**10. Uranium adsorption**

**9. Uranium solubility and precipitation**

(2 kPa) and an ionic strength standardized by 0.01 *M* NaNO3

Activity coefficients were determined by the Debye-Huckel equation.

**Table 10.** The MinteqA2 simulation of U(VI) species in the presence of CO2

**Species −log (activity)**

132 Uranium - Safety, Resources, Separation and Thermodynamic Calculation

is the crystalline phase [53] as:

goethite (α-FeOOH) in batch 0.1 mole NaNO<sup>3</sup>

UO2

UO2

UO2

UO2 (HPO4

UO2 H2

UO2 (H2 PO4

UO2 (H2 PO4

UO2

UO2

UO2 (CO3

UO2 (CO3

(UO2 )3 (PO4 )2 .

species.

Calcium and H3

PO4

(OH)2

UO2 (OH)

The solubility of U(VI) may be estimated from thermochemical data with the assumption that

**pH 4 pH 6 pH 8**

NO3 7.1 9.0 16.1

HPO4 6.4 7.4 11.0

PO4 7.2 10.2 15.8

PO4 8.8 7.8 9.4

CO3 7.1 5.1 (3.3%) 8.2

)2 10.1 14.1 18.1

)3 13.3 18.3 20.7

)2 12.0 6.0 5.1 (1.5%)

)3 19.3 9.3 4.4 (67.9%)

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

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

)2 3.8 (98.4%) 3.8 (96.0%) 3.8 (30.6%)

Hsi and Langmuir [56] investigated the adsorption of U(VI) onto noncrystalline Fe(OH)<sup>3</sup>

carbonate concentrations and pH intervals. Hsi and Langmuir documented that the opti-

U(VI)-carbonate complexes effectively reduced U(VI) adsorption. The effect of carbonate in the goethite suspensions broadened the pH of maximum U(VI) adsorption from pH 5.7 to pH 8.0, a feature attributed to the lack of U(VI)-carbonate complex desorption. Waite et al. [58] investigated U(VI) adsorption onto hydrous ferric oxides, noting that the maximum U(VI) adsorption occurred from pH 5 to pH 9; however, in the presence of carbonate, the U(VI) adsorption in the pH interval from pH 8 to pH 9 was limited. In general, U(VI) adsorption into

2+ + 2H2 O Log Ks1,0 = 5.6. (2)

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

PO4 .

(g) and H3

/liter suspensions prepared with different total

and

(g) at 2 × 10−2 bar

and in alkaline media,

<sup>2</sup> + 2H+ = UO2

mum adsorption pH was near pH 6.3–6.5 for noncrystalline Fe(OH)<sup>3</sup>

Fe-oxyhydroxides is greater than phyllosilicate minerals.

In a review, Langmuir [38] reported solution U(VI) speciation data from pH 7 groundwater at Yucca Mountain (Nevada, USA) with a total U(VI) concentration of 10−8 mol/L. The U(VI) percentage speciation was: (1) UO2 CO3 at 7.9%, (2) UO<sup>2</sup> (CO3 )2 at 83.1%, (3) UO<sup>2</sup> (CO3 )3 at 7.8%, (4) UO2 F at 0.007%, (5) UO<sup>2</sup> (OH)2 at 0.06% and (6) UO<sup>2</sup> PO4 at 0.8%. Pabalan and Turner [57] used a double layer model for simulating U(VI) adsorption on a smectite (montmorillonite). Their surface complexation constants were (1) > AlO<sup>−</sup> of −9.73, (2) > Al(OH)<sup>2</sup> + of 8.33, (3) > SiO− of −7.20, (4) AlO-UO<sup>2</sup> + of 2.70, (5) > SiO-UO<sup>2</sup> + of 2.60, (6) AlO-(UO2 )3 (OH)<sup>5</sup> of −14.95 and (7) SiO-(UO<sup>2</sup> )3 (OH)<sup>5</sup> of −15.29.

Uranium(VI) may be adsorbed onto Fe-oxyhydroxides which may subsequently pursue distinctive pathways: (1) U(VI) undergoes reduction to U(IV) by mobile Fe2+ or H2 S or (2) desorbed, especially in alkaline solutions at elevated pH levels. Surface properties of soil mineral phases have altered chemical's reactivity because of the presence of small quantities of noncrystalline Fe- and Al-oxyhydroxides. Thus, these alterations of chemical affinity may be attributed to differences in surface area, abundance and composition of Al-OH, Fe-OH and Si-OH groups, and other features that impact the structure of adsorption surfaces (**Table 11** and **12**).


**Table 11.** Adsorption site reactions and surface protonation/deprotonation reactions (McKinley et al. [46]).


UO2 , UO2

(UO2 )3 (OH)<sup>5</sup> + , (UO2 ) 4 (OH)7 +

SiO(UO<sup>2</sup>

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

**Author details**

USA

**References**

1984

Michael Thomas Aide

(OH)+

, UO2

(OH)2

and SiO(UO<sup>2</sup>

and UO2

)+

motion effect of phosphate was attributed to [≡SOH + UO<sup>2</sup>

Address all correspondence to: mtaide@semo.edu

Ellis Horwood Limited; 1988

layer, (4) Freundlich, (5) constant capacitance and (6) diffuse layer [59].

and (UO2

(OH)3 − . At UO2

> )3 (OH)7

aqueous species to be the same U species at 10−7 mol U/L with the addition of (UO2

at Si sites and AlO(UO<sup>2</sup>

pH range from pH 3.0 to pH 6.0, whereas sulfate had no measurable influence. UO<sup>2</sup>

Gao et al. [75] investigated U(VI) sorption on kaolinite using batch experiments to observe the effects of pH, U(VI) concentration and the presence of oxyanions. The sorption of U(VI) on kaolinite increased with pH increases from pH 4.0 to pH 6.5, thereafter, a sorption plateau was indicated. The presence of phosphate increased U(VI) sorption, especially in the

predicted as the major U(VI)-phosphate species from pH 4.0 to pH 6.0, thus, the sorption pro-

Barnett et al. [41] observed that U(VI) adsorption on naturally occurring media of mixed mineralogy was nonlinear, suggesting that preferential and finite binding sites exist. Adsorption increased strongly with pH transition from pH 4.5 to pH 5.5 and decreased sharply from pH 7.5 to pH 8.5. The reduced adsorption was associated with carbonate-U(VI) complexes. Hummel et al. [76] provided an excellent companion thermochemical database. The MinteqA2 is able to perform adsorption simulations using: (1) Langmuir, (2), ion-exchange, (3) triple

Department of Agriculture, Southeast Missouri State University, Cape Girardeau, Missouri,

[1] Greenwood NN, Earnshaw A. Chemistry of the Elements. New York: Pergamon Press;

[2] Keller C. Radiochemistry. Ellis Horwood Series in Physical Chemistry. Chichester, UK:

[3] Wanty RB, Nordstrom DK. Natural radionuclides. In: Alley WM, editor. Regional

[4] Aide MT, Beighley D, Dunn D. Soil profile thorium and uranium concentration distribution in southeastern Missouri soils. In: Jamison R, editor. Thorium, Chemical Properties,

Ground-Water Quality. New York: Van Nostrand Reinhold; 1995

Uses, and Environmental Effects. NY: NOVA Publishers; 2014

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

at 10−5 mol U/L, the model predicted the U

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

)+

2− = ≡SOUO<sup>2</sup>

1−. The sorption site species were proposed as

Chemical Thermodynamics of Uranium in the Soil Environment

and AlO(UO2

2+ + HPO4

)2 (OH)2 2+, 135

HPO4 is

<sup>−</sup> + H+ ].

at Al sites.

HPO4

**Table 12.** Surface reactions on surface adsorption modeling (Herbelin and Westall [74]).

Davis et al. [44] used the generalized composite model with variations of defining equilibria to model adsorption scenarios of UO2 2+ onto mixed mineralogy samples from the Koongarra W2 (Australia) U-impacted samples. The UO2 2+ initial equilibration concentration was 3.9 × 10−6 mole U/L with variable CO2 partial pressures. Given the different model equilibrium constraints, in general, the adsorption species dominance was (1) SO<sup>2</sup> UO2 (pH 5.2–5.6), (2) SO<sup>2</sup> UO2 CO3 2− (pH 8.3–8.5), (3) SO<sup>2</sup> UO2 CO3 HCO3 3− (pH 7.5–8.7), (4) SO<sup>2</sup> UO2 HCO3 1− (pH 6.5– 7.8), (5) SO<sup>2</sup> UO2 (HCO3 )3− (pH ≈ 8) and (6) SO<sup>2</sup> HUO2 (pH ≈ 6), where S is the surface site.

Waite et al. [58] investigated U(VI) adsorption onto ferrihydrite as a function of U(VI) concentration and the partial pressure of CO2 . Using the diffuse double layer model with two site surface complexes (weak and strong ≡FeOH), they hypothesized that UO<sup>2</sup> and at higher pH levels, UO2 (CO3 ) formed inner sphere mononuclear, bidentate complexes involving the Fe octahedron edge and the uranyl ion. The U-interacting surface reactions without CO2 participation were [≡Fe(OH)<sup>2</sup> ] + UO<sup>2</sup> 2+ = [FeO<sup>2</sup> ]UO<sup>2</sup> + 2H+ with log K = −2.57 for the strong site and log K = −6.28 for the weak site. The U-interacting surface reactions with CO<sup>2</sup> participation were [≡Fe(OH)<sup>2</sup> ] + UO<sup>2</sup> 2+ + CO2 = [FeO<sup>2</sup> ]UO<sup>2</sup> CO3 2− + 2H+ with log K = 3.67 for the strong site and log K = −0.42 for the weak site.

McKinley et al. [46] observed U(VI) hydrolysis and adsorption onto smectite (SWy-1) at three ionic strengths over a pH range of 4.0–8.5. At low ionic strength, U(VI) adsorption decreased from pH 4 to pH 7, whereas at higher ionic strengths, U(VI) adsorption increased with increasing pH, an attribute attributed to uranyl hydrolysis and cation exchange involving the background electrolyte. Aluminol surface sites were dominant with adsorption of UO2 2+, whereas (UO2 )3 (OH)<sup>5</sup> + was important in alkaline pH on SiOH edge sites. Turner et al. [49] employed a composite model based on gibbsite (α-Al(OH)<sup>3</sup> ) and silica (α-SiO<sup>2</sup> ) equilibrations under similar experimental conditions to investigate U(VI) adsorption onto ferruginous beidellite (smectite family) over a pH range from 4.0 to 10.0. The adsorption envelopes for both Al (gibbsite) and Si (silica) began near pH 4 and declined near pH 5.5. With the U(VI) concentration established at UO2 at 10−7 mol U/L, the model predicted the U aqueous species to be UO2 , UO2 (OH)+ , UO2 (OH)2 and UO2 (OH)3 − . At UO2 at 10−5 mol U/L, the model predicted the U aqueous species to be the same U species at 10−7 mol U/L with the addition of (UO2 )2 (OH)2 2+, (UO2 )3 (OH)<sup>5</sup> + , (UO2 )4 (OH)7 + and (UO2 )3 (OH)7 1−. The sorption site species were proposed as SiO(UO<sup>2</sup> ) 3 (OH)<sup>5</sup> and SiO(UO<sup>2</sup> )+ at Si sites and AlO(UO<sup>2</sup> )3 (OH)<sup>5</sup> and AlO(UO2 )+ at Al sites.

Gao et al. [75] investigated U(VI) sorption on kaolinite using batch experiments to observe the effects of pH, U(VI) concentration and the presence of oxyanions. The sorption of U(VI) on kaolinite increased with pH increases from pH 4.0 to pH 6.5, thereafter, a sorption plateau was indicated. The presence of phosphate increased U(VI) sorption, especially in the pH range from pH 3.0 to pH 6.0, whereas sulfate had no measurable influence. UO<sup>2</sup> HPO4 is predicted as the major U(VI)-phosphate species from pH 4.0 to pH 6.0, thus, the sorption promotion effect of phosphate was attributed to [≡SOH + UO<sup>2</sup> 2+ + HPO4 2− = ≡SOUO<sup>2</sup> HPO4 <sup>−</sup> + H+ ].

Barnett et al. [41] observed that U(VI) adsorption on naturally occurring media of mixed mineralogy was nonlinear, suggesting that preferential and finite binding sites exist. Adsorption increased strongly with pH transition from pH 4.5 to pH 5.5 and decreased sharply from pH 7.5 to pH 8.5. The reduced adsorption was associated with carbonate-U(VI) complexes. Hummel et al. [76] provided an excellent companion thermochemical database. The MinteqA2 is able to perform adsorption simulations using: (1) Langmuir, (2), ion-exchange, (3) triple layer, (4) Freundlich, (5) constant capacitance and (6) diffuse layer [59].
