Mechanistic insights into the migration behavior of cadmium (Cd) in the soil and groundwater systems at a construction site: Experimental and numerical analysis

The environmental fates of cadmium (Cd) in the soil and groundwater systems largely depend on its adsorption isotherms and kinetic with the soil matrix. Combined with HYDRUS-1D software, batch and column studies were conducted in this study to investigate the adsorption and movement process of Cd in soil and groundwater systems. The results from static adsorption experiment indicated that the second order model and Langmuir model could provide the better description for adsorption behavior of Cd. The results from SEM -EDS and XPS analysis revealed that the main adsorption mechanism of Cd were ion exchange, complexation and precipitation. The results from Tessier's sequential extraction suggested that more Cd would be diffused to the deeper soils, since the higher percentage of Cd were found in the exchangeable fractions. The results from soil column experiments and numerical analyses demonstrated that standard convection-dispersion equation (CDE) could better describe Cd breakthrough curve (BTCs) of column adsorption than the other three solute transport models within HYDRUS-1D software. Noteworthy, the horizontal and vertical migration distance of Cd (II) reached about 8 m after 50,000 days and 10 m after 15,000 days, respectively, indicating the easily mobile Cd (II) might be eventually released from contaminated soils to groundwater. Based on the above results, this study not only provided a new insight into the migration mechanism of Cd in the soil and groundwater environment, but was also beneficial to formulating remediation strategies of contaminated sites.