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Photochemical Generation of Reactive Species in Seawater: Analyses,
Kinetic Considerations, and Environmental Implications.

㸦ᾏỈ୰࡛ࡢάᛶ໬Ꮫ✀ࡢග໬Ꮫⓗ⏕ᡂ̿໬Ꮫศᯒࠊ཯ᛂゎᯒࠊ⎔ቃ⛉Ꮫⓗព⩏㸧                   ᗈᓥ኱Ꮫ኱Ꮫ㝔⏕≀ᅪ⛉Ꮫ◊✲⛉                   ⎔ቃᚠ⎔⣔ไᚚᏛᑓᨷ                   Ꮫ⏕␒ྕ 㹂151921                   Ặ  ྡ ADESINA, ADENIYI OLUFEMI
Abstract
Perspectives on photochemically generated reactive species in the environment, with particularemphasis on the hydrosphere was presented in Chapter 1. The role of solar radiation to provide
needed light energy to drive many photochemical reactions involving; geogenic, biogenic andanthropogenic chemicals from the atmosphere and land, found in natural surface waters, wasemphasized. A concise definition of reactive species, and discussion of major pathways to theirphotogeneration were also presented in this chapter. Environmental implications ofphotogenerated reactive species such as; redox reaction involving ions, dissolved oxygen recycling,generation of secondary reactive species, and photo-degradation of dissolved organic pollutants innatural waters, amongst others were highlighted. Potential technological applications ofphotochemical generation of reactive species in gas-phase photo-oxidation and photomicrobicidalsystems were also mentioned.
Chapter 2 reports about concurrent photo-generation and analyses of hydroxyl (·OH), nitric oxide(NO·) and superoxide (O2·−) radicals in seawater samples obtained during two consecutive

summers from the Seto Inland Sea, Japan. These were done with the sole aim of using the data
generated to predict the influence of interactive reactions among several photo-generated reactivespecies on the variety and fates of reactive species in seawater. Photo-generation rates of ·OH are[(6.98–35.27) × 10−12 M s−1], and those of NO· are [(1.20–58.25) × 10−12 M s−1]. Compared withthese generation rates, that for O2·− [(4.54–18.20) × 10−10 M s−1] was the highest, which suggeststhat O2·− is a very important photochemically generated reactive species in coastal seawater. The
average steady-state concentrations of the three reactive species are ·OH (7.23 × 10 −18 M), O2·−(3.79 × 10−12 M), and NO· (1.39 × 10−10 M). Based on kinetic considerations, estimated mutual
consumption/sink via interactive reactions between O2·− and NO· radicals are five to nine ordersof magnitude higher than any other radical pair considered in this study. Hence, the radical–radicalreaction between photochemical O2·− and NO· to form peroxynitrite (ONOO-) was predicted todominate above those involving; ·OH/ O2·− and ·OH/ NO· radical pairs.
A novel fluorescence analytical method developed to measure ONOO- in seawater was reported inChapter 3. Although there are existing methods for determination of other reactive species suchas; O2·− and NO· radicals (precursors of ONOO- in seawater), there was no analytical method formeasurement of ONOO- in any natural surface water system. This newly developed analyticalmethod is based on using β-(2-oxo-2H-1-benzopyran-7-yl)-boronic acid (CBA); a boronate probe
which reacts rapidly and stoichiometrically with ONOO- to yield fluorescent 7-hydroxycoumarin(COH). COH generation rate during irradiation of seawater with CBA probe was determined byreversed-phase isocratic high performance liquid chromatography (HPLC) with fluorescence
detection at excitation and emission wavelengths of 332 nm and 475 nm, respectively. COH
standards calibration plot is linear in concentration range of (0.25–100 nM) in seawater. The
detection limit, defined as 3σ (3 × standard deviation) of the lowest ONOO- photoformation rates

measured within 10 minutes’ irradiation, was 1.7 × 10-12 M s-1. This method is quite precise as
coefficients of variance are; 5.2%, 3.3% and 2.2%, for triplicate measurements of COH photoformation rates obtained by using 1 μM, 3 μM and 5 μM CBA probe concentrations, respectively.
Experimental procedures were optimized to handle potential interferents in measurements. To date,
this is the first report on the successful detection and measurement of photochemically generated
ONOO- in any natural water system. This modest achievement has, at least, established aqualitative empirical evidence in support of a classic example of interactive radical–radicalreaction involving photochemical O2·− and NO· in the marine environment.
Chapter 4 reports on the application of this novel analytical method to measure photogenerated
ONOO- in 13 surface seawater samples from the Seto Inland Sea, Japan. Existing analyticalmethods were used to concurrently measure photochemically generated NO· and O2·−, as well as,NO2¯, DOC, CDOM and pH in these samples. The average ONOO- photoformation rate, steadystate concentration and lifetime in those samples were; (9.0 × 10 -10 M s-1), (2.26 × 10-12 M), and(0.07 secs), respectively. The highest photochemical generation rates and steady-state
concentrations of ONOO-; (28.74 × 10-10 M s-1) and (6.11 × 10-12 M), respectively were measured
in Osaka Bay. On the other hand, the Kii channel region of the Seto Inland Sea has one of thelowest ONOO- photo-formation rates (2.4 × 10-10 M s-1), and steady-state concentrations (1.58 ×10-12 M). Comparative evaluation of the three reactive species’ data suggests that existence ofdetectable photochemically generated ONOO- in seawater depends to a large extent on, and is in
the limit of, photoformed NO‧ radical. There is good correlation between chemical stability ofphotoformed ONOO- and alkalinity of seawater. Correlation coefficients of salinity versus
photoformation rates and lifetimes of ONOO- are; (r = -0.455; P > 0.05) and (r = 0.403; P > 0.05),
respectively. These values suggest that seawater salinity or ionic strength has no effect on thephotoformation rate and lifetime of ONOO- in seawater. Consideration of both empirical evidences
and kinetic estimates reveal that less than 10% of photoformed O2.- and NO‧ radicals are involvedin ONOO- photoformation, suggesting that interactive radical-radical reaction between O2.- andNO‧ radicals is not a major sink of photoformed NO‧ radical in seawater. Due to the relatively
longer lifetime, lower reactivity and lower aqueous solubility of NO‧ radical, these results lend
credence to the sea–to–air NO‧ efflux phenomenon, which has been predicted as a major sink ofphotoformed NO‧ radical in seawater. Hence, this evidence–based inference on bulk diffusion ofNO‧ radical from seawater can be proposed as a non-anthropogenic contributor to daytime
atmospheric NOx concentrations, and this suggests that dynamics of atmosphere–hydrosphereinteractions do have significant environmental and health implications.
In Chapter 5, it was reported that catechin–glycerol impregnated filters illuminated by a 350
lumens LED light source can be used to photogenerate O2·− at a rate of 4.35 × 10-11 M s-1cm-2, inthe gas-phase with an oxygen gas stream maintained at a flow rate of 1 L min -1 for 1 hour. The
fact that the experimental conditions for this photogeneration are relatively inexpensive and easy
to achieve suggest that this system can be further optimized for technological applications wheremilder, biologically tolerant photo-oxidation or photomicrobicidal disinfection might be required.
Finally, Chapter 6 is a discussion of major findings, conclusions and recommendations for futurestudies. The chapter highlights the fact that successful detection and measurement of ONOO- byusing the novel boronate-based fluorescence analytical method validates the prediction thatphotogenerated NO· and O2·− would react together to yield ONOO- in the seawater milieu.
Consideration of kinetic estimates with experimental values obtained in this study, led to theconclusion that photoformation of ONOO- is not a major sink for both photochemical NO· andO2·− in seawater. However, the current study lends credence to earlier predictions, and recentfindings that bulk diffusion of NO· from sea to atmosphere would be a major sink of photoformed
NO· in seawater. Therefore, photoformed NO‧ radical in surface waters is being proposed as a
non-anthropogenic contributor to daytime atmospheric NOx concentrations, and this illustrates thesignificance of atmosphere–hydrosphere interaction dynamics on human health and theenvironment. Earlier investigators have predicted that reaction of ONOO- with CO2 can generate
up to 10 – 15% of total carbonate radicals (CO3– ‧) in seawater. Thus, ONOO- photogeneration canenhance to pollutant degradation by CO3– ‧ in sunlit seawater because the latter can react withelectron-rich substrates such as anilines, phenols and organic sulfur compounds which areubiquitous in natural surface water. Therefore, recommendation for further studies include;
concurrent measurement of photoformed O2·−, NO·, ONOO- and CO3‧–, in addition, to otherrelevant seawater parameters. This future research effort is expected to account for the fate androles of photochemical ONOO- sunlit seawater. Optimization of the newly developed gas-phaseO2·− generation system, and its deployment in relevant technological applications was alsorecommended.

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