Determinations of critical limits in ion transport in membrane electrodialysis and their current vs. voltage characteristics for ion concentration control in pulp and paper mill waste stream

Membrane electrodialysis (MED) has been used in several processes that require electrical potential as a vehicle to accomplish separation of mixed ionic components.; MED can be used as a second stage treatment to effectively control ion concentrations in the process stream. This can be accomplished by understanding the distributions of ion concentration and/or electrical potential in the membrane-electrolyte system, which can be related to the concept of limiting current density that plays an important role in enhancing the separation efficiency.; Experimentally, the measurement of current vs. voltage characteristics of dilute electrolytic solutions (KCl, NaCl, and LiCl) were carried out in a laboratory scale MED unit. The observed experimental results show that there is a critical change in the apparent conductance for all these systems. These critical phenomena suggest that in the low electrical field the separation efficiency is limited by the rate of ion exchange within the membrane. The ion exchange limit inside the membrane is due to the fact that the capability of ion transport in the membrane is not infinite. There will be ion accumulation inside the membrane near the surface where the number of ions entering the membrane is over its capacity to transport ions. Both the order of the critical point and the magnitude of the apparent conductance at the critical point indicate that the ion exchange limit is related to ion size, which is consistent with the results from the theoretical parametric analysis. This work provides a new understanding of ion transport in ion exchange membrane and electrolyte solution during estimation of separation efficiency of the MED process, such as the optimum usage of ion exchange membrane, energy consumption, and process stream recycling condition. (Abstract shortened by UMI.)...