Chemical Forms Transformation And Adsorption Of Pb And Cd In The Different Profiles Of Lou Soil

With the development of modernization and industrialization,soil heavy metal pollution has become a serious problem of our country.The contaminated soil often contains more than one kind of heavy metal ion,and these heavy metals interact with each other,thus affecting the biological effectiveness and migration.In this study,three layers of Lou soil as cultivated horizon,argillic horizon and calcic horizon were collected as the test materials.Two kinds of heavy metals,including lead(Pb)and cadmium(Cd)were involved in the experiment.The adsorption characteristics and chemical forms of single Pb,Cd and their coexistence,the variations of water content were studied by means of adsorption isotherm and incubation experiments.The main conclusions were as follows:1.For single Pb adsorbed by cultivated horizon,argillic horizon and calcic horizon of Lou soil,the distribution coefficients were 8.00,8.11 and 6.05,respectively.The maximum adsorption capacities calculated from Langmuir equation were 6.07,5.75 and6.01mmol/kg,respectively.For single Cd,the distribution coefficients were 4.19,7.05 and 4.72 for each soil samples,and the maximum adsorption capacities calculated from Langmuir equation were 5.54,4.77 and 2.95mmol/kg,respectively.These results showed that Pb had stronger adsorption capability than Cd in three soils.When Pb and Cd coexisted,it was found that the two metal ions competed with each other.Therefore,the maximum adsorption capacities of both Pb and Cd decreased.The affinity effect between Pb and Cd was 0.8-0.9.The competitive Langmuir adsorption equation could describe the competitive adsorption of Pb and Cd with high determined coefficient.2.The chemical forms of Pb and Cd in cultivated horizon,argillic horizon and calcic horizon of Lou soil was similar which were dominated by residual form,and theexchangeable form only accounted less about 10%.The exchangeable form of Pb and Cd decreased during the experimental period when exotic Pb and Cd were added.However,carbonated forms of Pb and Cd increased at the first stage,and then decreased during the incubation experiment.The contents of iron-manganese oxide and organic combination state of Pb and Cd increased over the whole incubation period.The residual forms of Pb and Cd kept at constant which might be due to the fact that heavy metal ions could not enter the lattice of minerals during a relatively short incubation time.3.Water conditions had great influence on the chemical form and redistribution of heavy metals.For Pb and Cd,the rate of exchange to the other forms was fastest under dry-wet alternate conditions,followed by field moisture capacity 75%,and then drought conditions.In general,the exchange rate ranged as follows: dry-wet alternate conditions > field moisture capacity 75% > field moisture capacity 50% > drought conditions.4.The form transformation was different between single Pb,single Cd and coexistence of Pb and Cd.Under competed conditions,the exchangeable form and carbonated form increased,residual forms decreased,but iron-manganese oxide and organic combination state of Pb and Cd remained constant.These results showed that Pb and Cd were inclined to converted to exchangeable carbonated form.Coexistence and competition of heavy metals in soils which can significantly affect the bioavailability and mitigation of heavy metals.Studying the characteristics of combined pollution of heavy metals are the basis of treatment of heavy metals.However,this thesis only done the preliminary work in such areas.If the technologies are used in treatment,further studies are needed.