Study of asymmetric capacitive deionization cells for water treatment applications

Resumo

Capacitive Deionization (CDI) is one of the emerging water technologies attracting interest in recent years. CDI works by removing ions present in water by applying a constant voltage or current between two electrodes immersed into solution. When the removal step is complete, the electrodes are regenerated by short-circuiting the system or reversing the sign of the voltage or current. During regeneration, the ions that were previously adsorbed on the electrodes undergo a desorption process and are returned to solution as a wastewater. One of the reasons for a high degree of interest in CDI research is the possibility of storing energy in the electrochemical double layer (EDL) of the electrodes while treating water. Hence, CDI systems might also be described as a non-ideal EDL capacitor that removes ions from water in the charging step and releases this energy during discharge. In addition to this attractive feature (saving energy while delivering clean water), CDI is a low-pressure method of water treatment in contrast with typical membrane technologies (Reverse Osmosis, Electrodyalisis). This results in a decrease in energy consumption. Moreover, aspects such as the high water recovery that diminishes the volume of brine, the low cost and high availability of carbon materials and the lack of posttreatment, that reduces the consumption of chemical compounds, are additional benefits of this technology. Thus, many studies have been performed in order to evaluate the potential of CDI being employed in a variety of applications such as: desalinating brackish waters, water softening and the removal of specific contaminants. In addition, extensive research has been performed focusing on the development of CDI electrodes having better performance with respect to the electrosorption of ions. In this thesis, new materials based on coating carbon substrates with nanoporous metal oxides developed by the Environmental Chemistry and Technology Program of University of Wisconsin-Madison were employed. Accordingly, the cathode was coated with a thin-film of nanoporous SiO2 whereas the anode with a nanoporous layer of aluminum oxide. This kind of configuration was named Asymmetric Capacitive Deionization (ACDI) in contrast with systems where the two electrodes are identical or can be described as being symmetric. In this thesis work, the application of this type of ACDI system for water treatment applications such as water softening and brackish water desalination has been evaluated. Firstly, the influence of metal oxide coatings on CDI performance was studied. This effect was evaluated in two different ways: material characterization (BET surface area, Pore Size Distribution (PSD), Microscopy) and electrochemical behavior (Cyclic Voltammetry (CV) and Zeta Potential analysis). Moreover, ion electrosorption mechanisms were investigated using methods of analysis specific for each of the ions being removed and by correlating this data to changes or lack of change in pH. This method allowed us to identify ion removal processes such as electrosorption in the EDL, specific adsorption on the metal oxides or carbon support. Furthermore, the effect of these different ion removal mechanisms on electrode regeneration was also studied. In this sense, different electrode regeneration strategies were attempted. In order to complete the research, the occurrence of faradaic (oxidation-reduction) reactions was examined. These can be parasitic in nature possibly reducing electrochemical efficiencies. Moreover, the impact of operational parameters such as applied voltage, the concentration and type of electrolyte in the inlet solution or the regeneration mechanisms on ion removal was widely analyzed using a medium size prototype (10 cells, 12 x 12 cm electrodes, 400 ml) reactor. These experiments included examining the symmetry of the removing of anions and cations, the occurrence of parasitic reactions and the effect of removal and regeneration on pH change. Besides the effect of these parameters on ion adsorption/desorption, an important part of the research was dedicated to the influence of those variables on the charge efficiency and the energy consumption of this ACDI device. As a final contribution, the effect of a long-term performance of an ACDI device was evaluated. Particular attention was paid to electrode stability (corrosion, surface acidification, electrode capacitance). This kind of analysis is essential if one wants to demonstrate the feasibility of CDI systems competing with other water treatment techniques in real world scenarios. The results from this thesis suggest that CDI systems outfitted with carbon electrodes coated asymmetrically with different metal oxides could be potentially applied for water softening as well as for brackish water desalination. In addition, this study has shown the ability of ACDI to be optimized by altering operational parameters according to the product specifications required for given treatment scenarios. Moreover, this thesis stresses the importance of CDI studies that are coupled to specific methods of analysis in coordination with pH measurements if one hopes to obtain useful information concerning ion removal mechanisms. Additionally, long-term experiments emphasize the role of the parasitic reactions with respect to the reduction of CDI performance and an increasing energy demand.