Reactive extraction for in-situ carboxylate recovery from mixed culture fermentation

Biorefinery wastewaters can be fermented to produce carboxylates which are high-value platform chemicals. However, the major challenges in this fermentation are limited product yields and productivities faced due to product inhibition and difficulty in carboxylate separation and recovery from fermentation broths. To mitigate the above problems, process optimization via integrated fermentation-separation i.e. in-situ product recovery (ISPR) systems can be considered. As a first step towards development of such coupled carboxylate bioprocesses, this study aimed to provide a detailed analysis of extraction behaviour of a wide array of extractants and diluents for C2-C6 carboxylates in synthetic solutions and real effluent from acidogenic fermentation. Compared to physical extraction without extractant, a 75–85 % increase was achieved when using reactive extraction (RE) and the difference was more pronounced for short chain carboxylates, particularly at pH 4.5. Distribution coefficients and extraction efficiencies increased with increasing extractant concentration and reached an equilibrium at molar ratio of 1:2. Aliquat 336 and tri-octylphosphine oxide solved in methyloctanoate emerged as the best RE systems and yielded high extraction efficiencies of 11.5 and 29.5 (acetic acid) to almost 100 (caproic acid) respectively. Testing with real fermentation effluent demonstrated similar high extraction yields as observed on synthetic solutions. Potential toxicity of RE on acidogenic fermentation was also investigated which suggested the application of an external ISPR configuration for these coupled bioprocesses.