Stabilization and Mineralization Mechanism of Cd with Cu-Loaded Attapulgite Stabilizer Assisted with Microwave Irradiation.

Cadmium (Cd) in soil was stabilized using copper loaded attapulgite (Cu/ATP) in a microwave (MW) system. Excellent Cd stability in soil was achieved with Cu/ATP addition due to higher adsorption energy (1.38 eV) of Cu/ATP for Cd than that of ATP (∼1 eV), confirmed by density functional theory calculations. The strong hybridization of the s, p-orbitals of Cd with the s, p, d-orbitals of Cu on ATP contributed to the strong interactions between Cd and Cu/ATP. The stability performance of Cd in Cu/ATP-treated soil was further enhanced after MW irradiation through a series of phase transformation to more stable Cd-bearing crystalline minerals. The transformation was initiated by MW-induced "hot spots", which created cationic vacancy on Cu/ATP surface and enhanced the solid-state reactions between Cd and Cu/ATP framework. The total bond orders of Cd in the formed CdAl4O7 crystalline mineral elevated to 3.38, which was 5-fold higher than that for Cd on Cu/ATP, ensuring the long-term stability of Cd even after 360 curing days. Cd contaminated soil from a former industrial electroplating site was successfully stabilized with the proposed strategy. The research provides an effective stabilization strategy as well as a comprehensive understanding of the mechanism of long-term Cd stabilization.