The Collaborative International Dictionary of English v.0.48:

deionize \de*i"on*ize`\ v. t.
   1. (Chem.) to remove ions from (a solution).
      [WordNet 1.5]

   2. to reassociate the ions of (an ionized gas). --RHUD
      [PJC]

WordNet (r) 3.0 (2006):

deionize
    v 1: remove ions from