4.7 Article

Near-neutral pH increased n-caprylate production in a microbiome with product inhibition of methanogenesis

期刊

CHEMICAL ENGINEERING JOURNAL
卷 446, 期 -, 页码 -

出版社

ELSEVIER SCIENCE SA
DOI: 10.1016/j.cej.2022.137170

关键词

Chain elongation; n -Caprylic acid; In-line extraction; Methanogenesis; Carboxylate platform; n -Caprylate

资金

  1. Alexander von Humboldt Foundation for the Georg Forster Research Fellowship
  2. Alexander von Humboldt Foundation
  3. Federal Min-istry of Education and Research in Germany
  4. U.S. Army Research Laboratory
  5. U.S. Army Research Office [W911NF-12-1-0555]

向作者/读者索取更多资源

pH is a critical parameter in chain-elongating bioreactors, affecting product concentration, thermodynamics, and kinetics. The effect of different pH levels on n-caprylate production was examined, and it was found that mildly acidic pH led to n-caproate production while near-neutral and neutral pH favored n-caprylate production. High concentrations of undissociated n-caprylic acid inhibited methanogenesis at near-neutral and neutral pH. The microbial communities at near-neutral and neutral pH were more diverse.
The pH is a critical parameter in chain-elongating bioreactors, affecting: (1) the concentration of inhibitory undissociated carboxylic acids, which in turn affects the efficiency of product extraction; (2) the thermodynamics; and (3) the kinetics. Here, we examined the effect of five different pH levels (5.5 to 7.0) on n-caprylate (C8) production using an anaerobic sequencing batch reactor (ASBR) with continuous membrane-based liquid-liquid extraction (pertraction). We found that the product spectrum was directed by pH: mildly acidic pH (5-6) led to n-caproate (C6) production, while near-neutral and neutral pH (6.75-7) favored n-caprylate production. In particular, the pH of 6.75 led to the maximum values of volumetric n-caprylate production rate (75.6 +/- 0.6 mmol C L-1 d-1; 0.06 g L-1 h-1) and n-caprylate concentration in the fermentation broth (420 mM C; 7.57 g L-1). Given that methane production remained low at near-neutral and neutral pH, we theorized that the high concentration of undissociated n-caprylic acid (5.71 mM C) inhibited methanogenesis. We then demonstrated such an inhibitory effect at neutral pH in: (1) microcosm experiments; and (2) the continuous bioreactor by adding methanogenic sludge. Furthermore, 16S rRNA gene sequencing analysis revealed that near-neutral and neutral pH led to more diverse microbial communities than at mildly-acidic pH. For the first time, we report predominant ncaprylate production in a microbiome at near-neutral and neutral pH conditions where methanogenesis was controlled by the inhibitory effects of undissociated n-caprylic acid. At the same time, extraction of this species occurred even at near-neutral and neutral pH.

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