Drying Induces Different Effects on the Selective Adsorption of Cd(II) and Zn(II) on Ferrihydrite and Kaolinite.

Soil minerals are important natural adsorbents that regulate heavy metal fate; yet, the influence of natural drying on their adsorption behavior remains poorly understood. This study compared how drying affects Cd(II) and Zn(II) retention and selectivity on two widespread soil minerals, ferrihydrite (Fhy) and kaolinite (Kln), using single/binary adsorption experiments. Results indicated that drying significantly enhanced Cd(II) and Zn(II) retention on Fhy by promoting inner-sphere complexation and enhancing the stability of metal-mineral bonds, as evidenced by extended X-ray absorption fine structure (EXAFS) spectroscopy analyses. In contrast, the effect on Kln was less pronounced, with Cd(II) and Zn(II) predominantly adsorbed via electrostatic interactions and drying having a minimal impact. Crucially, drying induced contrasting selectivity changes: it enhanced the selective adsorption of Zn(II) on Kln while favoring Cd(II) selectivity on Fhy. Density functional theory (DFT) computations corroborated this finding, showing lower adsorption energies for Cd(II) than for Zn(II) on Fhy, with the reverse trend observed on Kln. This preferential adsorption of Cd(II) on Fhy and Zn(II) on Kln after drying was attributed to the drying process facilitating the formation of more stable bonds between metal ions and mineral surfaces. These findings establish a theoretical framework for predicting the fate of metals in dehydration-affected soils and contribute to a systematic understanding of the environmental functionality of soil minerals.