Recent developments in forward osmosis membranes using carbon-based nanomaterials

Highlights

• Carbon nanoparticles were used to improve the performance of FO membrane.

• Incorporating nanoparticles improved water flux and mechanical strength of the FO membrane.

• Carbon nanoparticles reduced FO fouling, offering the opportunity for broader applications.

• Aggregation of particle is a critical problem faced in the course of nanocomposite membrane.

Abstract

Contamination and industrial development are among the reasons for water quality deterioration beyond treatability by conventional processes. Unfortunately, conventional water and wastewater treatment technologies are not always capable of handling industrial wastewaters, and hence more advanced treatment technologies are required. The new trend of osmotically driven membrane technologies has demonstrated an exceptional efficiency for water purification and treatment including seawater desalination. Compared to pressure-driven membrane processes, forward osmosis (FO) technology, as a standalone process, is more energy-efficient, and less prone to membrane fouling than its predecessor reverse osmosis (RO) technology. However, forward osmosis suffers a severe concentration polarization that is acting on both sides of the membrane and results in a sharp decline in water flux. A thinner support layer has been recommended to lessen the concentration polarization impact in the FO process but a very thin support layer compromises the membrane mechanical strength. Recently, researchers have applied different carbon-based nanomaterials to enhance water flux, fouling propensity, and mechanical strength of the FO membrane. This work reviews advancement in the FO membrane fabrication using carbon nanomaterials to improve the membrane characteristics. Despite a large number of laboratory experiments, carbon-based nanomaterials in the FO membrane are still at the early-stage of laboratory investigation and no commercial products are available yet. The study also reviews the main challenges that limit the application of carbon-based nanomaterials for FO membranes.

Introduction

Water contamination by organic and inorganic pollutants is a significant problem that has attracted considerable attention worldwide [[1], [2], [3], [4], [5]]. Contaminants removal from water requires robust and efficient decontamination technologies, which are capable of the treatment of a wide range of impurities. There are various existing physical and chemical technologies for water and wastewater treatment such as gravity separation [6,7], membrane filtration technologies [[8], [9], [10], [11], [12], [13], [14]], air flotation [15], absorption material includes fluorochemicals, chemical vapor deposition, coating mesh, carbon-based materials, hydrophobic aerogels, sol-gel process, and sponges [[16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]]. These technologies can remove a large proportion of dissolved matters [8], but they have some inherent limitations such as low separation or rejection rate, fouling, high energy consumption, reusability, and recyclability of the filtration media [9,17,21].

Membrane technologies stand out as the most widely recognized and advanced technologies for wastewater and water treatment and have a long history of excellent achievement in the field [[27], [28], [29]]. As of late, FO has stood out as competitive membrane technology for wastewater treatment with several advantages over the existing membrane technologies. These advantages include low power consumption, less fouling, and applicability for a wide range of feed solutions [30]. Additionally, the FO process includes the capability to concentrate low osmotic pressure feed solution with a concentrated draw solution [31]. This makes the FO process a potential technology for treating a wide range of water and wastewaters, including desalination. Despite the advantages of the FO process, it suffers from several drawbacks, for example, concentration polarization (CP), reverse solute flux (RSF), and low permeability [32]. Also, fouling materials accumulation on the membrane surface, especially in the treatment of complex feed solutions or when the pretreatment process is insufficient to provide a high-quality feed solution to the FO process [31].

One of the possible solutions for this problem is to apply nanotechnology to improve the performance of the FO membrane performance. Also, nanoparticles help to overcome the problems of low water permeability, low mechanical strength, and fouling propensity application of nanomaterials enhances permeability and lowers the phenomenon of CP [[33], [34], [35]].

The incorporation of hydrophilic nanomaterials in the substrate of a thin-film composite forward osmosis membrane will increase membrane porosity, hydrophilicity and decreases the tortuosity of the support layer, that collectively alleviate the effect of internal concentration polarization. Metal oxide nanoparticles and functionalized carbon-based nanomaterials are extensively utilized to accomplish this purpose. Among all the carbon-based nanomaterials, graphene oxide derivatives (hydrophilic additives) are becoming increasingly important as they can enhance the selectivity, performance, and productivity of the membranes by changing the mechanism of membrane formation. Therefore, carbon additives have been used in polymeric membrane fabrication to improve the characteristics of the membrane such as fouling, low flux, and poor mechanical-chemical stability of the FO membranes. The schematic representation of the influence of hydrophilic additives on non-solvent induced phase separation and membrane formation mechanism is shown in Fig. 1.

There is tremendous research related to the field of carbon nanoparticles, which has been shown by an enormous number of research articles published every year on the topic of FO technology. However, the major drawback of incorporating carbon additives is the instability and low efficiency of different membrane materials that have become a challenge over time [[36], [37], [38]]. To overcome this problem, an option must be developed, which gives the potential applicant as far as ease, high productivity at commercial scale with the simple technique.

Weiyi Li et al. [39] built up a theoretical model to understand the relationship between the structure parameter (S) and CP using the classical solution-diffusion theory. The model depicts internal concentration polarization (ICP), assuming a linear structure of the support layer while the intricate morphology is combined in the macroscopic phenomenological coefficients [39]. Studies demonstrated the reliance of the FO membranes fabricated by phase inversion technique on the porous structure, which is straightforwardly corresponding to the flux rate [[40], [41], [42], [43]]. Most recently, researchers focused on the preparation of FO membranes using functionalized graphene oxide [44,45] and ultrathin free-standing reduced graphene oxide [46] to reduce the CP. This is due to the fact that CP in the FO membrane is directly affected by the tortuosity and thickness of the membrane, and inversely with the membrane porosity and diffusion coefficient of the solute. The main disadvantage of the majority of the membrane-based water treatment processes is membrane fouling. Different aspects of mass transportation result in the attachment, adsorption, or accumulation of various particles onto pores and surfaces of membranes, leading to fouling of the membrane. In general, the three main categories of membrane fouling are organic, inorganic, and bio-fouling. Out of these, biofouling makes almost 40% of the membrane fouling in the reverse osmosis process that leads to an irreversible reduction in the salt rejection and the permeate flux [46]. Furthermore, the superior reversibility of fouling in FO enables its utilization in different applications in water treatment.

Our findings, concepts, and discussions in this review may be used to develop polymeric asymmetric membranes decorated with various functionalized carbon materials. Furthermore, the review also provides insights to reduce the CP effects, a field that still lacks a versatile strategy. As far as we could possibly know, the mechanisms involved in the free-standing fabrication and uniform GO, and rGO membranes are still unexplored. The existence of various oxygen-containing functional groups (epoxy, carbonyl, and hydroxyl) enhances the hydrophilicity of graphene-based membranes. In this review, detailed theoretical and experimental insights are discussed for the preparation of graphene-based membranes. Further, we have also examined the three types of fouling (organic, inorganic and biofouling), various factors controlling the fouling development, and research works performed with and without nanomaterials to confirm the fouling reversibility occurring in the FO membranes.