MIL-100(Fe)-derived Mg/Fe layered double hydroxides composite millimeter-scale aerogel beads for phosphate adsorption from real domestic wastewater

Abstract

Phosphate adsorption is an effective strategy for addressing phosphate pollution in wastewater, alleviating eutrophication, and recovering phosphorus resources. In this study, an environmentally friendly approach was employed to develop a new adsorbent material, MIL@MgFe-LDH/SA, derived from metal-organic frameworks (MOFs)-based layered double hydroxide (LDH) and fabricated into aerogel beads for phosphate removal from wastewater. By selecting MIL-100(Fe) as a template and utilizing a simple alkaline hydrolysis-assisted coprecipitation method, the excellent morphological characteristics of the MOFs were preserved in the aerogel beads, maintaining the distinct structure of the LDH nanosheets and significantly enhancing their adsorption performance. The aerogel beads, prepared via a crosslinking granulation method, effectively overcome common issues faced by powder adsorbents, such as compression, agglomeration, and difficulty in recovery during the adsorption process. These aerogel beads exhibit an extremely low density, enabling significant swelling in aqueous solution and recovery to a hydrogel state, thereby maintaining good stability and functionality. In a 200 mg P/L phosphate solution, MIL@MgFe-LDH/SA aerogel beads demonstrated an adsorption capacity of 60.26 mg P/g. The adsorption process followed the pseudo-second-order kinetic model and the Freundlich adsorption isotherm. Even in the presence of coexisting anions (e.g., Cl^–, NO₃^–, and CO₃^2–), the sample displayed excellent selectivity for phosphate removal. The microstructure and characteristics of the sample were thoroughly analyzed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption–desorption tests, and Zeta potential measurements. Furthermore, Density functional theory (DFT) calculations and molecular dynamics (MD) simulations were conducted to explore the phosphate adsorption mechanism. The results indicate that phosphate adsorption primarily occurs through electrostatic interactions and surface electron transfer between the LDH surface and phosphate species, demonstrating that MIL@MgFe-LDH/SA aerogel beads possess excellent phosphate removal capacity and can effectively eliminate phosphates from aqueous solutions