A Technical Proton Pump for Industrial Biotechnology: Electrochemical pH-Swing Extraction of Succinic Acid

Abstract

Industrial biotechnology established new routes for sustainable production of platform chemicals.[1] Succinic acid is a versatile building block for bio-based polymers and solvents.[2] Despite the progress made in up-stream development separation and product purification still account for up to 40% of total production costs.[3] The high separation costs originate from a mismatch between the optimal pH for fermentation and product recovery. While most bio-catalysts work best at neutral pH, acidic pH is advantageous for product recovery.[4] However, pH control by inorganic bases and acids comes with the drawback of producing large amounts of inorganic salt waste like gypsum.[5] Electrochemical separation methods have the potential to overcome the unsustainable production of salt waste and are thus regarded a more environmentally benign separation technology.[6] The integration of electrochemical separation methods showed great potential for overcoming the mismatch between fermentation pH and pH required for separation. With promising results reported for electrochemical unit operations applied for waste free product recovery from fermentation [7] integration of electrochemical unit operations into a complete downstream process sequence should be the next objective. [8]We recently proposed an electrochemical downstream concept applying electrochemical pH-swing extraction for the recovery of succinic acid from pH-neutral aqueous solutions followed by another pH-swing step for product stripping from the extractant and eventual product recovery by direct electrochemical pH-shift crystallization.[9,10] Thereby, generation of spatially separated regions of acidic and alkaline pH by water electrolysis is utilized to generate the pH-dependent driving force for reactive extraction of succinic acid by tri-n-octylamine. The acidic pH shifts the aqueous acid-base equilibrium towards the protonated succinic acid, which is then available for extraction.[4] Hydroxide ions simultaneously produced at the cathode create an alkaline catholyte, which can be used for pH control in succinic acid fermentation. The water electrolysis coupled to liquid-liquid extraction creates an integrated buffer-recycle between fermentation and product separation and overcomes the need for adding inorganic acids and bases for pH-control. In a second electrochemical pH-swing cell the alkaline solution produced by the cathodic reaction of a water electrolysis cell is used to strip the acid from the loaded extractant producing an aqueous solution with high succinic acid concentration. The anodic reaction of this second cell is eventually used to crystallize succinic acid from this mother liquor by the electrochemical pH-shift.[9] Recovery of solid succinic acid was achieved by a downstream process constituted of four electrochemical processing steps synergistically integrated into two electrochemical cells which did not require energy intensive evaporation of water.This contribution gives insights into experimental examples for succinic acid recovery via the proposed separation process and discusses opportunities for the design of membrane free cells, as separation selectivity is solely determined by liquid-liquid extraction. In extension to previous work [11] the dynamic modeling approach for electrochemical pH-swing electrolysis was extended to account for concentration dependent increase of ohmic losses and pH-dependent proton cross-over. The new model is used to identify good operational conditions with high current efficiencies. The model results are compared to experimental data recorded in a 100cm² electrolysis cell coupled to a lab-scale extraction column.