Green and sustainable membrane fabrication development

2.1. Recent advances for green and sustainable Membrane fabrication

MF and UF have grown into the comprehensive solution for removing suspended particles, emulsions, colloids, and microorganisms from virtually any water source, including fresh, saline, process, and wastewater, over the last two decades [17]. MF is commonly used to separate small particles with a diameter of 0.1–10 m and to reject microorganisms from water [18]. UF membrane has smaller hole diameters, ranging from 0.001 to 0.10 m [19]. The desire for highly chemically, thermally, and mechanically robust membranes with low fouling propensity remains a major aim for the membrane industry. Improvements in the preparation method and membrane material selection, including the discovery of novel materials, are critical in ensuring the long-term viability of membrane technology [20].

The ability of PES and PVDF to dissolve in common organic solvents makes them attractive for generating porous membranes using phase immersion techniques [7]. Despite various efforts to develop high-performance membranes, only a few studies have focused on developing a more prospective membrane preparation procedure, and most the companies continue to use conventional organic solvents such as N-Methyl-2-pyrrolidone (NMP) and N, N-dimethyl acetamide (DMA) [21]. The final point is critical when considering that the solvent constitutes the majority of the mass and component during membrane construction. Furthermore, the vast majority of the organic solvents utilized in the manufacture of membranes have been recognized as extremely hazardous to human health and the environment [22]. The chosen solvent plays an important role in the membrane preparation process, influencing the polymer precipitation rate and, as a result, affecting the final membrane shape and characteristics.

The environmental consequences of membrane preparation are becoming increasingly serious as more applications rely on membrane technology [23]. Solvents play an important role in membrane production, and the characteristics of a solvent and how it interacts with the polymer impact membrane shape and therefore performance [24]. As a result, finding a green solvent capable of dissolving the polymer of interest is only a first step; the resultant membrane must also function well in order to be embraced by the membrane industry. Furthermore, hazardous solvents are no longer allowed to be used in membrane manufacturing due to environmental laws. DMF, DMAc, and NMP, for example, have been designated as chemicals of very high concern by the European Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH), and their usage will be banned beyond May 2020 [23]. Table 1 summarizes recent research on green solvent options for membrane manufacturing.

Organic compounds derived from petroleum products are frequently used as solvents in the chemical industry, posing health and environmental concerns. Without a doubt, substituting water for organic solvents has significant advantages. Hanafia et al. [25] described the creation of a porous membrane from hydroxypropyl cellulose, a water-soluble biopolymer (HPC). To fix the membrane shape and avoid resolubilization, HPC was first dissolved in water at a concentration of 20, and then a 0.5 wt.% glutaraldehyde crosslinker was added in a dope solution after degassing. A catalyst of 1 wt.% HCl was introduced just before casting the membrane to start the crosslinking reaction. Chemical crosslinking was used to test membrane stability in a variety of organic solvents (DMSO, tetrahydrofuran (THF), methanol, and chloroform).

γ-Valerolactone (GVL) is primarily a solvent with a low melting point and a high flash point; it has a distinct herbal odor, and it has lately been utilized in the taste, fragrance, and food sectors [26]. Vankelecom et al. [27] used GVL and a series of glycerol derivatives to produce porous membranes through NIPS with common polymers (PI, PES, polysulfone (PSU), cellulose acetate (CA), and cellulose triacetate (CTA) with the goal of researching bio-based green solvents in membrane preparation. ,′-diglycerol, glycerol, monoacetin, glycerol carbonate, and diacetin were the bio-based cosolvents used. By raising the dope polymer concentration, the researchers were able to effectively produce microfiltration (MF), ultrafiltration (UF), and potentially nanofiltration (NF) membranes.

The bio-based dihydrolevoglucosenone (Cyrene) is one of the developing green solvents for substituting polar aprotic solvents (e.g., NMP, DMAc, and DMF) [28]. Cyrene is a nontoxic solvent produced from renewable waste and a nonfood cellulosic source. Clark et al. [29] utilized cyrene to make PES flat sheet membranes for water filtration using the NIPS method, using polyvinylpyrrolidone (PVP) as an addition. The membranes produced with Cyrene and NMP were compared extensively. Membranes made using bio-based Cyrene had reduced polymer loss, adjustable pore diameters, and contact angles, and were more durable [24]. In comparison to NMP-fabricated membranes, the authors stated that Cyrene-fabricated membranes had higher overall porosity and bigger pore diameters. The permeability of membranes produced with Cyrene was readily adjusted in the region between NF/RO and MF applications by altering the temperature of the dope solution, which influences the viscosity of the dope solution.

Sherwood et al. [28] found that cyrene may be made from biomass in two easy stages, reducing environmental impacts. Figoli et al. [2] used Cyrene in the fabrication of PES and PVDF membranes by phase inversion, where the membranes were produced using simultaneous VIPS and NIPS techniques. To get membranes with various pore diameters and permeability, the exposure period to ambient relative humidity was varied between 0 and 5 minutes in this study. Membranes manufactured using NIPS (23,400 L/m2/h/bar) and VIPS-NIPS (18,700 L/m2/h/bar) showed encouraging results in pure water permeability testing. The results showed that Cyrene may be utilized as a solvent for two of the most commonly used membrane polymers. The experimental findings were also reviewed in terms of dope solution viscosity, membrane shape, porosity, thickness, and contact angle.

More recently, Lee and Drioli et al. [30] proposed Polarclean as a promising eco-friendly option to replace polar aprotic solvents (e.g., NMP, DMAc, and DMF) for membrane preparation. The solvent is known by its IUPAC designation of methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate and has a high solvent power and moderate volatility. Polarclean has been used in cold solutions as a solvent for agrochemicals and as a crystal growth inhibitor. Furthermore, at 24 °C, its solubility in water is quite high, at 490 g/dm³. Wang et al. [31] used the Polarclean solvent as an NIPS solvent to produce PI, CA, PES, and PSU membranes. The experiment demonstrated that MF, UF, NF, and gas separation membranes may be made, demonstrating the Polarclean solvent's flexibility. Porous PES membranes with high pure water permeability were successfully manufactured in another study by Marino et al. [32]. Utilizing PolarClean as the solvent, the prepared membranes were produced using a combination of NIPS and VIPS techniques. The shape of the membrane was altered by common additions such as PVP and poly(ethylene glycol) (PEG).

The TIPS technique was used by Chaouachi et al. to manufacture membranes from recycled low-density polyethylene (LDPE) using butyl acetate as a greener solvent [33]. LDPE is a semi-crystalline polymer that is only soluble in hazardous chemical solvents such phthalate-based compounds at high temperatures. To test the differences, the scientists used xylene as a solvent, and the findings showed that the membrane produced using butyl acetate had ideal qualities in terms of thickness (0.363 mm), pore size (0.14 m), and contact angle (120°). Because the polymer was derived from recyclable materials, the findings were positive. Because the TIPS method works at higher temperatures (greater polymer solubility), there are more options for finding green solvents. Acetyl tributyl citrate (ATBC), for example, is a relatively new green solvent that has no health risks. ATBC has been documented and is commonly utilized in pharmaceutical coatings and food packaging [34]. ATBC is not water miscible, thus it is thought to be far more ecologically friendly than phthalate-based TIPS solvents. A PVDF membrane with competitive performance was created using ATBC. To maximize the mechanical strength of the membranes, up to 50 wt. % of polymer concentration was employed, and the produced membrane was then stretched to increase porosity and permeability.