Forward osmosis with direct contact membrane distillation using tetrabutylphosphonium p-toluenesulfonate as an effective and safe thermo-recyclable osmotic agent for seawater desalination

On the evaluating membrane flux of forward osmosis systems: Data assessment and advanced intelligent modeling

As an emerging desalination technology, forward osmosis (FO) can potentially become a reliable method to help remedy the current water crisis. Introducing uncomplicated and precise models could help FO systems' optimization. This paper presents the prediction and evaluation of FO systems' membrane flux using various artificial intelligence-based models. Detailed data gathering and cleaning were emphasized because appropriate modeling requires precise inputs. Accumulating data from the original sources, followed by duplicate removal, outlier detection, and feature selection, paved the way to begin modeling. Six models were executed for the prediction task, among which two are tree-based models, two are deep learning models, and two are miscellaneous models. The calculated coefficient of determination (R2 ) of our best model (XGBoost) was 0.992. In conclusion, tree-based models (XGBoost and CatBoost) show more accurate performance than neural networks. Furthermore, in the sensitivity analysis, feed solution (FS) and draw solution (DS) concentrations showed a strong correlation with membrane flux. PRACTITIONER POINTS: The FO membrane flux was predicted using a variety of machine-learning models. Thorough data preprocessing was executed. The XGBoost model showed the best performance, with an R2 of 0.992. Tree-based models outperformed neural networks and other models.

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.

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.

Influence of the anionic structure and central atom of a cation on the properties of LCST-type draw solutes for forward osmosis

Thermo-responsive ionic compounds were synthesized to examine if they have a powerful ability to draw solutes for forward osmosis (FO). The investigated compounds were tetrabutylammonium benzenesulfonate, tetrabutylphosphonium benzenesulfonate, tetrabutylammonium 2-naphthalenesulfonate, and tetrabutylphosphonium 2-naphthalenesulfonate (abbreviated as [N₄₄₄₄][BS], [P₄₄₄₄][BS], [N₄₄₄₄][NS], and [P₄₄₄₄][NS]). The lower critical solution temperature (LCST) characteristics of the materials that formed the monocyclic aromatic compound [BS] were not confirmed; however, the LCSTs of others that formed the bicyclic aromatic compound [NS] were confirmed to be approximately 37 °C ([N₄₄₄₄][NS]) and 19 °C ([P₄₄₄₄][NS]) at 20 wt% in aqueous solutions; this is valued in reducing the energy required for recovery of the draw solute. In addition, it suggests that ammonium-based ionic compounds have a higher recovery temperature than phosphonium-based ionic compounds. When an active layer was oriented to a draw solution (AL-DS mode) and using 20 wt% aqueous [N₄₄₄₄][NS] draw solution at room temperature, water and reverse solute fluxes were about 3.07 LMH and 0.58 gMH, respectively. Thus, this is the first study to investigate structural transformations of the anion and central atom of the cation and to examine prospective draw solutes of the FO system in this series.

Thermo-responsive ionic compounds having lower critical solution temperature were utilized as a draw solute for eco-sustainable forward osmosis.

Forward Osmosis Membranes: The Significant Roles of Selective Layer

Forward osmosis (FO) is a promising separation technology to overcome the challenges of pressure-driven membrane processes. The FO process has demonstrated profound advantages in treating feeds with high salinity and viscosity in applications such as brine treatment and food processing. This review discusses the advancement of FO membranes and the key membrane properties that are important in real applications. The membrane substrates have been the focus of the majority of FO membrane studies to reduce internal concentration polarization. However, the separation layer is critical in selecting the suitable FO membranes as the feed solute rejection and draw solute back diffusion are important considerations in designing large-scale FO processes. In this review, emphasis is placed on developing FO membrane selective layers with a high selectivity. The effects of porous FO substrates in synthesizing high-performance polyamide selective layer and strategies to overcome the substrate constraints are discussed. The role of interlayer in selective layer synthesis and the benefits of nanomaterial incorporation will also be reviewed.

EDTA-Na2 as a recoverable draw solute for water extraction in forward osmosis

Regeneration and reuse of draw solute (DS) is a key challenge in the application of forward osmosis (FO) technologies. Herein, EDTA-Na₂ was studied as a recoverable DS for water extraction by taking advantages of its pH-responsive property. The FO system using EDTA DS achieved a higher water flux of 2.22 ± 0.06 L m^-2 h^-1 and a significantly lower reverse salt flux (RSF) of 0.06 ± 0.01 g m^-2 h^-1, compared to that with NaCl DS having either the same DS concentration or the same Na⁺ concentration. The suitable pH range for the application of EDTA DS was between 4.0 and 10.5. A simple recovery method via combined pH adjustment and microfiltration was employed to recover EDTA DS and could achieve the recovery efficiency (at pH 2) of 96.26 ± 0.48%, 97.13 ± 1.03% and 98.56 ± 1.40% by using H₂SO₄, H₃PO₄ and HCl, respectively. The lowest acid cost for DS recovery was estimated from 0.0012 ± 0.0001 to 0.0162 ± 0.0003 $ g^-1 by using H₂SO₄. The recovered EDTA DS could be reused in the subsequent FO operation and the overall recovery efficiency was 94.4% for four reuse cycles. These results have demonstrated the feasible of EDTA-Na₂ DS and a potentially cost-effective recovery approach, and encouraged further exploration of using EDTA-based compounds as a draw solute for FO applications.

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.

Faradic capacitive deionization (FCDI) for desalination and ion removal from wastewater

Capacitive deionization (CDI) is one of the emerging desalination technologies that attracted much attention in the last years as a low-cost, energy-efficient, and environmentally-friendly alternative to other desalination technologies, such as multi-stage flash desalination (MSF) and multiple effect distillation (MED). The implementation of faradaic electrode materials is a promising method for enhancing CDI systems' performance by achieving higher salt removal characteristics, lower energy consumption, and better ion selectivity. Therefore, a novel CDI technology named Faradaic CDI (FCDI) that implements faradaic electrode materials arose as a high-performance CDI cell design. In this work, the application of FCDI cells in desalination and wastewater treatment systems is reviewed. First, the progress done on using various FCDI systems for saline water desalination is summarized and discussed. Next, the application of FCDI in wastewater treatment applications and selective ion removal is presented. A thorough comparison between FCDI and conventional carbon-based CDI is carried out in terms of working principle, electrode material's cost, salt removal performance, energy consumption, advantages, and disadvantages. Finally, future research consideration regarding FCDI technology is included to drive this technology closer towards practical application.