Article Cite This: J. Phys. Chem. C 2017, 121, 24548-24556
pubs.acs.org/JPCC
Radiolytic Dissolution of Calcite under Gamma and Helium Ion Irradiation Amaury Costagliola,*,† Johan Vandenborre,† Guillaume Blain,† Véronique Baty,† Ferid Haddad,‡ and Massoud Fattahi† †
SUBATECH, UMR 6457, Institut Mines-Télécom Atlantique, CNRS/IN2P3, Université de Nantes; 4, Rue Alfred Kastler, La chantrerie BP 20722, 44307 Nantes cedex 3, France ‡ GIP ARRONAX, 1 rue ARRONAX, CS 10112, 44817 Saint-Herblain Cedex, France ABSTRACT: Samples of calcite have been irradiated using both the 60.7 MeV helium ion beam of the ARRONAX (Nantes, France) cyclotron and 661.7 keV gamma radiations. The irradiation of calcite pellets results mainly in radiolytically generated gas (H2, CO2). The presence of superficial water during the calcite irradiation in a humid atmosphere is a key factor for the production of radiolysis gases. Besides, the irradiation of calcite−water biphasic media was performed. The gas released after these irradiations is quantified. No significant amount of CO2 has been detected, whereas carbonate ions act as OH• scavengers, thus inducing an increase of H2 production. The amount of hydrogen peroxide created confirms this scavenging mechanism. Finally, ionic chromatography experiments with irradiated solutions allow us to quantify the organic anions (formate HCOO−, acetate CH3COO−, propionate CH3-CH2COO−, and oxalate C2O42−) formed by calcite and/or carbonate ion radiolysis and study the variation of calcium carbonate solubility under irradiation. Formate and acetate ions are formed at low irradiation doses, whereas oxalate is favored at high irradiation doses. The increase of calcium ions in solution during the irradiation of calcite−water media indicates that calcite is progressively dissolved during the irradiation. The main difference between calcite−water media and carbonate ion solutions is that calcite acts as a carbonate ion supply in the radiolysis mechanisms.
1. INTRODUCTION The management of French Long Lived nuclear wastes is the application of the December 1991 Waste Act called “Loi Bataille”, later modified by the 2006 planning act on sustainable management of radioactive materials and waste. Therefore, extensive researches have been performed to investigate the reversible disposal in deep geological formations. CallovoOxfordian clay shows interesting properties such as low water permeability, and chemical, mechanical, and geological stability. Concrete waste disposal compartments are then designed to ensure the reversibility of the storage. Hence, the filling concrete leaves around 5% space inside the compartment, thus allowing decimetric movement for a potential container withdrawal. Moreover, ventilation is needed inside the compartment to avoid dihydrogen accumulation due to radiolysis effects on waste containers.1 Therefore, the presence of atmospheric carbon dioxide leads to the carbonatation of the cement2 (Figure 1), thus increasing the amount of calcite already present in the concrete. Hydroxyl ions are consumed following the dissolution of atmospheric carbon dioxide. Then, portlandite is dissolved to maintain pH around a value of 12.5. The overall process of the carbonatation of portlandite into calcite is described by the following reaction: (eq 1) Ca(OH)2 + CO2(g) = CaCO3 + H 2O © 2017 American Chemical Society
Figure 1. Portlandite (Ca(OH)2) carbonatation process.
The presence of radionuclides in the waste container induces radiations due to their radioactive disintegration. Moreover, numerous studies have dealt with the radiation chemistry effects on carbonate ions solutions.3−18 The first step of the degradation of carbonate ions leads to the formation of two radicals3,4,8,16−20 (CO2−• and CO3−•) either by direct effect of the ionizing radiation or by the reaction of carbonate ions with the species formed by water radiolysis (eaq−, H•, OH•, H2, and H2O2). Then, recombination reactions occur between these radicals (eq 2), or between these radical and molecular species (eq 3) or radical products of water radiolysis (eq 4). During this step, small-chain organic anions such as formate (HCOO−) and oxalate (C2O42−) can be produced.4,9 Received: July 24, 2017 Revised: October 5, 2017 Published: October 5, 2017
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DOI: 10.1021/acs.jpcc.7b07299 J. Phys. Chem. C 2017, 121, 24548−24556
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The Journal of Physical Chemistry C k 2 = 5 × 108 L mol−1 s−1
CO2−• + CO2−• ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ C2O4 2 −
that the particle energy on the other side of the entrance window is 60.7 ± 0.3 MeV. For solid state calcite irradiation, 10 mm diameter calcite pellets are placed into a slot inside the cell and irradiated with a vertical ion beam. The pellet diameter matches the beam diameter. The height of the pellets was set to 1.10 ± 0.05 mm because the range of a 60.7 MeV helium ion beam inside the pellet has been simulated to be 1.08 ± 0.05 mm. This way, the helium ion beam deposits its energy all the way across the pellet. The dose rate (Gy s−1) received by the target is calculated using the following expression derived from the physical dose calculated by Costa et al.:30 (eq 5)
(2)
k 3= 8 × 105 L mol−1 s−1
CO3−• + H 2O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CO32 − + HO2• + H+ (3) 8
−1 −1
k4 = 9 × 10 L mol s
CO2−• + eaq − ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ HCOO− + OH−
(4)
However, very little relevant information can be found about the radiolysis of calcite itself,21−26 and none on biphasic systems of calcite and water. A series of studies by Albarrán et al. have shown that the leaching of gamma- and self-irradiated Ca14CO3 yielded significant amounts of formic, oxalic, glyoxylic, glycolic, and acetic acid.21,22 However, G-values are determined for a single-dose irradiation. It is then difficult to understand the various formation mechanisms. More recent studies also state that samples of calcite irradiated under a gold ions flux show a modification of their X-ray diffraction (XRD) spectra as a function of dose.26 However, the authors could not find which chemical transformation was responsible for the new XRD peak observed. Moreover, the average linear energy transfer (LET) in calcite of gold ions used in this study (20.4 MeV μm−1) is strongly higher than the LET of helium ions (51.4 keV μm−1) considered in our study. Finally, evidence has been made for the formation of same kind of carbonyl radicals (•CO2−, • CO3−) as in irradiated aqueous carbonate solutions.25 Other more reactive carbonyl radicals (•CO33− and •CO−) are radiolytically generated as well as unknown organic radicals. Besides, there is no information available about the role of the atmosphere humidity on the irradiation of solid calcite samples. This could however be a critical parameter for the stability of calcite. Indeed, for 100% relative humidity, it has been shown that there are around 5 water monolayers at the surface of calcite.27 The first water layer is chemisorbed by bonding with Caδ+ and CO3δ− at the surface, leading to Ca(OH)(CO3H), while the following layers are superficial ones.28 The aim of this study is then to better understand the fundamental mechanisms of calcite degradation under ionizing radiation. During our study, samples of solid calcite are first irradiated by a helium ion beam to maximize the dose rate and damages to the calcite system. Then, the role of water is studied by irradiating a biphasic water−calcite medium. Finally, a comparison with the radiolysis of potassium carbonate solutions is made. In each case, various analyses have been carried out in order to quantify degradation products and describe the radiation chemistry of calcite. These same samples are then irradiated by gamma radiation to analyze the low dose range.
I ·E Ḋ = 2m
(5)
with I (nA) the intensity of the helium ion beam, E (MeV) the energy of the helium ion particle, and m (g) the mass of the target. The difference between hydrated and dried calcite pellets has also been studied. In a first series of experiments, calcite pellets have been dried for 24 h into a stove, then kept under argon pressure. Another series of pellets have been hydrated for 1 week into a sealed hot (90 °C) desiccant containing water. A control of the water adsorption on the calcite surface has been performed thereafter on some pellets by monitoring the O-H vibration peak using attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR).31 After the hydration phase, calcite pellets are conditioned under Ar pressure directly in the irradiation cell, with a small water tank which maintains a water pressure inside the cell. For biphasic water−calcite systems, 20 mL of water is set inside the cell with a small amount of calcite. Stirring of the solution is then necessary to ensure a homogeneous irradiation of the water. In this case, Fricke dosimetry32 was performed by in situ monitoring of the ferric ion concentration with a CARY4000 (VARIAN) spectrophotometer, 20 m long fiber optics, and a probe (HELLMA, optical path 10 mm) as previously described.33 The ferric ion radiolytic yield for this helium ion beam is30 G(Fe3+) = 11.7 × 10−7 mol J−1. 2.1.2. Gamma Irradiation. Gamma irradiation was performed with a GSM D1 (Gamma-Service Medical) irradiator containing a 123 TBq 137Cs source. This radionuclide disintegrates in 137mBa, which delivers 661.7 keV gamma radiations. A dose cartography has been performed by Fricke dosimetry inside the gamma irradiation chamber, and the average deposited dose inside the aqueous samples is between 7 and 9 Gy min−1. The dose in solid samples was derived from this cartography by applying a conversion factor based on the density ratio between the Fricke dosimeter and the solid. As gamma radiation is a penetrating radiation, the dose is displayed almost equally everywhere inside the sample. There is no need to put an entrance window to the cell, neither to stir the solution. 2.2. Analytical Procedures. Solutions described in this section are prepared with commercial chemical products as received with no further purification. All reactants are analytical grade and the aqueous samples are prepared with ultrapure (Milli-Q) water. The hydrogen peroxide (H2O2) concentration was determined by spectrophotometric analysis with a CARY-4000 (VARIAN) spectrophotometer. The method used was the Ghormley method34 in which the absorption of the triiodide complex resulting from the oxidation of the iodide ion by H2O2 is measured at 350 nm (ε = 25800 L mol−1 cm−1).35
2. EXPERIMENTAL METHODS 2.1. Irradiation Experiments. Gamma and helium ion irradiations were performed at the ARRONAX (Nantes − France) facility. The irradiation cells are made with PEEK (polyether ether ketone), mounted with a Rotulex 19/9 glass tube, and have an internal volume of 41 or 42 mL, respectively, for alpha and gamma cells. These cells are gas-tightened using a screwed joint in a glass-metallic valve. Specificity of each experiment is discussed thereafter. 2.1.1. Helium Ion Irradiation. The initial energy of the ARRONAX helium ion beam is 68 MeV. The PEEK irradiation cell is equipped with a 150 ± 15 μm thick borosilicate glass disk as entrance window in order to minimize energy losses as ions enter the cell. The SRIM2008 simulation code29 demonstrated 24549
DOI: 10.1021/acs.jpcc.7b07299 J. Phys. Chem. C 2017, 121, 24548−24556
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The Journal of Physical Chemistry C Table 1. Summary of the Various Irradiation Experiments Carried Out during This Study
Molecular hydrogen and carbon dioxide have been monitored by micro gas-chromatography (μGC) using a 490GC (VARIAN) micro gas-chromatograph. Two columns were used: a 5 Å molecular sieve for H2 and a poraplot Q for CO2. The injection system and the column were purged with argon beforehand. Calcium and potassium ions concentration was determined by ionic chromatography using a METROHM 850 pro IC chromatograph with a Metrosep C6 column and HNO3 8 mmol L−1, PDCA 1.3 mmol L−1 eluent. Another column Metrosep ASUP16 was used to monitor formate, acetate, propionate, and oxalate anions. This column was thermostated (55 °C) and a Na2CO3 75 mmol L−1, NaOH 0.75 mmol L−1 eluent was chosen. Finally, carbonate anions concentration was determined using a DIONEX chromatograph with a DIONEX IonPac AS18 column and 0.25 mmol L−1 NaOH eluent. The sample types, atmosphere, and irradiation characteristics of the experiments are summarized in Table 1. Figure 2. Dihydrogen production from the irradiation of calcite pellets (A11-A12) (atmosphere: Ar-100% RH; 60.7 MeV helium ion irradiation; I = 2 nA) and A13 (atmosphere: air-100% RH; 60.7 MeV helium ion irradiation; I = 50 nA).
3. RESULTS AND DISCUSSION 3.1. Helium Ion Irradiation. 3.1.1. Gas Production. A series of calcite pellets (A11-A12-A13-A21) are irradiated by the ARRONAX helium ion beam. Two cases are investigated depending on the hydration of the material. First of all, dried calcite pellets (A21) show no hydrogen production and a very low CO2 production. The CO2 concentration is indeed equal to 75 ppm inside the cell for a 4.6 MGy irradiation. In the literature, no study mentioned the release of carbon dioxide through calcite irradiation. Hydrated calcite (A11-A12-A13) displays a different result. Carbon dioxide is indeed observed for strikingly lower doses (about 210 ppm for 91 kGy). Furthermore, molecular hydrogen is detected in contrary to dry calcite experiments. Figure 2 shows the variation of dihydrogen production in the
case of the irradiation of hydrated pellets with a low intensity beam (A11-A12) (I = 2 nA). For doses between 4.55 and 91.1 kGy, H2 concentration increases almost linearly with the dose. The corresponding hydrogen radiolytic yield is G(H2) = 1 × 10−10 mol J−1. However, for higher doses (between 91.1 and 182 kGy), it seems that an equilibrium is reached and no more dihydrogen is produced. For higher intensity experiments (A13) (also in Figure 2), higher doses are reached (up to 2.12 MGy). No significant variation of the H2 concentration at such high intensity can be observed. The origin of the H2 radiolytically produced is 24550
DOI: 10.1021/acs.jpcc.7b07299 J. Phys. Chem. C 2017, 121, 24548−24556
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The Journal of Physical Chemistry C k 9 = 4 × 108 L mol−1 s−1
probably only the water radiolysis at the surface of the pellet. At low dose, H2 production is proportional to the absorbed dose, whereas, at high dose, the whole water at the surface has been consumed by the ionizing radiation; then H2 is constant. There is however a difference between the total H2 equilibrium production by the A11-A12 and the A13 samples. The geometry of the pellet in both cases is indeed slightly different. There is then less water on the surface of the A13 pellets. Besides, irradiation of empty cells with Ar - 100% RH atmosphere led to a constant H2 production of (1.48 ± 0.14) × 10−7 mol J−1. This dihydrogen production is due to the atmospheric water inside the cell. Radiolytic gases are also studied following the irradiation of biphasic calcite−water systems (B11-B12-B20). For this application, a low intensity helium ion beam is considered (I = 2 nA). First of all, no trace of carbon dioxide has been observed for 200−5000 mg calcite in 20 mL water systems irradiated with doses up to 60 kGy. This shows that, even if carbon dioxide is formed, the latter is dissolved in the water. Besides, as shown on Figure 3, the dihydrogen production yield
CO32 − + HO• ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CO3−• + OH− k10 = 8.5 × 106 L mol−1 s−1
HCO3− + HO• ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CO3−• + H 2O
CaCO3(s) = Ca(aq)2 + + CO3(aq)2 −
Figure 4. Hydrogen peroxide production from the irradiation of calcite suspensions (2 g) in water (B11) (atmosphere: air; 60.7 MeV helium ion irradiation; I = 2 nA).
the variation of H2O2 concentration after the irradiation of stirred water−calcite medium (B11). The hydrogen peroxide radiolytic yield is G(H2O2) = 0.75 × 10−7 mol J−1, whereas, in the same conditions, irradiation of pure aerated water yielded33 1.5 × 10−7 mol J−1. There is therefore a sharp decrease of hydrogen peroxide radiolytic production in the presence of calcium carbonate. There is no impact of carbonate ions on H2O2 as long as the latter is already present in solution.5 The only mechanism explaining the decrease of H2O2 yield is then the OH• radical scavenging by carbonate and bicarbonate ions (eqs 9 and 10) which competes with H2O2 formation reactions such as OH• recombination (eq 12).
(6)
k 7 = 3.6 × 107 L mol−1 s−1
H• + H 2O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ OH• + H 2O
(7)
H 2 + H 2O2 → 2H 2O
(8)
(11)
This leads, in the case of a carbonate solution, to an increase of the H2 radiolytic yield, because H2 reacts less with OH• than in the case of pure water. The calcium carbonate solubility is very low (0.013 g L−1 at 25 °C). The yield difference between 0.2 and 2 g calcite systems is then primarily difficult to understand if bulk reactions are only taken into account. The solubility limit of the calcium carbonate has indeed already been reached in both cases. Thus, interfacial reactions are more likely to occur for the more calcite-saturated medium. 3.1.2. Hydrogen Peroxide Production. Hydrogen peroxide is another major product from water radiolysis. Figure 4 shows
from the helium ion irradiation of a stirred biphasic water− calcite system (B11-B12) increases from G(H2) = 0.45 × 10−7 mol J−1 for 0.2 g of calcite to G(H2) = 0.68 × 10−7 mol J−1 for 2 g of calcite. Both values are higher than the dihydrogen radiolytic yield in pure water G(H2) = 0.26 × 10−7 mol J−1 obtained in the same conditions.36 The presence of calcite seems then to have a strong influence on the hydrogen radiolytic production. During the radiolysis of water, there is a recycling of dihydrogen by the Allen chain reaction:37 (eqs 6−8) k6 = 4.2 × 107 L mol−1 s−1
(10)
Hence, there is a competition between dihydrogen and carbonate/bicarbonate ions for the scavenging of OH•. Carbonate ions are present in solution because of the dissolution equilibrium of calcium carbonate: (eq 11)
Figure 3. Dihydrogen production from the irradiation of calcite suspensions B11-B12 (0.2−2 g) in water (atmosphere: air; 60.7 MeV helium ion irradiation; I = 2 nA).
H 2 + HO• ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H• + H 2O
(9)
k12 = 8.5 × 106 L mol−1 s−1
HO• + HO• ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H 2O2
2− 5
(12)
3.1.3. Organic Anions Production. Organic anions are also investigated by ionic chromatography. Figure 5 shows the production of organic anions by helium ion radiolysis of
It is also well-known that carbonate (CO3 ) and bicarbonate (HCO3−)3 ions are hydroxyl radical scavengers: (eqs 9 and 10) 24551
DOI: 10.1021/acs.jpcc.7b07299 J. Phys. Chem. C 2017, 121, 24548−24556
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The Journal of Physical Chemistry C k16 = 105 L mol−1 s−1
HCOO− + CO3−• ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CO2−• + HCO3− (16)
The steady-state oxalate concentration is explained by the oxalate formation which competes with the HO• scavenging by oxalate. k17 = 4 × 107 L mol−1 s−1
C2O4 2 − + HO• ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CO2−• + CO2
(17)
Irradiation of aqueous carbonate solutions is carried out in order to investigate the role of calcium ions in the formation of organic ions during the radiolysis of calcium carbonate. Results are shown on Figure 6. The maximum acetate concentration is
Figure 5. Organic anions production from the irradiation of calcite suspensions B20 (0.2 g) in water (atmosphere: Ar; 60.7 MeV helium ion irradiation; I = 10 nA).
biphasic water−calcite systems under an argon atmosphere (B20). At low doses (
