Green water-etching synthesized La-MIL-101(Fe)-NH2@SiO2 yolk-shell nanocomposites with superior pH stability for efficient and selective phosphorus recovery

Abstract

Although MIL-101(Fe) has a homogeneous distribution of active metal sites, the relatively low stability of its coordination bonds limits its potential for application, especially in harsh environments. Here, a novel lanthanum-doped MIL-101(Fe)-NH2@mesoporous SiO2 yolk-shell (LMFN-YS) material, synthesized by green water-etching, has effectively improved the adsorption capacity and stability of MIL-101(Fe)-NH2. Compared to traditional acid-base etching methods, water etching offered a more co-friendly and cost-effective approach to producing yolk-shell structures. Due to the mesoporous protective shell of mesoporous SiO2 and more exposed active sites, the yolk-shell materials showed a faster adsorption rate (30 min) and a larger uptake capacity (240.0 mg P/g) than those of MIL-101. The material also exhibited broad range of pH stability from 2 to 10 and displayed good cyclic regeneration ability, with a removal efficiency of 75.9 % after three cycles. Additionally, it demonstrated excellent selectivity towards various coexisting anions such as Cl−, NO3−, SO42−, CO32− and HCO3− at diverse ionic strengths ranging from 0.001 to 1 M. The superior performance of L1MFN-YS (yolk-shell materials of La: Fe = 1:1) was attributed to a combination of factors, including ligand exchange between La(Fe)-OH and phosphates, Lewis acid-base interaction and electrostatic attraction. Additionally, high phosphate removal performance was observed in the real petrochemical wastewater (initial concentrations of 5.48 mg P/g) with a residual effluent concentration of 0.05 mg P/L, which was below the A-level standard of phosphate effluent in China (0.5 mg/L). This work provides a green method for preparing yolk-shell materials with improved ability to treat phosphate wastewater.