Degradation of phenol with Horseradish Peroxidase immobilized on ZnO nanocrystals under combined irradiation of microwaves and ultrasound

Zinc oxide (ZnO) nanocrystals with different morphologies were synthesized by varying the hydrothermal reaction conditions. Horseradish peroxidase (HRP) was successfully immobilized on ZnO nanocrystals with different morphologies. The activities of different types of the immobilized enzyme (nanodiscs, nanoflowers, and nanorods) were found to be 42.3, 26.1, and 14.1%, respectively. All three types of the immobilized enzyme showed higher phenol removal efficiency (for nanodiscs: 86.09%; for nanoflowers: 79.46%; for nanorods: 77.03%) compared to the corresponding value for the free enzyme (61.52%). The removal of phenolic compounds from aqueous solution using ZnO-immobilized HRP was explored with additional five phenolic compounds used as model pollutants. In comparison with the soluble HRP, the ZnO-immobilized HRP exhibited higher removal efficiencies for these phenolic compounds. The results indicate that the technology combined with the irradiation of microwaves and ultrasound can further promote the oxidation of primary intermediates in the conversion experiment, while the immobilization showed an obvious protective effect on HRP against the inactivation/inhibition effects.