domain: phys chim

text:
Muriel Sicot
email: muriel.sicot@univ-lorraine.fr

Damien Tristant

Iann C Gerber

Bertrand Kierren

Frédéric Chérioux

Yannick Fagot-Revurat

Luc Moreau

Julien Granet

Daniel Malterre

Polymorphism of 2D Halogen Bonded Supramolecular Networks on a Graphene/Iridium(111) Surface

The properties of 2D supramolecular self-assemblies on surfaces depend on the fine balance between molecule-substrate and molecule-molecule interactions. In this article, we study the growth of 1,3,5-tri(4'-bromophenyl)benzene (TBB) monolayer on graphene epitaxially grown on Ir(111) by means of low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) combined with a fully atomistic 1 description of the molecules in interaction with the Gr/Ir(111) substrate, using density functional theory (DFT). In order to figure out the impact of the underlying metallic layer upon the self-assembling behavior of the molecules and their properties, we compare our results with those theoretically obtained on pristine graphene or experimentally achieved on HOPG. We demonstrated that the use of the Ir layer allows the formation of large extended, continuous and two-dimensional supramolecular networks laying even over Ir step edges like a carpet. In addition, we highlighted the obtention of two structural polymorphs never observed on HOPG. In the light of DFT simulations, we assumed that the formation of these polymorphs is driven by the balance between molecule-molecule interactions, due to Halogen bonds (XB), and the tailored molecule-surface interactions due to the presence of Ir layer.

Introduction

Molecular self-assembly is the spontaneous association of molecules into structurally stable well-defined aggregates joined by noncovalent interactions. This key concept allows for the engineering of molecular architectures with novel or targeted optical, electrical and magnetic properties. In supramolecular assembly, molecular building blocks are interconnected to each other by virtue of directional intermolecular interactions such as halogen bonding, [START_REF] Cavallo | The Halogen Bond[END_REF] hydrogen bonding, 2,3 dipolar coupling, [START_REF] Yokoyama | Selective Assembly on a Surface of Supramolecular Aggregates with Controlled Size and Shape[END_REF][START_REF] Wei | Self-Assembly of Patterned Monolayers with Nanometer Features: Molecular Selection Based on Dipole Interactions and Chain Length[END_REF] metal coordination, [START_REF] Lingenfelder | Towards Surface-Supported Supramolecular Architectures: Tailored Coordination Assembly of 1,4-Benzenedicarboxylate and Fe on Cu(100)[END_REF][START_REF] Stepanow | Surface-Assisted Assembly of 2D Metal-Organic Networks That Exhibit Unusual Threefold Coordination Symmetry[END_REF][START_REF] Giovannantonio | Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization[END_REF] van der Waals forces [START_REF] Qiu | Alkane-Assisted Adsorption and Assembly of Phthalocyanines and Porphyrins[END_REF][START_REF] Nath | Crystal Engineering in Two Dimensions: An Approach to Molecular Nanopatterning[END_REF][START_REF] Ger | Surface Noncovalent Bonding for Rational Design of Hierarchical Molecular Self-Assemblies[END_REF] or π-π interactions. [START_REF] Grimme | Do Special Noncovalent pi-pi Stacking Interactions Really Exist?[END_REF] Nevertheless, on a surface, the resulting molecular geometry does not depend only on intermolecular interactions but on a subtle balance between moleculemolecule and molecule-substrate interactions. Therefore, opting for the right combination composed of a molecular building unit and a substrate would lead to the desired functional nanomaterial. Moreover, tunability could be obtained by the subtle modifications of the substrate properties.

In this framework, epitaxial graphene on metal (Gr/M) has emerged as an appealing support for molecular self-assembly. [START_REF] Macleod | Molecular Self-assembly on Graphene[END_REF] First, the presence of a moiré nanopattern [START_REF] Marchini | Scanning Tunneling Microscopy of Graphene on Ru[END_REF][START_REF] N'diaye | Structure of Epitaxial Graphene on Ir(111)[END_REF][START_REF] Sicot | Nucleation and Growth of Nickel Nanoclusters on Graphene Moiré on Rh(111)[END_REF][START_REF] Land | STM Investigation of Single Layer Graphite Structures Produced on Pt (111) by Hydrocarbon Decomposition[END_REF] originating from a lattice mismatch between the metal and the carbon layer leads to an interesting ad-sorption energy landscape that can be exploited to grow novel molecular architectures such as nanoporous [START_REF] Cai | Adsorption Properties of CoPc Molecule on Epitaxial Graphene/Ru[END_REF] or Kagomé networks [START_REF] Mao | Tunability of Supramolecular Kagome Lattices of Magnetic Phthalocyanines Using Graphene-Based Moiré Patterns as Templates[END_REF] that could not be achieved using HOPG [START_REF] Scheffler | Structural Study of Monolayer Cobalt Phthalocyanine Adsorbed on Graphite[END_REF][START_REF] Åhlund | The Adsorption of Iron Phthalocyanine on Graphite: A Scanning Tunnelling Microscopy Study[END_REF] or other substrates. In addition, the coupling between Gr and the metal is an extra parameter to play with in order to modify the molecule-substrate interaction and therefore to obtain various and original molecular arrangements. For example, the growth of iron-phthlocyanine on Gr/Ru for which the carbon-metal interaction is strong leads to a Kagomé lattice whereas same molecules form densely packed 2D islands on Gr/Pt for which the coupling is weak. [START_REF] Yang | Molecule-Substrate Coupling between Metal Phthalocyanines and Epitaxial Graphene Grown on Ru(0001) and Pt(111)[END_REF] Moreover, the advantage of using Gr/M substrates relies on the fact that even more complex Gr/X/M interfaces can be built for fine adsorption tuning by means of controlled intercalation of a foreign species X (atoms or molecules) between the graphene layer and the metallic support. [START_REF] Shikin | Surface Intercalation of Gold Underneath a Graphite Monolayer on Ni(111) Studied by Angle-Resolved Photoemission and High-Resolution Electron-Energy-Loss Spectroscopy[END_REF][START_REF] Dedkov | Intercalation of Copper Underneath a Monolayer of Graphite on Ni(111)[END_REF][START_REF] Varykhalov | Surface Intercalation of Gold Underneath a Graphite Monolayer on Ni(111) Studied by Angle-Resolved Photoemission and High-Resolution Electron-Energy-Loss Spectroscopy[END_REF][START_REF] Enderlein | The Formation of an Energy Gap in Graphene on Ruthenium by Controlling the Interface[END_REF][START_REF] Sutter | Chemistry under Cover: Tuning Metal-Graphene Interaction by Reactive Intercalation[END_REF][START_REF] Rutkov | Intercalation of Graphene Films on Metals with Atoms and Molecules, Physics and Applications of Graphene -Experiments[END_REF][29][START_REF] Sicot | Copper Intercalation at the Interface of Graphene and Ir(111) Studied by Scanning Tunneling Microscopy[END_REF] Once again, the primary objective behind this interface engineering is to finely tune the electronic properties of the topmost graphene layer in order to have control on the resulting molecule-substrate interaction. In this vein, the possibility to manipulate molecular adsorption at the nanometer scale and to tailor molecular self-assembly using intercalation has been recently demonstrated. [START_REF] Bazarnik | Tailoring Molecular Self-Assembly of Magnetic Phthalocyanine Molecules on Fe-and Co-intercalated Graphene[END_REF][START_REF] Schumacher | The Backside of Graphene: Manipulating Adsorption by Intercalation[END_REF] Finally, the growth of epitaxial graphene is carpet-like such that the carbon layer continuously covers the metallic step edges. 29,[START_REF] Coraux | Structural Coherency of Graphene on Ir(111)[END_REF][START_REF] Sutter | Epitaxial Graphene on Ruthenium[END_REF] Hence, unlike any other substrate, one could expect to avoid the disruption of the molecular network and therefore, obtain low-defect interface for graphene-based organic electronic devices with high performances.

Although HOPG has been widely used for molecular self-assembly, there is no straightforward way to deduce from these existing studies the self-assembly behavior on Gr/M since the interaction of graphene with the underlying layers (carbon in the case of HOPG and metal in the case of Gr/M) plays an important role in the number and nature of phases even in the case of weakly coupled Gr/M interfaces. [START_REF] Scheffler | Structural Study of Monolayer Cobalt Phthalocyanine Adsorbed on Graphite[END_REF][START_REF] Åhlund | The Adsorption of Iron Phthalocyanine on Graphite: A Scanning Tunnelling Microscopy Study[END_REF][START_REF] Yang | Molecule-Substrate Coupling between Metal Phthalocyanines and Epitaxial Graphene Grown on Ru(0001) and Pt(111)[END_REF][START_REF] Hämäläinen | Self-Assembly of Cobalt-Phthalocyanine Molecules on Epitaxial Graphene on Ir(111)[END_REF] Therefore, in order to understand how molecular properties can be affected by the substrate, it is crucial to determine the impact of the underlying metal layer below graphene on the self-assembly.

Here, we investigate by means of LT-STM/STS and DFT the structural and electronic properties of self-assembly of 1,3,5-tri(4'-bromophenyl)benzene (TBB) molecules on Gr/Ir(111).

The particular choice of this molecule is motivated by the fact that it can form self-assembly driven by halogen• • • halogen bonding on various substrates. [START_REF] Gatti | Molecular Structure, and Solvent Effects in 2D Self-Assembly via Hydrogen and Halogen Bonding[END_REF][START_REF] Gutzler | Surface mediated synthesis of 2D covalent organic frameworks: 1,3,5-tris(4-bromophenyl)benzene on graphite(001),Cu(111)and Ag(110)[END_REF][START_REF] Silly | Selecting Two-Dimensional Halogen-Halogen Bonded Self-Assembled 1,3,5-Tris(4-iodophenyl)benzene Porous Nanoarchitectures at the Solid-Liquid Interface[END_REF] Moreover, halogen bonded molecular self-assembly on Gr/M substrates are lacking. As this molecule has been previously investigated on semi-conducting, 39 metallic [START_REF] Gutzler | Surface mediated synthesis of 2D covalent organic frameworks: 1,3,5-tris(4-bromophenyl)benzene on graphite(001),Cu(111)and Ag(110)[END_REF][START_REF] Walch | Material-and Orientation-Dependent Reactivity for Heterogeneously Catalyzed Carbon-Bromine Bond Homolysis[END_REF][START_REF] Blunt | Templating Molecular Adsorption Using a Covalent Organic Framework[END_REF][START_REF] Russell | Dimerization of Tri(4-bromophenyl)benzene by Aryl-Aryl Coupling from Solution on a Gold Surface[END_REF] or graphite (HOPG) substrates, [START_REF] Gutzler | Surface mediated synthesis of 2D covalent organic frameworks: 1,3,5-tris(4-bromophenyl)benzene on graphite(001),Cu(111)and Ag(110)[END_REF] a direct comparison with our results allows us to elucidate the role of the Gr/Ir interface in the assembly process. In this work, we demonstrate by LT-STM, the formation of a supramolecular carpet of high structural quality on Gr/Ir. In addition, the coexistence of two polymorphs is observed. We show that the two phases are composed of TBB dimers stabilized by Br• • • Br and Br• • • H bonds. The two polymorphs have been determined to be different than the single one previously obtained on HOPG 37 revealing the crucial role of the layer lying under the topmost graphene sheet. DFT demonstrates that the Ir has only little influence on the adsorption geometry of the molecule. However, LT-STS and simulated PDOS reveal that Ir acts on the energies of the HOMO and LUMO states of the adsorbed molecule via charge transfer with graphene. Finally, the comparison of our results with previous works shows that the use of Gr/Ir(111) results in original packings with unique electronic properties.

Results and discussion

System

The substrate is made of one single monolayer of graphene epitaxially grown on Ir(111) and prepared as described elsewhere. [START_REF] Sicot | Copper Intercalation at the Interface of Graphene and Ir(111) Studied by Scanning Tunneling Microscopy[END_REF] At the sample scale (about one square centimeter), lowenergy electron diffraction (LEED) patterns (not shown here) reveal a long-range ordered and single-domain graphene overlayer. A moiré superstructure resulting from a mismatch between graphene and Ir(111) lattices is also observed both with LEED and scanning tunneling microscopy (STM) (Fig. 1(a)) with a superperiodicity of about 2.5 nm corresponding to approximately 10 times of the lattice constant of graphene and 9 times that of Ir(111). This is in good agreement with earlier works. [START_REF] N'diaye | Structure of Epitaxial Graphene on Ir(111)[END_REF] As shown in Fig. 1(b), this graphene corrugation is responsible for the creation of three regions for possible preferential molecular adsorption: FCC-, HCP-and ATOP-type. They are named for whether the center of the carbon hexagons sites above an Ir fcc-, hcp-hollow or atop site (up triangle, down triangle and circles circumscribe these regions in Fig. 1(b)). Fig. 1(c) displays a high-resolution STM topograph showing the graphene overlayer that spreads across an Ir step without visible defect at the atomic scale. This carpet-like growth has already been described by Coraux et al. [START_REF] Coraux | Structural Coherency of Graphene on Ir(111)[END_REF] As we will show later on, this particular structural property is a key feature to obtain low defect density 2D self-assembled molecular networks (SAMNs). Indeed, this property to cover the Ir edge might lead to a reduction of the Ehrlich-Schwöbel barrier and therefore promote step edge crossing of the molecules during diffusion in comparison with diffusion on uncovered metallic surfaces such as Ag, Au. The TBB molecule is star shaped (Fig. 1(d)) and possesses a bromophenyl group at the extremity of its arms. In the gas-phase, a single molecule is not flat : each arm is rotated by a dihedral angle δ of about 39 • between the phenyl groups and the central ring as obtained by gas-phase DFT optimization of the molecule and shown in Fig. 1(d)(bottom view) (see the computational details section below).

Growth

The growth of TBB on Gr/Ir has been investigated in the submonolayer range. At early stages of growth, molecules adsorbed on the lower terraces along the Gr/Ir step edges as shown in Fig. 2(a). At increasing coverages, TBB molecules self-assemble starting from the Gr/Ir step edges and form one molecule thick nanoislands as pointed by a white arrow in Fig. 2(b). Large defects on terraces can anchor molecules and act as starting points for molecules to self-assemble as well. Fig. 2(c) displays an STM topograph of a 2D SAMN of TBB molecules that spontaneously formed at higher coverage. Molecular patches as large as 200 nm were observed by means of STM. Two different polymorphs, labeled 2MOL and 4MOL in the following, were identified (Fig. 2(c) and(d)). Their respective occurence and molecular packing will be discussed in details later in the text. White parallel lines in Fig. 2(c) outline one main crystallographic direction of the first polymorph. Those lines are not deviated when crossing the Gr/Ir step edge displayed as a vertical black line according to the color scale in the center of the STM image. This shows the continuity of the SAMN over the step edge. This feature can be easily understood considering the carpet-like growth of the graphene film on Ir as mentioned earlier: molecules adsorbed at any places on graphene independently of the vertical graphene lattice bending caused by the presence of Ir step edge. This feature is also well illustrated for the second polymorph in the 3D rendering of an STM topograph in Fig. 2(d) where the molecular network smoothly adopts the graphene deformation when crossing the Ir step edge.

Molecular packings

In the next paragraph, we will discuss the occurence and the molecular packing of the two TBB polymorphs on graphene. For each polymorph, three different domains rotated from another by 120 • are observed, in good accordance with the underlying 3-fold symmetry of the graphene lattice. The two phases are formed in extended domains in the surface together with nanoislands composed of the two coexisting phases as shown in Fig. 3(a). On this STM topograph, the two polymorphs are visible and form three domains separated by two domain boundaries highlighted with dashed lines. Submolecular resolution allowed us to identified adsorbed molecules as intact TBB molecules since under specific tunneling conditions such as in Fig. 3 Starting from the abovementioned lattice vectors, DFT calculations were performed to obtain the optimized structures of the TBB networks. The graphene-molecule interaction was taken into account through a starting block made of the optimized structure of a single TBB molecule adsorbed on pristine graphene as depicted in Supporting Information (Fig. S7). DFT calculations of the self-assembly on graphene couldn't be carried out considering the incommensurability with the substrate that does not allow for the use of a tractable calculation cell. Nevertheless, considering the substrate through the use of the abovementioned starting block is satisfactory since it leads to computed structures in excellent agreement with experimental measurements. The 3D representations of the DFT models in Fig. 4(d) help us to understand intermolecular interactions governing the self-assembly process. The nature of the C-Br...Br-C bond in molecular crystals has only been recently unravelled [START_REF] Capdevila-Cortada | The Nature of the C-Br...Br-C Intermolecular Interactions Found in Molecular Crystals: a General Theoretical-Database Study[END_REF] and properties of the halogen bond has been under debate for a long time. [START_REF] Cavallo | The Halogen Bond[END_REF] One crucial property of the halogen bond is its directionality originating from the electron density of the halogen atom that is anisotropically distributed whenever the atom is covalently bound to another one. It was shown that a higher electron density, where the electrostatic potential is negative forms a belt orthogonal to the covalent bond and a region of lower electron density, where the potential is positive (the so-called σ-hole) forms a cap of depleted electron density in the direction of the covalent bond. [START_REF] Murray | Expansion of the sigma-Hole Concept[END_REF][START_REF] Politzer | Halogen Bonding and Other sigma-Hole Interactions: A Perspective[END_REF] We have shown by calculating the electrostatic surface potential (ESP) map that, in the case of TBB, an electron density anisotropy exists as shown in Supporting Information (Fig. S8). Based on these results and recent works, [START_REF] Cavallo | The Halogen Bond[END_REF][START_REF] Capdevila-Cortada | The Nature of the C-Br...Br-C Intermolecular Interactions Found in Molecular Crystals: a General Theoretical-Database Study[END_REF] we have retained two criteria for the occurence of the C-Br...Br-C bond: (i) the distance between bromine atoms has to be smaller than 4.2 Å and it also requires that (ii) θ 1 ≈ 180 • and θ 2 ≈ 90 • (or, equivalently

θ 2 ≈ 180 • and θ 1 ≈ 90 • ) such that |θ 1 -θ 2 | > 50 •
where θ 1 and θ 2 refer to ∠C -Br 1 ...Br 2 and ∠C -Br 2 ...Br 1 angles. According to these criteria, we have identified two and four C-Br...Br-C intermolecular bonds in the 2MOL and 4MOL phase, respectively (see Tab. 1) Table 1: Computed interatomic distances (see text and Fig. 4(d) for labels) of the TBB molecular network. Table 2: Dihedral angles of TBB for adsorption situations described in Supporting Information (Fig. S7)

# d( Å) θ 1 ( • ) θ 2 ( • ) |θ 1 -θ 2 |( • ) 1 3.
# bromophenyl arm Gaseous phase on pristine Gr on Gr/Ir(111)

1 39 • 0 • 1 • 2 40 • 17 • 18 • 3 39 • 9 • 12 •
In order to fully understand the coexistence of the two polymorphs, binding energy calculations have been performed. First, the binding energies of the two free-standing polymorphs have been calculated and are equal to -0.52 eV. In order to estimate the influence of the substrate, we have calculated the binding energy of a single TBB molecule adsorbed on Gr/Ir on FCC and ATOP regions as defined in Fig. 1(b) and for two equilibrium orientations of the molecule called staggered and eclipsed where the γ directions are either parallel to zigzag < 1 210 > or armchair < 10 10 > directions of the graphene layer (see in Supporting Information, Fig. S7(b) for eclipsed geometry), respectively. We show that the binding energies for all these configurations are equal (-2.5 ± 0.1 eV). The first conclusion to draw is that there is no preferential adsorption site i.e. the graphene moiré has no influence on the molecular packing. The second point is that both orientations are energetically equivalent. One can understand that from the substrate point of view, positioning black/yellow or green dimers as depicted in Fig. 4(c) would cost the same amount of energy. These results demonstrate that the 4MOL phase is energetically equivalent to the 2MOL phase explaining the coexistence of the two polymorphs.

The 2MOL and 4MOL phases of TBB are strongly distinct from the single phase previously obtained on HOPG. [START_REF] Gutzler | Surface mediated synthesis of 2D covalent organic frameworks: 1,3,5-tris(4-bromophenyl)benzene on graphite(001),Cu(111)and Ag(110)[END_REF] Generally, it is not straightforward to predict whether the molecular packings will be identical or distinct than on HOPG due to the fact that the behaviour of molecules depends on both molecule-substrate and intermolecular interactions.

The difference between the two substrates lies in the interaction of the upmost carbon layer with the neighboring layer (carbon in the case of HOPG and Ir in the case of Gr/Ir). HOPG is composed of stacked graphene layers with interplanar Van der Waals forces whereas Gr/Ir is made of one single layer graphene interacting with a metallic substrate. A priori, we would have expected a weak influence of the metallic layer and therefore same arrangements for both substrates since Ir presents a weak interaction with the graphene sheet. [START_REF] Preobrajenski | Controlling Graphene Corrugation on Lattice-Mismatched Substrates[END_REF][START_REF] Pletikosić | Dirac Cones and Minigaps for Graphene on Ir(111)[END_REF] In the case of Co and Fe metallated phthalocyanines, a square lattice is observed on Gr/Ir as well as on HOPG despite a unit cell slightly contracted. Nevertheless, additional phases are observed in the case of HOPG. In our case, although the interaction with the underlying layers is weak like in HOPG, the arrangements of TBB are different than on HOPG. That shows that even minute modifications of the interface can have a non negligible influence on the molecular packings.

The building block composed of a molecular dimer has also been observed in other systems such as in the bulk compound, [START_REF] Beltran | 1,3,5-Tris(pbromophenyl)benzene[END_REF] in SAMNs on HOPG as discussed above, [START_REF] Gutzler | Surface mediated synthesis of 2D covalent organic frameworks: 1,3,5-tris(4-bromophenyl)benzene on graphite(001),Cu(111)and Ag(110)[END_REF] on Au(111) [START_REF] Russell | Dimerization of Tri(4-bromophenyl)benzene by Aryl-Aryl Coupling from Solution on a Gold Surface[END_REF] and for related molecules such as 1,3,5-tris(4-iodophenyl)-benzene (TIB) on HOPG [START_REF] Silly | Selecting Two-Dimensional Halogen-Halogen Bonded Self-Assembled 1,3,5-Tris(4-iodophenyl)benzene Porous Nanoarchitectures at the Solid-Liquid Interface[END_REF] where bromine is substituted by iodine and 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (TBT) on Au(111) for which the benzene ring has been replaced by a triazine core. [START_REF] Gatti | Molecular Structure, and Solvent Effects in 2D Self-Assembly via Hydrogen and Halogen Bonding[END_REF] Although the building block is the same, the 4MOL phase has never been reported before. Concerning the 2MOL phase, one can find packing similarities with the phase labeled "phase I" in reference [START_REF] Russell | Dimerization of Tri(4-bromophenyl)benzene by Aryl-Aryl Coupling from Solution on a Gold Surface[END_REF] of TBB on Au(111) and the one called "α phase" of TBT at the liquid/Au(111) interface. [START_REF] Gatti | Molecular Structure, and Solvent Effects in 2D Self-Assembly via Hydrogen and Halogen Bonding[END_REF] However, in both cases, the two molecules fit in larger unit cells showing once again the non negligible influence of the substrate on the molecular arrangement. Larger distances between molecules can lead to a different bonding scheme playing a role in properties such as reactivity. Moreover, other coexisting phases which are not observed in our case are present on the Au(111) surface and are imposed by the so-called herringbone surface reconstruction [START_REF] Gatti | Molecular Structure, and Solvent Effects in 2D Self-Assembly via Hydrogen and Halogen Bonding[END_REF] and will surely modifiy the overall properties of the molecule/substrate interface.

Electronic properties

Finally, we have investigated the electronic properties by LT-STS recorded on bare and TBBcovered Gr/Ir surface and compared those results with calculated PDOS for free and adsorbed molecules on different Gr/Ir site (ATOP,FCC) and different orientations with respect to the substrate (eclipsed or staggered). Experimental dI/dV spectra were averaged over several unit cells of the two polymorphs. The dI/dV spectrum in Fig. 5(a) of the uncovered Gr/Ir surface (full red line) reveals one single feature at about -0.280 eV that is attributed to the Ir surface state that is know to be preserved upon graphene adsorption. [START_REF] Pletikosić | Photoemission and Density Functional Theory Study of Ir(111); Energy Band Gap mapping[END_REF] The dI/dV spectrum recorded on the 4MOL SAMN is shown as a dotted line in Fig. 5(a). Note that the one obtained on the 2MOL phase is identical (not shown here). In the range of ±1 eV, the conductance is dominated by the local density of states of the substrate. Above this range, extra features that can be attributed to molecular states are observed. The conductance starts to increase faster than the Gr/Ir one below -1.1 eV and above +1.6 eV leading to a HOMO-LUMO gap of about 2.7 eV.

The observation than dI /dV spectra are identical for both polymorphs is in good agreement with the fact that the projected density of states (PDOS) of TBB on Gr/Ir varies only slightly with the adsorption site or the orientation of the molecule as shown in Supporting Information (Fig. S9 and Fig. S10). In order to measure the impact of the underlying Ir substrate upon the electronic properties of the system, we have compared the calculated PDOS of TBB adsorbed on Gr/Ir (Fig. the addition of Ir layers leads to a rigid shift of the HOMO and LUMO states leaving the HOMO-LUMO gap unchanged. This can be simply explained by the fact that Ir induces a charge transfer resulting in a p-doped graphene as obtained theoretically here and exper-imentally. [START_REF] Preobrajenski | Controlling Graphene Corrugation on Lattice-Mismatched Substrates[END_REF][START_REF] Pletikosić | Dirac Cones and Minigaps for Graphene on Ir(111)[END_REF] The measured HOMO-LUMO gap of 2.7 eV is much smaller than the one of a single molecule in gas-phase for which our DFT calculations gives a value of 3.5 eV (Note that, it is well known that DFT calculations based on semi-local exchange correlation functional severely underestimate HOMO-LUMO gaps. [START_REF] Zhang | Comparison of DFT Methods for Molecular Orbital Eigenvalue Calculations[END_REF] ). This comparison highlights the fact that the presence of the substrate has a non negligible influence on the electronic properties of the molecule. The experimental value is in good agreement with our calculated value of 2.8 eV of TBB on pristine Gr and Gr/Ir (HOMO-LUMO gaps are summarized in Tab.3). We obtain theoretically for TBB inside a 4MOL SAMN without any substrate, a HOMO-LUMO gap higher than 3 eV and an electronic state close to the Fermi level (see in Supporting Information Fig. S11). These two last features are not in agreement with our STS measurements for which an energy gap between the LUMO and HOMO states of 2.7 eV has been measured and no conductance peak at zero bias (Fermy energy) is observed.

Once again, this leads to the conclusion that the substrate is responsible for a drastic change in the electronic states. On the contrary, we obtain a very good agreement when considering the TBB molecule adsorbed on pristine graphene (see in Supporting Information, Fig. S11(b)) or Gr/Ir (Fig. 5(b)) regardless the adsorption site or the orientation of the molecule with respect to the graphene underlayer (see in Supporting Information Fig. S9 and Fig. S10). According to the corresponding PDOS, these states are mainly of p z character (see in Supporting Information Fig. S12). Experimental determination of HOMO and LUMO energies or HOMO-LUMO gaps of adsorbed TBB are lacking in the literature and do not allow for a comparative study with other substrate. Nevertheless, a HOMO-LUMO gap of 1.4 eV of TBB adsorbed on boron-doped Si (111) has been calculated. [START_REF] Boukari | Full DFT-D Description of a Nanoporous Supramolecular Network on a Silicon Surface[END_REF] It is smaller than the one obtained in our work for TBB on graphene leading to the conclusion that the Gr/Ir provides weaker molecular-substrate interactions than SiB.

Conclusions and Perspectives

In summary, low-temperature scanning tunneling microscopy (LT-STM) and density functional theory (DFT) were used to investigate the structural formation of 1,3,5-tri(4'-bromophenyl)benzene (TBB) self-assembled monolayers on epitaxial graphene on Ir(111). The molecules adsorb on the Gr/Ir(111) substrate at room temperature and give rise to two distinct and coexisting well-ordered phases. Submolecular resolution STM topographs show that the molecular building block of these packings is a pair of TBB molecules. DFT calculations reveal that in both phases, intermolecular interactions are of C-Br• • • Br and C-Br• • • H type. The advantages of using Gr/Ir as a support lies in the obtention of highly-ordered molecular arrangements unperturbated by surface step edges. The high interfacial uniformity is a key criterion to optimize operation and performances of organic devices. Moreover, the presence of the graphene moiré pattern induced by the presence of the Ir layer helped us to determine the molecular lattices with great accuracy. In addition, we show that the TBB HOMO-LUMO gap is highly reduced when adsorbed on graphene revealing the non negligible interaction with the substrate. The presence of Ir below graphene induces a charge transfer from graphene resulting in a rigid shift of the HOMO-LUMO states compared to pristine graphene. Nevertheless, Ir has only little influence on the adsorption geometry of the molecule. DFT reveals that the molecule flattens on graphene and that this flattening is goverened by π-π interactions. As a perspective, more systematic investigations such as the influence of the underlying metal using intercalation for example or the dependence of the halogen atoms on the molecular arrangements are now required. In addition, molecules with different symmetries and number of halogen termination could be used to grow novel molecular architectures. Considering the importance of halogen• • • halogen interactions in the growth of molecular crystals and the fact that graphene is a promising candidate for ture electronic devices, this work opens up new routes for the fabrication of next-generation organic devices.

Experimental section DFT calculations. In the present study, we have investigated using Density Functional Theory (DFT) as implemented in the Vienna ab initio simulation package (VASP), [START_REF] Kresse | Ab Initio Molecular Dynamics for Liquid Metals[END_REF][START_REF] Kresse | Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor Transition in Germanium[END_REF][START_REF] Kresse | Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set[END_REF][START_REF] Kresse | Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set[END_REF] the structural and energetic properties of the two main self assembly TBB structures on Graphene@Ir(111). The corrugated Graphene@Ir(111) structure was modeled with a (10×10) Graphene structure over a (9×9) Ir(111) surface cells with 4 atomic layers, as proposed in different recent studies. [START_REF] Brako | Graphene on the Ir(111) Surface: from Van Der Waals to Strong Bonding[END_REF][START_REF] Busse | Graphene on Ir(111): Physisorption with Chemical Modulation[END_REF][START_REF] Vita | Understanding the Origin of Band Gap Formation in Graphene on Metals: Graphene on Cu/Ir(111)[END_REF] A (10×10) graphene cell was used for the study of the TBB molecule on the free standing graphene. As usual in this type of calculations, we have applied a supercell approach, with a vacuum region of more than 20 Å to avoid interaction between periodic images in the z direction. To obtain the reference energy and the optimized structure of a single TBB molecule in gas phase we have used the same cell as the free standing graphene layer. It has a lattice constant equal to a 0 = 2.46 Å , close to the experimentally obtained value (2.459 Å ) for a graphite at low temperatures. [START_REF] Baskin | Lattice Constants of Graphite at Low Temperatures[END_REF] Ions cores were modeled with projector augmented wave (PAW) pseudopotentials. [START_REF] Blöchl | Projector Augmented-Wave Method[END_REF] The 2s and 2p states of carbon, 1s state of hydrogen and 4s and 4p states of bromine were treated explicitly in the valence. The plane-wave basis set cutoff energy is set to 400 eV with a Gaussian smearing method of 0.1 eV width, in order to assure well converged total energy and force values. To include van der Waals interactions in our calculations, we have used the optB86b-vdW scheme. [START_REF] Klimeš | Chemical Accuracy for the Van Der Waals Density Functional[END_REF][START_REF] Klimeš | Van der Waals Density Functionals Applied to Solids[END_REF] This particular choice of exchange-correlation functional is based on several previous works. [START_REF] Tristant | Theoretical Study of Graphene Doping Mechanism by Iodine Molecules[END_REF][START_REF] Hu | Theoretical Study of the Interaction of Electron Donor and Acceptor Molecules with Graphene[END_REF][START_REF] Hu | Band gap Modulation of Bilayer Graphene by Single and Dual Molecular Doping: A van der Waals Density-Functional Study[END_REF][START_REF] Kong | Molecular Adsorption on Graphene[END_REF][START_REF] Tristant | Optical Signatures of Bulk and Solutions of KC8 and KC24[END_REF] All the atoms were allowed to relax until the maximum of all forces acting on them became smaller than 0.02 eV. Å-1 . The k-point sampling was always based on a Γ-centered grid for all types of calculations. We have used, to optimize the structures, a single k-point calculation for the TBB molecule, a (4 × 4 × 1) grid for the TBB-graphene system, a (4 × 4 × 1) grid for 2-TBB, a (3 × 3 × 1) grid for 4-TBB case, when a (2 × 2 × 1) grid for the single TBB on graphene@Ir(111) system was used. To determine the density of states (DOS), the tetrahedron integration method with Blöchl corrections 68 was used, with a (6 × 6 × 1) grid for the TBB-graphene and for the TBB on Graphene@Ir(111), a (7 × 7 × 1) grid for 2MOL and a (5 × 5 × 1) grid for 4MOL.

We have compared the energy stability of two structures, 2MOL and 4MOL, via the cohesion energy E coh of n TBB molecules. This energy can be calculated from the following equation :

E coh = E nMOL -nE TBB , (1)
where E nMOL is the total energy of nMOL system and E TBB is the energy of single free TBB molecule.

Molecule and substrate preparation. The experiments were carried out in an ultra high vacuum system with a base pressure of 1×10 -10 mbar equipped with an low-temperature STM Omicron. The Ir(111) single crystal was cleaned by repeated cycles of Ar + sputtering at an energy of 1.5 keV followed by a flash annealing to 1100 K. An ordered graphene monolayer (ML) was prepared by cracking on Ir(111) held at 1100 K under a propene pressure of 3 × 10 -7 mbar for 5 min. 1,3,5-tri(4'-bromophenyl)-benzene was purchased from Aldrich and then purified by column chromatography on silica gel. Then, the molecules were sublimated using a Knudsen evaporation cell onto a substrate held at room temperature.

Figure 1 :

 1 Figure 1: (a) Atomically-resolved STM topograph of the moiré structure of the graphene layer on Ir(111) (U T = 0.15 V; I T = 2 nA). (b) Schematic illustration of the DFT optimized C(10 × 10)/Ir(9 × 9) unit cell. Carbon and Ir atoms are represented as green and yellow spheres respectively. The dashed white rhombus outlines the supercell of the moiré superstructure with three different regions marked by a full circle (ATOP), an dashed uppointing triangle (FCC) and a dotted down-pointing triangle (HCP) (see text). (c) 3D rendering of an atomically-resolved STM topograph showing the continuity of the graphene film across an Ir step edge. Three white hexagons schematically represent sp 2 bonds of the graphene film adsorbed on the upper, lower terraces and on the Ir step edge. (17 × 11 nm 2 , U T = 9 mV; I T = 12 nA). (d) ball-and-stick model of the 1,3,5-tri(4'-bromophenyl)benzene (TBB) molecule in gaseous phase. Top: top view showing the three bromophenyl arms labeled 1,2 and 3 and the central benzene ring labeled C. Bottom: side view showing the dihedral rotation of the side groups.

Figure 2 :

 2 Figure 2: Early stage of growth of TBB on Gr/Ir(111). STM topographs recorded after deposition of (a) 0.05 monolayer (ML) and (b) 0.2 ML of TBB. The white arrow in (b) points at a self-assembled nanoisland. (c) STM topograph of 1ML-thick nanoisland of molecules organized in the 4MOL phase. White dashed lines highlight the continuity of the molecular lattice across the Gr/Ir step edge. (d) 3D rendering of an STM topograph recorded on a 1ML-thick molecular nanoisland of the 2MOL phase. Blue and green rhombuses in (c) and (d) outline the unit cells of the 4MOL and 2MOL phases respectively. Parameters are: (a) (65 × 59 nm 2 , U T = 2.22 V; I T = 20 pA), (b) (70 × 56 nm 2 ,U T = 2.22 V; I T = 40 pA), (c) (17 × 11 nm 2 ,U T = 2.22 V; I T = 0.25 nA) and (d) (10 × 10 nm 2 ,U T = 2.22 V; I T = 0.09 nA).

  Fig.3(c). The six inner spots linked with white dotted lines forming an hexagon correspond to the moiré superlattice. The outer spots linked with pink dotted lines forming a rhombus are related to the 2MOL SAMN. On the basis of the superperiodicity of the moiré unit

Figure 3 :Figure 4 :

 34 Figure 3: (a) STM topograph of TBB on Gr/Ir(111) showing molecular domains and domain boundaries between the two TBB polymorphs that are delimited by white dashed lines. Full line green and dashed line blue rhombuses outline the unit cells of the 2MOL and 4MOL polymorphs respectively. Ball-and-stick model of molecules are overlaid (14 × 14 nm 2 , U T = 2.22 V; I T = 80 pA). (b) Scheme of the TBB dimer building block. Carbon atoms are green, bromine atoms red, and hydrogen atoms white. The dimer is characterized by the angle between the two directions γ in dashed lines made by central C rings and bromophenyl arms and the distance between C bromophenyl rings. (c) 2D Fast Fourier Transform of an STM topograph recorded on the 2MOL arrangement. White dotted and pink dashed lines link spots originating from graphene moiré and molecular cell respectively. (d) Structural model of the growth of TBB on Gr/Ir(111) extracted from the 2D Fast Fourier Transforms of STM images taken on the 2MOL and 4MOL arrangements. Red, green and blue rhombuses represent the unit cells of the graphene moiré on Ir(111), 2MOL and 4MOL polymorphs respectively. Orientations of the unit cells with respect to the graphene moiré have been respected. A graphene honeycomb lattice corresponding to the red moiré unit cell is overlaid.

in Fig. 4 (

 4 Fig.S7(c)). This flattening of TBB on Gr/Ir is as strong as on pristine graphene (see in Supporting Information, Fig.S7(b)) suggesting that the carbon layer is solely responsible for this effect. As a conclusion, TBB gets flat as a consequence of π-π interactions with the graphene layer overcoming repulsive interactions between hydrogen atoms.

  5(b)) and on pristine Gr (Fig.5(c)). As a result,

Figure 5 :

 5 Figure 5: (a) Spatially averaged tunneling spectra recorded over Gr/Ir and on a monolayer of 4MOL polymorph on Gr/Ir(111). The arrow points at the Gr/Ir surface state. Projected densities of states (PDOS) of one single TBB molecule adsorbed (b) on a FCC site of Gr/Ir(111) and (c) on pristine Gr. PDOS are projected on different rings 1, 2, 3 and C as labelled in Fig. 1(d) and on bromine atoms.

  STM/STS experiments. All microscopy and spectroscopy measurements were performed at 77 K. The STM images were recorded at a tunneling current I T and a bias voltage U T where its sign corresponds to the voltage applied to the sample. STS spectra were ac-quired with a PtIr tip and a lock-in current detection (bias modulation of 70 mV, frequency 1100 Hz) , in open feedback loop conditions. Acknowledgment D. Tristant thanks the Midi-Pyrénées Région and the PRES Université de Toulouse for PhD funding. I. Gerber and D. Tristant also acknowledge the Calcul en Midi-Pyrénées initiative-CALMIP (Project p0812) for allocations of computer time. Part of this work was also performed using HPC resources from GENCI-IDRIS and GENCI-CINES (Project x2015096649). I. Gerber thanks CNRS for financial support.Supporting Information availableRaw STM images showing both graphene moiré and molecular lattices. Projected densities of states of one single TBB molecule adsorbed on various surfaces and geometries. Dihedral angles of TBB for various adsorption geometries. Computed interatomic distances of the TBB molecular network. Experimental and computed HOMO-LUMO gaps of the adsorbed TBB. Optimized geometry of TBB calculated by DFT. Molecular ESP map of TBB in gas phase.

Table 3 :

 3 Experimental and computed HOMO-LUMO gaps of the TBB adsorbed on X/(Y) where X is the adsorption site and Y the surface. The symbol ./ is used when the adsorption site is not specified.

	Experimental
	./(Gr/Ir)	2.7 eV
	DFT
	gas-phase	3.5 eV
	./(pristine Gr) 2.8 eV
	FCC/(Gr/Ir)	2.8 eV
	TOP/(Gr/Ir) 2.8 eV


Marine Boulange

Catherine Lorgeoux

Coralie Biache

Julien Michel

Raymond Michels

Pierre Faure
email: pierre.faure@univ-lorraine.fr

Marine Boulangé

Aging as the

Keywords: Availability, Ionic strength, Oxygenated PACs, Nitrogenated PACs, Groundwater, Colloidal mobilization Cw Cct LMW PAHs & Polar PACs Coal

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In Europe, between the eighteenth and twentieth centuries, coal was a major source of energy [START_REF] Van Krevelen | Coal[END_REF]. In order to meet the increase in energy needs, thousands of coking and manufactured gas plants were built for coke and gas production respectively. During the twentieth century, the progressive transition from coal to petroleum and natural gas induced the dismantling or the abandonment of these industrial sites [START_REF] Mannion | Chapter 5 "The history and consequences of carbon domestication[END_REF]. Nowadays, due to these coal-related activities, about 200,000 sites have been diagnosed as polluted by polycyclic aromatic hydrocarbons (PAHs). Due to their toxicity, their mutagenic and carcinogenic potential, 16 PAHs have been listed as priority pollutants by the US Environmental Protection Agency (US-EPA). These 16 PAHs are organic pollutants derived from coal transformation which are under regulation and by consequence measured for site diagnosis or monitored during remediation treatment. However, others families of organic pollutants, especially polar PACs such as oxygenated and nitrogenated polycyclic aromatic compounds (O-PACs and N-PACs), occur on these industrial sites and could induce a risk for superficial and groundwater resources. O-PACs and N-PACs occur in the initial pollution (coal tar) associated with PAHs (Lundstedt et al. 2006a[START_REF] Lundstedt | Sources, fate, and toxic hazards of oxygenated polycyclic aromatic hydrocarbons (PAHs) at PAHcontaminated sites[END_REF], but can also be produced (especially O-PACs) during remediation treatments (Lundstedt et al. 2006b[START_REF] Lundstedt | Sources, fate, and toxic hazards of oxygenated polycyclic aromatic hydrocarbons (PAHs) at PAHcontaminated sites[END_REF] or during natural attenuation (through biotic or abiotic processes) [START_REF] Biache | Low temperature oxidation of a coking plant soil organic matter and its major constituents: an experimental approach to simulate a long term evolution[END_REF][START_REF] Lundstedt | Sources, fate, and toxic hazards of oxygenated polycyclic aromatic hydrocarbons (PAHs) at PAHcontaminated sites[END_REF][START_REF] Wilcke | Microbial formation and degradation of oxygen-containing polycyclic aromatic hydrocarbons (OPAHs) in soil during short-term incubation[END_REF]. Recent studies revealed that some O-PACs and N-PACs can be equally or even more toxic than their corresponding PAHs (Lübckevon et al. 2011;[START_REF] Wincent | Aryl hydrocarbon receptor activation and developmental toxicity in zebrafish in response to soil extracts containing unsubstituted and oxygenated PAHs[END_REF]. Moreover, due to their higher solubility and lower lipophilicity compared to PAHs [START_REF] Lundstedt | Sources, fate, and toxic hazards of oxygenated polycyclic aromatic hydrocarbons (PAHs) at PAHcontaminated sites[END_REF], polar PACs are leached in higher proportion [START_REF] Schlanges | Identification, quantification and distribution of PACmetabolites, heterocyclic PAC and substituted PAC in groundwater samples of tar-contaminated sites from Germany[END_REF]) and mainly as dissolved form [START_REF] Lundstedt | Sources, fate, and toxic hazards of oxygenated polycyclic aromatic hydrocarbons (PAHs) at PAHcontaminated sites[END_REF]). As a matter of fact, these polar PACs can be found downstream in the aquifer, while PAHs are only detected near the source, on the tarcontaminated site [START_REF] Schlanges | Identification, quantification and distribution of PACmetabolites, heterocyclic PAC and substituted PAC in groundwater samples of tar-contaminated sites from Germany[END_REF].

Based on the sole knowledge of PAHs, two main mechanisms are involved in the release of contaminants from coal tar polluted soil into water: (i) PAC dissolution from coal tar and (ii) PAC desorption from the contaminated soil matrixes (mineral or organic constituents). These two mechanisms can be influenced by environmental factors, such as temperature, ionic strength, pH, concentration, and nature of dissolved organic matter (DOM) as well as time. Temperature affects PAH mobilization by increasing (i) their water solubility [START_REF] Feitkenhauer | Biodegradation of aliphatic and aromatic hydrocarbons at high temperatures[END_REF] and (ii) the diffusion of the contaminants into coal-tar particles [START_REF] Benhabib | PAHs and organic matter partitioning and mass transfer from coal tar particles to water[END_REF] or in the pores of soil particles [START_REF] Feitkenhauer | Biodegradation of aliphatic and aromatic hydrocarbons at high temperatures[END_REF]. The increase in temperature can lead to a higher PAH mass transfer to water and generally to an enhanced desorption [START_REF] Enell | Desorption kinetics studies on PAH-contaminated soil under varying temperatures[END_REF]) and dissolution processes [START_REF] Benhabib | PAHs and organic matter partitioning and mass transfer from coal tar particles to water[END_REF]). On the contrary, increase in ionic strength tends to decrease PAH solubility [START_REF] Schwarzenbach | Environmental organic chemistry[END_REF], PAH dissolution [START_REF] Mahjoub | Phase partition of organic pollutants between coal tar and water under variable experimental conditions[END_REF], and desorption processes [START_REF] Lamichhane | Polycyclic aromatic hydrocarbons (PAHs) removal by sorption: a review[END_REF]). In addition, ionic strength and pH as well as DOM (dissolved organic matter) amount and structure control colloid mobilization in water [START_REF] Marschner | DOM-enhanced mobilization of benzo(a) pyrene in a contaminated soil under different chemical conditions[END_REF]. Colloids are known to be carrier and facilitator phases for PAH mobilization and transportation in water [START_REF] Marschner | DOM-enhanced mobilization of benzo(a) pyrene in a contaminated soil under different chemical conditions[END_REF]). If DOM concentration can increase with pH [START_REF] Curtin | pH-dependence of organic matter solubility: base type effects on dissolved organic C, N, P, and S in soils with contrasting mineralogy[END_REF], pH modification has only an indirect effect on PAH release as it has no or limited effect on neutral organic compounds, especially on their dissolution and sorption [START_REF] Lamichhane | Polycyclic aromatic hydrocarbons (PAHs) removal by sorption: a review[END_REF]. Another major parameter, scarcely discussed in the literature, controlling PAH mobilization into water is time. With time, the contaminant availability is gradually reduced, decreasing the PAH release from coal tar to water [START_REF] Luthy | Interfacial films in coal tar nonaqueous-phase liquid-water systems[END_REF]. This phenomenon, known as "aging" is an important parameter to take into account, especially in historically industrial contaminated site.

In literature, many studies discuss these different parameters in order to describe and predict PAH behavior in contaminated soils. However, industrial soils (from coking plant to manufactured gas plant) are characterized by the predominance of anthropogenic organic matter in which natural recent carbon is absent or limited [START_REF] Pernot | Protective role of fine silts for PAH in a former industrial soil[END_REF]). Thus, they strongly differ from natural (agricultural, forest…) soils. Most of the studies are performed on natural laboratory spiked soils or sediments [START_REF] Bi | Sorption of heterocyclic organic compounds to reference soils: column studies for process identification[END_REF][START_REF] Abu | Mechanistic characterization of adsorption and slow desorption of phenanthrene aged in soils[END_REF][START_REF] Kleineidam | Desorption kinetics of phenanthrene in aquifer material lacks hysteresis[END_REF] or on fresh pollution (coal tar) [START_REF] Endo | Evaluating coal tar-water partitioning coefficient estimation methods and solute-solvent molecular interactions in tar phase[END_REF], and may not be representative of historically contaminated soils subjected to aging processes [START_REF] Luthy | Interfacial films in coal tar nonaqueous-phase liquid-water systems[END_REF]. Furthermore, to our knowledge, only few studies have been published on the parameters affecting O-PAC and N-PAC water release.

The purpose of the present study is to improve our knowledge (i) on the parameters influencing O-PAC and N-PAC release from soil to water and (ii) on the involved mechanisms. Two industrial soils, sampled on coking plant sites were used to perform 24-h batch leaching tests according to a design of experiments (DOE) with three parameters (i) temperature, (ii) ionic strength, and (iii) aging (or availability level). The pH was not considered in the DOE as the two coking plant soils presented a strong buffering capacity (pH could not be set without affecting the ionic strength). After carrying out the batch leaching tests, the soil-water mixture was separated and the leachates were analyzed to determine the aqueous concentration at equilibrium of the 16 US-EPA PAHs, but also 11 O-PACs and 5N-PACs. The differences of behavior between the two families were studied, and the water release mechanisms were investigated.

Materials and methods

Soils samples

The PAC-polluted soils were sampled from two former coking plants in Lorraine (France): COK1 (Homécourt) and COK2 (confidential site). COK1 was in operation from 1922 to 1981 and COK2 from 1912 to 1970, the pollution having thus aged over periods from 30 to 100 years. After sampling, the soils were air-dried, sieved at 2mm, and freeze-dried. The chemical (pH, carbonate, and total organic carbon contents) and physical characteristics (Laboratoire d'Analyse des Sols, INRA, Arras, France), the mineral and maceral compositions (Weatherford Laboratories, Shenandoah, TX, USA) were determined for both soils (Table 1).

Thermal treatment

A thermal treatment was applied on both soils to increase the pollution availability without modification of the other main compartments (mineral fraction and insoluble organic matter) [START_REF] Biache | Effect of pre-heating on the chemical oxidation efficiency: implications for the PAH availability measurement in contaminated soils[END_REF][START_REF] Usman | Effect of thermal pre-treatment on the availability of PAHs for successive chemical oxidation in contaminated soils[END_REF]). The soils were preheated at 100 °C for 5 days in 2-L Schott bottles hermetically sealed under inert atmosphere (Ar) to prevent air oxidation. Analyses were performed on the initial and preheated soils to determine the thermal treatment impact on PAC content and distribution.

Batch leaching test

Water solution (500 mL), prepared using deionized water (ELGA PURELAB Option-Q) plus a biocide azide (NaN3, 200 mg/L) to prevent PAC biodegradation, was added to 50 g of soil (liquid/solid ratio of 10) in a 1-L Schott bottle. The soil-water mixture was shaken under dark conditions with a rotary-shaker (Heidolph, Reax 20) at 10 rpm during 24 h (NF ISO 18772). The soil-water mixture was then preseparated by centrifugation (5 min, 2500 G). The supernatant was filtered by vacuum filtration on a 0.70-μm pore-sized glass microfiber filter (Grade GF/F, Whatman). The leachates (20 mL) were kept for dissolved organic carbon (DOC) measurement, and the remaining volume (400 mL) was extracted by solid-phase extraction (SPE) for PAC quantification.

Experimental design

A DOE with three parameters and two levels was set up to study the influence of each parameter and their interactions on PAC release from soil to water. The parameters investigated were temperature (3 °C (t), 28 °C (T)), ionic strength evaluated by electrical conductivity (EC) (500 μS/cm (s), 5600 μS/cm (S) for COK1 and 2000 μS/cm (s), 5600 μS/cm (S) for COK2), and PAC availability (limited initial availability (a) or thermally increased availability (A)). High levels of ionic strength were chosen to match those found on field samples, mainly due to high sulfate content. The batch leaching tests were performed in temperature-controlled chamber, by adding a CaCl2 water solution to the initial solution to modify the ionic strength and with either the initial soil (low availability) or the preheated soil (high availability). The different conditions of the DOE are summarized in Table S1. To evaluate the repeatability of the experiments and measurements during the DOE, triplicates were carried out under intermediate conditions (central points of the DOE: 16 °C, EC of 3050, and 3800 μS/cm for COK1 and COK2 respectively, initial and preheated soils mixed with a 1:1 m:m ratio-Table S2).

Analytical methods

pH and electrical conductivity Before soil/leachate centrifugation, pH and EC were measured using a multi-probes pH-ECmeter (HANNA HI 255).

Dissolved organic carbon measurements

The leachate DOC was determined, within 24 h at the most after the experiment, with a TOC-V CSH analyzer (Shimadzu) according to a catalytic combustion oxidation method carried out at 680 °C. The DOC results were normalized to the mass of the leached soil.

Organic matter extraction for PAC quantification in soils

The organic matter was extracted from the soil (2 g) with dichloromethane using an accelerated solvent extractor (ASE 350, Dionex; 2 cycles of 5 min at 130 °C, 100 bars). Sodium sulfate and copper powder were added to each sample prior to extraction to eliminate residual water and molecular sulfur, respectively. After adjusting the volume at 20 mL with dichloromethane, aliquots of the solvent extracts (3 mL) were placed in preweighed vials. The extractable organic matter (EOM) content was determined by weighing the vials after solvent evaporation under a gentle N2 flow.

PAC extraction from the leachates

In order to quantify the PACs leached from the soils during the leaching tests, the filtered leachates were extracted by SPE using Oasis HLB cartridges (200 mg,6 mL,Waters). Before extraction, the SPE cartridges were conditioned with the following solvents: dichloromethane: ethyl acetate (v:v; 8:2), methanol, anddeionized water. Deuterated surrogates ([ 2 H8]dibenzofuran, [ 2 H10]fluorene, [ 2 H10]anthracene, [ 2 H8]anthraquinone, [ 2 H10]fluoranthene, and[ 2 H12]benzo[ghi]perylene, supplied by CIL Cluzeau; 100 μL at 24 μg/mL) were added to the leachates which were then eluted through the SPE cartridges at a flow rate of 10 mL/min. The SPE cartridges were air-dried, and the PACs were eluted three times with 4 mL of dichloromethane: ethyl acetate (v:v; 8:2). Eluted compounds were concentrated to 1 mL of dichloromethane after evaporation with a gentle N2 stream for GC-MS (gas chromatograph-mass spectrometer) analysis. Recoveries of the surrogates were determined after each SPE extraction and each analytical result was corrected accordingly.

PAC quantification

PAC quantification (in soils and leachates) is carried out using internal calibration. An internal deuterated PAC standard mixture ( and[ 2 H12]perylene, supplied by CIL Cluzeau; 20 μL at 12 mg/mL) was added to the organic extract (80 μL) before GC-MS analyses. The instrument was calibrated using internal calibration method for 16 PAHs, 11 O-PACs, and 5N-PACs at 12 different concentrations (low calibration range: 0.024, 0.048, 0.06, 0.12, 0.18, and 0.3 μg/mL; high calibration range: 0.3, 0.6, 1.5, 3, 6, and 9.6 μg/mL; deuterated internal standard: 2.4 μg/mL). The two calibration ranges were verified with control standard analyses (at 0.06 and 3 μg/ mL) every ten samples and a deviation lower than 20% was accepted. Experimental and analytical blanks were also monitored regularly to assess external contamination. Analyses were performed on an Agilent Technologies 7820A GC equipped with a silica DB5-MS column (60 m× 0.25mm id × 0.25 μm film thickness) coupled to an Agilent Technologies 5975 inert MS operating in single-ion monitoring (SIM) mode (Table S3). The oven temperature was programmed as follows: 70 °C (held 2 min) to 130 °C at 15 °C/min, then 130 to 315 °C at 4 °C/min, and then 315 °C (held 25 min). The carrier gas was helium at 1.5 mL/min constant flow. The injection was set in splitless mode at 300 °C.

[ 2 H8]naphthalene, [ 2 H7]quinoline, [ 2 H10]acenaphthene, [ 2 H8]fluorenone, [ 2 H10]phenanthrene, [ 2 H10]pyrene, [ 2 H12] chrysene,

Soil specific area

The specific area of the soil was determined before and after the thermal treatment (Belsorpmini II, BELJAPAN Inc). Prior adsorption, the samples were degassed at 30 °C under vacuum. The conventional B.E.T method was performed to complete step-by-step volumetric adsorption desorption cycles of nitrogen gas at 77 K. The soil specific area was calculated according to the NF X 11-621 method (AFNOR 1975).

Results

Initial soil characteristics

The soil textures (Table 1) were characterized by high sand (> 68%) and low clay (< 9.6%) contents and belonged to the sandy loam and loamy sand classes for COK1 and COK2, respectively, according to the FAO guideline for soil description (Food and Agriculture Organization of the United Nations 2006). The pH values were high (8.8 and 7.8 in COK1 and COK2 respectively) due to high carbonate content (especially in COK1) (Table 1). They also exhibited a high TOC content (11.0 and 8.5% in COK1 and COK2 respectively). In addition, the mineral and maceral compositions, determined by particle counts, revealed the predominance of anthropogenic organic matter (coal, coke, and coal tar), whereas the natural organic matter (NOM) was almost absent (< 1%). These characteristics are typical of coking plant soils [START_REF] Pernot | Protective role of fine silts for PAH in a former industrial soil[END_REF], making them clearly different from natural soils (forest, agricultural…).

The EOM content was higher in COK1 than in COK2 (21.4 and 14.3 g/kg respectively) and represented nearly 20% of the TOC for both soils (Table S4). The COK1 and COK2 soils also differed in their PAC content, with concentrations of 16 PAHs (2424 and 936 mg/kg respectively), 11 O-PACs (333 and 148 mg/kg respectively), and 5 N-PACs (31 and 18 mg/kg respectively), being 2.6, 2.3, and 1.7 times higher in COK1 than in COK2, respectively. However, the proportion of polar PACs compared to total dosed PACs was equivalent in both soils (12-13% of O-PACs and 1-2% of N-PACs) (Table S4). In addition, the PAC molecular distribution was characterized by the predominance of (i) 3-4 ring PAHs (from fluoranthene to chrysene in COK1 and from phenanthrene to chrysene in COK2); (ii) 3 O-PACs (benzo(a)fluorenone ,benzanthrone, and benzo(cd)pyrenone in COK1 and dibenzofuran, 9Hfluorenone, and benzo(a) fluorenone in COK2) among the 11 analyzed O-PACs; and (iii) 2 N-PACs (carbazole and benzo(c)acridine) among the five analyzed N-PACs.

Preheated soil characteristics

The thermal pretreatment performed to increase the availability of the pollutants neither affected the EOM concentration nor the 16 PAH and N-PAC contents and their respective distribution in both soils (Table S4). An increase (~30%) in O-PAC content was observed (from 333 to 442 mg/kg and 148 to 196 mg/kg for COK1 and COK2, respectively). The most affected O-PACs were 9Hfluorenone, benzo(a)fluorenone, and benzoanthracenedione, but their distributions were not modified. This higher O-PAC content could be an evidence of an air oxidation of the PAHs into O-PACs [START_REF] Biache | Low temperature oxidation of a coking plant soil organic matter and its major constituents: an experimental approach to simulate a long term evolution[END_REF][START_REF] Faure | Low temperature air oxidation of n-alkanes in the presence of Na-smectite[END_REF][START_REF] Hanser | Evolution of dissolved organic matter during abiotic oxidation of coal tar-comparison with contaminated soils under natural attenuation[END_REF] in spite of the use of an inert atmosphere during the preheating treatment. Nevertheless, the O-PAC proportion of the analyzed PACs remained similar between the preheated soils (15 and 17% in COK1 and COK2, respectively) and the initial soils (12 and 13% in COK1 and COK2, respectively) (Table S4). After preheating, the soil specific areas varied from 12.8 to 5.9 m 2 /g (54% decrease) in COK1 and from 15.1 to 11.4 m 2 /g (25% decrease) in COK2 (Table S4). As both soils did not present any microporosity, we assumed that when heated, the coal tar at the surface of the soil particles melted due to lower viscosity and either covered larger area (decreasing the surface roughness) or agglomerated the particles into larger-sized pieces (decreasing the free area forN2 adsorption). Simultaneously, no change of the mineral phases, confirmed by DRX analyses (not shown), was observed after the thermal pretreatment.

Relative standard deviation and validation of the design of experiments

Based on the results of the triplicates analyzed during the DOE, relative standard deviations (RSD) were calculated for DOC, PAHs, O-PACs, and N-PACs and expressed in percentage (Table S2). Globally, the RSD could be considered as acceptable considering the multiple steps required from the start of the batch leaching test to the final analysis. Naphthalene was excluded from the monitored PAHs as it is the most volatile of the studied PACs and the most sensitive to the number of experimental and analytical steps performed for the DOE. This was confirmed by the lower RSD found when considering 15 PAHs compared to the 16 PAHs (respectively 8 and 10% for COK1 and 4 and 8% for COK2). These low RSD validated the DOE and the comparison of the different responses obtained from the different conditions of the batch leaching tests.

Influence of availability, ionic strength, and temperature on DOC, PAH, O-PAC, and N-PAC release

To highlight the main trends, the DOC and PAC concentrations in the leachates were compared to those obtained under "raw" conditions defined as low-level conditions: availability (initial soil (a)), ionic strength (without CaCl2 addition (s)), temperature (3 °C (t)). In order to clarify experimental conditions used in the text, the following nomenclature will be used:

-Availability: initial soil (a), preheated soil (A) -Ionic strength: without CaCl2 addition (s), with CaCl2 addition (S) -Temperature: 3 °C (t), 28 °C (T) The effect of temperature (28 °C) was evaluated by comparing (ast) with (asT), the effect of ionic strength was evaluated by comparing (ast) with (aSt) and the influence of the availability level was evaluated by comparing (ast) with (Ast) (Table 2). The effects of the three parameters were then expressed in proportion of change (percentage of increase or decrease).

Availability For both soils, the increase in pollutant availability induced a strong increase in DOC, PAH, O-PAC, and N-PAC concentrations in water (Table 2). The effect of the increase in availability on DOC was similar for both soils (+ 177 and + 195% in COK1 and COK2,respectively). This increase was also observed in the same range for PAH and O-PAC concentrations for COK1 leachates (+ 154 and + 153%, respectively) and more pronounced for COK2 leachates (+ 365 and + 446% for PAH and O-PAC contents, respectively). N-PACs exhibited the more important increase in their water concentrations due to the enhanced availability (+ 398 and + 694% in COK1 and COK2, respectively) (Table 2). In COK2, LMWPAHs were more affected by the availability increase than HMW-PAHs (+ 368% for LMW and + 192% for HMW), whereas in COK1, LMW-PAHs increased by 177% while HMW-PAHs remained stable (Table 2).

Ionic strength For both soils, an increase in ionic strength caused lower DOC release, -33 and -8% in COK1 and COK2, respectively (Table 2). The same trend was observed for PAH, O-PAC, and N-PAC contents in both samples with a less pronounced decrease, except for N-PAC content in COK2 leachates. The effect of ionic strength was globally higher on COK1 compared to COK2. HMW-PAH concentrations were mostly affected by the increase in ionic strength with a 77 and 39% decrease for COK1 and COK2 leachates, respectively, while LMW-PAH quantities were reduced of 19 and 3% in COK1 and COK2 leachates, respectively (Table 2).

Temperature An increase in temperature (from 3 to 28 °C) induced higher DOC, PAH, O-PAC, and N-PAC releases into water, except for O-PACs in COK1 leachates where a minor decrease was observed (Table 2). The temperature effect was more intense on COK2 compared to COK1. For COK2, temperature increase had a similar effect on PAH and O-PAC mobilization (+ 22 and + 24%, respectively), and on DOC and N-PAC mobilization (+ 45 and + 42%, respectively). For COK1, temperature increase led to an increase in PAH and N-PAC contents (+ 17%), whereas O-PAC concentration remained stable (-3%). For both samples, the increase in temperature induced a much higher effect on HMW-PAHs than on LMW-PAHs (89 and 6% for COK1 and 100 and 20% for COK2 for HMW and LMW-PAHs respectively-Table 2).

Discussion

Global behavior and mechanisms involved in the release of PACs

PACs vs. DOC in water PAH and polar PAC concentrations which represent 0.5 to 2% of the DOC content were linearly correlated to the DOC into leachates (Fig. 1). A linear correlation between DOC and PAHs is also observed by [START_REF] Benhabib | PAHs and organic matter partitioning and mass transfer from coal tar particles to water[END_REF] and [START_REF] Totsche | Release of polycyclic aromatic hydrocarbons, dissolved organic carbon, and suspended matter from disturbed NAPL-contaminated gravelly soil material[END_REF] and suggests a link between DOC and PAH content as well as between DOC and polar PACs. Although the PACs analyzed in this study represented a tiny proportion of the DOC, it shows that PACs behave similarly to the other organic compounds released into water. The specific organic composition of our soils mainly inherited from coal transformation industries (Table 1) with marked polycyclic aromatic signature can explain the linearity observed between DOC and studied PACs Polar PACs vs. PAHs in water In both coking plant soils, the polar PAC proportions comprised between 13.0 and 16.1% (raw and preheated COK1, respectively) and between 15.1 and 19.1% (raw and preheated COK2, respectively) of the total PAC content (Table S4). Whatever the conditions applied to the leaching experiment (temperature, ionic strength, and availability), the polar PACs in the leachates represented 52 to 62% (COK1) and 51 to 58% (COK2) of the total PACs (Table S1). This difference of polar-PAC proportion between soils and leaching waters clearly indicates a preferential mobilization of polar PACs (enrichment by a factor 2 to 4 for O-PACs and 5 to 14 for N-PACs) compared to PAHs. [START_REF] Lundstedt | Sources, fate, and toxic hazards of oxygenated polycyclic aromatic hydrocarbons (PAHs) at PAHcontaminated sites[END_REF] also showed through column experiments the preferential mobility of O-PACs compared to PAHs. The presence of heteroatoms (O or N) in the structure of the polar PACs induces a higher solubility. Consequently, the final concentration of an individual PAC into water seems to be controlled by the combination of its water solubility and its initial soil concentration.

PAC release by dissolution

In coal-tar-contaminated soils, the main mechanism involved in the PAH release is known to be dissolution and is described by Raoult's law [START_REF] Endo | Evaluating coal tar-water partitioning coefficient estimation methods and solute-solvent molecular interactions in tar phase[END_REF][START_REF] Mahjoub | Phase partition of organic pollutants between coal tar and water under variable experimental conditions[END_REF]. This law defines the equilibrium concentration of a compound in water as function of the compound water solubility and its molar fraction in the initial phase in contact with water. Recent studies showed that Raoult's law could also describe the concentrations of O-PACs and N-PACs at equilibrium [START_REF] Endo | Evaluating coal tar-water partitioning coefficient estimation methods and solute-solvent molecular interactions in tar phase[END_REF]). Raoult's law is defined and simplified [START_REF] Endo | Evaluating coal tar-water partitioning coefficient estimation methods and solute-solvent molecular interactions in tar phase[END_REF] as follows:

C w-dissolution = (C 0 × MW ct × S PAC )/(MW PAC × EOM)
With Cw-dissolution, the compound concentration in water in equilibrium with the soil (mg/L), C0, the initial compound concentration in soil (mg/g soil), MWPAC and MWct, the PAC and coal-tar molecular weights occurring in soil respectively (g/mol), EOM, the extractable organic matter content (mg/g soil) and SPAC, the PAC solubility in the aqueous phase (mg/L) (Table S4). For compounds that are solid at ambient temperature (all studied PACs except quinoline), the subcooled solubility must be used instead of the crystal solubility as these compounds are already dissolved in the coal-tar phase [START_REF] Lee | Partitioning of polycyclic aromatic hydrocarbons from diesel fuel into water[END_REF]. For the calculations, the MWct was determined by size exclusion chromatography (Table S4).

The PAC concentrations found in leachates, whatever the conditions of availability and ionic strength applied on the leaching experiment, were plotted and compared to those predicted by Raoult's law (Fig. 2). To be consistent with the solubility data found in the literature (25 °C), only the leachates obtained at 28 °C (T) were considered. Whatever the soil, the concentrations obtained for high availability level show reasonable agreement with Raoult's law prediction for soluble compounds including LMW-PAHs and O-PACs (solubility > 0.2 mg/L). At low availability level, Raoult's law prediction overestimates the concentrations for these compounds. Raoult's law does not allow good prediction for N-PACs whatever the level of availability probably linked to the higher polarity of these compounds than PAHs and the studied O-PACs. Indeed, Raoult's law is generally applied to predict solubility of apolar compounds such as PAH [START_REF] Schwarzenbach | Environmental organic chemistry[END_REF]). Raoult's law predicts well the concentration of HMW-PAH leachates whatever the ionic strength in the case of COK2 leachates but only at high ionic strength levels for COK1 leachates (Fig. 2). This clear distinction observed for COK1 leachates at low ionic strength between soluble compounds (solubility > 0.2 mg/L) and HMW-PAHs (solubility < 0.2 mg/L) reveals two different mechanisms of PAC release into water according to their solubilities.

R 2 = 0,97 R 2 = 0,93 R 2 = 0,94 R 2 = 0,97 R 2 = 0,98 R 2 = 0,
Figure 2: Individual PAC (µg/L) concentrations (in Log scale) in leachates from (a) COK1 and (b) COK2 soils for AST (), AsT (), aST () and asT () conditions. Upper and lower cases correspond to high and low parameter levels, respectively. PACs are classified depending on their solubility (grey background: > 0.2 mg/L and white background: < 0.2 mg/L). The black line corresponds to the PAC concentrations deduced from the Raoult law. Perinaphtenone and benzo(a)acridine are not considered due to the lack of reliable data about their respective solubilities

PAC mobilization by colloid release

To distinguish the two mechanisms involved in the release of HMW-PAHs identified previously, only the results of COK1 are discussed.

The ionic strength increase induced a decrease in PAC concentration (Table 3). The presence of ions in solution is known to reduce the solubility of neutral or nonpolar compounds like PAHs [START_REF] Schwarzenbach | Environmental organic chemistry[END_REF]. During dissolution processes, a reduced PAH mobilization is observed as ions increase the polarity of the aqueous phase and repel PAHs from water molecules (Mahjoub et al. . In our experiment, this effect was increasingly marked with the decrease in PAC polarity (N-PACs < O-PACs < PAHs) (Table 3). It is consistent with previous works of [START_REF] Mahjoub | Phase partition of organic pollutants between coal tar and water under variable experimental conditions[END_REF] and reveals that more soluble compounds like polar PACs are less affected by ionic strength modification.

The increase in ionic strength caused mainly a reduction of the PAH release which was much higher forHMW-PAHs than for LMW-PAHs (Table 3). The clear distinction between these two classes of compounds, in regard to ionic strength changes, suggests different release mechanisms as previously discussed. Moreover, at low ionic strength, the HMW-PAHs were released in water above their solubility limits (Fig. S1), implying another mechanism than dissolution for HMW-PAH mobilization as already shown by the Raoult law prediction (cf. § 4.2).

It is known that low ionic strength can favor the mobilization of colloids or soil particles in water [START_REF] Ryan | Colloid mobilization and transport in groundwater[END_REF]. In our experiments, the increase in ionic strength stabilized the colloids or particles in soil by neutralizing the negative charges of mineral or organic particles and consequently decreased the repulsion between each other. In addition, the cations control the configuration of the colloids or soil particles by binding them to the solid phase through ternary surface complexes or by condensing them through intramolecular cation bridging [START_REF] Marschner | DOM-enhanced mobilization of benzo(a) pyrene in a contaminated soil under different chemical conditions[END_REF]. All these processes tend to decrease the release of colloids or soil particles into water [START_REF] Kanti | Review on subsurface colloids and colloid-associated contaminant transport in saturated porous media[END_REF]. On the opposite, the leaching with low ionic strength solution induced a release of cations from soil particles to the water leading to surfaces negatively charged, causing repulsion but also breaking of the ternary surface complexes and intramolecular cation bridging [START_REF] Marschner | DOM-enhanced mobilization of benzo(a) pyrene in a contaminated soil under different chemical conditions[END_REF].

In our case, the good correlation between HMW-PAH concentrations in water at low ionic strength and their concentrations in soil (Fig. S2) is compatible with a mobilization of these compounds in association with colloids or soil particles. Other authors already identified this mechanism as the main release mechanism for HMW-PAHs [START_REF] Chin | Binding of pyrene to aquatic and commercial humic substances: the role of molecular weight and aromaticity[END_REF][START_REF] Lundstedt | Sources, fate, and toxic hazards of oxygenated polycyclic aromatic hydrocarbons (PAHs) at PAHcontaminated sites[END_REF].

Thus, due to their low solubilities, HMW-PAHs are mainly released in association with colloids, and this release is controlled by ionic strength. Simultaneously, a part of polar PACs and LMW-PAHs is also probably released in association with colloids but the dissolution process conceals this mechanism due to their higher solubility compared to HMW-PAHs. Additionally, it has to be pointed out that mechanisms involved in the release of PACs depend on the soil characteristics, as shown by the differences of behavior between COK1 and COK2 due to different levels of initial ionic strength (initial ionic strength in COK2 leachates is four times higher compared to COK1 leachates). limited by diffusion) (Table 2) could be an evidence of higher diffusivities at more elevated temperature. However, while this seems clear for COK2 leachates, it is not so for COK1 leachates.

(%). ast→aSt -33,0 -1,2 -26,6 -4,5 -19,3 -3,3 -76,6 -6,2 -10,4 -2,5 -3,2 -0,1 asT→aST -25,4 -0,9 -23,2 -3,9 -7,0 -1,2 -84,5 -6,8 3,2 0,7 2,7 0,1 Ast→ASt -16,5 -0,6 -7,7 -1,3 -5,2 -0,9 -58,1 -4,7 1,0 0,2 0,4 0,0 AsT→AST -13,0 -0,5 -2,7 -0,5 0,3 0,1 -58,9 -4,8 6,5 1
Case of colloids When plotting the HMW-PAH concentrations in COK1 soil and leachates under low ionic strength conditions (s), similar slopes (Fig. S2) were obtained for low temperature (Ast and ast). A higher slope (× 2) was obtained for high temperature and low availability (asT) suggesting a higher amount of mobilized colloids not observed in the case of high availability (AsT). As mentioned in "Preheated soil characteristics" the procedure used to increase the availability induced a modification of the coal-tar surface within the soils (coal-tar melting) and could lead to an artifact limiting the effect of temperature on colloid generation. On the contrary, results from asT conditions (Fig. S2) reveal an increase of colloid release induced by a higher temperature. The lower salt (anions and cations) dissolution from soil to water at low temperature (attested by the lower ionic strength obtained at low temperature than at high temperature) (Table S1) could be an explanation for the lower release of colloids. As a matter of fact, low ionic strength combined with high temperature resulted in a higher release of HMW-PAHs in association with colloids.

Conclusion

This study confirms a preferential release into water of polar PACs (O-PACs and N-PACs) compared to PAHs whatever the PAC availability level and whatever the conditions of ionic strength and temperature applied during the leaching test. PAHs and polar PACs were released according to a dissolution mechanism except under low ionic strength where HMW-PAHs were mainly released in association with colloids. Due to the higher solubility of polar PACs, their release into water was less affected by an increase in ionic strength than PAHs. Raoult's law overpredicted the equilibrium concentrations obtained at low availability level of the contamination (aged soils). Reasonable prediction of the equilibrium concentrations could be obtained under high availability level of the contamination (thermally pretreated soils in our work). The deviation observed at low availability was explained by a limitation of the PAC diffusion within the coal-tar particles that become more and more viscous with aging by depletion of the most soluble compounds. LMW-PAHs and polar PACs were more affected by aging than HMW-PAHs due probably to a difference in the extent of their depleted zone at the coal-tar particle surface. Higher temperature increased the dissolution process as well as colloidal mobilization. Polar-PACs mobilization seems to be more sensitive to temperature.

In real-aged soils, Raoult's law is not able to take into account all the involved mechanisms. For most soluble compounds mainly released in dissolved form, aging is the main parameter controlling their release into water while for less soluble compounds, mainly released in association with colloids, ionic strength is the main parameter. The history (aging) and the properties (especially ionic strength) of the soil are important parameters to take into account as they control the mechanisms involved in PAC release. To predict the equilibrium concentrations and the PAC behavior in aged soils, it would be necessary to add to the prediction laws a correcting factor that would take into account aging (availability).

Our batch leaching tests allowed identifying the mechanisms involved in the release of PACs and evaluate the influence of several parameters (temperature, ionic strength) influencing their release. The thermal pretreatment applied on soils allowed partly erasing the effect of aging and allowed studying the mechanisms at different aging levels. The next step is to study these mechanisms under dynamic conditions via column leaching tests to understand the processes occurring at more complex scale.

Σ

  15 PAHs: Sum of the concentrations of the 16 PAHs listed by the US-EPA except naphthalene. LMW-PAHs: Sum of low molecular weight PAH concentrations (acenaphthylene to pyrene). HMW-PAHs: Sum of high molecular weight PAH concentrations (benzo(a)anthracene to benzo(ghi)perylene). Σ 11 O-PACs: Sum of the concentrations of the 11 O-PACs quantified in this study. Σ 5 N-PACs: Sum of the concentrations of the 5 N-PACs quantified in this study. The Relative Standard Deviations (RSD) are calculated from the results of the three replicates.

Figure 1 :

 1 Figure 1: Σ 15 US-EPA PAH, Σ 11 O-PAC and Σ 5 N-PAC concentrations in the leachates as a function of dissolved organic carbon (DOC) content in COK1 and COK2 leachates whatever the temperature (t or T), the ionic strength (s or S) and the availability (a or A) level. Σ 15 PAHs: Sum of the concentrations of the 16 PAHs listed by the US-EPA except naphthalene. Σ 11 O-PACs: Sum of the concentrations of the 11 O-PACs quantified in this study. Σ 5 N-PACs: Sum of the concentrations of the 5 N-PACs quantified in this study.

  : Sum of the concentrations of the 16 PAHs listed by the US-EPA except naphthalene. Σ 11 O-PACs: Sum of the concentrations of the 11 O-PACs quantified in this study. Σ 5 N-PACs: Sum of the concentrations of the 5 N-PACs quantified in this study. The Relative Standard Deviations (RSD) are calculated from the results of the three replicates.

Table 1 :

 1 Grain texture, agronomic parameters, maceral, and mineral composition of the two coking plant soils (COK1 and COK2)

COK1 soil COK2 soil Grain texture (without decarbonation)

	Clay (< 2 µm) (g/kg)	96	55
	Fine silt (2-20 µm) (g/kg)	138	114
	Coarse silt (20-50 µm) (g/kg)	83	94
	Fine sand (50-200 µm) (g/kg)	134	190
	Coarse sand (200-2000 µm) (g/kg)	549	547
	Agronomic parameters
	pH (water)	8.77	7.77
	Total carbon (g/kg)	150	95
	Organic carbon (g/kg)	110	85
	Total CaCO3 (g/kg)	329	80
	Phosphorus P2O5 (g/kg)	-	0.049
	Total nitrogen (g/kg)	1.82	1.55
	Maceral and mineral composition
	-Organic matter (% vol)	26.6	21.1
	Anthropogenic (% vol)	26.0	20.8
	-Coal (% vol)	9.3	6.9
	-Coke and char (% vol)	12.1	12.1
	-Tar and pitch (% vol)	4.6	1.7
	Natural (% vol)	0.6	0.3
	-Mineral matter (% vol)	73.4	78.9
	Anthropogenic (fly ash/bottom ash) (% vol)	15.8	19.9
	Sediments (% vol)	57.6	58.9

Table 2 :

 2 DOC, Σ 15 PAH, HWM PAH, LMW PAH, Σ 11 O-PAC and Σ 5 N-PAC concentration differences (in percentage) induced by the temperature (ast vs. asT), the ionic strength (ast vs. aSt) or the availability (ast vs. Ast) modifications in COK1 and COK2 leachates (upper and lower cases of a, s and t correspond to high and low parameter levels respectively)

			DOC (mg/L)	Σ 15 PAHs (µg/g)	LMW PAHs (µg/g)	HMW PAHs (µg/g) Σ 11 O-PACs (µg/g) Σ 5 N-PACs (µg/g)
	Parameter Modification	Soil	Conc.	RSD	Conc. Diff.	RSD	Conc. Diff.	RSD	Conc. Diff.	RSD	Conc. Diff.	RSD	Conc. Diff.	RSD
			Diff (%)	(%).	(%)	(%).	(%)	(%).	(%)	(%).	(%)	(%).	(%)	(%).
	ast→asT	COK1	33,9	1,2	16,7	2,8	6,0	1,0	89,1	7,2	-2,9	-0,7	16,7	0,3
	(increase in Temperature)	COK2	45,1	4,0	21,9	1,8	20,5	1,7	100,5	20,5	24,4	3,2	42,3	3,4
	ast→aSt	COK1	-33	-1,2	-26,6	-4,5	-19,3	-3,3	-76,6	-6,2	-10,4	-2,5	-3,2	-0,1
	(increase in Ionic Strenght)	COK2	-7,6	-0,7	-3,9	-0,3	-3,3	-0,3	-38,9	-7,9	-0,8	-0,1	-10,4	-0,8
	ast→Ast	COK1	177,2	6,2	153,8	25,7	177,3	30,2	-5,0	-0,4	153,2	36,2	397,9	7,5
	(increase in Availability)	COK2	194,8	17,4	365,3	29,7	368,4	30,2	192,1	39,2	446,4	59,4	694,1	55,6

Table 3 :

 3 Influence of ionic strength modification on the DOC, PAH, LMW-PAH and HMW-PAH, O-PAC and N-PAC concentrations released in COK1 leachates at low (t) and high (T) temperature conditions and at low (a) and high (A) availability level.

		DOC		Σ 15 PAHs	LMW PAHs	HMW PAHs	Σ 11 O-PACs	Σ 5 N-PACs
	Parameter Modification	Conc. Diff (%)	RSD (%).	Conc. Diff. (%)	RSD (%).	Conc. Diff. (%)	RSD (%).	Conc. Diff. (%)	RSD (%).	Conc. Diff. (%)	RSD (%).	(%) Conc. Diff.	RSD

Acknowledgements

The authors acknowledge Y. Duclos from the Agence de l'Environnement et de la Maîtrise de l'Énergie (ADEME) for many scientific discussions. Arcelor Mittal company (especially P. Charbonnier) and the GISFI (French scientific interest group on soil pollution (http://www.gisfi.univ-lorraine.fr) and especially N. Enjelvin) are acknowledged for providing the two coking plant soils used in this work. D. Billet and A. Razafitianamaharavo from the LIEC laboratory are acknowledged respectively for the dissolved organic carbon measurements and the specific area determination by BET.

-tar Coal-tar Water Water CT-BL W-BL CT-BL W-BL Cct Cw Coal-tar Coal-tar boundary layer (CT-BL) Water boundary layer (W-BL) PAC diffusion into the coal-tar PAC diffusion form the coal-tar to the water HMW PAHs PAC Conc.

Funding information

This work was supported by ADEME, INERIS, and CNRS.

Effect of aging on PAC dissolution

In this section, results obtained at high ionic strength were selected (i) to focus on the effect of availability on PAC dissolution and (ii) to overcome the bias induced by colloid mobilization.

As discussed earlier, polar PACs and LMW-PAHs were mainly released according to a dissolution process described by Raoult's law. The log-log plot of the partitioning coefficient Kd (division of the PAC concentration in soil by the PAC concentration in water) and the solubility of the polar PACs (O-PACs and N-PACs) and PAHs indicate good correlations for both soils whatever the availability level (Fig. 3). These results show that dissolution processes occur at high and low availability. For both soils, when availability is low, higher Kd values (lower concentrations in the leachates) are obtained for the most soluble PACs while Kd values for less soluble PACs are less impacted. This was previously observed (Table 2) especially in the case of COK2 for which increasing effect of availability lead to increasing solubility (with N-PACs > O-PACs > PAHs). Although the dissolution process did operate at low availability level, these observations tend to show that aging impacted the most soluble PACs (polar PACs and LMW-PAHs) by decreasing their concentrations in water to a higher extent compared to less soluble PACs (HMW-PAHs).

The Kd values measured at high availability present similar values to those predicted by Raoult's law (Fig. 3), while Kd values obtained for leachates with low availability present higher deviation, the deviations being higher for COK2 leachates. [START_REF] Luthy | Interfacial films in coal tar nonaqueous-phase liquid-water systems[END_REF] reported a deviation from the concentration at equilibrium predicted by Raoult's law in the case of aged coal tars. Our results (Fig. 3) show increasing deviation to Raoult's law from high to low availability and are thus confirming their observations. Considering the lower deviation at high availability level, we can consider that the operated thermal pretreatment allowed erasing or partially erasing the effect of aging.

Some authors studied the influence of aging on the DOM and PAH transfer to water [START_REF] Ghoshal | Reduction of benzene and naphthalene mass transfer from crude oils by aging-induced interfacial films[END_REF][START_REF] Luthy | Interfacial films in coal tar nonaqueous-phase liquid-water systems[END_REF][START_REF] Mahjoub | Phase partition of organic pollutants between coal tar and water under variable experimental conditions[END_REF][START_REF] Totsche | Preferential flow and aging of NAPL in the unsaturated soil zone of a hazardous waste site: implications for contaminant transport[END_REF]. The authors observed the formation of a higher viscosity film at the interface after only few hours of contact between the fresh coal tar and water. [START_REF] Ghoshal | Reduction of benzene and naphthalene mass transfer from crude oils by aging-induced interfacial films[END_REF] attribute the film formation to interactions between the constituents of the coal-tar and the water molecules. Under soil conditions (vadose zone), oxidation and polymerization processes can occur and be responsible for the formation of a film at the water/coal-tar interface [START_REF] Totsche | Preferential flow and aging of NAPL in the unsaturated soil zone of a hazardous waste site: implications for contaminant transport[END_REF]. In addition, the fast dissolution of the most soluble compounds outcompetes their diffusion within the coal tar. This prevents their renewal at the coal-tar water interface and is responsible for the depleted zone. It also explains the formation of the film and the higher viscosity at the interface [START_REF] Mahjoub | Phase partition of organic pollutants between coal tar and water under variable experimental conditions[END_REF]. The film occurrence at the interface mainly causes a decrease in diffusion and mass transfer coefficients rather than a decrease in the partitioning between the two phases [START_REF] Ghoshal | Reduction of benzene and naphthalene mass transfer from crude oils by aging-induced interfacial films[END_REF]). Inversely, [START_REF] Benhabib | PAHs and organic matter partitioning and mass transfer from coal tar particles to water[END_REF] observed a decrease in the PAH equilibrium concentration with aging of coal-tar particles without any contribution of a film at the interface. The decrease in the partitioning is attributed to limitation in PAC diffusion within coaltar particles that depends only on the particle size and on temperature. The main difference between these studies is the history of the material used for the experiments, namely the time scale of the contact between coal tar and water. [START_REF] Ghoshal | Reduction of benzene and naphthalene mass transfer from crude oils by aging-induced interfacial films[END_REF] and [START_REF] Luthy | Interfacial films in coal tar nonaqueous-phase liquid-water systems[END_REF] used fresh liquid NAPL or coal tar aged for few days in laboratory, while [START_REF] Benhabib | PAHs and organic matter partitioning and mass transfer from coal tar particles to water[END_REF] used aged solid-like coal tar sampled on former coking plants. [START_REF] Liu | Partition behavior of polycyclic aromatic hydrocarbons between aged coal tar and water[END_REF] observed an important depletion of the most soluble compounds after a 5-year in situ aging period which induced a solidification of the coal tar. These authors observed also a large decrease in the equilibrium concentration. Thus, while a viscous interfacial film controls the release of PAHs into water in the case of freshly aged liquid coal tars, the influence of this film becomes negligible in regard to the increased viscosity of the coal tar that occurs after larger aging periods. After such aging period, the diffusion within the coal-tar particles becomes the main process controlling the PAC release into water. This was confirmed by [START_REF] Ortiz | Organic phase resistance to dissolution of polycyclic aromatic hydrocarbon compounds[END_REF]. Thus, as real soils aged for decades were used in our experiments, the PAC release into water is probably limited by the low diffusion within the solid-like coal-tar particles. This would explain the important decrease observed in the equilibrium concentration at low availability level compared to Raoult's law prediction.

However, a more pronounced decrease in water concentration for polar PACs and LMW-PACs than for HMW-PACs was observed with aging (Fig. 3). It may actually be explained by a two-film model [START_REF] Schwarzenbach | Environmental organic chemistry[END_REF]. This model considers the presence of an aqueous (water-side) boundary layer and nonaqueous (coal-tar side) boundary layer in coal-tar water systems (Fig. 4). The release into water is controlled by diffusion within these two boundary layers. [START_REF] Ortiz | Organic phase resistance to dissolution of polycyclic aromatic hydrocarbon compounds[END_REF] and [START_REF] Schluep | Mechanisms affecting the dissolution of nonaqueous phase liquids into the aqueous phase in slow-stirring batch systems[END_REF] evidenced a contribution of the coal side boundary layer higher for the most soluble compounds than for the less soluble compounds. Due to their higher solubilities, the diffusion of LMW-PAHs and polar PACs to the water phase is faster than for HMW-PAHs. Conversely, their diffusion within coal-tar particles is more limited. The diffusion limitation within the coal tar creates a depleted zone at the interface [START_REF] Ortiz | Organic phase resistance to dissolution of polycyclic aromatic hydrocarbon compounds[END_REF]) that we assume to be more developed in the case of LMW-PAHs and polar PACs compared to HMW-PAHs. As the depletion zone size increases with aging [START_REF] Ortiz | Organic phase resistance to dissolution of polycyclic aromatic hydrocarbon compounds[END_REF] alongside with viscosity, the most soluble PACs are increasingly impacted and their release into water presents even higher deviation to Raoult's law compared to less soluble PACs. It can then be assumed that the application of the thermal pretreatment allowed erasing the effect of aging, by redistributing the contaminants at the interface between tar and water and removing the depleted zone. It led to a higher equilibrium concentration for high availability conditions, close to Raoult's equilibrium concentration. However, assuming that the coal-tar viscosity is not changed by the thermal pretreatment once back at room temperature, the diffusion limitation concerns also the thermally pretreated particles. This is why, even if the deviation to Raoult's law prediction is lower than in the case of low PAC availability, a deviation is still observed in high availability conditions.

Effect of temperature

The effect of temperature enhancement was similar on polar PACs and PAHs with a global increase of concentrations in the water.

Case of dissolution (Raoult's law) Higher temperatures induce an increase in aqueous solubilities of PACs [START_REF] Feitkenhauer | Biodegradation of aliphatic and aromatic hydrocarbons at high temperatures[END_REF] and higher diffusion coefficients [START_REF] Benhabib | PAHs and organic matter partitioning and mass transfer from coal tar particles to water[END_REF][START_REF] Schluep | Mechanisms affecting the dissolution of nonaqueous phase liquids into the aqueous phase in slow-stirring batch systems[END_REF]. [START_REF] Benhabib | PAHs and organic matter partitioning and mass transfer from coal tar particles to water[END_REF] and [START_REF] Schluep | Mechanisms affecting the dissolution of nonaqueous phase liquids into the aqueous phase in slow-stirring batch systems[END_REF] observed an enhancement of the PAH diffusion through coal tar or coal-tar particles at higher temperature leading to higher mobilization into water according to dissolution processes. The stronger influence of temperature on most soluble PACs (highly limited by diffusion within the coal tar) compared to less soluble PACs (less