Novel thin-film reverse osmosis membrane with MXene Ti3C2Tx embedded in

polyamide to enhance the water flux, anti-fouling and chlorine resistance for

water desalination

Xiaoying Wang1, Qingqing Li1, Jianfeng Zhang1,3*, Haimeng Huang1, Shaoyu Wu2, Yan Yang3

1College of Mechanics and Materials, Hohai University, Nanjing 211100, China2Nanjing Delnamem Technology Co., Ltd. Nanjing 210000, China

3Jiangsu Engineering Research Center on Utilization of Alternative Water Resources, Hohai

University, Nanjing 211100, China

1. Microstructures of Ti3C2Tx

Fig. S1 Zeta potential of Ti3C2Tx

The negative charged Ti3C2Tx was confirmed by testing its zeta potential (Fig.

S1). Zeta potential of Ti3C2Tx was measured three times at 25 , which was -31.1, -℃29.7 and -29.9 mV, calculating the average zeta potential was -30.2 mV.

2. Influence of reaction conditions

2.1. Aqueous phase reaction time

Fig. S2 Effect of reaction time on separation performance of PA membrane.

Fig. S2 shows effect of reaction time of the aqueous phase on the membrane

separation performance of polyamide membrane. When the time was short, the MPD

molecules did not react completely with TMC, the flux of membrane was high but

rejection was low due to bad cross-linking degree (CLD). As the CLD reached the

best, the polyamide layer became thicker and the flux became smaller and the

rejection became large. While the reaction time was long, the aqueous phase solution

gradually penetrated into the inner membrane pores so that some substances

remaining in the aqueous phase were present in the composite membrane, and the

water flux and the rejection were all reduced. Therefore, in the process of post-

treatment of the composite membrane, membrane should be heat treated and washed

with DI water to remove the monomers that were not completely reacted and other

additives. According to Fig. S1, 4 min is the best reaction time for aqueous phase.

2.2. pH value

Fig. S3 Effect of pH on separation performance of PA membrane.

Hydrogen chloride formed in the interfacial polymerization will react with the

residual m-phenylenediamine in aqueous solution and reduce the concentration of

amine so that pH value is very important for the reaction solution. Fig. S3 shows the

influence of pH on separation performance of polyamide membrane. The hydrogen

chloride will be neutralized when pH increases, which is beneficial to the further

progress of monomer and the salt rate of membrane. However, TMC and the polymer

are hydrolyzed under the condition of strong alkali, so the pH value should be in the

best range (pH=10).

3. Surface and cross-section SEM images

3.1. Polysulfone supports

Fig. S4 Surface and cross-section SEM images of polysulfone supports

(a) (b) surface; (c) (d) section.

Fig. S4 shows SEM images of the surface and cross-section of the PSU support

layer. The polysulfone support had a uniform spongy pore structure (Fig. S4 (d)) with

a pore size of 0.4 μm-0.6 μm and a smooth surface (Fig. S4 (a) (b)). The non-woven

fabric at the bottom was reticulated (Fig. S4 (c)), so the bottom membrane had

excellent permeability and high mechanical properties, and had been widely used in

reverse osmosis.

3.2. Polyamide separation layer

Fig. S5 Surface and cross-section SEM images of polyamide layers

(a) (b) surface, (c) (d) section.

Polyamide membrane formed by interfacial polymerization of m-

phenylenediamine and trimesoyl chloride was covered on polysulfone support layer.

Fig. S5 shows surface and section SEM images of polyamide layers. The holes in the

surface of the polysulfone-based film are covered by the polyamide layer generated

by the interfacial polymerization reaction (Fig. S5 (a) (b)). The surface of the ultrathin

separation layer of polyamide is a rough "peak and valley" structure. Fig. S5 (c) (d)

shows the section of polyamide, and the polyamide layer is more dense and has no

pore structure.

Fig. S6 Cross-section SEM images of (a) PA, (b) PA15 membranes.

Fig. S6 shows the cross-section SEM images of PA and PA15 membranes,

indicating the approximate thicknesses were 280-455 nm and 205-375 nm,

respectively. Therefore, the incorporation of Ti3C2Tx reduced the thickness of the

membrane, which is helpful to the improvement of membrane water flux.

4. Membranes with high Ti3C2Tx loading

4.1. Surface and cross-section SEM images

Fig. S7 SEM images of surface and cross section of 0.225 wt% PA-Ti3C2Tx reverse osmosis

composite membranes (a) (b) surface, (c) (d) section.

Fig. S6 is 0.225 wt% PA-Ti3C2Tx reverse osmosis membrane surface and cross

section SEM images. The surface of bare polyamide membrane shows “peak and

valley” shape and the morphology of lamellar coating on the surface of high content

Ti3C2Tx (Fig. S6 (b)) obviously. Fig. S6 (c) (d) shows the cross section morphology of

Ti3C2Tx -polyamide membrane. The density of the separation layer decreased and the

mambrane showed loose morphology.

4.2. Influence of interfacial polymerization conditions

Fig. S8 Effect of reaction time (a) and pH (b) on dealination performance of Ti3C2Tx polyamide

reverse osmosis membrane.

When the aqueous phase reaction time was 4 min and pH was 10, the water flux

(34 L·m-2·h-1) and salt rejection (98.67%) reached the best. The longer the reaction

time of aqueous phase, the more serious the substance in the solution block the

membrane hole, which led to the decrease of dealination performance. It can be seen

from Fig. S7 (b) that when pH was less than 10, the water flux and salt rejection of

composite membrane all increased. Owing to the formation of hydrogen chloride in

the interfacial polymerization, the larger the pH was, the faster the hydrogen chloride

was excluded and the worse the monomer cross-linking degree became, thus reducing

the desalination performance.

4.3. Dealination performance

Fig. S9 Effect of high Ti3C2Tx loading on dealination performance of PA reverse osmosis

membranes.

At the beginning of the reaction, the salt rejection of the membrane increased

gradually and then decreased with the increase of Ti3C2Tx loading content (the

maximum value of 98.71% at 0.0175%). Ti3C2Tx is a lamellar structure, water

molecules can form hydrogen bonds with oxygen atoms of Ti3C2Tx, and the flow of

water molecules increases without frictionless (at 0.225 wt% loading). However

Ti3C2Tx aggregates with higher loading on the surface of the membrane and hinders

the passage of water molecules.