Toward greener MOFs in phosphate adsorption: A performance-based approach to green MOF selection

Abstract

This study presents a performance-driven selection of green metal-organic frameworks (MOFs), integrating yield, cost-effectiveness, and adsorption efficiency, critical yet often overlooked criteria in MOF development. The selected MOFs were synthesized using environmentally benign solvents, ethanol (EtOH) and deionized water (DIW), under mild temperature conditions, offering a scalable and sustainable route. Three zirconium-based green MOFs were identified: one Terephthalic acid (TPA)-based (GR2-MOF(80)) and two Pyromellitic acid (PMA)-based (GR8-MOF(25) and GR10-MOF(25)). These materials outperformed commercial benchmarks such as UIO-66, exhibiting high phosphate adsorption capacities while following Langmuir isotherm behaviour, pseudo-second-order kinetics, and intra-particle diffusion mechanisms. Modifications in the activation process further enhanced adsorption, achieving up to 83 mg PO₄·g⁻¹ for the TPA-based MOF and 221 mg PO₄·g⁻¹ for the PMA-based MOFs. Structural characterization via scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) confirmed the crystalline nature of the TPA-based MOF and the amorphous character of the PMA-based ones, supporting chemisorption as the dominant mechanism. Beyond capacity, these MOFs demonstrated high selectivity in both binary and complex media and retained efficiency over multiple reuse cycles, reinforcing their practical potential for environmental applications. More than a case study on phosphate removal, this work demonstrates how a rational selection framework can guide the development of environmentally sustainable MOFs for broader applications in water treatment and beyond.