Insights into reaction mechanisms and kinetic characteristics of electron-beam irradiation-induced decomposition of 2,3,7,8-TCDD: a ReaxFF-based MD simulation study.

OBJECTIVE: Recognized by the International Atomic Energy Agency (IAEA) as a pivotal technology for the peaceful use of nuclear energy, electron beam (EB) irradiation holds significant potential for degrading persistent organic pollutants, such as polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). However, the atomistic reaction mechanisms involved in this process remain inadequately understood, impeding the rational optimization and practical application of the technology. METHODS: To address this knowledge gap, molecular dynamics (MD) simulations using reactive force field (ReaxFF-based MD simulation) were employed to investigate the EB-induced decomposition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) over a wide temperature range of 1000-3000 K. RESULTS: The simulations reveal that hydroxyl radicals (·OH) exhibit the highest reactivity among the key reactive species (·OH, O and O3) generated in EB irradiation. At 3000 K and ER=1 (defined as the stoichiometric ratio of ROS to 2,3,7,8-TCDD), the decomposition rate of 2,3,7,8-TCDD in the presence of ·OH is enhanced by factors of 2.0 and 4.3 compared to O and O3, respectively. Temperature is also shown to critically influence both the reaction pathways and product distribution. At 1000 K, 2,3,7,8-TCDD initially undergoes polymerization prior to fragmentation, while direct dechlorination and ring-opening prevail at 2000 K and 3000 K. Correspondingly, increasing the temperature from 1000 K to 3000 K reduces the time required for the complete decomposition of 2,3,7,8-TCDD by an order of magnitude, elevates the CO2 selectivity from 13% to 95% and also shifts the dominant chlorine-containing product from HClO to HCl. Furthermore, a stepwise degradation pathway of 2,3,7,8-TCDD initiated by ·OH is proposed based on the simulation trajectories, involving sequential dechlorination, ring cleavage and ultimate mineralization. The kinetic analysis shows that the reaction follows 1st-order dependence on 2,3,7,8-TCDD and 0.32-order dependence on ·OH, with an apparent activation energy of 28.70 ± 0.3 kJ·mol⁻1 and a pre-exponential factor of (4.72 ± 0.5) × 1012 mol·L⁻1·s⁻1. CONCLUSIONS: Overall, this work provides fundamental insights into the reaction mechanisms and kinetics governing the EB-induced degradation of 2,3,7,8-TCDD, offering a theoretical basis for the advanced application of EB technology in environmental remediation.