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 –OCH 3 , –CHO, –NHCOCH 3 , –NO 2 , and −Cl) at boron-doped diamond (BDD) anodes has been studied previously based on the removal efficiency and •OH detection. Innovatively, formations of CO 2 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, CO 2 transformation efficiency ranked as: no N or Cl contained phenols ( p -CHO, p -OCH 3 and Ph) > N-contained phenols ( p -NHCOCH 3 and p -NO 2 ) > Cl-contained phenols ( p -Cl and o,p -Cl). Carbon mass balance study suggested formation of inorganic carbon (H 2 CO 3 , CO 3 2− and HCO 3 − ) and CO 2 after organic carbon elimination. Inorganic nitrogen species (NH 4 + , NO 3 − and NO 2 − ) and chlorine species (Cl − , ClO 3 − and ClO 4 − ) 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 ( k 1 ) for phenols degradation was highly related to Hammett constant (∑ σ o , m , p ) and energy gap ( E LUMO - E HOMO ) of the compound ( R 2  = 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.