Efficient heavy metal removal by thin film nanocomposite forward osmosis membrane modified with geometrically different bimetallic oxide

Abstract

A novel thin film nanocomposite (TFN) forward osmosis (FO) membrane with a positively charged and nanofunctional substrate layer has been developed for effective heavy metal ions removal. The substrate layer is constructed by adding titania nanotubes and magnetite oxide hybrid nanoparticles (TNT–Fe₃O₄) in the polysulfone (PSf) matrix. The introduction of nanoparticles endowed the substrate layer with improved hydrophilicity and loose structure. The modified substrate layer also improved the affinity between the nanofillers and polymer matrix, hence maintaining the selectivity of membrane. Compared to pristine thin film composite (TFC) membrane, the TFN with 0.5 wt.% nanofillers loading showed enhanced water flux from 1.63 to 2.82 L m⁻² h⁻¹ without losing its selectivity in terms of J_s/J_v ratio when operated in FO mode. The enhancement was mainly attributed to the improved substrate hydrophilicity which has effectively reduced the internal polarization concentration (ICP). The best performing TFN-0.5 membrane increased the water flux with 73% compared to TFC and exhibited a high Cd²⁺ and Pb²⁺ heavy metal ion rejection of >98%. By designing the substrate layer, this study demonstrated the feasibility of enhancing the performance of FO membrane for treating heavy metal wastewater through a simple and efficient method.

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.