Advanced removal of persistent organic pollutants from diesel engine exhaust using a dual-catalyst system

Current aftertreatment systems can unintentionally promote the formation of halogenated toxic pollutants through soot accumulation and de novo synthesis, highlighting the need for improved emission control strategies. This study investigated the emission characteristics of toxic pollutants, including PCDD/Fs, PCBs, PCDEs, PBDD/Fs, PBBs, and PBDEs, from diesel engines and evaluated the effectiveness of a combined diesel oxidation catalyst and particle oxidation catalyst (DOC + POC) system. In raw exhaust, PBDEs exhibited the highest mass concentrations and emission factors, followed by PCBs, PCDD/Fs, and PBDD/Fs. In contrast, toxicity was dominated by PCDD/Fs, whose toxic equivalency (TEQ) levels were 11.9–41.6 times higher than those of PCBs and PBDD/Fs. The DOC + POC system consistently reduced mass and TEQ concentrations, even during regeneration. Removal efficiencies of PCDD/Fs, PCBs, PCDEs, PBDD/Fs, PBBs, and PBDEs reached up to 60.9%, 59.2%, 37.9%, 71.3%, 92.0%, and 71.8%, respectively, with TEQ reductions for PCDD/Fs, PCBs, and PBDD/Fs up to 81.6%, 95.2%, and 59.6%. These reductions are attributed to decreased soot and ash accumulation, which suppress de novo synthesis and heterogeneous precursor pathways. The DOC + POC system primarily influences gas-phase oxidation and post-formation transformation processes, with congener-specific responses governed by intrinsic chemical properties and dominant formation pathways. Notably, this is the first study to systematically evaluate the DOC + POC system's performance on a broad suite of halogenated toxic pollutants, including during regeneration phases. Passive regeneration occurred for approximately one-third of the operating time, enabling sustained high-temperature oxidation and demonstrating DOC + POC as a low-backpressure and effective option for mitigating toxic organic pollutant emissions from diesel engines.