Selective ion removal in electrochemical processes

Research output: Thesisinternal PhD, WU

Abstract

Ion selectivity is regarded as the ability of a system to remove specific ionic species from multi-ionic mixtures. In this PhD thesis, two electrochemical systems, namely capacitive deionization (CDI) and electrodialysis (ED), were studied for their potential to achieve selective ion separations relevant to a wide variety of applications. One such application is the removal of the most common chemical contaminant found in groundwater: nitrate. This PhD thesis combines the study of the selective separation of monovalent ions in CDI and ED. The research comprises a systematic evaluation of variables that influence ion selectivity (ion concentration, applied electrical potential, presence of competing ions). Additionally, theoretical frameworks were developed to describe ion adsorption in porous carbon electrodes and ion transport in ion-exchange membranes (IEMs).
Overall, results obtained in this PhD thesis indicate that selectivity results from the interplay between the properties of the ions, the properties of the main constituents of the electrochemical system, i.e., carbon electrodes in CDI and IEMs in ED, and operational parameters.
This thesis shows that during electrosorption, ion selectivity is time-dependent, which indicates that the electrical double layers (EDLs) constantly rearrange their structure. Overall, at equilibrium (when there is no transport of ions through the carbon pores), NO3 selectivity does not depend on the initial ion concentration in the bulk solution, but it does depend on the applied cell voltage. It is assumed that selectivity is not the result of rearrangements of the hydration shell, e.g., partial loss of water molecules, of the ion with lower hydration energy, i.e., NO3. The reasoning behind this assumption is that the mean size of the carbon pores is larger than the hydration shells of the adsorbed ions.  
Ion selectivity was also studied in newly-designed heterogeneous anion-exchange membranes (AEMs). The membranes were fabricated using a functionalized (charged functional groups) polymeric binder and three ion-exchange resins with alkyl groups with different hydrophobicity, i.e., methyl, ethyl, and propyl substituents, respectively. The study highlights the influence of hydrophobicity on membrane selectivity. Increasing the length of the alkyl group, and therefore hydrophobicity, leads to an increase in the selectivity for NO3 over other monovalent and multivalent ions. In electrolyte solutions containing monovalent ions with similar hydrated size, selectivity can be related to the differences in hydration energy of the ions. However, for monovalent ions of different size, selectivity trends cannot be reported based on hydration energy. In this case, it is assumed that the chemical structure of the membrane influences the observed selectivity.
The performance of the home-made AEMs was further studied and compared with that of two commercial AEMs. Using a theoretical model, some strategies to enhance the NO3 selectivity in AEMs were explored. Theoretical results point out that suitable strategies are: to increase the chemical affinity or thickness, or to decrease the charge density in the membranes.
This thesis also shows a novel approach for the fabrication of IEMs, which involves the use of alginate and extracellular polymeric substances (EPS) extracted from anaerobic granular sludge. The membranes fabricated with these two biopolymers showed preferential transport of cations (current efficiency ~80%), and most importantly preferential transport of potassium (K+) over sodium (Na+). These results are promising and give an indication of the potential of biopolymers for the selective removal of ions.
Finally, the removal of ions from aqueous solutions was evaluated in a CDI cell consisting of wire-shaped carbon electrodes, unlike the commonly used CDI cell with flat carbon electrodes. A thin layer of ion-exchange membranes was coated on the surface of each electrode, which leads to an increase in the desalination capacity of the CDI system. This study shows that the type of solvent used in the fabrication of carbon electrodes impacts their salt adsorption capacity.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • van der Wal, Bert, Promotor
  • Porada, S., Co-promotor, External person
  • Dykstra, Jouke, Co-promotor
Award date19 Feb 2021
Place of PublicationWageningen
Publisher
Print ISBNs9789463956673
DOIs
Publication statusPublished - 19 Feb 2021

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