Radical attack and mineralization mechanisms on electrochemical oxidation of p-substituted phenols at boron-doped diamond anodes

Degradation of phenols with different substituent groups (including -OCH₃, -CHO, -NHCOCH₃, -NO₂, and -Cl) at boron-doped diamond (BDD) anodes has been studied previously based on the removal efficiency and •OH detection. Innovatively, formations of CO₂ gas and various inorganic ions were examined to probe the mineralization process combined with quantitative structure-activity relationship (QSAR) analysis. As results, all phenols were efficiently degraded within 8 h with high COD removal efficiency. Three primary intermediates (hydroquinone, 1,4-benzoquinone and catechol) were identified during electrochemical oxidation and degradation pathway was proposed. More importantly, CO₂ transformation efficiency ranked as: no N or Cl contained phenols (p-CHO, p-OCH₃ and Ph) > N-contained phenols (p-NHCOCH₃ and p-NO₂) > Cl-contained phenols (p-Cl and o,p-Cl). Carbon mass balance study suggested formation of inorganic carbon (H₂CO₃, CO₃^2- and HCO₃^-) and CO₂ after organic carbon elimination. Inorganic nitrogen species (NH₄⁺, NO₃^- and NO₂^-) and chlorine species (Cl^-, ClO₃^- and ClO₄^-) were also formed after N- and Cl-contained phenols mineralization, while no volatile nitrogen species were detected. The phenols with electron-withdrawing substituents were easier to be oxidized than those with electron-donating substituents. QSAR analysis indicated that the reaction rate constant (k₁) for phenols degradation was highly related to Hammett constant (∑σ_o,m,p) and energy gap (E_LUMO - E_HOMO) of the compound (R² = 0.908), which were key parameters on evaluating the effect of structural moieties on electronic character and the chemical stability upon radical attack for a specific compound. This study presents clear evidence on mineralization mechanisms of phenols degradation at BDD anodes.