Migration of typical heavy metal As in the soil of underground coal fire

In the present paper,the authors conducted experiments on the samples of the soil of the Daquanhu coal fire at different temperatures (25,50,70 ℃ ) for analyzing the adsorption and desorption characteristics of typical heavy metal to the soil layers (Ⅰ,Ⅱ,Ⅲ,Ⅵ,Ⅴ,Ⅳ) at different depth for the same sampling site of the fire zone. Results show that the amount of adsorption of As to soil increases with the increase of As concentration added to the soil. And it also varies up and down with the increase and decrease of temperature. Desorption of As varies up and down with the same trend of the adsorption. With the experiment data,the authors fitted the trend of the isothermal adsorption of typi- cal As to these soil layers by using the Freundlich model. The key parameter KF for this model,which indicates the capacity of As adsorption to soil layers,was quantified as different equations for each soil layer. At the temperature of 25 ℃ ,the parameter KF for each layer has the trend of Ⅳ>Ⅴ>Ⅵ>Ⅲ>Ⅱ>Ⅰ,which shows the same change as the change of the CaCO3 content for each layer. At the temperature of 50 ℃ ,the parameter KF for each layer has the trend of Ⅳ>Ⅵ>Ⅲ>Ⅴ>Ⅱ>Ⅰ,which shows the same change as the change of the pH value and the CaCO3 content of soil layers. At the temperature of 70 ℃ ,the parameter KF for each layer has the trend of Ⅰ>Ⅲ>Ⅱ>Ⅵ>Ⅳ>Ⅴ,which shows the opposite change with the change of the pH value. With these soil samples,soil columns for each layer were established,including a column for the integrated soil layer with all layers. Result from leaching experiments show that the releasing rate of As was fast at the beginning of leaching which indicates the As move downward quickly with the leaching liquid movement. At the middle and late stage of leaching,the releasing rate reduces slowly to reach an equi- librium sate. In the present paper,numerical simulations also were made to study the change of migration of As at dif- ferent conditions,i. e. ,the inner As adding condition,the surface As adding condition,and the external As adding con- dition. Single layer simulation shows the same movement of As as the experiments which verified the reliability of simu- lation method. Integrated soil simulation shows that the As content within the soil column increases with the increase of temperature which indicates the soil temperature is the major factor for the As migration.