Elsevier

Bioresource Technology

Volume 165, August 2014, Pages 365-371
Bioresource Technology

Influence of headspace composition on product diversity by sulphate reducing bacteria biocathode

https://doi.org/10.1016/j.biortech.2014.03.075Get rights and content

Highlights

  • (Bio)electro/biochemically produced hydrogen determine the final product profile.

  • Several valuable chemicals were microbially electrosynthesized.

  • Headspace environment should be regulated to achieve the desired bioelectrochemical conversions.

Abstract

Mixed culture of sulphate reducing bacteria named TERI-MS-003 was used for development of biocathode on activated carbon fabric fastened to stainless steel mesh for conversion of volatile fatty acids to reduced organic compounds under chronoamperometric conditions of −0.85 V vs. Ag/AgCl (3.5 M KCl). A range of chemicals were bioelectrosynthesized, however the gases present in headspace environment of the bioelectrochemical reactor governed the product profile. Succinate, ethanol, hydrogen, glycerol and propionate were observed to be the predominant products when the reactor was hermetically sealed. On the other hand, acetone, propionate, isopropanol, propanol, isobutyrate, isovalerate and heptanoate were the predominant products when the reactor was continuously sparged with nitrogen. This study highlights the importance of head space composition in order to manoeuvre the final product profile desired during a microbial electro-synthesis operation and the need for simultaneously developing effective separation and recovery strategies from an economical and practical standpoint.

Introduction

Recently there has been an emerging class of study on microbes which are capable of taking up electrons from cathodic surfaces and utilizing them for a series of electrochemical transformations through which they reduce inorganic (e.g. CO2) or organic chemicals (e.g. volatile fatty acids) into extracellular organic compounds (Soussan et al., 2013, Schröder, 2011, Rabaey and Rozendal, 2010). Microbial electrosynthesis requires some external electrical input to drive the conversions and overcome cathodic over-potentials, since many of the coupled electrochemical reactions are usually not thermodynamically feasible (Harnisch and Schröder, 2010). This electrical enhancement manipulates the redox metabolism by generation of reduced NADH within the cell through microbial electrodic interface reactions (Pandit and Mahadevan, 2011). More recently, major advances have been made in this realm of microbial electrosynthesis signifying the urgent need of research in this sector for production of value added chemicals. The successful demonstration of directly feeding electrons to acetogens with electrodes and the concept of integration of photovoltaics with electricity driven microbial reduction to organics was pitched by Nevin et al. (2010). Besides, there have been reports where this process is used for the production of H2 (Rozendal et al., 2009, Sleutels et al., 2013), caustic soda (Rabaey et al., 2010), hydrogen peroxide (Rozendal et al., 2009), methane (Wagner et al., 2009, Villano et al., 2010, Cheng et al., 2009), caproate, caprylate (Van Eerten-Jansen et al., 2013) and combination of one or more of the above mentioned chemicals (Lovley and Nevin, 2013, Marshall et al., 2012, Angenent and Rosenbaum, 2013).

In our previous study, we reported the possibility of bioelectrochemically reducing acetic and butyric acids to a number of organic products such as alcohols and acetone by a mixed electroactive (EA) sulphate reducing bacteria (SRB, now designated as TERI-MS-003) based biocathode (Sharma et al., 2013a). Electrons used for such conversions are derived mainly from direct electron transfer (DET). Yet a minor role was attributed to H2 as energy carrier. Steinbusch et al. (2008) proved that increasing H2 partial pressure (HPP) by accumulation in the headspace would result in a metabolic shift from acidogenesis to alcohol production. Villano et al. (2010) showed that the product profile can be influenced by the gases present in the headspace mainly by hydrogen generation along with bioelectrochemical conversion of carbon dioxide to methane when cathode potential was poised more negative than −0.7 V vs. Ag/AgCl. However in our study, methane production was not observed, presumably due to high salinity and acidic pH of the electrolyte.

Following our earlier results and the rationale of such above mentioned citations, the effect of HPP is investigated here as a step further to elucidate the mechanistic features involved in SRB electrosynthesis in Bioelectrochemical systems (BES). This overall research aims to culminate in practical application to recycle and subsequently divert energy in the form of biochemicals, particularly from low grade organic carbon present in wastewaters like fermentation effluents.

Section snippets

Inoculum and electrolyte

Inoculum of a mixed EA-SRB, TERI-MS-003 consortium was taken from a previously running bioelectrochemical reactor (Sharma et al., 2013a).The inoculum (10% v/v) was added to the electrolyte used for reactor operation, that consisted of a synthetic feed composed of 572 mg NH4Cl, 416 mg KH2PO4, 8 mg CaCl2, 96 mg MgCl2·6H2O, 1.98 mg FeCl2·4H2O, 2.37 mg CoCl2·6H2O, 0.59 mg MnCl2·4H2O, 0.034 CuCl2·2H2O, 0.062 mg H3BO3, 0.073 mg Na2MoO4·2H2O, 0.069 mg Na2SeO3, 0.095 mg NiCl2·6H2O, 0.055 mg ZnCl2 and 10 g NaCl per

Result and discussion

One of the major bottlenecks for conversion of electrical energy into organic chemicals via microbial electrocatalytic systems has been the unavailability of biocathodes that can achieve selected transformations at relevant kinetic rates. In our recent study (Sharma et al., 2013a), the development of SRB biocathode was achieved. The same inoculum (TERI-MS-003) was used here to develop a biocathode in a low-cost, easy to assemble, single chambered reactor as shown in the schematic (ES 1),

Conclusions

Electroactive SRB biocathodes serve as efficient biocatalysts and the metabolic routes shifts with alteration of headspace environment. Though most of such metabolic interventions are generally carried out though metabolic engineering, this study demonstrates that a set of economically desirable (bio) chemicals can be bioelectrochemically synthesized without any genetic manipulations in the biocatalyst. Other bigger challenges like effective separation of these microbial electrosynthesized

Acknowledgements

This work was funded by the Department of Science and Technology, India (Sanction number DST/INTSPAIN/P-23/2009). M.S. also acknowledge Indo-Belgian scholarship from the Flemish Government (Vlaamse Gemeenschap). The authors thank Dr. R.K. Pachauri, Director General TERI for providing excellent infrastructure and research facility. The authors also thank Mr. Rambaran for his technical assistance and Mr. Pradeep from HEG Ltd. for providing electrode sample material.

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    These authors contributed equally to this manuscript and should be considered as co-first authors.

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