Fluoride Removal Using Capacitive Deionization (CDI) in Water Treatment

: (Alternative Format Thesis)

Abstract

Water scarcity is a global issue caused by climate change, industrialisation, rapid population growth, and energy shortages. Desalination has emerged as an alternative solution to ease the water crisis. Capacitive deionization (CDI) is a promising desalination technology and is gaining increasing attention due to its low environmental impact, high water recovery, high energy efficiency, and simple operation and regeneration. CDI has been reported as a feasible method to remove fluoride, whose excessive intake can lead to serious public health concerns such as dental and skeletal fluorosis. Therefore, efficient fluoride removal in CDI is crucial for water treatment.

The feasibility of fluoride removal in CDI is examined in this thesis, along with information on the electrochemical properties of electrode materials, the influence of natural characteristics, the effectiveness of different commercial membranes in the removal of fluoride, and the mechanisms of selective fluoride removal by fluoride selective electrodes.

Firstly, a non-destructive method to characterise activated carbon fibre (ACF) based materials is established, and electrochemical impedance spectroscopy (EIS) results are linked to CDI desalination performance. The study reveals that the weaving patterns of ACF significantly affect the electrochemical properties and desalination performance. The volume-salt adsorption capacities in CDI are found to be related to the volume-specific capacities in EIS.

Secondly, the effects of different substances present in water on fluoride removal in membrane capacitive deionization (MCDI) are investigated. The study shows that dissolved inorganic carbon (DIC: HCO3- and CO32-) has a negative effect on fluoride removal, but the DIC species act as a buffering agent, keeping the pH stable. A small amount of dissolved organic carbon (DOC: humic acid) does not affect fluoride removal and does not cause fouling in membrane capacitive deionization (MCDI).

Thirdly, the fluoride removal in CDI and MCDI with six commercial membranes is compared in a binary solution containing Cl-. The study demonstrates that while using a membrane could improve both fluoride and chloride adsorption capacities, it cannot selectively remove fluoride in CDI or MCDI. The different pH changes in CDI and MCDI during the adsorption and desorption processes revealed the presence of different Faradaic reactions in CDI and MCDI.

Lastly, the fluoride-selective electrode materials (metal oxides modified ACF) are synthesised via the atomic layer deposition method and drop-coating method, respectively. These materials are characterised in terms of their specific surface area, pore size distribution, surface morphology, crystallinity, chemical structure, chemical bonding, and electronic structure of the atoms before and after electrosorption and desorption. The fluoride selective mechanisms are investigated.

Overall, this thesis contributes to the development of efficient and sustainable water treatment technologies. It highlights the importance of understanding the electrochemical properties of electrode materials, the impact of water characteristics on CDI performance, and the efficiency of different commercial membranes for fluoride elimination. By synthesising selective fluoride removal electrode materials and understanding their mechanisms of selective removal, this thesis has the potential to help mitigate the adverse effects of excessive fluoride intake on human health.

Date of Award	4 Dec 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Davide Mattia (Supervisor), Frank Marken (Supervisor) & Junjie Shen (Supervisor)