Photolytic and photocatalytic decomposition of bromomethanes in irradiated aqueous solutions

摘要

The decomposition of bromomethanes has been studied by homogeneous photolysis and by heterogeneors photocatalysis. The photolytic degradation of tetrabromomethane has been investigated also for the effect of pH and oxygen presence. The photocatalytic degradation of CBr_4, CHBr_3 and CH_2Br_2 over irradiated TiO_2 has been investigated at pH5.5 under aerobic and anaerobic conditions.Bromomethanes degrade through combined reductive and oxidative processes. The importance of reductive pathways decreases from CBr_4 to CHBr_3 and CH_2Br_2. 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 scavengers was evaluated on CHBr_3 and CH_2Br_2 to elucidate the importance of reductive and oxidative pathways. Electron scavengers as periodate, enhance the rate of disappearance for CH_2Br_2. The hole scavengers as alcohols, have limited effect on the degradation rate of CHBr_3 but interestingly, have different effect on the degradation rate of CH_2Br_2. tert-Butanol does not significantly affect the degradation rate whilst methanol increases it. Methanol generates radicals that efficiently react with CH_2Br_2, notably if the system is oxygen-free.