Uranyl reduction by ferrous iron

The rate law is based on a surface complexation model Liger et al. (1999) developed for the hematite nanoparticles (see Chapter 10, Surface Complexation ). The FeOH surface sites react by protonation and deprotonation to form FeOII2h and FeO-, by complexation with ferrous iron to form FeOFe+ and FeOFeOH, and to make a complex Fe0U020H with uranyl. Table 28.1 shows the reactions and corresponding log K values. The nanoparticles are taken to have a specific surface area of 109 m2 g-1, and a site density of 0.06 per Fe2C 3. 

To see how we can use the surface complexation model to trace the kinetics of this reaction, we simulate an experiment conducted at pH 7.5 (Liger et al, 1999, their Fig. 6). They started with a solution containing 100 mmolar NaNC 3, 0.16 mmolar FeS04, and 0.53 g l-1 of hematite nanoparticles. At t = 0, they added enough uranyl to give an initial concentration of 5 x 10-7 molar, almost all of which sorbed to the nanoparticles. They then observed how the mass of sorbed uranyl, which they recovered by NaHCC 3 extraction, varied with time. 

To run the simulation, we save the surface complexation model to a dataset FeOH U02.dat , decouple the relevant redox reactions, set the system s initial composition, and define the rate law. The procedure in REACT is 

5e-4 mmolal U02++ sorbate include le-3 umolal U++++ swap Hematite for Fe+++ 

Reaction in the simulation follows the trend observed in the experiment for about the first hour. After an hour, however, the rate law (Eqn. 28.2) predicts the reaction will proceed to completion, consuming the uranyl in about three hours. In the 

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