Thermodynamic modeling of the behavior of Uranium and Arsenic in ...

ISSN 1028334X, Doklady Earth Sciences, 2015, Vol. 465, Part 1, pp. 1159–1163. © Pleiades Publishing, Ltd., 2015. Original Russian Text © O.L. Gaskova, V.P. Isupov, A.G. Vladimirov, S.L. Shvartsev, M.N. Kolpakova, 2015, published in Doklady Akademii Nauk, 2015, Vol. 465, No. 2, pp. 203–207.

GEOCHEMISTRY

Thermodynamic Modeling of the Behavior of Uranium and Arsenic in Mineralized ShaazgaiNuur Lake (Northwest Mongolia) O. L. Gaskovaa, b, V. P. Isupovc, A. G. Vladimirova, f, S. L. Shvartsevd, e, and M. N. Kolpakovad, e Presented by Academician N.Z. Lyakhov May 29, 2015 Received June 06, 2015

Abstract—Highly mineralized closed lakes on the territory of ore regions of Mongolia are of special interest in relation to the search for nonconventional sources of metals. Water of soda ShaazgaiNuur Lake contains ~1 mg/L U, and the content of the undesirable admixture of As is up to 300 μg/L. Uranium and Arsenic spe ciation in solution and in the bottom sediments of the lake was estimated using thermodynamic modeling, and a method of their separation was suggested. Calculation of the models of sorption of these elements by goethite and calcite showed that at pH 9.4 typical of natural water it could be effective only at a high concen tration of FeOOH sorbent. In this case, at pH 8 (the area of U sorption), As may be removed by sim ple filtering of solutions from the suspension upon additional coagulation. DOI: 10.1134/S1028334X15110094

Highly mineralized closed lakes on the territory of ore regions of Mongolia are of special interest in rela tion to the search for nonconventional sources of met als. Recently the saline lakes of West and Northwest Mongolia and Chuya Basin of the Mountain Altai (Russia) potential for searching the nonconventional sources of metals were studied in detail [1, 2]. ShaazgaiNuur Lake is the most interesting among soda lakes, because its water drainage area is located within the TsaganShibetinsk promising uraniumore zone. The composition of water in this lake with a total mineralization of ~15 g/L is Na–Ca hydrocarbonate– chloride (pH ~ 9.25). The concentration of U is ~1 mg/L in water and up to 13.8 × 10–2 g/kg in bottom sediments [2, 3]. Uranium was initially accumulated in this lake at the expense of incoming uraniumbear ing groundwater, which is released directly into the lake or into the KhargainGol River flowing into the lake

a

Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia b Novosibirsk State University, Novosibirsk, Russia c Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia d Tomsk Branch, Trofimuk Institute of Oil and Gas Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, Tomsk, Russia e National Research Tomsk Polytechnical University, Tomsk, Russia f Tomsk State University, Tomsk, Russia email: [email protected]

[1–4]. The daily intake of U into the lake is 0.65 kg. Calculations of the equilibria of different lake waters with major minerals of host rocks and likely reasons for accumulation of uranium and other elements were considered in [4], but the problems of sorption were not discussed. Our study continues the abovementioned investi gations and was aimed at estimation of U and As spe ciation in solution and bottom sediments of Shaazgai Nuur Lake using thermodynamic models. It suggests a likely method of their separation, since the content of As in solution reaches 300 µg/L. Calculations were performed in the heterophasic twentycomponent system H–O–Na–K–Ca–Mg– Cl–C–S–Al–Si–Sr–U–As–Ba–Cu–Mn–Fe–Pb– Zn at 25°С and a total pressure of 1 atm by the GIBBS algorithm based on the search for a minimum of the Gibbs isobaric–isothermal potential using an integral thermodynamic database UNITHERM in the “HCh” software [5]. The system was maintained as opened by P O2 , and P CO2 ; the values varied. The starting condi tions were preset by constant temperature and pres sure, the chemical composition of water in the lake, and the measured redox potential Eh. We applied the method of calculation of pH, the composition of equi librium solutions and solid phases, and the degree of undersaturation/oversaturation of solutions in rela tion to possible minerals. The boundary conditions of state of the system are characterized by the minimum of the Gibbs potential. At the next stage, sorption of U and As on minerals of the suspension (bottom sedi ment) was calculated. In addition to the abovemen tioned starting conditions, at the second stage, all

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GASKOVA et al. 2+

2–

Table 1. Logarithms of adsorption constants of U O 2 and As O 4 on the surface of goethite (>FeOH) and calcite (>CaOH) logK

Equation of the reaction +

>FeOH + H+ = >FeO H 2

7.29

>FeOH = >FeO– + H+ >FeOH +

2+ U O2

–8.93

+ H2O = >FeOHUO3 + 2H+

2+

–3.05

2+

>FeOH + U O 2

6.63

= >FeOHU O 2

3–

0

>FeOH + As O 4 + 3H+ = >FeH2As O 4 + H2O 3–

30.94

–

>FeOH + As O 4 + 2H+ = >FeHAsO 4 + H2O 3–

26.75

2–

>FeOH + As O 4 + H+ = >FeAs O 4 + H2O –

20.16

–

6.77

>CaOH + HC O 3 = >CaC O 3 + H2O –

>CaOH + HC O 3 + H+ = >CaHCO3 + H2O

–1.7

>CaOH = >CaO– + H+ +

>CaOH + H =

2+

>CaOH + U O 2 –

11.85

+ H2CO3 = >CaOHUO2CO3 + 2H+

2+

>CaC O 3 + U O 2 –

–12.0

+ >CaO H 2

–

–1.03

–

6.0

2–

–8.79

+ H2C O 3 = >CaCO3UO2C O 3 + H+ –

–

>CaC O 3 + H2As O 4 = >CaHAs O 4 + H+ + C O 3 –

0

2–

>CaC O 3 + CaHAs O 4 = >CaAsO4Ca0 + H+ + C O 3

–9.07

The symbol “>” indicates the surface sorption position.

sorption positions of minerals were accepted as ener getically equivalent; the stoichiometry of the com plexes and the constants of their stability were set using the reference data (Table 1). According to the data of natural sampling [2, 3], the coefficient of U partition between water and bot tom sediment in the lake system is quite high: K d = S = 138 l/kg, (1) C where S (g/kg) is the concentration of a component in the solid phase; С (g/L) is the concentration of the same component (sorbate) in solution. Under the conditions of local equilibrium or steady state in the closed lake system, the ratio of the concentrations of U and other minor components in two phases should be constant. However, upon permanent relaxation of uraniumbearing underground solutions into the lake, minor elements from lake waters will be constantly redistributed into the sediment until its sorption capacity is gained. The time factor of interaction between the components of the water–rock system plays an important role [6]. Because of this, the esti mate of the adequate water/rock (W/R) ratio is quite

uncertain in sorption models in geological environ ments. For example, J. Davis [7] modeling sorption of U(VI) in waterbearing horizons used the following proportions of precipitates in experiments (g/L): 25, 125, 250, and 820 (W/R from 40 to 1.2). We may use the approach of solution of inverse problems accepted in geochemistry: knowing the current composition of soils and underground waters on the studied areas, we may establish the reacting masses of meteoric waters and waterhosting rocks [8]. It was 100 in the case mentioned. In our model, we initially assumed that minerals saturating lake water (task 1) would sorb U and As during the formation of sediment in the lake. This was based on the results of step leaching of bot tom sediments and separation of a “labile fraction,” which determined the portion of U in the sorbed form [9]. According to [2], it exceeded 60%. However, the calculations showed that the portion of precipitated minerals was negligible (mg); because of this, it was set arbitrarily as 1 mg and 1 g. Then, after the equations of sorption reactions are written (Table 2), it is necessary to set the total number of sorption positions for each mineral (capacity). In DOKLADY EARTH SCIENCES

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Table 2. Minerals precipitated from a solution with a composition consistent to that of lake waters upon reaching equilibrium (pH 9.4, Eh 180 mV), indices of undersaturation in relation to some other phases (logIU = 0 ± 0.5 corresponds to the equilib rium) Minerals Goethite Rhodochrosite Strontianite KMontm. · H2O Quartz Calcite Magnesite

Weight, mg

Minerals

logSI

Uphases*

Weight, mg

0.04 FeOH for goethite and >CaOH for calcite, where the symbol “>” indicates the surface sorption position [10]. Numerous data on the values of the capacity of a sorbing complex of soils, shales, and clay minerals, suspended and involved river deposits, and bottom sediments of oceans, seas, and continental water reservoirs mainly range within 0.05–1.5 mg eq/g [11]. It may be accepted in this model that the sorption capacity is 0.1 mmol/g for goethite (the spe cific area of the surface is 50 m2/g; the density of sorp tion positions is ~2 mmol/m2), and 0.2 mmol/g for calcite (the specific area of the surface is 10 m2/g at a size of 1.8 μm and a density of sorption positions of 0.02 mmol/m2). A detailed description of the nonad justable and adjustable parameters of the models is given in [12]. Table 2 contains the results of calculations of equi libria between lake water and bottom sediments. It turned out that waters were oversaturated in relation to magnesite and calcite; precipitation of 50 and 17 mg of these phases was likely, respectively. As is evident from [3], this results in the absence of a correlation between mineralization and the Ca and Mg concentrations in waters of the studied soda lakes. Goethite, quartz, and hydrated K montmorillonite may be observed in sus pension; the appearance of strontianite and rhodoch rosite is likely. The calculated pH agrees with the mea sured value (9.3–9.4); the ionic strength of solutions is ~0.2. Solutions are only slightly undersaturated in relation to barite and copper arsenate, conichalcite CaCuAsO4(OH), since up to 300 μg/L As and 2 μg/L Cu were registered in lake water. At the same time, the solutions were significantly undersaturated in relation to the minerals containing Al (goethite, kaolinite), ferrohydrite, and CaUO4. The data of Xray phase analysis of bottom sediments from ShaazgaiNuur Lake [2] provide evidence for the presence of the phases that may accumulate U: calcite, kaolinite, montmorillonite; i.e., these data agree qualitatively with the results of thermodynamic calculations. There are prerequisites for the formation of an effective sorp DOKLADY EARTH SCIENCES

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tion clay–carbonate complex with goethite. Accord ing to the calculations, W/R is ~14 000 L/kg, when 71.74 mg of precipitate are formed from 1 L. At pH 9.4 and Eh 180 mV, U occurs as a highly charged carbon 4– ate complex UO2(СО3 ) 3 . According to the calculations, solutions of such type are aggressive in relation to Ubearing rocks and minerals and may leach up to ~3 g/L; this is the basis for the schemes of extraction of U from Urich sedi ments with the help of (bi)carbonate solutions of alkali metals. An equilibrium with βUO2(OH)2, ura nophane Ca[(UO2)(SiO3OH)]2 · 5H2O, and CaUO4 is established above ~3 g/L. This means that, up to n g/l U in solution, its behavior should be controlled by the processes of sorption, coprecipitation, isomorphism, and water vaporization (the Na4UO2(CO3)3 phase is registered in the model upon vaporization). Account ing for the probability of the formation of combined phases of U and As, NaAsUO6 · 3H2O may appear in these alkaline solutions. It is known that iron hydroxide is the most effective sorbent of anions and cations; quartz is less effective, and ionic exchange on clays at a calculated ionic force of I = 0.2 is poor [7, 12, 13]. Because of this, we decided to estimate adsorption on FeOOH and СаСO3 as major phases. Figure 1 shows detailed pH patterns of the prevalence of the different sorption positions on the goethite surface (1 g/L), as well as adsorption of U and As at 1 g/L and 1 mg/L of sorbent. The composition of lake waters of ShaazgaiNuur has pH of 9.4 (an end position in the figures). However, it is of interest to understand the situation in general, with account for the necessity of further extraction of valuable compo nents and removal of harmful admixtures. For exam ple, the total U in acid solutions at pH < 4, where the surface of goethite has a positive charge, occurs as 2+ 0 UO 2 and UO2S O 4 (lake water contains up to 800 mg/L of sulfateion). This means that only the area of 5 < pH < 8 is promising for removal of U from

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GASKOVA et al. Ci, g mol/L 1.2E–04 >FeOH2+

1.0E–04 8.0E–05 6.0E–05 4.0E–05

>FeOH

2.0E–05 0

3

Ci, g mol/L 3.5E–06

4

5

6

7

Ci, g mol/L 4.5E–06 (a) 4.0E–06 3.5E–06 >FeO– 3.0E–06 2.5E–06 2.0E–06 1.5E–06 1.0E–06 5.0E–07 0 3 8 9 10 pH

(c)

2.5E–06 >FeHAsO4

2.0E–06 1.5E–06

>FeAsO4

1.0E–06 5.0E–07 0

3

4

5

6

7

8

9

>FeOHUO3

>FeOHUO22+

4

5

6

7

8

9

10 pH

Usorb., Assorb, % Astot, sorb. 1 g/L FeOOH 100 90 (d) 80 70 UO22+ 60 UO2SO4 UO2CO34– >FeOHUO3 50 40 30 20 UO@, Astot, sorb. 1 mg/L FeOOH 10 0 3 4 5 6 7 8 9 10 pH

>FeH2AsO4

3.0E–06

(b)

10 pH

Portion of the U, %

Assorb., % Usorb., %

(a) 100 100 90 90 80 80 –1 >CaHAsO4 70 70 60 60 Astot 50 50 40 40 >CaAsO4Ca 30 30 –1 20 20 >CaUO2(CO3)2 10 10 0 0 9.0 7.5 8.0 8.5 9.5 pH

Portion of the As complex As, %

Fig. 1. Distribution of sorption positions on the goethite surface in dependence on pH (a); stoichiometry of the surface complexes of U (b) and As (c) bonded with 1 g/L FeOOH; the portion of U and As sorbed on 1 g/L and 1 mg/L FeOOH (d).

(b) 100 Ca2UO2(CO3)3 90 UO2(CO3)34– 80 70 60 50 40 30 20 UO2(CO3)22– CaUO2(CO3)32– 10 0 7.5 8.0 8.5 9.0 9.5 pH

Fig. 2. Contents of U (black circles) and As (squares) sorbed on 1 g CaCO3 and stoichiometry of the surface complexes of As (a). The portion of U complexes in the solution in dependence on pH (b).

solution using 1 g/L FeOOH (Fig. 1b). On the con trary, the negatively charged anions of arsenic acid are sorbed intensely in acidic conditions (Fig. 1c); how ever, if the content of As is only 300 μg and the con centration of FeOOH is sufficient (1 g/L), As remains bonded even in the alkaline conditions up to pH 11. In any case, certain pH values depend on the As/Fe ratio, but As captured by a sorbent may be removed by

simple filtering upon additional coagulation. With decreasing the sorbent weight up to 1 mg/L, sorption of elements does not exceed 2% (Fig. 1d). 2+

3–

The maximal sorption of UO 2 and As O 4 on cal cite should be observed in alkaline solutions, i.e., in the field of carbonate stability. The solid lines in Fig. 2a indicate the portion of sorbed U and As per 1 g DOKLADY EARTH SCIENCES

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СаСО3 (solid/liquid = 1/1000 exceeds the content of elements, which may be formed in suspension by more than a factor of 10). Sorption of As at pH 7.9 may reach 71%, whereas sorption of U may reach 9% only – as a complex >СаUO2(СO3 ) 2 . The stoichiometry of the predominant surface complexes of As changes with pH > 8, although, as a whole, their content becomes insignificant, since in alkaline solutions the surface of СаСО3 has a negative charge with domi –

nance of >СаС O 3 . The data in Fig. 2b allow us to understand the reason for poor sorption of U on cal cite within the whole pH range. At pH < 8.3, the solu tion is characterized by a high Ca content and by the formation of a stable complex Са2UO2(СO3)3 at the expense of partial dissolution of СаСO3, whereas 4–

UO2(СО3 ) 3 predominates in the alkaline solutions. –

Note that the complexes CaHAsO4 and CaAs O 4 are registered in solutions at pH < 8.3 as well; however, 2– HAs O 4 predominates. CONCLUSIONS (1) The model calculations performed show that the suspension formed in solutions of ShaazgaiNuur Lake and mostly composed of carbonates cannot play a significant role in U and As immobilization. It is likely that the high empirical coefficient Kd = 138 L/kg is typical of the system “bottom sediment–porous solutions” with low W/R ratios only. We revealed the probability of U and As separation in acidized and alkaline solutions upon introduction of a sufficient amount of FeOOH, i.e., ≥1 g/L. (2) Speciation of U and As, which may precipitate from alkaline solutions, is represented by a mixture of U oxides–hydroxides, As sorbed by Fe (hydr)oxides, and compatible U–As phases like NaAsUO6 · 3H2O. The data of calculations obtained for ShaazgaiNuur

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Lake support experiments [14], in which combined precipitation of U and As on the surface of Al oxides was studied. ACKNOWLEDGMENTS This study was supported by the Russian Science Foundation (project no. 151710003, thermody namic modeling) and by the Russian Foundation for Basic Research (project no.130500032, analytical data). REFERENCES 1. V. P. Isupov, A. G. Vladimirov, N. Z. Lyakhov, et al., Dokl. Earth Sci. 437 (1), 352 (2011). 2. V. P. Isupov, S. Ariunbileg, L. I. Razvorotneva, et al., Dokl. Earth Sci. 447 (2), 102 (2012). 3. V. P. Isupov, A. G. Vladimirov, S. L. Shvartsev, et al., Khim. Intereûþ Ustoich. Razvit., No. 1, 141 (2011). 4. S. L. Shvartsev, V. P. Isupov, A. G. Vladimirov, et al., Khim. Interesþ Ustoich. Razvit., No. 1, 43 (2012). 5. Yu. V. Shvarov, Geochem. Int. 46, 834 (2008). 6. S. L. Shvartsev, Litosfera, No. 6, 3 (2008). 7. J. A. Davis, D. E. Meece, M. Kohler, and G. P. Curtic, Geochim. Cosmochim. Acta 68, 3621 (2004). 8. B. N. Ryzhenko, Geochem. Int. 49, 196 (2011). 9. H. Meier, E. Zimmerhackl, G. Zeitler, P. Menge, W. Hecker, J. Radioanal. Nucl. Chem. 109, 139 (1987). 10. S. Goldberg, L. G. Criscenti, D. R. Turner, J. A. Davis, et al., Vadose Zone J. 6, 407 (2007). 11. V. S. Savenko and A. V. Savenko, Experimental methods of the study of lowtemperature geochemical processes (Geos, Moscow, 2009) [in Russian]. 12. O. L. Gaskova and M. B. Bukaty, Phys. Chem. Earth 33 (14/16), 1050 (2008). 13. O. L. Gas’kova, Geochem. Int. 47, 611 (2009). 14. Y. Tang and R. J. Reeder, Geochim. Cosmochim. Acta 73, 2727 (2009).

Translated by A. Bobrov