**1. Properties, sources, characteristics of soil uranium**

Uranium is the third element in the actinide series having an atomic number of 92 and an electronic configuration of [Rn] 5f<sup>3</sup> 6d1 7s2 . The 5f orbitals are less effective in penetrating the inner core electrons than the 4f orbitals (lanthanide series), thus permitting more favored covalent bonding character [1]. Uranium(IV) and uranium(VI) have ionic radii of 89 and 73 pm, respectively. The two, more abundant, long-lived isotopes of uranium are <sup>235</sup>U92 and <sup>238</sup>U92. The naturally occurring mass abundances of uranium isotopes are 234U (0.0057%), 235U (0.71%) and 238U (99.284%). 235U is fissile, whereas 238U in a breeder reactor will yield fissile <sup>239</sup>Pu [2].

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

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 halflife of 1.18 minutes) to yield 234U. 234U will undergo α-emission (half-life of 2.45 × 105 ) to yield 230Th90, whereas 235U decaying by α-emission to yield 231Th (half-life of 7.04 × 108 years) and later in the decay sequence to yield 227Th [2].

[33]. Sheppard et al. [30] noted that leafy vegetables could accumulate U(VI) to a greater extent than common grain crops. Chopping and Shambhag [35] showed U(VI) binding by soil organic matter, particularly if the soil organic materials acquired a negative charge density at or above pH 7. Organic complexes of U may be replaced by other cations, especially divalent

Chemical Thermodynamics of Uranium in the Soil Environment

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Phyllosilicates (clay minerals) typically manifest a net negative charge density because of isomorphic substitution and unsatisfied edge charges [36–38]. Al-, Mn- and Fe-oxyhydroxides have variable charged surfaces (amphoteric) that acquire a positive charge density when the pH is more acidic than the mineral's point of zero net charge density [39, 40]. Uranyl ions, along with its hydroxyl monomers and hydroxyl polymers, will participate in adsorption reactions with phyllosilicates and Mn- and Fe-oxyhydroxides [41–49]. The transport of U-bearing colloids by wind and water erosion is an important source of U transport from

There lies great interest in understanding the U transport in natural systems such as soil profiles, sediments and aquifers [4, 9, 10, 19, 40, 50–52]. Johnson et al. [51] investigated depleted uranium soil sites in Nevada (USA), observing that uranium retention is a function of (1) soil type, (2) soil binding site concentrations, (3) the presence of phyllosilicates and their associated Fe-oxyhydroxides, (4) the contaminant concentration, (5) the presence of competing ions and (6) the contaminant speciation based on pH and Eh. They noted that the estimated distribution coefficients (Kd = concentration of the sorbed contaminant/the contaminant in the aqueous phase) increased with soil reaction from pH 7 to pH 11. Roh et al. [16] investigated two U-impacted sites at Oak Ridge, TN using sequential leaching and demonstrated that soil

U was associated substantially with carbonates (45%) and Fe-oxyhydroxides (40%).

O = U(OH)3+ + H+ log K1,1 = −0.65

O = U(OH)<sup>4</sup> + 4H+ log K1,4 = −10.3

2+ + 2H+ log K1,2 = −0.2.6

+ 3H+ log K1,3 = −5.8

<sup>−</sup> + 5H<sup>+</sup> log K1,5 = −16.0

9+ + 15H<sup>+</sup> log K6,15 = −17.2

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

O + OH<sup>−</sup> = U(OH)5

− .

and trivalent cations [36].

impacted sites.

**3. Uranium hydrolysis**

UO2

U4+ + H2

U4+ + 2H2

U4+ + 3H2

U4+ + 4H2

U4+ + 5H<sup>2</sup>

6U4+ + 15H<sup>2</sup>

(log Ks1,5 = −3.77).

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

O = U(OH)<sup>3</sup>

O = U(OH)5

O = U<sup>6</sup>

+

(OH)<sup>15</sup>

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

is the crystalline phase, as: UO2 + 2H2

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