PNNL

Abstract

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+?Pu?^(3+)?=SO?Pu?^(2+)+H^+, Log K=-2.1 (-10) (1) =SOH+?Pu?^(4+)?=SO?Pu?^(3+)+H^+, Log K=15.3 (7.2) (2) =SOH+PuO_2^+?=SO?PuO?_2+H^+, Log K=-8.5 (-16.5) (3) =SOH+PuO_2^(2+)?=SOPuO_2^++H^+, Log K=1.2 (-6.5) (4) =SOH+?Pu?^(4+)+3H_2 O?=SOPu?(OH)?_3+4H^+, Log K=12.5 (4.6) (5) =SOH+?Pu?^(4+)+4H_2 O?=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: =SOPuO_2+H_2 O ?(?-k_1 )/?(?-k_2 )=SOPu?(OH)?_3+?(1/2) H_(2(g)), ?Log k?_1=-5.3 (7) Where the reaction rate is equal to: -d[=SOPuO_2 ]/dt=k_1·[?Pu?_ox]-k_2·[?Pu?_RED] (8) and [PuOX] and [PuRED] are the sums of the oxidized (V and VI) and reduced (III and IV) surface species, respectively. Predictions using the equilibrium and kinetic models were validated against previously published experimental results, which give credence to the validity of the proposed mechanisms controlling the sorption of Pu onto mineral oxide surfaces. Of importance, a reversible, rate-limited, reaction successfully predicted time dependent behavior associated with Pu sorption onto goethite. Previously, researchers have suggested desorption of Pu to these surfaces is extremely slow or even irreversible. Model predictions based on such suggestions would severely overestimate the kinetic stability of Pu sorbed species and the overall importance that Pu sorption kinetics, alone, has on pseudo-colloid transport mechanisms.