Cross-Coupling of 1,2,3,4-Tetrachlororodibenzo-<i>p</i>-dioxin with Six Coexisting Polycyclic Aromatic Hydrocarbons during Photodegradation on a Fly Ash Surface

The adverse conditions of the garbage incineration process can lead to the generation of dioxins and polycyclic aromatic hydrocarbons (PAHs). This study aimed to investigate the removal efficiency and possible cross-coupling effect of 1,2,3,4-tetrachlorodibenzo-<i>p</i>-dioxin (1,2,3,4-TCDD) and six coexisting low-molecular-weight PAHs during photodegradation on the fly ash surface. Due to their higher photoreactivity and light-shielding effect, the six PAHs exhibited inhibitory effects on the photodegradation of 1,2,3,4-TCDD, causing a reduction of 4.1%-21.2% in the removal efficiency. Common degradation products of 1,2,3,4-TCDD and PAHs were identified by LC-MS and GC-MS, and the formation of primary products was verified by theoretical calculations of bond dissociate energies, excitation energy, frontier electron densities, and transition states. In addition, high-molecular-weight coupling products of 1,2,3,4-TCDD and its interaction products with PAHs were observed in the mixed irradiation samples, and two coupling elimination mechanisms were proposed to illustrate their formation through C-O-C bonding and -COO- bonding, respectively. According to toxicity prediction analysis, the developmental toxicity and mutagenicity of most interaction products were higher than 1,2,3,4-TCDD. This study provided some new insights into the transformation, interaction, and related ecological risks of dioxins and PAHs coexisting on the surface of fly ash during the waste incineration process.