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ScienceDirect Procedia Earth and Planetary Science 17 (2017) 144 – 147

15th Water-Rock Interaction International Symposium, WRI-15

Competitive adsorption of organic molecules on clay rock Romain V.H. Dagneliea,1, Sabrina Rasamimananaa, Emilie Thorya, Gregory Lefèvreb a

DEN-Service d’Etude du Comportement des Radionnucléides (SECR), CEA, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France b PSL Research University, Chimie ParisTech — CNRS, Institut de Recherche de Chimie Paris, 75005, Paris, France

Abstract The processes governing adsorption of anthropogenic organic molecules on natural systems are complex. The presence of various phases in soils and mudstones may lead to various and simultaneous retention mechanisms. Adsorption experiments were performed on acetic, o-phthalic and citric acids on the Callovo-Oxfordian clay rock. Whereas the retention of inorganic species is dominated by clay minerals, small carboxylic acids were found to sorb on various phases or sites and not necessarily endured competition during co-adsorption. Data will be presented to illustrate the interest of various adsorption protocols to discriminate between mechanisms occurring beside adsorption onto soils and sediments: covalent bounding, chemical perturbation, phase dissolution, bacterial activity, etc.. © 2017 2017The TheAuthors. Authors. Published by Elsevier © Published by Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15. Peer-review under responsibility of the organizing committee of WRI-15 Keywords: Competitive adsorption; Clay rock; Callovo-Oxfordian; Organic pollutants

1. Introduction The adsorption of chemicals and contaminants on mineral surfaces is an important process limiting their mobility in soils and sediments. It is then crucial to understand the underlying mechanisms for environmental issues, hazardous wastes management or decontamination processes. To this aim, competitive adsorption experiments are valuable tools for both understanding phenomenology of natural systems and designing industrial processes for wastes management. This study presents competitive adsorption of several carboxylic acids, performed in support of a larger study dedicated to adsorption on a clay rich sedimentary rock. The chosen adsorbent was a sedimentary clay-rich rock, the Callovo-Oxfordian formation from the eastern Paris Basin) which is investigated by the French radioactive waste management agency (Andra). This material displays various phases (clay minerals, tectosilicates, carbonates, sulfurs, organic matter…), which makes adsorption mechanisms hard to predict with accuracy. Adsorption of inorganic ions is mostly dominated by ligand exchange * Corresponding author. E-mail address: [email protected]

1878-5220 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15 doi:10.1016/j.proeps.2016.12.033

Romain V.H. Dagnelie et al. / Procedia Earth and Planetary Science 17 (2017) 144 – 147

and electrostatic interaction with clay minerals. Still adsorption may also occur on minor phases such as sulfurs, oxides and on organic matter by hydrophobic interaction and Van der Waals interactions. The chosen adsorbates were small soluble carboxylic acids with various shapes and chemical functions. Since carboxylic acids are complexing agents, their species in solution might sorb simultaneously on clay minerals, oxides or even on hydrophobic domains depending on their speciation in solution and charge (anionic, cationic or uncharged). We will present competitive adsorption experiments with three carboxylic acids, namely acetate, ortho-phthalate, and citrate, on Callovo-Oxfordian clay rock. The results helped to confirm or deny which chemical functions and solid phases are responsible for adsorption. Such an approach is useful for prediction of organic plume migration or confinement of hazardous wastes. 2. Materials and Methods 2.1. Sedimentary rock The sedimentary rock comes from the Callovo-Oxfordian (COx) formation in the east of the Paris basin. Experiments were carried out on a clay-rich rock core (EST40471). It was collected from the borehole OHZ1705, argon-drilled downwards in the Meuse/Haute Marne Underground Research Laboratory, at a depth between 497.7 and 498.1 m below ground level. This sample is roughly composed of one third of clay minerals, one third of carbonates and one third of quartz. The composition of main minerals and minor phases is detailed in table 1. Table 1. Composition of clay rock sample (EST40471 from borehole OHZ1705). Composition of rock Clayey minerals

Quantity (%mass) 35 ± 5

Carbonates

29 ± 2

Quartz Minor phases

27 ± 5 99.7%). Adsorption experiments were performed with radiolabelled compounds to improve sensitivity and selectivity towards natural organic matter. Radiolabelled compounds were purchased from ARC: acetic acid [3H] sodium salt (ART 0202, 150 mCi/mmol), citric acid [1,5-14C] (MC-365, 112

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mCi/mmol), phthalic acid [2,3,4,5-3H] (ART 0445, 60 Ci/mmol) and homemade phthalic acid [ 14C] (synthesis detailed in5, 55 mCi/mmol). 2.3. Competitive adsorption protocol Adsorption batch experiments were carried out at 21±1°C with a liquid/solid ratio of 40 L.kg -1 for citrate and 8 L.kg-1 for acetate and phthalate. For each organic studied, at least 8 triplicates were performed with various competitors. A first phase of equilibration was performed by stirring during for four weeks the clay, the porewater and the non-labelled organic molecules. Afterwards, a 100 µL spike of radiolabelled molecule was added and the system was stirred for four weeks again. Supernatant was sampled after ultra-centrifugation to avoid colloidal effects (50 000 g for 1 hour). Adsorption was quantified after four weeks by measuring 14C-Orga or 3H-Orga using β liquid scintillation counting (Packard TRICARB 2500, ultima gold™). The solid liquid distribution ratio, R d (L kg−1), was estimated using the following equation:

Rd

>Orga@adsorbed >Orga@solution

· V § A0 ¨¨ 1¸¸ u © A(t ) ¹ m

(1)

where A0 and A are the initial and equilibrium activities in solution measured after four weeks. 3. Results 3.1. Langmuir isotherms and calculation of equilibrium concentrations Firstly, solid liquid distribution ratio, Rd(Orga/COx) in (L.kg-1), were quantified as a function of equilibrium concentration [Orga]solution without competitors. Experimental results were found to fit a Langmuir isotherm model (Figure 2, solid lines). Experimental isotherm was adjusted with two parameters, K and Q, using the following equation:

Rd (Orga)

>Orga@solid >Orga@solution

K uQ 1 K u >Orga@solution

(2)

Q(mol kg-1) represents the maximum amount adsorbed on the solid and K(L.mol-1) quantifies the affinity between adsorbent and adsorbate. Saturation was reached when R d value decreases at high concentration (i.e. K×[Orga]>1). Leaching of clay rock induced a partial dissolution of natural organic matter (fulvic acids) with concentrations below 10-5 mol L-1 for major species. Since saturation effects appeared above 10 -3 mol L-1, results were considered as not affected by natural organic matter leaching. The maximum adsorption, RdMAX=K×Q, was rather different for all three organic molecules. We observed MAX (acetate) 1 L kg-1. These results suggested either different adsorption phases, adsorption sites or even Rd different retention mechanisms. Given the low affinity of acetate for the material, we only discuss here the results of competitive experiments for citrate and phthalate.

Fig. 2. Adsorption of 14C-citrate (left) and 3H-phthalate (right) on COx clay rock after four weeks. Solid line: Experimental adsorption isotherm Rd(Orga1) = f[Orga1]eq. and corresponding variability (min, max). Dots : Results of competitive adsorption, Rd(Orga1)=f[Orga1+Orga2]eq., with 10-7 < [Orga1]0 < 10-6 mol L-1.

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3.2. Competitive adsorption isotherms Figure 2 compares the adsorption isotherms of sole organic molecules (solid lines) with experiments in presence of various competitors (dots). The distribution ratios, Rd(Orga1), were calculated using equation (1). The concentrations of competitors at equilibrium were estimated based on Langmuir isotherms. It was supposed to be Ceq.(Orga2)~C0/[1+Rd(Ceq.)×m/V]. This quadratic equation in Ceq, is resolved after expressing Rd(Orga2eq.) using eq.2. The equilibrium concentration is then estimated by equation 3:

Ceq

(1 KQm / V KC0 ) (1 KQm / V KC0 ) 2 4 KC0 2u K

(3)

The total concentration used in the x-axis of figure 2, is then [Orga]TOTeq. = [Orga1]eq.+[Orga2]eq., [Orga1]eq. being measured with the radiolabeled tracer and [Orga2]eq. being estimated by eq. 3. Results confirmed two trends. Firstly, the distribution ratios measured at low concentrations were not quantitatively affected by the presence of competitors. This result improved the confidence on RdMAX estimate. Secondly, higher concentrations of competitors, which led to saturation of the rock, did not decrease the distribution ratio of the measured species. This result suggested either different sorbing phases (or sorbing sites) for phthalate and citrate. This was in agreement with the rather different values obtained for RdMAX and Q for both species. 3.3. Discussion Competitive adsorption experiments were found to be complementary with studies on simple binary systems (organic / rock). Both methods highlighted various retention behaviours for carboxylic acids. A more detailed study, focusing on specific minerals of such an heterogeneous clay-rich rock, will be presented. It was performed in order to demonstrate which phase and chemical functions were responsible for adsorption of carboxylic acids on COx clay rock. Such data are useful for theoretical understanding of adsorption and migration mechanisms of anthropogenic organic matter in environment. It is also directly applicable for waste management such as hazardous chemicals industry or nuclear industry. The possibility to isolate adsorption mechanisms on natural samples will also be discussed. Quantifying only adsorption is a challenging task because high concentrations of organic molecules may alter the materials. By adding the adsorbates on a rock or a soil, one does not quantify only adsorption on a stable material but the sum of several mechanisms: reversible adsorption; irreversible covalent coating; chemical perturbation; phase dissolution, bacterial activity, etc. It may be necessary to perform various protocols in order to dissociate these mechanisms. Besides competitive experiments, complementary results will be presented on the use of bacterial inhibiting/promoting agent to prevent or enhance microbial activity, and the use of isotopic exchange methods to distinguish adsorption from chemical perturbation. Acknowledgements This work was supported by the French Atomic Energy and Alternative Energies Commission (CEA) and French radioactive waste management agency (Andra). We thank Virginie Blin and Jean-Charles Robinet for support on these studies and review of this manuscript. References 1. Gaucher E, Robelin C, Matray J, Négrel G, Gros Y, Heitz J, Vinsot A, Rebours H, Cassagnabère A, Bouchet A. ANDRA underground research laboratory: interpretation of the mineralogical and geochemical data acquired in the Callovian–Oxfordian formation by investigative drilling. Phys. Chem. Earth, 2004; 29: 55–77. 2. Savoye, S, Beaucaire, C, Grenut, B, Fayette, A, Impact of the solution ionic strength on strontium diffusion through the Callovo-Oxfordian clayrocks: an experimental and modelling study. Appl. Geochemistry, 2015; 61: 41–52. 3. Vinsot A, Mettler S, Wechner S. In-situ characterization of the Callovo-Oxfordian pore water composition. Phys. Chem. Earth, 2008; 33: S75S86. 4. Lundy, M, Garitte, B, Lettry, Y, Vinsot, A, Experimental Design for in situ Characterization of the Callovo-Oxfordian Pore Water Composition at 85°C. Procedia Earth Planet. Sci. 2013; 7: 533–536. 5. Dagnelie, RVH, Descostes, M, Pointeau, I, Klein, J, Grenut, B, Radwan, J, Lebeau, D, Georgin, D, Giffaut, E., Sorption and diffusion of organic acids through clayrock: comparison with inorganic anions. J. Hydrol. 2014; 511: 619–627.