Novel flow electrodes for increased ion removal rate and energy consumption reduction in flow-electrode capacitive deionization

• Autor(a) principal: Nelson Henrique Jardim Freire
• Data de Publicação: 2023
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Repositório Institucional da UFMG
• Texto Completo: http://hdl.handle.net/1843/64465; https://orcid.org/0000-0002-6519-0648
• topic: Flow electrodes; Flow-electrode capacitive deionization; Electrochemical separation; Desalination; Engenharia de minas; Tecnologia mineral; Hidrometalurgia; Aguas residuais - Purificação; Eletrodos

The separation and removal of ions from water is essential to comply with increasingly restricted wastewater pollutant emissions and water reuse. Furthermore, new technologies are necessary to provide the locals with quality water in drought-affected areas. Finally, ions usually discharged in the wastewater can be recovered as valuable by-products in industrial operations. Flow electrode capacitive deionization-FCDI is one of the techniques attracting attention due to potential advantages relative to the established technologies, such as specific ion removal, continuous operation, and the treatment of high salinity water streams. One major drawback of FCDI is the higher electrical resistance of the flow electrode compared to typical static electrodes. Few attempts to use other materials, such as Prussian-Blue Analogs, resulted in even worse desalination performance due to the lower conductivity of these materials relative to carbon-based materials. Hence, this work focused on reducing energy consumption and increasing the productivity of FCDI by improving the charging transfer in the system. The desalination performance achieved with the modified electrodes is compared to that of typical FCDI with activated carbon (AC) slurries. The application of nickel hexacyanoferrate (NiHCF) as an active material in the flow electrode was studied along with ferricyanide [Fe(CN)6]3- as a redox mediator. The flow electrodes with NiHCF and a redox mediator showed twice the current compared to typical flow electrodes with AC. Compared to the system where the redox mediator is also added to the AC flow electrode, the system with NiHCF and redox mediator also showed higher and more stable currents during desalination. Due to the improvement of adding a redox mediator to AC- flow electrodes, iron chloride (Fe-Cl), iron citrate (Fe-Cit), and activated carbon were also investigated. It is demonstrated that even at a concentration of 0.05 M Fe-Cl, the desalination rate is higher than AC-only slurries. Also, a synergetic effect is observed when AC and Fe-Cl are mixed. The desalination rate is enhanced up to 100% for the AC - 0.2 M of Fe-Cl mixture, and the addition of ferric citrate at concentrations of 0.2 M increased desalination by 23%. It is demonstrated that adding redox couples is a relevant strategy to improve the FCDI performance, and this will likely make it more competitive against other technologies.