Microfiltration pretreatment of polymer-flooding produced wastewater before desalination: Role of Ca2+ and Mg2+ in membrane fouling

To better control membrane fouling during microfiltration pretreatment of polymer-flooding produced wastewater (PFPW) before desalination, an accurate elaboration of the fouling mechanisms of anionic polyacrylamide (APAM) in high-salinity environments is vital. Herein, the effects of divalent cations (Ca2+ and Mg2+) on APAMinduced membrane fouling were comprehensively investigated. Both Ca2+ and Mg2+ aggravated membrane fouling. The pseudo-stable flux decreased from 0.38/0.38 to 0.35/0.32, 0.28/0.25, 0.23/0.20, and 0.19/0.15 as the ionic strength of Ca2+/Mg2+ increased from 0 to 1000, 2000, 5000, and 10,000 mg/L. Results of fouling resistances, morphology images of fouled membranes, and extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory supported this. In comparison with hydrated Mg2+, the charge density of hydrated Ca2+ is larger, which provides more efficient electrostatic screening. Moreover, Ca2+ can coordinate with APAM carboxylate to form APAM-Ca2+ complexes with a regular porous structure. Consequently, under identical ionic concentration, APAM-Ca2+ could provide weaker osmotic pressure. Accordingly, lower fouling resistances, rejection rates, and higher flux recovery rates were achieved in the case of Ca2+. This study distinguishes the roles of Ca2+ and Mg2+ in microfiltration membrane fouling by polymeric organics, which will be targeted to guide the efficient microfiltration process of PFPW before advanced desalination.

Automated summary

By studying the fouling mechanisms of anionic polyacrylamide (APAM) in high-salinity environments, it was found that both divalent cations Ca2+ and Mg2+ aggravated membrane fouling. Compared to hydrated Mg2+, hydrated Ca2+ has a larger charge density and can form regular porous complexes with APAM carboxylate, resulting in weaker osmotic pressure. Therefore, lower fouling resistances and higher flux recovery rates were achieved in the case of Ca2+.