An electrochemical system has a first reservoir receiving a feed stream. The feed stream includes a solvent and a solute different than the salt. A second reservoir receives a brine stream with a higher salt concentration higher than the feed stream. Electrodes contact a loop of redox-active electrolyte material causing reversible redox reactions. The reactions cause the loop to accept a first ion from the salt in the first reservoir and drive a second ion into the brine stream in the second reservoir. Three ionic exchange membranes of alternating type define the first and second reservoirs. A concentrate stream is output from the first reservoir, the concentrate stream having a second solute concentration greater than the first solute concentration.