Bejoy Thomas

Thibaud Coradin

Guillaume Laurent

Romain Valentin

Zephirin Mouloungui

Florence Babonneau

Niki Baccile
email: niki.baccile@upmc.fr

Biosurfactant-mediated one-step synthesis of hydrophobic functional imogolite nanotubes

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

Synthetic metal oxide Single-Walled NanoTubes (SWNTs) are expected to possess a number of interesting and unique properties (surface reactivity, mechanical strength, tailored length, insulating properties, etc.) and have generated a great deal of interest and potential use in a variety of applications such as molecular separation and storage, chemical sensing, drug delivery applications, photonics, biotechnology, catalysis, composite materials and anion/cation retention from water. 1 Although much less popular than the conventional carbon nanotubes, they represent attractive materials due to their potentially wide range of tunable compositions and properties accessible by low-temperature liquid-phase chemistry. An important goal in this area is to achieve a precise size control at the nanoscale over the three dimensions to get access to unique properties such as tunable band gaps, ballistic transport of charge/heat/mass, and quantum confinement phenomena. 2 Although the variety of such SWNT is not yet extensive, an attractive aspect of these materials is that they can be synthesized by liquid-phase synthesis under moderate conditions, often hydrothermal or solvothermal. An interesting member of this family is imogolite, 3 a natural aluminosilicate-based SWNT, whose synthesis has attracted substantial interest in recent years. 4,5 The cylindrical wall of the imogolite nanotubes can be visualized as a rolled-up sheet of gibbsite (aluminum hydroxide), with isolated silanol (Si-OH) groups linked to the inner surface of the nanotube wall. The nanotube wall is structurally ordered and is composed of an aluminum octahedron (AlO6) with three oxygen atoms shared by silicon (SiO4) tetrahedra, while the other three oxygen atoms are shared with other Al octahedra in the nanotube wall.

The nanotubes have an inner diameter of about 1.0 nm, while the outer diameter of the nanotubes varies with relative composition of Si atoms in the structure. The length of the tubes is in the 400 to 1000 nm range. Imogolite empirical formula was found to be (OH)3Al2O3-SiOH, with a [Al]/[Si] = 2. These paracrystalline hydrous silicate polymers are most commonly associated with weathering of non-crystalline volcanic materials, which have also been identified in pumice deposits, coating of primary particles, stream sediments, and soil derived from igneous and sedimentary rock sandstone. Taken together, 3,5 previous reports dedicated to the synthesis, structure and properties of imogolite lead to the following statements: i) synthetic imogolite always appears as SWNT with monodispersed diameter size; ii) imogolite formation is generally preceded and/or accompanied by that of proto-imogolite, whose XRD and NMR signatures are undistinguishable from imogolite but whose morphology is not well-defined (particles, platelets); TEM and FT-IR are commonly used to make the distinction between imogolite and proto-imogolite; c) boehmite and amorphous aluminosilicates are common side-products of the synthetic procedures; d) even if the theoretical Al/Si molar or atomic ratio is equal to 2, the experimental ratio may vary within a 15% range, depending on the synthesis method; e) nanotube packing occurs in a monoclinic space group rather than in a hexagonal one, as one would expect; f) imogolite outer surface is hydrophilic and its isoelectric point is close to pH= 9.

Preparation of the synthetic imogolite generally requires rigorous experimental conditions. 4,5 The solgel route employing an aluminium salt (mostly aluminium perchlorate or aluminium chloride) in presence of tetraethoxysilane (TEOS) in millimolar concentrations in water at T< 100°C is the most explored pathway. TEOS is preferred to sodium silicate for its slower condensation kinetics while perchlorates are preferred to chlorides due to their lower complexing ability, both reducing the uncontrolled homocondensation towards amorphous aluminium hydroxides and silica. The crystallization procedure is optimized at temperatures slightly below 100 ºC. Below 90°C, the formation rate strongly decreases while above 100 ºC boehmite rods or gibbsite platelets are formed. Synthesis of pure imogolite from more concentrated solution is also possible but with usually longer reactions times, going from 10 to 60 days to obtain long-range ordered nanotube packing. 5c,5d The functionalization of the outer surface of imogolites constitutes a new field of research due to the interest of dispersing imogolite nanotubes in hydrophobic media. Attempts to use organosilanes is not as efficient as expected and phosphonic acids were used instead. Using this procedure, poly(methylmethacrylate) and octadecyl groups were recently grafted on the surface of freshly synthesized imogolite nanotubes providing hydrophobic properties 5h,i . So far, no one-pot synthesis of surface modified imogolite synthesis has been reported, to the best of our knowledge.

In the present work, we have investigated, for the first time to our knowledge, an alternative hybrid organic/inorganic strategy to control the formation and organization of imogolite SWNTs. Indeed, the use of organic-templating strategies was already explored to obtain metal oxide nanotubes but all reported structures are multiwalled and are several tens of nanometers in outer diameter. 6 Our organic additive, glycerol α-monolaurate (MG) was selected for two main reasons: 1) monoacylglycerol derivatives (fatty acid chain covalently bonded to a glycerol head group through an ester linkage) are cheap, non toxic, commercial emulsifiers derived from both natural (animal, vegetable) and synthetic routes and routinely used in food products (bakery products, beverages, ice cream, chewing gum, shortening, whipped toppings, margarine, and confections) [7][8] ; 2) both glycerol and carboxylic acids can complex Al 3+ cations in solution and in the solid state, 9 potentially influencing morphological changes and/or process kinetics.

In line with our concern to establish a low-environmental impact synthesis, we kept conditions as simple as possible: i) use of chlorides instead of more toxic perchlorates; ii) synthesis at quasi-neutral pH and limited pH changes; iii) centimolar concentration range ([Al 3+ ]> 0.05 mol . L -1 ). Use of TEOS as silica precursor and of hydrothermal (T= 98°C) conditions are the only factors making our approach similar to previous works.

Through this approach, it is possible to obtain functionalized imogolite SWNTs in less than two weeks under neutral pH conditions. At the moment, boehmite constitutes the main impurity but preliminary results show that an optimization of the synthesis should drastically reduce its overall content.

In addition, we show that this process yields to surface-modified imogolite nanotubes with hydrophobic properties that make them promising candidates as nanoscale anisotropic fillers in polymer-based composite materials. Finally, surface modification induced an interesting two-dimensional packing of the nanotubes never reported before, as shown by small angle X-ray diffraction and TEM.

Experimental Section Preparation of the materials

Glycerol α-monolaurate (MG) was synthesized following a previously-reported procedure 10 on hydrophobic bed in continuous reactor at 80°C giving a 86 % hight purity. The resulting powder was equilibrated in 100 mL distilled water under stirring at room temperature overnight. The desired amount of AlCl3.6H2O was then added under stirring at pH ca. 2.5 at 80 °C. The solution turned into an emulsion and stirring was kept for 6 additional hours to homogenize the system. Finally, tetraethylorthosilicate (TEOS) was added under stirring and pH of the system was adjusted to ca. 7 using 1M NaOH solution about one hour after TEOS addition. The mixture was allowed to stir at the same temperature for 3 h after which the sample undergoes a hydrothermal treatment at 98 °C for a period of 72 h or 240 h. A white solid product was recovered by filtration, washed with distilled water and dried at room temperature under atmospheric conditions. Similar experiments were performed with lauric acid (LA) and glycerol, both purchased from Aldrich. Molar ratios are: additive:H2O:AlCl3:TEOS = 1: 5550: x: 3.5 where additive is MG, LA or glycerol and 1<x<7.5. Blank experiments refer to those conducted without any additive.

Characterization of the materials

Chemical analysis of the alumina-silica samples was obtained by Energy Dispersive X-ray Spectroscopy (EDAX) using an Oxford X-Max (area: 20 mm 2 ) detector installed on a Hitachi S3400N Scanning Electron Microscope. Calibration of the instrument was performed on the Ti Kα at 4.509 keV. Powder Xray diffraction (XRD) study was performed on Bruker D8 Advance diffractometer using Cu-Kα radiation (λ = 1.5418 Å) at 45 kV and 40 mA, 0.05° step size and 60 s step time over a range of 1° to 80°. Small angle X-ray scattering (SAXS) measurements were performed on a pinhole type Rigaku Nanoviewer instrument using Cu-Kα radiation equipped with a CCD detector over a range of 1 to 5 degrees (2θ) respectively. The applied voltage and filament current were 40 kV and 50 mA respectively. The finely powdered samples were sealed in metal cells with polyethylene terephthalate films and kept in a temperature controlled sample chamber for measurements. Bragg spacing is calculated by d = 2π/q, where q is the magnitude of the scattering vector. Transmission electron microscopy (TEM) images were acquired with a FEI Tecnai 120 Twin microscope operating at 120 kV and equipped with a high resolution Gatan Orius CCD 4k x 4k numeric camera. Sample holder is equipped with an alpha-tilt (+70°/-70°) goniometer to observe the sample at different tilt angles and avoid 2D projections artefacts.

Adsorption/desorption isotherms were recorded on a Belsorp-Max Instrument (BEL-Japan Inc.) using nitrogen as probe gas.

Solid-State Nuclear Magnetic Resonance (NMR) experiments were performed on a Bruker AVANCE III 300 (7.05 T) spectrometer using a wide-bore superconducting magnet. 1 H and 27 Al Magic Angle Spinning (MAS) NMR spectra were recorded with a 4 mm MAS probe at a spinning frequency of 14 kHz. For the 1 H and 27 Al spectra, 32 and 4200 transients were collected with π/2 and π/12 pulses of 6.5 and 0.95 μs and at repetition times of 10 s and 500 ms respectively. The chemical shifts were referenced to TMS and Al(NO3)3 at 0 ppm respectively for 1 H and 27 Al. For the 13 C{ 1 H} Cross-Polarization (CP), one pulse 13 C MAS, 29 Si CP{ 1 H} and one pulse 29 Si MAS NMR studies, 7 mm zirconia rotors spinning at 5 kHz were employed. A total of 4096 transients were collected with a proton π/2 pulse of 6.5 μs and recycle delays of 1 s for the CP spectra of both nuclei. A cross-polarization contact time of 3 ms was applied for all the experiments. For 13 C and 29 Si MAS NMR experiments 4096 and 1024 transients were collected with a π/4 pulse of 2.65 μs and at a relaxation delay of 10 and 60 s respectively.

The chemical shift values were calibrated with reference to tetramethylsilane (TMS) at 0 ppm. The 1 H-

Results and Discussion

Nanotube preparation

Imogolite was obtained by modification of earlier procedures, 4,5 involving the mixing of acidic solutions of an aluminium salt with tetraethoxysilane, followed by neutralization and hydrothermal treatment. Main variations compared to the literature were the selection of aluminium choride, the use of centimolar precursor concentration and the addition of glycerol α-monolaurate (MG).

During washing procedures, we observe a different behavior for samples prepared with or without biosurfactant: whereas the purely inorganic preparation was easily dispersed in water, as expected for hydrophilic aluminosilicate nanotubes, the sample prepared with MG remained located at the air-water interface, suggesting that it exhibits an hydrophobic character. To confirm this hypothesis, 1-nonanol was added to the water solution. As shown in Figure 1, after phase separation, the MG-free product was still in the bottom aqueous phase whereas the surfactant-templated sample was located in the top organic layer. This unexpected and unprecedented behavior of imogolite urged us to investigate in details the resulting materials first from an inorganic and organic point of view and then at the hybrid interface. Additional experiments led us to propose a reaction mechanism that is compatible with the one-step formation of hydrophobic imogolite nanotubes Figure 1. Behaviour of aluminosilicate powders in a water/nonanol biphasic system: MG-free sample is easily dispersed in the bottom aqueous media (left) whereas the MG-templated sample localizes in the top organic layer.

Characterization of the inorganic structure 3-days aging. XRD patterns of MG and MG-free materials at increasing Al/Si molar ratios after 3 and 10 days aging are given in Figure 2. At low Al/Si ratio (0.65) and after 3 days of aging at 98°C, the main broad diffraction peaks for MG (b) and MG-free (a) samples are at d = 0.43 nm, 0.34 nm and 0.23 nm (Figure 2a,b). These values can be related to the silicate/gibbsite layers belonging to the proto-imogolite structure, considered to be the precursor of imogolite. [3][4][5] At similar aging time but higher Al +3 concentration values (Al/Si= 2.14, Figure 2c,d), additional peaks are observed: the d= 0.14 nm distance corresponds to the (004) diffraction plane of imogolite while reflections at d= 0.32, 0.24, 0.18, 0.16 and 0.13 nm indicate the additional presence of boehmite, our major impurity. 12 The evolution from amorphous alumino-silicates to imogolite as a function of Al/Si molar ratio (3 days aging time) can also be followed by 29 Si CP and 27 Al solid state MAS NMR experiments in Figure 3.

At low Al/Si values (2a,b), a broad peak centered at -93 ppm characterizes an amorphous alumino-silicate network made of Al sites, ranging from Q 3 (4Al) to Q 4 (4Al). 13 At increasing Al 3+ values (3c,d), a characteristic peak at -78 ppm appears and, at Al/Si= 2.14 (Figure 2d), it constitutes more than 30% of the overall silicon spectrum. This peak, in good agreement with previous studies, 13a is extremely rare in aluminosilicates 13b and is commonly assigned to (AlO)3SiOH species in the imogolite environment, that is silicon tetrahedron connected to a gibbsite layer. A similar behavior occurs for the 27 Al spectra evolution (Figure 3e-h), where, at low aluminum amount (Figure 3e-g), a mixture of octahedral (δ= 2.5 ± 0.5 ppm) and tetrahedral (δ= 55 ± 0.5 ppm) sites characterizes the material. At Al/Si values close to 2 (3h), integration of the 27 Al spectrum shows that more than 90% of the material is composed of octahedral aluminum sites, as expected for imogolite, allophanes and boehmite compounds. 14 This is characteristic of the octahedral environment of aluminium polymerized in a gibbsite-like environment. A similar trend can be observed in MG-free samples (data not shown), showing that evolution from amorphous alumino-silicate to proto-imogolite does not actually depend on MG.

As mentioned earlier, the only way to distinguish between imogolite and proto-imogolite is either TEM or FT-IR spectroscopy. TEM images (Figure 4a 4) can be attributed to the (071), ( 004) and (006) (hkl) diffraction planes observed for imgolite. 5a On the contrary, the d(3)= 0.18 nm value corresponds to the (200) plane of the boehmite impurity, as discussed before.

MG-free samples (Figure 4d) only show agglomerates of particles whose morphology is not fibrous. Therefore, at a Al/Si = 2.14 ratio, the presence of MG favors the formation of imogolite nanotubes within a 3 days aging period. However, the absence of significant peaks in the low-angle domain of the XRD pattern suggests that the nanotubes are completely disordered with no bundle organization at this stage. 10-days aging. In a previous report, it was demonstrated that longer aging times favors threedimensional nanotube packing in Al-rich samples. 5c In our system, XRD patterns of the 10-days aged MG and MG-free samples for Al/Si = 2.14, given in Figure 2(e,f), show the striking effect of MG on the imogolite nanotube stacking. For MG-free samples, no significant change occurs in the XRD pattern with respect to the 3-days aged sample material, both at low (diffraction pattern not shown) and wide (Figure 2e) angles. On the contrary, in the presence of MG, more defined diffraction peaks appear in the lowangle region. In particular, as shown in Figure 2, a series of sharp low-angle diffraction peaks characterizes the MG-Al/Si= 2.14 -10 days sample. The calculated d-values indicate the presence of a clear lamellar stacking. This result is highly unexpected for imogolite for two reasons: 1) imogolite bundles are known to pack in a monoclinic crystalline system 5 ; 2) imogolite peaks at low angle are much broader than what is currently found in MG-derived system. The absence of the characteristic imogolite low-angle pattern and the presence of a lamellar pattern seem to be in contrast with the presence of imogolite nanotubes but rather support the presence of a layered aluminosilicate or a lamellar mesostructure material.

Before proceeding with a deeper analysis of the 10 days samples, we observe that the amount of residual boehmite is particularly low in this sample (Figure 2f), showing that our approach may lead to rather pure imogolite provided that synthetic conditions are optimized through a more systematic study.

We did not explore this aspect any further in this work.

Very interestingly, nitrogen adsorption/desorption isotherms are of particular use to exclude the presence of a mesostructure lamellar solid. As shown in Figure 1 in supplementary information, the MG-Al/Si= 2.14 -10 days sample displays combined micro and mesoporosity, as indicated, respectively, by the nitrogen uptake below P/P0= 0.2 and above P/P0= 0.5, where a hysteresis loop occurs. In particular, after calcination at 350°C, the amount of microporosity increases while the hysteresis loop remains intact.

The presence of a hysteresis loop for the MG-containing and calcined sample excludes the presence of a lamellar structure. In fact, mesostructured, surfactant-containing, materials do not generally display any porosity and, most importantly, the lamellar framework collapses after calcination, which is not the case here. Any additional hypothesis of a layered aluminosilicate is also excluded by the sharp 29 Si NMR peak at δ= -78 ppm, which cannot be attributed to any flat aluminosilicate surface. 13b Transmission electron microscopy images obtained for the MG and MG-free samples after 10 days are shown in Figure 5 (a-d). Evolution with time of the MG-free sample goes from morphologically undefined proto-imogolite (3 days aging, Figure 4d) to disordered imogolite nanotubes (10 days aging, Figure 5d). MG-containing samples, on the contrary, show imogolite nanotubes already at 3 days aging (Figure 4a,b) and an interesting planar lamellar packing after 10-days aging, as shown on several sections of the samples in Figure 5a-c). Throughout the sample, a large number of apparent sheets are actually composed of individual nanotubes arranged together in a parallel fashion. No three-dimensional order seems to exist between the nanotube sheets but superposition occurs rather in a non-organized way.

Further confirmation of the existence of the nanotubes can be found in Figure 5e-f, where we show an image of the same section, containing both individual nanotubes and sheets, before and after a 20° tilting of the sample holder. This manipulation allows to discriminate between a true, uni-dimensional, nanotube and a sideviewed, two-dimensional, platelet, which could actually be confused due to the projection of a 3D object on a two-dimensional surface. The black arrows point towards individual nanotubes before and after tilting and, as one can see, the indicated one does not change in lateral size upon tilting. This is of course expected for a nanotube but not for a platelet. TEM observations perfectly corroborate low-angle peaks in XRD shown in Figure 2 and attributed now to a long-range order of the imogolite nanotubes within a single sheet. This specific arrangement was never reported for imogolite and, at the moment, we can qualitatively attribute it to the presence of the MG on the surface of the tubes, as shown hereafter. Non-modified imogolite bundles are known to pack in a monoclinic crystal structure 5 but, unfortunately, we could not find any report describing the packing of surface-modified imogolite. Park and coll. 5i have reported the two-dimensional alignment of octadecylphosphonic acid modified imogolite but that was only possible by using post-synthesis modification of fresh imogolite combined with a Langmuir-Blodgett technique.

Nevertheless, despite the lack of experimental details, they briefly comment on the different packing of pure imogolite in bulk with modified imogolite nanotubes aligned on the surface as observed by XRD.

The faster formation of functionalized nanotubes in 2D lamellar packing in presence of MG is highly reproducible and also occurs at the stoichiometric Al/Si= 2 ratio. In this situation, the final material after 10 days aging shows a good quality XRD pattern (Figure 2g), typical for imogolite at wide angles, and of a lamellar phase, as described above, at low angles. As a complementary source of information, the Al/Si ratio of the samples was measured by SEM/EDAX and averaged on different sections of the material. The experimental values were very similar to the initial stoichiometric content, i.e. 2.1 ± 0.1 when MG was present. In contrast, for an initial Al excess (Al/Si = 2.14), MG-containing samples showed lower value (1.7 ± 0.1) whereas MG-free powders contain a more important Al excess (2.5 ± 0.1). These discrepancies (about 20%) could probably be justified by the presence of impurities in the final sample.

Characterization of the organic component

XRD and TEM have so far shown the important role of MG in favouring the formation of imogolite over proto-imogolite phase. In these conditions, TGA analyses (not shown) also revealed that the organic fraction constitutes between 20% and 30 wt% of the final materials. To understand better this process, we used a combination of multinuclear solid-state NMR experiments. Figure 6 shows the 13 C CP solid-state NMR spectra of MG-derived imogolite samples after 3 (Figure 6b) and 10 (Figure 6c) days aging. In a classical imogolite synthesis, one would not expect the presence of any major carbon species; here, both samples show the classical fingerprint of the carbon backbone of MG, as compared to the 13 C solution spectrum of this compound (Figure 6a). Close comparison between solution and solid-state spectra depicts some important differences: (i) The glycerol moiety (between 60 and 75 ppm) of MG is lost during the synthetic process, as indicated by the lack of peaks n°1-3 in the solid-state spectra (Figure 6b,c). This is probably due to the hydrolysis of the ester bond between lauric acid and glycerol due to the acidic conditions used at the stage of AlCl3 addition.

(ii) The ester group in MG has a typical chemical shift value of δ = 175 ppm (Figure 6a). The chemical shift of the same group in the imogolite samples is now around δ= 180 ppm, suggesting that the ester has been hydrolyzed into carboxylate groups COO -. This might result from the neutral pH conditions used for TEOS condensation and/or from complexation with Al 3+ ions.

(iii) The chemical shift of peak n°5, corresponding to the α-CH2 with respect to the ester function, shifts from 34 ppm (Figure 6a) to 37 ppm (Figure 6b,c), which may be due to the change in the chemical surrounding occurring on the carbonyl group. For instance, values at 37 ppm were reported for α-CH2 group of aliphatic esters. 15 Additionally, the relative intensities of peaks n° 5, 13 and 6 are affected at various aging times in imogolite. It was previously reported that adsorption of SDS on alumina provokes relative intensities variations with increasing SDS, but the authors could not provide a clear explanation. 16 This point is still to be clarified.

Investigation of the hybrid interface

13 C CP MAS NMR experiments show that MG undergoes hydrolysis during the material forming process.

Glycerol, being water-soluble, is washed away and the remaining fatty acid alone (lauric acid) in its basic form is retained in the final material. Possible conformational changes and the unshielded value δ= 180 ppm for the COO -group suggest that MG lays on the nanotube surface. This idea is confirmed by 29 Sior 27 Al-filtered 1 H NMR solid-state experiments using both 1 H{ 29 Si{ 1 H}} double cross polarization and 1 H{ 27 Al} cross polarization. The double cross polarization sequence uses the proton bath to polarize all atoms through an initial CP step at an optimized contact time, tCP1; then, 29 Si magnetization is transferred to its nearest proton neighbors by employing a second, variable, contact time, tCP2. The shortest the tCP2 value, the closest 29 Si-to-proton distance is actually explored.

Figure 7a shows the single-pulse and 1 H{ 29 Si{ 1 H}}double-CP experiments on the MG-derived sample after 3 days aging time. In the single pulse experiment, the methylene (CH2)x chain and the CH3 goups are detected between 0.5 and 2 ppm while H-bonded H2O is observed at 5 ppm. For double-CP experiments, the 29 Si{ 1 H} contact time (tCP1) is set to 3 ms while the second contact time (tCP2) varies between 500 µs and 10 ms. These values are optimized to explore both close and large internuclear distances. At all tCP2, only the H-bonded water (5 ppm) is detected while no MG-related signal is observed. The fatty acid is therefore not in close proximity with silicon, which excludes the direct functionalization of silicate layers of imogolite. However if an additional 10 ms delay time is set after cross-polarization, favoring uncontrolled long-range spin-spin interactions, the CH2 backbone signal is observed. The residual fatty acid is therefore intimately mixed with the imogolite sample and it is not phase separated. As a complementary experiment, the proton signal was also detected after a 1 H{ 27 Al} cross polarization step. 1 H{ 27 Al} CP experiments provide a clear-cut answer on the localization of the residual fatty acid. The proton signal related to the fatty acid backbone (Figure 7b) appears at tCP2= 500 µs and its intensity increases continuously up to 10 ms. Additionally, this experiment partially filters the SiOH signal with respect to AlOH, δ= 3.6 ppm. The interpretation of these experiments is quite straightforward: the fatty acid is very close to the Al 3+ centre, undoubtedly chelated by the carboxylate group (that cannot be detected directly via 1 H NMR experiments). These experiments are consistent with the specific nanotube morphology and structure parameters of imogolite, whose silicate layer is localized inside the tube while the gibbsite sheet is outside. Chemical and steric considerations help understanding that the C12 fatty acid preferentially complexes aluminium atoms on the nanotube external surface rather than the inner silicon atoms. Indeed, the inner tube diameter is only 1 nm, thus smaller than the theoretical length

(1.6 nm) of the lauric acid molecule in its all-trans configuration. 17 All together, these data suggest that laurate residues functionalize the external surface of the nanotubes, providing an hydrophobic character to the whole sample, in agreement with our initial observations (Figure 1).

Insights in the reaction mechanisms.

Previous experiments show that MG hydrolyzes during reaction and its laurate moiety finally ends as complexing agent for Al 3+ sites. In order to verify the role of glycerol versus laurate in the formation mechanism, we have performed three test experiments with pure lauric acid, pure glycerol and a mixture of lauric acid and glycerol at 10 days aging time. Figure 8 shows the wide angle X-ray diffraction patterns obtained for these samples: both samples containing lauric (b,c) acid have the typical pattern of imogolite nanotubes, whose packing is, once again, constituted of a 2D arrangement of nanotubes as shown by the diffractions peaks below 2θ= 15° and where the inter-tube distance, d, corresponds to 3.03 nm. On the contrary, no evidence of stacked imogolite nanotubes is obtained when glycerol is used alone (Figure 8a). These data are confirmed by the TEM image of the lauric acid-derived sample in Figure 8, showing a long-range order nanotube packing with an inter-tube distance of d = 3.2 nm, consistent with XRD data. These experiments support previous data on the MG system and indicate that lauric acid plays a key role in the formation of imogolite.

The synthesis of imogolite nanotubes has been reported so far under several conditions. 4,5 In general, good quality imogolite can be obtained at millimolar Al 3+ concentrations in presence of TEOS as silica source, two factors that contribute to favor hetero-(formation of Al-O-Si bonds) with respect to homocondensation (formation of Al-O-Al and Si-O-Si bonds). Functionalization of the imogolite outer surface could only be achieved by post-synthesis functionalization with phosphonic acids. In our system, we combine relatively high Al 3+ concentration probably due to the stabilizing action of the surfactant. In a second stage, when proto-imogolite forms, laurate strongly interacts with gibbsite sheets, thus favoring the formation of curved surfaces, already imposed by the silicate tetrahedra. 5a,18 Laurate-functionalized imogolite nanotubes are then formed. Finally, side-by-side interactions between nanotubes are assisted by van der Waals interactions between laurate alkyl chains, that favor nanotube packing. At the moment, even if we do not have a clear explanation of the reason why packing occurs only within a plane and not in the 3 rd dimension, providing so-called "raft-like" objects, we can nevertheless formulate some hypothesis.

In the domain of carbon nanotubes, it was shown that packing (hexagonal vs. collier-like) can be influenced by their chemical surrounding. [START_REF] Needleman | Proc. Nat. Acad. Sci[END_REF][START_REF] Jeong | [END_REF] In particular, Jeong at al. [START_REF] Jeong | [END_REF] have shown that single-walled carbon nanotubes can pack in two-dimensional plate-like sheets using a smectic liquid crystal template. In our system, the amount of MG is not enough to justify the formation of a liquid crystal. Nevertheless, in the field of surfactant-templated mesostructured silica and aluminosilica materials, the formation of lyotropic hybrid organic/inorganic phases occurs both under a liquid crystal like approach (high concentration of surfactant) 21 and under dilute conditions, where self-organisation and precipitation is driven in solution by the cooperative formation of inorganic-organic interfaces, as proposed by Monnier et al. 22 Chmelka and co. also reported on the formation of mesostructured aluminosilicate by the same approach and, interestingly, their advanced solid state NMR study showed that incorporation of aluminum in the inorganic framework did not induce a 29 Si chemical shift at -78 ppm, 23 which supports the fact that in our case the formation of a classical mesoporous aluminosilicate is most-likely excluded.

Plate, "raft" and nanosheets (nomenclature varies according to the authors) aluminosilicate materials have been described earlier. Barret 24 and Zhao, 25 for instance, reported, respectively, about the formation of pure and Ga-doped boehmite "rafts". Nevertheless, in their case, the "raft" population was described as being uniform in size and composed of either "units" or layered materials; in particular, XRD did not show any small-angle reflection at all, excluding any long-range order in the mesoscale range. In our work, the 2D plates are not homogeneous in size and are composed of nanotubes of different length, as clearly shown by TEM experiments at different tilt angles (Figure 5 e,f). Interestingly, "raft"-like objects have been reported for imogolite before, 26 and in particular several authors have pointed out the liquid crystal behavior of imogolite nanotubes at high concentration, as predicted by the Onsager and Flory theory. 26c

In our case, we think that the plate-like sheets composed of imogolite nanotubes is driven by a synergic interaction between the laurate and the imogolite nanotubes and assembly may occurs during the drying process at the liquid/air interface, as predicted by several authors for this type of objects. 27 Scheme 1 proposes a picture summarizing the main achievement of this work. In order to describe properly the sharp small-angle reflections in the XRD pattern, one has to assume that each plate contains a large number of nanotube units. Finally, uncontrolled side reactions, probably due to non-optimized MG/Al/Si stoichiometry, lead to the formation of boehmite.

Scheme 1. Schematic side and longitudinal tentative representation of the hydrophobic imogolite nanotubes. The outer surface of the tube is composed of gibbsite sheets whose Al 3+ sites are complexed by lauric acid. The longitudinal view is adapted from Ref. 5e. The size of the plates in this drawing is not to scale. In particular one has to consider the presence of a large number of nanotubes in order to account for the sharp XRD reflections observed in the small-angle range.

Conclusions

Among inorganic nanotubes, imogolite represents a very unique case due to its natural occurrence.

Therefore, in contrast to other metal oxide phases whose morphology and dimensions can be controlled by templating approaches, 28 the addition of organics is not expected to change the structure of individual nanotubes but may control their kinetics of formation and their packing, as demonstrated here. Of particular interest is the possibility to achieve functionalization of the imogolite surface in one-step and achieve a new type of nanotube arrangement, never observed before for imogolite; additionally, this is done in a short time and starting from high inorganic precursor concentration. This was possible by the selection of a surfactant molecule that exhibits good affinity for the aluminium species, both in the initial precursor solution and within the aluminosilicate framework. An additional, unexpected outcome of this specific affinity is that a direct surface functionalization of the nanotube is occurring, turning the hydrophilic aluminosilicate particles into a solvent-dispersable powder. Together with the fact that our synthetic pathway complies with several key requirements of green chemistry principles applied to inorganic materials (solvent-free, near neutral pH, biosurfactant, use of chloride instead of perchlorate aluminium salt,..), 29 it can be expected that these results open new perspectives for the use of imogolite as a functional filler in the field of (nano)-composite materials. 30

Figure 2 .

 2 Figure 2. Powder X-Ray patterns of MG (b,d,f,g) and MG-free (a,c,e) alumina-silica samples obtained after 3 days (a-d) and 10 days (e-g) aging time: (a,b) Al/Si = 0.65; (c,f) Al/Si = 2.14; (g) Al/Si = 2. Small and wide angle XRD of MG-Al/Si= 2.14 (10

Figure 3 .

 3 Figure 3. (a-d)29 Si CP and (e-h)27 Al MAS solid state NMR experiments performed on MG-derived imogolite samples after 3 days aging and at different Al/Si ratios.

  ,b) show that MG-derived powder is composed of a bundle of fibers whose characteristic dimensions, length L > 50 nm and diameter d = 2.5 nm, confirm them as imogolite nanotubes. The SAED pattern extracted from Figure 4a is given in Figure 4c. The dspacings calculated from the analysis of the diffraction rings numbered from (1) to (4) provide the following values: d(1)= 0.32 nm, d(2)= 0.21 nm, d(3)= 0.18 nm, d(4)= 0.14 nm. d(1), d(2) and d(

Figure 4 .

 4 Figure 4. (a, b) Transmission electron microscopy images obtained for MG-derived imogolite (tubular morphology); c) SAED pattern obtained on image displayed in (a); (d) MG-free proto-imogolite (particle morphology). The Al/Si ratio is 2.14 and aging time is 3 days for all samples.

Figure 5 .

 5 Figure 5. Transmission electron microscopy images obtained for (a-c) MG and (d) MG-free imogolite samples upon 10 days aging at Al/Si= 2.14. (e-f) Highlight on images observed at two different tilt angles of the sample holder with respect to the electron beam z-direction: 0° for (e) and +20° for (f). Arrows in both figure put in evidence the individual nanotubes before and after tilting showing no change in their thickness.

Figure 6 .

 6 Figure 6. 13 C solution NMR spectrum of MG (a) and 13 C CP MAS NMR spectra of MG-derived imogolite upon 3 (b) and 10 days (c) aging. Highlighted regions are discussed in the text. § refers to CDCl3. *: Spinning Side Bands

Figure 7 .

 7 Figure 7. (a) 1 H{ 29 Si{ 1 H}} double CP MAS NMR (B0= 7.05 T; νMAS= 14 kHz) spectra of MG-derived imogolite upon 3 days aging time. tCP1= 3 ms while tCP2 varies as indicated next to each spectrum. (b) 1 H{ 27 Al} CP MAS NMR (B0= 16.45 T; νMAS= 23 kHz) spectra of MG-derived imogolite upon 3 days aging time. Contact times are provided next to each spectrum. The corresponding single pulse 1 H MAS NMR spectra, recorded under comparable conditions (B0 and νMAS) for experiments shown in (a) and (b), are shown at the bottom of each set of experiments.

Figure 8 .

 8 Figure 8. Wide angle X-ray diffraction patterns of (a) glycerol, (b) Lauric acid (LA) and (c) glycerol/LA derived samples obtained after 10 days aging time (asterisks indicate the peaks due to boehmite structure). Values are given in nm and correspond to associated d-spacings. (C) TEM image of lauric acid-derived imogolite at 10 days aging time.

29

  Si-1 H double CP were performed on a Bruker AVANCE III 300 (7.05 T) spectrometer. Details for double CP experiment are given in ref 11. The 27 Al-1 H CP experiments were performed on a Bruker AVANCE III 700 (16.45 T) spectrometer using a 3.2 mm rotor spinning at 23 kHz.

Acknowledgements B. T. thanks the Conseil Régional d'Ile-de-France for his post-doctoral fellowship (Allocations postdoctorales Hors DIM "Chimie Verte et Eco-matériaux"). M. Selmane, G. Mosser and P. Le Griel (LCMCP, Paris, France) are acknowledged for their help in XRD, SAXS and TEM experiments. We thank Prof. JP Jolivet, C. Chanéac (LCMCP, Paris, France) and Dr C. Levard (CEREGE, Aix-Marseille,

Entry for the Table of Contents

One-step functionalized imogolite single wall nanotubes, obtained from centimolar precursor solutions within 10 days at neutral pH using a biosurfactant-mediated route, demonstrate an unusual 2D lamellar order packing and highly-demanded hydrophobic surfaces.

Paris, France) for enlightening discussions. This project was supported by an AIC CPDD-RDR1 from CNRS/INRA. We thank reviewer one for the helpful comments.

Supplementary data


Yosra Hichri
email: yosra.hichri@ifsttar.fr

Véronique Cerezo

Minh-Tan Do

Friction on road surfaces contaminated by fine particles: some new experimental evidences

Keywords:

Relevance of the test results

Acknowledgment

Role of mass of particles on lubrication mechanisms

Rain simulator

• Contaminants can be originated from different sources: temporary (pollution due to road works), seasonal (road salting in winter), accidental (hazardous chemicals) or chronic (vehicle exhaust, pavement and tire wear) [START_REF] Legret | Evaluation of pollutant loadings in the runoff waters from a major rural highway[END_REF].

• Research focused on their impact on environment : impact on road safety is missing.

• Accidents increase sharply during the first rain after a long dry period [START_REF] Eisenberg | The mixed effects of precipitation on traffic crashes[END_REF]. This trend is due to the accumulation of fine particles that contaminate the road surface.

• The friction drop is attributed to the high-viscosity mix of rain water and road debris [START_REF] Persson | Rubber friction on wet and dry road surfaces: the sealing effect[END_REF].

• Data from the three tested particle concentrations follow the same "master" trend.

• Friction starts from a minimum value (shearing mechanism) then increases with successive passes of the rubber slider (sliding mechanism with partial contact between the rubber slider and the surface asperities).

• The trapped particles can explain why the friction coefficient at the end of dry tests is lower than that of a clean surface.

The experimental program (contaminant collecting and preparation, laboratory measurements, etc.) was performed by Patrick Maisonneuve, Christophe Ropert and Jean-François Le Fur from IFSTTAR. • Friction values can be very low even when the surface is dry.

Effect of initial concentration of contaminants on friction

• Friction is lowest at the beginning then increases progressively with increasing number of slider passes.

• Further passes do not help to recover the value of a clean surface.

• Higher particle concentration induces lower friction values.

• Washing process starts rapidly for a concentration of 1 mg/cm 2 (from step 2). The time needed to wash the particles increase with the quantity of deposited particles.

• Water induces a viscous lubricant which reduces the tire traction.

• It removes the particles by first dislocating them and then removing them by washing and restores the friction value of a clean surface.

• A "critical" water mass (minimum friction irrespective of initial particle concentration) was found. The cohesion of the mixture is maximum and then the rubber pad slides on a rigid contaminant layer without any (or few) resistance due to heaping of particles • Adding water to a dry granular material (particles) can increase first their cohesion due the formation of water bridges between the particles then, when water saturates the space between the particles, decrease this cohesion [START_REF] Goldszal | Wet agglomeration of powders: from physics toward process optimization[END_REF].

The test protocol reproduces qualitative trend found in the literature [START_REF] Persson | Rubber friction on wet and dry road surfaces: the sealing effect[END_REF].

The simulated curve includes all phases involved in the variation of friction during dry phase (before rain), wet phase (during rain) and drying phase (after rain).

Literature [3]

Laboratory results


Guilhem Grimaud
email: guilhem.grimaud@ensam.eu

Nicolas Perry

Bertrand Laratte

Life Cycle Assessment of Aluminium Recycling Process: Case of Shredder Cables

Keywords: Life Cycle Assessment, recycling, aluminium, metal refining

Life cycle impact of European generic primary and secondary aluminium are well defined. However specific recycling processes are not available in literature. In this study, the environmental assessment of cable recycling processing is examined. The data come from a recycling plant (MTB Recycling) in France. MTB process relies only on mechanical separation and optical sorting processes on shredder cables. On the one hand, the study demonstrates huge environmental benefits for aluminium recycled in comparison with primary aluminium. On the other hand, the results show the harmful environmental influence of the heat refining by comparison with cold recycling process.

Introduction

The purpose of this study is to document the environmental impact of a recycling aluminium process, using the Life Cycle Assessment (LCA) methodology. Today, the life cycle impact of European generic primary and secondary aluminium are well defined through the work of the European Aluminium Association (EAA) [START_REF]Environmental Profile Report for the European Aluminium Industry 2008[END_REF]. However specific recycling processes are not available in literature. In this study, the environmental assessment of cable recycling processing is examined. The data come from a recycling plant (MTB Recycling) in France. The specific and innovative process was developed by MTB Recycling engineers and is sold as a process solution in different countries. The specificity of MTB process relies on the absence of fusion for metal refining. Nevertheless, it reaches standard aluminium purity up to 99.6%. This performance is obtained using only mechanical separation and optical sorting processes on shredder cables. Environmental impact assessment is done using ILCD Handbook recommendations [START_REF]The International Reference Life Cycle Data System (ILCD) Handbook[END_REF]. Three systems are compared: European primary aluminium data from EAA aggregated in Ecoinvent 3.1, secondary aluminium from European remelter data from EAA aggregated in Ecoinvent 3.1 and MTB cable recycling process.

On the one hand, the study demonstrates huge environmental benefits for aluminium recycled in comparison with primary aluminium. On the other hand, the results show the harmful environmental influence of the heat refining by comparison with cold recycling process. The study demonstrates the interest of recycling by sector rather than in blend.

The European demand for aluminium has been growing over the past few decades at a rate of 2.4% per annum [START_REF] Bertram | Aluminium Recycling in Europe[END_REF]. This increase is mainly supported by the rise of recycling which growth was in the same time about 5% per annum [START_REF] Bertram | Aluminium Recycling in Europe[END_REF]. The abundance and the versatility of aluminium in various applications have made it one of the top solutions for lightweight metal strategy in various industries [START_REF] Liu | Addressing sustainability in the aluminum industry: a critical review of life cycle assessments[END_REF]. In the cable industry, substitute copper for aluminium can considerably reduce the linear weight without degrading too much the electrical properties [START_REF] Bruzek | Eco-Friendly Innovation in Electricity Transmission and Distribution Networks[END_REF]. To obtain optimal electrical conductivity, aluminium use for cables has a purity above 99.7% [START_REF] Goodwin | Metals[END_REF]. Because secondary aluminium does not meet the quality requirements for aluminium cables manufacturers; only primary aluminium is used for the aluminium cables supply chain. Nevertheless, improvement in recycling could help reach quality targets, by using new sorting technologies.

Aluminium properties are not deteriorated by recycling. However, in most cases aluminium parts are mixed together at the end of life step without considering their provenance and use. According to this, the 7 series of aluminium are mixed together in waste treatment plant. All aluminium series do not have the same purity and alloying elements pollute aluminium. When aluminium series are mixed together, the cost-effective solution for refining use furnaces. As the metal is molten, the separation is done by using the difference of density and buoyancy (decantation methods, centrifugation, filtration, flotation, etc.). [START_REF] Rombach | Future Potential and Limits of Aluminium Recycling[END_REF] Despite the technology optimisation, a fraction of metal is unrecyclable [START_REF]Global Aluminium Recycling : A Cornerstone of Sustainable Development[END_REF]. Some alloying elements are lost in the process [START_REF] Paraskevas | Sustainable Metal Management and Recycling Loops : Life Cycle Assessment for Aluminium Recycling Strategies[END_REF] and this results in a drop of quality which is akin to a down-cycling [START_REF] Allwood | Squaring the Circular Economy : The Role of Recycling within a Hierarchy of Material Management Strategies[END_REF]. The solution lies in a better separation of aluminium series upstream from the recycling chain. This strategy should enable products to be guided through the best recycling path and maintain the quality of alloys. The cable is composed of an aluminium core cable (a), covered with a polymer thick layer (b) as illustrated in Fig. 1. Additional metallic materials (c) are coaxial integrated in order to finish the definition of this composite product. These cables are manufactured by extruding together all the materials that compose it.

Aluminium in cables represents between 35 and 55% of the total weight. Other metals are mainly steel, lead, copper, zinc. The variety of plastics contained in the sheath is even stronger than for metals: silicone rubber, Polyethylene (PE), crosslinking PE (xPE), Polychloroprene (PCP), vulcanised rubber, Ethylene Vinyl Acetate (EVA), EthylenePropylene Rubber (EPR), flexible PVC, etc. [START_REF]Union Technique de l'Électricité (UTE), UTE C30-202 -Insulated cables and flexible cords -System for cable designation[END_REF] Although aluminium cables represent about 8% of aluminium products in Western Europe [START_REF] Eaa | Aluminium for Future Generations : Progressing through Brussels[END_REF]. The inherent purity of aluminium used for cables justifies differentiate recycling channels to optimise processing steps and improve cost efficiency. At the end of life, the challenge concerns the separation of materials from each other. The most economical way to separate different materials relies on a smelting purification.

An alternative process for cables recycling uses only mechanical steps instead of thermal and wet separation, as developed for several years by MTB Recycling. The aluminium obtained by recycling cables is specially appreciated by the smelter. Its high purity makes it easy to produce a wide variability of aluminium alloys. Recycled aluminium can then be used in a large number of aluminium products and not only in applications requiring high alloy aluminium.

Numerous studies were conducted concerning the sustainability of aluminium recycling in comparison with primary aluminium. Outcomes about global and local environmental impacts show decrease up to 90% by using recycled aluminium [START_REF] Bertram | Aluminium Recycling in Europe[END_REF], [START_REF] Eaa | Sustainability of the european aluminium industry[END_REF]. However, systems modelling always relate to the standard melting solution for recycling aluminium. In contrast this study focuses on the environmental assessment of cable recycling in MTB specific process.

The objective of this study is, therefore, to examine the environmental benefits and energy savings of the MTB recycling system. Additionally, LCA was conducted in order to help the company to highlight environmental hotspots of the system and try to design new solutions to decrease environmental impact of aluminium produced. The data collection method does not allow the use of the results for other cables recycling processes. The results are representative only of recycling solutions developed by MTB.

Analytical Framework

Functional Unit Proposal

As part of this study, the functional unit used is as follows: producing one ton of aluminium intended for end-user applications, with a purity of > 97% using current industrial technologies (annual inbound processing > 10,000 t) located in Europe. The matching quality of the compared products can meet the same function as a high purity aluminium can be used for producing a large number of alloys without refining. We selected three scenarios that meet all the conditions of the functional unit:  Scenario 1 or primary: primary aluminium, resulting from mining.  Scenario 2 or secondary: secondary aluminium from recycling by melting.  Scenario 3 or MTB: MTB Aluminium, from recycling using MTB processes.

The primary aluminium production is used as a reference for guidance on the quality of production. Foremost, our analysis is intended to compare methods of recycling. Comparison with scenario 1 should help translate environmental benefits of recycling.

Sources of Data for the Life Cycle Inventory

The evaluation is designed by modelling input and output flows that describe different systems of aluminium recycling with the software SimaPro 8.04 [START_REF] Goedkoop | Introduction to LCA with SimaPro[END_REF]. All the flows are based on processes from Ecoinvent 3.1 library [START_REF] Wernet | Introduction to the ecoinvent version 3.1 database[END_REF]. The systems are developed according to the local context of Western Europe. To allow comparison all the inventory elements are compiled based on the Ecoinvent database boundaries and data quality check [START_REF] Weidema | Ecoinvent database version 3 -the practical implications of the choice of system model[END_REF], [START_REF] Weidema | Data quality guideline for the ecoinvent database version 3[END_REF]. Once modelling were done, the characterisation is conducted according to International Reference Life Cycle Data System (ILCD) Handbook [START_REF]The International Reference Life Cycle Data System (ILCD) Handbook[END_REF] recommendations. The following sections detail key aspect of the methodology.

This study compares two different modelling systems. Both systems modelling using Ecoinvent data. Scenarios 1 and 2 using available data in Ecoinvent library without any a b c modifications. And scenario 3 using Ecoinvent data to model MTB recycling process, the inventory data set was done using the inventory data sets recommendations from JRC [START_REF]Specific guide for Life Cycle Inventory data sets -EUR 24709 EN[END_REF]. The three modelling rely on the same system boundary.

Presentation of the Study Scope

This study is based on a life cycle approach, in accordance with the standards of International Organisation for Standardisation (ISO 14010/44) [START_REF]International Standard Organization, ISO 14040 -Environmental management -Life Cycle Assessment -Principles and Framework[END_REF], [START_REF]ISO 14044, Environmental management -Life cycle assessment -Requirements and guidelines[END_REF]. The Fig. 2 presents the study scope used for the life cycle analysis of MTB recycling process, the boundaries are based on the Ecoinvent modelling. The boundaries include cradle to exit gate stages [START_REF] Jolliet | Analyse du cycle de vie: Comprendre et réaliser un écobilan[END_REF], [START_REF] Grisel | L'analyse du cycle de vie d'un produit ou d'un service: Applications et mises en pratique[END_REF]. Life in use of aluminium in the products are not included in our study scope. The study only focuses on recycling transformation steps of aluminium. As shown on the Fig. 2 by-products are included in environmental impacts calculation, but no benefit of by-products recycling is integrated into the study. Cycle Inventory (LCI) [START_REF] Althaus | [END_REF]. For Ecoinvent v3.1 [START_REF] Moreno | Documentation of changes implemented in ecoinvent database 3.0[END_REF][START_REF] Moreno | Documentation of changes implemented in ecoinvent -Version 3[END_REF], the Aluminium processes are built with data collected by EAA in 2013 [START_REF]Aluminium Mass Flow Europe 2013[END_REF], [START_REF]Environmental Profile Report for the European Aluminium Industry April 2013[END_REF]. The Ecoinvent modelling using the average technology available on the market for Western Europe [START_REF] Weidema | Data quality guideline for the ecoinvent database version 3[END_REF].

The Fig. 3 presents the main steps taking into account in each scenario for the comparison. For scenarios 1 and 2, the final product is aluminium ingots, while for scenario 3 the final product is aluminium shot. In any case, scenarios meet the functional unit.

Scenario 1: Primary Aluminium

The Fig. 3 presents the intermediate steps required to obtain aluminium primary ingots (and included in the modelling) for primary aluminium datasets. The various steps shown in Fig. 3 can be carried out in multiple locations. This results in transport between sites, in the case of the study the total distance travelled is 11 413 km (11 056 km by sea, 336 km by road and 21 km by train).

The scenario for primary aluminium comes from Ecoinvent data. The data used for the study is aluminium production, primary, ingot. In this case, the final transport to the market is not taken into account, but all the upstream transformation steps are included in the boundaries. The choice falls on this data also for the aluminium purity. At this stage of the production process, the aluminium contains only 1,08% silicon and the overall purity is 98,9.

The modelling of primary aluminium is based on the average of primary aluminium production for the European market. The technologies take into account correspond to the up-to-date technologies used in Europe. The electricity mix used by the primary aluminium industry is a specific electricity mix. Modelling this mix relies on the compilation of specific data for all European primary aluminium producers. This mix is made up with over 80% from hydroelectric power, 8.7% of electricity from nuclear and the remaining part comes from fossil fuel.

Scenario 2: Conventional Aluminium Recycling

Scenario 2 provides the modelling of the traditional aluminium recycling solution. This scenario is based on shredding steps and melting purification step made by refiners. As for scenario 1, the scenario 2 is based on European remelter average values. The data was compiled by the EAA and provided in Ecoinvent database. The electricity mix used in the modelling is equivalent to the electricity mix provide by the European Network of Transmission System Operators for Electricity. It is mainly fossil fuel (48,3%), nuclear power (28,1%) and renewable energy (23,6%). The distance of transport takes into account for the scenario 2 is 322 km (20 km on water, 109 km by train and 193 km by road).

In Ecoinvent, there are 2 data collections. One data collection was done for production scraps (new scrap) and the other one for post-consumer scrap (old scrap). The processes used for recycling new and old scraps are not the same. New scrap needs less operation than old scraps. The inbound logistics is also different because some of the waste is recycled directly on production plants. For the study the ratio between old and new scrap is based on European aluminium mix [START_REF]Aluminium Mass Flow Europe 2013[END_REF]. In 2013, old scrap represents 46,3% of aluminium recycled in Europe and new scrap 53,7%. After the recycling process, there are 2 outlets possible: wrought or cast aluminium. For the study, the choice falls on wrought aluminium because it has sufficient purity required by the functional unit (97%). The data chosen for the study is Aluminium, wrought alloy {RER}| Secondary, production mix.

Scenario 3: MTB Cables Recycling

An intensive inventory analysis was developed during an internal survey conducted in collaboration with EVEA consultants firm at MTB Recycling plant during autumn 2014. Foreground data are based on measurement and on stakeholder interviews. Collection of background data comes from Ecoinvent v3.1 or relevant literature.

For this scenario, the distance of transport takes into account is 540 km for old scraps and 510 km for new scrap from various cable manufacturers. The intrinsic aluminium quality reaches at least 99.6% of aluminium purity (average quality check during the period 2012-2014).

MTB Recycling has an environmentally friendly strategy at a strategic level. As a consequence, they subcontracted with an energy provider that ensures an electricity mix from renewable energy source. Electricity comes almost exclusively from hydroelectric power (96,92% from alpine reservoirs and 2,4% from run of river). The remaining electricity comes from waste to energy plants (0,51%) and from cogeneration plants (0,17%). The Table 1 presents the models of selected indicator of the Life Cycle Impact Assessment (LCIA) method. The two models in italics in the Table 1 are models replace compared to the recommended ILCD 2011 impact assessment methodology [START_REF]Characterisation factors of the ILCD Recommended Life Cycle Impact Assessment methods -EUR 25167[END_REF], which was used throughout the study. The ILCD method is used with 2 modifications on calculation factors:  Human toxicity, non-cancer effects  Water resource depletion A sensitivity analysis on the characterisation method was conducted using the ReCiPe Midpoint v1.1 method and CML IA Baseline v3.01 methods. This analysis has not yielded conflicting results.

Results

Impact Assessment Method

Comparison of the 3 Scenario

The Fig. 4 draw the comparison for the three scenarios. The calculations were made using the specific electricity mix for each scenario. As expected the scenario 1 emerges as far more significant on all indicators with the exception of freshwater eutrophication where recycling aluminium (scenario 2) takes the lead. The scenario 2 (Al secondary) has the highest impact, even higher than primary aluminium (scenario 1) on the freshwater eutrophication impact category as can be seen in Fig. 4, because it requires the addition of alloying metals to aluminium production. The addition of alloying elements is required to supply the market with aluminium alloys that meet the market constraints. Copper is the principal alloying element modelled in Ecoinvent 3.1, indeed production chain requires sulphuric tailing representing 96.4% of the impact on the freshwater eutrophication impact category. This result seems to be a modelling error into Ecoinvent 3.1. The study does not account this result from LCA to draw conclusions. Average secondary aluminium reaches approximately 10% of the impact of the primary aluminium scenario. And MTB aluminium shot is close to 5% of the primary aluminium impact on all the set of indicators. Those results correspondent to evaluation already done and meet the values given by the Bureau of International Recycling (BIR) for aluminium recycling benefits [START_REF]A Report on the environmental benefits of recycling -A critical review of the data for aluminium[END_REF]. On the set of indicators, the impact of scenario 3 does not exceed the impact of scenario 2. In addition, the impact of MTB recycling scenario represents between 2% and 46% of the impact of recycling by melting. The average impact of the solution is halved.

Uncertainty Analysis for Recycling Scenarios

An uncertainty analysis was conducted between the 3 scenarios. With the specific electricity mix, the uncertainty between scenarios 2 and 3 do not exceed 5% on all the indicators. Except human toxicity indicators (8%) and the water resource depletion indicator (45%). With equivalent electricity mix, the uncertainty exceeds 5% on 3 indicators: ozone depletion (11%), human toxicity, non-cancer effects (9%) and water resource depletion (45%). The conclusions of the study are strong with respect to the weak uncertainties on the characterisations.

Details About MTB Recycling Process

The Fig. 6 shows the results for the characterisation of the MTB aluminium shot, with the specific renewable electric mix used by MTB. The values used for representation in Fig. 6 are given on the figure. The results show a very strong contribution from the upstream transport for the collection of waste in the total impact of the scenario. On the set of indicators, the MTB recycling steps represents between 11,4% and 79,7% of the total impact. The average of the 11 indicators brings up an average impact of 36,1% and a median of 33,0%.

When characterisation uses the European electricity mix, all the recycling stages of MTB scenario represents on average 50% of the total impact on the set of indicators.

Discussion and Conclusion

The shredding steps are on average 2 times more impacting than mechanical separation steps. Work on the efficiency of the shredder is necessary to reduce the electricity consumption of this step. However, LCA also allowed showing the shredder consumables (steel blades and screens) as elements with a high impact in proportion to their mass. MTB and subcontractors have launched tests with new alloys to increase longevity of screens and blades.

This study allows us to establish a hierarchy between environmental recycling solutions for aluminium cables. Whatever the electricity mix used by the recycling plant, the MTB mechanical recycling process is the most environmentally friendly.

This study also highlights the need to develop green recycling processes. MTB Recycling scenario has a black dot, plastics from the cable sheaths are not all recycled yet. It is essential to find recycling methods for these plastics. Following this study, the company has initiated a development approach to sort and recycle these plastics. A first prototype was developed in late 2015.

This study also demonstrates that recycling when driven without loss of quality is a relevant alternative to mining.

Fig. 1

 1 Fig. 1 Section of a cable having multiple beams of aluminium.

Fig. 2

 2 Fig. 2 Study scope for the cable recycling system boundaries

Fig. 3

 3 Fig. 3 Presentation of the 3 scenarios and main step of the production process

Fig. 4

 4 Fig. 4 Characterisation of the 3 scenarios with specific electricity mix4.3. Recycling Scenario Comparison

Fig. 5

 5 Fig. 5 Comparison of the characterisation of the 2 recycled scenarios, equivalent electricity mix In previous characterisation, the difference between scenarios 2 and 3 are not clearly shown on the representation. The Fig. 5gives the opportunity to compare more specifically the two recycling scenarios. The same values are used for the representation of Fig. 4 and Fig. 5. On the Fig. 5, the impacts are still presented using the specific electricity mix for the 2 recycling scenarios.

Fig. 6

 6 Fig. 6 Characterisation of MTB aluminium shot with purity of 99.6% specific electricity mix

Table 1

 1 Indicators selected for the life cycle impact assessment[START_REF]ILCD Handbook: Recommendations for Life Cycle Impact Assessment in the European context[END_REF]

	Indicators	Model
	Climate change	Baseline model of 100 years of the
		IPCC
	Ozone depletion	Steady-state ODPs 1999 as in WMO
		assessment
	Human toxicity, non-cancer	USEtox model v1.04 [29]
	effects
	Particulate matter	RiskPoll model
	Ionizing radiation HH	Human health effects model as
		developed by Dreicer
	Photochemical ozone	LOTOS-EUROS
	formation
	Acidification	Accumulated Exceedance
	Freshwater eutrophication	EUTREND model
	Freshwater ecotoxicity	USEtox model
	Water resource depletion	Pfister water scarcity v1.01 [30]
	Mineral, fossil & ren resource	CML 2002
	depletion

Acknowledgements

This work was performed within the help of Marie Vuaillat from EVEA Consultancy firm and with financial support from French Agency for Environment and Energy Efficiency (ADEME).


Guilhem Grimaud

Marie Vuaillat

David Ravet

Bertrand Laratte

Nicolas Perry

ANALYSE DE CYCLE DE VIE DU RECYCLAGE DE L'ALUMINIUM : ÉTUDE DE CAS DES CÂBLES

Keywords: Analyse de Cycle de Vie, ACV, Aluminium, Recyclage, Affinage des Métaux, Optimisation des Procédés Life Cycle Assessment, LCA, recycling, aluminium, metal refining, process optimisation

Le but de cette étude est de documenter l'impact environnemental d'un procédé de recyclage de l'aluminium, en utilisant la méthodologie d'analyse de cycle de vie (ACV). Aujourd'hui, l'impact environnemental des aluminiums primaires et secondaires est déjà bien défini par l'Association Européenne de l'Aluminium (EAA). Cependant, les processus de recyclage spécifiques ne sont pas disponibles dans la littérature. Dans cette étude, l'évaluation environnementale du traitement et du recyclage des câbles sont examinés. Les données proviennent de l'usine de recyclage MTB Recycling située en France. Le processus spécifique a été développé par les ingénieurs de MTB Recycling et est vendu comme solution de traitement dans les différents pays. La spécificité du procédé MTB repose sur l'absence de fusion pour l'affinage des métaux. Malgré tout, la pureté standard de l'aluminium atteint 99,6 %. Cette performance est obtenue en utilisant uniquement la séparation mécanique et des procédés de tri optique sur les câbles déchiquetés. L'évaluation de l'impact environnemental est effectuée en utilisant la méthode ILCD MidPoint. L'étude compare 3 systèmes : l'aluminium primaire, l'aluminium secondaire affiné par fusion et le processus de recyclage des câbles développé par MTB. D'une part, l'étude démontre les avantages environnementaux de l'aluminium recyclé par rapport à l'aluminium primaire. D'autre part, les résultats font apparaître la forte influence du recyclage à chaud de l'aluminium par comparaison avec le processus MTB de recyclage à froid. L'étude démontre l'intérêt du recyclage par filière vis-à-vis du recyclage en mélange.

INTRODUCTION

The purpose of this study is to document the environmental impact of a recycling aluminium process, using the Life Cycle Assessment (LCA) methodology, in accordance with the standards of International Organisation for Standardisation (ISO 14,010/44) [START_REF]-Environmental management -Life Cycle Assessment -Principles and Framework[END_REF]2]. Today, the life cycle impact of European generic primary and secondary aluminium are well defined through the work of the European Aluminium Association (EAA) [START_REF]Environmental Profile Report for the European Aluminium Industry[END_REF]. However specific recycling processes are not available in literature. In this study, the environmental assessment of cable recycling processing is examined. The data come from a recycling plant (MTB Recycling) in France. The specific and innovative process was developed by MTB Recycling engineers and is sold as a process solution in different countries. The specificity of MTB process relies on the absence of fusion for metal refining. Nevertheless, it reaches standard aluminium purity up to 99.6%. This performance is obtained using only mechanical separation and optical sorting processes on shredder cables. Environmental impact assessment is done using ILCD Handbook recommendations [START_REF]The International Reference Life Cycle Data System (ILCD) Handbook[END_REF].

Numerous studies were conducted concerning the sustainability of aluminium recycling in comparison with primary aluminium. Outcomes about global and local environmental impacts show decrease up to 90% by using recycled aluminium [START_REF] Bertram | Aluminium Recycling in Europe[END_REF][START_REF]Sustainability of the european aluminium industry[END_REF]. However, systems modelling always relate to the standard melting solution for recycling aluminium.

The objective of this study is, therefore, to examine the environmental benefits and energy savings of the MTB recycling system.

ANALYTICAL FRAMEWORK

Functional Unit Proposal

As part of this study, the functional unit used is as follows: producing one ton of aluminium intended for end-user applications, with purity above 97% using current industrial technologies (annual inbound processing> 10,000 t) located in Europe. The matching quality of the compared products can meet the same function as a high purity aluminium can be used for producing a large number of alloys without refining. We selected three scenarios that meet all the conditions of the functional unit:

 Scenario 1 or primary: primary aluminium, resulting from mining, data from EAA aggregated in Ecoinvent 3.1 [START_REF] Althaus | Life Cycle Inventories of Metals -Final report ecoinvent data v2[END_REF][START_REF]Aluminium Mass Flow Europe[END_REF][START_REF]Environmental Profile Report for the European Aluminium Industry[END_REF].  Scenario 2 or secondary: secondary aluminium from recycling by melting, data from European remelter collected by EAA and aggregated in Ecoinvent 3.1 [START_REF] Moreno | Documentation of changes implemented in ecoinvent database 3.0[END_REF][START_REF] Moreno | Documentation of changes implemented in ecoinvent -Version 3[END_REF][START_REF] Weidema | Data quality guideline for the ecoinvent database version 3[END_REF].  Scenario 3 or MTB: MTB Aluminium, from recycling using MTB processes [START_REF] Grimaud | Life Cycle Assessment of Aluminium Recycling Process: Case of Shredder Cables[END_REF].

Scenario development

The evaluation is designed by modelling input and output flows that describe different systems of aluminium recycling with the software SimaPro 8.04 [START_REF] Goedkoop | Introduction to LCA with SimaPro[END_REF]. All the flows are based on processes from Ecoinvent 3.1 library [START_REF] Wernet | Introduction to the ecoinvent version 3.1 database[END_REF]. The systems are developed according to the local context of Western Europe. To allow comparison all the inventory elements are compiled based on the Ecoinvent database boundaries and data quality check [START_REF] Weidema | Data quality guideline for the ecoinvent database version 3[END_REF][START_REF] Weidema | Ecoinvent database version 3the practical implications of the choice of system model[END_REF]. The boundaries include cradle to exit gate stages [START_REF] Jolliet | Analyse du cycle de vie: Comprendre et réaliser un écobilan[END_REF]. Life in use of aluminium in the products are not included in our study scope. The study only focuses on transformation steps of aluminium. Once modelling were done, the characterisation is conducted according to ILCD Handbook recommendations [START_REF]ILCD Handbook : General guide on LCA -Detailed guidance[END_REF][START_REF]Specific guide for Life Cycle Inventory data sets -EUR 24709 EN[END_REF]. The scenario development for this study is already present in [START_REF] Grimaud | Life Cycle Assessment of Aluminium Recycling Process: Case of Shredder Cables[END_REF]. The data collection method does not allow the use of the results for other cables recycling processes. The results are representative only of recycling solutions developed by MTB.

The Figure 1 presents the main steps taking into account in each scenario for the comparison. For scenarios 1 and 2, the final product is aluminium ingots, while for scenario 3 the final product is aluminium shot. In any case, scenarios meet the functional unit. The Figure 2 draw the comparison for the three scenarios. The calculations were made using the specific electricity mix for each scenario. As expected the scenario 1 emerges as far more significant on all indicators with the exception of freshwater eutrophication where recycling aluminium (scenario 2) takes the lead. The scenario 2 (secondary aluminium) has the highest impact, even higher than primary aluminium (scenario 1) on the freshwater eutrophication impact category as can be seen in Figure 2, because it requires the addition of alloying metals to aluminium production. The addition of alloying elements is required to supply the market with aluminium alloys that meet the market constraints. Copper is the principal alloying element modelled in Ecoinvent 3.1, indeed production chain requires sulphuric tailing representing 96.4% of the impact on the freshwater eutrophication impact category. This result seems to be a modelling error into Ecoinvent 3.1. The study does not account this result from LCA to draw conclusions.

RESULTS

Comparison of the 3 Scenario

Average secondary aluminium reaches approximately 10% of the impact of the primary aluminium scenario. And MTB aluminium shot is close to 5% of the primary aluminium impact on all the set of indicators. Those results correspondent to evaluation already done and meet the values given by the Bureau of International Recycling (BIR) for aluminium recycling benefits [START_REF]A Report on the environmental benefits of recycling -A critical review of the data for aluminium[END_REF].

The primary aluminium production is used as a reference for guidance on the quality of production. Foremost, our analysis is intended to compare methods of recycling. Comparison with scenario 1 should help translate environmental benefits of recycling.

Recycling Scenario Comparison

In previous characterisation, the difference between scenarios 2 and 3 are not clearly shown on the representation. The Figure 3 gives the opportunity to compare more specifically the two recycling scenarios, the impacts are still presented using the specific electricity mix for the 2 recycling scenarios.

On the set of indicators, the impact of scenario 3 does not exceed the impact of scenario 2. In addition, the impact of MTB recycling scenario represents between 2% and 46% of the impact of recycling by melting. The average impact of the solution is halved. Nevertheless MTB has launched an approach to reducing impacts by working on different subjects (blades, plastics processing, shredding steps, etc.) [32].

Uncertainty Analysis for Recycling Scenarios

An uncertainty analysis was conducted between the three scenarios. With the specific electricity mix, the uncertainty between scenarios 2 and 3 do not exceed 5% on all the indicators. Except human toxicity indicators (8%) and the water resource depletion indicator (45%). With equivalent electricity mix, the uncertainty exceeds 5% on 3 indicators: ozone depletion (11%), human toxicity, non-cancer effects (9%) and water resource depletion (45%). The conclusions of the study are strong with respect to the weak uncertainties on the characterisations.

CONCLUSION

This study allows us to establish an environmental hierarchy between recycling solutions for aluminium cables. Whatever the electricity mix used by the recycling plant, the MTB mechanical recycling process is the most environmentally friendly. Additionally, LCA was conducted in order to help the company to highlight environmental hotspots of the system and try to design new solutions to decrease environmental impact of aluminium produced [START_REF] Grimaud | Reducing Environmental Impacts of Aluminium Recycling Process Using Life Cycle Assessment[END_REF].

On the one hand, the study demonstrates huge environmental benefits for aluminium recycled in comparison with primary aluminium. On the other hand, the results show the harmful environmental influence of the melting refining in comparison with mechanical recycling process. The LCA revealed that the closed product loop option (considering aluminium cables) has lower environmental impact over the other recycling scenario using mixed aluminium scraps. This performance has already been demonstrated for aluminium cans [START_REF] Niero | Circular economy : to be or not to be in a closed product loop ? A Life Cycle Assessment of aluminium cans with inclusion of alloying elements[END_REF]. To conclude, recycling when driven without loss of quality is a relevant alternative to mining.

Figure 1

 1 Figure 1 Presentation of the 3 scenarios and main steps of the production process

Figure 2

 2 Figure 2 Characterisation of the 3 scenarios with specific electricity mix

Figure 3

 3 Figure 3 Comparison of the characterisation of the 2 recycled scenarios, equivalent electricity mix

Acknowledgements

This work was performed within the financial support from French Agency for Environment and Energy Efficiency (ADEME).