Irradiation of natural water samples

Vione D, Falletti G, Maurino V, Minero C, Pelizzetti E, Malandrino M, Ajassa R, Olariu R-I, Arsene C (2006) Sources and sinks of hydroxyl radicals upon irradiation of natural water samples. Environ Sci Technol 40 3775-3781... 

Photoreductiom in Aqueous Solution. Solvated electrons form on the irradiation of natural water samples or of aqueous solutions of natural organic matter isolated from surface waters (12, 45, 46). The solvated electron is a powerful reductant that reacts rapidly with electronegative substances such as chlorinated, brominated, and iodinated compounds. The interaction of pho-toejected electrons and a halocarbon is demonstrated by the laser flash photolysis data shown in Figure 3. 

When an aqueous solution of aldrin (0.07 M) in natural water samples collected from California and Hawaii were irradiated (7, <220 nm) for 36 h, 25% was photooxidized to dieldrin. By comparison, no loss was reported when aldrin in deionized water was subject to UV light for 10 h. 

Photolytic. When an aqueous solution of p,//-DDE (0.004 pM) in natural water samples from California and Hawaii were irradiated (maximum X = 240 nm) for 120 h, 62% was photooxidized to jD,p -dichlorobenzophenone (Ross and Crosby, 1985). In an air-saturated distilled water medium irradiated with monochromic light (>. = 313 nm), p,//-DDE degraded to jo,//-dichloro-benzophenone, l,l-bis(p-chlorophenyl)-2-chloroethylene (DDMU), and l-(4-chlorophenyl)-l-(2,4-dichlorophenyl)-2-chloroethylene (o-chloro DDMU). Identical photoproducts were also observed using tap water containing Mississippi River sediments (Miller and Zepp, 1979). The photolysis half-life under sunlight irradiation was reported to be 1.5 d (Mansour et al., 1989). 

Photochemical operations offer several routes of hydroxyl radical formation by UV irradiation. The formation of hydroxyl radicals by irradiation of samples doped with hydrogen peroxide or ozone is the state-of-the-art in water treatment. Two comprehensive reviews cover the historical development of the UV photo-oxidation technique as a pretreatment step in the inorganic analysis of natural waters, its principles and the equipment available, and its principal applications in the analytical field.3,4 They include tables summarizing the elements determined, the analytical techniques used, and the sample matrices studied. 

Acifluorfen (5-(2-chloro-o , a, a ,-trifluoro-p-tolyl)-2-nitrobenzoic acid) (IV) mainly undergoes photodecarboxylation upon irradiation at 254 or 313 nm. This compound was exposed to solar light, first in the presence of humic acids (lOmglr1), and then dissolved in a natural water sampled in an artificial lake [68]. No effect of humic acids was observed. The lack of a photo-inductive effect of humic substances can be explained by the fact that acifluorfen bears electron-withdrawing substituents rendering it poorly oxi-dizable by reactive triplet states. In contrast, the half-life of IV was found to decrease by a third in the natural water. This photo-inductive effect might be due to the photogeneration of hydroxyl radicals by chromophores other than humic components of DOM nitrate and nitrite ions are probable candidates. 

Analysis for DDE, y-l,2,3,4,5,6-hexachlorocyclohexane (lindane), and p,p -dichlorobenzophenone in natural water samples, solutions of humic substances, or suspensions of soils and sediments was conducted by first adding acetonitrile (25% by volume) to the irradiated solution or suspension, followed by the addition of an equal volume of isooctane. The resulting mixture was briefly agitated and then sonicated for 15 min. The isooctane layer was analyzed by gas chromatography with a Hewlett-Packard model 5890 gas chro-... 

Given other evidence that hydrophobic organic compounds such as DDE sorbed to the NOM aggregates are in a hydrocarbon microenvironmenl (30-34), the results in Table II may be attributable to a solvent effect oi direct photoreaction. This hypothesis is supported by the finding (eq 12) tha only direct photoreaction products were observed in organic extracts of thi irradiated natural water samples. 

Photoreductions of Sorbed Halocarbons. Comparisons of computed rates of halocarbon photoreduction by eaq with observed rates in natural water samples indicate that other reaction pathways are more important. For example, recent results obtained with continuous irradiations indicate that chlorinated acetates produce chloride more efficiently than chloroethanol in solutions of dissolved organic matter that was isolated from the Suwannee River. Observed quantum yields (355 nm) for chloride production at pH 6.2 in aque-... 

Superoxide is unstable in aqueous solution. Disproportionation occurs according to equation (2) and is a source of hydrogen peroxide which functions along with uv irradiation in a standard analytical method for the destruction of organic matter in natural water samples. 

A procedure for the analysis of natural iodine in aqueous solutions of iodohippuric acid (57) has utilized a freezing technique for the preparation of the sample for irradiation. Irradiation of a frozen sample rather than a solution has a number of advantages, the pressure in irradiation containers caused by the radiolysis of water is reduced substantially losses of iodine due to vaporization and also to adsorption on container walls are greatly minimized and interference from nuclides from container walls is avoided. 

Apart from acidification, UV irradiation is the only preliminary preparative step in voltammetric determinations it may be required to degrade organic substances binding trace metals in the form of inert complex species. The analysis of samples from natural waters may be significantly affected by the binding capacity of such dissolved organic substances. 

There have been reports of hidden arsenic species in natural waters. These are organic species that do not form arsine gas with NaBH4 and were, therefore, undetected in early speciation studies. Some, though not all, such arsenic species are detected after UV irradiation of samples (Hasegawa et al., 1999 National Research Council, 1999). 

However, these photochemical reactions may not be very important. Butler and Smith (10) noted that intense UV irradiation of seawater with excess peroxide added to the sample resulted in IOa formation. However, the reaction was retarded in the absence of added peroxide, and iodate also reacted with natural organic matter. Richardson et al. (26) noted that BrOa formation was retarded in natural estuarine waters during 03 oxidation of Br". 

Recently, Thomas-Smith and Blough [73] determined that quantum yields for the production of the aqueous solvated electron in irradiated coastal DOM samples were too low to account for the production of 02 (as determined from the yield of H2O2), suggesting that reaction (3) is the main source of 02 in natural waters. 

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