Thermodynamic Modelling of the Effect of Organic Complexants on Sorption Behaviour

Abstract

The results of an experimental study of the sorption of uranium onto clay at pH 8 and pH 11 have been successfully simulated using a triple layer sorption model. The model has been incorporated in a Harwell derivation (HARPHRQ) of the PHREEQE geochemical code. The model has been parameterized using data for sorption onto ferric oxyhydroxide.The experimental study also investigated the effects of hydroxycarboxylic acids on the sorption process. The model has satisfactorily simulated the competing sorption and complexation reactions at pH 8, but sorption at pH 11 is underestimated for low uranium inventories.

Experimental Studies of the Effects of Organic Materials on the Sorption of Uranium and Plutonium

“…These explanations are based on equilibria involving the sorption sites and the dominant species U0 2 (0H) + and (U0 2 ) 3 (0H) 5 + [15,16]. This work is discussed in detail in the companion paper [17] and emphasizes the importance of carrying out parallel experimental and modelling programmes.…”

Modelling Studies of the Sorption of Radionuclides in the Far Field of a Nuclear Waste Repository

“…At low concentrations of sorbed uranium, the mono- meric species, S0-U0 2 (0H), is dominant, and assuming 5% of the clay is ferric oxy-hydroxide, the predicted Rd is about 4000 ml g -1 . However, as the concentration of sorbed uranium increases, so does the importance of the trimeric sorbed species, SO-(U0 2 ) 3 (0H) 5 . This change in speciation results in the sorbed concentration increasing faster than the solution concentration, and thus, the predicted Rd increases.…”

“…A thermodynamic equilibrium approach was adopted for this work, and the PHREEQE code [3] has been extended to a version HARPHRQ [4] which includes several 'surface-complexation' models of sorption such as the 'triple-layer' model. The application of the model to uranium sorption onto London clay in the presence and absence of organic materials is described in a separate paper at this conference [5], The study has given confidence in the validity of this approach, by reproducing the major trends observed in batch sorption experiments.…”

“…In the model, the initial conditions were derived by equilibrating the reservoirs and clay with a solution fixed at pH 8 containing the following elemental concentrations: This is clearly an approximation for the acidified concrete water used in the experiments. However, it includes the elements that have been shown to be most important in determining uranium speciation and sorption behaviour (for example, in the modelling of batch sorption experiments competitive sorption with calcium ions was shown to be significant [5]). In addition, 10" 7 mol dm -3 uranium was added to the upstream reservoir, and this was assumed to remain constant throughout the calculation.…”

“…Following the modelling of the batch sorption experiments [5], it was assumed that sorption onto London clay could be approximated by sorption on to a ferric oxy-hydroxide phase. The work of Cremers and Maes [7] has shown that sorption onto clays can be dominated by the amorphous iron phases present in the clay.…”