Corrective term for convective transport

Hi,

i am modelling ion transport through a nanofiltration membrane with the Nernst-Planck equation. I am assuming that the membrane consists of small pores, so the ions are hindered while moving through the membrane. Because of this fact, I need to consider a corrective term for diffusive and convective transport for different ion species. Problem is, when using physics such as Nernst-Planck, the velocity is set for all species, so I cant consider a corrective term for different species. Is there a way to set different velocities for different species?

Thanks in advance!