sonochemistry in environmental remediation

Sonochemistry in Environmental Remediation.2. Heterogeneous Sonophotocatalytic Oxidation Processes for the Treatment of Pollutants in Water
Y U S U F G.A D E W U Y I*
Department of Chemical Engineering,North Carolina A&T State University,
Greensboro,North Carolina27411
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 environ-mental sonochemistry,which involves the application of ultrasound to induce in situ cavitation to destroy or accelerate the destruction of liquid-phase contaminants(7-12).Cavi-tation is the nucleation,growth,and sudden collapse of gas-or vapor-filled microbubbles formed from acoustical wave-induced 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 implo-sion 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(g5000K)and pres-sures(g1000atm),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 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 sum-marized in Table1[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 sonophotocata-lytic 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 TiO2or ZnO to catalyze redox reactions.For a semicon-ductor catalyst(i.e.,TiO2)the threshold or ideal wavelength corresponding to the band-gap energy of3.02eV is300nm (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≈1015s-1)in a well-illuminated reactor (37).The photogeneration and interaction of radical species can be represented by the reactions in Table1(eqs R28-R68).The e--h+pair of carriers could recombine at the surface of the semiconductor particle or in the bulk to produce
*Corresponding author phone:(336)334-7564;fax:(336)334-
7904;email:adewuyi@.
Environ.Sci.Technol.2005,39,8557-8570
10.1021/es0509127CCC:$30.25©2005American Chemical Society VOL.39,NO.22,2005/ENVIRONMENTAL SCIENCE&TECHNOLOGY98557 Published on Web10/11/2005
TABLE 1.Chemistry of Coupled Sonophotocatalytic Oxidation Processes
Section A:Sonolysis in the Absence and Presence of O 2,H 2O 2,O 3,or S 2O 82-(i)Ultrasound Only H 2O +)))f H •+•OH (R1)•OH +•OH f H 2
O +O •
(R2)•OH +H 2O f H 2O 2
+H •
(R3)H •+•OH f H 2O (R4)H •+H •f H 2(R5)O •+O •f O 2
(R6)O •+H 2O f 2•OH (R7)•OH +•OH f H 2+O 2(R8)•OH (aq)+•OH (aq)f H 2O 2(aq)(R9)H •+O 2f HO 2
•(R10)HO 2•+H •f H 2O 2
(R11)HO 2•+HO 2•f H 2O 2+O 2(R12)•OH +HO 2•f H 2O +O 2(R13)(ii)In the Presence of O 2and H 2O 2O 2+)))f 2O •
(R14)O •+HO 2•f •OH +O 2(R15)O 2+O •f O 3
(R16)H 2O 2+)))f 2•OH (R17)(iii)In the Presence of O 3O 3(g)98)))
O 2(g)+O(3P)(g)
(R18)O(3P)(g)+H 2O (g)98)))
2•OH (g)
(R19)•OH (aq)+O 3(aq)f HO 2(aq)•+O 2(aq)
(R20)•OH (aq)+H 2O 2(aq)f HO 2(aq)•+H 2O
(R21)HO 2(aq)•h O 2(aq)•-+H
+(R22)•OH +O 2(aq)•-f OH -+O 2
(R23)•OH +O 2(aq)•-+H (aq)+
f H 2O +O 2(aq)(R24)(iv)In the Presence of S 2O 82-S 2O 82-+)))f 2SO 4-•
(R25)SO 4-•+H 2O f HSO 4-+•OH (R26)SO 4-•+SO 4-•f S 2O 82-(R27)
Section B:TiO 2-UV -O 2-H 2O 2-O 3-S 2O 82--HSO 5-Reaction Systems
(i)TiO 2-O 2System
TiO 2+h ν(g 3.2eV,<387nm)f h VB(TiO 2)++e CB(TiO 2)-(R28)h VB(TiO 2)++e CB(TiO 2)-f TiO 2+heat (R29)h VB(TiO 2)++H 2O (adsorbed)f •OH TiO 2+H +(R30)2h VB(TiO 2)++2H 2O f H 2O 2+2H +(R31)h VB(TiO 2)++OH (adsorbed)-f •OH (R32)e CB(TiO 2)-+O 2(adsorbed)f O 2(TiO 2)•-(R33)O 2(adsorbed)+e -+H +f HO 2•(R34)O 2•-+H 2O 2f •OH +OH -+O 2
(R35)2O 2•-982H +
H 2O 2+O 2
(R36)O 2•-+e CB(TiO 2)-+2H +f H 2O 2
(R37)O 2(adsorbed)+2e -+2H +f H 2O 2(adsorbed)(R38)(ii)TiO 2-H 2O 2System
H 2O 2(adsorbed)+2h +f O 2+2H +
(R39)H 2O 2(adsorbed)+2H ++2e -f 2H 2O (R40)H 2O 2(adsorbed)+h +f HO 2•+H +(R41)HO 2•+HO 2•f H 2O 2+O 2(R42)O 2•-+HO 2•f HO 2-+O 2(R43)HO 2-+H +T H 2O 2
(R44)e CB(TiO 2)-+H 2O 2f •OH TiO 2+OH -(R45)O 2(TiO 2)•-+H 2O 2f •OH TiO 2+OH -+O 2(R46)H 2O 2+h νf 2•OH
(R47)•OH +H 2O 2f H 2O +HO 2•
(R48)(iii)TiO 2-O 3System O 3+H 2O 98h ν
H 2O 2+O 2
(R49)e CB(TiO 2)-+O 3(adsorbed)f O 3(TiO 2)•-(R50)O 3(TiO 2)•-+H +f HO 3•(R51)HO 3•f O 2+•OH
(R52)O 3+HO 2-f •OH +O 2+O 2•-(R53)HO 2-+•OH f HO 2•+OH -(R54)O 2•-+O 3(aq)f O 2(aq)+O 3(aq)•-(R55)O 3(aq)•-+H (aq)+f •OH (aq)+O 2(aq)(R56)O 3(TiO 2)•-+H 2O f •OH +OH -+O 2
(R57)H 2O 2(aq)+O 3(aq)f •OH (aq)+HO 2(aq)•+O 2(aq)(R58)O 3+•OH f O 2+HO 2•
(R59)
8558
9
ENVIRONMENTAL SCIENCE &TECHNOLOGY /VOL.39,NO.22,2005
thermal energy,or it could interact or be captured by other species or molecules absorbed or close to the surface of the semiconductor.An acceptor (A)capturing the excited e -is reduced while a donor (D)reacting with h +is oxidized,completing the redox cycle.That is,the surface electron deficiency,or holes,left behind in the valence band can either be filled by adsorbed organic donor,which thus undergoes direct oxidation,or react with water molecules or hydroxide anions (R30-R32)to form hydroxyl radicals and/or hydrogen peroxide.Thus,oxidation of the pollutant is mediated mainly by the three species,h +,•OH,and H 2O 2,with the hydroxyl radical widely thought to be required to initiate photocata-lyzed degradation (6).The reaction of surface-bound hyroxyl radicals with the adsorbed organic compound is usually considered the rate-determining step.
One practical problem in using TiO 2as a photocatalyst is electron -hole recombination,which in the absence of proper electron acceptors is extremely efficient (with char-acteristic times of 10×10-9to 100×10-9s)(24)and thus represents a major energy-wasting step and a limitant to achieving a high quantum efficiency.In oxygen saturated aqueous solutions,dioxygen (O 2)acts as a sink for photo-promoted conduction band electrons,trapping these elec-trons to reduce the recombination of electrons and holes and forming superoxide anions (O •-)and the protonated form,hydroperoxide radicals (HO 2•)(R33-R34).It is believed that •OH radicals from hole-trapping by surface hydroxyl groups (eq R32)are the primary oxidizing agents and oxygen is a scavenger for photogenerated electrons (21).It has been shown that inorganic peroxide such as H 2O 2,potassium peroxomonosulfate (commercially called oxone with active ingredient HSO 5-in aqueous solution),peroxydisulfate (e.g.,Na 2S 2O 8),and ozone (O 3)enhanced rates of photocatalytic degradation better because they trap the photogenerated conduction band electrons more efficiently than O 2,thereby lowering the electron -hole recombination rate and increas-ing the efficiency of the hole utilization for reactions such as eq R32and hence the concentration of hydroxyl radicals (42-48).Also,the addition of H 2O 2acts as extra source of •OH radicals via eq R47and might mitigate the problem of O 2defficiency in the solution phase resulting from O 2comsumption or slow O 2mass transfer (6).However,•OH radicals can also react with the added H 2O 2and O 3(R48and R59).The net effect depends on the type of water,the TiO 2specimen,and experimental conditions.It is also known that Na 2S 2O 8not only traps the electrons but simultaneously produces the sulfate radical by photolysis with a quantum efficiency of unity (eq R61).The sulfate radical is a very strong oxidant (E 0)2.6eV)and can participate in the degradation process (34-35).Upon accepting an electron,HSO 5-also dissociates in two different pathways producing sulfate and sulfate radical through reactions R67and R68(43).All these species can play relevant roles in the photocatalytic oxidation of organic compounds.
In general,ionic species,both anions and cations,influence the rates of photocatalytic oxidation by affecting the extent of adsorption of the pollutants on the photocatalyst,interfering with electron -hole recombination or by scav-enging of •OH radicals (49-63)as shown in Table 1(R69-R87).For example,a trapped hole reacts with an adsorbed halide,I -ion,to form I atom,which further reacts with I -to produce I 2-.In the presence of oxygen,electrons are effectively removed by transferring to adsorbed oxygen.When there is no oxygen or other efficient electron scavenger present trapped electrons will eventually recombine with holes trapped in I 2-to give back I -ions (22).Experimental studies have shown that Mn 2+enhanced photocatalytic oxidation rate of chlorophenols by surface reaction increasing the number of photogenerated electrons and holes and inhibiting e --h +recombination (49).Cu 2+also enhanced photocatalytic oxidation of phenol and monocrotophos at low concentrations (<10-5M)but inhibited degradation at
TABLE 1Continued
(iv)TiO 2-S 2O 82-System S 2O 82-+h νf 2SO 4•-(R60)S 2O 82-+e CB -f SO 42-+SO 4•-(R61)S 2O 82-+e aq -f SO 4•-+SO 42-(R62)SO 4•-+e CB -f SO 42-(R63)SO 4•-+H 2O f SO 42-+•OH +H +(R64)S 2O 82-+2•OH f 2HSO 5-(R65)S 2O 82-+SO 4•-f SO 42-+S 2O 8•-(R66)HSO 5-+e CB -f •OH +SO 42-(R67)HSO 5-+e CB -f OH -+SO 4•-(R68)Section C:Effects of pH,Cations,and Anions
I -(i.e.,Cl -,Br -)+h +f I (R69)I +I -f I 2-(R70)2I 2-f I 3-+I -(R71)I 2-+e -f 2I -(R72)h ++OH -f •OH
(R73)O 3+OH -f HO 2•+O 2•-(R74)•OH
+OH -{\}k f )1.2×1010M -1s -1
k b )9.3×107s -2
O •-+H 2O (R75)HCO 3-+•OH 98k )1.5×107M -1s -1
CO 3•-+H 2O
(R76)CO 32-+•OH f CO 3•-+OH
-(R77)h ++HCOO -f H ++CO 2•-(R78)h ++CO 2•-f CO 2
(R79)•OH +CO 2f CO 3
•-+H +
(R80)•OH +CO 2•-f HCO 3-(R81)H 2PO 4-+•OH f H 2PO 4•+OH -(R82)HPO 42-+•OH f HPO 4-+OH -(R83)HSO 4-+•OH f SO 4•-+H 2O
(R84)Cl -+•OH f Cl •+OH -(R85)Cu 2++2e -f Cu 0(R86)Cu 0+2h +f Cu 2+
(R87)
VOL.39,NO.22,2005/ENVIRONMENTAL SCIENCE &TECHNOLOGY
9
8559
higher concentrations(58).These effects were ascribed to the short-circuiting mechanism of coupled reactions(eqs R86-R87),which occurs only at Cu2+concentration above a certain level.The additions of oxyanion oxidants such as ClO2-,IO4-,S2O82-,BrO3-,and ClO3-have been shown to improve the photodegradation of4-chlorophenols in the following order:ClO2->IO4->BrO3->ClO3-(54).However, in general,the photocatalytic degradation rates are consid-erably reduced by impaired adsorption of pollution on the TiO2surface.The isoelectric point(pI)of TiO2is around6.3, so that the TiO2particles 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 TiO2particle surface and also partly absorb UV light,negatively impacting photocatalytic deg-radation 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 detri-mental 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 Cl-where 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(26-28,36):
where K a is the equilibrium adsorption coefficient of sub-strate/reactant,C is the concentration of reactant at any time t,k r 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 K a C could be considered insignificant and the rate is essentially first order(assuming no other reaction paths appear at the lower concentrations) and the apparent reactivity of a molecule is the product of two characteristics:its tendency to be converted when adsorbed,k r,and its apparent adsorption constant,K a.Also, eq1can be rewritten as
Thus,from eq3,it can be noted that the photodegradation rate of organic substrate depends on both values of chemical reaction constant(k r)and adsorption coefficient(K a).When both k r and K a increase,the photodegradation rate will increase ing 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 r0is measured for an arbitrary initial concentration C0,eq1can be written in a linearized form as
where the values of k r and K a are obtained from the intercept and slope of the straight line,obtained from a least-squares regression.Also,for a batch reactor,eq1is written in the form(26)
which can be integrated to yield C(t)
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 C0to C0/2is given by
where for C0,1/K a,the reaction is always first order and a reaction half-life is determined which is independent of reactant concentration but is dependent on catalyst con-centration(m/V)and apparent reactivity,k r K a(in turn dependent on intensity)and hence,t1/2and k r K a 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)
or linearized form for estimating parameters
where K s is the solvent adsorption constant and C s is the concentration of the solvent(in water,C s≈55.5M),and the slope is now dependent also on solvent properties.As C s.
C and C s remains practically constant,the part of the catalyst covered by water is unaltered over the whole range of concentration C and eq8can be integrated as follows:
where K a/(1+K s C s)is usually denoted by K app,and eq11 represents the sum of zero-order and first-order rate equa-tions 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 K a to an apparent value K app)K a/(1+K s C s).When K a C.(1+ K s C s),eq8reduces to zero order asymptote
r)-∂C
∂t
)k
r
K a C
1+K a C
(1) r)k r K a C(2)
-∂C
∂t
)k
r
C
1
K a
+C
(3)
C0
r0
)
1
k r
C0+
1
k r K a
(4)
-V
∂C
∂t
)
mAk r K a C
1+K a C
(5)
1
K a
ln(C C0)+(C-C0))-(mA V)k r t(6)
t1/2)
(0.693/k r K a+C0/2k r)
mA/V
(7)
r a)
k r K a C
1+K a C+K s C s
(8)
1
r0
)
1+K s C s
k r K a
1
C0
+
1
k r
(9)
ln
C0
C
+
K a
1+K s C s
(C0-C))
k r K a
1+K s C s
t(10)
ln
C0
C
+K app(C
-C))k
r
K app t(11)
(C0-C))k r t(12)
85609ENVIRONMENTAL SCIENCE&TECHNOLOGY/VOL.39,NO.22,2005
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 thatΦvaries with I as follows(6).(i)Low I:rate varies as I andΦis constant; (ii)Intermediate I:rate varies as I0.5andΦvaries as I-0.5;and (iii)High I(mass transfer limit):rate varies as I0.0(constant) andΦvaries 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 levelsΦdecreases as I-0.5,indicating an efficiency penalty for suf-ficiently 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 K I on I was K I)I/[a+I0] where a is constant.The photocatalytic activity of suspended TiO2in 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 improve-ments 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 pho-tocatalytic reaction and ultrasonic irradiation or the simul-taneous irradiation of ultrasound and UV light with a photocatalyst in the presence of oxygen or other chemical oxidants such as O3or H2O2.In addition to activation of the photocatalyst surface,enhancement of mass transport of organic compounds to the surface of the TiO2from 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,sonophotocata-lytic 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(Table1).Serpone et al.(69) studied the kinetics of2-,3-,and4-chlorophenol decom-position in air-equilibrated media by low-frequency ultra-sound 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-ra-diolysis,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 path-ways.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 TiO2and 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 com-pounds,with a long hydrocarbon chain,which interact poorly with TiO2in water,the use of ultrasound initially to ensure faster COD decrease and mineralization seems more ap-propriate.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 photoca-talysis and ultrasound are therefore of interest to treat waters polluted with numerous compounds having various hydro-phobic 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-pho-tolytic,and other chemical degradation methods.Ultrasonic degradation is several times(about10000-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&TECHNOLOGY98561
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 re-
mediation 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;2,4-
DCP;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(70W,20kHz),PC(0.25g/L Hombikat UV100
photocatalyst;450W,320-nm Hg lamp),or the combination
(sonophotocatalysis)at30(2°C).Hombikat UV100was
chosen for the study because it was determined to exhibit
the highest enhancement with US when tested against
Degussa P25and Ishihara ST-21.Significant enhancement
was reported for the reaction rate of phenol(1.0mM initial
concentration)for the sonophotocatalytic decomposition(k )7.0µM/min)compared to that with US alone(4.3µM/ min),an increase of about63%.The enhancement was also
intensified by reducing reaction volume or increasing US
power density within a reasonable range with an optimal
usage of0.7W/mL.It was reported that for a50-ml reaction
volume(with k)0.5µM/min for US and5.8µM/min for
UV),the synergistic effect from the combination of UV and
US achieved9times greater effectiveness(k)4.4µM/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(1000sccm)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 after140
min was reduced from96%(without the salt)to86%in the
presence of0.25M Na2SO4,and drastically to39%in the
presence of0.5M NaCl.
Johnston and Hocking(74)investigated the UV photolytic
and photocatalytic degradation of2,4-dichlorophenol(2,4-
DCP)at350nm with and without sonication.They found
that the use of sonication(1×10-3M2,4-DCP;0.2%or0.05%
TiO2)in photolysis resulted in the enhancement of chloride
release rate by a factor of4compared with UV irradiation
only.They also studied the photocatalytic and photolytic
degradation of a2.4×10-4M solution of pentachlorophenol
(PCP)with and without sonication.They demonstrated that
the initial rate of chloride formation due to photocatalysis
using TiO2was about2.7times faster with sonication than
without.The photocatalytic/photolytic degradation of PCB
isomer3-chlorobiphenyl at75ppm level(4×10-4M)was
also investigated by monitoring the chloride formation.TiO2
(0.2%w/w)and30-mL aliquots were subjected to UV 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 hydro-
phobic 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
of2-chlorophenol using sonication(at20kH and7.5W)in
different dissolved gas media(Ar,O2,O2/O3),and photoca-
talysis(using0.1g/L Degussa P25TiO2(80%anatase/20%
rutile,BET area)50m2/g;315-400nm/250W UV light)
individually and simultaneously.In general,the following
reactivity scale emerged from the results of the study:O3-
UV-TiO2g O3-US-UV-TiO2>O3-US>O3-UV>O3> US-UV-TiO2>.Typical first-order rate
constants for US,UV-TiO2,US-UV-TiO2in using air as
dissolved gas were0.61(0.05×10-5s-1,5.89(0.08×10-5
s-1,and4.92(0.11×10-5s-1respectively,and0.69(0.05×10-5s-1,13.4(0.8×10-5s-1,and9.14(0.13×10-5s-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 O2content of the gas mixture in equilibrium with the treated suspensions.The effect of stirring was also investigated by using a gas mixture of O2and 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 TiO2particles and maintained them in suspension,where they could be efficiently il-luminated.Shirgaokar and Pandit(76)observed that the sonophotocatalytic degradation of100ppm aqueous solution of2,4,6-trichlorophenol(TCP)using combined ultrasound (22kHz,40Wcm-2)and photocatalysis(using anatase-grade TiO2,0.1gL-1and15W UV,254nm light)at30(2°C after 5h.of sonication was substantially higher(increase of50to 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 of1,4-dioxane
in water(C0)50ppm)using sonolysis(20kHz,50W),
photocatalysis(365nm,144mW/cm2UV with TiO2and HF-
treated TiO2)and the combined sonophotocatalytic systems
at5°C.It was shown that HF treatment of TiO2surface
enhanced its absorption capabilities for both1,4-dioxane
and improved the overall decomposition rate of1,4-dioxane
by the sonophotocatalytic treatment.The pseudo-first-order
rate constants(min-1)obtained for the decomposition of
1,4-dioxane were2.8×10-5,1.4×10-4,1.6×10-4,4.7×10-4,
and5.4×10-4,respectively,for US,UV-TiO2,UV-HF-
TiO2,US-UV-TiO2,and US-UV-HF-TiO2systems.The
results suggest that the rate constant for the US-UV-HF-
TiO2system is slightly higher than that of US-UV-TiO2
despite a low specific surface area(66m2/g)of the HF-TiO2
powder compared that of the TiO2without HF treatment(84
m2/g).
Matsuzawa et al.(78)investigated the photocatalysis of
5×10-4M solutions of dibenzothiophene(DBT)and4,6-
dimethyldibenzothiophene(4,6-DMDBT)using0.2g of
different TiO2photocatalysts(Degussa P25,50m2/g;PC-1,
300m2/g;PC-2,250m2/g)and UV irradiation(Hg-Xe lamp, >290nm,19mW/cm2),and the effect of H2O2(3%)and/or ultrasound(45kHz,50W)in the presence of air.The
85629ENVIRONMENTAL SCIENCE&TECHNOLOGY/VOL.39,NO.22,2005。