title: Mechanisms of direct and TiO2-photocatalysed UV degradation of phenylurea herbicides.
abstract: Phenylurea herbicides undergo low-yield (phi(PI) <15 %) monophotonic photoionisation upon 193-nm laser flash excitation. The so-formed radical cations (phenylurea.+) are highly acidic (-1.5 < pKa <0.5) and deprotonate readily to yield the corresponding neutral radical (phenylurea.). Pulse radiolysis experiments allowed limitation of the reduction potential of phenylurea.+ within 2.22 V versus the normal hydrogen electrode (NHE) < E degrees (phenylurea.+/phenylurea) < 2.43 V versus NHE. The main photoproducts of UVC (lambda=193 nm) photodegradation of phenylureas correspond to a photo-Fries rearrangement. One-electron reduction with e-(aq) yields the corresponding radical anions (phenylurea.-), for which 4.3< pKa < 5.33. The rate constants for reaction with e-(aq) show that in photocatalysis the generation of phenylurea.- and O2.- on the surface of the photocatalyst may be competitive. High reactivity toward e-(aq) is predicted from linear free-energy relationships (LFER) for phenylureas bearing electron-withdrawing groups. Reaction with HO. takes place mainly via addition to the aromatic ring and/or H. abstraction from a saturated carbon atom (98 %), rather than one-electron oxidation (2 %). High reactivity toward oxidation by HO. is predicted from LFER for phenylureas bearing electron-donating groups. Adsorption studies for TiO2 in its polymorphic forms of rutile and anatase, as well as with the commercial mixture Degussa P-25, show photocatalysis is independent of the specific area of the catalyst. A variety of compounds are generated during the photocatalytic degradation of Diuron, while only two hydroxychloro derivatives are observed upon prolonged direct 365 nm irradiation. The photocatalytic degradation proceeds mainly by oxidation of the Me group of the side chain, hydroxylation of the aromatic ring, and dechlorination. The photoproducts of photocatalytic degradation differ from one polymorphic form of TiO2 to another.
authors.affiliation: Grupo de Reactividade Química e Fotorreactividade, Departamento de Química Física e Enxeñería Química I, Universidade da Coruña, Rúa Alejandro de la Sota 1, 15008A Coruña, Galicia, Spain. mcanle@udc.ess
authors.forename: Moisés
authors.initials: M
authors.lastname: Canle López
authors.forename: M Isabel
authors.initials: MI
authors.lastname: Fernández
authors.forename: Santiago
authors.initials: S
authors.lastname: Rodríguez
authors.forename: J Arturo
authors.initials: JA
authors.lastname: Santaballa
authors.forename: Steen
authors.initials: S
authors.lastname: Steenken
authors.forename: Emmanuelle
authors.initials: E
authors.lastname: Vulliet
mesh_terms: D000327:Adsorption; D002384:Catalysis; D002851:Chromatography, High Pressure Liquid; D006540:Herbicides / Q000737:chemistry / Q000528:radiation effects; D007700:Kinetics; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010671:Phenylurea Compounds / Q000737:chemistry / Q000528:radiation effects; D010777:Photochemistry; D014025:Titanium / Q000737:chemistry*; D014466:Ultraviolet Rays
chemical_list: D006540:Herbicides; D010671:Phenylurea Compounds; C009495:titanium dioxide; D014025:Titanium