Continuous desalination via redox flow desalination using sodium 4-sulfonatooxy-2,2,6,6-tetramethyl-piperidine-1-oxyl (NaSO4-TEMPO)

Abstract

• NaSO 4 -TEMPO was used as an efficient redox mediator in redox flow desalination. • NaSO 4 -TEMPO enabled continuous desalination at high salt removal rate and charge efficiency. • The desalination performance of NaSO 4 -TEMPO and mechanisms under different operational conditions was investigated. • NaSO 4 -TEMPO successfully desalted 35000 ppm NaCl solution to 15 ppm at low energy consumption. Redox flow desalination has garnered increasing attention owing to its continuous operation and high salt removal rate. In this study, we perform an efficient continuous desalination for water of different salinities with sodium 4-sulfonatooxy-2,2,6,6-tetramethyl-piperidine-1-oxyl (NaSO 4 -TEMPO). NaSO 4 -TEMPO continuously produces freshwater via the reversible transformation of nitroxide radical/oxoammonium cation, and removes NaCl at a significantly higher rate at higher operating voltages. A 2000 ppm NaCl solution could be desalted to 762 ppm in a single pass. An increase in NaSO 4 -TEMPO concentration significantly improved the desalination performance owing to the increase in redox-incorporated mediators and more opportunities for redox reactions; a higher NaCl concentration in NaSO 4 -TEMPO enhances desalination performance by providing a higher conductivity and more charge-compensating species. In addition, NaSO 4 -TEMPO can desalinate flows with salinity of 500–5000 ppm in a single pass; in fact, NaSO 4 -TEMPO can desalt brine (35000 ppm) to freshwater (500 ppm) in the batch mode within 163 min. Under different operating conditions, the charge efficiency of NaSO 4 -TEMPO exceeds 90% at voltages higher than 0.8 V. Using NaSO 4 -TEMPO in a single-pass desalination system, a high sate removal capacity (∼367 μg/cm 2 /min) and a high charge efficiency (>89%) for over 134 h at 1.2 V are achieved. The aim of this study is to develop a safe and low-cost total-organic redox desalination system. The desalination behavior of the redox flow system is investigated, and the results can facilitate further optimization of the desalination structure and operating parameters.