  We report a molecular dynamics (MD) simulation study of reverse osmosis
desalination using nanoporous monolayer graphene passivated by SiH$_2$ and
Si(OH)$_2$ functional groups. A highly accurate and detailed polarizable
molecular mechanics force field model was developed for simulating graphene
nanopores of various sizes and geometries. The simulated water fluxes and ion
rejection percentages are explained using detailed atomistic mechanisms derived
from analysis of the simulation trajectories. Our main findings are: (1) The
Si(OH)$_2$ pores possess superior ion rejection rates due to selective
electrostatic repulsion of Cl$^-$ ions, but Na$^+$ ions are attracted to the
pore and block water transfer. (2) By contrast, the SiH$_2$ pores operate via a
steric mechanism that excludes ions based on the size and flexibility of their
hydration layers. (3) In absence of ions, water flux is directly proportional
to the solvent accessible area within the pore; however, simulated fluxes are
lower than those inferred from recent experimental work. We also provide some
hypotheses that could resolve the differences between simulation and
experiment.
