Leaching behaviour of heavy metals from coal gangue under the impact of site Ordovician limestone karst water from Shandong closed coal mines, North China

Site Ordovician limestone karst water plays a key role in the release of heavy metals from abandoned coal gangue or filled-in goafs in closed coal mines. In this study, a series of immersion experiments was performed to evaluate the leaching behavior of heavy metals in coal gangue from the Baizhuang coal mine in Shandong Province. The experiment was conducted with different immersion solutions, including site Ordovician limestone karst water (pH 7.3) that was compared to three solution pH levels (pH 5.3, 7.3, and 9.3), over a maximum duration of 648 h. The surface structure, chemical composition of the original coal gangue and post-leached residue, and multiple water quality parameters [pH, oxidation reduction potential (ORP), electrical conductivity (EC), total dissolved solids (TDS), and dissolved oxygen (DO)] of the immersion solutions were analyzed using scanning electron microscopy–energy-dispersive spectroscopy (SEM–EDS), X-ray fluorescence spectrometry (XRF), and a Hach Hydrolab multiparameter wate...