Leaching characteristics and pollution risk assessment of potentially harmful elements from coal gangue exposed to weathering at different time

Abstract This study explored the fresh coal gangue present in the goaf of the Hongqi Coal Mine and the coal gangue present on the surface of the mining area (weathered over a period of one year) to study the migration mechanism of pollutants when different types of coal gangue backfill goaf. Static immersion comparative experiments were conducted to determine the dissolution characteristics of Fe3+, Mn2+, SO42–, and other major pollutants present in the two types of coal gangue. The results revealed that under the same soaking conditions, weathered coal gangue released more polluting elements than fresh coal gangue. Moreover, the ion concentration in the soaking solution fluctuated significantly. Meanwhile, the Fe3+ concentration presents a “wave-like” trend, and the Mn2+ and SO42– concentrations fluctuated significantly during the early stages (1–7 d). The trend changed during the middle and late periods (after 7d). The Mn2+ content in the soaking solution of the fresh coal gangue decreased slowly, while the Mn2+ content in the weathered coal gangue increased steadily. The changes in the pH exhibited a “logarithmic trend”, and the pH of the fresh group was slightly higher than that of the weathered group. Based on the evaluation of the geo-accumulation index, combined with the pollution level evaluation standard, it was found that the levels of heavy metal elements produced by the weathered coal gangue are significantly higher than the levels produced by fresh coal gangue, which are more likely to pollute groundwater and damage the ecological environment.