Hydrophobic halocarbons

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. 

Kinetic data for the hydrophobic halocarbon, DDE, provide another example of a photoreaction rate enhancement attributable to sorption on NOM. The direct photoreaction rates of DDE, corrected for light attenuation effects, are increased in filtered natural water samples (0.2 pm) or a solution of a soil fulvic acid (Contech) with high DOC (Table II). As shown in Table II, the degree of rate enhancement is approximately equal to the enhancement in direct photoreaction rates in going from an aqueous to hydrocarbon solution (Table I) (38). 

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. 

Aromatic compounds undergo carbonisation during sonication [44]. The reaction can occur either at the bubble interface or inside the cavity, according to the hydrophUicity of the substrate. Generally it would appear that apolar, hydrophobic compounds, e. g. benzene and halocarbons are pyrolysed inside the bubble [45,46]. 

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