Modelling the transport of ions and electrochemical regeneration of the resin in a hybrid ion exchange/electrodialysis process for As(V) removal

Abstract

This paper presents the 2D modeling of a laboratory scale ion exchange/electrodialysis (IXED) flow cell for removal of As(V) ions from water. The cell consists of a central compartment (DS) delimited by two anion membranes and packed with anion exchange resin, one compartment on each side of the central compartment (CC and AC compartment) lined with a cation exchange membrane and a rinse compartment at each end of the cell. The developed model comprises: the anion exchange in the resin bed in a process controlled by the mass transport rate; the ion transport in the solutions of resin-free compartments, in the membranes and resin, based on Nernst–Planck equation; and enhanced water dissociation at the anion membrane/solution interface. The obtained results show the potential profiles, Donnan potential, concentration polarization and the contribution of each mechanism (diffusion and migration) to ion transport rate as well as the effect of the potential difference on water dissociation rate along the membrane surface. The results show that, at typically low arsenic concentrations in arsenic removal processes, water dissociation plays a key role in ion exchange resin regeneration, process whose intensity grows as the cell potential rises. Moreover, the non-homogeneous distribution of current produces uneven resin regeneration that depends on design and operating parameters. The ion exchange/electrodialysis model is applied to the IXED cell operating in recirculation mode by using individual tanks connected to each cell compartment to describe the experimental batch behavior where arsenic concentration varied from (initial) 13.3 ppm to (final concentration) less than 0.01, 8.9 and 21.3 ppm in DS, CC and AC compartments, respectively, at an operating current density of 8.4 A m−2 removing 99.9% of arsenic in DS compartment with 18.9% of total current efficiency. The model results of As(V) concentration decline in the solution flowing over the ion exchange bed agree very closely with experimental data; however, the As(V) concentration results in the resin-free compartments show deviations from experimental data during the process with 6 and 16% deviations at the end of the batch. These deviations are attributed to model assumptions, in particular to the effect of diverse, non-considered, As(V) ion species.