Remediation Efficiency, Chemical Species, and Mineralogical Characteristics of High-Concentration Heavy Metal Contaminated Groundwater in Smelter Area Treated with Sodium Hydroxide

Concentrations of heavy metals in highly contaminated groundwater samples collected from a smelter area and used in the removal efficiency experiments were Cd 153.34 mg/L, Cu 29.38 mg/L, Fe 4.63 mg/L, Mn 1150.16 mg/L, and Zn 10062.27 mg/L. The amount of precipitate formed increases proportionally with the amount of NaOH added to raise the pH. A total of 863 mL of 1 M NaOH was required to increase the initial pH from 3.32 to 11.30. The total amount of precipitate was 52.8 g, with 46.57 g formed in the pH range of 5.55 to 7.34. The color of the precipitate generally corresponds to Munsell color 10YR. Based on removal efficiency evaluation, the order of heavy metal removal from highly contaminated groundwater is Fe > Cu > Zn > Cd > Mn. X-ray diffraction analysis shows that minerals in the precipitates consist of kieserite (MgSO₄·H₂O), namuwite [Zn₂Cu₂(SO₄)(OH)_6·4H2O], birnessite (MnO₂·nH₂O), hausmannite (Mn₃O₄), cadmium hydroxide [Cd(OH)₂], thenardite (Na₂SO₄), and brucite [Mg(OH)₂]. According to the MINTEQ modeling results, Zn precipitates as hydroxy-sulfate and hydroxide phases, Mn as sulfate phases, and Cd as hydroxy-sulfate and hydroxide phases.