Mechanistic insights and mitigation strategies for brominated dioxin formation from thermal treatment of hexabromocyclododecane waste.

The thermal treatment of hexabromocyclododecane (HBCD)-containing waste poses a risk of generating toxic polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs). Previous mechanistic studies on PBDD/F formation from brominated flame retardants have largely excluded HBCD, particularly under real-waste conditions, leaving key factors insufficiently understood. This study established one-stage and two-stage reactor systems to simulate primary combustion and post-combustion flue gas conditions, enabling systematic investigation of parameters influencing PBDD/F formation. With increasing temperature, HBCD underwent debromination, ring-opening and aromatization, with maximum PBDD/F yields observed at 600 °C. Following high-temperature incineration, secondary PBDD/F formation in post-combustion flue gas peaked at 300 °C, consistent with the optimal conditions for chlorinated dioxin formation. Bromobenzenes and bromophenols were generated during HBCD degradation and acted as potential key precursors for PBDD/F formation. CuO enhanced PBDD/F formation by 50.3%, while CaO inhibited formation by 33.0% through bromine capture. In real waste, additional carbon sources facilitated interactions between aromatics and bromine, amplifying PBDD/F generation by approximately 11-fold. From a mitigation perspective, optimizing combustion and post-combustion temperatures and the addition of alkaline sorbents (e.g., CaO) effectively reduces PBDD/F emissions, providing practical strategies for the environmentally sound disposal of HBCD-containing wastes.