Apparatus for purifying a fluid, which comprises an ion absorption cell (2) traversed by a fluid to be treated (F1) containing cationic particles and/or anionic particles, provided with a containment structure (3) defining at its interior two operating chambers (4, 5) traversed by the fluid to be treated (F1), and each delimited by a respective electrode (10, 11) that can be power supplied to opposite polarities. Between the two operating chambers (4, 5), a third evacuation chamber (13) is interposed, containing a porous electrical conductor (18) traversed by a washing fluid (F2) and delimited by two opposite walls (16, 17) which together with the electrodes (10, 11) define the aforesaid operating chambers (4, 5). The aforesaid walls (16, 17) have at least one ion-selective area associated, which is susceptible to being traversed by charged particles with corresponding polarity contained in the two contiguous operating chambers (4, 5) subjected to the action of the electric field generated by the electrodes. The porous electrical conductor (18) is in turn susceptible to reduce the electric field inside the third evacuation chamber (13) such that the ion-selective areas of the walls of the third evacuation chamber (13) are capable of repelling the charged particles with corresponding polarity contained in the third evacuation chamber (13) and subjected to the action of the electric field reduced due to the shielding effect of the porous electrical conductor (18).