Comparison of Electrostatic and Non‐Electrostatic Models for U(VI) Sorption on Aquifer Sediments

Abstract

A non-electrostatic generalized composite surface complexation model (SCM) was developed for U(VI) sorption on contaminated F-Area sediments from the U.S. Department of Energy Savannah River Site, South Carolina. The objective of this study was to test if a simpler, semi-empirical, non-electrostatic U(VI) sorption model (NEM) could achieve the same predictive performance as a SCM with electrostatic correction terms in describing U(VI) plume evolution and long-term mobility. One-dimensional reactive transport simulations considering key hydrodynamic processes, Al and Fe minerals, as well as H and U surface complexation, with and without electrostatic correction terms, were conducted. The NEM was first calibrated with laboratory batch H and U(VI) sorption data on F-Area sediments, and then the surface area of the NEM was adjusted to match field observations of dissolved U(VI). Modeling results indicate that the calibrated NEM was able to perform as well as the previously developed electrostatic model in predicting the long-term evolution of H and U(VI) at the site, given the variability of field-site data. The electrostatic and NEM models yield somewhat different results for the time period when basin discharge was active; however, it is not clear which modeling approach may be better to model this early time period because groundwater quality data during this period were not available. A key finding of this study is that the applicability of NEM (and thus robustness of its predictions) to the field system evolves with time and is strongly dependent on the pH range that was used to develop the model.

Influence of Streambed Heterogeneity on Hyporheic Flow and Sorptive Solute Transport

“…where ρ b is the bulk density of the subsurface medium (mg m −1 ), n is porosity (), and K d is the equilibrium sorption distribution coefficient (m 3 mg −1 ). The x denotes the spatial variability of K d in the x, y, and z directions, which results in spatial variability in R. More robust and mechanistic models (e.g., non-linear isotherms, Surface Complexation Models) could be used to model sorption (e.g., [67,68]), but our goal was to address the lack of understanding of how simple sorption processes affect solute transport within the hyporheic zone. Such knowledge is critical in designing remediation strategies for solutes that undergo sorption (e.g., perfluoroalkyl and polyfluoroalkyl substances (PFAS), 1,4-dioxane, perchloroethylene, trichloroethylene, and trace metals) as they are transported through porous media, particularly within the heterogeneous riverbed.…”

Reactive Transport at the Crossroads

“…A number of other studies have been carried out demonstrating the ability of the surface complexation models to describe field scale behavior, including those at the Rifle CO uranium contaminated site (Yabusaki et al 2007(Yabusaki et al , 2017Zachara et al 2013), and at the contaminated Savannah River site (Bea et al 2013;Arora et al 2018).…”

Abiotic and Biotic Controls on Soil Organo–Mineral Interactions: Developing Model Structures to Analyze Why Soil Organic Matter Persists

“…Flow and reactive transport processes influence SOM formation and decomposition. In particular, geochemical processes can affect the mobility of chemical species through different mechanisms of dissolution-precipitation, sorption-desorption, ion-exchange, redox-reactions, complexation, and colloidal interactions (Arora et al 2016(Arora et al , 2018Yabusaki et al 2017;Dwivedi et al 2018b, a). There are a variety of RTMs and numerical codes-such as TOUGHReact (Xu et al 2006;Maggi et al 2008;Gu et al 2009), PFLOTRAN (Hammond et al 2014), CRUNCH (Steefel et al 2015), ecosys (Grant 2013), BAMS (Riley et al 2014;Dwivedi et al 2017a;, and BeTR Tang and Riley 2018)-that are available to describe and can represent the interaction of various complex and competing biogeochemical processes across spatial and time scales.…”