Adsorption behavior and surface complexation models of plumbum and cadmium in basalt yellow soil

<bold>Objective</bold> This study aims to to understand the adsorption characteristics and mechanisms of Plumbum（Pb）and cadmium（Cd）elements，which have strong pollution toxicity，in Guizhou yellow soil，thus providing theoretical basis for the prevention and control of heavy metal Pb and Cd pollution in agricultural soil. <bold>Method</bold> Basalt parent yellow soil from different sites in Guizhou Province was selected as the research object.According to the relevant formulas of the 1-site/2-p<italic>K</italic>_a Generalized Complex Surface Complexation Model（SCM）theory，combined with soil physicochemical property determination experiments，potentiometric titration experiments，and Pb and Cd adsorption edge experiments，the surface acid-base property parameters，namely model parameters，of yellow soil were obtained.Then，using the 1-site/2-p<italic>K</italic>_a SCM，the morphological distribution of three active sites on the yellow soil surface and the adsorption behavior of Pb and Cd at different pH levels were simulated to explore the adsorption characteristics and mechanisms of Pb and Cd by yellow soil.Correlation analysis was conducted to analyze factors influencing the adsorption of Pb and Cd by yellow soil. <bold>Result</bold> According to the relevant formulas of the SCM theory，the charge zero pH_pzc，SCM of the four yellow soil samples were obtained as 5.040，5.549，7.984，and 4.297，respectively.The surface site concentrations（<italic>H</italic>_s）were 0.137，0.183，0.181，and 0.308 mol/kg，and the surface site densities（<italic>D</italic>_s）were 4.519，2.571，2.122，and 3.664 sit/nm²，respectively.Changes in the forms of ≡SOH₂⁺，≡SOH，and ≡SO^- on the yellow soil surface under different pH conditions were simulated by SCM，and the adsorption mechanisms of Pb and Cd on the yellow soil surface were preliminarily explained.The adsorption behavior of Pb and Cd on the yellow soil surface was fitted at different pH levels（correlation coefficient <italic>R</italic>≥0.96），indicating that SCM was suitable for describing the adsorption characteristics of Pb and Cd on the yellow soil surface.Adsorption edge experiments showed that the initial concentrations of Pb、Cd and the pH of the soil environment both affect the adsorption behavior of Pb and Cd on the yellow soil surface，and there are differences in the adsorption of Pb and Cd on the yellow soil surface；The adsorption capacity of yellow soil for Pb was greater than that for Cd，which was not only due to Pb having a smaller hydrated ion radius and a larger hydrolysis constant，but also because the calculated complexation equilibrium constant lg<italic>K</italic>_SOPb was greater than lg<italic>K</italic>_SOCd，indicating that the complex formed by Pb on the yellow soil surface was more stable than that formed by Cd.The correlation analysis indicated that Hs in yellow soil was primarily influenced by iron-aluminum oxides and organic matter.pH had a significant impact on pH_pzc and lg<italic>K</italic>_SOCd，as well as lg<italic>K</italic>_SOPb in yellow soil. <bold>Conclusion</bold> The 1-site/2-p<italic>K</italic>_a SCM is highly applicable for accurately describing the adsorption characteristics of Pb and Cd on the surface of basalt parent yellow soil in Guizhou Province and explaining the adsorption mechanisms of Pb and Cd，.It could provide reference for the study of the migration and transformation of heavy metals in agricultural soil.