Ion transport through microgel-modified membrane surfaces

  • Ionentransport durch mikrogel-modifizierte Membranoberflächen

Roghmans, Florian Georg; Wessling, Matthias (Thesis advisor); Lammertink, Rob G. H. (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020

Abstract

In membrane separation processes, the main limitations to mass transfer take place at the membrane-solution interfaces. Consequently, the modification of membrane surfaces is an effective and simple means to tailor the membranes’ performance. This thesis aims at the improvement of ion transfer through two different types of membranes via the deposition of microgels on their surface. The potential of microgels to influence ion transport in deionization processes is novel and so far unexplored. Within this work, cation-exchange membranes are modified for the use in electrodialysis. In addition to that, microgels are used to produce a widely unknown membrane class, charge mosaic membranes, for new and efficient pressure-driven water desalination processes. Confluent monolayers of positively charged microgels deposited on cation-exchange membranes enhanced the membranes’ monovalent ion selectivity. Because of the selective transfer of monovalent ions, these membranes exhibit a particular polarization behavior: high current densities do not induce the depletion of ions at the membrane solution interface as elucidated by direct numerical simulations. As a consequence, the experimentally obtained current-voltage curves did not exhibit plateau regions. Electrochemical impedance spectroscopy revealed that the charge of the modifications has a decisive impact on the dielectric response of the system, thereby fingerprinting monovalention selectivity and transport competition between different ions. Ink-jet printed patterns of uncharged microgels on cation-exchange membranes were proven to cause crucial mass transfer enhancements in the electrodialysis process. Direct numerical simulations and experimental results revealed that the pattern, the surface charge, as well as the distribution of the microgel entities within a pattern trigger an intensified and earlier onset of electroconvection, thereby increasing the limiting current density. Charge mosaic membranes were produced via coating of zwitterionic microgels coated on non-charged porous support membranes. These membranes exhibited an increasedion permeation induced by the electrical field arising between the differently charged arrays. A preferential ion transport as compared to water transport occurs at the charge mosaic membranes which was experimentally detected by negative retentions. All surface modification methods were proven to improve ion transport, both in electrically and pressure-driven membrane processes. The fundamental mechanisms by which microgels tailor and enhance ion transfer in membrane processes were identified and rigorously analyzed via experiments and direct numerical simulations. With this work it was demonstrated that microgels are stimuli-responsive materials with the potential to revolutionize different water desalination processes.

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