Zr6O8-porphyrinic MOFs as promising catalysts for the boosting photocatalytic degradation of contaminants in high salinity wastewater

The treatment of wastewater containing high salinity has been a pressing challenge due to the inhibition of reactive oxygen species by co-existing ions in the actual wastewater. In this work, four Zr6O8-porphyrinic MOFs (PCN-223, PCN-224, PCN-225, and MOF-525) with different topologies were selected to investigate their performance and mechanisms for the removal of contaminants from hypersaline waters in a visible photocatalytic degradation system. The results showed that the molar ratios of monocarboxylates, ligands, and Zr of the porphyrin-MOFs led to variations in the porosity, pore size distribution, crystal defects, and surface electrical properties of the as-obtained MOFs, resulting in differences in the adsorption and photocatalytic degradation performance of the materials towards bisphenol A. In-situ electron paramagnetic resonance (EPR) and quenching experiments showed that high salt surroundings can significantly increase the levels of O2∙-, 1O2 and ∙OH in Zr6O8-porphyrin-MOFs/visible-light systems. DFT calculations explain that the presence of anions not only promotes the adsorption of BPA by MOFs but also facilitates the conduction of photogenerated electrons and prevents the recombination of photogenerated electrons and holes during the photocatalytic process. The super-exchange charge transfer mechanism was shown to play a key role in the Zr6O8-porphyrin-MOFs/visible-light system, thereby accelerating the degradation of pollutants in high-salinity wastewater. The catalytic system can maintain consistent contaminant degradation efficiency over a wide pH range, with high concentrations of co-existing ions and in real water matrices. In addition, the catalyst also exhibits excellent stability and reusability. This work provides new insights into the application of porphyrin MOFs based visible photocatalytic processes for the treatment of highly saline wastewater.