Removal of Rare-Earth Elements from Aqueous Solutions by Microporous Titanosilicate Ets-4

Abstract

The unsustainable nature of the mining of Rare-Earth Elements (REE) raises environmental, economic, and societal concerns, prompting the development of suitable technologies for their removal from contaminated waters. Sorption is, in general, a clean and efficient method to decontaminate water. While microporous titanosilicates are promising materials for REE removal from water, the effect of solution parameters on this process is poorly understood. The present work assesses the influence of pH (4-8), concentration (1-5 µM) and sorbent amount (20-180 mg L-1) on the removal from water of a REE3+ mixture (Y, La, Ce, Pr, Nd, Eu, Gd, Tb, Dy) by the microporous titanosilicate ETS-4 (Na9Ti5Si12O38(OH)·12H2O). The experimental conditions studied were based on a Box-Behnken design, and the results were expressed as three-dimensional response surfaces. The removal mechanism relies on the ion exchange of Na+ by REE3+, with pH being the main process variable (i.e., competition with H+). Increasing pH results in higher removal, which reaches 100% for ETS-4 amounts of 180 mg L-1 and REE concentrations of 1 µM. Most REE3+ removal occurs in the first 15 minutes, with little difference between metals (except for Y, which shows lower removal). For ETS-4 amounts of 20 mg L-1, REE are highly concentrated in the sorbent, reaching 145 mg g-1 (15% of the sorbent’s mass, post sorption). The results show that even low ETS-4 amounts have a considerable potential to remove high concentrations of REE3+ from aqueous solutions with pH close to natural systems.

Keywords: Sorption, Response surface, REE, Recovery, Ion exchange