Effect of divalent heavy metal ions on the colloidal properties and microbial toxicity of MXene

2D Ti3C2Tx MXene has emerged as a promising material on the scientific stage owing to its advantageous physicochemical properties; however, its environmental behavior and toxicity have rarely been investigated. To elucidate the effect of divalent metal cations (namely Me2+ including Pb2+, Co2+ and Zn2+) on the environmental behavior and toxicity of Ti3C2Tx, this study investigated the adsorption behavior of Me2+ on Ti3C2Tx MXene; the destabilizing ability of Me2+ to Ti3C2Tx MXene; and the joint toxicity of Ti3C2Tx MXene and Me2+ on the survival of Escherichia coli (a Gram-negative bacterium) and Staphylococcus aureus (a Gram-positive bacterium). It was found that as a carrier, Ti3C2Tx reduced the concentration of free Me2+ to a certain extent. The aggregation behavior of Ti3C2Tx was changed by Pb2+, Co2+ and Zn2+, exhibiting critical coagulation concentration values of ~0.02 mM for Pb2+, ~0.3 mM for Zn2+ and ~0.5 mM for Co2+. Additionally, Co2+ accelerated the oxidation of Ti3C2Tx via special reactions. The joint toxicity of Ti3C2Tx and Me2+ to bacteria was significantly dependent on the interaction among Ti3C2Tx, Me2+ and bacteria. The toxicity of Me2+ in the presence of Ti3C2Tx against E. coli and S. aureus decreased in the order of Co2+ > Zn2+ > Pb2+. Finally, Density Functional Theory analyses fully revealed the essence of the binding between MXene and Co2+ from two aspects: the structural basis of multidentate coordination and the electronic essence of localized charge transfer. The results of this study provide important insights for assessing the fate and environmental impact of delaminated 2D Ti3C2Tx MXene in the aquatic environment.