Highly Permeable Double-Skinned Forward Osmosis Membranes for Anti-Fouling in the Emulsified Oil-Water Separation Process Phuoc H. H. Duong, † Tai-Shung Chung, †,‡, * Shawn Wei, § and Lana Irish § † Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore ‡ Water Desalination & Reuse (WDR) Center, King Abdullah University of Science and Technology, Saudi Arabia 23955-6900 § Kraton Polymers, LLC., 15710 John F. Kennedy Boulevard, Suite 300, Houston, Texas 77032, United States * S Supporting Information ABSTRACT: Forward osmosis (FO) has attracted wide attention in recent years. However, the FO performance may be restricted due to internal concentration polarization (ICP) and fast fouling propensity that occurs in the membrane sublayer. Particularly, these problems significantly affect the membrane performance when treating highly contaminated oily wastewater. Recently, double-skinned flat sheet cellulose acetate (CA) membranes consisting of two selective skins via the phase inversion method have demonstrated less ICP and fouling propensity over typical single-skinned membranes. However, these membranes exhibit low water fluxes of <12 LMH under 2 M NaCl draw solution. Therefore, a novel double-skinned FO membrane with a high water flux has been aimed for in this study for emulsified oil-water treatment. The double-skinned FO membrane comprises a fully porous sublayer sandwiched between (i) a truly dense skin for salt rejection and (ii) a fairly loose dense skin for emulsified oil particle rejection. The former dense skin is a polyamide synthesized via interfacial polymerization, while the latter one is a self- assembled sulfonated pentablock copolymer (Nexar copolymer) layer. The resultant double-skinned membrane exhibits a high water flux of 17.2 LMH and a low reverse salt transport of 4.85 gMH using 0.5 M NaCl as the draw solution and DI water as the feed. The double-skinned membrane outperforms the single-skinned membrane with much lower fouling propensity for emulsified oil-water separation. ■ INTRODUCTION Forward osmosis (FO), a naturally driven process for fluid transport, has attracted worldwide attention in recent years for applications in clean energy 1-9 and water production. 9-21 In FO processes, clean water is separated from contaminated sources via an asymmetric semipermeable membrane by the osmotic gradient across the membrane. Compared to other pressure- driven processes for water purification, FO processes have advantages of operations at no or low pressure, 3,16,22 less fouling propensity, and easy cleaning. 15,19,22,23 Due to the asymmetric structure of FO membranes, there are two modes in FO operations. When the active skin (i.e., dense skin) of the membrane faces the draw solution (referred to as AL-DS or the pressure retarded mode (PRO) mode), the feed solutes may accumulate inside the porous support. As a result, the solute concentration inside the support is higher than the bulk feed. This phenomenon is called as the concentrative internal concentration polarization (ICP). The other operating mode is the active skin facing the feed solution. This mode is often called as the AL-FS or FO mode. Since the draw solution inside the porous support is diluted by the permeated water in the AL-FS (FO) mode, it creates a dilutive ICP. Although both concentrative ICP and dilutive ICP counteract the effective driving force of the FO process and cause flux decrease, the concentrative ICP under the AL-DS (PRO) mode is less severe than the dilutive ICP under the AL-FS (FO) mode. 24-26 Therefore, the AL-DS (PRO) mode often results in a higher water flux than the AL-FS (FO) mode in typical FO processes. However, despite of the higher flux, the former has greater fouling propensity than the latter due to the easy entering of foulants from the feed solution into the porous support. 27,28 As a consequence of additional transport resistance, the flux of the AL-DS (PRO) mode drops significantly over time in real applications. In addition, it is harder to remove foulants inside the porous support (referred to as internal fouling) than the external fouling on the membrane surface during the cleaning process. Wang et al. were the pioneers in designing double-skinned FO membranes in order to take advantages of the high flux nature of the AL-DS (PRO) mode and to mitigate the internal fouling. 29 Their membranes consisted of a primary dense skin to reject the Received: December 18, 2013 Revised: March 11, 2014 Accepted: March 12, 2014 Published: March 12, 2014 Article pubs.acs.org/est © 2014 American Chemical Society 4537 dx.doi.org/10.1021/es405644u | Environ. Sci. Technol. 2014, 48, 4537-4545