Influence of the freeze–thaw process on coal gangue–based ecological restoration materials: performance and ecological risk assessment of heavy metals

Abstract

Materials made from coal gangue (CGEr) can be used for ecological restoration in mining areas. This paper comprehensively analyzed the influence of the freeze-thaw process on the performance of CGEr and the environmental risk of heavy metals. The safety of CGEr was assessed by sediment quality guidelines (SQGs), geological accumulation index (Igeo), potential ecological risk index (RI), and risk assessment code (RAC). The freeze-thaw process reduced the performance of CGEr, that the water retention of CGEr decreased from 1.07 (g water/g soil) to 0.78 (g water/g soil), and the loss rate of soil and water increased from 1.07 to 4.30%. The freeze-thaw process reduced the ecological risk of CGEr, the Igeo of Cd and Zn decreased from 1.14 to 0.13 and 0.53 to 0.3, respectively, and the RI of Cd decreased by 50% from 0.297 to 0.147. Reaction experiments and correlation analysis showed that the freeze-thaw process destroyed the pore structure of the material, resulting in the degradation of its properties. Water molecules undergo phase transformation during freeze-thaw, and particles were squeezed by ice crystals to form agglomerates. The formation of granular aggregates resulted in the enrichment of heavy metals in the aggregates. Influenced by the freeze-thaw process, specific functional groups such as -OH were more exposed on the surface of the material, which affected the occurrence form of heavy metals and thus reduced the potential ecological risk of the material. This study provides an important basis for the better application of ecological restoration materials of CGEr.