Bioelectrochemical chain elongation

Sanne M.T. Raes

Research output: Thesisinternal PhD, WU

Abstract

To decrease our society’s dependence on polluting fossil resources, alternative sources for chemical and fuel production need to be developed. Organic residual streams are a renewable feedstock that can be used to replace these fossil-based fuels and chemicals. In this thesis bioelectrochemical chain elongation (BCE) has been studied to convert short chain fatty acids (SCFAs, model substrates for acidified organic residual streams) into biobased intermediate chemicals. BCE is subtype of a microbial electrosynthesis (MES) system, in which microorganisms catalyse the elongation of SCFAs and/or CO2 towards medium chain fatty acids in an electrochemical cell.

Part 1 of this thesis studied the formation of valuable products from SCFAs using BCE systems. In chapter 2 it started with the proof of concept of using an electrode for the sustained chain elongation of CO2 and acetate in continuous BCE systems. Four BCE reactors were used to study the role of applied current: two were applied with 3.1 A m-2 (projected surface area of electrode) and the other two with 9.4 A m-2. n-Butyrate (nC4) was the main identified product in all reactors. The highest applied current led to the highest nC4 production rate of 0.54 g L-1 d-1 (24.5 mMC d-1). The highest concentration of nC4 reached under high current regime was 0.59 g L-1  (26.8 mMC). Trace amounts of propionate and n-caproate were also produced, but no alcohols were detected over the course of the experiments (163 days).

To improve BCE and enhance production, in chapter 2 as well a literature review is provided to give insights into all the reported pathways to produce nC4 in fermentations. In fermentative chain elongation soluble electron donors, like ethanol or lactate, supply reducing equivalents and drive microbial metabolism. Since such compounds were not detected in the BCE reactors, it was hypothesised that nC4 production was limited by intermediate production and subsequent fast consumption of ethanol or lactate.

This hypothesis of intermediary production of ethanol or lactate limiting BCE performance was verified in chapter 3. Both ethanol and lactate were separately introduced in triplicate BCE reactors applied with 9.4 A m-2. Both compounds did not significantly affect the rates of nC4 production. Next to these compounds, the effect of formate on nC4 production was tested. Formate injection led to acetate production and decreased nC4 production. The results suggested that formate conversion to acetate competed with acetate elongation to nC4 for electrons. This competition subsequently resulted in decreased production of nC4. To investigate role of the electrode as electron donor, the current was increased to 18.1 A m-2. This increase in applied current doubled the production rates of nC4. Hence, this chapter demonstrates that the nC4 production in our BCE systems was not limited by intermediate production of well-known electron donors, but was driven by electrode-derived electrons.

For BCE to become a feasible organic waste valorisation technology, the studied substrate range needs to extend beyond acetate reduction. Therefore, in chapter 4 four different substrate feeding strategies and the subsequent product spectrum were investigated: I) acetate, II) acetate and propionate, III) acetate and n-butyrate, and IV) a mixture of acetate, propionate and n-butyrate. In phase I, nC4 was produced at 0.9 g L-1 d-1 (39.7 mMC d-1). After introduction of propionate in phase II, n-valerate (nC5) production started and sustained until medium was changed at the start of phase III. The maximum concentration  of  nC5  reached  was  1.2  g L-1  (60.6 mMC), and the highest production rate was 1.1 g L-1 d-1 (57.5 mMC d-1) at a high carbon-based selectivity of 73.8 %. This seems contradictory to ethanol chain elongation studies in which acetate is concurrently formed leading to straight fatty acids as by-products. Upon introduction of acetate and n-butyrate, n-caproate (nC6) production started and reached a maximum concentration of 0.3 g L-1 (15.8 mMC). The nC6 formation selectivity was 83.4 % in phase III. When all the three SCFA were supplied as substrate in phase IV, nC5 was the main product (95.4%). The observed preference for propionate elongation over both nC4 formation or nC6 formation is in contrast to fermentative ethanol-based chain elongation studies.

Part 2 of this thesis focusses on the extraction of the bioproducts from dilute aqueous streams using ionic liquids. The conversion of organic waste streams as renewable feedstocks into carboxylic acids (such as SCFAs but as well the medium chain fatty acids (MCFAs)) results in relatively dilute aqueous streams. These relatively low concentrations are a major bottleneck for these bioprocesses to compete with the production of platform chemicals based on fossil resources. A way to overcome this bottleneck is to extract the carboxylates from the fermentation broths using liquid- liquid extraction. Hydrophobic ionic liquids (ILs) are novel extractants which can be used for this purpose. Ionic liquids are salts comprised of ions, with relatively low melting temperatures (often below 100°C). By varying the types of ion and, for example, the branching of the ions, the physical properties of the IL, such as its hydrophobicity, can be tailored. To integrate these ILs as in situ extractants in biotechnologies, the ionic liquid should be compatible with the bioprocess.

In chapter 5 the biocompatibility of the two hydrophobic ILs [N8888][oleate] and [P666,14][oleate] were investigated in a two-phase system (IL layer on top of water phase). Commonly, ILs are synthesized in organic solvents, such as toluene and ethanol. After synthesis some trace amounts of these solvents can remain in the IL. When that hydrophobic IL is placed on top of a water phase, the trace amounts of synthesis solvent can leak into the water phase. To circumvent possible toxic effects of the trace amounts of solvent in the IL, water was used as synthesis solvent. After synthesis of the two ILs, their bioprocess compatibility was assessed. Methanogenic granular sludge was placed in medium without carbon source, and on top of that medium the IL phase was placed. After 24 days the sludge was separated from the water phase and placed into fresh medium. Upon transfer of the sludge into fresh medium with acetate as substrate, [P666,14][oleate] exposed granules were completely inhibited. Granules exposed to [N8888][oleate] sustained anaerobic digestion activity, although moderately reduced. Co-ions of the starting materials of the ILs, bromide and oleate, could have remained in the IL after synthesis. Both bromide (5 to 500 ppm) and oleate (10 to 4000 ppm) were demonstrated to not inhibit methanogenic conversion of acetate. Conclusively, [P666,14] was identified as a bioprocess incompatible component and [N8888][oleate] as bioprocess compatible.

For an IL to become the envisioned in situ extractants for bioprocesses, the IL needs to be regenerated and reused. In chapter 6 a concept of an IL as transport liquid is presented, in which a product (from a bioprocess) is in situ extracted into a hydrophobic IL. The subsequent extraction of the product from the IL (i.e. regeneration) does not necessarily need to take place in/at the same physical location, time and/or medium as where the extraction of product into the IL occurred. Therefore, the IL can be regarded as transport liquid of the product.

To study the feasibility of this concept, the bioprocess compatible hydrophobic IL [N8888][oleate] was used for two successive cycles of i) extraction of SCFAs into the IL [N8888][oleate] and ii) regeneration of the IL. For the regeneration of the IL a novel method was described which employs microorganisms to assist in IL regeneration, naming it ‘microbial assisted regeneration’. Microbial assisted regeneration is beneficial as no additional salt is needed for both pH control of the bioprocess as well as for recovery of the products from the IL. The experiments in this chapter demonstrate the potential of using hydrophobic ILs as transport liquid between two bioprocesses. When the concept of an IL as transport liquid is coupled with the proposed microbial regeneration method, two distinct biological processes can be coupled.

For BCE to become an industrial waste valorisation technology, the production needs to be improved. Although the electron transfer pathways are not unravelled yet, chapter 7 gives an overview of all the nowadays described pathways. In this way, the coupling of microbial metabolism with an electrode can be understood more. Based on these insights, several recommendations are provided to improve BCE and to render the technology mature enough to prove its potential using real acidified organic residual streams.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Buisman, Cees, Promotor
  • Strik, David, Co-promotor
Award date29 May 2019
Place of PublicationWageningen
Publisher
Print ISBNs9789463439169
DOIs
Publication statusPublished - 2019

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    Raes, S. M. T. (2019). Bioelectrochemical chain elongation. Wageningen University. https://doi.org/10.18174/472842