Investigation of the interaction between sulfonate-based polyelectrolyte hydrogels and divalent ions for use in seawater desalination

In this study, the interaction of sulfonate-based polyelectrolyte hydrogels with divalent ions is explored to assess their potential for membrane-free desalination. Previous studies have demonstrated that polyelectrolyte hydrogels can be applied for desalination purpose as they behave similarly to semipermeable membranes. [1-6] The underlying physical principle is the Gibbs-Donnan equilibrium, describing the uneven distribution of mobile ions between gel phase and supernatant solution due to fixed charges on the polymer backbone. [7] Consequently, the counterions within the gel remain confined, and ions of the same charge from the external solution are excluded—a phenomenon referred to as salt rejection—resulting in a lower ion concentration within the gel phase. [1] By gradually deswelling the hydrogels through external stimuli or mechanical force, salt-deficient water can be extracted. [1,6] Polyacrylic acid (PAAc) hydrogels have been used to validate this principle, as they are inexpensive, widely used and exhibit a high charge density along the polymer chain. However, a key limitation of PAAc hydrogels is their strong electrostatic interaction with divalent ions in seawater, which significantly reduces their swelling capacity. [8,9] To overcome this limitation, sulfonate-based polyelectrolyte hydrogels are investigated as an alternative, aiming to enhance ion rejection while maintaining adequate swelling behaviour for effective desalination. Therefore, hydrogels with varying degree of crosslinking (1-5 mol%) and degree of ionization (25-75 mol%) based on different sulfonate monomers are synthesized and their swelling behaviour in salt solutions with varying concentrations (MgCl₂, CaCl₂, and NaCl) and artificial seawater is explored and compared to that of a PAAc hydrogel reference system. In addition, salt rejection measurements are performed to assess the desalination efficiency of these hydrogels.

References

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