Photocatalysis is a rapidly expanding technology for wastewater treatment. In this review the chemical effects of various variables on the rate of degradation of different pollutants are discussed in detail. The effects of adsorption, temperature, intensity of light, pH, and the presence of anions, cations, etc have been speci®cally covered. A critical analysis of the available literature data has been made and some general conclusions have been drawn concerning the above mentioned effects. The need for more work on speci®c points has been brought out.

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Photocatalytic (TiO 2 , air and solar radiation) and photochemical (H 2 O 2 and solar radiation) mineralizations of phenol solutions (up to 500 mg dm À3 ) in a batch reactor using concentric solar radiation were studied. A compound parabolic re¯ector as a concentrating re¯ector was used to obtain concentrated solar radiation in both cases. Considerable enhancement (Five-fold) in the rate of degradation was obtained when concentrated solar radiation was used instead of plain solar radiation in photocatalytic degradation. The effect of initial phenol concentration, intensity and presence of anions has been studied in both the cases. Low concentrations of intermediates were observed, during the reaction, over the entire range of phenol concentrations studied in the case of photocatalytic degradation using Degussa-P25 TiO 2 as photocatalyst and in the absence of anions; whereas signi®cant quantities of intermediates were observed in the case of the photochemical degradation. The presence of anions generally had a detrimental effect on the mineralization process in both cases. Different anions showed different effects. Total organic carbon (TOC) variations with time were also studied. A rational explanation has been given for the decrease in the rate in the presence of anions.

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