Adsorption of rare earth metals (Sr2+ and La3+) from aqueous solution by Mg-aminoclay-humic acid [MgAC-HA] complexes in batch mode

Abstract

The recoveries of Sr2+ and La3+ as rare earth metals (REMs) were studied using Mg-aminoclay-humic acid [MgAC-HA] complexes prepared by self-assembled precipitation due to electrostatic attraction between water-solubilized [MgAC] and water-soluble [HA], and were compared with the recoveries using [MgAC] and [HA]. The influences of pH and Sr2+ and La3+ concentrations in single and binary systems were evaluated. The adsorbents before/after adsorption of Sr2+ and La3+ were characterized by (1) scanning electron microscopy (SEM) micrographs, (2) Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS) spectra, and by (3) powder Xray diffraction (XRD) pattern analysis. After fitting Langmuir and Freundlich isotherms, the Langmuir model was found to present better matches than the Freundlich one: the maximum adsorption capacities of Sr2+ and La3+ were 0.12 mg g(-1) and 4.76 mg g(-1) in the binary system at room temperature, and the optimal recovery pH was similar to 8.0. In practical seawater meanwhile, the recoveries of Sr2+ and La3+ by [MgAC-HA] complexes were the highest in the binary system. However, with further recycling runs, the recoveries of Sr2+ and La3+ were critically diminished due to disassembly in [MgAC-HA] complexes under acidic conditions. Thus, for the purposes of industrial application, we are currently pursuing the enhancement of recyclability for [MgAC-HA] complexes by their encapsulation or direct hydrogel formation.