In this study, we demonstrated the influence of adding nanofiltration membranes to a CDI cell, when desalinating mono- and divalent salt solutions both theoretically and experimentally, and proposed a new procedure for the separation of mono- and divalent ions. It is revealed that the distinction of the diffusion coefficient between mono- and divalent ions is the key to the effective separation. Combined with the appropriate time of adsorption (1.5 h), the concentration ratio of NaCl to MgSO4 can be increased by a factor of 3, from a mixture with equal amount of NaCl and MgSO4. (C) 2019 Elsevier Inc. All rights reserved.