Marine Chemistry 97 (2005) 14–33 www.elsevier.com/locate/marchem
Photochemical production of hydrogen peroxide and methylhydroperoxide in coastal waters
Daniel W. O’Sullivan a,*, Patrick J. Neale b, Richard B. Coffin c, Thomas J. Boyd c, Christopher L. Osburn c
a United States Naval Academy, Chemistry Department, 572 Holloway Road, Annapolis, MD 21402, USA b Smithsonian Environmental Research Center, 647 Contees Wharf Road, Edgewater, MD 21037, USA c Marine Biogeochemistry Section, Code 6114, US Naval Research Laboratory, Washington, DC 20375, USA Received 27 April 2004; received in revised form 5 April 2005; accepted 19 April 2005 Available online 13 September 2005
Abstract
Hydrogen peroxide (H2O2) has been observed in significant concentrations in many natural waters. Because hydrogen peroxide can act as an oxidant and reductant, it participates in an extensive suite of reactions in surface waters. Hydrogen peroxide is produced as a secondary photochemical product of chromophoric dissolved organic matter (CDOM) photolysis. Apparent quantum yields for the photochemical production of hydrogen peroxide were determined in laboratory irradiations of filtered surface waters from several locations in the Chesapeake Bay and in Arctic coastal waters with varying levels of CDOM. The apparent quantum yield for H2O2 decreases by about an order of magnitude from 280 nm to 500 nm, and the majority of H2O2 production occurs at wavelengths less than 340 nm. The apparent quantum yield for H2O2 production at 290 nm ranged 4 6 1 from 4.2 10 to 2.1 10 mol H2O2 (mol photons) from freshwater to marine waters. A linear relationship was found between the production of H2O2 and change in CDOM absorbance characterized as photobleaching (loss of absorbance). No significant relationship was observed between DOC concentration and peroxide production. Methylhydroperoxide (CH3O2H) was the only short chain peroxide produced during the irradiations, and its production is at least an order magnitude less than that of hydrogen peroxide. Peroxide production was greatest in waters containing significant amounts of terrigenous C in the form of humic substances. Surface waters whose synchronous fluorescence spectra indicated the presence of polyaromatic and/ or extensive conjugated compounds exhibited the greatest peroxide production. CDOM photobleaching is not significantly linked to apparent quantum yields for peroxide production. 2005 Published by Elsevier B.V.
Keywords: Hydrogen peroxide; Organic peroxides; Photochemistry; Seawater
1. Introduction
* Corresponding author. Hydrogen peroxide has been of interest since it was E-mail address: [email protected] (D.W. O’Sullivan). observed in significant concentrations in natural
0304-4203/$ - see front matter. 2005 Published by Elsevier B.V. doi:10.1016/j.marchem.2005.04.003 D.W. O’Sullivan et al. / Marine Chemistry 97 (2005) 14–33 15 waters in the 1980s (Cooper and Zika, 1983). Because references therein). Surface and aircraft-based pro- hydrogen peroxide can act as either an oxidant or a grams have demonstrated that H2O2 is the dominant reductant, the suite of reactions involving hydrogen gas phase peroxide in the troposphere. The most peroxide in natural waters is quite extensive. Hydro- abundant organic peroxides are CH3OOH and hydro- gen peroxide influences the redox chemistry of natural xymethylhydroperoxide (HMHP, HOCH2OOH). waters in several potentially important ways; by alter- CH3OOH is the dominant organic peroxide in the ing the speciation of trace metals in natural waters remote marine troposphere (Heikes et al., 1996; (Moffett and Zika, 1987), by changing the fate of O’Sullivan et al., 1999), and HMHP is thought to be pollutants, and potentially influencing the biological more prevalent over continental regions (Hewitt and activity of the microbial community (Obernosterer et Kok, 1991; Tremmel et al., 1994; Weinstein-Lloyd et al., 2001). To fully characterize and model the effects al., 1998). of hydrogen peroxide and organic peroxides in natural A number of studies have determined the photo- waters, additional information regarding the mechan- chemical production of H2O2 in freshwater and isms of production and loss in natural systems is ground waters (Cooper et al., 1988, 1994, Scully et needed, in particular the factors influencing the photo- al., 1996). Zuo and Hoigne (1992) examined the chemical production of peroxides, for example DOC formation of H2O2 in atmospheric water that results concentration, humic acid or fulvic acid content. from iron(III)-oxalato complex photolysis. Although Appreciable levels of hydrogen peroxide have been observations of H2O2 in surface seawaters are abun- observed in all types of natural surface waters. Cooper dant, only a few studies have examined the wave- and Zika (1983) observed levels of hydrogen peroxide length dependence of H2O2 production (Moore et al., in different freshwater lakes and ground waters ran- 1993; Andrews et al., 2000; Yocis et al., 2000), and ging from 10 s to 100 s of nanomolar. Zika et al. apparently no studies have examined the wavelength (1985) and Miller and Kester (1994, and references dependence of organic peroxide formation in surface therein) have observed levels of hydrogen peroxide in waters. An understanding of the wavelength depen- surface ocean waters up to several hundred nanomo- dence of photochemical reactions is important for lar. Levels of H2O2 of 10 to 100 AM have been determining how photochemical processes might observed in atmospheric aqueous phases: rainwater, vary with changing environmental conditions, such cloud water, and fog waters (Zika et al., 1982; as enhanced UV irradiance due to depletion of strato- Schwartz, 1984). Peroxides in atmospheric aqueous spheric ozone, or alterations in the depth of the photic phases are of interest because of their role in dissolved zone where photochemical processes might dominate SO2 oxidation. Hydrogen peroxide is believed to be other mechanisms of DOM turnover (Del Vecchio and the dominant oxidant for SO2 in atmospheric aqueous Blough, 2002). phases with pHb5(Calvert et al., 1985). Although the H2O2 formation in surface waters is initiated by the presence of organic peroxides is often considered, absorption of sunlight by dissolved organic matter there have been only a few observations of aqueous (DOM). The fraction of the DOM pool that interacts phase organic peroxide levels in natural waters. Sauer with sunlight, referred to as chromophoric dissolved et al. (1997) observed 800 nM of hydroxymethyl organic matter (CDOM), impacts the optical proper- hydroperoxide (HOCH2O2H) and 400 nM 1-hydro- ties of surface waters. Photolysis of CDOM in surface xyethyl hydroperoxide (CH3CH(OH)OOH) in rain- waters leads to the destruction of chromophores, water collected in the marine boundary layer off the potentially influencing the optical transparency of coast of France. The sources of atmospheric aqueous some surface waters (Gao and Zepp, 1998). CDOM phase peroxides are in situ photochemical generation photolysis is coincident with the production of a and partitioning of gas phase peroxides into aqueous variety of oxygen radical species (OH, O2 ,HO2 phases. The latter mechanism is supported by a large and RO2) in surface waters. These reduced oxygen number of gas phase atmospheric peroxide observa- radical species can further influence the optical envir- tions. A variety of different analytical methods and onment by directly effecting aquatic organisms and/or field results involving gas phase peroxide observa- altering the redox state of metals. Absorption of ultra- tions have been reviewed by Lee et al. (2000, and violet radiation by CDOM generates an excited singlet 16 D.W. O’Sullivan et al. / Marine Chemistry 97 (2005) 14–33 state, which produces an excited triplet state through short chain organic peroxides in surface seawaters intersystem crossing (Eqs. (1) and (2)). with varying levels of CDOM. 1CDOM þ hy ð280 400 nmÞY1CDOM4 ð1Þ 2. Methods 1CDOM þ hyðÞ280 400 nm Y3CDOM4 ðÞðvia intersystem crossing 2Þ 2.1. Peroxide analysis
The excited triplet state (3CDOM*) reacts with mole- An analytical technique for the determination of cular oxygen to form superoxide (O2 ) and its conjugate gas phase hydroperoxide concentrations (Lee et al., acid (HO2). 1995; Kok et al., 1995) was modified for work in surface waters. Although a number of more sensitive 3 3 + CDOM*+ O2YCDOM +O2 (3) analytical methods are available for the determination of hydrogen peroxide in seawater (Miller and Kester, Superoxide dismutates to produce hydrogen peroxide 1988; Yuan and Shiller, 1999), these methods do not (H2O2) and oxygen (Eqs. (4) and (5)). differentiate between hydrogen peroxide and other short chain organic peroxides. HO2+HO2YH2O2+O2 (4) The mobile phase composition for the high perfor- mance liquid chromatographic (HPLC) technique of HO2+O2 +H2OYH2O2+O2+OH (5) Lee et al. (1995) was modified by the addition of methanol for application to surface seawaters. A Of the reactive oxygen species in seawater, super- mobile phase composition of 95% 1.00 10 3 M oxide has been observed in the highest steady state H2SO4 and 5% methanol was used for the analysis. 9 8 concentrations: 10 to 10 M. H2O2 is long-lived All other reagent concentrations were identical to those relative to superoxide, and relatively stable, making it described by Lee et al. (1995). Type II horseradish an ideal photochemical marker (Miller, 1994). peroxidase and p-hydroxyphenyl acetic acid (Sigma Photochemically formed organic radicals are Chemical Co.) were used as received, as were potas- expected to participate in reactions analogous to HO2, sium hydrogen phthalate and sodium hydroxide resulting in the production of short chain organic per- (Fisher Scientific, ACS Certified Reagent Grade). oxides, for example methylhydroperoxide (MHP). Kie- Surface seawater samples collected in acid-cleaned ber and Blough (1990) demonstrated the formation of glass or PFATeflon vials were filtered through 0.20 Am carbon centered radials during the photochemically MilliporeR filters and either analyzed promptly or induced cleavage of a Keto acids and ketones in aqu- refrigerated for later analysis (see Section 2.6). The eous solutions. Carbon centered radicals can react with content of C1 to C2 peroxides in each sample was oxygen to form RO2, leading to organic peroxide for- determined using the modified HPLC fluorometric mation. Another possible path for the formation of method (Lee et al., 1995). C1 to C2 hydroperoxides short chain organic peroxides is the addition of singlet were separated by reverse phase HPLC on a 5 Am oxygen to alkenes. Riemer et al. (2000) determined the Inertsil ODS-2 PEEK column (4.6 mm 250 mm, photochemical production rates for C2–C4 alkenes in a Alltech) followed by a post column derivatization variety of surface waters. These processes are not reaction that formed a fluorescent dimer. Peroxidase expected to lead to high levels of organic peroxides; catalyzed dimerization of p-hydroxyphenyl acetic acid however, they do provide several possible mechanisms occurs in the presence of peroxy functional groups at for the in situ formation of short chain organic perox- elevated pH (i.e., pHz10). Formation of the fluores- ides in surface waters. In an effort to develop quantita- cent dimer is proportional to the concentration of a tive relationships between CDOM photochemical given hydroperoxide and resulted in an average pre- transformation, loss of absorbance or fluorescence, cision of better than F8% for analysis of the C1 to C2 and peroxide production, we have begun work to char- hydroperoxides in this study. The detection limits, acterize the photochemical production of H2O2 and defined as three times the standard deviation of the D.W. O’Sullivan et al. / Marine Chemistry 97 (2005) 14–33 17
blank, for the two short chain organic peroxides ob- (l) cells. Ak was then corrected for the absorption of 8 served in this study were: 3.0 10 MH2O2 and pure water as a function of wavelength (k) using low- 1.5 10 8 MMHP. DOC 18 MV MilliporeR water and converted to absorption coefficients according to Kirk (1994),
2.2. Compound synthesis and standardization 1 aCDOMðÞk ¼ 2:303Akl ð6Þ
MHP was synthesized from 30% H2O2 (Sigma- CDOM absorption spectra were corrected for instrument Aldrich Co.) and dimethyl sulfate (Eastman Kodak, drift, fine particle scattering, and the difference between Rochester, NY) in the presence of potassium hydroxide refractive indices for Milli-Q and estuarine and seawater as described by Lee et al. (1995). In the presence of using the following equation (Johannessen et al., 2003): strong base, HO2 will undergo nucleophilic addition to Sk either methyl group of dimethyl sulfate. MHP was aCDOMðÞ¼k a0e þ C ð7Þ synthesized by mixing 10 mmol of dimethyl sulfate, where a , S (the spectral slope coefficient), and C 20 mmol of 30% H O and 20 mmol of 40% KOH in a 0 2 2 were calculated by nonlinear least squares regression round bottom flask that was immersed in an ice bath. (which used a Gauss–Newton algorithm). The para- The reaction was subsequently initiated by slowly heat- meter C was an offset value, and was subtracted from ing the mixture to 60 8C. The solution was maintained the entire absorption spectrum. at 60 8C until bubble formation stopped. During the Photobleaching of CDOM for the photolysis experi- reaction N was initially passed through the solution at 2 ments was characterized as the average loss of absor- 5 ml/min. N flow was controlled with a flow meter 2 bance (L ,m 1) over the range of 280 to 500 nm (Cole–Palmer), and increased in 50 ml increments avg (Osburn et al., 2001). every 10 min to a maximum 500 ml/min near the 500 reaction end point. The N2 gas passed in series through X the reaction flask and then through 50 ml of 18 MV aCDOMðÞk initial aCDOMðÞk final R k¼280 Millipore water in a West–Gaeke bubbler immersed Lavg ¼ ð8Þ in an ice bath to capture MHP. This procedure separates 220 MHP from H2O2 as a result of the large difference in the Changes in CDOM over the course of the photolysis MHP and H2O2 Henry’s law constants (O’Sullivan et experiments were also characterized by determination al., 1996). The resulting MHP solution has a concen- of the CDOM synchronous fluorescence (SF). Syn- tration between 10 3 and 10 4 M, and is stable for chronous fluorescence was measured with a Shimadzu about a year when stored at 4 8C in brown borosilicate RF-5301 spectrofluorometer. The excitation wave- glass bottles. MHP stock solutions were calibrated by length range was 236 to 600 nm, and emission wave- reaction with iodide. Iodine liberated by the reaction lengths ranged from 250 to 614 nm, maintaining a 14 was determined spectrophotometrically following the nm offset between excitation and emission wave- procedure of Banerjee and Budke (1964).H2O2 stock lengths. Several corrections were applied to raw spectra solutions were made by dilution of a 30–35% H2O2 in order to compare our results with those in the litera- solution obtained from Sigma-Aldrich. H2O2 stock ture. First, a correction for the inner-filter effect of the solutions were calibrated by titration with Ce(IV), sample’s absorption of source and emitted light were which was made directly from analytical reagent made following Belzile et al. (2002) and McKnight et grade cerium ammonium nitrate, a primary standard al. (2001). Second, we applied a correction for non- (Skoog et al., 1996). linearity in both excitation intensity and emission intensity per Shimadzu instructions. Third, we normal- 2.3. Absorbance, fluorescence and DOC ized each corrected SF spectrum to the water Raman measurements signal area to give SF in Raman units (nm 1; Laurion et al., 1997; Stedmon et al., 2003). Optical density (Ak) measurements for CDOM Concentrations of dissolved organic carbon (DOC) were made on a Shimadzu 160 UV spectrophotometer were determined by high-temperature catalytic oxida- from 230 nm to 700 nm with 1 or 10 cm path-length tion using a MQ1001 TOC analyzer (Shimadzu Instru- 18 D.W. O’Sullivan et al. / Marine Chemistry 97 (2005) 14–33