Shock electrodialysis (shock ED), an emerging nonlinear electrokinetic process for water treatment, has recently showed its capability to selectively remove multivalent cations from electrolyte mixtures. However, the mechanisms have not been understood yet. Based on the depth-averaged model developed in the first-part of the series paper for planer shock ED system, this work focuses on the mechanisms of selective ion removal. We first assume plug feed flow and do scaling analysis, and identify three possible mechanisms of selective removal of multivalent cations: smaller concentration left behind deionization zone, affinity to the charged surface where non-slip boundary slows down the velocity, and stronger push-back by streaming potential. Then we use numerical simulations to prove our hypothesis and show additional contribution from electroosmotic flow. Simulation results show that Mg is more removed from Na-Mg-Cl mixture at overlimiting current, which is qualitatively consistent with experiment results in literature. (c) 2020 Elsevier Inc. All rights reserved.