Production of Black Carbon-like and Aliphatic Molecules fromTerrestrial Dissolved Organic Matter in the Presence of Sunlight andIronHongmei Chen,† Hussain A. N. Abdulla,† Rebecca L. Sanders,‡ Satish C. B. Myneni,‡ Kenneth Mopper,†

and Patrick G. Hatcher*,†

†Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, Virginia 23529, United States‡Department of Geosciences, Princeton University, Princeton, New Jersey 08544, United States

*S Supporting Information

ABSTRACT: Photochemical processing of dissolved organic matter(DOM) in natural waters can alter its composition and structure, supplyparticulate organic matter (POM) to sediments, and deliver modifiedterrestrial DOM to the ocean. Our studies show that terrestrial DOMexposed to simulated sunlight is altered to produce POM with a markedlydifferent molecular composition enriched with newly formed aliphaticand condensed aromatic molecules. This process is closely tied to thechemistry of iron, which primarily exists as dissolved Fe(II) and Fe(III)−organic complexes in initial DOM and photochemically matures toFe(III) oxyhydroxides before coprecipitating out with POM. The newlyformed condensed aromatic compounds resemble black carbon, which until now was thought to be produced by onlycombustion. These new molecules contribute a pool of Fe-rich, aliphatic, and black carbon-like organic matter to sediments asthe terrestrial DOM is transported through rivers. We estimate that the annual global flux of this photoproduced black carbon,most of which may be preserved in sediments, is nearly equivalent to the estimated flux of dissolved black carbon to the oceanfrom all other sources.

■ INTRODUCTION

Rivers worldwide transport approximately 0.4 × 1015 g of Cyear−1 of terrestrial dissolved organic matter (DOM) to theocean,1 with the ratio of dissolved to particulate organic carbonbeing around 1.5.2 These DOM and particulate organic matter(POM) pools transported to oceans could be sufficient tosupport the annual turnover of marine dissolved organic carbon(DOC)3 and account for all organic matter buried in marinesediments.4,5 However, the photochemical transformation ofDOM in freshwaters changes the characteristics of both thedissolved and particulate pools6,7 and leads to aggregation oforganic matter, commonly termed photoflocculation.8,9 Herewe show that photochemical transformation of DOM producesnew molecules that contribute to DOM and POM, someresembling black carbon, an important global contributor tocarbon in both dissolved and sedimentary systems. Blackcarbon, generally thought to be exclusively from combustion,contributes between 50 and 270 × 1012 g of C year−1 to theglobal pool of carbon, and most of this is incorporated in notonly terrestrial sediments but also marine sediments.10,11

Dissolved black carbon has recently been estimated toconstitute 10% of DOM exported by rivers to the ocean.12 Inaddition, sunlight modifies DOM−Fe interactions and Fespeciation,13,14 which would significantly influence thecharacteristics of photoflocculated POM and dictate its stabilityand burial of organic carbon.

To evaluate the dynamics of phototransformation ofterrestrial DOM, we examined sterile-filtered (0.1 μm) waterfrom an important headwater to the North Atlantic, the GreatDismal Swamp (in the state of Virginia),15 and subjected it toabiotic photodegradation in a solar simulator for 60 days. Thesestudies offer a means of evaluating phototransformation ofDOM in fluvial ecosystems. DOM and POM from the labexperiments were separated by filtration at different intervalsduring the photoirradiation experiment (Supporting Informa-tion), and the bulk chemical characteristics of both particulateand dissolved fractions were determined using C, N, and Fe X-ray absorption spectroscopy. The molecular composition ofthese fractions was obtained using electrospray ionizationcoupled to Fourier transform ion cyclotron resonance ultrahighresolution mass spectrometry (ESI FTICR-MS). The chemicalcharacteristics of newly produced POM in the laboratoryphotochemical studies were compared with those of naturallyoccurring freshwater POM from Dismal Swamp and Pahokeepeat (in the state of Florida), as well as DOM from coastal(close to the shore and off-shore Chesapeake Bay) and openocean (Cape Verde Rise in the North Atlantic Ocean) samples.

Received: August 19, 2014Revised: September 22, 2014Accepted: September 24, 2014Published: September 24, 2014

Letter

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© 2014 American Chemical Society 399 dx.doi.org/10.1021/ez5002598 | Environ. Sci. Technol. Lett. 2014, 1, 399−404

■ MATERIALS AND METHODS

Water samples for our experiments were collected during 2012from the Great Dismal Swamp. The samples were filteredthrough filter capsules (0.1 μm pore size, Whatman PolyCapTC) to remove particles and bacteria. A sample of naturalparticles (i.e., not lab-irradiated) from the Dismal Swamp wascollected by filtering the water through precombusted (450 °Coven for 5 h) Whatman GF/F filters (0.7 μm pore size). Thesample from the Pahokee peat was humic acid provided as astandard by the International Humic Substances Society.Initial DOM from the filtrate was used for photoirradiation

experiments in a solar simulator system, as describedpreviously.16 Details of the photoirradiation experiments, themeasurements of dissolved organic carbon (DOC) and totaldissolved nitrogen (TDN) concentrations, elemental analyses(C, H, and N) of freeze-dried samples, and the analysis of totaliron concentrations are presented in the Supporting Informa-tion.DOM and POM samples were analyzed on a Bruker

Daltonics 12 Tesla Apex Qe FTICR-MS instrument with anApollo II ESI ion source in the negative ionization mode. Thefreeze-dried POM samples were weighed (∼1 mg each) andsuspended in 1 mL of ultraquality water on a shaker (120 rpm)overnight (>10 h). Subsequently, the pH of the solutions wasincreased to 12 by small additions of concentrated NaOH [50%(w/w)] for recovery determinations and NH4OH (≥25% inH2O) for FTICR-MS analysis, respectively. Dissolution wasfacilitated on a shaker (120 rpm) 2 h prior to analysis. Therecovery of OM was constantly in the range of 66−74%(organic C) and 53−66% (organic N) for all POM samples.Particles dissolved in NH4OH (pH 12) were mixed withmethanol [50% (v/v)] and allowed to stand for 0.5 h (to allowsettling of insolubles) before being injected into the ESI source.DOM samples collected at each time point during photo-irradiation were frozen (−20 °C) immediately upon samplingfor later analysis. DOM samples were thawed and mixed withMeOH [50% (v/v)] containing NH4OH [0.5% (v/v) in final toincrease the pH to 12] for FTICR-MS analysis conducted asdescribed previously.17 All mass spectra were calibrated andassigned molecular formulas to peaks following previousstudies.17 Assigned formulas included elements C, H, O, N,S, and P. Double bond equivalent (DBE) values were calculatedas DBE = 1 + C − 0.5H + 0.5N + 0.5P. Aromaticity indices(AIs)18 were calculated as AImod = (1 + C − 0.5O − S − 0.5H)/(C − 0.5O − S − N − P). Molecular formulas with DBE/C <0.3 and H/C = 1.0−3.0 are unambiguously assigned asaliphatics;19 formulas with AImod ≥ 0.5 and AImod ≥ 0.67 areassigned as aromatic compounds and condensed aromaticcompounds,18 respectively.C and N K-edge X-ray absorption near edge structure

(XANES) spectra were collected at Spherical GratingMonochromator beamline 11ID-1 at the Canadian LightSource (Saskatoon, SK). Fe K-edge extended X-ray absorptionfine structure (EXAFS) spectra were acquired at a bendingmagnet X-ray source (X18B) at the National Synchrotron LightSource (Brookhaven National Laboratory, Upton, NY). Spectraof the standard were collected frequently during the XANESdata measurements to correct for the systematic errorassociated with the drift of the monochromator. To ensurelittle to no contamination, sample preparation was conductedon a clean Kapton table, using polyfilm to prevent Fecontamination. Samples for Fe EXAFS were freeze-dried

immediately upon sampling to protect Fe speciation fromartificial changes during storage. The X-ray beam did notappear to change the Fe speciation. Details of the XANES andEXAFS analyses are given in the Supporting Information.

■ RESULTS AND DISCUSSIONUpon photoirradiation, Dismal Swamp DOM was partlyremineralized to gases (e.g., CO2), chemically modified, andflocculated to form particles. The total organic carbon (TOC)concentrations in samples exposed to photoirradiationdecreased dramatically with time (Figure 1), and ∼76% of

the initial organic carbon was lost in 60 days. Particulateorganic carbon (POC) concentrations increased during theinitial 30 days but decreased with longer irradiation to day 60.The pH in samples exposed to light increased gradually from3.9 initially to 5.5 on day 18, then 6.1 on day 30, and finally 6.9on day 60. Buffering by carboxylates controlled the pH initially,but as photoproduced CO2 accumulated during the irradiation,the pH became buffered at ∼5.7 by the bicarbonate system; atpH >5.7, photoproduction of ammonium (as supported by ourammonium measurements) and possibly proton-consumingphotoreactions were likely important in controlling the pH. Incomparison, flocculation, pH changes, and organic carbonlosses were not detectable in dark controls.The bulk organic carbon and nitrogen of DOM changed

significantly with photoirradiation, while the particles, onceformed, exhibited similar composition without much variationover time. The bulk C speciation in DOM determined using X-ray absorption spectroscopy showed a dominant proportion ofaromatic/unsaturated carbon as well as carboxyl and phenoliccarbons suggestive of carboxylated/hydroxylated aromaticstructures (Figure S1 of the Supporting Information). Withirradiation, the DOM pool showed a loss of aromatic/unsaturated C and a relative increase in the carboxyl C andO-alkyl C (alcohols or carbohydrates). The bulk elementalcomposition of the photoflocculated POM shows a H:C molarratio significantly lower than that of the corresponding DOM,suggesting that it contains a greater abundance of aromatic/olefinic structures (Table S1 of the Supporting Information).As photoirradiation continued, the POM maintained itselemental composition but the DOM showed an increase inits H:C molar ratio, an indication that the DOM was becomingmore aliphatic, consistent with nuclear magnetic resonance9

Figure 1. Changes in total organic carbon (TOC) and particulateorganic carbon (POC) concentrations during photoirradiation.

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and X-ray studies. The N X-ray absorption spectroscopy ofDOM also showed significant changes in the amide andoxidized forms of N (e.g., nitrosyls) as the photoirradiation ofDOM progressed (Figure S2a of the Supporting Information).The relative abundance of these groups did not vary with timein POM (Figure S2b of the Supporting Information). However,the newly formed POM is enriched with N compared to theinitial DOM (Table S2 of the Supporting Information), inagreement with previous studies,9 and exhibits the samespectral features as DOM without any detectable oxidized Nspecies.The molecular formulas of both DOM and alkali-soluble

POM (>60% is soluble at pH 12), obtained from ESI FTICR-MS, suggest 64−94% of all formulas contained only CHO units

(Table S3 of the Supporting Information). With an increasingphotoirradiation time of <30 days, the proportions of CHONformulas increase in DOM but are almost constant in POM,being consistent with the N XANES analysis (Figure S2 of theSupporting Information). The POM samples are enriched inCHON formulas compared to the corresponding DOM ondays 18 and 30. However, the percentage of CHON formulasand elemental N:C ratios in POM sample decrease by day 60(Table S3 of the Supporting Information), which will beexamined in a future study. Moreover, the FTICR-MS resultsshow that photoflocculated POM is chemically very differentfrom the respective photoirradiated and original DOM (Figure2a,b and Figure S3a,b of the Supporting Information). Byplotting the H:C ratios as a function of O:C ratios for all

Figure 2. Changes in the molecular composition of molecules identified by ultrahigh resolution mass spectrometry. (a) van Krevelen diagram withelemental H:C and O:C ratios of molecules in day 0 DOM. (b) van Krevelen diagram of day 60 DOM (red) and POM (black). Similar data forother time points are presented in Figure S3a−c of the Supporting Information. Areas we define as low-O:C molar ratio (O:C < 0.3) aliphatic,aromatic, and condensed aromatic molecules are marked by dashed lines. (c) Temporal changes in the relative abundance of aromatic (●) or lowO:C aliphatic (○) formulas (percent of total identified formulas) with photoirradiation. With increasing irradiation time, the emergence of newmolecules (shown next to data points as percentages of condensed aromatic and aliphatic molecules, compared to all identified formulas in DOM onday 0) increases steadily for DOM and is constant for POM. Also listed in the shaded area are the data for natural POM from the Dismal Swamp.

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molecular formulas associated with POM [on the van Krevelendiagram (Figure 2b)], we find there is a clear segregation of allmolecules into two chemically distinct regions: highly aromaticmolecules (low H:C molar ratio) and aliphatic molecules (highH:C and low O:C molar ratios). There are very few moleculesin regions more typically associated with the DOM.15,20 Inaddition, the aromatic molecules detected mainly in the POMbut also the DOM are primarily condensed aromatic structureswith hydroxyl and carboxyl substituents, such as those observedin black carbon produced from combustion.21 These newlyformed black carbon-like molecules from photoflocculation aretermed “photoBC” in this report. The abundances of moleculesfrom these two regions (defined by fields on the van Krevelendiagram in Figure 2b) suggest that the relative proportion ofphotoBC increases in POM in the first 30 days and thendecreases slightly with increasing photoirradiation time (Figure2c). This observation together with the decrease in POMconcentration after 30 days (Figure 1) suggests that a portionof photoBC in the POM is redissolved, contributes to DOMcomposition, and becomes photodegradable after beingirradiated for 30 days.The proportion of low-O:C molar ratio aliphatic molecules

in the photoflocculated POM (but not the photoBC fraction)appears to increase during the course of photoirradiation. Thephotoproduced aliphatic formulas in POM have H:C ratiosfrom about 1.5 to 2.0, and most have O:C ratios lower thanthose of the newly formed aliphatic formulas in the DOM(Figure S4 of the Supporting Information). The majority of thenew aliphatic formulas in the photoflocculated POM have H:Cratios around 1.5−1.7, which indicates that the structures arenot linear/branched aliphatic but rather either alicyclic or amixture of aliphatic and olefinic structures, possibly similar tocarboxyl-rich alicyclic molecules observed in natural waters.20,22

Newly produced aliphatic molecular formulas in DOMincrease in relative abundance with greater photoirradiationtime and are more oxygenated than their POM counterparts.The newly formed aliphatics in DOM are more soluble than thenewly formed aliphatics in POM because of the oxygenatedfunctional groups [probably carboxyl groups with higher O:Cmolar ratios (Figure S4 of the Supporting Information)]. In thecase of photoBC molecules in DOM, a major proportion isabsent from the original DOM [80−100% (Figure 2c)]. Itappears that photoirradiation is creating a group of newphotoBC molecules, not present initially, that become moreprevalent with an increasing degree of photodegradationrelative to total condensed aromatic DOM molecules. Theminor diminution over 30 days is probably because, in solution,these molecules are susceptible to continued photodegradationas shown previously.23 The increase in the amount of photoBCin the DOM pool after irradiation for 60 days is probably fromparticulate dissolution as this corresponds to the concomitantdecline in their abundance in POM (Figure 2c).In addition to these photochemical changes in organic

molecule composition, the concentration and speciation of theassociated Fe vary significantly. The Fe:C molar ratios of DOMslightly decreased (from 0.005 to 0.003), while this ratioincreased significantly for POM (from 0.040 to 0.244) withphotoirradiation time. In addition, the X-ray absorption spectraof Fe (both near-edge and extended X-ray absorption finestructure) associated with both dissolved and particulate poolsindicated the presence of both Fe(II) and Fe(III), with moreFe(III) in the POM than in the DOM (Figure 3a). Theinteractions of organic molecules with Fe in both DOM and

POM also changed with time: initial and day 18 samples at theFe absorption edge indicated a significant fraction of Fecoordinated to organic matter with traces of Fe(hydr)oxidepolymers (Figure 3b,c). However, with time, the level of Fe−organic molecule association decreased and the level of Fe−Fepolyhedral linkages increased markedly (days 30 and 60),suggesting the formation of amorphous ferric hydr(oxide)(Figure 3c). Interestingly, the Fe speciation of natural particlesfrom initial DS water was similar to the Fe speciation ofsamples from the early stages of photoflocculation withpredominant Fe−organic complexation and minimal Fehydr(oxide) polymerization (Figure 3). These observationsare consistent with a previous report that both Fe(II) andFe(III) form complexes with organic matter in natural watersand soils, often in combination with Fe(III) hydroxides.24 It isnoted that the complexation between Fe(III) and organicmatter prevents hydrolysis to polymeric Fe(III) hydroxides.25,26

Our study is the first to examine the changes in Fe speciationover time during photoirradiation. The changes in Fe speciationand production of photoBC were correlated with the onset ofparticle production. However, direct flocculation of DOM byaddition of soluble Fe(III) in the absence of light did not resultin the production of the same molecules (data not shown),suggesting the need for photochemical transformation ofDOM.The X-ray and mass spectral characteristics of the natural

POM (i.e., not lab-irradiated) obtained from the Dismal Swamp

Figure 3. Changes in DOM and POM Fe speciation with time. Thederivative energy of the white line from Fe X-ray spectra indicates Fein the DOM was more reduced (lower energy) than the Fe in thePOM (a). Fe−C correlations were observed in all samples except theday 60 POM (b). The coordination number for second neighbor Feincreased with time, indicating an increased level of Fe polymerizationin both the DOM and POM with time (c). The top schematicrepresents the overall trend of Fe speciation, transformation fromFe(II)/Fe(III)−organic complexes to small Fe(III)(hydr)oxideclusters with time.

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are similar to those of the POM generated in the laboratorystudies described above. Mass spectrometry indicated thatapproximately 70% of the condensed aromatic formulasobserved in natural POM (>60% soluble at pH 12) are alsofound in the laboratory photoflocculated POM (Figure 2 andFigure S3 of the Supporting Information). As a note of caution,molecular formulas identified among mass spectra fromdifferent samples do not necessarily mean the molecules haveexactly the same structure, because there could be numerousdifferent structural isomers attributed to a single formula.However, the X-ray spectra of natural POM are almost identicalto those of the POM produced during photoflocculation(Figures S1b and S2b of the Supporting Information),supporting the belief that the chemical structures of thesetwo POM samples are likely similar.Our new findings document that particulate and dissolved

photoBC collectively represent a previously unrecognizedsource of black carbon in all terrestrial and marine aquaticand sedimentary systems receiving input from freshwaterDOM. While a significant portion of the aromatic moleculeswithin dissolved photoBC are considered to be labile in thepresence of sunlight,23 photoBC preservation is tied to itsoccurrence in the particulate phase.27 However, dissolvedoxygen and photochemically produced reactive oxygen speciesoxidize DOM-associated Fe(II) to Fe(III), leading to theformation of insoluble Fe(III) oxyhydroxides13,28 and sub-sequent formation of particles which will scavenge photoBC.Thus, there appears to be an important connection betweenphotodegradation of DOM and changes in Fe chemistry.These results for the first time show that photoflocculation,

similar to that observed in the laboratory, is actively producingcarboxylated/hydroxylated photoBC compounds, and these areassociated with Fe and contributing to the BC POM and DOMin fluvial systems. The photoBC mechanism may partly explainthe finding that fluvial BC is isotopically young (compared toBC derived from fossil fuel combustion),29 as the 14C age ofriverine DOM ranges from modern to only a few thousandyears, with most rivers dominated by modern DOM (<500years).30 Approximately 5% of the initial DOM was removedfrom the water as photoflocculated POM by day 60. Aconservative estimate, based on the relative percent ofcondensed aromatic molecules in the POM (Figure 2),indicates that 10% of the POM is photoBC. When oneconsiders that approximately 0.4 × 1015 g of C year−1 ofterrestrial DOM is being transported to the ocean,1 we canestimate a flux of 0.02 × 1015 g of C year−1 of photoBC fromrivers to the ocean, mainly to coastal sediments. This flux isalmost equal to the estimated global flux of dissolved blackcarbon from land to the ocean.12 Moreover, the greatest flux ofdissolved BC was from wetlands12 where the increased levels ofDOM subjected to high solar irradiance will likely magnify theproduction of photoBC. Although photoBC molecules aremostly present in the particulate phase in our photoirradiationexperiment, we observed that they are redissolved afterextended photoirradiation (e.g., >30 days) and contribute toDOM [noted by an increase in the level of photoBC in theDOM (Figure 2c)]. It is reasonable to expect that redissolvedphotoBC will eventually be photodegraded further as shownpreviously.23 Any photoBC remaining as POM will likely beprotected from photodegradation.27

These newly discovered photoBC and aliphatic moleculesfound in our studies were observed in the mass spectralanalyses of a peat sample (the Pahokee peat from Florida) as

well as the ambient Dismal Swamp POM and peat (Figure S5of the Supporting Information). DOM from both coastal waters(Chesapeake Bay; in samples from immediately off-shore) andopen ocean (the Cape Verde Rise in the North Atlantic Ocean)(Figure S5 of the Supporting Information) also containscompounds similar to the photoproduced molecules but mainlythe aliphatic ones, most likely because the aromatic photoBCmolecules have been lost through photodegradation. Theseresults suggest that the photoflocculation phenomenonobserved in the Dismal Swamp is pervasive, and that thisorganic matter may be photoproduced globally in allterrestrially influenced coastal waters and adjacent seas.Moreover, the new molecules from natural organic matter arecomposed of condensed aromatics or alicyclic molecules,similar to those of humified organic matter. As a result, thesenew molecules are likely to be less biodegradable andcontribute to the stabilization/sequestration of carbon andmetals such as Fe.

■ ASSOCIATED CONTENT*S Supporting InformationAdditional details about materials and methods and supple-mentary tables and figures mentioned in the text. This materialis available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Department of Chemistry and Biochemistry, Old DominionUniversity, 4402 Elkhorn Ave., Physical Sciences Building II,Room 3100, Norfolk, VA 23529. E-mail: phatcher@odu.edu.Phone: (757) 683-6537. Fax: (757) 683-4628.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank the synchrotron beamline staff at the NationalSynchrotron Light Source and CLS and Dr. Bargar for some ofthe data collection at the CLS. R.L.S. and S.C.B.M. weresupported by a grant from the U.S. Department of Energy. Thisresearch was partly funded by National Science FoundationGrant OCE-0728634 to K.M. and P.G.H. and by the FrankBatten Endowment to P.G.H. We thank Luni Sun for herassistance during the photoirradiation experiment. We aregrateful to Susan Hatcher and Jared Callan at the COSMIC(College of Sciences Major Instrumentation Cluster) facility atOld Dominion University for their assistance with the FTICR-MS analyses. We thank Dr. Rachel Sleighter for providing theoriginal FTICR-MS data from the Pahokee peat sample.

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