Resolving experimental biases in the interpretation of diffusion experiments with a user-friendly numerical reactive transport approach.

The reactive transport code CrunchClay was used to derive effective diffusion coefficients (De), clay porosities (ε), and adsorption distribution coefficients (KD) from through-diffusion data while considering accurately the influence of unavoidable experimental biases on the estimation of these diffusion parameters. These effects include the presence of filters holding the solid sample in place, the variations in concentration gradients across the diffusion cell due to sampling events, the impact of tubing/dead volumes on the estimation of diffusive fluxes and sample porosity, and the effects of O-ring-filter setups on the delivery of solutions to the clay packing. Doing so, the direct modeling of the measurements of (radio)tracer concentrations in reservoirs is more accurate than that of data converted directly into diffusive fluxes. While the above-mentioned effects have already been described individually in the literature, a consistent modeling approach addressing all these issues at the same time has never been described nor made easily available to the community. A graphical user interface, CrunchEase, was created, which supports the user by automating the creation of input files, the running of simulations, and the extraction and comparison of data and simulation results. While a classical model considering an effective diffusion coefficient, a porosity and a solid/solution distribution coefficient (De-ε-KD) may be implemented in any reactive transport code, the development of CrunchEase makes it easy to apply by experimentalists without a background in reactive transport modeling. CrunchEase makes it also possible to transition more easily from a De-ε-KD modeling approach to a state-of-the-art process-based understanding modeling approach using the full capabilities of CrunchClay, which include surface complexation modeling and a multi-porosity description of the clay packing with charged diffuse layers.

Mechanistic Understanding of Uranyl Ion Complexation on Montmorillonite Edges: A Combined First-Principles Molecular Dynamics-Surface Complexation Modeling Approach.

Systematic first-principles molecular dynamics (FPMD) simulations were carried out to study the structures, free energies, and acidity constants of UO22+ surface complexes on montmorillonite in order to elucidate the surface complexation mechanisms of the uranyl ion (UO22+) on clay mineral edges at the atomic scale. Four representative complexing sites were investigated, that is, ≡Al(OH)2 and ≡AlOHSiO on the (010) surface and ≡AlOHOa and ≡SiOOa on the (110) surface. The results show that uranyl ions form bidentate complexes on these sites. All calculated binding free energies for these complexes are very similar. These bidentate complexes can be hydrolyzed, and their corresponding derived p Ka values (around 5.0 and 9.0 for p Ka1 and p Ka2, respectively) indicate that UO2(OH)+ and UO2(OH)2 surface groups are the dominant surface species in the environmental pH range. The OH groups of UO2(OH)2 surface complexes can act as complexing sites for subsequent metals. Additional simulations showed that such multinuclear adsorption is feasible and can be important at high pH. Furthermore, FPMD simulation results served as input parameters for an electrostatic thermodynamic surface complexation model (SCM) that adequately reproduced adsorption data from the literature. Overall, this study provides an improved understanding of UO22+ complexation on clay mineral edge surfaces.

Effect of fulvic acid surface coatings on plutonium sorption and desorption kinetics on goethite.

The rates and extent of plutonium (Pu) sorption and desorption onto mineral surfaces are important parameters for predicting Pu mobility in subsurface environments. The presence of natural organic matter, such as fulvic acid (FA), may influence these parameters. We investigated the effects of FA on Pu(IV) sorption/desorption onto goethite in two scenarios: when FA was (1) initially present in solution or (2) found as organic coatings on the mineral surface. A low pH was used to maximize FA coatings on goethite. Experiments were combined with kinetic modeling and speciation calculations to interpret variations in Pu sorption rates in the presence of FA. Our results indicate that FA can change the rates and extent of Pu sorption onto goethite at pH 4. Differences in the kinetics of Pu sorption were observed as a function of the concentration and initial form of FA. The fraction of desorbed Pu decreased in the presence of FA, indicating that organic matter can stabilize sorbed Pu on goethite. These results suggest that ternary Pu-FA-mineral complexes could enhance colloid-facilitated Pu transport. However, more representative natural conditions need to be investigated to quantify the relevance of these findings.

Effects of fulvic acid on uranium(VI) sorption kinetics.

This study focuses on the effects of fulvic acid (FA) on uranium(VI) sorption kinetics to a silica sand. Using a tritium-labeled FA in batch experiments made it possible to investigate sorption rates over a wide range of environmentally relevant FA concentrations (0.37-23 mg L(-1) TOC). Equilibrium speciation calculations were coupled with an evaluation of U(VI) and FA sorption rates based on characteristic times. This allowed us to suggest plausible sorption mechanisms as a function of solution conditions (e.g., pH, U(VI)/FA/surface site ratios). Our results indicate that U(VI) sorption onto silica sand can be either slower or faster in the presence of FA compared to a ligand-free system. This suggests a shift in the underlying mechanisms of FA effects on U(VI) sorption, from competitive sorption to influences of U(VI)-FA complexes, in the same system. Changes in metal sorption rates depend on the relative concentrations of metals, organic ligands, and mineral surface sites. Hence, these results elucidate the sometimes conflicting information in the literature about the influence of organic matter on metal sorption rates. Furthermore, they provide guidance for the selection of appropriate sorption equilibration times for experiments that are designed to determine metal distribution coefficients (Kd values) under equilibrium conditions.

Theoretical analysis of kinetic effects on the quantitative comparison of K(d) values and contaminant retardation factors.

Distribution coefficients (K(d) values) describe contaminant partitioning between liquids and solids for linear sorption at equilibrium conditions. If experimentally-determined K(d) values do not represent sorption equilibria, errors are introduced in contaminant transport models. These errors may be further propagated when K(d) values are used to compare contaminant mobility under different chemical solution conditions. Our theoretical analysis based on pseudo-first order sorption kinetics shows that, independent if two systems have the same or different sorption kinetics, relative comparisons of K(d) values and retardation factors are always affected by sorption times under non-equilibrium conditions. The time-frames required for attaining constant K(d) values are not only dependent on kinetic sorption characteristics, but also the equilibrium K(d) values approached. The type of kinetic errors introduced is affected by the specific differences in sorption kinetics and equilibrium K(d) values between the two systems. For systems with the same sorption kinetics, relative increases or decreases in contaminant velocities are always underestimated. In case of different kinetics, either an under- or overestimation of relative differences seems possible. Experimental sorption times should aim to equilibrate the system with the highest K(d) value for systems with comparable kinetics, and the system with the slowest sorption kinetics for different kinetics.

A new method to radiolabel natural organic matter by chemical reduction with tritiated sodium borohydride.

In this paper, we describe a new method for labeling NOM with the radioisotope tritium (3H) using fulvic acid (FA) as the target NOM fraction. During labeling, FA ketone groups are chemically reduced with tritiated sodium borohydride (NaBH4), while the chemical functionality of the carboxyl and phenol groups is preserved. The labeling procedure was optimized in efficiency experiments that determined the excess concentration of tritiated NaBH4 required for optimum reduction conditions. The chemical characterization of the labeled FA product using FTIR and 1H NMR spectral analysis confirms the proposed reaction mechanism and rules out any significant amounts of impurities or undesirable side reactions. Results from size exclusion chromatography indicate thatthe tritium label is distributed uniformly over the whole molecular size range of FA and that it is stable over time and under various pH conditions. Potential differences in FA sorption behavior onto mineral surfaces due to labeling were excluded based on experimental data. This method produces NOM of high specific activity (e.g., 1.9 mCi mg(-1) FA); this permits the tracing of FA at a detection limit of 0.3 microg L(-1) FA.