Phototransformation of 4-chloro-2-methylphenol in water : influence of humic substances on the reaction

The phototransformation of 4-chloro-2-methylphenol was studied in distilled, natural or humic substances containing waters. Samples were irradiated under different conditions: in monochromatic light at 280 nm, in polychromatic light with lamps emitting within the wavelength ranges 290–350 nm and 300–450 nm and in solar-light. When 4-chloro-2-methylphenol is irradiated in pure water, dechlorination occurs with a good efficiency (Φ = 0.66). Methylbenzoquinone is the main primary photoproduct in oxygenated solution; methylhydroquinone and methylhydroxybenzoquinone are produced via secondary photolysis of methylbenzoquinone, especially in polychromatic light. Humic substances enhance significantly the rate of photodegradation of 4-chloro-2-methylphenol at λ  > 300 nm. The photoinduced reaction yields 3-chloro- trans – trans - and 3-chloro- cis – trans -1-methyl-1-one-2,4-hexadienoic acids. The opening of the aromatic ring is explained by the abstraction of the phenolic hydrogen atom by reactive species produced by excitation of the humic substances followed by the reaction of the 4-chloro-2-methylphenoxyl radical with HO 2 · /O 2 −· radicals. When 4-chloro-2-methylphenol is dissolved in natural waters and exposed to solar-light, dechlorinated and ring opening products are produced. It is deduced that under environmental conditions 4-chloro-2-methylphenol can be phototransformed by both direct and photoinduced processes.