Sonochemistry in Environmental Remediation. 1. Combinative and

Critical Review

Sonochemistry in Environmental Remediation. 1. Combinative and Hybrid Sonophotochemical Oxidation Processes for the Treatment of Pollutants in Water YUSUF G. ADEWUYI* Department of Chemical Engineering, North Carolina A&T State University, Greensboro, North Carolina 27411

Sonoprocessing is the utilization of sonic and ultrasonic waves in chemical synthesis and processes. It is a new and rapidly growing research field with broad applications in environmental engineering, green chemical synthesis, and processing. The application of this environmentally benign technique in environmental remediation is currently under active research and development. Sonochemical oxidation is effective in treating toxic effluents and reducing toxicity. However, the ultrasonic treatment is highly energy intensive since sonication is relatively inefficient with respect to total input energy and is therefore not economically attractive or feasible alone. Hence, sonochemistry has not yet received much attention as an alternative for industrial and large-scale chemical and environmental processes. One of the most interesting topics in the recent advances in sonochemistry is the possibility of double or more excitations with ultrasound and other types of energy. The coupling of ultrasound with other free energy sources (i.e., UV) or chemical oxidation utilizing H2O2, O3, or ferrous ion presents interesting and attractive approaches. Therefore, many recent efforts have been devoted to improving the efficiency of sonochemical reactions by exploiting the advantages of combinative or hybrid processes involving the simultaneous or sequential use of ultrasonic irradiation and other advanced oxidation processes, electrochemical processes, and biological treatment. This paper provides a critical review of the applications of ultrasound in environmental remediation, focusing on recent developments and unifying analysis of combinative or hybrid systems, namely, sonophotochemical oxidation processes.

Introduction The removal of hazardous substances from industrial waste streams and remediation of contaminants in groundwater, soil, and rock is a major problem in the United States. Traditional techniques to remove contaminants from soil include landfilling, air stripping/carbon adsorption, incineration, biological activity, and chemical treatment. Incineration, adsorption, and landfilling merely transfer the contaminant to another phase or location (i.e., a pollution shift) and produce a potentially dangerous and toxic secondary disposal requirement. Biodegradation is very sensitive to numerous environmental factors, is slow, often produces unpredictable results, and is uneconomical for highly * Corresponding author phone: (336)334-7564; fax: (336)334-7904; e-mail: [email protected]. 10.1021/es049138y CCC: $30.25 Published on Web 04/08/2005

 2005 American Chemical Society

concentrated waste effluents. Recent treatment methodologies involving chemical oxidation have the potential to treat all types of organic and inorganic contaminants (volatile, semivolatile, and nonvolatile). These processes, which are all oxygen based, are usually termed Advanced Oxidation Processes (AOPs). An AOP is defined as the oxidation process that generates hydroxyl radicals in sufficient quantity to affect water treatment. The oxidative processes can entail complete mineralization, implying that the final products of degradation reactions are carbon dioxide, short-chain organic acids, and inorganic ions, typically less toxic and amenable to biodegradation. The AOPs generally use a combination of oxidation agents (O3, H2O2), irradiation (ultraviolet, ultrasound), and catalysts (TiO2) as a means of generating the excited hydroxyl (or •OH) radicals, which are more powerful oxidants than molecular O3 or H2O2. They include the nonthermal homogeneous systems without irradiation (O3/H2O2, O3/OH-, Fenton’s reagent - H2O2/Fe2+) and with irradiation (photo-Fenton, H2O2/UV, O3/UV, H2O2/O3/UV), nonthermal heterogeneous systems with irradiation (TiO2/O2/UV or solar energy) and without irradiation (electro-Fenton), and hydrothermal systems (wet air and supercritical water oxidation) (1-8). Hydrothermal oxidation involves the oxidation of organic matter in water under pressure and temperature. The end products are essentially water and mineral acids (HCl, H2SO4, H2PO4, etc.) in the liquid phase and carbon dioxide and molecular nitrogen in the gas phase. Wet Air Oxidation (WAO) is a process of subcritical oxidation of organic material in the aqueous phase with pure oxygen or air at elevated temperatures (150 °C e T e 325 °C) and pressures ranging from 0.5 to 20 MPa (350-3000 psig) in the absence or presence of homogeneous catalysts. The main limitations are the O2 diffusion in the liquid phase (diphasic system) and the ammonia produced as end products. Supercritical Water Oxidation (SCWO) techniques employing higher pressures (P g 22.1 MPa) and temperatures (374 °C e T e 600 °C) ensure extreme conditions under which both organic matter and oxidants (air, oxygen, or hydrogen peroxide) are completely miscible with water. This suppresses interfacial transport phenomena as in WAO but might lead to corrosion problems due to the extreme conditions. A rapidly developing field in advanced oxidation technologies for applications in environmental remediation is the use of cavitation (i.e., environmental sonochemistry), which is the application of ultrasound to destroy or accelerate the destruction of liquid-phase contaminants. Cavitation science and engineering is a field involving the application of ultrasonic waves to chemical processing (9-16). The application of sonochemistry in environmental remediation falls under three catagories: (i) the use of cavitation alone VOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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as a clean energy source; (ii) the use of cavitation to improve other treatments (e.g., advanced oxidation); and (iii) the use of cavitation to effect reduction in the amounts of chemicals required for conventional treatments (e.g., reduction in biocide levels) (17). Recent reviews and books have addressed the fundamentals; science and engineering aspects of sonochemistry; applications to catalytic reactions, organic electrochemistry, organometallic synthesis, and environmental remediation; and types of ultrasonic equipment in use (18-22). However, recent research efforts have focused on combinative or hydrid techniques (i.e., the methods of using ultrasonic irradiations in combination with other advanced oxidation methods and/or biological treatment) since such novel techniques have proven to be more advantageous than ultrasound alone in effectively degrading recalcitrant comtaminants, which are otherwise difficult to handle (23-26). However, the combinative or hybrid techniques are not fully understood and remain to be explored and exploited for efficiency and economic benefits. This paper provides an overview of the fundamentals of ultrasound and a critical review of the environmental remediation applications of sonophotochemical oxidation (defined here as the coupling of ultrasonic irradiations with chemical oxidation, UV photoysis, and hydrothermal oxidation techniques). Sonophotocatalytic, sonocatalytic, and sonoelectrochemical systems will be presented in follow-up papers.

Theoretical Aspects of Ultrasound Fundamentals of Ultrasound and Chemical Effects. The chemical effects of ultrasound are due to the phenomenon of acoustic cavitation, which involves the formation and subsequent collapse of microbubbles from acoustical waveinduced compression/rarefaction. The microbubbles formed during the rarefaction part of the wave contain vaporized liquid or gas, which was previously dissolved in the liquid. The microbubbles can be either stable oscillating, often nonlinearly, about their average or equilibrium size for many acoustic cycles or transient when they grow to a certain size in one or at most a few acoustic cycles and violently collapse during the compression part of the wave. The critical size of the bubble depends on the liquid and the frequency of sound; at 20 kHz, for example, it is roughly 100-170 µm. The lifetime of these microbubble are of the order of microseconds, and their sudden collapse leads to localized, transient high temperatures (g5000 K) and pressures (g1000 atm), resulting in the generation of highly reactive species including hydroxyl (•OH), hydrogen (H•), and hydroperoxyl (HO2•) radicals and hydrogen peroxide as shown in Table 1 (27-40). The formation of •H and •OH is attributed to the thermal dissociation of water vapor present in the cavities during the compression phase. The rate of hydrogen peroxide production during sonolysis of water from reactions in Table 1 is given by:

∂[H2O2] ) kOH,OH[OH][OH] + kHO2,HO2[HO2][HO2] ∂t kOH,H2O2[OH][H2O2] - kpyr[H2O2] (1) where kOH,H2O2 ) 2.7 × 107 M-1 s-1; kHO2,HO2 and kOH,OH are respectively 8.3 × 105 and 5.5 × 109 M-1 s-1 in solution at 25 °C and 3 × 10-12 and 1.5 × 10-11 cm3 molecule-1 s-1 in the gas phase (41). The actual production of •OH and H2O2 during sonolysis will be affected by the collapse temperature and pressure of the bubble and the bubble lifetime. Nagata et al. (42) estimated the rates of •OH radical formation to be 19.8 µM min-1 under argon and 14.7 µM min-1 under air and that of hydrogen formation to be 20 µM min-1 under argon in the sonolysis of pure water at 200 W and 200 kHz. In general, most studies involving the sonochemical degradation of chemical contaminants have adopted the “hot 3410

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TABLE 1. Chemistry of Coupled Sonophotochemical Oxidation Processes Section A: Sonolysis Only (Ultrasound)

H2O + ))) f H• + •OH •OH + •OH f H O + O• 2 •OH + H O f H O + O• 2 2 2 • • H + OH f H2O • • H + H f H2 O• + O• f O2 •OH + •OH f H + O 2 2 •OH(aq) + •OH(aq) f H O (aq) 2 2 H• + O2 f HO2• HO2• + H• f H2O2 HO2• + HO2• f H2O2 + O2 O2 f 2•O O2 + •O f O3

(R1) (R2) (R3) (R4) (R5) (R6) (R7) (R8) (R9) (R10) (R11) (R12) (R13)

Section B: SonoFenton and SonoPhoto-Fenton Systems Fe2+ + H2O2(aq) f •OH(aq) + Fe3+(aq) + OH-(aq) Fe2+(aq) + •OH(aq) f Fe3+(aq) + OH-(aq) Cu2+(aq) + H2O2(aq) f Cu+(aq) + HO2•(aq) + H+(aq) Cu+(aq) + H2O2(aq) f Cu2+(aq) + OH-(aq) + •OH(aq) Fe3+(aq) + H2O + hν f Fe2+(aq) + H+(aq) + •OH(aq) -H+

(R14) (R15) (R16) (R17) (R18)

))) or UV

Fe3+(aq) + H2O2 98 Fe-O2H2+ 98 Fe2+(isolated) + HO2• Fe2+ (isolated) + H2O2 f Fe3+ + OH- + OH• Fe3+ + HO2• f Fe2+ + O2 + H+ •OH + H O f HO • + H O 2 2 2 2 FeIII(OH)2+ + hν f Fe2+ + •OH 3III 2+ Fe (C2O4)3 + hν f Fe + 2.5C2O42- + CO2 RH + •OH f H2O + R• R• + Fe3+ f Fe2+ + R+ (products) R• + •OH f ROH R+ + H2O f ROH + H+ R• + H2O2 f ROH + •OH •OH + HO • f H O + O 2 2 2

(R19) (R20) (R21) (R22) (R23) (R24) (R25) (R26) (R27) (R28) (R29) (R30)

Section C: Ultrasound-OzonePeroxide-UV Systems )))

O3(g) 98 O2(g) + O(3P)(g) )))

O (P)(g) + H2O(g) 98 2•OH(g) •OH(aq) + O (aq) f HO •(aq) + O (aq) 3 2 2 •OH(aq) + H O (aq) f HO •(aq) + H O 2 2 2 2 HO2•(aq) h •O2(aq) - + H(aq)+ 2•OH(aq) f H2O2(aq) •OH(aq) + •O -(aq) f OH- + O 2 2 HO2•(aq) + •O2-(aq) + H+(aq) f H2O2(aq) + O2(aq) 2HO2• f H2O2(aq) + O2(aq) •O -(aq) + O (aq) f O (aq) + •O -(aq) 2 3 2 3 •O -(aq) + H+(aq) f •OH(aq) + O (aq) 3 2 • H2O2(aq) + O3(aq) f OH(aq) + • HO2 (aq) + O2(aq) H2O2 h HO2- + H+ HO2- + •OH f HO2• + OHO3 + HO2- f •OH + O2 + O2•O3 + UV f O2 + O(1D) O(1D) + H2O f [HO• ... •OH] f H2O2 H2O2 + UV f 2•OH H2O2 + 2O3 f 2•OH + 3O2 H2O2 + O3 + UV f •OH O3 + -OH f O•- + HO2• 3

(R38) (R39) (R40) (R41) (R42) (R43) (R44) (R45) (R46) (R47) (R48) (R49) (R50) (R51)

Section D: Wet Oxidation (WO) Only Systems

RH + O2 f + HO2• RH + HO2• f R• + H2O2 H2O2f 2OH• RH + OH• f R• + H2O R• + O2 f ROO• ROO• + RH f ROOH + R• R•

(R31) (R32) (R33) (R34) (R35) (R36) (R37)

(R52) (R53) (R54) (R55) (R56) (R57)

spot” concepts to explain experimental results. This theory considers a sonochemical reaction as a highly heterogeneous reaction in which reactive species and heat are produced from a well-defined microreactor, comprised of three regions where chemical reactions occur: (i) the inside of the cavitation bubble known as the hot gaseous nucleus; (ii) the interfacial region between the gas-phase cavitation bubble and the liquid-phase bulk solution, a region with radial gradient in temperature and local radical density; and (iii) the bulk solution at ambient temperature (20). The “structured hot spot” model has led to several modes of reactivity being proposed: thermal reaction or pyrolytic decomposition and hydroxyl radical attack and/or addition. Within the center of the bubble, harsh conditions generated on bubble collapse cause bond breakage and/or the dissociation of the water and other vapors and gases leading to the formation of free radicals or the formation of excited states. The radicals generated either react with each other to form new molecules and radicals or diffuse into the bulk liquid to serve as oxidants. Reactions involving free radicals can occur within the collapsing bubble, at the interface of the bubble, and in the surrounding liquid. Hoffmann and co-workers also proposed the occurrence of supercritical oxidation water (SCWO) on the basis of results obtained in the sonolytic hydrolysis of p-nitrophenyl acetate (p-NPA) at 115 W (96 W/cm2) using Ar, Kr, and He as irradiating gases (4, 37). But unlike classical SCWO techniques, ultrasonic processes form localized supercritical regions without the temperature and pressure of the bulk solution increasing much beyond ambient conditions. However, it is also important to note that subsequent studies have either disputed the formation of supercritical water or minimized the role of transient supercriticality. Tauber et al. (43) investigated the sonolysis of 4-NPA in argon-saturated aqueous solution at 321 kHz (170 W kg-1) and concluded that supercriticallity was not persistent enough to permit a sizable contribution relative to homolytic/pyrolytic decomposition pathway. Ando et al. (44) could not prove the formation of supercritical water in the ultrasonic decarboxylation of 6-nitrobenzisoxazole-3-carboxylate under argon at 20 kHz (12 ( 1 W). Bubble Dynamics and Mechanical Aspects. An acoustic pressure (Pa) is created when sonic vibrations are applied to a liquid. For a sinosoidal sound source, the acoustic pressure at any given time (t) is given by

Pa ) PA sin 2πft

(2)

where PA is the maximum pressure amplitude for a given intensity and f is the frequency of the sound wave (>16 kHz for ultrasound). By analogy with electrical vibrations, the intensity of a progressive planar or spherical wave (I) (i.e., energy transmitted per unit time per unit area of fluid) is described by

I ) PA2(2Fc)-1

Tmax ) To[Pm(γ - 1)/P] ) To[Ro/Rmin](3γ -1)

(5)

Pmax ) P[Pm(γ - 1)/P](γ/γ-1)

(6)

where To is the temperature of bulk solution (i.e., ambient or experimental temperature), Rmin is the bubble radius upon collapse, P ) PV + Pg (i.e., if gas enters the cavity in which case P will depend on Pg when the bubble is at its maximum size), and Pm () Ph + Pa) is the peak pressure in the bubble at the moment of transient collapse. If we assume that the sonochemical reactions follow Arrhenius behavior (k ) A exp(-Ea/RgTmax), then

ln k ) ln A -

]

PV 2σ (Pb - PA sin ωt) dR˙ 3R˙ 2 1 R3R Pgo 3R+1 + ) - 2dt FL R R 2R R R (4) with initial conditions t ) 0, R ) Ro, and R˙ ) 0, where R is the instantaneous radius of the bubble or cavity, R˙ is the bubble wall velocity, Pb is the atmospheric pressure (Ph), Pgo

Ea

P

(7)

RgToPm(γ - 1)

As with transient cavitation, high temperatures and pressures are also produced in stable bubbles as they oscillate in resonance with the applied acoustic field. The ratio To/Tmax is given by (48, 53)

{ [( )

To Ph ) 1+Q Tmax Pm

1/3γ

]}

3(γ-1)

-1

(8)

where Q is the damping factor, which is the ratio of resonance amplitude to static amplitude of the oscillating or vibrating bubble usually assumed to be 2.5. The maximum size of a cavitation bubble is dependent on the density of the liquid, the applied frequency, the hydrodynamic pressure, and the acoustic pressure as follows (50):

Rmax )

[ ][

2 4 (P - Ph) 3ωa A FPA

0.5

1+

2 (P - Ph) 3Ph A

]

0.33

(9a)

where ωa is the applied or acoustic frequency. For a bubble in an ultrasonic field under constant pressure, the maxiumum radius of the bubble is related to the collapse time by the expression (49, 50):

(3)

where F is the density of the fluid, c is the speed of sound in the fluid (1500 m/s in water), and the term Fc represents the acoustic impedance (z) of the medium (19). The Rayleigh, Plesset, Noltingk, Neppiras, and Poritsky (RPNNP) equation for the motion of the bubble wall is commonly used for bubble cavitation (45-54):

[ ( )

is the initial gas pressure inside the bubble, and R () κ) is the polytropic index of the saturated gas, which varies from γ (i.e., Cp/Cv ratio for the cavity medium) for adiabatic conditions to 1 for isothermal conditions. The RPNNP equation has been solved to estimate the radius and pressure-pulse magnitude and history of bubble oscillation for a variety of conditions and is also used to analyze the sonochemical and sonoluminescence effects of cavitation (55-63). The maximum temperatures and pressures attained during the collapse of transient cavitation bubbles are predicted by approximate solutions of the RPNNP equation (51, 52):

τ ) 0.915Rmax

[ ]( F Pm

0.5

1+

)

Pv T < Pm 2

(9b)

where τ ) time of cavitation bubble collapse and T ) ultrasonic period. The above equations show that at high acoustic intensities (i.e., large PA values) the cavitation bubbles are able to grow in size during a rarefaction cycle such that insufficient time is available for complete collapse of bubble during a single compression cycle. Therefore, there is an optimum power density (acoustic intensity) that can be applied during sonochemical irradiation to obtain maximum reaction rates (41). The resonance size (Rr) of an acoustically cavitating bubble is inversely correlated with the ultrasonic frequency as (38): VOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Rr2 )

3κPh

(10)

2

Fωr

where ωr is the resonant circular frequency (2πfr). As the frequency increases, bubble lifetime is shorter, but there are more cavitation events per unit of time and the bubble surface area-to-volume ratio is increased. This increases transport activities across the bubble interface. In general, lower frequency sonication results in greater degree of vapor-phase pyrolysis due to high temperatures that are achieved during bubble collapse conditions, while higher frequencies favor •OH radical production. At high frequencies, the resonant bubble size is not large enough to produce enough energy upon collapse to form sufficient numbers of •OH radicals from water and a point of diminishing return is reached (38). The critical or minimum rarefaction pressure, termed the Blake threshold pressure (PB), which must be applied in excessof the hydrostatic pressure to create a bubble is described by

x

4 PB ) Ph - PV + σ 3

2 × 3

σ (11) σ Ph + 2 - PV R O 3 RO

(

)

This equation assumes that the static gas pressure (Ph), the vapor pressure (PV), the surface tension (σ), and the equilibrium or resonance radius of the bubble (RO) determine the required negative pressure in the liquid to start the explosive growth of a cavity (54). The energy that is required to expand a population of bubbles in solution (i.e., energy stored in cavitating bubbles before collapse) can be estimated by (47):

4 E ) πRmax3PmN 3

(12)

where N is the number of bubbles in solution. This equation indicates that the number of bubbles will increase linearly with power density.

Applications of Sonophotochemical Oxidation in Wastewater Remediation Numerous investigators have examined the mineralization of pollutants by ultrasound and other advanced oxidation processes (AOPs). The results of most studies seem to demonstrate that while ultrasound is effective in degrading pollutants; like other AOPs or chemical methods, total mineralization is difficult to obtain with ultrasound alone, in particular, with recalcitrant pollutants or mixtures of pollutants (20). It is obvious that if two irradiations are operated in combination or if ultrasonic irradiation is coupled with H2O2, O3, or ferrous ion, an additional source of free radicals will be available (as shown in Table 1) for the reaction resulting in enhanced degree of oxidation for these combinative systems. Hence, combinative or hybrid processes involving the use of ultrasound with other AOPs are becoming popular in the treatment of waste streams since they are are found to be more effective in degrading some recalcitrant compounds, which are otherwise difficult to handle. Many efforts have been devoted recently to improve the efficiency of sonochemical reactions to ensure that the substantial amount of the energy employed in generating the radicals is effectively converted into optimum yield of desired product. For example, the combination of ozonolysis and ultrasonic irradiations may be a more effective oxidation system than ozonolysis alone since two •OH molecules are formed per O3 molecule consumed. The recombination of •OH to yield H O both in the gas phase within the bubbles 2 2 and in solution are two of the major processes that limit the 3412

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amount of reactive radicals accessible to the target molecules. The sonochemically generated H2O2 in most cases is not able to react with them and eventually decomposes. The addition of appropriate amounts of Fe(II) ions accelerates the degradation of pollutants due probably to its ability to regenerate •OH from H2O2, which would be formed from recombination of •OH radicals and which may contribute little to the degradation (42). Therefore, the combinative approaches are studied as a way to enhance the amount of • OH available in solution. For example, the classical Fenton’s reaction (eq R14 in Table 1) used in combination with sonication becomes a secondary source of •OH, recovering part of its chemical activity otherwise lost in the production of relatively large amounts of H2O2 during sonication. When Fe3+ is present in solution, it reacts with H2O2 and produces a complex intermediate, FeO2H2+. The complex intermediate, FeO2H2+, can be isolated as Fe2+, and the O2H radical can be isolated either by sonication or by UV light (64, 65). The isolated Fe2+ reacts with H2O2 and again produces the hydroxyl radical, resulting in a high concentration of •OH for the coupled system. However, these coupled processes have not been adequately explored and exploited, and the economics are poorly understood. Studies involving the use of homogeneous combinative sonophotochemical oxidations (i.e., coupling of ultrasonic irradiation with one or more of the following oxidative systems: chemicals such as H2O2 and/ or O3, UV photolysis, UV/H2O2/O3, Fenton reagent, or wet oxidation techniques) are reviewed here. Ultrasound-Peroxide-Fenton Systems. Lin et al. (66, 67) found that the combination of ultrasound with H2O2 increased the efficiency of the decomposition of 2-CPOH significantly and investigated the effect of H2O2 concentration on the decomposition. With the ultrasound/H2O2 process, they observed that with pH controlled at 3 the rate of decomposition of 2-CPOH (i.e., 99%) was enhanced up to 6.6-fold and TOC removal (i.e., 63%) was enhanced up to 9.8-fold as compared with values with the pH controlled at 11. It was also shown that the more H2O2 added, the greater the degradation efficiency. However, the effect of a catalyst (FeSO4) addition on decomposition was also found to be insignificant as compared to direct addition of H2O2. Using a coupled ultrasound (20 kHz, 160 W), Fe2+ (10 mg/L) and H2O2 (500 mg/L) process, Lin and Ma (68) also demonstrated that 99% of 2-chlorophenol was decomposed and 86% was mineralized as compared to ultrasound alone, which resulted in e31% decomposition and 10% dissolved organic carbon (DOC). Chen et al. (69) studied the dechlorination of aqueous chloroform (CHCl3 + 2•OH f CO2 + 3Cl- + 3H+) with ultrasound (40 kHz, 300 W) in combination with H2O2 with or with out addition of ferrous ions (20 mg/L). The addition of Fe2+ improved conversion rates, but an excess amount of H2O2 slightly retarded the decomposition of CHCl3. De Visscher and Langenhove (70) explored the influence of Fenton-type oxidants on the aqueous sonochemical degradation of trichloroethylene (TCE), o-chlorophenol (o-CP), and 1,3-dichloro-2-propanol (DCP). They observed that, in the presence of H2O2 and copper ions, the sonochemical degradation of o-CP is the sum of the effect of the ultrasound and the chemical oxidation effect and likened the overall degradation mechanisms to sonochemical switching effect (i.e., silent chemical and sonochemical degradations follow separate path). Entezari et al. (71) investigated the degradation of phenol (0.662-0.678 mM) using ultrasound alone (50 W, 0.143 W/m; 20, 35, or 500 kHz) and combination of ultrasound and Fenton-type reagent (60.5 mM H2O2/2.39 mM CuSO4) at 20 ( 2 °C to determine sonication conditions and oxidizing agent for maximum efficiency. They found that the rate of sonochemical destruction of phenol was higher at 500 kHz than at 35 or 20 kHz. However, the presence of the CuSO4 catalyst provided a different oxidative system

that proceeded more efficiently at 35 kHz (about three times the rate at 500 kHz) with faster elimination of the intermediate organic compounds as compared with the other two frequencies. With the combined ultrasound and Fenton-type system, the first-order rate constant was 5.8 × 10-2, 1.6 × 10-2, and 1.0 × 10-2 min-1, respectively, at 35, 500, and 20 kHz. The improved rates compared with corresponding values of 1.3 × 10-3, 5.7 × 10-3, and 0.3 × 10-3 min-1, respectively, at 35, 500, and 20 kHz for sonication alone was attributed to the combined production of •OH radical by cavitation and H2O2 decomposition catalyzed by CuSO4 (eqs R16 and R17). It was concluded that the enhanced efficiency at 35 kHz was due to the faster rates of decomposition of H2O2 leading to faster rates of formation of radicals at 35 kHz as compared with 500 or 20 kHz. Joseph et al. (72) investigated the effect of FeSO4 (0.1 or 0.01 M) on the sonochemical degradation of aqueous solutions of azobenzene (AB) and related azo dyes (methyl orange (MO), o-methyl red (o-MR), p-methyl red (p-MR)) saturated with air, O2, and argon at 15.0 ( 0.5 °C, 500 kHz, and 50 W (2W/cm2). The equation for the oxidation of the dye by •OH radical:

dye(aq) + •OH f products

(13)

The main oxidation products were carboxylic acids, carbon dioxide, quinones, and nitrate ions. It was shown with Arsaturated MO solutions (10 µM) that a maximum of 3-fold increase in the measured rate constant for bleaching and mineralization was attained at optimal Fe(II) concentration between 0.1 and 0.5 mM as compared with the sonochemical rate in the absence of Fe(II). The increment was attributed to the high •OH radical concentration produced through Fenton’s reaction (eq R14). However, it was also shown that further increases in Fe(II) concentration showed no further catalytic activity due to the direct reduction of •OH radicals by the metal ions as shown in eq R15. A kinetic model was explored to reproduce the experimental observation and to predict the magnitude of the improvement in the sonochemical efficiency that can be theoretically achieved if the •OH recombination could be partially diverted by Fenton-type reactions. The kinetic model for MO was based on the simplied reaction scheme (eqs 13, R8, R14, and R15) describing concentration changes of species involved in the bulk liquid phase and are given as:

∂[•OH] ) kOH - k13[MO][•OH] - kR8[•OH]2 + ∂t kR14[Fe2+][H2O2] - kR15[Fe2+][•OH] (14) ∂[MO] ) k13[MO][•OH] ∂t

(15)

∂[H2O2] ) kH2O2 + kR8[•OH]2 - kR14[Fe2+][H2O2] (16) ∂t ∂[Fe2+] ) - kR14[Fe2+][H2O2] - kR15[Fe2+][•OH] ∂t

(17)

where kOH ) 5.5 × 10-9 M s-1, kH2O2 ) 2.4 × 10-8 M s-1, k13 ) 2 × 1010 M-1 s-1, kR15 ) 3 × 108 M-1 s-1, 2kR8 ) 1.1 × 1010 M-1 s-1, and kR14 ) 76 M-1 s-1. The values of the zero-order constants for the •OH(aq) and H2O2(aq) sonochemical production, kOH and kH2O2, are adjusted parameters related to the density and rate of implosions and their capability to release •OH and H2O2 in solution. Beckett and Hua (73) investigated the ultrasonic decomposition of 1,4-dioxane (1.0 mM) in the presence of ferrous iron, Fe(II) [1.0-10 mM], and the resulting total organic

carbon (TOC) observed at 25 ( 1 °C and frequencies of 205, 358, 618, and 1071 kHz in the presence of 75% Ar/25% O2 mixture. It was shown that decomposition rate and mineralization efficiency improved at all frequencies. At 358 kHz and 10 mM Fe(II) concentration, 95% of 1,4-dioxane in a 88 mg/L (1.0 mM) solution was removed after 50 min using US irradiation (with no addition of H2O2) as compared with 90% removal for initial 100 mg/L after 8 h via the Fenton process alone. The first-order rate constants were 0.025, 0.049, and 0.069 min-1 at 358 kHz (5.1 W/cm2) with 0, 1.0, and 10 mM Fe(II), respectively. Also, a 2-fold increase in mineralization was observed at 205 kHz and 10 mM Fe(II), while the enhancement generally decreased with increasing frequency. Nagata et al. (42) studied the sonochemical degradation of chlorophenols (2-,3- and 4-CP) and chlorobenzene in the presence of Fe(II) under air and argon at 200 W and 200 kHz. They found that the degradation of 3-CP was enhanced in the presence of Fe (II): the rate increased to 2.4 times at 1 mM Fe(II) concentration and to 1.5 times at 2 mM. They also suggested that there was an optimum Fe(II) concentration for CPs sonolysis with a maximal efficiency and that the excessive amount of Fe (II) led to a decrease in the degradation rate due to the scavenging of •OH radicals by Fe(II) ions as in eq R15. Yim et al. (74) showed that the sonolytic degradation and mineralization of alkylphenols (APs) such as butylphenol, pentylphenol, octylphenol, and nonylphenol in the presence of Fe(II) [80-100 µM] and Fe(III) [g30 µM] at 200 kHz (6 W/cm2) under argon and oxygen resulted in significant enhancement. It was shown that the Fe(III)/US system under oxygen was more effective for mineralization than the Fe(II)/US system and US alone while the Fe(II)/US system provided faster degradation. In these systems, Fe(II) and Fe(III) react with H2O2, which is formed by the sonolysis of water, producing reactive radicals. Neppolian et al. (65) investigated the degradation of MTBE (1.13 × 10-1 mM) using coupled ultrasound (20 kHz), Fe2+ (1.08 × 10-3 mM), and H2O2 (0.05, 0.5, and 1 M) and compared results with the individual Fenton and ultrasonic methods. It was shown that the coupled process of US with Fenton using 0.5 M H2O2 completely degraded the MTBE as compared with 49% degradation in the case of Fenton process and 48% in the case of ultrasonic irradiation alone. The improved rate and efficieny of degradation was ascribed to the combined effects of higher amount of •OH radicals produced during H2O2 decomposition in the presence of Fe2+ ions and the thermolytic cleavage of the target compound (MTBE). In the SonoFenton process, the complex intermediate Fe-O2H2+ formed from the conventional Fenton process (reaction of generated Fe3+ with H2O2) is isolated by the effect of ultrasonic irradiation into Fe2+ and HO2•, the isolated Fe2+ reacting with H2O2 and again producing •OH radicals (eqs R19 and R20). The pseudo-first-order rate constant for the degradation was found greatest (k ) 3.7 × 10-4 s-1) at an optimal concentration of H2O2 (0.5 M) and decreased beyond that concentration due to the reaction of •OH with H2O2, which is faster than the rate of •OH with organic pollutants. The overall •OH radical concentration in the US + Fe2+/H2O2 system was expressed on the basis of reactions in Table 1 as follows:

∂[•OH] ) kUS[H2O][)))] + kFe2+/H2O2[Fe2+][H2O2] + ∂t kFeiso2+/H2O2[Fe2+ iso][H2O2] -

∑ OH radical consumption reaction, OH + •

H2O2 f H2O + HO2• (18)

2+ ion formed from FeO H2+ where Fe2+ 2 iso is the isolated Fe during ultrasonic irradiation denoted by [)))]. The MTBE

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degradation in the coupled process was described by four pathways: (1) direct thermolytic cleavage of methyl tertbutyl ether (MTBE) in the interfacial region of microscopic bubbles during ultrasonic irradiation, (2) reaction with •OH radicals from thermolytic cleavage of water, (3) reaction with •OH radicals from Fenton process, and (4) reaction of OH radicals from isolated Fe2+ reacting with H2O2 due to the coupled process. The overall rate of degradation of MTBE was described by:

-

∑

∂[MTBE] opt ) (kpyr + kMTBE,OH [•OH]Fe 2+/H O ,Fe /H O ) 2 2 iso 2 2 ∂t [MTBE] ) kIII[MTBE]o (19)

opt • where ∑[•OH] Fe 2+/H O ,Fe /H O is the optimum OH radicals 2 2 iso 2 2 concentration from H2O2 and kIII is the overall pseudo-firstorder rate constant for MTBE degradation. Ultrasound-Ultraviolet-Peroxide-Fenton Systems. Wu et al. (75) investigated the dechlorination of trichloroacetic acid (TCA) using ultraviolet (UV) photolysis, ultrasound (US) sonolysis, and their combination (photosonochemical process) by monitoring the yield of the Cl- ions. The efficiency of the sonochemical degradation of the substrate (2.89 × 10-4 M) at 30 kHz (20.4 W) was much lower as compared to that achieved by photolysis. The low sonochemical rate was attributed to the strong hydrophilicity of TCA, which makes it difficult to be transferred into the regions of the gas-liquid interface where it could be subjected to pyrolysis and free radical attack. However, it was found that the degradation by the combined processes was more significant than in the UV photolysis or sonolysis alone. Toy et al. (76, 77) observed the synergistic effects of sonolysis and photolysis in the decomposition of aqueous 1,1,1-trichloroethane. Ku et al. (78) found that the degradation of 2-chlorophenol in the presence of O2 by sonication alone was slow as compared with the use of a UV/H2O2 process. Poon et al. (79) studied the kinetic aspects of UV/H2O2/US treatment process for Cuprophenyl Yellow RL (0.1 g/L). The UV/H2O2 oxidation system was enhanced under ultrasonic irradiation. When lower H2O2 dosage (0.1 mg/L). But excess H2O2 amount (0.2 mL/L) reduced the first-order rate constant. Fung et al. (80, 81) using a bench-scale UV/US combined reactor (UVC, 253.7 nm; US, 320 kHz) also showed that the combined US/UV/H2O2 system improved the decolorization of reactive dyesCI Reactive Red 120seliminating sludge formation and leading to final products soluble in water. The rate of decolorization was enhanced by the addition of H2O2 and increasing the amount of the initial peroxide dosage from 0.1 to 0.2 M. However, peroxide dosage greater 0.2 M lowered the rate of the photosonochemical degradation. The decreased rate was ascribed to the competitive reaction between hydroxyl radical and peroxide when the optimum dosage of peroxide was exceeded, which resulted in the formation of hydroperoxyl radical (HO2•), a less reactive species than •OH (eq R22) in the oxidative degradation of organic substrates. The photochemical disappearance of 4,4′-dihalogenated benzyl in a homogeneous solution at 27 ( 3 °C was accelerated by the simultaneous irradiation of ultrasound (20 kHz, 75 W/cm2) in order of the halogen substituent ) I, Br > Cl > F (82). The CAV-OX process (83, 84) has been shown to mineralize trichloroethane (TCE), benzene, toluene, ethylbenzene, and xylene (BTEX) in contaminated groundwater with efficiencies better than 99.9% when varied principal operating parameters such as H2O2 dose, pH, and flow rate are used. This commercial process involves the use of sequential hydrodynamic cavitation and direct UV photolysis of H2O2 to generate hydroxyl and hydroperoxyl radicals. In this process,

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H2O2 is added to the contaminated groundwater, which is then pumped through a cavitation nozzle followed by UV radiation resulting in an increased rate of degradation. Naffrechoux et al. (85) evaluated the efficiency of sonochemical effect in conjunction with photochemical irradiation for the degradation of aqueous solution of phenol at pH of about 5.5 using ultrasound (485 kHz, 100W) alone, UV (low-pressure Hg vapor lamp, 15 µW cm-2 at 253.7 nm) alone, and the combined “SonoUV” reactor. They observed a significant enhancement in the pseudo-first-order degradation rate of phenol for the combined system: 8.7 × 10-4, 20.3 × 10-4, and 38.0 × 10-4 s-1, respectively, for the UV, US, and the “SonoUV” process. The only organic product detected by HPLC in the oxidized solution after 90 min in the SonoUV reactor was oxalic acid with concentration below 10-6 M. The synergistic effect was attributed to three oxidation mechanisms: photodecomposition, sonodecomposition, and ozone oxidation resulting from the transfer of O3 formed from O2 (due to emission of UV light shorter than 200 nm into air) into the aqueous phenol solution. Wu et al. (86) also found that the combination of ultrasound (30 kH, 100 W) and photochemistry (18 W, 253.7 nm UV radiation) was more effective in degrading aqueous phenol (1.06 × 10-4 mmol/L) and oxidizing its subsequent intermediates than ultrasound or ultraviolet light alone based on total organic carbon (TOC) removal and that the presence of Fe2+ ions as catalyst enhanced the TOC removal. For example, the combined effect of UV and US achieved 99% degradation in 80 min as compared to 54% achieved with sonication alone. The increase in percent phenol degradation and TOC removal in the presence of Fe2+ in the aqueous phenol solution was ascribed to the additional oxidation process induced by the reaction system comprising Fe2+, H2O2, and UV (eqs R14, R15, and R18-R22), which resulted in more •OH radicals produced. Ultrasound-Ozone-Ultraviolet Systems. Weavers and Hoffmann (87) conducted cyclohexene degradation experiments using sonication (in the presence of O2), ozonation, and sonolytic ozonation to determine the effect that ultrasound at frequencies of 20 and 500 kHz had on an O3 gas bubble diffusing into a solution. The degradation experiments using 2.5 mM cyclohexene indicated that rates were fastest when sonication combined with ozonation, followed by sonication with O2, and then ozonation alone in that order. The independent addition of the rate constants for sonication and ozonation of cyclohexene (kUS + kO3 ) 0.191 min-1) and comparison with the rate for the sonolytic ozonation proces (kUS/O3 ) 0.204 min-1) at 20 kHz indicated a slight increase in the degradation rate constant but the synergistic effect was not evident from the data at 500 kHz (kUS + kO3 ) 0.069 min-1 vs kUS/O3 ) 0.065 min-1). Barbier and Petrier (88) investigated the ultrasonic degradation and mineralization of 4-nitrophenol (4-NP) at two ferquencies (20 and 500 kHz) and in water saturated with oxygen or an oxygen-ozone gas mixture. They found that the coupling of ultrasound and O3 increased the potential of ozonation to mineralize 4-NP degradation products. At low pH (pH ) 2), where the ozone auto-decomposition radical pathway is suppressed, they observed 4-NP mineralization at 500 kHz was 1.8 times faster than at 20 kHz for the same O3 consumption and attributed the enhanced rate to increased O3 utilization occurring at the higher frequency. Hua and Hoffmann (89) studied the sonolytic destruction of aqueous CCl4 in the absence or presence of ozone (O3). The observed first-order degradation rates in argon-saturated solutions were found to be 3.3 × 10-3 and 3.9 × 10-3 s-1 with CCl4 initial concentration of 1.95 × 10-4 and 1.95 × 10-5 mol L-1, respectively. It was shown that the presence of O3 did not affect the degradation rate significantly but inhibited the accumulation of C2Cl4 and C2Cl6 intermediates.

Kang and Hoffmann (90) studied the sonolysis, ozonolysis, and combined sonolysis/ozonation of MTBE in the concentration range of 0.01-1.0 mM and demonstrated that the addition of O3 to the influent O2 gas ([O3]o ) 0.26-0.34 mM in solution) accelerated the degradation of MTBE by a factor of 1.5-3.9, depending on the initial concentration of MTBE. The sonochemical first-order degradation rate constant for the loss of MTBE was found to increase from 4.1 × 10-4 s-1 at [MTBE]0 ) 1.0 mM (90% conversion in 93 min) to 8.5 × 10-4 s-1 as the concentration of MTBE decreased to 0.01 mM (90% conversion in 45 min). They indicated that the observed reaction rate was either limited by •OH diffusion out of the interfacial regions of the collapsing bubbles or the interfacial surface areas for chemical reaction between •OH and MTBE. It was also shown that MTBE degraded slowly by ozonation alone under ambient conditions, but the direct reaction of MTBE with ozone appeared to be enhanced at the cavitation bubble interface due to zones of higher temperature or due to indirect reactions with ozone decomposition products. The O3-ultrasound system (Table 1) was shown to effectively degrade the MTBE into innocuous and biodegradable products with tert-butyl formate (TBF), tert-butyl alcohol (TBA), methyl acetate (MA), and acetone identified as the primary intermediates and byproducts of the degradation reaction. It was also shown that acetone, which was formed from the oxidation of TBF (degradation rate constant, kTBF ) 1.87 × 10-3 s-1) and TBA, was the most prevalent intermediate product with the highest yield of 12%. They obtained the following kinetic expression from a mechanism involving three main parallel pathways (i.e., direct pyrolysis, direct reaction with O3, and reaction with •OH), the measured values of the rate constants for MTBE and TBE, and the observed TBF concentration at a given [MTBE]o:

-

∂[MTBE] ) (kpyr + kI + kII + kIII) ) ko[MTBE] (20) ∂t

where kpyr, kI, kII, and kIII are the pseudo-first-order rate constant for direct pyrolysis, thermal degradation leading to O-CH3 bond breakage, direct reaction with ozone, and other direct routes to products formation, respectively. Kang et al. (41) also explored the role of O3, H2O2, frequency, and power density on the sonolytic destruction of MTBE (0.01-100 mM) over a wide range of frequencies (i.e., 205, 358, 618, and 1078 kHz) and power (50-240 W) at 23 ( 3 °C. They found the combination of O3 and US to be significantly more effective than either ultrasound or O3 alone, especially at 354 kHz. They also found the enhancement factor, ko(O3-US)/ko(US) where ko is the peudo-first-order rate constant to be similar for 205 and 354 kHz at higher initial MTBE concentrations and to decrease slowly as [MTBE]o decreases. For 0.01 mM MTBE, the enhancement factor was 3.9 at 205 kHz and 5.4 at 358 kHz as compared with 1.5 and 1.8, respctively, at 1.00 mM [MTBE]o. Olson and Barbier (91) studied the effectiveness of the “sonozone” process (i.e., combined ultrasound ozonolysis) in degrading refractory electrolytes such as humic materials using purified fulvic acid, FA (as a substrate), and naturally colored groundwater sample with pH 8.6. The combined system was found to significantly enhance TOC removal and mineralization rates. With an initial FA of 10 mg/L of FA and under constant ultrasonic irradiation and continuous bubbling of O3 for 60 min, TOC removal and mineralization were 91% and 87%, respectively, for combined O3 and ultrasound as compared with 40% and 28% for O3 alone. The enhanced mineralization efficiency obtained with sonolysis was partially attributed to the direct combustion of volatile intermediates in the cavitation bubble. An additional advantage found with the “sonozone” process was that virtually all TOC removed was mineralized (i.e., converted to CO2) whereas a significant

fraction of the TOC removed by ozone alone is lost as volatile organic carbon compounds. The decoloration rates of fulvic acid solutions were also found to be slightly more rapid than rates with ozone alone. However, the enhancement was attributed to an increase in the mass transfer rates of ozone into solution. Sierka and Amy (92) also studied the singular and combined effects of ultraviolet (UV) light and ultrasound (US) on the ozone (O3) oxidation of humic substances (trihalomethane precursors) using purified solution of a commercially available humic acid. The objectives of this study were to minimize trihalomethane formation potential (THMFP) and to destroy nonvolatile total organic carbon (NVTOC). They found that the combination of O3-US-UV proved to be most effective reaction condition, followed by O3-UV, O3 alone, and O3-US providing 93, 86, 75, and 71% reduction in the THMFP levels, respectively, in the reaction time of 20 min. They also found both mass transfer and reaction kinetics to be improved. Lall et al. (93) studied the decolorization of the dye Reactive Blue 19 by ozonation, ultrasound, and ultrasound-enhanced ozonation using a semibatch reactor. At dye concentration of 0.1 g/L and O3 concentration of 9.4 mg/L and in the presence of ultrasonic irradiations (20 kHz) at powers of 40, 80, and 120 W/L the apparent first-order rate constants for the ultrasound-enhanced ozonation were 0.0038, 0.0049, and 0.0067 s-1, respectively, as compared with ozone alone, which was determined to be 0.0028 s-1, denoting rate constant increases of 35.7, 75, and 139.9% for the respective ultrasonic powers. They also studied the effect of ultrasonic power input on the overall mass transfer coefficient of ozone in solution. Using 5.4 mg/L O3 and 0.03 g/L dye concentration at the maximum ultrasonic power input of 120 W/L, the ratio of mass transfer coefficient with ultrasound (kLa) to that without ultrasound (kLa*) increased by 90%. The increase in overall rate was attributed to the combined effects of an increase in mass transfer and intrinsic kinetics resulting from cavitation. An increase in ultrasonic powers increases the autodecomposition rate of ozone (eq R31), which increases the hydroxyl radical formation (eq R32) and, hence, the intrinsic kinetics of the reaction. An increase in ultrasonic power also increases turbulence, which reduces the liquid film thickness and, hence, increases the mass transfer rate of ozone in solution. Dahi (94) studied the ozonation process with and without simultaneous sonication at 20 kHz in regard to the disinfections of microorganism (Escherichia coli) and oxidation of organic dye (e.g., Rhodamine B). The ultrasonic treatment was found to intensify the action of ozone in the oxidation of chemicals and in the inactivation of microorganisms. It was shown that simultaneous ultrasonic treatment increased the half-order rate constant of decoloration of Rhodamine (k0.5) by 37% in distilled water and by 55% in sterilized secondary effluent water from biological sewage plant. They also found that ultrasonic treatment of the effluent from the biological sewage plant reduced the dosage of O3 needed for its sterilization by up to 50% and improved the all-round aeration constant, kLa (min-1) of the systems by 15-45%. Sierka (95) investigated the treatment of aqueous solutions of trinitrotoluene (TNT) and cyclotrimethylene-trinitramine (RDX) in the weight ratio of 70% to 30% (typical of munitions wastewater) at 25-59 °C using a combination of ozone and ultrasound in the pH range of 5.84-10.00. They demonstrated the synergistic effects of both ozone and ultrasound. While the maximum destruction of both TOC and TNT was observed at 859 kHz and 50 W, experiments performed without O3 at these same conditions resulted in no TNT or TOC removals. Weavers et al. (96) investigated the degradation kinetics of three similar aromatic compounds, nitrobenzene (NB), 4-nitrophenol (4-NP), and 4-chlorophenol (4-cp), at 25 ( 5 °C using sonication, ozonation, and sonolytic ozonation at frequencies of 20 kHz (56.1 W) and 500 kHz (48.3 W) and VOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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ozonolysis. They observed enhancement in the pseudo-firstorder rates of degradation for all three compounds at 20 kHz due to sonolytic ozonation as compared with the individual oxidation system but retardation effects at 500 kHz. Ozonolysis was observed to be slower than sonolysis combined with ozonolysis, and sonolysis was slowest for all compounds at both frequencies. The catalytic effect at 20 kHz was consistent with a pathway involving the thermolytic destruction of ozone in the vapor phase of the cavitation bubbles to form atomic oxygen, which subsequently reacts with water vapor yielding gas-phase hydroxyl radicals (eqs R31 and R32). However, O3 may also react with atomic oxygen or be scavenged by other reactive species in or near the bubble such as •OH formed from sonication, reducing both the production efficiency of •OH and the O3 available in solution to react directly with substrate. On the other hand, ultrasonically enhanced mass transfer of O3 has been observed, which may result in additional O3 being transferred to solution. Despite the complexity of the combined treatment system, the results supported the mechanism of •OH radical formation from O3 decomposition as the main mechanism for the sonolytic ozonation (eqs R31 and R32). The overall rate of substrate (S) disappearance in the sonolytic ozonation process was represented by a linear combination of contributing terms:

∂S ) kUS[S] + kO3[S] + kUS/O3[S] ) ktotal[S] ) ∂t

∑

ki[S] (21)

where kUS, kO3, and kUS/O3 are the first-order degradation rate constants for sonolysis, ozonolysis, and residual kinetic effect upon combining the system, respectively; ktotal is the overall pseudo-first-order reaction rate constant in the combined system. The kUS[S] term combines all pyrolysis and •OH radical reactions with substrates in the sonication alone (with O2) experiments and the term kO3[S] includes both direct O3 reaction and secondary reactions from O3 autodecomposition that occurred in the nonirradiated experiments. The remaining residual kinetic effect in the combined process after the terms kUS and kO3 were determined in separate sonolysis and ozonation experiments was attributed to sonolytic ozonation. The enhancement factor of sonolytic ozonation over the separate processes is given as:

enhancement )

kUS/O3 kUS + kO3

× 100

(22)

The enhancements were found to be 411, 227, and 315% at 20 kHz and 137, 35, and 64% at 500 kHz, respectively, for NB, 4-NP, and 4-CP. The rates constant at 20 kHz were found to be 109, 68, and 46% higher for NB, 4-NP, and 4-CP, respectively, for the combined system than for the linear combination of separate experiments. At 20 kHz, typical pseudo-first-order degradation rate constants determined for NB, kUS, kO3, ktotal, and kUS/O3 were 3.2 × 10-3 ( 8.4 × 10-5, 3.9 × 10-2 ( 1.1 × 10-3, 8.8 × 10-2 ( 3.9 × 10-3, and 4.6 × 10-2 ( 4.1 × 10-3 min-1, respectively. Also, typical pseudofirst-order degradation of TOC associated with NB, kUS, kO3, ktotal, and kUS/O3 were 3.2 × 10-4 ( 6.3 × 10-5, 2.2 × 10-3 ( 2.9 × 10-4, 1.3 × 10-2 ( 2.2 × 10-3, and 1.0 × 10-2 ( 2.2 × 10-3 min-1, respectively. Destaillats et al. (97) studied the sonochemical oxidation of 10 µM azobenzene (AB) or methyl orange (MO) at 500 kHz (50 W, 2 W/cm2) and found the extent of mineralizations measured as total organic carbon (TOC)sto increase to more than 80% after 150 min in the presence of O3 as compared to 20% for US alone and 30% for O3 (27-310 µM) alone. It was also shown that nitrobenzene (NB) and benzoquinone (BQ), two rather persistent byproducts of sonolysis, were rapidly and completely mineralized by the combined oxida3416

9


tion treatment. Similarly with MO, it was shown that ozonation or sonication led to about 50% TOC reductions, but their combination achieved a faster and more complete mineralization with TOC∝ e 0.2 TOC0. The abatement of the total organic load by the joint action of ultrasound and O3 was ascribed to chemical synergism, likely involving the fast oxidation by O3 of free organic radicals or unsaturated species generated by •OH radical attack on otherwise refractory products such as partially oxidized saturated mono- and dicarboxylic acid that are known to be resistant both to ozonation and ultrasonic irradiation perfomed separately. The chemistry of O3 under ultrasonic irradiation involves thermal decomposition of O3(g) in the vapor phase of the cavitation bubble, which leads to enhanced •OH radical (eqs R31 and R32). Although the ground-state O atoms produced are rather unreactive and mostly recombine to yield O2, they also contribute to increase in •OH radical (and their recombination product, H2O2) production by cavitation bubbles as shown in eqs R31-R36. Also, the aqueous chemistry involves the reaction of dissolved O3 directly with target substrates, their initial byproducts, or species originating from water (and O3) sonolysis as shown in eqs R31-R42 in Table 1. Assuming that the •OH radical is the only reactive species in the absence of O3, the pseudo-first-order rate constant is

∂[X] ) kx[X] ∂t

(23)

where X ) AB, MO, NB, or BQ and kx (0.041 min-1) includes the steady-state concentraction of •OH(aq) (3.4 × 10-11 M): • kx ) kOH x [ OH]ss

(24)

[•OH]

At the interface, is constant and has been estimated to be extremely high in the interfacial region of a cavitation bubble based on •OH reaction with iodide (98). To assess the role of •HO2 and •O2- in eqs R34 and R35 in the oxidation of BQ, Destaillats et al. (97) postulated a simple kinetic model using eqs 25, 26, and R34-R39 to reproduce experimental observations. The differential equations (eqs 27-30), which describe the evolution of the species concentrations in the liquid phase during ultrasonic irradiation under O2 saturation, were solved neglecting the direct oxidation of BQ by H2O2 as slow. The following rate constant values between 20 and 25 °C were used assuming that the temperature dependence of these diffusion-controlled rates is small: k25 ) 1.2 × 109 M-1 s-1, k26 ) 1.0 × 109 M-1 s-1, kR36 ) 6 × 109 M-1 s-1, kR37 ) 1.01 × 1010 M-1 s-1, kR38 ) 9.7 × 107 M-1 s-1, kR39 ) 8 × 105 M-1 s-1, and pKa of eq R35 ) 4.8. Also, the anormalous kinetic behavior of the sonochemical degradation of BQ (in the absence of O3) was accounted for by its particularly high reactivity toward relatively inert •HO2 and •O2- radicals:

BQ(aq) + •OH(aq) f products BQ(aq) + •O2- f products

(25) (26)

∂[•OH] ) kOH - k25[BQ][•OH] - 2kR36[•OH]2 ∂t kR34[•OH][H2O2] - kR37[•O2-][•OH] (27) ∂[•O2-] ) kR34[H2O2][•OH] - k26[BQ][•O2-] ∂t kR37[•O2-][•OH] - kR38[•O2-]2 (28) ∂[H2O2] ) kH2O2 + kR36[•OH]2 - kR34[•OH][H2O2] ∂t ∂[BQ] ) -k25[BQ][•OH] - k28[BQ][•O2-] ∂t

(29) (30)

Ultrasound-Wet Oxidation Systems. Dhale and Mahajani (99) studied the treatment of turquoise blue C125 (a

model dye compound) using a hybrid process consisting of nanofiltration membrane separation followed by sonication and wet oxidation (MEMSONIWO). Sonication at 30 °C and 40 kHz of the concentrate obtained after membrane filtration, in the presence of CuSO4 catalyst, made the waste stream amenable to wet oxidation at 170-215 °C. Dhale and Mahajani (100) also studied the subcritical mineralization of sodium salt of dodecyl benzene (800-1000 g/m3 COD concentration) via a hybrid processssonication (303 K, 40 kHz) followed by wet oxidation in the temperature range 473-503 K and 0.69-1.138 MPa oxygen partial pressure and in the absence and presence of homogeneous of CuSO4 (2.002 × 10-4-1.001 × 10-3 kmol/m3) as catalyst (SONIWO). It was shown that the hybrid system was effective in treating this surfactant at 483 K and higher and that both the sonication and CuSO4 catalyst enhanced COD reduction. Also, sonication eliminated the initial induction period experienced with the wet oxidation alone. Ingale and Mahajani (101) investigated the degradation of a refractory component in the industrial waste of a cyclohexene oxidation unit using a hybrid system, namely, sonication followed by catalytic wet oxidation (SONIWO). It was shown that sonication in the presence of CuSO4 as catalyst resulted in an accelerated degradation of the unknown compound, which was refractory to wet oxidation at 225 °C as compared with sonication without a catalyst, and the CuSO4 catalyst was superior to NiSO4. The SONIWO systems resulted in a 66% decrease in COD (1473 to 499 mg dm-3) as compared with 36% (1533 to 980 mg dm-3) for the simple CuSO4-catalyzed system, and a 16% decrease (1791 to 1495 mg dm-3) at the end of 3 h of oxidation reaction. Aymonier et al. (102) developed a hydrothermal sonochemical reactor with the objective of accelerating the oxidation rate of organic compounds in order to obtain comparable (if not better) conversion at lower temperature and pressure than are used in standard SCWO operating conditions. They evaluated the oxidation of acetic acid (1.5 g/L-1) as a model compound using 30% H2O2 in excess of stoichiometric ratio (1:1.3 H2O2:CH3COOH) as an oxidizer at 200-230 °C and P g 22.1 MPa in the absence and presence of sonication. They found that, at the optimal conditions of 2.8 MPa and 220 °C, ultrasonic activation (4000 W and 20 kHz) resulted in 83% degradation in a space time of 10 min, an increase in the acetic acid oxidation by 40% over oxidation without ultrasound. The sonichemical reactor at 2.8 MPa and 220 °C was also more efficient than the SCWO process with KMnO4 at 29 MPa and 400 °C without ultrasound. Using a residence time distribution (Rtd) method, it was shown that ultrasonic cavitation actually induced an oxidation effect on the oxidation reaction, speeding up its kinetic rate. They also demonstrated by oxidizing (at the same conditions) an industrial waste that contained salts and halogens (COD ) 3000, PO43- ) 145, SO42- ) 195, Na+ ) 300, Ca2+ ) 108, Cl) 120, NH4+ ) 96; all in mg L-1) that 86% reduction of COD (with only CO2 detectable in the gas phase) was obtainable by the sonochemical SCWO reactor with no significant salt precipitation and no corrosion problem with the titanium liner of the reactor. With the ultrasonic SCWO oxidation the percent of salt recovery at the reactor outlet was greater than 90%, and it was concluded that the ultrasound improved salt transport, decreasing salt precipitation considerably.

Future Research Needs, Challenges, and Opportunities Sonochemistry has the potential for use in environmental remediation as demonstrated in several studies (20). However, sonochemistry has not yet received much attention as an alternative for large-scale chemical processes probably because ultrasound when used alone is highly energy intensive, since not all of the cavitational energy produces chemical and physical effects. Sonication is therefore not

economically attractive or feasible alone. The utilization of other types of energy or the addition of H2O2, O3, air, and ferrous ions as additional sources of free radicals is needed for the intensification of the ultrasonic degradation process. More efforts are required to use available information on these coupled systems, which is mostly on a laboratory scale for the design of industrial-scale reactors applicable for the efficient treatment of industrial waste streams. The present challenge is to scale-up sonochemical processes in order to meet industrial needs in terms of volumetric flow rates, reaction energy rates, and overall cost. Further work is needed to improve the efficiency of energy conversion as well as methods to increase the cavitation sites and the rate of free radical generation for a given energy input. Ultrasound may become applicable in technical-scale reactors when every effectiveness factor in the energy transformation cascades is optimized for the final goal to achieve maximum caviation with minimum energy input. The properties of the primary reaction intermediates play a key role in the efficiency of the total coupled or hybrid process and provide insight into the synergism observed within the process coupling. Predicting the performance outcome requires knowledge of the physical and chemical properties of the major reaction intermediates and their susceptibility to degradation by each process. Hoffmann and co-workers have used some simplified mathematical approaches involving mainly liquid-phase chemistry to model and explain the liquid-phase enhanced oxidation chemistry of the coupled systems with some success (65, 72, 96-98). Much more research is needed to understand the advantages of these coupled systems. For example, more work is needed concerning the degradation kinetics within the combined process, from initial attack of the primary compound through the dynamics of intermediates and onto total mineralization. More complex mathematical models, which incorporate bubble dynamics and classical basic chemical engineering principles with the characteristics kinetic forms found in each of the individual processes, are needed for the exploration and prediction of the combined process performance under various assumed circumstances. Further insight will be gained by coupling ultrasonic parameters with bubble dynamics and gas-phase/liquid-phase chemistry, leading to a better understanding of the cause and effects of the ultrasound. The models for the coupled systems should also allow for the determination of optimal operating regions in terms of physical-chemical parameters and efficiency or economic cost for given effluent characteristics and appropriate practical limits. An individual AOP may be limited by slow rate or oxidative potential and the tendency to form harmful byproducts. If degradation rates can be enhanced, reaction times will be reduced for equivalent final contaminant concentrations, translating to reduction in reactor sizes. Consequently, the cost of application of the coupled technology to industrial or field-scale projects would be significantly reduced. The design key to such coupled or hybrid systems lies in choosing processes that complement each other and lead to a synergistic effect. Also, if ultrasound is to be used in the destruction of hazardous wastes on an industrial scale, the associated reactions must be ecologically acceptable. A quick minerilization of an organic contaminant should be the goal to minimize the survival time of toxic intermediates. Also, economic, physical, and technological limitations of the individual processes should be recognized to design a more effective and economical coupled processes. Given these limitations, effective treatment of difficult-to-degrade wastes may require a combination of available processes in such a way as to exploit their individual strengths and reduce waste treatment costs substantially over single-step processes. The global efficiency of the combined process is a key perforVOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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mance measure of these systems. More work is needed to understand the global efficiency and economics of the appropriate and complementary combinative or hybrid processes for a given ultimate treatment goal, whether specific pollutant removal or reduction of a global parameter such as total organic carbon (TOC). In the design of large-scale reactors for coupled gasliquid reaction system (e.g., coupled US-O3 system), the transfer rate of the gas into the liquid medium will be a critical factor in determining the overall efficacy of the process. Furthermore, ultrasonic irradiation has been demonstrated to increase the mass transfer of O3 to solution, allowing more O3 to enter solution than in a nonirradiated region (98). Some more recent studies have shown that ultrasonic irradiations result in the intensification of mass-transfer phenomena and that the extent of intensification depends strongly on the operating parameters including the type and geometry of reactors used for irradiation, gas flow rate, and power dissipation into the system (103). The presence of sodium chloride (as an electrolyte) is also shown to lead to significantly enhanced rates of mass transfer especially at high acoustic power due to its noncoalescing nature in aqueous system leading to the formation and maintenance of smaller bubbles, which subsequently increases the available gasliquid interfacial area and hence the magnitude of KLa (103). However, quantification of the mass transfer ratessthe allimportant information in the efficient design of coupled gasliquid sonochemical systemssis still lacking. Also, systematic analysis of the influence of the various process parameters is still missing. More detailed investigation needs to be done with coupled systems to improve our understanding of the extent of mass-transfer intensification resulting from ultrasonic irradiation as a function of process and operating parameters. Research efforts on the optimization of the process parameters using, for example, experimental statistical design approaches with a view of arriving at a set of conditions that yield optimal sonochemical effects and delineate the interaction between the process parameters are needed (104). The scale-ups of sonochemical reactors remain problematic because few studies have been done to determine which measures of reactor efficiency correlate best with pollutant destruction rates (47). Much work has to be done to design optimal combinative and hybrid sonophotochemical oxidation systems and reactors by exploiting the parameters, which influence the efficiency of the coupled systems. For example, it has been shown that the sonochemical SCWO process allows for a very efficient treatment to be obtained, which reduces considerably the working conditions of the reactor (2.8 MPa, 220 °C) allowing salt precipitation and corrosion to be overcome (102). The lowering of pressure and temperature translates into reduction in investment cost (high-pressure pump, reactor material, heat exchanger, etc.) and cost of energetic power supply (compression cost, pretreating cost, etc.). It is suggested that under supercritical conditions for water (P g 22.1 MPa and T g 374 °C), the cavitaion phenomena could not be expected because the medium remained monophasic despite the observation of ultrasonic effects in supercritical carbon dioxide (54). For sonoxidation in the pressure range 5-25 MPa, results observed were close to those of oxidation without ultrasound as no cavitation effects were observed. In the homogeneous subcritical domain (T e 374 °C) cavitation can be observed if the local pressure variation generated by the sonic horn vibration is greater than the difference between the hydrostatic pressure and the vapor pressure of water at a given temperature (100 °C e T e 374 °C). It was shown that if the local pressure variation generated by the sonic horn vibration is too low, then the hydrostatic pressure of the water must be reduced to obtain cavitation. The domain of pressure and 3418

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temperature needed to overcome salt precipitation and corrosion and the chemistry of the sonochemical wet oxidation systems should be exploited further. In summary, advanced modeling of combinative or hybrid process such as coupling of advanced oxidation reactions and kinetics with bubble dynamics, development of reactor models, and determination of optimal operating regions in terms of physical-chemical parameters and efficiency or economic cost are needed. Development of such complex models in combination with validation and critical evaluation could aid further development of the sonophotochemical oxidation processes and technologies. The implementation of the combinative and hybrid technologies in the marketplace will be determined by a balance obtained between increase in yield or mineralization and a detailed economic analysis relative to existing technologies.

Acknowledgments The author thanks the Air Force Office of Scientific Research (Grant F49620-95-1-0541) and the Department of Energy (Grant DE-FC04-90AL66158). Partial support from the STC Program of the National Science Foundation under Agreement CHE-9876674 is also acknowledged.

Nomenclature A

Arrhenius parameter

c

velocity of sound in the medium

E

energy stored in the cavitating bubbles before collapse

Ea

activation energy

f

frequency

fr

resonance frequency

I

intensity of wave

N

number of cavitation bubbles per unit time and unit liquid volume

P

pressure in the bubble at its maximum size

PA

pressure amplitude delivered by the transducer

Pa

acoustic pressure

PB

Blake threshold pressure

Pgo

initial gas pressure inside the bubble

Pg

partial pressure gas in the bubble

Pb, Ph

hydrodynamic or atmospheric pressure

Pm

liquid pressure at transient collapse (Pm ) Ph + Pa)

Pmax

maximum pressure developed at the moment of bubble collapse

Pv

vapor pressure in the bubble

Q

damping factor

R

radius of bubble

R˙

first derivative of the bubble radius with respect to time (m/s)

Rg

universal gas constant

Rm

maximum radius of the bubble before collapse

Rmax

maximum cavitation bubble radius

Rmin

maximum bubble radius

R0

equilibrium bubble radius

Rr

resonance radius of bubble

t

time

T

ultrasonic period

T0

equilibrium temperature

Tmax

maximum temperature developed at the moment of bubble collapse

To

ambient or experimental temperature (K) of bulk solution

Greek Letters R, κ, γ

polytropic index or specific heat capacity ratio of gas (Cp/Cv)

F

density of medium

FL

liquid density

σ

surface tension of the bulk liquid medium

τ

collapse time of cavitation bubble

ω

angular frequency

ωa

applied or acoustic angular frequency

ωr

resonance angular frequency

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Received for review June 8, 2004. Revised manuscript received January 20, 2005. Accepted February 21, 2005. ES049138Y

Sonochemistry in Environmental Remediation. 1. Combinative and

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