**3. Uranium hydrolysis**

Hydrolysis constants for U(IV) are presented in **Table 1**.

The solubility of U(IV) may be estimated from thermochemical data, with the assumption that UO2 is the crystalline phase, as: UO2 + 2H2 O + OH<sup>−</sup> = U(OH)5 − .

(log Ks1,5 = −3.77).

Uranium decay is an isotope function, with (i) 238U92 decaying by α-emission to 234Th90 (halflife of 4.45 × 10<sup>9</sup> years) and then by two successive β-emissions (half-life of 24.1 days and half-

230Th90, whereas 235U decaying by α-emission to yield 231Th (half-life of 7.04 × 108 years) and

The Earth's crustal uranium abundance is centered near 2.3 mg U/kg [1]. Soil parent materials vary substantially in their uranium concentrations, with granites (4.4 mg U/kg) and shales (3.8 mg U/kg) having greater abundances than basalts (0.8 mg U/kg) and K-feldspars (1.5 mg U/kg) [3]. The phyllosilicates, muscovite and biotite have U concentrations centering near 20 mg U/kg, and some zircon minerals may have up to 2500 mg U/kg [3]. Aide et al. [4] documented total uranium concentrations by soil horizon depth in numerous southeastern Missouri soils, noting that the uranium concentrations varied from 0.58 to 2.89 mg U/kg, with course-textured soils generally having smaller U concentrations. In their study, uranium in individual soil pedons was well correlated with Fe-oxyhydroxide concentrations. Birke et al. [5] reported that the amount of uranium in river waters in Germany varied from 0.007 to 43.7 μg U/L, with a median of 0.33 μg U/L. Mendez-Garcia et al. [6] observed that high uranium concentrations in sediment in the Rio Grande Basin in Mexico were of natural

and thucholite [uranium-bearing pyrobitumen]. Less abundant uranium-bearing minerals

) 2 (PO4 ) 2 ● 12 H<sup>2</sup>

Soils may become uranium impacted because of nuclear fuel production, nuclear weapons production, depleted uranium in munitions, coal combustion and most importantly by phosphorus fertilizer applications [7–19]. Stojanovic et al. [17] observed that maize and sunflower plants may be very useful for uranium phytoremediation, with the root mass acquiring much greater uranium accumulations than culms, leaves and grain. Stojanovic et al. [18] documented previous research showing that the use of phosphorus fertilizers may contribute 73% of the total anthropogenic uranium to the global soil resource. Echevarria et al. [20] observed that low pH levels favored increased uranium plant availability. Laroche et al. [21] in a hydroponic study observed that phosphorus reduced uranyl activity, especially at higher pH intervals.

Plant uptake of U has been investigated for phytoremediation of impacted soils [7, 13–15, 22– 34]. Sunflower (*Helianthus annuus*) has been shown to substantially phytoaccumulate U(VI)

● 8–10 H<sup>2</sup>

● 8–10 H<sup>2</sup>

O], carnotite [K<sup>2</sup>

) to yield

], pitchblende [U<sup>3</sup>

O], tyuyamunite [Ca(UO<sup>2</sup>

CO3

], davidite [(rare earth elements) (Y,U) (Ti,Fe3+) 20 O38]

O], uranophane [Ca(UO<sup>2</sup>

(UO2 ) 2 (VO4 ) 2

O], rutherfordine [UO<sup>2</sup>

O]. Uranium(V) species and associated minerals are comparatively rare

O8

) 2 (HSiO<sup>4</sup> ) 2 ●

● 1–3 H<sup>2</sup>

], coffinite

O], seleeite

) 2 (VO4 ) 2

] and schoepite

life of 1.18 minutes) to yield 234U. 234U will undergo α-emission (half-life of 2.45 × 105

later in the decay sequence to yield 227Th [2].

124 Uranium - Safety, Resources, Separation and Thermodynamic Calculation

Common uranium-bearing minerals include: uraninite [UO<sup>2</sup>

O6

O], torbernite [Cu(UO<sup>2</sup>

) 2 (PO4 ) 2

● 8–12 H<sup>2</sup>

1–x(OH)4x], brannerite [UTi<sup>2</sup>

O], uranocircite [Ba(UO<sup>2</sup>

) 2 (PO4)2

> ) 2 (AsO4 ) 2

because of disproportionation into U(IV) and U(VI) species [1].

include: autunite [Ca(UO<sup>2</sup>

O], zeunerite [Cu(UO<sup>2</sup>

● 12H<sup>2</sup>

) 2 (PO4 ) 2 ● 10 H<sup>2</sup>

**2. Introduction to soil uranium**

occurrence.

[U(SiO<sup>4</sup> )

[Mg(UO<sup>2</sup>

● 5–8 H<sup>2</sup>

5 H<sup>2</sup>

[(UO<sup>2</sup> ) 8 O2 (OH)2


**Table 1.** Hydrolysis constants for U(IV) (Baes and Mesmer [53]).


**Table 2.** Hydrolysis constants for U(VI) [53, 54].

The uranyl ion (UO2 2+) is an oxycation, given that the high charge polarization of U6+ prevents this aqueous species from being stable. Hydrolysis constants for U(VI) are presented in **Table 2**.

In low ionic strength media, the U(VI) polymers are not thermodynamically favored, with the exception of (UO2 )3 (OH)<sup>5</sup> + [41, 42, 44, 46, 49, 54–58].

#### **4. Simulation of uranium hydrolysis**

Using the MinteqA2 software [59], U(VI) speciation may be estimated from thermochemical data for pH intervals from pH 4 to pH 8. Setting the total U(VI) concentration at 10−8 mole/liter, the pCO2 pressure at 0 and then again at 0.02 bar (2 kPa) were the primary model vari able inputs. Establishing a constant ionic strength with 0.01 mole NaNO3 /liter, activity coefficients were estimated using the Debye-Huckel equation. In the CO<sup>2</sup> closed system, UO2 2+ is the dominant species in very acidic media, whereas UO2 (OH)+ is the dominant species from pH 6 to pH 8 (**Table 3**). The ion pair UO2 NO3 + is an important secondary species, particularly in acidic media. In the CO2 open system, UO2 2+ is the dominant species in very acidic media; however, the UO2 CO3 , UO2 (CO3 )2 2− and UO2 (CO3 )3 4− are U(VI) species increasingly dominant upon transition from acidic media to neutral and then to alkaline media (**Table 3**). Importantly, the uranyl carbonate complexes are stable at Eh conditions that would promote U(VI) reduction in CO2 closed systems. This MinteqA2 simulation of dilute U(VI) speciation closely corresponds with the analytical data and its MinteqA2 simulation as presented by Langmuir [38] and also the data analysis from Waite et al. [58].

Repeating the simulation at 10−3 mol U/L, with allowance for mineral precipitation yielded different U species distributions across the pH intervals (**Table 4**). At pH 4, the UO2 2+ species is increasingly converted by polymerization into the (UO2 )2 (OH)2 2+ species. At pH 5, the UO2 2+ and UO2 CO3 species similarly transitioned into the (UO2 )3 (OH)<sup>5</sup> + and the (UO2 )2 (OH)2 2+ species. The pH 7 and 8 simulations witnessed the expanding abundances of UO<sup>2</sup> (CO3 )2 2−. Rutherfordine (UO<sup>2</sup> CO3 ) was indicated to have precipitated at pH 4–7, whereas calcite (CaCO3 ) precipitated at pH 8.

**5. Uranium oxidation and reduction**

pH 4 to pH 7, whereas calcite was predicted to precipitate at pH 8.

ionic strength standardized by 0.01 *M* NaNO3

UO2

**Species −log (activity)**

Total U concentration was 10−8 mole/L.

**Species −log (activity)**

Activity coefficients were determined by the Debye-Huckel equation.

**Table 3.** The MinteqA2 simulation of U(VI) speciation.

were 0.001 mol/L. Within a pH column, ( ) indicates the percentage of the U species.

UO2

(UO2 )2

(UO2 )3

UO2

UO2

UO2 (CO3

UO2 (CO3

UO2

(UO2 )2

(UO2 )3

UO2

UO2

UO2 (CO3

UO2 (CO3

The presence of CO2

**pH 4 pH 5 pH 6 pH 7 pH 8**

Chemical Thermodynamics of Uranium in the Soil Environment

http://dx.doi.org/10.5772/intechopen.72107

127

UO2 8.21 (96.8%) 9.5 (44.8%) 10.3 13.6 19.2

(OH) 10.1 9.4 (4.1%) 10.2 12.5 17.1

(OH)2 14.0 12.7 14.2 18.8 27.9

(OH)<sup>5</sup> 20.2 16.2 16.6 21.4 33.1

)2 14.9 11.2 9.0 (14.8%) 8.3 (77.8%) 9.9 (2.5%)

)3 22.2 16.5 12.3 9.6 (17.9%) 9.1 (97.5%)

(g) at 2 × 10−2 bar (2 kPa) and an ionic strength standardized by 0.01 *M* NaNO3

UO2 2.61 (38.1%) 4.6 (14.5%) 6.6 8.6 13.6

(OH) 4.51 5.5 (1.3%) 6.5 7.5 11.7

(OH)2 2.79 (49.8%) 4.8 (19%) 6.8 8.8 17.1

(OH)<sup>5</sup> 3.41 (11.1%) 4.4 (48.6%) 5.4 (19.8%) 6.4 16.9

NO3 4.39 6.4 8.4 10.4 15.6

CO3 4.36 4.4(16.4%) 4.4 (66.6%) 4.4 (4%) 7.5

Total U concentration was 10−3 mole/L, which was allowed to equilibrate and allow precipitation.

)2 9.31 7.3 5.3 (11.8%) 3.3 (75.8%) 4.5

)3 16.6 12.6 8.6 4.6 (20.0%) 3.7 (99.5%)

**pH 4 pH 5 pH 6 pH 7 pH 8**

NO3 10.0 (1.2%) 10.3 12.1 15.4 20.9

CO3 9.9 (1.1%) 8.3 (50.3%) 8.1 (83.8%) 9.4 (4.3%)

The reduction of U(VI) to U(IV) may be presented as [53]:

Calcium concentrations were standardized at 10−3 mole/L. The presence of CO2

coefficients were determined by the Debye-Huckel equation. Rutherfordine (UO<sup>2</sup>

**Table 4.** The MinteqA2 simulation of U(VI) solubility by species in the presence of CO2

2+ + 4H+ + 2e<sup>−</sup> = U4 + 2H2

O E0 = 0.329volts (1)

(g) at 2 × 10−2 bar (2 × 10<sup>3</sup>

(g).

. Within a pH column, ( ) indicates the percentage of the U species. Activity

CO3

. Calcium concentrations

pascal) and an

) was predicted to precipitate from


Total U concentration was 10−8 mole/L.

The uranyl ion (UO2

O = UO<sup>2</sup>

O = UO<sup>2</sup>

O = UO<sup>2</sup>

O = UO<sup>2</sup>

O = (UO<sup>2</sup> )2 (OH)1

O = (UO<sup>2</sup> )2 (OH)2

O = (UO<sup>2</sup> )3 (OH)<sup>5</sup> +

(OH)+

126 Uranium - Safety, Resources, Separation and Thermodynamic Calculation

(OH)3

(OH)4

UO2 2+ + H2

UO2 2+ + 2H2

UO2 2+ + 3H2

UO2 2+ + 4H2

2UO2

2UO2

3UO2

2+ + 1H2

2+ + 2H2

2+ + 5H<sup>2</sup>

exception of (UO2

the pCO2

UO2

(CaCO3

)3 (OH)<sup>5</sup> +

**Table 2.** Hydrolysis constants for U(VI) [53, 54].

**4. Simulation of uranium hydrolysis**

pH 6 to pH 8 (**Table 3**). The ion pair UO2

larly in acidic media. In the CO2

CO3

) precipitated at pH 8.

CO3

media; however, the UO2

U(VI) reduction in CO2

2+ and UO2

Rutherfordine (UO<sup>2</sup>

2+) is an oxycation, given that the high charge polarization of U6+ prevents

**Baes and Mesmer [53] Davis [54]**

this aqueous species from being stable. Hydrolysis constants for U(VI) are presented in **Table 2**.

+ H+ log K1,1 = −5.8 = −5.20

<sup>−</sup> + 3H+ — = −20.00

2− + 4H+ — = −33.00

3+ + H+ — = −2.70

2+ + 2H+ log K2,2 = −5.62 = −5.62

+ 5H<sup>+</sup> log K3,5 = −15.63 = −15.55

(OH)2 + 2H+ — = −11.50

In low ionic strength media, the U(VI) polymers are not thermodynamically favored, with the

Using the MinteqA2 software [59], U(VI) speciation may be estimated from thermochemical data for pH intervals from pH 4 to pH 8. Setting the total U(VI) concentration at 10−8 mole/liter,

> NO3 +

2− and UO2

dominant upon transition from acidic media to neutral and then to alkaline media (**Table 3**). Importantly, the uranyl carbonate complexes are stable at Eh conditions that would promote

closely corresponds with the analytical data and its MinteqA2 simulation as presented by

Repeating the simulation at 10−3 mol U/L, with allowance for mineral precipitation yielded

different U species distributions across the pH intervals (**Table 4**). At pH 4, the UO2

species similarly transitioned into the (UO2

species. The pH 7 and 8 simulations witnessed the expanding abundances of UO<sup>2</sup>

open system, UO2

pressure at 0 and then again at 0.02 bar (2 kPa) were the primary model vari

(OH)+

(CO3 )3

closed systems. This MinteqA2 simulation of dilute U(VI) speciation

)2 (OH)2

)3 (OH)<sup>5</sup> +

) was indicated to have precipitated at pH 4–7, whereas calcite

/liter, activity coef-

2+ is

2+ spe-

2+ species. At pH 5, the

)2 (OH)2 2+

(CO3 ) 2 2−.

and the (UO2

closed system, UO2

is the dominant species from

4− are U(VI) species increasingly

is an important secondary species, particu-

2+ is the dominant species in very acidic

[41, 42, 44, 46, 49, 54–58].

able inputs. Establishing a constant ionic strength with 0.01 mole NaNO3

(CO3 )2

ficients were estimated using the Debye-Huckel equation. In the CO<sup>2</sup>

, UO2

Langmuir [38] and also the data analysis from Waite et al. [58].

cies is increasingly converted by polymerization into the (UO2

the dominant species in very acidic media, whereas UO2

CO3

Activity coefficients were determined by the Debye-Huckel equation.

The presence of CO2 (g) at 2 × 10−2 bar (2 kPa) and an ionic strength standardized by 0.01 *M* NaNO3 . Calcium concentrations were 0.001 mol/L. Within a pH column, ( ) indicates the percentage of the U species.

**Table 3.** The MinteqA2 simulation of U(VI) speciation.


Total U concentration was 10−3 mole/L, which was allowed to equilibrate and allow precipitation.

Calcium concentrations were standardized at 10−3 mole/L. The presence of CO2 (g) at 2 × 10−2 bar (2 × 10<sup>3</sup> pascal) and an ionic strength standardized by 0.01 *M* NaNO3 . Within a pH column, ( ) indicates the percentage of the U species. Activity coefficients were determined by the Debye-Huckel equation. Rutherfordine (UO<sup>2</sup> CO3 ) was predicted to precipitate from pH 4 to pH 7, whereas calcite was predicted to precipitate at pH 8.

**Table 4.** The MinteqA2 simulation of U(VI) solubility by species in the presence of CO2 (g).
