Effect Of Freeze-Thawing Cycles On Lead And Cadmium Adsorption-desorption In Soil And Mechanism

Freeze-thawing cycles(FTC) is a common climatic change in the northern area of China which affects the ecosystem biogeochemistry process in many aspects.Previous studies reported effects of FTC on soil physical and chemical properties.Its effects on soil heavy metal adsorption/desorption were not concerned enough.This paper studies the effects of FTC on soil heavy metal adsorption/desorption and elaborates mechanisms,pH changes of adsorption and desorption equilibrium solution,respectively.Investigating effects and mechanisms of exogenous DOM(Straw and Sewage Sludge) on dissolution of heavy metal in cantaminated soils by FTC.The main conclusions of this study are as follows:1.Effect of FTC on the ability of heavy metal absorbed/ desorbed to different soilsBoth adsorption values and desorption values increase with the concentration of Pb and Cd.Langmuir equation could fit well to the ability of soils with or without FTC to absorb Pb and Cd.Freundlich equation could describe both the adsorption and desorption process of Pb and Cd.Significant or extremely significant differences are observed between the equilibrium concentration of Pb and Cd in soil sulutions and adsorption values,the adsorption values and desorption values,desorption rates.The ability of soils treated with FTC to absorb Pb and Cd is smaller as compared to the untreated soil at the same content of added Pb and Cd under field capacity water of soil and the adsorption capacity of black soil to absorb Pb and Cd is always greater than the brown soil.The ability of soils treated with FTC to desorb Pb and Cd is bigger as compared to the untreated soils and at the same amount of heavy metal adsorption by soils,the desorption capacity of the black soil to desorb Pb and Cd is less than that of the brown soil,which suggest that FTC can reduce the ability of soils to keep Pb and Cd.Possible causes are changes in the nature of soil by FT,for example,soil pH,OM,CEC and free iron oxide,and equilibrium solution pH decrease in untreated soils is greater than that in treated soil.2. Effect of different FT conditions on the ability of heavy metal absorbed/desorbed to soilThe treated soils with FTC exhibited more ability to absorb Pb and Cd with the decrease of freeze temperature and FT frequency,but still less than untreated soils,desorption values and desorption rates also decrease gradually.Effect of FTC on Pb and Cd adsorption thermodynamics process in different moisture content soils is complexed.When the soil moisture content is 60%,the ability of soils treated with FTC to absorb Pb and Cd is smaller as compared to the untreated soil,however,the treated soils with FTC exhibites more ability to absorb Pb and Cd and less ability to desorb Pb and Cd with the increase of soil moisture content.When the soil moisture content was 120%,the adsorption ability of the treated soil to adsorb Pb is bigger than the untreated black soil.In this article within the moisture content scope,the adsorption ability of the treated soil to adsorb Cd was always smaller than the untreated black soil.T-test showed that the differences between soil properties reached a significant or extremely significant leval.3.Effect of FTC on the adsorption balance time of heavy metal adsorbed to different soilsAs the extention of adsorption time,adsorption amounts of heavy metals in soils increased firstly and then leveled off.The entire adsorption process was divided into two phases namely the fast adsorption stage and the slow adsorption stage.Regardless of whether through the freezing-thawing treatment,the time of reaching adsorption equilibrium of Pb in black soil and brown soil is greater than Cd.FT soils reduced the adsorption capacity of heavy metal Pb and Cd and extended the time for heavy metal adsorption.Whether or not FT,Elovich equation could fit well to the adsorption kinetics process of Pb and Cd adsorbed to soil.4.Effect of FTC on Pb and Cd dissolution in contaminated soilsThrough FTC,the dissolution rates increase in both black soil and brown soil and the dissolution rates of heavy metals in black soil was less than that in brown soil.At the same DOM concentration,heavy metal Pb dissolution using straw stalk is greater than that using sludge,Cd is the opposite trend.Along with the increase of contaminated time and pollution levels,heavy metal dissolution rates increase and showed significant differences.Causes not only with changes in soil properties (pH,OM,CEC and Eh) by FT but also with DOM molecular size and hydrophilic fractions.