Study On Cadmium Adsorption Behavior And Surface Complexation Model Of Two Parent Yellow Soils In Guizhou Province

The heavy metal cadmium（Cd）is a highly toxic inorganic substance that can remain in the soil for a long time and seriously threaten the growth and development of animals and plants.The acid yellow soil in Guizhou province is severely polluted with cadmium.The differences in soil main components and acid-base properties are caused by differences in geographic location and soil parent materials,which also could lead to differences of migration and transformation behavior of heavy metals,such as adsorption-desorption,solution-precipitation,oxidation-reduction.The migration and transformation behavior of heavy metals in soil is of great significance to the risk estimation and prevention of heavy metals in soil.At present,the research on soil heavy metal adsorption behavior and mechanism is mainly carried out by traditional empirical models in China,while,the surface complexation model（SCM）is a mechanistic model developed rapidly for the study of adsorption behavior at the solid-liquid interface in recent decades.Its applicability in the study of soil acid-base properties and heavy metal adsorption mechanism has been confirmed in typical soil studies in some areas of China.However,whether above study of SCM is also applicable to different parent materials of yellow soil in Guizhou province remains to be further explored.In this paper,the 1-site/2-p K_a generalized composite surface complexation model was applied to the 5 and 3 layers of acid yellow soil from different depths developed from the parent materials of sand shale and argillaceous limestone in Baihuahu Town,Guiyang city and Linka Village,Pingba District,Anshun city,Guizhou province.Firstly,the composition,surface functional groups,CEC,pH and other basic information of yellow soil samples were determined.Based on1-site/2-p K_a SCM,the yellow soil could be simplified as a unified hydroxyl surface,and the reaction of soil with the change of environmental pH could be expressed as the protonation-deprotonation process of hydroxyl.Secondly,based on the results of continuous acid-base potentiometric titration experiments,the surface acid-base characteristics of the yellow soils were explored,the distribution of 3hydroxyl species（≡SOH,≡SOH₂⁺,≡SO^-）on the yellow soil surface with pH was simulated by SCM.Based on the Cd adsorption experiments of the two parent yellow soils under different pH conditions,the SCM was used to simulate the Cd adsorption behavior to verify the applicability of the 1-site/2-p K_a SCM in fitting the Cd adsorption characteristics of the two parent yellow soils.Combined with the change of hydroxyl species,the adsorption and transfer mechanism of Cd in the two yellow soils was explained.Finally,through the correlation analysis of basic physical and chemical properties and components of yellow soil with H_sand pH_pzc,the important substances and parameters in soil acid-base properties and heavy metal adsorption were analyzed.The main conclusions are as follows:（1）For the two parent materials of yellow soil,the point of zero charge(pH_pzc,SCM)calculated by 1-site/2-p K_a SCM（S1～S5 in baihuahu Town of Guiyang city:4.83,4.71,4.61,4.96 and 4.98,S1～S3 in Pingba District of Anshun city:4.49,4.42 and 4.52）were highly close to the potentiometric titration experimental data pH_pzc,M（S1～S5 in baihuahu town of Guiyang City:4.78,4.69,4.63,5.03 and 5.05,S1～S3 in Pingba District of Anshun City:4.49,4.39 and 4.55）,and their change tendency were consistent,indicating that it was feasible for the application of1-site/2-p K_a SCM to research the acid-base properties of sand shale yellow soil and argillaceous limestone yellow soil from different depths.（2）Based on soil acid-base property parameters D_s、p K_a,SCM simulation results showed that with the increase of pH,≡SOH₂⁺decreased and≡SO^-increased on the surface of two yellow soils,≡SOH₂⁺and≡SO^-were the main surface species of two soils at pHpzc and pH>pH_pzc,respectively,thus Cd²⁺were gradually absorbed in the process of increasing≡SO^-.It was proved that 1-site/2-p K_a SCM was suitable for describing the morphological distribution and Cd adsorption and transfer characteristics of two kinds of yellow soil surface sites under different pH.（3）The variation of dissolved Cd and adsorbed Cd concentrations with pH in the two yellow soils simulated by 1-site/2-p K_a SCM was well matched with the Cd adsorption experimental data（sand shale yellow soil in baihuahu Town:R≥0.995,argillaceous limestone yellow soil in Pingba District:R≥0.991）,and the change curves of adsorbed Cd and dissolved Cd under different pH in the two parent soils or soils from different depths of the same parent material were almost the same for their close composition and pH_pzc,which proved that 1-site/2-p K_a SCM was suitable to analyze the adsorption and transfer characteristics of heavy metal Cd with pH in the two parent materials of yellow soil collected at two sites.（4）Correlation analysis showed that organic matter,clay minerals,amorphous iron and aluminum oxide,cation exchange capacity（CEC）and calcium carbonate were positively correlated with H_s,all of which could effectively increase the number of soil surface sites.For the two yellow soils with pH_pzc range of 4～5,the soil pH_pzc was mainly determined by clay minerals with lower pH_pzc value such as illite and muscovite.The 1-site/2-p K_a SCM fitting results indicated that the point of zero charge(pH_pzc)was a key factor affecting the acid-base properties of the soil surface.