Salt Partitioning in Cationic Thermoresponsive Hydrogels for Model-Seawater Desalination

Charged hydrogels partially reject salt ions during swelling in a salt solution reservoir. This feature suggests applicability for the separation of salt ions from saline water for water desalination in a membrane-free forward osmosis process. In this work, model charged cationic and thermoresponsive hydrogels are prepared and their potential for desalination of model seawater containing both monovalent and divalent ions with concentrations of 0.11-3.5 wt% is investigated. The recovery of adsorbed water, after partial salt rejection, is achieved by heating the hydrogels. The salt rejection increases upon increase of the charge density of the hydrogels and diminishes by increase of the concentration of the salt solution. In addition, the salt rejection during deswelling of the hydrogels is in the opposite of the overall process target, adding another challenge on the efficiency of the approach. Besides the experimental results, equations based on the Donnan theory are derived to predict the salt rejection of hydrogels during swelling and deswelling processes. While the theoretical values deviate from the experimental ones, the theory predict the overall trend well. Both experimental and theoretical results confirm that this approach has considerable potential for the desalination of saline water at low concentrations such as brackish waters.