Robust porous PAN-MIL-101(Fe) nanofiber composite membranes for highly selective phosphate removal and recovery from water

The water stability of metal–organic framework (MOF) adsorbents is critical for efficient adsorption and recovery of target pollutants while avoiding secondary contamination in wastewater. In this study, we employed electrospinning to encapsulate MIL-101(Fe) into polyacrylonitrile/polyvinylpyrrolidone (PAN/PVP) nanofibers. Afterward, PVP was removed to generate pores, resulting in the formation of porous PAN-MIL-101(Fe) nanofiber composite membranes (PPAN-M−NFMs) with excellent water stability. The mesoporous structure of the PPAN-M−NFMs exposes more active sites of MIL-101(Fe), enhancing its affinity for phosphate ions. As a result, the membranes exhibited excellent removal performance, reducing 5 mg·L -1 of phosphate to meet the WHO discharge standards within 60 min. Moreover, they demonstrated highly selective adsorption toward phosphate with separation factors exceeding 155 relative to interfering ions. Moreover, the stability of MIL-101(Fe) in the composite membranes was largely enhanced with almost no leaching of iron ions or the ligand because of the improved water resistance provided by the penetration of the PAN chains. The adsorption performance remained stable across eight cycles, with the phosphate recovery of 92.3 % after seven cycles. The recovered phosphate was precipitated with calcium salt to obtain calcium superphosphate, which could be utilized as a phosphate fertilizer. Phosphate adsorption by the PPAN-M−NFMs was attributed primarily to electrostatic attraction and ligand exchange between MIL-101(Fe) and phosphate ions. This study presents a promising approach for designing highly stable MOF-based adsorbents for the selective removal and recovery of phosphate from wastewater.