Peter Kovalsky

Fluoride and nitrate removal from brackish groundwaters by batch-mode capacitive deionization.

Capacitive deionization (CDI) is an emerging water desalination technology in which pairs of porous electrodes are electrically charged to remove ionic species from water. In this work, the feasibility of fluoride and nitrate removal from brackish groundwaters by batch-mode CDI was investigated. Initially, the effects of flow rate, initial fluoride concentration, and initial coexisting NaCl concentration on fluoride removal were studied. The steady-state fluoride concentration declined as the initial fluoride concentration decreased while initial NaCl concentration remained constant. Due to the competitive electrosorption between fluoride and chloride for limited pore surface sites, a higher initial chloride concentration resulted in a higher equilibrium dissolved fluoride concentration. A simplified one-dimensional transport model for dual anions was developed and found to reliably describe the dynamic process of removal of both fluoride and chloride ions in CDI cells over a range of well-defined operating conditions. Based on the ability of the model to describe fluoride removal, it was extended to description of nitrate removal from brackish groundwaters and also found to perform well. Thus, the approach to description of ion removal, at least in batch studies, appears robust and should assist in optimization of design and operating conditions such that optimal removal of trace ionic species is achieved even when high background concentrations of salt are present.

Comparison of Faradaic reactions in capacitive deionization (CDI) and membrane capacitive deionization (MCDI) water treatment processes

Capacitive deionization (CDI) and membrane capacitive deionization (MCDI) are the most common cell architectures in the use of CDI for water treatment. In this work, the Faradaic reactions occurring in batch-mode CDI and MCDI processes were compared by investigating the variation of H2O2 and dissolved oxygen (DO) concentrations, pH, conductivity and current during charging and discharging under different charging voltages. During charging, the H2O2 concentration in CDI increased rapidly and then decreased while almost no H2O2 was generated in MCDI due to the inability of oxygen to penetrate the ion exchange membrane. Chemical kinetic models were developed to quantitatively describe the variation of H2O2 concentration and found to present satisfactory descriptions of the experimental data. The pH drop during charging could be partially explained by Faradaic reactions with proton generation associated with oxidation of the carbon electrodes considered to be the major contributor. The electrode potentials required for the induction of Faradaic reactions were analyzed with this analysis providing robust thermodynamic explanations for the occurrence of carbon oxidation at the anode and H2O2 generation at the cathode during the ion adsorption process. Finally, electrochemically-induced ageing of the carbon electrodes and the resulting performance stability were investigated. The findings in this study contribute to a better understanding of Faradaic reactions in CDI and MCDI and should be of value in optimizing CDI-based technologies for particular practical applications.

Investigation of fluoride removal from low-salinity groundwater by single-pass constant-voltage capacitive deionization

Capacitive deionization (CDI) is attracting increasing attention as an emerging technology for the facile removal of ionic species from water. In this work, the feasibility of fluoride removal from low-salinity groundwaters by single-pass constant-voltage CDI was investigated and a model developed to describe the dynamic fluoride electrosorption behavior. Effects of operating parameters including charging voltage and pump flow rate as well as impact of fluoride and chloride feed concentrations on the effluent fluoride concentration and equilibrium fluoride adsorption capacity were studied and the obtained data used to validate the model. Using the validated model, the effects of various design parameters, including arrangement of multiple CDI cells, on fluoride removal were assessed. Single-pass constant-voltage CDI was found to be effective in removing fluoride from low-salinity groundwaters but, as expected, removal efficiency was compromised in waters of high salinity. The relatively simple electrosorption model developed here provided a satisfactory description of both fluoride removal and current evolution and would appear to be a useful tool for prediction of CDI performance over a range of operating conditions, cell arrangements and feed water compositions though scope for model improvement exists.

Integration of photovoltaic energy supply with membrane capacitive deionization (MCDI) for salt removal from brackish waters

Capacitive de-ionization (CDI) systems are well-known for their low energy consumption making them suitable for applications powered by renewable energy. In this study, CDI technology is, for the first time, integrated with a suitably-scaled, stand-alone, renewable power system comprising photovoltaic panels and battery storage. Guidelines for designing and sizing such power systems are proposed including determining electrode charging current, PV panels and battery capacity. A 1 kW pilot plant was designed, constructed and operated to verify the proposed guidelines. Using the pilot plant, the total energy consumption of the system has been evaluated with different electrode charging currents and influent flow rates and the relationship between these parameters analyzed. This analysis has enabled the development of practical design guidelines for bulk water treatment with MCDI electrodes. The results of this study show that use of photovoltaic-powered MCDI water treatment, particularly when combined with energy recovery, is competitive against more mature water-treatment technologies for particular applications and at particular locations.

Investigation of pH-dependent phosphate removal from wastewaters by membrane capacitive deionization (MCDI)

Membrane capacitive deionization (MCDI) is a promising technology for the removal of charged species from aqueous feed streams. In this study, the effects of initial pH on the removal of phosphate (P) species by MCDI were investigated. Two operational modes (constant voltage (CV) and constant current (CC)) were adopted with both the efficacy of P removal and the energy consumption between CV and CC modes compared at different initial pH values. Results indicated that, for feed water only containing phosphate species, the optimized P removal performance was achieved at pH 5.0–6.0 where the monovalent P species (i.e., H2PO4−) dominated in CC mode more energy-efficiently in terms of P removal than in CV mode. The competitive electrosorption between Cl− and P over a range of initial pH values was examined in CC mode with results indicating that at pH 5.0 and 6.0, where the monovalent P species (i.e., H2PO4−) dominated, Cl− was preferentially electrosorbed, presumably because of its smaller hydrated radius, while under more alkaline conditions (pH 8.0 and 9.0), divalent P species (i.e., HPO42−) were more selectively removed due to their charge. A Nernst–Planck equation, in which the ion flux ratio was expressed as a function of diffusion coefficient and charge, provides a good description of the ion selectivity between Cl− and P when the initial concentration of these two ions are equal.