Effect of grain size on the rate and extent of uranium (VI) sorption

Kathryn Elizabeth Draper

Defensible physicochemical parameters are essential for accurate predictions of the reactive transport of hexavalent uranium [U(VI)]. Most parameters, derived from homogenized (< 2 mm) samples, have proven inadequate for field-scale predictions. The main objective of this research was to develop methods for estimating and scaling these parameters using grain size data. Grain-size effects on sorption were explored using fine and coarse materials and their binary mixtures in synthetic groundwater containing 100 ppb of U(VI). The kinetics and equilibria of U(VI) sorption both depended on grain size distribution. Kinetics were first order in mixtures with less than 10% fines but became more complex with increasing fines. Sorption maxima in mixtures increased as fines increased but the relationship was nonlinear with the main inflection dependent on whether the system was grain-supported (fines in voids created by coarse) or matrix-supported (coarse dispersed in a fine matrix). Accusand and glass beads showed very low sorption. However, gravel of the same size as glass beads adsorbed more U(VI) than even silt, and equilibration was much slower. Both kinetics and isotherms can be predicted from the component grain size fractions by superposition but are best scaled by specific surface area (SA). Finding a link between grain size and SA is therefore critical as it allows use of data mining techniques on the readily available granulometric databases. Sand-, silt-, and clay-sized fractions were mixed to create 12 standard USDA textures. SA was measured on the mixtures and their component fractions by the BET (N2) method (SABET) and calculated using a geometric approach (SAG) and a component-additivity approach (SACA). SABET ranged from 9.82 m2/g to 19.30 m2/g; SAG ranged from 35.10 m2/g to 529.58 m2/g; whereas SACA ranged from 8.22 m2/g to 22.69 m2/g. SABET correlated strongly with SAG (r2 of 88%) and SACA (r2 = 94.3%). SAG was 3 to 30 times higher than SABET, whereas SACA was only 0.84 to 1.2 times SABET. Component additivity is recommended for estimating SA of polydisperse geologic materials and for scaling the rate and extent of U(VI) sorption over the wide range of particle sizes typical of geologic materials.

Effect of grain size on the rate and extent of uranium (VI) sorption

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