Unveiling the secondary pollution in the catalytic elimination of chlorinated organics: The formation of dioxins

Abstract

Since the discovery of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the process of municipal solid waste incineration (MSWI), a large number of researches have been conducted to reveal their formation mechanisms and emission characteristics. As one of national priority control pollutants, chlorinated organics are inclined to transfer into PCDD/Fs in the heterogeneously catalyzed process, which has been considered to be one of great challenges in environmental catalysis. However, so far direct evidences to support such a conversion process are insufficient, and the reaction mechanisms are lack of exploration. This study investigated the catalytic elimination of chlorobenzene (CBz) over a range of industrially applied active species including Pt, Ru, V, Ce and Mn oxides, and explored their reaction byproducts, chlorine adsorption/desorption behaviors and PCDD/F formations. We found that all of these species could generate the PCDD/Fs, amongst which, Mn species were the most active for PCDD/F formation. Approximately 140 ng I-TEQ g−1 PCDD/Fs were detected on the Mn-CNT surface after ageing at 250 °C for 30 h. Even using the dichloromethane (DCM) as a precursor, significant PCDD/Fs were still detected. The Ru and V species were shown to generate much less polychlorinated byproducts and PCDD/Fs, owning to their sufficiently high abilities in Cl desorption, which were through the semi-Deacon and Brønsted H reactions, respectively.