Immobilized horseradish peroxidase hybrid biocatalysts for degradation of pharmaceuticals and other emerging micropollutants.

With the increasing detection of emerging pollutants (EPs), such as pharmaceuticals, personal care products, and pesticides, in water bodies, there is a growing research priority to develop efficient methods for their degradation. In the present study, the degradation of 21 EPs from different classes was investigated using horseradish peroxidase (HRP) covalently immobilized onto two photocatalytic supports, TiO2 and ZnO. The hybrid biocatalysts (TiO2-HRP and ZnO-HRP) were used to degrade EPs with and without the presence of a redox mediator, 1-hydroxybenzotriazole (HOBT). The results revealed that not all EPs were equally degraded by HRP enzymes, with some pollutants showing complete, partial, or no degradation. For example, full degradation was achieved for MBT (100%), meloxicam (99.5%) and caffeic acid (98.6%). In contrast, prometryn and MCPA showed only partial degradation, reaching 19.2% and 20%, respectively, and no degradation was observed for fluometuron and atenolol. The addition of the model redox mediator, HOBT, improved the degradation of some pollutants, for example, lincomycin-HCl degradation increased to 36.9%, while trimethoprim degradation improved to 21.5%. Additionally, several pollutants, including roxithromycin, cimetidine, and caffeine, exhibited significantly higher degradation rates when treated with the immobilized HRP enzyme (TiO2-HRP or ZnO-HRP) compared to free HRP. For instance, roxithromycin degradation increased from 0.5% to 58.2% with ZnO-HRP, and for cimetidine, TiO₂-HRP improved degradation from 0.3% to 60.1% without HOBT and from 9.0% to 66.7% with HOBT. Molecular docking with the catalytic heme cofactor retained further suggested that degradation trends were not explained by docking score alone, but were influenced by ligand access to the distal heme pocket, molecular planarity, and contact topology near catalytic residues. Finally, an integrated HRP enzyme and chemical oxidation remediation strategy was applied to degrade two pollutants, trimethoprim and DEET. For trimethoprim, TiO2-HRP combined with H2O2 and UV achieved approximately 30% degradation, while for DEET, both TiO2-HRP + H2O2 + UV and ZnO-HRP + H2O2 + UV resulted in around 40% degradation, thereby highlighting a novel application for these hybrid peroxidase-photocatalysts.