posted on 2012-08-21, 00:00authored byMeiqin Hu, Yan Wang, Zhigang Xiong, Dongqin Bi, Yuhong Zhang, Yiming Xu
Molecular iodine has been studied, for the first time,
as a sensitizer for the degradation of 2,4,6-trichlorophenol (TCP)
in aqueous solution under visible light (λ ≥ 450 nm).
TCP was degraded in the presence of commercial I2, but
the reaction rate decreased significantly after 2 h. When a solution
of NaI and H2O2 was used as an iodine source
with phosphotungstic acid (PW) as a catalyst, TCP degradation was
not only fast but also followed zero-order kinetics. Importantly,
the I2 concentration remained unchanged with time, indicative
of I2 recycling as a kind of photocatalyst. During TCP
degradation, 2,6-dichloro-1,4-benzoquinone was produced as the main
intermediate (76%), which slowly degraded in the irradiated solution.
For every equivalent of TCP consumed at the 2 h time point, approximately
1.7 equivalents of chloride ions were produced. Further study of the
effect of variables including the type of polyoxometalates (POM) and
the initial concentration of each component revealed that the rate
of TCP degradation under visible light was determined by the rate
of I2 production in the dark. The optimum pH and apparent
activation energy for TCP disappearance were 4.5 and 42.8 kJ/mol,
respectively. It is proposed that TCP degradation is initiated by
iodine radicals produced from I2 photolysis, followed by
I2 regeneration through a POM-catalyzed oxidation of I3– by H2O2.