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 investig