Abstrakt
We model a process of seawater desalination that employs compression of a weak polyelectrolyte hydrogel as the drawing force for ion transfer. We model seawater as an aqueous solution containing mono- and divalent ions, Na(+), Cl(-), and Ca(2+).
We demonstrate that the compression of hydrogel in equilibrium with a bath containing such a solution creates two types of ion flow. The first type is the exchange of divalent Ca(2+) by monovalent Na(+).
The charge density during this process is preserved, so that per one absorbed Ca(2+) two Na(+) are released. The second type is the desalination itself which leads to the uptake of Na(+) and Cl(-) pairs from the surrounding solution.
The first flow predominates when Ca(2+) are in excess; however, as their fraction in the solution decreases, desalination flow becomes more significant. We have shown that although the gel is a weak polyelectrolyte, its compression in the presence of divalent ions does not influence its ionization degree.
This is caused by a significant screening of electrostatic interactions by divalent ions. When the density of Ca(2+) is small, the gel compression may lead to its discharge and to the release of counterions, which shows up in an increase of the solution ionic strength.