The high concentration heavy metal contaminated groundwater was treated with sodium hydroxide to increase the pH. As the pH increased, various types of heavy metal compounds precipitated. The precipitates were recovered at each increased pH and analyzed using X-ray diffraction analysis. Chemical species that precipitated at each pH were identified using Visual Mintaq3.1 computer program. The concentrations of heavy metals utilized for assessing the remediation efficiency of heavily contaminated groundwater were: Cadmium at 153.34 mg/L, Copper at 29.38 mg/L, Iron at 4.63 mg/L, Manganese at 1150.16 mg/L, and Zinc at 10062.27 mg/L. Cadmium showed a removal efficiencies of 22.35% at pH 6.36 and significantly higher efficiency of 99.84% at pH 10.31. At pH 6.36, Copper demonstrated a notable removal efficiency of 97.11%, while Iron showed a high initial removal efficiency of 94.60% at pH 5.55. Manganese removal gradually increased up to pH 9.32, reaching 99.70% efficiency at pH 10.31. At pH 7.34, high concentrations of zinc exhibited a removal efficiency of 99.59%, which increased to 100% at pH 10.31. According to the X-ray diffraction analysis results, the precipitates recovered at pH 4.30 and 5.55 were identified as Kieserite (MgSO₄H₂O). At pH 6.35, the recovered precipitates consisted of Namuwite [Zn₂(SO₄)(OH)₆4H₂O] and Brucite [Mg(OH)₂]. The precipitates at pH 7.34 are Namuwite, Birnesite, Hausmannite (Mn₃O₄), and Cadmium hydroxide (Cd(OH)₂). At pH 8.30 and 9.32, the precipitates consist of Thenardite (Na₂SO₄) and Hausmannite, while Brucite at 10.31. Finally, at pH of 11.30, Thenardite and Brucite precipitated and were removed. Chemical species identified by Mintaqu modeling analysis included ZnOH⁺, Zn(OH)₂(aq), Zn(OH)₃^-, Zn(OH)₄^2-, ZnF⁺, ZnSO₄(aq), Zn(SO4)₂^2-, and ZnNO₃⁺ for Zinc, Cd²⁺, CdOH⁺, Cd(OH)₂(aq), Cd(OH)₃^-, CdF⁺, CdCl⁺, CdSO₄(aq), Cd(SO₄)₂^2-, and CdNO₃⁺ for Cadmium, and MnOH⁺, Mn₂(OH)₃⁺, MnF⁺, MnSO⁴(aq), and MnNO₃⁺ for Manganese. Iron existed as FeSO₄ below pH 5.0 and as Fe(OH)₃ above pH 7.0, while Copper existed as CuSO₄ below pH 9.5, transitioning to Cu(OH)₂ above pH 7.5, with both species coexisting at pH 7.5-9.5.

Woo Taek Lim

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