3. HOMOGENEOUS SENSITIZED SYSTEMS170 3·1 General background 170 3·1·1 Singlet oxygen as oxidant 171 3·2 Sensitized oxidation of chlorophenols by singlet oxygen 172 3·3 Homogeneous reactions of chlorophenols with metal complexes 176 4. PHOTOINDUCED DEGRADATION OF CHLOROPHENOLS 180 4·1 Free radical oxidation of chlorophenols 180 4·2 Free radical processes in semiconductor-photocatalysed degradation of chlorophenols 182 4·2·1 TiO 2 -mediated degradation of chlorophenols 183 4·3 Sensitization of chlorophenol degradation by supported catalysts 188 4·4 Radiation induced degradation of 2,4,6-trichlorophenol 191 5. REACTIONS OF CHLOROPHENOLS WITH TRANSIENT INTERMEDIATES 193 6. CONCLUSIONS 197 7. ACKNOWLEDGEMENTS 198 8. REFERENCES 198 ABSTRACTThe photodegradation of mono-, di-, tri-and pentachlorophenols in aqueous solution is surveyed from several viewpoints, namely kinetic and mechanistic, the nature of the reactive intermediates and final products, and the potential of photochemical means of treating water contaminated by chlorophenols. In direct photolysis, the roles of heterolytic and homolytic processes are considered, and the appearance of carbene, in addition to ionic and radical, intermediates noted. Sensitized photolysis deals with the roles of singlet oxygen and of a variety of metal complexes. The induced degradation of chlorophenols refers to the oxidation of chlorophenols by free radicals generated from photo-and radiolytic systems, particularly illuminated semiconductors such as titanium dioxide. The article finishes with an overview of the reactions of various types of reactive intermediates with chlorophenols. 1. dIonization potentials of 2-chlorophenol (8·66 eV) and 4-chlorophenol (8·94 eV) in organic solvents have been determined from charge transfer spectra with TCNE [51] e Values obtained for 4-chlorophenol for OH bond dissociation energies by this method, photoacoustic calorimetry [52], pulse radiolysis [53] and theoretical calculations [54] vary over the range 84·4-90·3 kcal mol -1 f Calculated in [50] g From [55] h n-butanol solution [56]

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A survey is made of the iron-catalyzed oxidation of sulfite describing a conceptual framework to explain the key processes involved, with a focus on kinetics. Perhaps most importantly, the incorporation of the HSO5- + Fe(II) step into the regeneration of catalytically active ferric ions which does not deplete its role over the iron redox cycle. The radical-radical recombination SO5-• + SO5-•, which terminates the cycling between ferric and ferrous ions, represents a gross but not a net loss of the chain-carriers, because nearly all of them are reformed through the branching step HSO5- + Fe(II) → Fe2+ + H2O + SO4-•, [Formula: see text] in just a few seconds or somewhat longer. A branching mechanism is thus the only possible means of allowing the catalytic process to reach a stationary state. Observations that may be considered as evidence (fingerprints) of rate variations in sulfite depletion due to the branching mechanism are explored in detail, and the related dynamics of the chain-carriers and metal ions cycles are discussed. In particular, the most important is found to be the aspect related to the intrinsic limitation of the cycle of metal ions. This limitation governs the extent of the oxidative/reducing potential of sulfite solutions with respect to the Fe(III/II) couple, thereby governing the quasi-state partioning between ferric and ferrous ions. Such a view enables examination of those conditions under which the limitation to the rate of the catalytic reaction is controlled by the reduction or re-oxidation of ferric ions. Readily applicable kinetic criteria and kinetic diagrams to delimit the conditions are given. In such a framework, the majority of known anomalies of the catalytic reaction receive an explanation.

A combined mathematical model was developed for the regional-scale dynamics of gaseous admixtures and aerosols in the atmosphere. The model incorporates the following modules: thermo-hydrodynamic equations for meso-scale atmospheric processes in the non-hydrostatic approximation; transport of gaseous admixtures and aerosols, with allowance for photochemical transformation and binary homogeneous nucleation; and kinetic processes of condensation/evaporation and coagulation. Particular emphasis is given to the mechanisms of new-particle formation through binary homogeneous nucleation of drops of sulfuric acid and water vapor. By using this model, numerical experiments were performed to investigate spatio-temporal variations in the concentrations of gaseous admixtures and aerosols, as well as the formation of fine aerosol clusters in the Lake Baikal (Russia) and Antwerp (Belgium) areas due to strong industrial emission sources. The results of the numerical experiments are analyzed.