Role of nonspherical DLVO and capillary forces in the transport of 2D delaminated Ti3C2Tx MXene in saturated and unsaturated porous media

Abstract

The transport and retention of two-dimensional (2D) nanomaterials, such as graphene oxide, in porous media have attracted lots of attention. However, previous studies often simplified these 2D colloids as equivalent spheres for numerical simulations, which ignored the influence of particle shape on colloid retention at multiple interfaces. In this study, a novel 2D nanomaterial delaminated Ti3C2Tx (d-Ti3C2Tx) was adopted to fill this knowledge gap. Comprehensive analyses of the 2D colloid retention mechanisms were conducted based on colloid characterization, saturated and unsaturated column experiments, reactive transport modeling, 2D-based DLVO and nonspherical capillary energy simulations. Results show that d-Ti3C2Tx mobility in both saturated and unsaturated conditions enhanced with the increase in pH and decrease in ionic strength. The DLVO interaction energy of d-Ti3C2Tx at the sand-water-interface (SWI) decreased with the orientation angle of the colloidal major axis to the sand surface from 0° to 90°. The primary mechanism under saturated flow conditions was the irreversible attachment in the deep secondary minimum at the SWI with the major axis of d-Ti3C2Tx parallel to the sand surface. The attachment in the primary minimum at 0° was impossible due to the extremely high energy barrier, and the attachment in the primary and secondary minimum at other orientation angles were negligible. d-Ti3C2Tx only experienced repulsive electrostatic force when approaching the air-water-interface (AWI) no matter the particle orientation. The detaching capillary potential energy was 3 orders of magnitude larger than the attractive DLVO interaction energy of the SWI in the secondary minimum at 0°, suggesting that the capillary force-induced irreversible attachment at the AWI was the primary mechanism under unsaturated flow conditions. This study shows that the DLVO and capillary potential energies were significantly dependent on the particle-interface orientation and colloidal shape. A simplification of 2D colloids as spheres is not recommended.