Role of coke-bounded environmentally persistent free radicals in phenanthrene degradation by hydrogen peroxide

Emission of polycyclic aromatic hydrocarbons (PAHs) is accompanied with the discharge of carbonaceous particles during the coke production. To degrade the adsorbed PAHs, hydrogen peroxide (H₂O₂) was applied as an oxidising agent, which might be activated by the inherent environmentally persistent free radicals (EPFRs) on coke particles. The transformation of phenanthrene (PHE), selected as model molecule, was achieved in H₂O₂/coke particle system without the addition of additional activating agent. This process consumed the particle-bounded EPFRs, inducing the decreasing of spin density from 1.92 × 10¹⁸ to 4.4 × 10¹⁷ spins g⁻¹ in 30 min of reaction time. Electron paramagnetic resonance (EPR) technique coupled with spin-trapping agent 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) was used to probe the potential formation of reactive oxygen species. A higher capture [$\cdot \mathrm{OH}$] concentration was observed with larger decreases in EPFRs concentration, indicating that EPFRs were the main contributor to the formation of $\cdot \mathrm{OH}$. The obtained results suggested that the activation of H₂O₂ by EPFRs on coke particles resulted in the generation of hydroxyl radical ($\cdot \mathrm{OH}$), which then back-reacted with adsorbed PHE. The finding of this study shed light on a new remediation technology for toxic carbonaceous byproducts discharged during the coke production.