Enhancement of sulfate radicals degrading phenol in water by an external electric field: a study by DFT calculations

This study investigates the effect of an external electric field (EF) on the oxidative properties of sulfate radicals (SRs) generated from activated persulfate during the degradation of phenol by discharge plasma. In this paper, density functional theory (DFT) was employed to explore the variations in the reaction rate of phenol degradation by SRs under the influence of an external EF. Additionally, free energy barriers, spin densities, dipole moments (DMs), frontier molecular orbitals, and electrostatic potentials on van der Waals (vdW) surfaces were analyzed to elucidate the mechanisms driving the changes in reaction rate. It was found that discharge plasma with the +<i>X</i>-axis EF increased the degradation rate of phenol, while the degradation rate declines when the -<i>X</i>-axis EF is applied. As the strength of the +<i>X</i>-axis EF increased, a corresponding decrease in the free energy barrier and HOMO-LUMO gap was observed, alongside increases in the DM, spin density, and electrostatic potential. It can be concluded that the application of the +<i>X</i>-axis EF decreases the thermal stability of the phenol degradation system while enhancing electrostatic interactions, thereby leading to an increase in reactivity.