Influence of inevitable iron precursor anions on MIL-100(Fe) structure and its La(Ⅲ) adsorption performance

Abstract

The influence of iron precursor anions on the structure and properties of MIL-100(Fe) has often been overlooked. In this study, iron chloride hexahydrate and iron nitrate nonahydrate were used to synthesize MIL-100(Fe)-Cl and MIL-100(Fe)-N, respectively. Characterization comparisons revealed that MIL-100(Fe)-Cl exhibited a higher organic ligand content and a larger specific surface area. Subsequently, the adsorption properties of both materials for the rare-earth element lanthanum (La(Ⅲ)) were investigated. MIL-100(Fe)-Cl demonstrated superior La(Ⅲ) adsorption performance, with a maximum adsorption capacity of 54.7 mg·g−1, which is more than twice that of MIL-100(Fe)-N at 25.5 mg·g−1. The adsorption rate of MIL-100(Fe)-Cl for low-concentration La(Ⅲ) approached 100 % and sustained effective adsorption under weakly acidic conditions. The La(Ⅲ) adsorption processes on MIL-100(Fe)-Cl and MIL-100(Fe)-N were spontaneous and endothermic, involving both physisorption and chemisorption. Notably, a strong chemical interaction occurred during La(Ⅲ) adsorption by MIL-100(Fe)-Cl, whereas ion exchange was predominant in MIL-100(Fe)-N. Through further exploration of the adsorption mechanism via FT-IR and XPS characterizations, the main chemical active sites were the uncoordinated –COOH and terminal –OH groups of the Fe trimer. This study underscores the significant impact of different iron precursors on the synthesis of MIL-100(Fe), highlighting the need for careful selection of these materials in precursor choice.