Environ. Sci. Technol. 1993, 27, 1790-1795
Photocatalytic Degradation of Monuron in Aqueous Ti02 Dispersions Edmondo Pramauro' and Marco Vincent1 Dipartimento di Chimica Analitica, Universith di Torino, 10125 Torino, Italy
Vlncenzo Augugliaro and Leonard0 Palmisano Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universith di Palermo, 90128 Palermo, Italy
Reagents and Materials. High-purity Monuron was purchased from the laboratory of Dr. Ehrenstorfer (Ger-
many). Standards of the following compounds (from Aldrich) were used: 2-chlorophenol, 4-chlorophenol,4-chloroaniline, 1,4-dihydroxybenzene,1,2,3-trihydroxybenzene, 4-chlorophenyl isocyanate, and 1,2,4-trihydroxybenzene, 5-chloro-2-benzoxazolone.All solvents used were chromatographic grade (Lichrosolv, Merck). Doubly distilled water was filtered through 0.45-pm cellulose acetate membranes (HA, Millipore) before use. Stock solutions of Monuron (50-100 ppm) were prepared by dissolving the proper amount of the herbicide in water. These solutions were protected from light and stored at 5 "C. Ti02 P25 Degussa was used in most photodegradation experiments. The powder was preliminarly irradiated and washed in order to eliminate the adsorbed inorganic ions (17). Other semiconductor oxides were also employed for comparison purposes: SnOz 99% (BDH),ZnO 99% (Carlo Erba), and W03 99.7% (Ventron). Surface Area Determinations. Surface area measurements of the powders were performed by applying the dynamic BET method, using a Flowsorb 2300 apparatus (Micrometrics) and Nz as the adsorbate. The determined surface area values (expressed in m2g')were the following: TiOz, 44; ZnO, 30; WOa, 4; and SnOz, 8. Irradiation Experiments. Irradiations were performed in cylindrical Pyrex glass cells (40 mm i.d. x 25 mm high) (17) on 5 mL of aqueous suspensions containing the proper amount of dissolved Monuron (typically 10100 mg L-l) and the Ti02 powder, using a 1500-W Xenon lamp (Solarbox,from CO.FO.MEGRA, Milan) equipped with a 310-nm cutoff filter and simulating the AM1 solar light. Under these conditions the radiant power is ca. 60 mWJcrn2. The experiments were carried out under aerobic conditions, and the temperature within the Solarbox was 60 "C. Analytical Determinations. Concentration of unreacted Monuron was determined as follows: the cell irradiation was stopped at fixed times, then the dispersion was filtered through 0.45-pm cellulose membranes (HA, Millipore), and the filtrate was analyzed by HPLC. Usually 100pL of the filtered sample was injected into the chromatograph, composed by a L 6200 pump (Hitachi), a L 4200 UV detector, and a RP-Cla 5 pm (Lichrospher, Merck) 250 mm X 4 mm i.d. column, and isocratic elution with acetonitrile/water (35:65 v/v) at 1 mL min-l was performed. The Monuron absorption was monitored at 244 nm, and its measured retention time was ca. 5 min. C02 evolution during irradiation was followed by headspace gas chromatography according to a previously described procedure ( I & , using a Carlo Erba 4600 GC apparatus equipped with a Hayesep Q SO/lOO mesh (2 m x 6 mm id.) packed column and a TCD detector. The following working conditions were employed: flow rate, 30 mL min-l (He carrier); detector temperature, 150 "c (block) and 250 "C (filament); injector temperature, 120 "C; column temperature, 110 "C. Blanks obtained after
1790 Environ. Sci. Technol., Voi. 27, No. 9, 1993
0013-936X/93/0927-1790$04.00/0
The light-induced degradation of Monuron [ 3-(4-chloropheny1)-1,l-dimethylurea]under simulated solar irradiation has been investigated in aqueous solutions containing Ti02 suspensions as photocatalysts. The herbicide (present at ppm level) was mineralized to COZ and C1-, whereas oxidation of nitrogen to NOS-occurs more slowly, via the intermediate formation of "3. The primary degradation of the pollutant follows a pseudo-first-order kinetics. Several reaction intermediates were identified using HPLC and GC-MS techniques. The formation and fate of some of these compounds under irradiation were also investigated. Together with the expected formation of several hydroxyaromatic derivatives, the unexpected hydrophobic compound 4-chlorophenyl isocyanate was recognized as a major reaction intermediate. From the analytical data, a reaction scheme was proposed. Introduction
In recent years increasing attention has been paid to photocatalytic reactions occurring on semiconductor particles suspended in aqueous and nonaqueous solutions (13). In particular, the efficacy of such processes for the complete oxidation of organic pollutants contaminating aqueous media has been demonstrated by several authors (4-8). Among the semiconductors used, titanium dioxide is particularly efficient since the formation of electronhole pairs under illumination with sunlight is promoted by the near-UV component of this radiation, which encompasses energies higher than the corresponding band gap. The successive steps involve the oxidation of organics through reactions with the holes or with the radicals coming from the solvent, from the catalyst surface, and from adsorbed oxygen (9-15). In this work, the photocatalytic degradation of Monuron (1) over Ti02 suspensions was investigated.
1
This widely used herbicide is an inhibitor of photosynthesis and is recommended for the total weed control of noncrop areas. Since Monuron is subject to a quite slow transformation in moist soils, with a mean persistence in the environment of about 10 months (16),there are relevant risks of its leaching into groundwater. Therefore, the investigation of viable remediation treatments of polluted waters containing trace amounts of Monuron is of environmental interest. Experimental Section
0 1993 American Chemical Society
long-term irradiation experiments indicated a very low C02 contribution (less than 0.6 mM), which was used to correct the obtained data. The formation of chloride and nitrate ions was followed by suppressed ion chromatography, using a Biotronik IC 5000 apparatus equipped with a 100 mm long X 4 mm i.d. BTlAN column (Biotronik). The eluent was a mixture of NazC03 (1mM) and NaHC03 (2 mM), at a flow rate of 1.5 mL min-1. Blank experiments were performed by irradiating an aqueous suspension of the catalyst, in order to evaluate the amount of chloride ion leached from the semiconductor. This contribution was negligible under the working conditions. The presence of ammonium ions in the irradiated solution was detected spectrophotometrically, using the Nessler reagent (19). The absorbance of the reaction product was measured at 400 nm against a blank. The calibration curve, prepared using NH4C1, was linear in the NH4+ concentration range of 0.3-10 mg L-l. GC-MS experiments were run on a Finnigan-MAT 95Q double-focusing reverse geometry mass spectrometer interfaced to a Varian 3400 gas chromatograph. Samples of irradiated solutions (5 mL) were filtered through a 0.45pm cellulose filter and extracted with chloroform (3 mL). The organic phase was placed in 0.3-mL vials with glass inner cone (Supelco) and successively concentrated up to 20-fold by a stream of ultrapure nitrogen gas. One microliter of these extracts was injected splitless (30 s) into a 25 m X 0.22 mm bonded-phase BP5 column with a film thickness of 0.25 pm (S.G.E., Ringwood, Australia). The GC oven temperature was programmed as follows: isothermal at 50 "C for 3 min; 50 to 300 "C at 10 "C min-l; isothermal at 300 "C for 10 min. Injector and transfer line temperatures were kept at 260 "C. Electron impact (EI) mass spectra were obtained at 70 eV electron energy with the ion-source temperature at 220 "C. The magnetic analyzer was scanned at 0.8 s decade-' from 29 to 350 amu, and the resolution was 1000 (10% valley). The background-subtracted mass spectra were matched against those of authentic standards (when available) or NIST mass spectra library and interpreted on the basis of the observed fragmentation. In order to identify the possible impurities of the reagents, a sample was prepared from the nonirradiated starting material and successively analyzed by GC-MS under identical conditions as for irradiated samples. Experiments in desorption chemical ionization mass spectrometry (DCI-MS) were conducted by depositing 3-4 drops of a collected HPLC fraction on a rhenium wire and allowing the solvent to evaporate under vacuum. The wire was subsequently introduced in the center of the mass spectrometer ion-source and was rapidly evaporated (the wire is heated at a rate of 40 "C s-l) under a relatively high pressure (0.5mBar) of isobutane as CI reagent gas. Details on DCI-MS experimental procedures are reported elsewhere (20). Results and Discussion
Primary Degradation. Irradiation of air-equilibrated aqueous suspensions containing the semiconductor and Monuron withAM1 simulated sunlight (A > 310 nm) leads to the disappearance of the herbicide. Starting from solutions containing 10-20 mg L-1 of substrate and 100 mg L-l of dispersed catalyst, about 30-40 min are necessary
TiO,
0 1 0
n
n I
1
10
20
-
1
-
1
30
40
I
I
50
60
Irradiation time ( m i d Flgure 1. Plots illustrating the photodegradation of Monuron (20 mg L-1) at pH 5.5 as a function of irradiation time for some semiconductor oxides (100 mg L-').
for complete disappearancewhen working with TiO2, while longer irradiation times are required using other semiconductor oxides (see Figure 1). These results are in agreement with previous findings concerning the degradation rate of other organic pollutants (21, 22). Commercial Ti02 (rutile) specimens were not considered since they are practically inactive (23). From these observations and taking into account photostability, low cost and constant crystalline composition of anatase-based Ti02 powders, this catalyst was used throughout the work. Degradation of Monuron does not proceed significantly under direct irradiation (without semiconductor)or in the dark (in the presence of semiconductor suspension). In fact, we could demonstrate that, after about 40 min of irradiation, direct photolysis contributed less than 4 % to the degradation process, whereas after 40 min in the dark, we found that only 0.5% of the initial Monuron was decomposed. Thus, hydrolytic processes, which are predominant in the natural degradation of phenylureas (24), at pH 5.5 and under the experimental conditions adopted, are negligible. SinceTiO2 exhibits an amphoteric character with a zero charge in the pH range around 6 (251,working at pH 5.5 minimizes electrostatic effects between the semiconductor particles or between these particles and ionic substrates. The experiments were performed at constant Ti02 concentration (100 mg L-1). t Figure 2 reports typical results of photodegradation runs performed at different initial Monuron concentrations, which obey pseudo-first-order kinetics: is the herbicide concentration, and kobs is the observed pseudo-first-order rate constant. The decrease of kobs by increasing the pesticide concentration can be explained by assuming competition between intermediates and substrate for the semiconductor active sites (26-28). The kobs values show an inverse dependence on the initial Monuron concentrationand can be fitted to the LangmuirHinshelwood equation: 1 -=kobs
1 +-cc, '&Mon
kc
(2)
where COis the initial herbicide concentration, KMon is the adsorption equilibrium constant of Monuron, and kc is Environ. Sci. Technol., Vol. 27, No. 9, 1993 1791
2o 15
i N01-
-
NH4*
10 -
-2.5
4 0
100
200
300
400
Irradiation time ( m i d Flgure 4. Formation and evolution of nitrogen-containing products.
Experimental conditions as reported in Flgure 3.
co2
10
8 -
*-0
s -
8 0 _
6 -
6:
I
6 0
I
A
4s
4 .
0
10
20
30
40
irradiation time Iminl Flgure 3. Formation kinetics of carbon dioxide and chloride ion under simulated solar light Irradiation. Monuron, 20 mg L-I: TiOp, 100 mg L-';
pH: 5.5 The plateau region corresponds to the stoichiometric amount of final products.
the product of the second-order rate constant (k") by ORad (@Rad represents the coverage of the sites by the reactive radicals) (28). From the plots of l/k,b, vs CO(see inset in Figure 2) both kcand&,, have been estimated. By using a least-squares best-fitting procedure and taking into account the measurement uncertainties, the values k, = (5.1 f 0.6) X M s-1 and Khlon= (1.4 f 0.3) X lo5 M-1 were obtained. Analysis of Final Products. COpFormationKinetics. The formation of carbon dioxide, which confirms the complete mineralization of Monuron, reaches its stoichiometric value after about 40 min, t1/2 = 10 min (see Figure 3). No aromatic derivatives were detected by HPLC after ca. 30 min of irradiation, implying that compounds arising from ring opening (not absorbing at the chosen detector wavelength) can still be present in the system at that time. Chloride Formation Kinetics. Chloride ion formation is also shown in Figure 3. About 25 min of irradiation is required to obtain the stoichiometric amount of C1-, thus indicating that dechlorination of Monuron is faster than mineralization to COZ. Similar results have been previously obtained studying other monochloroaromatic compounds, such as 4-chlorophenol (29). Nitrate Formation Kinetics. Both ammonia and nitrate have been detected in different relative concentrations 1702 Envlron. Scl. Technol., Vol. 27, No. 9, 1993
starting from compounds having nitrogen-containing groups, such as primary amines or nitro derivatives (30). At long exposuretimes, conversionof ammonium to nitrate ions has been observed. Under the present experimental conditions, the quantitative recovery of nitrogen as nitrate is achieved after about 6 h of irradiation (Figure 4). Cells were opened at regular time intervals (typically every hour) to maintain a large oxygen excess. The presence of nitrite was not detected using either IC or Griess spectrophotometric methods (31). Oxidation to NO3- was found to be incomplete when working in closed cells. Ammonia Formation Kinetics. The presence of NH4+ was detected via the Nessler reaction after 50 min of irradiation, when the organic products were no more present in the solution (see Figure 4). Ammonium ion concentration significantly decreases after ca. 3 h of irradiation, with concomitant formation of nitrate. A similar sudden oxidation of NH4+ to NO3- was also observed by Low et al. (30) during the photocatalytic degradation of n-pentylamine, and it was attributed to the possible occurrence of autocatalytic reactions by the nitrate ion. The nitrogen mass balance of Monuron is initially slightly lower than stoichiometric, thus indicating that other nitrogen-containing inorganic compounds (different from nitrite) should be present in the solution. The possible formation of hydroxylamine during the ammonia photooxidation over Ti02 has been hypothesized (32,331, but it is still a subject of debate. Stoichiometry of Photodegradation Reaction. The followingoverall equation, valid after long-term irradiation and in the presence of oxygen excess, describes the mineralization process: C9H,,N,0C1
+ 13.50, 9C0,
-
Ti02 hu
+ 2NOc + C1- + 3H+ + 4H20 (3)
Analysis of IntermediateReactionProducts. HPLC Analysis. The HPLC pattern of aqueous Monuron after photooxidation showed agroup of peaks at retention times shorter than that of Monuron (Figure 5) which completely disappear after about 30 min of irradiation. Moreover, a shoulder or a little peak (2) adjacent to the Monuron peak (1)was also observed at low irradiation times.
153
100 -
w
*
c
I-
lA
c 0,
N
II
c
C
> C m
0
5 50a
/I
125
155
I-
-
90
0,
K
I
63
I1
,,/
.
I
/II
5
169 I
I
0
L I
I
I
10
5
0
78
51
I
171 113
~
I,, I l l ,
125
,I
140 I
,
,
,
,I
Retention time (min) Flgure 5. HPLC pattern of degradation intermediates after 7 min of irradiation. 1, Monuron; 2, 2-chlorophenol; 3, 1,4dihydroxybenzene (hydroquinone); 4, 1,2,34rihydroxybenzene (pyrogallol); 5, 1,2,4trihydroxybenzene; 8, 2-chlorophenyl isocyanate; 8, 4-chlorophenol. Eluent: acetonltrile/water (35.65 v/v). Detection wavelength: 244 nm. Flow rate: 1 mL min-I. AUFS: 0.02.
The chemical structure of some of these compounds was investigated by comparing their retention times with those of authentic standards: peak 2 was assigned to 2-chlorophenol, peak 3 was assigned to 1,2-dihydroxybenzene, while peaks 4 and 5 were assigned to 1,2,3trihydroxybenzene and 1,2,4-trihydroxybenzene, respectively. Since under our experimental conditions nitrate ion and 5 show the same retention times, chromatographic runs using detection at 300 nm were also performed in order to minimize the NOa- contribution to the absorption. At this wavelength, the nitrate absorbance becomes negligible (within the detector noise). Of particular interest was peak 6, having a retention time significantly higher than that of Monuron (and, consequently, a lower polarity), since the formation of nonpolar compounds is rather unusual in photocatalytic degradation of aromatics. The nature of this compound, which is less susceptible to oxidation than Monuron itself, was investigated by GC-MS. GC-MS Analysis. A set of compounds having structures related to Monuron were identified as possible degradation intermediates. Some of these compounds (6-9)were found
only in the irradiated sample, while others 10 and 11were detected both in the irradiated sample and in the blank. 4-Chlorophenyl isocyanate (6) (Figure 6a), 4-chloropheno1 (8), and 4-chloroaniline (10) were identified on the basis of coincidence of GC retention times and E1 mass spectra with those of authentic standards; 5-chloro-2imino-3,5-cyclohexadienone(9) and 4-chlorophenylformamide (11) were identified only on the basis of coincidence with library mass spectra and spectra interpretation. They are consistent with reasonable mechanisms. The mass spectrum of compound 7, identified as 4-chloro-2-benzoxazolone (Figure 6b) gave extremely high purity, fit, and retrofit scores when matched against that of 5-chloro-2-benzoxazolonein the NIST mass spectra library. The latter substance is available as an authentic standard. As expected,the mass spectrum of the 5-chloro2-benzoxazolonestandard is nearly identical with that of the unknown. Only the ion at mlz = 125 is slightly less abundant. However, since in the structure of Monuron the chlorine atom is located at position 4, and the formation of similar cyclic structures was observed in previous degradation studies (29), the unknown substance is identified as 4-chloro-2-benzoxazolone(7). Compounds 6 and 7 are the most abundant intermediates. Although compound 6 is one of the reactants used in the synthesis of Monuron, no evidence of the peak Environ. Sci. Technol., Vol. 27, No. 9, 1993 1783
eluting a t higher retention times than Monuron was found in the corresponding HPLC pattern of the starting material, even when a higher concentration of the herbicide was used. Compound 6 can also be produced by thermal degradation of Monuron in the GC injector of the GC-MS system. Thus, a significant amount of 6 is found also during the analysis of nonirradiated Monuron samples. Nevertheless, 6 has been inserted within the group of intermediatesformed only after irradiation because it was detected also in samples arising from much longer irradiation times, when the starting herbicide was no longer present. Further evidence that 6 is the most important degradation intermediate of Monuron was found in the DCI-MS analysis of the collected HPLC fraction containing the corresponding peak. In fact, the resulting mass spectrum showed a molecular ion corresponding to that of cornpound 6. Compound 7 is an abundant intermediate which might arise from 6 by an oxidative process involving the attack of 'OH radicals, followed by isomerization with transposition of a hydrogen atom and ring closure. Further evolution of 7 might proceed through hydrolysis, with COZ loss and formation of compound 9. Degradation of the latter reasonably occurs through fast kinetics, as inferred from its nonaromatic (o-quinoneimine) structure and its detection in very small abundance. Et has to be noted that compound 9 can also originatefrom the hydroxylation of 4-chloroaniline in ortho position, followed by an oxidation step. The compound 4-chlorophenol (intermediate 8) was also identified by GC-MS analysis of irradiated solutions. It may be produced by several degradation pathways, including the attack of 'OH on the C-NH bond. Its presence could not be clearly confirmed from HPLC data, where only a small peak (8) with a retention time corresponding to this compound can be observed in some cases (see Figure 5 ) . Compounds 7 and 9 could not be identified in the HPLC profile because the authentic standards were not available, whereas the polyhydroxylated intermediates (3-5) could not be confirmed by GC-MS analysis because they may not be extracted in the chloroform solvent used. 4-Chloroaniline (10) and 4-chlorophenyl formamide (1 1) were detected in both the irradiated sample and the blank, and their abundances in the two samples were similar, i.e., they are synthetic impurities of Monuron and not decomposition products. However, the hypothesis that compounds 10 and 11are intermediates ofthe degradation reaction cannot be excluded. As a matter of fact, the degradation kinetics of 4-chloroaniline is faster than that of Monuron when measured in the same experimental conditions, Working with 10 mg L-l of substrate and 100 mg E-1 of Tion,a t pH 5.5, the k&s values were 10.5 X s-1 for 4-chloroaniline and 8.1 x 10-3 s-l for Monuron, respectively. Therefore 4-chloroaniline should disappear quite soon from irradiated samples, which is not the case, unless its oxidation (asa low-concentrationcontaminant) is inhibited by competition with the bulk organics present in the system. The persistence of 4-chloroaniline in irradiated samples may indicate that it is also one of the degradation intermediates, whose concentration reaches a stationary state. 1794
Environ. Sci. Technol., Vol. 27, No. 9. 1498
I
0
I
5
10
15
I
20
30
25
Irradiation time lmin) Figure 7. Formation and degradation kinetics of some interrnediate products. The curve numbers identify the examined compounds (see text). The Monurondegradation (dashed curve) is given for comparison purposes.
Scheme I
If
Intermediate Degradation Kinetics. The kinetics of formation and decomposition of some of the degradation intermediates were followed by HPLC. In particular, the formation and fate of 4-chlorophenyl isocyanate, hydroquinone, and 1,2,4-trihydroxybenzene were investigated. Typical bell-shaped profiles were obtained (see Figure 71, with maximum concentrations reached after short irradiation times (within ca. 10 min). The kinetic behavior of the derivatives clearly confirms that these aromatic compounds undergo further transformation, their concentrations being negligible after ca. 30-35 min of irrwdiation. Nonaromatic products (arising from ring opening and successive oxidations) can still be present in the solution, but apparently only for a very short time since the COz formation becomes quantitative after ca. 40 min. Degradation Scheme. On the basis of the analytical and kinetic results, a degradation scheme can be proposed which accounts for the simultaneous attack of radical species on different points of the starting molecule (paths a, b, e, d). From abundance and persistence data concerning the identified intermediates, the sequence a-g, shown in Scheme I, is suggested as the main reaction path. Although the retention time of compound 2 was in excellent agreement with that of 2-chlorophenol, a complex sequence of events must be invoked to rationalize mechanistically the formation of this intermediate from Monuron. For this reason, the presence of 2 (indicated within brackets) is simply hypothesized. Similarly, compounds
12* and 13* are inserted as reasonable precursors of intermediates 7 and 9, respectively, but they were not
experimentally detected. Conclusions
Photocatalytic degradation of Monuron has been demonstrated in aqueous media. The complete degradation of this contaminant, initially present at concentrations of tens of milligrams per liter, is observed in less than 1 h working with moderate amounts of TiOn, under simulated sunlight irradiation. During this time substrate dechlorination is complete, whereas most nitrogen is found as ammonium ion, which is slowly oxidized to nitrate. Several intermediate derivatives were identified after low irradiation times (
