Ti3C2/PVDF membrane for efficient seawater desalination based on interfacial solar heating

The photothermal material of Ti3C2 was synthesized by etching Ti3AlC2 with hydrofluoric acid. The asprepared Ti3C2 was deposited on a polyvinylidene fluoride (PVDF) membrane via vacuum filtration to form a Ti3C2/PVDF membrane, which was used for seawater desalination in the next step based on interfacial solar heating. The water evaporation rate of the Ti3C2/PVDF membrane could be enhanced to 0.98 kg/m·h under 2 sun irradiance, which was 2.8 times and 5.4 times higher than that of pure water (0.35 kg/m·h) and PVDF (0.18 kg/m·h) respectively. The temperature difference between the two air–water interfaces with and without the Ti3C2/PVDF membrane was as high as 11.8 C, confirming the interfacial heating behavior. The water evaporation rate under 2 sun irradiance kept mostly in the range of 0.96–0.86 kg/m·h over 30 days under continuous operation, indicating the high stability of the Ti3C2/PVDF membrane. Finally, it was demonstrated that the typical water-quality indexes of the condensed fresh water were below the limit values of the Standards for Drinking Water Quality in China, WHO, and US EPA.


INTRODUCTION
Water shortage has become an urgent problem around the world, restricting social progress and economic development. The contradiction between supply and demand of water resources is sharp, with current predictions that more than half of the world's population (about 3.9 billion people) will live in water-scarce areas by 2025 (Elimelech & Phillip ). Therefore, solving the problem of water shortage is very important for achieving sustainable development of society and improving people's living standards. Compared with the other commonly used fresh water acquisition methods, seawater desalination is considered as the most feasible and economical way to solve the shortage of water resources because 96.5% of the Earth's water resources are distributed in the ocean.
The current seawater desalination technologies mainly involve reverse osmosis (RO) (Malaeb & Ayoub ) and multi-stage flash (MSF) (Khawaji et al. ). However, these two types of technologies require high energy consumption and advanced supporting infrastructure, as well as large centralized installations, which limit the application in distributed small villages or remote regions (Ghaffour et al. ).
A new concept named 'air-water interface solar heating' emerged to be applied for seawater desalination in the early 2010s (Zeng et al. , , ). The principle of this concept is that the transformation of water from liquid to gas only occurs on the surface of the water (Ghasemi et al. ; Zhang et al. ). Therefore, only the surface water needs to be heated instead of the whole water body to achieve desalination. Due to the poor light absorption capacity of water, light will penetrate into the solution when solar radiation falls on the water surface without materials (Zhang et al. ; Wang et al. ). This leads to inefficient photothermal conversion. Nowadays, a common method to raise the temperature of surface water is to float a layer of light absorbing material ( Herein, we report that Ti 3 C 2 is another new photothermal material because of its excellent electromagnetic wave absorption and subsequent heat generation operation (Ma et al. ; Ren et al. ). The Ti 3 C 2 was synthesized by etching Ti 3 AlC 2 with HF. For floating on the water surface, the Ti 3 C 2 was deposited on the PVDF membrane. The PVDF membrane was used as the support for floating because of its hydrophobic surface. Moreover, the Ti 3 C 2 / PVDF membrane has an advantage over other photothermal materials due to its convenient separation and recycling in liquid-phase reactions. Finally, the quality of the condensed fresh water was tested for evaluating the potential application of this new seawater desalination technology. The purposes of this study are: (i) to provide a Ti 3 C 2 /PVDF membrane for seawater desalination based on interfacial solar heating; (ii) to examine the water quality of the condensed fresh water obtained using this technology.

Chemicals
All chemicals were of analytical grade and used as received without further purification. Titanium aluminum carbide (Ti 3 AlC 2, 98%) was purchased from Fosman Co., Ltd. Sodium chloride, hydrogen fluoride and ethanol (99.7%) were purchased from Sinopharm. Polyvinylidene fluoride membrane was purchased from Amanda PVDF Nano Spraying Co., China.
Preparation of Ti 3 C 2 and Ti 3 C 2 /PVDF membrane After etching, the mixture was constantly washed with ethanol, shaken evenly and centrifuged for each cycle, until the pH value of the supernatant increased to 7. The Ti 3 C 2 was dried in an oven at 60 C for 12 h.
The as-prepared Ti 3 C 2 in different amounts ranging from 40 to 70 mg was dispersed in 50 mL of ethanol by ultrasonic crushing in a biosafer 650-92 Ultrasonic Cell Shredder at an ultrasonic power of 650 W for 5 min. Then, the Ti 3 C 2 was uniformly deposited on the surface of the PVDF membrane by vacuum filtration at 0.07 MPa. Finally, the Ti 3 C 2 /PVDF membrane was dried in an oven at 60 C for 12 hours.

Water evaporation
Water evaporation experiments were conducted at a temperature of 25 ± 1 C and air humidity of 50% ± 10%.
Firstly, 150 mL of water was filled in a cylindrical glass, then the Ti 3 C 2 /PVDF membrane was placed on the water surface. A 300 W CEL-S500 xenon lamp obtained from Beijing Zhong Jiao Jin Yuan Science and Technology Co, China, with an AM 1.5 filter was used to simulate the solar light.
The light intensity can be adjusted in the range of 1,000-4,700 W/m 2 , as measured by a laser power meter (LP-3A, Beijing Physcience Opto-Electronics, China). The weight of the evaporated water was measured using an electronic balance (FA2104, Shanghai Sunny Hengping Scientific Instrument Co., Ltd).

Materials characterization and water-quality analysis
The crystalline properties of the Ti 3 C 2 and Ti 3 AlC 2 were identified by an X-ray diffractometer (XRD, Rigaku, Japan) using Cu Kα radiation (45 kV, 40 mA). The morphologies of Ti 3 C 2 and Ti 3 AlC 2 were examined by an FEI FEG650 field-emission scanning electron microscope and a JEM 2010 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The contact angle of the Ti 3 C 2 /PVDF membrane was measured by an OCA20 contact angle measuring device (Dataphysics, Germany). A U-4100 ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrophotometer (Hitachi, Japan) in which BaSO 4 powder was used as the 100% reflectance standard and an integrating sphere accessory was equipped to obtain UV-Vis-NIR diffuse reflectance spectra of the Ti 3 C 2 and Ti 3 AlC 2 . The temperature of the Ti 3 C 2 /PVDF membrane surface was supervised by an IR camera (FTIR T650sc, USA). The concentration of anions and cations in the condensed fresh water were examined by a Dionex ICS-2000 ion chromatograph (Dionex, USA) and a PE NexION 300Q inductively coupled plasma mass-spectrometer (Perkin Elmer, USA), respectively. Excitation-emission matrix (EEM) fluorescence spectroscopy was performed by a fluorescence regional integration (FRI) method (HITACHI-F 4600, Japan).

RESULTS AND DISCUSSION
Synthesis and characterization of Ti 3 C 2 In order to verify whether aluminum was completely etched by HF, the crystalline structures of the Ti 3 C 2 before and after HF etching were analyzed by X-ray diffraction. The diffraction peaks originating from the Al element in the Ti 3 AlC 2 sample (Figure 1(a)) almost disappeared after HF etching, indicating that the Al element was gradually decreased.
The morphologies of the Ti 3 C 2 before and after HF etching were examined by SEM and TEM observation. Before HF etching, a distinct dense layered structure with a thickness of about 0.3 μm can be clearly observed in the pristine Ti 3 AlC 2 (Figure 1(b) and 1(e)). After HF etching, Ti 3 C 2 was obviously peeled off from the layered structure to form a single smooth plate crystal with a length of 3.0 μm and a width of 1.4 μm (Figure 1(c) and 1(f)). It can be seen that the gap between the Ti 3 C 2 particles increased, which is conducive for vapor to pass through, resulting in enhanced water yield. Moreover, a single flaky crystal can also reduce the dosage of Ti 3 C 2 needed for desalination.
The optical absorption property, which is one of the important properties of the photothermal conversion  As shown in the digital photo (Figure 2(a)), the Ti 3 C 2 / PVDF membrane has a smooth surface. In addition, 3D optical microscopy analysis was carried out to further measure the thickness and surface property of the Ti 3 C 2 /PVDF membrane. As measured (Figure 2(b)), the heights of the highest point and the lowest point of the Ti 3 C 2 /PVDF membrane differed by 41.65 μm, and the average thickness was 21.34 μm.
The surface of the Ti 3 C 2 /PVDF was smooth and flat, which ensures that the Ti 3 C 2 can be heated uniformly under the solar light while avoiding the surface structure damage caused by local high temperature. In order to float on the water surface, the Ti 3 C 2 was deposited on the surface of the hydrophobic PVDF membrane. The hydrophobicities of the PVDF and Ti 3 C 2 /PVDF membrane were characterized by measuring the water contact angle. The contact angles of PVDF (Figure 2(c)) and the Ti 3 C 2 /PVDF membrane ( Figure 2(d)) were 136.5 and 128.6 respectively, which indicate the hydrophobic surface of the Ti 3 C 2 /PVDF membrane.
As a result, the Ti 3 C 2 /PVDF membrane can be easily floated on the water surface due to the surface tension effect.

Water evaporation
The performance of the Ti 3 C 2 /PVDF membrane for water evaporation was explored. As references, water evaporation by PVDF membrane and evaporation of pure water itself were also conducted. As can be seen in Figure 3(a), all water evaporation processes can be simply modeled using zero-order kinetics, which can be described as: where k is the water evaporation rate, m 0 is the initial water mass, and m is the actual water mass at time t. The k value of Ti 3 C 2 /PVDF membrane was 0.98 kg/m 2 ·h, which was 2.8 times and 5.4 times higher than pure water (0.35 kg/m 2 ·h) and PVDF (0.18 kg/m 2 ·h) respectively. The enhanced water evaporation rate of the Ti 3 C 2 /PVDF membrane clearly proves that Ti 3 C 2 /PVDF membrane can enable effective interfacial solar heating and promote water evaporation.
The decreased k value of the PVDF was due to the hydrophobicity and light reflection effect of PVDF membrane.
The temperature change of the air-water interface can further confirm the heating effect of the Ti 3 C 2 /PVDF membrane on the solar interface. Before solar irradiation, the temperature of the air-water interface with and without Ti 3 C 2 /PVDF film measured by infrared thermal imager was 21.2 and 22.1 C, respectively (Figure 4). The surface temperature of the Ti 3 C 2 /PVDF membrane was raised to 43.3 C, which was 11.8 C higher than the pure water after only ten minutes of solar irradiance. It can be seen from the temperature change curve that the temperature rise of the pure water was relatively balanced throughout the water body, while the temperature rose rapidly within the first ten minutes after floating a Ti 3 C 2 /PVDF membrane on the water surface. After being irradiated for 60 minutes, the surface temperature of the Ti 3 C 2 /PVDF membrane reached a steady state. This was because a large amount of water evaporated and took away heat, making the temperature stable at 48 C.
In order to optimize the solar evaporation conditions, the effects of Ti 3 C 2 dose and solar irradiance intensity were investigated, and the results are shown in Tables 1 and 2. The water evaporation rate (k) increased from 0.59 to 0.98 kg/m 2 ·h with increase of Ti 3 C 2 dose from 40 to 60 mg. Further increasing the Ti 3 C 2 dose to 70 mg resulted in a deterioration of the k value (Figure 3(b)), which can be attributed to the excessive   0.42 kg/m 2 ·h, which was probably due to the specular reflection of the water surface (Figure 3(c)). However, after covering with a thin Ti 3 C 2 /PVDF membrane, the water evaporation rates were correspondingly enhanced to 0.71, 0.98, To further evaluate the quality of the condensed fresh water obtained by the Ti 3 C 2 /PVDF membrane, a real seawater sample obtained from the East China Sea was used as the source water for solar evaporation. Water-quality indexes including conductivity, turbidity, cations and anions of the East China Sea water before and after solar distillation by Ti 3 C 2 /PVDF membrane were investigated.
The conductivity (salinity) of the seawater was significantly reduced from 36,400 to 16.8 μs/cm ( Figure 5(a)), which is far below the value (2,500 μs/cm) defined by WHO. As an important test item in drinking water standards, the turbidity of the seawater was significantly reduced from 6.07 NTU to 0.63 NTU, which is far below the value (5 NTU) defined by WHO ( Figure 5(b)). Other typical cations and anions such as Na þ , K þ , Ca 2þ , Mg 2þ , SO 4 2-, NO 3 -, Cl -, Fand Brdecreased to <0.01-1.91 mg/L (as shown in Table 3  seawater is also volatile, thus the organic matter will evaporate together with the evaporation of water. However, after adding Ti 3 C 2 /PVDF membrane (Figure 6(c)), the organic matter in the condensed fresh water will be lower than that of the original East China Sea, which shows that the Ti 3 C 2 has a very significant adsorption and removal property on for organic matter removal.

CONCLUSION
In summary, a Ti 3 C 2 /PVDF membrane was prepared by HF etching of Ti 3 AlC 2 followed by vacuum deposition on a hydrophobic PVDF membrane surface. An enhanced water evaporation rate was achieved due to high solar absorption, uniform heating of the Ti 3 C 2 /PVDF membrane, and the interfacial solar heating effect. In addition, the enhanced water evaporation rate, the encouraging durability and stability, as well as the high quality of the condensed fresh water make the Ti 3 C 2 /PVDF membrane potentially useful in islands with insufficient fresh water supply.