Environ. Sci. Technol. 1990, 24, 990-996
Petterson, R. C.; Grzeskowiak, U. J. Org. Chem. 1959,24, 1414-1419. Zief, M.; Edsall, J. T. J . A m . Chem. SOC. 1937, 59, 2245-2248. Olson, D. L.; Shuman, M. S. Anal. Chem. 1983, 55, 1103-1107. Jensen, J. N.; LeCloirec, C.; Johnson, J. D. In Water Chlorination: Environmental Impact and Health Effects;
Jolley, R. L., Ed.; Lewis Publishers: Chelsea, MI; 1990; Vol. 6, Chapter 61. Received for review January 5,1990. Accepted March 12,1990. This work was supported by the Electric Power Research Institute (Contract No. RP2300- 7). Portions of this work were presented at the American Water Works Association Water Quality Technology Conference, Baltimore, MD, November 1987.
Photodegradation of 2- and 3-Chlorophenol in TiOp Aqueous Suspensions Jean-Christophe D’Oiivelra, Ghassan Ai-Sayyed,+ and Pierre Pichat URA au CNRS “Photocatalyse, Catalyse et Environnement”, Ecole Centrale de Lyon, BP 163, 69131 Ecully Cbdex, France ~
The photocatalytic degradations of 2-chlorophenol (2CP) and 3-chlorophenol (3CP) in Ti02 aqueous suspensions at X > 340 nm have been studied. They are faster than direct photolysis at X > 290 nm despite the additional number of photons available in this wavelength region and produce lower maximal concentrations of intermediates. Complete mineralization is achieved eventually; however, it requires a much longer time than dechlorination and dearomatization. Effects of radiant flux, wavelength, initial concentration, and pH are examined. Para hydroxylation is the main pathway, so that 2CP and 3CP both give rise to chlorohydroquinone (CHQ), whereas formation of catechol (CT) is a very minor pathway in the degradation of 2CP. Further hydroxylation of CHQ and CT forms hydroxyhydroquinone, which rapidly decomposes to carboxylic acids and carbonyl compounds not identified yet. Introduction Photocatalytic degradation of water organic pollutants appears to be a promising method, in particular when using titanium dioxide as a stable and inexpensive photosensitive material. Research in this field has recently been reviewed (I). References 2-12 indicate papers published since. Fundamental studies are still needed to assess the possibilities of this method. These include the screening of molecules typical of water pollutants to determine the degradation pathways and limitations, as well as the investigation of the effects of various parameters to optimize the process. In this work, we chose 2- and 3-chlorophenol as the representatives of chlorinated aromatic compounds. By contrast with many previous studies concerning other pollutants, the experiments were performed under conditions that allow one to distinguish photocatalytic degradation from direct photolysis. Effects of illumination characteristics, initial concentration, and pH were investigated, which is important for implementation of the method. To find out the degradation pathways, the main intermediates have been identified. Finally, the kinetics of dearomatization, dechlorination, and complete mineralization have been compared. Monochlorophenolsconstitute an important category of water pollutants. Their toxicity to mammalian and aquatic life is classified as moderate, but they have strong organoleptic effects and their taste threshold is 0.1 ppb (pg L-l) (13). Their presence in water stems principally from industry, which produces them as chemical intermediates or generates them during the chlorination of effluents containing phenolic compounds. They can also be formed by the degradation of phenoxy herbicides. They have been Present address: Chemistry Department, University College, Cork, Ireland. 990
Environ. Sci. Technoi., Vol. 24, No. 7, 1990
found in river water, usually at levels of a few ppb. Use of chlorination in the treatment of drinking water can result in the presence of monochlorophenols. These compounds are biodegradable; however, 3-chlorophenol is resistant enough (13). Experimental Section Materials. Monochlorophenolsand intermediates, such as chlorohydroquinone, hydroxyhydroquinone, resorcinol, and catechol, were obtained from Aldrich (purity >99% or >99.5%) and used without further purification. Rathburn HPLC-grade eluents were employed. The photocatalyst was Degussa P-25 Ti02 (mainly anatase, nonporous, 50 m2 g-l). Photoreactors. Two types of batch photoreactor opened to air were utilized. Photoreactor 1 (Pl) was a cylindrical flask of ca. 90 mL with a bottom optical window of ca. 4 cm in diameter. The suspension in P1 was magnetically stirred. Figure 1represents photoreactor 2 (P2). It was similar to P1 except for the volume (ca. 180 mL) and the circulation of the suspension (100 L h-l) via a peristaltic pump. P1 was used for studying the effects of various parameters and P2 for analyzing the intermediates and C02. Illumination was provided by a Philips HPK 125-W high-pressure mercury lamp through a 2.2-cm-thick circulating water cell. If a cutoff filter (A > 340 nm, Corning 0.52) was used, the radiant flux entering the photoreactor was 50 mW cm-2; without it, it was 60 mW cm-2 (ca. 4.6 X 1017photons/s in the 290-400-nm region). The corresponding numbers of photons per second potentially absorbable by the Ti02 sample were ca. 1.6 X lo1’ and 2.9 x 1017,respectively (for the calculation see below, paragraph 2.3 in Results and Discussion). Procedures. Solutions of 1.55 X lo4 M (20 ppm = 20 mg L-l) monochlorophenol were used unless otherwise indicated. In P1 the volume of suspension was chosen equal to 20 mL, which corresponded to 3.1 pmol of chlorophenol. As expected for a photocatalytic reaction, the initial rate of disappearance ro of the pollutant increased with increasing amounts of Ti02 up to a limit corresponding to the complete absorption of photons. From this relationship we chose to use 2.5 g L-’ Ti02, i.e., a concentration corresponding to the beginning of the plateau in the plot of ro vs Ti02 mass. Similarly, 1.25 g L-’Ti02 was used in P2. Figure 2 shows the effect of the volume of suspension for this concentration of TiOz. Therefore we chose the optimum volume of 135 mL (the nonilluminated volume was 40 mL), which corresponds to 21 pmol of chlorophenol. The aerated suspensions were stirred (Pl) or circulated (P2) in the dark for 90 min before illuminating. After this period the concentration of monochlorophenol in water was found nearly stable. The
0013-936X/90/0924-0990$02.50/0
0 1990 American Chemical Society
I
I.
A
4 i
'pump
, 1- f
r,
Figure 3. Degradation of 2CP in photoreactor P1
optical filters
r==1 -
U.V.
4 .
1
6
Figure 1. Batch photoreactor P2 with circulation of the suspension.
without Ti02 ; A 1290 nm
1-
t lm e/m in
Figure 4. Degradation of 3CP in photoreactor P1.
and COOwas trapped into three flasks in series containing a 5 X lo4 N Ba(OH)2solution. After filtration of BaCO,, the remaining OH- ions were titrated with a 0.05 N HC1 solution. During the experiment, the kinetic evolution of C02 was monitored by GC (Intersmat IGC 120 MB chromatograph, Porapak Q column, 3 m long, 6.3 mm i.d.).
v
tcm3
Figure 2. Effect of the volume of suspension (TiO,, 1.25 g L-') in photoreactor P2 upon the initial degradation rate of 3CP. Cond%ions: see text.
starting pH was ca. 4.5 or 4.7 for 2- or 3-chlorophenol, respectively. Analyses. Monochlorophenols and their organic intermediates were analyzed by HPLC using an LDC/Mdton Roy system, which comprised a Constametric 3000 isocratic pump and a Spectro Monitor D UV detector adjusted at 254 or 280 nm. A reverse-phase column, 25 cm long, 4.6 mm i.d., packed with Spherisorb 5 ODS2, was used. The mobile phase was a mixture of methanol (35%), deionized doubly distilled water (55%),and acetonitrile. Identification of the eluting compounds was made by comparing their UV spectrum to those of commercial compounds with a Varian 9065 Polychrom diode array detector. Cl- ions were also analyzed by HPLC using a Waters 501 isocratic pump, a Waters 431 conductivity detector, and an IC-PAK anion column, 5 cm long, 4.6 mm i.d. To determine the total quantity of COS formed, the reaction mixture was continuously flushed by a N2-02 flow
Results and Discussion 1. Kinetics of the Photocatalytic Disappearance of 2CP and 3CP. Comparisons with Direct Photolysis and with 4CP. The rates of the photocatalytic disappearance of 2-chlorophenol (2CP) and 3-chlorophenol (3CP) depend on various parameters such as initial concentration, pH, radiant flux, wavelength, mass and type of photocatalyst, and type of photoreactor. Consequently, their measurements have no absolute meaning. However, for a given set of conditions, they allow one to determine the effect of each of the experimental parameters or to compare the monochlorophenols between them (or with other pollutants). To obtain relevant information about the photocatalytic transformation, it is necessary to carry out such experiments that direct photolysis is excluded. Monochlorophenols possess a maximum absorption at X = ca. 275 nm, so that illumination through a 290-nm cut-off filter gives rise to some photochemical transformation even in very diluted aqueous solutions (Figures 3 and 4). However, the use of a filter transmitting wavelengths of >340 nm produces a decrease of 290 nm, because of the competition of absorption between the pollutant and the powder semiconductor. Nevertheless, fixing the lower limit of wavelengths to 340 nm provides surer conditions to study the photocatalytic transEnviron. Scl. Technol., Vol. 24, No. 7, 1990 001
Table I. Initial Rates of Photocatalytic Degradation ( r o ) and Illumination Times Required To Decompose 50% ( t 1/2) or 99% (tnsb)of the Pollutant separate degradationn pollutant
2CP 3CP 4CP
ro, pmol h-'
4.20 4.10
4.30
li
1031mo1.1
/
10
5
simultaneous degradation*
t1p
teepa? ro, min pmol h-'
min
tee%, min
34 37.5 26.5
120 110
2.8
80
2.4
30 45 32
125 150 95
min
c,
1
5
1
1.7
t1p
Cn = 20 mg d ~ n - ~C., = 10 mg dm3 for each CP.
formation in the absence of any significant contribution of direct photochemical transformation of monochlorophenols. Indeed, as shown in Figure 4 the disappearance of 3CP over Ti02 is slower at X > 290 nm than at X > 340 nm, despite the greater number of photons potentially absorbable by Ti02 (see Experimental Section), because of the effect of the photochemical intermediates, as will be discussed later on. The disappearance of 2CP (Figure 3) or 3CP (Figure 4) in the presence of Ti02 at X > 340 nm is faster than in its absence at X > 290 nm in spite of the reduced radiant flux (see Experimental Section). The same result was also found for 4CP (14). Note that direct photolysis is more efficient for 2CP than for 3CP, since 99% disappearance of the pollutant is obtained after 175 or 245 min, respectively, under our conditions. This is due to differences in the polarization of the excited molecules. Photochemical transformations of monochlorophenols have been studied in detail previously (15). The photocatalytic disappearance of 2CP or 3CP at X > 340 nm (Figures 3 and 4) apparently follows a fmt-order kinetic law for the initial concentration chosen, as previously reported for 4CP (8)and phenol (16,17) under similar conditions. These apparent relationships are due to the small concentrations (see paragraph 2.1 for more detail). Table I sums up some kinetic results of the photocatalytic transformation of monochlorophenols over Ti02. The 99% disappearance of 2CP is obtained within a time slightly longer than that of 3CP despite a somewhat greater initial rate. These facts point to competitive adsorptions of the pollutant and of its products of degradation. They also demonstrate that the initial rate, although useful, can be misleading when used as the sole piece of information to estimate the efficiencies of the photocatalytic degradation of a series of pollutants. The photocatalytic treatment of a mixture of the three monochlorophenolsat equal initial concentrations results in increased kinetic differences between these pollutants (Table I). In fact, not only the three reactants compete for the adsorption sites of Ti02,but also their degradation intermediates, so that changes in the kinetics (see the relative values of ro, tlIz, tss%)are not unexpected relative to the situation in which each monochlorophenol is treated individually. These competition processes made it difficult to predict the kinetics for water containing several pollutants. 2. Effects of Various Parameters on 3CP Photocatalytic Degradation. 2.1. Effect of the Initial Concentration. Curve A of Figure 5 shows that the initial rate ro of the photocatalytic transformation of 3CP first increases sharply and then progressively levels off with increasing initial concentrations C,. The plot of r0-' as a function of C0-'(Figure 5B) yields a straight line (r = 0.985) as expected if ro obeys the Langmuir-Hinshelwood relationship ro = kKCo/(l + KCo) (absence of adsorption competition between 3CP and H20) or ro = kKCo/(l + 992
Environ. Sci. Technol., Vol. 24, No. 7, 1990
0.2
1
c
d
mmoll.1
Flgwe 5. (A) Initial rate f o of 3CP photocatalytic degradation against the initial concentration C oin photoreactor P l . (B) Linear transform of (A), A > 340 nm.
c
I
I
2
4
6
8
10
12
PH
Figure 8. Dependance of the initiil rate r o of 3CP photocatalytic degradation as a function of the initial pH in photoreactor P1, X > 340 nm.
KCo + K,C,)
where k is the rate constant, K is the adsorption constant of 3CP on Ti02from water, i.e., the ratio of the rate constant of adsorption of 3CP to its rate constant of desorption, K , is the equivalent constant for water, and C, is the concentration of water, which does not vary. The Langmuir-Hinshelwood relationship shows the importance of the catalytic steps in the degradation of 3CP. From Figure 5B, the values of k = 21 pmol h-' and K = 1620 L mol-' can be calculated by assuming the simplest model ro = kKCo/(l + KCo) to be valid. The value of k depends on the conditions and accordingly has no absolute meaning. To determine the effects of other parameters and to analyze the main intermediates, we have arbitrarily chosen Co = 0.155 mM (20 ppm) because it is a representative level of pollution in industrial effluents and it allows significant analyses. This concentration is situated in the first part of curve A of Figure 5 and corresponds to an initial surface coverage of the adsorption sites of 3CP equal to ca. 0.2 as calculated from 6 = KCo/(l + KC,). An error of less than 10% in the denominator 1+ KC of the Langmuir-Hinshelwood relationship occurs if KC is neglected when C is smaller than O.l/K, Le., 6.17 X 10" M or ca. 40% of C,. This explains the apparent first-order kinetics of the disappearance of 3CP. 2.2. Effect of pH. Changing the pH affects the relative proportions and not the nature of the products of 3CP degradation detected by HPLC. Variations in ro (Figure 6) and time of 99% disappearance (ca. 90, 115, and 190 min at pH 10.8, 4.5, and 2.5, respectively) are observed. Similar changes in the efficiency of the degradation of phenol (16a, 17) or the hydroxylation of sodium benzoate (18)over Ti02have been reported. The increase in ro with
Scheme I Q1
I
' .
1
05 "
"
'I"
OH
OH
1 HO &ci
0.1
O S
l4
OH
OH
OH
OH
1
0 100
Figure 7. Variations in the initial rate of 3CP photocatalytic disappearance in photoreactor P1 with the relative radiant flux (stars) or its square root (dots). 4 refers to the nonattenuated radiant flux (4.5 X 10'' photons s-l absorbable by TiO,), A > 340 nm.
increasing pH above the pK, of 3CP can be attributed to the increased number of OH- ions at the surface of Ti02, since OH' radicals (presumably involved in the formation of the primary intermediates; see paragraph 3) can be formed by trapping photoproduced holes. Also the dissociation of 3CP probably changes its reactivity. Similarly, the decrease at the lowest pH can be explained by the lack of OH- ions. However, the absence of pronounced changes in ro in the 3.5-9 pH range (Figure 6), i.e., on each side of the point of zero charge of Ti02[6.3 for the sample we used (l9)], indicates that another phenomenon compensates for the variation in OH- ions. Oxygen (or another electrophilic species) is required for photocatalytic oxidations in order to trap the photoproduced electrons. From measurements carried out for the system gaseous 02-UV-illuminated Ti02,the existence of several types of negatively charged adsorbed oxygen species has been derived: 02-,03-,0-, 022-.The formation of H02' radicals from 02-and H', giving rise to H202has been proposed (8, 16a) as another way of producing OH' radicals. This way would be favored on decreasing the pH. In any case, the present results indicate that the efficiency of the process is not much affected over a wide range of pH, which is quite satisfactory in view of applications. 2.3. Effect of Radiant Flux. As shown in Figure 7, ro is proportional to the radiant flux 6 for values smaller than ca. 20 mW cm-2,i.e., ca. 6.4 X 10l6efficient photons/s (see below). Above this value, it is proportional to its square root, which means that the recombination of the photoproduced charges predominates (20) and, accordingly, the initial quantum yield of the degradation decreases. This is obviously detrimental and difficult to control since the means of decreasing recombination rates are not well established. To determine the initial quantum yields cpo, the number of photons potentially absorbable by Ti02 per time unit is calculated from the lamp spectral distribution, the transmission curves of the filter, the absorbance spectrum of TiOz (whose amount was sufficient to completely absorb the incident flux), and the reflectivity of Ti02 (0.15, after ref 21). This gives lower limits of cpo since no allowance was made for scattering by the solid particles. For starting concentrations of 1.55 X lo4 M (20 ppm) and in the region (2CP) or 6.6 X where ro = &I, cpo is equal to 6.9 X (3CP).
/ min
time
Figure 8. Photocatalytic degradation of CHQ or HHQ or CT in photoreactor P1 and variations in the amount of HHQ formed from CHQ or CT. Conditions: as for 3CP degradation, A > 340 nm.
3. Intermediates. 3.1. 3-Chlorophenol. Ten intermediates are detected by HPLC under the conditions indicated. By far, the two main ones are chlorohydroquinone (CHQ) and hydroxyhydroquinone (HHQ). None of the other chromatographic peaks corresponds to dihydroxybenzenes or benzoquinone. As the maximum areas of these peaks are at least 10 times smaller than that of the CHQ peak, we did not attempt to identify them further. Formation of CHQ and presumably of HHQ has also been found upon illumination of an aqueous solution of 3CP in which a sample of ZnO was dispersed (22). CHQ could result from the attack of an OH' radical at the para position with respect to the OH group of 3CP, as shown in Scheme I. An attack at the ortho position is less probable. Under the same conditions, CHQ is less stable than 3CP and even decomposes in the dark. Within ca. 60 min, 3.25 pmol of CHQ in 20 cm3 disappears almost completely (Figure 8), whereas the elimination of 3.1 pmol of 3CP in 20 cm3 requires ca. 110 min (Figure 4). The weaker polarity of CHQ is in favor of an easier attack by OH' radicals. The main product of the degradation of CHQ is by far HHQ, so that the sequence 3CP CHQ HHQ can be proposed. This implies that the C-C1 cleavage occurs in the second step only. Figure 8 shows that HHQ is still more rapidly degraded than CHQ; no intermediate was detected by HPLC under our conditions. By contrast, direct photolysis of 3CP at X > 290 nm and pH < pK, produces 1,3-dihydroxybenzene(resorcinol, Rs). The cleavage of the C-Cl bond takes place because of the polarization of this bond in the excited state of 3CP (23) (Scheme 11). Figure 9 shows that the maximum concentration in RS formed by direct photolysis is greater than the total of the maximum concentrations in CHQ and HHQ obtained during the photocatalytic degradation of 3CP at X > 340 nm. Furthermore, RS remains after the
- -
Environ. Sci. Technol., Voi. 24, No. 7, 1990 903
Scheme 111 OH +OH
\
A /
HHO+CHO
c.5
t
w l t h T102,I L 340 nm
60
Y OH
a
240
180
120
300
t I m I/m 1"
Figure 9. Photocatalytic (A > 340 nm) or photochemical (A > 290 nm) degradation of 3CP in photoreactor P2. Variations in the amounts of the main intermediates (starting amount of 3CP, 3.1 pmol).
Scheme I1 OH
OH
OH
OH
A
6*
disappearance of 3CP. These comparisons further demonstrate the greater efficiency of the photocatalytic degradation of 3CP with respect to direct photolysis, despite the reduced radiant flux employed. Moreover the slower disappearance of 3CP in the presence of Ti02 at X > 290 nm than at X > 340 nm, although the number of photons potentially absorbable by the semiconductor is increased by a factor ca. 1.8, can be explained by the simultaneous occurrence of photocatalytic and photochemical processes. Indeed, less CHQ is formed at X > 290 nm than at X > 340 nm, while RS was also detected (however [RS]
