The introduction of modifying molecules can effectively regulate the surface characteristic of two-dimensional (2D) membranes, thus optimizing the water permeation and ion rejection performance. In this work, the MoS2-based 2D membrane was prepared and modified by molecules with different structures and sizes─poly(vinyl alcohol) (PVA, polymer, contains abundant −OH groups), poly(acrylic acid) (PAA, polymer, contains abundant −COOH groups), tannic acid (TA, macromolecule, contains abundant phenolic −OH groups), and gallic acid (GA, micromolecule, contains phenolic −OH and −COOH groups)─to improve the surface hydrophilicity and regulate the interlayer structure. In addition, the modifying molecules polyethylenimine (PEI, polymer, contains abundant −NH2 and −NH– groups) and cetyltrimethylammonium bromide CTAB, chain, features a positively charged end −(CH3)3N+ were used to further investigate the influence of positive charge on the ion-rejection capability of the membrane. When the dosage of GA is 20 wt %, the GA–MoS2 membrane enables a water flux of roughly 3870 L/m2·h·bar, which is approximately 9.68 times higher than that of the MoS2 membrane with the same thickness reported in the previous research. The MoS2 membranes can achieve a rejection rate of over 95% for typical metal cations, and molecules with different structures and dosages exert distinct impacts on the membrane structure. Meanwhile, the permeation processes of monovalent and divalent ions tend to be governed by the balance between the ionic hydration size, hydration energy barrier, and electrostatic interaction.
Zhao et al. (Mon,) studied this question.