Bio-electro-elongation of small and medium chain fatty acids

Details

Supervisors

Gerin, Patrick A.

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en chimie et bioindustries

Abstract

With the need to rethink our fossil carbon-based societies, numerous alternatives emerge all around the globe. Among the exploited fossil resources, fossil oil is the most widespread energy source for transportation, with a growing demand at least until 2040. It also underlies the majority of the current chemicals. To replace sustainably fossil oil as a raw material in these sectors, one of the most promising answers are bio sourced chemicals produced by Biorefineries. By integrating the model of the oil refineries and petrochemical industry into a bio-based extraction and transformation industry, biorefineries are able to produce both bio- based fuels and bio-based platform chemicals to replace fossil-fuel derivatives. These platform chemicals serve as the chemical building blocks to produce a wide range of more complex chemicals. One of these chemical platforms is the carboxylate platform. Carboxylates, especially short and medium chain fatty acids (S-MCFAs, from 2 to 10 carbons in the chain), have a very wide range of applications and can be produced from biowaste streams, making them the perfect candidate for a sustainable bio-based chemistry. In this work, the aim was to test the ability of a bio-electro-fermenter (EF) inoculated with a natural microbial community to produce in an inexpensive way S-MCFAs with acetic acid as a feed, and electricity as an electron donor. The anaerobic microbial pathway producing S- MCFAs is supposed to be the β-reverse oxidation cycle which elongates S-MCFAs by the addition of an acetyl-coA, adding two carbons to the chain. The second aim of the work was to test the influence of in situ produced hydrogen through water electrolysis on the elongation of S-MCFAs. The EF was built with a stainless steel cathode submerged in the buffered fermentation broth and a platinum-iridium coated anode submerged in a 0.1 M K2SO4 solution. Both compartments were separated by a cation exchange membrane and a constant current of 0,04A was applied to the system. To test the influence of molecular hydrogen, another type of reactor was created where the same type of fermentation broth was exposed to dihydrogen (or dinitrogen for the control reactor) in a pressurised reactor. Maximum concentrations of the targeted S-MCFAs during the EF experiment were : 0,74, 0,54 and 1,2 g.LML-1 (= 1,36, 1,2 and 2,96 gCOD.LML-1) respectively for C4, C6 and C8 carboxylates. They were observed around day 55 of the incubation. To our knowledge, such high levels of C8 were never reported with bio-electro-fermentation processes before. Control reactors with pressurised with N2 went through methanogenesis while methanogenesis was inhibited in the EF and H2 reactors where elongation occurred. The H2 reactors showed concentrations up to 1,77 ±0,007, 0,37±0,002 and 0,44±0,15 g.LML-1 (2,9 ±0,45, 0,82 ±0,005 and 1,06±0,38 gCOD.LML-1) of respectively C4, C6 and C8 observed around day 55 as well. Hence, H2 instead of electrons was able to play the role of an electron donor but MCFAs were produced at lower levels than in the EF. While these results are very encouraging, a lot of improvements have still to be made, especially regarding to the reproducibility of the results of the EF. The long pre-production period should also be investigated in order to be shortened and to make S-MCFAs bio-electro-synthesis an economically and sustainable alternative solution to fossil-fuel based chemistry. On the long term, this type of system could be further implemented into a broader process were the acetic acid will be produced in situ by the combination of anaerobic digestion and inhibition of the methanogenesis.