Uranyl-sulfate graphite intercalation compounds—I. Their synthesis and stability

Carbon, Vol. 33, No. 6, pp. 809-815, 1995 Copyright 0 1995 ElsevierScienceLtd Printedin Great Britain. All rightsreserved 0008.6223/95 $9.50 + .OO

Pergamon 0008-6223(95)00003-8

URANYL-SULFATE GRAPHITE INTERCALATION COMPOUNDS -1. THEIR SYNTHESIS AND STABILITY ALAIN ‘CREGU

MOISSETTE,‘**

JEAN DUBESSY,’

ANDRE BURNEAU,* GDR-CNRS 077 BP 23

and

HERV~

MICHBLE

FUZELLIER,~

LELAURAIN~

54501 Vandoeuvre les Nancy Cedex, France; ‘Laboratoire de Chimie Physique pour I’Environnement (LCPE), UMR 9992 CNRS-UniversitC Nancy I, 405 rue de Vandoeuvre, 54600 Villers les Nancy, France; de chimie du solide minCra1 (URA 158), Universitt 54506 Vandoeuvre les Nancy Cedex, France

3Laboratoire

(Received

28 February

1994; accepted

in revised form

Nancy

12 December

I,

1994)

Abstract-Stability of new graphite-uranyl-sulfate intercalation compounds was studied experimentally as a function of temperature and concentration. The action of uranyl-sulfuric acid aqueous solution leads to the formation of two mixed-phase intercalation products: the usual sulfate phase (d, = 798 pm) and a new uranyl-sulfate phase (d, = 855 pm). This latter phase is obtained alone after desorption of the sulfate phase, due to the lower thermal stability of the sulfate phase. Key Words-Graphite,

intercalation,

uranium,

sulfate,

X-ray

diffraction.

periments were also performed at 1 kbar pressure using a technique in which samples of HOPG (10 x 3.5 x 0.2 mm3) and reagents (0.4 cm3) were sealed in gold capsules and placed in an autoclave. Pressure and temperature were continuously monitored using a pressure gauge and a thermocouple[5]. X-ray diffraction measurements of [OOl]reflections were performed using a 0-20 goniometer (MO K,) with a quartz monochromator and scintillation counter to determine the stage and the interlayer distance (d,) of graphite intercalation compounds (GIC). Raman spectra were recorded with a XY Dilor multichannel microspectrometer (magnification: x80), using ionized Ar laser 514.5 nm radiation with about 50 mW power on the GIC sample. In order to quantify the thermal stability of graphite intercalation compounds, differential scanning calorimetry was carried out. This technique is based on the thermal changes induced in materials during heating or cooling. Indeed, during variations in temperature, many physical or chemical changes may occur in compounds, including allotropic transformations, state changes, and even total decomposition.

1. INTRODUCTION

The present study was prompted by a major geochemical problem, namely genesis of graphite rich-uranium deposits, the largest in the world, as represented by those in Saskatchewan, Canada[l-31. A previous study had shown that graphite was strongly altered[4] and spatially connected with acid sulfate fluids derived from alteration of pyrite[2]. The aim of this study is to describe the reactivity of uranyl ion in sulfuric acid solutions to investigate if intercalation of uranium (as uranyl) and (or) sulfuric acid can be a process for uranium precipitation. A previous study carried out with pure sulfuric acid had shown that HzS04 was spontaneously intercalated between the graphite layers, in the temperature range 190”-28O”C[5,6]. The decomposition of H2S04 into S03, a powerful oxidizing agent, becomes efficient above 190°C and initiates this intercalation. To relate these results with geological observations and to quantify the possible role of uranium under natural conditions, uranium trioxide (i.e., UOz+ in solution) was added to the sulfuric acid solution.

2. EXPERIMENTAL 3. RESULTS

The experiments were carried out under atmospheric pressure in a Pyrex@ glass apparatus using highly oriented pyrolytic graphite (HOPG, average dimension: 10 x 7 x 0.2 mm3) in hot sulfuric acid (10 cm3) in the presence of uranium trioxide. A reflux condenser was attached to the glass reactor to avoid variations in concentration when dilute solutions were used. This method allows the use of concentrated acid up to 300°C. To determine the reactivity of graphite with dilute solutions for temperatures higher than lOO”C, and thus to approach geological conditions, some ex-

AND DISCUSSION

3.1 Influence of temperature at constant concentration To determine if uranium oxide is able to react with graphite in sulfuric acid solution, experiments were, at first, carried out with 18 M HzS04 and UO, (0.5 M in sulfuric acid). Pure sulfuric acid ([H2S04] = 18 M) was used to compare these results with the previous study[5,6]. The uranium trioxide concentration was selected to be above the solubility limit for temperatures higher than 100°C. 809

810

A. MOISSETTE et al.

Contrary to the experiments carried out only with sulfuric acid[5,6], sulfate intercalation occurs at temperatures lower than 200°C. Even at room temperature, very rich GIC are formed; a mixture of sulfate stage 2 and stage 3 GIC is obtained, although the uranium oxide is not completely dissolved. Stage 1 sulfate, which was never obtained with pure sulfuric acid, is formed at 110°C. Chemical analysis of these compounds proves that such conditions do not allow the intercalation of uranium. Thus, addition of U03 increases the oxidizing potential of solution with respect to graphite, such that SO3 formation is no longer necessary to induce the intercalation process. While sulfate GICs are formed up to 9O”C, above this temperature the normal graphite-sulfate phase is accompanied by a new intercalation phase, unknown before this study. The chemical analysis results given below corresponding to pure stages 1 and 2 of this new phase show that sulfate is associated with uranyl ion to form a graphite-uranyl-sulfate (GUS) phase. The interlayer distance of the GUS phase is di = 855 pm and is larger than that of the sulfate GIC, dj = 798 pm. The composition and structure of this new phase are detailed elsewhere[7,8]. Each mixed product is characterized by the proportion of the two phases, as well as by their stage (Fig. 1). A qualitative estimation is given by the ratio of the main [OOl] reflections of GUS phase relative to that of sulfate phase obtained by X-ray diffraction (Fig. 2). However, absorption coefficients of each phase are different and, therefore, the ratio of [OOl] reflections does not give exactly the abundance ratio but only their relative proportion. Between 90” and 15O”C, the m-any1 ions act mainly as an oxidizing agent by favoring the sulfate intercalation; the proportion of the GUS phase being very weak. The GUS phase becomes dominant for temper-

A

Diffraction

angle

8Mo

(degree)

Fig. 1. X-ray [Wl] diffraction pattern of a mixed compound comprising a sulfate phase (stage 3, di = 798 pm) and the new GUS phase (stage 2, d, = 855 pm); black triangles = GUS phase; white triangles = sulfate phase.

0

50

100 150 200 250 Temperature PC)

Fig. 2. Relative proportion tained by X-ray diffraction,

300

of GUS and sulfate phases, obversus temperature of reaction.

atures above 17O”C, with a maximum intercalation around 195°C. Stage 2 compounds of GUS phase are preferentially formed above 150°C independently of experimental time, whereas stage 1 GUS is only obtained below this temperature. This probably indicates different thermodynamic stability fields and possible variation in oxidation potential as a function of temperature. Higher stages of GUS phase could not be obtained under these conditions. The stages of the sulfate phases vary from 2 to 5 and do not depend on reaction time. 3.2

Synthesis of a pure sulfate-uranyl phase

Direct preparation of the pure GUS compound was not possible; thus, its synthesis was carried out by taking advantage of the low thermal stability of the sulfate phase. When the mixed compounds, synthesized at 195”C, are maintained under air atmosphere for several weeks, the sulfate phase decomposes and evolves towards high stage compounds. No desorption of the GUS phase was observed. To expel completely the sulfate phase, the two mixed-phase product is heated in an oven at 200°C for 2 or 3 hours. By this procedure, graphitic domains take the place of the sulfate phase, Fig. 3 (a), while the GUS phase remains in the form of a very stable stage 2. Finally, the sample containing only graphite and GUS phases is added to the uranyl-sulfate solution at 195°C and reacts again to increase the proportion of GUS stage 2. This procedure must be repeated several times to obtain a pure stage 2 GUS compound, Fig. 3 (b). It is worth noting that the sulfate phase does not intercalate again during this procedure. No definitive interpretation was found to explain this mechanism, but we could assume that a drop of the Fermi level, due to the first intercalation of sulfate, would make easier a massive intercalation of the GUS phase. The same procedure carried out at 130°C leads to the formation of a pure stage 1 GUS compound, Fig. 3 (c). By X-ray diffraction, the [OOl] reflections of stage 1 GUS can be observed only by increasing the detector sensitivity because of the absorption by uranium atoms (high mass percentage of uranium). This also explains why the sulfate reflections were always much larger than the GUS reflections, whatever their

Uranyl-sulfate

10021 graphite

(a)

graphite

intercalation

compounds

1

811

too31

0

10

20

30

Time (days) Fig. 4. Relative proportion of GUS and sulfate phases, obtained by X-ray diffraction, versus the reaction time.

/ 10031

(b)

be readily detected about stage 12).

as shoulders

to the peaks

(until

3.3 Influence of reaction time

stage 2

on GIG formation

(c) stage 1

15 Diffraction

10

5

angle 8Mo (degree)

Fig. 3. X-ray [OO]]diffraction pattern of (a) stage 2 GUS compound containing graphitic domains, (b) pure stage 2 GUS compound, (c) pure stage 1 GUS compound.

proportions in the mixed compounds. The chemical analyses obtained for stages 1 and 2 of GUS phase are as follows: the mass percentage of carbon is 25% -+ 5% for stage 1 and 33% + 5% for stage 2; that of sulfur is 12% + 1% for stage 1 and 9% f 1% for stage 2. The uranium mass percentage is too imprecise to differentiate between stages (8% to 17%). Moreover, the higher sensitivity response of the X-ray detector indicates clearly the purity of these compounds because sulfate-phase intercalation could

In these experiments, sulfate and GUS-phase formation was studied as a function of reaction time (Fig. 4). The temperature was kept constant at 195°C corresponding to the maximum GUS-phase formation relative to the sulfate phase, as demonstrated above. A solution of UOs (0.5 M) in sulfuric acid (18 M) was used. After several hours of heating, the sulfate phase is clearly dominant, as estimated from X-ray reflections ratios. Thus, for a short period, the uranyl ions act principally by their oxidizing capacities and initiate sulfate intercalation. The amount of GUS phase increases progressively with reaction time and reaches the same proportion as the sulfate phase after four days of heating. The proportion of the GUS phase becomes progressively more important with time, and a maximum is obtained after 13 days. For reaction times longer than 13 days, the GUS-phase proportion seems to remain nearly constant and even to decrease weakly. This evolution may indicate a parasitic reaction of overoxidation that blocks the intercalation process during prolonged heating. This effect was also noted in the case of pure sulfate compounds. For excessive oxidation[5]: C==O and C-OH bonds are formed and prevent the intercalation process.

3.4 Stability field of GUS and sulfate phases as a function of UO, and H2S04 concentration The minimum concentrations of sulfuric acid and of uranyl ions necessary to form sulfate and GUS graphite-intercalation compounds were determined. Experiments were carried out at 95°C and 195°C under 1 bar and 1 kbar, for different concentrations.

3.4.1 H2S04 (18 M) and UO, (concentration variable), T = 195°C and P = 1 bar. The first experiments were carried out at 195°C under 1 bar to compare them with the previous results. The influence of uranyl concentration was studied in the range 0.5-4 x lo-’ M. Only pure sulfuric acid (18 M) was used (Table 1). For high UO, concentrations (0.08 M zz [UO,] 5 0.5 M), simultaneous intercalation of sulfate and GUS

A. MOISSETTEet al.

812 Table

1. Intercalation

Temperature (“C)

of GUS and sulfate phases into graphire as a function concentration [H,SO,] = 18 M, P = 1 bar Time (days) 13.0 12.0 4.0 6.0 9.0 8.0 7.5 5.5 13.0 7.5 7.5 8.0 8.0 8.0 11.0 11.0 11.0 9.0 7.0 7.0 9.0 7.0

190 195 190 190 190 180 195 19s 195 195 195 195 195 195 195 200 200 190 190 195 190 195 *Indicates iIndicates

wo31

(mole/l)

of UO,

Stage of GUS phase

Stage of sulfate phase

2* 2* 2* 2* 2 2* 2 2 _

4 4 + 37 3 + 4t 4+5 2* + 3* 2 + 3* 2’ + 3* 3’ 2 + 3t 3* + 2t 3 3 3 3+4 3+4 3+4 3+4 3+4 9 4+5 3+4

0.500 0.500 0.500 0.500 0.280 0.240 0.170 0.170 0.139 0.087 0.035 0.026 8.7 x 1O-3 3.5 x 10-3 2.1 x 10-3 1.2 x lo-’ 4.5 x lo-” 4.4 x 10--J 2.7 x 1O-4 1.4 x lo-” 4.5 x lo-’ 0

2 _ _ _ _ _ _ _ _ _ _ _ _

9t

the main phase formed. small amount of a given stage

phases occurs. The GUS phase is always synthesized as a stage 2; even with lower U03 concentrations. Higher stages of GUS phase were never observed. This confirms that temperatures around 195°C correspond to the thermodynamic stability zone of stage 2 GUS. The stage of the sulfate phase increases if U03 concentration decreases because the oxidizing potential of the solution becomes less important. For U03 5 0.035 M, GUS-phase intercalation does not take place and only the sulfate phase forms. Sulfuric acid intercalation becomes very weak for small concentrations of U03. However, even with a very low concentration of uranium oxide, such as 4.5 x 10e5 M, a mixture of stage 3 and 4 is obtained demonstrating the oxidizing effect of UO,. It is worth noting that only traces of a stage 9 of sulfate phase were obtained when sulfuric acid was used without UOj. 3.4.2 H,SO, (concentration variable) and r/O, (0.5 M), T = 95°C and P = I bar. The following experiments were carried out at 95”C, a temperature lower than the boiling point of water, to study the intercalation process as a function of H2S04 concentration at I bar. The U03 concentration was held constant and equal to 0.5 M (Table 2). Except for pure acid, the GUS-phase proportion is always more important than the sulfate phase. Progressive dilution of HzS04 involves formation of sulfate compounds of high stages: for [H,SOJ 2 13.6 M, stage 2 sulfate is obtained. Lower concentrations lead first to the formation of mixtures of stages 2 and 3 and, finally, traces of stage 4 are detected for 11.5 M, which corresponds to the intercalation threshold of the

sulfate phase instead of about 5 M at room temperature in such conditions[9]. Concerning the GUS phase, HzS04 dilution also reduces the intercalation process. Indeed, stages 1 to 6 of GUS phase are formed for acid concentrations above 5.4 M. For acid concentrations between 4 and 5.4 M, the GUS phase intercalation is very weak and takes place in localized domains and the main part of the sample is comprised of graph-

Table 2. Intercalation of GUS and sulfate phases into graphite as a function of H,SO, concentration [UO,] = 0.5 M, 7’=95”C,P= I bar Time (days)

[H,SO,I

Stage of

(mole/l)

GUS phase

6.5 7.0

18.0 16.2

It2 1* +2t

7.0 8.0 12.5 8.0 6.5 7.0 12.5 12.5 6.5 14.5 14.5 14.5 6.5

14.4 13.6 13.3 12.6 12.6 il.5 10.8 9.1 9.0 8.45 7.55 5.75 5.4

2’ 2* 2 2* 3* 4* 4 4+5 6 + 77 + St 6t 6t 6t traces

*Indicates tlndicates

the main phase formed. small amount of a given stage.

Stage of sulfate

phase 2* _

2 2 2+3 2 4t 41 _ _ _ _ _ _ _

Uranyl-sulfate graphite intercalation compounds I ite. Intercalation was only detected by Raman microspectroscopy, which allows pinpoint analyses at the micrometric scale. Thus, the decrease of the molar ratio S/U does not only limit sulfate intercalation, but also allows the formation of high-stage GUS compounds. However, the fact that reaction time influences GUS-phase formation suggests that GUS could be formed at the expense of the sulfate phase. Thus, at low HzS04 concentration, the reaction rate for the sulfate-phase formation could be lower than the reaction rate for the GUS phase.

3.4.3 Experiments

under pressure (IO00 bar).

To determine the influence of pressure, experiments were carried out under 1000 bar at 95” and 195°C for various acid and uranium oxide concentrations. High pressure permits the reaction in the liquid phase at 195”C, even for dilute sulfuric acid solutions. 3.4.3.1 T = 195°C: First, experiments were performed at 195°C with constant concentration of U03 (0.5 M) and various acid concentrations (Table 3). The results obtained are quite similar to those obtained under atmospheric pressure. Indeed, whatever the pressure (1 or 1000 bar), only the stage 2 GUS is synthesized with U03 in 18 M sulfuric acid. For increasing dilution, the same trend is observed as at 1 bar and lOO”C, demonstrating the small effect of pressure. This suggests, however, that the formation of higher stages is probably due to a temperature increase rather than a pressure effect. For 9.7 and I I .5 M acid concentrations, only traces of intercalation were detected; whereas, for lower concentrations, no intercalation was observed. Moreover, the presence of domains of pure graphite remains important in most of the samples, as indicated by X-ray diffraction. This is possibly due to sulfate-phase desorption, which gives rise

813

to pure graphitic domains because of delay between the end of the experimental and X-ray diffraction analysis. This explanation agrees with the absence of a sulfate phase in most of the experiments. Similar experiments were also carried out at different UO, concentrations: 0.35 and 0.125 M (Table 3). For 0.35 M U03, the GUS phase (stage 2) is intercalated for acid concentrations higher than 16.2 M, whereas the sulfate phase is observed for [H2S04] 2 14.4 M. For the lower U03 concentration (0.125 M), the GUS phase never appears, even with U03 in 18 M sulfuric acid, and homogeneous sulfate GIC was only obtained for [H,SO,] above 15.1 M. 3.4.3.2 T = 95°C: At this lower temperature, with 0.125 M U03, the GUS and the sulfate phases are intercalated for [H,SO,] 2 9.7 M. For [H,SO,] = 7.2 M, traces of intercalation were observed for both phases (Table 4). The experiments performed with [UO,] = 0.035 M demonstrate that uranium oxide acts only by its oxidizing capacity, by favoring sulfate intercalation if the H,SO_, concentration is higher than 14.4 M. Thus, experiments carried out at high pressures suggest that pressure would not affect the intercalation process under geological conditions.

3.5 GUS phase stability and the existence of higher distance phase

23 23 23 23 23 23 14

18.0 16.2 15.1 13.3 11.5 9.7 5.4

0.500 0.500 0.500 0.500 0.500 0.500 0.500

2 I* + 2* + 37 2+3 3 + 4t traces traces -

-

X-ray diffraction and Raman microspectroscopy analysis have shown that pure, highly intercalated sulfate compounds decompose rapidly (after about one week) under air to form high-stage GICs. The desorption of these high-stage GICs evolves more slowly. By contrast, the results presented above demonstrate the high stability of the GUS phase in air up to 200°C. Stage 1 and stage 2 GUS compounds remain stable in their initial forms for several weeks. A stage 1 GUS compound was kept for three weeks in air, immersed in water during 2 days with stirring and then centrifuged, and still exhibited no change. For a longer time (several months), stage 1 gradually evolved to a stage 2 GUS compound. However, higher stages of GUS phase were never obtained even after very long periods of time.

5 _ _ _ _

Table 4. Intercalation of GUS and sulfate phases into graphite as a function of H,SO, concentration [UO,] = 0.125 and

20 20 20 20

18.0 16.2 14.4 12.6

0.350 0.350 0.350 0.350

2* 2* _

4 3 _

14 14 14 14 14 14

18.0 16.2 15.1 13.3 11.5 9.1

0.125 0.125 0.125 0.125 0.125 0.125

Table 3. Intercalation of GUS and sulfate phases into graphite as a function of H,SO, concentration [UO,] = 0.5, 0.35, and 0.125 M; T = 195°C; P = 1000 bar Time (days)

[H,SO,] (mole/l)

*Indicates tIndicates

[UO,] (mole/l)

Stage of GUS phase

Stage of sulfate phase

_ _ -

the main phase formed. small amount of a given stage.

4+5 4 4 traces traces traces

0.035 M; T = 95°C; P = 1000 bar Time

WzSO,I

(days)

(mole/l)

wo31

(mole/l)

6.5 6.5 6.5 6.5 7.5

18.0 16.2 12.6 9.7 7.2

0.125 0.125 0.125 0.125 0.125

7.5 7.5

18.0 14.4

0.035 0.035

*Indicates

Stage of GUS phase 1+2 1+2 3 4 traces _ _

the main phase formed.

Stage of sulfate

phase

5* 6 4 4 traces 1+2+3 2+3+4

A.

814

MOISSETTE el al.

After one year, X-ray diffractograms for samples stored in solution or air, show not only very intense [OOl] reflections similar to the [002] and [004] lines of graphite, but also weak reflections of unknown origin (Fig. 5). Chemical analysis performed on these products show that they contain an important amount of uranium and sulfur: the mass percentages are in the range of 0.85%-9.32% for uranium and 2.86%-5.41% for sulfur, respectively. The carbon mass percentage is =70%. Moreover, Raman microspectroscopy analysis indicates stage 4 or 5 GIC. Two lines are observed relative to the EZgcZJvibrations of graphite for bound and internal layers at 1607 and 1586 cm-i respectively[lO-121. Analysis of the GIC surface shows that the same spectra are obtained everywhere. There was no detectable increase in intercalation related to the edges. This pinpoint analysis illustrates the homogeneous composition of these compounds and demonstrates the absence of graphite domains. Indeed, in such a case, we should observe a single line at 1580 cm-’ corresponding to pure graphite. This line was not observed, either alone for possible graphite areas or superposed upon the two previous lines at 1586 cm-’ and 1607 cm-‘. Furthermore, to obtain internal analysis, the GIC surface layers were removed using sellotape. Therefore, if we take into account the previous chemical analysis and the Raman spectra, it is surprising that no intercalation can be detected by X-ray diffraction ([OOl] reflections). Thus, these samples seem to be reorganized during desorption with formation of a new phase; its interlayer distance being a multiple of that of pure graphite. Some domains were completely desorbed to graphite. This phenomenon has already been observed by Fuzellier et al. and Clinard

15 Diffraction

10

et al. for graphite-HN03 compounds of 01 type, the general formula of which is Cs, HN03 with n = 1, 2, 3, . [ 13-151. These compounds, kept in air or under dry conditions, change into a new variety of graphitenitrate compounds, the /3 type: Cs,HNO, with n = 2, 3, 4. The P-type GIC exhibit an interlayer distance of d, = 655 pm, which is lower than that of the Q!type: d, = 780 pm. As for reorganized GUS compounds, all the X-ray diffractograms for stages 2, 3, 4 of nitrategraphite /3 phase display two very intense [OOl]reflections at the same position as for [OOl] graphite lines, while the other reflections are clearly weaker. From the [OOI]diffraction angles of “reorganized” GUS compounds, the value of the identity period compatible with all the reflections is 2040 pm +- 10. The interplanar distance di deduced from this value could be 365, 700, 1035, 1070, and even 2040 pm. However, as Raman microspectroscopy documented stage 4 or 5 compounds and as the minimum geometric disposition of sulfate corresponds to 790 pm, the interplanar distance must be at least equal to 1035 pm (which corresponds to a stage 4). Increase of d, to 1370 pm would be unlikely because the larger value known of d, is 1340 pm for metalic alloy graphite Rb-Tl[16]. Thus, the evolution of GUS compounds involves reorganization of the intercalated species with a large increase of the interlayer distance (1035 pm) and a variation of the stage number: 2 to 4. Further investigations are in progress to determine the structure of the intercalated layers. 3.6

Differential scanning calorimetry

Sulfuric acid GICs, when progressively heated, decompose very fast. Total desorption and graphite formation is reached at around 33O”C, which is to be expected because the boiling point of pure sulfuric acid is equal to 338°C. The thermograms of the sulfate compounds display no peak, demonstrating the progressive and continuous nature of the desorption until stages 8-10 formation, after which exfoliation takes place[5]. Stage 1 and 2 GUS compounds were also heated to 500°C. The thermogram represented (in Fig. 6) corresponds to the evolution of a stage 1 GUS compound when heated to -430°C. To characterize compound evolution during temperature increase, other experiments were performed at various temperature from 160°C to 430°C. To determine possible transforma-

5

angle 8Mo (degree)

Fig. 5. X-ray [OOl]diffraction pattern of a sample that was stage 2 GUS compound, after one year of exposure to the air; the stars indicate the main reflections and the triangles indicate the weak reflections.

TEAIPERATURE

(“C)

initially

Fig. 6. Thermogram of a stage I GUS compound to -400°C.

heated

Uranyl-sulfate

graphite

Table 5. Differential enthalpic analysis results obtained stages 1 and 2 GUS compounds

Temperature (“C) 160 230 261 321 390 427 500 *Indicates TIndicates

with

Initial stage of the GUS compound

Final stage of the compound after heating

1 1 1 1 1 1 2

I* +2t 1* + 2 1+2 2* + reorganized phase 2 + reorganized phase reorganized phase + graphite

1

the main phase formed. small amount of a given stage.

tions, the stage is verified by X-ray diffraction after each temperature cycle (Table 5). Traces of stage 2 only occur at 230°C. From 270°C to 33O”C, the amount of stage 2 increases slowly to become equal to stage 1. The low slope variation corresponds to a transformation process that extends over a wide temperature range and leads to the formation of pure stage 2 compound around 400”-430°C. The large peak around 240°C is probably associated with sulfuric acid decomposition and SO1 formation. This observation correlates with the results obtained during the spontaneous sulfate intercalation study; maximum intercalation occurring at =24O”C[5,6]. Indeed, this process only takes place because of formation of SO,, which is a powerful oxidizing agent. These two results lead us to propose that intercalated sulfate of the GUS phase progressively changes from HzS04 to SO,, through an intermediate state corresponding to oleum (SOs, xH,SO,). This transformation can occur without any significant change of interplanar distance because the d, value of G-SO, compounds is quite similar to that of G-H,SO, compounds: 790 to 800 pm[17,18]. Accordingly, at high temperatures, the GUS compound composition would be G-UOz+S03. When stage 2 compounds are heated to 5OO”C, they reorganize to form a new phase, the main [OOI] reflection of which is located at the same value as the [002] graphite reflection. This phase corresponds to the compound obtained by progressive desorption in air of stage 2 or 1 GUS compounds. These experiments confirm the high stability of GUS compounds (stages 1 and 2) and their reorganization to give a new phase with a large interplanar distance of 1035 pm.

4. CONCLUSION

From tions on nyl ions facilitate calation peratures with the

a study of uranyl ions in sulfuric acid solugraphite reactivity, it can be shown that uraact not only by their oxidizing capacity to sulfate intercalation, but also by direct interwith sulfate ions forming a new phase at temhigher than 90°C. This new phase coexists sulfate phase and their proportion is a func-

intercalation

compounds

I

815

tion of the reaction temperature. A complete study of the intercalation process has been carried out as a function of sulfuric acid and uranium oxide concentrations to determine the different intercalation thresholds. With pure sulfuric acid at 195°C and 1 bar, the GUS-phase intercalation requires at least a concentration of 0.08 M UOs; below this concentration, only pure sulfate phase is formed. At 95°C 1 bar and constant UO? concentration (0.5 M), the GUS phase is obtained for H$O, concentrations lower than 5.4 molar and the sulfate phase for 11.5 M. Experiments performed under 1000 bar indicate the same trend as that at normal pressure. Moreover, the high stability of the uranyl phase contrasts markedly with the low thermal stability of the sulfate phase. It should be noted that these concentrations are incompatible with geological fluid compositions. However, at lower concentrations, oxidation of the graphene layers by uranyl-sulfate solutions appears to be in agreement with graphite surface corrosion. Thus, intercalation cannot be wholly excluded because the effects of time and radiolysis are still unknown. Acknowledgemenf-This work has been supported by INSUCNRS (DBT n”92/ATP/641 and DBT n”93/ATP/615). We are also pleased to thank A. W. Moore (Union Carbide, Parma, Ohio) for providing the HOPG samples used during this work.

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