Characterization of Post-Mining Soil and Solid Waste from Silica Sand Purification

Post-mining soil and solid waste from the silica sand refining industry is widespread and the potential long-term impact of toxic metals and metalloids is a significant and under-appreciated issue. This study presents the characteristics of post-mining soil and solid waste resulting from silica sand purification to observe its physical, chemical, and biological composition. Analysis of the physical properties was carried out with reference to ASTM 112-10 and the results show that post-mining soil contains 36.95% sand, 18.80% clay, and 42.74% silt, with coefficients of permeability and porosity of 0.69×10^-6 cm·s^-1 and 35.84%, respectively. Meanwhile, the solid waste contains 43.35% sand, 35.96% clay, and 20.68% silt with coefficients of permeability and porosity of 1.49×10^-6 cm·s^-1 and 51.12%. The overall mineralogy and morphology of both samples showed that they have the same chemical composition as gehlenite (Ca₂Al₂SiO₇), spinel (MgAl₂O₄), akermanite (Ca₂MgSi₂O₇), monticellite (CaMgSiO₄), aluminum oxide (Al₂O₃), magnetite (Fe₃O₄), and hematite (Fe2O3) supports this data. The chemical composition of both samples is SiO₂, Al₂O₃, CaO, and MgO, but the post-mining soil has lower heavy metal and nutrient contents compared to solid waste. Meanwhile, solid waste has a high content of heavy metals and nutrients due to washing and bonding from the silica sand purification process. The abundance of bacteria (Colony Forming Unit) for the 10^-4 and 10^-5 dilutions in post-mining soil was 1.59×10³ and not detected, while in the solid waste, 4.10×105 and 1.64×10⁵ were found, respectively.