Environ. Sei. Technol. 1992, 26, 1059-1064
Long-Term Ozone Consumption by Aquatic Fulvic Acids Acting as Precursors of Radical Chain Reactions Feng Xlong, Jean-Phlllppe CrouO, and Bernard Legube"
Laboratoire de Chimie de I'Eau et des Nuisances (UA CNRS No. 14681, ESIP, Universite de Poitiers, 86022 Poitiers, France
A preozonated commercial humic acid has been reported to undergo radical chain reactions with ozone. This study reports the ozone consumption by a number of aquatic fulvic acids. In the presence of OH-radical scavengers, it was found that the relative ozone consumption increased as the initial ozone dose, pH, and relative UV absorbance of fulvic acids increased. This consumption became greater, however, in the absence of scavengers, demonstrating undoubtedly the occurrence of radical chain reactions between the fulvic acids and ozone. The intermediate fulvic acid ozonation products, namely, hydrogen peroxide and glyoxalic acid, were also identified as precursors of radical chain reactions. Introduction Humic substances comprise the major component of dissolved organic matter in water ( I ) . Compared to the amount of literature concerning the chlorination of humic substances, however, only a few studies have been carried out on the reactions of ozone with these substances. The results of such work in the literature have usually been expressed in terms of the mass ozone consumption versus humic or carbon mass (2-4), although some investigators have reported the determination of apparent first-order rate constants for ozone consumption (5-7). In water, ozone may either react directly with dissolved substances or decompose to form secondary oxidants such as OH radicals, which may then lead to a series of radical chain reactions accompanied by further ozone consumption. Solutes are classified into three categories: initiators, which lead to the formation of OH radicals; promoters, which maintain radical chain reactions; and scavengers, which cause the termination of radical chain reactions (for mechanistic details see, e.g., ref 5). According to the reaction mechanism, the presence of radical scavengers in solution reduces the rate of ozone consumption where ozone radical chain reactions would occur. Our previous work (7) has demonstrated than an aquatic fulvic acid may be involved in radical chain reactions with ozone from the very early stages of the reaction. In addition, Staehelin and Hoign6 (5) have reported that a preozonated commercial humic acid may participate in the propagation of radical reactions. The exact mechanism has, however, not been elucidated. The objective of this research was to determine both the relative ozone consumptions of a number of aquatic fulvic acids and the effects of different reaction parameters, such as initial ozone dose, pH, relative UV absorbances of the fulvic acids, and the presence of radical scavengers, on this consumption. Particular emphasis was placed upon the effect of radical scavengers on the ozone consumption to verify the presence of radical chain reaction precursors. Efforts have also been undertaken to identify some of these precursors. Experimental Section Fulvic Acids. Fulvic acids were extracted from nine French surface waters using the XAD-8 macroreticular resin procedure, as developed by Thurman and Malcolm 0013-936X/92/0926-1059$03.00/0
(8) and recommended by the International Humic Substances Society (9). Table I presents the origins and DOC of waters, relative UV absorbances, and elemental analyses of the fulvic acids under study (for further details, see ref 10). Ozonation Method. The ozonation experiments were carried out in 25-mL obturated glass flasks, which had been previously soaked in dichromic acid solution (HzS04 + K2Cr207)and rinsed with high-purity water (Milli-Q system). Fulvic acid stock solutions were prepared by dissolving a preweighed mass of the fulvic extracts in the high-purity water, so that upon addition of a phosphate-buffered solution, with or without a radical scavenger (tert-butyl alcohol or bicarbonate ions), and an ozone stock solution, the experimental conditions shown in the figures and tables might be obtained. Under neutral pH conditions (pH 7.5 and [HCO,] = M, as quoted in the following section), initial bicarbonate concentration was actually -0.94 X M while initial carbon dioxide concentration was nearly 6 X lo4 M. Since COz is slightly oversaturated M), a small portion of the (with solubility of -2 X initial radical scavenger (HC03-) is likely to volatilize as C02 during experimentation, despite the low temperature and use of flask stoppers. Ozone stock solutions were also prepared in the highpurity water. Ozone was produced from oxygen by a Trailigaz ozone generator (Labo 76). Ozone concentrations in solution were determined by the indigo method (11). Once the aqueous ozone concentration had been determined, certain volumes of ozone solutions were quickly added to the flasks containing the buffered fulvic acid solutions. The reaction was stopped after a preset reaction time by adding an indigo solution, which quenched the residual ozone. The indigo decoloration measured at 610 nm indicated the amount of residual ozone (11). Specific Analytical Methods. The DOC was measured with a DC 80 Dohrman carbon analyser, using UVpersulfate oxidation. The concentration of hydrogen peroxide formed during ozonation of the fulvic acids was determined by a spectrophotometric method (12) in which N,N-diethyl-p-phenylenediamine(DPD) is oxidized by Hz02in the presence of a peroxidase. Humic substances do not interfere with this method of quantification (12). The analytical procedure for the determination of aldehydes and glyoxalic acid in the ozonated fulvic acid solution is shown in Figure 1. This method, similar to that proposed by Yamada and Somiya (13), is based on a double-derivatization procedure using 0-(2,3,4,5,6-pentafluorobenzy1)hydroxylamine (PFBHA) as a derivatizing agent for the carbonyl function in aqueous solution and diazomethane as a methylating agent for the carboxyl function in organic solution. The extracts obtained after diazomethane derivatization were analyzed by gas chromatography (Chrompack Model 437A) equipped with a 63Ni electron capture detector (GC/ECD) or Varian-Finnigan ITS 40 GC/MSD. A 50 m X 0.32 mm CPSIL 5 column (Chrompack) was used for GC/ECD analysis with a temperature program as follows: 50 "C isothermal for 2 min, rising to 300 "C at 3 "C/min,
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26, No. 5, 1992
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Table I. Characteristics of the Aquatic Fulvic Acids
a
fulvic acid (water origin, DOC content mg L-I)
re1 UV abs"
%C
%H
70 0
%N
Cebron, CFA (dam, 15.1) Gartempe, GFA (river, 9.9) Landes, LFA (lake, 8.0) Moulin Papon, MPFA (dam, 9.0) Pinail, PFA (pond, 15.0) Seine, SeFA (river, 2.4) Sorme, SOFA (dam, 8.6) Vienne, VFA (river, 6.9) Villejean, VjFA (dam, 7.2)
0.0309 0.0322 0.0215 0.0278 0.0358 0.0224 0.0298 0.0339 0.0258
50.7 52.2 53.4 51.8 54.6 51.7 51.8 50.2 52.3
5.1 5.1 5.3 5.3 4.8 5.7 5.7 4.9 5.2
38.9 37.8 30.0 37.9 37.7 38.4 37.9 38.5 38.8
1.8 1.7 1.1 1.8 0.8 1.5 1.7 1.0 1.3
7 0
s
1.3 1.6 1.3 2.4 1.1
5.1 0.7
Relative UV absorbance was measured at 270 nm and is quoted in units of crn-'/mgL-' DOC.
2 rnL PFBHA, HCI(1 rng/mL)
i
ume(min)
&
0
0.75
1.20
u2
I
3
1.20 1.15
5
8
1.15
1.20 1 . 1 5
10
c
Storage (2 hrs)
8 drops HZSO4
4 mL ether with internal standard (200 kg)
Extraction and Phase Separation
6 -iEther Layer
Anhydrous Na2SO4 Diazornethane
< GCECD or
GC,MS>
Figure 1. Analytical procedure for identification of glyoxalic acid in ozonated fulvic acid solution using a double derivatization (PFBHA and CH"
hold at 300 "C for 4 min. A 25 m X 0.25 mm OV1701 column (Chrompack)was used for GC/MSD analysis with a temperature program as follows: 50 "C isothermal for 2 min, rising to 300 "C at 4 "C/min, hold at 300 OC for 5 min. Results Long-Term Ozone Consumption (LTOC). Our previous work (14,15) has demonstrated that a slight acidiM phosphate) and the presence of fication (pH 4-5, M tert-butyl alcohol as an OH-radical scavenger in solution allowed a satisfactory stability of molecular ozone. The present experiments of ozone consumption by fulvic acids were undertaken using similar conditions, but at 2 "C, the objective being to reduce the reaction rate, thus allowing the measurement of ozone concentrations from the very early phases of the reaction. Figure 2 presents ozone consumption in the case of Pinail fulvic acid (PFA) as an example. Independent of the fulvic acids under study, two phases of ozone consumption could be identified: a rapid initial phase lasting approximately 30 s and a second much slower phase which may last up to 10 min. The initial phase was previously investigated by means of kinetic modeling (7). The present paper deals with the overall ozone consumption of both phases, which will be referred to as the long-term ozone consumption. Under the experimental conditions (pH 4.7, 2 "C, [t-BuOH] = M), the ozone consumption after 5-min reaction corresponds approximately to the autodecomposition of ozone in solution without fulvic acids (see blank, Figure 2). This 5-min ozone consumption was thus chosen for determining the long-term ozone consumption in the present study. 1060
Envlron. Sci. Technol., Vol. 26, No. 5, 1992
Ozonation time (min) Flgure 2. Ozone consumptlon by different concentrations of PFA versus time: pH 4.7, [t-BuOH] = M, 2 "C, [0310/[PFA], = 0.6 mg of O,/mg of DOC.
2-
1Slope : 0.35 mg O3img DOC
0
2
3
PFA
6
(mg/L)
Figure 3. Long-term ozone consumption versus initial fulvic acid (PFA) concentration: pH 4.7, 2 "C, [t-BuOH] = M, 5 mln, [O,],/[FA], = 0.6-0.8 mg of O,/mg of DOC.
As shown in Figure 2, the LTOC depends upon the initial fulvic acid concentration. Furthermore, this LTOC has been determined for other initial fulvic acid concentrations, while the initial ozone dose ([03]O/[FA]o)remained nearly the same. Figure 3 presents as an example the results obtained with PFA. As shown in Figure 3, the LTOC increases linearly with initial fulvic acid concentration. The slope (mg of 03/mg of DOC) refers to the relative long-term ozone consumption for a fixed initial ozone dose under the given conditions. Experiments were also undertaken on six other fulvic acids. The results concerning the relative LTOCs are summarized in Table 11. The values in Table I1 demonstrate that the relative LTOCs vary from one fulvic acid to another. It was found
Table 111. Effect of tert-Butyl Alcohol on the Relative Long-Term Ozone Consumption at pH 2.6"
Table 11. Relative Long-Term Ozone Consumption of Aquatic Fulvic Acids fulvic acids
re1 O3 consumption (n)"
fulvic acids
re1 O3 consumption (n)"
GFA LFA MPFA PFA
0.31 f 0.04 (20) 0.21 f 0.04 (5) 0.27 f 0.06 (5) 0.35 f 0.03 (20)
SeFA VFA VjFA
0.23 f 0.03 (16) 0.32 f 0.03 (20) 0.26 f 0.03 (20)
fulvic acid
M, 2 "C, 5 min, [FA], "Determined a t pH 4.7, [t-BuOH] = = 1-6 mg L-' DOC, [03],/[FA], = 0.73 f 0.12 mg of 03/mg of DOC and quoted in units of mg of 03/mg of DOC. n, number of experiment runs.
CFA GFA LFA MPFA PFA SeFA SoFA VFA VjFA
[t-BuOH] = lo-' M (n) 0.07 f 0.01 0.07 f 0.02 0.04 f 0.01 0.05 f 0.01 0.15 i 0.02 0.03 (2) 0.05 f 0.01 0.06 f 0.02 0.04 f 0.02
(4) (4) (4) (4) (4) (4) (4) (4)
[t-BuOH] = 0 (n) 0.15 f 0.01 (4) 0.12 f 0.01 (4) 0.09 f 0.01 (4) 0.24 f 0.01 (4) 0.08 (1) 0.11 f 0.01 (4) 0.13 f 0.01 (4) 0.08 f 0.02 (4)
nozone consumption was determined at 2 "C, 5 min, [FA], = 1-6 mg L-' DOC, [03],/[FA], = 0.5-0.6 mg of 03/mg of DOC and quoted in units of mg of O,/mg of DOC. n, number of experiment runs.
Table IV. Effect of Bicarbonate Ions on the Relative Long-Term Ozone Consumption at pH 7.5" fulvic acid
I
04 0
1
2
3
4
5
6
7
Relative ozone dose (mg 03/mg DOC) Flgure 4. Relative long-term ozone consumption by PFA versus relative ozone dose: pH 4.7, [t-BuOH] = lo-' M, 2 O C , 5 min.
that fulvic acids possessing higher relative UV absorbances (Table I) had larger relative LTOCs, the correlation coefficient (CC) being 0.987 when relative LTOC was plotted versus relative UV absorbance of fulvic acids. Effect of Relative Ozone Dose (Ratio of Initial Concentrations). The relative LTOC was found to be proportional to the initial ozone dose (ratio of initial concentrations) for three fulvic acids (LFA, PFA, and VjFA). Figure 4 presents the results obtained with PFA. Anderson et al. (2) also observed similar results for the Black Lake fulvic acid ozonated in a semibatch reactor. Compared to the chlorine demands of fulvic acids, these observations are particularly surprising. I t has been established that, with an increasing chlorine dose, the chlorine demand increases and then tends toward an almost constant value (16). The relative LTOC, however, seems to rise consistently as the relative ozone dose increases. It is worth noting that relative LTOC does not appear strictly linear with relative ozone doses below 0.4 mg of 03/mg of DOC (doses largely applied in practice for water treatment). Effect of pH. Shown in Tables 11,111, and IV are the relative LTOCs for several fulvic acids at three pHs in the presence of OH-radical scavengers (tert-butyl alcohol at pHs 2.6 and 4.7 and bicarbonate ions at pH 7.5). By comparison of Table I1 and the second column of Table I11 on the one hand, and the third columns of Table I11 and Table IV on the other hand, it is evident that the higher the pH the greater the reactivity of fulvic acids with ozone. Note that ozone loss in the absence of fulvic acid is insignificant at pHs 2.6 and 4.7. However, ozone decomposition in the blank at pH 7.5 is significant but low compared to the values given Table IV. Indeed, for similar experimental conditions (1.03 mg/L Os,pH 7.5 (phosphate buffer), 2 "C), the ozone consumptions after 5 min with M) were found reand without bicarbonate ion (spectively to be 0.05 and 0.19 mg/L 03.As indicated above
CFA GFA MPFA PFA SeFA SoFA VFA VjFA
[HC03-] =
M (n)
0.43 i 0.01 (4) 0.48 f 0.01 (4) 0.34 f 0.01 (4) 0.44 f 0.05 (4) 0.39 f 0.03 0.39 f 0.01 0.44 i 0.01 0.43 f 0.01
(4) (4) (4) (4)
[HCOJ = 0 (n) 0.50 f 0.01 (4) 0.49 f 0.02 (4) 0.43 f 0.01 (4) 0.46 f 0.01 (4) 0.41 f 0.01 (4) 0.45 f 0.01 (4) 0.46 f 0.01 (4) 0.46 k 0.01 (4)
"Ozone consumption was determined a t 2 "C, 5 min, [FA], = 1-6 mg L-l DOC, [O,]O/[FA]o = 0.5-0.6 mg of 03/mg of DOC and quoted in units of mg of 03/mg of DOC. n, number of experiment runs.
for pH 4.7, the relative LTOC at pH 2.6 is dependent upon the nature of the fulvic acids. The linear relationship between the relative LTOC and relative UV absorbance (with CCs of 0.76 and 0.79 in the presence or absence of scavenger) is far less apparent than that for pH 4.7 (with a CC of 0.99), probably due to the fewer experiments repeated at pH 2.6 (four times for each fulvic acid). The linear relationship appears even worse for pH 7.5 when radical chain reactions take place. Effect of OH-Radical Scavengers. The effect of OH-radical scavengers on the relative LTOC of fulvic acids was investigated at acidic pH (tert-butyl alcohol as scavenger) and at neutral pH (bicarbonate ions as scavenger). The results obtained are summarized in Tables I11 and IV. I t is observed that scavengers reduce more or less the relative LTOC. At acidic pH, the average reduction in relative LTOC is -50%. Since no radicals are produced in pure water at this pH, fulvic acids must participate directly and/or after preozonation in the radical chain decomposition of ozone in water. At neutral pH, however, the average reduction in relative LTOC is much lower or perhaps nil, since the stability of ozone at pH 7.5 in the absence of fulvic acid was found to be enhanced by the presence of bicarbonate ion (see above). This observation could be explained by the fact that, in fulvic acids, the precursor sites initiating and promoting the radical chain decomposition of ozone are probably not the same at acidic pH as those at neutral pH. Identification of Glyoxalic Acid as a Precursor of Radical Chain Reactions at Acidic pH. Among the simple organic compounds listed by Staehelin and Hoign6 (5) as initiators and/or promoters of radical chain reactions with ozone, glyoxalic acid is the only one that is both an initiator and a promoter at acidic pH. By use of a double-derivatization procedure (PFBHA and diazomethane), Environ. Sci. Technol., Vol. 26, No. 5, 1992
I061
Table V. Mass Spectra of the Main GC/MS Peaksa compounds formaldehyde acetaldehyde unknown PFBHA derivative glyoxalic acid oxo acid 1 oxo acid 2 oxo acid oxo acid oxo acid oxo acid
3 4 5 6
MW RT
mass spectra, m / t (RI)
225 7.9 181 (loo), 195 (20.9), 161 (9.4), 182 (6.6), 117 (6.1), 99 (4.3), 93 (3.2), 167 (2.5) 239 12.8 181 (loo), 209 (12.5), 182 (8.4), 161 (5.4), 117 (3.5), 195 (3.3), 99 (3.1), 167 (3.0) 253 13.4 181 (loo), 182 (19.8), 254 (14.7), 253 (14.6), 206 (11.2), 56 (8.9), 72 (8.9), 223 (8.3), 434 (8.11, 236 (7.5), 161 (6.6) 283 22.1 181 (loo), 211 (8.8), 182 (7.21, 464 (5.51, 161 (4.8), 59 (4.5), 195 (4.2), 284 (3.41, 252 (2.2) 297 21.2 181 (loo), 59 (7.9), 182 (6.8), 57 (6.6), 161 (6.6), 117 (5.31, 99 (4.9), 195 (4.7), 71 (4.5), 298 (2.41, 112 (1.2) 267 (2.7), 297 22.8 181 (loo), 59 (17.51, 182 (6.7), 161 (4.9), 195 (4.3), 298 (4.2), 478 (4.0), 266 (3.9), 117 (3.51, 99 211 (2.3) 341 33.9 181 (loo), 59 (10.7), 211 (10.6), 182 (6.96), 522 (4.7), 195 (3.9), 161 (3.7), 342 (3.5), 117 (3.0), 99 (2.3), 119 (1.9) 351 34.9 181 (1001, 352 (18.41, 59 (15.41, 182 (6.6), 161 (5.4), 292 (4.9), 99 (3.8), 81 (3.81, 117 (3.31, 353 (2.8), 320 (1.9) 325 42.2 181 (loo), 309 (8.9), 506 (8.11, 182 (6.81, 161 (5.1), 311 (4.71, 195 (3.31, 117 (3.0), 99 (2.81, 59 (2.41, 325 (0.9) 339 42.9 181 (loo), 323 (13.4), 520 (lo&), 161 (6.5), 99 (3.9), 521 (3.7), 117 (3.4), 265 (3.4), 59 (3.2), 195 (3.0), 208 (3.01, 339 (0.8)
"MW, molecular weight of PFBHA and CHzNzderivatives; RT, retention time (min); RI, relative intensity with respect to the basic peak ( m / z 181).
' 0
-
CH,-0-h=CH-C
'0-CH,
B 0.
-. ? 5
. I
A
6
I
I
d l
'
"
Figure 5. GUMS (Ion trap) chromatogram of carbonyl and oxo acld compound derlvatlves from an ozonated fulvic acid analyzed using a double-derlvatizatlon procedure (PFBHA and CH,N,). GC conditions: 50 OC isothermal for 2 mln, rising to 300 OC at 4 OC/mIn, hold at 300 OC for 5 min.
glyoxalic acid was identified in an ozonated fulvic acid solution. Figure 5 shows a GC/MS total ion chromatogram for Sorme fulvic acid ozonated at acidic pH (pH 3.8, unbuffered, 7 mg/L DOC, ozone dose 2 mg of 03/mg of DOC). The 22.06-min retention time peak was identified as glyoxalic acid and confirmed with a standard. The electron impact mass spectrum of the methylated PFBHA-glyoxalic acid derivative is presented in Figure 6. As for all the PFBHA derivatives of carbonyl comthe mass spectrum pounds investigated by Glaze et al. obtained shows a base peak at m/z 181 and other specific peaks at m/z 161,117, and 99. The m/z 284 peak corresponds to (M l)+.The fact that this peak was present whatever the electron impact mode is probably due to the ion trap system used (Finnigan ITS 40). The mass shift of 1unit cannot be attributed to a chemical self-ionization or an ordinary chemical ionization, since no hydrogen occurred in these experiments with the exception of probably very low picogram to fentogram quantities of background water vapor. The unusually abundant (M + 1)+ions are probably caused by the data acquisition algorithm and broad-shifted ion peak, as already observed and mentioned by Slivon (18)with the same ion trap detector. The (M + l)+ions were not detected using a Finnigan INCOS 50 quadrupole GC/MS (not shown). Notice that the molecular ion m / z 283 was detected, but represented only 0.25% ( m / z 181,100%). The peak at m / z 59 corresponds to the (COOCH3)+ion from CHzNz methylation, and the peak at mlz 252 is apparently due
(In,
+
1082
Environ. Scl. Technol., Vol. 26, No. 5, 1992
.".
1
50
188
150
288
250
388
358
488
458
Flgure 8. Electron impact mass spectrum (ion trap MSD) of methylated PFBHA-glyoxalic acid derivative.
Table VI. Concentration of Glyoxalic Acid Formed during Ozonation of Sorme Fulvic Acida initial FA concn, mg L-I
glyoxalic acid concn, FM (clg L-l)
5.5 15
1.7 (130) 2.3 (170)
pH 3.8, unbuffered, 20 OC, relative a Ozonation conditions: ozone dose 1 me of Ol/me of FA. ~~~~
~
to the loss of OCH, from the molecular ion. The presence of the peak at m/z 464 is probably the result of a rearrangement between the molecular ion and the C6F6CH2+ ion. As shown in Figure 5, together with glyoxalic acid, six other oxo acids were formed from Sorme fulvic acid after ozonation. Table V summarizes mass spectra of the main GC/MS peaks. The unknown oxo compound could be either acetone or propanol, while oxo compounds 1-6 are, undoubtedly, oxo acids because of the presence of their two characteristic peaks (mlz 59 due to methylation of the carboxyl group and mlz 181 due to PFBHA derivatization of the carbonyl group). Those apparently similar oxo acids (compounds 1and 2) could be isomers of oxo-3-propanoic acid. Compound 3 has a molecular formula OHC(C3H6O)COOH, while compound 4 is OHC(C5H8)COOH(an unsaturated oxoacid). Compounds 5 and 6 may be oxo-5and oxo-6-pentanoic acids. Using a GC/ECD method and the standard curve, we were able to quantify the glyoxalic acid formed during the
Table VII. Concentration of Hydrogen Peroxide (PM) Formed during the Ozonation of Sorme Fulvic Acid" ozone dose, mg/L 0 1.0 4.2
pH 2.6
pH 7.5
0
0
3.4 8.8
1.1 1.2
"Ozonation conditions: [SoFAIo = 3 mg/L DOC, phosphate buffer I = M, with no OH-radical scavenger.
ozonation of SOFA. Table VI shows that for a relative ozone dose (1mg of 03/mg of FA), the amount of glyoxalic acid formed increases with the initial fulvic acid concentration. Identification of Hydrogen Peroxide as a Precursor of Radical Chain Reactions. Hydrogen peroxide in its dissociated form (HOC) has been speculated to be an initiator of radical chain reactions with ozone in water (19) and a promoter in both its protonated and dissociated forms (20). Therefore, attempts were made to analyze this specific compound. Subsequently, hydrogen peroxide was quantified in fulvic acid solutions ozonated at both acidic and neutral pHs (pHs 2.6 and 7.5) in the absence of an OH-radical scavenger. Table VI1 presents the results obtained with Sorme fulvic acid. The formation of hydrogen peroxide after the ozonation of Sorme fulvic acid at acidic pH was always greater than that at neutral pH. At neutral pH hydrogen peroxide, being slightly ionized, could be involved in the initiation step of the radical chain reactions with ozone. Moreover, once ionized, the peroxide ion (H02-) reacts much more rapidly with OH radicals than H202in the propagation step of radical decomposition of ozone. At acidic pH, the larger the ozone dose, the greater the concentration of Hz02 formed, which is probably due to the reactions of molecular ozone with organic compounds.
Discussion Relative Long-Term Ozone Consumption in the Presence of OH-Radical Scavengers. Initial results (Figures 2-4) were obtained in the presence of OH-radical scavengers at pH 4.7. Under these conditions, the radical decomposition of ozone is substantially inhibited (14,15), and ozone can be considered to be consumed through direct reactions and initiation reactions with no further ozone consumption by the propagation step. The results shown in Figure 3 demonstrate that, for a constant relative ozone dose, there are a certain amount of specific sites on the fulvic acid available for consuming ozone (the LTOC versus the initial fulvic acid concentration is linear). These specific sites strongly relate to the aromatic moieties or other conjugated double bonds (reflected by the relative UV absorbance at 270 nm). Furthermore, since an increasing amount of ozone is absorbed as more ozone is added (Figure 4),it appears that additional primary sites (e.g., slower reacting sites) become kinetically available to ozone at higher ozone doses. The results in Tables I11 and IV indicate that pH plays a large role in the LTOC of fulvic acids. Some acidic groups in the fulvic acids may dissociate with increasing pH and then may become more reactive with ozone (21). In addition, as pH increases, the decomposition of ozone could be further initiated by OH-. Both would explain a much higher ozone consumption at pH 7.5 than at pH 2.6. Presence of Ozone Radical Chain Reactions. In order to simplify the discussion, we will initially deal with the results obtained at pH 2.6 (Table 111). During the initial phase of reactions with ozone (a few seconds), the
nucleophilic sites of fulvic acids react quickly and selectively with molecular ozone, which refers to direct reaction of ozone. In this initial phase, the presence of an OHradical scavenger does not exert any significant effect on the ozone consumption, as detailed previously (7). However, during the second long-term phase of the reaction, the addition of an OH-radical scavenger dramatically reduces the ozone consumption (by as much as 50%), as shown in Table 111. These results indicate that, as the reaction moves forward, radical chain reactions with ozone take place. From the above, we may hypothesize that the radical chain reactions may be initiated and propagated by the slower reacting sites. These sites could be the primary ones (present in fulvic acids), but they could also be the secondary ones produced by an early reaction with ozone. This is confirmed by the identification of glyoxalic acid (initiator and promoter) as an ozonation product of fulvic acids at acidic pH. A preozonated humic acid has been similarly speculated to be involved in radical chain reactions with ozone (5). Staehelin and Hoign6 (5) have suggested that many solutes in water can react with ozone and then initiate the radical chain reactions of ozone. The main reactions known are O3+ OH- 02'-+ HOz' hi = 70 M-l s-l (19) O3 + HOz-
- +
02'- HOz'
O,+M03'-+ M,,
O3+ M
-
+ O2
ki = 2.8 X lo6 M-l s-l (19)
ki[HOOC-CHO]= 0.2 M-l s-l (21)
03'+ M,,
ki[-OOC-CHO]= 2 M-l s-l (21) According to the kinetic data and the concentration of ozonation products found in this study, at pH 2.6 glyoxalic s-l (though very acid with ki[glyoxalic acid] of 5 X small) is considered as an initiator of OH-radical production, compared to OH- with a product k,[OH-] of 6 X s-l and H02- with ki[H02-] of 2 X s-l. OH radicals, formed during the initiation by glyoxalic acid and probably other oxo acids, react with promoters (glyoxalic etc.) to maintain the radical acid, fulvic acid, H202,03, chain reactions by the following mechanisms: 'OH + M ... 0 2 ' - + M,, k, = 108-1010 M-1 s-l (5) *OH + O3 ... H02' + O2 k, = 3 X lo9 M-l s-l (5) *OH + H202 ... -* HOz' + H2O It, = 2.7 x 107 M-I s-l (20) As the scavengers result in the reduction of ozone consumption only by blocking the propagation step, the important ozone consumption decrease (50%)at pH 2.6 when scavenger is added (Table 111) signifies that the ozone consumption through the propagation reactions appears to be significant in this case. At pH 7.5, ozone radical chain reactions may take place from the very early period of the reaction (7). Moreover, the dissociation of fulvic acid and its ozonation products (H202/H02-,glyoxalic acid/glyoxalate ion, etc.), together with OH-, results in considerable increase in ozone consumption. At this pH, not only fulvic acid and its secondary organic species but also inorganic species might contribute to the initiation of radical chain reactions of ozone (e.g., ki[glyoxalate ion] = 4 X lo4 s-l, ki[OH-] 4 X s-l, ki[HO,] 2X s-l, etc.). In this case, ozone
-
--
-
-
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is essentially consumed through the initiation reactions and direct reactions rather than the propagation reactions. We actually observed only a small decrease in ozone consumption at pH 7.5 (10%) by adding scavenger, as shown in Table 1V. Literature Critique. The role of bicarbonate ions during ozonation has been frequently highlighted in the literature. It has been speculated that the degradation of humic substances as natural organic halide precursors by ozonation is enhanced in the presence of bicarbonate ions (3,22,23),as is the oxidation by ozone of manganese ions in the presence of humic substances (24). Since radical chain reactions occur in the humic substance solutions under ozonation, bicarbonate ions will inhibit the decomposition of molecular ozone, which, unlike its secondary species (OH radicals), acts as a specific oxidant on the natural organic halide precursors and manganese ions. As a result, the degradation of the precursors and the oxidation of manganese ions could be enhanced. Finally, since glyoxalic acid and hydrogen peroxide, precursors of radical chain reactions with ozone, were often described as byproducts during the ozonation of numerous organic compounds, in particular of aromatic substances (25-27), we may speculate that radical chain reactions should occur in the ozonation of these organics. Registry No. 03,10028-15-6.
Literature Cited .... .. - ..... Thurman, E. M. Developments in Biochemistry: Organic Geochemistry of Natural Waters; M. Nijhoff, Dr. W. Junk Publishers: Dordrecht, The Netherlands, 1985; Chapter 10. Anderson, L. J.; Johnson, J. D.; Christman, R. F. Environ. Sci. Technol. 1986, 20, 739. Reckhow, D. A.; Legube, B.; Singer, P. C. Water Res. 1986, 20, 987. Legube, B.; CrouB, J.-P.;De Laat, J.; DorB, M. Ozone: Sci. Eng. 1989, 11, 69. Staehelin, J.; HoignB, J. Environ. Sci. Technol. 1985, 19, 1206. Yurteri, C.; Gurol, M. Ozone: Sci. Eng. 1988, 10, 277. Xiong, F.; Legube, B. Ozone: Sci. Eng. 1991, 13, 349.
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(8) Thurman, E. M.; Malcolm, R. L. Enuiron. Sci. Technol. 1981, 15, 463. (9) MacCarthy, P.; Malcolm, R. L.; Hayes, M. H. B.; Swift, R. S.; Schnitzer, M.; Campbell, W. L. Presentation at 3rd International Meeting of Humic Substances Society, Oslo, Norway, 1986. (10) Legube, B.; Xiong, F.; CrouB, J.-P.; DorB, M. Reu. Sci. Eau 1990, 3, 399. (11) Bader, H.; HoignB, J. Water Res. 1981, 15, 449. (12) Bader, H.; Sturzenegger, V.; HoignB, J. Water Res. 1988, 22, 1109. (13) Yamada, H.; Somiya, I. Ozone: Sci. Eng. 1989, 11, 127. (14) CrouB, J.-P.;Xiong, F.; Legube, B.; DorB, M. Proceedings of 67th Congrds de I’Association G6nBrale des Hygi6niste.s et Techniciens Municipaux; Nice, France; AGHTM Paris, 1987. (15) Xiong, F. Thesis, UniversitB de Poitiers, France, 1990; No. 383. (16) Reckhow, D. A. Ph.D. Dissertation, University of North Carolina, 1984. (17) Glaze, W. H.; Koga, M.; Cancilla, D. Enuiron. Sci. Technol. 1989, 23, 838. (18) Slivon, L. E. Anal. Chem. 1987, 59, 2730. (19) Staehelin, J.; HoignB, J. Enuiron. Sci. Technol. 1982, 16, 676. (20) Christensen, H. S.; Sehested, H.; Corfitzan, H. J . Phys. Chem. 1982,86, 1588. (21) HoignB, J.; Bader, H. Water Res. 1983, 17, 185. (22) Legube, B.; CrouB, J.-P.;Beltran-Novillo, F.; DorB, M. Rev. Fr. Sci. Eau 1987, 6 , 435. (23) Lienhard, H.; Sontheimer, H. Ozone: Sci. Eng. 1979, I,61. (24) Paillard, H.; Legube, B.; Bourbigot, M. M.; Lefebvre, E. Proceedings of 8th ZOA Ozone World Congress; Zurich, Switzerland; International Ozone Association: Zurich, Switzerland, 1987. (25) Niki, E.; Yamamoto, Y.; Shiokawa, H.; Kamiya, Y. J. Org. Chem. 1979,44, 2137. (26) Legube, B.; Guyon, S.; Sugimitsu, H.; Dor6, M. Ozone: Sci. Eng. 1983, 5, 151. (27) Singer, P. C. Proceedings of 5th ZOA Ozone World Congress and Wasser Berlin ’81; Berlin, Germany; International Ozone Association: Vienna, VA, 1981.
Received for review June 25,1991. Revised manuscript received December 23, 1991. Accepted December 26, 1991.
