Study On Transport Mechanism Of Nitrogen Pollutants In Saturated Clay Layer Under Vertical Infiltration

The clay layer in the groundwater system has been seen as impermeable layer. Only in recent years, it has been sometimes recognized as the weak impermeable layer (or aquitard). Therefore, the transfer kinetics law of chemical elements in the clay layer has been the weak point of the study. In the technology of underground water pollution prevention and repairation, clay as block walls or impermeable layer was usually used to intercept the transfer of pollutants. However, clay is not absolutely impermeable, at a certain osmotic pressure, it may be flooding, Simultaneously, pollution in the water may also penetrate the clay layer. Therefore, the permeability of clay and contaminants's transfer action, transfer kinetics in the clay layer become an important research question in groundwater pollution control and management.This paper, basic on a large number of domestic and foreign literature , using nitrogen the most common groundwater pollutants as the research object. By understanding the sources,species distribution ,transfer mechanism and hazard of nitrogen in groundwater. By learning on the research of pollutants in clay layer and nitrogen pollutants in groundwater at home and abroad , founding there are two deficiency in the study of nitrogen in Clay layer. (1)Generally,consider the migration law of nitrogen in clay as the same as the migration law in other Soil. (2)The study on migration ,transform law of nitrogen in Clay layer at different reducibility conditions is little.For the deficiency above,this paper through static adsorption / desorption , static conversion,dynamic conversion of nitrogen in clay, more comprehensive understanding the transfer mechanism and concentration change of nitrogen pollutants in the saturated clay layer under vertical penetration. Provide a basis and foundation for preventing nitrogen flow transfer, protecting and restorating of groundwater environment. The content of this paper is divided into four parts: 1 Through static adsorption / desorption experiments, determing static adsorption/desorption amount, mechanism,kinetics equation of nitrogen when does not change the reducibility conditions of solution.Determing static adsorption capacity, adsorption mechanism, adsorption characteristics of nitrogen when change the reducibility conditions of solution.The results of this part show that:clay have a strong adsorption for ammonia. Absorption rate is 60%, and absorption finished in 2 hours. About absorption mechanism, consider the main force is ionic bond . The desorption of ammonia in the clay is slow, five hours later the desorption is in balance,the desorption rate is low.Through the experimental results of different reduction conditions impact on the nitrogen adsorption process , conclusion that under the conditions of Eh: -375 ^-150mv, pH: 7 10 ,the stronger of the solution reduction, the higher of its adsorption capacity and adsorption rate.2 Through static conversion experiment, determing nitrification, denitrification transformation process, conversion rate, conversion mechanism, transforming dynamic equations of nitrogen when does not change the reducibility conditions of solution. Determing conversion trend, conversion rate, conversion mechanism of nitrogen when change the reducibility conditions of solution. The results of this part show that: the nitrification and denitrification rate in the clay is rapid, 90 percent of the response is completed in 6 days. In the environment of Eh for 100-300mv, pH for 9-10, nitrification rate constant is less than denitrification rate constant, that is, this conditions is more conducive to the conduct of denitrification. Through the experimental results of different reducibility conditions on the nitrogen transformation process, obtained in the environment of Eh between -375^-150mv,enhance the Solution reduction can reduce the nitrogen pollution in groundwater.3 Through dynamic soil column experiments,revealing migration and transformation mechanism of nitrogen pollutants in the saturated clay at the time of occurring vertical migration ,through experiments,determing oxidation-reduction,infiltration height,with the ion on the impact of nitrogen concentration when pollutants through Saturated clay layer.The results of this part show that: The more thicker of the layer, the more stronger of it's blocking ability on ammonia.Before ammonia penetrate the clay layer,the interception rate is more than 90%.When nitrogen migrate in the saturated clay layer,the main action is adsorption, nitrification. denitrification accounted for a small part. Clay adsorption for ammonia mainly concerned with the cation exchange. Ca²⁺, Mg²⁺ concentration is the main factors for adsorption. We divide the chemical transformation of ammonia in the clay into three stages: (1) Weak period of nitrification, leaching time for 1-10 days, at this stage, the effluent nitrate concentration is low, pH is low. (2)Strong period of nitrification, leaching time for 10-20 days, at this stage, the effluent nitrate concentrations is high, H + increased, the effluent pH decreased.(3)Denitrification period , 20 days later, at this stage, the effluent nitrate concentration reduced again, this process will produce a large number of OH-, the effluent pH increased. The change and intensity of this three-stage are impacted by several factors. (1)dissolved oxygen. (2)soil and solution pH, Eh. Through the measure of retardation factor and water chemistry transport parameters of NH4+-N, NO3--N,NO2--N in the clay, conclusion that:With the increasing of clay thickness, the retardation factor of NH4+-N,NO3--N,NO2--N will be increased.That is, the more thick of layer,the block capacity of NH4+-N, NO3--N,NO2--N is greater .And the barrier capable of clay on NH4+-N is stronger than NO3--N,NO2--N. The water chemistry transport parameters of NH4+-N, NO3--N,NO2--N decreased with the increasing of height,that is, the more thick of layers, the more slow of NH4+-N, NO3--N,NO2--N. And soil thickness has little effect on the migration of NO3--N,NO2--N,but has a great impact.on the migration of NH4+-N.4 Establishing transport model of nitrogen in the saturated clay layer. The results show that :The migration process of NH4+-N and nitrogen in the saturated clay layer are in line with the one-dimensional mathematical model. So, we can use the model to predict the concentration trends of nitrogen when it occurring vertical migration in the saturated clay layer .