Biotransformation of furanic and phenolic compounds with hydrogen production in microbial electrolysis cells
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Lignocellulosic biomass is an abundant, renewable energy source for biofuel production, providing an important alternative to fossil fuels. However, pretreatment of biomass for biofuel production produces furanic and phenolic compounds, contributing to the corrosiveness, instability, and toxicity of various biomass-derived streams, thus presenting a significant challenge in downstream processes and waste disposal. Microbial electrolysis cell (MEC) is an emerging bioelectrochemical technology, which converts organic wastes in the bioanode and produces H2 in the abiotic cathode. Integration of MEC in biofuel production not only offers an alternative method for waste handling, but also production of renewable H2 for the downstream hydrogenation of biomass-derived bio-oil, thus reducing the external H2 supply currently derived from natural gas (i.e., methane), a non-renewable fossil fuel. Considering that furanic and phenolic compounds are among the most problematic components of biomass-derived waste streams, it is critical to understand the biotransformation of these compounds in MEC for H2 production. This study focused on two furanic (furfural, FF; 5-hydroxymethyl furfural, HMF) and three phenolic (syringic acid, SA; vanillic acid, VA; 4-hydroxybenzoic acid, HBA) compounds, representative of predominant furan derivatives from biomass carbohydrates and phenolic acids derived from major lignin units. The objectives of this study were to: a) achieve efficient conversion of the selected furanic and phenolic compounds with H2 production in a MEC; (b) elucidate the metabolic fate of the furanic and phenolic compounds in the MEC bioanode; (c) assess the inhibitory effect of the furanic and phenolic compounds on MEC bioanode microbial processes; and (d) delineate the specific role of microorganisms in different physiological groups in the MEC bioanode. The five furanic and phenolic compounds as a mixture were utilized as the sole carbon and energy source in the MEC bioanode, resulting in cathodic H2 production at promising Coulombic efficiency (44 − 69%) and H2 yield (26 - 42%). The two furanic compounds (FF and HMF) contributed to the cathodic H2 production to a higher degree than the three phenolic compounds (SA, VA, and HBA). The biotransformation of furanic and phenolic compounds in the MEC bioanode occurred via fermentation followed by exoelectrogenesis. The fermentative transformation proceeded independently from exoelectrogenesis, but the extent of fermentation controlled the exoelectrogenic activity. The aromatic ring of SA was cleaved, resulting in acetate production, which was further used by exoelectrogens, whereas VA and HBA were transformed to catechol and phenol, respectively, without aromatic ring cleavage. The different extent of biotransformation of SA, VA, and HBA is related to the difference in their number and position of hydroxy (–OH) and methoxy (–O–CH3) substituents. The furanic and phenolic compounds inhibited exoelectrogenesis, but not fermentation in the bioanode, with IC50 values in the range of 1.9 – 3.0 g/L. The inhibition was primarily caused by the parent compounds, as opposed to their transformation products. An additive, but not synergistic inhibitory effect on exoelectrogenesis was observed with the mixture of the five parent compounds. In a continuous-flow bioanode MEC, complete transformation of the parent furanic and phenolic compounds was achieved at a short hydraulic retention time of 6 h. An increased H2 production rate was achieved by increasing the organic loading rate (OLR) or the applied voltage, but the trade-offs were lower biotransformation extent of the parent compounds, lower MEC effluent quality, and lower overall energy efficiency. The MEC anode biofilm microbial community consisted of three major phyla: Proteobacteria, Bacteroidetes, and Firmicutes. Phylogenetic analysis identified species closely related to putative exoelectrogens, furanic and phenolic degraders, and other fermentative bacteria, supporting the observed fermentative/exoelectrogenic biotransformation in the MEC bioanode. This research is the first systematic, comprehensive study on the metabolic fate, contribution to cathodic H2 production, and inhibitory effect of furanic and phenolic compounds in MECs. Results of this study can be used to guide the design and optimization of MECs converting biomass-derived waste streams to renewable H2. Reducing the discharge of organic wastes and minimizing external H2 supply, currently derived from natural gas, will promote carbon-neutral, sustainable biofuel production.