Ryosuke Takagi

Membrane potential of separation membranes as affected by ion adsorption

Abstract Recently it has been reported that membrane charge density depends on the bulk concentration. In this paper, we investigate the effect of ion adsorption on membrane charge and on membrane potential by using the selective ion adsorption model, which considers that a membrane which has no fixed charge can adsorb anions selectively. We find that the membrane potential depends on the bulk concentration as if the membrane itself had a fixed charge. However, the magnitude of the membrane potential in the region of low bulk concentration is determined by the adsorption coefficient and the saturated ion concentration of adsorbed ions. These parameters can be determined by analysis of the membrane potential using our model. After obtaining values of parameters which characterize the membrane, the variation of the membrane charge density and the interfacial potential difference caused by ion adsorption can be predicted theoretically.

Ionic dialysis through amphoteric membranes

A membrane charge is one of the most important factors that affects the permeation of ions through the membrane. For amphoteric membranes such as inorganic membranes, the membrane charge depends on the pH and the electrolytes in the bulk solution. In this paper, we theoretically discuss the permeation of ions through the amphoteric membrane using the advanced amphoteric membrane model. In this model, the membrane charge is determined by the dissociation of the amphoteric membrane material as well as the selective adsorption of ions. We assumed that the system was in the steady-state and there was no volume flow. We also assumed that no ions were adsorbed selectively in order to clearly show the effect of the dissociation of membrane material. As a result of the discussion, it was theoretically predicted that the flux through the amphoteric membrane was maximized with the pH. It is, therefore, possible to select the best condition for a large flux by adjusting the pH of the bulk solution.

Characterization of the membrane charge of Al2O3 membranes

In order to characterize the membrane charge of the alumina membrane, we saw about the information on the membrane charge at first. Then, we pointed out that the effective membrane charge of alumina consists of two parts. One part is the membrane charge due to the dissociation of the ionizable groups of membrane, and the other part is the membrane charge affected by the electrolytes in the bulk solution. Then, we proposed the new membrane model that is the combination of the selective ion adsorption model and the amphoteric membrane model. In this model, the effective membrane charge is determined by the dissociation of the ionizable groups of the membrane and the selective adsorption of ions. We theoretically discussed the effect of the electrolyte concentration, the ionic species and the pH on the effective membrane charge. Then, we showed that this new model, the advanced amphoteric membrane model, can qualitatively explain the phenomena related to the membrane charge. We applied the advanced amphoteric membrane model to the membrane potential of alumina membrane and concluded that the effective membrane charge of the alumina membrane was determined by the dissociation of the ionizable groups and the adsorption of anions.

Characterization of inorganic membranes as amphoteric membranes

It has been reported that the membrane charge of amphoteric membranes such as inorganic membranes depends on the pH and the electrolyte in the bulk solution. To explain these phenomena, we have proposed the advanced amphoteric membrane (AAM) model where the effective membrane charge is determined by the dissociation of the amphoteric membrane material as well as by the selective adsorption of ions. We applied the AAM model to the alumina membranes and microporous glass membranes to characterize their membrane charges. It was concluded that the effective membrane charge of these membranes was determined by the dissociation of membrane material besides the selective adsorption of ions. It was also shown that the dependency of the effective membrane charge on the pH and the electrolyte was well explained by the AAM model.

Removal performance of NO3− ion from groundwater by electrodialysis

An excess amount of nitrate in drinking water is harmful to body health. A batch type of electrodialysis (ED) which is consist of cation and anion exchange membrane has been used to separate a model of nitrate ion from groundwater by using ten pairs of cation/anion exchange membrane. The objective of this research is to investigate the effect of initial concentration feed solution on the separation performance of nitrate ion by electrodialysis. Also, the effect of applied current in ED on the nitrate permeability is also studied. Experimental results indicated that the decrease of nitrate concentration in the dilute compartment is significantly influenced by the applied current. The amount of nitrate passing through the membrane increases linearly by increasing amount of current. The initial feed concentrations do not influence on the removal rate of nitrate as a function of membrane surface area.