Transport mechanism of ammonia nitrogen in the simulated gob area packed with coal gangue

In many coal mining areas in western China, waste water produced during coal mining is often dominant in mine water, resulting in high ammonia nitrogen content. The pretreatment of mine water by filling gangue in coal mine gob areas has become an important measure for the resource utilization of mine water and gangue. The research took the coal gangue from Baode Coal Mine of Shenhua Shendong Coal Group Corporation as the filling medium and the ammonia nitrogen in mine water as the research object. Based on CXTFIT2.1, column simulation experiments were carried out to study the migration mechanism of ammonia nitrogen in the filling gangue by simulating the hydrogeological environment of the studied gob area. The results show that the migration of Cl⁻ through the filled gangue could be well described by the convection-dispersion equation (CDE) at a Darcy flux of 3.12 cm/h at 25℃(r² =0.999). The high values of D, D_f, D_h and λ were related to the large grain size and long migration distance of the filling gangue. There was a significant linear relationship between the contents of total dissolved solids (TDS) and Cl⁻ in the effluent samples. The two-site adsorption solute transport model could well describe the migration process of ammonia nitrogen in the filling coal gangue. The arrest coefficient R of ammonia nitrogen was 23.79, and its equilibrium adsorption points on the coal gangue only accounted for 46% of the total adsorption points. The first-order kinetic adsorption rate coefficient α was 3.5×10⁻⁴ h⁻¹. The pore velocity and hydrodynamic dispersion coefficient of ammonia nitrogen were much lower than those of Cl⁻, which was mainly related to the adsorption of ammonia by the high content of clay mineral kaolinite in the studied matrix. The variations of nitrite and nitrate nitrogen contents and pH were not obvious during the test duration of 649 h. The TDS in the effluent samples was stable in the middle and later stages of the experiment. The above results indicate that the transport of ammonia nitrogen in the simulated gangue column was dominated by convection, dispersion and adsorption while the biotransformation of ammonium ion could be ignored. The findings could provide an important theoretical basis for evaluating the water quality pretreatment technology of underground reservoirs in the coal gob area in China.