MÖSSBAUER STUDIES ON THE GAMMA RADYOLYSIS OF SOLID IRON (II) SULFATE AND ITS HYDRATES

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MÖSSBAUER STUDIES ON THE GAMMA

RADYOLYSIS OF SOLID IRON (II) SULFATE AND

ITS HYDRATES

J. Meyers, J. Ladrière, M. Chavee, D. Apers

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, suppldment au no 12, Tome 37, Dtcembre 1976, page C6-905

MOSSBAUER STUDIES

ON

THE

GAMMA RADYOLYSIS

OF

SOLID IRON (11) SULFATE AND ITS HYDRATES

J. MEYERS, J. LADRIERE, M. CHAVEE, D. APERS Laboratoire de Chimie NuclCaire, UniversitC de Louvain, 2, chemin du Cyclotron, B-1348 Louvain-la-Neuve, Belgique

Rbum6.

-

Les complexes FeS04.7 H20, FeSO 4.4 H20, FeS04. H20, FeS04 ont et6 irradies aux rayonnements y du 60Co SOUS azote et B temperature normale. Les Bchantillons sont analyses par spectroscopie Mossbauer. I1 a et6 montre que tous les composes hydrates se transforment en sulfate basique de Fe I11 hydrate tandis que le sulfate anhydre reste insensible aux irradiations. Un mecanisme d'oxydation est propose sur base d'un modele cinetique simple tenant compte du fait que la probabilite de formation des radicaux OH est directement proportionnelle au nombre de molecules #eau dans la premiere sph6re de coordination du Fe 11.

Abstract. - The complexes FeS04.7 H20, FeS04.4 H20, FeS04. H20, FeS04 have been y

irradiated (60C0 source) under nitrogen atmosphere at room temperature. The samples were analyzed by means of Mossbauer spectroscopy. AI1 the hydrated compounds are converted into hydrated basic ferric sulfates, whereas the anhydrous sulfate is insensitive to irradiation. A mechanism of the oxydation is proposed on the base of a simple kinetic model taking into account that the formation probability of OH radicals is directly proportional to the number of water molecules surrounding the Fe2+ ion.

1. Introduction.

-

The effects of the 1 radiolysis of Iron (11) sulfate solutions have been studied inten- sively. It is well known that Iron (11) is oxidized into Iron (111) by the oxidizing radicals, mainly OH, originated from the radiolysis of water [l].

In y radiolysis of solid heptahydrated Iron (11) sulfate it has been suggested that Fe2+ is also oxidized into Fe3+ by the same radical OH originating from the crystallisation water [2,3].

In this work we have extended the study to several hydrates of Iron (11) sulfate in order to determine the influence of the hydration number of Fe2+ ion on the oxidation rate.

The radiolytic decomposition has been followed using Mossbauer spectroscopy and other techniques such as spectrophotometry, X ray diffraction and thermogravimetric analysis.

The Mossbauer parameters are used to identify the oxidation states, coordination and relative abundances of the radiolytic products.

2. Experimental part.

-

All investigated compounds have been synthetized from solid Iron (11) heptahydrated sulfate Merck P. A.. FeS04.4 H 2 0 has been prepared by dehydration at 60 OC during 85 hours [4]. FeS04.H20 has been precipitated at room temperature from a saturated solution of Iron (11) sulfate and 15 M. H,S04 [5]. The anhydrous sulfate is obtained by heating the heptahydrate at 400 OC under nitrogen atmosphere.

The irradiations have been performed under nitrogen atmosphere at room temperature using 60Co gamma rays with a dose rate of 0.65 Megarads hours- l .

The Mossbauer spectra were recorded a t 78 K, using a constant acceleration spectrometer and fitted in Lorentzian lines by means of least squares iterative program.

3. Results. - The Mossbauer parameters of unirradiated and irradiated

FeS0,. 7 H 2 0 , FeS0,. 4 H 2 0 , FeSO,. H 2 0 and FeSO, are listed in table I.

The relative abundances of ~ e and Fe3+ have ~ + been calculated from the peak areas by assuming that the Lamb-Mossbauer factor f are the same for the two species. This assumption is particularly reasonable for spectra recorded a t low temperature. It has been found to be in good agreement with the results obtained by chemical analysis (potentiometric titrations with Ce4+ and Cr2+).

The corresponding spectra are shown in figure 1

and figure 2. All the initial compounds (Fig. l a , Figs 2a, 2c, 2e) give rise respectively to a doublet with large quadrupole splitting (Q. S.) and isomer shift (I. S.) due to high spin Iron (11).

On irradiation of the hydrated compounds broad peaks appers wits a Q. S. and I. S. characteristic of an high spin Iron (111).

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C6-906 J. MEYERS, J. LADRIERE, M. CHAVEE, D. APERS

Mossbauer parameters of the unirradiated and irradiated compounds. All the spectra are recorded at 78 K dose Compound Mrads A (")

a

("* b,

r

("9 C ) % F e 2 ' + 5 % % F e Z f + 5 % - - - FeS04 0 3.10 1.11 0.33 100 - 2 500 3.10 1.12 0.33 100 0 FeSO,. H 2 0 0 3.07 1.18 0.34 100 - 1 386 3.07 1.18 0.37 90 0.74 0.33 0.41 10 FeS0,. 4 H,O 0 3.48 1.17 0.35 100 1381 3.48 1.20 0.39 67 0.74 0.34 0.55 33 FeS0,. 7 H 2 0 0 3.45 1.19 0.37 100 1 210 3.43 1.20 0.35 47 0.96 0.33 0.45 53

( a ) All values are given in mm/s-1 with an accuracy of 4~ 0.05 mm/s-l.

( b ) 6 is relative to 57Co(Pd).

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r

is the full width at half maximum.

- L - 2 0 2 &-L -2 0 2 t V E L O C I T Y ( m m / s )

FIG. 1.

-

Mossbauer spectra of FeS04.7 Hz0. a ) Unirra- diated sample. b) 666 Mrads. c) 907 Mrads. d) 2 030 Mrads.

e ) 2 958 Mrads. f ) 3 658 Mrads.

On the other hand the irradiated anhydrous sulfate doesn't show such a doublet indicating that this compound is insensitive to irradiation. From this observation it is concluded that the radiolytic oxidation of Fe2+ occurs only in the presence of water

VELOCITY (rnrnls)

FIG. 2. - Mossbauer spectra of a) Unirradiated FeS04 ; b) FeS04 : 2 065 Mrads ; c) Unirradiated FeS04.H20 ;

d ) FeS04.H20 : 1 802 Mrads ; E ) Unirradiated FeS04.4 H20

f ) FeS04.4 H 2 0 ; 1 596 Mrads.

molecules and that the SO; - anions are unable to produce radicals that can oxidize Fe2

+.

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THE GAMMA RADIOLYSIS OF SOLID IRON (11) SULFATE A N D ITS HYDRATES C6-907

0 0 S E f Mrads I

FIG. 3. - Relative abundance of Fe3+ as a function of the dose

for the different hydrates.

centage of ~ e increases as a function of the dose ~ + and the hydration number (Fig. 3).

In the investigated compounds FeS04.7H20, FeS04.4 H20, FeSO,. H 2 0 and FeSO, the Fe2+ ion is respectively coordinated to 6, 4, 1 and 0 water molecules.

4. Discussion. - The radiolytic oxidation of Fe2+ in Fe3+ occurs only in presence of water molecules. The oxidation process is therefore depending on the radiolysis of water molecules. The radiolytic decomposition of water and aqueous solution has been explained in terms of the ionization of water and the subsequent formation of H atoms and OH radicals [6,7].

H 2 0 * O H

+

H . (1)

In the case of aqueous solutions the combination of radicals in the region of high concentration of ions or free radicals produces finally molecular hydrogen, hydrogen peroxide and water. With radia- tions with low linear energy transfer (LET), e. g. X-rays or gamma rays, most of the radicals escape from the region of high radical concentration and react with the molecular products, hydrogen and hydrogen peroxides, to give water, H,O, OH and H. In solid state, however, only the primary radicals, e. g. OH and H, are expected to be formed because the radical diffusion is strongly reduced or inhibited. For the same reason recombination of the primary radicals must be much more important than in liquid phase. However, the presence of a scavenger of OH or H radicals in their immediate neighbourhood will decrease this recombination. If water molecules are bound to a Fe2+ ion, a very rapid stage which is complete in a few microseconds involves the oxidation of Fe2+ ion by OH radicals [g].

Fe2+

+

OH + Fe3+

+

OH-

.

₍₂₎

In agreement with Ph. Giitlich [3] the following oxidation mechanism is proposed : in a primary reaction the radiolytic decomposition of water molecules produces OH and H radicals. This step is followed by the immediate oxidation of the Fe2+ by OH and a subsequent substitution of H 2 0 by 08- in the ligand field (reactions 1 and 2). The remaining H

atoms are small enough to diffuse through the lattice and escape as H, molecules after recombination with other H atoms [g].

This mechanism is summarized in the following equation

The presence of hydroxyl ions has been detected during the thermal decomposition of the irradiated compounds. Around 5000C there appears indeed a new step which has been assigned to the liberation of water arising from the OH- ions. The X-ray diffraction pattern of the irradiated compounds is also characteristic of an hydrated hydroxy Iron (111) sulfate.

The increasing of the percentage of Fe3+ with the number of water molecules (Fig. 3) can be attri- buted to the radiolytic oxidation of ~ e by OH ~ + radicals, the formation probability of which increases with the number of water molecules bounded to the Fe2+ ion (n). This supposition is supported by our experimental results : from figure 4 it is seen that the

FIG. 4. -Verification of the first order reaction for the inves-

tigated compounds.

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J. MEYERS, J. LADRIBRE, M. CHAVEE, D. APERS

where cp is the dose rate ant t the time (cpt = Dose). After integration this equation becomes

1 - a = exp(- Kcpt) where a is the ratio of Fe3+ on Fe2+ t Fe3+.

FIG. 5.

-

Plot of the rate constants versus the hydration number of the ferrous sulfates.

This equation is well verified in figure 4.

The reaction rate constants calculated from the slopes of the different straight lines are listed in table I1 as a function of the number of water molecules (n) in the first coordination sphere.

These reaction rate constants are indeed directly proportional to the hydration number n, as shown in figure 5.

K = knHZo

.

By assuming that the radiolysis of the n water molecules produces a OH radical stationary concentration which is also proportional to n

[OH] = k'nHZo

the final expression of the reaction rate can be written as

which demonstrates clearly that the oxidation of

FeZf is only due to the OH radicals originated from the coordination water.

References

[l] FRICKE, H., HART, E. G., Radiation dosirnetry, (Academic [4] FRAENKEL, F., 2. Anorg. Chern. 55 (1907) 288. Press, New York and London) 1966, vol. I1 [5] FRAENKEL, F., 2. Anorg. Chem., 55 (1907) 225.

[2] KELLERSHOHN, C., SOUBIROU, F., HUBERT, C., J. Physique [6] WEISS, J., Nature, 153 (1944) 748.

Colloq. 35 (1974) Cl-73. [7] ALLEN, A. O., J. Phys. Colloid Chem. 52 (1948) 479. [3] GUTLICH, Ph., ODAR, S., FITZSIMMONS, B., ERIKSON, N., [8] THOMAS, J. K., Radiolysis of water, Advances in radiation