Effects Of Iron Oxides On Surface Properties Andadsorption-desorption Of Lead By Several Loessial Soils

Iron oxide contained in soil is an important component of secondary minerals, is commonly present in the soil, it includes amorphous adhesive film state and a crystalline state, is a major component of nano-soil minerals. In view of the universality of soil containing iron oxide and the important influence of iron oxide to the soil physical and chemical properties, a further study about function and influence of iron oxide on the soil surface properties as well as the adsorption and desorption of heavy metal ions do will greatly promote the research and application of iron oxide in the soil. Although a lot of researches on the nature and function of iron oxide in the soil have already been done, there are still a lot of work to be carried out. In the past most scholars regarded iron oxide as a whole and didn’t distinguish the influence of different forms of iron oxide on heavy metal adsorption and desorption as well as soil surface properties.This paper takes paleosol, eluvial cinnamon soil and yellow cinnamon soil that developed form the loess parent material as the research object, using chemical selective solvent to remove the different forms of iron oxide in soil sample, Mehlich method was used to study the changes of soil surface charge after removal of the different forms of iron oxide; we can compare the changes of the isothermal adsorption- desorption curve and adsorption-desorption hysteresis effect in the soil By isothermal adsorption and desorption test before or after the removal of the different forms of iron. Aims to reveal the different roles that the various forms of iron oxide played in loess soil surface properties and the adsorption – desorption of Pb²⁺. Besides that, it also can provide scientific basis about the behaviors of the Pb²⁺ in the soil and the impacts on the ecological environment. Get the following main conclusions:（1） The influence of different form s of iron oxide on the soil surface propertiesMorphology of ferric oxide in soil greatly affects the soil specific surface area and the amount of charge, it’s related to the interaction between ferric oxide and layered silicate minerals. iron oxide without fixed form and free iron oxide in the soil is the main carrier of variable charge soil.The vast majority of the soil specific surface area is on the decline after the removal of various forms iron oxide in the soil. after the removal of free iron oxide, the specific surface area of the soil decreases to the most obvious. Removal of complexing state iron oxide can make increase of quantity of soil total negative charge quantity and permanent negative charge, it also can make the variable negative charge falling; Removing functional, free iron oxide can make soil permanent negative charge quantity increase in different degrees, soil variable negative charge quantity decreases, but the impact on total negative charge amount is not identical.（2） The influence of different form iron oxide on soil adsorption of Pb²⁺Iron oxide interacting with layered silicate minerals in nature will influence the adsorption capacity of heavy metals. The content of different forms iron oxide is not the key factor adsorption Pb²⁺ in the soil, and forms of iron oxide in the soil is largely affects the ability of adsorption of Pb²⁺ in the soil.The adsorption ability of Pb²⁺of original soil without removing iron oxide is a gradually increasing process, its adsorption curve more accord with the Freundlich model, and Pb²⁺ affinity（KL） difference is not obvious, but the Pb²⁺ adsorption quantity had a obvious different. Using selective solvent extracting technology, respectively, to remove all kinds of iron oxide in soil, the soil adsorption of Pb²⁺ curve changes obviously, the stage characteristics, especially the rapid growth stage is very obvious, it conforms to the Langmuir adsorption curve model, the soil affinity of Pb²⁺（KL） multiplied. Compared with the original soil, the adsorption capacity of Pb²⁺ on the soil increased in different degrees after the removal of complexing state iron and amorphous iron oxide, and the adsorption capacity of Pb²⁺ increased after the removal of free iron oxide from paleosol and eluvial cinnamon soil, the adsorption capacity of Pb²⁺ reduced in yellow cinnamon soils.（3） The free iron oxide influenced Pb²⁺ desorption characteristics and the hysteresis effect.As to three kinds of loess soil, whether removed free iron oxide or not, it’s Pb²⁺ adsorption process are in line with the Langmuir model, but the adsorption isothermdesorption isotherm didn’t overlap, there is a lag between adsorption- desorption. the original soil witnout the removal of free iron oxide, it’s desorption hysteresis level is larger. Removal of free iron oxide, can significantly enhance the desorption effect of Pb²⁺ and make soil desorption hysteresis decreases. After removal of free iron oxide, the desorption percentage of Pb²⁺ were greater than the original soil.With the increase of Pb²⁺ concentrations in the solution, desorption hysteresis value of each soil increased. The lag value of the soil after removal of free iron oxide（or Kd/Ka values） is less than the original soil of lag values（or Kd/Ka value）; As to Loess soil, the conclusion come from Pb²⁺ adsorption-desorption hysteresis characteristic parameters（Kd/Ka values） and the size of the lag of the area（Hv） size were consistent, it’s confirmed that Kd/Ka values represent as characterization of Pb²⁺ adsorption- desorption hysteresis degree is feasible.