Environmental risk assessment of industrial byproduct gypsum utilized for filling abandoned mines

In response to the basic policy of green and low-carbon circular development to solve resource, environmental and ecological problems, gypsum is considered to be a filling material for mine backfilling. To explore the potential risks of gypsum to the groundwater environment due to the backfilling of abandoned mines, a sequential batch leaching experiment was carried out in this paper, which used three types of industrial waste gypsum, namely, phosphorus gypsum (PG), titanium gypsum (TG) and flue gas desulfurization gypsum (FGDG). COMSOL Multiphysics 5.4 software was used to simulate and solve the migration process of the leached metal elements in the mine floor when these three gypsum types were used as filling materials to observe the concentration distributions and diffusion distances of the metal elements from these three gypsum types in the mine floor. The results show that (1) during repeated contact of the three types of industrial waste gypsum with the leaching medium, the pH levels changed, and the changes in pH affected the leaching patterns for the heavy metal elements in the gypsum. (2) Based on the concentrations of the metal elements that were leached from the three types of gypsum, it can be determined that these three types of gypsum are not classified as hazardous solid wastes, but they cannot be ruled out with regard to their risk to the groundwater environment when they are used as mine filling materials. (3) When the three types of gypsum are used as filling materials, the concentration distributions of the metal elements and their migration distances all exhibit significant changes over time. The concentration distributions, diffusion rates and migration distances of the metal elements from the different gypsum types are affected by their initial concentrations in the leachate. The maximum migration distances of Zn in the floor from the PG, FGDG and TG are 8.2, 8.1 and 7.5 m, respectively.