2634
J . Phys. Chem. 1994,98, 2634-2640
Ultrasonic Induced Dehalogenation and Oxidation of 2-, 3-, and 4-Chlorophenol in Air-Equilibrated Aqueous Media. Similarities with Irradiated Semiconductor Particulates Nick Serpone' and Rita Terzian Laboratory of Pure and Applied Studies in Catalysis, Environment and Materials, Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec, Canada H3G I M8
Hisiao Hidaka Department of Chemistry, Meisei University, 2- I -I Hodokubo, Hino, Tokyo 191, Japan
Ezio Pelizzetti Dipartimento di Chimica Analitica, Universita di Torino, 10125 Torino, Italy Received: August 23, 1993; In Final Form: November 17, 1993'
Three chlorophenols {2-chlorophenol (2-CPOH), 3-chlorophenol (3-CPOH), and 4-chlorophenol (4-CPOH)) were examined under pulsed sonolytic conditions (frequency, 20 kHz; power, 50 W)in air-equilibrated aqueous media. These phenols are totally transformed to dechlorinated, hydroxylated intermediate products via firstorder kinetics in about 10 h for 2-CPOH and 3-CPOH and about 15 h for 4-CPOH; rate constants for the disappearance of these phenols are (4.8 f 0.4) X m i d , (4.4 f 0.5) X le3 m i d , and (3.3 f 0.2) X m i d , respectively, for approximately 80 pM initial concentration. Dechlorination is nearly quantitative and occurs soon after initiation of the disappearance of the initial substrate. Comparison of the intermediate products formed by the sonochemical technique with those reported earlier from oxidation of thesexubstrates by direct photolysis, flash photolysis, UV/peroxide, and irradiated semiconductor (SC) particulatea (Ti02 and ZnO) suggests that the sonochemical oxidation process finds strong similarities with and physically mirrors the heterogeneous photocatalytic process with S C particulates, in particular, where k's are 1-2 orders of magnitude greater; this infers the need for the substrate to diffuse to the bubble/liquid interface in contrast to preadsorbed substrates on the semiconductor particles. The kinetics show two regimes: a low-concentration regime where the rate is zero order in [CPOHIr, and a second regime at higher concentrations where the rate displays saturation-type kinetics reminiscent of Langmuirian type behavior in solid/gas systems. The relevant mechanistic significance is that the reaction takes place in the solution bulk at low concentrations of chlorophenol, while at the higher concentrations the reaction occurs predominantly a t the gas bubblo/liquid interface.
Introduction Under specified conditions, ultrasounds accelerate several reactions and lead to formation of novel Used less discriminately,however, ultrasounds (sonolysis) can convert organic substrates of environmentalinterest, ideally to COZand HzO, or convert them to compounds which are less harmful than theoriginal substrates. Used aloneor inconjunctionwith another technique (e.g., photochemistrygJO),sonolysis could prove industrially useful, especially if it were found to be efficient and economical to decontaminateindustrial organicdischarges before their input into aquatic ecosystems. The earliest reports in the sonolysis of organics appeared in the 1950sby Zechmeister and co-workers11-13 whodemonstratedthat halobenzenes and heterocyclic rings (pyridine, halothiophenes, pyrrole, and halofurans) together with benzene and phenol can be cleaved by ultrasounds, most efficiently in the presence of AgNO3 in oxygen-free solutions. In air-equilibrated media, C02 evolution predominated over acetylene f0rmation.1~ Specific reference to utilization of ultrasounds to detoxify waters contaminated with phenol and o-chloronitrobenzene and actual wastewaters was reported by Chen and co-workers as early as 1971.l5J6 The past decade has witnessed a resurging interest in applications of ultrasounds to the breakup of molecules of environmental concern in aquatic ecosystems: for example, c h l ~ r o f o r macetate,ls ,~~ acetylene.19 polycyclic aromatic hydrocarbons, a dioxin, and chlorinated solvents in ethanol/water
* Address all correspondence to this author.
Abstract published in Advance ACS Abstracts. February 15, 1994.
0022-3654/94/2098-2634$04.50/0
solutions,20 methano1,ZI methylene chloride and carbon tetrachloride122pnitrophcnol,23hydrogen sulfide.24 pentachlorophenate125 and phenol, catechol, hydroquinone, and benzoquinone26 have been examined. Over the past decade, we have focused much of our efforts on the development and applicationof advanced oxidation processes (AOP) to environmental detoxification, in particular on a heterogeneous photocatalytic technique that employs irradiated semiconductors (SC),e.g. titania in the anatase form, to accomplish the mineralization of refractory organics to carbon dioxide. Various fundamental and applied aspects, and some milestones of heterogeneous photocatalysis have been reviewed recentl~.2~-3~ In this particular AOP process, illuminationof the semiconductor photocatalyst generatea electron/holepairs which, subsequent to a variety of photophysical events,38migrate to the SC particle and get trapped at the surface: electronsas T P and holes as surface-bound'OH radicals for Ti02.399'0 These trapped charge carriers are then poised to initiate redox reactions with adsorbed substrates, which in many cases oxidation has led to complete and quantitative mineralization of organics to carbon dioxide. The early events in this oxidative mineralization of various halo- and methylated phenols have been probed by pulsed radiolysis methods.41-43 In every instance, the primary product between the *OH radical and the aromatic substrate was the 'OH adduct.43 Subjecting a liquid of relatively high vapor pressure and dynamic tensile strength (such as water) to high-frequency ultrasonic waves (few to several tens to hundreds of kilohertz) will cause acoustic cavitation in the liquid (formation, growth, 0 1994 American Chemical Society
Dehalogenation and Oxidation of Chlorophenols and implosive collapse of smallgas bubbles) which is accompanied by high temperature (2000-2500 K) and high pressure (hundreds of atmospheres) gradients in the cavity interior.1-6 Under an argon atmosphere, water thermolyses to 'OH radicals and 'H atoms, as evidenced by spin-trapping ESR technique~,25*~9~5 and by formation of the luminescent hydroxyterephthalate from sonochemical treatment of terephthalate in aqueous media.1° Under oxygen, only 'OH was observed by DMPO trapping in the bulk s0lution,2~~~ and H' atoms, though also formed, are scavenged by 0 2 or 0 atoms to give additional HO2' or 'OH radicals. These observations find similarities with the radiation chemistry of aqueous solutions,' and with irradiated Ti02 particulate suspensions (see above).30 However, unlike radiation chemistry, solvated electrons are not produced during the sonolysisof water.47 The wide range of redox reactions observed in aqueous sonochemistry is the result of secondary reactions of radical intermediates.48 The common thread in all the above oxidative modes is the implication of 'OH radicals. Inasmuch as the sonolytictechnique also offers these and other radicals and since much of our focus has been directed at clarifying the primary oxidizing agent@)in heterogeneous photocatalysis, it was of interest to examine this technique in oxidizing the three monochlorophenols (2-CPOH, 3-CPOH, and 4-CPOH) and identify the intermediates formed as a follow-up of our recent examination of phenol and some of its derivatives under otherwise identical conditions.26 We herein report our results of the sonolytic decomposition of monochlorinated phenols. We show that these chlorophenols are decomposed and dechlorinated almost quantitatively to give hydroxylated aromatic intermediate productsin the first instance and, subsequently,probably species with a lesser number of carbon atoms which remained undetectable under our conditions.
Experimental Section The three chlorophenols examined here, 2-chlorophenol (2CPOH; pK,, 8.1 l), 3-chlorophenol (3-CPOH; PK,, 8-80), and 4-chlorophenol (CCPOH; pK,, 9.20), were reagent grade from Aldrich and were used as received. Water was deionized and doubly distilled; acetonitrile was HPLC grade solvent. The pH of the stock aqueous solutions of these chlorophenols was not adjusted as done earlier for phenol;26 acids interfered with detection of intermediates at the monitoring wavelength of the HPLC technique. Studies were therefore carried out at the natural pH of the solutions (defined here as the pH of the solutions on dissolution of the phenols in aqueous media): 5.1 (CCPOH), 5.4 (3-CPOH), and 5.7 (ZCPOH). Ultrasonicirradiation of aqueous solutions of the chlorophenols was carried out with a VibraCell Model VC-250 direct immersion ultrasonic horn (Sonics C Materials) operated at a frequency of 20 kHz with a constant power output of -50 W (the actual insonation power at the solution was 49.5 Wand the power density was 52.1 W cm-2 49). Reactions were done in a glass sonication cell (4.4 cm i.d. by 10 cm) similar to the one described by Suslick;50 the horn titanium tip (1.1 cm dia.) was immersed -3 cm into the solution leaving 3.6 cm between the titanium tip and the bottom of the glass cell. The reaction volume was 100 mL. The upper part of the cell was maintained in constant equilibration with air. The cell was encased in a water jacket and was cooled with a recirculating bath of water/ethylene glycol mixture set at 15 O C ; the temperature in the sonoreactor was 33 f 2 O C . The temporal course of the sonochemical process(es) was monitored by high-performanceliquid chromatography (HPLC) using a Waters Associates liquid chromatograph consisting of a 501 HPLC pump and a 441 absorbance UV detector. The detection wavelength was 214 nm; the column was a Whatman reverse phase C-18 (Partisil-lO,ODS-3). The mobile phase was a water/acetonitrile mixture (70/30, v/v). The identity of some of the intermediateswas confirmed by comparing retention times with known, commercially available standards. The flow rate
The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 2635 was 2 mL min-1. All aliquots used for HPLC analysiswere filtered through MSI Nylon 66 filters (0.2-pm pore size) to remove titanium metallic particles produced during sonication by corrosion of the titanium tip of the sonication horn. Chloride ion determinations were carried out with a Fisher C1- ion specific electrodecalibratedwith dry reagent grade NaC1; detection limit, 0.18 mg L-' or -5 pM in chloride (manufacturer stated limits). RCSultS
Ultrasonic irradiation (ca. 50 W cm-2) of a 100-mL airequilibratedaqueous solution of 4-chlorophenol (pH, 5.1; 10ppm or mg L-l; 75.1 pM) resulted in the first-order disappearance of the phenol (0.0033 f 0.0002 min-l), accompanied after a 1-h delay by the first-order growth of C1- (0.0026 min-1; 60.2 pM or -80% after 11 h) and the appearance and subsequent transformation of hydroquinone as the major intermediate (5.7 pM after 6 h) and 4-chlororesorcinol (0.8 pM after 6 h); these are depicted in Figure la. Two other intermediate species formed with retention times 3.2 and 4.8-5.2 min; the latter was identified as 4-chlorocatechol by comparison of retention times of HPLC signals between the three chlorophenols examined. The pH of the insonated solution dropped gradually from the initial value of 5.1 to 3.5 after 11 h, corresponding to the formation of 308 pM H+ during the process. This quantity is far greater than the -75 pM H+ expected from eq 1,where ))))) denotes the
6C0,
+ 2 H 2 0 + H+ + C1-
(1)
ultrasounds for complete oxidative sonolytic decomposition. The excess increase in acidity of the solution must arise from processes involving, to some extent, oxidation of nitrogen to nitrite and nitrate (see below) and from the remaining complex chemistry inherent between the initial solute and intermediates with the various radical species formed under the prevailing conditions. Insonation of the aqueous solution of 3-chlorophenol (77.8 pM; pH, 5.4; Figure lb) showed an induction period5' of -90 min following which its concentration decreased via first-order kinetics (0.0044 f 0.0005 min-l) with concomitant nearquantitative -94% generation of chloride ions (0.004 min-') together with formation of chlorohydroquinone (- 18 pM) and its subsequent conversion after 14 h to unspecified carbonaceous species. Two additional intermediates formed and disappeared; we tentatively identify them with 4-chlorocatechol and 3-chlorocatechol. It is interesting that the pH of the solution in the present case dropped from 5.4 to only ca. 4.2; that is, -59 pM of H+ were generated, lower than expected from eq 1 and in contrast to the sonochemical decomposition of its 4-chloro homolog. The pH of the insonated solution of 2-chlorophenol (83.1 pM; initial pH 5.7; Figure IC) decreased at first to ca. 4.9 and then recovered to its near initial value until -9 h of insonation when it dropped abruptly to pH 4.4 and remained constant. The initial drop in pH occurred during the induction period also seen for the first-order disappearance of the phenol (0.0048 f 0.0004 min-l) and the singly exponential growth of C1- ions (0.0038 min-1) to 95% quantity after 12 h of insonation. The two principal intermediates were chlorohydroquinone (1 3 p M after 5 h) and catechol (2 pM after 2 h) which formed and degraded totally after ca. 12 h. Also formed was a small quantity of a hardly perceptible intermediate in the prevailing mobile phase that we attribute to 3-chlorocatechol, by comparison with a similar retention time for a species in the sonolysis of aqueous 3-chlorophenol. The various intermediate species formed and identified by HPLC techniques in the sonochemical transformation of chlorophenols are portrayed in Table 1.
Serpone et al.
2636 The Journal of Physical Chemistry, Vol. 98, No. 10, 1994
8 0 50 ppm 20 ppm
f 60
A
3 300
z
& 250-
E:
c
U Ly I-
I O ppm
5 ppm
40
z w
0
z
0 0
20 n
"0
100
200
300
"0
400 500 600 706
200
100
300
400
500 600 700
INSONATION TIME, min.
INSONATION TIME, min.
Figure 2. Plots of the concentration of 4chlorophenol as a function of insonation time depicting the decomposition of this substrate at various initial concentrations in air-equilibrated aqueous media.
SCHEME 1 OH
"0
200
400 600 INSONATION TIME, min.
806
TABLE 1: Intermediates Identified in the Sono-oxidation of Monochlorophenols htul subtmtc
hermediate Rcducts A
B
C
D
E
P
0
f z
s
I-
U Ly
c z w 0
z u
0
"
0
200
400
600
801)
INSONATION TIME, min. Figure 1. Plots of concentrations as a function of insonation times illustrating the disappearance of chlorophenols and the formation and subsequentdecomposition of variousintermediatesat constantinsonation power of 50 W (see text) in air-equilibrated aqueous media; for experimental details, see text: (a, top) 4-chlorophenol, (b, middle) 3-chlorophenol, and (c, bottom) 2-chlorophenol.
Earlier we reported the power dependence in the sonolytic transformation of a phenolic aqueous solution and found that the rate of disappearance of phenol was first order in power P.26 Herein we examinedthe concentrationdependenceof the sonolytic process using 4-chlorophenol as the test substrate in the concentration range 2-50 mg L-1 for constant insonation power density of -50 W cm-2; the data are depicted in Figure 2. In all cases, the disappearance followed first-order kinetics; these can be affected by variations in pH and in insonation power.26 Discussion Monochlorophenols represent an important class of environmental water pollutants of moderate toxicity to mammalian and aquatic life; they possess relatively strong organoleptic effects, with taste threshold -0.1 rcg L-' (ppb).52 Their principal sources originate with natural degradation of chlorinated herbicides (e.g., chlorophenoxyaceticacids), chlorination of phenolic substances
in waste effluents, and chlorine treatment of drinking waters. Of the three chlorophenols examined here, 3-chlorophenol displays the greatest resistance to biodegradation.48 Not surprisingly, the decomposition of these halophenols has been examined extensively by direct or sensitized photoly~is,53-5~ by UV/H2O254 or via Fenton's reagent Fe2+/H202 oxidation?8 and by heterogeneous photocatalytic methods employing either Ti02551sW3or Zn0.64+65In most cases, complete mineralization of these halophenols was demonstrated particularly when utilizing semiconductor particulates; in others, cyclic intermediate products were identified. It is noteworthy that variations do exist in the nature of the immediate intermediates depending on the oxidative mode (Schemes 1,55362,S3J5J6*6)*65and 35495639.62). By contrast, once the aromatic ring is cleaved, the generated aliphatic unsaturated and saturated intermediate species have tended to elude detection and identification. Sonolysis adds an added dimension to AOP processes since ultrasounds can be transmitted through opaque systems (contrary to light) and the oxidative entity (*OHradical) is identical to those in other AOP's. As well, a gas bubble/solution interface is provided that might find similarities with the interface available when semiconductor photocatalysts are considered. Except for the cavity interior, the possible location(s) for redox reactions in sonochemistry mirrors closely that addressed in
Dehalogenation and Oxidation of Chlorophenols
The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 2631
SCHEME 2 9H OH
OH
3CPOH
I
bH
SCHEME 3 0
I
OH
OH
0
@-I I
I
I
CI
OH
OH qiorinmnudi
OH
a
heterogeneous photocatalysis with irradiated semiconductor particulates. With the latter, reactions may occur either at the interface and/or in the solution bulk. Evidence has mounted and strong arguments have been presented to suggest that, in the latter AOP method, reactions occur at the surface of the semiconductor particulates, that is, at the solid/solution interface.27.28 It is therefore relevant to consider the various possible site(s) where reactions may occur in sonochemistry. In this regard, the 'hot spot" model is worth n ~ t i n g . ~ . ~ Reaction Sites and the Interface. The "hot spot hypothesis"66 has accounted for most chemical effects of ultrasounds; high temperatures and pressures generated at the sites of cavity implosion have been implicated. Reactions may occur at three possible locationsduring sonolysis;they are (i) the gaseous interior of the cavity, (ii) the liquid shell immediately surrounding the cavity, and (iii) the bulkof the solution. A pictorial representation of these sites has been described.' In the first site, high temperature (-5000 K in some organic solvents66 and -22000-2500 K in water") and pressure (several 100 atm) gradients exist that solutes trapped within the cavity interior undergo reactions typical of high-temperature combustion.19.68 Combined EPR and spin-trapping studies showed that solvent vapor and ambient gases (e.g., air) decompose to atoms or free radicals in the gaseous bubble interior. Water vapor is thermally dissociated into *OHradicals, H' atoms, and 0 atoms. The latter interconvert with 'OH radicals at the high temperatures in the cavity and recombine in the cooler interfacial region to form 0 2 and H202.' Scavenging of H*atoms in the bubble (and/ or at the interface) by 0 atoms and 02 molecules increases the concentration of the oxidizingradicals ('OH and HO29; H' atoms do not escape.25 This adds credence to earlier reports which noted that, in addition to formation of oxidized intermediate
products, sonolysis of aqueous phenol also produces the reduction products 2-cyclohexen-1-01and/or cyclohexanonelsand sonolysis of an aqueous solution of benzoquinone produced hydroquinone26 nearly exclusively at first. Such processes could only occur via reduction with H atoms at the bubble/liquid interface. Volatile solutes in high concentration which diffuse into the bubble can also thermolyze to free radicals.19168 At the frequency of 20 kHz used here, these events must occur in -50 ~ s . Solutes present in high concentrationscan be pyr0lyzed6~~~ at the cooler (T< 2000 K) bubble/liquid interface (site ii; -0.2 pm thick) and where pressure gradientsalso exist.66 The efficiency of pyrolysis of nonvolatile solutes at the interface depends on their hydrophobicity which dictates their ability to accumulate at the interfa~e.~ To the extent that the concentration of *OH radicals is greatest at the interface, scavenging of these radicals should predominatewhen the nonvolatile solutes are present even at low concentrations. The processes occurring at the interface would seem to account for many of the chemical effects produced by ultrasounds.8 In the third region, composed of the bulk of the solution at ambient temperature,those radicals produced in thecavity interior and which escaped scavenging by solutes at the interface react with solutes in the bulk (except H atoms) with kinetics that might parallel those observed in radiation chemistry of aqueous solutions.7J2 The fate of air nitrogen is also worth noting since the systems examined here were continually air-equilibrated. Nitrogen inside the cavity interior can react with 'OH radicals and oxygen atoms to give N20 and NO, respectively. However, the former product is unstable under the conditions prevailing in the bubble interior and is further transformedtoNO, which is subsequently converted to the more stable oxidized forms of nitrogen, NO2- and NO,ions, by reaction with the active oxygen species in the solution bulk.25 Sonolysisof Chlorophewls. The three chlorophenolsexamined here are transformed in reasonable time via dechlorination to intermediate species with a decreased carbon to oxygen ratio; the half-lives are 133 min (2-CPOH), 158 min (3-CPOH), and 210 min (4-CPOH). This order mimics somewhat the observations from oxidative mineralization of these sustrates via irradiation of semiconductor particulates of Ti02.s5,62 Dechlorination is nearly quantitative and occurs soon after initiation of the disappearance of the initial substrate (Figure la-c). The intermediates that formed under sonochemical conditions are presented in Table 1 and indicated in Figure la-c. For 2-chlorophenol, the major intermediate observed is chlorohydroquinone with a small quantity of catechol; chlorohydroquinone is also the major intermediate product produced from sonolysis of 3-chlorophenol solutions. By contrast, the principal intermediate from the ultrasonic irradiation of 4-chlorophenol is hydroquinone with a small amount of 4-chlororesorcinol also produced. Other intermediateswere identified but not confirmed (Table 1). Formationof these intermediateproductsare depicted in Figure 3, a, b, and c, where we infer that the primary product generated from 'OH radical attack on the substrate is the OH adduct, chlorodihydroxycyclohexadienylradical, in keeping with earlier pulsed radiolytic observations on 2,4,5-trichlorophenol,41 pentahalophenols,4*and dim ethyl phenol^.^^ At high temperatures (T 1 400 K), *OH radicals appear to react with phenols and aromatics predominantly via H-atom abstraction;" however, formation of the phenoxyl radical subsequent to OH-adduct production is not to be excluded even at the high temperatures prevailing at the bubble/liquid interface. Irrespective of the mode of action of the *OH radicals, both the OH adducts and the phenoxyl radicals lead to otherwise identical hydroxylated produ~ts.2~-2~ Comparison of the intermediates formed by the sonochemical technique (Table 1) with those reported earlier (see Schemes 1,
Serpone et al.
2638 The Journal of Physical Chemistry, Vol. 98, No. 10, 1994
b"" OH
0
D
.-C
E
3
I
n
VI
I] Y v
OC
I
100
0
I
I
200 [4-CIPhOH],
300
400
DM
Figure 4. Plot of (kob)-l versus [CCIPhOH] showing the concentration dependence of the sonochemical transformation of 4-chlorophenolused as the test substrate.
and C (= 1.45 f 0.05 pM-l min) are the slopes of the respective lines and B (=202 f 14 min) is the intercept of the line in regime 2. The overall rate for the process is74
dH OH
h
Figure 3. Schemes illustrating the reaction of chlorophenolswith 'OH
radicals and the subsequent intermediates formed and identified (see text): (a, top) 2-chlorophenol, (b,middle) 3-chlorophenol,and (c. bottom) 4chlorophenol. TABLE 2 Observed Rate Constants in the Temporal Disappearance of 4-Chlorophenol at Various Concentrations in Air-EquilibratedAqueous Media 394 154 75.1 38.9 18.2
1.3 f 0.2 2.3 f 0.2
3.3 f 0.1
5.9 0.8 29 f 9
533 296 207 118
where ko = 0.25 f 0.03 pM min-I, k = 0.69 f 0.03 pM min-I, and K is (7.2 f 0.5) X 10-3 pM-*. Expression 2 indicates that the rate of disappearance of the chlorophenols follows a concentration independent path and a [CPOHI-dependent course. At the higher concentrations of 4-CPOH (Figure 4), the sonochemical process illustrates saturation-type kinetics reminiscent of Langmuirian type behavior displayed in solid/gas systems and suggested by several workers in photocatalytic processes implicating semiconductor particulates [see e.g., refs 27-37]. By analogy, K is the equivalent of adsorption/desorption coefficientsof chlorophenols at the gaseous bubble/liquid interface. These kinetics and the nature of the intermediates noted inTable 1call immediateattention to possible similaritiesbetween the highly hydroxylated and hydrated surfaces of metal oxide semiconductors (e.g., TiOz) and the imploding bubble surface in aqueous media. Mechanistic Significanceand ReactionSites. Our earlier report on the sonolysis of aqueous solutions of phenol, catechol, hydroquinone, and benzoquinone26 noted that these substrates were oxidized principally by their interaction with the 'OH radicals produced during the cavitational collapse (eq 3),where )))))
24
2, and 3) from oxidation of these substrates by direct or sensitized photolysis,53,ss~56 flash photolysis," UV/~eroxide,5~ and irradiated semiconductor (SC) particulates (Ti02 and Zn0)5s.59,62,64865 suggests that the sonochemical oxidation process finds strong similarities with and physically mirrors the heterogeneous photocatalytic process with SC partic~lates.*~-~~ A relevant connection is made between a solid/liquid interface and a gaseous bubble/liquid interface (also see below). ConcentrationDependence and KineticAspects. The temporal disappearance of 4-chlorophenol at various concentrations is illustrated in Figure 2. The solid lines through the data points were drawn using the observed rate constants, kob, summarized in Table 2 and obtained from the expression: [CPOH] = [CPOHIi exp(k,bt). These kob are depicted in Figure 4 as l/kob versus initialconcentrationof the chlorophenol, [CPOHIi. The kinetics show two regimes: a regime 1 at low concentrations of 4-CPOH (175 pM) and a regime 2 at higher concentrations of 4-CPOH (275 pM). In regime 1, l/k(I),b = A[CPOH]i, while in regime 2, l/k(Z),b = B + C[CPOH]i, whereA (=4.00 f 0.27 pM-' min)
H,O
+ )))
ki
'OH
+ H'
(3)
denotes the ultrasounds at power P. Subsequently, recombination can occur at the interface (reaction 4) and/or formation of hydrogen peroxide at the interface and/or in the solution bulk (reaction 5 ) . Oxidation of the chlorophenols can take placeeither
'OH
- --
+ H'
ki
H,O
(4)
k3
2 'OH
H20,
(5)
at the interface and/or in solution. Whence,
+
*OH (ch1oro)phenol-
k4
products
(6)
Using the steady-state condition for production of 'OH radicals under continuous insonation and with the rate of conversion of the chlorophenol given by rate = k4[*OH][CPOH] yieldsz6
Dehalogenation and Oxidation of Chlorophenols rate =
The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 2639
k,k, [CPOH],P (k,[H']
+ k,[*OH])+ k,[CPOH],
(7)
If the reaction occurred at the bubble/liquid interface where the concentration of 'OH radicals is greatest and the concentration of the chlorophenol lowest (chlorophenols are relatively hydrophilic while the bubble/liquid interface is hydrophobic75), the rate of conversion is expected to be first order in [CPOHIi and first order in P (see e.g., ref 25) at low initial concentrations of the chlorophenol, Le., when (kz[H'] + k,['OH]) >> k4[CPOH]. However, since the rate is experimentally zero order in [CPOH], (ko term in eq 2), we conclude that even at low [CPOHIi (a) conditions are such that ks[CPOHIi >> (kz[H*] + kg[*OH]) so that (b) the rate = k l P = ko for constant insonation power P.The only location where condition a exists is in the solution bulk; we conclude, therefore, that the chlorophenol reacts with hydroxyl radicals in solution. At higher [CPOHIi (175 pM), more of the chlorophenol can accumulate at the interface (increased adsorption) such that the process now follows reaction 8,for which a kinetic expression is
0+
CPOH
K
{CPOH}hwm
k
bubble
[ Idechlorinated intermediates
produds
(8)
hydroxylated
intermediates
(4) (a) Suslick, K. S.;Doktycz,S. J. AdvancesinSonochemistry; Mason, T. J., Ed.; JAI Press: London, 1990; Vol. I, p 197. (b) Suslick, K. S., Ed. Ultrasound-Its Chemical, Physical and Biological Effccts; VCH Publishers: New York, 1988. (5) Bremner, D. In ref 4a, p 1. ( 6 ) Luche, J. L. In ref 4a, p 119. (7) Riesz, P.; Kondo, T. Free Radical Biol. Med. 1992, 13, 247. (8) Serpone, N.; Colarueso,P.; Hidaka, H.; Peliuetti, E., tobe submitted for publication (taken in part from the Senior Undergraduate Thesis of P. Colarusso, April 1992, Concordia University). (9) Toy, M. S.;Stringham, R. S . In Organic Phototransformations in Nonhomogeneous Media;ACS SymposiumSeries; Fox, M. A., Ed.;American Chemical Society: Washington, 1985; Vol. 278, Chapter 18, pp 287-295. (10) Mason, T. J. Sonochemistry-A Technology for Tomorrow. Chem. Ind. (London) 1993, Jan 18, 47-50, and references therein. (11) Zechmeister, L.; Wallcave, L. J . Am. Chem. Soc. 1955, 77, 2953. (12) Zechmeister, L.; Magoon, E. F. J. Am. Chem. Soc. 1956,78,2149. (13) Currell, D. L.; Zechmeister, L. J. Am. Chem. Soc. 1958, 80, 207. (14) Currell, D. L.; Wilheim, G.; Nagy, S . J. Am. Chem. Soc. 1%3,85, 127. (15) Chen, J. W.; Smith, G. V. 'Feusibility Studies of Applications of (sonochemical) Catalytic Oxidation in Wastewater". U S . Environmental Protection Agency, Water Pollution Control Research Series, 17020 ECI 11/71, 1971. Chen, J. W.; Chang, J. A.; Smith, G. V. Chem. Eng. Prog. Symp. Ser. 1971, 67, 18. (16) Chen, J. W.; Hui, C.; Smith, G. V. 'Oxidation of Wastewaters with OzonelCatalyst and Air~Catalyst~Ultrasound Systems". In Proceedings of the First International Symposium on Ozone for Wastewater Treatment; Rize, R. G., Ed.; International Ozone Institute, Inc.: Waterbury, CT, 1975; pp 121-131. (17) Henglein, A,; Fischer, Cg.-H. Ber. Bunsenges. Phys. Chem. 1984, 88, 1196. (18) Gutierrez, M.; Henglein, A,; Fischer, Ch.-H. Int. J . Radiat. Biol. 1986, 50, 313. (19) Hart, E. J.; Fischer, Ch-H.; Henglein, A. J. Phys. Chem. 1990, 94, 284. (20) #Silva, A. P.; Laughlin, S.K.; Weeks, S. J.; Buttermore, W. H. Polycycl. Arom. Hydrocarbons 1990, 1 , 125. (21) Buttner, J.; Gutierrez, M.; Henglein, A. J . Phys. Chem. 1991, 95, 1528.
represented by the second term in the experimental rate law, eq 2. We conclude that at high [CPOH] the reaction takes place predominantly at the interface. (24) Kotronarou, A.; Mills, G.; Hoffmann, M. R. Environ. Sci. Technol.
Conclusions Ultrasounds induce the dechlorination and the oxidation of the three monochlorophenolsexamined in air-equilibrated media. Major intermediate products have been identified; they parallel those observed in the heterogeneous photocatalyzed mineralizations of these same substrates in aqueous dispersions of TiOl anatase, but contrast those reported by other AOP processes. Similar kinetics and analogous processes occur with these two entirely contrastingtechniques in forming the redox species, calling attention to possible analogies with and similar characteristics of the interfaces. Two reaction sites have been inferred: the solution bulk at low concentrations and the gas bubble/liquid interface at high concentrations in phenol. The industrial pertinence of ultrasounds in environmental detoxification has not been demonstrated to the same degree that heterogeneousphotocatalysis has. Yet the potential exists either alone or in combination with photochemical methods. Future work should address these issues further.
1992. 26. 2420. (25) Petrier, C.; Micolle, M.; Merlin,G.;Luche, J.-L.;Reverdy,G. Environ. Sei. Technol. 1992, 26, 1639.
(26) Serpone, N.; Terzian, R.; Colarusso, P.; Minero, C.; Pelizzetti, E.; Hidaka, H. Res. Chem. Intermed. 1992, 18, 183. (27) Serpone, N.; Pelizzetti, E.; Hidaka, H. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Ak-Ekabi, H. Eds.; Elsevier: Amsterdam, 1993; pp 225-250. (28) Serpone, N.; Pelizzetti, E.; Hidaka, H. In Photochemical Conversion and Storage of Solar Energy (IPS 9);Cai, S.M.; Cao, Y., Xiao, X.R., Eds.; Academia Sinica International Academic Publishers: Beijing, 1993; pp 3374. (29) Bahnemann, D.; Cunningham, J.; Fox, M. A,; Pelizzetti, E.; Pichat, P.; Serpone, N. In Aquatic and Surface Photochemistry; Crosby, D., Helz, G., Zepp, R., Eds.; Lewis Publishers: Boca Raton, FL, in press. (30) Ollis, D. F.; Pelizzetti, E.; Serpone, N. Enuiron. Sci. Technol. 1991, 25, 1522. (31) Serpone, N.; Lawless, D.; Terzian, R.; Minero, C.; Pelizzetti, E. In Photochemical Conversion and Storage of Solar Energy (IPS 8);Pelizzetti, E., Schiavello, M., Eds.; Kluwer Academic Publ. Inc.: Dordrecht, The Netherlands, 1991; pp 451-475. (32) Hidaka, H.; Zhao, J.; Suenaga, S.;Peliuetti, E.; Serpone, N. In Surfactants in Solution; Mittal, K. L., Shah, D. O., Eds.; Plenum Press: New York, 1991; pp 335-348. (33) Pelizzetti, E.; Pramauro, E.; Minero, C.; Serpone, N. Waste Management 1990, 10,65. Acknowledgment. This work was supported by a grant from (34) Ollis, D. F.; Pelizzetti, E.; Serpone, N. In Photocaralysis-fundamentals and applications; Serpone, N., Pelizzetti, E., Eds.; Wiley-Interthe Natural Sciences and Engineering Research Council of science: New York, 1989; pp 604-637. Canada. We are also grateful to the North Atlantic Treaty (35) Serpone, N. In Photochemical Energy Conversion; Norris, Jr., J., Organization for an exchange grant between our respective Meisel, D., Eds.; Elsevier: Amsterdam, 1989; pp 297-315. (36) Pelizzetti, E.; Minero, C. Electrochim. Acta 1993, 38, 47. laboratories (Montreal and Torino together with those of the (37) Matthews, R. W. In PhotochemicalConversionandStorageofSolar Ecole C e n t r a l e de Lyon, Dr. P. Pichat, and of t h e University of Energy (IPS 8);Pelizzetti, E., Schiavello, M., Eds.; Kluwer Acadmic Publ. Texas, Prof. M. A. Fox). Inc.: Dordrecht. The Netherlands. 1991: DD 427-449. (38) Rothenberger, G.; Moser, J.; Graiiel, M.; Sharma, D. K.; Serpone, N. J. Am. Chem. SOC.1985, 107, 8054. References and Notes (39) (a) Lawless, D.; Serpone, N.; Meisel, D. J . Phys. Chem. 1991,95, (!) Mason,T. J. Inchemistry with Ultrasound;Mason,T. J., Ed.; Elsevier 5166. (b) Serpone,N.;Lawless, D.;Terzian, R.;Meisel, D. In Electrochemistry Applied Science: London, 1990; Chapter 1, pp 1-26. in Colloids and Dispersions; Mackay, R. A., Pexter, J., Eds.; VCH (2) Mason, T. J.; Lorimer, J. P. Sonochemistry: Theory, A p p l i ~ a r i ~ t ~ Publishers: New York; 1992; pp 399416. and Uses of Ultrasound in Chemistry; Ellis Honvood Ltd.: Chichester, U.K., (40) Micic, 0.I.; Zhang, Y.; Cromack, K. R.; Trifunac, A. D.;Thurnauer, 1988. M. C. J. Phys. Chem. 1993, 97, 7277. (3) Levy, S. V.; Low, C. M. R. Ultrasound in Synthesis; Springer(41) Draper, R. B.;Fox, M. A.; Pelizzetti, E.; Serpone, N. J. Phys. Chem. Verlag: London, 1989. 1989, 93, 1938.
2640 The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 (42) Terzian, R.; Serpone, N.; Draper, R. B.; Fox, M. A,; Pelizzetti, E. Lungmuir 1991, 7, 3081. (43) (a) Terzian, R.Ph.D. Thesis, ConcordiaUniversity, Montreal, Canada, January 1993. (b) Terzian, R.; Serpone, N.; et al., to be published. (44) Makino, K.; Mossoba, M.; Riesz, P. J. Phys. Chem. 1983,87, 1369. (45) Riesz, P.; Berdhal, D.; Christman, C. Enuiron. Health Perspect. 1985, 64, 233. (46) Hart, E. J.; Henglein, A. J . Phys. Chem. 1985, 89, 4342. (47) Gutierrez, M.; Henglein, A,; Dohrmann, J. K. J . Phys. Chem. 1987, 91, 6687. (48) Suslick,K. S.In Ultrasound-Its Chemical,Physical, and Biological Effects; Suslick, K. S., Ed.; VCH Publishers: New York, 1988; pp 123-163. (49) The procedures used to determine the ultrasonic power entering the reaction medium are described in: Mason, T. J. Practical Sowhemistry; Ellis Horwood Ltd.: Chichester, England, 1991; pp 4546. (50) Suslick, K. S.Sci. Am. 1989, 80. (51) The induction period we observe for the 3-chlorophenoland to some extent also for the 2-chlorophenol is somewhat puzzling, since the 'OH and H' radicals are presumably formed immediately on applying ultrasound to the aqueous solution. A few remarks can be made in this regard. First, the oxidationof the chlorophenolsis a bimolecular reaction with the *OHradicals whether the process takes place at the interface or in solution; second, the chlorophenolshave to compete for the 'OH radicals with rwmbination with H' atoms, with couplingto give H202, with nitrogen to form nitrite and nitrate ions and with whatever other substrate is adventitiously present in the system. Unlikesemiconductorparticulate?where theorganicsubstratesarepreadsorbed to the particle surface (even in the dark), no such preadsorption can take place in sonochemical events until such time as the gas bubble interface is formed by the applied ultrasounds. As well, the surface of Ti02 particulates (as normally uscd) is hydrophilic while the gas bubble surface is hydrophobic. Not surprisingly then that there are some distinctions between the properties of the two surfaces, but yet the redox chemistry is surprisingly identical. (52) Krijgsheld, K. R.; van der Gen, A. Chemosphere 1986, 15, 825. (53) Boule, P.; Rossi, A.; Pilichowski,J. F. New J. Chem. 1992,16, 1053. (54) Lipczynska-Kochany, E.; Bolton, J. R. Ewiron. Sci. Technol. 1992, 26, 259. ( 5 5 ) D'Oliveira, J.-C.; AI-Sayyed, G.; Pichat, P. Enuiron. Sci. Technol. 1990, 24,990. (56) Boule, P.; Guyon, C.; Tissot, A,; Lemaire, J. ACS Symp. Ser. 1987, 327, 10. Boule, P.; Guyon, C.; Lemaire, J. Toxicol. Enuiron. Chem. 1984, 7, 97. (57) Lipczynska-Kochany, E. Chemosphere 1992, 24, 91 1.
Serpone et al. (58) Barbeni, M.; Minero, C.; Pelizzetti, E.; Borgarello, E.; Serpone, N. Chemosphere 1987, 16, 2225. (59) Stafford, U.; Gray, K. A.; Kamat, P. V.; Varma, A. Chem. Phys. Lett. 1993, 205, 55. (60) (a) Al-Ekabi, H.; Serpone, N. J. Phys. Chem. 1988,92,5726. (b) AI-Ekabi, H.; Serpone, N.; Pelizzetti, E.; Pramauro, E.; Minero, C.; Fox, M. A.; Draper, R. B. Lungmuir 1989, 5, 250. (61) Lichtin, N.; Dong, J.; Vijayakumar, K. M. Water Poll. Res. J. Can. 1992, 27, 203. (62) AI-Sayycd, G.; D'Oliveira, J.-C.; Pichat, P. J. Photochem.Photobiol. A: Chem. 1991, 5 4 9 9 . (63) Mills, A.; Morris, S.J. Photochem. Photobiol. A: Chem. 1993, 71, 75. (64) Sehili, T.; Bonhomme, G.; Lemaire, J. Chemosphere 1988,17,2207. (65) Bonhomme, G.; Lemaire, J. C. R. Acad. Sci. Paris 1986,302,769. (66) Suslick, K. S.;Hammerton, D. A.; Cline, R. E., Jr. J. Am. Chem. Soc. 1986,108, 5641. (67) We thank a referee for pointing this out. (68) Krishna, C. M.; Lion, Y.; Kondo, T.; Riesz, P. J . Phys. Chem. 1987, 91, 5847. (69) Hart, E. J.; Henglein, A. J . Phys. Chem. 1986, 90, 3061. (70) Gutierrez, M.; Henglein, A. J. Phys. Chem. 1988, 92, 2978. (71) Gutierrez, M.; Henglein, A,; Fischer, Ch.-H. Inr. J. Radiat. Biol. Relat. Stud. Phys. Chem. Med. 1986, 90, 222. (72) Kondo, T.; Krishna, C. M.; Riesz, P. In?. J. Radiat. Biol. 1988,53, 891. (73) (a) He, Y. Z.; Mallard, W. G.; Tsang, W.J. Phys. Chem. 1988,92, 2196. (b) Atkinson, R. Chem. Reu. 1985,85,69. (74) Noteregardingeq2. Thatthesecond terminthisequationisanalogous to the rate equation forthcomingfrom consideringthe Langmuir-Hmhelwood model for a solid/gas system is simply fortuitous. It cannot be concluded from such similarities that in the prcscnt instance where a gas bubble/solution interface is formed that necessarilythe processes examined here are interfacial processes. In fact, the second term of eq 2 is also found for many reactions in purely homogeneous phase, where no such interfaces exist, and also found in the Michaelis-Menten model for enzyme kinetics. This type of expression as exemplified by the second term of q 2 is nothing more than a manifestation of saturation type kinetics whether in a homogeneous or heterogeneousphase. Hence no immediate conclusions can be drawn simply on the basis of kinetics alone. Other data and added arguments must be prcscnted to argue for or against interfacial processes. (75) Riesz, P.; Kondo, T.; Krishna, C. M. Ultrasonics 1990, 28, 295.
