Fluoride Removal from Brackish Groundwaters by Constant Current Capacitive Deionization (CDI)

Charging capacitive deionization (CDI) at constant voltage (CV) produces an effluent stream in which ion concentrations vary with time. Compared to CV, charging CDI at constant current (CC) has several advantages, particularly a stable and adjustable effluent ion concentration. In this work, the feasibility of removing fluoride from brackish groundwaters by single-pass constant-current (SPCC) CDI in both zero-volt and reverse-current desorption modes was investigated and a model developed to describe the selective electrosorption of fluoride and chloride. It was found that chloride is preferentially removed from the bulk solution during charging. Both experimental and theoretical results are presented showing effects of operating parameters, including adsorption/desorption current, pump flow rate and fluoride/chloride feed concentrations, on the effluent fluoride concentration, average fluoride adsorption rate and water recovery. Effects of design parameters are also discussed using the validated model. Finally, we describe a possible CDI assembly in which, under appropriate conditions, fluoride water quality targets can be met. The model developed here adequately describes the experimental results obtained and shows how change in the selected system design and operating conditions may impact treated water quality

Faradaic Reactions in Water Desalination by Batch-Mode Capacitive Deionization

Non-Faradaic (ion electrosorption) and Faradaic (oxidation–reduction) effects in a batch-mode capacitive deionization (CDI) system were investigated, with results showing that both effects were enhanced with an increase in charging voltage (0.5–1.5 V). Significant concentrations of hydrogen peroxide (H₂O₂) were observed with the generation of H₂O₂ initiated by cathodic reduction of O₂ with subsequent consumption occurring as a result of cathodic reduction of H₂O₂. A kinetic model of the Faradaic processes was developed and found to satisfactorily describe the variation in the steady-state concentration of H₂O₂ generated over a range of CDI operating conditions. Significant pH fluctuations were observed at higher charging voltages. While the occurrence of Faradaic reactions may well contribute to pH fluctuations and deterioration of electrode stability and performance, the presence of H₂O₂ could provide the means of inducing disinfection or trace contaminant degradation provided H₂O₂ could be effectively activated to more powerful oxidants (by, for example, ultraviolet irradiation)

Development of Redox-Active Flow Electrodes for High-Performance Capacitive Deionization

An innovative flow electrode comprising redox-active quinones to enhance the effectiveness of water desalination using flow-electrode capacitive deionization (FCDI) is described in this study. The results show that, in addition to carbon particle contact, the presence of the aqueous hydroquinone (H₂Q)/benzoquinone (Q) couple in a flowing suspension of carbon particles enhances charge transfer significantly as a result of reversible redox reactions of H₂Q/Q. Ion migration through the micropores of the flow electrodes was facilitated in particular with the desalination rate significantly enhanced. The cycling behavior of the quinoid mediators in the anode flow electrode demonstrated a relatively high stability at the low pH induced, suggesting that the mediator would be suitable for long-term operation