Insights into the mechanisms of REE removal by structurally analogous gypsum and brushite phases

This study reports the effects of precipitation of sulfate and phosphate phases with very similar crystal structures (i.e., gypsum and brushite) on the mobility of critical raw materials such as rare earth elements (REEs) dissolved in aqueous solution. It aims to establish the potential mechanisms of REEs removal and compare the REEs immobilization capacities of the studied mineral phases, where these valuable elements could be concentrated for possible recovery. To induce gypsum and brushite precipitation, solutions of CaCl 2 ∙2H 2 O and Na 2 SO 4 or (NH 4 ) 2 HPO 4 were added simultaneously to aqueous solutions containing different concentrations of REEs (i.e., La, Eu, Gd or Lu). The content of REEs in the sulfate and phosphate phases increased progressively as solutions with higher concentrations of these target metals were used. REE concentrations in brushite were significantly greater than in gypsum, generally ranging from 2 to 26 times those in gypsum, with an average of approximately 10-fold. In this sense, the REEs elimination rates by gypsum (less than 65% in all cases studied) were significantly lower than those by brushite precipitation (around 100%). The results of this study suggest that removing REEs from aqueous solutions through brushite precipitation could be an effective strategy for concentrating these valuable metals prior to potential recovery. • Removal of aqueous REEs by precipitation of sulfate and phosphate phases was tested. • Gypsum precipitation achieved higher removal rates for La and Eu than for Gd and Lu. • Brushite precipitation removed around 100% of the REEs from the aqueous solution. • REE concentration in brushite was 2 to 41 times higher than that measured in gypsum-. • Potential mechanisms of REE incorporation into gypsum and brushite were studied.