A simple, universal, biofouling-inspired surface modification to improve separation efficiency in membrane systems

Abstract

A challenge facing widespread implementation of membranes for water purification is fouling, which is the deposition of organic contaminants in a membranes pores (internal fouling) or on its surface (external fouling). Fouling leads to a catastrophic decrease in water flux which, in turn, results in increased energy costs (and, therefore, higher operation costs) due to increased pressure requirements and membrane cleansing and replacement protocols. To alleviate fouling from many types of contaminants, including proteins, natural organic matter, oily wastewater emulsions, and bacteria (biofouling), many studies have focused modifying conventional membrane surfaces to increase hydrophilicity, reduce roughness and charge density, and generally reduce attractive forces between the membrane and contaminant. A simple, universal modification technique is reported here. This modification uses benign monomers and polymers, and can be used to effectively treat all wetted parts of a membrane module, providing enhanced fouling resistance not only to the separation membrane itself, but also to all other wetted parts in a membrane system. A recent study reported that dopamine (4- (2-aminoethyl)benzene-1,2-diol) self-polymerizes under mild conditions and deposits from solution to virtually any surface with which it came into contact, including traditional non-stick materials such as PTFE [1]. This new polymer (termed polydopamine) mimics mussel adhesive plaque by incorporating two chemical groups (catechols and amines) prevalent in the mussel- secreted protein. Previously, the catechol-like structure in polydopamine was shown to adhere to a TiO2 surface with a strength that is approximately one-half that of a silicon-carbon covalent bond [2]. Because polydopamine can polymerize to reach molecular weights on the order of several million, multiple attachment points between the polydopamine chain and the membrane surface should be achieved, producing an extraordinarily strong adhesion to the membrane surface. Multiple membranes were successfully modified using polydopamine deposition, as indicated by increased membrane surface hydrophilicity and improved membrane fouling resistance to oil emulsions. Additionally, most polydopamine-modified membranes had higher flux recovery following mild cleaning protocols, so polydopamine modification has the potential to enhance membrane cleaning efficacy.