Effects of Acid Mine Water on the Long‐Term Release Characteristics of Heavy Metal in Coal Fly Ash Backfill Material

ABSTRACT Coal fly ash, a by‐product of coal‐fired power plants, is widely used in mine backfilling. However, acidic mine water and sudden surges can destabilize the backfill, promote heavy metal release, and threaten soil and groundwater. Here, we examined long‐term heavy metal leaching from coal fly ash backfill under different pH conditions to evaluate risks during acidic mine water surges. Leaching showed strong pH dependence. At pH 2, manganese, zinc, and arsenic dissolved during the mid‐leaching stage, exceeding Class III limits by 1.85–42.71 times. Lead was mainly released in the early stage due to surface scouring, with an initial concentration 1.31 times above the limit. In contrast, at pH 4 and 5.6, key elements remained below the limits. Scanning electron microscopy indicated that acidic conditions erode gelling minerals, weaken the matrix, and create pathways for heavy metal migration. These findings provide mechanistic insight into the release of heavy metals from coal fly ash backfill under acidic conditions.