Concentration effects of calcium ion on polyacrylamide fouling of ion-exchange membrane in electrodialysis treatment of flue gas desulfurization wastewater

Residual anion polyacrylamide (APAM) and fluctuating calcium ion (Ca²⁺) in flue gas desulfurization wastewater after pretreatment could result in ion-exchange membrane fouling during electrodialysis process. Concentration effects of Ca²⁺ on APAM fouling were systematically investigated and the mechanisms were illustrated. Results of desalination performance indicated that Ca²⁺ alleviated APAM fouling and the mitigating effect increased firstly and then declined, obtaining the mildest fouling phenomenon in the presence of 1.0 mM Ca²⁺. Zeta potential and particle radius of APAM in the feed solutions increased as Ca²⁺ changed the charge balance of APAM molecules and complexed with them. Morphology and chemical structure analysis concluded that APAM molecules were adsorbed on the membrane surface and the fouled layer changed with increasing Ca²⁺ concentration, from dense layer to reticular structure and then to sophisticated multilayer structure. Calculation of interfacial free energy based on Extended Derjaguin-Landau-Verwey-Overbeek theory confirmed that electrostatic interaction dominated the membrane fouling process. Concentration effects of Ca²⁺ were more remarkable on membrane-APAM attraction and APAM-APAM repulsion at 0.0–1.0 and 1.0–10.0 mM, respectively. The results contribute to establishing comprehensive understanding of the concentration effects of Ca²⁺ on polymer fouling of membranes and provide new insights into cleaning process optimization.