Study On The Adsorption Behavior Of Cadmium And Chromium In Soil Aggregates Of Different Grain Grade

The formation of natural soil depends on the cementation of inorganic-organic complexes and consists of soil aggregates formed by complex physical,chemical,and biological cross-reactions.Soil aggregates of different grain grade have different physical and chemical properties due to their different internal component and binding ways.Their physical and chemical properties play a decisive role in the adsorption behavior of heavy metal.In the past,the adsorption behavior of heavy metals in soil was mainly studied in soils of different textures,but the behavior in aggregates of different grain grade of the same soil was seldom studied.It is great significance to clarify their migration,transformation and fate in soil by studying the adsorption behavior of heavy metals in the soil aggregates of different grain grade.The heavy metals cadmium and chromium were taken as research objects,and5 types of soil aggregates(clay grade?0-0.002mm?,powder grade?0.002-0.05mm?,fine sand grade?0.05-0.25mm?,medium sand grade?0.25-1mm?,and coarse sand grade?1-2 mm?were sieved and extracted,to study the differences in physical and chemical properties of soil aggregates at different grain grade.Based on the properties of soil in liquid phase and the composition of soil in solid phase,explore its effect on the adsorption of cadmium and chromium in soil aggregates of different grain grade and study the adsorption behavior and aging process of cadmium and chromium in soil aggregates of different grain grade.By studying the environmental behavior of cadmium and chromium in soil aggregates,we can provide a theoretical basis for accurately assessing the risk of cadmium and chromium in soil and a prevention and control method.The main research results obtained are as follows:?1?Physical and chemical properties of soil aggregates.The aggregates in the tested soil were mainly fine-grained and coarse-grained,which manifest alkalescent and stable in structure.The organic carbon,free iron oxide and amorphous iron in the soil aggregates tend to be aggregated into clay aggregates in the group-level distribution.What's more,the cation exchange capacity?CEC?of the soil aggregates with smaller particle diameters is higher.?2?The properties of soil in liquid phase affect the adsorption of Cd and Cr by soil aggregates of different grain grade.The higher solution acidity results in the smaller the amount of Cd adsorption and the greater the amount of Cr adsorption on soil aggregates;the increase of solution ion strength will inhibit the adsorption of Cd and Cr by the aggregates;Fe³⁺has competitive adsorption with Cd.Fe³⁺can inhibit the adsorption of Cd,but promote the adsorption of Cr by aggregates.As the increase of water-soil ratio,the adsorption capacity of the aggregates to Cd increases first and then decreases,and the amount of Cr continues to increase.?3?The solid phase composition of soil affects the adsorption of Cd and Cr by the aggregates.After removing organic matter,free iron oxide and amorphous iron respectively,the adsorption capacity of Cd and Cr in each grain grade aggregates decreased.The relationship between the removal of the three components on the total Cd is in the following order:organic matter>amorphous iron>free iron oxide,and the relationship on the total influence degree of Cr shows:free iron oxide>amorphous iron>organic matter.In terms of unit influence,the tend Cd is amorphous iron?3.40-3.84mg/mg?>free iron oxide?0.023-0.086mg/mg?>organic matter?0.040-0.056mg/mg?;the order for Cd is amorphous iron?0.165-0.305 mg/mg?>free iron oxide?0.008-0.014mg/mg?>organic matter?0.001-0.003mg/mg?.?4?Adsorption and desorption characteristics of cadmium and chromium in soil aggregates of different grain grade.The adsorption capacity of clay grade aggregates to Cd and Cr is the largest,while the adsorption capacity of fine sand aggregate to Cd and Cr is the smallest.The amount of adsorption has no obvious relationship with the aggregate grade.Both Cd and Cr have desorption hysteresis in the desorption process,and the overall performance is that the greater the amount of adsorption,the greater the amount of desorption.?5?Aging process of cadmium and chromium in soil aggregates of different grain grade.After Cd and Cr enter the soil aggregates,they are mainly in the acid-soluble and reducible states,and gradually change from the unsteady state to the stable state with time.The early morphological distribution changes rapidly,and the late morphological distribution changes slowly.After 30 days,the distribution tends to be stable and after 60 days,Cd is mainly in oxidizable state and residue state,and Cr is mainly in reducible state and residue state.By investigating the adsorption and aging process of cadmium and chromium in soil aggregates of different grain grade,we provides a theoretical basis for evaluating the environmental risks of cadmium and chromium and providing control methods.