Selective adsorption of rare earth elements onto functionalized silica particles

Rare earth elements (REE) are essential components of equipment used for renewable energy, green technologies, and more traditional sectors such as chemical catalysis and metallurgy. Interest has been growing in alternative REE sources to supplement current ore sources and related refining, which have historically supplied the bulk of REE oxides. This study investigated the capability of adsorbent silica particles functionalized with three different ligands – phosphonoacetic acid (PAA), N,N-bisphosphono(methyl)glycine (BPG), and diethylenetriaminepentaacetic dianhydride (DTPADA) – to selectively extract REE from acidic to circumneutral aqueous brine solutions in equilibrium conditions. Maximum REE removal from 0.5 M NaCl solutions was displayed at pH 7 for PAA, pH 2 for DTPADA, and in both acidic and basic conditions for BPG functionalized materials. The REE adsorption performance for functionalized materials was largely unimpeded by the presence of competing ions (Ca, Mg, Zn, Fe, Al). Tests with real brines (I ∼ 3 M) showed >90% efficiency in REE recovery, which improved at higher temperatures (up to 100 °C). Effective elution of REE was accomplished with 0.7 N HNO3, and performance of the adsorbents improved with additional usage cycles.