Ion transport and chemical reactions in nanoporous materials for environmental engineering
In many electrochemical systems amphoteric ions are present, which undergo acid-base reactions. In these systems the ion transport rate is dependent on, amongst others, the local pH. In my talk I will discuss several theoretical approaches to describe the transport of amphoteric ions because of an electrical field. I will show that some of these approaches result in many difficulties, and require to make assumptions when it comes to which ionic species within a group of amphoteric ions exactly participates in a certain reaction (e.g. HCO3- or CO2). I will show that, by grouping amphoteric ions, one can derive an elegant description of ion transport and electrochemical reactions.
In my presentation I will illustrate the theory focussing on two processes: I) an electrochemical separation process for the removal of boron, a toxic ion, from water, and II) a(n) (bio)electrochemical process for the production of acetate from CO2.
In the first process, electrochemical separation for the removal of boron from water, porous carbon electrodes are employed for the electrosorption of boron from water. Several processes must be described. Firstly, the transport of ions due to advection, diffusion and migration must be included. Secondly, the electrosorption of ions in electrical double layers formed in porous carbon electrodes must be described. Thirdly, in these double layers chemical phenomena, such as the binding of protons to surface groups, must be considered. Lastly, acid-base equilibria must be included. I will present a theoretical description of these joint processes and I will show how this description can contribute to the development of an effective process to remove amphoteric ions, including boron, from water.
In the second process, electrochemical cells are applied that rely, for the production of acetate, on conductive biofilms covering an electrode. I will show how we can derive a theoretical model to describe the transport of amphoteric ions and electrons, coupled to biochemical conversions, through a current-producing biofilm. This theoretical approach can be extended and can be employed to describe ion transport, interfacial phenomena (faradaic and non-faradaic processes), and chemical reactions in other (porous) media as well.