Systems and methods for use of water with switchable ionic strength

A method for removing a solute from an aqueous solution or concentrating a dilute aqueous solution, comprising the steps of:

• (a) contacting a switchable water comprising water and an additive switchable between a first form and a second form, wherein said second form of the additive is water-miscible or water soluble and includes at least one ionized functional group that is neutral in said first form of the additive, such that switching the additive from the first form to the second form increases the ionic strength of the switchable water, with an ionizing trigger to ionize at least one functional group in the additive and thereby increase the ionic strength of the switchable water;
• wherein the additive is a polymer and wherein the at least one functional group is an amine;
• (b) providing a semi-permeable membrane that is selectively permeable for water and has on one side a draw solution that comprises said switchable water, wherein the step of contacting the switchable water with the ionizing trigger to switch the additive to the second form is performed before or after association of the switchable water with the semi-permeable membrane to increase the osmotic pressure of the draw solution;
• (c) contacting the other side of the semi-permeable membrane with a feed stream of the aqueous solution to permit water to flow from the aqueous solution through the semi-permeable membrane into the increased ionic strength draw solution;
• and
• (d) optionally, removing the additive from the resulting diluted draw solution.