Impact of Iron-Reducing Bacteria on the Corrosion Rate of Carbon

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Impact of iron-reducing bacteria on the corrosion rate of carbon steel under simulated geological disposal conditions Marta K. Schütz, Michel L. Schlegel, Marie Libert, and Olivier Bildstein Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00693 • Publication Date (Web): 19 May 2015 Downloaded from http://pubs.acs.org on May 19, 2015

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Impact of iron-reducing bacteria on the corrosion rate of carbon steel under simulated geological disposal conditions Marta K. Schütz§,ǁ,¥,†,*, Michel L. Schlegel #,ǂ,¥, Marie Libert §,¥, Olivier Bildstein §,¥ §

CEA, DEN, DTN/SMTA/LMTE, 13108 Saint Paul lez Durance, France

#

CEA, DEN, DANS/DPC/SEARS/LISL, F91191 Gif-sur-Yvette, France

* Corresponding author: [email protected] KEYWORDS: biocorrosion, iron-reducing bacteria, magnetite, hydrogen, carbon steel, corrosion rate, geological disposal 1

ABSTRACT. The current projects for the disposal of high-level radioactive waste rely on

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underground burial and confinement by metallic envelopes that are susceptible to corrosion

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processes. The impact of microbial activity must be fully clarified in order to provide biological

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parameters for predictive reactive transport models. This study investigates the impact of

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hydrogenotrophic iron-reducing bacteria (Shewanella oneidensis strain MR-1) on the corrosion

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rate of carbon steel under simulated geological disposal conditions by using a geochemical

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approach. It was found that corrosion damage changes mostly according to the experimental

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solution (i.e. chemical composition). Magnetite and vivianite were identified as the main

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corrosion products. In the presence of bacteria, the corrosion rate increased by a factor 1.3

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(according to weight loss analysis) to 1.8 (according to H2 measurements) and the detected

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amount of magnetite diminished. The mechanism likely to enhance corrosion is the

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destabilization and dissolution of the passivating magnetite layer by reduction of structural Fe(III)

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coupled to H2 oxidation.

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INTRODUCTION

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The disposal of high-level radioactive waste (HLW) in deep geological formations is the

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solution investigated for HLW management in many countries. In these repositories, long-term

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confinement of HLW would be ensured by a multi-barrier system involving several natural (e.g.

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clay minerals) and/or engineered barriers (e.g. carbon steel, stainless steel, glass, concrete).1 An

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assessment of the long-term behavior of the radioactive waste and confining materials in the

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geological environment is crucial to demonstrate the disposal safety. This assessment entails an

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understanding of corrosion of the successive metallic barriers in contact with water coming from

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the geological formation.

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Corrosion can be described as a process coupling the oxidation of metals (anodic reaction) and

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the reduction of an oxidizing agent (cathodic reaction) in the presence of an electrolyte. For

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example, iron corrosion by water under anoxic conditions usually lead to the release of Fe2+

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which precipitates as Fe(OH)2 according to:

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Fe + 2H2O → Fe2+ + 2OH- + H2 → Fe(OH)2 + H2

(1)

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Fe(OH)2 is metastable in solution and oxidizes to form magnetite (Fe3O4).2 The overall chemical reaction describing iron oxidation and magnetite formation can be written:

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3Fe + 4H2O → Fe3O4 + 4H2

(2)

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However, this simple description does not apply in the geological environment, where

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physicochemical conditions change with time and promote the formation of several distinct solid

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corrosion products at the metal surface. In addition, these solid products can further protect the

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metal from corrosion, depending on their chemistry and morphology. The current consensus is

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that the most aggressive forms of corrosion (e.g. localized, stress corrosion cracking) will occur

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during initial transient conditions in the repository, and more predictable generalized corrosion

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will take place later.3 Several studies have suggested that decreasing long-term corrosion rates

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may result from the presence of a passivating layer made of magnetite in direct contact with the

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metal surface.4-10 However, changes in the environmental conditions (e.g. pH, redox potential,

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CO2 pressure, presence of microorganisms) can affect the stability of passivating layers and,

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therefore, the overall susceptibility of the metal to corrosion.

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Corrosion associated to the action of microorganisms is known as either microbiologically

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influenced corrosion (MIC) or biocorrosion. MIC is usually related to the presence of biofilms

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and to bacterial metabolism11,12 which can control physicochemical properties at the metal

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surface,13-15 alter the properties and stability of passivating layers,16-18 and eventually modify

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(increase or decrease) the kinetics of cathodic and/or anodic reactions.19,20 Of specific concern is

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the role of iron-reducing bacteria (IRB), for which either inhibitory or enhancing effects have

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been hypothesized.21-24 Indeed, magnetite destabilization and dissolution under anaerobic

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conditions has been observed as a result of the activity of bacteria using hydrogen (H2) to reduce

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structural Fe(III).17,18,25 This mechanism is therefore believed to influence the corrosion rate. In

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fact, corrosion processes can provide all the necessary chemical compounds for IRB anaerobic

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bioactivity, such as H2 as electron donor and Fe(III) from Fe hydr(oxides) as electron acceptor.

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Hydrogen is a potent energetic substrate for anaerobic microbial communities, especially in

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geological formations containing small amounts of biodegradable organic matter.26-30 In addition,

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the extreme conditions (e.g. radiation, desiccation, high temperature and pressure) that will

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prevail during the early lifetime of the geological repository should not prevent microbial

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survival and subsequent growth when conditions will become more favorable: radiation dose rate

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< 60 Sv.h-1, temperature < 70°C, and water saturation > 0.97.20,31 Therefore, the effect of

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microbial activity on corrosion needs to be investigated and quantified in the framework of

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safety assessment of HLW geological disposal. Our previous work demonstrated the impact of

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hydrogenotrophic IRB on corrosion process by combining geochemical and electrochemical

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techniques.18 Short term corrosion rates (20 days) were estimated from aqueous Fe concentration

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(in the case of geochemical measurements) and from exchanged charge (in the case of

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electrochemical measurements).

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The objective of the present study is to assess the impact of hydrogenotrophic IRB on carbon

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steel corrosion mechanisms and corrosion rate on a longer timeframe (5 months) and using

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additional techniques. Corrosion rates were determined by weight loss analysis and H2

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measurements in order to compare these two methods. The presence and nature of corrosion

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products was assessed by spectroscopic and structural techniques to relate the discrepancies in

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corrosion rate to the nature of corrosion products. Shewanella oneidensis strain MR-1 has been

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chosen as model organism for this study because it is able not only to oxidize organic matter but

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also hydrogen, an abundant energy source in such a disposal environment.28

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MATERIALS AND METHODS

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Experimental solutions. Two experimental solutions with distinct chemical compositions

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were used in this study: M1 and S1 (see chemical composition in Table 1). M1 is a minimal

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medium used to cultivate IRB species but with a chemical composition slightly modified in order

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to be representative of the groundwater from the argillaceous Callovo-Oxfordian formation of

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the French Underground Research Laboratory (FURL, Bure, France).18,32 For comparison

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purposes, a solution (S1) with simplified chemical composition was also used. Its composition

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consists in a mixture of 10 mM Na2SO4, 2 mM NaHCO3, and vitamins (0.08 mM nicotinic acid,

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0.01 mM thiamine-HCl, 0.40 µM biotine). The sulfate concentration is similar to that measured

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in the FURL groundwater.32 The pH in the M1 and S1 was adjusted to ~7 with NaOH, and

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solutions were sterilized by autoclaving (121°C, 20 min), except for the thermolabile

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components (e.g. vitamins) which were filter-sterilized through a 0.22 µm filter and added to the

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autoclaved solutions.

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Table 1. Chemical composition of the M1 and S1 solutions. Concentration Compound M1

S1

Na2SO4

-

10 mM

(NH4)2SO4

9 mM

-

Na2SeO4

11 µM

-

HEPES

17 mM

-

NaHCO3

2 mM

2 mM

K2HPO4

0.5 mM

-

KH2PO4

0.3 mM

-

CoSO4.7H2O

5 µM

-

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NiSO4.6H2O

5 µM

-

NaCl

10 µM

-

H3BO3

45 µM

-

ZnSO4.7H2O

0.8 µM

-

Na2MoO4.2H2O

3 µM

-

CuSO4.5H2O

0.2 µM

-

MnSO4.H2O

1 µM

-

MgSO4.7H2O

0.8 mM

-

CaCl2.2H2O

0.4 mM

-

FeSO4.7H2O

4 µM

-

arginine

0.11 mM

-

glutamate

0.13 mM

-

serine

0.19 mM

-

nicotinic acid

0.08 mM

0.08 mM

thiamine-HCl

0.01 mM

0.01 mM

biotine

0.40 µM

0.40 µM

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Bacterial culture. Shewanella oneidensis strain MR-1 (ATCC 700550TM) was used as model

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of hydrogenotrophic IRB in the biotic experiments. Pre-cultures were obtained aerobically at the

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beginning of stationary growth phase in Luria Bertani Broth (LB) medium (5 g.L-1 NaCl, 10 g.L-

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1

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medium by centrifugation (4000 rpm, 20 min), washed once with sterile M1 or S1 solutions and

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then inoculated into the batch reactors. The initial bacterial concentration was determined by

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epifluorescence method with LIVE/DEAD® BacLight™ kit33,34, and equalled 108 cells.mL-1.

tryptone, 5 g.L-1 yeast extract) after 24 h at 30°C. Bacterial cells were harvested from the LB

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Corrosion experiments. One- and 5-months experiments were performed in 250 mL batch

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reactors at 30oC under anaerobic atmosphere (N2:CO2 ratio of 90:10%) and abiotic (without

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bacteria) or biotic (with bacteria) conditions. The experimental conditions were detailed in our

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previous study.18 All experiments were started under aseptic conditions by introducing 140 mL

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of experimental solution (either M1 or S1) and one cylindrical coupon sample (A37 low carbon

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steel polished up to 600 grit) into glass reactors without agitation. The coupons were laterally

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embedded in a diallyl phthalate glass-fiber resin in order to expose only a basal surface of 0.8

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cm2. Biotic conditions were achieved by adding 1 mL of bacterial inoculum (108 cells.mL-1).

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Then, the reactors were sealed with rubber plugs and aluminum caps and flushed with anaerobic

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gas that was sterilized by passing through a 0.22 µm filter.

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Sample analyses. Aqueous Fe concentration ([Fe]aq) was analyzed by inductively coupled

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plasma optical emission spectrometry (ICP-OES, Varian VISTA-MPX) after 0.02 µm filtration

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and 2% (v/v) HNO3 acidification. Analysis of H2 in the headspace was carried out by micro gas

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chromatography (GC, Varian CP-4900) using a thermal conductivity detector with N2 as carrier

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gas. The total H2 concentration ([H2]) was calculated as the sum of the contributions from the

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gaseous and aqueous phases (determined using Henry’s law), normalized to the solution volume

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to allow direct comparison with [Fe]aq. The experiments were usually performed in triplicate,

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with two coupons used for the weight loss analysis, and one for the characterization of solid

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corrosion products.

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After experiment, all coupon samples were dried in an M-Braun LABstar MB10 anaerobic

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glovebox (O2 concentration