Effects of calcium ions on anionic polyacrylamide fouling of ion-exchange membranes in desalination of polymer-flooding wastewater by electrodialysis

The effects of calcium ions (Ca²⁺) on anionic polyacrylamide (APAM) fouling of ion-exchange membranes (IEMs) in the treatment of polymer-flooding wastewater (PFW) from oil production industry by electrodialysis were systematically investigated by associating the desalination performance, physicochemical and electrochemical characteristics with the electrostatic interaction and interfacial free energy. Bench-scale experiments were performed under constant current conditions to investigate the fouling mechanisms with different molecular weights (MWs). It was found that APAM fouling mainly occurred on the anion-exchange membranes (AEMs) due to electrostatic attraction, which significantly deteriorated the membrane desalination performance. The membrane fouling of APAM with a MW of 6 million Dalton (600APAM) was more serious than that of 0.5 million Dalton (50APAM), and APAM fouling on homogeneous membranes was more serious than that on heterogeneous membranes. Ca²⁺ would connect with the carboxylate ions of APAM by coordination. The electrostatic attraction between APAM molecules and AEM weakened due to increasing zeta potential after CaCl₂ addition. And as a result, the APAM+Ca fouling formed intricate and compact aggregates sparsely distributed on the surface of AEMs compared to the APAM fouling layer. Both electrostatic interactions and interfacial free energy affected APAM fouling of IEMs, but electrostatic interactions dominated the fouling process.