Novel nanocomposite forward osmosis membranes for treating highly saline and oily wastewater with low fouling, high water flux and high selectivity
Date of Issue2016-06-01
Interdisciplinary Graduate School (IGS)
Energy Research Institute @ NTU (ERI@N). The degree should be Doctor of Philosophy (IGS)
This study focuses on the design and synthesis of a new nanocomposite forward osmosis (FO) membrane that is able to separate highly saline and oily wastewater with low fouling, high water flux and high selectivity, in order to battle against global freshwater scarcity. A new design rationale of FO membrane is developed, which highlights that a genuine FO membrane must possess high antifouling capability as the requisite besides high water flux and high selectivity. Guided by this rationale, as-designed new nanocomposite FO membrane consists of a novel antifouling selective layer on top of a three-dimensionally (3D) interconnected porous support layer. Particularly, graphene oxide (GO) nanosheets assisted phase inversion technology is developed to fabricate the 3D interconnected porous support layer, on top of which dip-coating technique is employed to further construct the hydrogel selective layer in ultrathin thickness (~100 nm). The structures and properties of hydrogel selective layer are finely tuned towards both high antifouling capability and high selectivity, wherein the key role of chemical crosslinking is revealed. The best crosslinking agent is identified as glutaraldehyde; the optimum molecular weight of hydrogel is found to be 93 kDa; the optimum concentration of hydrogel solution is 0.25 wt%; the optimum coating time is 20 min; and more importantly, the optimum crosslinking degree is determined as 30%. Based upon all these optimized results, as-synthesized hydrogel FO membrane even with conventional phase inversion constructed support layer can already demonstrate the evident advantages in high selectivity and high antifouling capability, with its water flux/reverse salt flux ratio (JW/JS) 2.4 times higher than that of commercial HTI FO membrane (cellulose triacetate, woven). Furthermore, the support layer is optimized through introducing GO nanosheets to finely adjust the phase inversion process. Here, support layer in highly interconnected porous structure is the key to minimize FO’s intrinsic limitation on water flux i.e. internal concentration polarization (ICP) problem. For the first time, hydrophilic 2D graphitic nanomaterial is demonstrated able to transform the interior pore structure of the support layer from 1D connected to 3D interconnected. Based upon systematic optimization of GO assisted phase inversion process, an entirely new support layer structure with its interior pores highly interconnected in all three dimensions at micrometer scale is created. The formation mechanism of this 3D interconnected porous support layer is attributed to GO induced viscosity difference. Compared with conventional 1D pore connected support layer, this 3D pore interconnected support layer can reduce FO membrane structural parameter (S) by as much as 41.4%, leading to the enhancement in FO water flux (JW) by 72%. Meanwhile, the JW of as-synthesized nanocomposite membrane arrives at 30.5 L m-2 h-1 at FO mode with draw solution of 1.5 M Na2SO4, which is 3.1 times higher than that of HTI membrane under identical operational conditions. Therefore, for the first time, micrometer-scale 3D interconnected porous support layer that is able to break the ICP bottleneck and thus achieve high FO water flux is successfully synthesized with dominant membrane manufacture process (i.e. phase inversion). Most importantly, as-synthesized nanocomposite FO membrane is systematically investigated for its ability to accomplish simultaneous desalination and oil/water separation of highly saline and oily wastewater. FO separation results indicate that this nanocomposite membrane can simultaneously desalinate and deoil hypersaline oil-in-water emulsion with more than three times higher water flux, higher removal efficiencies of both oil and salts (>99.9% for oil and >99.7% for multivalent salt ions), and significantly lower membrane fouling (>80% lower water flux reduction ratio) compared with HTI membrane. The further operation results reveal that this new FO membrane is remarkably superior to HTI membrane in both resistance to salinity induced fouling aggravation and long term antifouling durability. In summary, this is the first study that explores and optimizes the capability of hydrogel macromolecule as a new selective layer for FO membrane. Furthermore, it creates a micrometer-scale 3D interconnected porous nanocomposite support layer to break ICP bottleneck with dominant membrane manufacture process (i.e. phase inversion). Moreover, it also achieves simultaneous desalination and oil/water separation of highly saline and oily wastewater by as-synthesized new FO membrane with low fouling, high water flux and high selectivity. This study points out a new direction for the development of genuine FO membrane and makes a significant impetus to the industrialization of FO technology in order to address global freshwater scarcity.