Experimental study and modeling of the U(VI)-Fe(OH) 3 surface precipitation/coprecipitation equilibria
The kinetics and thermodynamics of the coprecipitation of U(VI) with Fe(III) oxyhydroxides have been studied under controlled laboratory conditions, by means of chemical solution and spectroscopic techniques. The monitoring of uranium concentration after the simultaneous precipitation of Fe(III) and U(VI) from a homogeneous solution indicates that a fast and quantitative sorption of uranium on the Fe(III) oxyhydroxide surface occurs. This is followed by the gradual release of a part of the uranium from the solid phase which can be attributed to a structural rearrangement of the solid surface. Equilibrium uranium concentrations are attained after fifteen days of contact. Several surface complexation models are tested to model the behavior of the system and a simple thermodynamic approach has been proposed in order to describe this post-adsorption phenomena. A conditional solubility constant for the uranium limiting phase is defined as K SO∗ = K SO · χ , where KSO is the solubility constant for the individual solid phase and χ stands for the molar fraction of the uranium in the bulk of the solid. Treatment of data obtained at different U(VI) to Fe(III) initial concentration ratios and at fixed pH value has allowed us to propose the surface precipitation of schoepite on Fe(III) oxyhydroxide as the uranium solubility limiting phase. It has been shown that the behavior of the system can be equally described by both surface complexation and conditional solubility approaches. However, the surface complexation modelling requires the knowledge of the surface properties of the solid. The much simpler coprecipitation approach is only based on mass balances and stoichiometric relationships, which can be a useful predictive tool for the trace metal concentrations in natural systems.