Desalination using polyelectrolyte hydrogels

When the gel is put into contact with aqueous salt solution, it absorbs a solution with the ion composition different from the original one. The absorbed solution can be easily squeezed out from the gel by means of sieve or microfiltration membrane. In our previous work we proposed a fully reversible desalination cycle made of compression and swelling steps, which can in principle work on ideal thermodynamic efficiency. In this work we simulate the desalination process using theoretical and coarse-grained models of gel and prove the concept by experiment. We used Monte Carlo and molecular dynamics molecular simulations in the reaction ensemble to predict the degree of ionization of the weak polyelectrolyte hydrogel when it is put in contact with salt solution, and calculate the salt partitioning between the gel and bulk salt solution. We constructed laboratory apparatus based on swelling and pressing cycles of the gel. First, we let the polyelectrolyte gel swell with salt solution of defined concentration. Then we press the gel and the liquid is released. Due to the ion exchange in polyelectrolyte hydrogel, this released liquid has lower salt concentration than the initial one. We measure the salinity of the solution before and after this procedure and we compare the results with theoretically obtained salt partitioning. We also measure the pressure applied on the gel and corresponding gel volume and compare these results with respective computational results

Water Desalination Using Polyelectrolyte Hydrogel: Gibbs Ensemble Modeling

Polyelectrolyte hydrogels can absorb a large amount of water across an osmotic membrane as a result of their swelling pressure. On the other hand, the insoluble cross-linked hydrogel network enables dewatering under the influence of external (thermal and/or mechanical) stimuli. Moreover, from a thermodynamic perspective, a polyelectrolyte hydrogel is already an osmotic membrane. These properties designate hydrogels as excellent candidates for use in desalination, at the same time avoiding the use of expensive membranes. In this article, we present our recent theoretical study of polyelectrolyte hydrogel usage for water desalination. Employing a coarse-grained model and the Gibbs ensemble, we modeled the thermodynamic equilibrium between the coexisting gel phase and the supernate aqueous salt solution phase. We performed a sequence of step-by-step hydrogel swellings and compressions in open and closed systems, i.e., in equilibrium with a large and with a comparably small reservoir of aqueous solution. The swelling in an open system removes ions from the large reservoir, whereas the compression in a closed system decreases the salt concentration in the small reservoir. We modeled this stepwise process of continuous decrease of water salinity from seawater up to freshwater concentrations and estimated the energy cost of the process to be comparable to that of reverse osmosis