Continuous Ion Separations by Insertion Processes

Abstract

Humanity is reliant on many important chemical separations which are performed daily at the industrial scale. With a historical dependence on thermal distillation techniques, which require large amounts of energy, chemical separations account for 10-15% of energy consumption worldwide. Membrane-based alternatives mitigate separation energy demand, yet prove challenging to scale-up and possess intrinsic limitations. Membranes usually separate species based on size and charge exclusion mechanisms. Importantly, however, improvements in the selectivity of membranes almost always leads to lower permeability. Much of the fundamental research in the field of membrane separations is focused on breaking the scaling relationship between selectivity and permeance, and improving membrane lifetime and stability. As discussed next, our group is developing electrochemically active membranes that can be actuated by application of an electric field. The long-term goal is to control selectivity and permeance. The specific goal of this project was to investigate the exploitation of ion insertion processes for the continuous and potential-controlled separation of ions. We proposed that an intercalation material operated as a bipolar electrode (BPE) would insert ions at one pole while simultaneously deinserting ions at the other pole. This results in the continuous transport of ions through the electrochemically-actuated and ionically-conductive material. This is significant because selective ion transport through the material can be toggled “on” and “off” by controlling the electrochemical state(s) of the material. In addition, we propose that insertion of ions into an intercalation material will result in a local ion depletion zone (IDZ) and a corresponding electric field gradient, which could be utilized for the redirection of secondary ions (defined as ions that are redirected as a consequence of their interaction with a local electric field gradient). Success in this approach would enable electrochemical control over separation, continuous ion transport, improved transport specificity compared to conventional membranes, and the elimination of undesirable faradaic side products like H+ which have proved problematic for previously reported systems. This aspect of our project has had three main focuses. The first focus is the design and fabrication of a microfluidic system containing a Prussian blue (PB) deposit to be actuated as a BPE. The second focus explores the quantitative study of ion transport through the PB BPE within the microfluidic system and characterization of local IDZ formation and the corresponding electric field gradient. The third focus seeks to confirm the actuation of PB as a BPE by use of simple macroscale systems. A second key aspect of our research has related to removal of microplastics from water. For this part of the project, we used experiments and finite element simulations to investigate electrokinetics within straight microchannels that contain a BPE and an unbuffered electrolyte solution. Our findings indicated that in the presence of a sufficiently high electric field, water electrolysis proceeds at the bipolar electrode and leads to variations in both solution conductivity and ionic current density along the length of the microchannel. The significance of this finding is twofold. First, the results indicate that both solution conductivity and ionic current density variations significantly contribute to yield sharp electric field gradients near the BPE poles. The key point is that ionic current density variations constitute a fundamentally new mechanism for forming electric field gradients in solution. Second, we showed that the electric field gradients that form near the bipolar electrode poles in unbuffered solution are useful for continuously separating microplastics from water in a bifurcated microchannel. This result expands the potential scope of membrane-free separations using bipolar electrodes.