The different reaction mechanisms by which chlorine and chlorine dioxide react with polycyclic aromatic hydrocarbons (PAH) in water

The removal of PAH from surface water by disinfectants like chlorine or chlorine dioxide is important where contamination by these compounds is concerned and no other water treatment processes are available. Our particular interest in these reactions arise from the fact that PAH can be used as an excellent model for the investigation of the different mechanisms by which the two oxidants react with aquatic organics. The vast differences between the rates of Cl₂ and ClO₂ reactions with various PAH, as well as the physical and chemical factors influencing those reactions indicate that chlorine reacts with PAH by several possible mechanisms, e.g. addition, substitution and oxidation. Chlorine dioxide on the other hand reacts mainly as a pure oxidant and a one-electron acceptor. As a consequence, chlorine dioxide reacts much more specifically with those PAH that undergo facile oxidation. Therefore, some PAH that react quite easily with Cl₂, do not react at all with ClO₂, while other PAH react with ClO₂ much more rapidly than with Cl₂. The widespread and highly carcinogenic benzo(a)pyrene and benzo(a)anthracene for example react with ClO₂ much faster than with Cl₂.