The Adsorption Behavior of Cd at the Iron-Clay Mineral Interface Simulated by the Surface Complexation Model with Component Addition

In order to reveal the adsorption behavior of mineral components in soil on heavy metals and their contribution, this study constructed a binary system of iron minerals and clay minerals, represented by goethite and illite, respectively. The morphology and distribution of Cd on the surfaces of goethite and illite were investigated by combining macroscopic experiments such as potentiometric titration experiment and pH adsorption edge experiment with surface complexation models (SCM), including CD–MUSIC and DLM, and their contributions in the binary system was analyzed by the Component Addition method in Surface Complexation Model (CA–SCM). The results showed that the binary system exhibited variable surface charge, with the point of zero charge between 8.70 and 10.21. The adsorption of Cd on the surface of the system increased with pH until complete adsorption, and the adsorption capacity of goethite was greater. CD–MUSIC revealed that the complexation constants of the bidentate complexes of goethite ≡(FeOH)2Cd+ on the surface of goethite were significantly greater than those of the hydrolyzed state ≡(FeOH) 2CdOH. The form ≡(FeOH)2Cd+ was the dominant adsorbed species in the pH range of 4-10, while DLM showed that illite was dominated by the monodentate complex ≡SOCd+ and the hydrolyzed form ≡SOCdOH in the pH ranges of 3–6.5 and 6.5–10, respectively. The CA–SCM analysis of the binary system revealed that the ≡SOCd+ and ≡SOCdOH forms increased with increasing illite content, but both were lower than the ≡(FeOH) 2Cd+ and ≡(FeOH)2CdOH forms, with ≡(FeOH)2 Cd+ remaining the dominant form. In the pH range of 6 to 10, the CA–SCM calculations revealed that the combined content of the ≡(FeOH)2 Cd+ and ≡(FeOH)2CdOH species in goethite ranged from 62% to 100%, which demonstrated quantitatively goethite contributed more to cadmium adsorption in the binary system from a microscopic perspective. Moreover, the study of the effect of Cd concentration and ionic strength showed that, due to the stronger activity of the ≡FeOH0.5– group compared to the ≡SO – group, the CdOH+ produced by Cd hydrolysis were more likely to bind to ≡FeOH0.5–, resulting in an increase in both Cd concentration and ionic strength that favors the formation of ≡(FeOH)2CdOH. The modeling data obtained in this study using CD–MUSIC, DLM, and CA–SCM methods were consistent with the macroscopic experimental results (r​>0.98), which could accurately simulate the distribution and contribution of Cd complexation patterns in the soil mineral fraction community, providing a theoretical basis for the prevention and control of heavy metal pollution in soils.