Abiotic nitrate reduction induced by carbon steel and hydrogen: Implications for environmental processes in waste repositories

Applied Geochemistry 28 (2013) 155–163

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Abiotic nitrate reduction induced by carbon steel and hydrogen: Implications for environmental processes in waste repositories Laurent Truche a,⇑, Gilles Berger b, Achim Albrecht c, Léo Domergue d a

Université de Lorraine, G2R, 54506 Vandoeuvre-Lès-Nancy, France CNRS, IRAP, 14 av. E. Belin, 31400 Toulouse, France c ANDRA, 1/7 rue Jean Monnet, 92290 Châtenay-Malabry, France d Université de Toulouse, IUT Mesure Physique, 115 Route de Narbonne, 31077 Toulouse Cedex 4, France b

a r t i c l e

i n f o

Article history: Received 18 April 2012 Accepted 11 October 2012 Available online 23 October 2012 Editorial handling by J. Routh

a b s t r a c t Reducing conditions induced by steel canister corrosion and associated H2 generation are expected in nuclear waste repositories. Aqueous NO 3 present in the aquifers will become thermodynamically unsta ble and may potentially be converted to N2 and/or NHþ 4 . However, NO3 reduction by H2, in the absence of bio-mediators, is generally thought to be kinetically hindered at low temperature, although the reaction may be promoted by the concomitant oxidation of Fe. In this study the reduction rate of aqueous NO 3 is quantified in the presence of H2 and carbon steel surfaces from waste canisters and construction materials, as well as magnetite as their possible corrosion by-products. A parametric study (0 < P(H2) < 10 bar, 0.1 < [NO 3 ] < 10 mM, 90 < T° < 180 °C, 4 < pHin situ < 9) reveals that even at 90 °C the reaction can occur within hours or days and leads to the formation of NHþ 4 and pH increase. Different mechanisms may be potentially involved. It is shown that NO 3 reduction in the presence of carbon steel does not require H2, since steel constitutes an electron donor by itself, as does metallic Fe. The reaction rate is strongly pHdependent. Activation energy in the 90–180 °C range is found to be 45 kJ/mol. Magnetite is the main corrosion by-product and specific experimental runs demonstrate that it can serve as a catalyst for the  NO 3 —H2 reaction. Hydrogen alone, without the presence of steel, is not sufficient to reduce NO3 under the temperature and pressure conditions used in this study. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Nitrate reduction induced by different electron donors represents a major reaction of interest, particularly in the context of nuclear waste disposal (Honda et al., 2006; Albrecht, 2008; Katsounaros et al., 2009) or remediation and decommissioning of nuclear sites containing such oxyanions (Zachara et al., 2007; Thorpe et al., 2012). Nitrates are amongst other oxyanions byproducts of fuel-rod dissolution and sequential extraction during reprocessing. Hence, they are present in significant amounts in a wide range of nuclear waste matrices (e.g. bitumen, concrete, compacted salts). The bituminous wastes, which are part of the intermediate level, long-lived wastes (MAVL), is used here as a relevant example, with the work focusing entirely on abiotic NO 3 reduction. Bituminous wastes are characterized by the coexistence 3 2 of oxyanions (e.g. NO 3 , PO4 , SO4 ), and potentially reducing agents such as organic matter, native metals and H2 gas present in the waste mixture. In deep geological repositories, H2 would be produced both by radiolysis and by anoxic corrosion of Fe present in the steel containers (Ortiz et al., 2002; Lassabatère et al., ⇑ Corresponding author. Tel.: +33 3 83 68 47 13; fax: +33 3 83 68 47 01. E-mail address: [email protected] (L. Truche). 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2012.10.010

2004). In addition to the large number of reactants present in bituminous wastes, metallic components present in the armored concrete over packs as well as concrete in the MAVL waste cell itself need to be considered. Together these constituents form an architecture that guarantees long-term mechanical stability of both the waste container and the waste cell. From a thermodynamic point of view, the coexistence of NO 3 and numerous potential electron donors (e.g. organic matter, H2, Fe, steel) favor redox reactions, which would lead to the generation of more reduced N species þ such as: NO 2 , N2, NH3 or NH4 , and pH increase (Eqs. (1)–(4)). þ The production of NH4 , which is an excellent exchangeable cation, can affect the adsorption capacity of clay minerals, and compete with radionuclides such as 135Cs for sorption sites (Missana et al., 2004). Formation of NHþ 4 will also enhance degradation of concrete and reduce its durability (Carde et al., 1997). In addition, steel corrosion and H2 generation could be affected by Fe oxidation induced by NO 3 reduction.

NO3 þ 4H2 þ 2Hþ ! NHþ4 þ 3H2 O

ð1Þ

4Feð0Þ þ 10Hþ þ NO3 ! NHþ4 þ 4Fe2þ þ 3H2 O

ð2Þ

8Fe2þ þ NO3 þ 10Hþ ! 8Fe3þ þ NHþ4 þ 3H2 O

ð3Þ

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NO3 þ 8Fe2þ þ 13H2 O ! NHþ4 þ 8FeOOH þ 14Hþ

ð4Þ

It is well known that the NO 3 ion has a high chemical stability. In order to react with suitable reducing agents to form N2 or NH3, presence of catalysts or high temperature are required (Fanning, 2000). There is a clear distinction between enzymatic reactions controlled by microbial activity leading to N2 formation (Van Loosdrecht and Jetten, 1998), and surface reactions leading to the formation of NHþ 4 (Fanning, 2000). In order to provide a better understanding of these processes, here the focus is on abiotic NO 3 reduction. The question of the reaction preferences in the case of competition between abiotic vs. biotic reaction will have to be investigated subsequently. In the case of abiotic NO 3 reduction in a MAVL bituminous waste cell, the role of Fe(0), Fe(II) (in solution, adsorbed, complexed, or structural), and of some Fe-oxides can be considered both as catalysts and electron donors. Under anoxic conditions, granular or colloidal zero-valent Fe (Singh et al., 1996; Cheng et al., 1997; Huang et al., 1998; Zawaideh and Zhang, 1998; Devlin et al., 2000; Huang and Zhang, 2002; Alowitz and Scherer, 2002; Rodriguez-Maroto et al., 2009), freshly precipitated Fe oxy-hydroxyde (Postma, 1990), wüstite (Rakshit et al., 2005), and green rust (Hansen et al., 2001) are reducing agents for NO 3. Hence, carbon steel, which constitutes a part of the waste canister material and engineered barriers, can be considered as an electron donor. However, oxic (Fe oxy-hydroxyde, maghemite) and anoxic (ferrihydrite, magnetite, green rust) corrosion by-products may also contribute to NO 3 reduction. In addition to the different types of steel and metallic phases that are used (e.g. carbon steel, stainless steel 316L, special alloys), it is necessary to consider the presence of H2, which is both a corrosion by-product (Eqs. (5)–(7)), and a potential NO 3 reducing agent (Eq. (1)).

Feð0Þ þ 2H2 O ! Fe2þ þ H2 þ 4OH

ð5Þ

Feð0Þ þ 2Hþ ! Fe2þ þ H2

ð6Þ

3Feð0Þ þ 4H2 O ! Fe3 O4 þ 4H2

ð7Þ NO 3

It is hypothesized that abiotic reduction can occur in disposal cells for intermediate-level nuclear waste. However, its prediction remains ambiguous because of the variety of available catalysts, electron donors, their interdependence and uncertainties regarding waste cell evolution in time and space. In this contribution, the influence of carbon steel and magnetite as its possible corrosion by-product on NO 3 reduction as a function of: H2 pressure (0– 7.5 bar), aqueous NO 3 concentration (0.1–10 mM), temperature (90–180 °C), and pH (4–9), are evaluated. The given ranges are based on estimations made for nuclear waste storage safety assessment. Notably, in the case of H2, the pressure can reach a theoretical, but unlikely value of 90 bar (Talandier et al., 2006; Xu et al., 2008). Nitrate, in a normal scenario, is not considered to reach concentrations above 10 mM because of the slow diffusive transport within a bitumen matrix (Sercombe et al., 2006). The pH range has been restricted to values possibly occurring in clay-rich host rocks. In addition, the decrease in pH as a consequence of pyrite oxidation and formation of H2SO4 (Konhauser, 2007), as well as increase in pH because of the close vicinity to concrete barriers (Gaucher and Blanc, 2006), has been considered. Only the temperature range does not reflect values likely for a bituminous waste cell, because temperatures are not expected to increase above 40 °C. Higher temperatures enhance reaction kinetics and reduce reaction times; hence, they have been chosen to study the reaction kinetics within a specified timeframe. 2. Literature review: nitrate/iron(0)/iron-oxide systems This section presents an overview of the abundant literature in the context of groundwater or sewage pollution and early Earth

prebiotic syntheses, in order to draw a parallel for NO 3 reduction pathways under deep geological conditions in the presence of H2 gas, steel, and various corrosion products. It has been clearly demonstrated that reduction of NO 3 to NH3/ NHþ 4 occurs at room temperature and pressure under oxic or anoxic conditions in the presence of un-corroded and pure metallic Fe. Magnetite forms as a by-product and coats the Fe surface. The reactions and associated kinetics are complex and dependent on numerous factors such as: pH, temperature, initial NO 3 concentration and specific surface area of Fe particles. In acidic solutions (pH < 4), NO 3 reduction in Fe(0)–H2O systems is fast and efficient (>95%); (Singh et al., 1996; Zawaideh and Zhang, 1998; Huang and Zhang, 2004; Yang and Lee, 2005). Above pH 5, NO 3 reduction efficiency decreases and is usually below 50% (Singh et al., 1996; Cheng et al., 1997; Huang et al., 1998; Zawaideh and Zhang, 1998; Huang and Zhang, 2002; Alowitz and Scherer, 2002; Xiaomeng et al., 2009). These studies however do not reveal a general consensus concerning the reaction rate and mechanism. Therefore, different models have been proposed e.g., first order reaction rate with time (Cheng et al., 1997; Choe et al., 2000, 2004; Alowitz and Scherer, 2002), superior (Huang et al., 1998; Rodriguez-Maroto et al., 2009) or inferior orders (Yang and Lee, 2005), Langmuir adsorption formulation (Huang and Zhang, 2002), and others. These kinetic models depend on the nature of electron donors. For example, research conducted these past few years suggests that the reducing agent could be: (1) the Fe(0) – mixed-oxide layer assemblage, whereby electrons are transferred through the conductance bands of semi-conductive oxide layers or (2) the atomic H produced by Fe(0) oxidation through an acid-driven process (Scherer et al., 1998; Huang et al., 2003; Huang and Zhang, 2004; Mishra and Farrell, 2005). 2+ Other studies have focused on NO 3 reduction by Fe , Fe(OH)2, and some Fe oxy-hydroxides (Bremner and Shaw, 1955; Brown and Drury, 1967; Buresh and Moraghan, 1976; Van Hecke et al., 1990; Hansen and Bender Koch, 1998; Hansen et al., 2001; Rakshit et al., 2005; Choi and Batchelor, 2008). They indicated that under alkaline pH conditions, Fe(OH)2 is the reactive species rather than a free ion, and that magnetite, is produced as a by-product. The enhanced reactivity of Fe2+ when it forms a complex as Fe(II)-bearing minerals or adsorbs to the surface is well known during the reductive transformation of organic pollutants (Stumm, 1992; Scherer et al., 1998; Schultz and Grundl, 2000). The most interesting consequence for such a mechanism is that precipitation of the newly formed Fe2+ phase could speed up NO 3 reduction by providing additional reactive sites. Data for NO 3 reduction at elevated temperatures are rare and little attention has been paid to the activation energy of these reactions. Rakshit et al. (2005) estimated an activation energy of 47 kJ/mol for NO 3 reduction in the presence of FeO(s) (wüstite) at 3–44 °C. Hansen and Bender Koch (1998) reported higher (84 kJ/mol) activation energy for NO 3 reduction by green rust (mixed oxide Fe(II)–Fe(III)) at 15–50 °C. Activation energy can be related to the elementary reaction controlling the overall reaction because it is proportional to the energy threshold that needs to be crossed to drive the elementary reaction. It is often considered that activation energy values below 20 kJ mol1 indicate a diffusion-controlled kinetic regime, whereas values between 20 and 70–80 kJ mol1 suggest a kinetic regime more likely controlled by a combination of surface reactions and diffusion (Lasaga, 1984). In the presence of steel, only two studies are available in the literature. These studies indicate the production of NO 2 , and NH3 in stagnant water inside non-galvanized steel tubing (Künzler and Schwenk, 1983), and in the presence of carbon steel under alkaline conditions at 50 °C (Honda et al., 2006).

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powder (Alfa Aesar, 99.997%, 1.68 ± 0.01 m2/g) was reacted in the presence of aqueous NO 3.

3. Experimental methods Batch experiments on NO 3 reduction by H2 in the presence of carbon steel (main part of the experiments), magnetite or FeCl2 were carried out at 90–180 °C (±1 °C) in three-phase systems: liquid (aqueous NO 3 -bearing solution, 0.1–10 mM) + solid (powder or steel chips) + gas (0–7.5 bar H2 partial pressure). In some experiments bentonite MX80 (mainly composed of smectite; see Table S2 in Supplementary data) or gaseous CO2 were added as pH buffer.

3.1. Chemicals All aqueous solutions were prepared using ultrapure water with resistivity up to 18.3 MX cm (Milli-Q water). Nitrate stock solution was prepared with KNO3 (99%+, Aldrich). For the experiment dedicated to evaluating the effect of Fe2+ as a potential electron donor for NO 3 reduction, FeCl2, (99%+, Aldrich) was used. To avoid any oxidation before the experiments, FeCl2 feed solution was stocked in an anoxic glove box (PlasslabÒ, equipped with Pd catalyst and filled with Ar/10%H2, [O2(g)] < 2 ppmv). Carbon steel tubes (see chemical composition displayed in Table S1; Supplementary data) were mechanically stripped to remove surface oxidation products and surfactants. They were subsequently filed or milled to obtain either a powder or metal chips. The specific surface areas of the steel samples are, respectively, 1.9(±0.2)  102 m2/g (krypton BET surface measurements on a Coulter SA 3100 apparatus – solids outgassed overnight at 100 °C down to 2 lm Hg pressure before analysis with Kr as sorbent gas) and 0.1(±0.05)  102 m2/g (geometrical consideration). The metal particles obtained were immediately used for experiments or stored in the anoxic glove box to prevent oxidation of the material. During the experiments the increase in specific surface area of the powders did not exceed 20%, and the reaction rates were calculated on the basis of the initial steel powder surface area. In addition to carbon steel, the potential catalytic effect of magnetite was also investigated. For this experiment, fine magnetite

3.2. Apparatus The experiments were conducted in a 450 mL ParrÒ stirred hydrothermal reactor made of pure Ti grade 4 (Ti-reactor) with a maximum working pressure and temperature of 100 bar and 300 °C, respectively. The reactor was filled to 66% of its volume by solution, continuously stirred by an impeller driven by an external magnetic driver. For safety reasons, 10% H2 in Ar (grade 5.0) was used rather than pure H2 as the source of H2 in the system. As demonstrated in preliminary experiments conducted without steel, Ti is an inert material with respect to NO 3 reduction in presence of hydrogen. 3.3. Experimental procedure Steel powders or chips were weighed and added to the solution, so allowing varying the specific surface area/water ratio. The solution was poured directly intyo the pressure vessel and then flushed with Ar for at least 30 min to remove dissolved O2. The reactor was pressurized with Ar–10%H2 gas mixture at room temperature in order to obtain a H2 partial pressure of between 0.2 and 7.5 bar at the specified temperature of the experiment. Based on Henry’s law constants for H2 gas in water (Pray et al., 1952; Alvarez et al., 1988; Stefánsson and Seward, 2003), the dissolved H2 concentration ranged from 0.2 mM (150 °C – 0.2 bar H2) to 8 mM (150 °C – 7.5 bar H2). Blank experiments were set up for NO 3 to react with carbon steel in an Ar atmosphere (Ar, grade 5.0) free of H2 gas, in order to evaluate the potential of carbon steel as an electron donor. In most of the experiments the pH was free to change during the course of the reaction (free pH conditions, no buffer added). However, some specific runs were performed at 150 °C under controlled pH. For the acidic condition, a 10 bar pressure of CO2, an acid gas which dissolves to HCO 3 and controls the pH around 4, was imposed. To mimic pH condition of a clay host-rock, MX 80 bentonite was added (Bradbury and Baeyens, 2002; see Table S2 in Supplementary data), its exchange and surface properties buffer

Table 1 Summary of experimental conditions. P(H2) (bar)

[NO 3 ]° (mmol/L)

Steel/solution ratio (g/L)

Steel loadinga

pH control or specific solution compositionb

Experiments without steel #ACN11 150 #TiNO3 200 #FeCl 150

7.5 7.5 7.5

0.2 8.4 1.0

– – –

– – –

Free pH condition Free pH condition +10 mM FeCl2

Carbon steel #ACN0 #ACN3 #ACN6 #ACN7 #ACN8 #ACN9 #ACN10 #ACN19 #ACN23 #ACN26 #ACN27 #ACN28 #ACN29 #ACN37 #ACN38 #ACN39

150 150 150 150 150 150 150 120 90 150 122 182 90 150 150 150

7.5 0 (Ar) 7.5 0 (Ar) 7.5 7.5 7.5 7.5 7.5 0 (Ar) 7.5 7.5 7.5 7.5 7.5 7.5

9.7 9.7 9.1 1.0 1.0 1.0 1.0 1.0 1.0 9.0 1.0 1.0 1.0 0.3 1.0 1.0

20 20 20 20 20 20 20 20 20 20 20 20 20 20 30 10

Metal chips Metal chips Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder Powder

Free pH condition Free pH condition Free pH condition Free pH condition pHini25 = 3 (HCl) MX80 at 6.7 g/L Free pH condition 10 bar CO2 10 bar CO2 Free pH condition Free pH condition Free pH condition Free pH condition Free pH condition Free pH condition Free pH condition

Magnetite #Mag0 #Mag1

150 150

7.5 0

1.0 1.0

10 10

1.68 m2/g 1.68 m2/g

Free pH condition Free pH condition

Run no.

a b

T (°C)

Specific surface areas are given in Section 3. Free pH condition means no pH buffer added. Partial pressures given in this paper are all for the given T values.

L. Truche et al. / Applied Geochemistry 28 (2013) 155–163

the pH around 8 at 150 °C (Gaucher et al., 2004; Tertre et al., 2006). Acidic conditions, such as those imposed by CO2(g) pressurization are not expected in a nuclear waste repository, but are used here to investigate the reaction mechanism and its sensitivity to pH. A summary of each experiment with specific values of different parameters used is given in Table 1.

During each kinetic experiment, aliquots of solution were sam þ pled during the course of the runs for analyses of NO 3 , NO2 , NH4 , 2 PO3 , SO and Fe concentration. Prior to each sampling event, stir4 4 ring was stopped for 1 min to allow the steel particles to elutriate to facilitate the sampling of a clear solution. Two separate aliquots were sampled with a total of about 8 mL (purging included). pH was measured at room temperature in the quenched solution immediately after sampling. After the pH measurement, one part of the sampled solution was acidified to pH 2 using HCl, in order to convert volatile NH3(aq) into NHþ 4 and to avoid eventual precipitation of Fe. Concentrations of NHþ 4 and Fe were measured in the acidified aliquot using high-pressure ion chromatography (DionexÒ ICS-1000, CS12 column, methyl sulfonic acid (MSA) eluent at 20 mM) and flame atomic emission spectrometry (Perkin-Ele merÒ AAnalyst 400), respectively. Concentrations of NO 3 , NO2 , 3 SO2 and PO were determined in the unacidified aliquots by 4 4 high-pressure ion chromatography (HPLC, DionexÒ ICS-2000, AS18 column, 30 mM KOH eluent). The analytical precision was 3% at the 95% confidence level. The given in situ pH values of these experiments are those recalculated from values obtained at room temperature on quenched sampled aliquots of solution, using the PHREEQC geochemical software package V2.17 (Parkurst and Appelo, 1999) and the associated LLNL database (Johnson et al., 2000). Thermodynamic modeling was applied to investigate the possible presence of NH3 in the gaseous phase. Based on the gas phase volume (equal to the volume of liquid phase), the measured NHþ 4 concentration, and the recalculated in situ pH value, it appears that NH3(g) represents less than 0.1% of total N in the system, even under slightly alkaline conditions such as those measured when pH is free to evolve. Ammonia is thus negligible in the mass balance for N species. After reaction, the autoclaves were quenched in cold water and opened. Solids were immediately transferred into a glove box, rinsed with Milli-Q water and dried overnight at 40 °C. They were stored in a glove box before conducting mineralogical, textural and chemical analyses using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS), respectively.

12 10

1.0

8

0.8 nitrate

0.6

6

nitrite

pH

3.4. Analytical techniques for characterization

150°C, P(H2 ) = 7.5 bar

1.2

Concentration (mmol/L)

158

ammonium

0.4

pH

4 2

0.2 0.0 0

50

100

150

200

250

300

0 350

Time (Hours) Fig. 1. Typical profile of pH and N species concentration vs. time as a consequence of nitrate reduction in the presence of carbon steel powder (run #ACN10). T = 150 °C, P(H2) = 7.5 bar, ½NO 3 ini ¼ 1 mM, carbon steel loading: 20 g/L at 1.9  102 m2/g.

In contrast, in the presence of both carbon steel and H2, nitrates fully reduced to NHþ 4 within a few days or hours depending on experimental conditions. Typical time-concentration profiles of  þ NO 3 , NO2 , NH4 and pH are illustrated in Fig. 1 for a representative experiment (#ACN10: 150 °C, PH2 = 7.5 bar, powdered carbon steel at 20 g/L), where NO 3 reduction was complete within 400 h and NHþ 4 was produced in stoichiometric quantity (Fig. S1 in the Supplementary data). Nitrites were produced in minor amounts; their concentrations increased slightly over the course of the experiments, but remained below 5% of the total amount of N present in the system. The pH value of the aqueous solution increased quickly from 7 to 11 during the first hour, while conversion of NO 3 was still low; in the latter stage of the reaction pH changed only slightly. After 300 h of reaction, the solution pH was 11.4 at 25 °C in the final aliquot. The NO 3 concentration profile (Fig. 1), displayed two kinetic regimes: (1) a distinct decrease occurred during the first 24 h of reaction, and (2) a slower and quasi-constant NO 3 consumption (see Section 5 for further discussion). At the end of all the experiments, the particles were coated with a black layer. X-ray diffraction analysis (Fig. S2 in Supplementary data) revealed the predominance of magnetite (Fe3O4) in this coating. In addition, scanning electron microscopy of the reacted particles at 150 °C and 350 h suggested a thin and discontinuous magnetite coating (Fig. 2). In experiments performed without particular pH control or in the presence of MX80 bentonite, lepidocrocite (FeO(OH)) and goethite were occurred, but to a lesser extent. In the case of experiments #ACN19 and #ACN23, performed under a CO2 pressure of 10 bar, siderite (FeCO3) was the dominant byproduct.

4. Results

4.2. Parameterization of reaction kinetics

4.1. General observations

4.2.1. Partial pressure of H2 and steel surface concentration effects The effect of H2 partial pressure was examined by carrying out runs under 7.5 bar H2 partial pressure or in a H2-free atmosphere (Ar). Three series of experiments were performed using different steel surfaces and NO 3 concentrations (Figs. 3 and 4). The comparison of the two series of experiments performed at 1 and 9 mM NO 3 concentration using 20 g/L of carbon steel powder (steel surface concentration of 0.38 m2/L) demonstrated that the presence of H2 was not essential to reduce NO 3 with large surface area of powder (Fig. 3, experiments #ACN6 and #ACN26, and Fig. 4, experiments #ACN10 and #ACN7). In both cases, the NO 3 reduction rate was similar. However, at higher (9.8 mM) NO 3 concentration and lower (0.02 m2/L, metal chips) steel surface concentration, carbon

Two NO 3 reduction experiments were performed without steel at 150 °C and 200 °C in Ti reactors under 7.5 bar H2 and free pH conditions (#ACN11 and #TiNO3). Measurable signs of NO 3 reduction were absent over the 300 h that elapsed: NO 3 concentration remained unchanged, there was no NH3 production, and pH increase did not occur. pH is an extremely sensitive indicator of  NO 3 reduction and conversion of 5 ppm NO3 into NH3 induces a pH increase from 6 to 9.5 at 25 °C. These experiments demonstrate  that Ti is inert with respect to NO 3 and that NO3 thermal decomþ position into NO , NH or other N compounds is negligible over 4 2 the 300 h reaction period at 150 and 200 °C.

L. Truche et al. / Applied Geochemistry 28 (2013) 155–163

159

Nitrate concentration (mmol/L)

Fig. 2. SEM pictures of the carbon steel powder particles: (A) and (B) before reaction, (C and D) after 350 h elapsed time at 150 °C and 7.5 bar hydrogen partial pressure in run #ACN10. Images A and C were obtained in secondary electron mode and show the morphology of the original and reacted steel particles. After reaction, a magnetite layer covers the steel particles. Images B and D were obtained in backscattered electron mode to underline the chemical contrast between the pristine carbon steel (light gray) and magnetite (dark gray).

#ACN0: 20 g/L metal chips, 7.5 bar hydrogen #ACN3: 20 g/L metal chips, no hydrogen (Ar) #ACN6: 20 g/L steel powder, 7.5 bar hydrogen #ACN26: 20 g/L steel powder, no hydrogen (Ar)

9.8 9.6 9.4 9.2 9.0 8.8 8.6 8.4 8.2 0

50

100

150

200

250

300

350

Time (Hours) Fig. 3. Effect of carbon steel loading on nitrate reduction at 150 °C at high initial nitrate concentration (9–9.8 mM) with and without hydrogen.

steel alone did not reduce NO 3 (Fig. 3, experiment #ACN3). Hydrogen supply to the low surface area experiment enhanced the reaction rate remarkably, in particular the first transitory reaction stage (Fig. 3, experiments #ACN0 and #ACN3). Figs. 3 and 4 also show the two rate regimes. During the first stage of reaction, the pristine carbon steel surface was extremely reactive resulting in rapid NO 3 reduction. After 24 h of reaction, the reaction rate slowed reflecting a residual reaction rate probably associated with the development of a protective magnetite layer. This secondary rate regime could be fitted with a zero order rate law. Fig. 4 also compares the NO 3 reduction rate during three different steel loadings of 10, 20 and 30 g carbon steel powder/L corresponding to steel surface concentrations of 0.19, 0.38 and 0.57 m2/L, respectively. The first transitory reaction stage was more pronounced during a high steel surface loading, but the long-term rate regime appeared to be weakly affected by this parameter as shown by the associated zero order rate constants reported in Table 2.

Fig. 4. Effect of carbon steel loading on nitrate reduction at 150 °C at low initial nitrate concentration (0.3 and 1 mM) with and without hydrogen.

4.2.2. Initial nitrate concentration effect The effect of initial NO 3 concentration was studied in three experiments (at 0.3 mM, 1 mM and 9 mM NO 3 ) conducted under the same conditions (150 °C, 20 g/L carbon steel powder at 1.9  102 m2/g, 7.5 bar H2 partial pressure). The NO 3 concentration decreased as a function of time following the two rate regimes previously mentioned (Figs. 3 and 4). The zero order rate constant corresponding to the residual or long term rate regime was quasiindependent of the initial NO 3 concentration in the range 1–9 mM (Table 2, experiments #ACN6 and #ACN10). At low initial NO 3 concentration (0.3 mM), the secondary rate regime was not visible because the entire NO 3 was reduced within the first 24 h of reaction corresponding to the first transitory reaction stage (Fig. 4). 4.2.3. pH and other possible effects Three experiments were performed under different in situ pH conditions: free, buffered by bentonite or buffered by CO2. Under

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Table 2 Zero order rate constants measured at 150 °C and 7.5 bar H2 with carbon steel powder (#ACN) and magnetite (#Mag).

#ACN38 #ACN10 #ACN39 #ACN6 #Mag0

Surface concentration (m2/L)

Zero order rate constant (mmol/m2/h)

0.57 0.38 0.19 0.38 16.8

(3.3 ± 0.5)  103 (6.7 ± 0.5)  103 (9.1 ± 0.5)  103 (4.3 ± 0.5)  103 (7.6 ± 0.8)  105

Nitrate concentration (mmol/L)

free pH conditions, in situ pH changed from 6 to 8.6 at 150 °C. Under 10 bar CO2, in situ pH was calculated from the available thermodynamic data to be between 4 and 5 according to the amount of NHþ 4 produced. For the bentonite-buffered systems, pH was estimated to be 8 based on the acid–base properties reported by Tertre et al. (2006) at 60 °C, and considering a negligible temperature effect on the smectite PZC (point of zero charge). The results of these experiments are plotted in Fig. 5. The NO 3 reduction profiles are comparable regardless of the pH change in the range 6–8.6 (at 150 °C) as demonstrated by the free pH and bentonite-buffered experiments. Hence, the changes in pH as well as presence of bentonite-associated minerals such as montmorillonite, quartz and pyrite did not have an impact on the reaction rate. This implies that aqueous chemical perturbations induced by the presence of bentonite did not affect the reaction 3 kinetics. Likewise, anions (4 mM Cl, 0.6 mM SO2 4 , 0.3 mM PO4 ) supplied to the solution by dissolution of bentonite, did not influence the reaction rate at these low concentrations. However, the concentration of NHþ 4 measured in the solution was 3 times lower in the presence of bentonite. This result is not surprising because NHþ 4 can easily substitute the exchangeable cations within the interlayers in bentonite. Hydrogen sulfide was detected both in solution (0.2 mM as measured by the iodine back titration method) and in the overlying gas phase (olfactive detection), when bentonite was added to the batch experiment. BSR (bacterial sulfate reduction) and TSR (thermochemical sulfate reduction) hypotheses can be discarded as possible explanations given the low temperature (Truche, 2009; Truche et al., 2009), and the short duration of the run. Hence, it is suggested that pyrite reduction (0.3 wt% of bentonite) to pyrrhotite by H2 is the most likely explanation of H2S production, as demonstrated by Truche et al. (2010). Under 10 bar CO2 partial pressure and at 150 °C, the reaction was extremely fast and NO 3 was fully reduced within less than 1 h. In contrast, more than 300 h were required under free pH con-

ditions or in the presence of a bentonite buffer. At 90 °C, the reaction was slow but still faster by 2 orders of magnitude than experiments performed under free pH conditions at 150 °C. In the presence of CO2, the solution aliquots had a brownish color containing 3.0 mM Fe, which decreased to nearly 1 mM Fe at the end of the experiment. In comparison, the Fe concentration was below 1 lM in all other experiments. Acidic pH, induced by CO2 pressure, caused intense corrosion of the steel powder. XRD analysis performed on the steel powder indicated siderite (FeCO3) as the main solid corrosion product. Siderite formation may explain the measured decrease in Fe concentration. At the end of the experiments, siderite covered the steel particles and formed a protective layer. 4.2.4. Temperature effect The effect of temperature on NO 3 reduction was monitored in the range 90–182 °C in the presence of carbon steel powder and 7.5 bar H2 partial pressure (Fig. 6). The rate of NO 3 reduction increased with temperature. A zero order kinetic model was used to fit the secondary rate regime; the slope of the regression line provided the rate constant in mmol/m2/h. The temperature dependence of the rate constants was plotted in an Arrhenius diagram (Fig. S3 in Supplementary data), which allowed extracting the activation energy and the frequency factor. The values for these parameters were 45 kJ/mol and 2.06 mol m2 h1, respectively. At 90 °C, the reaction rate was measurable on the basis of data collected over several hundreds hours. From the Arrhenius equation, the half-life of NO 3 was expected to be in the range of 1 year at 25 °C under the conditions of the experiments. 4.2.5. Effect of other potential electron donors: aqueous Fe2+ and magnetite To investigate the effect of aqueous Fe2+ on NO 3 reduction rate, one batch test was conducted without carbon steel under the following conditions: 1 mM KNO3, 1 mM FeCl2, 7.5 bar H2 partial pressure, 150 °C and free pH condition. Under these experimental conditions, NO 3 reduction and pH increase were absent after 200 h of reaction. These results demonstrated that carbon steel was necessary for the reaction to occur and that the aqueous byproducts of steel corrosion (i.e. Fe2+ and H2(aq)) were insufficient to maintain the reaction without the catalytic effects of the carbon steel surface.

1.2 #ACN10: pH around 8.6 (free pH condition)

1.0

#ACN9: pH=8 (MX80 bentonite) #ACN19: pH=4 (P(CO2)=10 bar)

0.8 0.6 0.4 0.2 0.0 0

50

100

150

200

250

Time (Hours) Fig. 5. Nitrate concentration vs. time profiles obtained at 150 °C and P(H2) = 7.5 bar for different in situ pH conditions. Under free pH condition, the in situ pH evolved between 6 and 8.6, bentonite MX 80 buffered the pH at a value around 8, and a CO2 partial pressure of 10 bar at pH = 4.

Fig. 6. Nitrate remaining in the system (mmol/m2) as a function of time (h) and temperature. Linear regressions of the data were calculated assuming a zero order rate law, discarding the first measurements (<24 h) corresponding to the initial transitory regime.

L. Truche et al. / Applied Geochemistry 28 (2013) 155–163

Fig. 7. Nitrate concentration as a function of time in the presence of 10 g/L magnetite powder (16.8 m2/g) at 150 °C with 7.5 bar H2 partial pressure compared with an H2-free equivalent system. No reaction is observed without H2. In the presence of H2, nitrate reduction occurs and the slope of the linear regression line gives the zero order rate constant.

Magnetite was a common solid phase by-product of anoxic steel corrosion on steel, which could by itself act as an electron donor (Fe(II)), or as a catalyst for NO 3 reduction. The effect of magnetite on NO 3 reduction was studied by introducing 20 g/L of magnetite powder (surface area 1.68 m2/g) either with 7.5 bar H2 or without H2 (Ar atmosphere). The two experiments were conducted at 150 °C, with 1 mM KNO3, without carbon steel and pH control, and for 130 h. As shown in Fig. 7, no NO 3 reduction was measured in the absence of H2, and suggested that magnetite did not serve as an electron donor under these conditions. In the presence of H2, NO 3 reduction was measurable, but at a much lower rate (two orders of magnitude lower) than in the presence of carbon steel (Table 2). 5. Discussion The experimental results clearly indicate that H2 is insufficient to reduce NO 3 under temperature and pressure conditions applied in this study. However, with the addition of carbon steel powder to the system, NO 3 reduction occurs readily, even in the absence of H2. Considering the effects of H2 and pH as well as the formation of various by-products, the similarities between reaction mechanisms and chemical kinetics of NO 3 reduction by carbon steel and Fe(0) become apparent (Scherer et al., 1998; Huang et al., 2003; Huang and Zhang, 2004; Mishra and Farrell, 2005). In both cases, the reaction rates decrease with increases in pH, and NHþ 4 and magnetite are the main reaction products. Magnetite coating tends to decrease the reaction rates because it reduces access to the pristine reactive surface of steel. Although considerable research has been conducted with Fe(0), there is an absence of a general consensus on reaction mechanisms involved during NO 3 reduction by Fe-mediated processes, because Fe(0) acts both as a surface catalyst and as an electron source either directly or indirectly through the corrosion products. The possible reducing agents in the carbon steel/H2 system include: Fe(0), Fe2+ in solution, Fe(II) associated with Fe-oxide (mainly Fe3O4), carbon steel coated with magnetite, and H (atomic or molecular). Based on the experimental results and data available in the literature for NO 3 reduction in the presence of Fe(0) (Singh et al., 1996; Zawaideh and Zhang, 1998; Alowitz and Scherer, 2002; Huang and Zhang, 2004; Yang and Lee, 2005; Xiaomeng et al., 2009), it is possible to compare the NO 3 reduction mechanism mediated either by carbon steel or Fe(0), and discuss the type of electron donor. From previous studies, it is apparent that aqueous Fe2+ cations are efficient electron donors because of their reactivity and

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accessibility (Stumm and Morgan, 1996). However, Fe2+ concentration is three orders of magnitude lower than NO 3 concentration in the experiments: the alteration of steel with precipitation of magnetite or green rust controls the aqueous Fe2+ content at a low (typically below 1 lM) and roughly constant concentration. Hence, there is poor availability of Fe2+ ions are for NO 3 reduction, and they probably do not take part in the reaction. Moreover, this hypothesis is further supported by experiments conducted with 2+ NO in the solution and in the absence of carbon steel 3 and Fe (Experiment #FeCl2, see Table 1). In this case, even in the presence 2+ of H2, NO does not act 3 reduction is absent. This suggests that Fe as an electron donor in the absence of a catalyst. Magnetite is an inefficient catalyst and it does not serve as an electron donor (Fig. 7). Nitrate reduction only occurs when H2 and magnetite co-exist, but at a rate of two orders magnitude slower than in the carbon steel/H2 system (Table 2, Experiment #Mag0). Hence, magnetite can only act as a catalyst for NO 3 reduction in the presence of H2, whereas carbon steel appears to be both an electron donor and a catalyst. This weak catalytic effect of magnetite, when compared to the effect of freshly cut or ground carbon steel, explains the low reduction of NO 3 by steel coated with magnetite. This is most likely a consequence of a semi-conductive mechanism between the unaltered carbon steel and magnetite coating. As suggested by the strong correlation with pH, corrosion of steel is probably needed for NO 3 reduction. This conclusion implies that NO 3 reduction in the presence of carbon steel is an acid-driven process, and that protons either directly participate in the reaction or indirectly facilitate it. It is likely that aqueous protons are first reduced by carbon steel to form atomic H, which can either immediately react with NO 3 or combine to form molecular H2. Hydrogen spillover from the pristine carbon steel surface to the magnetite coating can also activate the reaction (Conner and Falconer, 1995). Spillover, in heterogeneous catalysis, is defined as the transport of adsorbed species from a primary surface on which adsorption occurred to an adjacent surface. The role of H2 in NO 3 reduction by carbon steel is observed in the case of small reactive surface areas in the particles. Siantar et al. (1996) reported that Fe powder pre-exposed to H2 reduces NO 3 faster than without H2 pretreatment. Huang et al. (1998) speculate that Fe(0) converts sorbed H2 gas or aqueous protons to atomic H, which reduces NO 3 . Carbon steel, which can be assimilated to Fe(0), is likely to behave identically as demonstrated by the experiments conducted at high NO 3 and low steel surface concentration with and without H2 (Fig. 3, experiments #ACN0 and #ACN3). The discrimination into two rate regimes in these experiments is sustained by the change of the carbon steel surface from a pristine surface to a magnetite-coated surface. Any change in the nature of the pristine steel–water interface will have a direct influence on the NO 3 reduction mechanism, and thus, on the apparent reaction rate and activation energy. In the present case, the controlling reaction likely changes with time from a surface- to a diffusioncontrolled mechanism, but the electron donor may also change with time. Hence, the following critical steps can control the NO 3 reduction kinetics: (i) coupling between carbon steel corrosion and magnetite precipitation leading to a complex growth rate of magnetite coating, (ii) production of two electron donors, Fe2+ and H2, at the corrosion front, (iii) H2 and Fe2+ diffusion from the corrosion front to the bulk solution through the porous magnetite structure, the composition and porosity of which change with time and reaction progress, and (iv) heterogeneous catalysis involving NO 3 and or H2 sorption or spillover either on the carbon steel pristine surface or at the magnetite surface. The overall reaction rate is controlled by the slowest elementary reaction among these multiple processes. The complexity of the reaction sequence is not easy to formulate and the consideration of two simple regimes may be

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an oversimplification. However, this phenomenon describes the transition operating at the beginning of the reaction. The zero-order regime is considered as the best approximation of the longterm rate, even if the extrapolation to geologic time-scales of such a simplified kinetic formulation may be inaccurate. Concerning the comparison of NO 3 reduction rate between Fe(0) and carbon steel, it appears that the NO 3 reaction rate is about four times faster at 25 °C using Fe(0) than at 150 °C in the presence of carbon steel. For example, Huang and Zhang (2002) found a rate constant of 2.1  102 mmol m2 h1 for NO 3 reduction at near neutral pH and at 25 °C in the presence of Fe(0) powder pre-coated with magnetite. This particular mineral assemblage is relevant to this discussion as the focus is on the secondary rate regime corresponding to a carbon steel surface coated by magnetite. At 150 °C the zero-order rate constants for the long-term rate regime are in the range of 3.3  103 to 9.1  103 mmol m2 h1 depending on the initial NO 3 concentration (see Tables 1 and 2). Extrapolation to 25 °C based on an activation energy of 45 kJ/mol leads to a rate constant of 104.5 mmol m2 h1. Thus, at 25 °C, NO 3 reduction is about 600 times faster in the presence of Fe(0) than in the presence of carbon steel under comparable experimental conditions. This difference is probably related to the corrosion resistance which is lower for Fe(0) than for carbon steel. In fact, as Fe(0) corrodes faster, it produces more H2, which is capable of reacting with NO 3 , compared to carbon steel.

The authors are aware that the results cannot be unambiguously applied to a defined waste disposal setting, such as a waste-cell for intermediate level radioactive waste. However, this study clearly indicates the possibility of steel-catalyzed NO 3 reduction with NHþ 4 production. Uncertainties in this assessment will be reduced as soon as more results are available on (1) abiotic reduction at lower temperatures, higher pH and NO 3 concentration, and (2) competition between surface- and enzyme-catalyzed NO 3 reduction has been evaluated. Acknowledgments This work was supported by the French National Radioactive Waste Management Agency (ANDRA). We appreciate the help of two anonymous reviewers for their valuable comments and recommendations on an earlier version of this article, which helped to improve the paper. Dr. Joyanto Routh is thanked for editorial corrections and very constructive review of the manuscript. We thank M. Toplis for insightful English improvements. We are extremely grateful to C. Causserand (GET, Toulouse), S. Mounic (GET, Toulouse), E. Gardrat (LA, Toulouse) for technical assistance. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apgeochem.2012. 10.010.

6. Implications for environmental processes References This experimental study in combination with reaction rate extrapolation to lower temperatures demonstrates that abiotic NO 3 reduction induced by the combination of carbon steel and H2 is likely to be a relevant process under natural conditions. Thus, it can be important in situations where NO 3 -bearing waste (i.e. nuclear) is disposed near the surface (Zachara et al., 2007; Thorpe et al., 2012) or in deep geological settings (Mariën et al., 2011; Albrecht et al., 2012). When carbon steel is used in repositories where wastes are characterized amongst others by the presence  of oxyanions (i.e. NO 3 in bituminous wastes), abiotic NO3 reduction can take place under a wide range of pH conditions together or in competition with biotic NO 3 reduction not discussed here (Spain and Krumholz, 2011). Hydrogen is not required for the reaction to take place, but can facilitate the reaction under certain conditions, most notably: enhanced NO 3 concentrations and reduced carbon steel reactive surface area. Iron(0) and carbon steel are comparable reducing agents for aqueous NO 3 in terms of reaction mechanism, but the reaction rate is two to three orders of magnitude faster in the presence of Fe(0). The reaction exhibits different kinetic regimes depending on the corrosion state of the steel. Three reaction steps can be distinguished. The first step is the direct reaction between the pristine surface of the steel and aqueous NO 3 and corresponds to an initial faster transitory rate regime. This initial stage is many orders of magnitude faster than the second kinetic regime. However, this faster rate may not have an impact on NO 3 reduction under natural conditions if the steel is corroded before its encounter with NO 3 . The second step corresponds to the reduction of aqueous NO 3 when the carbon steel surface is coated with magnetite. This reaction stage is important for environmental settings and can be evaluated using a zero order rate law together with an activation energy of 45 kJ/mol, as demonstrated in this study. The reaction is dependent upon pH, and is likely to proceed even under midly-alkaline pH. The final step will be the reaction of aqueous NO 3 with magnetite only (i.e. carbon steel is fully coated or corroded). The kinetics of this reaction stage are an order of magnitude slower than the previous one, and this process requires the presence of H2.

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