Mechanisms of plutonium sorption to mineral oxide surfaces: new insights with implications for colloid-enhanced migration

New equilibrium and kinetic models have been developed to describe rate-limited sorption and desorption of Pu onto and off of mineral oxide surfaces using a generic approach to estimate sorption constants that require minimal laboratory calibrations. Equilibrium reactions describing a total of six surface species were derived from a combination of empirical relationships previously described in the literature and generated as part of this work. These sorption reactions and corresponding equilibrium constants onto goethite (and silica) are: ≡SOH+Pu³+↔≡SOPu²++H+ , log K =-2.1(-10) (1) ≡SOH+Pu⁴+↔≡SOPu³++H+ , log K = 15.3(7.2) (2) ≡SOH+PuO₂+↔≡SOPuO ₂+H+ , log K =-8.5(-16.5) (3) ≡SOH+PuO² ₂+↔≡SOPuO₂++H+ , log K = 1.2(-6.5) (4) ≡SOH+Pu⁴++3H₂O↔≡SOPu(OH)₃+4H+ , log K = 12.5(4.6) (5) ≡SOH+Pu⁴++4H₂O↔≡ SOPu(OH)₄-+5H+ , log K = 5.0(-2.3) (6) The kinetic model decouples reduced (III, IV) and oxidized (V, VI) forms of Pu via a single rate-limiting, but reversible, surface mediated reaction: ≡SOPuO₂+H ₂O+1/2H_2(g) k1↔ k2 ≡SOPu(OH)₂ log k₁ =-5.3 (7) Where the reaction rate is equal to: d[≡SOPu ₂] dt = k₁[Puox]-k₂[Pured] (8) and [Pu _ox] and [Pu_red] are the sums of the oxidized (V and VI) and reduced (III and IV) surface species, respectively.