Environ. Sci. Technol. 2008, 42, 3271–3276
Relating Carbon Monoxide Photoproduction to Dissolved Organic Matter Functionality A R O N S T U B B I N S , * ,†,‡,§ VESPER HUBBARD,† GUENTHER UHER,‡ C L I F F S . L A W , §,| ROBERT C. UPSTILL-GODDARD,‡ GEORGE R. AIKEN,⊥ AND K E N N E T H M O P P E R * ,† Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, Virginia 23529, Marine Science and Technology, Armstrong Building, Newcastle University, NE1 7RU, U.K., Plymouth Marine Laboratory, The Hoe, Plymouth, PL1 3DH, U.K., NIWA, 301 Evans Bay Parade, Wellington, 6021, New Zealand, and U.S. Geological Survey, 3215 Marine Street, Boulder, Colorado 80303
Received December 3, 2007. Revised manuscript received February 11, 2008. Accepted February 15, 2008.
Aqueous solutions of humic substances (HSs) and pure monomeric aromatics were irradiated to investigate the chemical controls upon carbon monoxide (CO) photoproduction from dissolved organic matter (DOM). HSs were isolated from lakes, rivers, marsh, and ocean. Inclusion of humic, fulvic, hydrophobic organic, and hydrophilic organic acid fractions from these environments provided samples diverse in source and isolation protocol. In spite of these major differences, HS absorption coefficients (a) and photoreactivities (a bleaching and CO production) were strongly dependent upon HS aromaticity (r2 >0.90; n ) 11), implying aromatic moieties are the principal chromophores and photoreactants within HSs, and by extension, DOM. Carbonyl carbon and CO photoproduction were not correlated, implying that carbonyl moieties are not quantitatively important in CO photoproduction. CO photoproduction efficiency of aqueous solutions of monomeric aromatic compounds that are common constituents of organic matter varied with the nature of ring substituents. Specifically, electron donating groups increased, while electron withdrawing groups decreased CO photoproductivity, supporting our conclusion that carbonyl substituents are not quantitatively important in CO photoproduction. Significantly, aromatic CO photoproduction efficiency spanned 3 orders of magnitude, indicating that variations in the CO apparent quantum yields of natural DOM may be related to variations in aromatic DOM substituent group chemistry.
Introduction Carbon monoxide (CO) is quantitatively the second largest photoproduct of dissolved organic matter (DOM) photom* Corresponding authors e-mail:
[email protected] (K.M.),
[email protected] (A.S.). † Old Dominion University. ‡ Newcastle University. § Plymouth Marine Laboratory. | NIWA. ⊥ U.S. Geological Survey. 10.1021/es703014q CCC: $40.75
Published on Web 03/26/2008
2008 American Chemical Society
ineralization (1, 2) and a significant term in the aquatic carbon cycle. Oceanic CO photoproduction mineralizes 30–90 Tg of dissolved organic carbon annually (3, 4), driving CO emission to the atmosphere (5) where it plays an important role in climate regulation (6). Low background levels and precise and accurate analytical techniques allow precise and accurate quantification of CO photoproduction rates. Consequently, CO is used as a proxy for the photoproduction of dissolved inorganic carbon (DIC) (1, 7, 8) and biolabile organic carbon (9, 10), which account for the majority of DOM photomineralization in natural waters, but are considerably more difficult to measure than CO. CO is also a key tracer for testing and tuning models of mixed layer processes (11–14). Despite widespread interest, little is known about the chromophoric sites responsible for DOM photoreactivity (15–17). Although previous studies have hypothesized that CO is produced by direct photocleavage of carbonyl groups from DOM (18–20), the chemical controls governing CO production have not been empirically determined. Humic substances (HSs) typically account for 40-90% of the DOM pool (21) and have photoreactivities and optical properties comparable to natural DOM at similar carbon and chromophore concentrations (19, 22) making them suitable DOM surrogates in photochemical experiments (23). In addition, HS isolates are amenable to 13C nuclear magnetic resonance spectroscopy (13C NMR) providing a level of chemical characterization presently unattainable for nonisolated DOM. HSs can be referred to as aquatic (aHSs) or terrestrial (tHSs). tHSs prevail in freshwaters receiving significant DOM through soil leaching and surface runoff and contain high levels of aromatic carbon derived from lignin and lignin degradation products (24, 25). In contrast, aHSs are produced in situ from microbial sources and are usually highly aliphatic with lower aromaticity (24, 26, 27). They are prominent in oceans, eutrophic lakes, and lakes receiving limited terrestrial input. The impact of DOM chemistry upon CO photoproduction was investigated using HSs from a diverse selection of natural waters. In addition, sets of monomeric, structurally related aromatic compounds were used as proxies for DOM photoreaction sites. Chosen aromatics consisted only of carbon, hydrogen, and oxygen. Although DOM includes a wide variety of biochemical residues (16, 28) and condensation products (29), our experiments focused on monomeric aromatics for the following reasons: (a) Natural DOM and HSs are prohibitively complex for preliminary mechanistic studies. (b) The absorption spectra of many naturally occurring aromatics extend into the UV-B and UV-A, the main wavelengths for CO photoproduction (19). (c) Syringyl, vanillyl, cinnamyl, and quinone moieties occur in marine DOM (30–32) and tHSs (33, 34). (d) Algal polyphenols are major constituents of DOM in coastal surface waters (35, 36) and estuarine phenol concentrations can be well correlated with CDOM absorbance (36). (e) Many aromatic compounds are photoreactive in dilute aqueous solutions at environmentally relevant wavelengths (34, 37, 38). (f) The number, type, and location of functional groups (ring substituents) can be systematically changed, facilitating evaluation of substituent impact upon photoreactivity. And (g) A large number of natural and synthetic aromatic compounds are commercially available and relatively inexpensive. The objective of this study was to relate the light absorbance and CO photoproduction of a suite of diverse HSs to source, mode of extraction, spectral properties, and VOL. 42, NO. 9, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Humic Substance Source, Isolation Protocol, Percent (%) Carbon, Aromatic and Ketonic Carbon (mgC L-1) by 13C NMR, Absorption Coefficient (a350), Absorption Photobleaching (δa350), and CO Photoproduction Rates (CO) (All Samples 5 mg L-1 Humic Substance in Ultrapure Water) sample Pacific Ocean HPOAa Lake Fryxell HPOA Williams Lake HPOA F1 Everglades HPOA F1 Everglades HPIAb 2BS Everglades HPOA 2BS Everglades HPIA U3 Everglades HPOA S10E Everglades HPOA IHSS Suwannee River HAc IHSS Suwannee River FAd
source aHSe aHS mixed mixed mixed mixed mixed mixed mixed tHSh tHS
aHSf tHSg tHS tHS tHS tHS tHS
% carbon
aliphatic (mg C L-1)
aromatic (mg C L-1)
ketonic (mg C L-1)
a350 (m-1)
δa350 (m-1 hr-1)
CO (nmol L-1 hr-1)
57.5 55 53.9 51.9 47.7 51.7 47.2 54.7 54.5 53.4 53.5
2.06 1.80 1.94 1.76 1.68 1.24 1.64 1.71 1.78 0.93 1.25
0.21 0.42 0.34 0.47 0.31 0.36 0.30 0.49 0.47 1.12 0.75
0.05 0.00 0.05 0.07 0.04 0.22 0.05 0.11 0.09 0.19 0.16
0.65 2.48 1.43 6.07 2.48 3.84 1.65 5.03 5.50 12.83 6.36
0.03 0.07 0.04 0.10 0.04 0.06 0.03 0.09 0.10 0.19 0.12
5.0 14.6 9.3 22.2 15.1 14.2 8.1 16.7 21.0 43.4 24.3
a HPOA ) Hydrophobic organic acid. b HPIA ) Hydrophilic organic acid. c HA ) Humic acid. d FA ) Fulvic Acid. e aHS ) Aquatic humic substance derived from aquatic microbial sources. f Mixed aHS ) Predominantly aHS with some tHS inputs. g Mixed tHS ) Predominantly tHS with some aHS inputs. h tHS ) Terrestrially derived humic substance.
chemical composition (based on 13C NMR). Subsequent experiments with monomeric aromatics explored the relationships between CO photoproduction and the substituent chemistry of chromophores potentially present within natural DOM.
Experimental Section Humic Substance Isolation, Characterization, and Sample Preparation. Whole water samples were collected from terrestrial and marine environments (described below) and filtered immediately (0.45 µm, AquaPrep 600, Pall Gelman). Samples were acidified to pH 2 with hydrochloric acid (HCl) passed through XAD-8 and XAD-4 resins (Amberlite) to extract the HPOA (hydrophobic organic acid) and HPIA (hydrophilic organic acid) fractions, respectively (30, 39). These fractions were then eluted from the resins with 0.1 N sodium hydroxide. Eluates were immediately acidified with reagent-grade HCl to pH 3 (minimizing sample alteration at high pH), desalted using H+ saturated AG-MP 50 cation exchange resin (BioRad), lyophilized, and stored in a desiccator. For some samples, the HPOA fraction was further separated into humic acid (HA) and fulvic acid (FA) fractions by acidifying the XAD-8 eluate to pH 300 nm than monomeric analogs). The tHS samples had higher a350 per unit carbon than aHSs, and HPOA and HA samples had higher a350 per unit carbon than corresponding HPIA and FA extracts from the same water samples (Table 1) indicating that source and isolation procedure affect a350. However, the strong, linear regression between a350 and aromatic carbon for all samples (Figure 2a) suggests that HS a350 is controlled primarily by the concentration of aromatic carbon in the sample and that HS source, isolation protocol, and chemistry have only secondary impact. The statistical relation between aromatic carbon and a350 (mg aromatic carbon L-1 ) 0.0710 × a350 + 0.1646; r2 ) 0.90, p ) 7.13 × 10-6, n ) 11) can be used to predict aromatic carbon concentrations from absorption coefficients of natural waters (note that in this regression a350 is the independent variable as the slope is designed to allow prediction of aromatic carbon from DOM absorption). No relation was found between a350 and ketonic, heteroaliphatic, anomeric, or carboxylic carbon levels (data reported in ref (23)). However, aliphatic and aromatic carbons (Table 1) were negatively correlated (r ) -0.91, p ) 6.53 × 10-6, n ) 11), resulting in a negative correlation between a350 and aliphatic carbon (r ) -0.87, n ) 11). Humic Substance Photoreactivity. CO photoproduction rates ranged from 5.0 to 43.4 nmol L-1 hr-1 (Table 1; Figure 2b) and HS photobleaching rates ranged from 0.03 to 0.19 a350 m-1 h-1 (Table 1; Figure 2c). As previously reported for DOM samples (19, 20, 50), CO photoproduction and HS a photobleaching were related to initial a. CO production related most strongly with initial a at 350 nm, where a linear 3274
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regression best described the relationship (r2 ) 0.97, p ) 4.30 × 10-8, n ) 11). Both CO production and photobleaching rates were higher for tHSs than aHSs, while the rates for HPOA and HA samples were higher than those of corresponding HPIA and FA samples (Table 1). However, linear regressions of initial a350 (Figure 2a), CO production (Figure 2b; r2 ) 0.91, p ) 4.30 × 10-6, n ) 11) and photobleaching at a350 (Figure 2c; r2 ) 0.93, p ) 2.27 × 10-6, n ) 11) versus aromaticity indicate that light absorption, CO production, and CDOM photobleaching are determined by aromatic chromophore concentrations. While it has been proposed that CO is photoproduced through photolytic cleavage of carbonyl groups from DOM (18, 51), we observed no relation between total ketonic carbon and CO photoproduction (Table 1), indicating that carbonyl moieties are apparently not important in controlling the rate of CO photoproduction. This conclusion is supported by experiments with model aromatic chromophores (below). Carbon Monoxide Photoproduction from Monomeric Aromatic Compounds. CO photoproduction efficiency for the 27 monomeric aromatics varied by over 3 orders of magnitude (0.01–34.67; Table 2), demonstrating the major, quantitative impact that substituent chemistry has upon CO photoproduction efficiency. When just one substituent group was present (Figure 3), variations in CO production efficiency followed a broad trend of increasing CO production efficiency with the increasing electron donating power of the substituent group (e.g., benzaldehyde < ethoxybenzene < methoxybenzene < benzoic acid < phenol). CO production efficiency was lowest for benzaldehyde and 3-phenyl-propenal, indicating that the presence of a carbonyl group (particularly an aldehyde group) is not a primary determinant of CO production potential. However, the most efficient CO producer was acetophenone, suggesting the ketonic carbonyl group in this configuration is an efficient precursor for CO. The photoreactivity (i.e., photoreduction) of acetophenone is known to be significantly enhanced in hydrogen-donating solvents due to pronounced red shifting of the π,π* state (52) and possibly resonance stabilization of the excited-state with the ground state, followed by a Norrish type I cleavage reaction to yield CO (53). For the disubstituted aromatics, the trend of electron donating substituents increasing, and electron withdrawing groups decreasing CO production efficiency is even more apparent. Figure 4 presents a series of phenolic compounds with varying substituent groups. In this series, the phenols with electron donating methoxy and ethoxy substituents clearly enhance CO production efficiency, whereas phenols with aldehyde or ketone substuituents (hydroxy-benzaldehydes and hydroxy-phenyl-ethanones) exhibited marked drops in CO prodution efficiency (Figure 4). The presence of
anisms and chemical controls upon the efficiencies of CO photoformation and other photoreactions.
Acknowledgments
FIGURE 4. Carbon monoxide photoproduction efficiency for phenol and a series of monosubstituted phenols. Photoproduction efficiency was calculated as the cross-product of the spectral aromatic solution optical density (absorbance) per nm (OD nm-1) and solar simulator spectral irradiance per nm (W m-2 nm-1). electron donating (e.g., hydroxy, methoxy, and ethoxy) and withdrawing (e.g., aldehyde, ketone) substituents have been shown to cause similar increases and decreases in e-aq and OH production efficiency relative to phenol (39, 54). Grossweiner and Joschek (54) found that compounds with low gas-phase photoionization potentials have the greatest e-aq yields, and concluded that monosubstituted benzene derivatives with electron donating substituents in the ground state favor photoionization in solution, while the presence of electron withdrawing substituents inhibit it. Thus, it seems likely that the trends for CO photoproduction have a similar basis. Possible explanations of these trends include the increased acidity of electron donating groups in the excited singlet state (55) and/or the increased stability of the excited triplet state and reaction intermediates imparted by electron donating groups, particularly alkoxy groups (56). Surprisingly, the position of substituent groups (ortho, meta, or para) did not cause significant or consistent trends in CO production efficiency (Figure 4), indicating that, for the current data set, steric effects have only secondary influence upon CO photoproduction. Atmospheric studies have shown that a variety of volatile organic compounds are readily mineralized through direct or photosensitized degradation, with CO as an end product (57). Therefore, observed high CO production by methoxyaromatics could result from the reaction of formaldehyde with hydroxyl radicals (57), as both are photoproducts of aqueous solutions of methoxy-phenolic compounds (38) and DOM (8). CO photoproduction efficiencies for studied monomeric aromatics vary by more than 3 orders of magnitude (Table 2). Therefore, although the exact mechanisms of CO production are not apparent, it is clear that variations in aromatic substituent chemistry have a profound influence upon CO photoproduction efficiencies. CO photoproduction efficiencies (apparent quantum yields) for natural DOM vary by over an order of magnitude at a given wavelength depending upon source, environment and previous light exposure (3, 5, 14, 19). Our results suggest that these natural variations in DOM apparent quantum yields may be related to variations in the substituent chemistry of aromatic chromophores within the DOM pool. For instance, the rapid photodegradation or rearrangement of aromatics with substituent chemistries which confer high photoreactivity may be responsible for the rapid decreases in CO apparent quantum yields with increasing absorbed photon dose (58). Continued work with simple aromatics is required to further explore the mech-
We thank Dr. Vassilis Kitidis, Malcolm Woodward and John R. Helms for help running the irradiation system and the three anonymous ES&T reviewers for their valuable comments. This work was funded by the UK National Environmental Research Council (grant GR3 11665), the Plymouth Marine Laboratory, the Marine Biological Association of the UK via a Ray Lankester Investigatorship to K.M., the U.S. National Science Foundation (OCE0196220, OCE0327446), and the U.S. Geological Survey National Research Program. Opinions, findings, conclusions, and recommendations expressed are those of the authors and do not necessarily reflect those of the funding bodies. Brand names are for identification purposes only and do not imply endorsement.
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