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

Abstract

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 (Ti3C2Tx 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 Ti3C2Tx 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 Ti3C2Tx followed an SH > MT > NS sequence. Increasing SH concentration led to Ti3C2Tx 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 Ti3C2Tx 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 Ti3C2Tx 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 Ti3C2Tx via their carboxyl and hydroxyl groups, respectively, rendering the Ti3C2Tx surface more reactive and faster degradation. NS showed a weak promotion effect because of its less affinity with Ti3C2Tx and limited organic matter and clay contents with hydroxyl and carboxyl groups. This study demonstrated the unstable environmental behaviors of Ti3C2Tx in the groundwater and mitigated its environmental risk concerns.