  Ion partitioning between different compartments (\emph{e.g.} a porous
material and a bulk solution reservoir), known as Donnan equilibrium, plays a
fundamental role in various contexts such as energy, environment, or water
treatment. The linearized Poisson-Boltzmann (PB) equation, capturing the
thermal motion of the ions with mean-field electrostatic interactions, is
practically useful to understand and predict ion partitioning, despite its
limited applicability to conditions of low salt concentrations and surface
charge densities. Here, we investigate the Donnan equilibrium of coarse-grained
dilute electrolytes confined in charged slit-pores in equilibrium with a
reservoir of ions and solvent. We introduce and use an extension to confined
systems of a recently developed hybrid nonequilibrium molecular dynamics /
grand canonical Monte Carlo simulation method ("H4D"), which enhances the
efficiency of solvent and ion-pair exchange via a fourth spatial dimension. We
show that the validity range of linearized PB theory to predict the Donnan
equilibrium of dilute electrolytes can be extended to highly charged pores, by
simply considering \textit{renormalized} surface charge densities. We compare
with simulations of implicit solvent models of electrolytes and show that in
the low salt concentrations and thin electric double layer limit considered
here, an explicit solvent has a limited effect on the Donnan equilibrium and
that the main limitations of the analytical predictions are not due to the
breakdown of the mean-field description, but rather to the charge
renormalization approximation, because it only focuses on the behavior far from
the surfaces.
