Ion Transport & Electrokinetics in Colloidal Networks
Harm Wiegerinck is a PhD student in the Department Soft matter, Fluidics and Interfaces. (Co)Promotors are prof.dr.ir. R.G.H. Lammertink and dr. J.A. Wood from the Faculty of Science & Technology.
Currently around 10 percent of the people in the world are living under conditions of extreme water scarcity and it is expected that this will only become worse in the near future for various reasons such as climate change. For this reason, it is important that we find alternative water resources to reduce the current and future water scarcity. One option to reduce this problem is to remove the salt of brackish (slightly saline) waters. This could be done with a technique called electrodialysis, which removes salt from water with electricity and cat- and anion selective membrane pairs. However, to implement this technique it is important to optimize the ion transport to make this technique more attractive for industrial applications. In this thesis the ion-transport a fundamental study is conducted for electrodialysis applications using a model system composed of colloidal particle networks. Colloidal particle networks are porous materials composed of close-packed nano- or microparticles and could be useful to study a wide variety of different electro-driven processes. In this thesis we explore these opportunities by means of experimental as well as numerical studies. In Chapter 1, some background information on various types of electrodialysis processes and their ion transport limitations is provided as well as for instance the difference between ion-exchange membranes and colloidal networks. Next to this the origin of several relevant electrokinetic phenomena are discussed. The scope of all the other chapters is provided below.
In Chapter 2, the use of alternating conductive and non-conductive patched model membranes is explored to optimize the transport of ions for electrodialysis. In this study colloidal particle networks made from silica are used in a miniature electrodialysis system to study the influence of electro-osmotic mixing at the fluid-silica interface on the ion transport by studying the effect of the conductive patch size on the electric current. Next to this, visualizations of the velocity profile, pH, and the ion concentration in the channel were performed. In this study it was found based on experiments and numerical simulations that the performance of electrodialysis can be improved by a combination of electroosmotic mixing, transport from the bulk towards the depleted layer above the non-conductive patches, and slower development of the depleted layer due to the smaller absolute amount of ions removed per patch for smaller patches. Furthermore, it was found that most of the resistance was located in the colloidal silica networks, which masks the influence of the electroosmotic mixing on the ion transport. Finally, it was found that the influence of charge regulation of the silica on the ion transport was substantial
In Chapter 3 a relatively new process, which is known as shock electrodialysis was numerically studied. In this electro-driven process a salt solution flows through a porous material such as silica. This arrangement results in a relatively large depleted layer due to enhanced transport of counterions along the surface of the charge silica material, which is known as surface conduction in literature. Due to the size of the depleted layer, it can be physically separated at the outlet of the setup. In literature it was up to now assumed that the charge on the silica remains constant regardless of the salt concentration it is exposed to. However, it is well known that the surface charge of silica reduces when it is contacting lower concentrations of salt, known as charge-regulation. In this work the existing model was made more realistic by including a charge-regulating porous material. Since the transport through the depleted layer is related to the surface charge of the silica, the electric driving force required for the removal of ions increases substantially and therewith the energy consumption. Furthermore, when the energy consumption for shock electrodialysis is compared to conventional electrodialysis, even in the most ideal case where the surface charge remains constant, the energy consumption is orders of magnitude larger for shock electrodialysis. This makes it challenging to make shock electrodialysis an industrially attractive process.
In Chapter 4 both anion and cation particles are packed in a capillary to fabricate a thick and porous model bipolar membrane system. Due to the arrangement of both anion and cation selective membranes, bipolar membranes block the transport of salt ions. Instead when the electric field is applied in the correct direction it is able to split water at the interface between both layers to make an acidic and alkaline product on both sides of the bipolar membrane. In this study, the function of the bipolar membrane is validated by visualizing the pH over time with a pH-sensitive dye. While this bipolar membrane has a substantially higher resistance compared to conventional bipolar membranes, due to its porosity water transport is facilitated towards the interface and higher current densities are reached than would be possible when water would only be transported by diffusion. Finally, due to large resistance of the bipolar membrane, the water in between both layers is heated by Joule heating as was shown by IR-camera visualizations, which could potentially further enhance ion transport and water splitting reactions.
In Chapter 5 a commercial anion-exchange material is coated with an colloidal particle network material known as metal-organic frameworks (MOFs) via a novel modified fabrication method. This was done by contacting a metal ion and a (charged) organic ligand precursor solution on both sides of the membrane. Since the anion-exchange membrane only allows anions to pass the membrane, the positively charged metal ions are prevented from passing, while the organic ligand can pass the membrane. This allows to coat the membrane only on a single side, which means that the conditions for crystallization only have to be optimized for a one side of the membrane. In this study, we present the working principle of this method for two different types of MOFs where the precursors are both dissolved in water as well as in methanol. Since both experimental conditions show that there are virtually no MOF crystals formed on the metal solution side, it could be concluded that this method is promising for future studies to fabricate monovalent ion selective anion-exchange membranes for a variety of different MOFs.
When an alternating current electric field (AC) is applied across a microchannel with slightly bent sidewalls, it was found that when a dispersion with particle flows through this channel, the particles move towards the center and bottom of the channel, while still following the fluid flow. This would be an interesting technique to continuously separate particles from water. In Chapter 6, this phenomenon was studied experimentally and numerically in more depth to find what forces could potentially play a role. It was found that a combination of alternating current electroosmosis as well as induced dipole forces are very likely candidate forces to contribute to this phenomenon. However, solely based on these two forces, the particles would stick to the bottom. Therefore, a third force is required such as electrostatic repulsion between the bottom wall and the particle. While the theoretical force model did not completely line up with the experimental data, this study provides a solid foundation for future studies to further explore this phenomenon.
In Chapter 7, the main conclusions are provided. Furthermore, more research ideas and recommendations for future studies are given. It presents for instance ideas to translate the more fundamental insights obtained throughout this thesis to more practical and realistic systems to potentially improve the performance of various electro-driven separation techniques to make them more attractive for industrial applications.
