Geochemical modeling of rare earth element adsorption onto aluminum hydroxide and hydroxysulfate under variable sulfate conditions

Aluminum (Al) hydroxides and hydroxysulfates commonly precipitate and sequester rare earth elements (REE) at pH 4.5 to 6 within acid mine drainage (AMD) treatment systems. However, sulfate, the predominant anion in AMD, can alter Al solubility and REE mobility through aqueous and surface complexation reactions. Here, we apply aqueous and surface speciation models to predict Al precipitation and potential for REE enrichment with Al phases. We conducted laboratory titration experiments to investigate the effects of pH and varying sulfate concentrations (0-, 10- and 30- mM) on the attenuation of dissolved REE during and after the precipitation of Al solids. The stoichiometry of the Al phases in the sulfate-free and sulfate experiments is consistent with that of Al(OH)₃(a)) with a solubility constant (logK) of 11.8 and basaluminite with variable composition and logK of 6.22 or 4.09, depending on stoichiometry. Using PHREEQC and the parameter estimation software, PEST, we calculated binding constants for each of the lanthanide REE onto basaluminite and amorphous Al hydroxide (Al(OH)₃(a)). These binding constants were applied with PHREEQC to simulate staged titration of field-sampled AMD. Experimental results indicate that the REE are attenuated through adsorption rather than coprecipitation. The presence of sulfate enhances the removal of REE as demonstrated by decreases in the sorption edge by 0.75 pH unit due to (1) the precipitation of basaluminite at lower pH than Al(OH)₃(a) and (2) the formation of Hao_OHSO₄^2- surface which adsorbs REE³⁺ more effectively than the uncharged Hao_OH surface in the sulfate-free experiment. However, as sulfate concentration increases, the activities of AlSO₄⁺ and REESO₄⁺ complexes increase, reducing the potential for Al precipitation and REE adsorption at pH 4 to 6. By applying the newly calculated constants for REE adsorption, major metal solubility and REE sequestration to solids formed from untreated or neutralized AMD can be accurately simulated.