Evolution of anion-exchange membrane properties in a full scale electrodialysis stack

In this paper, the physicochemical, structural and mechanical properties of an anion-exchange membrane were investigated, during its lifetime in industrial electrodialysis for whey demineralization. Several analytical techniques permitted to disclose the membrane structure and to describe the evolution with time of the membrane properties. The homogeneous anion-exchange membrane, AMX-SB, is constituted of a semi-interpenetrating polymer network of poly(vinyl chloride) and functional poly(styrene-co-divinylbenzene). No significant loss of the ion-exchange capacity or degradation of the functional poly(styrene-co-divinylbenzene) chains were detected. However, fouling caused a decrease in the counter-ion mobility within the membrane, which produced a reduction of the electrical conductivity. A progressive loss of poly(vinyl chloride), which was degraded or washed out from the membrane during the cleaning-in-place process, was clearly evidenced. This led to the formation of non-charged pores available for electroneutral electrolyte solution and large molecules, such as lactose and proteins. The occurrence of such pores resulted in an increase in electrolyte permeability through the membrane and a rebound of the conductivity at the last stage of electrodialysis. The microheterogeneous model was applied to well account for these changes. By means of tensile strength tests, it was possible to investigate the mechanical properties of the membrane, which turned gradually from a rigid and tough material, to a rigid and brittle one, due to the loss of poly(vinyl chloride), leading to the end of the membrane lifetime.