Photolytic and photocatalytic decomposition of bromomethanes in irradiatedaqueous solutions

The decomposition of bromomethanes has been studied by homogeneous photolys is and by heterogeneous photocatalysis. The photolytic degradation of tetra bromomethane has been investigated also for the effect of pH and oxygen pre sence. The photocatalytic degradation of CBr4, CHBr3 and CH2Br2 over irradi ated TiO2 has been investigated at pH 5.5 under aerobic and anaerobic condi tions. Bromomethanes degrade through combined reductive and oxidative processes. T he importance of reductive pathways decreases from CBr4 to CHBr3 and CH2Br2 . Brominated and oxygenated intermediates have been quantified until comple te degradation or debromination was achieved. Reactions accounting for the observed results are presented based on presumed transient species and iden tified intermediates. The effect of hole and electron scavengers was evaluated on CHBr3 and CH2Br 2 to elucidate the importance of reductive and oxidative pathways. Electron scavengers as periodate, enhance the rate of disappearance for CH2Br2 The hole scavengers as alcohols, have limited effect on the degradation rate of CHBr3 but interestingly, have different effects on the degradation rate of CH2Br2. tert-Butanol does not significantly affect the degradation rate wh ilst methanol increases it. Methanol generates radicals that efficiently re act with CH2Br2, notably if the system is oxygen-free. (C) 1999 Elsevier Sc ience B.V. All rights reserved.