Mechanisms of Direct and TiO2-Photocatalysed UV Degradation of Phenylurea Herbicides

• Published in: Chemphyschem Vol. 6; no. 10; pp. 2064 - 2074
• Main Authors: Canle López, Moisés, Fernández, M. Isabel, Rodríguez, Santiago, Santaballa, J. Arturo, Steenken, Steen, Vulliet, Emmanuelle
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 14.10.2005; WILEY‐VCH Verlag; Wiley
• Subjects: Adsorption; Catalysis; Chemistry; Chromatography, High Pressure Liquid; Exact sciences and technology; General and physical chemistry; herbicides; Herbicides - chemistry; Herbicides - radiation effects; Kinetics; Molecular Structure; Oxidation-Reduction; phenylurea; Phenylurea Compounds - chemistry; Phenylurea Compounds - radiation effects; Photochemistry; photolysis; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); radical ions; Titanium - chemistry; Ultraviolet Rays; radical ions herbicides; Pesticides; Transition element compounds; Ions; photolysis, radical ions; Herbicide; Mechanism; Degradation; Ultraviolet radiation; Photolysis; phenylurea; Ureas; Titanium oxide; Photochemistry; herbicides
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200500004

Phenylurea herbicides undergo low‐yield (ϕPI <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° (phenylurea.+/phenylurea)<2.43 V versus NHE. The main photoproducts of UVC (λ=193 nm) photodegradation of phenylureas correspond to a photo‐Fries rearrangement. One‐electron reduction with e${{{- \hfill \atop aq\hfill}}}$ yields the corresponding radical anions (phenylurea.−), for which 4.3<pKa<5.33. The rate constants for reaction with e${{{- \hfill \atop aq\hfill}}}$ show that in photocatalysis the generation of phenylurea.− and O2.− on the surface of the photocatalyst may be competitive. High reactivity toward e${{{- \hfill \atop aq\hfill}}}$ 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. ion 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. Shedding light on herbicides: Phenylurea herbicides undergo photoionisation to form acidic radical cations that deprotonate to the neutral radical. One‐electron reduction by pulse radiolysis yields radical anions. The mechanisms of direct UV photodegradation and of TiO2 photocatalytic degradation of phenylurea compounds in aqueous solution are discussed (see picture) to give a better understanding of advanced oxidation processes.