The Oxidative Precipitation of Thallium at Alkaline pH for Treatment of Mining Influenced Water

Thallium (Tl) may exceed regulatory limits in mining-influenced water (MIW) associated with processing cadmium, copper, gold, lead, and zinc ores. It is a toxic metal that is soluble over a wide pH range, resulting in both persistence in the environment and poor removal by conventional lime precipitation. This study evaluated the effect of potassium permanganate (KMnO₄) at alkaline pH on Tl removal from MIW in batch experiments. The oxidation of Tl⁺ to Tl³⁺ by KMnO₄ and subsequent Tl removal was explored at Tl concentrations of ≤1 mg/L in synthetic and actual MIW. In addition to Tl, the synthetic MIW contained ≈5 mg/L of Mn, while the actual MIW contained >10 mg/L of Al, Cu, Fe, Mn, and Zn and had a pH ≈ 2.5. Dissolved Tl <2 μg/L in synthetic MIW was achieved at a pH ≈ 9 (CaO addition) and ≥5 mg/L of KMnO₄. In the actual MIW, dissolved Tl <2 μg/L was achieved at pH ≈ 9 and ≥12 mg/L of KMnO₄. The Tl removal mechanism is complicated due to the presence of reduced Mn in the synthetic MIW and multiple metals in the actual MIW. However, effective Tl removal was achieved by adding KMnO₄ to synthetic and actual MIW at alkaline pH.