Assisted reverse electrodialysis for CO2 electrochemical conversion and treatment of wastewater: A new approach towards more eco-friendly processes using salinity gradients

In this paper, the utilization of assisted reverse electrodialysis (A-RED), recently used for pre-desalination, is proposed as a general method to reduce the energy requirements of electrolysis processes and evaluated for two model processes: (i) the cathodic conversion of carbon dioxide to formic acid; (ii) the anodic treatment of water contaminated by organics. In A-RED, two solutions with different salt content and an external potential difference, applied in the direction of the natural salinity gradient, are both used to drive redox processes. It was shown, for the first time, that the cathodic conversion of CO2 to formic acid can be performed by both reverse electrodialysis (RED) and A-RED, saving electric energy with respect to electrolysis processes. A-RED allows to increase significantly the production of formic acid with respect to RED (after 4 h, 2 and 6.3 mM were produced for RED and A-RED with an additional external cell potential of 0.8 V, respectively, for a salinity ratio SR = 3300). For the anodic treatment of wastewater, A-RED allowed to accelerate the removal of organics and/or to use smaller salinity gradients with respect to RED (as an example, after 2 h with a SR of 4.4, an abatement of TOC of 55 and 92% was obtained with RED and A-RED with 1.5 V, respectively) and to save electrical energy with respect to electrolysis. A simplified economic analysis performed for the anodic treatment of the adopted synthetic wastewater has shown that A-RED presents the most appealing economic data with respect to both electrolysis and RED in most of investigated cases.