Stability and transport of titanium dioxide nanoparticles in three variable-charge soils

Purpose The exposure pathways and environmental impacts of titanium dioxide nanoparticles (TiO2 NPs) released into soils could be significantly influenced by their stability and transport behaviors. The aim of this study was to investigate the stability and transport of TiO2 NPs in three variable-charge soils and to determine the key factors controlling these behaviors. Materials and methods Three surface (0~15 cm) variable-charge soils derived from quaternary red clay, humid ferralsols and stagnic anthrosols collected from Yingtan, Jiangxi Province (YT-H and YT-S, respectively), and humid ferralsols collected from Fuyang, Zhejiang Province (FY-H), were used in this study. Batch sedimentation experiments of TiO2 NPs in soil suspensions were performed for 10 h to quantify their stability. Transport of TiO2 NPs in soil columns was conducted with and without the presence of fulvic acid (FA). Results and discussion Apart from soil organic matter (SOM) and iron oxides, there was no significant difference between the tested soils. Batch experiments showed that TiO2 NPs were more stable in the YT-S soil suspension with high dissolved organic matter than in the YT-H and FY-H soil suspensions. In the column experiments, all TiO2 NPs were retained in YT-H and YT-S, while 11% of the TiO2 NPs were eluted from FY-H with a low amorphous iron oxide content. The significant retention of TiO2 NPs in the soils could be attributed to the straining and adsorption of TiO2 NPs on the surface of soil particles. FA enhanced the transport of TiO2 NPs in YT-H and FY-H by dispersing the TiO2 NPs and reducing their adsorption onto soil particles, while all the TiO2 NPs dispersed in the FA solution were still deposited in YT-S with a high amorphous iron oxide content. Conclusions The stability of TiO2 NPs in three variable-charge soil suspensions was dependent on the SOM. However, the mobility of TiO2 NPs in soils was not directly related to their stability in the soil suspensions. The difference in amorphous iron oxide content could induce the disparity in mobility of TiO2 NPs in soils.