A new method is employed to determine the rates of photochemical hydroxyl radical (OH) formation in aqueous solutions and in natural waters under both aerobic and anaerobic conditions. Quantum yields for OH formation from the photolysis of nitrate and nitrite obtained by this method are in good agreement with previous measurements. Photolysis of Suwannee River fulvic acid (SRFA) solutions produced the hydroxyl radical under anaerobic conditions in proportion to the SRFA concentration. Under aerobic conditions, the quantum yields for OH formation were slightly higher and exhibited a different wavelength dependence than those obtained under anaerobic conditions. Experiments employing catalase indicate that Fenton chemistry can account for at most 50% of the total signal under aerobic conditions for SRFA irradiated at 310 and 320 nm. These results indicate the presence of a dioxygen-independent pathway of hydroxyl radical production that cannot be assigned to nitrate/nitrite photolysis or to Fenton chemistry. Results from the preliminary application of this method to natural waters are also presented.
Photochemical reactions involving colored dissolved organic matter (CDOM) in natural waters are important determinants of nutrient cycling, trace gas production and control of light penetration into the water column. In this study the role of the hydroxyl radical ( ∑ OH) in CDOM photodegradation was explored as well as the contribution of photo-Fenton chemistry to ∑ OH formation. Photochemically produced ∑ OH was observed under aerobic and dioxygen-depleted conditions in highly colored, acidic natural water samples obtained from a freshwater reach of the Satilla River, a river in the southeastern United States. Net aerobic ∑ OH formation along with the production of hydrogen peroxide and Fe(II) provided evidence of photo-Fenton produced ∑ OH. A reduction in ∑ OH production in the presence of iron chelators further suggests the Aquat. Sci. 65 (2003) 402-414 Aquatic Sciences importance of iron and the photo-Fenton reaction in this water. Apparent quantum yield values for the photochemical production of ∑ OH were determined from 300-320 nm. In addition, the relationship between ∑ OH photoproduction and effects of irradiation on the optical properties of CDOM was examined. Changes in the light absorption and fluorescence properties of water samples from the Satilla River and other natural waters were compared to ∑ OH production rates. The ability of constituents of Satilla River water, principally the dissolved organic matter, to scavenge ∑ OH was also considered. Results indicate that the photoFenton reaction accounts for more than 70% of total photochemical ∑ OH production in Satilla River water. Given the significant levels of ∑ OH produced in this water, it is possible that ∑ OH influences CDOM photobleaching.
Photolysis of 2-methyl-1,4-benzoquinone (toluquinone) in aqueous solution results in the oxidation of water to create either hydroxyl radical or some species capable of transferring a hydroxyl radical. Trapping of the latter with dimethyl sulfoxide (DMSO) creates a methyl radical which in turn can be trapped by the stable radical 3-amino-2,2,5,5-tetramethyl-1-pyrrolidinyloxy. Competitive trapping studies using DMSO and either nitrite anion or salicylate anion show that the hydroxylating species is much more selective in its reactions than free hydroxyl radical. Laser flash photolysis experiments on toluquinone in aqueous solution show formation of a transient species immediately (<150 ns) following the excitation pulse that had previously been assigned to the excited triplet state of the quinone. This spectrum differs from the authentic triplet state spectrum generated in less reactive organic solvents (carbon tetrachloride and acetonitrile). The same intermediate is shown to react with the hydroxyl radical traps, cupric ions, and benzoate anion to yield the semiquinone radical. On the basis of these experiments it is argued that this transient species is a hydroxylating intermediate, probably best described as a complex between the semiquinone radical and the hydroxyl radical. It is further argued that this species is responsible for the hydroxyl radical trapping reactions.
Chemical trapping studies combined with optical and electron paramagnetic resonance measurements were employed to examine the mechanisms of the aqueous photochemistry of methyl-benzoquinone (mBQ) at both low and high quinone concentrations. At low [mBQ], dimethylsulfoxide (DMSO) reacted with a photogenerated intermediate to form a methyl radical, but methane did not, thereby unequivocally excluding the hydroxyl radical. DMSO at concentrations between 50 mM and 2 M completely suppressed the formation of the hydroxylated quinone, while only slowing the formation of the hydroquinone, suggesting reaction with either the triplet state or an intermediate arising from the triplet. Addition of Cl-, a putative physical quencher of the triplet, inhibited the DMSO reaction both noncompetitively and competitively in a fashion similar to that observed previously with nitrite, formate, and salicylic acid, thus providing further evidence for a reactive intermediate distinct from the triplet. This intermediate is attributed to a water-quinone exciplex. The relative yield of the methyl radical from the DMSO reaction decreased with increasing [mBQ], suggesting that at high concentrations, a bimolecular reaction of the triplet with the ground-state quinone outcompetes the formation of the quinone-water exciplex.
To determine effects of photochemical weathering of petroleum, surrogate and Macondo (MC252) crude oils were exposed to solar radiation during the formation of Water Accommodated Fractions (WAFs) in sterile seawater. Samples were incubated in either unfiltered sunlight, with ultraviolet radiation blocked (Photosynthetically Active Radiation [PAR] only), or in darkness. WAFs were collected at two time points over the course of a week. Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) analyses of water soluble species formed during exposure to sunlight were compared for the different treatments. Photochemical alterations resulted in differences in compound class distributions. In general, surrogate oil was photo-oxidized across a wider carbon number range compared to MC252. While photochemical differences were observed between MC252 and surrogate oils, microbial production in seawater responded similarly to both WAFs from both types of oils with the majority of the inhibition resulting from oil exposure to visible light.
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