Discrepant effects of monovalent cations on membrane fouling induced by colloidal polymer: Evaluation and mechanism investigation

Understanding how ionic conditions affect membrane fouling induced by anionic polyacrylamide (APAM) is important for achieving long-term and stable operation of a polymer flooding produced wastewater (PFPW) membrane separation process. However, there is lack of studies on the effects of monovalent cations (Na+ and K+) on APAM-based membrane fouling. In this work, the effects of Na+ and K+ on filtration efficiency, flux decline behavior, fouling resistance, and cleaning efficiency were studied through a series of microfiltration tests. Moreover, the influencing mechanism of membrane fouling was further comprehensively revealed from the aspects of the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, the hydration force, and the microstructure characterizations. The XDLVO theory agreed well with membrane fouling behavior at various ionic strengths. The increase in ionic strength (0–10,000 mg/L) of Na+ and K+ exacerbated the reduction of relative flux (J/J0) and the accumulation of fouling resistance, as well as made the porous APAM-induced fouling layer denser and more compact, boosting removal efficiency. Furthermore, K+ had a stronger aggravating effect on membrane fouling than Na+. Specifically, the final value of J/J0 for APAM+K+ (0.08) was lower than that for APAM + Na+ (0.12), and the fouling resistance for APAM+K+ (12.25 × 1011 m−1) was higher than that for APAM + Na+ (12.01 × 1011 m−1) at an ionic strength of 10,000 mg/L, which was owing to the larger hydration force caused by Na+ with a smaller ionic radius. This research offers practical guidance for the PFPW membrane filtering process.