Hydrogenotrophic Methanogenesis Under Alkaline Conditions.

Richard M Wormald,Helena Gomes,Simon P Rout,Paul N Humphreys,William Mayes

doi:10.3389/fmicb.2020.614227

Abstract

A cement-based geological disposal facility (GDF) is one potential option for the disposal of intermediate level radioactive wastes. The presence of both organic and metallic materials within a GDF provides the opportunity for both acetoclastic and hydrogenotrophic methanogenesis. However, for these processes to proceed, they need to adapt to the alkaline environment generated by the cementitious materials employed in backfilling and construction. Within the present study, a range of alkaline and neutral pH sediments were investigated to determine the upper pH limit and the preferred route of methane generation. In all cases, the acetoclastic route did not proceed above pH 9.0, and the hydrogenotrophic route dominated methane generation under alkaline conditions. In some alkaline sediments, acetate metabolism was coupled to hydrogenotrophic methanogenesis via syntrophic acetate oxidation, which was confirmed through inhibition studies employing fluoromethane. The absence of acetoclastic methanogenesis at alkaline pH values (>pH 9.0) is attributed to the dominance of the acetate anion over the uncharged, undissociated acid. Under these conditions, acetoclastic methanogens require an active transport system to access their substrate. The data indicate that hydrogenotrophic methanogenesis is the dominant methanogenic pathway under alkaline conditions (>pH 9.0).

Highlights

Geological disposal is largely the preferred choice for the management of intermediate level radioactive wastes (ILWs; IAEA, 2011) with a number of countries pursuing the establishment of geological disposal facilities (GDFs; Faybishenko et al, 2016)
The trend toward hydrogenotrophic methanogenesis at alkaline pH was confirmed by the establishement of hydrogen and acetate enrichments from the cellulose degradation products (CDPs) fed Control and New Lime enrichments at each pH
The data presented here suggest that an alkaline pH >9.0 results in a methanogen community that is dominated by hydrogenotrophs, with populations enhanced by the activity of syntrophic acetate oxidation (SAO) bacteria

Summary

Introduction

Geological disposal is largely the preferred choice for the management of intermediate level radioactive wastes (ILWs; IAEA, 2011) with a number of countries pursuing the establishment of geological disposal facilities (GDFs; Faybishenko et al, 2016). The presence of metallic and organic materials means that abiotic and biotic gas generating processes (NEA, 2014) are likely to proceed within a GDF resulting in gas generation being evaluated in a number of international waste management programs (Felicione et al, 2003; NDA, 2010; Avis et al, 2011; Summerling, 2013; Poller et al, 2016). The alkaline conditions generated by the cementitious materials will promote the alkaline hydrolysis of cellulosic wastes (Humphreys et al, 2010). This abiotic process generates a range of cellulose degradation products (CDPs; Rout et al, 2015b; Charles et al, 2019) collectively which are dominated (>70%) by isosaccharinic acids (ISAs; Almond et al, 2012)

Methods

Results

Conclusion