Ho(III)-Based Metal–Organic Framework for Water Pollution Treatment: Insights into Sensitive Phosphate Removal and Sensing in Aqueous Solution

The existence of phosphate in aquatic ecosystems generates eutrophication. As a novel alternative to tackling this problem, this study reported a Ho(III)-based metal–organic framework (MOF) for removing and detecting phosphate molecules. Ho-bpdc MOF shows an outstanding maximum adsorption capacity of 311.9 mg g–1, with high pH (4–10) stability. PSO and Freundlich's models reveal that the chemical interaction between the phosphate molecules and Ho-bpdc plays the main role during the removal process. The combination of FTIR and XPS characterizations confirmed the possible interaction mechanism involving a ligand-exchanged process. Thus, the formation of Ho–O–P sites was determined due to the relatively strong base character for the phosphate ion. Moreover, Ho-bpdc exhibited outstanding fluorescence properties for the sensing of phosphate molecules. The fluorescence experiments showed a LOD of 2.57 ppm, based on the solid-state emission spectra of Ho-bpdc at different phosphate concentrations (5–100 ppm). Ho-bpdc exhibited a characteristic turn-on effect after the phosphate interaction due to the increments of the electronic π* → π transition of ligands. This work demonstrated the remarkable properties of Ho-bpdc in the adsorption and sensing of phosphate.