Study On The Impact Of Soil Colloid On Heavy Metals Migration In Vadose Zone

With the rapid development of industry and agriculture,the contamination of heavy metals in the underground is becoming increasingly serious.The migration depth of heavy metals in the soil is much larger than the theoretical predictive value.Because there is a lot of soil colloid between 1nm to 2 m in the underground environment,which can be effectively absorbed contaminants, as well as migrate similar to the rate of water. In suitable conditions,they will be separated from the soil and carry heavy metals to migrate as one of the main forms in the underground,thereby promoted the migration capacity of heavy metals.Therefore,in this experiment,clay loam,fine sand and silt loam collected from the farmland,floodplain and Loess platform in Changchun seperately were chosen for the matrix to study the adsorption and desorption capacity of Pb,Cr(VI) and Hg by soil colloid.The turbidimetric method was used to study the colloid stability,as well as the methylene blue indicator papers were used to study the colloid mobility influenced by the solution chemistry of the infiltration water taking rainning for example.And the leaching clumns were also used to study the impact of rainfall on the release and migration compacity of the soils colloid which can co-migrate in matrix and preferential flow with heavy metals influenced by the same conditions as colloid stability and mobility.Finally mathematical statistics were used to obtain the co-migration parameters of the complexes.This is important to establish the precise transportion model of heavy matals,characterize the migration rule of heavy metals in Vadose zone,evaluate the pollution of the aquifer, and provide more precise theoretical basis for scientific forecasts of heavy metals migration in the underground.So as to propose the governance effective strategy and treatment technology for preventing and controlling groundwater pollution,and ultimately to protect the underground environment.Through system methods at each level progressively,the experiments were studied in order to provid evidence for the selection of parameters which verified the experiments were reasonable.Although the conclusions and water chemistry parameters have a certain "spatial scale effects",they also indirectly reflected the actual complexity of heavy metals migration in the underground and proved that the role of the promotion of preferential flow and soil colloid on the migration of heavy metals can not be ignored.The results show that the soil colloid play different role in the adsorption of soils to heavy metals.The adsorption contribution rate of colloid in the soil adsorption to heavy metals is Cr(VI)>Hg>Pb.The adsorption capacity of soil colloid to heavy metals in dynamic adsorption experiments is lower than static experiments, but the result is closer to the actual environment.In the underground,the mobility of the soil colloid is closely related to its stability.Generally,the more stability of the soil colloid is,the more sustainable colloid mobility is,so the colloid can migrate farther.When the pH is between 5 and 8,Na+ strength is not more than 50 mmol/L or Ca2 + strength is not more than 5 mmol/L of the rainfall,colloid is the best dispersion and has stronger mobility.Also a certain pollution level of Pb,Cr(VI) and Hg enhance the stability and mobility of the soil colloid in a certain ionic strength.In the underground,the colloid has a certain stability and mobility, so the release and migration of the three kinds of soil colloid should not be ignored, and their release and migration have a "pulse" rule with rainfall cycles.The release concentration between 40 and 250 mg/L of the colloid extracted from the soils is floodplain soil column>farmland soil columns>loess soil column. The release amount between 30 and 120 mg/m3 of the soils is farmland soil column>floodplain soil column>loess soil column.Since the release and migration ability is different,the soil colloid has different promotion for the migration of heavy metals.The experiments show that the promotion capacity of the colloid to Cr(VI) is N1>H1>T1,the promotion capacity of the colloid to Pb is T1> N1>H1,and the promotion capacity of the colloid to Hg is H1>T1>N1.In the course of promoting migration of heavy metals,colloid-heavy metal complexes formed, CrO42-, CaCrO4(aq), HCrO4-and Cr2O72--colloid complexes account 10 ~ 50 percent of the Cr(VI) moved out of soil columns, Pb2+, PbOH+, PbSO4(aq) and PbCl+-colloid complexes account 20 ~ 30 percent of the Pb leaching from soil columns,and 25 ~ 35 percent of the Hg leaching as the form of Hg(OH)2, HgClOH(aq) and HgCl2(aq) colloid complexes,but under different migration conditions the complexes share different percents. Because of the colloidal promotion, three heavy metals in the soils have larger hydrochemical rate,migration concentration and amount,and front migration rate than in the control groups which have not colloidal promotion, so they can migrate to the same depth of groundwater in relatively short time with larger amount.Time consumpting of heavy metals migrating to the same depth of groundwater is in floodplain soi>farmland soil>loess soil.Thus the stronger the heavy metals or colloid- heavy metals complexes migrate,the greater risk of the groundwater being polluted is.And the role of the preferential flow in the co-migration process can not be ignored.Preferential flow has a significant impact on both the hydrodynamic conditions and the migration of heavy metals in solution.The preferential flow is faster than the matrix flow.And the fracture of loess soil has better unblocked preferential flow path than the root holes or wormholes of farmland soil, so it can reach the maximum water yielded earlier.The average flow velocity in farmland soil and loess soil is 2.07 and 1.63 times faster than the control groups respectively.Regardless of colloid existence,the migration ratio of heavy metals from the soil columns with preferential flow is larger than non-preferential flow soil columns.In farmland soil columns,promotion capacities of the preferential flow and colloid to Cr(VI),Hg and Pb are 2.18,1.9 and 2.09 times larger than the control groups with no preferential flow and colloid,and the promotion capacity is that Cr(VI) is the strongest,then Hg, and Pb is the smallest.The same as in the loess soil columns,promotion capacities of the preferential flow and colloid to Cr(VI),Hg and Pb are 1.81,1.26 and 3.71 times more than the control groups,and the promote capacity is that Hg is the strongest,then Cr(VI), and Pb is the smallest.The study shows the role of preferential flow,further down the role of co-migration of heavy metals with the colloid.Whether preferential flow exists,heavy metals or the complexes have certain penetration capacities in the soils,and the capacities of the floodplain soil column is the largest,then the farmland soil column and the loess soil column is the smallest.Generally,the lower the solution ionic strength and pH higher, the colloid is more stable, the higher the breakthrough concentration of heavy metals is.Whether how the mechnism between colloid and heavy metals acts, the processes exist in common:First,through some sort of adsorption, heavy metals form complexes with colloid,and co-migrate in the underground environment,which is called a co-migrate mechanism.Second,the heavy metal complexes with colloid have a certain stability and mobility.Third,in migrating process, the presence of colloid increases the migration concentration and amount of heavy metals.Forth,the presence of colloid reduces the reduction ability of colloid layer to heavy metals,and shortens the consumpting time for heavy metals migrating to a certain depth of groundwater.Fifth,if large pores exsiting in the soil,preferential flow can also promote colloid-heavy metals complexes to migrate.. In this paper,the innovation is use natural soils and hydrogeochemical simulation methods to quantitatively study the average release concentration, the average release rate,the release amount and the release migration rule of the soil colloid from the vadose zone soils under different natural conditions taking rainfall for example,as well as to identify the source of colloidal source for soil colloid and heavy metals co-migration research.At the same time a combination of static and dynamic simulation method was used,and through the identification of the soil the complexes amount to evaluate the formation and stability of colloid-heavy metals complexes,on the basis of which to use the hydrogeochemical simulation methods to quantitatively study the co-migration composition and amount,the hydrochemical rate, and redartation factor of the colloid-heavy metals taking Pb,Cr(VI) and Hg for example ,and forecast the time consumption of heavy metals migrating to groundwater,and study the migration law and capacities,and further research on the role of preferential flow,the migration of heavy metals-colloid complexes,which can make up for the shortcomings of the previous studies,and provide a basis of parameters for the actual simulation, prediction,prevention and control of heavy metals in the underground.