Photolytic and Photocatalytic Decomposition of Bromomethanes in Irradiated Aqueous Solutions

The decomposition of bromomethanes has been studied by homogeneous photolysis and by heterogeneous photocatalysis. The photolytic degradation of tetrabromomethane has been investigated also for the effect of pH and oxygen presence. The photocatalytic degradation of CBr4, CHBr3 and CH2Br2 over irradiated TiO2 has been investigated at pH 5.5 under aerobic and anaerobic conditions. Bromomethanes degrade through combined reductive and oxidative processes. The importance of reductive pathways decreases from CBr4 to CHBr3 and CH2Br2. Brominated and oxygenated intermediates have been quantified until complete degradation or debromination was achieved. Reactions accounting for the observed results are presented based on presumed transient species and identified intermediates. The effect of hole and electron scavengerswas evaluated on CHBr3 andCH2Br2 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 whilst methanol increases it. Methanol generates radicals that efficiently react with CH2Br2, notably if the system is oxygen-free.