Experimental study of mechanisms of fixation and reduction of uranium by sedimentary organic matter under diagenetic or hydrothermal conditions

0016703?/84/%3.00

Gexhimkw n Cosmcxhimicw &a Vol. 48. pp. 2321-2329 &, Rrlpmon Prcs Ltd. 1984. Rinted in U.S.A.

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Experimental study of mechanisms of fixation and reduction of uranium by sedimentary organic matter under diagenetic or hydrothermal conditions S.

NAKASHIMA’,

J. R. DISNAR’,A. F%RRUCH~T~ and J. TRKHE?

‘Centre de Recherches surla Spthbc et la ChimietiesMin&raux G.I.S. CNRS-BRGM. IA, rue de la Rrollerie, 45045 Orlkans Cedex,Franceand E.R.A. 601 du C.N.R.S. *B.R.G.M.,Dtpartement Gites Mineraux, B.P. 6009,45060 Orleans Cedex, France and E.R.A. 601 du C.N.R.S. ‘Centre de Recherches sur la Synthese et la Chimie des Miniraux G.I.S. CNRSBRGM, 1A, me de la Ferollerie 45045 Orl&ms Cedex, France %Jnivcrsid d’OrI&ans, Laboratoire de Geologic Appliqu&e, 45045 Orlhns Cedex, France and E.R.A. 601 du C.N.R.S. (Received March 28, 1984; accepted in revisedform August 9, 1984) Abstract-Interactions between lignite and soluble uranyl species have been investigated experimentally at d&rent temperatures from 20” to 4OO’C. Fixation of uranyl species by lignite (45” to 25OT) and their reduction to uraninite (120’ to 400°C) were observed The fixation of uranyl species by lignite results in the formation of stable organo-many1 compounds. The reduction of many1 species by lignite results in a stoichiometric liberation of H+ in the solution medium and in a dehydrogenation of lignite. This dehydration can be attributed to two different procmses. The first is an oxidation of alcohol functional groups into aldehyde or ketone functions accompanied by a ~rn~n~~ reduction of uranyl species. The second is a caption of h~~~~ aliphatic moieties induced by the uranium species. The molecular hydrogen produced during this process is subsequently used for an additionat reduction of uranyl species.

with simultaneous oxi&tion of alcohols and aldehydes (ANDREYEVand CIW~ACHENKO,1964); (c) reduction of uranyl species with simultaneous release of 2 protons from organic matter (dehydrogenation) and/ or fixation of 1 oxygen (oxidation) (ROUZWD et al., 1979). This experimental study aims to explore the mechanisms of direct reduction of uranium by a natural organic material, under diagenetic or hyd~~~~ conditions. Considering the fquent association of uranium with coaly carbonaceous materials, a lignite was chosen as organic substrate.

INTRODUCITON THE FREQUENTLY observed association of uranium with various sedimentary carbonaceous substances (coals, kerogens, bitumens, petroleum) stresses the important role that these organic materials may play in the concentration of this metal (VINE, 1962; HREGER, 1974). VariOus PRXXSXS have been sugg&ed to explain this association: (a) the direct fixation of uranium ion species by various prooesses: ion exchange (SZALAY, 1964); ~mplexation and chelation (SCHMIDT-C• LLERUS, t 969; DLSNAR,198 1); physical

adsorption (MOORE,1954); (b) the indirect reduction of soluble U(VI) species to insoluble U(W) species by elemental sulfur and/or hydrogen sulfide produced by sulfate-reducing bacteria associated with detrital organic matter (SCHMIDT-C• LLERUS, 1969; BREGER, 1974); (c) the direct reduction of U(W) species by organic matter (GARRELSand POMMER,1959; ANDREYEVand CHUMACHENKO, 1964; ROUZWD ez al., 1979; DISNARand TRICHET,1983). All these pmcesses have been suggest& to occur at various stages of the sedimentary cycle; syngenetic, diagenetic, or epigenetic stages (GARRELSand POMMER,1959, VINE, 1962; BREGER,1974). However, these processes have not been investigated to the same extent. Important insights into the mechanisms of the direct fixation and the indirect reduction of uranium have been obtained from laboratory experiments (see references cited above). On the other hand, the ditcct reduction of uranium by organic matter has only given rise to theoretical considerations: (a) complete oxidation of woody materials as schemati& by IZUt$+ + CdH,t,O, + 7H20 - 12UO2 + 6CC& + 24H+ (C%H,005represents a celh~lose monomer, GARRELS and POMMER,1959); (b) reduction of U(W) to U(W)

EXPERIMENTAL

METHODS

In order to avoid the masking of transformation of organic matter due to its reaction with uranium by its normal heat-induced tltermal evolution, we have chosen an organic substrate which has already reached a certain degree of evolution in the course of its natural d&genes& the lignite of Gardanne @ouches du RhBne, France). This material was initially purified by sucEcssive washings with ethanol, HCl I N and distilled water. A&r separation of the heavy fraction s-essentially Fe&-by divan in CC&, the purlIied lignite was dried (at 4O”C), homogeneixed, and sieved. The 63-100 pm fraction was finally used for the experiments The purified lignite presents the following elemental composition (wcigbt percent): C 64.04; H 5.12; N 1.40; 0 17.93; S 4.93: Ash 6.58. Experimental procedures 100 to 400 mg of lignite and 10 to 40 ml of 0.1 M uranyl chloride solution (initial pH = 1.8) were placed in closed vessels at temperatures of 20” to 400°C for 1 to 262 hours. At lower temperatures, the experimentswere conducted in PTFE-lined steel vessels, and at temperatures of more than 2OO”C, they were conducted in gold t&es placed in autoclaves After the experiments, tbe solii phases were separated by filtration, then analyzed by X-my di&actometry and inI?arecI

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S. Nakashima n al

(IR) spectroscopy. The IR spectra were recorded using a Perkin-Elmer 180 spectrometer on KBr pellets (2 mg of sample for 200 mg of KBr) which have been previously conserved for at least 7 days in a desiccator (ROBINand

ROUXHET.1976). The elemental analysis of the same solid products were performed by 8oci&&ATX (Nanterra, Fiance). The uranium content of the solutions was determined by calorimetry using the acetone-thiocyanate method modified by SCHMIDT-C• LLERUS (1969).The H+ concentration was

determined by conductimetric and potentiometric titrations. RESULTS AND DISCWSSION

1) Evolution of the system uranium-lignite as a function of temperature The results of a first series of experiments (run duration: 262 hours) show that the quantity of uranyl cations free in solution (curve 1) decreases progressively from 45T to 150°C and very quickly from 15O’C to 200°C. At 200°C 99.5% of the uranium introduced initially in the medium is eliminated from the aqueous solution. The decrease in UOP ion content of the solution is accompanied by a nearly proportional simultaneous increase in its H+ ion content (the final pH of the solution is varying from 1.7 to 0.8) (Fig. 1, curve 3). X-ray diffraction analysis of the solid phases obtained from experiments carried out at temperatures exceeding 120°C reveals the presence of utaninite (UOr). This result, indicating the reduction of U(VI) to U(W), is confirmed in IR spectroscopy of the same solid samples by the presence of an absorption band at 350 cm-’ attributable to UOz (NYQUIST and KAGEL, 1971). For the experiments carried out between 45 and 250°C this method also indicates the existence of an absorption band at 920 cm-‘, attributable to many1 species (BULLOCK, 1967). The association of these non-reduced uranium (VI) species with the carbonaceous residues may result from their direct fixation (adsorption ?) by the organic substrate. 010

0

In the temperature range 45” to 120°C. where only the fixation is observed, the surface area of the band at 920 cm-’ (uu=o) is directly proportional to the amount of UOF species extracted from the solution. This correlation permits one to evaluate, by extrapolation, the quantities of UOY fixed by the organic substrate in the temperature range where the reduction occurs simultaneously with the fixation (120 to 250°C). The results thus obtained are presented in Fig. 2. The quantities of fixed uranyl species mcrease progressively from 45” to 120°C the temperature at which a maximum is attained. They then decrease progressively from 120” to 250°C with a step around 150°C. This decrease may result from a competition between the two phenomena considered (fixation of uranyl species and reduction of them to uraninite), since the maximum of fixation occurs at the temperature at which the reduction begins (120°C). and since the quantities of fixed uranyl species becomes negligible when the reduction terminates (- 250°C). The step around 150°C may indicate a complex nature of the fixation mechanisms. The quantities of non-reduced many1 species can be determined by adding the amounts of uranyl species fixed by the organic substrate (Fig. 2) to the amounts of UOP free in solution (Fig. 1; curve 1). for all the studied temperatures. These results are presented in Fig. 1 (curve 2). The shift between curves 1 and 2 (shaded area in Fig. 1) corresponds to the quantities of uranyl species fixed by lignite. The very close coincidence now observed between curves 2 and 3, corresponding respectively to nonreduced many1 species and H+ ions liberated, shows that the reduction of uranyl species is accompanied by a stoichiometric (2: 1) liberation of protons in the aqueous medium. On the other hand, this also shows that the fixation of many1 species is not accompanied by any liberation of protons in the aqueous medium. Four stages in the evolution of the uranium-lignite system (run duration: 262 hours) may be distinguished from these results as a function of temperature:

UO:’

bohll)

0.10

am

a) 20”-45°C: no reaction; b) 45”-120°C: exclusive fixation of many1 species by lignite; c) 120°-250°C: coexistence of fixation and reduction of uranyl species; d) 250”-400°C: exclusive reduction of many1 species to uraninite. 2) Fixation mechanism of uranyl species by lignite

0

100

300

4

FIG. I. Variation of UOP and H+ content (mole- I-‘) in aqueous phase aa a function of temperature after 262 hours of heating of 200 mg of lignite with 20 ml of aqueous solution UQC12 0.1 M. (1): IJW free in solution. (2): uranium not reduced (=UO$+ in solution + UO? fixed by lignite). (3): H+ liberated. (shaded area): UO$+ fixed by lignite (see Fig. 2).

The absence of any release of protons during the fixation of uranyl species is in contradiction with a classical ion exchange process. Such a process has been studied by SZALAY (1964) with peat samples and UOi+ or other cations. In this case, the liberated H+ ions originated from protonated reacting sites (= functional groups). If such sites existed originally in the Gardanne lignite, primarily in the form of salts

Organic fixation of U

no. 2. Variation of fixed UO$+cmtent(in mitimole - 1-l and milkquivalent per gram liite) in solid phase a5 a function of temperature far the same series of experiments as Fig.1,basedon the f.R. absorption band at 920 nn-’ fvu=ok

or compkxer& they ShotsIdhave been reiamd to

ttie

H* form by the pr&nkry acid washing (see experimental part). Moreover, the strongly acidic character of the reaction medium (pH = 0.8 to 1.8) is also unfavorable to the ion exchange me&an&m propo& by Szu&~ (op. cir.f. The stability of the association of umnyl species with the organic substrate w&palso tested by submitting the solid residue, obtained from an ex.periment conducted at 120% to an acid treatment (HCI 1 N; 2 hours). This operation leads to a removal of only about 25% of the originally fixed uranyl species. Therefon, the major part of the fixed Vof‘ (- 1 meq. g-’ organic matter) should be in the form of stable organo-uranyl combinations. The thermal and chemical stability of these moieties allows to compare them with natural substances, such as (a) the organouranium chelates, whose existence was proposed by SCHM~DT-COLLERUS(1969) based on pyrolysis experiments of organic materials originating from uranium ore deposits, and (b) the “true organo-uranium compounds” whose existence was proposed by WI~ASSILIOW (I 980) after examination of uraniferous lignite sample.

Among the reduction pmcesses of uranium in the presence of organic materials which were briefly reviewed in the intm&ction, we can exclude, in our case, the intervention of sulfur moieties. Indeed, the total sulfur content of lignite in both organic and mineral forms is much smaller than the amounts of uranium used in our experiments (about a f&or of IO). R&me of protons. As was noticed before, the reduction of uranyl species is accompanied by a sto~hi~rn~c liberation of protons (2:1). This can be futier ilhzstra&d by the very good correlation observed between the quantities of UO$+ species reduced and the quantities of H+ liberated (Fig. 3; correh&m coefikient 0.995). Efementaf ~~~~~. The main results of the elemental anaIysis of the carbonaceous residues obtained

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from our experiments are presented in a H/C-OK VAN KREVEJ..EN (196 1) type diagram (Fig. 4, curve 1). In order to distinguish the consequences of the reduction process on the organic substrate &om its simpje thermal evolution, the results of the elemental analysis of lignite samples having been submitted to a similar thermal treatment, in the presence of water but without uranium, are ako repnsented (Fi& 4, curve 2). These reference sampks apparently undergo no transformation up to 200°C Above this temperature, they start to evolve following the same path as diagenetically altered natural organic materials of ligneous origin ftyEK: IIf: DtmWD and @PflmE, 1976). Water seems to have no signiftcant effect on this evolution, since similar transformations of Comparable materials can be reproduced by a simpie heating without water under an inert atmosphere (ROBINetal., 1977). Concerning the lignite samples heated in the presence of uranium, it was first verified that the presence of UC& had no in%tence on the results of elemental analysis. This test was can&5 out by analyxing samples of the starting organic material, to which known amounts of UO, had been previously added. This absence of influence of Ui& on the results Of the ekmental ana&& of the associated organic materials is also supported by the great thermal stability of this mineral (SUITH et al., 1982). From 120” to 2oO*C, the atomic H/C ratio of the Iignite samples heated in the presence of many1 solution decreases progressively (Fig. 4, curve I), in relation to the increasing reduction of uranium (see Fig. 1, curve 2). Above 2OO”C, where the reduction is almost complete, a thermal maturation effect adds to the dehydrogenation of lignite. This fact is shown in Fig. 4 by the diminution of the O/C ratio which starts after the diminution of the H/C ratio due to the reduction. With increasing treatment tempera_ tures, the chemically altered IigGte samples tend progressively to rejoin the evolution path of lignite

UOT

Fez. 3. comelation hetwcen reduced ucg+content and liberated H* content in the aqueous medium (in mole- I-*) for the same series of experiments as Fig. I.

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S. Nakashima et a/ 1.0. 0.9. 0.8 0.7 0.6.

H/C 0.5. 04. 0.3 0.2. 0.1

I 1 0

w5

0.10

o,c

0.15

a20

0.25

FIG. 4. Elemental compositions of lignite plotted on a H/C-O/C diagram. 0: purified Gardanne lignite (starting material). 0: lignite heated with an aqueous solution of UO#Zl~ 0.1 M (results of the experiments shown in Fig. 1); (-: curve 1). 0: lignite heated with distilled water, (-----: curve 2). The numbers are the temperatures of heating. All the experiments were carried out using 200 mg of lignite and 20 ml of aqueous solution, and heated for 262 hours.

heated without uranium, i.e.: evolution path of type III materials (Fig. 4). It must be noted that the reduction of uranium is not accompanied by any increase of O/C ratio, contrary to what was proposed by ROIJZAIJD et al. (1979)after a reduction mechanism implicating the intervention of water. The relationship observed between the amounts of protons liberated in the aqueous medium as a result of the reduction process (AH+) and those of hydrogen lost by lignite as a result of the dehydrogenation process (AH) is presented in Fig. 5. The AH values were calculated by means of the elemental composition of the lignite samples: the decrease of atomic H/C ratio (A(H/C)) was multiplied by the carbon content of the lignite sample (m mole *g-‘) to give the amount of lost hydrogen (AH; m mole *g-l). A direct I: 1 correlation between the calculated AH+ and AH values is observed for the experiments conducted at relatively high temperatures (I 65” to 200°C) (Fig. 5, line 1). Considering that the liberation of protons is the result of the uranyl reduction process (Fig. 3), the following reaction scheme can be written: 2 {H} lignite + UO:+ -

2H+ + U02. 1

hydrogen lost by lignite in these conditions is eliminated under a neutral form, e.g. molecular hydrogen (Hz). The hydrogens lost during the dehydrogenation of lignite were thus used totally (165°-2000C) or partly ( 120°-1 50°C) to reduce uranyl cation. This point will be discussed later. I.R.spectroscopy. The structural transformations undergone by lignite as a result of the reduction of uranium were studied by I.R. spectroscopy. TWO

(1)

({H} lignite: hydrogen atom in lignite) The conversion of hydrogen atoms to hydrogen ions appears to be total for these high temperature experiments. However, for the experiments conducted at lower temperatures ( 120” to 15O”C), the amounts of hydrogen lost by lignite (AH) exceed, by a factor of approximately 2, the amounts of protons liberated in the aqueous medium (Fig. 5, line 2). This indicates that the reduction process proceeds at a slower rate than the dehydrogenation process at relatively low temperatures. It can be assumed that the excess of

AH (mmole/g llgmte I FIG. 5. Correlation between hydrogen ions liberated in solution (AH*) and hydrogens released from lignite (AH). l : results of the experiments shown in Fig. 1; (numbers” indicate temperatures (“C) of heating). 0: results of the experiments using 100 mg of lignite and 10 ml of aqueous solution UO& 0. I M, heated at 1gO’C; (numbers indicate durations (hours) of heating), (-----: evolution path of the results at 180°C with two SW I and II). Lines (1) and (2) represent the relations AH+ = AH and AH+ = % AH respeclively.

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Organic iixationof U

80

K K,

,X100

( SA;)

Time

(hours

1

FIG. 6. Variations in reduced uranium and rciatin importances of LR. absorption coefficients as a function of the heating duration for the experiments conducted at 18o”C, using 100 mg of lignite and IO ml of aqueous solution UO,Cl, 0.1 M. (a): reduced uranium (mmole per gram lignite). (b): Percentages of absorption coefficients Kim (( 1)). Klmo ((2)) and Kpm ((3)) for their sum (&). (K2920:aliphatic C-H;

K,nro: ,‘C = 0, KIW: aromatic and oiefmic C=C, bridged quinonic C=O and HzO).

series of experiments with different initial U/lignite ratios were conducted at 1SO* and 2OO“C, mspectively (Figs. 6 and 7). Effectively, at such temperaturesz (a) from the results of elemental analysis, only a very small thermal alteration of lignite is to be expected (Fig. 4): lb) the reduction of uranium pmceeds at a notable rate (Fig. 1, curve 2). The results discussed here concern the main I.R. absorption bands observed for sedimentary carbonaceous materials @SITTALE et al., 1973), except for the bands characteristic of hydroxyl groups. The reasons for this exception are: (a) the interference of water moisture for the band culminating around 3400 cm-‘, and this in spite of the precautions taken for preparing the sample pellets (ROBIN and ROUXHET, 1976); (b) the bands characteristic of C-O stretching of alcohols, phenols, ethers, esters and acids which appear between 1300 and 1000 cm-’ were too broad and too weak to be measured. Therefore, the bands studied here are the following (BELLAMY,1975): -2920 cm-‘: two bands around 2920 cm-’ and 2850 cm-’ due to the asymmetric and symmetric stretching of aliphatic C-H groups. -1700 cm-‘: C=O stretching of carbonyl (aldehyde, ketone) and carboxyl (acid, ester) groups. -1600 cm-‘: C=C stretching of olefins, aromatic and polyaromatic nuclei and C=O stretching of quinones bridged to acidic hydroxyls (plus bending vibrations of molecular water). -720 cm-‘: skeletal vibration of straight chains with more than 4 -CHrgroups.

However, the absorption coefficients determined in this way are not correct, in the present case, because of the presence of uraninite associated with the organic phase. This mineral introduces an error in the real we&ht of organic matter analyzed. In order to overcome this difficulty, we considered initially the importance of each main absorption coefficient relative to the sum of the coefficients (~KT= K-20 + K,TW + K,,&. The coefficient K720 was not taken into account in this sum, because it is measurable only for the experiments carried out with a high U/ lignite ratio and which required relatively long heating duration at 18O’C (Fig 813, and even in this case, this band remains very small (Km Q 0.01X Krsoo). The evolution of the Kllu, ratios as a fimction of the duration of heating are presented in Figs. 6b and 7b. They show that the bands at 2920 cm-’ (uC+) 0

80

K -x KT

100

80

5

-.-

10

15

0

20

b

I

(%)

Time

< hours 1

The integrated absorption coefficients K of these bands were calculated after the method described by ROBINand ROUXHET(1976):

FIG. 7. Variations in &wed uranium and r&Uive importances of 1.R. absorptioncoetheientsas a function of the heating duration for tbc exprrimcnts conducted at 2OO*C, using 400 mg of lignite and 10 ml of aqueous sohttion

K = unit of absorbance X wave number (cm-l) X (mg organic matter)-’ X cn?.

UQCI, 0.1 M. (a): reduced uranium (mm& per gram lignite). (b): percentages of absorption eoetbeients ?&, (l)), K,, ((2)) and Kn20((3)) for their sum (Kz).

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S. Nakashima PI al.

AK

__ AK,

1

0.3

QZ

1

2

3

I

4 5 6 7 0 9 10 UOP reduced (mmole/g) FIG. 8. Variation of I.R. absorption coefficients LS,,~ (a), aK,, (b) and AK,20(c) as a function of the amount of uranium reduced (mmole per gram lignite). 0: data based on the experiments presented in Fig 6 with a high initial U/lignite ratio (UOxC120.1 M 10 ml for lignite 100 mg) and with a long heating duration (until 232 hours). 0: data based on the experiments presented in Fig. 7 with a low initial U/lignite ratio (UO~C&0.1 M 10 ml for lignite 400 mg) and with a short heating duration (until 9 hours). I, II: two stages of the reduction process. 0

and 1700 cm-’ (~+o) evolve in an opposite way during the reduction of uranium, while the band at 1600 cm-’ (uczc mainly) remains apparently constant (Kim/K7 = 67 k 2%). However, for the experiments carried out with a high U/lignite ratio, the reduction of the metal seems to continue after the end of the evolution of the bands situated at 2920 and 1700 cm-’ (Fig. 6a,b). This is better shown in Fig. 8, where the changes of the absorption coefficients of the bands studied (AK) am reported as a function of the amount of uranium reduced. The AK values were calculated in the following way: considering the constancy of the importance of Kjm in the whole set of samples, we can take it as a reference coefficient to represent the evolution of the other coefficients in the form of W&MM values; the change of these values A(K/K,& were then multiplied by the K1a value determined on the initial lignite sample (which is assumed to remain constant for all samples) to give AK values [(AK) sample = A(K/K,& sample X (KM,& reference]. Two stages in the course of the reduction process may be distinguished from these results (Fig. 8). During the first stage the changes of the coefficients K,700 and Kzyzo seem to be directly proportional to the amounts of uranium reduced (from 0 to about 3

millimole of uranium per gram lignite) (Fig 8a.b). The second stage is observed only for the experiments carried out with a high U/lignite ratio (Fig. 8a,b). During this second stage, the reduction of uranium goes on with no change of the band at 1700 cm ’ and practically also. no change of the hand at 2920 cm-‘. The increase of the band at 1700 cm-’ should be attributed to the formation of carbonyl groups (ketones, aldehydes) and/or carboxyl groups. Since the results of elemental analysis have shown that the reduction of uranium was accompanied by a dehydrogenation of lignite, but was not accompanied by any increase of atomic O/C ratio of this material (Fig. 4, curve l), it must be concluded that these new carbonyl function originate from a dehydrogenation of preexisting hydroxyl groups (e.g. alcohol groups). Considering the stoichiometric liberation of protons (2:l) in the aqueous medium during the course of uranium reduction, the following reaction scheme can be proposed for the first stage of the reduction process (Fig. 8. I):

:CHOH+

uo:+-:c=o + UOz ---l

+ 2H’.

(2)

This mechanism could explain the principal phenomena observed: (a) increase in the amount of carbonyl functions (&M) (Fig. 8a); (b) simultaneous decrease of aliphatic protons (Kzqm) (Fig. 8b) without any increase of the atomic O/C ratio of lignite (Fig. 4); (c) stoichiometric (2:l) liberation of protons (Fig. 3). Despite the fact that aliphatic alcohols are explicitly represented in this reaction scheme (2), the particination of hydroquinones in the reduction process cannot be excluded. However, the reaction scheme proposed conforms with the results of NOZAKJand INAMI (1974) who have studied the oxidation, over U03, of 2-propanol and 1-butanol to the corresponding carbonyl products. Uranium oxide was reduced in the coume of the reaction. The increase in the amount of carbonyl groups terminates at about 3 m mole-g-’ of the reduction of uranium (Fig. 8a). By the reaction scheme (2). 3 m mole of alcohol groups per gram lignite are necessary for this first stage of the reduction process. This value is in very good agreement with those which have been reported for similar materials: 3 to 2 m mole-g-i of alcohol functions in lignites and brown coals (BLOM et al., 1957; MARINOV, 1977b). From this calculation, for the experiments carried out with a low U/lignite ratio, the system contains initially about 1.2 m mole of alcohol functions. There being an excess of these reducing moieties to react

Organic fixation of U with all the uranium introduced (0.9 m mole) in the system, it is thus logical not to observe the second stage of the reduction process in this case (Fig. 8a,b). For the second stage of the experiments carried out with a high U/Iignite ratio, where all the alcohol functions seem to have been already consumed (Fig. 8a), another process must be taken into account for the reduction of uranium. Data on the correlation between the amoitnts of protons liberated in the aqueous medium (AH+) and the amounts of hydrogen released from lignite (AH) for this series of experiments are represented in Fig. 5. Two different stages can be observed in this figure, and they coincide with the two stages of the reduction process (Fig. 8a,b). During the first stage (Fig. 5; I), AH+ and AH increase simultaneously, but there am more hydrogens lost by lignite than protons liberated (AH+/AH = 0.5). This observation confirms that the dehydrogenation proceeds at a faster rate than the reduction process. The excess hydrogen lost from lignite, compared to the liberation of protons, may reflect a release of molecular hydrogen from aliphatic hydmcarbonaceous moieties. Various studies have demonstrated the ability of uranium oxide catalysts to promote the dehydrogenation of aliphatic (NOZAKI and kHIN0, 1973; DELVALLEZ et al., 1978), alicyclic (SKUNDRICef al., 1977) and alkyl-aromatic hydrocarbons (NOZAKI and ICHIKAWA, 1973; HEYNEN aiid VAN DER BAAN, 1974). The dehydrogenation of lignites and coals has also been studied in the presence of oxygen (JOSEPH, 1982; MARINOV, 1977a,b) or under the influence of various catalysts: metal oxides (YOKONO et al., 1982a,b) and metal chlorides (MATSUURAet al.. 1974; BODILY et al.,1974). In our case, this dehydrogenation can be partly responsible for the observed decrease in the amount of the lignite aliphatic protons Fig. 8b). However, any change in the IR t&920, spectra could be attributed to the appearance of ethylene functions which may result from the dehydrogenation of lignite. This may be explained in two different hays: (a) the absorption of these new functional groups may be obscured by the stronger absorption of preexisting similar moieties (band at 1600 cm-‘); (b) the other bands characteristic of these groups may be too weak to be observed. During the second stage of the reduction (Fig. 5; II) lignite does not lose any more hydrogen, but one can observe a gradual oxidation into H+ of hydrogens PR~OUSly lost by lignite. This oxidation of the hydrogen can be naturally related to the reduction of uranium at this stage (Fig. 8; II) by the following reaction scheme: Hz + Ua+

+ 2H+ + U02. 1

This reaction has been already reported by authors

various

(LE PAGEand FANE, 1974; EKSTROMet al.,

1974; NOZAKI and SODESAWA,1979). The appearance of a new band at 720 cm-‘, attributable to paraffinic chains, is a relatively minor

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and slow process (Fig. 8~). Various differentreactions could explain the formation of these new moieties: hydrogenation and/or polymerization of olefinic groupsof lignite, and/or structural rearrangements in the polymer chains. Despite the difficulty of verifying these mechanisms, it should be noted that the ap pearance of paraffinic chains should be related to the transformations of the organic matrix induced by the reduction of uranium, since no band at 720 cm-’ is observed in the corresponding lignite samples heated without uranium. CONCLUSIONS The evolution of the acidic many1 solution-lignite system after about 10 days of heating at 20” to 400°C was studied and shows four stages as a function of temperature: a) 20”-45”C: no reaction; b) 45”-120°C: exclusive fixation of uranyl species by lignite; c) 120”-250°C: coexistence of fixation and reduction of many1 species; d) 250”-4OO”C: exclusive reduction of uranyl species to uraninite. Under diagenetic or hydrothermal conditions, a natural organic matter of ligneous origin having reached the lignite stage can exert its reactivity towards many1 species in two different ways. 1) A first process (T> 45’C) is the lixation of uranyl species by lignite without any reduction. This process does not proceed through a classical ion exchange mechanism but results in a formation of stable organo-metallic combinations. Though the real nature of these products has not yet been elucidated, they can be compared with the natural urano-organic materials studied by WHMIDT-COLLERUS ( 1969) and HAJI-VASSILIOU (1980).

2) A second process (T > 120°C) is the reduction of uranyl dissolved species, which results in the precipitation of uraninite. Two distinct mechanisms appear to be responsible for this process: (i) the first is a redox reaction in which hydroxyl (alcohol) groups are the reductant moieties In the course of the reaction, these groups are oxidized to carbonyl (ketone and/or aldehyde) functions. (ii) this first reaction proceeds in competition with a dehydrogenation of hydrocarbonaceous moieties of lignite, leading to a release of molesular hydrogen. This dehydrogenation is assisted (catalyzed) by the uranium species. (iii) when there is an excess of many1 species compared with the reducing capacity due only to the hydroxyl groups of organic material, the molecular hydrogen formed during the dehydrogenation of lignite can be responsible for the further reduction of uranium.

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Both these mechanisms result in a stoicbiometric liberation Of protons (2:1) in the aqueous medium and a decrease of the H/C ratio of lignite (while its O/C ratio remains constant). The re~~1t.s of this study provides the following constraints for the genetic schemes proposed to explain uraniferous mineralization associated with sedimentary oganic matter (BREGERand DEUL, 1956; VINE, 1962; BREGER,1974): (a) hydroxyl groups appear to be the most active functional groups in the reduction process; (b) the reduction process results in a decrease of H/C ratio of organic substances, while their O/C ratio remains constant; (c) the low hydrogen content of carbonaceous substances associated with uranium mineralization was previously thought to be a consequence of the dehydrogenating and demethanating radioactivity of natural uranium and its daughter products (BREGER, 1974). This study shows that this hydrogen loss can also be a result of an oxidation-reduction process; (d) the quick reduction of important quantities of uranium (>2.4 g U-g-’ organic matter) by natural organic materials requires only mild thermal conditions, which may be reached either during natural diagenesis or during low temperature hydrothermal events, and permits the deposition of uranium from aqueous solutions. Their kinetic and thermodynamic aspects will be reported in a further paper; (e) the acidification of the medium resulting from the reduction process also favors the deposition of uranium by destabilizing the various complexes that it forms in aqueous solution with either inorganic (LANGMUIR, 1978) or organic lignands (SZALAY, 1964; DISNAR, 198 1); (f) even in such a strongly acidic medium, some uranium can he simply fixed by organic materials to form particularly stable organo-uranyl combinations. Acknowl~ents-The authors Bratefuliyacknowl~ M. Reynard of the HouiI&es de Provence for providing stunpIes of lignite of Gardanne. We thank also Centre de Rechemhes sur Ies Organisations CristaIlines Imparfaites, C.N.R.S. for their permission to use the I.R. spectrometer. REFERENCES ANDREYEV P. F. and CHUMACHENKO A. P. ( 1964) Reduction of uranium by natural organic substances. Geochem. Inf. 1, 3-7. BELLAMY L. J. (1958, 1975) The lt@ared Spectra c?fcda Molecules. 26 ed., Methuen. 1958. 3” ed., Chapman and

Hall. 1975. BL~M L., EDELHAUSENL. and VAN KREVELEND. W. (1957)ChemicaI structure and properties of Coal-XVIII: oxygen groups in coal and related products. Fuel 36, I35-

153. D. M., LEE S. H. D. and WISER W. H. (1974) Dehydrogenation of coal by metaI sah catalysts. Amer. Chem. Sot.. Div. Fuel Chem.. Prepr. 19, 1, 163-166. BREGERI. A. (1974) The role of organic matter in the accumulation of uranium: the organic geochemistry of the coal-uranium association. Proc. Symposium I.A.E.A. Athens, 99- 124.

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BREGER I. A. and DEULM. ( 1956) The organic geochemistry of uranium. Int. Con/ Pea&i/ Uses AI. Energy (Proc. Conf. Geneva, 1955), 6, UN, New York, 418-422. BULLOCK J. I. (1967) Infrared spectra of some uranyl nitrate complexes. J. Inorg. Nuci. Chem. 29, 2257-2264. DELVALLEZ. H., GADELLEC. and SEREEDE ROCH 1. (1978) Oxydishydrogenation des m&hylbut&s en &opt&e par les systema U-Sb-0. Bull. Sot. Chim. I, 127-130. DISNARJ. R. (1981) Etude exdrimentale de la fixation de metaux par un ma&au s&limentaite actuel d’origine alaaire II. Fixation “in vitro” de UO?. Cu2+. Ni2+. Zn2’. Pb*+, Co’+, Mn*+,ainsi que de VO,‘, ‘M&i- et r&O-: Geochim. Cosmochim. Acta 45, 363-379. DISNARJ. R. and TRICHETJ. (I 983) Pyrolyze de complexes organo-m&alliques form&s entre un mat&au organique acmeI d’otigine algaire et divers cations mitaIli&s hivalents fUof’. Cu*+. Pb*+. Ni’+. Mn*+. Zn’+ et Co’+). Chem. &0~.-46. 2031223. ’ DURAND B. and E~PITALIE J. (1976) Geochemical studies on the organic matter from the Douala Basin (Cameroan)-II. Evolution of kerogen. Geochim. C’osmochim. Acta 40, 801-808. EK~TROMA., BATLEYG. E. and JOHNSON D. A. (1974) Studies of topochemicai heterogeneous catalysis-l. The cat&tic effect of olatinum on the reaction of UF. with 02, and of U02F2’and UO, with H2. J. Catnl. 34: 106116. ESPITALIEJ., DURAND B., ROUS~ELJ. C. and SOURONC. (1973) Etude de la mat&e organique insoluble (k&gene) des argiles du Toarcien du Bassin de Paris-II. Etudes en spectroscopic infrarouge, en anaiyse thermique differentielle et en analyse thermogravimCtrique. Rev. Insf. Franc. Pktrole 28, 37-66. GARRELSR. M. and POMMERA. M. (1959) Some quantitative aspects of the oxidation and reduction of the ores. U.S. G. S. Prof Paper 320. 157-164. HAJI-VASSILIOU A. (1980) The form of occurrence of uranium in deposits associated with organic matter. &on. Geol. 75, 609-6 17. HEYNENH. W. G. and VAN DER BAAN H. S. (1974) The catalytic dehydrogenation of ethylbenzene and cumene over catalysts containing uranium oxide. J. Catal. 34, 167-174. JCXEPH D. (I 982) L’oxydation des mat&es carbonees. These d’Etat, Univ. OrlQns, 82 p. LANGMUIRD. (I 978) Uranium solution-mineral equilibria at low temperatures with applications to sedimentaiy ore deposits. Geochim. Cosmochim. Acta 42, 547-569. LE PAGE A. H. and FANE A. G. (1974) The kinetics of hydrogen reduction of UO, and UsOs derived from ammonium diuranate. J. Inorg. Nuci. Chem. 36, 87-92. MARINOVV. N. (1977a) Self-ignition and mechanisms of interaction of coal with oxygen at low tetttpmtutes. 2. Changes in weight and thermal effects on gradual heating of coal in air in the ranae 20-300°C. Fuef 56. 158-164. MARINOVV. N. (I 977b) -Self-ignition and me&anisms of interaction of coal with oxy8en at low temperatures. 3. Changes in the composition of coal heated in air at 60°C. Fuel 56, 165-170. MAT~UURAK., BODILYD. M. and WISERW. H. (1974) Active sites for coal hydrogenation. Amer. Chem. Sot.. Div. FuelChem.. Preur. 19, 1, 157-162. MOOREG. W. (1954) Extraction of uranium from aqueous solution by coal and some other materials. Econ. Geol. 49.652-658. NOZAKIF. and ICHIKAWA F. (1973) Catalytic activities of uranium oxide and its mixed catalysts for oxidative dehydrogenation of ethylbenzene. J. Chem. Sot. Japan. Chemistry and Industrial Chemistry 2, 254-259. NOWKI F. and ICHINOM. (1973) Activity and selectivity of U -Sb-0 catalyst for oxidative dehydrogenation of I-butene to 1,3-butadiene. J. Chem. Sot. Japan, Chemistry and Industrial Chemistrv 8. 1397-1402.

Organic iixation of U NOZAKI F. and INAMI 1. (1974) Dehydrogenation and dehydration of 2-propanol and I-butanol over utanium oxide catalysts. J. Chem. Sot. Japan, Chemistry and Industrial Chemistry 10, 1856-l 860. NOZAKIF. and SODESAWA T. (1979) Reduction-reoxidation behavior of the catalysts containing uranium oxide and their catalytic activities. Rep. Asahi Glass, Found. Ind. Technol. 34,69-82. NYQUIST R. A. and G&EL R. 0. (1971) Infrared Spectra ofInorganic Compounds: 3800-45 cm-‘. Academic Ptess. ROBIN P. L. and ROUXHET P. G. (1976) Contribution of molecular water in the infrared spectra of kerogens and coals. Fuel 55, 177- 183. ROBIN P. L., ROUXHET P. G. and DURAND B. (1977) Caracttisation des k&g&s et de leur ivolution par spectroscopic infrarouge: fonction hydrocarbon&s. In Advances in Organic Geochemistry, 1977. (eds. R. CAMand J. &NI), pp. 693-716. Enadimsa, Madrid. ROUZAUDJ. N., OBERLINA. and TRICHETJ. (I 979) Interaction of uranium and organic matter in uraniferous sediments. In Advances in Organic Geochemistry, 1979. (cds. A. G. DOUGLASand J. R. MAXWELL),pp. 505-516. Pergamon Press. SCHMIDT-C• LLERUS J. J. (1969) Investigations of tbe relationship between organic matter and uranium deposits:

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