Salt partitioning in ionized, thermo-responsive hydrogels: perspective to water desalination

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.

Swelling of Thermo-Responsive Gels in Aqueous Solutions of Salts: A Predictive Model

The equilibrium degree of swelling of thermo-responsive (TR) gels is strongly affected by the presence of ions in an aqueous solution. This phenomenon plays an important role in (i) the synthesis of multi-stimuli-responsive gels for soft robotics, where extraordinary strength and toughness are reached by soaking of a gel in solutions of multivalent ions, and (ii) the preparation of hybrid gels with interpenetrating networks formed by covalently cross-linked synthetic chains and ionically cross-linked biopolymer chains. A model is developed for equilibrium swelling of a TR gel in aqueous solutions of salts at various temperatures T below and above the critical temperature at which collapse of the gel occurs. An advantage of the model is that it involves a a small (compared with conventional relations) number of material constants and allows the critical temperature to be determined explicitly. Its ability (i) to describe equilibrium swelling diagrams on poly(N-isopropylacrylamide) gels in aqueous solutions of mono- and multivalent salts and (ii) to predict the influence of volume fraction of salt on the critical temperature is confirmed by comparison of observations with results of numerical simulation.