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.

Comparative analysis of membrane fouling mechanisms induced by colloidal polymer: Effects of sodium and calcium ions.

Polymer (anionic polyacrylamide, APAM) flooding produced wastewater has a relatively high degree of mineralization and abundant ionic species. A comprehensive and systematic investigation of the influence of ion identity on APAM-induced membrane fouling is extremely necessary but has not been conducted to date. A comparative investigation was performed herein to reveal the underlying mechanisms of the influence of Na+ and Ca2+ (1000 mg/L) on APAM-induced membrane fouling in the adsorption and microfiltration (MF) processes. Na+ and Ca2+ exhibited contrasting influences on the filtration efficiency, cleaning efficiency, and fouling resistance. Compared to Na+, Ca2+ promoted reversible fouling and the formation of a loose cake layer; moreover, a higher removal rate and flux recovery were achieved. Additionally, simulations based on adsorption kinetic and membrane fouling models, and a series of microscopic analyses were performed to validate the contradictory influences. During the APAM-based MF process, the membrane fouling was effectively mitigated at the applied ionic strength because of the stronger hydration repulsive force generated by hydrated Ca2+ compared to that by Na+. This study provides vital guidance for membrane fouling control in the microfiltration of polymer flooding produced wastewater.

New mechanistic insights into the effect of cations on membrane fouling caused by anionic polyacrylamide.

HYPOTHESIS: Understanding the effect of cations on membrane fouling is crucial for the widespread application of the membrane technology. However, contradictory results have been reported based on different studies. Moreover, although the effect of the ionic strength has been studied extensively, limited information is available on the effect of the ion type on membrane fouling. EXPERIMENTS: The physicochemical properties of the membrane and anionic polyacrylamide (APAM) were evaluated to calculate the APAM-membrane and APAM-APAM interfacial interaction energies under different conditions. Moreover, a series of microfiltration (MF) experiments was conducted to investigate the effects of the ionic conditions on the flux decline, pore blockage and cake layer resistances, and the flux recovery rate of APAM during the MF process. FINDINGS: As the ionic strength increased, the rate of decrease in the normalized flux increased, the total and cake layer resistances increased significantly, the pore blockage resistance was affected slightly, and the recovery rates of the water flux after physical and chemical cleaning decreased gradually, which could be clearly explained using the Derjaguin-Landau-Verwey-Overbeek theory. Furthermore, compared with Na+, Ca2+ could effectively mitigate the membrane fouling at an identical ionic strength, which is attributed to the hydration forces of APAM-membrane and APAM-APAM.