**2. Introduction to soil uranium**

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 occurrence.

Common uranium-bearing minerals include: uraninite [UO<sup>2</sup> ], pitchblende [U<sup>3</sup> O8 ], coffinite [U(SiO<sup>4</sup> ) 1–x(OH)4x], brannerite [UTi<sup>2</sup> O6 ], davidite [(rare earth elements) (Y,U) (Ti,Fe3+) 20 O38] and thucholite [uranium-bearing pyrobitumen]. Less abundant uranium-bearing minerals include: autunite [Ca(UO<sup>2</sup> ) 2 (PO4)2 ● 8–12 H<sup>2</sup> O], carnotite [K<sup>2</sup> (UO2 ) 2 (VO4 ) 2 ● 1–3 H<sup>2</sup> O], seleeite [Mg(UO<sup>2</sup> ) 2 (PO4 ) 2 ● 10 H<sup>2</sup> O], torbernite [Cu(UO<sup>2</sup> ) 2 (PO4 ) 2 ● 12 H<sup>2</sup> O], tyuyamunite [Ca(UO<sup>2</sup> ) 2 (VO4 ) 2 ● 5–8 H<sup>2</sup> O], uranocircite [Ba(UO<sup>2</sup> ) 2 (PO4 ) 2 ● 8–10 H<sup>2</sup> O], uranophane [Ca(UO<sup>2</sup> ) 2 (HSiO<sup>4</sup> ) 2 ● 5 H<sup>2</sup> O], zeunerite [Cu(UO<sup>2</sup> ) 2 (AsO4 ) 2 ● 8–10 H<sup>2</sup> O], rutherfordine [UO<sup>2</sup> CO3 ] and schoepite [(UO<sup>2</sup> ) 8 O2 (OH)2 ● 12H<sup>2</sup> O]. Uranium(V) species and associated minerals are comparatively rare because of disproportionation into U(IV) and U(VI) species [1].

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) [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 and trivalent cations [36].

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 impacted sites.

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%).
