Mechanism of bromoxynil phototransformation: Effect of medium and surfactant

Bromoxynil (BXN, 3,5-dibromo-4-hydroxybenzonitrile) is a herbicide that is classified as a highly hazardous chemical, toxic for the reproduction. The processes and mechanisms regarding the fate of this compound in the environmental compartments subject to solar light are poorly documented. In this work, we investigated the photochemical transformation of BXN and its main photoproducts on two relevant compartments: the surface of plant leaves that are contaminated during the crop treatments and water as the medium receiving pollutions. In a first step, BXN was deposited on wax films mimicking the surface of leaves and exposed to simulated solar light. Deposited as a pure molecule, BXN was photostable whereas deposited in the presence of an ethoxylated surfactant (5-10% in weight) BXN underwent a fast photoreduction yielding 3-bromo-4-hydroxybenzonitrile (MBr) as the unique primary photoproduct. Using muconic acid as a trapping agent for bromine atoms, it was shown that photodebromination was partly homolytic with release of bromine radical. In a second step, we studied the photolyses of BXN and MBr in water, because both should be able to reach surrounding aquatic media by wash off and leaching. The primary reactions were the heterolytic replacement of Br- by OH- but photoreduction, photooxidation and formation of adducts were also observed at higher extent of the reactions. These results demonstrate that the mechanism of phototransformation of BXN depends drastically on the medium. They also stress the potentially important role of surfactants in the fate of BXN on leaves, and the need to investigate the photochemical reactions in a wide range of conditions to cover the diversity of situations.