Relating Carbon Monoxide Photoproduction to Dissolved Organic Matter Functionality

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<ul><li><p>Relating Carbon MonoxidePhotoproduction to DissolvedOrganic Matter FunctionalityA R O N S T U B B I N S , * , , , </p><p>V E S P E R H U B B A R D , G U E N T H E R U H E R , </p><p>C L I F F S . L A W , , |</p><p>R O B E R T C . U P S T I L L - G O D D A R D , </p><p>G E O R G E R . A I K E N , A N DK E N N E T H M O P P E R * , </p><p>Department of Chemistry and Biochemistry, Old DominionUniversity, Norfolk, Virginia 23529, Marine Science andTechnology, Armstrong Building, Newcastle University, NE17RU, 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</p><p>Received December 3, 2007. Revised manuscript receivedFebruary 11, 2008. Accepted February 15, 2008.</p><p>Aqueous solutions of humic substances (HSs) and puremonomeric aromatics were irradiated to investigate the chemicalcontrols upon carbon monoxide (CO) photoproduction fromdissolved organic matter (DOM). HSs were isolated from lakes,rivers, marsh, and ocean. Inclusion of humic, fulvic, hydrophobicorganic, and hydrophilic organic acid fractions from theseenvironments provided samples diverse in source and isolationprotocol. In spite of these major differences, HS absorptioncoefficients (a) and photoreactivities (a bleaching and COproduction) were strongly dependent upon HS aromaticity (r2&gt;0.90; n ) 11), implying aromatic moieties are the principalchromophores and photoreactants within HSs, and by extension,DOM. Carbonyl carbon and CO photoproduction were notcorrelated, implying that carbonyl moieties are not quantitativelyimportant in CO photoproduction. CO photoproductionefficiency of aqueous solutions of monomeric aromaticcompounds that are common constituents of organic mattervaried with the nature of ring substituents. Specifically, electrondonating groups increased, while electron withdrawinggroupsdecreasedCOphotoproductivity,supportingourconclusionthat carbonyl substituents are not quantitatively important inCO photoproduction. Significantly, aromatic CO photoproductionefficiency spanned 3 orders of magnitude, indicating thatvariations in the CO apparent quantum yields of natural DOMmay be related to variations in aromatic DOM substituent groupchemistry.</p><p>Introduction</p><p>Carbon monoxide (CO) is quantitatively the second largestphotoproduct of dissolved organic matter (DOM) photom-</p><p>ineralization (1, 2) and a significant term in the aquatic carboncycle. Oceanic CO photoproduction mineralizes 3090 Tg ofdissolved organic carbon annually (3, 4), driving CO emissionto the atmosphere (5) where it plays an important role inclimate regulation (6).</p><p>Low background levels and precise and accurate analyticaltechniques allow precise and accurate quantification of COphotoproduction rates. Consequently, CO is used as a proxyfor the photoproduction of dissolved inorganic carbon (DIC)(1, 7, 8) and biolabile organic carbon (9, 10), which accountfor the majority of DOM photomineralization in naturalwaters, but are considerably more difficult to measure thanCO. CO is also a key tracer for testing and tuning models ofmixed layer processes (1114). Despite widespread interest,little is known about the chromophoric sites responsible forDOM photoreactivity (1517). Although previous studies havehypothesized that CO is produced by direct photocleavageof carbonyl groups from DOM (1820), the chemical controlsgoverning CO production have not been empiricallydetermined.</p><p>Humic substances (HSs) typically account for 40-90% ofthe DOM pool (21) and have photoreactivities and opticalproperties comparable to natural DOM at similar carbonand chromophore concentrations (19, 22) making themsuitable DOM surrogates in photochemical experiments (23).In addition, HS isolates are amenable to 13C nuclear magneticresonance spectroscopy (13C NMR) providing a level ofchemical characterization presently unattainable for noniso-lated DOM.</p><p>HSs can be referred to as aquatic (aHSs) or terrestrial(tHSs). tHSs prevail in freshwaters receiving significant DOMthrough soil leaching and surface runoff and contain highlevels of aromatic carbon derived from lignin and lignindegradation products (24, 25). In contrast, aHSs are producedin situ from microbial sources and are usually highly aliphaticwith lower aromaticity (24, 26, 27). They are prominent inoceans, eutrophic lakes, and lakes receiving limited terrestrialinput. The impact of DOM chemistry upon CO photopro-duction was investigated using HSs from a diverse selectionof natural waters.</p><p>In addition, sets of monomeric, structurally relatedaromatic compounds were used as proxies for DOM pho-toreaction sites. Chosen aromatics consisted only of carbon,hydrogen, and oxygen. Although DOM includes a wide varietyof biochemical residues (16, 28) and condensation products(29), our experiments focused on monomeric aromatics forthe following reasons: (a) Natural DOM and HSs areprohibitively complex for preliminary mechanistic studies.(b) The absorption spectra of many naturally occurringaromatics extend into the UV-B and UV-A, the mainwavelengths for CO photoproduction (19). (c) Syringyl,vanillyl, cinnamyl, and quinone moieties occur in marineDOM (3032) and tHSs (33, 34). (d) Algal polyphenols aremajor constituents of DOM in coastal surface waters (35, 36)and estuarine phenol concentrations can be well correlatedwith CDOM absorbance (36). (e) Many aromatic compoundsare photoreactive in dilute aqueous solutions at environ-mentally relevant wavelengths (34, 37, 38). (f) The number,type, and location of functional groups (ring substituents)can be systematically changed, facilitating evaluation ofsubstituent impact upon photoreactivity. And (g) A largenumber of natural and synthetic aromatic compounds arecommercially available and relatively inexpensive.</p><p>The objective of this study was to relate the lightabsorbance and CO photoproduction of a suite of diverseHSs to source, mode of extraction, spectral properties, and</p><p>* Corresponding authors e-mail: kmopper@odu.edu (K.M.),aron.stubbins@gmail.com (A.S.).</p><p> Old Dominion University. Newcastle University. Plymouth Marine Laboratory.| NIWA. U.S. Geological Survey.</p><p>Environ. Sci. Technol. 2008, 42, 32713276</p><p>10.1021/es703014q CCC: $40.75 2008 American Chemical Society VOL. 42, NO. 9, 2008 / ENVIRONMENTAL SCIENCE &amp; TECHNOLOGY 9 3271Published on Web 03/26/2008</p></li><li><p>chemical composition (based on 13C NMR). Subsequentexperiments with monomeric aromatics explored the rela-tionships between CO photoproduction and the substituentchemistry of chromophores potentially present within naturalDOM.</p><p>Experimental SectionHumic Substance Isolation, Characterization, and SamplePreparation. Whole water samples were collected fromterrestrial and marine environments (described below) andfiltered immediately (0.45 m, AquaPrep 600, Pall Gelman).Samples were acidified to pH 2 with hydrochloric acid (HCl)passed through XAD-8 and XAD-4 resins (Amberlite) to extractthe HPOA (hydrophobic organic acid) and HPIA (hydrophilicorganic acid) fractions, respectively (30, 39). These fractionswere then eluted from the resins with 0.1 N sodium hydroxide.Eluates were immediately acidified with reagent-grade HClto pH 3 (minimizing sample alteration at high pH), desaltedusing H+ saturated AG-MP 50 cation exchange resin (Bio-Rad), lyophilized, and stored in a desiccator. For somesamples, the HPOA fraction was further separated into humicacid (HA) and fulvic acid (FA) fractions by acidifying theXAD-8 eluate to pH </p></li><li><p>DOM (8). Simulated sunlight was chosen above naturalsunlight due to the need for reproducible irradiation condi-tions between sample sets. Samples were run over a 2-monthperiod (Summer 2005) at Old Dominion University (VA)during which time the solar simulator output was monitored(Biospherical PUV 2500) and remained stable. Samples wereirradiated under oxic conditions. CO quantum yields werenot measured; however, CO production rates were dividedby the absorbed light (the cross-product of the aromaticsolution optical density spectrum (OD nm-1) and solarsimulator spectral irradiance (W m-2 nm-1)) to account fordifferences in molar absorption coefficients, yielding COphotoproduction efficiencies (Table 2).</p><p>Carbon Monoxide. For HSs, 60-mL crimp top vials wereflushed and filled with sample immediately after irradiation,sealed with Teflon faced butyl septa (Supelco), and analyzedfor CO. CO scrubbed (Hopcalite, Supelco) carrier gas (zerograde air) was introduced through the vial septa to create a25-mL headspace. Following 30-min dark equilibration usinga wrist action shaker, 15 mL of headspace gas was collectedin a gastight syringe (Hamilton). For aromatic compounds,sample was drawn directly from the gastight flasks into 100-mL gastight glass syringes (Hamilton), ensuring neitheratmospheric contact nor bubbling occurred. Syringes werefilled with 70 mL of sample. Then, 30 mL of CO-free headspacegas was added to bring the total volume to 100 mL. Sampleswere equilibrated for 30 min using a wrist action shaker beforethe headspace was injected directly onto the GC drier.</p><p>CO in sample gaseous headspace was measured using areduction gas detector (RGD2) with UV-photometer (HSs:Trace Analytical; aromatics: SRI) following separation by gaschromatography (4, 5). For all samples, atmospheric pressureand aqueous sample temperature were recorded for use incalculating CO solubility.</p><p>Absorption Coefficient Spectra. Samples were stored inirradiation flasks, kept in the dark, and allowed to reach roomtemperature before processing. Absorbance spectra wereobtained using a scanning UVvisible, double-beam, spec-trophotometer (Kontron, Uvicon 923) with DIW as a referencebeam blank. Samples were measured in 1-cm quartz cuvettes,cleaned with HCl (0.1 M), copious DIW, and then triple rinsedwith sample before use. Wavelengths from 250 to 800 nmwere measured and corrected for offsets due to scattering,particulate absorbance, and instrument drift by subtractingthe average absorbance between 700 and 800 nm. Data outputfrom the spectrophotometer were in the form of dimension-less absorbance or optical density (OD) at wavelength . ODwas converted to the absorption coefficient, a, defined as a) (OD ln10)/l where l is the optical path length of thecuvette (44).</p><p>Results and DiscussionRelating Chemical and Absorption Characteristics to HumicSubstance Source and Isolation Protocol. Terrestrial humicsubstances (tHSs; Suwannee River HA and FA) containedhighest levels of aromaticity (2842% by carbon; Table 1).Aquatic humic substances (aHSs) had lower aromatic carboncontents (715% by carbon) due to their dominantly microbialsources (Table 1). HPOA and HA isolates had higher aromaticcarbon levels than HPIA and FA extracts from the same waters(Table 1).</p><p>HS absorption coefficient (a) spectra (Figure 1) exhibitedapproximately exponential increases with decreasing wave-length, in agreement with previous studies (45). The absorp-tion coefficient at 350 nm (a350) ranged from 0.65 to 12.83m-1 (Table 1; Figure 2a), compared to ranges of 0.5 to 1.0 a350m-1 in coastal seawaters and 20 to 104 a350 m-1 in freshwaters(4547). As HS concentrations were identical for all samples(5.0 mg L-1), large differences in a350 (Table 1) are attributedto differences in aromatic carbon concentration (mg ArCL-1) with which HS a350 was linearly related (r2 ) 0.90, p )7.13 10-6, n ) 11; Figure 2a). These results agree withprevious studies that demonstrated the dominant role ofaromatic chromophores in colored dissolved organic matter(CDOM) (48, 49). The nonzero x-intercept of the a350 versusaromatic carbon linear regression (1.67( 0.74 mg aromaticC L-1; Figure 2a) may be due to unconjugated olefin moieties,which absorb in the same region of the 13C NMR spectrumas aromatic carbons (110160 ppm) but do not absorb lightat wavelengths &gt;250 nm (49). Alternatively, this could beattributable to more extensive charge transfer for HS withgreater aromatic composition or increased resonancethroughout the pi-electron system at higher aromatic con-</p><p>TABLE 2. Carbon Monoxide (CO) Photoproduction Efficiency fora Suite of 27 Monomeric Aromatic Compounds Dissolved inUltrapure Water (Aromatic Concentrations 10 mM) andIrradiated under Simulated Sunlight in Order of Increasing COPhotoproduction Efficiency</p><p>monomeric aromaticcompound</p><p>CO production efficiency(production/light absorbed)</p><p>1-(2-hydroxy-phenyl)-ethanone 0.011-(3-hydroxy-phenyl)-ethanone 0.112-hydroxybenzaldehyde 0.11benzaldehyde 0.113-phenyl-propenal 0.113-hydroxybenzaldehyde 0.134-hydroxybenzaldehyde 0.213-phenyl-acrylic acid 0.28ethoxybenzene 0.391-(4-hydroxy-phenyl)-ethanone 0.504-hydroxybenzoic acid 0.57methoxybenzene 0.83benzoic acid 0.893-methoxybenzaldehyde 1.08phenol 1.244-methoxybenzaldehyde 2.57acetophenone 3.222-methoxybenzaldehyde 3.461-(4-hydroxy-3-methoxy-phenyl)-</p><p>ethanone3.58</p><p>benzene-1,3-diol 4.55benzoquinone 5.502-methoxyphenol 11.164-methoxyphenol 15.803-methoxyphenol 17.054-ethoxyphenol 19.122-methoxy-4-methylphenol 20.012-ethoxyphenol 34.67</p><p>FIGURE 1. Ultravioletvisible absorption coefficient spectra forhumic substances dissolved in deionized water.</p><p>VOL. 42, NO. 9, 2008 / ENVIRONMENTAL SCIENCE &amp; TECHNOLOGY 9 3273</p></li><li><p>centrations (e.g., unsubstituted dimeric aromatics havegreater absorption coefficients &gt;300 nm than monomericanalogs).</p><p>The tHS samples had higher a350 per unit carbon thanaHSs, and HPOA and HA samples had higher a350 per unitcarbon than corresponding HPIA and FA extracts from thesame water samples (Table 1) indicating that source andisolation procedure affect a350. However, the strong, linearregression between a350 and aromatic carbon for all samples(Figure 2a) suggests that HS a350 is controlled primarily bythe concentration of aromatic carbon in the sample and thatHS source, isolation protocol, and chemistry have onlysecondary impact. The statistical relation between aromaticcarbon 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 topredict aromatic carbon concentrations from absorptioncoefficients of natural waters (note that in this regressiona350 is the independent variable as the slope is designed toallow prediction of aromatic carbon from DOM absorption).</p><p>No relation was found between a350 and ketonic, het-eroaliphatic, anomeric, or carboxylic carbon levels (datareported 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 a350and aliphatic carbon (r ) -0.87, n ) 11).</p><p>Humic Substance Photoreactivity. CO photoproductionrates ranged from 5.0 to 43.4 nmol L-1 hr-1 (Table 1; Figure2b) and HS photobleaching rates ranged from 0.03 to 0.19a350 m-1 h-1 (Table 1; Figure 2c). As previously reported forDOM samples (19, 20, 50), CO photoproduction and HS aphotobleaching were related to initial a. CO productionrelated most strongly with initial a at 350 nm, where a linear</p><p>regression best described the relationship (...</p></li></ul>

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