Roles of a mixed hydrophilic/hydrophobic interface in the regulation of nanofiltration membrane fouling in oily produced wastewater treatment: Performance and interfacial thermodynamic mechanisms

Abstract

Nanofiltration (NF) membrane fouling during the treatment of oily produced wastewater remains the main technical challenge of crude oil (CO) exploitation. Here, a mixed hydrophilic/hydrophobic interface was introduced by adding anionic polyacrylamide (APAM) to CO solutions to mitigate NF membrane fouling. Interfacial free energies were applied to reveal how the mixed interface and coexisting metal ions regulate membrane fouling. Notably, in the initial stage of NF, both the flux decline rate and reversible fouling resistance (Rrev) were aggravated by adding APAM, and these changes were closely related to the higher osmotic pressure between the concentration polarization layer and permeation. With further addition of calcium ions (Ca2+), the strong coordination effects rendered the highest osmotic pressure in all test cases, and thus, the most serious Rrev and flux loss in the initial stage. Fortunately, in the subsequent pseudo-stable stage, the flux loss and irreversible fouling resistance (Rirr) were mitigated with the addition of APAM, which could be attributed to the formation of a mixed hydrophilic/hydrophobic interface (i.e., APAM/CO interface) with lower APAM–CO cohesive energies than the CO–CO/membrane free energies and the consequent retardation of irreversible fouling layer formation and growth. Moreover, the lower APAM–CO free energies for the mixed interface likely represented a more unstable thermodynamic system of the irreversible fouling layer and promoted the flux recovery of fouled membranes after chemical cleaning. In addition, the CO rejection of the NF membranes was also enhanced by the addition of APAM because the hydrophilic APAM molecules formed an energy barrier that inhibited the adsorption of hydrophobic CO.