Degradation synergism between sonolysis and photocatalysis for organic pollutants with different hydrophobicity: A perspective of mechanism and application for high mineralization efficiency

Despite extensive studies, the fundamental understanding of synergistic mechanisms between sonolysis and photocatalysis for the abatement of persistent organic pollutants (POPs) remains uncertain. As different phases formed under ultrasound irradiation, hydrophilic POPs, sulfamethoxazole (SMX, Kow: 0.89), predominantly resides in bulk liquid and is ineffectively degraded by sonolysis (kUS = 3.33 x 10-3 min-1) since 10% of hydroxyl radicals (.OH) formed at the gas-liquid interface of cavitation is diffused into the bulk, whereas the other fraction rapidly recombines into hydrogen peroxide (H2O2). This study provides a proof-of-concept for the mechanism by presenting various analytical results, endorsing the synergistic role of photoexcited electrons in splitting sonolysis-induced H2O2 into .OH, particularly in the bulk phase. In a sonophotocatalytic system, the hydrophobic POPs such as bisphenol A (BPA) and atrazine (ATZ) were mainly degraded in gas-liquid interface indicated by the low synergistic values correlation compared to SMX [i.e., SMX has a higher synergistic factor, fsyn (3.26) than BPA (1.30) and ATZ (1.35)]. Also, fsyn was found linearly correlated with the contribution factor of photocatalysis to split H2O2. Three times of consecutive kinetics using an effluent of municipal (MP) wastewater spiked by POPs presented 98% POPs and >96% total organic carbon (TOC) removal.

Automated summary

Despite extensive studies, the synergistic mechanisms between sonolysis and photocatalysis for the removal of persistent organic pollutants (POPs) remain uncertain. This study provides a proof-of-concept for the role of photoexcited electrons in splitting sonolysis-induced hydrogen peroxide into hydroxyl radicals, particularly in the bulk phase. The synergistic degradation of hydrophobic POPs in a sonophotocatalytic system varies compared to hydrophilic POPs, with different degradation patterns observed in the gas-liquid interface and bulk liquid phase.