Tuning the structure flexibility of metal-organic frameworks via adjusting precursor anionic species for selective removal of phosphorus

Abstract This study proposes a new strategy for manufacturing metal-organic frameworks (MOFs) with respective topologies and properties by adjusting the anionic species in the precursor solution. Three MOFs (e.g. Fe/Al (Cl-), Fe/Al (NO3-) and Fe/Al (SO42-)) were successfully synthesized and used for removal of phosphate from water. Surprisingly, when chlorinated salt was used as the precursor, only Al was detected in the Fe/Al (Cl-). However, when nitrate and sulfate were used as the precursors, Fe and Al were simultaneously detected in the Fe/Al (NO3-) and Fe/Al (SO42-). These MOFs exhibit ultrafast adsorption kinetics and high uptake capacity for phosphate. The maximum phosphate uptake capacity (130 mg P/g) of Fe/Al (NO3-) is much higher than that of most phosphate removal materials reported in the literature. More importantly, Fe/Al (NO3-) can still effectively remove phosphate from water bodies after multiple cycles of adsorption/desorption. When there are a large number of common interfering ions (Ca2+, Mg2+, CO32-, HCO3-, Cl-, NO3- and SO42-) in the water, the prepared MOFs can still specifically remove phosphorus from water. In addition, the prepared MOFs can adapt to the removal of phosphorus in a wide pH water body. Furthermore, based on FTIR and XPS spectra, chemical adsorption and ligand exchange were identified as the main phosphorus removal mechanisms by Fe/Al (NO3-). These findings prefigure that MOFs synthesized by regulating the anionic species can efficiently and economically remove phosphate from water.