Diffusion dialysis (DD) is an ion-exchange membrane (IEM) separation process driven by concentration gradient and has been applied for separation and recovery of acid/alkali waste solutions in a cost-effective and environmentally friendly manner.

Diffusion dialysis with ion-exchange membranes is mainly used today to recover acids or bases from a mixture with salts. The process is based on the fact that ion-exchange membranes generally show a high permeability for counterions while being more or less impermeable for co-ions. Exceptions are protons and hydroxide ions which can easily permeate both cation- and anion-exchange membranes. The transport mechanism in diffusion dialysis is more complex than in conventional dialysis due to the electrostatic interaction between positive and negative charges and the electroneutrality requirement。