Core-shell PPy@TiO2 enable GO membranes with controllable and stable dye desalination properties
Graphical Abstract
Introduction
The production of salty wastewater from textile, paper, electroplating, oil, mining, pharmaceutical and food industries has been continuing to increase with the population and economic development [1], [2]. However, the associated effects such as biological toxicity and high salinity make the salty wastewater unsuitable for the ordinary water treatment process composed of activated sludge and reverse osmosis (RO) units [3], [4]. More than this, the salty wastewater has also been forbidden to discharge into rivers arbitrarily for damaging the ecological environment or accelerating the corrosion of bridges and hulls [5], [6]. For a long time, salty water processing has been relying on distillation, dye/salt solid waste burning, and mixed salt solid landfill [7], [8], [9], but on the other hand causing significant economic burdens owning to the high processing cost [6], [10], [11].
In recent years, GO-based nanofiltration membranes (GOMs) have attracted numerous attention to separate organic solutes and inorganic salts completely, attributed to their superior lattice strength [12], controllable layer distance spacing (D-spacing) [13], [14], high efficiency [15], [16], [17], and low energy consumption [18]. As a result, industry-grade salt products and clean water can be potentially generated by distilling the salty wastewater through the GOMs [19], [20], [21], [22], [23], [24]. However, the low selectivity towards the dye/salt separation of GOMs (e.g. > 90% for dyes and 30% - 90% for salts) inhibits the practical implementation [18], [24], [25], [26]. Besides, the continuous nanofiltration results in the formation of the filter cake block over the membrane surface and the expansion of the nanochannel size, which is known as the membrane fouling issue [7], [27], [28], [29]. It remains the main hinder to practical applications of GOM, causing the short usage life and deteriorated long-term performances [27], [30], [31], [32], [33].
Compositing nano-photocatalysts with GOM is an efficient strategy to regulate membrane selectivity and relieve membrane fouling, as the separation performance responding to the membrane D-spacing can be enlarged and organic pollution responding to the filter cake block formation can be degraded in-situ during the nanofiltration processing [19], [34]. TiO2 has been the most studied photocatalytic material for the incorporation into GOM [19], [34], [35], [36], [37], [38]. However, the features of high surface energy and wide bandgap make TiO2 nanoparticles agglomerated within the membrane and only responsive to the narrow UV excitation region, which greatly degrades the membrane structure and anti-fouling performances. The replacement of visible-light-driven photocatalysts such as g-C3N4 certainly expands the optical response region [17], [39]. However, the highly hydrophobic feature of g-C3N4 also results in the agglomeration and twist of the layer-by-layer membrane structure [39], [40]. Such an observation is also found in GOMs composited with visible-light-driven three-dimensional (3D) MOFs [41], [42]. Therefore, the surface engineering of embedded photocatalysts is critical in developing nanocomposite nanofiltration membranes with photocatalytic self-cleaning capabilities.
In this work, we focus on the investigation of GOMs composited with TiO2, in which the surface engineering and energy band modification of TiO2 is realized by the functionalization of polymeric polypyrrole (PPy) through a single-step approach. Firstly, the coating of PPy polymeric shell over TiO2 nanoparticles (PPy@TiO2) leads to the homogeneous dispersion of the nanocomposite in an aqueous solution [43], [44], attributed to the electrostatic repulsion. Secondly, the semiconducting PPy expands the optical absorption range of the PPy-TiO2 core-shell heterojunction towards the visible light region [45], therefore improving the pollutant photo-degradation performances [44]. We intercalate PPy@TiO2 heterostructures with three different sizes into GOMs for investigating their dye nanofiltration and visible-light-driven self-cleaning performances. In addition, as the PPy@TiO2 dosage significantly affects the membrane surface morphological and structural properties, we particularly analyze the water permeability and the selectivity of dye/dye and dye/salt of both the high-dosed and low-dosed GO-PPy@TiO2 composite membranes. Through comprehensive investigations, we consider that our composite membranes with visible light photocatalytic and controllable filtration properties can be of great potentials for efficient resource utilization of salty wastewater and seawater, accelerating the development and practical application of 2D nanofiltration membranes.
Section snippets
Materials
A commercial GO slurry with a lateral size of less than 1.2 μm was bought from the Sixth Element Materials Technology Co., Ltd. (Changzhou, China). The PVDF membranes with the 20 nm pore size were produced by Tianjin Navigator Lab Instrument Co., Ltd. (Tianjin, China), and the SEM, EDS, hydrophilicity, and nanofiltration performance of that were listed in Fig. S1. In comparison, the PVDF membrane solely acts as support with distinctly different features to other nanofiltration membranes. The
Characterizations of TiO2 and PPy@TiO2 particles
From the transmission electron microscopic (TEM) images in Fig. 1a, TiO2 particles are agglomerated due to their large surface energy. With the modification of the PPy shell, such high surface energy of TiO2 is reduced and the agglomeration of TiO2 is hence effectively inhibited, resulting in the uniform dispersion of PPy@TiO2 nanoparticles (Fig. 1b). From a more zoom-in TEM image in Fig. 1c, the TiO2 core- PPy shell- structure is also observed.
Subsequently, we used DLS to reveal the size
Conclusion
We successfully developed the PPy@TiO2 heterostructure-enabled GOMs with controllable filtration and self-cleaning properties. The polymeric coating of PPy over the TiO2 nanoparticles facilitated the homogenous dispersion of the heterostructures in GOM. Through the tuning of the TiO2 particle size, the ratio between PPy and TiO2, and the synthesis duration of PPy@TiO2, the optimized heterostructure was found in the combination of TiO2 with sizes of <10 nm and the PPy: TiO2 ratio of 20: 2 at the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank the National Natural Science Foundation of China (51774245, 52172155), the scientific and technological projects for Distinguished Young Scholars of Sichuan Province (2020JDJQ0028), and the Fundamental Research Funds for the Central Universities (Grant No. 2682021CX107 and 2682021CX118) for financial support and ‘ceshigo’ (www.ceshigo.com) for providing the testing service.
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