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

Highlights

• Transport of organic compounds is quantified in a soft clay rock (Tégulines)

• Low content of organic matter drives the retention of lipophilic compounds

• Retention of ionizable sorbates decreases with lipophilicity (log P_OW < 0)

• Apparent partition coefficient, P_APP, are reported for ionizable compounds

• Diffusion coefficients provide contribution of charge and size to porosity exclusion

Abstract

The migration of organic compounds in soils is a major concern in several environmental issues. Contaminants display distinct behaviours as regards to their specific affinities towards soils constituents. The retention mechanism of hydrophobic compounds by natural organic matter is well known. The retention of ionizable compounds is mainly related to oxides and clay minerals, even if less documented in reductive media. In this work, we investigated the migration of organic compounds in a soft clay-rich sedimentary rock (Tégulines clay, France). The aim was to determine the relative contributions of natural sorbents on retention, and eventual correlations with solutes properties. Both hydrophobic compounds (toluene, benzene, naphthalene) and hydrophilic species (adipate, oxalate, ortho-phthalate, benzoate) were investigated, using batch and diffusion experiments. The retention of neutral aromatic compounds correlates with their lipophilicity (log P_OW), confirming that absorption mechanism prevails, despite a low content of natural organic matter (≤0.5%). A low retention of ionizable compounds was quantified on Tégulines clay. The eventual discrepancies between data acquired on crushed rock and solid samples are discussed. Low effective diffusion coefficients are quantified. These values hint on the relative contributions of steric and electrostatic exclusion, despite a large pore size in such “soft” clay-rock. Overall, the dataset illustrates a general scheme for assessing the migration over a wide variety of organic compounds. This approach may be useful for predictive modelling of the fate of organic compounds in environmental media.

Introduction

The transport of organic compounds in environmental media is studied for various applications: extraction of fossil fuels, fate of emerging organic contaminants, waste disposal and remediation of soils. The term “organic” may be misleading, as it corresponds to a large diversity of compounds: small ionizable compounds such as amino-acids, hydrophobic or hydrophilic hydrocarbons, large synthetic or natural geopolymers and natural organic matter (NOM) such as fulvic and humic acids. This work focuses on the transport of small soluble molecules in the near surface of geological systems. Both ionizable carboxylic acids and neutral aromatic compounds are investigated. The migration of such solutes in sedimentary rocks is highly sensitive to sorption leading to diffusive retardation (Altmann et al., 2012; Shackelford and Moore, 2013). Sorption-influenced transport is largely described in literature for organic compounds (Borisover and Davis, 2015; Schaffer and Licha, 2015) indicating that the retention part may originate from various mechanisms. Hydrophobic compounds, such as alkanes and aromatic compounds are mainly absorbed by NOM (Borisover and Graber, 1997; Karickhoff, 1981) with Freundlich-type sorption isotherm, eventually leading to intra-particulate slow diffusion mechanism (Pignatello and Xing, 1996). On contrary, hydrophilic compounds such as ionizable molecules are adsorbed on the surfaces of minerals, e.g. clays and oxides (Gu et al., 1994; Hwang et al., 2007; Johnson et al., 2004; Kang and Xing, 2007). Adsorption may be assumed in this case, with instantaneous and reversible Langmuir-type isotherms, leading to increased apparent porosity and diffusive retardation factor. Both mechanisms of adsorption and absorption, may occur simultaneously on the various mineralogical components of a natural medium, but the relative contribution of these mechanisms is rarely discussed.

Clay rock geological formations are considered in several countries for hosting a radioactive waste disposal, e.g. Callovian-Oxfordian (COx) and Tégulines in France, Opalinus Clay (OPA) and Helvetic Marl in Switzerland, Boom Clay in Belgium, etc. (Altmann et al., 2012; Appelo et al., 2010; Maes et al., 2011). The COx clay rock has been extensively studied in the context of the French Geological Radioactive Waste Disposal (Cigéo project). Corresponding studies investigated the interaction between inorganic or organic solutes and COx clay rock (Descostes et al., 2008; Melkior et al., 2007; Rasamimanana, 2017a; Savoye et al., 2012). Absorption of neutral hydrophobic compounds occurs in NOM (Vinsot et al., 2017), with an eventual contribution from clay minerals (Willemsen et al., 2019). Similarly, the adsorption of organic anions occurs on clayey minerals, despite a positive charge on clayey mineral surfaces (Rasamimanana et al., 2017b). 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. For that reason, the quantification of diffusive retardation factors seems mandatory.

This work focuses on the Tégulines clay, from the Albian Gault geological formation (East Paris Basin, France) under investigation for a potential near surface geological radioactive waste repository (Lerouge et al., 2018; Missana et al., 2017). This repository would confine low-activity long-lived waste, such as ¹⁴C-graphite, remain from old natural uranium graphite gas nuclear power reactor, developed in France until the 90s. In absence of exhaustive characterization, the organic source term potentially released by radiolytic lixiviation of graphite waste makes both neutral and anionic reference compounds being of interest (Andra, 2015; Pageot et al., 2016, 2018; Poncet and Petit, 2013). The purpose of this work is to quantify sorption and retardation factors of various organic species, in order to assess the confinement properties of the geological barrier toward potential release of ¹⁴C bearing compounds. To that aim, diffusion experiments were performed and compared to sorption experiments or predictive model based on media mineralogy. Moreover, the comparison between results on Tégulines “soft” clay rock and COx “hard” clay rock, displaying different compactions, are interesting to assess potential porosity exclusion and effects on sorption-influenced transport of anions.