Relationships between the structure of natural organic matter and its reactivity towards molecular ozone and hydroxyl radicals

PII: S0043-1354(98)00447-3

Wat. Res. Vol. 33, No. 10, pp. 2265±2276, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter

RELATIONSHIPS BETWEEN THE STRUCTURE OF NATURAL ORGANIC MATTER AND ITS REACTIVITY TOWARDS MOLECULAR OZONE AND HYDROXYL RADICALS M M PAUL WESTERHOFF1** , GEORGE AIKEN2, GARY AMY3* and 3 JEAN DEBROUX

Department of Civil and Environmental Engineering, Arizona State University, Box 5306, Tempe, AZ 85287, U.S.A.; 2United States Geological Survey, 3215 Marine Street, Boulder, CO 80303, U.S.A. and 3 Department of Civil, Environmental, and Architectural Engineering, University of Colorado, Box 428, Boulder, CO 80309, U.S.A. 1

(First received April 1998; accepted in revised form October 1998) AbstractÐOxidation reaction rate parameters for molecular ozone (O3) and hydroxyl (HO) radicals with a variety of hydrophobic organic acids (HOAs) isolated from di€erent geographic locations were determined from batch ozonation studies. Rate parameter values, obtained under equivalent dissolved organic carbon concentrations in both the presence and absence of non-NOM HO radical scavengers, varied as a function of NOM structure. First-order rate constants for O3 consumption (kO3 ) averaged 8.8  10ÿ3 sÿ1, ranging from 3.9  10ÿ3 sÿ1 for a groundwater HOA to >16  10ÿ3 sÿ1 for river HOAs with large terrestrial carbon inputs. The average second-order rate constant (kHO,DOC) between HO radicals and NOM was 3.6  108 l (mol C)ÿ1 sÿ1; a mass of 12 g C per mole C was used in all calculations. Speci®c ultraviolet absorbance (SUVA) at 254 or 280 nm of the HOAs correlated well (r>0.9) with O3 consumption rate parameters, implying that organic p-electrons strongly and selectively in¯uence oxidative reactivity. HO radical reactions with NOM were less selective, although correlation between kHO,DOC and SUVA existed. Other physical±chemical properties of NOM, such as aromatic and aliphatic carbon content from 13 C-NMR spectroscopy, proved less sensitive for predicting oxidation reactivity than SUVA. The implication of this study is that the structural nature of NOM varies temporally and spatially in a water source, and both the nature and amount of NOM will in¯uence oxidation rates. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐnatural organic matter structure, ozone, hydroxyl radical, fulvic acid, kinetics INTRODUCTION

Natural organic matter (NOM) in drinking water supplies poses signi®cant concerns during water treatment due to its reactivity with oxidants and disinfectants. The presence of NOM can decrease the e€ectiveness of oxidants or disinfectants, and may lead to the formation of inorganic and organic disinfection by-products (DBPs) of health concern (e.g., bromate, organo-bromine, aldehydes) (Glaze et al., 1993; Renken, 1994; Owen et al., 1995). For the case of chlorination, correlations have been observed between chlorine demand and the generation of halogenated DBPs and the unsaturated carbon bond content of well characterized NOM fractions (Reckhow et al., 1990). The e€ects of NOM on ozonation of drinking water are less well understood. During ozonation of water, both molecular ozone (O3) and hydroxyl (HO) radicals exist (Hoigne and Bader, 1975, 1979; Staehelin and *Author to whom correspondence should be addressed. [Tel.: +1-602-9652885; fax: +1-602-9650557; e-mail: p.westerho€@asu.edu].

Hoigne, 1985; Langlais et al., 1991). The reactivity of NOM with molecular ozone (O3) and hydroxyl (HO) radicals has been related to the amount of quanti®ed NOM as dissolved organic carbon (DOC, mg/l) or the ultraviolet absorption (UVA) at 280 nm (cmÿ1) with a constant speci®c-UVA (SUVA, cmÿ1 (mg/l)ÿ1) (Hoigne and Bader, 1979; Haag and Yao, 1993). However the variable e€ect of NOM structure was not well de®ned. For example, the authors observed an order of magnitude di€erence in the half-life for ozone for several samples with a DOC of 4 mg C/l. This may be explained by variable SUVA values. Rate constants for the reaction of model organic compounds with O3 and HO radicals have been shown to depend upon the chemical structure (e.g., ole®nic structure) and functional group content (e.g., R±OH, R±COOH, or R±NH2) of the compounds of interest (Hoigne and Bader, 1979, 1983a,b; Hoigne, 1997). Reacting as an electrophile, ozone preferentially oxidizes electronrich moieties such as ole®nic structures (i.e., aromatic carbon±carbon double bonds) and aromatic alcohols (Hoigne and Bader, 1983a,b). Studies of

2265

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Paul Westerho€ et al.

HO radical reactions with model organic compounds suggest that HO radicals react relatively unselectively; however, most carbon±carbon double and triple bonds react quicker than carbon±carbon single or carbon±hydrogen bonds, with the latter still reacting at a fast rates (i.e., 108±109 Mÿ1 sÿ1) (Buxton et al., 1988; Haag and Yao, 1992). For large macromolecules, such as DOC and HO, radical rate constants only vary over a one-log range (107±108 l/mol C s), compared to several order of magnitude di€erences in rate constants for smaller molecules (Peyton and Bell, 1993). HO radical reaction rate constants with reactive sites at the center of large molecules approach di€usional limitation rates, even though some sites may be ``blocked'' as larger molecules fold on itself. In general, reaction rates increase with increasing molecular size based upon a rule of additivity. Due to the internal blocking of sites that can occur in large molecules, such as DOC, it is unlikely that a linear additivity would be observed, as is common in smaller molecules (Atkinson et al., 1979; Peyton and Bell, 1993). NOM contains a heterogeneous mixture of organic compounds that complicates the quanti®cation and identi®cation of its structural characteristics. The chemical nature, or structure, of NOM in a drinking water supply is determined by its source materials and by biogeochemical processes that take place in the watershed (Aiken and Cotsaris, 1995). The diculty associated with de®ning its nature has been a limitation in understanding its reactivity. In this paper, we present results of research designed to identify properties of NOM that can be used to predict its oxidation reactivity with ozone. Ozonation experiments were carried out using well characterized hydrophobic organic acids (HOAs) isolated from various geographic locations. These NOM fractions were selected for study because they represent a substantial percentage (40±60%) of the DOC of most waters (Thurman, 1985), and are known to be reactive with ozone (Reckhow et al., 1992). Our speci®c objectives are to evaluate the importance of chemical structure on the reactivity of HOAs under constant experimental conditions (O3, pH, DOC, temperature, inorganic matrix), and to identify properties of NOM that are indicative of its reactivity with ozone.

CONCEPTUAL FRAMEWORK

In the absence of NOM, ozone consumption in water occurs through free radical processes (Gurol and Singer, 1982; Staehelin and Hoigne, 1985; Langlais et al., 1991) represented by reaction (1) below. Reactions of NOM with ozone are given by reactions (2)±(5)

ozone consumption in NOM-free water O3 ‡ …H2 O† ÿ4products

…1†

direct ozone consumption O3 ‡ NOM ÿ4products

…2†

initiator for ozone consumption O3 ‡ NOM ÿ4…Oÿ 2 ‡ H2 O2 † ÿ4HO ‡ products …3† promoter of ozone consumption HO ‡ NOMOÿ 2 ‡ products

…4†

inhibitor of ozone consumption HO ‡ NOM ÿ4products

…5†

Reactions (1)±(3) represent initiation of chain reactions that also consume O3. Hydrogen peroxide (H2O2) and superoxide (Oÿ 2 ) rapidly react with O3 and promote ozone consumption. Based on the reactivity of model compounds with ozone, it is known that organic carbon structure and functionality in¯uence the mechanisms of ozone consumption. For example, unsaturated bonds can directly consume ozone, alkyl compounds can inhibit ozone consumption, and aryl compounds can promote or inhibit ozone consumption. Given its heterogeneous nature, NOM can be involved in any or all of these mechanisms. The consumption of ozone, described by reactions (1)±(3), can be represented by an overall rate expression, rO3 . Reaction orders from one to two have been reported in the literature (Langlais et al., 1991; Chin et al., 1994; Hoigne and Bader, 1994), and ®t with the following expressions: first-order: rO3 ˆ ÿkO3 ‰O3 Š

…6†

three-halves-order: rO3 ˆ ÿk3=2 ‰O3 Š1:5

…7†

second-order: rO3 ˆ ÿk2nd ‰O3 Š2

…8†

where kO3 , k3/2, and k2nd represent ®rst-, threehalves-, and second-order rate constants, respectively. For most the analysis in this paper, however, ozone consumption is modeled as a two-stage process to account for both fast and slow reacting organic matter compounds. This is accomplished by analyzing the kinetics of ozone consumption in two steps: (1) the initial reaction de®ned as through the ®rst minute of reaction (D01), and (2) from a ®rstorder kinetic analysis on O3 from the ®rst minute to the end of the reaction (k2). The consumption of HO radicals by NOM, described by reactions (4) and (5), is represented by an overall rate expression, rHO,DOC. The term rHO,DOC is a function of the second-order rate constant (kHO,DOC, l (mol C)ÿ1 sÿ1) for HO radicals and DOC. Calculated rate parameters based on ozone consumption with di€erent NOM isolates are then used to determine

Structure and reactivity of NOM

2267

the relative signi®cance of NOM structural characteristics with its reactivity towards O3 and HO radicals.

concentrations of t-butanol (t-But) were determined using the expression X 10  ki,HO ‰Si Š , …9† ‰t-ButŠ ˆ kt-But,HO

METHODS

where ki,HO and kt-But,HO are second order rate constants for the reaction of HO radicals with a speci®c solute (Si) and t-butanol, respectively. The solutes (Si) considered were the following: O3 (125 mM), DOC (0.2 or 3 mg/l), and a phosphate (1 mM) bu€er at pH 7.5. Rate constants were taken from the literature (Haag and Yao, 1992, 1993; Buxton et al., 1988). All ozonation was performed at 20228C. In selected ozonation experiments, parachlorobenzoic acid (PCBA) was used as an HO radical probe compound for determining reaction rate constants between the NOM isolates and HO radicals, based on the methodology described by Haag and Yao (1993). In brief, at least six di€erent ozone doses (5±125 mM) were added to pH 7.5 bu€ered model solutions containing HOA isolates (3 mg C/l) and PCBA (1±3 mM) in glass vials, and the volumes adjusted to 14 ml. PCBA was measured 24 h after complete ozone decomposition. The rate constants between the aqueous solutes and HO radicals were determined from the oxidation-competition (O) value as follows: kd,DOC=OZkPCBA, where O is de®ned as the concentration of O3 consumed, after depletion of the probe begins, that resulted in a one-log (37%) reduction in the original PCBA concentration. Through direct reactions (equation 2), DOC consumes ozone without reducing the HO radical probe concentrations. Log±linear regressions were calculated to assist in estimate the amount of ozone directly consumed by DOC and O. The term kd,DOC (sÿ1) is the ®rst-order rate constant for HO radicals with aqueous solutes, kPCBA (5  109 Mÿ1 sÿ1) is the second order rate constant for HO radicals with PCBA, and Z (0.4±1.0) is the stoichiometric yield of HO radicals from ozone after its decomposition. A Z value of 0.67 was chosen since it represented the base-catalyzed HO radical yield without promotion and falls within reported ranges (0.5 < Z < 1.0, Haag and Yao, 1993; Peyton and Bell, 1993). Second-order rate constants (kHO,DOC (l (mol C)ÿ1 sÿ1)) for rHO,DOC were obtained by normalizing kd,DOC (sÿ1) to the molar concentration of DOC, assuming 12 g C per mol C.

NOM isolates and reagents NOM isolates were obtained from a variety of source waters (Table 1) according to the methods of Aiken et al. (1992). Hydrophobic organic acids (HOAs), operationally de®ned as the fraction of NOM retained on XAD-8 resin at pH 2 that can be eluted from the resin at pH 13, were obtained from all water sources. A single hydrophilic acid fraction of NOM, operationally de®ned as DOC retained on XAD-4 resin at pH 2 and eluted at pH 13, was also obtained from California State Project water. Well characterized humic and fulvic acids, both subsets of HOAs, from the Suwannee River were obtained from the International Humic Substances Society (Golden, CO) (USGS, 1989). Lyophilized NOM fractions were dissolved in Milli-Q water (Millipore Inc.) at least 24 h prior to use. All other reagents were reagent grade or better.

Ozonation experiments Ozonation experiments were conducted in a batch reactor consisting of a modi®ed 500 ml graduated glass cylinder with a sampling port at its base and an adjustable Te¯on cover to minimize volatile loss of ozone. Model solutions containing NOM isolates (03 mg C/l), 1 mM pH 7.5 phosphate bu€er and de-ionized Milli-Q water (NOMfree water) were ozonated by spiking with aqueous ozone solution prepared daily according to Bader and Hoigne (1981). NOM-free water contained 0.2 mg C/l (Westerho€ et al., 1997). Tertiary butanol (t-butanol) was added in some experiments to scavenge HO radicals. Concentrations of 0.28 mM and 2 mM t-butanol were used to achieve >90% removal of HO radicals in the absence and presence of NOM isolates, respectively. The

Table 1. Geographic location where NOM isolate samples were obtained, DOC fraction isolated, and acronyms for identifying NOM isolates Geographic location Coal Creek, CO Groundwater sample near Lake Shingobee, MN Lake Fryxell, Antarctica Yakima River at Kiona, WA Lake Michigan, IL Missouri River, IA Ogeechee River, GA Ohio River, OH Calif. State Project water, CA Calif. State Project water, CA Shingobee River, MN (1992) Shingobee River, MN (1993) Lake Shingobee, MN Suwannee River, GAa Suwannee River, GAa Williams Lake, MN Yakima River at CleElum, WA

Fraction

Acronym

XAD8

CCK

XAD8 XAD8 XAD8 XAD8 XAD8 XAD8 XAD8 XAD4 XAD8 XAD8 XAD8 XAD8 fulvic acidb humic acidb XAD8 XAD8

GW8 FXL KNR LM2 MSR OGR OHR SP4 SP8 SR1 SR2 SHL SRF SRH WLL YKR

a Obtained from International Humic Substances Society (School of Mines, Golden, CO).bHumic and fulvic acid separation from XAD8 isolate by pH 1 precipitation.

Analytical methods Dissolved ozone in NOM-free waters was measured spectrophotometrically (Shimadzu UV-1606), e = 2950 Mÿ1 cmÿ1 at 254 nm; equivalent to ÿ1 ÿ1 e = 3100 M cm at 258 nm, or via the indigo method for NOM-containing solutions (Langlais et al., 1991; Peyton and Bell, 1993). DOC (mg/l, mmol C/l) was measured by high temperature combustion analysis (Shimadzu TOC-5000). Ultraviolet absorption (UVA) of NOM isolates was measured at 254 and 280 nm. Speci®c ultraviolet absorbance (SUVA) was calculated by normalizing UVA to the DOC concentration and is analogous to molar absorptivity ((mg C/l)ÿ1 cmÿ1). Speci®c ¯uorescence (S¯uor, intensity/(mg C/l)), ¯uorescence intensity normalized to DOC concentration, was measured with a multiwavelength ¯uorimeter (Hatichi F-3010 with xenon lamp) at excitation and emission wavelengths of 320 and 430 nm, respectively (Miano et al., 1988). Solution pH was measured with a glass probe (Corning) using an Orion Model 701 m calibrated daily with pH 4, 7, and 10 bu€ers (Fisher Scienti®c Inc.) PCBA was measured by HPLC (Hewlett Packard 1050) with a multiple wavelength detector and Deltabond Octyl column (Keystone Sci., Inc.); con-

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ditions included 300 ml sample volume, an eluent mixture of water (60%), pesticide grade methanol (30%), 0.1 M ammonium acetate (10%) was used at a ¯owrate of 1 ml/min, and UV detection at 234 nm (e = 12,000 Mÿ1 cmÿ1). Coecient of variation equaled 0.6% for 20 PCBA analyses run in duplicate. Elemental analyses (C, H, O, N, S and ash) were performed by Hu€man Laboratories (Golden, CO). Weight average molecular weight (Mw) was obtained by highpressure size exclusion chromatography (SEC) measurements (Chin et al., 1994). Quantitative liquid state 13 CNMR spectra were obtained on a Varian XL300 with samples at pH 7.0 according to methods described elsewhere (Aiken et al., 1996). 13 C-NMR results were analyzed by dividing the area beneath the spectra into the following shift ranges: . aliphatic-I (0±62 ppm: unsubstituted saturated aliphatic carbons including methyl groups); . aliphatic-II (62±90 ppm: carbon singly bonded with oxygen including ether groups); . anomeric (90±110 ppm: carbon singly bonded to two oxygen's including acetal or ketal compounds); . aromatic (110±160 ppm: unsaturated carbon); the aromatic region is also subdivided into aromatic-I (110± 140 ppm: protonated and alkyl-substituted aromatic carbon) and aromatic-II (140±160 ppm: aromatic carbon substituted by oxygen and nitrogen including phenol, ether, and amine groups); . carboxyl (160±190 ppm: carboxyl groups); . ketone (190±230 ppm: carbonyl, amide, and ester groups). Based upon the statistical occurrence relationship for unsaturated carbon bonding and electron donating hydroxyl or amine functional groups (represented by molar carbon, C, and nitrogen, N, content) the expression derived by Reckhow et al. (1990) was used to estimate the activated aromatic carbon content (act-arom):

"  6 aromatic-I ‡ -II aromatic-II  1ÿ act ˆ 6 aromatic-I ‡ -II   # N 6 :  1ÿ C

…10†

RESULTS AND DISCUSSION

Properties of NOM isolates Physical±chemical properties of the NOM isolates used in this study are presented in Table 2. The NOM isolates were obtained from a diverse set of source waters and represent a broad range of structural con®gurations. The FXL and SRH isolates represent structural end-members and exhibit distinctly di€erent 13 C-NMR carbon bonding arrangements, elemental composition, SUVA, and S¯uor. Correlation analyses between the physical±chemical properties of the isolates indicate that a number of these properties are related (Table 3). For instance, SUVA determined at either 254 or 280 nm appears to correlate well with aromatic carbon content, aliphatic carbon content (inversely correlated), and Mw of the NOM isolates. These absorbance wavelengths fall within the range where p-electron interactions occur for a number of aromatic substances and SUVA at these wavelengths has been found to be a good indicator of aromatic carbon content for both soil and aquatic humic substances (Triana et al., 1990; Chin et al., 1994). The S¯uor value is generally not as robust an indicator of NOM properties as SUVA, except in the case of

Table 2. Summary of physical±chemical properties of NOM isolates obtained from di€erent locations Isolate

SUVAa 254 nm

SUVAa 280 nm

S¯uorb

Mw c

Al-Cd

ArI-Cd

ArII-Cd

Total Ar-Cd

Carb-Cd

Ketone

Actarom

Ce

He

Oe

Ne

CCK GW8 FXL KNR LM2 MSR OGR OHR SP4 SP8 SR1 SR2 SHL SRF SRH WLL YKR

0.051 0.013 0.019 0.035 0.018 0.029 0.045 0.042 0.026 0.040 0.032 0.049 0.028 0.042 0.107 0.017 0.039

0.038 0.009 0.014 0.027 0.012 0.020 0.034 0.032 0.019 0.030 0.024 0.039 0.021 0.033 0.087 0.012 0.030

4.82 1.55 1.89 3.49 1.20 2.77 5.26 5.07 2.30 3.39 3.10 3.65 2.57 3.13 2.43 1.27 2.93

2230 1000 1080 1690f 1060f 1460 2000 1330 1350f 1940f 1910f 2660f 1730f 2310f 3320f 1020f 1560

38.4 61.0 60.0 47.2 N/A 51.9 46.9 45.2 N/A N/A 52.2 45.9 53.7 44.0 28.0 67.2 47.5

18.1 8.1 9.3 16.8 N/A 14.3 16.7 16.2 N/A N/A 14.2 16.3 13.4 13.2 N/A 7.1 N/A

9.3 3.8 3.8 8.5 N/A 6.1 8.1 8.1 N/A N/A 6.2 8.2 5.1 6.8 N/A 3.3 N/A

27.4 11.9 13.1 25.3 14f 20.4 24.8 24.3 17f 25f 20.4 24.5 18.5 20.0 37.0 10.4 26.6

20.2 17.2 20.1 18.7 N/A 18.8 18.3 18.6 N/A N/A 20.4 19.4 22.0 21.1 N/A 20.0 17.2

7.2 4.7 0 4.6 N/A 4.4 3.3 6.4 N/A N/A 4.4 7.5 3.7 7 8 1.5 4.1

4.23 1.80 1.98 3.79 N/A 3.05 3.80 3.75 N/A N/A 3.05 3.78 2.70 3.81 N/A 1.59 N/A

52.8 52.7 55.0 56.1 48.5 55.4 54.0 55.5 N/A 48.1 51.1 50.6 51.1 53.5 54.3 51.8 57.2

4.5 5.7 5.5 5.0 5.3 5.3 4.0 5.4 N/A 5.1 5.7 5.1 5.9 4.2 4.1 6.3 4.9

38.4 40.6 34.9 35.5 35.5 35.0 38.5 35.9 N/A 34.3 40.7 41.8 40.7 41.3 39.4 37.7 35.9

0.95 0.7 3.1 2.2 1.4 1.3 1.4 1.5 N/A 2.4 1.6 1.7 1.6 0.69 1.1 1.6 1.0

Average Std. dev.

0.037 0.021

0.028 0.018

3.00 1.23

1740 640

49.2 9.8

13.6 3.6

6.4 2.1

21.2 6.8

19.5 1.5

4.8 2.3

3.11 0.91

53.0 2.6

5.1 0.7

37.9 2.6

1.5 0.6

a Speci®c ultraviolet absorption ((mg C/l)ÿ1 cmÿ1).bSpeci®c ¯uorescence excitation wavelength of 320 nm and emission wavelength of 430 nm (intensity/(mg C/l)).cWeight averaged molecular weight (Mw) by size exclusion chromatography (Da).dFrom 13 C-NMR analysis as % carbon content: aliphatic (Al-C) = 0±90 ppm, aromatic-I (ArI-C) = 110±140 ppm, aromatic-II (ArII-C) = 140±160 ppm, total aromatic (Ar-C) = 110±160 ppm, carboxyl (Carb-C) = 160±190 ppm.eWeight percentage of elemental species: C = carbon, H = hydrogen, O = oxygen, N = nitrogen.fEstimated from SUVA data and linear approximations for hydrophobic acids (Chin et al., 1994); N/A = not analyzed.

Structure and reactivity of NOM

2269

Table 3. Matrix showing coecients (r-values) obtained by correlation analysis of physical±chemical properties of NOM isolates Property SUVA S¯uor Mw C/H C/O C/N Al-C Type 1 Ar-C Type 2 Ar-C Total Ar-C Act-Ar-C Ketone Carb-C

SUVA

S¯uor

Mw

C/H

±

0.93 ±

0.95 0.66 ±

0.59 0.64 0.58 ±

C/O

±

C/N

±

Al-C

Type 1 Ar-C

Type 2 Ar-C

Total Ar-C

ÿ0.86 ÿ0.88 ÿ0.83 ÿ0.78

0.85 0.82 0.79 0.73

0.80 0.87 0.68 0.62

0.73 0.87 0.61 0.70

±

ÿ0.95 ±

ÿ0.93 0.99 ± ±

ÿ0.94 0.99 0.96 0.97

Actarom 0.89

Ketone

Carb-C

0.71 0.56

0.62 ÿ0.67 0.97 0.95

0.77 0.64 0.72

±

0.75 ±

±

Only correlations with r>0.5 values are presented. Negative r-values indicate inverse correlations.

activated aromatic carbon content. Elemental composition re¯ects di€erences in bonding arrangements and hybridization status. Elemental ratios are indicative of degree of saturation, relative polarity, and the presence of electron donating functional groups. The degree of unsaturation, indicated by the C/H ratio, is, therefore, related to aliphatic and

aromatic carbon content as determined by 13 CNMR analysis, and aliphatic carbon content is inversely correlated to aromatic carbon content. Ozonation of NOM isolates In the absence of t-butanol, ®rst-order rate analysis of the NOM-free water data has a low r2-value

Fig. 1. Comparison of O3 consumption pro®les in the (a) absence and (b) presence of t-butanol (2 mM) (pH = 7.5, ozone dose = 125 mM, DOC of HOA isolates = 3 mg C/l).

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Paul Westerho€ et al.

Fig. 2. Observed (symbols) and ®rst-order data ®ts (lines) of O3 consumption for selected HOA isolates (pH = 7.5, ozone dose = 125 mM, DOC = 3 mg C/l).

(kO3 =1.2 2 0.2  10ÿ3 sÿ1, r2=0.92). A three-halves reaction order with respect to ozone yielded the best data ®t (k3/2=0.172 2 0.005 Mÿ0.5 sÿ1, 2 r >0.99). This observation, and the magnitude of k3/2, is consistent with literature reports (Gurol and Singer, 1982). The rate of ozone consumption is faster in the presence of NOM isolates than for the NOM-free (Milli-Q water) solutions (Fig. 1(a)). The ozone consumption data for the NOM solutions are best described by ®rst-order reaction reactions, compared to three-halves- or second-order data ®ts.

The ®rst-order ®t for ozone consumption by three separate model solutions and a NOM-free solution is shown in Fig. 2. Statistical goodness of ®t (R2) values for ®rst-order treatment of the data are generally >0.98 (Table 4). Values of kO3 for solutions of the HOA isolates averaged 9.5  10ÿ3 sÿ1 with a range of 3.9  10ÿ3 to >12  10ÿ3 sÿ1 (Table 4). Variations in these rates re¯ect the in¯uences of organic matter composition on ozone consumption. The ®rst order rate expressions determined using the entire data set were found to overestimate

Table 4. Ozone decomposition (D01, k2, kO3 ) and HO radical (kHO,DOC) reactivity parameters for NOM isolates from various sources (pH = 7.5, [O3] = 125 mM, DOC = 0.23 20.02 mM). Coecients of determination (r2-values) for k2 and kO3 values are given in parentheses Isolate

Absence of arti®cial HO radical scavenger (no t-butanol) D01 (mM/min)

MQW CCK GW8 FXL KNR LM2 MSR OGR OHR SP4 SP8 SR1 R2 SHL SRF SRH WLL YKR Averaged Std. dev.d

332 0.8 96 38 68 812 0.4 55 702 5.4 88 85 55 742 0.8 76 90 88 84 106 64 872 1.2 75 16

k2 (103 sÿ1)

kO3 (103 sÿ1)

1.1 20.17 (0.92) 1.2 20.21 (0.92) 15 (0.98) 16 (0.98) 3.9 20.16 (>0.99) 3.9 2 0.15 (>0.99) 6.4 (0.99) 6.7 (0.99) 9.7 20.95 (0.97) 10 21.1 (0.98) 6.7 (0.99) 6.9 (0.99) 6.2 20.34 (0.98) 6.5 20.36 (0.97) 11 (0.91) 11 (0.93) 9.7 (0.99) 11 (0.98) 7.1 (0.99) 7.2 (0.99) 9.2 2 1.6 (0.98) 9.9 21.0 (0.98) 8.7 (>0.99) 9.3 (0.99) 14 (0.96) 16 (0.97) 5.9 (0.95) 6.4 (0.95) 9.7 (0.96) 11 (0.95) TFc TFc 7.6 (>0.99) 7.8 (>0.99) 11 22.4 (0.99) 12 22.0 (0.98) 8.8 2.9

9.5 3.4

Presence of arti®cial HO radical scavenger (with t-butanol) D01 (mM/min)a 2.8 21.2 58 24 22 35 22 45 44 36 41 39 24 35 58 38 74 26 39 23 37 12

k2 (103 sÿ1)a

kO3 (103 sÿ1)a

0.53 20.03 (0.99) 0.54 2 0.04 (0.99) 1.3 (0.99) 1.3 (0.99) 0.53 (0.98) 0.54 (0.98) 1.1 (0.98) 1.1 (0.97) 1.1 20.12 (0.99) 1.22 0.13 (0.98) 0.77 (0.97) 0.81 (0.96) 0.82 (0.99) 0.91 (0.99) 1.1 (0.99) 1.2 (0.98) 1.3 (0.99) 1.3 (0.98) 0.84 (0.99) 0.89 (0.97) 1.1 (0.99) 1.1 (0.99) 1.1 (0.99) 1.2 (0.98) 1.6 (0.99) 1.7 (0.98) 0.85 (>0.99) 0.90 (0.98) 2.2 (0.99) 2.3 (0.98) 4.3 (0.95) 4.4 (0.95) 0.91 (0.97) 0.94 (0.96) 1.8 20.46 (0.99) 1.82 0.46 (0.99) 1.1 0.41

1.2 0.42

Presence of HO radical probe KHO,DOC (10ÿ8 l (mol C)ÿ1 sÿ1)e N/A 4.3 2.6 3.5 3.6 3.0 3.5 4.5 4.12 0.1 3.1 4.0 3.3S 4.5 3.4 3.7 8.1 3.5 4.0 3.6 0.54

For n = 2 for data shown with 2.at-Butanol is used as a HO radical scavenger with concentrations of 0.28 mM in MQW and 2 mM in the presence of NOM isolates (DOC = 0.25 mM).bMilli-Q contains less than 0.02 mM of organic carbon.cReaction occurred too fast (TF) to collect enough data for determining rate constants.dMilli-Q and SRH are excluded from average and standard deviation.eLow PCBA concentrations used a HO radical probed; no t-butanol present.

Structure and reactivity of NOM

ozone concentrations during the early stages of the reactions of ozone with NOM isolates. The consumption of ozone during the ®rst minute was, therefore, modeled separately. It was found that the early part of the reaction followed pseudo-®rst order reaction kinetics. The rate term, D01, de®ned as the amount of ozone consumed during the ®rst minute of the reaction, was determined for each isolate (Table 4). The range of values for D01 for the isolates was 38 (GW8)±106 mM/min (SRH). On average, 60% of the ozone was consumed during the ®rst minute in the presence of the NOM isolates, whereas only 26% was consumed in the same period in the NOM-free water. It is possible that by-products of oxidation reactions (e.g., ring cleavage) could lead to the formation of addition reactive sites over time, during ozonation. These byproducts, plus the slower-reacting NOM sites, are probably responsible for the ozone decomposition during the second-phase in the observed ozonation experimental results. It is likely that the faster-reacting NOM sites are responsible for the ®rst-portion (<1 min) of the observed ozone decomposition. However, quantifying and accounting for these numerous organic reactions would be dicult given the heterogeneity of NOM (e.g., carbon bonding and functionality). Since all the reactions will depend, in some fashion, upon the initial structural characteristics of the NOM the analysis between rates of reaction and initial NOM structure is appropriate. Hoigne and Bader (1976) examined the slower phase of ozone consumption through evaluating the second half-life of ozone. Our data indicate that a ®rst order decay function also ®ts the slower, later phase of the reaction (starting at 1 min after ozone dosing). Therefore, a ®rst-order rate constant (k2) was evaluated for each solution using the observed ozone concentration at one minute as the initial concentration. Values for k2 ranged from 3.9  10ÿ3 (GW8) to 11  10ÿ3 sÿ1 (YKR) with a mean value of 8.82 2.9  10ÿ3 sÿ1 (n = 17) (Table 4). Ozone decomposed too rapidly to calculate a k2 value for SRH, the humic acid isolate. The single hydrophilic organic acid isolate (SP4) was less reactive than the corresponding HOA isolate (SP8) obtained from the same water sample. Ozonation decomposition in the presence of t-butanol Ozone decomposition pro®les for various solutions, in the presence of sucient t-butanol to scavenge greater than 90% of the HO radicals, are presented in Fig. 1(b). Addition of t-butanol a€ects overall O3 consumption by reducing the ability of HO radicals to react with NOM according to reactions (4) and (5). In the presence of t-butanol, therefore, the observed rO3 re¯ects the initiation rate of O3 depletion by DOC, as represented by reactions (1)±(3), since chain reactions are inhibited. In the presence of t-butanol, a ®rst-order rate con-

2271

stant of 5.42 0.4  10ÿ4 sÿ1 (r2>0.99) was calculated for NOM-free waters, and compares well with the predicted value of 5  10ÿ4 sÿ1 calculated by data from Staehelin and Hoigne (1982). For solutions with HOA isolates, the average values and standard deviation for kO3 , k2, and D01 were 1.2 2 0.4  10ÿ3 sÿ1, 1.1 2 0.4  10ÿ3 sÿ1, and 37 2 12 mM/min, respectively (Table 4). These values are lower than similar rate parameters collected in the absence of t-butanol and re¯ect slower ozone consumption (Fig. 1); note longer reaction times in Fig. 1(b). Solutions containing an NOM isolate and t-butanol still exhibited a two-phase O3 consumption pro®le (Fig. 1(b)), although the reaction was slower than in the absence of t-butanol. Addition of tbutanol to the NOM solutions led to an eight- to ten-fold increase in the ®rst half-life of ozone (i.e., a decrease in the value of kO3 ) compared with only a two- to three-fold increase observed in NOM-free water. The ratios of D01 and k2 obtained in the absence and presence of t-butanol are presented in Table 5. The initial, rapid loss of ozone (inferred from D01) is reduced by 50% in the presence of tbutanol. These results suggest that, in addition to the direct consumption of ozone by NOM oxidation, NOM also plays a signi®cant role as a promoter of ozone consumption, even during the early stages of the reaction. The presence of t-butanol also e€ected the later stages of ozone consumption. Decreases in the value of k2 average 87% (Table 5). This reduction is greater than the 50% observed during the ®rstphase and could imply that promotion reactions (i.e., reaction (4)) were more dominant during the later, slower phase. The e€ect of t-butanol on the percentage reduction is consistent, exhibiting a standard deviation of only 3%. The magnitudes of D01 and k2 still di€er among the NOM isolates in the presence of t-butanol, implying that the nature of the NOM direct ozone consumption and promotion reactions. HO radical scavenging by NOM isolates During ozonation of solutions containing HOA isolates, NOM served as the dominant HO radical scavenger. Graphical relationships for changes in the concentration of PCBA (the HO radical probe) as a function of increasing ozone dosages are illustrated in Fig. 3. Values for O, and subsequently kHO,DOC, were computed for each isolate. The average O value obtained in the presence of the HOA isolates was 25 mM, which was higher than the O value of 10 mM obtained in a NOM-free system. NOM scavenged HO radicals, increasing the ozone dose required to achieve a one-log reduction in PCBA concentrations. The mean value of kHO,DOC for the HOAs was 3.6 20.5  108 l (mol C)ÿ1 sÿ1 (3.1  104 (mg C/l)ÿ1 sÿ1), excluding SRH, which exhibits a very high HO radical scavenging rate

2272

Paul Westerho€ et al. Table 5. A€ect of t-butanol addition on D01 and k2

NOM isolate

MQW CCK GW8 FXL KNR LM2 MSR OGR OHR SP4 SP8 SR1 SR2 SHL SRF SRH WLL YKR Average Std. Dev.

Ratio of rate parameters

Percent decrease in rate parameters

D01 a

k2 b

D01 c

k2 d

12 1.7 1.6 3.1 2.3 2.6 1.5 2.0 2.3 1.3 1.9 3.1 2.6 1.5 2.2 1.4 2.5 2.2 2.1 0.6

2.2 11 7.3 6.1 8.7 8.7 7.5 9.7 7.6 8.5 8.5 7.7 9.0 6.9 4.5 ± 8.3 6.1 7.9 1.6

91% 40 36 68 57 61 35 50 57 25 48 68 62 34 55 30 60 55 49 14

54% 91 86 84 88 88 87 90 87 88 88 87 89 86 78 ± 88 83 87 3

a D01=(D01)with t-But/(D01)without t-But.bk2=(k2)with t-But/ (k2)without t-But.cD01=[(D01)without t-Butÿ(D01)witht-But]/ (D01)without t-But.dk2=[(k2)without t-Butÿ(k2)with t-But]/(k2)without t-But.

(8.1  108 l (mol C)ÿ1 sÿ1) compared to the other NOM isolates (Table 4). Humic acids tend to be more aromatic and have higher molecular weights than fulvic acids, and consequently may have a larger number of reaction sites. The kHO,DOC values obtained for NOM isolates were on the same order of magnitude as those reported by others for natural waters using a variety of analytical techniques and di€erent HO radical probes (Table 6). Overall, di€erent methods tend to yield similar HO radical reactivities for isolated NOM or natural waters and the magnitude of these HO radical reactivities fall within a narrow range (1  108±10  108 Mÿ1 sÿ1). In our work,

kHO,DOC values varied, depending upon the source of the HOA isolates, ranging from 2.6  108 (GW-8) to 4.5  108 l (mol C)ÿ1 sÿ1 (SR-1), not including the SRH sample. The absolute range of kHOA values is only a factor of 3. Given the uncertainty in the measurements and selected use of a constant Z-value (0.67), it may be concluded that most fulvic acids behave similarly HO radicals although larger and more aromatic humic acids appear to have a higher rate of reaction. E€ect of NOM structure on oxidation reactivity parameters A correlation matrix between ozone decomposition rate parameters (kO3 , k2, and D01) and HO radical reaction rate constants (kHO,DOC) with NOM physical±chemical properties is presented in Table 7. The SRH isolate has not been included in regressions with rate parameters since it was a signi®cant end-member. High correlation coecients were obtained between SUVA data determined at either 254 or 280 nm with ozone decomposition and HO radical rate parameters (Table 7), indicating that increasing SUVA of NOM isolates results in increasing reactivity with ozone (Fig. 4). Extrapolation of the line to a higher SUVA value for the SRH isolate (SUVA = 0.107 (mg C/ l)ÿ1 cmÿ1) suggests that this relationship is valid over a wide range of absorbances. High correlation coecients were obtained in both the presence and absence of t-butanol (Fig. 4 and Table 7). Ozone reacts selectively with certain moieties of NOM. The strong positive correlations indicate the importance of electron rich carbon±carbon double bonds on O3 and HO radical reactions. Unsaturated bonds are highly reactive in model compounds and their reactivities appeared to dominate the reactivity of hydrophobic and hydrophilic organic acids.

Fig. 3. Representative ®rst-order ®ts of PCBA consumption as a function of O3 dose and representative signi®cance O for the SP4 isolate (pH = 7.5, DOC = 3 mg/l).

Structure and reactivity of NOM

2273

Table 6. Comparison of kHO,DOC values obtained from di€erent studies employing variable HO radical sources and HO radical probe (PCBA, TCE, BuCl, and benzene) compounds (kHO,DOC values are computed for a DOC of 3 mg C/l and assume a Z = 0.67) Calculated O (mM of O3)

KHO,DOC (10ÿ8 l (mol C)ÿ1 sÿ1)

Reference

Ozone with HOA isolates and PCBA

25 28

this work

Ozone with SRF isolate and PCBA Ozone with bulk waters and PCBA Ozone with bulk waters and benzene

26 24a 33b

3.62 0.5 (range: 2.6±8.1) 3.7 3.4 4.4

± ± ±

1.9 10±16 2.3

Oxidation system

H2O2 and UV irradiation with Fluka humic acid and BuCl Ozone at high pH with SRF/SRH isolates and TCE H2O2 and UV irradiation with SRF/SRH isolates and TCE

this work Haag and Yao, 1993 Haag and Yao, 1993; Hoigne and Bader, 1979 Liao and Gurol, 1995 Peyton and Bell, 1993 Peyton and Bell, 1993; Peyton et al., 1998

a Based on the model for Californian waters: O (mg O3/l) = ÿ 0.42 + (0.57 20.05)[DOC].bBased on the model for Swiss surface, ground, and waste waters: O (mg O3/l) = 0.39 + 0.39[DOC].

The regression line for the kHO,DOC data in Fig. 4 excludes the SRH isolate, and extrapolation to the higher SUVA or aromatic carbon content closely approximates the measured SRH value. However, excluding the SRH isolate, the data could also be viewed as having some deviation about a mean kHO,DOC value. This interpretation may actually have greater theoretical support since HO radicals are generally considered to react relatively unselectively with organic compounds. The slight observed dependency upon SUVA may actually be an artifact, and actually represent the dependency of kHO,DOC on molecular weight; SUVA, aromaticity, and molecular weight (Mw) are correlated (Table 3 and Peyton and Smith, 1988; Peyton and Bell, 1993). Given the colinearity between Mw and SUVA, it is dicult to assess the importance of Mw in controlling the reactivity of the organic matter with ozone or HO radicals. Peyton and Smith (1988), however, observed a positive correlation between molecular weights of organic solutes (e.g., proteins, DNA, and NOM) and HO radical reaction rates, using both low and high molecular weight compounds. Fluorescence intensity normalized to DOC (S¯uor) correlated well with NOM reactivity

towards O3 only when HO radicals were not scavenged by t-butanol (Table 7). Values of S¯uor did not correlate well with kHO,DOC. The lack of a signi®cant correlation between ¯uorescence and kO3 , in the presence of t-butanol, might be related to inhibition of free chain promotion reactions involving carboxyl functional groups present on NOM. Fluorescence of the HOA isolates was inversely related to carboxyl functionality in organic compounds, which is consistent with other reports for model compounds (Seitz, 1981). Ozone consumption and HO radical rate constants were found to be positively correlated with aromatic carbon content, and inversely correlated with aliphatic carbon content as determined by 13 CNMR analyses (Table 7). This observation provides further evidence that ozone reacts preferentially with the aromatic constituents of NOM, and speci®cally electron enriched aromatics (aromatic-II and activated aromatics). However, the correlation of reactivity with the aromatic carbon content is weaker than that noted for SUVA of the isolates. The aromatic region of the 13 C-NMR spectra is actually a measure of sp2 hybridized carbon atoms bonded to other carbon atoms and includes both aromatic and unsaturated aliphatic carbon such as

Table 7. Correlation coecients (r-values) of physical±chemical properties of NOM isolates obtained by correlation analysis with ozone decomposition (D01, k2, kO3 ) and HO radical (kHO,DOC) reactivity parameters Parameter

D01 a

k2 a

kO3 a

D01 a,b

k2 a,b

kO3 a,b

kHO,DOC c

SUVA 254 nm SUVA 280 nm S¯uor Mw Al-C Total Ar-C Type 1 Ar-C Type 2 Ar-C Carb-C Act-Ar-C Ketone C/H C/O C/N

0.92 0.92 0.85 0.85 ÿ0.77 0.89 0.92 0.89 0.80 0.91 0.53 0.81 ± ±

0.91 0.92 0.81 0.85 ÿ0.76 0.85 0.85 0.89 0.57 0.88 0.56 0.73 0.59 ±

0.92 0.93 0.81 0.85 ÿ0.79 0.87 0.85 0.89 0.65 0.88 0.56 0.72 0.74 ±

0.83 0.81 0.74 0.76 ÿ0.61 0.82 0.84 0.78 ± 0.81 0.54 ± ± ±

0.89 0.91 0.53 0.66 ÿ0.53 0.70 0.69 0.74 0.57 0.72 0.59 ± ± ±

0.90 0.91 0.54 0.70 ÿ0.54 0.71 0.70 0.74 0.58 0.73 0.58 ± ± ±

0.97 0.97 ± 0.81 ÿ0.73 0.87 0.72 0.75 ± 0.75 0.52 0.61 ± ±

± indicates ÿ0.5 < r < 0.5.aSRH not included in analysis.bFor experiments with 2 mM t-butanol.cSRH included in analysis; the correlation coecient (r) values are based on linear regressions with approximately 20 data sets. Negative r-values indicate an inverse relationship between the NOM property and the rate parameter.

2274

Paul Westerho€ et al.

Fig. 4. Relationships between oxidant rate parameters with (a) SUVA and (b) total aromatic carbon content of HOA isolates.

ole®ns, and as a group have been correlated with SUVA (Chin et al., 1994). 13 C-NMR analysis provides information on the number or distribution of speci®c types of carbon atoms. The response of a given carbon atom is in¯uenced by other functional groups attached to it, such as carboxyl and phenolic groups. The HOA samples are complex mixtures, and the NMR spectra are complicated and dicult to interpret. Activated aromatic carbon content values ranged from 1.6 (WLL) to 4.2% (CCK), with an average of 3.2%. In each case resulting correlations with the ozone rate parameters were comparable to those obtained with aromatic carbon alone (Table 7). The relative selectivity of UV absorption can be an advantage when studying mixtures of compounds, as characteristic groups or structural features may be recognized in molecules of widely varying complexities (Silverstein et al., 1974). Whereas 13 C-NMR provides information about the

distribution of carbon atoms in a sample, absorption in the ultraviolet (UV) range is dependent on the electronic structure of the molecule and, at l = 254 nm, is largely limited to conjugated systems. In the case of ozonation, ozone is an electrophile and is very reactive with electron donating functional groups. SUVA, therefore, provides speci®c information about the isolates that is more related to its reactivity with ozone than is the 13 CNMR analysis. Elemental composition of NOM re¯ects bonding arrangements (Perdue, 1984), and variations in these bonding con®gurations may in¯uence oxidative reactivity. The only signi®cant relationship between molar ratios of elemental analytes (C, H, O, and N) with oxidation rate parameters is between the ratio of C/H, an indicator of unsaturated carbon, and molecular ozone (no t-butanol) and HO radical reaction rates (Table 7).

Structure and reactivity of NOM PRACTICAL IMPLICATIONS

A major result of the research presented in this paper is the demonstration that the chemical nature of NOM exerts a strong control on its reactivity with ozone, and will a€ect ozone consumption and possibly the rate of competition for HO radicals in ozonated waters. Although NOM is heterogeneous in nature, easily measured parameters, such as SUVA at 254 nm, were found to be powerful indicators for its reactivity toward oxidants. The rate parameters obtained in this work were determined to evaluate relationships between oxidation processes and the structure of NOM. However, the absolute values and mechanistic insights for the role of NOM on O3 and HO radical concentrations present during ozonation can be used in numerical ozone consumption models. Such numerical models can be used in turn to examine the e€ect of variable NOM structure on the disinfection or oxidation performance of ozonation systems. The relationships that exist between organic matter structure and its reactivity are useful in predicting potential impacts on ozonation dynamics associated with changes in the quality of organic matter in a given system. The chemical nature of NOM is often dependent on hydrologic and seasonally variable factors such as storm/agricultural runo€ events, reservoir turnover, and greater than normal algal productivity. The resulting changes in both the nature and amount of NOM in a system can signi®cantly a€ect ozonation process performance. Consider the reactivity of the two HOA isolates (SR1, SR2, SHL) obtained from the Shingobee River, MN. Although SR1 and SR2 were both collected from the Shingobee River at the same location, the SR2 isolate was collected in April during a snowmelt runo€ event (Aiken and Cotsaris, 1995). The SR2 isolate was signi®cantly more aromatic than the SR1 isolate as evidenced by both the SUVA absorbance and 13 C-NMR data. The SR2 isolate was found to consume ozone at a much faster rate, and had a higher rate constant for HO radical reactions than the SR1 sample. Changes in the SUVA of the isolates are re¯ected in the SUVA of the whole water samples (Aiken and Cotsaris, 1995). Therefore, although without direct experimental data, changes in reactivity of the whole water with ozone may be anticipated by monitoring SUVA, a relatively simple measurement.

2275

The relationship between the reactivity of a whole water sample and the isolates of NOM obtained from that location are demonstrated by examining the reactivities of California State Project water samples. In the case of this sample, the HOA fraction (SP8 isolate) accounted for 34% of the DOC and hydrophilic organic acid fraction (SP4 isolate) for 17%; the remaining DOC in the raw water was not characterized. The SP8 isolate has a higher SUVA than the SP4 isolate, while the raw water had a SUVA similar to the SP4 isolate (Table 8). Ozonation studies conducted on these solutions at similar DOC concentrations, pH, ozone doses, and t-butanol concentrations indicate that ozone consumption was fastest with the SP8 isolate solutions and slowest in the bulk water. The slow rate of ozone consumption in the raw water re¯ects, in part, the presence of less reactive organic matter and inorganic constituents in the sample. Ozone consumption in the presence of t-butanol resulted in similar rate parameters for these waters, although SP8 still exhibited the fastest, and the raw water the slowest, rate of ozone consumption. The SP8 isolate may contain a greater percentage of chain promoters and less scavengers than SP4 or bulk DOC. Finally, the results presented in this paper o€er insight into the reactivities of NOM that have been obtained from di€erent source materials, or have undergone processes in the watershed that have in¯uenced the chemical nature of the NOM. For instance, the NOM isolate from Lake Fryxell, Antarctica (FXL) is known to have been derived from algae and bacteria (Aiken et al., 1996). This type of microbially derived NOM may be representative of autochthonous organic matter in large lakes and reservoirs. The SUVA of this sample, and corresponding oxidative reactivity, is lower than the NOM isolates in this study obtained from other sources. In contrast with this sample are those obtained from the Suwannee River, which is thought to be derived primarily from higher plants and have not been signi®cantly altered by interactions within the soil environment. The SUVA data and the rates of ozone consumption are signi®cantly greater for SRF and SRH compared to FXL. These results suggest that increased understanding of watershed and water source dynamics with regard to the nature and generation of NOM will improve the ability to model and predict reactivity of NOM with oxidants such as ozone.

Table 8. Comparison of NOM fractions and raw water for California State Project water NOM source

SUVA ((mg C/l)ÿ1 cmÿ1)

D01 b (mM/min)

k2 b (103 sÿ1)

kHO,DOC (10ÿ8 l (mol C)ÿ1 sÿ1)

SP4 isolate SP8 isolate Raw watera

0.026 0.040 0.025

71 (41) 74 (39) 48 (42)

7.1 (0.84) 9.2 (1.08) 0.93 (0.67)

3.1 34.0 3.6c

DOC = 3.1, pH = 7.5, alkalinity = 80 mg/l as CaCO3, bromide = 250 mg/l.bValues in parentheses were

a

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Paul Westerho€ et al.

AcknowledgementsÐThe authors gratefully acknowledge the partial support of the AWWA Research Foundation. The assistance of Dr Robert Wershaw in performing 13 CNMR analysis is appreciated. Werner Haag, Phil Singer, Rengao Song, Kenan Ozekin, Mohammed Siddiqui, and several anonamous reviewers provided helpful discussions. Use of trade names in this report is for identi®cation purposes only and does not constitute endorsement by the U.S. Geological Survey. REFERENCES

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