Discovery of Significant Emission of Halogenated Polycyclic Aromatic Hydrocarbons From Secondary Zinc Smelting

Chlorinated and brominated polycyclic aromatic hydrocarbons (Cl/Br-PAHs) are persistent organic pollutants, and controlling their adverse effects requires the identification of their sources. However, current knowledge on the formations of Cl/Br-PAHs is far insufficient for source control, much less on their formation mechanisms. In this field study, we firstly discovered secondary zinc smelting as a significant source of Cl/Br-PAHs. The mean concentration of Cl/Br-PAHs emitted in stack gas from secondary zinc smelting was 9,553 ng/m3, exceeding concentrations from other metal smelting sources by one or two orders of magnitude. Cl/Br-PAHs with fewer rings were the dominant congeners in stack gas from secondary zinc smelting and secondary copper smelting. However, emissions from secondary aluminum smelting were dominated by congeners with more rings. The differences in congener profiles were attributable to the catalytic effects of metal compounds during smelting activities. Zinc oxide and copper oxide dominantly catalyzed dehydrogenation reactions, contributing to the formation of intermediate radicals and subsequent dimerization to Cl/Br-PAHs with fewer rings. Aluminum oxide induced alkylation reactions and accelerated ring growth, resulting in the formation of Cl-PAHs with more rings. The newly proposed mechanisms can successfully explain the emission characteristics of Cl/Br-PAHs during smelting activities and the higher concentrations of Cl/Br-PAHs from secondary zinc smelting, which should be useful in targeted source control.