Polyelectrolyte-grafted Ti3C2-MXenes stable in extreme salinity aquatic conditions for remediation of contaminated subsurface environments

Abstract

MXenes, an emerging class of two-dimensional materials, are recently gaining significant attention for numerous environmental applications owing to their superior hydrophilicity and unique surface functionalities, which are suitable for adsorptive removal of various aqueous contaminants. However, it has recently been shown that MXenes have poor colloidal stability in both synthetic or natural waters containing small amounts of salt ions, which will limit the potential uses of MXenes in remediation of subsurface environments that might sometimes contain considerable amounts of salt ions, and other relevant environmental applications. Herein, we develop Ti₃C₂-MXenes grafted with highly salt-resistant polyelectrolytes (PEs), MXene-g-PEs, which are colloidally stable in extreme salinity aquatic environments and have low adsorption to soil mineral substrates. The MXenes grafted with zwitterionic PEs are found to have superior mobility properties to those with anionic PEs, which are attributed to the anti-PE behavior of the grafted polymer brushes. The MXene-g-(zwitterionic) PEs show long-term colloidal stability over 6 months in American Petroleum Institute (API) brine with extreme salinity (ionic strength of 2 M with 182.2 mM Ca²⁺), and little adsorption (0.5 mg m⁻²) against α-alumina surfaces (2.3 m² g⁻¹). Furthermore, the MXene-g-PEs retained the excellent adsorption capacity for methylene blue as a model aqueous organic pollutant. The results suggest the great potential of the MXene-g-PEs as an aqueous pollutant scavenger for various environmental applications including the combined ex situ/in situ remediation, and other relevant subsurface applications.