Automated Electrospinning Equipment| Robust porous PAN-MIL-101(Fe) nanofiber composite membranes forhighly selective phosphate removal and recovery from water'

Introduction

In recent decades, rapid urbanization, industrialization and agricultural activities have exacerbated water pollution, with eutrophication being a major cause. Eutrophication, triggered by excessive discharge of phosphorus-containing wastewater, can lead to sharp declines in dissolved oxygen, threatening aquatic life. Therefore, strict control of phosphorus emissions and efficient removal of phosphorus from water bodies are crucial. Moreover, phosphorus is an essential nutrient for all organisms and plays an irreplaceable role in boosting agricultural yields, primarily in the form of phosphate fertilizers. Researchers have developed various technologies to recover phosphorus from wastewater, converting it into forms such as struvite, vivianite and hydroxyapatite, which can be used in slow-release fertilizers, chemical additives and other fields. Thus, phosphorus removal and recovery represent a dual-effect strategy for both environmental protection and resource conservation.

Metal-organic frameworks (MOFs) can be tailored through rational design of metal centers and organic ligands, offering significant advantages such as large specific surface area, high porosity and strong chemical tunability. These properties endow MOFs with high adsorption capacity, stable porous structures and catalytic active sites, making them widely applicable in phosphate adsorption. Among them, iron-based MOFs demonstrate remarkable phosphate adsorption capabilities. For instance, graphene oxide (GO)/MIL-101(Fe, Cu) composites prepared through electrostatic attraction, coordination bonds and hydrogen bonding achieve excellent phosphate adsorption; MIL-100(Fe) can be used for efficient phosphate removal from eutrophic water samples, with a maximum adsorption capacity of 93.6 mg·g⁻¹. However, existing MOF adsorbents are mostly in powder form, prone to particle agglomeration and difficult to recover for large-scale applications. Furthermore, iron-based MOFs such as Fe-MOF-74 and MIL-Fe-MOFs exhibit poor water stability, with morphology damage and structural unit leaching occurring after prolonged water exposure. Therefore, there is an urgent need to develop MOF-based materials that combine high water stability, anti-agglomeration properties and easy recovery to achieve efficient phosphate adsorption and enrichment.

Electrospinning technology is widely used to prepare highly porous functional nanofiber membranes, which can serve as flexible substrates for loading MOF materials, effectively preventing MOF particle agglomeration. Typically, MOF-polymer fiber composites can be achieved through two methods: in-situ growth (growing MOF crystals on fiber surfaces) or co-spinning (blending MOF particles with polymer solutions before spinning). Among these, the co-spinning method has attracted significant attention due to its tight integration, low cost and operational simplicity. However, since MOF particles are wrapped by polymer fibers, their specific surface area and number of active sites may be reduced. In our previous research, we successfully prepared porous nanofibers by introducing appropriate pore-forming agents during electrospinning. Therefore, combining MOFs with highly porous nanofibers can not only increase the exposure of MOF active sites but also inhibit particle agglomeration and facilitate adsorbent recovery and regeneration, which is of great significance for constructing efficient adsorption materials.

Recently, the research teams of Xianbiao Wang from Anhui Jianzhu University and Huanting Wang from Monash University, Australia, employed electrospinning technology to encapsulate MIL-101(Fe) in polyacrylonitrile/polyvinylpyrrolidone (PAN/PVP) nanofibers, subsequently forming a porous structure by removing PVP, successfully preparing porous PAN-MIL-101(Fe) nanofiber composite membranes (PPAN-M-NFMs) with excellent water stability. The related research, titled "Robust porous PAN-MIL-101(Fe) nanofiber composite membranes for highly selective phosphate removal and recovery from water," was published in the journal Separation and Purification Technology. The mesoporous structure of PPAN-M-NFMs exposes more active sites of MIL-101(Fe), enhancing its affinity for phosphate ions. Experiments showed that this composite membrane could reduce phosphate concentrations from 5 mg·L⁻¹ to World Health Organization (WHO) discharge standards within 60 minutes, demonstrating outstanding removal performance; its selectivity for phosphate adsorption was remarkable, with separation factors exceeding 155 relative to interfering ions. Additionally, due to the significant improvement in water resistance from PAN chain penetration, the stability of MIL-101(Fe) was greatly enhanced, with almost no leaching of iron ions or ligands. After 8 cycles of use, the adsorption performance remained stable, with phosphate recovery rates still reaching 92.3% after 7 cycles. The recovered phosphate could be converted into calcium superphosphate through precipitation with calcium salts for reuse as phosphate fertilizer. The primary adsorption mechanism of PPAN-M-NFMs involves electrostatic attraction and ligand exchange between MIL-101(Fe) and phosphate ions. This study provides an effective strategy for designing highly stable MOF-based adsorbents for selective phosphate removal and recovery from wastewater.

Scheme 1. Schematic illustration of the preparation of PPAN-M-NFMs

Figure 1. SEM images of (a) PAN/PVP-NFMs, (b) PPAN-1.0M-NFMs, (c) PPAN-1.5M-NFMs, (d) PPAN-2.0M-NFMs and (e) PAN/PVP-2.0M-NFMs, (f) TEM image of PPAN-2.0M-NFMs

Figure 2. (a) Nitrogen adsorption–desorption isotherms, (b) corresponding DFT pore size distributions and (c) water contact angles of different samples

Figure 3. (a) Effect of pH on phosphate adsorption by PPAN-2.0M-NFMs, (b) zeta potential of PPAN-2.0M-NFMs at various pH values, (c) adsorption capacity of PPAN-M-NFMs loaded with different masses of MIL-101(Fe), (d) phosphate concentration after adsorption with varying amounts of PPAN-2.0M-NFMs

Figure 4. (a) Effects of coexisting anion concentrations on selective phosphate adsorption by PPAN-2.0M-NFMs, (b) iron ion leaching and (c) XRD patterns of MIL-101(Fe) and PPAN-2.0M-NFMs after washing with water, (d) cycling performance and phosphate desorption efficiency of PPAN-2.0M-NFMs

Figure 5. (a) P2p spectra, (b) Fe2p spectra, (c) O1s spectra, (d) FTIR spectra and (e) XRD patterns of the PPAN-2.0M-NFMs before and after phosphate adsorption

Scheme 2. Possible mechanisms for phosphate adsorption on PPAN-M-NFMs and bare MIL-101(Fe)

Research significance

This study successfully developed a stable and recyclable porous PAN nanofiber-encapsulated MIL-101(Fe) composite membrane (PPAN-M-NFMs) and systematically evaluated its phosphate adsorption and recovery performance. Structurally, MIL-101(Fe) was embedded within the fibers while directional pores were introduced on the fiber surfaces, increasing the exposure of MIL-101(Fe) active sites while significantly enhancing the membrane's water stability. Experiments showed that the adsorption behavior of PPAN-2.0M-NFMs followed the Freundlich isotherm model and pseudo-second-order kinetics, indicating heterogeneous surface chemical adsorption as the dominant mechanism. Furthermore, the material demonstrated excellent selective phosphate capture capability in coexisting anion environments, maintaining over 80% adsorption capacity after 8 adsorption-desorption cycles with phosphate desorption efficiency as high as 92.3%, fully demonstrating its potential as a phosphate fertilizer recovery carrier. Although the presence of the polymer matrix slightly reduced active site density compared to bare MOFs, PPAN-M-NFMs' high structural stability, easy aqueous-phase recovery and excellent reusability make them highly promising for practical applications in phosphate removal and recovery from water environments.

Paper link: https://www.sciencedirect.com/science/article/pii/S138358662502338X?via%3Dihub