Acids. Davydov et al. (88) studied the role of ultrasound (at 20 kHz and power ) 100-110 W/L) and photooxidation (28 W, 375 nm UV system) at 30 ( 2 °C on the photocatalytic degradation of salicylic acid by performing individual and coupled experiments using four commercial titania powders (of different sizes), UV-light, ultrasound, UV-light + titania, and UV-light + US + titania. It was shown that the use of ultrasound during photocatalysis has a pronounced effect on the rate and efficiency of salicylic acid destruction compared with UV-light photocatalysis alone. It was also shown that the combination of the action of ultrasonic waves and UV-assisted PC yielded synergistic effects for the photocatalysts with smaller particle size (e.g., Hombikat (HK) < 10 nm size, BET area ) 313 m2/g), while no enhancement was observed for the largest particle (Aldrich Anatase ) 100-200 nm, BET area ) 100 m2/g). They also investigated the effects of catalyst mass by comparing the behavior of HK photocatalyst at 0.1, 0.25, and 0.4 g/L and found the rate enhancement to be negligible for the case of low catalyst concentration and highest for the intermediate concentration (0.25 g/L). They attributed the lack of synergy for the case of low solid concentration to the presence of smaller aggregate sizes difficult to break to expose light and insufficient number of centers of bubble disruption in the solution such that the ultrasound passed through the slurry without imparting sufficient energy into it to result in a sufficient formation of active radicals. The enhanced activity for the intermediate concentrations was ascribed to the combined effect of full attenuation of light, aggregate breaking by ultrasound due to the action of its shear stress, and generation of radicals by ultrasound itself upon impingement onto solid particles. Gogate et al. (89) reported the synergistic effect of photocatalysis and sonolysis on the degradation of formic acid (as a model compound) in solution (100-1000 ppm) using a 7.5-L hexagonal sonochemical reactor equipped with variable-frquency transducers (900 W when all the transduc-
ers 20 + 30 + 50 kHz are operated simultaneously). By adopting two modes of operation in their experimental procedures: sequential operation with the photocatalyst (Anatase-grade TiO2, 100-100 ppm) predispersed using ultrasound to increase surface area by fragmentation/ deagglomeration and clean catalyst surface followed by UV irradiation (8 W at 254 nm) and simultaneous operation with ultrasound and UV irradiation performed simultaneously, it was shown that the simultaneous irradiation produced about 30% more degradation of formic acid compared to the sequential technique. It was suggested that the improvements observed for the simultaneous technique was due in part to the continuous cleaning and increasing of the catalyst surface by the ultrasound and production of an additional number of hydroxyl radicals during the process of simultaneous photoactivation and ultrasonic irradiation. It was also demonstrated that the degradation rate of formic acid using the combination technique was four times more compared with sonication alone, whereas with photocatalysis alone the degradation was reduced initially followed by enhanced rate. The observed results were explained in terms of the important factors governing the synergistic effects of hybrid sonophocatalytic systems: fragmentation of catalyst (due to large catalyst size) under ultrasound leading to the formation of a large number of solid particles and hence available surface, which initially resulted in scattering of incident UV light and US wave and also inhibited efficient transmission of both US and UV through the medium; cleaning of the fouled catalyst as time proceeded; and enhancement in the number of free radicals generated. It was suggested that the fragmentation mechanism dominated initially but the two other mechanisms took over as the reaction proceeded resulting in the overall enhanced degradation in the sonophotocatalytic system compared with photocatalysis alone. It was also shown that a hybrid technique of UV/US/H2O2 offered the best results compared to individual techniques. Chlorinated Aliphatic Hydrocarbons. Hirano et al. (90) examined the effects of ultrasonic irradiation (45 kHz, 125 W) on the photocatalytic mineralization of dichloroethane, tri- and tetrachloroethylenes, chloroacetic acids, and chloromethanes (5-12.7 mmol‚dm-3) in oxygen saturated aqueous-suspended TiO2 particles (0.1 g in 100 mL solution). For carbon tetrachloride (CCl4) and trichloroacetic acid (Cl3CCOOH), which are comparatively stable to photocatalysis since they have no R-carbon for the initial step of Habstraction at the positions of R-C by the •OH radical, presonication of sample solution prior to photoirradiation significantly enhanced the subsequent photocatalytic oxidation to completion. For example, pre-sonication of CCl4 solutions produced intermediate products, hexachloroethane and tetrachloroethylene, which are photocatalytically more reactive than the original compound. Also, for CCl4 and Cl3CCOOH, the CO2 measured from the simultaneous sonochemical and photocatalytic reactions was higher than the sum of each of the two reactions, suggesting some interactions for the reaction mechanisms between the photocatalysis and sonolysis leading to synergistic effect in the mineralization. It was also shown that pre-sonication of aqueous TiO2 suspension (0.1 g/100 mL) also improved mineralization due to increased catalytic capability of TiO2 resulting from sonolytic dispersion of particles, which might have been coagulated. However, pre- and simultaneous sonolysis showed no significant effects on the photocatalytic mineralization of chloromethanes, chloroethenes, and chloroacetic acids. Oxygenates and Alcohols. Kado et al. (91) clarified further the mechanistic aspects of ultrasound in a sonophotocatalytic process by examining the photocatalytic and sonophotocatalytic oxidations of 2-propanol to acetone and ethanol to acetaldehyde as model reactions in an aqueous suspension VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of anatase-grade TiO2 powder (5-13 µm average size, 10 mg stirred in Ar atmosphere at 1100 rpm). The experiments were performed with Hg-Xe lamp (0.5 Wcm-2 at 365 nm) and ultrasonic generator (20 kHz, 20 W) at 20 °C. It was demonstrated that the rate of formation of acetone was significantly increased under ultrasonic irradiation. It was observed that the particle size of the TiO2 photocatalyst powder increased due to the particle agglomeration by ultrasonic irradiation but the agglomeration of the particles caused reduction of the total surface area. On the basis of these observations it was rationalized that the acceleration in the formation rate of acetone under ultrasonic irradiation was due to activation of the solid photocatalyst together with enhancement of mass transfer. By assuming the rate was controlled by Langmuir-type adsorption of substrate on the catalyst surface (eq 1), it was demonstrated that Ka is not influenced by ultrasound (Ka ) 4.8 mol-1 dm3 with US vs 5.1 mol-1 dm3 without), kr under ultrasonic irradiation is about three times larger than silence (kr ) 3.8 × 10-3 mol‚dm-3 with US vs 1.2 × 10-3mol‚dm-3 without), suggesting further that ultrasound did not affect the adsorption step but activated the catalyst. Selli and co-workers (92-93) investigated the kinetics of methyl tert-butyl ether (MTBE) degradation using photocatalysis (UV-TiO2) with a concentration of 0.1 g L-1 TiO2 either under UV-Vis (315-400 nm) or UV light only (254 nm, 70.4 W), sonolysis (20 kHz, 165 W), sonocatalysis (UVTiO2), and sonophotocatalysis (simultaneous use of US and PC, US-UV-TiO2) at (30 ( 1) °C, and the effects of H2O2 (1.8 × 10-2 M) addition under photolysis on these systems. It was shown that simultaneous sonolysis and photocatalysis exhibited just an additive effect on MTBE degradation probably because the US-induced degradation of the rather volatile substrate occurred in the vapor phase of the cavitation and not in the bulk aqueous phase. Typical first-order rate constants obtained for MTBE (C0 ) 1.0 × 10-3 M) for the US, US-TiO2, UV-TiO2, US-UV-TiO2, UV-TiO2-H2O2, USTiO2-H2O2, and US-UV-TiO2-H2O2 were 2.74 ( 0.08 × 10-5, 3.14 ( 0.09 × 10-5, 9.8 ( 0.4 × 10-5, 12.3 ( 0.3 × 10-5, 13.2 ( 0.2 × 10-5, 3.71 ( 0.09 × 10-5, and 21 ( 2 × 10-5 s-1, respectively. The synergistic effect of US and PC (US-UVTiO2-H2O2) was attributed to US-induced increased reactivity involving both MTBE and its degradation intermediates. Under these conditions, •OH radicals are simultaneously produced by US- and UV-induced scission of H2O2 in the aqueous phase and by the interaction of hydroxide anions with photoinduced holes on the semiconductor surface. Also, ultrasound assists in maintaining H2O2 in the aqueous phase by inhibiting its adsorption and subsequent photoinduced scission on the TiO2 surface. They also investigated the kinetics of MTBE degradation under O2/Ar (80:20) atmosphere, US (165 W) and UV lamp (607 W), and with/without continuous or intermittent stirring (using 46 W magnetic stirrer) in order to ascertain the optimal operating conditions in terms of power consumed (effective energy supply) and the time required for 90% MTBE degradation. Typical 90% degradation times and the corresponding energy consumptions are 148, 280, 322, 1460, and 2900 min and 1.96, 3.82, 3.50, 4.01, and 10.2 kWh, respectively, for the US-UV-TiO2 (intermittent stirring), US-UV-TiO2 (stirring), UV-TiO2 (stirring), US (no stirring), and US (stirring) systems. Thus, it was clearly demonstrated that sonophotocatalytic oxidation with intermittent stirring led to MTBE degradation in the shortest time and with the lowest energy consumption. Surfactants. Suzuki et al. (94) examined the effect of ultrasound (200 kHz, 200 W) at 298 K on the degradation rate of the surfactant polyoxy-ethylene-alkyl-ether (C14H29O(CH2CH2)7H) under three different conditions: ultrasonic irradiation only (sonoprocess), photocatalytic reaction using TiO2 (0.6 g L-1 of Degussa P25), and low-pressure mercury 8566
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(20 W, 253.7 nm) lamp (photolysis), and the combination of the two processes (photosonoprocess). Since hydrophobic substances are less adsorbable on somewhat hydrophilic particle surfaces, it was speculated that the hybrid sonophotocatalytic technique should provide highly efficient decomposition process for chemical substances having both hydrophobic and hydrophilic groups in their structure, such as surfactants. The results of the study indicated that the decomposition rate in the photosonoprocess was faster than the photoprocess or sonoprocess alone. The significantly enhanced rate in the photosonoprocess compared to the sonoprocess alone was attributed to the redispersion of the agglomerated catalyst particles under ultrasonic irradiation.
Future Research Needs, Challenges, and Opportunities The application of sonophotocatalysis for environmental remediation requires a thorough multidisciplinary understanding of both ultrasound parameters and liquid/solid properties including aspects from physics, chemistry, and engineering. Most of the effects (e.g., cavitation, reactions from cavitation-induced radicals, surface cleaning, mass transport, adsorption surface increase by fragmentation) are likely to occur simultaneously and the global effect should result in substantial enhancement in the rates of pollutant destruction. Hence, sonophotocatalytic oxidation processes eliminate some of the main disadvantages of photocatalytic oxidation such as fouling of the catalyst and mass transfer resistance, leading to enhanced rates of degradation. However, the relative contribution of these effects to the enhancement and overall efficacy of the heterogeneous sonophotocatalytic process is far from understood. While it is usually assumed that all the light is absorbed and efficiency quoted as apparent quantum yield is assumed to be unity for the case of no recombination (21), the ability to measure the actual absorbed light is very difficult in heterogeneous systems due to scattering of light by the semiconductor surface. Also, the application of this technology on a large scale is hampered by high cost and/or lack of suitable design reactor strategies. The ground-level solar spectrum contains approximately 1% near-UV photons of sufficient energy to photoexcite the TiO2 catalyst (29). The use of solar light could offer economically reasonable and practical solution for photocatalytic processes. But, a detailed experimental and economical analysis has to be made to arrive at a conclusion concerning the most appropriate technology and method for application (65, 95-96). In general, the photocatalytic degradation rates are considerably reduced by impaired adsorption of pollutant on the TiO2 surface (49-57). For example, the reaction of surface holes with the undesirable chloride ions can decrease the formation of •OH radical resulting in low photocatalytic efficiencies. However, it is known that the sonochemical method offers the significant advantage of improved functionality in the presence of non-oxidizable materials such as salts and inorganic oxides (97-98). Using 20 kHz ultrasound, Seymour and Gupta (97) reported large salt-induced enhancements in oxidation/destruction rates of some organic compounds: 6-fold for chlorobenzene, 7-fold for p-ethylphenol, and 3-fold for phenol oxidation, in sodium chloride solutions (0, 0.17, 0.67, and 1.38 mol/L). The addition of salt was thought to increase the ionic strength of the aqueous phase, which drives the organic pollutants toward the bubble-bulk interface under ultrasonic irradiation. Mahamuni and Pandit (98) also studied the effect of 2% (wt/vol, 0.3419 M) and 8% (wt/vol, 1.3675 M) on the sonochemical degradation of phenol. They found that the degradation products of phenol in the presence of NaCl were the same as those without NaCl and attributed the 1.5 times increase in the rate of degradation in the presence of the higher concentration of NaCl compared to the lower concentration
or absence of NaCl to the creation of more salting out effect, which increases the interfacial phenol concentration and the possibility of hydroxyl radical attack. However, only a few studies have looked at the effects of anions on sonophotocatalytic oxidation. Chen and Smirniotis (64) showed that both Na2S04 (0.25 M) and NaCl (0.5 M) reduced the sonophotocatalytic conversion of phenol, and that the chloride had a more drastic effect. Harada (99), in the sonophotocatalytic isolation of hydrogen from water, showed that the sonochemical process was influenced by the addition of NaCl (as Cl- ions scavenged •OH radicals and also reduced H2O2 production) but could not confirm the effect of addition of NaCl on the photocatalytic process. NaCl is the most common salt in the environment and Cl- is the most usual anoin produced when bio-recalcitrant organics are degraded. Considering the prevalence of different types of inorganic salts in real industrial effluents, additional research is needed in this area with expanded reaction systems to generalize and quantify the effect of ionic strength, cations, and anions on the sonophotocatalytic oxidation process. The effect of pH on the rate of sonophotocatalytic oxidation appears to be complex and is dependent on the type of molecule (acidic or alkaline nature as well as pKa value for the dissociation of the pollutant), and presence of H2O2 and O3 as a process intensifying parameter. For photocatalytic oxidation only, it is well-known that the pH of the aqueous solution significantly affects TiO2, including the charge on the particles, the size of the aggregates it forms and the band-gap energy, which controls the photocatalytic efficiency (25). In sonochemical oxidation, the variations in decomposition rates of the molecule at different pH values are attributed to the pKa value of the molecule since this determines the state of the molecule and the mode of decomposition. The molecule is almost completely in the ionic form when the pH exceeds the pKa but exists in molecular form when the pH < pKa. In ionic state the molecule does not vaporize into the cavitation bubble and oxidation reaction occurs with •OH radicals outside the bubble film. However, in the molecular state, reactions occur by both thermal cleavage inside the bubble and oxidation with •OH radicals outside, leading to more effective decomposition. Also, the detrimental effect of an increase in the solution pH during degradation is partly ascribed to the rapid dissociation of •OH in alkaline solutions as illustrated by eq R75 (100). Because the effects of pH cannot be generalized, more laboratory-scale studies for a variety of different contaminants at different operating conditions (concentrations of pollutants as well as H2O2 or O3) are required to assess the effects of the operating pH on sonophotocatalytic oxidation processes. It is well-known that reactive oxygen species commonly identified as hydroxyl radicals (•OH), superoxide radical anion (O2•-, a reductant and weak nucleophile), and hydroperoxide anion (HO2-, a reductant and strong nucleophile), play main parts in photocatalytic degradation. However, the importance of quantifying reactive oxygen species generated by sonophocatalytic oxidation cannot be overemphasized. Spin trapping methods using nitroso spin traps (e.g., water-soluble, nonvolatile 3,5-dibromo-4-nitrosobenzene sulfonate) combined with electron spin resonance (ESR) have been used to identify hydrogen atoms and hydroxyl radicals conclusively (101). Also, an attempt has been made to develop molecular probes such as 1-hexanol, carbon tetrachloride, and 1,3,5trinitrobenzene for hydroxyl radical, superoxide, and hydroperoxide, respectively (102). Still, better probes specifically reactive to individual species are needed to track oxygenated transient species under varying conditions of photocatalytic and sonophotocatalytic reactions. Commercial reactors for photocatalytic reactions should have the following: (i) high catalyst surface area per unit
volume and maximum penetration of the incident radiation to all parts of the reactor volume to increase the quantum yields, which have remained rather low for TiO2-based photocatalysts; and (ii) high rates of oxygen and substrate mass transfer from the respective phases to the catalyst surface. Also, the nature of the light source and the nature of the catalyst (immobilized or powdered TiO2) affect the cost of the operation. The convenience of catalyst immobilization on progressively larger particles for heterogeneous phase reactors using immobilized TiO2 is achieved at the expense of mass transfer limitation. The photocatalytic efficiency of TiO2 materials in principle could be increased by enlarging the active surface area. On the other hand nanoparticles have large surface area, crystalline structure, the advantage of being more stable, and can exhibit unique and enhanced chemical reactivity not observed for larger particles because of their unusual crystal shapes and lattice order. Davydov et al. (88) demonstrated the advantage of using nanosized particles for sonophotocatalytic oxidation. Nanoparticles that are activated by light should be further evaluated in sonophotocatalytic systems for the ability to remove organic contaminants from various aqueous media. Photocatalytic or sonophotocatalytic oxidation over TiO2 is a “green” sustainable process for the treatment and purification of water and wastewater. However, their applications for wastewater treatment on an industrial scale are hindered by lack of simple but efficient mathematical models for photocatalytic and sonophotocatalytic reactor design, scale-up, and optimization. Studies to select suitable dosages of TiO2 and optimal quantum fluxes in appropriate reactors are also essential to the commercial success of the sonophotocatalytic process. Development of models that take into consideration the coupled effects of ultrasonic parameters, catalyst dosage, particle fragmentation (and the resulting particle size distribution), surface activation, impact of light intensity, dissolved oxygen and other oxidants, and chemical reaction in the heterogeneous sonophotocatalytic systems may lead to a better understanding of the cause and the effects of ultrasound (95-96, 103-104). The need to devise photocatalytic reactor systems to increase the efficiency of photon utilization and overcome the problem of filtration and reuse of powdered TiO2 in slurry reactors, which limits the application of aqueous photocatalysis in practice, is urgent. Another challenge is to design sonophotocatalytic reactors capable of harnessing most of the energy losses in the ultrasonic processes to useful purposes (i.e., pollution treatment) (7). These aspects and demonstrations of economic feasibility might prove to be decisive for large-scale applications and the long-term use of sonophotocatalytic techniques.
Acknowledgments The author is grateful to the Air Force Office of Scientific Research for financial assistance (Grant F49620-95-1-0541) and the Department of Energy (DOE) (Grant DE-FC0490AL66158). Partial support from the STC Program of the National Science Foundation under Agreement CHE-9876674 is also acknowledged.
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Received for review May 12, 2005. Revised manuscript received August 31, 2005. Accepted September 1, 2005. ES0509127
