Thermal catalytic degradation of α-HBCD, β-HBCD and γ-HBCD over Fe3O4 micro/nanomaterial: Kinetic behavior, product analysis and mechanism hypothesis

Abstract

The new persistent organic pollutant (POP), 1,2,5,6,9,10-hexabromocyclododecane (HBCD), has been widely detected in various environmental media and proved to be biotoxic. However, the research on catalytic degradation of HBCD is in its infancy. Herein, we examined the degradation of α-HBCD, β-HBCD and γ-HBCD, over Fe3O4 micro/nanomaterial at 200 °C. The pseudo-first-order kinetic rate constants were in the range of 0.04–0.15 min−1, with half-life values of 5–19 min. γ-HBCD is slightly less stable than β-HBCD, but both of them readily convert into α-HBCD, as consistent with the Gibbs free energies of isomers themselves. The four products containing pentabromocyclododecene, two isomers of tetrabromocyclododecene and 1,5,9-cyclododecatriene were detected by conventional GC–MS. Interestingly, a high-throughput non-target product detection were performed by ESI-FT-ICR-MS, where up to 59 types of intermediate products were determined. It is tentatively proposed that different types of bromine-removed products (C12H17Br5, C12H18Br4, C12H18, C12H19Br5, C12H24 and C12H19Br5O) and cyclododecane ring-opened products (C12H19Br7, C12H20Br6O and C12H20Br6) form via elimination reaction, nucleophilic substitution, hydrodebromination and addition reaction. Besides, most of the products that were detected contained oxygen. The average carbon oxidation state (OSc¯) of the products indicate that the oxidation reaction is the dominant reaction type. Deep oxidation products, such as small molecular organic acids (formic, acetic, propionic, and butyric acids) and gas-phase oxidation products (CO2 and CO) were further detected by ion chromatography and GC-FID, respectively. This study might provide an alternative technique for the low-cost treatment of HBCD waste.