**6. Simulation of uranium reduction**


At a pe of 5 (296 mv), indicative of suboxic soil redox conditions, and at a total U concentration of 10−8 mol/L, the MinteqA2 simulation of U(VI) reduction to U(IV) indicates that U(IV) would

> be undetectable by inductively coupled plasma emission spectroscopy-mass spectroscopy or other comparable analytical technologies (**Table 5**). At pH 4, the dominant U(IV) species was

> Total U concentration was 10−3 mole/L, which was allowed to equilibrate and allow reduction at a pe of −3 (−177 mv). Activity coefficients were determined by the Debye-Huckel equation. Calcium and sulfate were present initially at 0.001 mole/L. Uraninite was precipitated and established the U equilibria (saturation index 0.00). No carbonate, sulfate or sulfide minerals were documented to precipitate. Calcium concentrations were standardized at 10−3 mole/L. The

> At a pe of −3 (−177 mv), indicative of anoxic soil redox conditions, and at a total U concentration of 10−3 mole/L at pH 6, the simulation of U(VI) reduction to U(IV) indicates that the

> Uranium complexation pairs a central cation (coordination center) with a surrounding array of molecules and ions. Phosphorus interactions with U(VI) have been studied to assess whether phosphorus may reduce the availability and mobility of U(VI) [12, 65−67]. Stojanovic et al. [18] reported that phosphorus may readily form uranyl phosphates and subsequently precipitate autunite. They noted that at pH levels greater than 6.0, the dominant U(VI)-phosphorus spe-

> species. Grabias et al. [65] studied uranyl acetate immobilization in ferruginous soils amended with phosphates. In acidic pH ranges, a strong U(VI) sorption was observed in the presence of phosphate, supporting their premise that adsorption was promoted by the formation of

> > )3 (PO4 )3 4H2 O.

CO3

(14.8%). The MinteqA2 predicted that uraninite(UO<sup>2</sup>

−

(**Table 6**). The dominant U(VI) species were UO2

species, whereas at more acidic soil reactions, UO2

were more abundant and are not considered as plant-available U-phosphate

(CO3 )2

Chemical Thermodynamics of Uranium in the Soil Environment

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

(pH 6), UO2

pascal) and an ionic strength standardized by 0.01 *M* NaNO3

. The dominant U(VI) species

(g) at pH 6 at a pe of −3 (−177 mv).

(pH 7) and UO2

(CO3 ) 3

. Within a column,

129

CO3

HPO4

) formed as a

, whereas at pH 5–8, the dominant species was U(OH)5

−

**7. Uranium complexation with an emphasis on phosphorus**

PO4

and (UO2

(pH 5), UO<sup>2</sup>

**U(VI) Speciation −log (activity) U(IV) Speciation −log (activity)**

UO2 19.9 U4+ 28.7

(OH) 19.8 U(OH) 23.3

(OH)2 33.3 U(OH)2 18.9

(OH)<sup>5</sup> 45.2 U(OH)3 15.6

NO3 21.6 U(OH)4 13.2 (3.6%)

CO3 17.6 (83.7%) U(OH)<sup>5</sup> 11.8 (96.4%)

CO3

**Table 6.** The MinteqA2 simulation of U(VI) reduction to U(IV) in the presence of CO2

and UO2

U(OH)4

UO2

(UO2 )2

(UO2 )3

UO2

UO2

UO2 (CO3 )

UO2 (CO3 )

(pH 8).

were UO2

presence of CO2

(83.7%) and UO<sup>2</sup>

solid phase.

and UO2

UO2 (H2 PO4 )(H3 PO4 )+ , UO2 (H2 PO4 )2

H2 PO4 +

(pH 4), UO2

dominant U(IV) species was U(OH)<sup>5</sup>

cies was the plant-available UO2

(CO3 )2

<sup>2</sup> 18.6 (14.8%)

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

<sup>3</sup> 21.8

( ) indicates the percentage of the U species.

Total U concentration was 10−8 mole/L, which was allowed to equilibrate and allow reduction at a pe of 5 (296 mv). Activity coefficients were determined by the Debye-Huckel equation.

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.

**Table 5.** The MinteqA2 simulation of U(VI) reduction to U(IV) in the presence of CO2 (g) at 2 × 10−2 bar (2 kilopascal) and an ionic strength standardized by 0.01 *M* NaNO3 .


At Eh values less than 0.2 volts, U(VI) reduction to uraninite (UO2

128 Uranium - Safety, Resources, Separation and Thermodynamic Calculation

**6. Simulation of uranium reduction**

**Species −log (activity)**

U(VI) Speciation

UO2

(UO2 )2

(UO2 )3

UO2

UO2

UO2 (CO3

UO2 (CO3

U(IV) Speciation

[60] observed that U(VI) reduction to U(IV) is inhibited in the presence of ferrihydrite. Yajima et al. [61] also observed that U(VI) reduction to U(IV) limited mobility. Goldhaber et al. [62] observed that coffinite formed via reduction processes in sedimentary rocks. Fendorf et al. [63] reviewed the biotic and abiotic pathways for U(VI) reduction in anaerobic soils, and they noted that U(IV) has more limited mobility and binds more preferentially to substrates than U(VI). Uranyl reduction is facilitated by bacterially mediated reactions [64]; however, noncrystalline ferric oxides and nitrate may be effective terminal electron acceptors. Similarly,

At a pe of 5 (296 mv), indicative of suboxic soil redox conditions, and at a total U concentration of 10−8 mol/L, the MinteqA2 simulation of U(VI) reduction to U(IV) indicates that U(IV) would

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

Burgos et al. [63] observed that soil humic acid partially inhibits U(VI) reduction.

UO2 8.2 (98.6%) 8.5 (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

NO3 10.0 (1.2%) 10.3 12.0 15.4 21.0

CO3 10.0 (1.1%) 8.3 (50.5%) 8.1 (83.8%) 9.4 (4.3%) 12.9

U4+ 25.0 29.3 35.1 42.4 52.0 U(OH) 21.7 25.0 29.8 36.0 44.6 U(OH)2 19.3 (1.6%) 21.6 25.4 30.7 44.6 U(OH)3 17.9 (24.1%) 19.3 (1.1%) 22.0 26.3 32.9 U(OH)4 17.1 (58.7%) 17.8 (26.9%) 19.6 (3.6%) 22.9 28.5

U(OH)<sup>5</sup> 18.1 (15.7%) 17.4 (72.0%) 18.2 (96.4%) 20.5 (99.6%) 25.1 (100%)

Activity coefficients were determined by the Debye-Huckel equation.

ionic strength standardized by 0.01 *M* NaNO3

an ionic strength standardized by 0.01 *M* NaNO3

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

**Table 5.** The MinteqA2 simulation of U(VI) reduction to U(IV) in the presence of CO2

.

Total U concentration was 10−8 mole/L, which was allowed to equilibrate and allow reduction at a pe of 5 (296 mv).

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

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

pascal) and an

(g) at 2 × 10−2 bar (2 kilopascal) and

)2 14.9 11.3 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%)

) is favored. Stewart et al.

Total U concentration was 10−3 mole/L, which was allowed to equilibrate and allow reduction at a pe of −3 (−177 mv). Activity coefficients were determined by the Debye-Huckel equation. Calcium and sulfate were present initially at 0.001 mole/L. Uraninite was precipitated and established the U equilibria (saturation index 0.00). No carbonate, sulfate or sulfide minerals were documented to precipitate. 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 column, ( ) indicates the percentage of the U species.

**Table 6.** The MinteqA2 simulation of U(VI) reduction to U(IV) in the presence of CO2 (g) at pH 6 at a pe of −3 (−177 mv).

be undetectable by inductively coupled plasma emission spectroscopy-mass spectroscopy or other comparable analytical technologies (**Table 5**). At pH 4, the dominant U(IV) species was U(OH)4 , whereas at pH 5–8, the dominant species was U(OH)5 − . The dominant U(VI) species were UO2 (pH 4), UO2 and UO2 CO3 (pH 5), UO<sup>2</sup> CO3 (pH 6), UO2 (CO3 )2 (pH 7) and UO2 (CO3 )3 (pH 8).

At a pe of −3 (−177 mv), indicative of anoxic soil redox conditions, and at a total U concentration of 10−3 mole/L at pH 6, the simulation of U(VI) reduction to U(IV) indicates that the dominant U(IV) species was U(OH)<sup>5</sup> − (**Table 6**). The dominant U(VI) species were UO2 CO3 (83.7%) and UO<sup>2</sup> (CO3 )2 (14.8%). The MinteqA2 predicted that uraninite(UO<sup>2</sup> ) formed as a solid phase.
