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

Abstract

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 APAM-induced 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.