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 HaoOHSO 4 2− surface which adsorbs REE 3+ more effectively than the uncharged HaoOH surface in the sulfate-free experiment. However, as sulfate concentration increases, the activities of AlSO 4 + and REESO 4 + 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. • Moderate concentrations of SO 4 (10- to 30-mM) enhance REE adsorption by Al–OH (-SO 4) minerals. • Without SO 4, amorphous Al (OH) 3 precipitates and attenuates REE at pH 5-7. • In SO 4 matrix, basaluminite forms, increasing sorbent availability and REE adsorption at pH 4-6. • New binding constants for REE adsorption on Al–OH (-SO 4) minerals consider SO 4 contributions. • PHREEQC with new constants predicts Al and REE partitioning during AMD neutralization.
Boothe-Lordon et al. (Fri,) studied this question.