Electrochemically generated ion depletion zones for continuous separations in microelectrochemical devices

Davies, Collin David
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The separation of chemical mixtures into pure and purer constituents is essential to humankind. However, the most common techniques for chemical separations are energy intensive and improvements in their efficiency are only incremental. To meet the rising demands of an ever-increasing global population, new techniques that separate chemicals on the basis of phenomena fundamentally different than that of the existing methods must be developed. To that end, we set out nearly five years ago with the goal to continuously separate charged objects within ion depletion zones formed by electrochemical processes in microfluidic channels. Ion depletion zones yield co-located electric field gradients that interact with charged objects in solution in a manner related to the electrophoretic mobilities of the objects. Importantly, by judiciously tuning the forces of electromigration and convection in microchannels, the motion of charged objects can be controlled in useful ways. Herein, we report three studies that describe our scientific progress thus far toward the stated goal. The first study outlines the processes fundamental to controlling the flow of charged objects with a local electric field gradient. The key finding from this study is that an electric field gradient in the vicinity of a channel bifurcation directs the flow of nearly 100% of charged microplastic particles into a specific outlet channel. The second study introduces a more sophisticated microelectrochemical device than that used in the first study. In this case, two electric field gradients formed within a trifurcated microchannel continuously sort and separate two microplastics having different electrophoretic mobilities. The third study investigates electrochemically oxidizing Cl⁻ to neutral Cl₂ to form ion depletion zones in Cl⁻-containing solutions like seawater. Success in this endeavor would make it possible to leverage the discoveries from the first two studies in which the ion depletion zones formed in Tris buffer solutions to chemical separations in an environmentally relevant solution. The main finding from the third study, however, is that electrochemically generated Cl₂ rapidly reacts in water to form an ion enrichment zone, rather than an ion depletion zone, in solution. Notwithstanding, these findings represent significant advancements in our understanding of the processes fundamental to continuously separating charged objects within ion depletion zones and electric field gradients formed by electrochemical processes