Efficient cadmium immobilization by organic loaded Na-montmorillonite in a contaminated soil

Clay minerals are effective sorbents for toxic metal immobilization in contaminated soils and waters. However, their Cd immobilization efficiency is unclear when they are loaded with organics. In this study, sodium montmorillonite (Na-M) was successfully loaded with potassium humate, chitosan, and glycine to adsorb Cd(II) in solution. Potassium humate loaded Na-M (Na-M-HA), which had the highest specific surface area and cation exchange capacity (CEC), showed the highest Cd(II) adsorption capacity (73.7 mg g^-1), 22.5 % and 81.8 % higher than that of chitosan loaded Na-M (Na-M-CTS) and glycine loaded Na-M (Na-M-G), respectively. The pseudo-second-order kinetic model best described (R² > 0.98) the adsorption kinetics of Cd(II) on the three Na-Ms, indicating that the adsorption processes were of chemisorption nature. The adsorption isotherm of Cd(II) on Na-M-HA was of the Freundlich type, suggesting multilayer adsorption. In contrast, the isothermal adsorption of Cd(II) on Na-M-CTS (R² = 0.99) and Na-M-G (R² = 0.89) was better described by the Langmuir model, suggesting the dominance of monolayer adsorption in the adsorption process. High temperature, high pH, low background ionic strength, and low valence competing cations favored Cd(II) adsorption on Na-M-HA. The underlying mechanisms of Cd(II) sorption on Na-M-HA were electrostatic attraction, ion exchange and complexation. Na-M-HA was applied to a Cd polluted soil planted with lettuce (Lactuca sativa L.). in a pot experiment. Compared to the control with no adsorbent application, Na-M-HA application at 2 % effectively reduced the available Cd content in soil and Cd accumulation in plant by 36.0 % and 56.8 %, respectively. This work demonstrated that Na-M-HA is a green, low-cost and excellent adsorbent for Cd stabilization, and that its application in Cd-polluted soils can efficiently reduce Cd bioavailability and thereby Cd transfer along the food chain and eventually reduce the threat of Cd pollution to human health.