Photochemical effects of humic substances on the degradation of organophosphorus pesticides

Photochemical effects of humic substances on the aquatic degradation of organophosphorus pesticides were studied on the basis of photo-induced radical generation abilities of humic substances and the increase percentage of the humin-sensitized photodegradation rate constants relative to those measured in the humin-less system. The increase percentages above stated were enhanced depending on the radical generation abilities of humic substances. Also, the degree of the sensitization effects of humic acids to be expressed by the increase percentages of the rate constants tended to enhance with the decrease in the inherent photodegradation reactivities of pesticides in the humin-less system. This suggests that the sensitization action of humic acids depends on binding affinities of pesticides to the radical generation source of humic acids, as the affinities may hinder the pesticide photodegradation in humin-less aquatic systems possibly initiated by any route other than the radical mechanism. (C) 1998 Elsevier Science Ltd. All rights reserved.