Neutralization of acid and base solutions by Reverse Electrodialysis with Bipolar Membranes: a sustainable way to recover energy

The large amount of acidic wastewaters produced in various industrial processes can have a large economic and environmental impact. Companies producing waste acid solutions have to send them to specific sites for the neutralization with alkaline solutions. However, efforts have been devoted so far to promote the acid recovery or its reuse. In the perspective of a more circular manufacturing approach, the present work proposes for the first time the use of the novel Bipolar Membrane Reverse Electrodialysis technology as a viable on-site way to convert the chemical energy associated to the pH gradient of waste acid/base solutions into electrical energy. Bipolar Membrane Reverse Electrodialysis is able to produce electric energy by converting the mixing energy of two solutions at different pH by neutralisation in Bipolar Membranes. The process is carried out in a conventional bipolar membrane electrodialysis stack, composed by several repeating units, named triplets. Each triplet is composed by an anion-exchange membrane, a cation-exchange membrane and a bipolar membrane (made by a cation-exchange layer and an anion-exchange layer), separated by net spacers creating the channels where the acid, base and salt solutions flow. The electrochemical equilibrium between membranes and adjacent solutions establishes an electric potential difference over the membranes. Redox reactions arise at the electrode end compartments, thus providing an electric current to the external load, while a selective ion transport takes place within the stack through the membrane, with H+ and OH- ions neutralization in the interlayer of the bipolar membrane. A first attempt to quantify the potential of Bipolar Membrane Reverse Electrodialysis in acid neutralization is performed in this work via an extensive experimental campaign. Tests were performed in a 10×10 cm2 lab-scale unit fed with HCl, NaOH and NaCl solutions. The effect of operating conditions (concentration and velocity of the feed solutions), was studied, mimicking different stages of neutralization of the acid waste. The effect of the number of triplets was also studied in view of a future scale-up. Collected results suggest that among the main detrimental phenomena, the impact of parasitic currents may be dramatic when a large number of triplets is employed. Conversely, concentration polarization and pressure losses were not an issue compared to the energy produced by the neutralization. This work was performed in the framework of the BAoBaB project, funded by the EU H2020 program.