Indirect Photochemical Formation of Carbonyl Sulfide and Carbon

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Environmental Processes

Indirect Photochemical Formation of Carbonyl Sulfide (COS) and Carbon Disulfide (CS) in Natural Waters: Role of Organic Sulfur Precursors, Water Quality Constituents, and Temperature 2

Mahsa Modiri Gharehveran, and Amisha Dilip Shah Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01618 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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Environmental Science & Technology

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Indirect Photochemical Formation of Carbonyl Sulfide

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(COS) and Carbon Disulfide (CS2) in Natural Waters:

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Role of Organic Sulfur Precursors, Water Quality

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Constituents, and Temperature

6 Mahsa Modiri Gharehveran§ and Amisha D. Shah§,†,*

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Lyles School of Civil Engineering, Purdue University, West Lafayette, Indiana, USA

Division of Environmental and Ecological Engineering, Purdue University, West Lafayette, Indiana, USA

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* Corresponding author phone: 765-496-2470; fax: 765-494-0395;

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e-mail: [email protected]

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7613 words

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1 Scheme

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2 Figures

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Abstract

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critical precursors to sulfate aerosols, which enable climate cooling. COS and CS2 stem from the

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indirect photolysis of organic sulfur precursors in natural waters, but currently the chemistry

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behind how this occurs remains unclear. This study evaluated how different organic sulfur

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precursors, water quality constituents, which can form important reactive intermediates (RI), and

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temperature affected COS and CS2 formation. Nine natural waters ranging in salinity were

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spiked with cysteine, cystine, dimethylsulfide (DMS) or methionine and exposed to simulated

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sunlight over varying times and water quality conditions. Results indicated that COS and CS2

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formation increased up to 11× and 4×, respectively, after 12 h of sunlight while diurnal cycling

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exhibited varied effects. COS and CS2 formation were also strongly affected by the DOC

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concentration, organic sulfur precursor type, O2 concentration, and temperature while salinity

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differences and CO addition did not play a significant role. Overall, important factors in forming

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COS and CS2 were identified, which may ultimately impact their atmospheric concentrations.

Carbonyl sulfide (COS) and carbon disulfide (CS2) are volatile sulfur compounds that are

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Introduction Carbonyl sulfide (COS) and carbon disulfide (CS2) are important trace gases in the

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atmosphere because they can contribute to the stratospheric sulfate aerosol layer which enables

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climate cooling1 and ozone destruction.2 COS can reach the stratosphere due to its > 1 year

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tropospheric lifetime1 while CS2 can react with hydroxyl radicals (•OH) to form COS.3 The

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ocean is one major source of COS and CS2, but previous studies have varied in accurately

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predicting their flux from the ocean (39-639 Gg as S/year4,5 for COS and 90-700 Gg as CS2/year

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for CS26,7). Consequently, large uncertainties remain in balancing their global budgets,8 which

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suggests that a greater understanding is still needed towards how they form in natural waters.

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One major pathway for forming COS and CS2 in water occurs through indirect photolysis

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from sunlight9–11 since organic sulfur precursors typically present in natural waters (e.g. cysteine)

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do not undergo direct photolysis.1 Sunlight is important since COS and CS2 concentrations have

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been found to decrease by up to 0.5× (i.e. 50% decrease) and 0.8× (i.e. 20% decrease) with

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decreasing sunlight intensity and increasing ocean depth of 1 to 3 m and 0 to 10 m,

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respectively.9,10 COS formation also follows a pronounced diurnal cycle in surface waters where

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a 2× increase is reached in early afternoon.12 Current COS flux models incorporate this light

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effect but only do so using two parameters, the ocean surface sunlight intensity and the seawater

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absorbance at 360 nm (UV360),13 a surrogate for the chromophoric dissolved organic matter

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(CDOM) content.

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Given this, much is still unknown about the photochemical processes involved in COS and

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CS2 formation in such waters and what specific precursors, reactive intermediates, and reaction

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mechanisms are involved. This is especially evident given that nearshore waters exhibit up to

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40× higher COS concentrations than open ocean waters.12,14,15 This may be due to the higher 4 ACS Paragon Plus Environment

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CDOM concentrations found in the nearshore waters,16 but the influence of other factors remains

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unclear. One factor may be the organic sulfur content which has mostly been quantified for

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individual compound (e.g. cysteine and dimethylsulfide (DMS)) concentrations, which range

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from pM to nM for freshwater and seawater.1,17 Alternatively, total dissolved organic sulfur

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(DOS) content has been challenging to measure due to analytical constraints, but one study

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indicated that it ranged in the low ( DMS (1.5 nM) > methionine (0.92 nM) (Fig. 2a). This trend was not solely

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due to the dark controls since the dark values were then subtracted from the light-generated COS,

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where the net formation again following the order of where cysteine (2.4 nM) > cystine (1.6 nM)

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> DMS (1.5 nM) > methionine (0.92 nM) (Fig. 2a). Two studies also found that cysteine >

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methionine by 2.8-3:1 (cysteine: methionine) when forming COS11,15. Alternatively, CS2

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formation, while similarly matching COS in terms of its dark formation (only cysteine led to >

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d.l. concentrations (Fig. 2b)), behaved differently with different precursors with light (Fig. 2b).

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In this case, CS2 decreased in formation where cysteine (0.62 nM) > cystine (0.4 nM) >

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methionine (0.23 nM) > DMS (< d.l) (Fig. 2b) but this order slightly changed for net formation

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where cystine (0.38 nM) > cysteine (0.32 nM) > DMS (0.17 nM) > methionine (< d.l.) (Fig. 2b).

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Other studies observed that CS2: (i) qualitatively formed from cystine10, (ii) did not form 10 or

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formed at 1-4 nM with methionine 11, or (iii) decreased according to cysteine > methionine 11. In

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parallel, no methionine decay was observed over time, unlike cysteine and DMS (Fig. 2).

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Overall, these trends indicated that organic sulfur precursor type impacted COS and CS2

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formation, but their sequences for net formation were not identical (thiols > disulfides >

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thioethers for COS and disulfides > thiols > thioethers for CS2). In both cases, the thiol (cysteine)

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consistently generated greater levels of COS and CS2 than the thioethers (DMS and methionine),



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but the disulfide (cystine) formed higher levels of CS2 than the other precursors. Different

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functional groups adjacent to the sulfur atom thus appear to alter COS and CS2 formation. These

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functional groups are not directly linked to S oxidation number since the sulfur oxidation states

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for COS and CS2 are -2 which is the same for cysteine, methionine, and DMS and in fact lower

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than cystine (sulfur oxidation state of -1).47 Instead, these patterns are controlled by a more

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complex set of reaction mechanisms (see later sections).

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Role of O2

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The presence of O2 ([O2]0 ≈ 8.9 mg/L at 20˚C) inhibited COS and CS2 formation under both

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dark and light conditions in the no spike and cysteine-, DMS-, methionine-, and cystine- spiked

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LA-B1 water (Fig. 2). This effect was especially dramatic with light where the presence of O2

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decreased COS by 0.2× with no precursor added but decreased by 0.3, 0.6, 0.6, and 0.9× with

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cysteine, cystine, DMS, or methionine, respectively (Fig. 2c). For methionine, the decrease

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observed with O2 was considerably lower than that observed without the precursor (Fig. 2c).

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This result indicated that O2 quenched methionine or the intermediates generated from it to some

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extent, although this likely occurred to a small degree since no observable % methionine loss

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occurred with O2 (Fig. 2c). Previously, studies found that O2 either decreased11,15 or increased

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COS formation,9 but they used a 254 nm light source9,11. O2 also decreased CS2 levels to < d.l,
60% and 80%, respectively, with O2 (Fig. 2c) than without O2. Thus, O2 does

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not appear to be directly involved in forming the COS and CS2, but more likely appears to be

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indirectly involved by quenching the various reactants involved in their formation. For COS, this

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is opposite to a previously proposed mechanism, although no direct experimental evidence was

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provided in this case to support it.11

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Role of CO

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COS and CS2 concentrations remained relatively unchanged and exhibited no trend when

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amending CO to the cysteine-spiked LA-B1 or cysteine-spiked synthetic water (Fig. 2). These

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results were expected for CS2 since no oxygen is needed but less expected for COS, which was

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proposed to form when thiols react with CO.30 Interestingly, a former study did find that the CO

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concentration simultaneously decreased with increasing COS formation in a cysteine-spiked

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natural water29. However, this study further proposed that this correlation was not due to having

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CO serve as a reactant for COS, but instead having it serve as a competing product from a

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common precursor29. One precursor tested was acetylacetonate which reacted with bisulfide to

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form both CO and COS.29 Our results further support the claim that CO is not involved in COS

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formation.

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Proposed Pathways Based on these results, several photochemical and dark pathways for COS and CS2 formation

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have been proposed that depend on the structure of the organic sulfur precursor (Scheme 1). The

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photochemical reactions are: (i) not expected to involve O2 or CO, (ii) likely involve DOM-



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based moieties, which are predicted to form key RIs (e.g. 3DOM*)) and for COS, contribute

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oxygen, and (iii) less likely involve halides or carbonate species.

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Cysteine: Cysteine is hypothesized to form considerable levels COS and CS2, as compared

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to the other precursors, through several steps which involve either the thiol (-SH) or thiolate (R-

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S-) moieties, given that its pKa of 8.425 falls within the pH range of the waters tested (Table S1).

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In fact, the thiolate moiety is expected to be more reactive than the thiol since R-S- is a better

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nucleophile than SH.48 Both the thiol and thiolate are then expected to undergo one electron

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transfer with RIs potentially including 3DOM*, •OH, or ROO•, as observed previously in

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biological matrices,48 to form R-S• (Scheme 1). Notably, the thiol may also undergo hydrogen

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abstraction by the RI, as observed previously with 3DOM* and •OH (Scheme 1), 30,49 and

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especially in low O2 environments50 (Scheme 1). Hydrogen abstraction has been shown to either

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form R-S• or a carbon centered radical (-αC• ) by attacking the S-H or αC-H moiety,

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respectively..49,51,52 αC• is predicted to form more readily than R-S• since the C-H bond exhibits a

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lower bond dissociation energy than the S-H bond.49 Although, they can convert back and forth

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by rearranging via hydrogen transfer (Scheme 1).53 After these radicals are formed, COS is

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proposed to form through some sequence of: (i) hydrogen abstraction, (ii) disproportionation,

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which is known to occur in peptides containing cysteine,51 and/or (iii) β-cleavage51 (Scheme 1).

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Within this sequence, oxygen also becomes bonded to carbon through some unknown steps but

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which likely include attack by various DOM-derived oxygen based radicals (e.g. •OH, RO•,

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ROO•, and R-C•(O)29) (Scheme 1).

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CS2 is also likely generated from the -αC• or R-S• radicals but where different steps are

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required to form a S-C-S linkage and then generate CS2. One possibility is that the -αC• and/or R-

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S• radicals undergo some sequence of reactions involving: (i) reaction with each other to form a


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S-C bond, as previously proposed,11 (ii) hydrogen abstraction, (iii) disproportionation and/or (iv)

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β-cleavage (Scheme 1). A second possibility is that the thioaldehyde (-C(S)HR1) moiety,

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generated via the COS pathway, undergoes nucleophilic attack by another cysteine thiolate

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moiety (R-S-), as also observed in multiple cysteine-containing peptide chains54, to form the S-C-

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S bond. Following this, it becomes less evident how the S-C-S linkage then forms CS2. Clearly,

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several bonds need to be cleaved, but this is not hypothesized to occur by radical pathways

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induced by RIs since this is not supported by the CS2 diurnal cycling results. Instead, some long-

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lasting intermediate is involved (e.g. , (R-S-)) (Scheme 1), but further research is required to

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confirm its identity. In addition, it should be noted that CS2 has been observed to react with •OH

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in the aqueous phase to form COS.55 However, the linear formation of CS2 during diurnal

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cycling and the fact that CS2 formation after 12 h light exposure and 12 h of diurnal cycling

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closely matched each other (Fig. 1b) indicated that this was not likely the dominate route for

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forming COS.

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Moreover, in the dark, cysteine is hypothesized to form COS and CS2 via two possible routes

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where: (i) R-S• is generated through metal-catalyzed auto-oxidation.56 Trace concentrations of

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metals are expected in these waters, even though < d.l. concentrations of Cu, Pb, Hg were

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measured (Table S1) or (ii) through other pathways, since cysteine can react with DOM in the

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dark.57–59

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Cystine: Cystine is believed to form COS and CS2 through two major proposed pathways.

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The first pathway suggests that its C-S bond can undergo homolytic bond cleavage when

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reacting with various sensitizers (e.g. n, π* and π, π* type- 3DOM* sensitizers) to form the RSS•

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intermediate, as similarly observed for other disulfides60. In the second pathway, cystine could

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react with 3DOM* through a one electron transfer to form the disulfide radical anion, RSSR•-, 21 ACS Paragon Plus Environment


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which is known to readily dissociate and form R-S-, and R-S•.48,52,61 Once R-S- and R-S• are

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formed, pathways similar to cysteine would be adopted. Both proposed pathways support the fact

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that cystine formed greater levels of CS2 than other precursors, since the two reactive S moieties

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formed from one cystine molecule would be in close proximity to each other.

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DMS and Methionine: The mechanisms driving DMS and methionine to form COS and CS2

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are less clear, although their attached methyl groups and the absence of R-S- implies that forming

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R-S• is more difficult. This is especially true if R-S• is formed through homolytic bond cleavage,

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where DMS and methionine would generate less stable radicals as compared to cystine62.

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Alternatively, hydrogen abstraction could occur at the C-H bond adjacent to the sulfur group,50

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leading to a carbon centered radical. In addition, methionine has previously been reported to

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react with 3DOM* to form a sulfur centered radical cation, R-S•+.63 More detailed investigations

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are needed to assess how the carbon centered radical. and/or R-S•+ would then form COS or CS2.

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Environmental Implications

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constituents, and temperature affected COS and CS2 formation. These efforts were made to

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better elucidate which factors were important and how this could affect their volatilization into

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the atmosphere and inform global sulfur models. This is a critical issue since these models only

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use the ocean surface sunlight intensity and UV36013,36 to predict COS photoproduction rates.

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While our results further confirmed that DOM and UV360 affected COS, other influential factors

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for COS and CS2 formation included: (i) length of sunlight exposure, especially for CS2 which

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required only a brief period of light to continue formation, (ii) O2 concentration, which can vary

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from 4 to 9 mg/L in surface waters depending on salinity and temperature.64 (iii) temperature,

This study aimed to link how sunlight exposure, various organic precursors/water quality


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which can fluctuate from -1.9˚C to 30˚C depending on latitude and seasonal variations65 and (iv)

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organic sulfur precursor type. In addition, these findings provided greater mechanistic insights

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towards the key RIs and other radicals that are involved such as those derived from DOM and

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quenched by O2 such as 3DOM*,24,33 R•,34 and sulfur-centered radicals (e.g. R-S•)11. One next

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research step is to further elucidate the role of individual RIs in more controlled, synthetic-based

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systems. Moreover, one major impedance in linking COS and CS2 formation to a more robust set

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of water quality parameters falls in the inability to quantify total DOS content in natural waters.

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Recent work has made significant progress in this area but has only estimated upper boundary

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limits (

Indirect Photochemical Formation of Carbonyl Sulfide and Carbon

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