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

Abstract

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.

Role of interlayer porosity and particle organization in the diffusion of water in swelling clays

“…Diffusion is the main transport process in compacted clayey rocks owing to their low hydraulic conductivity (Pusch, 1979;Bourg et al, 2003;Bourg and Tournassat, 2015). This process has been extensively studied, especially in the context of the storage of nuclear waste and CO2, and resulted in an abundance of experimental data obtained by various methods (such as through-diffusion (TD) experiments using tracers or quasi-elastic neutron scattering) and models describing the diffusion of different elements in clayey media (e.g., Sato and Suzuki, 2003;Van Loon et al, 2004;Andra, 2005;García-Gutiérrez et al, 2006;Landais, 2006;Bachu, 2008;González Sánchez et al, 2008a and b;Glaus et al, 2010;Gimmi and Kosakowski, 2011;Altmann et al, 2015;Charlet et al, 2017;Bestel et al, 2018;Dagnelie et al, 2018). Note that these data were obtained for various media constitutive of mono-or multiclayey phases and considering the presence or not of non-clayey minerals (Shackelford and Moore, 2013;Bourg and Tournassat, 2015;Charlet et al, 2017;Bestel et al, 2018 and references therein).…”

Mobility of organic compounds in a soft clay-rich rock (Tégulines clay, France)

“…Previous studies evidenced anion exclusion in COx clay rock for both inorganic (Descostes et al, 2008) and organic anions (Dagnelie et al, 2018), with exclusion factors down to 0.1 (i.e.…”

“…Still, electrostatic interactions between mineral surfaces and solutes leads to a partial exclusion of anions from rock porosity. This so-called "anion exclusion" decreases both effective diffusion coefficient and retardation factor of anions, as compared to neutral solutes (Chen et al, 2018;Dagnelie et al, 2018). However, some major discrepancies are observed between data measured by sorption on crushed clay rock and retardation factor observed in solid samples.…”

Reactive Transport Modeling of Coupled Processes in Nanoporous Media

“…In classical isotopic tracer diffusion experiments (Molera and Eriksen 2002;Van Loon et al 2003Descostes et al 2008;Wersin et al 2008;Tachi and Yotsuji 2014;Glaus et al 2015;Dagnelie et al 2018), the concentration of the diffusing species i is very low compared to the constant electrolyte background concentration. Under these conditions, Equation 35reduces to:…”