Adsorptive separation processes are ubiquitous in chemical, biological, pharmaceutical and environmental processes, driven by specific interactions between the adsorbate molecules and the adsorbent surfaces. Reversal of the adsorption to recover the adsorbed species or to regenerate the adsorbent is accomplished by many means, including pH, ionic strength, solvent, temperature and pressure swings. In general, these methods require either the addition of chemicals, which may increase the environmental burden, or the heating/cooling of the bulk system, or significant changes in pressure, both of which are energy intensive. We have explored a variety of electrochemically-driven separation systems that are very attractive due to their flexibility of control through modulation of electrochemical potential, fast kinetics and possibility of energy recovery and integration with renewable sources. Our approach relies on heterogeneous redox-active species to mediate the specific interactions with target solutes, as well as take advantage of the large pseudocapacitive charge to provide faster kinetics, higher adsorption capacity and energy recovery performance.
We report the use of heterogeneously supported redox-organometallics, namely metallocenes, for the sorption and release of specific anions from the liquid-phase under oxidizing and reducing conditions, respectively, in both organic and aqueous media. Rather than rely solely on adsorbate charge or size, these redox-functionalized electrodes, through their electrochemical response to an applied potential, regulate the specific chemical interactions of the supported-organometallics with target functional groups of anions to achieve reversible and stoichiometric binding. We explore the mechanisms of this selective interaction through various spectroscopic methods, electrochemical characterization as well as electronic structure calculations. This technology is of scientific and economic interest for (1) sustainable water remediation and waste control, through the removal of highly toxic aqueous contaminants such as organic anions present in agricultural or industrial waste, and (2) improving chemical processes, as a powerful method for direct purification of charged products in chemical synthesis.
In a second example of redox-mediated separation processes, we describe a facile approach for the electrochemically-controlled capture and release of CO2 from gas streams. In this instance, the metallocene electrode serves as an electron source for the reduction of a poly-quinone on the cathode side of the cell to foster the reaction of CO2 with the reduced quinone moieties. The CO2 is released during the stripping cycle on reversal of the inter-electrode potential. Again, the electrodes serve as a separation medium, while also allowing for some energy recovery during the swing operations.
The presentation will provide an overview of these new redox-mediated separation processes, including a description of the various redox-electrode preparation methods, and conclude with general observations on the strengths and limitations of such electrochemically-driven separation processes relative to more accepted separation operations.