Removal of norfloxacin from high salinity wastewater by Hf-porphyrin MOF with missing linker defects: Insights into anion trapping and photoinduced charge transfer effects

Treatment of saline wastewater with advanced oxidation technologies has been an urgent challenge due to the inhibitory effect of coexisting ions on reactive oxygen species in real wastewater. Herein, monocarboxylic acids with varying acidity coefficient were employed to modulate the number of missing-linker defects in Hf-porphyrin MOFs (Hf-TCPP-X) to regulate their performance in removing norfloxacin (NFX) from complex aqueous matrices. The missing-linker defect density in Hf-MOFs promoted the removal efficiency of NFX from 8.5 % to 93.6 %. In various anion surroundings, the adsorption and photocatalysis performance of Hf-TCPP-FA with missing-linker defects was enhanced significantly. Quenching tests and the electron paramagnetic resonance (EPR) exhibited that O2∙−, 1O2, and ∙OH dominated NFX removal and their content rising in Hf-TCPP-FA/NaCl/visible-light (Hf-TCPP-FA/NaCl/vis) system. Density functional theory (DFT) calculations displayed that the adsorption energy of Hf-TCPP-FA for NFX decreased from −12.128 eV to −19.157 eV after adding Cl−. The missing-linker defects of catalysts facilitated the photoinduced charge transfer (PICT) under the visible-light irradiation and increased the number of active sites (Hf) in high salinity water. Additionally, favorable NFX removal efficiency could be sustained in Hf-TCPP-FA/visible-light (Hf-TCPP-FA/VIS) system from a wide pH range of aqueous solutions, high coexisting ion concentrations, and in the actual water matrices, and be performed well in cycling tests. This work suggests a promising route to tailor MOF-based photocatalysts by defect engineering and thus improve the removal performance of contaminants in highly saline water.