A novel electro-sorption and acidic capturing method for continuous volatile fatty acid recovery

Abstract

Volatile fatty acids (VFAs) can be sustainably produced from waste streams by anaerobic microorganisms. However, VFA separation or purification from complex mixtures is still challenging in terms of selectivity and costs. A new electrochemical method was presented to continuously capture VFAs from a mixture using a porous activated carbon (AC) membrane electrode by two steps: electro-sorption and acidic capturing. Dissolved VFAs near neutral pH conditions (carboxylate form: R-COO-) can be collected near the electrode surface by electrostatic force. Using an acidic environment, the electro-sorbed VFAs can be continuously harvested on the other side of the electrode by converting carboxylates to acid form (R-COOH). Higher (5.5 times) VFA concentrations (456 +/- 108 mg/L) were obtained at the applied current of 7.2 A/m2 than control (no current), and VFA recovery fluxes were further improved (44 %) under optimal pH conditions across the AC membrane electrode that were substantiated by the numerical modeling. Acetate was recovered the most among the tested VFAs, likely due to the shortest chain length. The oxidation of the electrode surface was unavoidable under given cell voltages (2.0-2.9 V) which resulted in the higher oxygen group content after multiple cycles. This study has proved a new potential VFA recovery mechanism to realize selective and continuous VFA recovery from mixtures.