Chemical Removal of Toxic Heavy Metals in Acid Mine Drainage Using Hydrazine as Reductant

Abstract

Acid mine drainage (AMD) wastewater generated from coal field mining activities going on in Mpumalanga Province in Republic South Africa contained toxic heavy metals which is harmful to human health and the environment, as it is required to be removed before discharged into the natural water body. The aim of this study is to remove toxic heavy metals (Hg, Th, Cr, Mn, Pb, As, Cd and Ba) from AMD using hydrazine (N 2 H 2 ) as reductant. Chemical reduction method was used to remove the toxic heavy metals. Physicochemical analysis was carried out on the mine wastewater. Digestion method was also conducted on the acid mine wastewater. Quantification techniques used in this study are inductively coupled plasma optical emission spectroscopy (ICP-OES), ion chromatography (IC), X-ray diffraction (XRD), X-ray Fluorescence (XRF), transmission electron microscopy (TEM), FTIR and scanning electron microscopy energetic dispersion spectroscopy (SEM-EDS). The results of the maximum concentration and percent removal of heavy metals removed in the AMD using 1.0 M of N 2 H 2 reductant solution are Hg (0.00019 mg/L, 82.49%), Th (3E-06 mg/L, 99.98%), Cr (0.0078 mg/L, 96.53%), Mn (22.31 mg/L, 82.54%), Pb (0.0004 mg/L, 89.04%), As (0.0009 mg/L, 98.32%), Cd (0.0028 mg/L, 99.09%), and Ba (0.0021 mg/L, 92.00%). The results of the maximum contact time of 120 minutes and percent removal of heavy metals removed in the AMD are Hg (0.00012 mg/L, 89.17%), Th (0.0014 mg/L, 92.49%), Cr (0.013 mg/L, 94.34%), Mn (10.53 mg/L, 91.68%), Pb (0.002 mg/L, 94.91%), As (0.005 mg/L, 90.93%), Cd (0.014 mg/L. 95.37%), and Ba (0.002 mg/L, 90.90%). The optimum concentration was 0.6 M NaBH 4 reductant solution and contact time was 30 minutes. The concentration removal of the heavy metals in the treated AMD revealed that the maximum concentrations of some metals are within the WHO while the others are above WHO limits. In conclusion, N 2 H 4 reductant removed most of the toxic heavy metals effectively in the AMD solution with brownish precipitate formed without generating sludge was identified to be Fe 0 nanoparticles.