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Geochemical effects of millimolar hydrogen concentrations in groundwater - an experimental study in the context of subsurface hydrogen storage Marton Berta, Frank Dethlefsen, Markus Ebert, Dirk Schafer, and Andreas Dahmke Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05467 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018
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Geochemical effects of millimolar hydrogen
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concentrations in groundwater - an experimental
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study in the context of subsurface hydrogen storage
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Marton Berta*, Frank Dethlefsen**, Markus Ebert, Dirk Schäfer, Andreas Dahmke
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Department of Applied Geology, Aquatic Geochemistry and Hydrogeology,
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Institute of Geoscience, Kiel University
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Ludewig-Meyn-Straße 10, 24118 Kiel, Germany
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*
[email protected], **
[email protected] 9
ABSTRACT
Hydrogen storage in geological formations is one of the most promising
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technologies for balancing major fluctuations between energy supply from renewable energy
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plants and energy demand of customers. If hydrogen gas is stored in a porous medium or if it
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leaks into a shallow aquifer, redox reactions can oxidize hydrogen and reduce electron acceptors
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such as nitrate, FeIII and MnIV (hydro)oxides, sulfate, and carbonate. These reactions are of key
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significance, because they can cause unintentional losses in hydrogen stored in porous media and
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they also can cause unwanted changes in the composition of protected potable groundwater. To
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represent an aquifer environment enclosing a hydrogen plume, laboratory experiments using
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sediment-filled columns were constructed and percolated by groundwater in equilibrium with
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high (2-15 bars) hydrogen partial pressures. Here we show that hydrogen is consumed rapidly in
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these experiments via sulfate reduction (18±5 µM h-1) and acetate production (0.030±0.006 h-1),
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while no methanogenesis took place. The observed reaction rates were independent from the
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partial pressure of hydrogen and hydrogen consumption only stopped in supplemental
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microcosm experiments where salinity was increased above 35 g l-1. The outcomes presented
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here are implemented for planning the sustainable use of the subsurface space within the
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ANGUS+ project.
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Introduction
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The energy production by wind turbines and solar panels fluctuates due to variable wind
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velocity and solar radiation, while the energy demand of consumers also varies creating a
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significant challenge for the stability of future energy supplies based mainly on renewable
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sources. One concept to address this issue is the “power to gas” approach, where the generation
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of a gas with high energy content uses the excess electrical energy at times where the public
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energy demand is smaller than the produced energy. Large scale gas storage is a potentially
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necessary element in this concept, allowing the gas to be available for reuse when the demand
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for energy exceeds the supply. The overall difference between the amount of energy supplied by
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renewable energy sources and the amount of energy demanded usually varies in time, but this
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gap may reach 10 GWh - 10 TWh, calculated as an example on a time scale of days to months on
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a German regional scale power network1. Electrochemical generation, storage, and consumption
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of molecular hydrogen (in the following just “hydrogen”) has the potential to balance such
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fluctuating gaps1, 2. The volume of hydrogen corresponding to the amount of energy to be
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balanced in an energy network ranges between millions to billions normal cubic meters2. The
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subsurface may offer a comparatively cost-efficient and safe option for storing such large
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quantities of gaseous hydrogen2-9. Moreover, sustainable, renewable-based energy systems will
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likely require a series of subsurface energy storage technologies simultaneously.
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technologies include chemical (hydrogen, synthetic methane), mechanical (compressed air), or
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heat energy storage, facing the considerable amount of energy that needs to be stored and the
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lack of other available storage technologies in the short or medium term fulfilling volume, cost
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and response time requirements1.
Such
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Power-to-gas energy storage including geological hydrogen storage is expected to play a
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potentially deciding role in future energy networks, considering the following reasons. First,
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large-scale development of renewable energy sources became a cornerstone of all national
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energy policies following the Paris Agreement10, even if those actions are still unsatisfying
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compared to the volume of development required to reach climate protection goals already
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agreed on internationally11. Second, consequently, estimations on for instance German energy
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supplies project up to 100% renewable power generation by 205012. Third, if more than ca. 80%
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of the energy is produced by renewable sources in a power network then the inclusion of large-
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scale storage facilities, like subsurface hydrogen storage, is suggested to be a prerequisite for
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stable and profitable operation1. Overview studies on technical aspects of subsurface hydrogen
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storage were published in the last few years13-16 as a part of an increasing research and industrial
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interest, but field or experimental studies specifying potential environmental impacts or
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operational issues linked to storing large amounts of pure hydrogen at elevated pressures in
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geological media are missing so far. However, studies on hydrogen-containing gas mixtures
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stored in porous town gas reservoirs hint that hydrogenotrophic redox reactions potentially
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consume hydrogen at such conditions14, 17-21. These reactions may cause losses from the stored
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gas and they may also change the composition of reservoir water. Furthermore, accidental
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leaking of hydrogen into shallow aquifers22 may result in conditions where an enlarged hydrogen
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partial pressure (i.e. equal to the hydrostatic pressure in case of a pure hydrogen gas phase) will
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cause a high dissolved hydrogen concentration in the shallow groundwater, probably initiating
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typical redox reactions associated with hydrogen oxidation.
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Hydrogen-driven redox reactions are predominantly microbiologically catalyzed23 and well
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known from aquatic geosystems, where hydrogen is the electron donor and O2, NO3-, FeIII and
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MnIV(hydr)oxides, SO42-, or dissolved CO2 are usually the terminal electron acceptors (Table 1).
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These redox reactions may produce NO2-, N2O, N2, NH4+, MnII, FeII, H2S, CH3COOH, or CH4,
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which might be released into the pore water or precipitated into various mineral phases.
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Reaction products such as NO2-, H2S or CH4 may have a negative effect on the composition of
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the groundwater in terms of its usability for other applications, e.g. drinking water production24.
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Moreover, groundwater itself is usually a protected natural good.
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fermentative degradation of peptides, saccharides, and further organic substances produces
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hydrogen25, while other microorganisms consume it, usually resulting in nanomolar
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concentrations of dissolved hydrogen in a steady state25-41. A competition for hydrogen may
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then result in the development of a typical redox sequence (i.e. decreasing redox potential with
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increasing time or flow path length) because the electron accepting processes coupled to
In pristine aquifers,
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hydrogen oxidation with the higher energy yield (e.g. sulfate reduction) outcompete the others
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(e.g. acetogenesis) by keeping the hydrogen concentrations below a threshold concentration
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required for that process (Table 1)42, 43. Amongst other possibilities, such reactive environments
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may be numerically simulated assuming a dual substrate limitation by hydrogen concentration on
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the one hand side and the concentration of sulfate or total dissolved inorganic carbon species on
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the other. Furthermore, not only the occurrence but also the kinetics of the redox reactions is
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usually dependent on the concentration of dissolved hydrogen in those hydrogen-limited reactive
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environments and such processes are often modeled using Monod kinetics44-48.
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In-situ remediation of contaminated aquifers may lead to elevated dissolved hydrogen
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concentration, e.g. when applying zero valent iron (ZVI)49-51 or hydrogen releasing compounds52,
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in hydrogenotrophic denitrification systems53-57, or during the sulfidogenic treatment of
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wastewaters containing dissolved metal ions58, 59. Increased dissolved hydrogen concentration is
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thereby either an aim of the technology (e.g. denitrification or sulfidogenic treatment) or a side-
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effect (e.g. ZVI application for reductive dehalogenation). Usually, those systems also show the
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establishment of redox sequences known from pristine aquifers, and the kinetics of the occurring
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redox reactions also depend on the hydrogen concentrations created by hydrogen-producing
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primary reactions46.
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environments is mostly limited to a few hundred micromoles per liter.
Furthermore, the hydrogen concentration found in such reactive
In the context of the storage of pure gaseous hydrogen at pressures between 30 and 400 bars60-
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per liter groundwater due to the roughly linear correlation between increasing depth and
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increasing hydrostatic pressure as well as between increasing pressure and increasing gas
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solubility following Henry’s law. Also an unintended leakage of pure hydrogen gas into a
in the subsurface, dissolution of hydrogen may result in concentrations of several millimoles
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shallow aquifer (up to 140 m depth) will result in a much higher dissolved hydrogen
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concentration than the nanomolar quantities usually present in aquifers (Table 1). Elevated
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hydrogen concentrations will probably result in a surplus in hydrogen supplies for the microbial
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community, which usually strives for hydrogen.
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consuming hydrogen rapidly through a series of consequent redox reactions46,
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simultaneous redox processes64 without the establishment of typical redox sequences. On the
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other hand, the groundwater above a gas storage reservoir may have an elevated salt content and
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a saltwater or brine intrusion might accompany a gas leakage into a shallow aquifer. Elevated
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salinity potentially inhibits the microbiologically catalyzed redox reactions due to a limited
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halotolerance of the microorganisms present65-72.
Therefore, the microorganisms may start 63
or through
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In order to estimate the potential effects the subsurface storage of hydrogen could have on
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groundwater, including the effects following an unintended leakage into a shallow aquifer, an
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improved understanding of hydrogenotrophic redox reactions at high dissolved hydrogen
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concentration is required, but investigations on those systems are missing so far, especially
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regarding reaction kinetics. Available models on the spatial or temporal zonation of redox
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processes arise in part from electron donor limitation, while after a gaseous hydrogen intrusion
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into an aquifer the availability of hydrogen as electron donor is not limiting any more. The
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knowledge on the establishing sequence of redox reactions, their reaction kinetics, and reaction
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products is necessary for any risk assessment or spatial planning attempts and for modeling
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efforts at reservoir or regional scales7, 8. Furthermore, monitoring and operation strategies for a
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gas storage site also require knowledge about the fate of the stored gas in shallow aquifers
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completed by an estimation of the impact the stored gas may have through volatile products of
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redox reactions (e.g. H2S or CH4).
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The study presented here shows results of a high pressure column experiment using natural
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aquifer sediment and groundwater equilibrated with hydrogen partial pressures of 2-15 bars,
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representing a scenario of hydrogen gas leakage into a shallow or medium deep aquifer. The
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results enabled the development of a simplified reaction model scheme describing simultaneous
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redox reactions at elevated dissolved hydrogen concentrations at flow through conditions,
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including reaction kinetics and rate coefficients. Furthermore, results of a simple testing routine
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for estimating the halotolerance of a microbial community are shown. The outcome of the study
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is a first step in an improvement of the understanding of hydrogen oxidizing redox reactions in
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the context of hydrogen gas storage. The contributions of this work will also be integrated into
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the Compendium on Concepts and Methods for Quantitatively Deducting the Spatial Demand of
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Subsurface Energy Storage prepared by the ANGUS+ research project facilitating future
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improvements in planning the sustainable use of the subsurface space6-8.
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Table 1. Hydrogen oxidizing reactions with associated Gibbs free energy yields under standard
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conditions and neutral pH; and the concentrations of hydrogen typical for environments with
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characteristic metabolic processes.
Reactions Aerobic hydrogen oxidation Hydrogen oxidation coupled to denitrification producing N2 Hydrogen oxidation coupled to nitrate reduction producing NH3 Hydrogen oxidation
2 H2+O2 2 H2O
5 H2+2 H++2 NO3- N2+6 H2O
4 H2+2 H++NO3- NH4++3 H2O H2+MnO2 Mn(OH)2
∆G0
H2
[kJ·mol(H2)-1]
[nmol·l-1]
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n.d.
73
-224
0.03
25, 46, 73
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