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+, logߙK = −2.1(−10) (1)
≡SOH + Pu4+ ↔ ≡SOPu3+ + H+, logߙK = 15.3(7.2) (2)
≡SOH + PuO2+ ↔ ≡SOPuO2 + H+, logߙK = −8.5(−16.5) (3)
≡SOH + PuO22+ ↔ ≡SOPuO2+ + H+, logߙK = 1.2(−6.5) (4)
≡SOH + Pu4+ + 3H2O ↔ ≡SOPu(OH)3 + 4H+, logߙK = 12.5(4.6) (5)
≡SOH + Pu4+ + 4H2O ↔ ≡SOPu(OH)4− + 5H+, logߙK = 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:...