Marine Chemistry 62 Ž1998. 89–101
Carbonyl sulfide žOCS / and carbon monoxide žCO / in natural
waters: evidence of a coupled production pathway
Willer H. Pos 1, Daniel D. Riemer ) , Rod G. Zika
UniÕersity of Miami, Rosenstiel School of Marine and Atmospheric Science, 4600 Rickenbacker Causeway, Miami, FL 33149-1098, USA
Accepted 7 January 1998
Abstract
The mechanisms for the photoproduction of carbonyl sulfide ŽOCS. and carbon monoxide ŽCO. in natural waters were
studied by evaluating experimental results from different aqueous systems. A coupled photoproduction mechanism was
observed operating on CO and OCS. For CO photoproduction, the presence of a carbonyl group is necessary, while for OCS,
a source of reduced sulfur in addition to the carbonyl is required. An acyl radical is postulated to be the key intermediary for
OCS and CO photoproduction while a sulfur-centered radical Žthiyl or sulfhydryl radical. is likely to be the key species that
reacts with acyl radicals to produce OCS. Laboratory experiments indicated that addition of reduced sulfur to seawater and
subsequent irradiation leads to a decrease in CO and an increase in OCS photoproduction rates relative to original water.
Furthermore, treatment of Suwannee fulvic acid ŽSFA. or Aldrich humic ŽAH. aqueous solutions with sodium borohydride
ŽNaBH 4 . decreased photoproduction of CO compared to untreated samples. A metal redox system ŽCeŽIV.rCeŽIII.. was
also used to generate radicals in solution and demonstrate radical participation in the production processes of OCS and CO.
Based on these results, potential pathways are proposed for the photoproduction of both gases in natural waters involving the
formation of free radicals. In natural waters, the anti-correlation COrOCS is likely to be seen in areas with high biological
productivity in which reduced sulfur compounds and dissolved organic matter are abundant. This study furthers our
understanding of sulfur chemistry in aqueous systems and provides another demonstration of the complex link of the
biogeochemical cycles of carbon and sulfur. q 1998 Elsevier Science B.V. All rights reserved.
Keywords: carbonyl sulfide; carbon monoxide; photoproduction
1. Introduction
Carbonyl sulfide ŽOCS. and carbon monoxide
ŽCO. are two important atmospheric trace gases with
an oceanic source. OCS is the most abundant sulfur
)
Corresponding author. Present address: NCARrACD, P.O.
Box 3000, Boulder, CO80307-3000, USA. E-m ail:
[email protected]
1
Present address: Departamento De Engenharia Sanitana E
Ambiental, Belo Horizonte, Brazil.
compound in the atmosphere with average concentration of 500 " 50 ppt ŽTorres et al., 1980; Rasmussen
et al., 1982.. Due to its atmospheric lifetime of
around two years, OCS can reach the stratosphere
where it can undergo photolysis and oxidation to
sulfate. Sulfate is the predominant component in the
formation of the stratospheric aerosol and during
periods of volcanic quiescence, OCS is thought to be
the major source of this sulfate in the stratosphere
ŽTurco et al., 1980; Crutzen, 1978.. The stratospheric
sulfate layer plays an important role in the earth’s
0304-4203r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII S 0 3 0 4 - 4 2 0 3 Ž 9 8 . 0 0 0 2 5 - 5
90
W.H. Pos et al.r Marine Chemistry 62 (1998) 89–101
radiation budget and hence OCS is directly involved
in the earth’s climate balance. The oceanic source of
OCS is believed to comprise around 28% of the total
sources Žfor a review, see Chin and Davis, 1993..
Thus, the ocean makes a significant contribution to
atmospheric OCS.
In contrast, CO is a reactive gas serving as a
major sink for OH Žhydroxyl radical. in the atmosphere ŽThompson and Cicerone, 1986.. The oceanic
source of CO amounts to 0.4–9% of the total sources
to the atmosphere ŽBates et al., 1995; Johnson and
Bates, 1996.. Despite the small amount of oceanically derived CO, it is thought to play a major role in
controlling the atmospheric oxidation conditions in
the remote marine atmosphere ŽEricson and Taylor,
1992..
The major oceanic source of OCS and CO is the
photodegradation of dissolved organic matter ŽDOM.
by the UV portion of solar radiation ŽRedden, 1983;
Valentine and Zepp, 1993; Zepp and Andreae, 1995..
Field studies have demonstrated that photoproduction of OCS and CO in surface seawater follows a
diurnal cycle with maximum photoproduction concentrations in middle afternoon ŽConrad et al., 1982;
Ferek and Andreae, 1983; Bates et al., 1995; Weiss
et al., 1995.. Field measurements have also shown
that the surface seawater in coastal regions is usually
supersaturated with respect to the atmospheric concentrations of both gases. The supersaturation of
these compounds in the photic zone affects their
exchange with the atmosphere. Thus, photoproduction of OCS and CO in seawater has a direct impact
upon their concentration in the atmosphere. Recently, several authors have suggested that the decrease of the ozone layer and the consequent increase in the UV-B reaching the ground could have
an impact on OCS and CO photoproduction in seawater ŽZepp and Andreae, 1995; Najjar et al., 1995..
Experiments have demonstrated that the photosensitized reactions of organosulfur compounds in
seawater result in OCS production ŽFerek and Andreae, 1984; Zepp and Andreae, 1995.. Similar studies demonstrated that irradiation of different natural
waters Žlake, wetland and coastal waters. led to
production of CO ŽRedden, 1983; Valentine and
Zepp, 1993.. In these studies, the authors pointed to
DOM as being indirectly ŽOCS. or directly ŽCO.
involved in the photoproduction process. In the OCS
photoproduction mechanism, a photosensitized reaction via DOM is thought to mediate the photoreaction of dissolved organic sulfur compounds and
the subsequent production of OCS ŽZepp and Andreae, 1995; Ferek and Andreae, 1984., while for
CO, direct DOM photooxidation is believed to be the
major pathway ŽRedden, 1983; Valentine and Zepp,
1993.. Therefore, only generic and independent
mechanisms Ži.e., photosensitized and photooxidation reactions. are postulated as the main processes
that govern OCS and CO photoproduction, respectively.
This study was intended to assess basic mechanisms involved in the photoproduction of CO and
OCS in seawater. Model carbonyl bearing compounds Že.g., potassium acetylacetonate, sodium
pyruvate and sodium glyoxylate. were used as
sources for carbonyl functionality. Cysteine and
bisulfide were used as a source of reduced sulfur.
Our results indicate that CO and OCS may share a
common photoproduction pathway. The proposed
mechanisms are potential pathways by which photoproduction of OCS and CO occurs in natural waters.
2. Experimental
Biscayne Bay seawater ŽBBSW. was collected at
the RSMAS dock, filtered Ž0.2 m m filter. and refrigerated until used. A UV-quartz immersion well
photoreaction unit ŽAce Glass, Vineland, NJ. was
used to produce DOM-free seawater. Sodium sulfide
nonahydrate ŽNa 2 S Ø 9H 2 O., potassium-2,4-pentadionate ŽKCH 3 CO5CHCOCH 3 . commonly known as
potassium acetylacetonate, sodium pyruvate
Ž C H 3 C O C O 2 N a . and sodium glyoxylate
ŽHCOCO 2 Na. were purchased from Aldrich Chemicals ŽMilwaukee, WI. and cerium sulfate ŽCeŽSO4 . 2,,
0.1 N solution. was from Supelco ŽBellefonte, PA..
All chemicals were used without further purification.
Highly purified Suwannee fulvic acid extract supplied by Dr. E.M. Perdue ŽGeorgia Institute of Technology. was dissolved in distilled water and diluted
with DOM-free seawater to concentrations ranging
from 150 to 250 m M C, measured by TOC analysis,
on a Shimatzu 5000 TOC analyzer. Laboratory photochemical studies were done using a Kratos–
W.H. Pos et al.r Marine Chemistry 62 (1998) 89–101
Schoeffel irradiation system equipped with a 1000 W
Xe lamp. Sunlight was simulated by removing radiation shorter than 300 nm with a cutoff filter and
excessive IR radiation with a 1 cm circular quartz
cell filled with distilled water. Typical irradiation
exposures consisted of a 20, 30, 40, 60 min sequence. Most of the irradiation experiments were
duplicated for each time sequence. A quartz cell with
a 10 cm pathlength Ž; 30 ml. was used in the
irradiation experiments. The cell was adapted to hold
a fritted glass fitting and a luer connector to facilitate
the sparging of OCS and the handling of the solutions. All the irradiation procedures were performed
at room temperature Ž23.58C.. OCS concentrations
were measured by stripping the volume of the irradiated sample cell with helium followed by collection
on a cryogenic trap and subsequent gas chromatographic analysis using a Hewlett Packard 5890II gas
chromatograph and flame photometric detector ŽFPD.
fitted with a Carbopack BHT 100, 1.4 m = 0.32 cm
column ŽSupelco, Bellafonte, PA.. OCS standards
were generated from certified permeation devices
ŽVICI-Metronics. held at 308C and diluted via a
three-stage dilution system. A small polycarbonate
filter holder with a paper filter impregnated with
silver nitrate solution was used in-line when analyzing seawater with added sulfide. The purpose of the
filter was to retain H 2 S, that otherwise would interfere with OCS separation. CO was measured in the
headspace gas sample after transferring the irradiated
solution to a 60 ml gas tight syringe. Carbon monoxide analysis was performed by flushing the headspace
gas Ž30 ml. into a 0.32 cm Teflon loop filled with
molecular sieve 5-A 60r80 mesh kept at y1868C
with liquid nitrogen. Upon heating the loop with hot
water, the trapped gases were injected on a HewlettPackard 5890 II gas chromatograph fitted with a
methanizer-FID. Carbon monoxide calibrations were
performed using a certified gas mixture of 1% CO,
1% CH 4 in helium ŽScott Specialty Gases, Plumsteadville, PA. after dynamic dilution in a helium
flow. Sulfide concentrations were determined by the
spectrophotometric methylene blue method according to Cline Ž1969.. Two different dilution procedures were used to cover a sulfide range from 1 to
40 m M. The absorbance at 666 nm was monitored
spectrophotometrically with a Hewlett Packard 8452
A UV–vis spectrophotometer. Cysteine was deter-
91
mined by HPLC after derivatization with a bimane
derivative, mono-bromotrimethylammoniobimane
ŽThiolite w , Calbiochem, La Jolla, CA. using procedure adapted from Fahey and Newton Ž1987.. The
Fig. 1. Photoproduction of CO and OCS in BBSW exposed to the
solar simulator. Ža. OCS photoproduction, Žb. CO photoproduction; Žcircle. BBSW, Žsquare. BBSW with 10 mM cysteine added,
Žtriangle. BBSW with 10 m M HSy added. Note the increase in
OCS and decrease in CO photoproduction upon addition of reduced sulfur compounds.
92
W.H. Pos et al.r Marine Chemistry 62 (1998) 89–101
3. Results
Fig. 2. Pseudo-first order plot of the photoconsumption of cysteine
Žsquare. and bisulfide Žcircle. added to BBSW.
eluent was a mixture of 5% methanol in water
delivered in isocratic mode at a flow rate of 1.2 ml
miny1 . The column used was a C 18 reversed-phase
ODV ŽBurdick and Jackson, California.. An F100
fluorometer ŽSpectra Vision. with 360 nm excitation
and 460 nm emission filters was used as the detector.
Signal output was directed to a HP3689 A integrator.
Results of OCS and CO photoproduction from
BBSW and BBSW with 10 m M cysteine added and
exposed to Xe lamp irradiation system are shown in
Fig. 1a and b, respectively. With a typical irradiation
time sequence of 20, 30, 40, and 60 min, photoproduction of OCS and CO in BBSW followed relatively linear increases with time. These results also
showed an up to 10-fold increase in OCS and a 30%
decrease in CO photoproduction rates in BBSW with
added cysteine as depicted in Fig. 1a and b, respectively. Exchange of the sulfur source to HSy by
adding Na 2 S Ø 9H 2 O in the same concentration range
of cysteine led to similar decrease in CO but a more
efficient OCS photoproduction. Sulfide and cysteine
consumption followed pseudo-first order kinetic rates
with half-lives of 41 min and 63 min, respectively
ŽFig. 2.. Initial estimates considering the amount of
sulfide consumed and OCS produced indicate a less
that 5% conversion efficiency. Considering that both
gases produced have in common a carbonyl functionality ŽC5O., this anti-correlation is suggestive of
a coupled pathway with a common transient species
operating in both OCS and CO photoproduction
mechanisms. To address the role of carbonyl moiety
Table 1
Relative photoproduction rates under Xe lamp system
Solution type
CO
ŽnM hy1 .
COS
Žnm hy1 .
BBSW
Bbsw q 10 m M cysteine
BBSWq 10 m M HSy
BBSWq 10 m M acetylacetone
BBSWq 10 m M sodium pyruvate
BBSWq 10 m M sodium glyoxylate
DOM y free seawater
DOM-free seawaterq 10 m M acetylacetonea
DOM-free seawaterq 10 m M acetylacetoneq 10 m M HSya
Sowance Fulvic acid solutiona
Sowance Fulvic acid solutionq 10 m M cysteinea
Sowance Fulvic acid solutionq NaBH 4 treatment a
Aldrich Humic solution
Aldrich Humic solution qNaBH 4 treatment a
28.1 " 7.1
14.8 " 1.2
15.9 " 3.1
40.4 " 6.7
32
30
2.3
31
17
58.3 " 6.7
33.2 " 4.9
41.0 " 12.0
89.0 " 10.5
23
0.6 " 0.1
8.1 " 1.5
13.5 " 2.2
2.4 " 0.7
1.3
bdl
bdl
bdl
0.8
0.4 " 0.1
2.7 " 0.5
bdl
5.3
0.9
BBSWs Biscayne Bay water; Sowanee fulvic acid solutions 237 m M carbon, pH 8.0; Aldrich Humic acid solutions 281 m M carbon, pH
8.0.
a
s 40 min irradiation.
W.H. Pos et al.r Marine Chemistry 62 (1998) 89–101
on the photoproduction of OCS and CO, three carbonyl bearing compounds Žsodium pyruvate, sodium
glyoxylate and potassium-acetylacetonate. were used
as water soluble, non-volatile models for naturally
occurring carbonyl molecules in seawater. Using
these model compounds, production rates of both
gases were evaluated. A summary of the production
rates obtained are shown in Table 1. Compared to
BBSW, without an added carbonyl or reduced sulfur
source the addition of 10 m M of acetylacetone to
seawater resulted in an almost 5.3-fold increase in
CO photoproduction rate, while a 10 m M addition of
pyruvate or glyoxylate to seawater led to 4.3 and 3.5
fold increases in CO production rate, respectively
ŽTable 1.. Exchange of BBSW for DOM-free seawater in the above experiment led to a similar CO
photoproduction pattern. Again, the addition of reduced sulfur led to an increase in the photoproduction rate of OCS and a decrease on CO. Interestingly, addition of these carbonyl compounds to doubly distilled water and the subsequent Xe light exposure Ž40 min. did not produce CO. These results
implicate a seawater constituent in the photoprocess,
possibly a transition metal or an inorganic free radiyP
2yP .
cal Že.g., Xy
.
2 , HCO 3 , CO 3
In a further attempt to assess the participation of
carbonyl functionality on OCS and CO photoproduction, two 2.5 ml samples of a solution of highly
purified SFA were added to two 10 ml glass tubes.
To one tube, 2 ml of 5 mM NaBH 4 ŽpH 10. was
added and allowed to react. In a third tube, without
fulvic acid, 2 ml of 5 mM NaBH 4 ŽpH 10. was also
added. Tubes were left for 1 h at room temperature.
Subsequently, 100 m l of HCl Žconcentrated. was
added to the samples with NaBH 4 . This step decomposes NaBH 4 into borate and NaCl. The sample
without NaBH 4 and the one without SFA were
mixed together. Next, distilled water was added to
both samples to a final volume of 10 ml. The pH was
adjusted to around 8 with NaOH solution. A similar
procedure was repeated with AH samples. The above
method is known to promote the reduction of carbonyl groups present in humic material to hydroxyl
groups ŽStevenson, 1992.. UV–vis absorption spectra showed a decrease in absorbance in the samples
that were treated with NaBH 4 as is shown in Fig. 3a
and b. Treated and untreated SFA and AH samples
were exposed to Xe lamp system and the CO photo-
93
Fig. 3. Ža. UV–vis spectra of Suwannee fulvics ŽSFA. treated with
NaBH4 and SFA untreated Žb. UV–vis spectra of Aldrich humics
ŽAH. treated with NaBH 4 and AH untreated. In both cases
treatment with NaBH 4 led to decrease in absorption, indicating
carbonyl reduction.
production was evaluated. The results are also shown
in Table 1 and in Fig. 4a and b. Samples treated with
NaBH 4 had lower CO production rates than untreated samples.
To demonstrate the participation of sulfur-centered
radicals in the production of OCS, a classical oxidation procedure using CeriumŽIV. sulfate was performed ŽGraceffa, 1983.. CeriumŽIV. is a typical
one-equivalent oxidant that removes one electron
94
W.H. Pos et al.r Marine Chemistry 62 (1998) 89–101
CeŽIV. and sulfide did not generate OCS. By varying
the concentration of acetylacetonate from 100 to 400
m M in reactions with and without added sulfide Ž100
m M. and subsequent quantification of OCS ŽFig. 5a.
and CO ŽFig. 5b. led to a similar effect as observed
before with BBSW.
Fig. 4. Photoproduction of CO in solutions of AH and SFA,
Žsquare. the dissolved organic material were treated with NaBH 4 ,
Žcircle. untreated sample.
from a substrate. In a one-electron oxidation of
bisulfide ŽHSy. a sulfhydryl radical ŽHSP. is generated without light participation ŽGraceffa, 1983.. In
this experimental set, sodium sulfide Ž10 m M. and
potassium acetylacetonate Ž10 m M. were added to
distilled water and transferred into a 30 ml purging
cell as described previously in Section 2. Addition of
100 m l of cerium sulfate Ž0.5 M. solution and subsequent stirring of the content of the cell led to an
almost instantaneous production of OCS. Control
reactions without sulfide generated CO. Reaction of
Fig. 5. Ža. Production of CO as a function of acetylacetonate
Žacac. concentration from a redox reaction with CeŽIV., Žsquare.
solution of acacqCeŽIV.qHSy, Žcircle. solution of acacq
CeŽIV.. Žb. Production of OCS from the solution with added
sulfide. Note that the addition of reduced sulfur leads to production of OCS and a decrease in CO. See text for more details.
W.H. Pos et al.r Marine Chemistry 62 (1998) 89–101
4. Discussion
The results from the above experiments give a
strong indication that photoproduction pathways of
OCS and CO involve a common transient species.
The decrease in CO production upon addition of a
reduced sulfur source suggests that the sulfur species
is either competing with or depleting a key transient
in the photoproduction of CO. The fact that the dark
control reactions for each experiment did not generate either OCS or CO points towards photochemically generated radical species as being intermediary
in the process. The significant increase in CO photoproduction upon addition of carbonyl containing
molecules to BBSW, the sharp decrease in CO production upon treatment of SFA with NaBH 4 and the
covariance of CO and OCS upon addition of reduced
sulfur suggest that carbonyl moieties Žketonic, aldehydic and quinonics. present in biological molecules
and DOM are potential common substrates involved
in CO and OCS production in natural waters. Early
investigations by Redden Ž1983. demonstrated the
possible participation of carbonyl compounds on CO
photoproduction. In addition, several more recent
studies have demonstrated the direct involvement of
humic substances in the photoproduction of CO
ŽMopper et al., 1991; Valentine and Zepp, 1993..
Processes that lead to CO production via photochemical decarbonylation are well known and essentially
involve an excited carbonyl molecule ŽKagan, 1993..
In one of these photoprocesses, also called Norrish
type I mechanism, a bond adjacent to the carbonyl
group is broken Ž a-fission. and a pair of radicals is
formed. Norrish type I mechanism can occur by two
distinct electronic excitation processes. In the first
one, an electron from the highest occupied molecular
orbital ŽHOMO. can be promoted to the lowest
unoccupied molecular orbital ŽLUMO. of the p-system. This process is known as p ,p ) excitation. In
the second process, one of the non-bonding electrons
can be promoted into the vacant p-orbital giving
origin to n y p ) excitation. The rate constants for
Norrish type I reactions are much faster for n y p )
as compared to the p ,p ) excited states. However,
in aqueous solution, the n y p ) excitation mode
might have a lower rate constant due to either protonation or other weak but relevant interaction of the
carbonyl non-bonding electrons with water
95
molecules. This is not the only obstacle for CO
formation from carbonyl groups in aqueous systems.
Other factors such as the stability of the radicals
formed after the a-cleavage are pivotal in determining the pathway by which the acyl radical ŽScheme
1. will react. In condensed media such as seawater,
these radicals are kept in close proximity Žcage
effect. and recombination is likely to occur. Decarbonylation of the acyl radical following a-fission
occurs only if a very stable radical is formed Že.g.,
benzyl, t-alkyl. or if a different radical reaction leads
to a more stable molecule. The former assumption is
likely to be the case in seawater where a variety of
transient species can be present at the same time.
This pathway for CO production is depicted in
Scheme 1. Evidence supporting the formation of
these transient species proposed in Scheme 1 has
recently been demonstrated for natural waters ŽKieber
and Blough, 1990; Blough, 1990..
There could be fundamental differences in the
mechanisms that generate CO from the added carbonyl molecules. Acetylacetonate, for example, has a
strong absorption band at wavelengths above 300 nm
in seawater which make this compound a candidate
for direct photolysis via Norrish type I mechanism
with production of CO. Additionally, the ability of
this molecule to chelate metals is well known ŽPearson and Anderson, 1970.. Therefore, metal participation cannot be disregarded as a potential mechanism
in the photodecomposition process. Indeed, the addition of potassium acetylacetonate to distilled water
led to less CO photoproduction compared to that of
potassium acetylacetonate added to DOM-free seawater. A plausible explanation for these differences
is that transition metals present in seawater would be
complexed to a certain extent by this ligand. Absorption of light by the metal-complex formed could lead
Scheme 1.
96
W.H. Pos et al.r Marine Chemistry 62 (1998) 89–101
to a ligand-to-metal charge transfer ŽLMCT.. The
nature of the LMCT photochemical transition process involves promotion of an electron from an
orbital largely localized on the ligand to an orbital
which is essentially metal-localized, leading to the
metal reduction and formation of radical species
from the ligand ŽZika, 1982.. Due to the abundance
of electron-rich structures that can complex metal
ions such as carboxylates, enolates, carbonyls, phenolics, and amines ŽStevenson, 1992. and even structures like b-diketones ŽPiccolo and Stevenson, 1981.
present in the DOM structure, metal-mediated photodecomposition of DOM is likely to play an important role in the production of CO and OCS.
Increased photoproduction of CO upon addition
of a-ketoacids to seawater points toward photodecomposition via a sensitized reaction. This assumption is based on evidence that 10 m M of sodium
pyruvate or glyoxylate added to distilled water or
seawater did not lead to any significant increase in
absorption above 300 nm. Studies on direct photolysis of a-ketoacids in pure water have demonstrated
that the neutral ŽRCOOH. and not the anionic
ŽRCOOy. species are responsible for the pyruvate
absorption and decomposition ŽLeermakers and Vesley, 1963.. Because the majority of the biologically
relevant ketoacids have p K a values lower than four
ŽKortum et al., 1961., at seawater pH Ž7 to 8.2.,
a-ketoacids will be in the anionic form. There have
been several studies on the mechanism and products
of a-ketoacids photolysis. However, most of these
studies were conducted in organic solvents Žbenzene,
acetonitrile, methanol, etc.. or in mixtures of
waterrorganic solvent, and substantial differences in
mechanism and final products are possible in photolytic process occurring in these solvents compared
to those in pure water ŽSteenken et al., 1975; Davidson and Goodwin, 1980; von Sonntag and Schuchman, 1991; Budac and Wan, 1992.. Lower CO photoproduction upon glyoxylate addition compared to
that of pyruvate addition can be explained by the
equilibrium of hydrated and unhydrated glyoxylate
in seawater. At seawater pH, more than 90% of
glyoxylate is in the hydrated form ŽKieber, 1988., a
form that is not suitable to photogenerate CO. A
proposed mechanism for CO generation from pyruvate-like structure is depicted below ŽScheme 2.. All
the intermediary species ŽŽCO P2 ., acetyl and methyl
radicals. shown in Scheme 2 have been detected in
irradiated solutions containing either carbonyl mixtures, pyruvic acid or Suwannee fulvic acid ŽWeber
et al., 1996..
Recent studies on the photoproduction of methyl
iodide ŽCH 3 I. ŽMoore and Zafiriou, 1994. have
demonstrated that addition of pyruvate to seawater
followed by irradiation led to a substantial increase
in CH 3 I. The authors speculated that acetyl radical
ŽScheme 2. would be the key intermediary Žmethyl
donor. in CH 3 I formation. The fate of the carbonyl
group was not addressed, though it has been sug-
Scheme 2.
W.H. Pos et al.r Marine Chemistry 62 (1998) 89–101
gested that reaction with oxygen would be the predominant sink for these radicals Žvon Sonntag and
Schuchman, 1991. producing CO 2 . These results,
added together, support the speculation that acetyl
radical reactions play a important role in natural
water photoprocesses.
Despite the indication that the addition of carbonyl bearing molecules to seawater leads to increase in CO photoproduction, contribution from CO
ketoacids to photoproduction can be neglected in
natural water. This is due to the following: Ž1. the
concentration of both ketoacids used Ž10 m M range.
was at least one order of magnitude higher than the
values found in natural waters Žlow m M range.
ŽKieber, 1988., Ž2. the photoproduction rates upon
addition of fulvic acids were at least one order of
magnitude higher at the same carbon concentration
range and Ž3. relatively fast microbial uptake of
ketoacids occurs relative to the direct photolysis
rates ŽKieber, 1988.. Therefore, we can anticipate
that low molecular weight ketoacids would be an
insignificant source of carbon monoxide in seawater.
Redden Ž1983. came to a similar conclusion using
different carbonyl substrates Žacetone and acetaldehyde. to enhance CO photoproduction. It is important to keep in mind that the carbonyl compounds
used were only intended to simulate the behavior of
carbonyl functionality present in DOM.
The results from SFA samples are highly suggestive of the involvement of carbonyl constituents in
both CO and OCS photoproduction. CO photoproduction rates upon light exposure of untreated SFA
and SFA that was treated with NaBH 4 are illustrated
in Fig. 4a and b. NaBH 4 is a mild reductant that will
preferentially reduce carbonyl groups Žaldehydes, ketones and quinones. to alcohols. It has been successfully used in several studies of carbonyl group reactivity in humic materials ŽWeber et al., 1996; Thorn
et al., 1996.. The decrease in the UV–vis absorbance
spectra of DOM samples ŽSFA and AH. treated with
NaBH 4 could be a indication that the carbonyl available for reduction plays only a minor role in the
chromophoric character of the organic material used.
But other interpretations are possible, such as the
low extinction coefficient of the lone carbonyl structure, etc. Regardless of the structural modifications,
it is clear that treatment of SFA with NaBH 4 leads
to decrease in CO production. Lower but constant
97
CO production for irradiation after 30 min was also
observed in Aldrich humics under the same conditions. Prolonged exposure of both samples ŽSFA and
AH. to light led to an increase in the photoproduction rates as can be seen in Fig. 4a and b. This
behavior possibly indicates that re-oxidation of the
alcohol functions Žlikely hydroquinone systems. can
mediate radical reactions regenerating carbonyl
groups. Therefore, a decrease in CO production upon
treatment with NaBH 4 , and a similar decrease upon
addition of reduced sulfur, is an indirect but powerful indication of a carbonyl mediated photoprocess in
CO and OCS production.
A plausible mechanism for the involvement of
sulfur in the photoproduction of OCSrCO is more
complex, but sulfur radicals Žthiyl, RSP; and
sulfhydryl, HSP. are potential candidates in this process. The results of experiments on BBSW amended
with reduced sulfur Žcysteine or bisulfide. indicate
that reactive sulfur species are generated upon irradiation of the solutions. The photoproduction of CO
and OCS in DOM-free seawater upon addition of
acetylacetonate and bisulfide is direct evidence of a
common pathway for both OCS and CO. Considering that the only light absorbing species in this
system is acetylacetonate, it is reasonable to assume
that acetylacetonate generated radicals Žacetyl or
other carbon-centered radical. react with the reduced
sulfur compound to produce sulfur-centered radicals
Žthiyl. which in turn react with the carbonyl group
producing OCS. By analogy, DOM generated radicals can follow a similar pathway and also generate
CO and OCS. The reaction of bisulfide with CeŽIV.
in the presence of a carbonyl source Žacetylacetonate. makes apparent the participation of a sulfur
radical in the OCS mechanism ŽFig. 5.. Electron spin
resonance ŽESR. of the trapped thiyl radical generated by this redox process has been well characterized ŽGraceffa, 1983.. The driving force behind this
reaction is the redox potential of the constituents
involved, CeŽIV.rCeŽIII. Ž E 0 ; 1.45 V. and
HSyrHS Ž E 0 ; y0.9 V. ŽSurdhar and Armstrong,
1986, 1987. that leads to the generation of sulfhydryl
radical. The radical generated can propagate a series
of free radical reactions involving acetylacetonate
that, in turn, generates transient species such as
acetyl and oxoacetyl radicals. The high yield of OCS
in this reaction ŽFig. 5b. may be a direct conse-
98
W.H. Pos et al.r Marine Chemistry 62 (1998) 89–101
quence of the high concentration of thiyl radical
generated almost instantaneously in the redox process. Although this is indirect evidence for the involvement of a sulfur-centered radical in the production of OCS, this experiment is the first attempt to
demonstrate the radical character of the OCS production mechanism in natural waters. This experiment
demonstrates that, while light can efficiently generate free radicals in natural waters, any other process
that generates radicals is also a potential source of
OCS. Examples of non-photochemical processes capable of free radical formation are metal–redox processes, similar to these described above, and enzymatic processes ŽPos et al., 1998 manuscript in
preparation.. Interestingly, Cutter and RadfordKnoery Ž1993. observed high OCS production just
below the oxicranoxic layer in the Pettaquamscutt
River Estuary and in the sediments of Chesapeake
Bay. These authors speculated that mechanisms other
than photochemistry might be responsible for such a
high OCS production. These environments typically
have high bacterial density and low oxygen levels
ŽBarry Taylor, personal communication. that favor
high concentrations of reduced sulfur compounds
ŽLuther and Church, 1992..
A great deal of work has recently been reported
on the role of thiols as radical scavengers in biologi-
cal processes ŽDunster and Willson, 1993.. In these
systems, thiyl radicals can be formed by either transferring an electron to an oxidizing Žradical or metal.
species ŽReaction 1. or by donating a hydrogen atom
to more reactive radicals, often a carbon-centered
radical ŽReaction 2. ŽWardman and Sonntag, 1995..
Since C–H bonds Ž99 kcal moly1 . are generally
stronger than S–H bonds Ž; 89 kcal moly1 ., equilibrium in Reaction 2 are usually shifted to the right
side of the reaction and high yields of thiyl radicals
are expected ŽZhao et al., 1994..
RY Pq RSy™ RYyq RSP Ž Reaction 1 .
RXPq RSH ™ RX H q RSP Ž Reaction 2 .
Thiyl radical formation from cysteine and bisulfide and many other organic thiols in aqueous solution has been extensively reported ŽHuston et al.,
1992a,b; Zhao et al., 1994.. Addition reactions of
thiyl radicals to carbonyl groups have also been
demonstrated ŽPetrova and Gasanov, 1981.. More
interestingly, formation of OCS from molecules containing thiocarbonyl groups Žalso called Barton–McCombie reaction. have been shown to proceed via a
radical chain mechanism ŽCrich and Quintero, 1989..
Based on the literature and experimental results presented, a pathway that generates CO and OCS and
Scheme 3.
W.H. Pos et al.r Marine Chemistry 62 (1998) 89–101
also explains the competitive behavior seen for both
CO and OCS is shown in Scheme 3.
The photochemical generation of thiyl radical in
natural surface waters is an indirect process since
most of the reduced sulfur compounds available in
aquatic environments Žglutathione, methanethiol,
bisulfide, cysteine etc.., do not absorb light above
. carbonate radical
300 nm. Dibromine radical ŽBryP
2
ŽCO 32yP. and carboxyl radical ŽCOyP
. Ž PRY in Scheme
2
3. are possibly the most important radicals that act to
generate thiyl radical in seawater ŽZepp and Andreae, 1995.. These reactive transients have been
observed in a set of well documented reactions via
electron transfer with hydroxyl Ž POH. radical in
aqueous systems ŽWardman and Sonntag, 1995;
Armstrong, 1990.. In natural waters DOM has been
implicated in the photoprocess that lead to the formation of these transient radicals ŽKieber and Blough,
1990; Blough, 1990; Blough and Zepp, 1995..
If sulfur-centered radicals are indeed the intermediaries of OCS production, as we have indirectly
demonstrated, any process that generates thiyl species
should be a potential source of OCS if a carbonyl
group is available. Furthermore, if the anti-correlation between CO and OCS is a real feature in natural
photoprocesses, it is most likely to be apparent in
regions of high biological productivity, where biogenic reduced sulfur would be widespread. Finally,
the fast disappearance of both bisulfide Ž t 1r2 s 41
min. ŽPos et al., 1998. and cysteine Ž t 1r2 s 63 min.
in BBSW upon irradiation ŽFig. 2. and a mass
balance on sulfide consumed and OCS produced
demonstrated that a minor Ž- 5%. percentage of the
reduced sulfur added is converted to OCS. A possible explanation for this behavior can be found in the
complex set of competing reactions involving oxygen, hydroxyl radicals, and other free radicals that
could scavenge the sulfur-centered radicals formed
in the process ŽDunster and Willson, 1993; Sonntag,
1993..
5. Conclusion
Evidence has been presented supporting a coupled
pathway involving CO and OCS photoproduction.
Carbon monoxide photoproduction from fulvic and
99
humic material and carbonyl bearing organic compounds points to a carbonyl moiety present in large
molecular weight structures of DOM rather than in
low molecular weight carbonyl compounds as being
responsible for the CO photoproduction pattern in
natural waters. The inverse covariance of CO and
OCS production upon addition of reduced sulfur
compounds points toward a common intermediary in
the photoprocess. We presented indirect evidence
that the key sulfur intermediary is a sulfur-centered
radical Žthiyl or sulfhydryl. generated in seawater in
a similar manner as it is generated in biological
systems.
Acknowledgements
We would like to thank Cindy Moore for her help
with TOC measurements, Dr. Peter Milne for helpful
suggestion during early stage of this work and Dr.
Mike Perdue ŽGeorgia Tech. for sample of Suwannee fulvic material. Thanks are also due to Dr. Paul
Wine ŽGeorgia Tech. for helpful discussion and partial support to W. H. Pos through grant from NSF
ŽAC897645. and to Neil Blough for providing us
with a copy of a manuscript prior to publication
ŽBlough, 1997.. This work was supported by a grant
from the US Office of Naval Research ŽN000149510207..
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