Variation Profiles, Formation Mechanisms, and Emission Risks of Brominated Flame Retardant Compounds during Cement Kiln Co-processing of Hexabromocyclododecane-Containing Waste

Abstract

Cement kiln co-processing technique has been suggested as a promising disposal method for hexabromocyclododecane (HBCD)-containing construction wastes. However, concerns persist regarding the potential emissions of secondary brominated flame retardant (BFR) compounds. To address this, we conducted both field and laboratory experiments to elucidate the emission characteristics and formation mechanisms of BFRs during the co-processing of HBCD-containing waste in cement kilns. In the field experiments, which examined a range of HBCD disposal dosages from 0 to 400 kg/day, the concentrations of new brominated flame retardants (NBFRs), polybrominated diphenyl ethers (PBDEs), and polybrominated biphenyls (PBBs) in the stack gas were 0.57–0.80, 0.68–51.56, 0.62–1.79 ng/Nm³, respectively. Over 77% of the emitted BFRs can be sequestered within solid materials. Further laboratory experiments revealed that the alkaline substances present in cement kilns can absorb HBr thus inhibiting the formation of BFRs. The transformation mechanisms from HBCDs to BFRs were further explored to involve processes including structural re-arrangement, de novo synthesis, and precursor formation. Furthermore, the national annual emission risk associated with the disposal of HBCD-containing construction wastes via cement kilns has been assessed. The findings of our study furnish a critical scientific basis for the development of strategies for managing HBCD-containing waste in the future.

Environmental Implication

HBCDs had been widely used in construction materials before 2021. The service life of these previously used construction materials is approaching expiration in the coming years. China is thus faced with the formidable task of managing tens of millions of tons of such waste. In this study, we conducted a demonstration project for the co-processing of waste containing HBCDs in cement kilns and conducted laboratory simulation experiments to elucidate the mechanisms of BFR formation during this thermal process. The findings of our study therefore furnish a critical scientific basis for the management of HBCD-containing waste in the future.