Modeling of ion transport through nanofiltration and ion-exchange membranes

  • Modellierung des Ionentransports durch Nanofiltrations- und Ionenaustauschmembranen

Evdochenko, Elizaveta; Wessling, Matthias (Thesis advisor); Nikonenko, Victor V. (Thesis advisor)

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

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020

Abstract

Nanofiltration and electrodialysis membrane separation processes are used for wastewater treatment and clean water production. In spite of numerous experimental studies directed to better understand the selective performance of membranes, so far, the relation between membrane structural characteristics and transport properties remains a difficult question to answer. This thesis aims to attain a deeper phenomenological understanding of ion transport through charged nanofiltration and ion-exchange membranes by means of conducting numerical simulations. The proposed numerical frameworks solve the one- and two-dimensional Poisson-Nernst-Planck equations for the transport of ions through n electrolyte layers (En) and n polyelectrolyte layers (PEn). The electrically driven transport model is called EnPEn for electrically, while pEnPEn describes the pressure (p) induced mass transfer. Coupling of the pEnPEn formulation with the Navier-Stokes and Brinkman equations establishes a completely novel model. The coupled model is able to describe ionic transport through nanofiltration multilayer films forming not only on top of the porous support but also inside the support capillaries. The charge distributions of new multilayer membranes were interpreted via numerical simulations. For an intermixed neutral compensated bulk layer, the impact of the top layer on rejection and selectivity is predominant. When the bulk charge is overcompensated, the rejection and selectivity can increase significantly due to the strong influence of the overall charge on salt transport. An asymmetric architecture of multilayers regarding each layer thickness and charge can essentially improve the selectivity for multi-ionic solutions. The increase in selective transport of monovalent ions was proven to result from the enhanced repulsion of multivalent species. The porous material, used as a support for a multilayer nanofiltration film, was also modeled. The modeling results reveal that the support contributes significantly to the salt rejection. The enhanced rejection refers to a selective layer forming inside the support capillaries during the coating procedure. Surface modification of cation-exchange membranes surface coated with polymer patterns, and confluent layers of positively charged microgels were modeled, and their influence on the current density and selectivity was analyzed. The surface modification reveals the enhancement in mass transfer for the electrodialysis process, which also agrees with experimental data. The developed modeling frameworks give insightful details on ion rejection and selectivity, enable us to explain important influences of membrane structural parameters, solution properties, preparation, and operation conditions on mass transport properties of membranes used for nanofiltration and electrodialysis applications.

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