**9. Uranium solubility and precipitation**

The solubility of U(VI) may be estimated from thermochemical data with the assumption that UO2 (OH)2 is the crystalline phase [53] as:

$$\text{UO}\_2\text{(OH)}\_2 + 2\text{H}^+ = \text{UO}\_2^{3+} + 2\text{H}\_2\text{O} \qquad \text{Log}\,\text{K}\_{\text{sl}\downarrow} = 5.6. \tag{2}$$

Hsi and Langmuir [56] investigated the adsorption of U(VI) onto noncrystalline Fe(OH)<sup>3</sup> and goethite (α-FeOOH) in batch 0.1 mole NaNO<sup>3</sup> /liter suspensions prepared with different total carbonate concentrations and pH intervals. Hsi and Langmuir documented that the optimum adsorption pH was near pH 6.3–6.5 for noncrystalline Fe(OH)<sup>3</sup> and in alkaline media, 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 Fe-oxyhydroxides is greater than phyllosilicate minerals.

Typically, the pH range of minimal U(VI) mineral solubility coincides with the pH range for optimal U(VI) adsorption. U(IV) complexes are frequently less soluble and less mobile than U(VI) complexes [73]. Duquene et al. [23] noted that U(VI) reduction to less soluble U(IV), by either biotic or abiotic processes, influenced uranium mobility. Stojanovic et al. [17] confirmed that soil temperature, pH, oxidation–reduction potentials and the presence of complexing agents were important factors influencing uranium bioavailability and plant uptake. Shahandeh and Hossner [33] employed a selective sequential extraction protocol to show that U(VI) partitioned into exchangeable, carbonate, manganese, iron, organic and residual fractions. In soils where the carbonate fraction was expected to be important, appreciable plant uptake of U(VI) into the roots and culms of a wide variety of plants was demonstrated. In soils having U(VI) partitioning into iron, manganese and organic fractions, the U(VI) plant uptake was substantially smaller.

Sandino and Bruno [68] provided the solubility estimate for (UO<sup>2</sup> )3 (PO4 )2 4H2 O(s) = 3UO<sup>2</sup> 2+ + 2PO4 3− + 4H2 O as log Kso ± 2σ = 48.48 ± 0.16.
