Environ. Sci. Technol. 2008, 42, 1933–1937
A Photochemical Source of Methyl Chloride in Saline Waters ROBERT M. MOORE* Department of Oceanography, Dalhousie University, Halifax, NS Canada B3H 4J1
Received August 1, 2007. Revised manuscript received November 30, 2007. Accepted December 6, 2007.
It is shown experimentally that the methoxy group in simple ligninlike molecules can be the source of the methyl group in CH3Cl produced by a photochemical reaction in an aqueous solution of chloride. Terrestrially derived colored dissolved organic matter (CDOM) in river water also yields CH3Cl through a photochemical process in a chloride solution. CDOM extracted from subsurface ocean waters showed some ability to enhance photochemical production of CH3Cl while CDOM from surface water showed no effect. Reactions of the kind described in this paper may be contributors to the marine source of methyl chloride and possibly other alkyl halides.
Introduction Methyl chloride is one of the most abundant sources of atmospheric chlorine, having a mixing ratio of approximately 600 pptv (1). It is relevant in atmospheric chemistry as a natural background source of chlorine atoms in the upper atmosphere that play an important role in the regulation of stratospheric ozone concentrations. Its sources, once thought to be predominantly oceanic, are now known to include terrestrial plants and biomass burning, as well as a relatively small marine component (6.4 – 8 × 109 mol/y; (2)). Little has been published on the origin of marine methyl chloride, though a fraction must come from the reactions of both methyl iodide and bromide with chloride ion in seawater, particularly in warm waters. There is also laboratory evidence for some production from phytoplankton (3–5). There are a number of reports of the photochemical production of methyl halides (chiefly methyl iodide) in seawater (6–8). There is little known about the mechanism or mechanisms involved, and specifically nothing is known of the source of the methyl moiety. It seems plausible to suppose that it is a component of the light absorbing molecule and, as such, may occur within marine colored dissolved organic material (CDOM). At least in the case of terrestrially derived CDOM it is conceivable that the methyl groups have their origin in the aromatic methoxy groups that are abundant in lignin precursors of CDOM. To assess whether such a process is feasible, selected simple methoxy substituted aromatic compounds were tested by irradiation in aqueous chloride solution.
were normally tested at concentrations of 50 µM either in 0.5 molar chloride solutions or in coastal seawater. The solutions were filtered through a GF/F filter and irradiated in an Hereaus Suntest CPS solar simulator at 2900 µmol quanta m–2 s-1. The simulator contains a xenon arc lamp with an adjustable output from 400 to 765 W m-2 between 300 and 800 nm; in this work it was operated at maximum power. A UV filter removed wavelengths below 290 nm. Samples were held in quartz tubes (3 cm o.d.; volume ca. 125 mL) for periods between 20 min and 3.5 h. Although the tubes were cooled by a fan during irradiation, the samples warmed by about 10 °C; the dark controls in quartz tubes wrapped in Al foil were also placed in the solar simulator. After irradiation, samples were withdrawn from the tubes into a 100 mL glass syringe via a long needle and introduced to the measuring pipet (40 mL) of a purge and trap apparatus that stripped out gases in a stream of helium (40 mL/min). The stream was first reduced in moisture content by passage through a short condenser held at 4 °C, and then thoroughly dried over magnesium perchlorate. Carbon dioxide was removed as the gas passed through a bed of Ascarite and condensable gases were trapped in a steel tube (i.d. 0.5 mm) at –150 °C over liquid nitrogen. Analytes were separated in a pair of DB 624 columns that allowed backflushing of less volatile components, and transferred to a Trace MS quadrupole mass spectrometer operating in single ion monitoring mode. Except when deuterated methyl chloride was a product of experiments, the stability of the instrument was checked by injection of a known volume of deuterated methyl chloride. The precision for this method for CH3Cl as determined by coefficient of variation for repeated measurement of a sample of seawater is 1.2%. When CD3Cl is measured, the necessary omission of the internal standard would change this estimate to approximately 5.7%. As detailed below, some experiments used syringic acid that was deuterated on the methoxy group. Deuterated trimethoxybenzoic acid was synthesized from gallic acid (9) and was then demethylated at the para position by the method of Roth et al. (10) to give syringic acid which was recrystallized from toluene and acetone. Its purity with respect to the starting material (trimethoxybenzoic acid) was estimated as 95% by 13C NMR. River water used in experiments was collected from the Ingram River, Nova Scotia, the waters of which are high in CDOM (absorption coefficient at 350 nm, 24 m-1). The water was filtered through a glass fiber filter before use and, in cases where it was mixed with seawater or sodium chloride
Materials and Methods Compounds tested for production of methyl chloride were 4-methoxy-1-naphthol(CAS84-85-5),4-hydroxy-3,5-dimethoxybenzoic acid (syringic acid, CAS 530-57-4), 2-methoxyphenol (CAS 90-05-1), 3,4,5-trimethoxy benzoic acid (CAS 118-412), and 2-methoxyhydroquinone (CAS 824-46-4). Compounds * Corresponding author e-mail:
[email protected]. 10.1021/es071920l CCC: $40.75
Published on Web 02/14/2008
2008 American Chemical Society
FIGURE 1. Results of two experiments showing production of CH3Cl during the irradiation of 50 µM syringic acid in 0.5 M aqueous NaCl. VOL. 42, NO. 6, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Absorption coefficient (m-1) as a function of time (up to 90 min) during irradiation of 50 µM syringic acid in 0.5 M aqueous NaCl: (A) wavelengths 250–320 nm; (B) wavelengths 320–700 nm.
FIGURE 3. Irradiation of deuterated syringic acid at a concentration of 50 µM in 0.5 M NaCl solution.
FIGURE 5. Comparison of the effects of light and dark on an intermediate formed by irradiating a 50 µM solution of syringic acid in 0.5 M NaCl.
FIGURE 4. Methyl chloride concentration in samples of 50 µM syringic acid + 0.5 M Cl- irradiated at pH 5 and 7.7 for 1 h and dark control. solution, it was filtered after the addition to remove any precipitate. Seawater, salinity 29, was collected from a coastal location (Sandy Cove, NS). Two experiments were performed with seawater that was enriched with marine-derived dissolved organic matter. One utilized organic matter extracted from surface waters, and the other from water collected at 600 m; both came from a location off Kona, HI; the extraction procedure and characterization are given in ref (11); the material was 34% C. Seawater supplemented (by 5.4 mg C L-1) with extracted DOC was irradiated for periods up to 1.5 h in a solar simulator before being analyzed for methyl halides. Optical absorbance measurements were made using a Cary 3 Visible-UV spectrophotometer.
Results and Discussion The preliminary experiments, involving irradiation of five methoxy aromatic compounds in seawater, all yielded enhancements in CH3Cl concentrations relative to untreated seawater and dark, amended samples. Syringic acid, which 1934
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FIGURE 6. Increase in concentration of methyl chloride on irradiating solutions of H-SA + D-SA and preirradiated H-SA + D-SA. Total syringic acid concentration was 50 µM; Cl- was 0.5 M, and pH was buffered at 7.6. gave the greatest enhancement in CH3Cl, was selected as the model compound for the subsequent experiments. This compound is formed during lignin breakdown either by chemical oxidation or the action of white-rot fungi; it has been used as a model monomer in the study of lignin degradation (12). It is proposed that lignin, altered by oxidation and demethylation, is among the reactants that combine to form soil humic acids (12). Figure 1 shows the results of duplicate experiments in which 50 µM syringic acid was irradiated in a medium of 0.5 M aqueous NaCl. The experiments showed that the rate of methyl chloride production was much lower during the first 10 min of irradiation than at 1 h and beyond. Optical absorbance measurements
FIGURE 7. Production of CH3Cl during irradiation of river water to which Cl- has been added. In the right panel, a sample of the irradiated water was buffered at 7.6 (cf. pH 5.2 for unbuffered samples).
FIGURE 8. Methyl chloride production in an irradiated mixture of river water and seawater (ratio 1:3) (squares); and in dark samples (filled squares). Also shown are irradiated and nonirradiated seawater (open triangles and filled triangles, respectively). (Figure 2) on solutions from the same experiment (“Expt 1”) showed that there was a rapid increase in absorbance at 300 nm over the first 10–20 min of irradiation, while the absorbance below 275 nm declined dramatically due to destruction of the phenol. It is likely that at longer wavelengths there is a contribution from scattering as a result of polymeric material produced during the irradiation. As experiments described below confirm, these results point to production of the CH3Cl, not directly from syringic acid, but from a product of its photolysis. Experiments were performed to confirm that the methyl chloride had its origin in the methoxy groups of the initial syringic acid. Syringic acid deuterated on the methoxy groups was synthesized and photolyzed at low concentrations in aqueous chloride solutions (0.5 M). Figure 3 shows that CD3Cl is produced and at a rate similar to that seen in the experiments described for nondeuterated syringic acid; again the rate was initially low. The results confirm that the methyl group has its origin in a methoxy group. Because the deuterated syringic acid is not free of deuterated trimethoxybenzoic acid, a test was made of the relative rates of methyl chloride production; it was found that trimethoxybenzoic acid yielded methyl chloride at 20% the rate of syringic acid. Therefore the presence of this material as an impurity does not invalidate the finding that the methyl group of CH3Cl has its origin in a methoxy group during the photolysis of syringic acid. It became apparent from experiments like this one that the rate of methyl chloride production was variable, consequently experiments were conducted to establish whether the reactions were pH dependent. Results shown in Figure 4 show that there is a strong pH dependence with CH3Cl production being promoted at higher pH. Following this finding all experiments involving syringic acid in distilled water plus Cl- had the pH of the medium adjusted to between 7 and 8 either by addition of NaOH or by use of a 0.05 M
phosphate buffer. Seawater and river water experiments were at natural pH unless otherwise indicated. Based on the acceleration in production rate of methyl chloride during irradiation, it was hypothesized that an intermediate was being produced that played a direct or indirect role in CH3Cl formation. An attempt was made to establish whether the reaction undergone by the intermediate to yield CH3Cl was photochemical or thermal. By first irradiating a solution of syringic acid + Cl- (buffered at pH 7.2) for 1 h and then holding it in the dark (Figure 5) it was shown that no further production of CH3Cl occurred; this suggeststhatthesecondphaseofthereactionisphotochemical. It appears that the hypothesized intermediate could have two possible roles: (A) as the direct precursor of CH3Cl through reaction with Cl-, or (B) as a substance that catalyzes CH3Cl production, for example, through absorbing light energy and transferring it to a different CH3Cl precursor that reacts with Cl-. These two roles should be distinguishable in an experiment that uses one isotopic form of syringic acid to generate the intermediate during photolysis, followed by the addition of the second isotopic form and further photolysis. If role B applies, the presence of the precursor would yield both CH3Cl and CD3Cl in a solution containing similar quantities of the two forms of syringic acid. (The actual ratio would depend on proportion of the substrate lost during the first phase of irradiation.) In contrast, if role A applies, a larger quantity of one form of methyl chloride would be produced: that containing the same isotope as the preformed intermediate. This experiment was performed using a solution of syringic acid and Cl- that had been initially irradiated for 70 min. A sample of the solution was measured for CH3Cl and the remainder was purged with zero air and supplemented with deuterated syringic acid at the same concentration. Two samples of this mixture containing nondeuterated intermediate, unreacted nondeuterated syringic acid, and deuterated syringic acid were irradiated for 20 min. A fresh, unirradiated mixture of the two forms of syringic acid was also irradiated for 20 and 75 min. The results (Figure 6) show that the presence of preirradiated syringic acid does not enhance production of CD3Cl in the 20 min irradiations. This experiment thus indicates that the postulated intermediate acts directly as a precursor of methyl chloride. During 20 and 75 min irradiations, CH3Cl and CD3Cl from the fresh mixture of the two forms of syringic acid are produced in more similar amounts, but 9 times more at 75 min than at 20 min, consistent with the increasing rate of production seen in other experiments. The experiment also demonstrates that the intermediate has a lifetime of at least several minutes, since the intermediate was not lost during a 3 min purging that was done between the two periods of irradiation. While the nature of the intermediate is unknown, some possible types of reaction can be suggested. One would be decarboxylation and oxidation to give a quinone derivative, and a second would be polymerization of photochemically VOL. 42, NO. 6, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 9. Effect of addition of marine DOC extracted from 600 m water on photochemical production of CH3Cl in seawater. produced phenoxyl radicals (13). Both could potentially account for the appearance of color in the irradiated solutions. Since the CH3Cl was typically formed at subnanomolar concentrations, it is possible that the relevant intermediate was a minor product. Do reactions of this kind occur in natural waters? The experiments described so far indicate that a simple ligninlike model compound reacts photochemically in an aqueous chloride solution to give CH3Cl. Taking the ocean to atmosphere flux of CH3Cl for supersaturated waters as 100 nmol m-2 d-1 (2) and assuming that the production occurred over a water column thickness of 10 m (an arbitrary but low estimate), gives a production rate of 0.4 pmol L-1 h-1, or twice that value if production occurred only during daylight. Since this is 3 orders of magnitude less than that observed in experiments described above, the photochemical production in these experiments is potentially environmentally significant, certainly in waters containing terrestrial humic substances and possibly in open ocean waters. (It should be noted that CH3Cl is only a minor product in the photolysis of syringic acid in aqueous chloride media; other studies have shown that methanol is formed with higher efficiency, P. Wan, personal communication). The first step in determining whether this type of reaction also occurs in natural waters involved experiments with highly colored water from the Ingram River, Nova Scotia. Irradiation of this water with added chloride ion (0.5 M) showed a 3-fold increase in CH3Cl over 2 h (Figure 7, left panel); the production rate is estimated as 60 pmol L-1 h-1. A second experiment (Figure 7, right panel) showed an initial rapid increase in CH3Cl followed by a sustained production rate of 60 pmol L-1 h-1. No induction period is discernible within the resolution permitted by these experiments suggesting that the reactive moiety is already present in the river water. These results also differed from those seen with syringic acid in that changing the pH of the water from 5.2 to 7.6 with a phosphate buffer (Figure 7, right panel) did not enhance the production rate. Two irradiation experiments were performed using a mixture of river water (25%) and coastal seawater. The results of one are shown in Figure 8; in both cases the production rate of CH3Cl was 30 pmol L-1 h-1. These experiments support the case for photochemical production of CH3Cl in estuarine and coastal waters. Again no induction period can be discerned, suggesting that the direct precursor of CH3Cl is present in riverine CDOM or is formed rapidly. It must be noted that these experiments cannot demonstrate that the same mechanism that occurs with the model 1936
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methoxy compound occurs also in terrestrial runoff rich in lignin-derived organic matter. The same limitation applies to interpretation of the experiments described below that used marine-derived organic matter. Nevertheless, the absence of production of CH3Cl in such experiments would have argued against the importance of the photochemical process that is being proposed. While a number of field studies (e.g., (14)) have had the objective of determining the emission to the atmosphere of CH3Cl from the surface ocean, it appears that there are no data on fluxes from estuarine or coastal waters. The question of whether this type of reaction could account for any production of CH3Cl in pelagic waters is even more difficult to address since in such waters the CDOM is not primarily derived from terrestrial material and so is expected to be less similar to lignin. In this study the question was addressed by enriching seawater with two types of organic material that had been extracted from surface and deep (600 m) ocean waters (11). When the added material had a surface ocean origin, there was a small difference between samples supplemented with DOC and those that were not; both types of sample showed a modest increase in CH3Cl (9–13%) after irradiation for 1.5 h. A larger effect was seen for water supplemented with DOC extracted from a depth of 600 m and irradiated for the same time; it yielded a 22% growth in CH3Cl (production rate 8 pmol L-1 h-1) compared with a 13% increase for the unsupplemented water (Figure 9). However, with the addition of 5.4 mg of organic carbon, the dissolved organic carbon content of the amended water was brought to approximately 6 times the normal value for ocean water. In view of the fairly large increase in DOC, the observed effect is not large. It should be noted that these experiments also provide evidence for a modest rate of photochemical production of CH3Cl in the coastal seawater used in these experiments. Riverine and marine CDOM possess structural differences reflecting their different sources. Lignin-derived structures contribute to the terrestrial form of CDOM, but the absence of lignin production in marine organisms means that marine CDOM contains a lower concentration of methoxyphenol groups characteristic of lignin breakdown products such as syringic and vanillic acids (15). Based on analysis of lignin phenols in ultrafiltered marine dissolved organic matter (UDOM), Opsahl and Benner (16) estimated that ca. 2.4% of Sargasso Sea UDOM is terrigenous. Dissolved lignin has been shown to be photochemically reactive (17) with ca. 75% of the lignin in a sample of Mississippi water being photodegraded by sunlight over a period of 28 days. That study also
showed that the ratio of syringyl to vanillyl phenols decreased by a factor of 2, demonstrating the higher photoreactivity of the syringyl subunits. The foregoing experiments, while demonstrating photochemical CH3Cl production, reveal little about the mechanism except that, where syringic acid is the reactant, there is an induction period during which an intermediate is apparently produced which undergoes further photochemical reaction with Cl- to give CH3Cl. A motivation for this study was the knowledge that veratryl alcohol (3,4-dimethoxybenzyl alcohol) forms a relatively long-lived radical cation on oxidation (18): could such a species activate an ether methyl group to nucleophilic attack, for example by chloride ion? The mechanism is apparently more complex than this, as shown by the intermediate formation. There are a few other studies that have shown that alkyl halides can derive their alkyl group from ether moieties. It has been reported (19, 20) that the methoxy group in the model compound 2-methoxyphenol would react with ferric ion in aqueous halide solution to yield methyl halide. In this case the authors propose that the mechanism involves oxidation of the methoxyphenol by ferric ion and nucleophilic reaction of the halide ion with the methyl group. The reaction of methylated pectin with chloride at elevated temperature has been shown to yield methyl chloride (21); curiously, this reaction occurs less readily as the water content of the reaction mixture increases. In this case the methyl groups are derived from methyl esters of the galacturonic acid polymer. Keppler et al. (22, 23) have shown evidence, through stable isotope studies, that such reactions which yield methyl groups strongly depleted in C-13 are consistent with isotopic budget calculations of atmospheric CH3Cl. Reactions of the kind described in this paper are expected to contribute to methyl chloride production in estuarine and coastal waters and to a lesser extent in open ocean waters; other alkyl halides may also be produced. Other contributors to the marine source of methyl chloride include nucleophilic displacement reactions between methyl bromide and methyl iodide and chloride ion, biological processes involving both phytoplankton and macroalgae as sources for CH3X (X ) Cl, Br, I), and a photochemical process in the case of CH3I (6, 7).
Acknowledgments I thank D. Repeta for providing extracted marine CDOM, A. Thompson and T. Woods for their generous assistance with the syntheses of deuterated syringic acid, and P. Wan, E. Arctander, J. Hamilton, F. Keppler, J. Pincock, and O. Zafiriou for helpful discussions. The work was supported by a grant to the author from the Natural Sciences and Engineering Research Council of Canada.
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