Melamine-derived carbon sponges for oil-water separation

Accepted Manuscript Melamine-derived carbon sponges for oil-water separation Aude Stolz, Sylvie Le Floch, Laurence Reinert, Stella M.M. Ramos, Juliette TuaillonCombes, Yasushi Soneda, Philippe Chaudet, Dominique Baillis, Nicholas Blanchard, Laurent Duclaux, Alfonso San-Miguel PII:

S0008-6223(16)30425-0

DOI:

10.1016/j.carbon.2016.05.059

Reference:

CARBON 11023

To appear in:

Carbon

Received Date: 11 March 2016 Revised Date:

24 May 2016

Accepted Date: 25 May 2016

Please cite this article as: A. Stolz, S. Le Floch, L. Reinert, S.M.M. Ramos, J. Tuaillon-Combes, Y. Soneda, P. Chaudet, D. Baillis, N. Blanchard, L. Duclaux, A. San-Miguel, Melamine-derived carbon sponges for oil-water separation, Carbon (2016), doi: 10.1016/j.carbon.2016.05.059. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Melamine-derived carbon sponges for oil-water separation

Aude Stolza,b , Sylvie Le Flocha , Laurence Reinertb , Stella M.M Ramosa , Juliette Tuaillon-Combesa , Yasushi Sonedac , Philippe Chaudetd , Dominique Baillisd , Nicholas Blancharda , Laurent Duclauxb,∗, Alfonso San-Miguela,∗ a

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Institut Lumi`ere Mati`ere, UMR5306 Universit´e Claude Bernard Lyon 1-CNRS, Universit´e de Lyon, 69622 Villeurbanne cedex, France. b Univ. Savoie Mont-Blanc, LCME, F-73000 Chamb´ery, France. c National Institute of Advanced Industrial Science and Technology, Energy Technology Research Institute, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan. d Laboratoire de M´ecanique de Contacts et des Structures UMR5259, INSA, Villeurbanne 69621, France.

Abstract

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The elaboration and characterization of hydrophobic melamine-based sponges are presented. Sponges were obtained by single-step carbonization of commercial melamine foam having a 3D interconnected network. We show that optimized sponges can be elaborated from a simple pyrolysis treatment with rather low temperatures of 500-600 ◦ C. These materials exhibited excellent absorption capacities (they absorbed 90 to 200 times their own weight), a very high porosity of 99.5%, a low density around 7 mg/cm3 and water contact angles ranging from 120 ◦ to 140 ◦ close to superhydrophobicity. The relationship between hydrophobicity and physicochemical evolution on heat treatment (carbonization process, diffusion of additives, porosity evolution) was studied in detail. The as-prepared carbon sponges are compressible up to 80% with a Young’s modulus ranging from 0.58 kPa to 0.80 kPa, and keep part of their elastic properties after a hundred compression-decompression cycles. The carbonized sponges were characterized by thermogravimetric analysis (TGA), infra-red spectroscopy (FTIR), Raman spectroscopy, elemental analysis (EA), X-ray photoelectron spectroscopy (XPS) and scan∗

Corresponding author Email address: [email protected] ; [email protected] (Alfonso San-Miguel) Preprint submitted to Carbon

May 25, 2016

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ning electronic microscopy (SEM). These characteristics make these materials promising absorbents for water depollution: oil-spill clean-ups or removal of oils and organic solvents from water, especially for the recovering of the pollutant by simple squeezing of the absorbent. Keywords: porous carbon, hydrophobic sponge, oil absorption, melamine sponge, recyclability, water depollution, oil-spill clean-up.

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1. Introduction

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With the expansion of oil production and transportation, risk of oil spills and other accidental pollution is more and more present. Several techniques exist to solve the problem of pollution including the use of chemical dispersants, and physical techniques such as combustion, pumping and creaming. Among all these methods, the use of sorbent materials to remove oil from water is considered to be one of the most efficient. Furthermore, common sorbents are low cost and made of various raw materials including (porous) inorganic materials (zeolites, activated carbons, clays), natural organic fibers (straw, wool, cotton)[1, 2, 3, 4, 5, 6, 7, 8] and synthetic organic polymers (polypropylene, alkyl acrylate copolymers)[9, 10, 11]. However, these materials have some limitations such as low absorption capacity, poor recyclability, poor selectivity and are not environmental friendly. New advanced absorbent materials have been developed to remedy these drawbacks, including polymer foams [12, 13, 14], graphene and/or carbon nanotubes sponges [15, 16, 17, 18, 19, 20], graphite foams [21], carbon foams [22, 23], ultralight carbon aerogels or hydrogels [24, 25, 26, 27], carbon materials grafted or deposited on polymer sponges [28, 29, 30], hydrophobic polymer coatings on polyurethane sponges [31, 32, 33, 34, 35], etc, with superhydrophobic properties. Recently, absorbents based on commercial melamine sponges start to be investigated [36, 37, 38, 39, 40, 41, 42, 43, 44, 45]. This melamine raw material is interesting due to its characteristics : low cost, low density, high porosity, good elasticity, flame retardancy property and environmental friendliness. Nevertheless, pristine melamine is hydrophilic and chemical modifications are needed for oil-water separation applications. Recently, several authors reported the interest of applying a pyrolysis treatment to melamine sponges under inert atmosphere in order to obtain a (super)hydrophobic absorbent. 3

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This included pyrolysis treatments at temperatures above 700 ◦ C [37] or pyrolysis pre-processing with coating additives [43, 44] or pyrolysis postprocessing with additives [40]. In this work, we present a simple, one-step, cost effective and scalable pyrolysis method to elaborate elastic hydrophobic carbon sponges from melamine foams with excellent absorption capacities. We tried to characterize and understand the effect of pyrolysis temperature on the sponge properties (structural, chemical, mechanical) to determine the best process of carbonization at low temperatures in order to obtain the best absorption capacity. Our results show that both from absorption and mechanical point of view a pyrolysis treatment limited to 500-600 ◦ C and without the introduction of additives or further processing, is sufficient to obtain hydrophobic sponges suitable for water depollution or oil-spill clean-up applications. In fact, the superhydrophobic property is not mandatory to obtain a good absorbent for oil-spill depollution. The as-produced materials were tested for oil and solvent absorption. We studied the mechanical response of the sponges as well as their chemical evolution as a function of temperature. 2. Materials and experimental section

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2.1. Materials Melamine foams, melamine-formaldehyde (MF) sodium bisulfite copolymer (Figure 1) were commercially available units manufactured by Wako R according to BASF’s patents (Basotect ) and used as raw material. The melamine structure corresponds to a s-triazine cycle where hydrogens on carbon are replaced by -NH2 groups. The commercial foam is a melamineformaldehyde-sodium bisulfite copolymer prepared from melamine mixed with formaldehyde and a blowing agent to create the porosity (such as hydrocarbons, chlorinated and/or fluorinated hydrocarbons, alcohols, esters, isocyanates, carbonates and bicarbonates with acids, etc). A hydromethylation reaction occurs [46, 47, 48] where the hydrogens of -NH2 groups are transformed in methylol groups (-CH2 OH), followed by a reticulation reaction between methylolmelamines, and results in an insoluble and infusible three-dimensional network (Figure 1). The solvents and oils used in the absorption experiments are : chloroform (≥99%, Sigma-Aldrich), dimethylformamide (≥99.8%, DMF - Riedelde Ha¨en), ethanol (≥96%), n-hexane (≥95%) and toluene (≥99.5%, Acros

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Figure 1: Chemical architecture of the polymerized melamine sponge.

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Organics); an Algerian crude oil (d = 0.76) and a commercial sunflower oil (d = 0.9). All these chemicals were used as received.

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2.2. Preparation of carbonized foams The carbon sponges were obtained by placing the MF foam in a quartz tube inside an electric furnace heated to a defined temperature between 300 ◦ C and 800 ◦ C, for 1h under nitrogen atmosphere, with a heating ramp of 10 ◦ C/min. A flow of nitrogen was used during 20 minutes before the pyrolysis to ensure a neutral atmosphere. The volume shrinkage for carbonized sponges can reach 86% of the original foam volume (Figure 2). Samples are referred to their heat treatment temperature values in ◦ C.

Figure 2: Photograph of the raw foam (white) and carbonized foams (brown and black) between 300 ◦ C and 800 ◦ C (from left to right). On top, pictures of a water droplet deposited on top of the corresponding sponge in the contact angle experiments. For raw, 300 ◦ C and 800 ◦ C samples, water was immediately absorbed by the sponge.

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2.3. Characterization techniques Physical (wetting, compressibility) and chemical (thermogravimetric analysis - TGA, infra-red spectroscopy - FTIR, Raman spectroscopy, elemental analysis - EA, X-ray photoelectron spectroscopy - XPS) probes as well as imaging techniques (scanning electronic microscopy - SEM) were used to characterize the sponges properties. Thermogravimetric Analysis was performed under nitrogen atmosphere with a heating rate of 10 ◦ C/min from 20 ◦ C to 1000 ◦ C. Fourier Transform Infrared spectra were collected in the 4004000 cm−1 range to identify molecules and the chemical composition of samples. The Raman spectroscopy was used with blue wavelength excitation at 473 nm to determine the molecular composition and in particular the type of carbon component. The elemental composition was determined by Elemental Analysis using a combustion method at 950 ◦ C. The XPS experiments were performed on an ESCALB220i-XL apparatus (Fisons Instruments) equipped with monochromated Al Kα as X-ray source and hemispherical analyzer, to identify the nature of atoms and chemical bonds by the energy shift. The pass energy was set at 20.0 eV with the energy step of 0.1 eV. The micromorphologies were observed by Scanning Electron Microscopy with a Nova NanoSEM 450 (FEI). The sessile drop method was used to characterize the wetting properties of the as-processed samples. For such experiments a home-made device was used. The compressive mechanical tests were performed with two parallel flat-surfaces and a 100N detection cell at various deformation speed. More details on the characterization techniques are presented in the Supplementary information.

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3. Results and discussion

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3.1. Characterization of carbonized sponges As shown in Figure 2, pyrolysis leads to be a strong evolution of the sponge volume. The apparent density decreased from 8.3 mg/cm3 for the raw sponge to 6.7 mg/cm3 after thermal treatment at 800 ◦ C (Supplementary information, Figure SI.1). The skeleton densities of the carbonized sponges have been measured to be around 1.5 g/cm3 using Archimede’s principle with sunflower oil. The porosity of the carbonized foams was found then to be ∼ 99.5%. The TGA curve of the raw foam (Supplemantary information, Figure SI.2) shows three weight losses under nitrogen atmosphere at the following temperatures : 20-150 ◦ C, 350-400 ◦ C and 400-800 ◦ C. The first mass loss 6

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(7.5 wt%), up to 150 ◦ C corresponded to the evaporation of the absorbed water and moisture contained in the sponge. The main mass loss (30.5 wt%) occurred between 350 ◦ C and 400 ◦ C and could be associated to the breakdown of the methylene bridge (HN-CH2 -NH) of the melamine structure. Over 400◦ C, the mass loss (52.7 wt%) was attributed to the thermal decomposition of the triazine ring [49, 50, 51, 48, 39]. To obtain a deeper insight on the sponge chemical evolution due to the temperature treatment we combine vibrational spectroscopy techniques (FTIR, Raman spectroscopy) with EA and XPS probes.

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Figure 3: ATR-FTIR spectra (a) and Raman spectra (b) of raw and carbonized sponges.

Figure 3a. shows the ATR-FTIR spectra of the untreated melamine sponge and the carbonized sponges at different temperatures. The spectrum 7

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of the raw melamine sponge presents peaks at 812, 1548, 2929 and 3374 cm−1 that were assigned to the triazine ring bending, whose hydrogen atoms were substituted by nitrogen atoms in the normal melamine, C=N stretching in triazine ring, C-H stretching, and N-H(of the primary and secondary amine or -OH groups) respectively [52, 53, 54, 48, 39]. The peaks at 1340 and 1481 cm−1 have been differently assigned by different authors. The peaks in the range 1300-1500 cm−1 can either be attributed to the C-H bending [48, 39], or to the C-N stretching modes of the s-triazine ring [49, 53, 54, 55]. The spectrum of the 300 ◦ C sample was very similar to the one of the raw sponge. The peaks in the 1050-1700 cm−1 region disappeared when heating above 400 ◦ C to form a very broad band. The intensity of the peak at 812 cm−1 (s-triazine bending) gradually faded during the thermal treatment. This disappearance seemed to be in relation with the growth of one or two peaks in the 2150-2215 cm−1 range attributed to the C≡N stretching (or C≡C) [53, 56, 54]. By combining the TGA and FTIR results, many authors suggested a mechanism for the thermal degradation of melamine [50, 51, 47, 48]. Up to 350 ◦ C, several reactions can take place such as water evaporation, the elimination of formaldehyde from ether links to form methylene bridges, the degradation of free melamine, the degradation of residual methylol groups. Between 350 ◦ C and 400 ◦ C the breakdown or scission of methylene bridges has occured. The by-product has undergone some dimerization reactions together with the N2 gas removal. From 400 ◦ C to 600 ◦ C, these degradation reactions have continued such as melamine has undergone inter-ring condensation to melem, melam and other condensation products [49]. Thus, the melamine thermal condensation leads to the rejection of ammonia (NH3 ). Above 600 ◦ C, the complete decomposition of the triazine rings and other organic products led to the vanishing of all peaks which become a very broad band in the infrared spectra. The breaking of the triazine rings has been reported to occur between 600 and 675 ◦ C [50, 47, 48]. The Raman spectrum (Figure 3b) of the raw sponge shows an intense peak at 972 cm−1 , characteristic of the breathing modes of the triazine ring of melamine [46, 57, 58]. Two Raman bands are characteristic of melamine : 673-676 cm−1 and 975-982 cm−1 . The first corresponds to the in-plane deformation vibrational mode and the second to the triazine ring breathing vibrational mode [58, 59]. In general, the 673-676 cm−1 peak is used for melamine identification, but this peak can disappear if the melamine ring is substituted [57] in particular upon methylolation. Upon the pyrolysis 8

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temperature of 400 ◦ C, the 972 cm−1 peak vanished and two new peaks assigned to the carbon D-band (1343-1358 cm−1 ) and G-band (1538-1573 cm−1 ) appeared. The D-band corresponds to the disorder-induced due to breathing modes in rings, and, the G-band to the C-C stretching modes in both rings and chains [56]. So, the carbonized sponge is made of a sp2 carbon network. The decrease of the width of the D-band and G-band was also observed with the increase of the pyrolysis temperature, which constitutes a signature of the carbonization process.

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3.1.2. Elemental Analysis and XPS. The elemental analysis has revealed the presence of C, N, H and O. For low pyrolysis temperature (up to 400 ◦ C), the most abundant component is nitrogen (>42 wt%) followed by carbon (between 34 and 41 wt%). Above the pyrolysis temperature of 400 ◦ C, the main component is carbon (4550 wt%). For pyrolysis temperatures higher than 500 ◦ C, the quantity of nitrogen decreases to 12 wt%, whereas the oxygen content increases to 28 wt%. This is in accordance with the pyrolysis treatment where ammonia is rejected as described previously. On the whole, the ash content increases with the pyrolysis temperature, but is greater above 600 ◦ C. The sodium bisulfite contained in the initial copolymer was probably turned into oxidized forms of sodium at temperatures higher than 600 ◦ C and these forms are present in the ash. The same tendency is observed in XPS experiments. The quantity of nitrogen has decreased strongly to 6.6 at.% and the carbon amount is higher than 60 at.% for pyrolysis temperatures higher than 500 ◦ C. Howerver the quantity of oxygen has strongly increased for pyrolysis temperature higher than 600 ◦ C. Moreover, the sodium content has decreased between 300 ◦ C and 600 ◦ C, but has increased again above 700 ◦ C. This might be due to some chemical transformations of the sodium bisulfite contained in the sponge such as its thermal decomposition in oxidized sodium, with a diffusion towards the surface of fibers for high temperatures (T>700 ◦ C), as it will be later discussed. Indeed, the XPS is a surface technique with a penetration depth around 10 nm, contrary to FTIR and Raman spectroscopies (0.5-2 µm). To calibrate the XPS spectra, the C-C graphitic or adventitious carbon in C 1s electron binding energy was referenced at 284.8 eV [60, 61, 62, 63, 64, 65, 66]. For pyrolysis treatment below 500 ◦ C, the samples are electrical insulators. Thus a shift in binding energy was necessary to interpret the spectra of raw and 400 ◦ C samples. The XPS spectra in Supplementary 9

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information (Figure SI.3) indicated the presence of five elements, C, N, O, Na and S, which correspond to the composition of the commercial sponges containing formaldehyde-melamine-sodium bisulfite copolymer [61, 63, 64, 66, 67].

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Figure 4: Fitting of the XPS C1s peak of the melamine spinge (raw) and after heat treatment at 500 ◦ C and 800 ◦ C. Graphitic carbon component became majority from 500 ◦ C.

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In the following paragraph, we consider in more detail the C 1s XPS signal. In Figure 4 are shown the corresponding C 1s XPS band for the raw sponge and those obtained from 500 ◦ C and 800 ◦ C heat treatments. The C 1s spectra was composed of several peaks, between three and five depending on the treatment temperature. The different observed peaks correspond to : C-C graphitic or adventitious C (284.8 eV); C-O alcohol, phenolic, ether (285.7-287.0 eV); C=O carbonyl, quinone (287.2-288.1 eV); O-C=O carboxylic, ester (288.6-290.0 eV); carbonate, adsorbed CO and π-π transition due to the conjugaison (290.0-291.0 eV). Moreover, C-N structures could be assigned in the range 285.8-287.7 eV, that is in the same range as C-O and C=O functions [61, 64, 65, 66, 68]. The main component in C 1s for raw sample was assigned to C-N structures preferentially, which corresponds to MF sponge structure (Figure 1) [60, 68]. The 400 ◦ C sample has an intermediate behavior between the raw and the 500 ◦ C sample. The fit of the C 1s spectra shows that the main component is graphitic 10

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carbon (at 284.8 eV) above 500 ◦ C and this accentuates until 800 ◦ C. The carbonaceous sponge nature obtained by heat treatment above 500 ◦ C is both confirmed by Raman spectroscopy (Figure 3b) and XPS. The last types of carbon functions (carbonate, adsorbed CO and π-π transition) appear for pyrolysis treatments above 600 ◦ C. The N 1s region was fitted with four peaks (more details in Supplementary information). The increase in temperature promotes the formation of the graphitic configuration as reported by Ding et al. [68]. The oxidized nitrogen appearing above 600 ◦ C is correlated with the increase in oxygen from elemental analysis for the higher temperatures of pyrolysis. The three peaks issued from the fit of the O 1s spectra were attributed to : O=C ; O-C, C-OH and C-O-C ; chemisorbed O and/or adsorbed water. Sodium showed two peaks : around 500 eV for Na Auger and in the range of 1071.4-1074.0 eV. Finally, the raw sample showed one peak around 167.5 eV corresponding to S 2p. This S 2p peak disappeared with the pyrolysis treatment and corresponds to a certain form of sodium bisulfite contained in the raw sponge. The details about the fit of N 1s, O 1s, Na 1s and S 2p are reported in Supplementary information. For pyrolysis temperatures above 600 ◦ C, by combining the O 1s peak at 531-531.3 eV, the Na 1s peak at 1071.4-1071.8 eV and the C 1s peak at values greater than 289 eV, the presence of sodium carbonate Na2 CO3 was supposed [69, 70, 71, 72, 73, 74] rather than sodium oxide Na2 O as an oxidized form of sodium [75, 72]. The detailed values and assignments for the XPS peak fit of raw, 500 ◦ C and 800 ◦ C samples are reported in Supplementary information (Table SI.1). The comparison of the ratios between C, N and O by elemental analysis and XPS is shown in Figure 5. We observe that the N/C ratio decreases for temperatures above 400 ◦ C, whereas the O/C ratio increases for temperatures above 500 ◦ C of thermal treatment. This confirms that carbon is the main component of the carbonized sponge in spite of a strong oxidation observed at temperatures higher than 600 ◦ C. This oxidation might have been the result of the contact between carbonized sponges and air, when the sponges were removed from the furnace. The Na/C ratio increased strongly above 600 ◦ C and could be the result of a sodium diffusion phenomenon towards the surface of sponges. To summarize, above 500 ◦ C a partial carbonization of melanine becomes dominant. Beyond 600 ◦ C the presence of hydrophilic surface sodium (most probably Na2 CO3 ) is evidenced by XPS. 11

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Figure 5: Ratios N/C, O/C and Na/C from the composition (wt.%) of elemental analysis and the peak areas of XPS.

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In the following subsection the effect of pyrolysis on the microstructure and the hydrophobic behavior of our melamine-based sponges will be presented.

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3.1.3. Morphology and hydrophobic behavior of sponges. Whatever the temperature used for the pyrolysis treatment, the sponges have kept an alveolar structure consisting of a three-dimensional interconnected network with concave triangular fibers as shown in Figure 6. The structure is highly porous, like a disorganized 3D honeycomb, i.e., a clathratelike structure [76]. By comparison of the structure obtained at different pyrolysis temperatures (Figure 6), a densification of samples was observed : the higher the pyrolysis temperature is, the smaller the alveoli are. Thus, the pore size (diameter of alveoli) decreased by a factor of 1.7 for the 400 ◦ C sample and by 2.9 for the 800 ◦ C sponge compared with the raw foam, and, the average value of the sides of alveoli (edges) decreased from 72 ± 24 µm (for 400 ◦ C), to 55 ± 22 µm and finally reached 41 ± 16 µm (for 800 12

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Figure 6: SEM images of carbonized sponges at 400 ◦ C, 600 ◦ C and 800 ◦ C (from left to right). The inset for 800 ◦ C sample shows a deposit on the fibers contrary to 500 ◦ C and 600 ◦ C. Observation at higher magnification for 400 ◦ C sample was not possible due to the decomposition of the sponge under the electron beam. The densification of samples with the thermal treatment can be observed. ◦

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C). The evolution of the alveolar geometry observed in the SEM images is shown in Figure 7a, showing a progressive decrease with the pyrolysis temperature. The pore size evolution correlates with the porosity (obtained from the apparent volume measurement) and the skeleton volume (from skeleton density) and is shown in Figure 7b. In fact, with the increase in the thermal treatment temperature, the porosity decreased between 400 ◦ C and 700 ◦ C (Figure 7b), and, a deposit has been observed on the sponge fibers of samples treated at temperatures higher than 600 ◦ C (inset in Figure 6). This deposit is attributed to the thermal decomposition of sodium bisulfite at high temperatures (temperatures above 700 ◦ C) into oxidized sodium on the sponge surface. The SEM images in Supplementay information (Figure SI.4), display the densification states at various magnifications of samples treated at 600 ◦ C and 800 ◦ C and the effects of compressive strain on the sponges. After a compressive strain at 80%, most of the alveoli fibers were broken. In the case of the 800 ◦ C sample, a delamination of fibers was observed creating a needlelike morphology (Supplementary information, Figure SI.4), where the deposit on the fibers had disappeared. This was also observed in the 700 ◦ C sample. This confirms the important impact of the compressive strain on the structure of carbonized sponges such as the skeleton breakdown, evidenced by SEM images and mechanical measurements. By combining the XPS experiments and SEM observations, the deposit on fibers can be assigned to oxidized forms of sodium such as sodium carbonate Na2 CO3 . 13

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Figure 7: Average alveoli diameter obtained from SEM images (a) and porosity obtained by apparent and skeleton volumes (b) for raw and carbonized sponges. Both parameters evolve correlately with the thermal treatment.

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The wetting response of the sponges will be discussed in the following paragraph. The water contact angle tests demonstrated the hydrophobic properties of samples pyrolysed at 500 ◦ C and 600 ◦ C for which the contact angles (CAs) were in the 120-140 ◦ range (Supplementary information, Table SI.2, and, Figure SI.5), with a maximum value for the 500 ◦ C material. The dispersion on the contact angle values resulted from statistical errors. In the case of 400 ◦ C and 700 ◦ C samples, a balanced average contact angle was calculated to take into account the special behavior of these samples (neither completely hydrophobic nor hydrophilic). For the 800 ◦ C sample, the water droplet was completely and rapidly absorbed by the sponge (CA ' 0◦ ), like for the raw (untreated melamine) and 300 ◦ C samples. For pyrolysis temperatures of 400 ◦ C and 700 ◦ C, less than 30% of the droplets were stable when we place a water droplet on the sample surface. These samples thus present 14

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heterogeneous wetting properties, with some hydrophobic patches distributed on a hydrophilic substrate. The Na2 CO3 deposit on fibers might lead to a hydrophilic behavior of 700 ◦ C and 800 ◦ C samples, thereby modifying the surface energy of the sponges. The strong hydrophobicity and oleophily of the 500 ◦ C and 600 ◦ C samples can be attributed to the surface chemistry having a high content of sp2 carbon (hydrophobic component), and to the surface roughness owing to the nanostructure [77, 78, 44]. Indeed, the hydrophobicity can be attributed to two factors, the surface topography and the surface energy. Even if the pore size decreased with the pyrolysis treatment (topography effect), the fibers surface appeared to stay smooth (apart from the hydrophilic sodium deposit at higher temperatures) and indicated that the surface energy is the predominant effect. At 300 ◦ C, the melamine chemical functions are preserved on the infrared spectra, and the water contact angle was zero. We can observe from XPS that the carbon atom component becomes dominant above a temperature of 500 ◦ C. We can reasonably postulate that the structure tended to become organized in a disordered arrangement of aromatic carbon planes (graphene-like) with heteroatoms, confirmed by Raman spectroscopy (presence of D and G-band) and the water contact angle becomes hydrophobic. We can estimate that the surface energy value of the sponge decreased by heat treatment through the carbonization process, and should progressively tend to the graphene one (around 40-60 mJ/m2 ) [79, 80, 81, 82] . Thus, the surface energy of carbon sponges was lower than water surface energy, preventing wettability. We have verified the role played by the sodium deposit in the hydrophobicity loss by washing the samples treated at 700 ◦ C and 800 ◦ C with water for fifteen days using a Soxhlet extractor. The SEM images showed no presence of deposit after this treatment and the samples exhibited superhydrophobic contact angles close to 150 ◦ . This confirms that the hydrophilic deposit of Na2 CO3 or other oxidized forms of sodium on the sponge surface is responsible of the loss of hydrophobicity. To complete the analysis, the applied pressure required for the liquid intrusion into the pores was calculated (Figure 8). The values of the intrusion pressure obtained for samples processed at 500 ◦ C and 600 ◦ C are higher than the atmospheric pressure, confirming thus the hydrophobic property of such surfaces. This is in good agreement with their water absorption capacities as will be discussed later.

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Figure 8: Pressure of intrusion for different pyrolysis temperatures. The 500 ◦ C and 600 C samples having positive intrusion pressures are hydrophobic.

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Figure 9: Untreated melamine (white) and 500 ◦ C sample in water. a) The raw sponge sinks immediately whereas 500 ◦ C sample floats on the water surface. b) The hydrophobic surface appears like a silver/mirror surface. c) 500 ◦ C sample after the release of an external force : sample floats again.

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3.2. Absorption capacity of the carbonized sponges In Figure 9, the photographs show the behavior of pristine melamine (white) and the carbonized one at 500 ◦ C (black) in water. For other treatment temperatures, see the photographs in Supplementary information (Figure SI.6). The raw melamine sinked immediately, whereas 500 ◦ C (also 600 ◦ C) sample floated on the water surface (Figure 9a). Maintaining the sample immersed in the water, a silver mirror-like surface was observed (Figure 9b). This indicates a non-wetting behavior, in which air was trapped between the water and the hydrophobic surface (Supplementary information, Video.1). This has also been described in many articles about (super)hydrophobic materials [83, 36, 84, 39, 40]. After releasing, the sample floated again (Figure 16

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9c). The absorption capacity Q was calculated using the following equation : (1)

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where mf is the weight of the sponge after absorption (g), and m0 is its initial weight (g). Our carbonized sponges exhibited absorption capacities from 90 to 200 times their initial weight for various organic solvents and oils (Figure 10, and, Supplementary information Figure SI.7), depending on the viscosity and the polarity of liquids but primarily on their density (Supplementary information Table SI.3).

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Figure 10: Absorption capacities for various organic solvents and oils for different samples : raw sponge, 500 ◦ C, 600 ◦ C and 800 ◦ C. The first bar (cased in blue) corresponds to water absorption and shows a marked decrease for 500 ◦ C and 600 ◦ C, i.e the hydrophobic samples. Other absorption capacities for carbonized samples are better than the raw sponge.

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Especially for the chloroform absorption, the 500 ◦ C sample showed an uptake of 200 times its weight. This value is higher than for most of the absorption capacities of other absorbent materials outlined in the literature (Table 1). Concerning water absorption (blue frame on Figure 10), the carbonized sponges at 500 ◦ C or 600 ◦ C absorbed 5 to 6 times less than untreated sponge. Consequently these pyrolysis temperatures have produced hydrophobic and oleophilic sponges, as shown also by their water contact angles (120-140 ◦ ). Moreover, their organic solvent absorption capacities were better than for the untreated sponge. The hydrophilic behavior of the samples 700 ◦ C and 800 ◦ C, which absorbed much more water, can be explained 17

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Table 1: Chloroform absorption capacities for various absorbent materials.

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Reference [26] [37] [44] this work

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[38]

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[15]

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[42]

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[36]

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[39]

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[29]

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[40]

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104

[13]

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[17]

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[34]

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Graphene-carbon nanotube sponge Carbon foam Lignin coated melamine sponge Carbon sponge from melamine sponge Mercapto-functionnalized polydopamine coated melamine sponge Carbon nanotube sponge Polyphenol and dodecanethiol coated melamine sponge Graphene and poly(dimethylsiloxane) coated melamine sponge Octadecyltrichlorosilane coated melamine sponge Reduced graphene oxide coated polyurethane sponge Trichlorosilane on 400◦ C carbonized melamine sponge Twisted carbon fibers from cotton Glutaraldehyde coated polyvinyl alcohol-formaldehyde sponge Spongy graphene Polypyrrole and palmitic acid coated polyurethane sponge Poly(dimethylsiloxane) sponge with NaCl Poly(dimethylsiloxane) sponge with sugar

Absorption capacity (g/g) for chloroform 568 411 217 200

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by the hydrophilic deposit of Na2 CO3 or other oxidized forms of sodium on the sponge surface. Indeed, when we wash these samples with water for fifteen days using a Soxhlet extractor, they become hydrophobic. Finally, the oils or organic solvents could be recovered by a simple squeezing of the sponge. With a total (100% of deformation) and manual compression, the amount of residual solvent inside the sponge has reached a maximum of 15% of the initial solvent volume after one absorption (depending on the pyrolysis temperature and the solvent used). After one hundred cycles of compression-decompression, the carbonized sponges have continued to absorb 81 ± 2.5% of the initial absorbed weight of crude oil (to the initial weight of dry sponge). So in average Q = 93 ± 3 g/g, that is in the range of the absorption capacity of other absorbent materials [85, 36, 29, 39, 40, 44, 34, 43, 42] (Supplementary information, Figure SI.8). With a test of separation efficiency (a layer of crude oil on water), we can estimate that the 500 ◦ C sample absorbed around 75% of oil (and 25% of water), although the 800 ◦ C sample without any washing treatment absorbed between 66% and 75%.

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3.3. Mechanical behavior of carbonized sponges The carbonized samples submitted to a manual compressive force keep their apparent macroscopic morphology demonstrating an elastic behavior (Figure 11), although the alveolar structure of sponges (skeleton) has been partially disrupted at a microscopic scale as evidenced by the SEM images (Supplementary information, Figure SI.4).

Figure 11: Manual compressive force on sample carbonized at 500 ◦ C without any solvent absorption, demonstrating the relative elasticity of the sponge.

In order to evaluate precisely the mechanical behavior, a specific rate of deformation (%strain) was applied to the sponge, and the stress response was measured. Therefore the stress-strain curves of raw and carbonized sponges were obtained with a maximum strain of 20%, 50% and 80%. The loading process showed three different regions (Figure 12) as described by several 19

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Figure 12: Mechanical behavior of dry 600 ◦ C sample with 50% and 80% of deformation at 8 mm/min.

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studies of other sponge materials [15, 37, 25, 40]. The first regime is an initial linear region, which corresponded to the elastic domain (reversible deformation). Thus, the stress increased linearly with the strain until a rate of deformation of about 10%. Then, a plateau region (plastic region) and finally a rapid increase of stress due to the densification of material until an irreversible deformation was observed [86, 87]. Between the loading and unloading process, an hysteresis was present on the stress-strain curves due to the dissipation of the mechanical energy. This energy could come from the fracture of fibers under the stress as shown by SEM images (Supplementary information, Figure SI.4) or due to the viscoelasticity. The second cycle of compression-decompression was very different from the first one after this irreversible deformation. However, the other cycles seemed to tend towards a mechanical behavior with a certain ”elasticity” (Figure 13). Indeed, the carbonized samples kept at least 62-70% of their macroscopic thickness after 100 cycles of compression-decompression at 80% of strain. Moreover, the hydrophobic behavior of 500 ◦ C sample seems to be preserved after 100 cycles of compression-decompression at 80% of strain (Supplementary information, Video.2). As a conclusion, the mechanical properties of the raw (Supplementary information, Figure SI.9) and carbonized sponges were modified after one cycle of compression and rapidly converged to a new mechanical profile, keeping a partially “elastic” behavior (recovery of a major part of their initial shape) up to deformation of 80%, but with a reduced mechanical resistance. The 20

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Figure 13: Mechanical behavior of 500 ◦ C sample with 80% of strain at 8 mm/min, dry (full lines) or with crude oil (dotted lines). The stress-strain curves have the same behavior as the 600 ◦ C sample, but the sponge carbonized at 500◦ C is more rigid than at 600 ◦ C. The number of cycles reduces the mechanical resistance of the sponge.

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SEM images in Supplementary information (Figure SI.4) show the presence of fractures on the “skeleton” after one cycle of compression at 80% of strain. The fibers in the stress direction were submitted to the highest local strain. The carbonization process has reduced the mechanical resistance either of the individual fibers (constituting the skeleton), or their interfiber links. Thus the fibers submitted to the highest strain (in a particular percolation path) would fracture in the first cycles, whereas other fibers would bend. Subsequently, the as-modified sponge structure was preserved even if some pores were bigger. Beyond 80% of strain, the skeleton could not support the stress and induced the collapse of the sponge. Our carbon sponges became more flexible for a stronger thermal treatment as can be quantified by the Young’s modulus evolution which decreased from 1.0 kPa (untreated sponge) to 0.58 kPa (800 ◦ C) (Supplementary information, Figure SI.10). This was confirmed by the decrease in the maximum measured stress values on each cycle, when the carbon sponges were submitted to a larger number of compression-decompression cycles (Supplementary information, Figure SI.11). For example after one hundred cycles, the maximum stress decreased from 32 kPa to 17 kPa for the dry 500 ◦ C sample, and from 29.1 kPa to 13.5 kPa for the wet sample embedded with crude oil. Theses values of Young’s modulus can be compared with other hydrophobic and oleophilic sponges : carbon aerogel (4 kPa) [26], graphene aerogel (10-17 21

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kPa) [25], carbon nanotube sponge (30-100 kPa) [88], graphene hydrogel (130 kPa) [24]. Finally, after ten compression-decompression cycles at 50% of strain, the samples lost less than 10% of their initial thickness, whereas at 80% they lost between 7 and 20%. After one hundred cycles at 80% of strain, the loss of thickness could reach up to 38%. The higher the thermal treatment temperature is, the bigger the loss of thickness is. The thermal treatment has made the carbon sponges more flexible and therefore easier to deform and to compress, but also more brittle. According to the thickness of sponges during the compression experiments, the elastic recovery could be determined using the following equation [89, 90] :

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where ti is the initial thickness of the sponge before the first compression (mm), tload is the thickness under load for the cycle n (at 80% of strain in our experiments, mm) and tf is the final thickness after n cycles of compressiondecompression (mm). The carbonized sponges exhibit an elastic recovery of 60% for the 500 ◦ C sample and around 38% for the 600◦ C sample after one hundred cycles of compression-decompression (Figure 14). Nevertheless, the

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carbon sponges still have a good absorption uptake after one hundred cycles (Q = 93 ± 3 g/g for crude oil, Q defined previously in 3.2 section).

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4. Conclusion

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Acknowledgements

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In summary, a simple method was developped to elaborate a flexible and hydrophobic carbon sponge by the simple pyrolysis of a commercial melamine sponge without further chemical treatment. A thermal pyrolysis around 500600 ◦ C, results in hydrophobic sponges having a water contact angle in the 120 ◦ -140 ◦ range. They present excellent absorption capacities for solvents (Q = 90 to 200 g/g), and exhibit a capacity of 93 g/g for crude oil uptake after one hundred cycles of compression-decompression. Moreover, the fabrication of these sponges at 500-600 ◦ C is easy to scale-up, low cost, and could be achieved without using additional chemical products or steps. The sponge obtained by pyrolysis at 500 ◦ C presents the best performances both from absorption and mechanical point of view (Supplementary information, Video.3). We have shown that for temperatures lower than ∼ 500 ◦ C the carbonization process does not yield the targeted hydrophobicity. On the other side, for pyrolysis temperatures beyond ∼ 600 ◦ C hydrophobicity starts to be lost in relation to the sodium migration towards the surface of the fibers. The prepared carbon sponges have a very low density, high porosity and preserve a certain “elastic” behavior after the first cycle of compression-decompression until 80% of strain. For all these reasons these sponges could be excellent candidates as absorbents for the clean-up of solvent and oil polluted water.

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This project is financially supported by the Region Rhˆone-Alpes (France) through the ”ARC Environnement” program. Suppementary information

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Details on characterization techniques. Figures showing the apparent density, TGA curve, XPS survey, SEM images, the values of water contact angle, the behavior of samples towards solvents, and mechanical behavior data. Three videos showing the sponges fluid absorption behavior of : 500 ◦ C sample with water, 500 ◦ C sample after one hundred cycles with water, and 500 ◦ C sample with crude oil on water.

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Captions for videos

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Video 1 : “500 °C treated sponge in water” We can see the 500 °C sample floating on the water surface due to its very light density and its hydrophobic behavior. When the sponge was immersed in water, a silver mirror-like surface appeared which corresponds to the air trapped between water and the hydrophobic sponge surface. After removing external forces, the sponge continued to float on the water surface.

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Video 2 : “500 °C treated sponge in water after 100 compression cycles” We can see the 500 °C sample after one hundred cycles of compression-decompression without liquid inside. As the as-prepared 500 °C sample, this sample floated on the water surface and a silver mirror-like surface appeared in water. Again, when the force was removed, the sponge continued to float on the water surface. This shows that the hydrophobic properties are not modified by the mechanical cycles.

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Video 3 : “500 °C treated sponge separating crude oil from water” A vessel was filled with water and a layer of crude oil. A 500 °C sponge sample was deposited on the crude oil surface. We can see that the sponge could absorb the crude oil and that it could be retrieved by simple squeezing. Six steps of absorption-squeezing were enough to remove completely the crude oil. Even if the sponge was a little deformed (it was very strongly squeezed), it continued to float on the water surface like in the previous videos. We can conclude that this material is well adapted for oil-spill clean-up applications.