Determination of photochemically produced hydroxyl radicals in seawater and freshwater

Marine Chemistry, 30 (1990) 71-88

71

Elsevier Science Publishers B.V., Amsterdam

Determination of photochemically produced hydroxyl radicals in seawater and freshwater* Xianliang Zhou** and Kenneth Mopper*** University of Miami, Rosenstiel School of Marine and Atmospheric Science, Division of Marine and Atmospheric Chemistry, 4600 Rickenbacker Causeway, Miami, FL 33149-1098 (U.S.A.) (Received August 23, 1989; revision accepted January 10, 1990)

ABSTRACT Zhou, X. and Mopper, K., 1990. Determination of photochemically produced hydroxyl radicals in seawater and freshwater. Mar. Chem., 30:71-88. A variety of short-lived, reactive chemical species (i.e. free radicals and excited state species) are known to be photochemically produced in natural waters. Some of these transients may strongly affect chemical and biological processes, and they have been implicated in the degradation of organic pollutants and natural organic compounds in aqueous environments. Previous studies demonstrated that the highly reactive hydroxyl radical (OH) is photochemically formed in seawater. However, the quantitative importance of this key species in the sea has not been previously studied because of past analytical limitations. By using a highly sensitive probe based on a-H atom abstraction from methanol, we were able to measure production rates and steady-state concentrations of photochemically produced OH radicals in coastal and open ocean seawater and freshwaters. The validity of the method was tested by intercalibrating with an independent, OH-specific reaction, hydroxylation of benzoic acid, and also by competition kinetics experiments. Our OH production rates and steady-state concentrations for freshwaters are in excellent agreement with those measured by previous investigators for similar waters. In contrast, for seawater, the values we measured are 1-3 orders of magnitude higher than previously predicted by models, indicating that there is a major unknown photochemical OH source (s) in seawater.

INTRODUCTION

In natural waters, absorption of sunlight, especially at UV-B wavelengths, by dissolved organic matter (DOM) and inorganic compounds leads to production of a variety of transient species. These include excited state DOM *Presented at the section on Atmospheric and Marine Chemistry of the 32nd IUPAC Congress in Stockholm, Sweden, August 2-7, 1989. **Present address: Environmental Chemistry Division, Brookhaven National Laboratory, Upton, LI, NY 11973, U.S.A. ***Author to whom correspondence should be addressed at: Chemistry Department, Washington State University, Pullman, WA 99164-4630, U.S.A.

0304-4203/90/$03.50

© 1990 - - Elsevier Science Publishers B.V.

72

X. ZHOU AND K. MOPPER

(Zepp et al., 1985; Faust and Hoign6, 1987), hydrogen peroxide (Cooper et al., 1988 ), singlet oxygen (Zepp et al., 1977, 1985; Haag and Hoign6, 1986 ), hydrated electrons (Fischer et al., 1985; Zepp et al., 1987a ), superoxide ion (Baxter and Carey, 1983; Petasne and Zika, 1987), organoperoxy radicals (Mill, 1980; Mill et al., 1980), hydroxyl radicals (OH) (Zafiriou, 1974, 1977; Zafiriou and True, 1979; Mill, 1980; Mill et al., 1980; Haag and Hoign6, 1985; Zafiriou and Bonneau, 1987; Zepp et al., 1987b; Mopper and Zhou, 1990), and in seawater, bromine-containingradicals (Zafiriou, 1974; Zafiriou et al., 1987 ). These transients are reactive and play important roles in chemical and biological processes in aquatic environments, such as photodegradation of organic pollutants and natural DOM, redox reactions, enzyme deactivation, and acceleration of otherwise sluggish reactions (Zafiriou, 1974; Mill, 1980; Haag and Hoign6, 1985; Zepp et al., 1987b; Mopper and Zhou, 1990; Cooper et al., 1989). Of these transient species, OH is by far the most reactive (Mill, 1980; Zafiriou, 1984; Zafiriou et al., 1984). However, because of its high reactivity, OH occurs in natural waters at very low steady-state concentrations and thus is difficult to monitor directly by spectroscopic methods, e.g. electron paramagnetic resonance (EPR). The use of probe molecules to react with or scavenge OH radicals is the most frequently employed technique to study photoproduction of OH in natural waters. These probes include cumene (isopropylbenzene) (Mill et al., 1980), n-butyl chloride (Haag and Hoign6, 1985; Zepp et al., 1987b), methyl mercury, nitrobenzene and anisole (Zepp et al., 1987b). Using these techniques, these investigators determined photoproduction rates and steady-state concentrations of OH radicals in the range of 10-12_ 10-10 M s- 1and 10-17 10- ~5 M, respectively, in nitrate- and DOMrich lake water and fiver water. However, no direct measurements have been made in seawater because of the inadequate sensitivity of these techniques. OH photoproducti,on rates are about one order of magnitude lower, and natural OH scavenger concentrations are about one order of magnitude higher in seawater than in freshwaters; thus, more sensitive probes are needed. In this study we used two probes to study photochemical production of OH radicals in natural waters, especially seawater. The first is based on a wellcharacterized reaction of OH radicals, i.e. hydroxylation of aromatic rings (Armstrong et al., 1960; Loeff and Swallow, 1964; Matthews, 1980; Mill, 1980). Benzoic acid was used in this study and the formation rates of its hydroxylated products, o-, m-, and p-hydroxybenzoic acids, and the destruction rate of benzoic acid itself, were monitored. The second technique is based on ot-H atom abstraction of methanol by OH radicals (Asmus et al., 1973; Buxton et al., 1988; Hess and Tully, 1989 ), and monitoring of the formation rate of the main stable product, formaldehyde (Asmus et al., 1973 ). Benzoic acid is a more specific OH probe (Mill, 1980); however, the methanol probe is

PHOTOCHEMICALLY PRODUCED HYDROXYL RADICALS

73

about 20 times more sensitive and is thus more useful for the study of OH photoproduction in natural waters, especially open oceanic waters. EXPERIMENTAL

Materials Reagent-grade hydrogen peroxide, potassium bromide, formic acid, sodium hydroxide and sodium bicarbonate were obtained from Fisher (Pittsburgh, PA). HPLC-grade methanol and acetonitrile, and reagent-grade benzoic acid and o,m,p-hydroxybenzoic acids were from Baker (Philipsburg, NJ ). Formaldehyde was from Sigma (St. Louis, MO). All chemicals were used as received. Deionized water was obtained from a Millipore Q-water system with an Organex attachment (Millipore, Milford, MA). 2,4-Dinitrophenylhydrazine ( D N P H ) (Sigma) was recrystallized twice from acetonitrile. Seawater samples from various depths in the Sargasso Sea and Gulf Stream were collected using Teflon-lined Go-Flo bottles (General Oceanics, Fort Lauderdale, FL ) during two cruises on the R / V "Columbus Iselin" (CI 8902, March 1989; CI 8913, November 1989 ). Samples were stored in 4-1 clear glass bottles with Teflon-lined caps in the dark at 4 ° C. Coastal surface seawater was collected from Biscayne Bay, FL, and Vineyard Sound, MA. Filtered (0.22 #m Nylon 66, MSI, Honeoey Falls, NY; < 200-mbar pressure differential) and unfiltered seawater were used in the experiments. Other samples used included brackish pond water (S%0 ~ 3 ) from the Coral Gables Campus of the University of Miami and Tamiami Trail freshwater from the Everglades National Park (filtered through Whatman G F / C glass fiber filters, precombusted at 400 ° C).

Irradiation procedure Methanol (2-30 m M ) or benzoic acid ( 1/zm or 0.2-5 m M ) was added to water samples. Most irradiations were done in 250-ml quartz flasks (with ground quartz stoppers) with natural sunlight using standard conditions (4 h, solar noon, cloudless sky, 26 °N). In addition, a few experiments were carried out using 80-ml quartz tubes (with ground quartz stoppers) in a merrygo-around irradiation system with a 450-W medium-pressure mercury lamp in a borosilicate immersion well (about 290-nm wavelength-cut-off) (Ace Glass, Vineland, N J). The results from the irradiation system were normalized to natural sunlight by comparing OH production rates from nitrate photolysis in seawater using the irradiation system and natural sunlight. Nitrate photolysis was used for this purpose because its action spectrum (Zepp et al., 1987b) is similar to the OH photoproduction action spectrum in natural waters (Mopper and Kieber, 1990; Mopper and Zhou, 1990).

74

X. ZHOU AND K. MOPPER

All results have been corrected for the blank, if necessary. Controls consisted of irradiated water samples with no added probe, irradiated deionized water with probe, and water samples containing probe stored in the dark. The results for the methanol probe had to be corrected for minor photochemical production of formaldehyde in seawater with no added methanol (Mopper and Stahovec, 1986 ). This correction was usually < 5% of the OH photoproduction rate. A similar correction was not necessary for the benzoic acid probe results. The experiments were run in the pH range 7.0-8.5. In this range, pH had no observable effect on measured rates constants. The production of formaldehyde (when methanol was used as the probe) or hydroxybenzoic acids (when benzoic acid was used) was monitored by measuring its concentration before and after irradiation by HPLC with UV-absorbance detection. The two probes were intercalibrated by irradiation of a known OH source, 100/tM H202 (Haag and Hoignr, 1985 ), added to deionized water and open ocean seawater which contained the respective probes.

Measurement of formaldehyde and hydroxybenzoic acids Formaldehyde was measured by HPLC with UV-absorbance detection at 370 nm, after forming its 2,4-dinitrophenylhydrazine derivative, as described by Mopper and Stahovec ( 1986 ). The derivative was eluted isocratically with 45% acetonitrile aqueous solution as the mobile phase. Samples for hydroxybenzoic acid analysis were acidified to pH ~ 2.5 with 5 M HCI and then injected into the HPLC system. A two-solvent gradient elution was used to separate benzoic acid and isomers of hydroxybenzoic acid. The weak solvent was water at pH 2.5 (A) and the strong solvent was acetonitrile (B). The gradient program was as follows: isocratic at 15% B for 3 min, 15-50% B in 10 min, and then 100% B in 2 min. Hydroxybenzoic acids were detected by UV absorbance at 255 nm for the p-isomer, and 300 nm for the o- and m-isomers. The HPLC system used in this study consists of an E-Lab Model 2020 gradient programmer and data-acquisition system (OMS Tech, Miami, FL) installed in an IBM-compatible PC, an Eldex Model AA-100 S pump (Eldex Laboratories, Menlo Park, CA) and a six-port Valco injector (Valco Instruments, Houston, TX ) with a 2-ml sample loop. The mobile phase was generated by an inert solenoid valve (OMS Tech) placed on the low-pressure side of the pump and controlled by the E-Lab system. Compounds were separated on a Radial Compression Separation System with a reverse-phase C 18 Radial-Pak cartridge (Type 8NVC184) (Waters Assoc., Milford, MA) at room temperature, and detected by an ISCO Model V4 variable-wavelength absorbance detector (ISCO, Lincoln, NE). 'On-the-fly' UV-visible absorption spectra of eluting compounds were obtained by an HP 1040A Diode-Array

PHOTOCHEMICALLY PRODUCED HYDROXYL RADICALS

75

Detector controlled by an HP 9000 data system (Hewlett-Packard, Avondale, PA). RESULTS AND DISCUSSION

Detection limit, reproducibility and controls The detection limit for formaldehyde (as the D N P H derivative) by HPLC with UV-absorbance detection is ~ 1-2 nM (Mopper and Stahovec, 1986), and the reproducibility is better than 5% at 40-nM level, which corresponds to the lowest OH production observed after a 4-h irradiation. The detection limit for p-hydroxybenzoic acid is ~ 10 nM, and 30 and 50 nM for the o- and m-isomers respectively, by the H P L C / U V absorbance detection method, with a reproducibility of~ 5% at the 1-#M level. The reproducibility of OH production rates, based on the methanol probe, was better than 5% for coastal seawater (n = 4) and organic-rich freshwater (n = 3 ), and ~ 10% (n = 8 ) for open ocean surface water from the Sargasso Sea. For all water types used in this study, we detected no OH production either in irradiated deionized water with probe or in dark controls. This suggests that thermal production of OH radicals under our experimental conditions is not important. In addition, we found no differences in production rates between filtered and unfiltered water samples ( n = 4 ) (in general only highly turbid samples, e.g. Tamiami Trail freshwater, needed to be filtered before irradiation). Therefore, we conclude that photobiological and surface catalysed processes are insignificant and that homogeneous photochemical processes dominate OH radical production in natural waters. This is in agreement with conclusions of previous investigators (Zafiriou, 1974; Mill et al., 1980; Haag and HoignG 1985; Zepp et al., 1987b).

Linearity of OH photoproduction with time Most experiments were carried out with two time points, and the photoproduction rate of OH radicals was calculated by the difference of concentrations of products between the final irradiation time, time = t (usually 4 h), and the initial time, time = 0, divided by the irradiation time. To evaluate the validity of this calculation, we performed time-series experiments with various water types, including natural seawater and seawater with added H202 and NO~-. We obtained linear correlations between product accumulation (formaldehyde or hydroxybenzoic acids) and time for all experiments, with r 2 better than 0.985 (Fig. 1 ). Therefore, we conclude that it is valid to calculate OH production rates from two time points for irradiation periods up to at least 4 h.

76

X. ZHOU AND K. MOPPER 600

400

200 < ~

0

i

0

i

,

100

,

i

200

TIME OF IRRADIATION

,

300

(min)

Fig. 1. Time-course studies showing the linearity of photoproduction of formaldehyde from the reaction of OH with the methanol probe as a function of sample irradiation time. [3, Sargasso Sea deep water (2000 m), using artificial light; ~ , Sargasso Sea surface water+ 50 aM N O ; , using artificial light; ×, Gulf Stream surface water+ 15/~M H202, using natural sunlight.

Selection of probe scavengers Methanol and benzoic acid were chosen as probe scavengers for several reasons: (1) they react with OH by completely different but well-characterized mechanisms (Loeff and Swallow, 1964; Asmus et al., 1973; Mill, 1980; Buxton et al., 1988; Hess and Tully, 1989); (2) their rate constants for reaction with OH in aqueous media are accurately known and significantly different, 0.98 × 109 M-1 s- ~for methanol and 6 . 0 X 109 M -1 s - l for benzoic acid (Buxton et al., 1988); (3) both reactants are transparent and thus photochemically inert in the solar radiation spectrum incident at the Earth's surface; (4) the products, formaldehyde, which exists predominantly ( > 99.9%) as methylene glycol in aqueous solution (Walker, 1975 ), and p-hydroxybenzoic acid, are also transparent and thus stable in near-UV-visible light; ( 5 ) the products are not significantly degraded by reactions with OH over at least a 4-h experiment; and (6) the products can be readily measured with liquid chromatography, with a low nM sensitivity for formaldehyde (Mopper and Stahovec, 1986). We tested points ( 3 ) - ( 5 ) experimentally, and found no formaldehyde or hydroxybenzoic acid production when methanol (10 m M ) or benzoic acid ( 1 mM ) in deionized water was exposed to sunlight. Also, no changes in concentrations (within the analytical error, < _+5%) were observed when formaldehyde (0.2 a M ) and p-hydroxybenzoic acid (1 ~tM) in deionized water and seawater were exposed to sunlight for 4 h. When benzoic acid (BA) reacts with OH radicals, three isomers of hydroxybenzoic acid (HBA) are formed (Armstrong et al., 1960)

PHOTOCHEMICALLY PRODUCED HYDROXYL RADICALS

H -OH + H - B A ~ Ho/~BA

77

o,m,p-HBA + HO2-

In addition, ring fission (Loeff and Swallow, 1964; Cox et al., 1980) and decarboxylation products (Matthews and Sangster, 1965; Matthews, 1980) are also formed. The hydroxybenzoic acid products were identified by comparing retention times on chromatograms (Fig. 2a) and 'on-the-fly' UV-visible absorbance spectra (Fig. 2b) with those of standards. We studied benzoic acid hydroxylation using hydrogen peroxide photolysis as an OH source (Haag and Hoignd, 1985 ). Similar percentages of isomers (o,m,p) were obtained using seawater (36, 33 and 31%) and deionized water (36, 34 and 30%) as the reaction media, which indicates that there are no significant matrix effects in the hydroxylation reaction. These percentages agree reasonably well with literature values based on different experimental conditions (Armstrong et al., 1960). Furthermore, we obtained similar percentages of isomers for natural waters without added OH sources, e.g. 36-34-30% for a 50-50 mixture of Tamiami Trail freshwater with surface Sargasso Sea water, and 36-33-31% for pond water from the University of Miami Coral Gables Campus. This good agreement between samples with and without added OH sources indicates that the benzoic acid probe is detecting predominantly OH radicals in natural samples. As p-HBA is the photochemically stable product and the sensitivity of its measurement is significantly higher than that of its isomers, only this compound was measured for most experiments. The detection limit ofp-hydroxybenzoic acid is~ 10 nM by HPLC/UV absorbance detection, therefore the corresponding detection limit for OH production is ~ 60 nM because of the formation of isomers and decarboxylation and ring fission products. The conversion factor for calculating OH production from the p-hydroxybenzoic acid formation rate is 5.87 + 0.18 ( n = 4 ) . This conversion factor was obtained by intercalibration with the methanol probe using a pure OH source (H202) and is in excellent agreement with G value data ( ~ 6) for products formed from reaction of benzoic acid with OH (Armstrong et al., 1960; Matthews and Sangster, 1965 ). Although benzoic acid is a more specific OH probe than methanol, its lower sensitivity prevents direct application to open oceanic water. By measuring formaldehyde (Fig. 3a), the dominant product formed by reaction of methanol with OH, the detection limit is lowered by about 20-fold compared with the benzoic acid probe (Fig. 3b). The main reaction (> 93%) of OH with methanol is t~-H atom abstraction (Asmus et al., 1973; Hess and Tully, 1989 ) -OH+CH3OH~ "CH2OH+H20 In the presence of oxygen, formaldehyde is formed as the dominant stable product (Asmus et al., 1973)

"]8

X. Z H O U A N D K. M O P P E R

•CH~ OH-F O2 ~ •O2 CH2OH-~ CH20-!- HO2" OH photoproduction rates measured under the same conditions agreed

within ~ 20% between these two independent probes for all samples tested (Table 1 ). This good agreement is strong evidence that OH is the main reactive species being measured by the probes. In addition, using the methanol 20

o

A

I

18' 16' 14'

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12 10

© r~

1-

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8

6

10

12

14

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A

{~.a . . . . . . . . . 220

24~

L 260

2B~

3G~I

320

340

360

380

W A V E I , E N G T I I (rim Fig. 2. (a) Chromatograms of hydroxybcnzoic acids produced in Biscayne Bay seawater+ 50 /zM H202 (benzoic acid as probe) after 2-h irradiation in sunlight. Line l: detected at 255 nm; line 2: detected at 300 nm. Peaks A, B, and C are p-, m-, and o-hydroxybcnzoic acids, respectively. (b) Spectra of hydroxybenzoic acids. Upper lines, standards; lower lines, products produced in 50:50 seawater-Everglades freshwater (benzoic acid as probe) after 4-h irradiation. A, B, and C are p-, m-, and o-hydroxybenzoic acids, respectively.

79

PHOTOCHEMICALLY P R O D U C E D H Y D R O X Y L RADICALS 28"

Z

0

1

lai Is! 14i la:

t=5

\

4!

2

8

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TIME (rain)

2°1

b

,B'

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.

.

.

.

.

2

I

!-!,_

. . . . . . . .

4

S

......

8

1~

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12

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14

TIME (min) Fig. 3. Chromatograms of BiscayneBay seawater sample before (t = 0) and after (t = 5) a 5-h irradiation in sunlight. (a) Methanol as probe; (b) benzoic acid as probe. OH photoproduction was 30 × 10- '~ M s-'. HPLC conditions for benzoic acid analysis: gradient elution starting at 10% acetonitrile, isocratic for 2 min, then ramped up to 30% in 8 min, followedby an immediate step to 100%, and isocratic at 100% for 2 min. probe, we obtained excellent agreement with the results o f Zepp et al. (1987b) for the production o f O H from nitrate photolysis; i.e. 3.0 × 10-13 M s - ' (/zM NO~- ) - ' (present s t u d y ) vs. 2 . 5 × 10 -13 M s - ' (gM NO~- ) - ' (Zepp et al., 1987b). Further supporting evidence is discussed in the next section.

80

X. ZHOU AND K. MOPPER

TABLE 1

Comparison of hydroxyl radical photoproduction rates measured with methanol and benzoic acid probes Water type a

S S S W + H202 ( 100/tM) QW+NO~(300/tM) BBSW-TTFW (50:50) BBSW

(low tide) SSDW (2900 m )

OH Photoproduction rates ( 10-1o M s - 1) Methanol probe b

Benzoic acid probe b

7.7 + 0.1 (n=4) 0.60 (n= 1) 1.3 (n---2) 0.18 (n=2) 0.14+0.01 (n=3)

8.0 + 0.2 (n=4) 0.65 (n= 1) 1.3 (n=2) 0.17 (n=2) 0.11 +0.02 (n=3)

Ratio MeOH/BA OH production 0.96 0.92 1.0 1.1 1.3

aSSSW - Sargasso Sea surface water; Q w - Milli-Q deionized water; BBSW - Biscayne Bay surface water; SSDW - Sargasso Sea deep water; T T F W - Tamiami Trail freshwater (humic-

rich Everglades water). bThe two probes were intercalibrated using H102 photolysis as OH source added in deionized water. The conversion factor for calculating OH production rate (Poll) from p-hydroxybenzoic acid production rate is 5.87 +0.18 ( n = 4 ) : P o l l = 5.87 Pp-HBA-

Competition kinetics experiments and use of high versus trace scavenger probe addition In a typical competition kinetics experiment, an OH radical scavenger probe, e.g. methanol, is added to a water sample over a wide range of concentrations to compete with the natural OH scavengers. The purpose of these experiments is three-fold: first to determine the extent of reaction of OH with natural scavengers (ns) by measuring the overall OH scavenging rate constant,/~s. This constant is needed to calculate OH steady-state concentrations from measured OH photoproduction rates. Second, comparison of k~s values measured with and without known OH sources added to natural water samples serves as an additional test of whether the probes are predominantly measuring OH in these samples. Finally, these experiments are required to determine the appropriate level of added probe needed to compete effectively with natural scavengers. When a water sample (with or without an added OH source) is exposed to sunlight, OH radicals are photochemically produced at a rate o f P o l l , and react with natural scavengers (ns) in the sample and the added probe (p) scavenger, at rates of Rns and Rp, respectively

81

PHOTOCHEMICALLY PRODUCEDHYDROXYLRADICALS

•OH sources

h

POH

, •OH Rns

•OH

natural scavengers probe scavenger

I

mostly Br~- in seawater

HBA or formaldehyde Rp

As these free radical reactions are rapid steady state with respect to OH radical concentration [ OH ]'~ is established within seconds after exposure to sunlight, i.e. OH photoproduction rate equals the consumption rate by the scavengers

(i)

POH= Rns + Rp In which R,s=k~s[OH]'ss

(2)

Rp=/q,[probe ] [OH]'s~=k~[OH]'s~

(3)

where k'nsis the apparent OH scavenging rate constant of natural scavengers, kp the reaction rate constant of the probe scavenger with OH, and [ OH ] '~ is the steady-state OH concentration in the presence of added and natural OH scavengers. Because of the presence of an added scavenger probe. [OH]'~ is less than the natural OH steady-state concentration, [ OH ] ~s. Combining the three equations above, one obtains the reaction rate of OH with the scavenger probe Rp (which is equal to the formation rate of scavenging product) as a saturation-type function of added probe concentration (Fig. 4a) [ probe ] Rp = POH × k n j k ° + [ probe ]

(4)

The saturation or plateau level represents the probe concentration where all natural OH scavengers have been out-competed by the added probe. The above equation can be linearly transformed as follows 1

1

k~s

Rp --Poll + ~ X

1

[probe]

(5)

Plotting 1/Rp against 1/[ probe ], a straight line is obtained (Fig. 4b). As the reaction rate constants of the probes, /q,, are accurately known, the OH

82

X. ZHOU A N D K. M O P P E R

30 0

211

1

10

0

l).Ol

OJ)2

o.q)3

0.04

0.05

iMe()ll[ IM)

90

t/3

70 ,t--4

5(I

3O 0

260 1/[MeOH]

4iJ0

600

(M-l)

Fig. 4. (a) Relationship (eqn. (4)) between formaldehyde production rate (RM) and concentration of added methanol ( [MeOH ] ) in sunlight-irradiated Biscayne Bay seawater. (b) Relationship (eqn. ( 5 ) ) between 1/RM and l / [ MeOH ] for the same experiment as (a); r 2 = 0.994.

photochemical production rate, POH, and the OH scavenging rate constant by natural scavengers, k~s, can be calculated from the y-intercept and slope, respectively. Using this kinetic approach, we estimated k'nsof seawater to be 2.5 _+0.3 × 106 s - ~ (n = 5 ), with 2 0 - 6 0 gM H202 and 100 pM NOj- as added OH sources, and methanol or benzoic acid as probe (Table 2). A few experiments were run with added formate (0.8 mM, k o n = 3 . 2 × 109 M -~ s-~; Buxton et al., 1988) as a third OH competitor. The/(ns measured without any added OH source and using methanol as the probe was 2.4 _+0.2 × 106 s - ~ ( n = 5 ) (Table 2). This excellent agreement is further evidence that the OH radical is the predominant reactive species measured by the two probes. Using the above value of 2.4 × 106 s - ~ for k'nsand the known values for/%

PHOTOCHEMICALLYPRODUCEDHYDROXYLRADICALS

83

TABLE 2 Evaluation of the overall scavenging rate constant (k~, s - l ) of OH by natural constituents in seawater and the corresponding rate constant for reaction of OH with bromide in seawater (kB, M-Is-I) Samplea SSDW CSSW (high tide) CSSW (low tide) CSSW (low tide ) CSSW (low tide) GSSW CSSW (high tide) CSSW (low tide) CSSW (low tide) CSSW

OH sources

Probe b

k~s

kBr

()<10 6)

( X I 0 9)

Natural Natural

MeOH MeOH

2.2 2.6

2.6 3.1

Natural

MeOH

2.7

3.2

Natural

MeOH

2.5

3.0

Natural

MeOH c

2.2

2.6

Average + sd

2.4 + 0.2

2.9 + 0.3

+ 20 #M H202 + 60 #M H202

MeOH MeOH

2.4 2.9

2.9 3.4

+ 30 #M H202

MeOH ¢

2.4

2.9

+ 100/~M NO~-

MeOH

2.2

2.6

Average__ sd

2.5 __0.3

3.0 _+0.3

Benzoic acid

2.5

3.0

+ 50 gM H202

aSSDW - Sargasso Sea deep water; CSSW - coastal surface sea water; GSSW - Gulf Stream surface water. bProbe concentrations were varied between 1 and 20 mM for methanol (MeOH) and 0.2 and 5 mM for benzoic acid. cSample also contained 0.8 mM formate as an additional competitor.

(for either methanol or benzoic acid), natural OH steady-state concentrations [OH ]ss and photochemical production rates can be determined from single-point probe additions to water samples as follows: without probe addition (Rp = 0), OH production rate (Poa) equals OH consumption rate by natural scavengers (R.s). Thus, from eqns. ( 1 ) and (2)

eoH=R.s=kns[OHlss

(6)

With addition of a probe scavenger, the OH steady-state concentration is shifted to a lower value, [OH]'s. but its production rate (PoH) remains unchanged. Thus, POH=/~ [OHls~= (k~+k~) [OHl'ss

(7)

84

x. ZHOU AND K. MOPPER

Substituting Rp=k~[OH]'~ (eqn. ( 3 ) ) into eqn. (7) and rearranging the terms, one obtains [OH]s~= [ (k~+k~)/(k,~,X/~)

] XRp

(8)

e.g. [OH]~s=5.0× 10-7×RM for a 10-mM methanol probe addition, where RM is equal to the photoproduction rate of formaldehyde from added methanol. In seawater, the main sink for OH ( > 93% ) is reaction with Br- (Zafiriou, 1974; Zafiriou et al., 1987). Thus, for seawater, k'ns (Table 2) is approximately equal to khr. Using [Br- ] = 0.84 mM (salinity range 34-36 ) (Stumm and Morgan, 1981 ), the reaction rate constant of Br- with OH can be estimated as 3.0___0.3× 109 M - l s -~ ( n = 10) (Table 2) in seawater medium, which is within the range of values reported in the literature of 0.5-11 X 109 M -~ s -~ (Buxton et al., 1988; Ross and Ross, 1977). The reaction kinetics of OH with Br- are complex and are sensitive to variations in experimental parameters, e.g. pH, dissolved oxygen concentration and other matrix effects, as described in detail by Matheson et al. (1966) and Zehavi and Rabani ( 1972 ). According to these studies, Br-+-OH ~

B r O H - k3 Br. + O H - ~...products

Conditions favoring the backward or forward dissociation of the intermediate product, B r O H - , will strongly affect the resultant rate constant. Thus, the wide range of kBr values reported in the literature is undoubtedly a result of the widely different experimental approaches (i.e. species being monitored) and conditions used in previous studies (O.C. Zafiriou, personal communication, 1989). The kBr measured in our study represents an apparent or overall rate constant for reaction of Br- with OH in a complex medium, seawater. Our kinetic analysis appears to be valid because, from a low, single-point probe addition where the ambient [OH ]ss is perturbed by < 20% by the probe, our rate constant accurately predicts ( _+5%) the saturation (or plateau) OH production rate (Fig. 4a) in seawater where competition from all natural scavengers, predominantly Br-, is negligible (Table 3 ). It also agrees, within a factor of 2, with OH production rates measured with trace levels of added probe, e.g. 1/~M benzoic acid, where the [ OH ] s~is perturbed by < 0.5% (Haag and Hoignd, 1985; Zafiriou et al., 1990). In this case, [OH ] ~ was determined from the loss of probe after irradiation of Everglades freshwater mixed ( 1 : 1 ) with Sargasso surface seawater. As this loss was usually < 10% in our experiments with low probe addition, the precision of this method is significantly worse ( ~ _+30%) than for high probe addition (e.g. 0.2-5 m M ) and is thus not suitable to seawater.

PHOTOCHEMICALLY PRODUCED HYDROXYL RADICALS

85

TABLE 3 OH photoproduction rates and steady-state concentrations in seawater determined from onepoint probe additions (predicted) and from multipoint saturation kinetics experiments (measured) Sample ~

SSDW (2000 m ) BBSW (high tide) BBSW (low tide) BBSW (low tide) BBSW (low tide)

Porl ( × 10- lo M s-I )

[OH]ss ( × 10 - l s M )

Predicted from onepoint addition b

Measured by multipoint additions

Predicted from onepoint addition c

Measured by multipoint additions

0.15

0.14

6.0

5.6

0.21

0.24

8.5

9.5

0.30

0.29

11.9

11.5

0.32

0.34

12.7

13.5

0.29

0.30

11.4

11.9

aSSDW Sargasso Sea deep water; BBSW - Biscayne Bay seawater. bpon = [ (kn$-Jt-kp)kp] XRp~-~- 1.25 Rp (for a 10 mM-methanol probe addition). c [ OH ] ss= POll/k~s = [ (k~s + k~, ) / (k~s xk~, ) ] × Rp = 5.0 X 10- 7× Rp (for a 10 raM-methanol probe addition). -

Comparison of typical results with those of previous studies We applied the methanol probe method to various natural waters, including organic-rich freshwater, coastal surface seawater, deep oceanic water and Sargasso Sea surface water (Mopper and Zhou, 1990). The OH photoproduction rate ranged from 3.2 × 10-11M s- 1 in pond water to 4.2 × 10-10 M s- 1 in organic-rich Everglades water, corresponding to steady-state concentrations of 1×10 -16 and 8.4×10 -16 M, which is in good agreement with 1 × 10-17-5 × 10-15 M in various freshwaters measured by previous workers (Mill et al., 1980; Haag and Hoign6, 1985; Zepp et al., 1987b). In contrast, the values we measured in surface oceanic seawater, e.g. 3 × 10 -12 M s -l (Mopper and Zhou, 1990), are significantly higher than those predicted by models based on nitrate and nitrite photolysis, e.g. 10-14-10-15 M s-1 (Zafiriou and True, 1979; Zepp et al., 1987b). The discrepancy between our measured OH photoproduction rates and the previously estimated values from models indicates that there is a major unknown OH source (or sources) in seawater, which, as in freshwaters (Mill, 1980; Mill et al., 1980), appears to be DOM (Mopper and Zhou, 1990).

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x. ZHOU AND K. MOPPER

CONCLUSIONS

In this study, we used two probes to measure photoproduction of OH radicals in natural waters, methanol and benzoic acid. Benzoic acid is a more specific OH probe through hydroxylation of its aromatic ring by OH radicals. The constancy of the proportion of the hydroxylated products in different sample types and under different experimental conditions is strong evidence that only the OH radical is being scavenged by this probe. The good agreement between the results using benzoic acid and methanol probes indicates that the methanol probe is also measuring predominantly OH photoproduction. This is further supported by competition kinetics experiments, which showed excellent agreement between the apparent OH scavenging rate constant by natural constituents in seawater with and without OH source addition. In addition, using the methanol probe, we obtained excellent agreement with the results of Zepp et al. ( 1987b ) for the production of OH from nitrate photolysis. By measuring the formation rate of formaldehyde, the main stable product from the reaction of methanol probe with OH, a detection limit in the low nanomolar level is attained. With this probe, OH photoproduction rate could be accurately measured even in oceanic surface seawater exposed to sunlight for only a few hours. The OH photoproduction rates measured using methanol and benzoic acid probes are similar to those reported by previous investigators using similar or different techniques for freshwaters. However, there is a large discrepancy between our measured values in seawater and calculated values using photolysis models, which suggests that there is a major unknown source for OH photoproduction in seawater, probably marine DOM. Discussion of photochemical pathways involved in the production of OH radicals from DOM, as well as the impact of this production on carbon cycling and biological processes, is presented elsewhere (Mopper and Zhou, 1990) and in future communications.

ACKNOWLEDGMENTS

We thank T. Mill, O.C. Zafiriou, J.M.C. Plane, M. Ehrhardt, C.H. Langford, E.S. Saltzman, N.V. Blough, R.G. Zika, and D.J. Kieber for valuable discussions of data. Financial support was provided by the U.S. National Science Foundation's Chemical Oceanography Program and the U.S. Office of Naval Research's Ocean Chemistry Program.

PHOTOCHEMICALLY PRODUCED HYDROXYL RADICALS

87

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