An Investigation of the Role of Rapid Chlorine Hydrolysis on the Performance of Electrochemically-Mediated Desalination

Abstract

A recent United Nations report estimates that more than two billion people presently lack access to clean drinking water and more than four billion people lack access to proper sanitation. Furthermore, steadily rising populations, a changing climate, and growth in the agriculture, energy, and manufacturing sectors jeopardize future water security in many parts of the world. Accordingly, energy-efficient separations of salts and trace contaminates from water represent one of the most significant (and difficult) challenges facing humankind. To address this problem, we have previously reported the development of a promising approach to the separation of salts from seawater which we refer to as electrochemically-mediated desalination (EMD). EMD is an electrokinetic technique which utilizes the electric field gradient formed local to an anode during chloride oxidation to separate charged species from pure water. Despite experimental and computational study of EMD, a deep understanding of phenomena fundamental to the technique has yet to be attained. To this end, we performed time-resolved high resolution electric field gradient measurements and direct conductivity measurements during electrochemical chloride oxidation within a simple system composed of microelectrodes embedded in a straight microchannel. We were surprised by the findings of these measurements. Specifically, a large electric field gradient was measured upstream of the anode during chloride oxidation, however, a local increase in solution conductivity was measured at the same axial position along the channel length during chloride oxidation. To address these seemingly contradictory findings, the experimental system was simulated by finite element modeling. Simulation results showed that electrochemically generated chlorine undergoes rapid hydrolysis and produces substantial quantities of HOCl, H+, and Cl- near the anode. This finding corroborates the experimentally observed increase in solution conductivity near the anode during chloride oxidation. Consequently, we hypothesize that the observed electric field gradient is not indicative of local solution conductivity, but rather is related to a surface interaction between a product of chloride oxidation and the Pt microband electrode used to probe the electric field gradient. Ongoing research seeks to confirm this claim. Ultimately, we conclude that complex aqueous chlorine chemistry significantly limits the utility of chlorine generation for the formation of a local electric field gradient and seawater desalination.