Nanomaterial-Enhanced Phytoremediation of Centella asiatica (L.) Urban in Cd-Contaminated Soils

This study investigates the effectiveness of titanium dioxide nanoparticles (TiO₂ NPs) on the uptake, accumulation, translocation, and phytoremediation efficiency of Cd byCentella asiatica-(L.) Urban (C. asiatica). TiO₂ NPs at a concentration of 100 mg/kg were applied to soils contaminated with Cd at varying levels (10, 100 mg/kg). The Cd concentration in the roots ofC. asiaticawas increased by 4 times when grown in soils containing both TiO₂ NPs and Cd, compared with soil containing only Cd, indicating that TiO₂ NPs play an important role in facilitating Cd removal from the soil. TiO₂ aggregates were observed in the cortex cells and xylem regions in the roots. The adsorption of Cd onto TiO₂ aggregates was confirmed by scanning transmission electron microscopy (STEM)-energy-dispersive X-ray spectroscopy (EDS) analysis. Once translocated, TiO₂ aggregates were found to be distributed within the air space of mesophyll cells, and there is an internalization of TiO₂ aggregates within the cytoplasm ofC. asiaticaleaves. TiO₂ NPs were observed to be transformed into rodlike aggregates when they penetrated through barriers such as cell walls and cell membranes in the transportation process. In addition, it is found that negatively charged TiO₂ NPs were more efficiently translocated from roots to shoots ofC. asiaticathan positively charged ones. These findings suggested that the uptake, translocation, and distribution of TiO₂ NPs were closely related to the properties of TiO₂ NPs.