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Radiogenic uranium-234 in humus acids of graptolitic argillite/[TEXT NOT REPRODUCIBLE IN ASCII.

In the present work, graptolitic argillite (Dictyonema shale) taken from the core of borehole 191-a at a deposit in North-West Estonia was investigated. The sample 2304 was taken from the bottom quarter of the layer, approximately 75 ([+ or -] 15) cm above its bottom. Alpha-ray spectrometric analysis data of the sample in question show the following activity ratios (AR) in the investigated sample: [sup.234]U/[sup.238]U = 1.03 [+ or -] 0.02 and [sup.230]Th/[sup.234]U = 0.99 [+ or -] 0.06, the average uranium content being ~100 [micro]g/g. These values suggest that uranium and its decomposition products present in the shale are in the state of radioactive equilibrium or rather close to it.

In the investigated sample, the total content of organic carbon (by Tjurin method) and fraction and group composition of humus (by Plotnikov-Ponomarjov method) were determined. It was established that the shale sample contains 14.4% organic carbon, but only 0.26% of this amount (i.e. 0.018% of the total organic matter of shale) is related to humic and fulvic acids, in other words to humus acids themselves. The rest of organic matter (98.2%) is presented by humins insoluble in the conditions of standard analysis, and also by individual substances oxidizing with difficulty, which represent components of kerogen organic matter. Humus acids are presented by three fractions of humic acids and four fractions of fulvic acids.

In humic and fulvic acids [sup.238]U and [sup.234]U content was determined. The influence of decalcination of humates and fulvates on isotopic composition of uranium was studied. After triple extractions, more than 30% of uranium present in shale is transferred into alkaline solutions. Its distribution is as follows: 20.2% in humic acids and 10.56% in fulvic ones. This sedimentation is not attributable to adsorption, occlusion or any other mechanism of uranium capture from a solution as proved by differing isotopic composition of uranium in the separated fractions. Uranium transfer into the deposit definitely proves the formation of chemical compounds of uranium with corresponding ligands in the humus phase of Dictyonema shale. The presence of radiogenic uranium in humic acids proves that, despite of significant destruction of organic matter in the track of the recoil atom, not only the molecule of humic acid is recreated from fragments and radicals, but also strong links of uranium-234 with both humic acids and fulvic acids form. They allow radiogenic uranium to follow a corresponding organic acid at alkaline extraction and sedimentation.

On the background of common radioactive equilibrium in the uranium series in the studied shale, an increase in the value of [sup.234]U/[sup.238]U ratio in organic phases over unity indicates the presence of an uranium-rich matter. Presumably, it is uranium black which directly reacts with organic phases of shale. The grains of this oxide phase were rated to be of 25-35 nm, and their [sup.234]U/[sup.238]U ratios within 0.22-0.25 [20]. Although fulvic acids prevail in the humus of the shale studied (the ratio of fulvic acids to humic ones is 1.37), the amount of uranium-238 associated with humic acids extracted by alkali is twice more than that in fulvic acids. However, uranium in fulvic acids is enriched with radiogenic uranium: the average value of AR for three consecutive alkaline extracts makes 1.17 [+ or -] .03 for fulvic acids and 1.08 [+ or -] 0.03 for humic acids. Obviously, it is related to higher content of isotope [sup.238]U in humic acids.

Experiments on decalcination of uranium humates and fulfates by hydrochloric acid solutions showed that molecules of humus acids keep radiogenic uranium by one and a half or two times weaker as compared with parent uranium. It points at different character or different quantity of chemical bonds produced by both uranium isotopes.

The conclusions are as follows:

1. The molecules of humus acids destroyed as a result of nuclear disintegration of uranium (and other nuclides) are able to restore their initial structure in the shale organic matter or a structure identical to the initial one.

2. Radiogenic uranium reacts with functional groups of humus acids in the solid phase forming respective chemical compounds (uranium humates and fulvates).

3. Chemical bonds of radiogenic uranium in the composition of a humate complex are weaker than those of its parental isotope possibly because of different valent state of uranium isotopes.

4. By stability of the bonds of uranium atom with humus acids, the complexes formed at their interaction can be ranged as follows:

* weakest--complexes formed at short-term interaction of uranyl ions with humus acids in the liquid phase;

* relatively strong--compounds formed with participation of radiogenic uranium (VI) in the solid phase of shale organic matter

* strong--humate and fulvate complexes formed with participation of uranyl ion of parental isotope [sup.238]U in the solid phase;

* very strong--complexes formed with participation of tetravalent uranium, especially in close association of humin and clay minerals.

Presented by J. Kann Received April 17, 2006

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Author:Bogdanov, R.; Ozernaya, S.; Pihlak, A.-T.; Timofeev, S.
Publication:Oil Shale
Date:Mar 1, 2007
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