  Using molecular dynamic simulations we show that single-layers of molybdenum
disulfide (MoS$_2$) and graphene can effectively reject ions and allow high
water permeability. Solutions of water and three cations with different valence
(Na$^+$, Zn$^{2+}$ and Fe$^{3+}$) were investigated in the presence of the two
types of membranes and the results indicate a high dependence of the ion
rejection on the cation charge. The associative characteristic of ferric
chloride leads to a high rate of ion rejection by both nanopores, while the
monovalent sodium chloride induces lower rejection rates. Particularly, MoS$_2$
shows 100\% of Fe$^{3+}$ rejection for all pore sizes and applied pressures. On
the other hand, the water permeation did not varies with the cation valence,
having dependence only with the nanopore geometric and chemical characteristic.
This study helps to understand the fluid transport through nanoporous membrane,
essential for the development of new technologies for pollutants removal from
water.
