Halocarbon photoreactions

The rate of any type of halocarbon photoreaction at a certain wavelength X, rx, depends upon the rate of light absorption by the photoreactive chro-mophore, Iak, and the quantum efficiency of the reaction, < >r (eq 1). 

KzX is usually expressed per meter and depends on wavelength and water composition. Typically, ultraviolet radiation penetrates less deeply than visible radiation. Attenuation coefficients vary over many orders of magnitude in natural waters, with the highest values (least light penetration) in inland water bodies and the lowest values (highest penetration) in open seawater. The photic zone for solar ultraviolet radiation, which is very important for halocarbon photoreactions, ranges from tens of meters in the open ocean and clear lakes to only a few centimeters in some inland wetlands. The spectral properties of water bodies are linked to water composition. Baker and Smith (20) developed algorithms that relate K,k to certain parameters such as chlorophyll a concentrations. 

Types of Halocarbon Photoreactions. In the following sections, we discuss several types of halocarbon photoreactions that provide sinks in aquatic environments. Emphasis in this chapter is placed on photoreactions that result in dehalogenation, in which halide ions (X ) are produced from halocarbons (RX). Here the term direct photoreactions indicates reactions that involve direct light absorption by the halocarbon itself (eq 4). 

Negligible photoreaction was observed for p,p -dichlorobenzophenone (DCB), a DDT oxidation product, in air-saturated, distilled water (half-fife >15 h at 313 nm). Nevertheless, this halocarbon photoreacted (313 nm) with half-lives corrected for light attenuation of about 3 h in a filtered natural-water sample and a solution of Contech fulvic acid (Table II). The greater than four- to fivefold enhancement in photoreaction rate in this case probably results from hydrogen atom abstraction from the natural organic matter by the DCB in its excited triplet state (eq 14). 

Indirect Photoreactions. Enhancements of halocarbon photoreaction rates are not limited to compounds such as DDE and DCB, which directly absorb solar radiation. Such effects have also been observed with hah ocarbons that have little or no absorption of solar radiation. These reactions most likely occur through indirect mechanisms, in which the radiation is absorbed by natural chromophores. In this section, we provide evidence for such indirect photoreactions. Then, in subsequent sections, we discuss possibk mechanisms for indirect photoreactions of halocarbons. 

Natural substances, especially natural organic matter, have important effects on halocarbon photoreactions in the environment. These effects include the initiation of indirect photoreductions of dissolved or sorbed halocarbons via the intermediacy of solvated electrons or excited states. Evidence is presented here that sorption enhances the quantum efficiencies of these indirect photoreactions, although more studies are required to better define these processes. 

Zepp, R. G., and L. F. Ritmiller, Photoreactions providing sinks and sources of halocarbons in aquatic environments . In Aquatic Chemistry - Inerfacial and Interspecies Processes, C. P. Huang, C. R. O Melia and J. J. Morgan, Eds., American Chemical Society, Washington, DC, 1995, pp. 253-278. 

The long lifetimes of CT excited states of the Pt(diimine)(dithiolate) complexes allow for bimolecular photochemistry, often involving oxidation of the complex. The earliest report of photoreactivity of these complexes dealt with the photooxidation of Pt(bpy)(tdt) (20) following excitation at 577 nm in chloroform (118). The reaction proceeds with a quantum yield of < ) = 0.03 and was attributed to ET to the halocarbon solvent (Eq. 8) similar to the CTTS photooxidation chemistry of the platinum bis(dithiolate) dianions described above. 

Finally, mechanisms of secondary thermal reactions involving the ferricenium produced in the primary photoreaction of ferrocene in ethanol/halocarbon solvents have also been investigated (155-157). 

Photoreactions Providing Sinks and Sources of Halocarbons in Aquatic Environments... 

Photochemical reductions and oxidations in aquatic environments provide sinks or sources for halocarbons. Such photoreactions are an important process in the dissipation of low-volatility halocarbons, such as halogenated agrochemicals, in aquatic environments (reference 9 contains lead references). For example, field studies of Crossland and Wolff (10) demonstrated the rapid dissipation of pentachlorophenol residues by its photoreaction in English ponds. Evidence emerged that volatile halocarbons such as 1,1,1-trichloroethane (methylchloroform) may have significant sinks in the aquatic environment... 

This chapter reviews past studies of other investigators and presents previously unpublished laboratory studies relating to photochemical reactions of halocarbons in aquatic environments. Kinetics considerations relevant to the modeling of halocarbons in natural waters are briefly considered. Then direct and indirect photoreactions that provide sinks for halocarbons are examined with consideration of the effects of sorption onto dissolved NOM and suspended sediments. Photoreductive dehalogenation is emphasized. Finally, the role of photochemically produced reactive oxygen species in the production of halocarbons in the aquatic environment is discussed. The main emphasis is on field and laboratory studies that provide predictive capability and a mechanistic understanding of the processes. 

Procedures. Continuous irradiations of halocarbon solutions were conducted with monochromatic radiation in a merry-go-round apparatus (12) or in a Schoeffel reaction chemistry system. Reactions were followed through analysis for remaining halocarbon or analysis of chloride ions produced by the photoreactions. Dark controls were used in all cases to correct for thermal production of chloride ions. Ferrioxalate actinometers were used to determine the irradiance (16). The irradiance at the photoreaction cell surface was typically about 10 nanoeinsteins/cm2,s. The Fe(II) concentrations were determined by using a modified version of the ferrozine procedure described by Stookey (17). Electronic absorption spectra were obtained by using a Shi-madzu model 265 spectrophotometer. 

U represents the scalar irradiance, ex is the molar absorptivity (or molar extinction coefficient) of the chromophore, l is the light path length, and [P] is the concentration of the chromophore that initiates the photoreaction (e.g., the halocarbon itself, a natural substance, or a complex of the halocarbon with a natural substance). The rate of light absorption depends on the spectral overlap between the light source and the spectrum of the chromophore that initiates the photorcaction. 

Direct photoreactions are mediated by halocarbon excited state(s) that react to form products. Dehalogenations can involve either photohydrolysis (i.e., photonucleophilic replacement of halogen by OH) or homolysis of the carbon-halogen bond to form free radicals. Photohydrolysis is the most important dehalogenation pathway for aromatic halocarbons. 

Indirect photoreactions of halocarbons involve reactions with reactive transients that are produced on absorption of light by natural substances, with NOM playing a key role as the source of the transients. These transients include reductants such as solvated electrons (eaq ) (eq 5). 

Complexation of halocarbons with natural substances can enhance the rates of photoreactions that provide sinks. Ionizable halocarbons, such as hal-ogenated organic carboxylic acids, potentially could form complexes with pho-toreactive transition metals, such as iron. In addition, dissolved NOM and sediments are known to sorb or bind ionic and nonionic halocarbons, and sorbed halocarbons may photoreact more efficiently (eq 7). 

Evidence is presented here that sorption to NOM enhances the photoreaction rates of halocarbons. Possible enhancement mechanisms are photoinduced electron transfer involving photoejected electrons or NOM excited states and/or formation of photoreactive complexes with NOM-associated electron donors, such as nitrogen bases. 

The overall photoreaction rate of a given halocarbon in a certain aquatic environment is the sum of the rates of the direct photoreactions of the uncom-plexed halocarbon, indirect photoreactions involving reactive transients that are produced by natural substances, and photoreactions of halocarbon complexes. After first discussing the effects of sorption on photoreaction kinetics, we then discuss these various reaction pathways in more detail. 

Sorption of hydrophobic halocarbons onto suspended sediments, biota, or NOM can have complex effects on photoreaction rates and quantum efficiencies. Hydrophobic or ionic halocarbons, with their great tendency to sorb on sediments or NOM, are most likely to be affected by heterogeneous photoreactions. A flurry of publications (e.g., 30-34 and references cited therein) provided abundant experimental evidence that extremely hydrophobic pollutants (e.g., polycyclic aromatic hydrocarbons, DDT, and mirex) have a strong tendency to associate with the particulate and dissolved organic matter in water bodies. 

Scheme 1. Conceptual model for direct and indirect photoreactions in heterogeneous systems. Symbols P represents the photoreactive halocarbon in solution, R-P represents halocarbon sorbed in reactive components of the sorbent, and U-P represents the halocarbon sorbed in unreactive components of the sorbent. Sorbents are sediments, natural organic matter, or biota. The types of reaction are direct with light absorption by P and indirect with light absorption...

Direct photoreaction (eq 4) is important only for halocarbons (e.g., aromatic compounds) that significantly absorb radiation at wavelengths >295 nm, the cutoff for solar spectral irradiance at the earth s surface. Because saturated chlorinated and fluorinated organic compounds, including methylchloroform and chlorofluorocarbons, absorb solar radiation very weakly, their direct photoreaction is very slow in the sea and in fresh waters. As discussed in a later section, photoreactions of these compounds may be accelerated by sorption and indirect photoreactions in natural waters. Saturated and olefinic polv-brominated and iodinated organic compounds have long absorption tails that extend beyond 295 nm. Direct photoreaction of such compounds in aquatic environments may be significant. 

Although Zika and co-workers (36) investigated the direct photoreaction of methyl iodide, there are few other environmentally relevant studies of direct photoreactions of saturated halocarbons in water. Kropp (37), however, re-... 

Table I. Direct Photoreaction Rates of Selected Halocarbons in Sunlight...

Halocarbons Sorbed on Natural Organic Matter, Photoreactions of aromatic halocarbons that strongly absorb solar radiation can be greatly accelerated in natural water samples or in aqueous solutions of NOM or humic substances (Table II). Although these effects can be due in part to indirect photoreactions or formation of photoreactive complexes, the results in Table II can be most simply explained in terms of increases in direct photoreaction rates of sorbed halocarbon in comparison to halocarbon in aqueous solution. 

In Table II, k is the pseudo-first-order photoreaction rate constant, DOC is the dissolved organic carbon concentration, and Sx is the light attenuation factor. Sx, which represents the fractional reduction in photoreaction rate of a very dilute halocarbon in a well-mixed system, was calculated by... 

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