Competitive adsorption of lead and cadmium on soil aggregate at micro-interfaces: Multi-surface modeling and spectroscopic studies

Aggregates are the basic structural units of soils and play a crucial role in metal migration and transformation. Combined contamination of lead (Pb) and cadmium (Cd) is common in site soils, and the two metals may compete for the same adsorption sites and affect their environmental behavior. Herein, the adsorption behavior of Pb and Cd on aggregates of two soils and contributions of soil components in single and competitive systems were studied by combining cultivation experiments, batch adsorption, multi-surface models (MSMs), and spectroscopic techniques. The results demonstrated that < 2 mu m size aggregate was the dominant sink for Pb and Cd competitive adsorption in both soils. Compared with Pb, the adsorption capacity and behavior of Cd were affected greatly under competition. MSMs prediction revealed that soil organic matter (SOM) contributed the most to Cd and Pb adsorption on aggregates (> 68.4%), but the dominant competitive effect occurred on different sites for Cd adsorption (primarily on SOM) and Pb adsorption (primarily on clay minerals). Further, 2 mM Pb coexistence caused 5.9 - 9.8% of soil Cd conversion to unstable species (Cd(OH)2). Thus, the competitive effect of Pb on Cd adsorption cannot be ignored in soils with high content of SOM and fine aggregates.

Automated summary

Aggregates are important structural units of soils and have a significant impact on metal migration and transformation. This study investigated the adsorption behavior of lead (Pb) and cadmium (Cd) on soil aggregates in both single and competitive systems. The results showed that aggregates with a size of < 2 μm were the primary adsorption sites for Pb and Cd competition in both soils. Further analysis revealed that soil organic matter (SOM) contributed the most to the adsorption of Cd and Pb on aggregates, but the competitive effect occurred on different sites for Cd and Pb adsorption. The presence of Pb also caused the conversion of a portion of soil Cd to unstable species. Therefore, the competitive effect of Pb on Cd adsorption cannot be ignored in soils with high SOM and fine aggregates.