Experimental And Numerical Simulation Of Transfer Process Of Mercury In Soil Driven By Freezing-thawing And Soil Leaching Technology

With the development of urbanization and industrialization,the pollution of soil by mercury and its compounds generated by human production and life has threaten people’s livelihood,and the efficient and economic removal of Hg pollution in soil to reduce the negative impact on human health and ecosystem is an important research topic.Chemical leaching of heavy metal contaminated soils has attracted much attention as an efficient remediation technology.This study focuses on the migration process and influence mechanism of freezing-thawing and leaching technology on Hg（Ⅱ）in soil.In this paper,the indoor test was used to select the best eluent for soil Hg（Ⅱ）removal,and the removal effect of chemical leaching on Hg（Ⅱ）in soil column was investigated by laboratory soil column test.The leaching test was further optimized to explore the effects of leaching and freezing-thawing leaching on the migration and transformation of Hg（Ⅱ）and soil quality.HYDRUS-1D was used to establish Hg（Ⅱ）transport model in soil to clarify the migration process of Hg（Ⅱ）in soil.It could provide reference for more efficient and stable remediation of Hg（Ⅱ）contaminated soil.The main research results are as follows:（1）The morphological transformation and influencing factors of exogenous mercury in soil are clarified.The whole aging process is divided into three stages:rapid change（0-25d）,slow change（25-100 d）and equilibrium（100-150 d）during 150 d aging incubation period.As the aging time increases,the effective state of soil Hg（Ⅱ）decreases and then stabilizes.The mobile Hg（Ⅱ）in soil first decreases and then tends to equilibrium,the semi-mobile Hg（Ⅱ）first increases and then equilibrium,and the immobile Hg（Ⅱ）shows a slow decreasing trend.The fixation mechanism of exogenous Hg（Ⅱ）in soil includes the adsorption and micropore diffusion.Soil clay content,organic matter content and p H are important factors affecting the morphological transformation of soil heavy metal Hg（Ⅱ）.（2）The species and conditions of eluants that are more effective in removing Hg（Ⅱ）from soils are identified,and the removal mechanism and desorption process of Hg（Ⅱ）by potassium iodide（KI）are revealed.The average removal rates of Hg（Ⅱ）from three soils by the four eluents are CA（1.39%）<EDTA（1.58%）<Na₂S₂O₃（33.30%）<KI（76.88%）,and KI is highly effective in leaching Hg（Ⅱ）from soil because of its strong binding ability to Hg（Ⅱ）.The screening shows that the best condition of KI leaching effect concentration of0.125 mol L^-1,the extraction time of 20 h,the solid-liquid ratio of 1:5,and the extraction time of 1 time.The removal rates of Hg（Ⅱ）from the three soils are close with sierozem（78.75%）>Lou soil（78.34%）>dark loessial soil（76.00%）.The desorption process of Hg（Ⅱ）in soils is consistent with the intra-particle diffusion model（R²>0.95）,and the desorption process is the result of a combination of mechanisms of the diffusion of particles and the diffusion of membrane diffusion.（3）The effect of freezing-thawing and leaching technology on the migration and transformation characteristics of Hg（Ⅱ）in soil and the physicochemical properties of soil is elucidated.KI leaching promotes the migration of Hg（Ⅱ）in soil and effectively reduces the residual Hg（Ⅱ）in soil,but the activity and mobility of residual Hg（Ⅱ）are high and the rate of KI is low.The use of KI saturated with water leaching effectively removes Hg（Ⅱ）from the soil and reduces the mobility and activity.Compared with KI saturated with water leaching,KI saturation,freezing-thawing and leaching changes the soil structure to promote the leaching of water-soluble Hg from the soil and reduces the mobile Hg（Ⅱ）concentration in the soil.The basic physical and chemical properties of soil are affected by freezing-thawing and leaching technology,especially soil p H and electrical conductivity.The soil p H increases significantly and soil conductivity decreased significantly after leaching.The solute transport model developed by HYDRUS-1D is able to simulate and predict the variation of Hg（Ⅱ）in soil.HYDRUS-1D model is well to respond the penetration process of Hg（Ⅱ）in the liquid and the change of mercury in the soil profile.The fitting parameters and error evaluation value are within the error range.Based on the fitted inversion parameters,the time required for the Hg（Ⅱ）concentration in leaching of the soil column to reach the safe discharge standard and the change of the liquid-phase Hg（Ⅱ）concentration in the soil column at different times are predicted.In summary,the freezing-thawing and leaching technology can effectively reduce the toxicity and bioavailability of Hg（Ⅱ）in the heavy metals contaminated soil of the seasonal frozen soil in northwest China,and the restored soil after the freezing-thawing and leaching technology meets the remediation objectives of high efficiency and stability.This is of great significance to the safety of soil and ecological environment.At the same time,it provides theoretical guidance for the remediation of contaminated soil and provides technical ideas for the actual project of seasonal frozen soil area.