Study on the controlling effect of biochar on heavy metals in the mining-induced subsidence area after gangue reclamation

The gangue reclamation materials employed in mining-induced subsidence areas have potential risks of heavy metal pollution to the mining environment. The agricultural waste biochar can be used as an adsorption material to control the migration of heavy metals. Based on the pore size, pore volume, and density of biochar, a microstructure of the heavy metal adsorption layer was randomly generated, and a numerical model for heavy metal migration and adsorption in the mining-induced subsidence area after gangue reclamation was constructed. The influence of adsorption layer thickness and material type on heavy metal migration and adsorption behavior was analyzed. The results show that the ratio of migration speed of heavy metals in the neutral zone, compression zone and tension zone is 12: 3: 38. In the absence of an adsorption layer, heavy metals tend to be more concentrated and migrate downward. The working cycles of the microstructure of cow manure biochar and Pleurotus ostreatus residue biochar are 10 s and 48 s. Under macroscopic conditions, the adsorption efficiency of heavy metals is higher than that under microscopic conditions, but its decreasing amplitude gradually decreases. Under the influence of a macroscopic high heavy metal concentration environment, the porous structure and large surface area of the biochar are fully utilized for adsorption, and the negative charges and functional groups inside are gradually occupied. This research provides a reference for the clean utilization of coal gangue and the ecological rehabilitation of mining areas.