Phototransformation of metobromuron in the presence of TiO
2
Amina Amine-Khodja, Abdelaziz Boulkamh, Claire Richard
*
Laboratoire de Photochimie Mole
´
culaire et Macromole
´
culaire, UMR No. 6505, CNRS-Universite
´
Blaise Pascal, 63177 Aubie
`
re Cedex, France
Received 10 November 2004; received in revised form 29 December 2004; accepted 8 January 2005
Available online 26 February 2005
Abstract
The photocatalytic degradation of the herbicide metobromuron was studied, using TiO
2
photocatalysts (Degussa P25 and Millennium
PC50, PC100, PC105 and PC500). Intermediate products were identified by means of liquid chromatography–mass spectrometry (LC–
MS–MS) using electrospray (ES) interfacing technique. In order of importance, the main reaction pathways were demethoxylation of the
urea chain, ring oxidation leading to ortho hydroxylation with respect to the N atom, substitution of Br by OH, ring oxidation with
formation of the meta hydroxylated compound. Demethylation of the demethoxylated product occurred as a secondary reaction. Formation of
bromide ions was observed too. In the presence of 2-propanol used as a hydroxyl radical scavenger, demethoxylation was drastically inhibited
while the other reactions were only partly inhibited. It suggests that demethoxylation was exclusively due to the oxidation by hydroxyl
radicals, whereas the other reactions involved positive holes too. The degradation rate of metobromuron varied in the order: PC50 <
PC100 < PC105 < PC500 < Degussa P25. TiO
2
Degussa P25 was therefore the most efficient photocatalyst although its surface area was
lower than that of most of Millennium TiO
2
.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Photocatalysis; Photodegradation; TiO
2
; Phenylurea; LC–MS–MS
1. Introduction
Metobromuron (MB) belongs to the chemical group of
phenylureas. These compounds are intensively used as plant
products owing to their herbicidal properties. As a
consequence, they are found in the aquatic environment,
harming the quality of water [1]. As the other phenylurea
herbicides, MB undergoes direct photolysis when irradiated
at l < 300 nm [2]. However, this transformation pathway
cannot be of relevant importance in environmental
conditions due to the poor absorption of solar radiations
by MB [3]. Techniques have therefore to be put in work to
ensure the elimination of this type of compounds from
drinkable water. Photocatalytic transformation is a promis-
ing way for organic compounds. The mechanism of the
process is well described in the literature [4].Briefly,
electron–hole pairs are formed upon illumination of the
photoactive oxide in the near UV. Oxygen is reduced into
O
2
À
by the electron promoted in the conduction band, and
valence band positive holes oxidize OH
À
or water into
hydroxyl radicals:
TiO
2
þ hn ! TiO
2
ðe
À
þ h
þ
Þ
e
À
þ O
2
! O
À
2
h
þ
þ H
2
O ! HO
þ H
þ
Substrates can be oxidized either by hydroxyl radicals or
directly by positive holes. The photocatalytic degradations of
phenylurea herbicides were already reported. A recent work
compared the photocatalytic degradation of four phenylurea
herbicides (metobromuron, isoproturon, chlortoluron and
chlorbromuron) in aqueous TiO
2
suspensions to assess
structure-reactivity relationships [5]. Others works were
mainly devoted to analytical aspects. The photocatalytic
degradation of fenuron (3-phenyl-1,1-dimethylurea) [6],
monuron (3-(4-chlorophenyl)-1,1-dimethylurea) [7], diuron
(3-(3,4-dichlorophenyl)-1,1-dimethylurea) and metoxuron
(3-(3-chloro-4-methoxyphenyl)-1,1-dimethylurea)[8,9]were
www.elsevier.com/locate/apcatb
Applied Catalysis B: Environmental 59 (2005) 147–154
* Corresponding author. Tel.: +33 4 73 40 71 42; fax: +33 4 73 40 71.
E-mail address: claire.richard@univ-bpclermont.fr (C. Richard).
0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2005.01.010