| Industrial modernization had been accelerated recently,while severe heavy metal contamination in farmland soils was found due to the exploit,smelt and waste discharge of zinc ores and have aroused extensive attention.Adsorption and desorption were one of the main functions controlling the concentration of heavy metal ions in the soil solution and affecting the behavior and fate of heavy metals in the soil.It determined the buffer ability and capacity of the soil for metal ions,and also restricted the quantity and capacity of heavy metals entering the food chain.Therefore,the study of Zn adsorption/desorption in stratified soil in mollisols area of Northeast China(NEC)and their response mechanism of different factors will help to find an effective control of the environmental behavior of Zn in soil,and provide a theoretical basis for future research on Zn contaminated soil and remediation.The adsorption/desorption behavior of Zn in different layered soils in the black soil region of NEC and influence of the mass concentration of nitrogen and phosphorus,pH value,organic matter content and freeze-thaw action on it was studied in this thesis.The main research results were as follows:(1)There were three stages: rapid adsorption/desorption,slow adsorption /desorption,and equilibrium adsorption/desorption of Zn in four types of black soils.The quasi-second-order kinetics equation fits better than the quasi-first-order kinetics equation on the adsorption/desorption process.The adsorption/desorption capacity of the soil for Zn was positively correlated with the initial concentration of Zn,and the Langmuir equation could better fit the isothermal adsorption/desorption process of the four kinds of black soils.The order of Zn adsorption capacity of the four soil layers is plough layer,plough bottom layer,subsoil layer and parent material layer,since the content of soil organic matter decreases with the increase of soil depth.The desorption amount of Zn was directly proportional to the adsorption amount,and the maximum desorption amount varies decrease as the profile depth increases,and there is a hysteresis effect.The order of the hysteresis effect is plough layer,plough bottom layer,subsoil layer,and parent material layer.The adsorption/desorption capacity of Zn increases with the increase of temperature,ΔG<0(ΔG is the change of Gibbs free energy),ΔH>0(ΔH is the standard enthalpy change),ΔS>0(ΔS is the standard entropy Change),indicating that the adsorption/desorption process of Zn in the four stratified soils is a spontaneous,endothermic,and disordered process.(2)Within the range of N and P mass concentrations of the experimental design,with the increase of N and P mass concentrations,the adsorption capacity of four kinds of layered black soils showed a downward trend,and the desorption capacity of zinc increased with the increase of N and P mass concentrations.However,the addition of N and P can improve the ability of soil to fix zinc and reduce the heavy metal harm of zinc to soil crops and groundwater.(3)The pH value can affect the existence form of heavy metal ions in the soil and the surface characteristics of the soil.Within the range of the tested pH value(2.0～6.0),the zinc adsorption capacity of the four layered black soils all increases with the increase of pH value.Gradually increase,and the influence on the amount of zinc adsorption is tillage layer> plough bottom layer> core soil layer> parent material layer;the desorption amount decreases by 23.7%,28.2%,29.0% and 30.3%,respectively.The increase of pH is not conducive to the desorption of heavy metal Zn.(4)The removal of organic matter in the four layered soils reduced the adsorption amount of Zn in the soil,which was far less than that in the soil without removal of organic matter,reduced the adsorption potential of the soil for heavy metals,and weakened the fixation ability of the four stratified soil for heavy metals zinc;After the removal of organic matter,the desorption rate increased,which further confirmed that the organic matter in the soil could inhibit the desorption of Zn.Therefore,appropriate increase of organic matter in soil or application of organic fertilizer can reduce the migration activity of Zn and reduce its pollution harm to groundwater.(5)Adsorption/desorption behavior of soil changed under freeze-thaw action.The adsorption amount of Zn to the soil decreases with the increase of freeze-thaw cycles;when the temperature,cycles,and water content of the freeze-thaw were the same,the adsorption amount of the 4 types of soil were tillage layer> plow bottom layer> subsoil layer> the parent material layer,and were all smaller than the unfreeze-thaw soil.The amount of Zn desorption increases with the number of freeze-thaw cycles.As the temperature difference between freeze-thaw increases,the amount of soil Zn desorption increases.After freezing and thawing,the soil desorption rate increases,and the ecological risk of Zn increases. |
