Mineralogical Controls on Element Mobility During Weathering of Alkaline Coal Gangue

Coal gangue is a complex mineral assemblage in which weathering processes regulate mineral dissolution, alkalinity evolution, and element mobility. Understanding these processes is important for evaluating the environmental behavior of gangue-derived materials exposed to seasonal weathering conditions. This study investigated the leaching behavior of alkaline coal gangue from Xuzhuang Coal Mine in Pei County, Xuzhou City, Jiangsu Province, China, under laboratory-simulated rainfall, wet-dry cycling, and freeze–thaw cycling. Initial solution pH strongly affected leaching behavior: more acidic conditions increased the release of Cr, Mn, Fe, Cu, and Zn, whereas higher initial pH resulted in greater cumulative alkalinity release. Under initial solution pH values of 5.0, 6.0, and 6.5, leachate pH decreased from 9.67–9.70 after the first leaching cycle to 8.28–8.45 after the fifteenth cycle, indicating persistent alkaline buffering during repeated leaching. Applied water volume affected alkalinity and elemental release differently: cumulative alkalinity release was highest at 400 mL per cycle, whereas the release of individual elements varied with water volume. Wet-dry cycling produced limited changes at early stages but, after prolonged treatment, increased Cr, Mn, Fe, and Cu release, while Zn showed rapid initial release followed by a decline. Freeze–thaw cycling also altered element mobility, with the largest changes generally observed at intermediate stages of cycling. Under freeze–thaw conditions, Cr release reached 652.57 mg kg−1 after 3 cycles, suggesting enhanced Cr mobility during early-stage freeze–thaw weathering. The observed trends are consistent with combined mineral dissolution and physical weathering, although direct mineralogical and microstructural evidence after cycling is still required. These results identify rainfall acidity and repeated weathering as key environmental factors that should be considered when evaluating alkaline coal gangue as a reclamation substrate in temperate mining areas.