Effect of temperature on the osmotic behavior of LCST type ionic liquid solutions as draw solutions in the forward osmosis process

The characteristics of temperature-responsive ionic liquids on the integrated operational effectiveness of water reclamation from semiconductor wastewater using forward osmosis

Large amounts of wastewater are produced from semiconductor manufacturing, and the production energy consumption has skyrocketed with its global demand in recent years. Forward osmosis (FO) provides unique merits in reclaiming the wastewater if suitable draw solutes with high water flux, low leakage, and limited energy requirement in regeneration are available. Two lower critical solution temperature-ionic liquids (LCST-ILs), tetrabutylphosphonium trimethylbenzensulfonate ([P4444][TMBS]) and tetrabutylphosphonium maleate ([P4444][Mal]) were synthesized and systematically assessed as recycled draw solutes in FO for the water reclamation from the wastewater of Si-ingot sawing. The water flux of [P4444][TMBS] and [P4444][Mal] was 4.12 and 2.67 LMH in the FO for the authentic sawing wastewater, respectively. The characteristics of relatively higher hydrophobic and the structure configuration ensure the lower thermal-stimulus separation energy (Ets) of the spent [P4444][Mal] solution and its relative ease in recycling because of the higher regeneration ratio. The more hydrophilic [P4444][TMBS] based draw solution (DS) exhibits higher osmotic pressure, which is beneficial for water filtration but leads to higher Ets. The estimated energy required for the integrated processes, including FO filtration for the sawing wastewater, thermal separation of draw solute, and the reclaimed water polishing, for the system with [P4444][Mal] as draw solutes is 14.22 kWh m-3. The value significantly reduced to 1.33 kWh m-3 if low-grade waste heat (<100 °C) was applied for the thermal separation.

Modeling and Elucidating the Behavior of a Thermoresponsive LCST Ionic Liquid

Desalination of seawater by forward osmosis is a technology potentially able to address the global water scarcity problem. The major challenge limiting its widespread practical application is the design of a draw solute that can be separated from water by an energetically efficient process and then reused for the next cycle. Recent experiments demonstrate that a promising draw solute for forward-osmosis desalination is tetrabutylphosphonium 2,4,6-trimethylbenzenesulfonate ([P4444][TMBS]). When mixed with water, this ionic liquid (IL) is thermoresponsive and exhibits a lower critical solution temperature (LCST), above which it phase-separates into an IL-rich phase and a water-rich phase. Elucidating the physical mechanism of the liquid-liquid phase separation, as well as rationally designing optimized derivatives, necessitates an accurate model to describe this and related ILs. In this paper, we resort to explicit-solvent all-atom molecular dynamics simulations and adopt AMBER-based force-field parameters for the cation whose partial charges were assigned by the RESP fitting procedure. Utilizing the same methodology, we parametrize the anion. The simulations' results indicate the IL/water mixture, at the experimental critical composition, can unambiguously phase-separate only when the partial charges of the ions are scaled down. Nevertheless, the best-performing charge scaling factor is found to be 0.95, a value much milder than those reported for ILs in neat phases. This can be explained by a diminished charge transfer, or induced dipoles, within the ions when the IL is in a mixture with water. With this charge scaling, the simulations reproduce well the LCST composition-temperature phase diagram, albeit overestimation of the critical temperature by 10 K. In particular, very good agreement is obtained for the composition of the two segregated phases. Estimation of viscosity points to IL/water mixture that is almost twice as viscous in simulations than that reported experimentally. Furthermore, we analyze changes in energy between different components in the mixture and find that the driving force for phase separation is, at least, enthalpic. Structural analyses of the ions and their interactions with water molecules corroborate the importance of the latter in mediating structural organizations of the anions, as well as in strengthening the interactions between the cations.

Design, synthesis, and application of thermally responsive draw solutes for sustainable forward osmosis desalination: A review

Forward osmosis (FO) is an emerging sustainable desalination technology; however, it is not a stand-alone process and requires an additional step to recover the water or regenerate the draw solute (DS), making it energy extensive. Therefore, incorporating inexpensive energy sources for DS regeneration is a viable solution to compete with reverse osmosis desalination technology. Hence, selecting suitable DS and its regeneration became a crucial research focus in FO desalination. Among various DSs reported, thermally responsive DSs (TRDS) provide an opportunity to integrate low-grade energy sources for DS regeneration. Utilizing such inexpensive energy will reduce fossil fuel energy demand, lower the cost of desalination, and minimize the carbon footprint. Hence, this review explores the TRDS for FO-based desalination with its design, synthesis, and applications. The manuscript has discussed the classification and selection criteria for the DSs, and how traditional and new-generation TRDSs are designed and synthesized from cationic and anionic moieties of ionic liquids, hydrogels, and other chemicals. The manuscript has also given importance to design criteria such as osmotic strength, viscosity, toxicity, and thermal stability for TRDSs. Furthermore, a detailed discussion on the FO performance, energy, and economic aspects of TRDSs has been reviewed, along with a discussion on the possible low-grade energy sources for the recovery of TRDS. Finally, the challenges and future directions for TRDSs have been discussed to drive FO toward sustainable desalination technology.

Thermoresponsive Ionic Liquid with Different Cation-Anion Pairs as Draw Solutes in Forward Osmosis

We synthesized various phosphonium- and ammonium-based ionic liquids (ILs), using benzenesulfonate (BS) and 4-methylbenzenesulfonate (MBS) to establish the criteria for designing an ideal draw solute in a forward osmosis (FO) system. Additionally, the effects of monocationic, dicationic, and anionic species on FO performance were studied. Monocationic compounds ([P₄₄₄₄][BS], [P₄₄₄₄][MBS], [N₄₄₄₄][BS], and [N₄₄₄₄][MBS]) were obtained in one step via anion exchange. Dicationic compounds ([(P₄₄₄₄)₂][BS], [(P₄₄₄₄)₂][MBS], [(N₄₄₄₄)₂][BS], and [(N₄₄₄₄)₂][MBS]) were prepared in two steps via a Menshutkin SN₂ reaction and anion exchange. We also investigated the suitability of ILs as draw solutes for FO systems. The aqueous [P₄₄₄₄][BS], [N₄₄₄₄][BS], [N₄₄₄₄][MBS], and [(N₄₄₄₄)₂][BS] solutions did not exhibit thermoresponsive behavior. However, 20 wt% [P₄₄₄₄][MBS], [(P₄₄₄₄)₂][BS], [(P₄₄₄₄)₂][MBS], and [(N₄₄₄₄)₂][MBS] had critical temperatures of approximately 43, 33, 22, and 60 °C, respectively, enabling their recovery using temperature. An increase in IL hydrophobicity and bulkiness reduces its miscibility with water, demonstrating that it can be used to tune its thermoresponsive properties. Moreover, the FO performance of 20 wt% aqueous [(P₄₄₄₄)₂][MBS] solution was tested for water flux and found to be approximately 10.58 LMH with the active layer facing the draw solution mode and 9.40 LMH with the active layer facing the feed solution.

Effect of the Alkyl Chain Length on Assessment as Thermo-Responsive Draw Solutes for Forward Osmosis

A series of thermo-responsive tetrabutylphosphonium 1-alkanesulfonates (abbreviated as [P₄₄₄₄][C _n S], n = 6, 8, 10, and 12), where n is the number of carbon atoms in the alkyl group on the 1-alkanesulfonate anion, were prepared by an ion-exchange reaction to investigate their potential ability toward the application of draw solutes in forward osmosis (FO). We systematically studied the recovery properties and FO performance of [P₄₄₄₄][C _n S]. This series exhibited lower critical solution temperature (LCST) characteristics, which offer a clear advantage of being energy-efficient for recovering draw solutes; however, [P₄₄₄₄][C₆S] was only observed at 20 wt %. The LCSTs of the 20 wt % [P₄₄₄₄][C₆S], [P₄₄₄₄][C₈S], [P₄₄₄₄][C₁₀S], and [P₄₄₄₄][C₁₂S] draw solutions were approximately 83, 54, 49, and 56 °C, respectively. Moreover, when the orientation of the active layer was heading toward the draw solution (AL-DS mode), the water and reverse solute flux of [P₄₄₄₄][C₁₀S] were about 1.58 LMH and 0.81 gMH, respectively, at 20 wt % aqueous solutions. When the membrane was used in the active layer facing the feed solution (AL-FS) system, the water and reverse solute flux of [P₄₄₄₄][C₁₀S] were approximately 0.71 LMH and 0.38 gMH, respectively, at 20 wt % aqueous solutions. Thus, this study is the first to examine the structural transformations of the bulkier alkyl group on the sulfonate anion moiety and its feasibility as the new draw solute for the FO system.

Thermoresponsive Ionic Liquid/Water Mixtures: From Nanostructuring to Phase Separation

The thermodynamics, structures, and applications of thermoresponsive systems, consisting primarily of water solutions of organic salts, are reviewed. The focus is on organic salts of low melting temperatures, belonging to the ionic liquid (IL) family. The thermo-responsiveness is represented by a temperature driven transition between a homogeneous liquid state and a biphasic state, comprising an IL-rich phase and a solvent-rich phase, divided by a relatively sharp interface. Demixing occurs either with decreasing temperatures, developing from an upper critical solution temperature (UCST), or, less often, with increasing temperatures, arising from a lower critical solution temperature (LCST). In the former case, the enthalpy and entropy of mixing are both positive, and enthalpy prevails at low T. In the latter case, the enthalpy and entropy of mixing are both negative, and entropy drives the demixing with increasing T. Experiments and computer simulations highlight the contiguity of these phase separations with the nanoscale inhomogeneity (nanostructuring), displayed by several ILs and IL solutions. Current applications in extraction, separation, and catalysis are briefly reviewed. Moreover, future applications in forward osmosis desalination, low-enthalpy thermal storage, and water harvesting from the atmosphere are discussed in more detail.

Using Reverse Osmosis Membrane at High Temperature for Water Recovery and Regeneration from Thermo-Responsive Ionic Liquid-Based Draw Solution for Efficient Forward Osmosis

Forward osmosis (FO) membrane process is expected to realize energy-saving seawater desalination. To this end, energy-saving water recovery from a draw solution (DS) and effective DS regeneration are essential. Recently, thermo-responsive DSs have been developed to realize energy-saving water recovery and DS regeneration. We previously reported that high-temperature reverse osmosis (RO) treatment was effective in recovering water from a thermo-responsive ionic liquid (IL)-based DS. In this study, to confirm the advantages of the high-temperature RO operation, thermo-sensitive IL-based DS was treated by an RO membrane at temperatures higher than the lower critical solution temperature (LCST) of the DS. Tetrabutylammonium 2,4,6-trimethylbenznenesulfonate ([N4444][TMBS]) with an LCST of 58 °C was used as the DS. The high-temperature RO treatment was conducted at 60 °C above the LCST using the [N4444][TMBS]-based DS-lean phase after phase separation. Because the [N4444][TMBS]-based DS has a significantly temperature-dependent osmotic pressure, the DS-lean phase can be concentrated to an osmotic pressure higher than that of seawater at room temperature (20 °C). In addition, water can be effectively recovered from the DS-lean phase until the DS concentration increased to 40 wt%, and the final DS concentration reached 70 wt%. From the results, the advantages of RO treatment of the thermo-responsive DS at temperatures higher than the LCST were confirmed.