EFFECTS OF SULFATE ON RARE EARTH ELEMENT ADSORPTION AND COPRECIPITATION BY AL HYDROXIDE AND HYDROXYSULFATE PHASES: BATCH EXPERIMENTS AND EQUILIBRIUM MODELS

An understanding of the mechanism(s) of trace metal interactions with acid mine drainage (AMD) solids is needed to develop effective strategies to recover valuable constituents from this unconventional source. In this study, we are investigating the importance of pH and aqueous concentrations of sulfate (SO₄) and aluminum (Al) on the attenuation of rare earth elements (REE) by Al hydroxide and hydroxysulfate phases such as gibbsite (Al(OH)₃) and basaluminite (Al₄SO₄(OH)₁₀.5H₂O). These phases precipitate within a pH range (4.5-6) that is commonly attained in AMD treatment systems and are important for removing Fe, Al, and trace metals as AMD is neutralized.

We are conducting experiments involving Al precipitation and REE coprecipitation or adsorption from solutions with varying SO₄ concentrations at pH levels relevant to AMD treatment systems. Preliminary modeling of the proposed experimental conditions using PHREEQC indicates that the presence of 10 mM SO₄ results in Al precipitation at a lower pH (4.0-4.5) than in SO₄-free experiments owing to relatively low solubility of basaluminite compared to Al(OH)₃. However, when the SO₄ concentration is increased 3-fold, less Al is precipitated at the same low pH value, presumably due to the formation of aqueous complexes with SO₄ which results in a decreased mass of precipitate. For the high-SO₄ scenario, less efficient adsorption of REEs is indicated. Both the availability of sorbent and the potential formation of REE-SO₄-ternary surface complexes explain why REEs are more effectively adsorbed at pH 4.5 in the low-SO₄ solutions compared to the high-SO₄ and SO₄-free solutions. Data from ongoing batch experiments will inform these geochemical models and further elucidate the importance of adsorption and coprecipitation as mechanisms for REE enrichment in Al-rich phases. In addition to simulating empirical data, the modeling will be used to extend results to field conditions and potential scenarios for recovery of REE from AMD treatment systems.