Solidification/stabilization and immobilization mechanism of Pb(II) and Zn(II) in ettringite

Abstract

Ettringite, as the dominant hydration product in cement materials, plays a significant role in the solidification/stabilization of heavy metals. However, the combination and immobilization mechanism of heavy metal ions in ettringite crystals have yet to be fully understood. In this study, ettringite crystals were synthesized using the solution reaction method to investigate the adsorption and chemical encapsulation reactions of Pb2+ and Zn2+. In addition, the microstructures of the ettringite crystals doped with heavy metals were characterized by XRD, FTIR and SEM/EDS to elucidate the immobilization mechanism of Pb2+ and Zn2+ on ettringite crystals. The results indicated that ettringite has a high specific surface area of 64.456 m2/g and considerable mesopore and macropore volumers. The optimal adsorption efficiency of Pb2+ and Zn2+ in ettringite, accounting for 98.7 % and 99.9 %, respectively, was realized in an initial liquid with a pH of 10. The adsorption kinetics results showed that the equilibrium adsorption capacities of Pb2+ and Zn2+ were 196.04 and 185.57 mg/g, respectively. In addition, the adsorption reaction of Pb2+ in ettringite was predominantly characterized by a chemisorption process; whereas the adsorption reaction of Zn2+ in ettringite was characterized by the simultaneous action of physisorption and chemisorption. The multi-peaks Gaussian fitting curves of the FTIR patterns indicated that Pb2+ and Zn2+ promoted the formation of ettringite and induced more H-O-H groups. Moreover, this study proposed and confirmed that significant differences exist between the immobilization mechanisms of Pb2+ and Zn2+ in ettringite. The lattice constant and coordination chemistry analysis revealed that Pb2+ mainly substituted Ca2+ and Al3+ in ettringite via lattice substitution. In contrast, Zn2+ predominantly induced some lattice distortion in the ettringite crystal structure and formed a solid solution via interstitial doping. Furthermore, the micro channel architecture and hydrogen bond network significantly contributed to the adsorption and solidification of heavy metals. Overall, the chemical bonding mechanism was more prominent for the solidification of Pb2+ than that of Zn2+ in ettringite, and ettringite exhibited a superior adsorption capacity and stronger binding affinity for Pb2+.