Enhanced continuous desalination performance with iron-complexed malonate redox couples

Abstract

Redox mediators facilitate electrochemical deionization with higher energy conversion efficiency and open new opportunities for efficacious water desalination. Among the various redox-active species employed for electrochemical deionization, the typical anionic iron-based [Fe(CN)₆]^3−/4− redox couple is well-known for its high salt removal rate, compatibility at neutral pH, fast kinetics and low membrane permeability. However, [Fe(CN)₆]^3−/4− forms precipitates with the Mg²⁺, Ca²⁺ and Fe³⁺ ions present in seawater and poses safety issues due to the release of the highly toxic HCN(g) in an acidic environment. Herein, we propose the complexation of Fe(II/III) with the malonate ligand (i.e., [Fe(Mal)₂]^−/2− redox couple) to perform both redox flow desalination (RFD) and flow-electrode capacitive deionization (FCDI). In the FCDI architecture, carboxylated multi-walled carbon nanotubes (MWCNT-COOH) were mixed with the [Fe(Mal)₂]^−/2− redox couple, which served as the electrochemically active electrode. The average salt removal rate of the RFD and FCDI processes reached up to 4.43 and 4.84 μg cm⁻² s⁻¹ at an applied current density of 7 and 11 mA cm⁻², respectively. Furthermore, the ferrous malonate redox couple was tested for the desalination of seawater (feed: 35 000 ppm NaCl), which showed a reasonable 0.7-fold decrease in salinity. Our work presents the desalination performance of the [Fe(Mal)₂]^−/2− redox couple in an effort to develop appropriate redox couples beyond the commonly utilized redox couples in electrochemical deionization technology.