Intercomparison and Refinement of Surface Complexation Models for U(VI) Adsorption onto Goethite Based on a Metadata Analysis

Abstract

Adsorption of uranium onto goethite is an important partitioning process that controls uranium mobility in subsurface environments, for which many different surface complexation models (SCMs) have been developed. While individual models can fit the data for which they are parameterized, many perform poorly when compared with experimental data covering a broader range of conditions. There is an imperative need to quantitatively evaluate the variations in the models and to develop a more robust model that can be used with more confidence across the wide range of conditions. We conducted an intercomparison and refinement of the SCMs based on a metadata analysis. By seeking the globally best fit to a composite dataset with wide ranges of pH, solid/sorbate ratios, and carbonate concentrations, we developed a series of models with different levels of complexity following a systematic roadmap. The goethite−uranyl−carbonate ternary surface complexes were required in every model. For the spectroscopically informed models, a triple-plane model was found to provide the best fit, but the performance of the double-layer model with bidentate goethite−uranyl and goethite−uranyl−carbonate complexes was also comparable. Nevertheless, the models that ignore the bidentate feature of uranyl surface complexation consistently performed poorly. The goodness of fitting for the models that ignore adsorption of carbonate and the charge distributions was not significantly compromised compared with that of their counterparts that considered those. This approach of model development for a large and varied dataset improved our understanding of U(VI)−goethite surface reactions and can lead to a path for generating a single set of reactions and equilibrium constants for including U(VI) adsorption onto goethite in reactive transport models.

Effects of Phosphate Competition on Arsenate Binding to Aluminum Hydroxide Surfaces

“…Although our EXAFS results suggested bidentate surface reactions, for simplicity, the model adopted monodentate surface reaction stoichiometry as in Karamalidis and Dzombak . We believe that such a simplification would not weaken the ability of the model to assess the competitive effect of phosphate and that more advanced model formulation considering multidentate and/or multisurface reactions would yield equivalent conclusions while at the cost of having more fitting parameters and numerical complications . Considering the variation in sorbent preparation and properties, the equilibrium constants for gibbsite derived from the present study mostly fell near the upper bound of the range of equilibrium constants complied by Karamalidis and Dzombak (Table S4).…”

“…While the effect of phosphate competition on arsenate adsorption can be assessed by the differences in the curvature and plateau of their adsorption isotherm data, such empirical descriptions lack the ability to extrapolate to water compositions outside the experimental conditions as the parameters of simple isotherm models (e.g., Langmuir) are not intrinsic. To address this ambiguity, surface complexation models (SCMs) can provide a unified framework with a set of mole balance and mass action equations that can reproduce the data of both arsenate and phosphate adsorption, individually or coexisting. ,− In addition, SCM can account for the surface electrostatic effects through which the surface complexation of oxyanions could introduce negative surface charge rendering further oxyanion binding less favorable. − Karamalidis and Dzombak compiled the double-layer SCM parameters for gibbsite, which include phosphate and arsenate. However, these thermodynamic equilibrium constants in the model were calculated based on the experimental data of single ion adsorption.…”

Investigation of Potential Drivers of Elevated Uranium Prevalence in Indian Groundwaters with a Unified Speciation Model

“…In spite of the above considerations, goethite was also included as an alternative Fe-containing adsorbent in surface complexation models (SCM) with the Mn-containing adsorbent unchanged as MnO 2 (s). A three-plane CD-MUSIC model for ferrihydrite and goethite and a diffuse layer model for manganese dioxide , were used to quantify the surface charge and its electrostatics. The average bulk density (2.07 g cm –3 ), porosity (0.44), and concentrations of Fe (64.2 mg g –1 ) and Mn (221.1 μg g –1 ) from reported total elemental concentrations in Indian aquifer sediments − were used to make estimates of the average concentrations (±1σ) of ferrihydrite as 123 ± 69 g kg –1 , goethite as 102 ± 57 g kg –1 , and of manganese dioxide as 350 ± 401 mg kg –1 .…”

U(VI), Np(V), Eu(III) sorption on goethite: A wide-ranging multiradionuclide dataset and uncertainty-aware parametrization of surface complexation models

“…Despite exhaustive guidelines (Payne et al, 2013), thermodynamic modeling of sorption is still challenging and controversial. Some of the most recent works offer applying different approaches to the modeling of large experimental datasets taken from literature for U(VI) sorption onto goethite (Satpathy et al, 2021) and quartz (Zavarin et al, 2022) correspondingly. The proposed approaches make it possible to obtain a more robust model.…”