A new insight into the CaO-induced inhibition pathways on PCDD/F formation: Metal passivation, dechlorination and hydroxide substitution

Xiaoxiao Wang; Jiabao Lv; Yuxuan Ying; Yunfeng Ma; Angjian Wu; Xiaoqing Lin; Ang Cao; Xiaodong Li; Jianhua Yan

doi:10.1016/j.scitotenv.2023.163782

A new insight into the CaO-induced inhibition pathways on PCDD/F formation: Metal passivation, dechlorination and hydroxide substitution

Sci Total Environ. 2023 Aug 10:885:163782. doi: 10.1016/j.scitotenv.2023.163782. Epub 2023 May 4.

Authors

Xiaoxiao Wang¹, Jiabao Lv¹, Yuxuan Ying¹, Yunfeng Ma¹, Angjian Wu², Xiaoqing Lin³, Ang Cao⁴, Xiaodong Li², Jianhua Yan¹

Affiliations

¹ State Key Laboratory for Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China.
² State Key Laboratory for Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China; Jiaxing Research Institute, Zhejiang University, 314031, PR China.
³ State Key Laboratory for Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China; Jiaxing Research Institute, Zhejiang University, 314031, PR China. Electronic address: linxiaoqing@zju.edu.cn.
⁴ Department of Physics, Technical University of Denmark, 2800 Kongens Lyngby, Denmark.

PMID: 37149162
DOI: 10.1016/j.scitotenv.2023.163782

Abstract

Ca-based inhibitors (especially CaO) for PCDD/F (polychlorinated dibenzo-p-dioxin and dibenzofuran) formation are considered as economic inhibitors with low toxicity and strong adsorption of acidic gases (e.g., HCl, Cl₂, and SO_x), whereas the insight understanding of its inhibition mechanisms is scarcely explored. Herein, CaO was used to inhibit the de novo reaction for PCDD/F formation (250-450 °C). The evolution of key elements (C, Cl, Cu, and Ca) combined with theoretical calculations was systematically investigated. The concentrations and distribution of PCDD/Fs demonstrated the significant inhibition effect of CaO on I-TEQ (international toxic equivalency) concentrations of PCDD/Fs (inhibition efficiencies: > 90 %) and hepta~octa chlorinated congeners (inhibition efficiencies: 51.5-99.8 %). And the conditions (5-10 % CaO, 350 °C) were supposed to be the preferred conditions applied in real MSWIs (municipal solid waste incinerators). CaO significantly suppressed the chlorination of carbon matrix (superficial organic Cl (CCl) reduced from 16.5 % to 6.5-11.3 %) and the formation of unsaturated hydrocarbons or aromatic carbon (superficial CC decreased from 6.7 % to 1.3-2.1 %). Also, CaO promoted the dechlorination of Cu-based catalysts and Cl solidification (e.g., conversion of CuCl₂ to CuO, and formation of CaCl₂). The dechlorination phenomenon was validated by the dechlorination of highly chlorinated PCDD/F-congeners (via DD/DF chlorination pathways). Density functional theory calculations revealed that CaO facilitated the substitution of Cl by -OH on the benzene ring to inhibit the polycondensation of the chlorobenzene and chlorophenol (Gibbs free energy reduced from +74.83 to -36.62 and - 148.88 kJ/mol), which also indicates the dechlorination effect of CaO on de novo synthesis.

Keywords: CaO; Dechlorination effect; Density functional theory; Hydroxide substitution; Inhibition mechanisms; Metal passivation.