Conversion Characteristics And Mechanism Of Typical VOCs Purification By UV₂₅₄/O₃ Coupled System

After the pollutions of NOx and SO₂ had been given enough controlled, other air pollutions such as BTEXs and CVOCs which came from petrochemical, pharmaceutical and other industrial production process emissions, become increasingly serious. Single biological treatment technology showed low efficiencies for some hydrophobic and recalcitrant VOCs, because they were difficult to be degraded or even non-degraded. Recently, ultraviolet photodegradation for VOCs began to get attention for their advantages, such as high efficiency, short cycle and easy operation. A large number of studies have shown that the strong oxidizer ozone played a better role in the degradation of VOCs, not only the removal efficiency were much higher, but also most of the organic matter could be transferred into higher oxidation state. However, the utilization rate of ozone was much lower.This research combined UV and ozone technology for the conversion of pollutants, increasing the utilization of ozone along with the conversion efficiency of target. This combined technology could make up the shortcomings of single technology. The study also investigated the potential of this combined technology as a pretreatment before the subsequent biopurification.This study used UV254 and ozone oxidation to convert ethylbenzene （EB） and chlorobenzene （CB） which were thought as two typical VOCs. Some process characteristic such as initial concentration, relative humidity, ozone concentration and light intensity were investigated. At the same time, the mutual influence of these two kinds was also analyzed. By the detection of intermediates, a detailed conversion pathway was built up. Finally, the biological evaluation ecological toxicity analysis of the intermediates were also processed.Through the study, we found that in UV254/O₃ system, the influence of relative humidity on the conversions of EB and CB were most obvious. The degradation rate of CB increased with the increase of relative humidity, and the optimum relative humidity was75-80%, the best conversion was 70%. But with the increase of relative humidity, the degradation of EB firstly increased to the highest rate about 49% and then began to decrease, and the optimal relative humidity of 45-50%. For these two typical gas, proper ozone concentration was important for the maximum conversion efficiency, and higher or lower ozone concentration was negative for the conversions. Kinetics model which based on the initial concentration and ozone consumption had been set up. By GC/MS and IC analysis of photodegradation products,11 types of main intermediates from EB conversion and 5 types of main intermediates from CB conversion were detected. The main intermediates of EB is acetophenone, the main intermediates of CB is chlorophenol. The water soluble and biodegradability of the off-gas changed with the different photolysis process parameters. The major products of EB were more biological and less ecological toxicity compared with the target, but the chlorophenol which was one of main products from CB conversion still had high ecological toxicity.