The kinetics of rare earth element (REE) uptake by iron oxides and manganese oxides were investigated in high ionic strength solution. Batch experiments were conducted at pH 7.8 using radiotracer concentrations of 10(-9)-10(-11) M, close to those found in the environment. The effect of solution carbonate complexation on sorption kinetics and on distribution coefficients (K(d)) was also evaluated. Fast uptake and large K(d) (10(5)-10(7)) were observed with both phases, but carbonate complexation exerts a greater influence on REE uptake by delta-MnO2 than by FeOOH. Kinetic models were applied to fit laboratory data. An evaluation of the sensitivity of the kinetic models was enhanced by using the REE whose chemical properties change gradually as a function of increasing atomic number. Critical assessment of the kinetic models suggests, however, that current experiments do not allow discrimination between sequential and simultaneous mechanisms and that further chemical information regarding the actual reactions taking place at the solid/solution interface is required to complement the kinetic data. Nonetheless, rate constants reveal regular trends throughout the lanthanide series.
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