The influence of pH and cadmium sulfide on the photocatalytic degradation of 2-chlorophenol in titanium dioxide suspensions

The influence of pH and cadmium sulfide on the photocatalytic degradation o f 2-chlorophenol (2-CP) in titanium dioxide suspensions was investigated to evaluate the feasibility of mixed semiconductors on the photodegradation o f chlorinated organics in aqueous solution. Apparent first-order rate const ants (k(obs)) and initial rate constants were used to evaluate the degradat ion efficiency of 2-CP. Higher degradation efficiency of 2-CP was observed at higher pH values. The apparent pseudo-first order rate constant was 0.03 6 min(-1) at pH 12.5 in TiO2/UV system, while a 2- to 9-fold decrease in k( obs) was observed over the pH range of 2.59.5. The addition of phosphate bu ffer solutions at different pH values have different effects on the degrada tion of 2-CP. H2PO4- has little effect on the photodegradation of 2-CP, whi le HPO42- could inhibit the photodegradation efficiency of 2-CP. Chlorocate chol, hydroquinone, benzoquinone and phenol were identified as the predomin ant aromatic intermediates for the photocatalytic degradation of 2-CP. More over, less aromatic intermediates at higher pH were observed. Direct oxidat ion contributed significantly to the photodegradation of 2-CP. An addition of a semiconductor decreased the initial and apparent first-order rate cons tants of 2-CP. The cutoff of wavelength of 320 nm could diminish the contri bution of direct photolysis of 2-CP. The combination of cadmium sulfide and titanium dioxide can lead to an enhanced rate of disappearance of 2-CP com pared to those in single semiconductor system. A 1.2 to 2.5-fold increase i n rate constant in coupled semiconductor system relative to the single semi conductor system was obtained. (C) 2001 Elsevier Science Ltd. All rights re served.