relating carbon monoxide photoproduction to dissolved organic matter functionality

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  • Relating Carbon MonoxidePhotoproduction to DissolvedOrganic Matter FunctionalityA R O N S T U B B I N S , * , , ,

    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 ,

    C L I F F S . L A W , , |

    R O B E R T C . U P S T I L L - G O D D A R D ,

    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 * ,

    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

    Received December 3, 2007. Revised manuscript receivedFebruary 11, 2008. Accepted February 15, 2008.

    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>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.

    Introduction

    Carbon monoxide (CO) is quantitatively the second largestphotoproduct of dissolved organic matter (DOM) photom-

    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).

    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.

    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.

    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.

    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.

    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

    * Corresponding authors e-mail: kmopper@odu.edu (K.M.),aron.stubbins@gmail.com (A.S.).

    Old Dominion University. Newcastle University. Plymouth Marine Laboratory.| NIWA. U.S. Geological Survey.

    Environ. Sci. Technol. 2008, 42, 32713276

    10.1021/es703014q CCC: $40.75 2008 American Chemical Society VOL. 42, NO. 9, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3271Published on Web 03/26/2008

  • 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.

    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

  • 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).

    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.

    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.

    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