Environ. Sci. Technol. 1989,23,1380-1385
Photocatalytic Degradation of Nonylphenol Ethoxylated Surfactants Etio Peiitzettl
Istituto di Chimica Fisica Applicata, Unhrersit2 di Parma, 43100 Parma, Italy Claudio Minero and VaRer Maurino
Dipartimento di Chimica Analitica, Universita di Torino,
10125
Torino, Italy
Antonino Sciafani
Dipartimento di Ingegneria Chimica, dei Processi e dei Materiali, Universita di Palermo, 90128 Palermo, Italy Hisao Hidaka
Department of Chemistry, Meisei University, 2-1-1 Hodokubo, Tokyo
191, Japan
Nlck Serpone
Department of Chemistry, Concordia University, Montreal
H3G 1M8, Canada
The photocatalytic degradation of p-alkylphenols and the nonionic nonylphenol ethoxylate surfactants has been investigated by using Ti02particulates as photocatalyst. Degradation processes have been monitored through liquid chromatography, C02 evolution, dissolved organic carbon (DOC), and particulate organic carbon (POC) measurements. Complete conversion to C02 has been demonstrated. The degradation routes involve, in the early part of the process, a competitive attack of hydroxyl radicals on the ethoxy chain and on the aromatic ring, which can be rationalized in the framework of the kinetic model herein proposed. 4
Introduction Surface active agents such as nonionic and anionic surfactants reach domestic and industrial wastewater in increasing amounts (1). The biodegradability of these substances is one of the more important constraints in their use in detergent formulations (2). The metabolic behavior of the nonionic surfactants of the alkylphenol polyethoxylate type is dependent on the conditions of wastewater or sludge treatment (3) and often leads to formation of more persistent and even more toxic metabolites. In fact, experiments have shown that the biodegradation of nonylphenol polyethoxylates (NPEn) leads to the formation of intermediates with one and two oxyethylene groups (4) as well as the completely deethoxylated nonylphenol (5). Recently, (nony1phenoxy)carboxylicacids were also identified (6, 7). To the extent that nonylphenol (NP) is a persistent (8) and toxic (9) substance (toxicity tests with Daphnia mugnu have shown similar effective levels for NP and cadmium), it was of interest to investigate the photocatalytic degradation of NPE-n. In a earlier note (IO), the behavior of NPE-n (with n = 6,7,17,50) in the presence of irradiated TiOa was followed through surface activity variation, UV spectrophotometry, and NMR. To achieve a more thorough and exhaustive picture of the photocatalytic degradation process, this paper reports our recent investigations on the fate of alkylphenols (unsubstituted phenol, 4-n-propylphenol, 4-nonylphenol) and of NPE-n (n = 2,5,12), when exposed to irradiated Ti02 particulate. The time evolution of the degradation products was followed by liquid chromatography (HPLC), COz evolution, and total organic carbon (TOC) measurements. Experimental Section Materials and Reagents. Polyethoxylated 4-nonylphenol with an average number of 2, 5, and 12 ethoxy 1380 Environ. Sci. Technoi., Vol. 23, No. 1 1 , 1989
units, Igepal CO-210, CO-520, and CO-720, respectively, were obtained from Aldrich-Chemie GmbH and were used as received. Igepal CO-210 and CO-520 are barely soluble in water, and stock solutions were prepared by weight in hexane. A stock solution of Igepal CO-720 was prepared just prior to use in doubly distilled filtered water. The stock solutions of phenol (Carlo Erba RPE), 4-npropylphenol (97%, Fluka AG), and nonylphenol were prepared by weight in filtered doubly distilled water, ethyl acetate, and hexane, respectively. Methanol (Lichrosolv, Merck), 2-propanol, isooctane, dichloromethane, hexane, and ethyl acetate (Pro Analysi, Merck) were used as received. Doubly distilled water was filtered over a 0.45-pm cellulose acetate membrane (type HA, Millipore). Titania, Ti02 P25 (BET area ca. 55 m2/g) Degussa AG (Frankfurt FRG), was utilized in all photodegradation experiments. Reagents used in the actinometric determinations were as described in ref 11. All reagents and solvents used were of analytical grade quality or better. Irradiation Experiments. The irradiations were carried out in 40 mm i.d. X 25 mm high Pyrex glass cells using a 1500-W xenon lamp equipped with a 340-nm cutoff filter (Solarbox, CO.FO.MEGRA, Milan Italy), simulating AM1 solar light. The cylindrical irradiation cell has two parallel plane faces and a lateral open-top screw cap with Teflon-faced silicone rubber septum. For aqueous soluble organic compounds the required amount of semiconductor powder was added to 5 mL of a previously prepared aqueous solution of the substrate. Compounds insoluble or barely soluble in aqueous solvents were deposited on the semiconductor powder by addition, to a weighed amount of semiconductor, of the required volume of the stock solution of the substrate in hexane. This waa followed by elimination of the solvent in a rotary evaporator under a N2 stream and subsequently by suspending the resulting powder in 5 mL of water. The sample slurry was magnetically stirred during irradiation. The total photonic flux in the irradiation cell, as measured by ferrioxalate actinometry and corrected by the fraction of light absorbed and the quantum yield of the actinometer in the range 340-546 nm (11),was found to einstein min-'. be 5.8 X Analytical Determinations. The HPLC determinations were carried out on a Hitachi Model L6200 pump with UV detection (Model L4200) at 277 nm. The procedure employed in the determination of the NPE-n was slightly different from that reported (12). Oligomers with different numbers of ethoxy units were separated by normal-phase HPLC. A bonded-phase diolic column
0013-936X/89/0923-1380$01.50/0
0 1989 American Chemical Society
(Lichrospher DIOL-100 Merck, 250 mm X 4 mm i.d., 10pm packing) and a two-step gradient elution were used. The mobile phase was as follows: eluent A, isooctane/2propanol/methanol, 94.5/5/0.5 in percent volume; eluent B, 2-propanol/methanol, 80/20 in percent volume; gradient from 100% A to 80% A over 15 min and then to 50% A over 10 min. The use of a ternary mixture was proved essential. Elimination of methanol gives poorly shaped chromatograms and poor selectivity. The use of 2-propanol is necessary to solubilize methanol in isooctane. A small amount of methanol in eluent A was required to avoid the socalled "solvent demixing" (13). The separation achieved was similar to that reported in the literature for a bonded-phase aminosilica column (12). Nonylphenol was analyzed by the same procedure. In order to obviate direct injection of an aqueous sample onto a direct-phase HPLC column, the irradiated samples were extracted with three aliquots of 5 mL of dichloromethane, after salting out with solid NaCl(100 g L-l). The extraction solvent was eliminated by purging the solution with a Nz stream, following which the residue was redissolved in eluent A for subsequent analysis. The determination of phenol and 4-n-propylphenol was performed by use of reverse-phase HPLC with isocratic elution and direct injection of the filtered irradiated suspension. A bonded-phase octadecylsilica column (Bondapak C18Waters, 250 mm X 4 mm i.d., 10-pm packing) was used. The eluent was a mixture of methanol and water (50/50 for phenol and 75/25 for 4-n-propylphenol, respectively). Determination of Carbon Dioxide and Total Organic Carbon. The C02evolution during irradiation was followed by gas chromatography. Carlo Erba Model 4600 gas chromatograph equipped with a Hayesep Q 80/100 mesh, 2 m X 6 mm i.d. packed column and a thermal conductivity detector was used. The analytical conditions were as follows: carrier gas He, 30 mL/min; column temperature, 110 "C; injector temperature, 120 "C; detector block temperature, 150 "C, filament temperature, 250 "C. The volumes of the cells were determined accurately to ascertain the headspace volumes. After irradiation, 0.5 mL of 1M sulfuric acid was added to the sample through the rubber septum via a syringe. The cells were thermostated at 25 "C over 30 min before headspace gas chromatographic analyses. A solution of Na2C03, prepared by weight in freshly doubly distilled water, was used as the analytical standard. Aliquots of 5 mL of the proper dilutions were put in the irradiation cells and subsequently treated and analyzed by the procedure followed for the irradiated samples. Typical values for the blanks after 3 h of irradiation ranged from 0.6 to 0.7 mmol L-l of equivalent COz. All the results reported herein are corrected for this contribution. Analysis of particulate organic carbon (POC) and dissolved organic carbon (DOC) were performed after filtering irradiated samples through a 0.45-pm cellulose acetate membrane (type HA, Millipore). The solids were desiccated over silica gel and analyzed with a Carlo Erba Elemental Analyzer, Model 1106. The determinations on liquids were carried out with a Carlo Erba Model 480 Total Carbon Monitor. Results
Data reported in Figure 1,which depicts COz evolution as a function of time for the three different NPEs, indicate
g .4 n
d .2 0 0
1
2 TIE(hours)
3
Flgure 1. COz (0) evolution and DOC (0)and POC (M) variation as a function of irradiation time for Igepal CO-720. Unlts are relative to the stoichiometric value: for COP,expected from eq 1; for DOC and POC, relatlve to total carbon. Inset: COPevolution as a function of irradiation time for NPEs. Initial concentratlons (a) Igepal CO-720,2 X lo4 M; (b) Igepal CO-5202.1 X lo4 M; and (c) Igepal CO-210, 2.1 X lo4 M. COP evolved is relative to the stoichiometric value calculated by using eq 1; [TiO,] = 2 g L-'.
that the complete mineralization of the surfactants, represented stoichiometrically by eq 1 in the case of Igepal CgH1gCGH,O(CHzCHz0)5H + 3302 25COz+ 22H30 (1) CO-520, is achieved (>95%) in a relatively short time (from 1 to 2 h, in the reported experimental conditions). As far as the dependence on the number of ethoxy units is concerned, the COzmeasurements indicate that the time required for evolution of half of the expected C02 is very similar for Igepal CO-520 and -720 (ca. 20-25 min), whereas a slightly longer time (35 min) is required for Igepal CO-210. Figure 1 also shows the balance of the carbon content through the measurement of DOC and POC, along with the quantity of COz evolved. The photocatalytic degradation initially results in reaction products of lower solubility (decrease of DOC and corresponding increase in POC). Figure 2 shows the variation in the chromatograms of Igepal CO-720 at different irradiation timw in the presence of TiOz particles. It is significant that no substantial changes of the substrate concentration are observed after 2 h of illumination in the absence of Ti02 or over the particulate in the dark. The commercial products herein investigated are mixtures of differently ethoxylated derivatives. It is evident from Figure 2 that, as far as the degradative route for the ethoxylated chain is involved, the oxidation pathway consists of random attacks by OH' radicals, subsequently leading to formation of compounds possessing fewer numbers of ethoxy units. The generation of compounds with no surface activity, such as (nonylphenoxy)ethanol, was evidenced previously by the lowering of surface activity (IO)and in the present case by the increase of the POC. Chromatogram D of Figure 2 supports this hypothesis, in as much as the two peaks growing after the oligomers of Igepal CO-720 have virtually disappeared arise from more hydrophilic derivatives of (nonylphenoxy)ethanol,probably hydroxylated derivatives at the aromatic ring. Figure 3 reports the variation of the peak area corresponding to the commercial mixture of differently ethoxylated derivatives (CO-720) as a function of the irradiation time. The concentration of the compounds with lower numbers of ethoxy (EO) groups shows a maximum Environ. Sci. Technol., Vol. 23, No. 11, 1989
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Figure 2. Chromatograms before (A) and after irradiation of 10 min (B), 30 min (C), and 70 min (D) for Igepal CO-720 (6.0 X IO-‘ M). [TiO,] = 2 g L-‘. Numbers over each peak represent the number of oxyethylene units. For (a) and (b) in part D, see text.
as a function of time; the same behavior was observed in the case of Igepal CO-520; for Igepal CO-210, however, an immediate decrease is observed for each ethoxylated species, supporting the view that other competitive degradation routes are also operating. In fact, the variations 1382 Environ. Sci. Technol., Vol. 23,No. 11, 1989
(
40 mlnutesl
Flgwe 3. Variation of the peak area for Igepal CO-720 as a function of the irradiation time. Peak areas are normalized to n = 10. The numbers on the different lines correspond to the number of ethoxy units. [TiO,] = 2 g L-’: Igepal CO-720, 6.0 X lo4 M.
in the UV spectra at 230 nm indicated that, in addition to the ethoxylate chain cleavage, the OH’ radicals do attack the benzene ring. The cumulative peak areas reported in Figure 4, which are proportional to concentration, show that there is an “induction period” during which the aromatic ring is not cleaved. From a kinetic standpoint, this period may result from a competitive attack of OH’ on the ethoxylated chains, or from some consecutive reactions on the aromatic part of the substrate, which lead to the ring opening. NMR evidence of ethylene glycol and acetic acid formation after long irradiation periods, as well as the increase in proton content of the solution, suggests that a series of lower molecular weight products are formed during the photocatalytic degradation. Discussion An ethoxylated alkylphenol possesses three principal units, which can be oxidatively attacked by OH’ radicals,
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Flgure 0. Kinetic scheme used In deriving eq 2 and 3. Horizontal atrows indicate hydroxyl attack on the aromatic rhg and vertical arrows attack on the poiyoxyethyienated chains. 0
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IRRADIATION TIME (minutes) Figure 4. Peak cumulative area (UV detection, 277 nm.) of the indMdual olig" of Igepai CO-720 as a function of the irradiatlon time. Full circles are the experimental points: lines a-c show the calculated behavior for different values of j m (see text): (a) jmu = 10, k, = 0.02 min-l, kR = 0.20 (---) or 0.24 min-' (-): (b)jy = 4, kc = 0.02 min-I, kR = 0.12 min-I; (c)j, = 2, kc = 0.02 min- , kR = 0.12 min-'. Peak areas are normalized to the value before Irradiation.
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Flgure 5. Photodegradation routes for NPEs. Degradation of the aliphatic chain is omitted for clarity.
namely, the EO chain, the aromatic ring, and the alkyl chain. It has been reported that OH' radicals react toward polyethylene glycols with high specific rate constants (15). As far as the reactivity toward the hydrocarbon moiety is concerned, it has been found that OH' addition to an aromatic ring is generally several times faster than hydrogen abstraction. Although OH' radicals are not very selective, pulse radiolysis studies showed that less than 10% of OH' radicals abstract hydrogen from methylated benzenes (16,17), methoxylated benzenes (I@, and phenylacetic acid (19,20),whereas the remaining OH' radicals add to the aromatic ring. Preferential hydroxyl adduct formation at the aromatic moiety has been proposed in the pulse radiolysis experiments in aqueous solutions of polystyrene sulfonate (21). The attack on the alkyl chain represents an occurring and competitive route [see NMR experiments (10)and COz evolution after long irradiation times], but for the analysis of the intermediate evolution, only the other two routes will be taken into account. In fact, data in Figure 3 refer to HPLC separations based on the partitioning of the ethoxylated chain, being monitored through UV response of the benzene ring. Then each peak corresponds to the convolution of the components bearing the same number of EO units attached to the benzene ring, irrespective of the alkyl chain length. Figure 5 reports the degradation scheme suggested for the photocatalytic degradation of nonylphenol polyethoxylates (NPEs). In this scheme, the sequence of reactions giving rise to formation of oxidizing radicals as a consequence of interaction of light with semicondudor particles
DEGRADATION T I E (minutes) Flgure 7. Photocatalytic degradation of akylphenols as a function of irradiation time. (0)Phenol (2.1 X io4 M), A)Cn-propylphend (2.2 X lo4 M), (m)nonylphenoi(2.1 X lo4 M). [TiO,] = 2 g L-l. Inset: COPevolution for phenol (O),Cn-propyl-phenol (A)and nonylphenol (m)as a function of the irradiation time.
(14) has been omitted for clarity.
Consequently, the kinetic scheme reported in Figure 6, based on the reaction mechanism proposed in Figure 5, is assumed. In Figure 6, Aji is a generic RPh,,O(EO)iH molecule, where Ph,, (oxidized phenyl fragment) is the species at the jth step, which leads to the aromatic ring opening. Each species Aji may be subject to an attack on the chain in every position among the i possible sites, with a probability that each event will succeed given by k C j , leading to formation of species Ajz with z < i. In addition, a species Aji can undergo change on the aromatic ring with a specific rate constant kRji,irrespective of the aromatic ring position considered [e.g., all the three possible isomers have been found in OH' addition to benzoic acid (22)]. These specific rate constants (in min-') include the concentration of OH' radicals, and a steady OH' concentration can be assumed during the degradation process. Other possible pathways (e.g., oxygen addition to the hydroxy cyclohexadienylradical, which may likely be the first intermediate to give directly ring opening) are not considered herein, for the sake of simplicity. The main factor that can change the global rate of degradation is probably the adsorption on the semiconductor surface, where local OH' concentration is higher than in solution. Pure A 'i compounds adsorb to a different extent, but they are difhcult to prepare, and in any case, they give complex degradation kinetics like that under study here. This effect was then tested by degrading nonylphenol, p-n-propylphenol, and phenol. The resulta are illustrated in Figure 7. The photocatalytic degradation Environ. Sci. Technol., Voi. 23, No. 11, 1989 1383
Table I. Best Fit Values for the Parameters kR and kc of eq 3 (mid)
no. of steps for ring opening (jmJ 3 4 5
compd
CO-520 CO-720
k, kR
0.019 f 0.002 0.15 f 0.02
kc
0.020 f 0.002
0.020 f 0.002 0.20 f 0.03 0.020 f 0.002
kn
0.11 f 0.01
0.13 f 0.02
4
0.020 f 0.002 0.23 f 0.02 0.020 f 0.002 0.15 f 0.02
of phenol was previously investigated in detail (23). It is evident that the presence of the aliphatic chain increases the adsorption and the global rate of the process, being faster by a factor of 2-3 for strong adsorption. Under the experimental conditions of Figure 7 , phenol, p-n-propylphenol, and nonylphenol are adsorbed to 95%, respectively. The inset of Figure 7 shows the C02 evolution for these phenols, confirming the complete mineralization. By assuming that chromatographic peaks correspond to the sum of Aji, at constant i, and by taking as fitting parameters the whole set of kcji and k the curves illustxated in Figure 3, and those for other NYEs not reported here, can be fitted by solving, through numerical techniques, the system of differential equations that evolve from the kinetic scheme of Figure 6. Under the hypothesis that kcji = kc (for all i) and kR.i = kR (for all j ) , furtherly discussed and extended elsewhere (24),the general differential equation for a species Aji of Figure 6 is
The system of linear first-order differential equations (2) for all the species i and j can be solved by using Laplace transforms and standard techniques. Although the algebra is quite tedious, finally one obtains the following recursive formula (24):
where 1 < i n and 0 < w
