Fossil fuels have a variety of harmful environmental effects by the emission of various toxins such as sulfur oxides (SOx) and nitrogen oxides (NOx), VOCs and greenhouse gases (Zhu et al, 2010; Alami et al, 2020). Researchers are investigating the potential of generating energy from wind, sun, water, biomass and thermal heat to minimize the use of fossil fuels (Hussain et al, 2017;Tawalbeh et al, 2020; Tawalbeh et al, 2020). Electricity has recently been produced from osmotic pressure using an emerging power generation technology called pressure retarded osmosis (PRO). The mixing of two aqueous solutions of different salinities releases the Gibbs free energy of mixing process, which can be transformed to hydraulic pressure by means of a pressure retarded osmosis (PRO).The electrical power will then be produced by means of hydro-turbines (Tawalbeh et al, 2020; Jia et al, 2014; Skilhagen, 2008).
Pressure retarded osmosis (PRO) has attracted significant interest as a possible technology capable of extracting sustainable osmosis energy from salinity gradients (Cui et al, 2014; Bui and McCutheon, 2014). PRO uses a osmotic pressure difference between feed (FS) and draw solution (DS) pressurized by hydraulic pressure (Loeb, 1976; Yip et al, 2011; Kwon et al, 2021). The energy recovery method relies on placing on the draw stream a higher absolute pressure than that added to the feed stream. As a result, osmosis results in a rise in the flow rate of the high pressure stream – as long as the osmotic pressure differential (Δπ= πD – πF) exceeds the difference in the applied absolute pressure (ΔP= PD – PF), where π and P are the osmotic and hydraulic pressure of the fluids, and D and F are the draw and feed streams, respectively. This illustrates a classic optimization problem that is essential for the implementation of PRO units, in that higher applied pressure results in higher energy recovery per mole of solvent transfer, but also at a lower rate of solvent transfer at steady state (Manzoor et al., 2020).
The osmotic gradient will determine the quantity of energy generated, similarly as other FO applications. By merging river and seawater, the estimated osmotic energy produced worldwide is between 1750 and 2000 TWh per year, which goes over the limit one-tenth of the world's energy demand with a working pressure of 13.5 bar, equal to a 135 MWC in a hydroelectric power plant (Sikdar, 2014; Chia et al, 2020). The average electricity generation in 2017 is about 481 TWh from biofuels, about 4197 TWh from hydraulics and about 1127 TWh from wind energy, compared to other renewable energy technologies (Tawalbeh et al, 2020; Chia et al, 2020; IEA, 2010).
Many of the studies published in the literature is aimed at improving the viability of using PRO for generating electricity, although on small-sized systems. Figure 1 shows recent PRO research focus directions. In order to prove its feasibility in extraction of energy back in 1976 using hollow fiber membranes, Loeb et al. (1976) are experimentally testing the PRO technique. Mehta and Loeb (1978) examined the influence of osmotic pressure on high pressures. Water transport was modeled in the PRO method in 1981. In this model, the internal effects of concentration polarization were considered (Lee et al, 1981). In 1990, Loeb and his collaborators perfomed an analysis evaluating the mechanical efficiency of the continuous operation of the PRO plant. The continuous operation of the underground PRO plant and the alternating flow configuration of the terrestrial PRO plant (Loeb et al, 1990). In 2009, the Norwegian company (Statkraft) built Loeb's first design of the PRO plant in Oslo, Norway (Statkraft, 2009). The prototype of Statkraft is situated approximately 60 km south of Oslo, where the capital of Norway. Because of the expensive pre-treatment of river water needed and the limited capacity of electricity due to the small osmotic differential pressure between the seawater and the river, work at Statkraft terminated in December 2013 (Sarp et al, 2015). The internal concentration polarization model was developed by Xu et al. (2010) to integrate the effect of drawing solution using spiral wound FO membrane modules. Increasing water permeation across the membrane has been shown to raise the concentration of draw solution; nevertheless, the negative impact of internal polarization of concentration has also been enhanced (Xu et al, 2010). Much of the studies performed up to 2011 centered on the investigation and enhancement of membranes in PRO operations. It should be mentioned that a thin film consisting of an active layer of polyamide and a support layer of polysulfone for the PRO process was developed by Yip et al. (2011) to enhance their hydrophilic properties and improve water permeability, polydopamine was applied over the supporting layers of two suitable RO membranes (Arena et al, 2011).
Despite promising developments in PRO technology, owing to a lack of membranes with suitable high power density and acceptable durability under highly pressurized PRO operating conditions, the PRO process has yet to be commercialized (Skilhagen et al, 2008; Cai et al, 2016). For instance, because of their poor water flow and low power density, prototype commercial cellulose acetate (CTA) membranes have not been shown to be commercially viable for use in PRO (under the conditions of smaller than 1 W m−2 using river water as feed and seawater as draw solution) (Kwon et al, 2021, Song et al, 2013; Chou et al, 2012; Vos, 1966).
Apart from the recent PRO research tendency we focused on the strengthening the membrane material cope with creeping phenomena in this study. One of the most important purposes is to use less petro-chemical materials during the production of membranes, while increasing the strength in the use of the membrane in the PRO process.
We concentrate on the biocompatible approach in this study by adding cellulose nanocrystals (CNCs) to the TFC membranes based on nanofiber. Natural needle-rod fragments arising from acid hydrolysis of raw cellulose are CNCs, also known as nanocrystalline cellulose (NCC) (De Souza Lima, 2004). The amorphous segments of cellulose, consisting of cellulose fibres, lignin, waxes, etc., degrade and remain extremely crystalline as biomass is processed in a harsh acid environment (Asempour et al, 2018). Cellulose can be defined as a high molecular weight homopolymer of β-1,4-linked anhydro-D-glucose units, irrespective of their source, in which each unit is 180° corkscrew relative to its neighbors and the repeat section is commonly used as a glucose dimer known as cellobiose (Figure 2). With respect to the termini of its molecular axis, each cellulose chain has dimensional chemical asymmetry: one end is a group that is chemically reduced (i.e. hemiacetal unit) and the other end is a pendant hydroxyl unit, the nominal non-reducing end. The number of glucose units or the degree of polymerization (DP) is up to 20 000, but shorter cellulose chains may occur and are often located in the primary walls of the cells (Habibi, 2010).
Depending on the source of cellulose and the conditions of acid treatment, the size of the CNCs varies, but typically they are a few hundred nanometers long and a few nanometers in diameter (Habibi et al, 2010; Peng et al, 2011). They have a high specific density, a large specific surface area and a large negative zeta potential, as well as Young's modulus. They also have a highly reactive surface that makes them favorable for chemical functionalization due to the presence of the single bond -OH groups (Habibi et al, 2010; Peng et al, 2011). CNCs are biodegradable, renewable, have very low impacts on the ecosystem and are generally referred to as non-toxic and harmless particles (Habibi et al, 2010; Peng et al, 2011; Canada 2020; Kovacs et al, 2010; Roman et al, 2009; Hanif et al, 2014; Mahmoud et al, 2010; Bai et al, 2012; Li et al, 2011; Daraei et al, 2017. For a wide variety of possible uses, these appealing properties of CNCs have attracted considerable interest in PRO membrane fabrication.
Usually, two categories of membranes have been used extensively for PRO membranes. First one is the thin-film composite (TFC) membrane with selective layers on porous supports and the second one is an engineered shelled membrane developed using cellulose acetate (CA) and cellulose triacetate (CTA) (Sun and Chung et al, 2013; Lee et al, 2020). It is feasible to fabricate TFC membranes to ensure support, and selective layers have a greater flux of water than CA or CTA membranes (Gonzales et al, 2019). PRO membranes using a TFC membrane have also been studied in several researches (Lee et al, 2020) . In other experiments, Bui and McCutcheon, (2014) and Song et al. (2013) developed nanofiber-based PRO membranes with nanofiber support for polyacrylonitrile (PAN). Nanofibers are known to have a high potential for use in the development of osmotically-driven membranes due to their high porosity and low tortuosity (Bui et al, 2014; Shirazi et al, 2017; Son et al, 2018). In fact, the produced membranes achieved a power density of 21.3 W/m2 @ 15.2 bar and 8.0 W/m2 @ 11.5 bar, when 1.06 M NaCl and 0.5 M NaCl were used as drawing solutions, respectively (Lee et al, 2020).
Mentioned studies have proved the feasibility of nanofibers for use in the production of PRO membranes with high power density. However, owing to their comparatively lower mechanical flexibility, at high hydraulic pressures in nanofiber oriented PRO membranes, the risk of membrane deformation still exists. In order to address the poor mechanical stability of nanofiber-based PRO membranes at different ratios, the mechanical stability of polyacrylonitrile (PAN) nanofiber support was enhanced by CNC composites and the nanofiber structure was controlled.