Elsevier

Process Biochemistry

Volume 102, March 2021, Pages 213-219
Process Biochemistry

Influence of Trace Metals concentration on Methane generation using Microbial Electrochemical Systems

https://doi.org/10.1016/j.procbio.2020.12.021Get rights and content

Highlights

  • Optimum concentrations of trace metals enhanced biocatalyst’s metabolic activity.

  • Higher concentrations of trace metals inhibited the overall performance of MES.

  • CH4 production in TM2 was almost 3.9 folds higher than the control without metal addition.

  • Cyclic voltammograms showed higher reduction profiles over oxidation in TM operations.

Abstract

The biomethane generation in microbial electrosynthesis systems (MESs) was affected by the addition of trace metals (TMs) during biocatalyst’s metabolic activity. The functional role of various TMs (Mg2+, Fe2+, Ni2+, Zn2+, Co2+, Mn2+, and Mo2+) in regulating the CH4 production potential of a biocatalyst was evaluated under three different ranges of TM concentrations, and their performances were compared with the control operation (no trace metals). The TM level in a relatively medium concentration range exhibited the best efficiency and could enhance the CH4 production and currents generation by 3.9 and 7.7 folds higher than the values from the control. Cyclic voltammogram profiles depicted increment in redox catalytic currents during MES operation with TMs and also supported the involvement of mediators towards CH4 generation. The optimum TM concentrations could enhance MES performance as a constituent of ferredoxin and hydrogenase linked to energy metabolism.

Introduction

From an industrial perspective, CO2 is a valuable feedstock for the generation of building block chemicals such as formate, alcohols (e.g., methanol and ethanol), and gases such as CH4 [1]. However, as CO2 is a stable molecule, its conversion in the abiotic process to a value-added product requires a more extensive energy input due to overpotentials and multielectron reduction steps. In this regard, several researchers had exploited the electrochemical and biological systems to catalyze the CO2 [2,3]. In particular, the biological reduction of CO2 with the use of microorganisms and enzymes assists in an eco-friendly sustainable approach. Moreover, the biological CO2 reduction is highly interested in the industrial sector due to ambient operational conditions such as temperature and pressure. Also, the microbes are self-regenerative catalysts, and therefore, they are much more suitable for long term operations of CO2 reduction systems [1,4].

In this regard, several studies have tested microbes' usage for the conversion of CO2 towards biosynthesis along with equivalent supplementation of H2 artificially. In the early 1980s, several researchers had grown numerous microbes using CO2 as a carbon source in the anaerobic fermentation process. Tanner et al. [5] demonstrated the growth of Clostridium lungdahlii for acetate generation by using H2 and CO2. Liou et al. [6] exhibited Clostridium species' growth for the generation of alcohols and products such as acetate from CO2 and H2 along with other sugars as carbon support. In further, Logan [7] suggested applying these microbes in microbial electrochemical systems (MES) to produce sugars, alcohols, and CH4 without external H2 supply. Biological reduction of inorganic carbon (CO2) has been piqued from the last decade in the field of microbial electrochemical systems as a valuable strategy for cyclic carbon reuse [8]. Also, MES technology opens a new prospect in the tunability of product formation with variation in potential applied [9]. Furthermore, MES can open new possibilities in combining renewable energy, viz., solar and wind power, to the biological systems to achieve a self-sustainable biorefinery process.

The external supplementation of H2 to generate the CH4 makes the anaerobic fermentation process unfavorable due to high operational cost and energy loss from storage and transport [10,11]. As an alternative method, the generation of CH4 by direct electrochemical reduction of CO2 by microbes without any external addition requirement of H2 in MES is preferable. This would offer a bioelectrocatalytic direct CO2 reduction without any need of additional mediators and supplementary processes such as water splitting to enable energy storage in the form of valuable products. To our knowledge, the first study on the CO2 reduction to generate CH4 bioelectrochemically in MES without the addition of any external mediators was reported by Cheng et al. [12]. The long-term studies (188 days) had exhibited a maximum energy efficiency of 51 %. In a similar study, Villano et al. [13] demonstrated high CH4 production rates with electron capture efficiencies of more than 80 % using the hydrogenophilic methanogenic biocathode. Also, Sato and co‐workers had discussed the possibility of using the geological storage of CO2 to value-added chemicals by using bioelectrochemical systems [14]. Thus, making the MES a viable technological alternative to the conventional anaerobic fermentation process. In simple, the reactive metabolic pathways for both direct and indirect electron transfer mechanisms of hydrogenotrophic methanogens in MES can be summarized as follows [15]:CO2 + 8H++ 8e- → CH4 + 2H2O (Direct electron transfer)CO2 + 4H2 → CH4 + 2H2O (Indirect electron transfer)

In a recent study, simultaneous CH4 and acetate production from CO2 and H2 has been shown using the methanogenic culture in bioelectrochemical systems [11]. Likewise, Bajracharya et al. [16] tested CO2 reduction in MES by using the Clostridium ljungdahlii and mixed cultures to generate acetate and methane. Nevertheless, all these studies are performed using bicarbonate salts into the cathode medium and limited with testing with pure CO2, except for the recent research by Bajracharya et al. [16] in which the CO2 was directly provided at gas diffusion biocathode electrode. Moreover, these studies are limited in testing the influence of micronutrients, i.e., trace metals, on the biocathode performance for biogas generation. Based on earlier studies, it was proven that the variation in divalent metal concentration had altered the performance of hydrogenic reactors in terms of product formation [17]. Furthermore, divalent metals are essential for many functions related to enzyme activation, energy metabolism, and biomass growth [17,18].

In this study, the MES was operated at various trace metals to investigate their effects on methane production from CO2. In this MES system, the in situ generated electrons or H2 was used for the supplemental energy source for proceeding CO2 reduction reaction forward. The optimum concentrations of trace metals were determined for hydrogenotrophic methanogens to maximize the performance of MES in methane fermentation. The biochemical functions of three different divalent metal compositions (TM1, TM2 and TM3) and control (no trace metals) in regulating the CH4 production process in MES were evaluated concerning the bioprocess parameters such as biogas generation. The influence of divalent metal ions on bio electrogenic performance of biocathode was further analyzed by cyclic voltammetry and Tafel analysis.

Section snippets

Inoculum source

Initially, the mixed culture obtained from an anaerobic digestion plant (Suwon Wastewater Treatment Plant; South Korea) effluent was used as an inoculum source. The hydrogenotrophic methanogenic culture required for reduction of CO2 to CH4 was initially enriched using bicarbonate followed by CO2 gas as the sole carbon source.

Bioreactor construction

Single chambered bioreactor fabricated using transparent polycarbonate material (total and working volume of 500 & 350 ml respectively) was utilized for the tests.

Currents generation at different trace metal concentrations

During the startup phase, four MES bioreactors at different trace metal concentrations (TM1, TM2, TM3, and control) were fed similarly with GM solution and CO2 in continuous mode at a rate of 5 ml/min and operated until stable currents, and uniform methane concentrations were obtained. After obtaining stable MES performance, the effect of different trace metals concentrations on overall MES performance of CO2 reduction to CH4 was evaluated, as shown in Fig. 1. The steady currents and uniform

Conclusions

The effect of trace metals on the CH4 generation efficiency in MES was evaluated utilizing CO2 as a feedstock. The trace metal concentrations improved CH4 production at optimum conditions (TM2), while both lower (TM1) and higher (TM3) concentrations showed comparatively less performance. The presence of metal ions could enhance the overall MES performance as the metalloenzymes are involved in the methanogenic pathway as Fe-S clusters and other enzyme activities. Redox catalytic currents

Author contributions

C. Nagendranatha Reddy: Conceptualization of experiment, Original draft preparation Methodology preparation, Experiment Investigation, Data analysis, revision of manuscript.

Sanath Kondaveeti: Original draft preparation, Data analysis, revision of manuscript.

Booki Min: Methodology preparation, Experiment supervision, revision of manuscript and submission.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

The study was carried out with research grants from the National Research Foundation of Korea (2018R1A2B6001507) and the Korea-India S & T Cooperation Program (2016K1A3A1A19945953).

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