Investigation of the Interaction of the Uranyl Group UO22+ and Fe(III) with Natural Humic Acid in Aqueous Solutions by X-Ray Photoelectron Spectroscopy

Complexes of uranyl UO(2)(2+) and Fe(III) with natural humic acid (NHA) were synthesized. The reaction is heterogeneous, the NHA reacting as a particle. The concentrations of the elements on the surface and inside this particle are different. In the absence of iron, uranyl reacts with NHA more completely than Fe(III) with NHA in the absence of uranyl. However, in the joint reaction of NHA with Fe(III) and uranyl, the NHA preferentially forms complexes with Fe(III). The UO(2)(2+) complexes in comparison with the Fe(III) complexes are preferentially formed inside the NHA; consequently, when the particles are ground, the U/Fe concentration ratio is increased. Accordingly, Fe(III) salts may be used to prevent migration of uranyl in the form of soluble complexes with NHA. Natural humic acid and its complexes also contain small amounts of sulfate and RSH-type groups, the sulfate/sulfide ratio on the surface being 0.5, but the ratio decreases by a factor of two after grinding the particles. The NR(2) group in the complex is found mainly in the ionic form HN R(2)(+).