Understanding the pH-responsive behavior of graphene oxide membrane in removing ions and organic micropollulants

This paper reports the systematic performance evaluation and mechanism elucidation of a multi-layered graphene oxide (GO) membrane in removing ionic and organic species from water at different pH levels. The rejections of representative monovalent (Na+, Cl-) and multivalent ions (SO42-, Mg2+) as well as organic dyes (methylene blue, rhodamine-WT) and pharmaceutical and personal care products (triclosan, triclocarban) by the GO membrane were tested in a reverse osmosis membrane system. It was found that, at pH 7, the GO membrane exhibited high removal of multivalent cations/anions and all tested organics, regardless of their charges, sizes, or hydrophobic properties, but low removal of monovalent ions. Such removal behavior is dramatically different from that of traditional nanofiltration membranes, which are often negatively charged and can only remove multivalent ions and organics that also bear negative charges. As pH varied, the key properties (e.g., charge, interlayer spacing) of the GO membrane were significantly changed, leading to different pH-dependent interfacial phenomena and separation mechanisms, and demonstrating the promise of using the GO membrane as a pH-responsive membrane. It was also revealed that the molecular shape of some organics (e.g., triclocarban) greatly affected their removal due to the mobility and thus p-p interaction of such organics with the carbon surface of the GO membrane. Finally, implications for the future development of high-performance GO membranes are discussed.