| Mining causes a lot of environmental pollution,which is a global environmental problem.Ammonia nitrogen pollution in ionized rare earth mining areas is due to the loss of a large number of solutions of leaching agents such as ammonium sulfate,ammonium carbonate and leaching liquid containing ammonia nitrogen during in-situ leaching,results in excessive ammonia nitrogen in groundwater and surface water in the mining area.The pollution barrier technology is an effective and timely remediation technology for contaminated sites such as mines,and has positive benefits for prevention and control of groundwater pollution.Therefore,the pollution barrier technology is applied to the ionic rare earth mine with ammonia nitrogen pollution to block the migration and diffusion of ammonia nitrogen pollutants,so as to achieve the purpose of pollution control.Based on silica fume and cement modified soil-bentonite as barrier material,the effect of modified barrier wall on ammonia nitrogen pollution in situ leaching of ion rare earth ore was studied.The permeability and mechanical properties of barrier wall materials were studied,and the microstructure and structure of the barrier wall were studied by means of scanning electron microscopy,X-ray diffraction,CT scanning and LBM numerical simulation.Combined with the research results of adsorption mechanism,migration characteristics and engineering characteristics of the barrier materials to ammonia nitrogen,the resistance and control performance of the modified soil-bentonite barrier materials to ammonia nitrogen pollution was discussed.The specific findings are as follows:?1?The results showed that the optimal mass ratio of silica fume modified soil to bentonite barrier material was silica fume:soil=1:10,and the optimal moisture content was 67.80%.Modified barrier material,the generated aluminosilicate improved the permeability of barriers,hydraulic conductivity was 2.36×10-9m/s,Ca CO3improved the mechanical properties of the material and the compressive strength reaches 0.896 MPa.?2?According to CT scan and LBM numerical simulation,the pores of the modified soil-bentonite barrier wall were mainly micropores with radius less than 4?m.The radius of the pore throat is mainly between 0 to 3?m,and the radius of the pore throat less than 2?m,which helps to improved the permeability of the barrier wall.The simulated permeability of the barrier wall was 1×10-9m/s,agreement with the measured value.The minor flow paths within barrie wall were obviously tortuous and complex.?3?The adsorption kinetic process of modified soil-bentonite barrier to ammonia nitrogen conforms to the quasi-second-order kinetic model,which indicated that the adsorption process is mainly chemical adsorption.The thermodynamic process of adsorption conforms to the Langmuir isothermal model,which indicated that the adsorption is an exothermic process.The adsorption block factor was calculated?between 1.051?1.313?.?4?The effective diffusion coefficient of ions in the barrier wall was 1.65×10-6cm/s2.The relationship of seepage velocity between barrier walls of different solutions was:500 mg/L?NH4?2SO4solution<100 mg/L?NH4?2SO4solution<20mg/L?NH4?2SO4solution<Na Cl solution<distilled water.The size relation of ammonia nitrogen content in barrier wall is:water soluble ammonium>exchange ammonium>fixed ammonium.?5?The mass change of barrier wall sample is proportional to the concentration of ammonia nitrogen in solution.The SO42-reacts with the cementin bodies such as calcium hydroxide in the sample,resulting in the expansion of the spacing between the skeleton structures and the mass of the sample.The ionic reaction between NH4+and the calcium oxide and magnesium oxide in the sample,resulting in partial dissolution of the sample and decreased strength.Under the same condition,it can be calculated that different breakdown concentration standards have great influence on the service life of barrier wall.The breakdown time of barrier wall will be prolonged with the increase of barrier factor and barrier wall thickness.The head height has little effect on the breakdown time of barrier wall.?6?According to the hydrologic environment,engineering geological conditions and the present situation of ammonia nitrogen pollution in the polluted site of an ionic rare earth mine,the location and scale of the barrier wall were determined in the site.The model obtained by Visual MODFLOW numerical simulation showed that the diffusion range of ammonia nitrogen is small and the barrier wall is not penetrated after 3650 days of setting barrier wall in the polluted site,so the resistance and control effect of ammonia nitrogen is suitable better. |
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