Cement-Based Immobilization of Pb and Cd in Contaminated Soils: Enhancement via Novel Carbon Nanodots

Heavy metal contamination in soils poses serious threats to the environment and human health. Cement-based solidification/stabilization (S/S) is a common method for treating heavy metal-contaminated soils due to its simplicity and effectiveness. However, the immobilization efficiency of conventional cement systems is limited. In this study, low-cost and highly dispersible novel carbon nanodots (CNDs) are synthesized from citric acid and urea by a microwave-assisted method and added to cement systems to improve the immobilization of Pb and Cd. Leaching results show that, with 15 wt.% cement, the addition of 0.4 wt.% (by weight of cement) CNDs reduces the leaching concentrations of Pb and Cd by 77.3% and 71.8%, respectively. Leaching results and Tessier analyses show that CNDs promote the transformation of Pb and Cd from exchangeable to stable forms and improve chemical stability. XRD, FTIR, DTG, and SEM results show that CNDs accelerate cement hydration and promote the formation of C–S–H gels, which enhances physical encapsulation. FTIR, UV-Vis, and adsorption results further indicate that oxygen- and nitrogen-containing groups on CNDs complex with Pb 2+ and Cd 2+ and strengthen chemical immobilization. This study provides a cost-effective nanomaterial for improving the immobilization of Pb and Cd in cement-based systems and offers new directions for the application of CNDs in sustainable environmental materials.