(381n) Electrochemical Separation of Carboxylates with Capacitive Deionization

Capacitive deionization (CDI), an emerging electrochemical separation process, facilitates ion adsorption within the electric double layers that form upon the application of a potential difference across two electrodes. In this study, CDI technology is employed for the separation and recovery of carboxylate salts, i.e., sodium acetate or sodium butyrate, which are precursors to biofuels and bioproducts. Pretreated activated carbon cloth serves as the electrode to store the charged carboxylate species during the charging step under a constant potential difference. Subsequently, these charged species are desorbed from the electrodes during the discharging step with no potential applied.

All experiments are carried out using a constant influent flowrate of either 4, 6, or 8 mL/min with an initial concentration of 10 mM and an initial pH of 6.2. Experiments are conducted with one of three applied voltages: 0.8, 1.0, and 1.2 V. The experimental setup is equipped with a data acquisition and control program to monitor system variables including temperature, current, effluent conductivity, and effluent pH throughout the experiment. To compute effluent concentration profiles from output variables and evaluate performance metrics, the Debye-Hückel-Onsager Limiting Law (DHOLL) is used to account for the nonideality in dilute electrolyte conductance. The adsorption capacity increases as the flowrate rises from 4 to 6 mL/min, but then it drops at 8 mL/min for both carboxylate salts. However, the adsorption capacity reduces if the applied voltage is lowered from 1.2 to 0.8V. The maximum adsorption capacity for acetate or butyrate is estimated to be approximately 0.5 mmol/g of activated carbon cloth under a flowrate of 6 mL/min with an applied potential of 1.2V. With this CDI process, a minimum of 90% ionic recovery is achieved for both carboxylates. In summary, the utilization of sustainable CDI technology for carboxylate recovery offers new insights on mass transfer, electrosorption and desorption in charging and discharging steps at different flowrates and applied voltages. The experimental data collected in this work will form the basis of a future CDI modeling framework that will enable more accurate CDI process models.