Photocatalytic degradation kinetics and mechanism of pentachlorophenol based on superoxide radicals

The micron grade multi-metal oxide bismuth silicate (Bi₁₂SiO₂₀, BSO) was prepared by the chemical solution decomposition technique. Photocatalytic degradation of pentachlorophenol (PCP) was investigated in the presence of BSO under xenon lamp irradiation. The reaction kinetics followed pseudo first-order and the degradation ratio achieved 99.1% after 120 min at an initial PCP concentration of 2.0 mg/L. The pH decreased from 6.2 to 4.6 and the dechlorination ratio was 68.4% after 120 min at an initial PCP concentration of 8.0 mg/L. The results of electron spin resonance showed that superoxide radical (O₂^·-) was largely responsible for the photocatalytic degradation of PCP. Interestingly, this result was different from that of previous photocatalytic reactions where valence band holes or hydroxyl radicals played the role of major oxidants. Some aromatic compounds and aliphatic carboxylic acids were determined by GC/MS as the reaction intermediates, which indicated that O₂^·- can attack the bond between the carbon and chlorine atoms to form less chlorinated aromatic compounds. The aromatic compounds were further oxidized by O₂^·- to generate aliphatic carboxylic acids which can be finally mineralized to CO₂ and H₂O.