Efficient heavy metal removal by thin film nanocomposite forward osmosis membrane modified with geometrically different bimetallic oxide
Graphical abstract
Introduction
Heavy metal contamination is a critical environmental issue that caused by the uncontrolled discharge of heavy metal. The electronics, electroplating, machinery manufacturing, and metallurgy industries have produced an extensive amount of wastewater containing heavy metals which is eventually discharged into water bodies. Consumption of water body containing trace amount of heavy metals is enough to severely threaten the health of human. Metal ions tend to penetrate inside human body through accumulation pathway and cause severe health effects and body dysfunction because it is unable to be metabolized by human body or decomposed easily [1]. Although heavy metal ions could be eliminated from wastewater by several conventional methods including adsorption, chemical precipitation and ion exchange [2], the major issues with these technologies are their effectiveness, energy consumption and long-term sustainability.
Forward osmosis (FO) has gained interest as a membrane-based water treatment alternative owing to its low membrane fouling tendency and low energy consumption, compared to other pressure-driven membrane techniques [3,4]. However, the common issue encountered by most TFC FO membranes is the ICP [5]. The reverse diffusion of salt and the intrinsically porous structure of the substrate layer are the major contributors to ICP. ICP results in the decrease in the effective driving force and eventually declines the water flux. The most common and effective strategy to reduce the ICP phenomenon in FO membrane involves the incorporation of inorganic nanoparticles to alter the membrane substrate properties [6]. The incorporated nanoparticles also transfer their essential properties to the membrane which maintain the membrane performance while addressing the ICP issue [7].
Currently, the inorganic nanomaterials that have been most widely used for FO membrane modification include zeolite, graphene oxide (GO), carbon nanotube (CNT) and metal oxides such as titanium dioxide (TiO2) [8]. These nanomaterials are introduced into polymeric membranes matrices to improve the membrane hydrophilicity and reduce the surface roughness [9,10]. Magnetite oxide (Fe3O4) is one of the widely explored nanomaterials for wastewater treatment as it is cheaply and easily available, low in toxicity, and highly hydrophilic [7,11]. Darabi, et al. [12] were the first to add Fe3O4 nanoparticles in polyethersulfone (PES) substrate to mitigate ICP. The results revealed that the porosity and the hydrophilicity of the PES substrate were improved after the addition of Fe3O4, hence leading to lower structural parameter (S) and enhanced water flux. As both hydrophilicity and ICP are equally important factors to be considered for a high-performance FO membrane, it is necessary to fabricate a membrane that can simultaneously address both issues. Nevertheless, most of the efforts in FO membrane modification were focused on using single nanomaterial to bring about the desired properties for desalination and wastewater treatment. Although the large specific surface area (SSA) of Fe3O4 endowed the material with good adsorptivity towards heavy metal, it also led to the high agglomeration tendency of the nanoparticle [13,14]. Hybridization of two geometrically different nanoparticles has been reported to effectively avoided this agglomeration issue [6,15]. To this end, titania nanotube (TNT) could serve as a great deposition platform for the Fe3O4 particles. Apart from possessing all the beneficial characteristics of TiO2 such as good hydrophilicity and high SSA, the fine aperture of the tubular TNT which is saturated with oxygen functional groups could favour the diffusion of water through the nanotube, leading to high water flux of the resultant nanocomposite [16]. All these features encouraged the employment of TNT-Fe3O4 hybrid to improve the performance of polymeric membrane for water separation application.
Currently, there are still limited studies on the synthesis of hybrid nanoparticles used for the modification of FO membrane substrate to mitigate ICP issue and improve the membrane performance. Sirinupong, et al. [17] developed a TFN FO desalination membrane in which its PSf substrate incorporated with the hybrid of TiO2/GO. The roles of hybrid nanoparticles were discussed in their study and significant improvement in the performance of TFN FO membrane was observed. The TFN FO membrane incorporated with TiO2/GO hybrid nanoparticles achieved a higher water flux of 23 L m−2 h−1 compared to that of incorporated with single TiO2 (18 L m−2 h−1) or GO (10 L m−2 h−1) nanoparticles. All the membranes showed salt rejection of over 90%. The great potential of synergizing multiple nanomaterials to improve membrane permeability and mitigate ICP can be achieved via nanomaterial hybridization approach. This study aims to determine the roles of synthesized TNT-Fe3O4 hybrid nanoparticles performance to mitigate ICP of FO membranes when they were incorporated into the membrane substrate. The loading of hybrid nanoparticles was optimized to achieve the best separation performance. Finally, the membrane regeneration capability and the leaching tendency of nanoparticles were also studied to give a general idea on the practical usage of the fabricated TFN.
Section snippets
Materials
Titanium dioxide (P25), ferric chloride (FeCl3·6H2O, >98%, Merck), ferrous chloride (FeCl2·4H2O, >98%, Merck) and ammonia hydroxide (NH4OH, 25%, Merck) was used to synthesize TNT–Fe3O4 hybrid nanoparticles. NH4OH was used to increase the alkalinity and oxidize the precursor to form a suspension. The ferrous chloride and ferric chloride were used as precursor for coprecipitation on TNT nanoparticles. Polysulfone (PSf Udel P-3500, Amoco Chemical) was used to form the membrane support layers.
Morphological Analysis of TNT–Fe3O4 Hybrid Nanoparticles
Fig. 3 shows the FESEM and TEM images as well as EDX analysis of the synthesized TNT–Fe3O4 hybrid nanoparticles. The synthesized TNT–Fe3O4 hybrid comprised of a mixture of tubular and spherical nanoparticles which were similar to the structures of the TNT and Fe3O4 synthesized by Raeisi, et al. [28] and Taufik, et al. [29], respectively. This confirmed that the TNT–Fe3O4 hybrid nanoparticles exhibited combination physical morphology of their pure counterparts. Based on the EDX spectra in Fig. 3
Conclusion
In summary, a novel TNT–Fe3O4 hybrid nanoparticle was synthesized through hydrothermal and co-precipitation method that incorporated into membrane substrate. TFN FO membrane had been developed via phase inversion to cast the membrane substrate which incorporated TNT–Fe3O4 hybrid nanoparticles and the formation of polyamide layer on the substrate through interfacial polymerization method for heavy metal removal. An optimized substrate layer was designed by adding TNT–Fe3O4 hybrid nanoparticles
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgement
This work was supported by the Higher Institution Centre of Excellence Grant (4J435) received from the Ministry of Higher Education Malaysia.
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