Application of coagulation-microfiltration for membrane fouling mitigation in highly saline dyeing wastewater: Anti-fouling modification of membrane and novel insight of salt into regulation of cake layer formation

Abstract

Cake layer formation is the dominant microfiltration (MF) membrane fouling mechanism after long-term operation. The regulation of cake layer characteristics is less clarified in the coagulation-microfiltration (CMF) technique due to an incomplete understanding of flocculation mechanism; however, gaining such insight will contribute to improving MF efficiency for water purification. Herein, an antifouling MF membrane for high saline textile wastewater was constructed using polydopamine (pDA) co-deposition with sodium p-styrenesulfonate (SS), and methoxypolyethylene glycols (mPEG). Thereafter, the cake layer formation in the CMF technique could be precisely regulated by adjusting salt concentration (NaCl or Na2SO4). Results indicated that the inorganic salt-induced transformation of flocculation mechanism endows a desired regulation of cake layer characteristics, consequently leading an increase in membrane flux. Various properties and formation of flocs were caused via different flocculation mechanisms (from predominant charge neutralization to integrated interaction) in response to a salt variation, giving rise to a directed transition behavior of cake layer formation from compact to porous structure. Particularly, the coagulation mechanism was validated by the experimental analysis and molecular dynamics simulations. Furthermore, the high salt permeation (>90%) and acceptable dye rejection (>98%) of two commercial reactive dyes could be obtained by the proposed CMF technique, exhibiting a satisfactory fractionation behavior of dyes/inorganic salt. A beneficial contribution is the direct reuse dyeing of NaCl from the spent dyeing bath in new dyeing operations. These results could provide a further understanding of the underlying mechanisms of coagulation-enhancing MF behavior and inspire the precise regulation of cake layer characteristics to achieve efficient MF.