Removal of tetracycline by a photocatalytic membrane reactor with MIL-53(Fe)/PVDF mixed-matrix membrane

This work demonstrates the synergistic effect of two-unit operations: filtration and photocatalysis to remove tetracycline (TC) from wastewater by immobilizing MIL-53(Fe) photocatalyst into the poly (vinylidene fluoride) (PVDF) matrix to create a mixed-matrix membrane (MMM) in a photocatalytic membrane reactor (PMR). Herein, 1.0–5.0 wt% of MIL-53(Fe) was blended into a PVDF-based casting solution to obtain MMMs after phase-inversion. The addition of MIL-53(Fe) resulted in an increased viscosity of the casting solution which caused the macrovoids decorating the cross-section of the polymer matrix to be suppressed, consequently decreasing the bulk pore size from 0.06 to 0.014 µm. Moreover, it also increased the bulk tortuosity by suppressing the macrovoid formation which enhanced the depth-filtration mechanism of the modified membranes, particularly P10M5 which delivered the highest rejection of 87 % against TC. Meanwhile, LC/MS technique confirmed the degradation of TC under UV illumination and revealed that P10M5 provided optimal condition for degrading TC with 93 % degradation efficiency. The trapping experiments revealed that the dominating reactive oxidation species responsible during the degradation process were h+ and •OH species. Our results indicate that TC was photocatalytically degraded through a series of de-amidation, de-hydroxymethylation, and ring opening reactions. Through the integration of size-exclusion/depth-filtration mechanisms offered by the membrane, and degradation process provided by MIL-53(Fe), the PMR system using P10M5 successfully removed tetracycline in the permeate and < 1 % undegraded TC remained. The findings of this study offer a viable approach to solve the crises of pharmaceutical products in wastewater by simultaneously rejecting and degrading these compounds.