Environ. Sci. Technol. 1004, 28, 2176-2183
Kinetics and Products of the Ti02 Photocatalytic Degradation of Pyridine in Water Catherlne Malllard-Dupuy, Chantal Gulllard, Henrl Courbon, and Pierre Plchat'
-
URA au CNRS Photocatalyse, Catalyse et Environnement, Ecole Centrale de Lyon, B.P. 163, 69131 Ecully CEtdex, France
Pyridine (Pyr), a noxious chemical whose ring is part of the chemical structure of many pesticides, is more rapidly eliminated in water by photocatalysis over Ti02 than benzamide, whose nucleus has also a relatively low electron density and whose extent of adsorption is equivalent. Hydroxylation of Pyr first occurs only at position 2. Beside acetate and formate, whose formation from Pyr is derived from ion chromatography analysis, seven aliphatic intermediates with one to five C atoms are identified by GCMS. They all contain one or several C-0 groups, and wherever the N atom subsists, it is as an amide, Le., with an unchanged oxidation number. For relatively high initial Pyr concentrations (16.5 mmol/L), dipyridyl and carbamoyl pyridine isomers are also detected as intermediates, at much lower concentrations, however, than that of 2-hydroxypyridine; this shows the existence of coupling reactions. Within an UV irradiation period about 2.5 times as long as that necessary to eliminate Pyr, organic nitrogen is almost entirely mineralized, mainly into NH4+ ions that are very slowly oxidized to nitrate.
Introduction Pyridine (Pyr) is an odoriferous and toxic compound. Its maximum workplace concentration value in the air is 15 mg/m3 (I). The average lethal dose (LD 50) orally in rats is 1.58 g/kg (2). In water, the toxicity limit for Alburnus is 100 mg/L (3). Pyr gives rise to an unpleasant taste to water at an average concentration of 0.82 mg/L, but depending on the person, the concentration limit can be as low as 7 pg/L (3) while the threshold odor concentration can even be 0.3 pg/L ( 4 ) . In addition, the substituted nucleus of pyridine is present in a great number of common pesticides. Consequently, it was deemed of interest to study the destruction of Pyr in water by heterogeneous photocatalysis. Also, in the framework of our studies aiming at correlating the Ti02 photocatalytic degradability of water aromatic pollutants with their molecular structure (5-7), it was deemed pertinent to include in the list of these pollutants a heterocycle with an electron-withdrawing atom. Furthermore, the nitrogen atom was expected, on behaving as an analytical marker, to facilitate the identification of the intermediate products by gas chromatography-mass spectrometry (GC-MS). To our knowledge, the only previous study (8) about the degradation of aqueous Pyr solutions by heterogeneous photocatalysis was principally concerned with the formation of ammonium and nitrate ions. Any process that produces hydroxyl radicals should generate l-azahydroxycyclohexadienyl radicals (9),which are oxidized to hydroxypyridines (OH-Pyrs) even in the absence of an added chemical oxidant (IO). Irradiation at 254 nm of Pyr causes
* Corresponding author; Fax: (33) 78-33-03-37; Telephone: (33) 72-18-64-95. 2178
Envlron. Scl. Technol., Vol. 28, No. 12, 1994
the n T* electronic transition and, in diluted aqueous solutions, leads to the radical addition of H and OH from H2O at ring positions 1and 2, respectively. The resulting intermediate could produce by ring opening 5-amino-2,4pentadienol which cannot be isolated and would yield pentenedial (glutaconic aldehyde) and ammonia by acid hydrolysis (11).
Experimental Section Materials. Pyr and its degradation intermediates, hydroxypyridines, whose kinetics of photocatalytic disappearance were studied in control experiments, as well as ethylenediamine were purchased from Aldrich. Acetronitrile was Rathburn HPLC grade. The photocatalyst was Ti02 Degussa P-25 (mainly anatase, 50 m2/g,nonporous). Reactor, Light Source, and Procedures. The aqueous suspensions were UV-irradiated in a cylindrical flask (total volume: ca. 90 mL), open to air, with a bottom optical window whose surface was approximately 11cm2. They were magnetically stirred. The output of a Philips HPK 125-W high-pressure mercury lamp was filtered by a 2.2-cm path circulstingwater cuvette to absorb the infrared wavelengths and by a 340-nm cutoff filter (Corning 0.52) to avoid the direct photolysis of the organic pollutants. The radiant flux thus entering the reactor was 45 f 4 mW cm-2, which corresponded to about 1.4 X lo1' photons s-l potentially absorbable by TiOz. The solutions were made with deionized,doubly-distilled water. For HPLC analyses, the volume of Pyr aqueous solution (0.165mmol/L) introduced into the photoreactor was 20 mL. Therefore, the starting amount of Pyr was ca. 3.3 pmol. The minimal amount of Ti02 necessary to completely absorb the UV photons entering the reactor was 50 mg, i.e., 2.5 g L-l. The degradation was carried out at room temperature and at natural pH, Le., 6-6.5. The aerated suspension was first stirred in the dark for 60 min, which was sufficient to reach equilibrated adsorption of Pyr or hydroxypyridines, as was deduced from the steadystate concentration of these compounds. Analyses. The HPLC apparatus was a LDC/Milton Roy System, which comprised a Constametric 3000 isocratic pump and a Spectro-Monitor D UV detector set at 254 nm. A chromspher B column, 15 cm long, 4.6 mm i.d., 5 pm film thickness, was used. The mobile phase (flowrate: 1.4mL/min) wasamixtureofacetonitrile(5%), ethylenediamine (1.25 X IO6 %), and deionized, doublydistilled water. To remove Ti02 particles before HPLC analysis, the water samples were filtered through 0.45-pm Millipore disks. Although nonagglomerated particles may pass through these membranes, our experience showed the performance of the chromatographic column was not impaired over a very long period. Identification of the organic intermediate products was performed both by comparing their UV spectrum, obtained with a Varian-9065Polychrom diode-array HPLC detector and the LC Star Varian System, to those of commercial 0013-936X/94/0928-2176$04.50/0
0 1994 American Chemical Society
compounds and by GC-MS (HP 5890 and 5971 A). In the latter case, a CP,il5 CB column, 25 m long, 0.32 mm i.d., 1.2 pm film thickness, and more concentrated starting solutions of Pyr (16.5 mmol/L) were employed. After 5-h irradiation, the reaction mixture was filtered, extracted with 2 X 20 mL of dichloromethane first at the pH of the experiment and then with the same CH& quantity at pH 1, and finally concentrated with a rotary evaporator at ambient temperature. The GC-MS analysis was performed either by chemical ionization (CI) in the presence of methane as the reagent gas or by electronic impact (EI). Separations were carried out by temperature programming from 308 to 323 K at 20 K/min and then to 473 K at 5 K/min. Nitrate, ammonium, acetate and formate ions were analyzed by HPLC by use of a Waters 501 isocratic pump and a Waters 431 detector. For the anions, an IC-PAK anion-exchange column (Waters), 5 cm long, 4.6 mm i.d. was employed. Sodium borate/gluconate and lithium benzoate were used as the eluents (1mL/min) for nitrate and the carboxylates, respectively. The formation of ammonium was first revealed with Nessler reagent and was quantitatively analyzed with an IC-PAK cationexchange columm, 5 cm long, 4.6 mm i.d. The eluent was nitric acid (2.5 mmol/L; flow rate: 1.5 mL/min). Results and Discussion
Photocatalytic Disappearance of Pyridine. The adsorption equilibrium of Pyr over Ti02 in the dark was reached within 1h. Therefore, the suspension was stirred for this duration before starting UV irradiation. About 9% of the Pyr initially introduced was adsorbed, which corresponded to ca. 0.07 molecule/nm2. Considering that the surface occupied by one Pyr molecule is approximately equal to that corresponding to a benzene molecule, i.e., 0.4 nm2 (12), it may be inferred that the coverage of titanium dioxide by Pyr was quite low. Under our conditions, the initial disappearance rate of mol/s. An apparent first-order Pyr was (1.7 f 0.1) X decay was observed with a rate constant k 0.049 f 0.001 min-I. The complete disappearance was obtained within 70 f 5 min. All these values depend on the experimental conditions and, thus, have no absolute meaning. However, they can be compared to those obtained for the photocatalytic disappearance of other nitrogen-containingchemicals under the same conditions. The constant k is higher for Pyr than for benzamide (ca. 0.029 min-I), N-phenylethanamide (ca. 0.025 min-I), and nitrobenzene (0,019 min-I), which have a nitrogen atom in the substituent. Several factors can be at the origin of these differences, among which are the surface coverage by the pollutant, the electron density over the aromatic nucleus, and other structural characteristics. Correlation with the rate constants of reaction of hydroxyl radicals with these compounds in aqueous solution is not significant, unfortunately, because of the uncertainties of the literature values. The coverage of the titanium dioxide surface by Pyr or benzamide in the dark was the same within the experimental accuracy, in agreement with the close values of the logarithm of the l-octanol/water partition coefficient L0.65 for Pyr; 0.64 for benzamide (1311. Consequently, the k values for these two compounds should be affected by the structure only. The aromatic nucleus is deactivated toward electrophilic substitution by the amido group or
the built-in N atom, which in principle should have slowed down the formation of hydroxylated derivatives (see below and ref 5). However, since the disappearance rate of Pyr or benzamide likely corresponds to several degradation pathways, the reason for the difference in 12 may not arise only from differences in the overall electron density over the nucleus. In particular, it is conceivable that the deformation of the delocalized r orbitals resulting from the heteroatom in the aromatic nucleus decreases the stability and facilitates the ring opening. By contrast, the 1,3,5-triazabenzene(s-triazine)ring was found to withstand the photocatalytic treatment (14,15) even in the presence of inorganic peroxides (16);note that this latter structure is very stable with respect to any common oxidative process. Identification of Degradation Intermediates. The identification of the intermediates of Pyr photocatalytic degradation was carried out by GC-MS under the conditions indicated in the Experimental Section, Le., for a relatively high initial concentration of Pyr and only one irradiation duration; this duration was expected to correspond to comparatively high concentration of organic intermediates. Figure 1 shows a typical chromatogram; the peaks denoted by letters have been attributed as is detailed in Tables 1-3. These attributions were made by comparison with standards (Table 1)or with mass spectra reported in the literature (Table 1) or finally by interpretation of mass spectra (Tables 2 and 3). Discussion on the nature of the corresponding intermediates is presented below. Kinetic Variations of 2-hydroxy pyridine, Acetate, Formate, Ammonium and Nitrate. High-performance liquid chromatography (HPLC)analysis with appropriate detectors allowed us to monitor the kinetic variations of the subtitle compounds (Figure 2). The maximum amount of 2-hydroxypyridine (2-HOPyr) was reached within about 10 min, and the maximum conversion of Pyr to 2-HOPyr was only ca. 3.7 % This low percentage was not inconsistent with the photocatalytic degradability of 2-HOPyr (apparent first-order rate constant of disappearance: 0.065 f 0.001 min-' compared with 0.049 f 0.001 min-l for Pyr). Two high-performance ionic chromatography (HPIC) peaks were attributed to acetate or formate ions. The temporal variations of Figure 2 show (i) that these ions were formed well before the complete disappearance of Pyr (initial concentration: 0.165 mmol/L) and (ii) that they did not accumulate but were progressively destroyed; however, about 0.1 pmol of formate was still detected when the suspension had been irradiated for 8 h. Ammonium ions were also detected by HPIC. A significant amount was already formed within 10min, and a maximum of ca. 2.2 pmol (Figure 2) was reached within about 2 h. Within 1h, i.e., the duration needed to eliminate Pyr, a low amount of nitrate was also formed. Figure 2 shows that about 95 ?' 4 of organic nitrogen was mineralized within 2.5 h. A very slow transformation of ammonium into nitrate could take place (curve F, Figure 2). Discussion of Nature and Formation Pathways of Nonionic Degradation Intermediates. Hydroxylated Pyridinic Intermediates. 2-HOPyr was detected by HPLC equipped with a photodiode array, which allows one to record UV spectra for each chromatographic peak. In contrast, its isomers were not detected, as well as 2,3-, 2,4-,and 2,6- dihydroxypyridine,N-oxide-Pyr, and N-oxide-
.
Environ. Sci. Technol., Vol. 28, No. 12, 1994
2177
G
\ L K 1
I
t
F
I
J
5.00
10'.00
15'.00
20.00
25'. 00
time /min Figure 1. Typical GC-MS chromatogram of the products of the photocatalytic degradation of pyridine. The capital letters refer to the identified compounds, as indlcated in Scheme 1 and in Tables 1-3. Conditions: see Experimental Section.
Table 1. Names, Chromatographic Retention Times ( h )and Main Mass Peaks (Listed in Order of Intensity) Corresponding to Intermediates (Identified by Comparison with Authentic Compounds') of Photocatalytic Degradation of Pyridine compoundb
(A) formamide
tR
(min)
mass peaks (mlz)and relative intensitiesc
3.7
CI, none EI, 45129144128 reaction, 100165/30/26 reference, 100/53/33/78 (C) pyridine 4.5 CI, 801108/140 EI, 79152/51150 reaction, 100/73/47/45 reference 100161/3I/19 (E) 2-furfural 5.8 CI, 97/69 EI, 39196195138/29167150 reaction 100/98/96/46/41/8/5 reference 100199199/51/57111/1I (H)4-cyanopyridine 9.6 CI, 10511331145 EI, 104/77/51/50/52138126 reaction, 100146/34/34/18/1I l l 1 reference, 100158/1212419/8111 (I) maleimide 10.3 CI, 9811261138 EI, 97126154153/69144128 reaction, 100175/69/45/36128/13 reference, 100183/771421371211I (K) 2-cyanopyridine 12.7 CI, 105/1331145 EI, 104/77150151/26138 reaction, 100/45/41/39/12/11 reference, 100180177176/24/21 (M) 2-hydroxypyridine 15 < t R < 18 CI, 9611241136 EI, 67/95139141/28 reaction, 100/54/37/18115 reference, 100/62137/26/16 (P)2,2'-dipyridyI 24.9 CI, none EI, 156l1281155/51/1291781102 reaction, 100/30147/2812212017 reference, 100/47/71138133l31I8 (4)2,4'-dipyridyl 27.2 CI, 15711851197 EI, 156112915111281781102 reaction, 100/52/34125122/17 reference, 100/50/23/24/17/I4 a Except for formamide and 4-cyanopyridine whose spectra are those of ref 25. The capital letters in parentheses refer to the chromatographic peaks in Figure 1. Relative intensities were obtained by analyzing the mixture of the degradation products, some of which were at very low concentrations; therefore, they are not accurate (see the case of Pyr, the original compound). Relative intensities of peaks at mlz = 44 and 28 are prone to substantial uncertainties. Relative intensities are italic.
2HOPyr. We had no authentic sample of 2,Ei-dihydroxypyridine; however, given the chromatographic character2178
Environ. Sci. Technol., Vol. 28, No. 12, 1994
istics of the isomers, no HPLC peak could be attributed to that compound. To determine whether 3- and 4-HOPyr
Table 2. Names, Chromatographic Retention Times (h)and Main Mass Peaks (Listed in Order of Intensity) of Intermediates of Photocatalytic Pyr Degradation8 CI and E1 mass peaks (rnlz) and E1 spectrum interpretation relative intensities of E1 peaksc compoundb t R (min) 3.75
(B)butenedial
CI, 8511131125 EI, 29/55/84/27/28 100158154151I23
84
/
55
29
28
27
(D) N-formylformamide
(
>NH~:
5.4
CI, 741114 EI, 29/45/44/28173 100177152144119
)'
, (H$4--CK))",
(HN-CHO)',
45
44
(CHO)',
29
'
(CO)' 28
73
(F) 2(lH)-pyridinone
6.6
CI, 9611241156 EI, 95/39141/67129128 100178173158149139
95
67
9.1
(G) 3-formamidopropenal
CI, 100/128/140 EI, 70142171129141128199 100194192177171I62146
71
99
(J)N-formyl-3-~arbamoylpropenal~
43
11.8
CI, none EI, 55/29/98/44/84/71/26 100168149144135125125
(L) dihydroxy-2(1H)-pyridinoned
13.9
CI, 110112818411561168 EI, 55129198/44/84/71/26
(N) 2-carbamoylpyridine
19.0
CI, 1231163 EI, 79/52/51/78/122/44
100151144132131130119 100/39/35/34/24/14
122
(0)4-carbamoylpyridine
22.3
CI, 1231151 EI, 791781122/51/521106 100/56/40/38/34/22
122
106
Proposed attributions of the mass peaks. The capital letters in parentheses refer to the chromatographic peaks in Figure 1. Relative intensities of E1 peaks are italic. The interpretations of the mass spectra are discussed in Table 3.
were not detected because of their photocatalytic instability, control experiments were carried out. They showed that both isomers were more stable than 2-HOPyr: firstorder disappearance apparent rate constant 0.065,0.049, and 0.046 min-l (accuracy f 0.001 min-1) for Pyr substituted at the 2, 3, or 4 positions, respectively. In other words, if 3- or 4-HOPyr were formed, they would have been detected since their GC or HPLC response coef-
ficients were close to those of 2-HOPyr. A possible explanation for the greater instability of 2-HOPyr with respect to its isomers might be the existence of its tautomer 2(1H)-pyridinone,which would facilitate the ring opening. Electrophilic substitutions on the Pyr ring are difficult and normally occur at the 3 position. For example, the addition of OH radicals to Pyr in aqueous solution was shown, by in situ radiolysis ESR technique, to take place Environ. Sci. Technol., Vol. 28, No. 12, 1994
2179
Table 3. Interpretations of Mass Spectra of (J) and (L) Compounds (Figure 1) Obtained in Photocatalytic Degradation of Pyridine Interpretation of Mass Peaks Observed for Compound ( J ) O
I I
(CH=CH-CO-NH)
I
(R)
(CH=CH --CONH2)'70
*+ 69 (RS)
t
(CHO)' 29
A
I
OHC-CH=CH)+ (CO-NHz)*
t
(OHC-CH
(CH=CH -CO-NH+CHO)+
98
5s
44
=CH +CO-NH~
9 '
(CO) '+ 23
I
3
I
0w-w
OHC+cH =cH)+ 55 ( K f - N H -CHO)'72
ca
=cH+co+
(HNCOH)' 44
I
I
(OHC)' 29
(CH=CH)+ 26 (CO)' 28
I (OHC-CH
I
OHc-cH
=CH+O-98
(CHO)' 29
I
I
=CH)+ 55
(co) *+ 28
OHC-CH
=CH)'55
(CO-NH)
'43
(NHCHO)+44 (CONH) .t 44
Existence of Fragment at m/z = 84 Could Be Accounted for As Shown
,
0'
127
84
Interpretation of Mass Peaks of Compound (L)b E1 detection Wz99
-(:,,,,
,
A
HO LLIOH
)
\
'+
,
WZM
Wz71 Wz55
w.44
---)
bss d -NH-CO-:
-(
(
m+
..BOH)
from Nz=98, IOSS d HCN from wz=99, lossof co fm WZ=84, loss d CHO Nio =H c\+)
CI detection WZllO,l~dH20
WZIIZ,IOSS~NH~ Wz 82, from w z = 110, loss d CO Wz84,lrom Wz= 112, loss d C 0
Eluted at 11.8 min and tentatively suggested to be N-formyl-3-carbamoylpropenal.Eluted at 13.9 min and tentatively suggested to be dihydroxy2(1H)-pyridone. However, since an aromatic ring has an intense molecular peak, which was not observed for this compound, this attribution remains tentative. a
2180
Envlron. Scl. Technol., Vol. 28, No. 12, 1994
I
3b
4 2
!i 1
0 0
30
60
120
90
150
180
T I M E (MIN) Figure 2. Kinetics of the disappearance of pyridine (A) and of the resulting appearance/disappearance of 2-hydroxypyridlne (B), formate (C), and acetate (D) as well as of the resulting formation of ammonium (E) and nitrate (F). Note that the ordinate scale is enhanced 10 times for curve B. Conditions: see Experimental Section.
at positions 5 and, above all, 3 in a percentage 280% (9). The present result may be explained by assuming that some Pyr molecules are bound to the Ti02 surface sites by the electron lone pair of the N atom; that adsorption mode would favor substitution at the 2 position nearer to the photocatalyst surface. Existence of such bonds was demonstrated for partly dehydroxylated titanium dioxide, which was exposed to Pyr vapor (17). I t would explain why Pyr is so far the only pollutant for which only one hydroxylated derivative has been detected. Monochlorophenols (18, 19), dimethoxybenzenes (20), benzamide (21, 22), N-phenylethanamide (7), and nitrobenzene (5) were all hydroxylated at several positions of the aromatic nucleus, under the same degradation conditions. However, there is no direct evidence that Lewis acid sites exist at the surface of thermally unpretreated titanium dioxide immersed in liquid water. Finally, it is worth noting that the formation of 2-HOPyr was corroborated by GC-MS analysis (Table 1) . Also, another mass spectrum was assumed to originate from a dihydroxylated derivative of 2( 1H)-pyridinone(Table 3); however, this attribution was less certain. Aliphatic Intermediates. Only one aliphatic compound, which still contained five C atoms and one N atom was identified-however with some uncertainty-by GC-MS analysis of the UV-irradiated solution. That compound was N-formyl-3-carbamoylpropenal (FCP; Table 3). Its formation could result from the opening of the ring of 2,3-dihydroxy-6(lH)-pyridinone (Scheme 1). Accordingly, that allows one to tentatively specify the positions of the hydroxy groups over the nucleus of the (1H)-pyridinone detected (see preceding paragraph). As is indicated in Scheme 1, FCP would yield 3-carbamoylpropenoic acid (no GC peak could be attributed to the corresponding aldehyde) by loss of one C atom; in fact, maleimide was identified and assumed to be produced by dehydration of this acid during the GC-MS analysis, since authentic 3-carbamoylpropenal was detected as maleimide. Butenedial and formamide could be the two moieties of another splitting of FCP. Finally, N-formylformamide could also derive from FCP (Scheme l), although no GC peak has been assigned to the other expected moiety.
Perhaps because the N atom behaves as an analytical label facilitating the identifications based on the mass spectra, 2-furfural was the only compound detected by GC-MS that did not contain nitrogen. I t was presumably formed by dehydration of 2-hydroxypentenedial during the analysis. However, pentenedial was not detected, and the HPLC analysis also failed to show the presence of the corresponding monocarboxylic acid (glutaconic acid) despite a detection limit of ca. 8 pmol/L. As mentioned in the Introduction, pentenedial is formed by direct photolysis at 254 nm of Pyr in liquid water (11). Although the irradiation was limited to wavelengths >340 nm in our experiments, direct excitation of Pyr cannot be completely excluded and might perhaps explain the formation of 2-hydroxypentenedial which, alternatively, could be due to the photocatalytic process. Note that all these aliphatic intermediates contained one or several C=O groups and that nitrogen was included in an amide functionality, i.e., its formal oxidation number was unchanged. However, the formation of amido groups or ammonium ions implied that hydrogen atoms were added to nitrogen by the photocatalytic process. Similar addition to a carbon atom was also shown by the formation of acetate. Intermediates Formed by Coupling Reactions. GCMS analysis of the Ti02 aqueous suspension, containing initially 16.5 mmol/L Pyr, which was UV-irradiated for 5 h, revealed the presence of 2,2’- and 2,4’-dipyridyl. The usual preparation of these compounds from Pyr implies the intermediate formation of the pyridinyl radical owing to either a dehydrogenating catalyst such as Raney nickel for the 2,2’-isomer or sodium for the other isomer (23).In the absence of a group VI11 metal deposited on TiO2, the abstraction of a hydrogen atom from Pyr can only be a minor phenomenon (24). Furthermore, the coupling of the radicals thus formed was favored by the relatively high Pyr concentration which was used, and it would be restricted for waters containing the concentrations usually observed in the environment. In a control experiment, we verified that the concentration of each pyridyl isomer was below the HPLC detection limit of 1 mg/L for a Pyr initial concentration of 200 mg/L (2.53 mmol/L) whatever the irradiation time. Envlron. Sci. Technol., Vol. 28, No. 12, 1994
2181
Scheme 1. Various Products of Photocatalytic Degradation of Pyridine and Suggested Pathwaysa
HCONH,
Y I
f
[QJ I
i I I
I
f
t (L)'
QH
I
i I
f o
OH
Ho
I I
I
I I
I I
rc7
"I
HO-CH=CH-CH=CH-C
