Environ. Sci. Techno/. 1995, 29, 2235-2242
Photocatalytic Degradation of Spectroscopic, Chromatographic, and Mass Spectrometric Studies KIM B. SHERRARD, PHILIP J . M A R R I O T T , ' R . G A R Y A M I E T , RAY COLTON,+ MALCOLM J. MCCORMICK, AND GEOFF C . S M I T H i Department of Applied Chemistry, RMIT City Campus, GPO Box 24761/: Melbourne, Australia, 3001
Secondary alcohol ethoxylate (SAE) was photocatalytically degraded using a suspension of Ti02 particles irradiated with long wavelength ultraviolet light. High starting concentrations of SAE were used (ca. 2000 mg/L), and changes in both SAE oligomer distribution and production of intermediates were monitored by using GC, GUMS, NMR, and ESMS. Initial preferential cleavage of the ethoxyl groups over HO' reactions with the aliphatic portions of the molecule is followed by cleavage at the secondary carbon. Relatively hydrophilic intermediates remained in solution, while the hydrophobic products adhered to the TiOz, causing agglomeration of the particles.
Introduction Secondary alcohol ethoxylates (SAE) are a class of nonionic surfactants widely used in detergents for a variety of industrial and domestic applications. They possess superior physical and surface active properties relative to nonyl phenol ethoxylates (NPE) and primary alcohol ethoxylates (PAE),and their lower toxicity, more rapid biodegradation, and lower metabolite toxicity relative to these latter surfactants (1)make them an increasingly favored choice. Existingwater treatment processes must adequately remove or degrade SAE before discharge or reuse; if they are not effective, alternative treatments must be investigated. Photocatalytic degradation is a promising water purification process that has been demonstrated to mineralize or degrade a multitude of compounds (2-8) including NPE (9-13) and PAE (12, 14, 15). Photoexcitation of the TiOz semiconductor produces electron-hole pairs that may then migrate to the surface of the Ti02 particles and undergo redox reactions. Hydroxyl and superoxide radicals are produced by oxidation of the appropriate ions and molecules in solution by the positive hole. The reactive radicals attack compounds present in the Ti02 slurry, and ideally mineralization is the eventual result. Most previous work * Corresponding author fax: 61-3-9639-1321. School of Chemistry, La Trobe University, Bundoora, Australia,
+
3083. CSIRO Division of Wool Technology, Belmont, Australia, 3216.
*
0013-936X/95/0929-2235$09.00/0
a 1995 American
Chemical Society
on photocatalytic degradation of nonionic surfactants has employed collective techniques such as dye extraction and W spectroscopy (11-13,15-18) to quantify the extent of degradation. If photocatalytic degradation is to be used as a future effluent treatment, the mineralization of macromolecules must occur in conjunction with smaller molecules that are often degraded more readily. The aim of this research is to photocatalytically degrade SAE at concentrations typically found in an industrial effluent, such as wool scour effluent, and to identify degradation intermediates. The general structure of SAE is CH~(CH~),-CH-(CHZ)~CH~
I
O(CH~CHZO),-H
m+n=9,10,11 x = 9 (average)
SAE, which is produced by the ethoxylation of a complex mixture of oxidized n-paraffins, presents a significant analytical challenge (19). Methods that have been reported to be successful and are relevant to this work include electrospray mass spectrometry (201, HPLC/MS (211, gas chromatography with a variety of detectors (22-24), and 'H and 13CNMR techniques (25). It is desirable to mineralize these surfactants because they are a major component of many effluents, such as wool scour effluent. The present work reports photocatalytic degradation of one component of wool scour effluent, SAE, which to our knowledge has not been reported. It specificallyaddresses identification of intermediates, using GC-FID,GUMS, NMR, and electrospraymass spectrometry (ESMS). A detailed analysis of SAE is provided to show that the starting material is a complex mixture of isomers and homologues. When this mixture was photocatalytically degraded, the spectra and chromatograms revealed intermediates and changing homologue distribution by comparison with the initial analyses. A comprehensive assay of each photodegraded sample was obtained, which enabled identification of intermediates and illustrated howthe bulkmaterialvaried as photodegradation proceeded.
Experimental Section Photocatalytic Degradation. The photoreactor and sampling system is shown in Figure 1. SAE was obtained from a commercial source. Milli-Q water (Millipore Australia) was used throughout. TiOzpowder (DegussaP25, Frankfurt, Germany) had manufacturer's specifications of a surface area of 50 k 15 m2/gand an average primary particle size of 30 nm. The 25-W U-shaped blacklight lamp ( U V S Ultraviolet Pty Ltd., Scoresby, Victoria, Australia) emitted a wavelength range of 300-400 nm with a maximum of 365 nm. Samples (100mL) were taken at regular time intervals for a total of 102 h. p-Chlorophenol (internal standard, 100 mg/L stock solution) was purchased from Merck Chemicals, AR grade. Dichloromethane was HPLC grade. The sample workup is shown in Chart 1. The sample taken after 96 h of irradiation was prepared in a different way than the other samples to reduce losses of low boiling intermediates. Gas Chromatography. Gas chromatographic analyses were performed using a Shimadzu GC-14A fitted with an AOC-14 autosampler and a flame ionization detector. A BPX5 capillary column (5% phenyl bonded phase, SGE International, Ringwood, Australia) of dimensions 12 m x
VOL. 29, NO. 9.1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
2235
overhead stirrer
2000mgiLSAE
2L Solution
--
1
-_ -
Is/LTiO2
/
FIGURE 1. Photoreactor and sampling system. CHART 1
centrifugation
1
supernatant
volumetric flask
containing 100 mL in1 std and
liquid-liquid enracted once with 10 mL CH2C12 5 mL NaCl (saturated, lab reagent), 5 mL HCI (ZM. lab reagent)
I
orgaiuc layer
I
supernatant
1
'r: e a r n e d once with I 5 mL
organic layer
&idover MgS04,
filtered, reduced to dryness under vacuum
dmolved in CDCI3 (0 7 mL)
filtered. reduced to dryness at mom Lssolved in
reduced to *ness at rwm temp d~ssolvedin carrier phase
ESMS Analysis
liquid-liquid earacted twice mthl5mL
ramping at 20 "C/min to 190 "C, with a second temperature ramp from 190 to 340 "C at 2 "Clmin. The aqueous samples were injected directly. Organic components present in the agglomerated TiOz sample were extracted into CHzClzand injected and analyzed under the same conditions as the aqueous samples. Gas Chromatography/Mass Spectrometry. Analysis was performed using a Kratos MS25RF mass spectrometer with a Carlo Erba 5160 Mega Series gas chromatograph. A BP1 capillary column of dimensions 25 m x 0.32 mm i.d. with a film thickness of 0.5pm was used. The compounds identified had relatively short retention times, and a longer column was used for greater resolution. The series of peaks observed using GUMS was matched with that obtained using GC-FID. The injector temperature was 280 "C, and the column temperature program began at 30 "C, was held for 20 s, and was ramped at 3 "C/min to 300 "C. The sample, dissolved in CH& was injected with splitless injection mode for 20 s. Standard E1 (70 eV) and CI (50 eVwith NH3 and CH4 reagent gases) conditions were used with a source temperature of 250 "C, with mass range from 20 to 400 m / z units for E1 and from 40 to 400 m / z units for CI. NMR Spectroscopy. Samples were dissolved in chlororoform-d, approximately5-10% (w/v)for 'Hand 13Cspectra. Spectra were acquired on a Varian Gemini 200 MHz instrument. Heteronuclear correlation (HETCOR)spectra and distortionless enhancement by polarization transfer (DEPT) spectra were used to confirm assignments. ESMS. ES mass spectrometry of nonionic surfactants has been recently reported (20). ES mass spectra were obtained using a VG Bio-Q triple quadrupole mass spectrometer WG Bio-Q, VG Bio-Tech, Altrincham, Cheshire, U.K.). A water/methanol/acetic acid (50:50:1%) carrier phase was used with a flow rate of 5 pL/min. A drop of the surfactant (-17 mg, 1.132 g/mL) was dissolved in water (1 mL), and this was then diluted 1/10 with a methanol/ acetic acid mixture resulting in an approximate concentration of the surfactant of 1700 mg/L. The sample solution was introduced into the carrier stream via a Rheodyne injector fittedwith a 10-pLloop. The potential applied to the first skimmer (Bl) was set at 40 V to preclude collisionallyactivated decomposition (20). NZwas used as both the drying gas (3 mL/min) and as the nebulizer gas (100 mL/min). The pressure in the mass analyzer region was typically 3 x Torr. To achieve a good signal to noise ratio, usually 8- 12 signal-averaged spectra were required.
reduced to drymess
Results and Discussion
reduced to 0 S mL at room
0.32 mm i.d. with a film thickness of 0.5 pm was used. Helium carrier gas was used with a column head pressure of 0.5kg/cm2. The split ratio was 34:1, volumetric column flow rate was 2.4 mL/min, and split vent flow was 80 mL/ min (average). Samples were introduced onto the column with splitless injection mode with splitless time of 3 min. The injector temperature was set at 300 "C, and the FID temperature was 340 "C. The temperature program was as follows: initial temperature of 80 "C, held for 3 min, then 2236 D ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 9. 1995
Analysis of SAE. Starting concentrations of SAE were reasonably high in keeping with the levels typically found in an industrial effluent such as that arising from wool scouring. Gas chromatograms of SAE starting material are shown in Figure 2 (and Figure 5a) with two expanded sections of the gas chromatogram of SAE shown in panels b and c. Several early eluting homologues ( x = 2-4) are shown in panel b and later eluting homologues ( x = 8 and 9) are shown in panel c. Comparison of the two parts of the chromatogram show that resolution is good for the early eluting homologues but deteriorates for later eluting homologues. While subject to mass discrimination, GC does show that SAE comprises a complex mixture of hydrocarbon chain lengths together with the structural isomers produced because the tertiary carbon may be present at any position along either of the three hydrocarbon
Il I
15
2o Time (rnin?
55
0
60
65
FIGURE 2. Gas chromatogram of SAE (a) statting material, with two expanded sections; (b) x = 2-4, and (c) x = 8 and 9 oligomers.
chainlengths (C12,C13, andC14). Themass discrimination observed in GC-FID may be overcome by analysis of SAE using appropriate spectroscopic techniques where discrimination is expected to be minimal. Proton NMR spectroscopyallows effectivedetermination of the ratio of average numbers of ethoxyl chain protons to hydrocarbon chain protons; Figure 3a shows the proton NMR spectrum of SAE. The shoulders on the peak at 3.5 ppm are shown by HETCOR to correspond to the protons at the hydroxide end of the ethoxyl chain as these protons are slightly different to the protons closer to the center of the chain. The small doublet at 1.0 ppm represents the methyl protons in the isomer where the secondary carbon (>CH-OR) is adjacent to the methyl group (Le., m = 0). Peaks are broadened because although the protons are very similar they are not identical. The manufacturer specifies that SAE is composed of a mixture of hydrocarbon chains with lengths of 12, 13, and 14 carbons, with an average ethoxylate mole number of x = 9. The ratio of ethoxyl protons to hydrocarbon chain protons is found to be 39:27 from the integrals of the corresponding peaks, which means SAE on average is composed of 9.75 ethoxyl units if the relative area of the hydrocarbon chain protons is taken to correspond to those on 13 carbons (an average value for the hydrocarbon chain). The carbon-13 NMR spectrum and DEPT spectrum of SAE, shown in Figure 3b, display many more peaks; these can be assigned on the basis of estimated shifts calculated with the aid of standard tables (26). Assignments of SAE with an average of 13 carbons in the hydrocarbon chain are based on the general formula CH3-(CHz),-CH-(CHp)jo_,-CH3
I
O-CH~-CH~-O-(CH~-CH~-O)~CH~-CH~-OH A 0 C'S D E 1
The carbon- 13 spectrum shown in Figure 3b exhibits many individual peaks that can be assigned to particular carbon atoms, as summarized in Tables 1 and 2. There are two types of peaks, relatively strong absorptions due to common carbons and weak peaks due to unique carbons, where the influence of the nearby ether linkcauses marked differences in chemical shift. The DEPT spectrum in Figure 3b shows four of the five peaks between 9 and 20 ppm to be methyl carbons, but the peak at 18.4 ppm is the particular methylene carbon adjacent to the methyl carbon when m = 2. Similarly,the DEPT spectrum proves that all the peaks above 75 ppm are methine carbons, Le., the unique carbon at the point of attachment of the ethoxyl chain to the backbone carbon chain. There are three unique oxygenated carbons in the ethylate chain, as assigned in Table 2; these are the hydroxyl terminal carbons (D and E) and the ethoxylate carbon (A) nearest to the hydrocarbon chain. (The solvent, CDC13, produces three peaks at 77 pprn). The third technique used for characterization of S A E was ESMS. The ES mass spectrum of the protonated form of SAE [M HI+ is given in Figure 4. The main peaks of the series range from mlz = 377 to mlz = 905 at intervals of 44 mlz units, which represents the x = 4- 16homologues. The nominal value (most abundant oligomer) occurs at rnlz = 597 corresponding to x = 9. There is no evidence for multiply charged species, and it is assumed that mass discrimination effects are minimal due to the similar nature of all compounds present in a single sample of surfactant so the spectrum gives a realistic representation of relative proportions of each compound present. The nominal value can be assumed, therefore, to represent the true value of the most abundant oligomer. On either side of the main peaks are two smaller peaks. Each group of three peaks corresponds to one homologue containing a mixture of aliphatic chain lengths 12, 13, and 14 carbons long (Le., m n = 9,10, and 11). For example, the peaks at m l z values
+
+
VOL. 29. NO. 9,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 12237
a
TABLE 2
Estimated Chemical Shifts for Carbons in the Ethoxylate Chain of SAE" carbon A
carbon B
carbon C's
carbon D
carbon E
67.8
-7 1
70.8
72.6
61.3
a
Refer to 1 in text.
14 carbons long. The C11 aliphatic chain length species is also present at concentrations about one-third of C12 and C14 and one-seventh of C13. Photocatalytic Degradation of SAE. (A) Gas Chromatography. Gas chromatograms of the aqueous samples are shown in Figure 5 after (a) 0, (b) 48, and (c) 96 h of irradiation. The gas chromatogram of the sample taken after 48 h of irradiation shows that between retention times of 8 and 15 min, new peaks have appeared, and the relative amounts of the original x = 1 and 2 homologues have increased. The first observation suggeststhat new, relatively low molecular weight compounds are being produced during the photodegradation process, and the second observation suggests that the higher molecular weight homologues (some ofwhich are not detected using GC) are being degraded to shorter ethoxylate chain compounds. This gives the appearance of the series being skewed toward lower molecular weight homologues. Because the pattern of peaks within each homologue cluster appears relatively unaltered, it is thought that between 0 and 48 h of irradiation there is little degradation of the hydrocarbon chain occurring. Significant changes are observed in the chromatogram of the sample irradiated for 96 h shown in Figure 5c. Traces of the originalsurfactant series may still be present (though onlythe lower homologues),but the pattern for each cluster is now quite different to that noted in panels a and b; the new, early eluting compounds dominate the chromatogram. This sample was analyzed by GClMS. GClMS analysis indicated the presence of three series of peaks, which had characteristic ions mlz = 73, 85, and 104, respectively. Molecular ions for the related peaks in each series differed by 14 mlz units. The mass spectrum of each peak in the first series matched library spectra for furanones with the general structure
b
n
C
FIGURE 3. (a) Proton NYR spectrum of SAE starting material. (b) C-13 and DEPT NMR spectra of SAE starting material (spectra slightly offset). (c) C-13 NMR spectrum of SAE after 72 h of irradiation. ~~~
~
TABLE 1
Estimated Chemical Shifts for Carbons in the Aliphatic Chain of SALa Cl
W
C3
C4
C5
C6
C7-Cll
C12 C13
33.3 22.5 13.9 m = 0 19.5 75.8 36.3 25.1 -30 -30 m = 1 9.4 27.4 81.0 36.0 25.4 -30 -33 22.5 13.9 22.5 13.9 m = 2 14.1 18.4 36.3 79.6 33.7 25.4 -33 a
Refer to 1 in text.
of 583,597, and 611 marked with an asterisk (*) in Figure 4 represent the C12, C13, and C14 mixture of carbon chain lengths for the x = 9 homologue. The homologue with an
aliphatic chain length of 13 carbons shows approximately twice the abundance of the aliphatic chains that are 12 and
-
2238 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL.
29. NO. 9 . 1 9 9 5
where y varies from 3 to 7, giving molecular ions at mlz 128,142,156,170,and 184. Confirmation of this series was obtained from ESMS data where molecular weights of a series of [M + HI+peaks were consistent with those found using CI-MS; however, ESMS suggests a more extended series ranging from y = 0 to y = 8. No definitive identification nor ESMS confirmation has been made on the other two series of peaks observed at similar concentrations in the GC/MS data. Two of the earliest eluting peaks in the chromatogram produced E1 mass spectra that corresponded well to library matches; 1,2-ethanediol, diformate (MW = 118), and propyl acetate ( M W = 102). After approximately48h of irradiation,the slurrybecame nonuniformly dispersed with agglomeration of the TiOz
*
r 100
I-
I
597
x 400
500
700
600
800
900
m/Z
FIGURE 4. ES mass spectrum of SAE starting material displayed over the range m/z 350-950. Nominal masses of some ions are given. Starred peaks have m/z values of 583, 597, and 611.
particles observed. It appears that as the surfactant molecule is degraded, it is less able to stabilize the Ti02 dispersion. As a result, the slurry became less opaque with irradiation time. Similar observations have been reported elsewhere (14).The TiOzclumps and some remainingliquid were extracted with CH2ClZ at the end of the irradiation experiment in a similar way to the liquid-liquid extraction described above. The gas chromatogram of the TiOzextract is shown in Figure 5d and contains predominantly early eluting compounds, indicative of relatively low molecular weights. There are also low levels of the original SAE series present with x = 1-4 approximately. A quantitative comparison of the changes in amounts of each homologue present in the aqueous phase and determined by GC during photodegradation is shown in Figure 6. The ratio of (total homologue cluster peak area)l (internal standard peak area) for each homologue from x = 1 to x= 9 is shown separatelyforirradiation times between 0 and 102 h. The aqueous concentration of all homologues is significantly reduced after 72 h of irradiation. Clearly, the homologues are not degraded at the same rate; concentrations of homologues x = 1-7 initially increase over the first 48 h before decreasing with extended photocatalysis, with the intermediate homologues x = 4 and 5 increasing by the greatest amount. In contrast, no increase is observed for the larger homologues x = 8 and 9; rather, steady reduction in concentration occurs as irradiation proceeds. These observations suggest that the ethoxyl chains of the largest homologues of SAE from about x = 8 to x = 16, which includes those not detected by GCFID, are being degraded by photocatalytic activity,resulting in production of the shorter chain homologues. This is observed as an initial increase in concentration of the x = 1-7 homologues shown in Figure 6. Because the hydrocarbon chain pattern within each homologue was not significantlyaltered, as seen in Figure 5a,b,it can be inferred that cleavage of ethoxylateunits is the main process between 0 and 55 h of irradiation (approximately),and no appreciable photocatalytic activity occurred at the hydrocarbon chain. Furthermore, the ethoxylate units must be cleaved from the terminal end to retain the observed repeating pattern resulting from the attached aliphatic chain portion of the molecule, without which a series of single peaks would be observed representingfree ethoxylate chains. These results agreed with those found for NMR and ESMS with respect
to the ethoxylate chain length reduction during the initial stages of irradiation (see below). The rate of this degradation appears to be relatively slow to begin with (0-55 h) and followingthis appears extremely rapid (55-72 h). The rapid degradation occurs concurrently with the agglomeration of the Ti02 particles, and it appears that adsorption of intermediates affects the apparent degradation rate. Similar observations are made for degradation experiments run under different conditions. A higher concentration of SAE (5000 mglL) also resulted in agglomeration and rapid degradation at around 72 h. Rapid degradation was observed between 7 and 24 h for a smaller volume (80 mL) containing 5000 mg/L SAE. A 400-W blacklight lamp resulted in earlier onset of the rapid degradation phase than a 25-Wlamp, although not as great an increase as was expected. Decreasing the volume of surfactant solution from 2 L to 80 mL significantlyincreased the degradation rate and lead to rapid agglomeration and degradation. Two other nonionic surfactants, a primary alcohol ethoxylate and an allcylphenol ethoxylate, were degraded from a starting concentration of 2000 mg/L (2 L) using a 400-W blacklight lamp. Agglomeration and degradation were again observed but at different irradiation times to SAE. (B) NMR and ESMS. A ratio of 1.0:0.7was found for the integrated areas of the peaks corresponding to ethoxyl protons compared with aliphatic chain protons prior to irradiation ( t= 0 h) from Figure 3a. After 48 h of irradiation, the ratio is 1.O:l.O (spectrum not shown), indicating that the number of ethoxyl units has been reduced with respect to the aliphatic chain. This supports the suggestion of loss of ethoxyl groups. Assuming, from the GC results, no reduction in the length of hydrocarbon chain (average length 13carbons), from the proton ratio it can be calculated that an average of 7 ethoxyl units (x = 7) remain compared with x = 9 initially. The ES mass spectrum of SAE after 48 h of irradiation, shown in Figure 7b, shows a shift in the envelope of peaks to lower mlz values, with a new nominal value of 7 (Le,, consistent with NMR results). This is illustrated by the relative increase in the mlz509 peak (x = 7) and the decrease of the mlz 685 peak ( x = 11). These observations suggest that after 48 h of irradiation there is a greater relative abundance of shorter chain homologues and a smaller relative abundance of the longer chain homologues, and the inference is that the larger homologues degrade to form their smaller counterparts, resulting in a shift of the most abundant oligomer from x = 9 to x = 7. It is likely that all homologues are being degraded, but it is difficult to determine exactly how many ethoxyl units each oligomer has lost. Although reduction in the ethoxyl chain has occurred, the proportions of the hydrocarbon chain lengths (i.e.,C12, C13, C14) areunaffected, suggestingno substantial photocatalyticactivity is occurring along the aliphatic chain. These observations support the assumption made concerning the NMR ratio. The spectroscopic data of the two techniques are in good agreement. Further photocatalytic degradation is apparent after 72 h of irradiation. The ratio of 1.0:0.4calculated from the ‘H NMR integral areas of the t = 72 h sample (spectrum not shown) indicates that the length of the alkane chain has decreased with respect to the ethoxyl chain, resulting in compounds that are deficient overall in aliphatic chain protons and are hence relatively more hydrophilic. This VOL. 29, NO. 9, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY 12239
0
40
20
60
20
0
60
40
Time (min) FIGURE 5 Gas chromatograms of the supernatant from the photocatalytic experiment SAE after (a) 0. (b) 48, and (c) 96 h of irradiation and (d) Ti& extracted sample (96 h).
R.hO or
9 8
I
O
200
403
CCO
d T h e (h)
'1"
102
FIGURE 6. Gas chromatographic quantitation of SAE homologue abundance variation with time during photocatalysis. The plotted ratio isthntofltotal homologue cluster peak arsall(internal standard peak areal against irradiation time. x rden to the particular homologue. See text for details.
contrasts with the results of the sample taken at 48 h where the proportion ofethoxylprotonswas reducedwithrespect to aliphatic chain protons. The change in ratios suggests that thereis competitivephotooxidationoccurringbetween the ethoxyl moiety and the aliphatic chain portion of the molecule, with the ethoxyl chain being most susceptible to initial attack. For NPE and PAE, similar conclusions have been drawn (12). ES mass spectral data support the N M R results. The ES mass spectrum ofthe sample taken at 72 h, shown in Figure 7c. reveals three main groups of peaks that are consistent with compounds that are overall deficient in the hydrocarbon chain relative to the starting material. These are proposed to be (i) ethanoic acid, propanoic acid, and 2240 m ENVIRONMENTAL SCIENCE LL TECHNOLOGY I VOL. 29.
NO. 9,1995
I
804 0
1W
4W
Ma
8W
Z
FIGURE 7. ES mass spectra of SAE after (a) 0. (bl48, and (c) 72 h of irradiation and (d) Ti02 emacted sample (% hl. (All spectra nonnalized to 100).
butanoic acid at mlz values of 61,75, and 89, respectively; (ii) IHO(CHZCHZO)P HI+ where x = 2-13 (marked *); and (iii) [CllHz30(CHzCHzO),H HI+ where x = 1-11 (markedO). Itisinterestingtonotethatthestartingmaterial was composed of a mixture of aliphatic chain lengths of 12, 13, and 14 carbons, and after 72 h of irradiation, ESMS reveals that only one aliphatic chain length of 11 carbons is detected. It is not apparent why such a series should appear. Short-chain carboxylic acids have been reported elsewhere (12,16,1B)as intermediatesof photodegradation of NPE. The mixture of compounds i, ii, and iii are consistent with the signals observed using 'H NMR as they are low in hydrocarbon chain content and hence are reasonably hydrophilic. The two spectrometric methods give very similar results. The positions ofcleavagemaybedeterminedfromC-13 NMR spectra and ESMS data. Comparison of the NMR
+
+
spectra of SAE irradiated for 0 h shown in Figure 3b with that for 72 h shown in Figure 3c shows that in many ways the spectra are similar. The most notable differences are the relative reduction in the size of the non-oxygenated carbon peaks ( ~ 4ppm) 0 and the large reduction in the size of the secondary carbon (CH) peaks (75-80 ppm). The four main CH peaks in the spectrum of nonirradiated SAE have been replaced by a much smaller (broad) peak at 79.6 ppm. The peaks that are lost (or significantly reduced) at 75.8 and 81.0 ppm in Figure 3c represent the secondary carbons close to the ends of the hydrocarbon chain, Le., m = 0 and m = 1 in 1, and the remaining peak (79.6 ppm) is caused by the secondary carbons toward the center of the chain, i.e., m 2 2 in 1. This suggests that the structural isomers of SAE in which the secondary carbons are close to the center of the chain have been much less affected by photocatalytic activity than the isomers in which the secondary carbon is toward the end of the chain. These changes indicate activity around the secondary carboneither cleavage at the secondary carbon-oxygen bond or at a secondary carbon-carbon bond. Evidence from ESMS after 72 h of irradiation suggests two main target areas for cleavage. The poly(ethy1ene glycol) series, marked * in Figure 7c, must result from cleavage at the secondary carbon-oxygen bond or very close to it because the series is only a little shorter (x = 2-13) than it was as part of the SAE molecule (x = 4-16) and has no residual aliphatic chain. The other intermediate series [CllH230(CH2CH20)xH+ Hl+wherex= 1-11, marked 0 in Figure 7c, results from some shortening of the ethoxylate chain where the average length is reduced from x = 9 to x = 6 (approximately), and a slightly truncated hydrocarbon chain, now 11 carbons in length, which was originally a mixture of 12, 13, and 14 carbons. This is probably indicative of sequential cleavage of ethoxyl units from the terminal end of the chain and limited photocatalytic activity toward the aliphatic chain. Because both series retain a regular distribution pattern, it is expected that the radical HO' attack of the SAE molecule is a nonrandom process. In other words, according to ES mass spectral data, cleavage is most commonly occurring at the secondary carbon-oxygen bond, or terminal ethoxyl units are sequentially cleaved (or both) to produce the intermediates that are consistent with observed m/z values. The extract from the agglomerated TiOz taken after 96 h was analyzed by 'H NMR and ESMS. The ratio of integrated peak areas for ethoxyl protons to hydrocarbon chain protons of 1:5 means that the compounds are high in hydrocarbon chain proton content and relatively low in ethoxyl protons. In other words, the compounds are no longer surfactants but rather overall relatively hydrophobic compounds, explaining their tenacity in binding to TiOz particles in the aqueous medium. The ES mass spectrometric data (Figure 7d) was found to support the conclusions drawn from 'H NMR results. Two series of compounds can be proposed that are consistent with observed mlzvalues, as previously reported (20). They are [HO(CH2CH20),H + HI+; x = 2-4 marked 0 and [CH~(CHZ),CH(CHZ),CH~ + HI+
I
O(CH&HzO),H
+
+
where x = 1-7 and m n = 9-11; m n = 10 is marked * in Figure 7d. The second series is likely to adhere to the
Ti02particles rather than remaining in solution because of its significantly shortened ethoxyl chain and resultant decrease in hydrophilic nature. Agglomeration is believed to be caused by these compounds, which are less watersoluble than the starting material. Observations suggesting that relatively hydrophilic intermediates remained in the aqueous phase and relatively hydrophobic intermediates were found adsorbed onto the Ti02 particles are consistent with those of the other two photodegraded nonionic surfactants mentioned previously. It is apparent that this separation of intermediates according to hydrophobicity causes agglomeration of the Ti02 particles, which is related to the phenomenon of rapid degradation observed after the initial slower degradation. The agglomeration of the TiOa particles is associated with the rapid reduction in the concentration of some of the surfactant homologues.
Acknowledgments The authors are extremely grateful to Frank Antolasic for his help with the GUMS work and would also like to thank La Trobe University for providing a SCAEF grant to assist in the purchase of the electrospray mass spectrometer and the International Wool Secretariat for provision of a student scholarship (K.S.) that made this research possible.
Literature Cited (1) Kurata, N.; Koshida, K.; Yokoyama, H.; Goto, T. In Monohydric
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Received for review October 28, 1994. Revised manuscript received April 1 7, 1995. Accepted May 12, 1995.@ ES940667J @Abstractpublished in Advance ACS Abstracts, June 15, 1995.
