Sorbed halocarbons

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). 

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. 

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-... 

In addition to the chloroacetates, as shown by comparisons with the estimates in Table V, the half-lives for both lindane and mirex in the natural water samples would have been considerably longer than those observed, had reaction with solvated electrons in bulk solution been the dominant mechanism for photoreaction. The higher efficiency of these halocarbon reactions may be attributable to sorption of the chloroacetates on the NOM, which permits more facile electron capture. Other possible pathways for reactions of sorbed halocarbons include direct photoreduction by excited states of the NOM, which, like solvated electrons, also are quenched by oxygen. Alternatively, the enhancement may involve other direct electron-transfer mechanisms such as amine-halomethane reactions. These alternative possibilities are examined in the following section. 

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. 

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...

Halocarbons Sorbed on Sediments. If a significant fraction of halo-carbon is sorbed in an unreactive microenvironment, then the kinetics can become limited by exchange between the unreactive (U-P) and photoreactive (R-P) parts of the system. Intrasorbent transport limitations have been observed for extremely hydrophobic halocarbons sorbed on soils and sediments suspended in water. The photoreactions of DDE in sediment suspensions provide a good example of such transport limitation (Figure 2) (43). Plots of log concentration versus time were linear for DDE photoreaction in water, but nonlinear in the sediment suspensions (Figure 2). The degree of nonlinearity depended upon the equilibration time of the suspensions prior to irradiation. 

The enhanced photoreactivity of sorbed nonionic halocarbons may involve photoreactive complexes with amines and other electron-donating substances. The enhanced photoreactivity of ionic halocarbons (e.g., chloroacetates) may involve complexes with DOM and transition metals. Additional studies are needed to examine the role of complexation in the aquatic photochemistry of halocarbons. 

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