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)Tx 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)Tx 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)Tx followed an SH > MT > NS sequence. Increasing SH concentration led to Ti(3)C(2)Tx 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)Tx 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)Tx 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)Tx via their carboxyl and hydroxyl groups, respectively, rendering the Ti(3)C(2)Tx surface more reactive and faster degradation. NS showed a weak promotion effect because of its less affinity with Ti(3)C(2)Tx and limited organic matter and clay contents with hydroxyl and carboxyl groups. This study demonstrated the unstable environmental behaviors of Ti(3)C(2)Tx in the groundwater and mitigated its environmental risk concerns.