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.

Ionizing radiation induced degradation of diuron in dilute aqueous solution

“…In general, • OH is a highly reactive electrophilic radical, in its reaction with diuron, however, direct oxidation (electron transfer) hardly occurs [ 5 ]: the basic reactions are addition to the ring and H-abstraction from one of the CH 3 groups or from the N-H group. The absorption band at ~360 nm is typical for the hydroxycyclohexadienyl radicals: e.g.…”

New insights into the primary phototransformation of acetaminophen by UV/H2O2: photo-Fries rearrangement versus hydroxyl radical induced hydroxylation

“…P1 was generated from the photo-Fries rearrangement of APAP and similar reactions have been observed for acetanilide derivatives, phenylurea herbicides, and aromatic esters (Amine-Khodja et al, 2004;Canle L opez et al, 2005;Shizuka, 1969a,b). The concentration of P1 changed during the reaction, as shown in Fig.…”

Coating-free TiO2@β-SiC alveolar foams as a ready-to-use composite photocatalyst with tunable adsorption properties for water treatment

“…, one of the most used contact herbicides from the family of substituted phenylureas, was taken as a model substrate to degrade for evaluating the liquid-phase photocatalytic activity of the TiO 2 @b-SiC composite foams under simulated solar light. Diuron is known to be degraded by photocatalysis via a well-accepted multi-pathway degradation route, involving mainly hydroxyl OHc radicals as active species, [17][18][19] and consisting rst in the attack of the alkyl function or in the chlorine substitution, followed by hydroxylation steps leading to the formation of acetic, formic and oxalic acid as the last short-chain acid reaction intermediates. 1,15,20,21 Experimental part…”