Thermo-responsive copolymers with ionic group as novel draw solutes for forward osmosis processes

Enhanced forward osmosis desalination of brackish water using phase-separating ternary organic draw solutions of hydroxypropyl cellulose and propylene glycol propyl ether

This study introduces draw solutions for application in forward osmosis (FO) processes, combining mono propylene glycol propyl ether (PGPE) with the cellulose derivative hydroxypropyl cellulose (HPC). A total of 16 unique single-solute and ternary organic draw solutions were prepared and evaluated, leading to the selection of three promising solutions for further investigation. Notably, eight of the initial organic draw solutions demonstrated osmotic pressures exceeding 2.4 MPa. The dynamic viscosities of all draw solutions exhibited a significant reduction with increasing temperature. Among the investigated solutions, the 0.25HPC-3.75PGPE demonstrated the most favorable FO performance, achieving average experimental water fluxes of 11.062 and 9.852 Lm-2 h-1 (LMH) against a 1 g/L NaCl brackish feed solution across two FO runs. PRACTITIONER POINTS: Hydroxypropyl cellulose (HPC, MW ~100,000) was mixed with propylene glycol propyl ether (PGPE) as draw solutes for FO processes. Seven combinations of HPC and PGPE produced osmolalities greater than 1000 mOsm/kg. 0.5HPC-7.5PGPE ternary draw solution achieved experimental water fluxes of 11.062 and 9.852 LMH against 1 g/L NaCl brackish feed solution. Leveraging the LCSTs of these ternary organic solutions holds promise for improved separation and regeneration processes.

Anion Effect on Forward Osmosis Performance of Tetrabutylphosphonium-Based Draw Solute Having a Lower Critical Solution Temperature

The applicability of ionic liquids (ILs) as the draw solute in a forward osmosis (FO) system was investigated through a study on the effect of the structural change of the anion on the FO performance. This study evaluated ILs composed of tetrabutylphosphonium cation ([P₄₄₄₄]⁺) and benzenesulfonate anion ([BS]^-), para-position alkyl-substituted benzenesulfonate anions (p-methylbenzenesulfonate ([MBS]^-) and p-ethylbenzenesulfonate ([EBS^-]), and methanesulfonate anion ([MS]^-). The analysis of the thermo-responsive properties suggested that the [P₄₄₄₄][MBS] and [P₄₄₄₄][EBS] ILs have lower critical solution temperatures (LCSTs), which play a beneficial role in terms of the reusability of the draw solute from the diluted draw solutions after the water permeation process. At 20 wt% of an aqueous solution, the LCSTs of [P₄₄₄₄][MBS] and [P₄₄₄₄][EBS] were approximately 36 °C and 25 °C, respectively. The water flux and reverse solute flux of the [P₄₄₄₄][MBS] aqueous solution with higher osmolality than [P₄₄₄₄][EBS] were 7.36 LMH and 5.89 gMH in the active-layer facing the draw solution (AL-DS) mode at osmotic pressure of 25 atm (20 wt% solution), respectively. These results indicate that the [P₄₄₄₄]⁺-based ionic structured materials with LCST are practically advantageous for application as draw solutes.

Effects of the Structure of Benzenesulfonate-Based Draw Solutes on the Forward Osmosis Process

A series of phosphonium-based ionic liquids (ILs) based on benzenesulfonate derivatives (tetrabutylphosphonium benzenesulfonate ([TBP][BS]), tetrabutylphosphonium 4-methylbenzenesulfonate ([TBP][MBS]), tetrabutylphosphonium 2,4-dimethylbenzenesulfonate ([TBP][DMBS]), and tetrabutylphosphonium 2,4,6-trimethylbenzenesulfonate ([TBP][TMBS])) were synthesized via anion exchange with tetrabutylphosphonium bromide ([TBP][Br]). Then, we characterized the ILs and investigated their suitability as draw solutes for forward osmosis (FO), focusing on their thermoresponsive properties, conductivities, and osmotic pressures. We found that aqueous [TBP][BS] was not thermoresponsive, but 20 wt% aqueous [TBP][MBS], [TBP][DMBS], and [TBP][TMBS] had lower critical solution temperatures (LCSTs) of approximately 41, 25, and 21 °C, respectively, enabling their easy recovery using waste heat. Based on these findings, 20 wt% aqueous [TBP][DMBS] was tested for its FO performance, and the water and reverse solute fluxes were found to be approximately 9.29 LMH and 1.37 gMH, respectively, in the active layer facing the draw solution (AL-DS) mode and 4.64 LMH and 0.37 gMH, respectively, in the active layer facing the feed solution (AL-FS) mode. Thus, these tetrabutylphosphonium benzenesulfonate-based LCST-type ILs are suitable for drawing solutes for FO process.

Recent Developments and Future Challenges of Hydrogels as Draw Solutes in Forward Osmosis Process

Forward osmosis (FO) has been recently regarded as a promising water treatment technology due to its lower energy consumption and lower membrane fouling propensity compared to the reverse osmosis (RO). The absence of suitable draw solute constraints the wide-range application of the FO. Hydrogels are three-dimensional hydrophilic polymer networks that can absorb a huge amount of water. Particularly, stimuli-responsive polymer hydrogels can undergo a reversible volume change or solution-gel phase transition in response to external environmental stimuli, including temperature, light, pressure, solvent composition, and pH. These intrinsic properties indicate the lowest regeneration cost of draw solutes compared to the thermal method and other membrane processes. This review aims to introduce the research progress on hydrogels as draw solutes, clarify the existing problems and point out the further research direction.

Polymer Having Dicationic Structure in Dumbbell Shape for Forward Osmosis Process

The thermal-responsive polymers, poly(alkane-1,#-diylbis(tri-n-butylphosphonium) 4-vinylbenzenesulfonate) (PSSBP#, # = 8, 6, and 4), where # is the number of carbon atoms in the central bridge structure of the dicationic phosphonium moiety, were synthesized to examine their potential application as draw solutes in forward osmosis (FO). The polymers exhibited low critical solution temperature (LCST) characteristics in aqueous solutions, which is essential for recovering a draw solute from pure water. The LCSTs of the 20 wt% aqueous solutions of PSSBP8, PSSBP6, and PSSBP4 were confirmed to be approximately 30, 38, and 26 °C, respectively, which is advantageous in terms of energy requirements for the recovering draw solute. When the concentration of the PSSBP4 draw solution was 20 wt%, water flux and reverse solute flux were approximately 1.61 LMH and 0.91 gMH, respectively, in the active layer facing the draw solution (AL-DS) system when the feed solution was distilled water. The PSSBP# thermal-responsive draw solute has considerable potential for use as a next-generation draw solute because of its excellent osmotic performance and efficient recovery. Therefore, this study provides inspiration for novel ideas regarding structural transformations of polymers and their applicability as draw solutes.

Thermo-responsive draw solute for forward osmosis process; poly(ionic liquid) having lower critical solution temperature characteristics

A poly(ionic liquid) having lower critical solution temperature characteristics was synthesized to investigate its suitability as a draw solute for forward osmosis.

Recent Advance on Draw Solutes Development in Forward Osmosis

In recent years, membrane technologies have been developed to address water shortage and energy crisis. Forward osmosis (FO), as an emerging membrane-based water treatment technology, employs an extremely concentrated draw solution (DS) to draw water pass through the semi-permeable membrane from a feed solution. DS as a critical material in FO process plays a key role in determining separation performance and energy cost. Most of existing DSs after FO still require a regeneration step making its return to initial state. Therefore, selecting suitable DS with low reverse solute, high flux, and easy regeneration is critical for improving FO energy efficiency. Numerous novel DSs with improved performance and lower regeneration cost have been developed. However, none reviews reported the categories of DS based on the energy used for recovery up to now, leading to the lack of enough awareness of energy consumption in DS regeneration. This review will give a comprehensive overview on the existing DSs based on the types of energy utilized for DS regeneration. DS categories based on different types of energy used for DS recovery, mainly including direct use based, chemical energy based, waste heat based, electric energy based, magnetic field energy based, and solar energy based are proposed. The respective benefits and detriments of the majority of DS are addressed respectively according to the current reported literatures. Finally, future directions of energy applied to DS recovery are also discussed.

Binding TiO2 nanoparticles to forward osmosis membranes via MEMO–PMMA–Br monomer chains for enhanced filtration and antifouling performance

Forward osmosis (FO) has attracted increasing interest in various applications for water and wastewater treatment and reuse. However, drawbacks caused by its lower-than-expected flux performance and fouling issues remain bottlenecks that limit the wider applications of FO technology. In this research, titanium dioxide (TiO₂) nanoparticles were grafted onto two commercially available FO membranes, a cellulose triacetate (CTA) membrane and an aquaporin (AqP) membrane, through a specially designed 3-(trimethoxysilyl)propyl methacrylate–polymethyl methacrylate–bromide (MEMO–PMMA–Br) monomer chain, to improve the filtration performance with regard to pure water flux and organic fouling resistance. The success of the surface coating method was verified using FT-IR, SEM-EDX, and AFM. Approximately 30% titanium coverage was obtained for both FO membranes. A reduction of the contact angle on the modified CTA membrane surface indicated enhanced water permeability and antifouling performance. An adverse effect on the surface hydrophilicity of the modified AqP membrane may be attributed to the obstruction of aquaporins from the feed solution due to the coverage of MEMO–PMMA–Br monomers and TiO₂ nanoparticles. The pure water flux of both membranes was significantly improved, with average flux increases of 73.4% and 13.6% identified for the modified CTA and AqP membranes, respectively. In addition, the antifouling performance of the AqP membrane was greatly enhanced after surface modification, attributed to the integrated effects of foulant photodegradation (catalyzed by TiO₂ nanoparticles at the interface) and the prevention of functional water channels being blocked by organic foulants due to TiO₂ coverage.

Commercial forward osmosis membranes were modified by TiO₂ nanoparticles. A designed MEMO–PMMA–Br monomer chain to reduce the damage on polymer membrane by TiO₂. Water permeability and fouling resistance were significantly improved.

Synthetic draw solutes for forward osmosis: status and future

Forward osmosis (FO) has developed rapidly over the past decade. The development of draw solutes, a key component of FO processes, has also progressed remarkably. A wide range of synthetic draw solutes have been explored in recent years. Synthetic draw solutes exhibit superiority over the conventional draw solutes obtained commercially in terms of lower reverse solute fluxes and less energy consumption in draw solute recycling. However, there are still some big challenges for synthetic draw solutes, such as complicated synthetic procedures, low water fluxes, severe concentration polarization (CP) and decreased water recovery efficiency when recycled draw solutes are reused in FO. These challenges are also the current research focus on the exploration of novel draw solutes. This article aims to review the recent progress especially on synthetic draw solutes. Their design strategies, synthesis routes and FO performance are assessed. Some representative applications involving the synthetic draw solutes-facilitated FO processes are exemplified. The advantages and disadvantages of the existing synthetic draw solutions are evaluated. The challenges and future directions in exploring novel draw solutes are highlighted.

Osmotic Heat Engine Using Thermally Responsive Ionic Liquids

The osmotic heat engine (OHE) is a promising technology for converting low grade heat to electricity. Most of the existing studies have focused on thermolytic salt systems. Herein, for the first time, we proposed to use thermally responsive ionic liquids (TRIL) that have either an upper critical solution temperature (UCST) or lower critical solution temperature (LCST) type of phase behavior as novel thermolytic osmotic agents. Closed-loop TRIL-OHEs were designed based on these unique phase behaviors to convert low grade heat to work or electricity. Experimental studies using two UCST-type TRILs, protonated betaine bis(trifluoromethyl sulfonyl)imide ([Hbet][Tf₂N]) and choline bis(trifluoromethylsulfonyl)imide ([choline][Tf₂N]) showed that (1) the specific energy of the TRIL-OHE system could reach as high as 4.0 times that of the seawater and river water system, (2) the power density measured from a commercial FO membrane reached up to 2.3 W/m², and (3) the overall energy efficiency reached up to 2.6% or 18% of the Carnot efficiency at no heat recovery and up to 10.5% or 71% of the Carnet efficiency at 70% heat recovery. All of these results clearly demonstrated the great potential of using TRILs as novel osmotic agents to design high efficient OHEs for recovery of low grade thermal energy to work or electricity.

Zwitterionic polymers showing upper critical solution temperature behavior as draw solutes for forward osmosis

We investigated the suitability of homopolymer with UCST characteristics as draw solutes for the FO process for the first time.

Using UCST Ionic Liquid as a Draw Solute in Forward Osmosis to Treat High-Salinity Water

The concept of using a thermoresponsive ionic liquid (IL) with an upper critical solution temperature (UCST) as a draw solute in forward osmosis (FO) was successfully demonstrated here experimentally. A 3.2 M solution of protonated betaine bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]) was obtained by heating and maintaining the temperature above 56 °C. This solution successfully drew water from high-salinity water up to 3.0 M through FO. When the IL solution cooled to room temperature, it spontaneously separated into a water-rich phase and an IL-rich phase: the water-rich phase was the produced water that contained a low IL concentration, and the IL-rich phase could be used directly as the draw solution in the next cycle of the FO process. The thermal stability, thermal-responsive solubility, and UV-vis absorption spectra of the IL were also studied in detail.

Magnetic thermoresponsive ionic nanogels as novel draw agents in forward osmosis

Magnetic thermosensitive ionic nanogels were prepared based on strong ionic monomer AMPS and thermosensitive monomer NIPAM via precipitation polymerization in the presence of Fe₃O₄ nanoparticles and investigated as draw solutes in FO.