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Irradiation of an aqueous suspension of TiCh induces ring hydroxylation and redox
reactions on nitrogen 1 of the urea chain of metoxuron (MX) [3-(3-chloro-4-méthoxyphényl)-
1,1-diméthylurea] or metobromuron (MB) [3-(4-bromophényl)-l-méthoxy-l-méthylurea].
Hydroxylation is predominant in ortho and para positions with respect to urea in the
case of MB and in para for MX. Déméthylation and oxidation of methyl are the main
reactions of the urea chain of MX. Déméthoxylation proccds déméthylation in MB.
Three molecules of MX or MB over four are oxidised by 110' Radicals. All of the urea
chain reactions take this route.
The kinetics of the photocatalysis of both herbicides are adequately modelised by the
Langmuir-Hinshelwood equation, MB and MX showing close values for the rate constants as
well as for the adsorption constants. Mineralization of MB is however much faster than that of
MX.
A pH increase accelerates the substrates degradation.
Chlorides moderately inhibit the photocatlysis of MX and MB while sulfates and
nitrates slightly accelerate MB degradation but do not affect that of MX.
TiC>2 Degussa P25 is the most active amongst studied semi-conductors. The TiOî
photocatalyses eliminated of MB end MX is improved by the introduction of H2O2.
The photoreactivity of or/Zro-phnylphenol (OPP) is considerably affected by Humics
Acids (HA), irradiation wavelength and pH.
According to operating conditions, there is formation of products of oxidation
(phenylbenzoquinone), of ring closure (2-hydroxybenzofuran) or hydroxylation
(phenylhydroquinone or 2,2’-dihydroxybiphenyl).
The 11A photoinduccd transformation of OPP involves HO' radicals at 253,7 nm but
not at 365 nm.
Depending on pH and irradiation wavelength, HA may accelerate OPP degradation by
photoinductive effect or slow it down by screen effect. |
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