Experimental Study On Freezing Temperature And Unfrozen Water Content Of Heavy Metal Contaminated Loess

With the increasing human activities such as resourc e development,engineering construction and nuclear waste storage in frozen soil area,excessive heavy metal elements will enter the cold soil circle through material circulation and energy flow,and then through material migration and transformation into hydrosphere,biosphere,atmosphere and other circles,which will have a profound impact on the fragile and sensitive ecological environment of frozen soil.The migration of salt,harmful chemicals and heavy metal elements in frozen soil or ice during soil freezing and thawing is a hot issue in recent years,and the freezing temperature and unfrozen water content are the key factor to study the solute migration in frozen soil.Because of the change of unfrozen water content,the hydraulic characteristics and thermal parameters of frozen soil are changed,which will affect the migration ability of heavy metal elements in frozen soil.At the same time,the freezing temperature is an important factor to determine the unfrozen water content,and it is also an imp ortant factor to affect the water migration and the frost heave and thawing settlement in frozen soil.Therefore,the freezing temperature and unfrozen water content have a crucial effect on the migration of pore water,heat and solutes in frozen soil.In order to accurately understand the migration law of heavy metal elements in frozen soil and to effectively control the migration of heavy metal elements in frozen soil,we should first study the influence factors of freezing temperature and unfrozen water content of heavy metal contaminated soil and the relationship between them.Taking artificially prepared heavy metal contaminated loess as the research object,on the basis of laboratory tests combined with theory analysis,this paper studied the factors affecting freezing temperature and unfrozen water content of heavy metal soils contaminated with zinc,nickel,copper,chromium,cadmium and lead,discussed the relationship between freezing temperature and unfrozen water content of heavy metal loess contaminated with zinc,nickel,copper,chromium,cadmium and lead,and established the forecasting model of unfrozen water content.The main results for this study are as follows:(1)Different cooling methods,rapid cooling and slow grading,have no significant effect on freezing temperature of heavy metal contaminated loess.The initial water content has a great influence on the freezing temperature of heavy metal contaminated soil.The increase of freezing temperature will accompany with the enlargement of the initial water content when the initial water content between 8%~22%,while the freezing temperature is almost independent of the initial water content when the initial water content between 22%~28%.The freezing temperature of the loess contaminated with zinc element,nickel element and cadmium element decrease linearly with the increase of their respective element content,while the freezing temperature of the loess contaminated with lead element,chromium element and copper element does not change obviously with the increase of their respective ion content.Effects of these heavy metal ions on freezing temperature of the loess are Zn(Ⅱ),Ni(Ⅱ),Cd(Ⅱ),Cr(Ⅲ),Pb(Ⅱ)and Cu(Ⅱ)in descending order of impact degree.(2)Temperature is the major factor affect ing the unfrozen water content of heavy metal contaminated loess.The decrease in temperature will decrease the unfrozen water content,and heavy metal elements will change the initial temperature of liquid water phase transition.The presence of heavy met al element has a great influence on the freezing temperature of loess.For the loess contaminated with the same heavy metal element except for copper and chromium element at the same temperature,the higher the content of heavy metal element is,the greater the unfrozen water content is.However,the unfrozen water content of chromium and copper contaminated loess remained basically unchanged with the increase of heavy metal element content.Under the same condition,the volume unfrozen water content of heavy metal contaminated loess increased slightly in the process of water content changing from unsaturation to saturation.W hen the content of heavy metal element in loess is equal to 0.5%,these heavy metal elements are arranged as Cu(Ⅱ),Cr(Ⅲ),Pb(Ⅱ),Cd(Ⅱ),Ni(Ⅱ)and Zn(Ⅱ)in descending order of their influence degree on the unfrozen water content in loess.While when the content of heavy metal elements in loess is equal to 3%,these heavy metal elements are arranged as Cu(Ⅱ),Zn(Ⅱ),Cr(Ⅲ),Ni(Ⅱ),Pb(Ⅱ)and Cd(Ⅱ)in descending order of their influence degree on the unfrozen water content in loess.(3)By using the modified generalized Clapeyron equation to calculate the freezing temperature of heavy metal contaminated soil,it is found that the error between the calculated results and the experimental results is almost negligible.A forecasting model of unfrozen water content in heavy metal contaminated loess is established based on the calculated results and the experimental results,w hich can well predict the change of unfrozen water content in heavy metal contaminated loess.In addition,the theoretical relationship between unfrozen water content and heavy metal element content was established,which changed in a cubic polynomial law under the experimental conditions in this article.