Physics

Geology

Electrode

Water softening

Membrane

Mechanics

Brackish water

Salinity

Oceanography

Engineering

Physical chemistry

Composite material

Chemistry

Metallurgy

Materials science

Desalination

Flow (mathematics)

Capacitive deionization

Softening

Divalent

Chemical engineering

Electrochemistry

Biochemistry

Environmental Science & Technology

While flow-electrode capacitive deionization (FCDI) is an emerging desalination technology, reduction in water hardness using this technology has so far received minimal attention. In this study, treatment of influents containing both monovalent and divalent cations using FCDI was carried out with flow-electrodes operated in short-circuited closed-cycle (SCC) configuration. Divalent Ca2+ cations were selectively removed compared to monovalent Na+ with the selectivity becoming dominant when the FCDI unit was operated at lower current densities and hydraulic retention times. Results showed that SCC FCDI operation was much more energy-efficient for brackish water softening compared to operation in isolated closed-cycle (ICC) mode, particularly with implementation of energy recovery. This finding was largely ascribed to (i) charge neutralization of the flow-electrodes in SCC configuration and (ii) regeneration of the active materials to maintain pseudo "infinite" capacity during electrosorption. In addition, mixing of the flow-electrodes in SCC operation significantly inhibited pH excursion in the flow-electrode with resultant alleviation of calcium precipitation on the carbon surface.

Short-Circuited Closed-Cycle Operation of Flow-Electrode CDI for Brackish Water Softening