Grain-Size Based Additivity Models for Scaling Multi-rate Uranyl Surface Complexation in Subsurface Sediments

Abstract

The additivity model assumed that field-scale reaction properties in a sediment including surface area, reactive site concentration, and reaction rate can be predicted from field-scale grain-size distribution by linearly adding reaction properties estimated in laboratory for individual grain-size fractions. This study evaluated the additivity model in scaling mass transfer-limited, multi-rate uranyl (U(VI)) surface complexation reactions in a contaminated sediment. Experimental data of rate-limited U(VI) desorption in a stirred flow-cell reactor were used to estimate the statistical properties of the rate constants for individual grain-size fractions, which were then used to predict rate-limited U(VI) desorption in the composite sediment. The result indicated that the additivity model with respect to the rate of U(VI) desorption provided a good prediction of U(VI) desorption in the composite sediment. However, the rate constants were not directly scalable using the additivity model. An approximate additivity model for directly scaling rate constants was subsequently proposed and evaluated. The result found that the approximate model provided a good prediction of the experimental results within statistical uncertainty. This study also found that a gravel-size fraction (2 to 8 mm), which is often ignored in modeling U(VI) sorption and desorption, is statistically significant to the U(VI) desorption in the sediment.

A Generalized-Rate Model for Describing and Scaling Redox Kinetics in Sediments Containing Variable Redox-Reactive Materials

“…The additivity model for reaction rates , can be described using the following equation: where r i A is the reaction rate for rate component i in the sediment assemblage, r i m is the reaction rate for the rate component i in individual sediment m, and β m is the mass fraction of sediment m in the assemblage. Applying eq into eq , and because all of the sediments have the sediment-independent rate constants ( i = 1, 2, and 3), the rate expression for the assemblage (eq ) has, thus, the same form as for the individual sediment.…”

“…A sediment is a composite of minerals or reactive components that have different properties of reactivity with respect to mineral dissolution and precipitation, redox transformation, and interaction with metals and contaminants. − The heterogeneity of geochemical reactivity in the sediment poses a significant challenge for scaling geochemical reaction parameters from laboratory to field-scale applications when a reactive transport model is used to predict geochemical processes in natural system. ,,− A multi-rate model has been widely used in reactive transport models to address the scaling challenge. ,,,,− Generalized composite (GC) and component additivity (CA) are two such concepts for scaling sorption processes in heterogeneous sediments. ,− The GC approach assumes that the sorptive properties of a sediment can be described using generic sorption reactions with parameters determined by fitting experimental data. However, the model parameters obtained from one field site cannot be applied to the others. − The CA approach assumed that sorption to a sediment can be described by linearly adding the sorption to the individual components in the sediment. ,,,, However, the applicability of the CA approach is limited because of the complexity of minerals and their interactions in a sediment that are often difficult to identify in the field. An effective approach for scaling the sorption process and parameters has yet to be developed.…”

“…29−31 The CA approach assumed that sorption to a sediment can be described by linearly adding the sorption to the individual components in the sediment. 1,26,27,32,33 However, the applicability of the CA approach is limited because of the complexity of minerals and their interactions in a sediment that are often difficult to identify in the field. An effective approach for scaling the sorption process and parameters has yet to be developed.…”