Effects of soil colloids on the aggregation and degradation of engineered nanoparticles (Ti3C2Tx MXene)

Soil colloid is a nonnegligible factor when evaluating the environmental risk of engineered nanoparticles (ENPs) in the groundwater. In this study, the environmental fate of an emerging ENP (Ti 3 C 2 T x MXene) in the groundwater was investigated for the first time, which currently poses a severe environmental risk due to its cytotoxicity but has received little attention. The colloidal dispersion stability and degradation kinetics of Ti 3 C 2 T x MXene in the groundwater were evaluated by considering the effects of soil colloids prepared from sodium humate (SH), montmorillonite (MT), and a natural soil (NS) under variable solution chemistry. The results showed that the affinity of soil colloids with Ti 3 C 2 T x followed an SH > MT > NS sequence. Increasing SH concentration led to Ti 3 C 2 T x disaggregation by enhancing the electrical and steric repulsive forces, while MT and NS resulted in hetero-aggregation because of the elevated collision frequency. SH and MT enhanced the critical coagulation concentrations of Ti 3 C 2 T x by 100 and 10 folders, respectively, via surface coating process, while NS slightly reduced due to the bridging effects induced by the soluble cations. The soil colloids promoted Ti 3 C 2 T x degradation compared with their absence and in an SH > MT ≫ NS sequence. SH and MT were through forming Ti–O–C and Si–O–Ti bonds with Ti 3 C 2 T x via their carboxyl and hydroxyl groups, respectively, rendering the Ti 3 C 2 T x surface more reactive and faster degradation. NS showed a weak promotion effect because of its less affinity with Ti 3 C 2 T x and limited organic matter and clay contents with hydroxyl and carboxyl groups . This study demonstrated the unstable environmental behaviors of Ti 3 C 2 T x in the groundwater and mitigated its environmental risk concerns.