Study On Soil Adsorption And Desorption Of Ammonium Nitrogen In A Typical Agricultural Irrigation District

With the increasing of agricultural fertilizer applications,the nitrogen pollution of groundwater is becoming more serious and threats the environment and human health.Ammonium nitrogen is a main form of nitrogen pollutants.At present,the studies mainly focused on influence of soil physical and chemical properties on the adsorption and desorption of ammonium nitrogen.However,the influence of groundwater ion properties and irrigation were less studied.The study site was People's Victory Canal Irrigation District which suffered the problems of conflict between water supply and demand and groundwater pollution.The mechanism of adsorption and desorption of ammonium nitrogen in three soil types?silt,silty loam,and sandy loam?under groundwater irrigation were studied based on the field measured data of soil and groundwater properites in September 2018,laboratory experiments,and model simulation.This study had significance and application value for improving nitrogen fertilizer utilization rate and preventing and controlling ammonium nitrogen pollutant in ground water.Main research contents and conclusions are as following:?1?Through the fitting analysis using Origin and the mathematical statistics method,the kinetics and thermodynamic characteristics of ammonium nitrogen adsorption and desorption were analyzed,and the changes and laws of ammonium nitrogen adsorption and desorption in irrigation area were revealed,and the applicability of the model was investigated.The kinetics of ammonium nitrogen adsorption and desorption conforms to the nonlinear pseudo-second-order kinetic model and exhibits the relationship of“rapid adsorption-slow desorption”in the studied soils.The thermodynamic process of ammonium nitrogen adsorption is consistent with three common isotherm adsorption models,i.e.,Freundlich model,Langmuir model and Temkin model.For the adsorption process,the adsorption capacity degree constant k_f value?Freundlich model?was positively correlated with clay and silt,and negatively correlated with sand;the theoretical maximum adsorption amount k_m value?Langmuir model?was positively correlated with clay and silt,and negatively correlated with;the equilibrium constant K value?Temkin model?was significantly positively correlated with the theoretical maximum adsorption amount and the actual maximum adsorption amount.For the desorption process,the capacity degree constant k_f value was negatively correlated with sand;the theoretical maximum desorption amount k_m value was positively correlated with sand;the equilibrium constant K value was significantly positively correlated with the theoretical maximum desorption amount and the actual maximum desorption amount.?2?By using SPSS software and linear regression method,the correlations and partial correlations between maximum adsorption and desorption amount in different soils,isothermal model parameters and influencing factors were analyzed,and the main factors affecting the process of adsorption and desorption in irrigation area were revealed.For the maximum adsorption amount,silt and silty loam had higher values,followed by sandy loam.The maximum adsorption amount was positively correlated with clay and silt contents.For the maximum desorption amount,sandy loam had higher values,followed by silt and silty loam.The adsorption amount of ammonium was significantly higher than that of desorption,and the maximum adsorption amount was about 1.8 to 19.5 times higher than that of desorption.The adsorption model parameters had higher correlation with Mn²⁺in groundwater.The desorption model parameters had higher correlation with Fe²⁺,Fe³⁺and Mn²⁺,especially with Mn²⁺in groundwater.?3?The effects of different concentrations of Mn²⁺,Fe³⁺and Fe²⁺on the adsorption and desorption process of ammonium nitrogen under groundwater irrigation were simulated according to the measured groundwater data,and the effects of various factors on the adsorption and desorption process of ammonium nitrogen were revealed.For the concentration of 50 mg/L NH₄⁺solution,the low concentration?0.5mg/L?of Fe²⁺promoted adsorption,especially for silty loam.The high concentration?10mg/L?of Fe²⁺had both promoting and inhibiting effects,the inhibition of silt was obvious and the promotion of silty loam was obvious.The low concentration?0.5mg/L?of Fe³⁺was mainly promoted adsorption,especially for silty loam.The high concentration?10mg/L?of Fe³⁺was mainly inhibited adsorption,especially for silt and sandy loam.The low concentration?0.2mg/L?and medium concentration?0.8mg/L?of Mn²⁺were mainly promoted adsorption,and the high concentration?10mg/L?of Mn²⁺was mainly inhibited adsorption.Withing the increase of Mn²⁺concentration,the correlation between the adsorption amount and particle size decreased.For each high concentration?10 mg/L?ion,the adsorption strength in a certain range was Mn²⁺,Fe³⁺,and Fe²⁺based on the rate of adsorption.