Continuous Desalination with Metal-free Redox-mediator
Deionization using electrochemical redox reactions has been the latest desalination technique potentially with a high salt removal capacity and lower energy cost due to capacity of electrode materials and electrochemical energy release. Herein, we presented a metal-free continuous electro-dialysis desalination technology, which can directly desalt high concentration of feed to drinking water level during continuous treatment with an unlimited salt removal capacity. The device consists of recirculating 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) stream between flowing positive and negative electrodes in series, and salt feed stream. During the charge process, the flowing TEMPO was oxidized at the positive reservoir, where anion in feed was captured through anion exchange membrane while the TEMPO recovery at the negative reservoir drives the cation removal in feed through cation exchange membrane by its reduction. Due to the redox reaction of TEMPO between the cyclical flow electrode streams, it acts as “catalyst” function while the salt ions in middle feed are continuously extracted. Our experiments demonstrate that the feed water with a 5860 ppm salt concentration can be directly desalted into 136 ppm, a drinking water level for a single batch treatment during the continuous charge process (salt removal of up to 97.7%). Furthermore, a three-membrane device is demonstrated to stop the salt concentration increase in TEMPO electrode reservoirs, realizing fully continuous cycling of TEMPO catalysts without any salt change in electrode reservoirs. Our research will supply a route for metal-free continuous desalination based on the molecular redox mediator of electrode material, and can be applied directly in open air environment and room temperature.