Separation and Purification Technology, Vol.210, 636-650, 2019
Characterization and cleaning of anion-exchange membranes used in electrodialysis of polyphenol-containing food industry solutions; comparison with cation-exchange membranes
This paper concerns the mechanisms of aging of ion-exchange membranes (IEMs) during their use in electrodialysis (ED) of food industry solutions containing polyphenols (PPs), as well as their cleaning. The study focuses on anion-exchange membranes (AEMs); their behavior is compared with that of cation-exchange membranes (CEMs). First, physicochemical static characteristics, structural, morphological and tensile strength parameters are determined for new AEMs and two batches of used AEMs at different duration of their use in industry, subjected to regular "Cleaning In Place". Second, non-aggressive and economic ex-situ static cleaning methods involving the application of NaCl at 35 g L-1, a reconstituted seawater and a water-ethanol mixture acidified with H2SO4 were examined. During the cleaning process, the evolution of physicochemical parameters, such as ion-exchange capacity (IEC), electrical conductivity (xm) and contact angle (8), were followed. It is shown that the application of NaCl solution has a negligible effect on IEC and Km; when soaking the membranes in the reconstituted seawater, xm even slightly decreases; however, there is a significant increase in these parameters when soaking the membranes in the acidified water-ethanol solution. As for the mechanism of fouling, PPs are the main responsible constituents. Apparently, they form dense colloidal nanoparticles not permeable for ions within membrane meso- and macropores, not penetrating into micropores. A modification of the micro heterogeneous model under this assumption allows an adequate description of membrane conductivity and explains the fact that the membrane pore size increases with the duration of membrane utilization, while the apparent volume fraction of the inter-gel solution (f(2app)) decreases. CEMs are found less prone to fouling. The soaking of CEMs in the water-ethanol solution leads to an increase in IEC and f(2app) by 33% and 60%, respectively, as well as to doubling xm and decreasing 8 by 23%, after a 120 h. treatment.