Study of the Mobilization of Uranium Isotopes in a 2 Sandstone Aquifer Using Methods for the Extraction 3 of Uranium with Different Strength Reagents in 4 Combination with Groundwater Data

: A partial extraction procedure was used to study the distribution of uranium in the 11 mineral phases of rocks of an aquifer of sandy-clay deposits of the Vendian in the northwest of 12 Russia. This work is a part of a research project to develop a method for combined radiocarbon and 13 uranium-isotope dating of groundwater. Representative aliquots of each core sample were 14 subjected to five "partial" extractions by treatment with: distilled water, low mineralized fresh 15 natural groundwater, minopolycarboxylic acid chelating agent (0.05M EDTA), 0.5M HCl, 15M 16 HNO 3 , and a total digestion, with U isotopes reported in this study for each procedure. The 17 following mineral phases of core samples: adsorbed material, carbonate minerals, amorphous iron 18 oxides, aluminosilicates partial digestion and a crystalline iron oxides, aluminosilicates total 19 digestion and a clay/quartz resistate were characterized. Red-colored siltstones depleted in 20 uranium in relatively readily soluble mineral phases. The concentration of adsorbed uranium was 21 established in the amount of 15.8±2.1 - 30.5±3.9 μg/kg. Carbonate minerals contain even less of this 22 element. In iron hydroxides and the most readily soluble aluminosilicates, its concentrations are in 23 the range 168±24 - 212±28 μg/kg. The most insoluble fraction contains 1.65±0.21 - 4.32±0.45 mg/kg of 24 uranium. In green-colored siltstones, the concentration of adsorbed uranium is much higher: 25 106±14 - 364±43 μg/kg. Carbonate minerals and amorphous iron oxides contain 1.91±0.21 - 2.34±0.26 26 mg/kg of uranium. In aluminosilicates and a clay/quartz resistate, uranium concentrations are 27 5.6±0.5 - 16.8±1.4 mg/kg. Elevated values of 234 U: 238 U activity ratio prevail in the adsorbed material 28 and iron hydroxides. In aluminosilicates and clay/quartz resistate, the values decrease. This 29 indicates the replacement of primary sedimentogenic uranium by secondary hydrogenic uranium 30 adsorbed on the surface of minerals and coprecipitated with iron hydroxides. The results obtained 31 made it possible to carry out preliminary quantitative estimates of the retardation factor and recoil 32 loss factor of uranium in the groundwater of siltstones of the studied Vendian aquifer.


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indicate the opposite process: the transition of nonequilibrium nuclides from water to rock with an 51 increase in the ratio of U isotope activities in the rock [20]. Under such conditions, it is difficult to 52 estimate the rate of chemical weathering by using the currently developed methods [21][22].

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Therefore, for a more complete understanding of the behavior of uranium isotopes in the 54 water-rock system, a transition from the presentation of solely total concentration and activity data 55 to an additional analysis for finding individual "weak" leach extractants or sequential extractions is 56 necessary. In recent decades, leach data, rather than total decomposition data are widely used in a 57 variety of fields of geoecology (see for example [23][24][25][26]). Therefore, in the present work, an attempt is 58 made to use the partial extraction procedure for reconstructing the processes of redistribution of 59 uranium isotopes in certain mineral phases of an aquifer of sandy-clay deposits of the Vendian.

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In a previous papers [27][28], the possibility of sharing uranium and carbon isotopes for dating 61 groundwater was discussed. The transport of uranium in solution is reasonably well described with

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Fröhlich and Gellermann [30], Ivanovich et al. [9], and Porcelli [5] to provide a 234 U-238 U dating 69 method for groundwater under oxidizing conditions. However, the values of R, Rd (a -1 ) and p needed 70 to be determined. In turn, the recoil loss factor value p depends on the SSA (specific surface area).

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Accordingly, the ranges of R and Rd can be calculated using these formulas. Therefore, we consider it 73 more appropriate to show R, Rd and p in the form of their ratios: Rd:p (a -1 ), R:p and Rd:R (a -1 ) . The

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In this case, two unknown parameters remain in formula (1), which cannot be directly 80 measured in water and rock samples: t and p R / . Therefore, in order to use equation (1), it is 81 necessary to make several determinations of the groundwater age by other methods, for example, 82 using isotopes of carbon. Then, we need to find out whether there is an increase in uranium 83 concentrations in groundwater with a decrease in the concentration of radiocarbon. If this is the case, 84 then the mean values p R / for the studied aquifers are determined. After this, uranium-isotopic 85 dating of other groundwater samples is carried out, which is less labor-intensive and more accessible 86 than radiocarbon dating. The average value of the retardation factor/recoil loss factor ratio (

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In this work, it was planned, by determining the amount of uranium adsorbed on the surface of 90 mineral particles, to proceed to a separate assessment of the values of the retardation factor and 91 recoil loss factor of uranium in the groundwater of siltstones of the studied Vendian aquifer.

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The samples were quickly packed in airtight polythene bags. The sample mass collected in each 100 case was about 1500 g. Sub-samples of the material were oven dried at 40 °C for 7 days and 101 homogenized by grinding with an agate mortar and pestle to pass through a 125 μm sieve. The 102 prepared material was stored in glass bottles for sequential extractions and isotopic analyses.

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The chemical and mineralogical composition of the Vendian deposits in the area of the diamond    ratio of Fe2O3 and FeO forms of iron in "red" siltstones is ~ 17: 1. In "green" siltstones, the Fe2O3 117 content is 2.7 times lower (Table 1)

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Representative aliquots of each core sample were subjected to five "partial" extractions and a 132 total digestion, with U isotopes reported in this study for each procedure. The various procedures

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The   shaken at room temperature for 1 hr on an end-over-end shaker.

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Nitric acid is an oxidizing agent that is not as powerful in its attack on aluminosilicates as HF,

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and is thus said to be a partial digestion. Also, crystalline iron oxide is digested by hot HNO3. In the 163 present study, 500 mL of 15М HNO3 was mixed with 25 g of core sample and heated for 1 hr while 164 stirring. The resulting solution was evaporated to wet salts, topped up with 500 ml of distilled water 165 and acidified with HCl to pH 1-2.

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Measurement uncertainties for U are reported individually (

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The undissolved residue was filtered off and washed three times with 5-10 cm 3 portions of hot 7M

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HNO3 and added to the solution. The U solution was transferred to a separatory funnel and a 30%  As can be seen from

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Reduced values of 234 U: 238 U activity ratio (less than 1) are characteristic of the most difficultly soluble 270 fractions. In them, the dissolution of uranium by groundwater was practically absent and only 271 depletion of 234 U atoms occurred due to recoil loss factor. In green siltstones, there is also a tendency 272 towards a decrease in 234 U: 238 U activity ratio in the direction from easily soluble fractions to sparingly 273 soluble fractions (Fig. 3, interval 99.5-99.6 m). However, for a sample taken from the interval 66.7 -

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color of the strain is light greenish gray. In terms of composition and structural-textural features, 437 siltstone does not differ from the rocks that are described in the above.