Abstract:

Objective With the goal of reducing pollution and carbon, efficient and low-cost photothermal wastewater recycling technology has attracted more attention. Solar interfacial evaporation is considered a green and sustainable water treatment technology for treating wastewater by absorbing solar energy to convert light energy into heat energy. However, the photothermal efficiencies of most photothermal carbon materials are dissatisfactory. Therefore, in order to improve the photo-thermal utilization rate, a metal organic framework(MOF)/graphene photothermal composite material with good photothermal performance was prepared by a vacuum filtration method. The prepared material was used to treat dyeing wastewater and recycled pure water by interfacial evaporation.

Method The graphene-based MOF material was prepared by in-situ growth method and deposited on a hydrophilic polyvinylidene difluoride (PVDF) based membrane surface as a photothermal layer to facilitate the evaporation of fresh water and rejection of pollutants. Because of the selectivity of the prepared G-ZIF membrane, only allowed water vapor was to pass through the membrane pores, and non-volatile organic matters were thus rejected. In addition, the microstructure and optical properties of the membrane materials were characterized, and the photothermal properties and wastewater evaporation performance were studied.

Results The results showed that the graphene surface changed from a two-dimensional layer-layered structure to a three-dimensional regular polyhedral crystal structure after in-situ growth of ZIF-8, which nucleated uniformly on the graphene surface and tightly encapsulated flake graphene (Fig. 1). According to the results of X-ray diffraction and Raman spectroscopy, ZIF-8 has been successfully loaded on the surface of graphene and has similar characteristic peaks to ZIF-8 (Fig. 2). The loading of ZIF-8 greatly increases the specific surface area of graphene up to 1 096.50 m²/g, thus providing more evaporation interfaces (Fig. 4). Furthermore, the optical performance analysis showed that the PVDF membrane had poor light absorption capacity, while the G-ZIF absorbance was about 2 times higher than that of the original graphene membrane, indicating the good optical property. Under the light radiation of 1.0 kW/m², the G-ZIF membrane surface temperature rose to 97.6 ℃ that is far higher than that of the PVDF membrane, demonstrating its excellent photothermal conversion property (Fig. 5). The test of pure water evaporation performance showed that the pure water evaporation rate of G-ZIF membrane reached 1.34 kg/(m²·h) under one sun illumination, and the photothermal efficiency was 91.2% (Fig. 6). The recycling treatment of printing and dyeing wastewater showed that each square meter of G-ZIF membrane could recover 3.19, 3.37 and 2.99 kg of pure water from the three types of printing and dyeing wastewater, respectively, with photothermal utilization rates of 83.3%, 87.9% and 78.4% (Fig. 7). After photothermal treatment, almost all dyes were removed, the color retention reached 99.9%, and the COD removal rate of wastewater was over 99.6%. After evaporation, the concentration of salt ions in distilled water was reduced to 0.01-0.84 mg/L, which is far lower than the concentration of ions in drinking water set by the World Health Organization. Meanwhile, the salt rejection reached 99.9% (Fig. 8). In addition, the photothermal performance of the G-ZIF composite membrane was stable, and the flux did not decrease significantly after 7 times consecutive operations (Fig. 9).

Conclusion A graphene/MOF-based photothermal material (G-ZIF) was prepared, which can efficiently produce pure water from synthetic dyeing wastewater. The porous microstructure of the G-ZIF membrane not only provides more surface area for water vapor but also improves light absorption. In the process of dyeing wastewater treatment, the concentrations of organic-inorganic pollutants and salt decreased significantly after treatment. The results showed that the G-ZIF membrane has the advantages of high evaporation rate, high photothermal conversion efficiency, and good performance stability. By simply modifying graphene materials with MOF, the photothermal properties of two-dimensional carbon-based materials are significantly improved, implying potential application values for textile wastewater purification.

Key words: interfacial evaporation, graphene membrane, metal organic framework, printing and dyeing wastewater, photothermal conversion, photothermal material