Electrically Enhanced Hydroxyl Radicals Degrading Malachite Green in Water: A Study by DFT Calculation and Experiment

Here, density functional theory (DFT) simulations and experimental studies were conducted to explore the effect and mechanism of an external electric field (EF) on the decolorization efficiency and degradation rate of malachite green (MG) in water by hydroxyl radicals generated in the discharge plasma process. The simulation results indicated that the external EF had a significant impact on the degradation rate. Under the action of the EF along the reaction axis direction, the free energy barrier of the reaction decreased and the reaction rate increased. As the EF strength increased from 0 to 0.002 a.u., the free energy barrier of reaction 1 decreased from 9.164 kcal/mol to 7.097 kcal/mol, and the reaction rate increased from 1.499 × 106 s−1M−1 to 4.764 × 107 s−1M−1; the free energy barrier of reaction 2 decreased from 2.318 kcal/mol to −1.262 kcal/mol, and the reaction rate increased from 1.313 × 1011 s−1M−1 to 5.457 × 1013 s−1M−1; the free energy barrier of reaction 3 decreased from 7.755 kcal/mol to 7.011 kcal/mol, and the reaction rate increased from 1.764 × 107 s−1 to 6.242 × 107 s−1. Under the action of the EF in the opposite direction along the reaction axis, the opposite effect was presented. In addition, we analyzed the surface electrostatic potential, dipole moment, condensed Fukui function and spin density under different-strength EFs to explore the mechanism of the influence of the external EF on the reaction and further clarified the feasibility of an external EF promoting the degradation of MG in water by hydroxyl radicals. The experimental study results showed that EF can promote the degradation of MG under a discharge plasma process. This is consistent with the trend of the results obtained from DFT simulations under the action of a positive EF with higher electric field intensity, verifying the feasibility of the method of EF-assisted discharge plasma degradation. In addition, we experimentally explored the changes in the decolorization efficiency of MG under different initial conditions.