Importance of Interlayer Equivalent Pores for Anion Diffusion in Clay-Rich Sedimentary Rocks

Wigger, Cornelia; Loon, Luc R. Van

doi:10.1021/acs.est.6b03781

Cited by 46 publications

(24 citation statements)

References 54 publications

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“…steric and charge effects. It seems clear that formal charge is not a good indicator of electrostatic effects, as observed with inorganic anions (Descostes et al, 2008;Wigger and Van Loon, 2017). Still, charge effects are expected in clay rich media, and most probably linked to charge density, polarizability, or even hardness of anionic Lewis bases.…”

Section: Origin Of Exclusion From the Porositymentioning

confidence: 98%

Diffusion of organic anions in clay-rich media: Retardation and effect of anion exclusion

et al. 2018

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The transport of emerging organic contaminants through the geosphere is often an environmental issue. The sorption of organic compounds slows their transport in soils and porous rocks and retardation is often assessed by extrapolation of batch experiments. However, transport experiments are preferable to strengthen migration data and modelling. In this context, we evaluated the adsorption of various organic acids by means of through-diffusion experiments in a sedimentary clay-rich rock (Callovo-Oxfordian, East of Paris Basin, France). A low diffusivity of organic anions was quantified with effective diffusion coefficients, D, ranged between 0.5 and 7 10 m s. These values indicated an organic anion exclusion. As for chloride, the porosity accessible to organic anions was lower than that of water: ε(organic anions) < ε(water). The partial exclusion of organic anions from rock porosity was linked to both charge and size effects. A significant retardation was observed for organic anions such as oxalate, citrate or α-isosaccharinate. Yet, retardation measured by diffusion experiments was significantly lower than expected from batch experiments on crushed samples. An empirical correction factor is proposed to account for a possible decrease of retardation with accessible porosity of diffusing solute. This feature has significant implications for the estimation of migration parameters of organic compounds in the environment.

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Section: Origin Of Exclusion From the Porositymentioning

confidence: 98%

Diffusion of organic anions in clay-rich media: Retardation and effect of anion exclusion

et al. 2018

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“…In a Donnan region cations are enriched and anions are depleted compared to the bulk solution. As a consequence, anion exclusion is typically observed when investigating the transport of anions through clays (Descostes et al, 2008;Van Loon et al, 2007;Tournassat & Appelo, 2011;Wigger & Van Loon, 2017). An accessible porosity smaller than the total, water-accessible porosity (more generally: a lower capacity) is assigned empirically to the transport of anions while assuming continuity of their concentration across the clay interface.…”

Section: Cations Anionsmentioning

confidence: 99%

“…5 and 7 are larger than those simulated for the full Donnan case and are better described, for instance, by a mixed ion exchange/Donnan model with K NaX ≈ 10. One has to keep in mind, though, that (i) the composition of the pore solutions derived for Opalinus Clay are dominated by Na and Cl, but other ions such as Ca, Mg, and Sr are also important, and that (ii) in addition to Donnan pores, charge-free pores and possibly inaccessible pores may also be present, requiring a multi-porosity model (Appelo & Wersin, 2007;Tinnacher et al, 2016;Wigger & Van Loon, 2017). Charge-free pores increase the limiting ξ a at low concentrations (ξ a > 0), inaccessible pores decrease the limit at high concentrations (ξ a < 1).…”

Section: Accessibility In Opalinus Claymentioning

confidence: 99%

Simulating Donnan equilibria based on the Nernst-Planck equation

Gimmi

Alt‐Epping

2018

Geochimica et Cosmochimica Acta

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Understanding ion transport through clays and clay membranes is important for many geochemical and environmental applications. Ion transport is affected by electrostatic forces exerted by charged clay surfaces. Anions are partly excluded from pore water near these surfaces, whereas cations are enriched. Such effects can be modeled by the Donnan approach. Here we introduce a new, comparatively simple way to represent Donnan equilibria in transport simulations. We include charged surfaces as immobile ions in the balance equation and calculate coupled transport of all components, including the immobile charges, with the Nernst-Planck equation. This results in an additional diffusion potential that influences ion transport, leading to Donnan ion distributions while maintaining local charge balance. The validity of our new approach was demonstrated by comparing Nernst-Planck simulations using the reactive transport code Flotran with analytical solutions available for simple Donnan systems. Attention has to be paid to the numerical evaluation of the electrochemical migration term in the Nernst-Planck equation to obtain correct results for asymmetric electrolytes. Sensitivity simulations demonstrate the influence of various Donnan model parameters on simulated anion accessible porosities. It is furthermore shown that the salt diffusion coefficient in a Donnan pore depends on local concentrations, in contrast to the aqueous salt diffusion coefficient. Our approach can be easily implemented into other transport codes. It is versatile and facilitates, for instance, assessing the implications of different activity models for the Donnan porosity.

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“…One important characteristic of clays is the negatively charged surface, which significantly affects the distribution and movement of porewater species (e.g., Birgersson & Karnland, ; Gimmi & Kosakowski, ; Glaus et al, ; Jougnot et al, ; Tertre et al, ; Tinnacher et al, ). The electrostatic forces exerted by the surface charge attract cations and repel anions, leading to the formation of a diffuse layer (DL) adjacent to the surface where anions are partly excluded and cations are enriched (e.g., Descostes et al, ; Tournassat & Appelo, ; Van Loon et al, ; Wersin et al, ; ; Wigger & Van Loon, ). Therefore, pore space in such clay systems can be characterized by a diffuse layer that counterbalances the surface charge and a charge‐balanced “bulk” or “free” water region unaffected by the mineral surface charge (e.g., Appelo & Wersin, ; Charlet et al, ; Tournassat & Steefel, ).…”

Section: Introductionmentioning

confidence: 99%

Multicomponent Ionic Transport Modeling in Physically and Electrostatically Heterogeneous Porous Media With PhreeqcRM Coupling for Geochemical Reactions

Muniruzzaman

Rolle

2019

Water Resources Research

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Low-permeability aquitards, such as clay layers and inclusions, are of utmost importance for contaminant transport in groundwater systems. Although most dissolved species, contaminants, and clay surfaces are charged, the role of electrostatic interactions in subsurface flow-through systems has not been extensively investigated. This study presents a two-dimensional multicomponent reactive transport investigation of diffusive/dispersive and electrostatic processes in homogeneous and heterogeneous clay systems. The proposed approach is based on multiple continua and is capable to accurately describe charge interactions during ionic transport in the free water, diffuse layer, and interlayer water of charged porous media. The diffuse layer composition is simulated by considering a mean electrostatic potential following Donnan approach, whereas the interlayer composition is calculated by adopting the Gaines-Thomas convention. Diffusive/dispersive fluxes within each subcontinuum (free water, diffuse layer, and interlayer) are calculated solving the Nernst-Planck equation while maintaining a net zero-charge flux. Furthermore, the multidimensional flow and transport model is coupled with the geochemical code PHREEQC, by utilizing the PhreeqcRM module, thus enabling great flexibility to access all PHREEQC's reaction capabilities. The code is benchmarked in 1-D systems against other software and previously published experimental data. Successively, reactive transport simulations are performed in 2-D clayey-sandy flowthrough domains with spatially variable physical and electrostatic properties at both laboratory and field scales. The results reveal that different properties of surface charge, diffuse layer, and Coulombic interactions impact the transport of charged species and lead to distinct spatial distribution of the ions in the different subcontinua and to significantly different breakthrough curves.

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Importance of Interlayer Equivalent Pores for Anion Diffusion in Clay-Rich Sedimentary Rocks

Cited by 46 publications

References 54 publications

Diffusion of organic anions in clay-rich media: Retardation and effect of anion exclusion

Diffusion of organic anions in clay-rich media: Retardation and effect of anion exclusion

Simulating Donnan equilibria based on the Nernst-Planck equation

Multicomponent Ionic Transport Modeling in Physically and Electrostatically Heterogeneous Porous Media With PhreeqcRM Coupling for Geochemical Reactions

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