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Reactivity of two models of non-ionic surfactants with ozone
Pergamon S0043-1354(96)00269-2
Wat. Res. Vol. 31, No. 8, pp. 1839-1846, 1997 © 1997 ElsevierScienceLtd. All rights reserved Printed in Great Britain 0043-1354/97 $17.00+ 0.00
REACTIVITY OF TWO MODELS OF NON-IONIC SURFACTANTS WITH OZONE A N N A BRAMBILLA% EZIO BOLZACCHINP, MARCO O R L A N D I l, STEFANO POLESELLO 2 and BRUNO RINDONE~*~ 'Department of Environmental Sciences, University of Milano, Via Emanueli 15, 1-20126 Milano, Italy and 2Department of Inorganic, Organometallic and Analytical Chemistry, University of Milano, Via Venezian 21, 1-20133 Milano, Italy (First received September 1995; accepted in revised form August 1996)
Abstract--The ozonation of ethylene glycol and diethylene glycol mono-n-octylether (models of non-ionic surfactants) has been performed in aqueous solution at pH 4 and pH 9.5. The experiments at acidic pH show that the methylene groups adjacent to the ethereal oxygens are transformed into carbonyl groups. At alkaline pH the aliphatic chain is cleaved. Kinetic experiments allow to suggest a reaction mechanism. © 1997 Elsevier Science Ltd Key words--aqueous ozonation, non-ionic surfactants, mono- and diethoxylated alcohols, trimethylsilyl derivatives
INTRODUCTION Many studies are intended to reveal the by-products of the ozonation of several polluting organic compounds. These might be even more dangerous than the substrates themselves (Legube et al., 1989; Reynolds et al., 1989). The identification of ozonation products is also important to investigate the reaction mechanisms that lead to their formation (Caprio et al., 1989; Andreozzi et al., 1990; Takanashi and Katsuki, 1990; Takanashi, 1990). Non-ionic surfactants are not destroyed in conventional biological degradation plants, and a chemical treatment is required to allow the concentration of these compounds to fit regulations. In the treatment of non-ionic surfactants ozonation causes alterations in the molecular structure, resulting in changes in characteristics such as foaming ability, complexation with bismuth or cobaltothiocyanate and increased water solubility. These surfactants are not completely broken down into CO2 and HzO, as reflected in the high TOC and COD residual concentrations (Narkis et al., 1987). As a consequence, although surface activity is destroyed by ozonation a large organic load remains and creates potential environmental pollution (Glazer, 1987). Thus ozone treatment alone, despite its many advantages, is inadequate for the total removal of non-ionic surfactants. However ozonation can be used to increase the biodegradability of the biologically resistant compounds: small doses of ozone can be sufficient to change the structure of the *Author to whom all correspondence should be addressed [Fax: +39 2 6447 4300].
non-ionic surfactant, making it more amenable to bacteriological breakdown and enhancing the biodegradable properties of the material. Previous work (Niki et al., 1983) investigated the ozonation in buffered solutions of small aliphatic alcohols, ketones and acids and small bifunctional molecules containing carboxyl groups. These studies showed that alcohols were oxidized to the corresponding carbonyl compounds and that aliphatic chains were functionalized by introduction of hydroxyl and keto groups. Fragmentation was also observed, with formation of carboxylic acids. In all cases, the rate of ozonation increased with increasing the pH of the medium. At low pH, small saturated acids were stable toward ozone at 30°C, but were attacked at an appreciable rate at higher pH. The presence of organic peroxides and hydrogen peroxide was also revealed in the reaction medium. A 1,3-dipolar attack of ozone to C - H bonds and hydrotrioxide intermediates was postulated. Very recent studies about the fate of the fragments deriving from the ozonation of non-ionic surfactants have also been made (Andreozzi et al., 1995). In order to better understand the ability of ozone to degrade non-ionic surfactants in aqueous solutions we studied the behaviour of a mixture of ethoxylated phenols toward ozone (Calvosa et al., 1991). A normal phase HPLC analysis showed decreased overall polarity of the mixture after ozonation as a result of the partial or total loss of the oxyethyl chain of these compounds. Inspection of the ozonation products from an ethoxylated alcohol mixture (Brambilla et al., 1993a) showed the formation of linear carboxylic acids. Some of these arose from aliphatic chain fragmenta-
1839
A. Brambilla et al.
1840
tion. Other products deriving from carbon chain fragmentation were also present: a) hydroxy and dihydroxy acids, b) ketoacids as 4-ketopentanoic acid and 5-ketohexanoic acid, c) dicarboxylic acids. Other products can be attributed to oxyethyl chain modification: a)
Glycolic acid might derive from acetic acid oxidation, but can more easily be considered as the oxidative depolymerization product of the oxyethyl chains, b) di-, tri- and tetraethyleneglycol, c) the mono- and diacid corresponding to di- and triethyleneglycol. These results stimulated a more systematic study intended to evaluate the ozonation mechanism at different pH. Pure starting alkylethers were needed for this purpose. Hence, two models of non-ionic surfactant, ethylene glycol mono-n-octylether and diethylene glycol mono-n-octylether were chosen as the starting material. MATERIALS
AND METHODS
Sodium acetate, acetic acid, sodium carbonate, sodium bicarbonate and anhydrous sodium sulfate were all analytical grade from Merck (Bracco S.p.A., Milan, Italy). Hexamethyldisilazane (HMSD) and trimethylsilylchloride (TMSC) were reagent grade from Aldrich (BH-Schilling, Milan, Italy). Ethyl acetate was purified by distillation. Benzene was distilled and dried over sodium in nitrogen atmosphere. Sample ozonation, extraction and preparation Ozone was produced in a Fisher 501 generator fed with oxygen. The ozonation was carried out using 1 L of an aqueous solution 10-3M in compounds ethylene glycol mono-n-octylether (1) or diethylene glycol mono-noctylether (9) adjusted at the required pH. Before ozonation the solution of the substrate was saturated with oxygen for 2 min. A 70 L/h stream of oxygen was subjected to a 200 mA electrical current and then passed through the solution. The amount of ozone produced in these conditions was 3.3 g/h. After the given time nitrogen was bubbled into the solution for 5 rain. Acidic conditions (pH = 4.0) were obtained by addition of a dilute solution of sulphuric acid and testing the pH with a Toptronic digital pH-meter: in these conditions the pH remained constant all over the reaction time. Alkaline conditions (pH = 9.5) were obtained by addition of a dilute solution of potassium hydroxide. During the reaction, acidification occurred due to the formation of carboxylic acids. The alkaline pH was held constant by addition of a 0.0713 N solution of potassium hydroxide. The reaction solutions were acidified to pH = 3 4 and extracted three times with 750 mL of ethyl acetate. The collected extracts were dried over anhydrous sodium sulphate for 1 night, evaporated under reduced pressure, and weighed. The samples were diluted with anhydrous benzene, derivatized via reaction with HMDS and TMSC (Cross, 1987) and analyzed by high resolution gas chromatography (HRGC), and HRGC-positive ion electron impact mass spectrometry (PIEIMS).
High resolution gas chromatography (HRGC), high resolution gas chromatography/mass spectrometry (HRGC/MS) HRGC was performed on a Varian instrument (Model 6000) equipped with a flame ionization detector (FID) and a split/splitless injector, connected to a fused silica capillary column (Supelco, SPB-5, 30m, film thickness 0.25#, internal diameter 0.32 ram). Samples of 1 pL containing 10 #g of silylated compounds were injected onto the column at room temperature; stream splitting was activated 2.0 min after the injection. Nitrogen was used for the carrier gas with a flow rate of 1.2 mL/min. The temperature program was 70°C (2 rnin), 70-270°C (4°C/min)and 270°C (isothermal). Injector and detector were kept at 250°C. Electronic integration of the gas chromatographic peak areas was performed with a Varian integrator (Model 4270). A VG mass spectrometer (Model 707 EQ) was connected with a DANI gas chromatograph (Model 3800). The capillary GC column was directly coupled to the ion source. GC column and procedures were as described above for the GC analyses. For the PIEIMS, the conditions were the following: reactant gas isobutane, mass range, 40-600 m/z. An HP mass selective detector (Model 5971 A) was coupled to an HP gas chromatograph (Model 5890) using helium as the carrier gas with a head pressure of 8.0 psi. GC column and procedures were as described above. Kinetic experiments Forty mililiters of water (grade Milli Q) were adjusted at pH 4 with sulphuric acid were treated with a 20 L/h stream of oxygen subjected to a 2.2 amp electric discharge for 5 min. Ten milliliters of a solution of ethylene glycol mono-n-octylether (1) in water pH 4 for sulphuric acid were then added. The final concentration of the compound was
~OH
•f•,O
~
o
/
(1)
~
/
~
<2) O
(3)
HO~ ° H (6)
(s) O
(7) (s)
o
(9) o
(1o)
,~o/~/o,,~o. (11)
Scheme A.
Non-ionic surfactants and ozone
184i
100 90 80 70 60 >-
50 40 30 20 J
10
0 0
10
20
30
I
I
I
40
50
60
time, min Fig. 1. Time course of the reaction of compound (1) ( 0 ) with ozone at pH 4 and of the formation of compounds (2) (1) and (3) (0). 5.6 x 10-4 mol/L. Oxygen subjected to a 2.2 amp electric discharge was then admitted at 298°K in the reaction vessel at a flow of 40 L/h. Two milliliters of solution were withdrawn at known intervals. Nitrogen was bubbled for 5 rain. Addition of sodium chloride and extraction with three 2 mL portions of methylene chloride to which the internal standard diethylene glycol mono-n-octylether (9) had been added allowing recovery of the organic material after drying with sodium sulphate and evaporation under reduced pressure at room temperature. The sample was then treated for 15rain with hexamethyldisilazane and trimethylchlorosilane, evaporated with a nitrogen stream and analyzed by GLC-MS.
Saturation of the solution with ozone Fifty milliliters of water adjusted at pH 3 with sulphuric acid was treated with a 10 L/h stream of dioxygen subjected to a 2.2 amp electric discharge at a flow of 20 lh and 40 L/h. Two milliliters of solution were withdrawn at 1 min intervals and the absorbance at 260 nm was monitored (UV Jasco Mod 7800),
Preparation of 2-hydroxyethyl octanoate (2) Two hundred microliters (1.2 mmol) of octanoic acid (4) was added to 2 mL (36 mmol) of ethylene glycol (6). One drop of concentrated sulphuric acid was added and the resulting mixture was stirred at 200°C overnight. After that, the mixture was poured in 10 mL of water and extracted three times with 10 mL portions of ethyl acetate and with one I0 mL portion of methylene chloride. The organic extracts, washed with a saturated sodium hydrogenocarbonate solution and with a sodium chloride solution, dried over sodium sulphate and evaporated at reduced pressure gave a mixture containing 50% of 2-hydroxyethyl octanoate (2).
Preparation of n-octyl glycolate (3) A solution containing 200 mg (0.37 retool) of glycolic acid (8), 3 ml of n-oetanol (7) and two drops of concentrated sulphuric acid was refluxed for 3 h. After that, the mixture was poured into 10 mL of water and extracted three times with 10 mL portions of ethyl acetate and with one 10 mL portion of methylene chloride. The organic extracts, washed with a saturated sodium bicarbonate solution and with a sodium chloride solution, dried over sodium sulphate and evaporated at reduced pressure gave a mixture containing 90% of n-octyl glycolate (3).
Preparation of 2-n-octyloxy acetic acid (5) Two hundred microliters of ethylene glycol mono-n-octyl ether (I) in 10 ml acetone were cooled to 0°C and a solution of potassium dichromate in concentrated sulphuric acid was added dropwise until the brown colour persisted. The excess of oxidant was consumed adding a few drops of methanol and the resulting suspension was concentrated at reduced pressure to give a suspension which was poured in 10 mL of water and extracted three times with 10 mL portions of ethyl acetate, and with one 10 mL portion of methylene chloride. The organic extracts, washed with a saturated sodium bicarbonate solution and with a sodium chloride solution, dried over sodium sulphate and evaporated at reduced pressure gave a mixture containing 80% of 2-n-octyloxy acetic acid (5). RESULTS AND DISCUSSION
Product studies The first model compound of a non-ionic surfactant was ethylene glycol mono-n-octylether (1). The preparative ozonation at pH 4 was followed by extraction, silylation and H R G C - M S analysis, showing that oxygenation of the carbon atoms adjacent to the oxygen atoms had occurred. 2-hydroxyethyl octanoate (2, 87% of the converted starting material), and its hydrolysis product octanoic acid (4), n-octyl-glycolate (3, 13%) and 2-n-octyloxy acetic acid (5, 1%) were obtained. Confirmation of their structure was obtained by independent synthesis. 2-hydroxyethyl octanoate (2) was prepared by reaction of octanoic acid (4) with ethylene glycol (6); n-octyl-glycolate (3) from the reaction of glycolic acid (8) with n-octanol (7), and 2-n-octyloxy acetic acid (5) was obtained by chromic oxidation of (1). The time course of the reaction showed that the oxygenation of ethylene glycol mono-n-octylether (1) to 2-hydroxyethyl octanoate (2) and octanoic acid (4) derived from three competitive pathways (Fig. 1). The second model of a non-ionic surfactant was
1842
A. Brambilla et al.
(12)
(14)
Fig. 2. Gas chromatographic analysis of the reaction mixture from the ozonation of compound (1) at pH 9.5 after silylation.
diethylene glycol mono-n-octylether (9). Under the same conditions a much slower reaction occurred and the major reaction product (85% selectivity) was diethylene glycol mono-n-octanoate (10). Its structure was demonstrated by the formation of n-octanoic acid (3) on hydrolysis. A minor component was probably O-(n-octyloxyethyl)glycolic acid (11), which was responsible for the formation of ethylene glycol mono-n-octylether (1) in the reaction mixture. A very different reactivity was exhibited by ethylene glycol mono-n-octylether (1) on ozonation at pH 9.5. A very complex mixture resulted from the much faster reaction (Fig. 2). HRGC-MS after silylation showed the presence of unreacted compound (1) and of reaction products deriving from fragmentation of the carbon skeleton of the n-octyl chain. The most abundant reaction product was butanedioic acid (12). Other reaction products were pentanedioic acid (13), 4-oxopentanoic acid (14) and glycolic acid (8).
The reaction in acidic solution is: C + zO3 ---, intermediates where z is the stoichiometric ratio, z was found to be generally 1 (Staehelin and Hoigne, 1983, 1985; Kalmaz and Trieff, 1986). According to this reaction
OH
HO
\
OH
/
1
O H O ~
G~
H O ~ C ~
(ts)
(12)
t
Kinetic studies
The selective regiochemistry of the reaction at low pH and the encouraging fragmentation of the carbon chain at high pH could be better understood by performing a mechanistic study.
116)
Scheme B.
1843
Non-ionic surfactants and ozone
0.9
e
0.8 0.7 0.6 E ¢0 ',0
<
0.5 0.4 0.3 0.2 0.1
I
0 1
-0.1
2
3
4
5
6
7
8
time, min Fig. 3. Time course of the dissolution of ozone in water pH 4 at a flow rate of 40 ( 0 ) and 20 (0) L/h.
the expression of the rate for the ozonation of the substrates is - d[C]/dt =
kd/z[C][03]
first-order in the substrate and first-order in ozone. Kinetic measurements are frequently carried out using a pseudo-first-order approach with respect to ozone. This implies using excess substrate. A very precise measurement of the concentration of ozone is also necessary. We used a different approach: our kinetic experiments were performed using a pseudofirst-order approach with respect to the substrate. This should minimize the experimental error. The kinetic expression is d[C]/dt =
k'd/z[C]
where k'd = kd[O3] The dissolved ozone concentration reaches its stationary and maximum value within 3-6 min of presaturation with either a 40 L/h or 20 L/h stream of ozone (Fig. 3). The validity of the assumption that the concentration of ozone was constant throughout the experiments was assayed measuring the change of the concentration of ozone in the solution on addition of the substrates. Here it could be shown that if the solution is presaturated with a 20 L/h or 40 L/h stream of ozone for 6 or 3 rain, respectively, no decrease of ozone concentration is noticed on addition of ethylene glycol mono-n-octylether (1), at a final concentration of ca. 10 4 mol/L (Fig. 4). In
0.7
0.5 E 0.4 *o ¢q
,,, 0.3
<
0.2 0.1
0
I
I
I
I
I
I
I
I
50
100
150
200
250
300
350
400
tlme, sec Fig. 4. Effect of addition of ethylene glycol mono-n-octylether (l) to a solution presaturated for 3 rain with a 40 L/h stream of ozone at pH 4.
1844
A. Brambilla et al. 2.5
E 01.5 N
( 0.5
0 278
I
I
I
I
I
I
I
I
I
I
280
282
284
286
288
290
292
294
296
298
T a'q~ccdure,
oK
Fig. 5. The influence of the temperature on the concentration of ozone at pH 4.
fact, a high excess of ozone with respect to the substrate is in solution in these experiments. Under these conditions it was possible to obtain the pseudo-first-order reaction rates of compound (1) at pH 4 for different temperature. The measure of the concentration of ozone at the different temperatures as reported in Fig. 5 allowed calculations of the second-order rate constants. The results are shown in Table 1. This allowed us to calculate a value of F = 0.962 and l n A = 5 4 . 5 , AH + = 3 0 . 6 K c a l m o l , AS + = 47.8 cal tool °K. A kinetic experiment performed at pH 9.5 at 20°C gave the pseudo-first-order rate constant of 3.70 + 0.03 x 10-3 s. DISCUSSION
The ozonation of the two model compounds at pH 4 introduces one oxygen atom at the carbon atoms to oxygens. This chemoselectivity is related to the mode of transfer of that oxygen atom. A second feature is the preference for functionalization of the methylene groups located in the central part of the molecule. In other words, in the ozonation of ethylene glycol mono-n-octylether (1) at acidic pH only bonds a, b and c are broken and the reaction of bond a is a preferred pathway. Insert scheme here The chemoselective functionalization of the carbon atoms ~ to oxygen may be understood in terms of an Table 1. The dependenceof k on the temperature,in the reactionof ethylene glycolmono-n-octylether(1) with ozone at pH 4 t (°K) 103 [ozone],,(mol/L)k(L mol s) 287.7 0.254 1.26 ± 0.08 292.9 0.209 2.40 ± 0.10 297.9 0.172 4.05 ± 0.22 302.8 0.144 16.6 ± 3.22 307.7 0.121 40.6 ± 1.13
electrophilic attack of ozone resulting in a rate-determining hydride transfer such as that depicted in Scheme 1 and the final formation of a hydrotrioxide (18). This mechanism has been postulated for the reaction of ethers with ozone in organic solvents (Giamalva et al., 1981). The reason for the preference of the reactions at the carbon atoms adjacent to oxygen is that the intermediate carbenium ion (17) is stabilized by resonance if it is in position ~ to oxygen. The acid-catalyzed rearrangement of this produces 2-hydroxyethyl octanoate (2) and hydrogen peroxide. Attack of ozone and hydride shift could also occur in a concerted way. The concerted alternative is ruled out by the positive value of the entropy of activation. Any concerted mechanism should have less flexibility in the transition state and a consequent negative entropy of activation. A negative entropy of activation is in fact noted in the low pH ozonation of aminotriazines, where a rate-determining coordination of ozone to the amino nitrogen atom occurs (Bolzacchini et al., 1994; Brambilla et al., 1993b),
~o
(~/H .O ~..~v~oJ~O
9-O-o-
I H ,=
(18)
J~V'V~o~OH
+H202
(2)
Scheme B.
(17)
~ ~ . . . O H +
Non-ionic surfactants and ozone whereas the ozonation of phenylureas is a non-concerted pathway since it has a positive entropy of activation (Brambilla et al., 1995a,b). The preference for the reaction of the more internal of the C-H bonds ~ to oxygen may be speculated to derive from the fact that ozone tends to occupy the lipophilic internal space of the mycelles generated by ethylene glycol mono-n-octylether (1) in aqueous solution. This should result in a proximity of ozone with the lipophilic region of ethylene glycol mono-n-octylether (1), where bond a and to a minor extent bond b are located. At pH 9.5 a much faster reaction and the presence of products deriving from the fragmentation of the alkyl chain indicates that a radical chain reaction is operating. Ozone is in fact decomposed at alkaline or neutral pH according to equations (1) and (2). The hydroxyl radical OH' is formed and is thought to be the most reactive species according to the SBH model (Hoigne, 1982): 03 + O H - ---*0~- + OHf
(1)
03 + OH2' -'-* 202 + OH'
(2)
1845 REFERENCES
Andreozzi R., Caprio V., D'Amore M. G. and Insola A. (1990) Quinoxaline ozonation in aqueous solution. Ozone Sci Eng. i2(3), 329-340. Andreozzi R., Caprio V. and Insola A. (1995) Kinetics and mechanism of polyethyleneglycolfragmentation by ozone in aqueous solution. 12th World Congress of the International Ozone Association, Lille, May 15-18, Vol. 1, pp. 53-63. Bolzacchini E., Brambilla A., Orlandi M., Polesello S. and Rindone B. (1994) Oxidative pathways in the degradation of triazine herbicides: a mechanistic approach. Wat. Sci. TechnoI. 30, 129-136. Brambilla A. M., Calvosa L., Monterverdi A., Polesello S. and Rindone B. (1993a) Ozone oxidation of polyethoxylated alcohols. Wat. Res. 27, 1313-1322. Brambilla A., Rindone B., Polesello S., Galassi S. and Balestuni R. (1993b) The fate of triazine pesticides in River Po water. Sci. Tot. Environ. 132, 339-348. Brambilla A., Bolzacchini E., Meinardi S., Orlandi M., Polesello S. and Rindone B. (1995a) Reactivity of organic micropollutants with ozone: a kinetic study. 209th American Chemical Society Meeting--Divisione of Environmental Chemistry, Anaheim, April 2-7, Vol. 35,
pp. 192-195. Brambilla A., Bolzacchini E., Meinardi S., Orlandi M., Polesello S. and Rindone B. (1995b) Reactivity of organic micropollutants with ozone: a kinetic study. 12th Worm Congress of the International Ozone Association, Lille,
This radical chain causes more decomposition of ozone and increased formation of hydroxyl radicals (Staehelin and Hoigne, 1983, 1985). Thus secondary oxidants such as hydroxyl radical, hydroperoxyl radical and hydrogen peroxide are produced by decomposition of ozone at high pH. Organic compounds react with hydroxyl radicals (OH') or hydroperoxyl radicals (HO2") and form secondary radicals (R') (equations (3) and (4)). Radicals R' react in ozonation conditions with excess oxygen and form peroxy radicals (equation (5) which behave as radical chain carriers to give hydroperoxides (equation (6)): OH' + R - H ~ R" + H20
(3)
HO2 + R - H ~ R' + H202
(4)
R' + 02 ~
ROO"
ROO" + R ' - H ~ ROOH + R"
(5)
(6)
The rearrangement of hydroperoxides gives carbonyt and carboxylic compounds which are responsible for chain breakdown. Two dicarboxylic acids, butanedioic acid (12) and pentanedioic acid (13) and a ketoacid, 4-ketopentanoic acid (13) are formed in the alkaline ozonation of ethylene glycol mono-n-octylether (1). These findings are in line with the suggestion (Brambilla et al., 1993a) that keto-acids such as 4-ketopentanoic acid (13) and 4-ketoheptanedioic acid (15) are the precursors of butanedioic acid (12), while 5-ketohexanoic acid (16) might be the precursor of pentanedioic acid (13) (Scheme 2).
May 15-18, Vol. 1, pp. 43-52. Caprio V., Insola A. and Silvestre A. M. (1989) Glyoxal ozonation process in aqueous solution. Ozone Sei. Eng. 11, 271-280. Calvosa L., Monteverdi A., Rindone B. and Riva G. (1991) Ozone oxidation of compounds resistant to biological degradation. War. Res. 25, 985-994. Cross J. (1987) Gas-Liquid Chromatography of non-ionic surfactants and their derivatives.In Non-ionic Surfactants (Edited by Cross J.), pp. 169 224, Vol. 19, Chemical Analysis, Surfactant Science series, Marcel Dekker Inc., New York. Giamalva D. H., Church D. F. and Pryor W. A. (1981) Kinetic of ozonation. 5, reactions of ozone with carbon-hydrogen bonds. J. Am. Chem. Sac. 108, 7678-7681. Glaze W. H. (1987) Drinking-water treatment with ozone. Environ. Sci. Technol. 21, 224~230. Hoigne J. (1982) Mechanisms, rates and selectivities of oxidations of organic compounds initiated by ozonation of water. In Ozone Technology and its Practical Applications (Edited by Rice R. G. and Netzer A.), Vol. l, pp. 341-379. Ann Arbor Science, Ann Arbor, M1. Kalmaz E. E. and Trieff N. M. (1986) Kinetics of ozone decomposition and oxidation of a model of organic compound in water. Chemosphere 15, 183-194. Legube B., Croue J. P., De Laat J. and Dare M. (1989) Ozonation of an extracted aquatic fulvic acid: theoretical and practical aspects. Ozone Sci. Eng. 11, 69-91. Narkis N., Ben-David B. and Schneider-Rotel M. (t987) Non-ionic surfactants interactions with ozone. Tenside Surf. Det. 24, 200-210. Niki E., Yamamoto E., Saito T., Nagano K., Yokoi S. and Karniya Y. (1983) Ozonization of organic compounds. VII. Carboxylic acids, alcohols and carbonyl compounds. Bull. Chem. Sac. Jpn. $6, 223-228. Reynolds G., Corless C., Graharn N., Pelry R., Gibson T. M. and Haley J. (1989) Aqueous ozonation of fatty acids. Ozone Sci Eng. 11, 143-154. Staehelin J. and Hoigne J. (1983) Mechanism and kinetics of decomposition of ozone in water in presence of organic solutes. Vom Wass. 61, 337-348.
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Staehelin J. and Hoigne J. (1985) Decomposition of ozone in water in the presence of organic solutes acting as promoters and inhibitors of radical chain reactions. Environ. Sci. Technol. 19, 1206-1213. Takanasbi N. (1990) Ozonation of several organic
compounds having low molecular weight under ultraviolet irradiation. Ozone Sci. Eng. 12, 1-17. Takanashi N. and Katuski O. (1990) Decomposition of ethylene glycol by the combined use of ozone oxidation and electrolytic methods. Ozone Sci. Eng. 12, 115-131.
Wat. Res. Vol. 31, No. 8, pp. 1839-1846, 1997 © 1997 ElsevierScienceLtd. All rights reserved Printed in Great Britain 0043-1354/97 $17.00+ 0.00
REACTIVITY OF TWO MODELS OF NON-IONIC SURFACTANTS WITH OZONE A N N A BRAMBILLA% EZIO BOLZACCHINP, MARCO O R L A N D I l, STEFANO POLESELLO 2 and BRUNO RINDONE~*~ 'Department of Environmental Sciences, University of Milano, Via Emanueli 15, 1-20126 Milano, Italy and 2Department of Inorganic, Organometallic and Analytical Chemistry, University of Milano, Via Venezian 21, 1-20133 Milano, Italy (First received September 1995; accepted in revised form August 1996)
Abstract--The ozonation of ethylene glycol and diethylene glycol mono-n-octylether (models of non-ionic surfactants) has been performed in aqueous solution at pH 4 and pH 9.5. The experiments at acidic pH show that the methylene groups adjacent to the ethereal oxygens are transformed into carbonyl groups. At alkaline pH the aliphatic chain is cleaved. Kinetic experiments allow to suggest a reaction mechanism. © 1997 Elsevier Science Ltd Key words--aqueous ozonation, non-ionic surfactants, mono- and diethoxylated alcohols, trimethylsilyl derivatives
INTRODUCTION Many studies are intended to reveal the by-products of the ozonation of several polluting organic compounds. These might be even more dangerous than the substrates themselves (Legube et al., 1989; Reynolds et al., 1989). The identification of ozonation products is also important to investigate the reaction mechanisms that lead to their formation (Caprio et al., 1989; Andreozzi et al., 1990; Takanashi and Katsuki, 1990; Takanashi, 1990). Non-ionic surfactants are not destroyed in conventional biological degradation plants, and a chemical treatment is required to allow the concentration of these compounds to fit regulations. In the treatment of non-ionic surfactants ozonation causes alterations in the molecular structure, resulting in changes in characteristics such as foaming ability, complexation with bismuth or cobaltothiocyanate and increased water solubility. These surfactants are not completely broken down into CO2 and HzO, as reflected in the high TOC and COD residual concentrations (Narkis et al., 1987). As a consequence, although surface activity is destroyed by ozonation a large organic load remains and creates potential environmental pollution (Glazer, 1987). Thus ozone treatment alone, despite its many advantages, is inadequate for the total removal of non-ionic surfactants. However ozonation can be used to increase the biodegradability of the biologically resistant compounds: small doses of ozone can be sufficient to change the structure of the *Author to whom all correspondence should be addressed [Fax: +39 2 6447 4300].
non-ionic surfactant, making it more amenable to bacteriological breakdown and enhancing the biodegradable properties of the material. Previous work (Niki et al., 1983) investigated the ozonation in buffered solutions of small aliphatic alcohols, ketones and acids and small bifunctional molecules containing carboxyl groups. These studies showed that alcohols were oxidized to the corresponding carbonyl compounds and that aliphatic chains were functionalized by introduction of hydroxyl and keto groups. Fragmentation was also observed, with formation of carboxylic acids. In all cases, the rate of ozonation increased with increasing the pH of the medium. At low pH, small saturated acids were stable toward ozone at 30°C, but were attacked at an appreciable rate at higher pH. The presence of organic peroxides and hydrogen peroxide was also revealed in the reaction medium. A 1,3-dipolar attack of ozone to C - H bonds and hydrotrioxide intermediates was postulated. Very recent studies about the fate of the fragments deriving from the ozonation of non-ionic surfactants have also been made (Andreozzi et al., 1995). In order to better understand the ability of ozone to degrade non-ionic surfactants in aqueous solutions we studied the behaviour of a mixture of ethoxylated phenols toward ozone (Calvosa et al., 1991). A normal phase HPLC analysis showed decreased overall polarity of the mixture after ozonation as a result of the partial or total loss of the oxyethyl chain of these compounds. Inspection of the ozonation products from an ethoxylated alcohol mixture (Brambilla et al., 1993a) showed the formation of linear carboxylic acids. Some of these arose from aliphatic chain fragmenta-
1839
A. Brambilla et al.
1840
tion. Other products deriving from carbon chain fragmentation were also present: a) hydroxy and dihydroxy acids, b) ketoacids as 4-ketopentanoic acid and 5-ketohexanoic acid, c) dicarboxylic acids. Other products can be attributed to oxyethyl chain modification: a)
Glycolic acid might derive from acetic acid oxidation, but can more easily be considered as the oxidative depolymerization product of the oxyethyl chains, b) di-, tri- and tetraethyleneglycol, c) the mono- and diacid corresponding to di- and triethyleneglycol. These results stimulated a more systematic study intended to evaluate the ozonation mechanism at different pH. Pure starting alkylethers were needed for this purpose. Hence, two models of non-ionic surfactant, ethylene glycol mono-n-octylether and diethylene glycol mono-n-octylether were chosen as the starting material. MATERIALS
AND METHODS
Sodium acetate, acetic acid, sodium carbonate, sodium bicarbonate and anhydrous sodium sulfate were all analytical grade from Merck (Bracco S.p.A., Milan, Italy). Hexamethyldisilazane (HMSD) and trimethylsilylchloride (TMSC) were reagent grade from Aldrich (BH-Schilling, Milan, Italy). Ethyl acetate was purified by distillation. Benzene was distilled and dried over sodium in nitrogen atmosphere. Sample ozonation, extraction and preparation Ozone was produced in a Fisher 501 generator fed with oxygen. The ozonation was carried out using 1 L of an aqueous solution 10-3M in compounds ethylene glycol mono-n-octylether (1) or diethylene glycol mono-noctylether (9) adjusted at the required pH. Before ozonation the solution of the substrate was saturated with oxygen for 2 min. A 70 L/h stream of oxygen was subjected to a 200 mA electrical current and then passed through the solution. The amount of ozone produced in these conditions was 3.3 g/h. After the given time nitrogen was bubbled into the solution for 5 rain. Acidic conditions (pH = 4.0) were obtained by addition of a dilute solution of sulphuric acid and testing the pH with a Toptronic digital pH-meter: in these conditions the pH remained constant all over the reaction time. Alkaline conditions (pH = 9.5) were obtained by addition of a dilute solution of potassium hydroxide. During the reaction, acidification occurred due to the formation of carboxylic acids. The alkaline pH was held constant by addition of a 0.0713 N solution of potassium hydroxide. The reaction solutions were acidified to pH = 3 4 and extracted three times with 750 mL of ethyl acetate. The collected extracts were dried over anhydrous sodium sulphate for 1 night, evaporated under reduced pressure, and weighed. The samples were diluted with anhydrous benzene, derivatized via reaction with HMDS and TMSC (Cross, 1987) and analyzed by high resolution gas chromatography (HRGC), and HRGC-positive ion electron impact mass spectrometry (PIEIMS).
High resolution gas chromatography (HRGC), high resolution gas chromatography/mass spectrometry (HRGC/MS) HRGC was performed on a Varian instrument (Model 6000) equipped with a flame ionization detector (FID) and a split/splitless injector, connected to a fused silica capillary column (Supelco, SPB-5, 30m, film thickness 0.25#, internal diameter 0.32 ram). Samples of 1 pL containing 10 #g of silylated compounds were injected onto the column at room temperature; stream splitting was activated 2.0 min after the injection. Nitrogen was used for the carrier gas with a flow rate of 1.2 mL/min. The temperature program was 70°C (2 rnin), 70-270°C (4°C/min)and 270°C (isothermal). Injector and detector were kept at 250°C. Electronic integration of the gas chromatographic peak areas was performed with a Varian integrator (Model 4270). A VG mass spectrometer (Model 707 EQ) was connected with a DANI gas chromatograph (Model 3800). The capillary GC column was directly coupled to the ion source. GC column and procedures were as described above for the GC analyses. For the PIEIMS, the conditions were the following: reactant gas isobutane, mass range, 40-600 m/z. An HP mass selective detector (Model 5971 A) was coupled to an HP gas chromatograph (Model 5890) using helium as the carrier gas with a head pressure of 8.0 psi. GC column and procedures were as described above. Kinetic experiments Forty mililiters of water (grade Milli Q) were adjusted at pH 4 with sulphuric acid were treated with a 20 L/h stream of oxygen subjected to a 2.2 amp electric discharge for 5 min. Ten milliliters of a solution of ethylene glycol mono-n-octylether (1) in water pH 4 for sulphuric acid were then added. The final concentration of the compound was
~OH
•f•,O
~
o
/
(1)
~
/
~
<2) O
(3)
HO~ ° H (6)
(s) O
(7) (s)
o
(9) o
(1o)
,~o/~/o,,~o. (11)
Scheme A.
Non-ionic surfactants and ozone
184i
100 90 80 70 60 >-
50 40 30 20 J
10
0 0
10
20
30
I
I
I
40
50
60
time, min Fig. 1. Time course of the reaction of compound (1) ( 0 ) with ozone at pH 4 and of the formation of compounds (2) (1) and (3) (0). 5.6 x 10-4 mol/L. Oxygen subjected to a 2.2 amp electric discharge was then admitted at 298°K in the reaction vessel at a flow of 40 L/h. Two milliliters of solution were withdrawn at known intervals. Nitrogen was bubbled for 5 rain. Addition of sodium chloride and extraction with three 2 mL portions of methylene chloride to which the internal standard diethylene glycol mono-n-octylether (9) had been added allowing recovery of the organic material after drying with sodium sulphate and evaporation under reduced pressure at room temperature. The sample was then treated for 15rain with hexamethyldisilazane and trimethylchlorosilane, evaporated with a nitrogen stream and analyzed by GLC-MS.
Saturation of the solution with ozone Fifty milliliters of water adjusted at pH 3 with sulphuric acid was treated with a 10 L/h stream of dioxygen subjected to a 2.2 amp electric discharge at a flow of 20 lh and 40 L/h. Two milliliters of solution were withdrawn at 1 min intervals and the absorbance at 260 nm was monitored (UV Jasco Mod 7800),
Preparation of 2-hydroxyethyl octanoate (2) Two hundred microliters (1.2 mmol) of octanoic acid (4) was added to 2 mL (36 mmol) of ethylene glycol (6). One drop of concentrated sulphuric acid was added and the resulting mixture was stirred at 200°C overnight. After that, the mixture was poured in 10 mL of water and extracted three times with 10 mL portions of ethyl acetate and with one I0 mL portion of methylene chloride. The organic extracts, washed with a saturated sodium hydrogenocarbonate solution and with a sodium chloride solution, dried over sodium sulphate and evaporated at reduced pressure gave a mixture containing 50% of 2-hydroxyethyl octanoate (2).
Preparation of n-octyl glycolate (3) A solution containing 200 mg (0.37 retool) of glycolic acid (8), 3 ml of n-oetanol (7) and two drops of concentrated sulphuric acid was refluxed for 3 h. After that, the mixture was poured into 10 mL of water and extracted three times with 10 mL portions of ethyl acetate and with one 10 mL portion of methylene chloride. The organic extracts, washed with a saturated sodium bicarbonate solution and with a sodium chloride solution, dried over sodium sulphate and evaporated at reduced pressure gave a mixture containing 90% of n-octyl glycolate (3).
Preparation of 2-n-octyloxy acetic acid (5) Two hundred microliters of ethylene glycol mono-n-octyl ether (I) in 10 ml acetone were cooled to 0°C and a solution of potassium dichromate in concentrated sulphuric acid was added dropwise until the brown colour persisted. The excess of oxidant was consumed adding a few drops of methanol and the resulting suspension was concentrated at reduced pressure to give a suspension which was poured in 10 mL of water and extracted three times with 10 mL portions of ethyl acetate, and with one 10 mL portion of methylene chloride. The organic extracts, washed with a saturated sodium bicarbonate solution and with a sodium chloride solution, dried over sodium sulphate and evaporated at reduced pressure gave a mixture containing 80% of 2-n-octyloxy acetic acid (5). RESULTS AND DISCUSSION
Product studies The first model compound of a non-ionic surfactant was ethylene glycol mono-n-octylether (1). The preparative ozonation at pH 4 was followed by extraction, silylation and H R G C - M S analysis, showing that oxygenation of the carbon atoms adjacent to the oxygen atoms had occurred. 2-hydroxyethyl octanoate (2, 87% of the converted starting material), and its hydrolysis product octanoic acid (4), n-octyl-glycolate (3, 13%) and 2-n-octyloxy acetic acid (5, 1%) were obtained. Confirmation of their structure was obtained by independent synthesis. 2-hydroxyethyl octanoate (2) was prepared by reaction of octanoic acid (4) with ethylene glycol (6); n-octyl-glycolate (3) from the reaction of glycolic acid (8) with n-octanol (7), and 2-n-octyloxy acetic acid (5) was obtained by chromic oxidation of (1). The time course of the reaction showed that the oxygenation of ethylene glycol mono-n-octylether (1) to 2-hydroxyethyl octanoate (2) and octanoic acid (4) derived from three competitive pathways (Fig. 1). The second model of a non-ionic surfactant was
1842
A. Brambilla et al.
(12)
(14)
Fig. 2. Gas chromatographic analysis of the reaction mixture from the ozonation of compound (1) at pH 9.5 after silylation.
diethylene glycol mono-n-octylether (9). Under the same conditions a much slower reaction occurred and the major reaction product (85% selectivity) was diethylene glycol mono-n-octanoate (10). Its structure was demonstrated by the formation of n-octanoic acid (3) on hydrolysis. A minor component was probably O-(n-octyloxyethyl)glycolic acid (11), which was responsible for the formation of ethylene glycol mono-n-octylether (1) in the reaction mixture. A very different reactivity was exhibited by ethylene glycol mono-n-octylether (1) on ozonation at pH 9.5. A very complex mixture resulted from the much faster reaction (Fig. 2). HRGC-MS after silylation showed the presence of unreacted compound (1) and of reaction products deriving from fragmentation of the carbon skeleton of the n-octyl chain. The most abundant reaction product was butanedioic acid (12). Other reaction products were pentanedioic acid (13), 4-oxopentanoic acid (14) and glycolic acid (8).
The reaction in acidic solution is: C + zO3 ---, intermediates where z is the stoichiometric ratio, z was found to be generally 1 (Staehelin and Hoigne, 1983, 1985; Kalmaz and Trieff, 1986). According to this reaction
OH
HO
\
OH
/
1
O H O ~
G~
H O ~ C ~
(ts)
(12)
t
Kinetic studies
The selective regiochemistry of the reaction at low pH and the encouraging fragmentation of the carbon chain at high pH could be better understood by performing a mechanistic study.
116)
Scheme B.
1843
Non-ionic surfactants and ozone
0.9
e
0.8 0.7 0.6 E ¢0 ',0
<
0.5 0.4 0.3 0.2 0.1
I
0 1
-0.1
2
3
4
5
6
7
8
time, min Fig. 3. Time course of the dissolution of ozone in water pH 4 at a flow rate of 40 ( 0 ) and 20 (0) L/h.
the expression of the rate for the ozonation of the substrates is - d[C]/dt =
kd/z[C][03]
first-order in the substrate and first-order in ozone. Kinetic measurements are frequently carried out using a pseudo-first-order approach with respect to ozone. This implies using excess substrate. A very precise measurement of the concentration of ozone is also necessary. We used a different approach: our kinetic experiments were performed using a pseudofirst-order approach with respect to the substrate. This should minimize the experimental error. The kinetic expression is d[C]/dt =
k'd/z[C]
where k'd = kd[O3] The dissolved ozone concentration reaches its stationary and maximum value within 3-6 min of presaturation with either a 40 L/h or 20 L/h stream of ozone (Fig. 3). The validity of the assumption that the concentration of ozone was constant throughout the experiments was assayed measuring the change of the concentration of ozone in the solution on addition of the substrates. Here it could be shown that if the solution is presaturated with a 20 L/h or 40 L/h stream of ozone for 6 or 3 rain, respectively, no decrease of ozone concentration is noticed on addition of ethylene glycol mono-n-octylether (1), at a final concentration of ca. 10 4 mol/L (Fig. 4). In
0.7
0.5 E 0.4 *o ¢q
,,, 0.3
<
0.2 0.1
0
I
I
I
I
I
I
I
I
50
100
150
200
250
300
350
400
tlme, sec Fig. 4. Effect of addition of ethylene glycol mono-n-octylether (l) to a solution presaturated for 3 rain with a 40 L/h stream of ozone at pH 4.
1844
A. Brambilla et al. 2.5
E 01.5 N
( 0.5
0 278
I
I
I
I
I
I
I
I
I
I
280
282
284
286
288
290
292
294
296
298
T a'q~ccdure,
oK
Fig. 5. The influence of the temperature on the concentration of ozone at pH 4.
fact, a high excess of ozone with respect to the substrate is in solution in these experiments. Under these conditions it was possible to obtain the pseudo-first-order reaction rates of compound (1) at pH 4 for different temperature. The measure of the concentration of ozone at the different temperatures as reported in Fig. 5 allowed calculations of the second-order rate constants. The results are shown in Table 1. This allowed us to calculate a value of F = 0.962 and l n A = 5 4 . 5 , AH + = 3 0 . 6 K c a l m o l , AS + = 47.8 cal tool °K. A kinetic experiment performed at pH 9.5 at 20°C gave the pseudo-first-order rate constant of 3.70 + 0.03 x 10-3 s. DISCUSSION
The ozonation of the two model compounds at pH 4 introduces one oxygen atom at the carbon atoms to oxygens. This chemoselectivity is related to the mode of transfer of that oxygen atom. A second feature is the preference for functionalization of the methylene groups located in the central part of the molecule. In other words, in the ozonation of ethylene glycol mono-n-octylether (1) at acidic pH only bonds a, b and c are broken and the reaction of bond a is a preferred pathway. Insert scheme here The chemoselective functionalization of the carbon atoms ~ to oxygen may be understood in terms of an Table 1. The dependenceof k on the temperature,in the reactionof ethylene glycolmono-n-octylether(1) with ozone at pH 4 t (°K) 103 [ozone],,(mol/L)k(L mol s) 287.7 0.254 1.26 ± 0.08 292.9 0.209 2.40 ± 0.10 297.9 0.172 4.05 ± 0.22 302.8 0.144 16.6 ± 3.22 307.7 0.121 40.6 ± 1.13
electrophilic attack of ozone resulting in a rate-determining hydride transfer such as that depicted in Scheme 1 and the final formation of a hydrotrioxide (18). This mechanism has been postulated for the reaction of ethers with ozone in organic solvents (Giamalva et al., 1981). The reason for the preference of the reactions at the carbon atoms adjacent to oxygen is that the intermediate carbenium ion (17) is stabilized by resonance if it is in position ~ to oxygen. The acid-catalyzed rearrangement of this produces 2-hydroxyethyl octanoate (2) and hydrogen peroxide. Attack of ozone and hydride shift could also occur in a concerted way. The concerted alternative is ruled out by the positive value of the entropy of activation. Any concerted mechanism should have less flexibility in the transition state and a consequent negative entropy of activation. A negative entropy of activation is in fact noted in the low pH ozonation of aminotriazines, where a rate-determining coordination of ozone to the amino nitrogen atom occurs (Bolzacchini et al., 1994; Brambilla et al., 1993b),
~o
(~/H .O ~..~v~oJ~O
9-O-o-
I H ,=
(18)
J~V'V~o~OH
+H202
(2)
Scheme B.
(17)
~ ~ . . . O H +
Non-ionic surfactants and ozone whereas the ozonation of phenylureas is a non-concerted pathway since it has a positive entropy of activation (Brambilla et al., 1995a,b). The preference for the reaction of the more internal of the C-H bonds ~ to oxygen may be speculated to derive from the fact that ozone tends to occupy the lipophilic internal space of the mycelles generated by ethylene glycol mono-n-octylether (1) in aqueous solution. This should result in a proximity of ozone with the lipophilic region of ethylene glycol mono-n-octylether (1), where bond a and to a minor extent bond b are located. At pH 9.5 a much faster reaction and the presence of products deriving from the fragmentation of the alkyl chain indicates that a radical chain reaction is operating. Ozone is in fact decomposed at alkaline or neutral pH according to equations (1) and (2). The hydroxyl radical OH' is formed and is thought to be the most reactive species according to the SBH model (Hoigne, 1982): 03 + O H - ---*0~- + OHf
(1)
03 + OH2' -'-* 202 + OH'
(2)
1845 REFERENCES
Andreozzi R., Caprio V., D'Amore M. G. and Insola A. (1990) Quinoxaline ozonation in aqueous solution. Ozone Sci Eng. i2(3), 329-340. Andreozzi R., Caprio V. and Insola A. (1995) Kinetics and mechanism of polyethyleneglycolfragmentation by ozone in aqueous solution. 12th World Congress of the International Ozone Association, Lille, May 15-18, Vol. 1, pp. 53-63. Bolzacchini E., Brambilla A., Orlandi M., Polesello S. and Rindone B. (1994) Oxidative pathways in the degradation of triazine herbicides: a mechanistic approach. Wat. Sci. TechnoI. 30, 129-136. Brambilla A. M., Calvosa L., Monterverdi A., Polesello S. and Rindone B. (1993a) Ozone oxidation of polyethoxylated alcohols. Wat. Res. 27, 1313-1322. Brambilla A., Rindone B., Polesello S., Galassi S. and Balestuni R. (1993b) The fate of triazine pesticides in River Po water. Sci. Tot. Environ. 132, 339-348. Brambilla A., Bolzacchini E., Meinardi S., Orlandi M., Polesello S. and Rindone B. (1995a) Reactivity of organic micropollutants with ozone: a kinetic study. 209th American Chemical Society Meeting--Divisione of Environmental Chemistry, Anaheim, April 2-7, Vol. 35,
pp. 192-195. Brambilla A., Bolzacchini E., Meinardi S., Orlandi M., Polesello S. and Rindone B. (1995b) Reactivity of organic micropollutants with ozone: a kinetic study. 12th Worm Congress of the International Ozone Association, Lille,
This radical chain causes more decomposition of ozone and increased formation of hydroxyl radicals (Staehelin and Hoigne, 1983, 1985). Thus secondary oxidants such as hydroxyl radical, hydroperoxyl radical and hydrogen peroxide are produced by decomposition of ozone at high pH. Organic compounds react with hydroxyl radicals (OH') or hydroperoxyl radicals (HO2") and form secondary radicals (R') (equations (3) and (4)). Radicals R' react in ozonation conditions with excess oxygen and form peroxy radicals (equation (5) which behave as radical chain carriers to give hydroperoxides (equation (6)): OH' + R - H ~ R" + H20
(3)
HO2 + R - H ~ R' + H202
(4)
R' + 02 ~
ROO"
ROO" + R ' - H ~ ROOH + R"
(5)
(6)
The rearrangement of hydroperoxides gives carbonyt and carboxylic compounds which are responsible for chain breakdown. Two dicarboxylic acids, butanedioic acid (12) and pentanedioic acid (13) and a ketoacid, 4-ketopentanoic acid (13) are formed in the alkaline ozonation of ethylene glycol mono-n-octylether (1). These findings are in line with the suggestion (Brambilla et al., 1993a) that keto-acids such as 4-ketopentanoic acid (13) and 4-ketoheptanedioic acid (15) are the precursors of butanedioic acid (12), while 5-ketohexanoic acid (16) might be the precursor of pentanedioic acid (13) (Scheme 2).
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Staehelin J. and Hoigne J. (1985) Decomposition of ozone in water in the presence of organic solutes acting as promoters and inhibitors of radical chain reactions. Environ. Sci. Technol. 19, 1206-1213. Takanasbi N. (1990) Ozonation of several organic
compounds having low molecular weight under ultraviolet irradiation. Ozone Sci. Eng. 12, 1-17. Takanashi N. and Katuski O. (1990) Decomposition of ethylene glycol by the combined use of ozone oxidation and electrolytic methods. Ozone Sci. Eng. 12, 115-131.