**4.2 PEG in the support layer of an FO membrane**

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specific MD membranes [15, 16].

**4.1 PEG used as/in draw solute**

**4. PEG associated with forward osmosis**

viscous draw solute from entering the pores of the middle layer.

feed solution is usually heated up, and the produced vapor passes through the pores of the membrane and condenses on the distillate side by cooling. MD is considered cost-effective and promising because it can achieve almost 100% dissolved solid rejection specifically in desalination. Similar to an FO module, an MD module can also be constructed as flat sheet, hollow fiber, and spiral wound forms. According to different configurations of the distillate side [14], MD can be classified into direct-contact MD (DCMD), air-gap MD, sweeping-gas MD (SGMD), and vacuum MD (VMD) shown in **Figure 3**. Conventional microporous membrane with pore size (0.1–1 μm) can be used for MD, and there also are some membranes specifically designed for MD. PEG and its derivatives can be used to improve the hydrophilicity of the membrane surface facing the feed solution during the fabrication of some

One of the challenges of FO applications in wastewater treatment and desalination is the selection of high-performance draw solutes. Among several hundreds of draw solute explored in the FO process, PEG is also evaluated for the FO process. Beside electrolyte NaCl, neutral PEG polymers at different molecular weights (M.W. = 100, 200, 600, 2000, 3000, 8300, and 10000) were used as model draw solute to evaluate the performance of a commercially available cellulose triacetate (CTA) FO membrane and a homemade porous UF-like FO membrane [17, 18]. Wei et al. fabricated a double-skinned selective thin film composite (TFC) FO membrane consisting of a top thin polyamide (PA) layer, a middle porous cellulose ester layer, and another bottom thin PA layer and tested its performance using several viscous draw solutes such as PEG monolaurate (PEG 640ML), sucrose, and ferric citric acid complex (Fe-CA) [19]. The novel membrane can minimize the effects of internal concentration polarization (ICP) because the bottom thin PA layer prevents

Hydrophilic magnetic nanoparticles (HMNPs) are a type of promising draw solutes, which may easily be recycled under a magnetic field. There exist some reports that HMNPs were fabricated from PEG and magnetic nanoparticles. Ge et al. synthesized a series of PEG-(COOH)2-coated MNPs with narrow size distribution through a thermal decomposition process [8]. Mishra et al. specifically synthesized HMNPs with PEG 400 and evaluated their performances in an FO process where synthetic saline water (NaCl solution) at different concentrations of 0, 5, 10, 20 and 35 g/L was used as feed solution. About 35 g/L is close to the level of total dissolved solids (TDS) in seawater. When these HMNPs were used as draw solute in a fundamental FO process of deionized water, they could significantly eliminate the draw solute reverse diffusion problems which are common in the applications of general salts, such as NaCl, KCl, MgCl2, MgSO4, etc., as draw solute [20, 21]. Biodegradable and biocompatible temperature-sensitive triblock copolymer hydrogels PEG-PLGA-PEG/GO-0.09 wt%, PEG-PLGA-PEG/GO-0.18 wt%, PEG-PLGA-PEG/G-0.09 wt%, and PEG-PLGA-PEG/G-0.18 wt% were fabricated and used as draw solute in FO by Nakka and Mungray [22], where GO represents graphene oxide and G is graphene. PEG-PLGA-PEG was synthesized from D,Llactide, 1,4-dioxane-2,5-dione, methyl ether polyethylene through ring-opening polymerization using stannous octane as catalyst. However, much smaller water fluxes were achieved when feed solutions are DI water and 2 g/L NaCl solutions

**60**

than the previous HMNPs as draw solute.

In order to improve the performance of an FO membrane, the support layer can be reconstructed, and the active layer can be modified with PEG or its copolymer. Addition of PEG 400 to the support layer was conducted to fabricate a TFC FO membrane, and it was found that the addition of 6 wt% PEG was needed to reach the highest water flux [23] when DI water and 2 M NaCl were used as feed and draw solution. PEG 400 and dimethyl sulfone (DMSO2) were used as an additive and a crystallizable diluent to fabricate the CTA support layer of an FO membrane through thermally induced phase separation, and the FO membrane exhibited better antifouling properties than PSf-based FO membranes [24]. Sharma et al. used PEG 4000 and 6000 as additive to prepare cellulose acetate flat asymmetric FO membranes, and the modified FO membranes were used to evaluate power density performance in pressure-retarded osmosis [25].

Liu et al. fabricated the support layer from PSf with 5-, 10-, and 15-wt% PEG or PEGMA (poly(ethylene glycol) methyl ether methacrylate) and evaluated the corresponding FO membranes by using DI water and 1 M NaCl as feed and draw solution [26]. The PSf-PEG support layer was made by blending PEG with PSf, and in the second type, it was PEGMA grafted on PSf. The FO membrane containing 10 wt% PEG achieved relatively steady performance for a long time operation process due to its better salt rejection, and the FO membrane with 5% PEGMA grafting possessed a high intrinsic permeability and a low structural parameter. Recently, amphiphilic PEG-block-PSf-block-PEG copolymers were used to cast the support layer, and the fabricated TFC FO membrane achieved some significant improvements on water flux, antifouling, and permeability selectivity [27].

#### **4.3 PEG in the active layer of an FO membrane**

When the active layer of a TFC FO membrane is modified with PEG, the surface hydrophilicity of the membrane can be improved, thus enhancing the membrane antifouling properties. Elimelech et al. functionalized the active layer of a TFC FO membrane with PEG diepoxides through surface grafting, and their dynamic experiments showed that the membrane fouling was significantly reduced when testing with alginate as model organic foulant [28]. The same research group later used a post-fabrication technique to graft a PEG-block copolymer on the active layer of commercial TFC FO membranes, and the PEG density was optimized to the best membrane performance by compromising the increased membrane hydrophilicity and reduced water flux [29]. Interestingly, a novel design of FO membranes by impregnating the support layer with hydrophilic cross-linked poly(ethylene glycol) diacrylate (PEGDA) was proposed by Zhao et al., and there is no additional PA layer needed [30]. The newly designed FO membrane had the ability to mitigate internal concentration polarization which is commonly for typical two-layer FO membranes and to improve the performance ratio by 50% compared to those of state-of-the-art commercial FO membranes. Recently, Chen et al. tethered the active PA layer of a TFC FO membrane with PEGMA, and the membrane ICP was greatly mitigated with only slightly flux reduction from 10.99 to 9.32 LMH during synthetic sewage treatment [31].

The antifouling ability and performance of CTA FO membranes can also be improved by applying PEG to the membrane surface. The surface of a CTA FO membrane was modified by firstly coating polydopamine (PD) and then grafting PEG, and the submerged osmotic membrane bioreactor using the FO membrane possessed better flux behaviors than the pristine reactor and anti-adhesion abilities of biopolymers and bio-cake [32].
