Preparation Of Modified Vermiculite And Its Adsorption Behavior For Ammonia Nitrogen In Rare Earth Wastewater

The high content of ammonia nitrogen in wastewater from rare earth mines is harmful to the environment and human health.Therefore,it is very necessary and urgent to find one or several low cost and high efficiency environmental protection methods to remove high concentration ammonia nitrogen.Vermiculite is a type of clay,a kind of 2:1 layered aluminosilicate,a product of biotite or mica altered by weathering or hydrothermal action.The structure is silico-oxygen tetrahedron and alumino-oxygen octahedron.In addition,vermiculite has the characteristics of permanent negative charge,large specific surface area,many active sites and pores on the surface,strong cation adsorption capacity,low price and non-toxicity.The interior of natural vermiculite is full of tiny holes and channels,which have strong electrostatic adsorption and ion exchange capacity.These unique properties of vermiculite make it a promising ion exchanger,providing more options for the development of adsorption materials with low cost and high adsorption performance.In this paper,natural powdery vermiculite（NV）was modified by 7 kinds of sodium chloride（Na Cl）solutions,and 7 kinds of Na-V were obtained.The adsorption capacity of Na-V for NH₃-N was significantly better than that of NV.The adsorption capacity of 180g/L Na Cl modified vermiculite（180-Na-V）for ammonium was the highest（Qm was11.569 mg/g）,which was 63.43%higher than that of NV（Qm was 7.079 mg/g）.The characterization of 180-Na-V confirmed the mechanism of NH₃-N removal by modified vermiculite.The improvement of mesoporous volume and ion exchange capacity of modified vermiculite was the result of sodium ion exchange and interlayer effect of high concentration of Na Cl.The adsorption isotherm and adsorption kinetics are in good agreement with Langmuir model and pseudo-second-order（PSO）model respectively.The adsorption capacity of vermiculite after 6 cycles（3.808 mg/g）can still maintain 75.09%of the initial adsorption capacity（5.071 mg/g）.A large amount of Na-V has little effect on the p H value of water,which meets the requirements of practical application.Finally,the effects of p H,dosage and coexisting ions on the adsorption of ammonium salt were investigated.Before using NaCl to modify vermiculite,natural vermiculite was treated with strong alkali to prepare Na OH/Na Cl modified vermiculite（Na OH-180-Na-V）successfully.The adsorption performance of Na OH-180-Na-V for NH₃-N is obviously better than that of NV and 180-Na-V.The saturation adsorption capacity of Na OH-180-Na-V for ammonium reached 17.851 mg/g,which was 54.3%higher than that of 180-Na-V（Qm was 11.569mg/g）.The characterization of Na OH-180-Na-V confirmed the mechanism of NH₃-N removal by modified vermiculite.Na OH not only removed the impurities in the internal pores of vermiculite,widened the pores and increased the pore size,but also made its surface more rough and increased the specific surface area,thus increasing the adsorption site.The improvement of mesoporous volume and ion exchange capacity of modified vermiculite is the result of sodium ion exchange and interlayer effect of high Na Cl concentration.The adsorption isotherm and adsorption kinetics are in good agreement with Langmuir model and pseudo-second-order（PSO）model respectively.The adsorption capacity of vermiculite after 7 cycles（4.242 mg/g）can still maintain 70.05%of the initial adsorption capacity.Finally,the effects of p H,dosage and coexisting ions on the adsorption of ammonium salt were investigated.Natural powdery vermiculite（NV）was modified with Na HCO₃solution and dried at high temperature to obtain Na HCO₃modified vermiculite（Na-100-V）.The adsorption performance of Na-100-V for NH₃-N is obviously better than that of NV and 180-Na-V.The saturated adsorption capacity of Na-100-V for ammonium reached 28.514 mg/g,which was 302.8%higher than that of NV（Qm 7.079 mg/g）.The characterization of Na-100-V confirmed the mechanism of NH₃-N removal by modified vermiculite.CO₂decomposed by Na HCO₃under the action of high temperature and drying further increased the average pore size and mesoporous volume,as well as the micropore surface area and volume,thus increasing the adsorption site of ammonia nitrogen and improving the ion exchange capacity of Na⁺and NH₄⁺.The adsorption isotherm and adsorption kinetics are in good agreement with Langmuir model and pseudo-second-order（PSO）model respectively.The adsorption capacity of vermiculite after 7 cycles（4.400 mg/g）can still maintain 73.55%of the initial adsorption capacity.Finally,the effects of p H,dosage and coexisting ions on the adsorption of ammonium salt were investigated.