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

This study investigates the effectiveness of titanium dioxide nanoparticles (TiO₂NPs) on the uptake, accumulation, translocation, and phytoremediation efficiency of Cd byCentella asiatica(L.) Urban (C. asiatica). TiO₂NPs 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 TiO₂NPs and Cd, compared with soil containing only Cd, indicating that TiO₂NPs play an important role in facilitating Cd removal from the soil. TiO₂aggregates were observed in the cortex cells and xylem regions in the roots. The adsorption of Cd onto TiO₂aggregates was confirmed by scanning transmission electron microscopy (STEM)-energy-dispersive X-ray spectroscopy (EDS) analysis. Once translocated, TiO₂aggregates were found to be distributed within the air space of mesophyll cells, and there is an internalization of TiO₂aggregates within the cytoplasm ofC. asiaticaleaves. TiO₂NPs 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 TiO₂NPs were more efficiently translocated from roots to shoots ofC. asiaticathan positively charged ones. These findings suggested that the uptake, translocation, and distribution of TiO₂NPs were closely related to the properties of TiO₂NPs.