Physico-chemical and elemental investigation of aqueous leaching of high sulfur coal and mine overburden from Ledo coalfield of Northeast India

Acid mine drainage (AMD) is one of the major problems in high sulfur coal mining areas generating acidic water. The acidic mine water generated contain hazardous elements in varying concentrations. The Northeastern Indian coalfield produces considerable amounts of AMD. The AMD and metal leaching from coal and mine over burden (OB) are the two important naturally occurring processes. In order to know the AMD potential, the aqueous leaching experiment of a few coal and OB samples from the Ledo colliery of the Northeastern coalfield, Mergherita (India), at different time periods (1, 3, 5, and 8 h) and at different temperatures (25, 45, 65, and 90 °C) were performed in the laboratory. The physico-chemical analysis of the aqueous leachates shows the pH, electrical conductivity (EC), and total dissolved solid (TDS) in the range of 1.62–3.52, 106–2006 µs/cm, and 106–1003 ppm for the raw coal samples respectively. The OB samples produced pH, EC, and TDS in the range of 3.68–6.92, 48.6–480 µs/cm, and 69.5–240 ppm respectively. From the study, it was revealed that the concentrations of major (Si, Al, K, Na, Fe, Ca, Mg), minor (Mn) and trace/hazardous elements (As, Ba, Cd, Co, Cr, Cu, Hg, Ni, Pb, Se, Zn) considerably change with leaching time as well as with leaching temperature. Out of these elements As, Cd, Hg, Pb, Cr, and Se are of greater environmental importance. Alteration of the physico-chemical structure of the coal and OB samples resulting from leaching was also studied by field emission scanning electron microscope- energy-dispersive X-ray spectroscopy method. The release of the potentially hazardous elements from the raw coal and OB during leaching time periods to the leachates was detected by inductively coupled plasma-atomic emission spectroscopy and ion-chromatographic analyses. The major minerals found in coal and OB are quartz (SiO2), pyrite (FeS2), hematite, marcasite, and kaolinite. The association of different functional groups in minerals and their mode of association were studied by Fourier-transform infrared spectroscopy and X-ray diffraction analytical techniques. The present laboratory study will be useful in relating the characteristics of aqueous leaching from coal and mine OB with the natural weathering condition at the coal mine area. © 2018, The Author(s).