Coordination of arsenic and nickel to aluminum and magnesium phases in uranium mill raffinate precipitates

Abstract The Key Lake U mill uses a stepwise neutralization process (pH 4.0, 6.5, 9.5, and 10.5) to treat raffinate (acidic, metal-rich wastewater) prior to safely releasing effluent to the environment. This process generates a complex mixture of precipitates that are deposited to a tailings facility. In this study, the coordination environments of As and Ni with respect to Al-Mg phases precipitated in the presence and absence of Fe in mill-generated and synthetic precipitates were defined using bulk X-ray absorption spectroscopy complemented with bulk X-ray diffraction. In low pH (pH 4.0–4.6) samples, As(V) precipitates as ferric arsenate and adsorbs to AlOHSO 4 (an amorphous hydrobasaluminite-like phase) and ferrihydrite via bidentate-binuclear complexes. Nickel(II) predominantly adsorbs to amorphous Al(OH) 3 via edge-sharing bidentate-mononuclear complexes. In high pH (pH 9.5–9.9) samples, As(V) adsorbs to amorphous Al(OH) 3, ferrihydrite, and MgAlFe-hydrotalcite (bidentate complex). Nickel(II) octahedra adsorb to amorphous Al(OH) 3 and likely form a Ni-Al layered double hydroxide (LDH) surface precipitate on MgAlFe-hydrotalcite via Al dissolution-precipitation. In the final solids (blended low and high pH precipitates) discharged at ∼ pH 10.5, As(V) adsorbs to amorphous Al(OH) 3, ferrihydrite, and MgAlFe-hydrotalcite. Nickel(II) adsorbs to amorphous Al(OH) 3 and forms Ni-Al LDH surface precipitates on hydrotalcite. This study demonstrates that neutralization of chemically complex wastewater precipitates multiple phases capable of controlling dissolved As and Ni concentrations. Knowledge gained from this study will aid investigations in understanding the long-term fate of these potential contaminants in the environment and can be applied to other industries and environmental systems with similar conditions.