SEMICONDUCTOR-SENSITIZED PHOTODEGRADATION OF 4-CHLOROPHENOL IN WATER

The effects of three non-porous powder samples of TiO2, ZrO2 and MoO3, with surface areas between 30 and 60 m2 g-1, on the kinetics of disappearance of 4-chlorophenol (4-CP) in water, under illumination at lambda greater-than-or-equal-to 340 nm (to avoid the direct photolysis of 4-CP) and 293 K, were determined. The TiO2 sample was by far the most active with an initial quantum yield of ca. 0.01 for an initial 4-CP concentration C0 = 0.155 mM in the pH range 3-6. The photocatalytic character of the 4-CP disappearance in its presence was shown by (i) the effects of the catalyst mass and radiant flux on the initial rate r0, (ii) the Langmuirian-type relationship between r0 and C0, (iii) the very small activation energy of 4-CP disappearance, (iv) the number of 4-CP molecules degraded per unit of surface area, (v) the stability of TiO2, and (vi) the quasi-suppression of the activity by substitutional doping with Cr3+ ions which behave as recombination centres of the photoproduced charges. Hydroquinone (HQ), the main intermediate in the degradation, corresponds to para hydroxylation and its maximum concentration was ca. 100 times smaller than C0 for C0 = 0.155 mM. 1,4-Benzoquinone, formed from HQ, and 4-chloro-1,2-dihydroxybenzene were also detected. None of the toxic biphenyl derivatives produced by direct photolysis was found in the presence of TiO2. Complete mineralization of 4-CP was eventually obtained. Without TiO2, at lambda greater-than-or-equal-to 290 nm, the degradation produced more numerous and more stable aromatic intermediates.