Mechanisms of chemical cleaning of ion exchange membranes: A case study of plant-scale electrodialysis for oily wastewater treatment

Abstract Chemical cleaning of ion exchange membranes (IEMs) in plant-scale electrodialysis (ED) for treating oily wastewater was investigated. Bench-scale ED experiments were conducted to elucidate the effects of chemical cleaning on the desalination performance of the IEMs and results showed that HCl was a more efficient cleaning agent for both cation exchange membrane (CEM) and anion exchange membrane (AEM) than NaOH. By analyzing the composition of the cleaning solutions, we suggested that NaOH could effectively remove anionic polyacrylamide (APAM) fouling on CEMs while HCl was better in removing APAM on AEMs, which can be reasonably explained by the deprotonation/protonation effects of NaOH/HCl on the electrostatic interactions between APAM molecules and IEMs. Results also demonstrated that NaOH was more effective in removing oil on IEMs; inorganic precipitations could be eliminated by HCl cleaning through double decomposition and neutralization reactions. Moreover, morphologies analysis of the cleaned IEMs along with the fouled ones were conducted, and results confirmed that the inorganic scales were mostly removed by HCl cleaning. Furthermore, X-ray diffraction (XRD) analysis of the scales revealed that the inorganic fouling on the IEMs was mainly caused by the deposition of calcite.