A diffusion dialysis system for decontaminating acids containing metal ions is disclosed. The system includes a plurality of vertically stacked, horizontally disposed anion exchange cells. Each cell includes an anion exchange membrane adapted to permit the diffusion of anions and to block the diffusion of cations therethrough. Each cell of the stack includes channels for supplying contaminated acid to one side of the membrane and water to the opposite side of the membrane and at the opposite ends thereof to provide for counterflow of acid and water across the upper and lower surfaces of each membrane. Means are provided to draw water and acid through the vertical stack of horizontally disposed exchange cells to provide a uniform flow rate of acid and water through the system.