Cyril Bergeron

Catherine Lamy-Bergot

Wassim Hamidouche

William Puech

SELECTIVE SECRET SHARING SCHEME FOR PRIVACY OF IMAGE AND VIDEO COMPRESSED IN MPEG-LIKE FORMATS

Keywords: Multimedia Security, Privacy Protection, Secret Video Sharing, Secret Image Sharing, Selective Encryption, Cloud Storage, CSE, MPEG VIMAF, AVC, HEVC, HEIF

This paper presents a reliable image and video secret storing method, aiming at ensuring the protected storage of multimedia files without risk for their owner to have their privacy violated. It relies on a particular property (standardized in MPEG-A Part.21 VIMAF) of video and image standards such as H.264/AVC, H.265/HEVC and MPEG-HEIF: a non negligible portion of the bitstream (so-called cipherable bits) can be altered without modifying the decodability of the stream. Such a modification results in an image or a video that is both a valid and visually encrypted media file. Not willing to be bothered by a complex key management system, we take inspiration from Shamir's secret sharing scheme, and propose a method where the secret to be shared is the original (unciphered) set of cipherable bits, that are encoded and divided in n shares, among which k -1 or less do not permit to reconstruct the secret, while k or more allow to recover it. Those n shares are then used to generate n ciphered fully standard compliant versions of the media. The rightful owner of the file will easily store these n versions in various Cloud storage locations, and recover them all when the fully deciphered file will be needed, while a hacker will most generally obtain only some of those files, hence not be capable to decrypt the file.

INTRODUCTION

With the generalisation of digital cameras, mobile phones and other recorders, and their use in public and private space, the issue of privacy and capability for the end-users to store in a secured manner images or videos containing private information is needed. In 2014, with the "Celebrity photo hack" has proven that even a security concerned Cloud service could be hacked, with another similar episode following in 2017, and presumably many others. Some may argue that the best option is not to upload a content one does not want to leak outside of one's own private sphere, but the ease of use for storing, sharing with other (a priori selected friends) and saving without risk of data loss is a huge motivation for most people to place private information in the Cloud. As a consequence, in our modern world, the main issue is no longer the available storage space but privacy and resistance to hacking, which remains difficult for end users that are not all digital natives or simply attentive. People understand the interest of the Cloud for safe keeping but the security issues (bugs, backdoors... regularly found and used by hackers) remain, leading to wonder if the best for end users is really to blindly trust storage service to keep their private life... really private. One may be interested is a solution allowing not to store everything with the same service and apply the old saying of not putting all your eggs in the same basket. This leads us back several decades ago, when the issue was to share a secret without risking the misfortune of loosing a key, but also the risk of somebody finding the complete key with only limited partial information. In 1979, Shamir [START_REF] Shamir | How to share a secret[END_REF] and Blakley [START_REF] Blakley | Safeguarding cryptographic keys[END_REF] designed independently secret sharing methods, that protect information by splitting it into smaller pieces (or shares) that are distributed among multiple of locations. More recently, it was extended by different Secret Image Sharing (SIS) strategies [START_REF] Yang | New visual secret sharing schemes using probabilistic method[END_REF][START_REF] Liu | Colour visual cryptography schemes[END_REF][START_REF] Li | Improvements of a two-in-one image secret sharing scheme based on gray mixing model[END_REF][START_REF] Thien | Secret image sharing[END_REF], research topic in multimedia security applied to different fields such as visual authentication and identification, data sharing, secured Cloud storage. In each case, similarly to the original Shamir approach, the original image or 'secret' is transformed into n different shadow images or image shares. Each share of the secret can be then distributed and the secret image can be reconstructed if k (or more) image shares are accessible and combined (with 1 < k < n), while less than k image shares combinations will reveal nothing about the original image. Such a system is called a (k, n)-threshold scheme (see Fig. 1).

Fig. 1: (k,n)-threshold Secret Sharing scheme

Several drawbacks exit in the SIS methods proposed in the literature: they work mostly on uncompressed image files (except for [START_REF] Yriarte | A joint secret image sharing and JPEG compression scheme[END_REF]), the reconstruction phase can be important due to the size of shares [START_REF] Yang | New visual secret sharing schemes using probabilistic method[END_REF], and some do not permit lossless reconstruction (though it is possible [START_REF] Li | Improvements of a two-in-one image secret sharing scheme based on gray mixing model[END_REF][START_REF] Thien | Secret image sharing[END_REF]). Furthermore, those solutions consider image only, and no application to the video field. We hence propose here to generalize the SIS approach to both image and video compressed data, taking advantage of the selective encryption method standardized in [START_REF]Multimedia application format (MPEG-A)-part 21: Visual identity management application format (VIMAF)[END_REF] that comes from a consistent work has been made last decades [START_REF] Shi | An efficient MPEG video encryption algorithm[END_REF][START_REF] Dufaux | Scrambling for Privacy Protection in Video Surveillance Systems[END_REF][START_REF] Bergeron | Compliant selective encryption for H.264/AVC video streams[END_REF][START_REF] Shahid | Fast protection of H.264/AVC by selective encryption of CAVLC and CABAC for I&P frames[END_REF][START_REF] Hamidouche | Selective Video Encryption using Chaotic System in the SHVC Extension[END_REF][START_REF] Stutz | A survey of H.264 AVC/SVC encryption[END_REF][START_REF] Boyadjis | Extended Selective Encryption of H. 264/AVC (CABAC) and HEVC Encoded Video Streams[END_REF][START_REF] Farajallah | ROI Encryption for the HEVC Coded Video Contents[END_REF] in the visual cryptography field. The secret consist of the bits selected for encryption, with a lossless (k, n)-threshold scheme based on a well-known Reed-Solomon code which can be considered as a Lagrange interpolation method in GF (2 p ) and which is already presented as a method to satisfy the Shamir's scheme in [START_REF] Mceliece | On sharing secrets and Reed-Solomon codes[END_REF]. This Secret Sharing Scheme is applicable to compressed video such as H.264/AVC [START_REF]Coding of audio-visual objects -Part 10: Advanced Video Coding (AVC)[END_REF] or H.265/HEVC [START_REF]High efficiency coding and media delivery in heterogeneous environments -Part 2: High Efficiency Video Coding (HEVC)[END_REF] and compressed image format such as HEIF [START_REF]MPEG systems technologies -part 12: High Efficiency Image File Format (HEIF)[END_REF] (based on Intra mode of AVC and HEVC codecs), and can be used both to cipher either the complete stream or only selections of it (e.g. faces of people).

This paper is organized as follows: Section 2 introduces video encryption and summarizes the principle of Content Sensitive Encryption, followed by Section 3 which details the proposed approach, and its possible application in a secured Cloud storage. Then Section 4 presents experimental results highlighting the privacy offered by the encryption and the losslessness of the reconstruction. Finally, Section 5 draws some conclusions.

PRIVACY FOR VIDEO: ENCRYPTION TECHNIQUES

Video Encryption

The increased usage of image or video compression in various services (medical imaging, social networking, media sharing platforms. . . ) has led to a high security-related issue for copyright protection, confidentiality or user's privacy. In the state of the art we find several video encryption methods that can be grouped in three major categories: Visual Encryption methods working at pixel level before compression, Crypto-Compression methods that operate while the compression is applied and take the bitstream structure into account and Naive Encryption methods that manipulate the compressed bitstream as generic data, depending on the stage of the video transmission they are applied, respectively in the Spatial-Domain, Video Coding Layer or the system level, as illustrated in Fig. 2. Working at pixel level before compression without a specific method generally generates a compression process degradation, as the ciphering (as any other scrambling) reduces of the coherence in the image or the video stream, hence creating larger bitstreams and more complex operations of compression and file management. On the other hand, treating the compressed video bitstream as an opaque data without taking into account the compressed video structure like the Full sample encryption standardized in MPEG Common Encryption (CENC) [START_REF]Common encryption in iso base media file format files[END_REF] does present the major drawback to generate a non-negligible computational complexity as the whole video data must be in general deciphered to allow video decoding. As a consequence, two approaches have been proposed in the literature, that rely on the knowledge of either the video structure or the stream structure to operate and ensure that said structure is maintained: the Partial and Perceptual Encryption methods. Partial encryption protects a portion (in bytes or bits) of the original bitstream while other portions are left as plaintext (i.e. unchanged). By limiting the number of bits ciphered, it reduces the complexity due to the inherent XOR operations and the usage of ciphering algorithm itself and the compression degradation due to limiting the scrambling. Perceptual Encryption operates with taking into account the stream structure, while keeping the bitstream decodable, whether to degrade some regions (e.g. region encryption) or the totality of the content.

MPEG Content Sensitive encryption (CSE) approach

Work on Partial or Perceptual encryption has emerged as an effective cryptographic security methods for different video and image codecs [START_REF] Dufaux | Scrambling for Privacy Protection in Video Surveillance Systems[END_REF][START_REF] Bergeron | Compliant selective encryption for H.264/AVC video streams[END_REF][START_REF] Shahid | Fast protection of H.264/AVC by selective encryption of CAVLC and CABAC for I&P frames[END_REF][START_REF] Hamidouche | Selective Video Encryption using Chaotic System in the SHVC Extension[END_REF][START_REF] Stutz | A survey of H.264 AVC/SVC encryption[END_REF][START_REF] Boyadjis | Extended Selective Encryption of H. 264/AVC (CABAC) and HEVC Encoded Video Streams[END_REF], and the MPEG committee has worked to defined a selective encryption process efficient from a cryptographic point of view but also maintaining a format compliant bitstream (perceptual encryption methods do not necessarily maintain that capability [START_REF] Lian | Secure advanced video coding based on selective encryption algorithms[END_REF]) and a good compression rate [START_REF] Van Wallendael | Encryption for high efficiency video coding with video adaptation capabilities[END_REF]. As a matter of fact, degrading too much the compression efficiency would be problematic and could even be a bias for statistical attacks based on occurrence probability of compressed symbol. Some authors provided solutions for formatcompliant selective encryption with bitrate constraint by modifying bits inside the entropic coding process, for instance by providing a scheme to provide selective encryption for AVC bitstreams coded with Context-Adaptive Variable-Length Coding (CAVLC) entropy coding mode by selecting some bits in different codewords in [START_REF] Bergeron | Compliant selective encryption for H.264/AVC video streams[END_REF], or for Context Adaptive Binary Arithmetic Coding (CABAC) coding for AVC [START_REF] Shahid | Fast protection of H.264/AVC by selective encryption of CAVLC and CABAC for I&P frames[END_REF] and for HEVC [START_REF] Hamidouche | Selective Video Encryption using Chaotic System in the SHVC Extension[END_REF]. As shown in Fig. 3, these solutions ensure bitstream format compliancy without compression efficiency loss and can be performed during the compression process (or by transcoding as proposed in [START_REF] Boyadjis | A Real-Time Ciphering Transcoder for H. 264 and HEVC Streams[END_REF]) which ensures a minimum latency and a negligible computing cost. Furthermore, slices (tiles in HEVC) can be used to split the video frame into areas with an integer number of blocks where Intra prediction and the entropy coding dependencies are reset at the slice/tile boundaries. Thus, as proposed in [START_REF] Bergeron | Real-time selective encryption solution based on ROI for MPEG-A visual identity management AF[END_REF], it is possible to perform a selective region encryption only on specific slices and tiles. The application of ciphering on a Region of Interest (RoI) only is also illustrated in Fig. 3, where only the face of a colleague (contained in one specific tile of HEIF) is protected .

Fig. 3: Constant bitrate Selective Encryption principle

Based on these works, the Content Sensitive Encryption defined by MPEG standardises which part of the syntax can be 'selected for encryption' (the so called cipherable bits) for different codecs without changing the coding efficiency and without disrupting a standard decoding process [START_REF] Bergeron | Real-time selective encryption solution based on ROI for MPEG-A visual identity management AF[END_REF]. In each case, the bits to be ciphered are chosen with the respect to the considered video standard to ensure full compatibility, which is achieved by selecting the bits (generally parts of code-words) for which each of the encrypted configuration modifies the decoding process contexts, which then makes it difficult for cryptanalysis attacks to find a weakness to break the cipher, as there is no redundancy to rely upon to break the code. Exhaustive definition can be found in the last edition of MPEG VIMAF [START_REF]Multimedia application format (MPEG-A)-part 21: Visual identity management application format (VIMAF)[END_REF] standard for video formats AVC, HEVC and image format HEIF, and a simple illustration is given below with example of the value of Motion Vector Difference (MVD) used in AVC with CAVLC entropic coding in Table 1. Noticing that the change of the MVD has no influence on the rest of the decoding (except visually), the suffix bits of the Exp-Golomb code are considered as cipherable bits (i.e. the bits following the first '1'), as illustrated by the highlighting (bold font). In CABAC (used in HEVC and in some AVC profiles), due to the arithmetic encoding step and the Context Modeling, bits in compressed streams are very interdependent. However, only by-passed coding does not update probability models during the arithmetic coding, so that them being ciphered has not impact on the CABAC engine. Thereby, for example, signs of MVD are cipherable bits since they are binarized in Fixed-Length code and by-passed.

PROPOSED METHOD FOR SELECTIVE SECRET SHARING FOR IMAGE AND VIDEO COMPRESSED FILES

The proposed method for ensuring the privacy of image or video streams stored in the Cloud is illustrated in Fig. 4, and consists of 5 different steps, that are detailed in the subsections hereafter. This can be done for complete media ciphering or partial ciphering when one uses Region selective encryption. To ensure the privacy of multimedia files stored in various public Cloud storage providers, different versions of the original file, each ciphered, are produced for storage in different Cloud services (under assumption that no transcoding is done in said Clouds), without totally relying on the security of various storage services as illustrated with 3 shares in Fig. 5. The resulting region selective encrypted pictures illustrate how the user could then browse them files without decoding when searching for particular content (e.g. a picture with specific attributes and given background context). Even if one of the different Cloud services (which all guarantees a given security level) is attacked by malicious persons, the media files can not be deciphered, which permits to maintain privacy for the user, while the user can access to sufficient shares (k or more) and consequently can decipher the original compressed file.

Selecting a secret for image or video compressed files

As explained in Section 2, the MPEG Content Sensitive Encryption scheme defines a set of cipherable bits, which permits to keep a compliant bitstream while ensuring that said ciphered stream is not visually meaningful for an end-user without the original cipherable bits information. Since the CSE scheme is standardized [START_REF] Bergeron | Real-time selective encryption solution based on ROI for MPEG-A visual identity management AF[END_REF] and fully systematic, it is easy to find and extract those cipherable bits for each frame, slice, or tile, and the same is true of the deciphering operation. The number of bits, (noted x bits in Fig. 4-1), corresponding to the initial secret obviously depends on the original media and the compression parameters, but on average, for the numerous tests carried with different codecs and obtained bitrates by these authors [START_REF] Bergeron | Compliant selective encryption for H.264/AVC video streams[END_REF][START_REF] Shahid | Fast protection of H.264/AVC by selective encryption of CAVLC and CABAC for I&P frames[END_REF][START_REF] Hamidouche | Selective Video Encryption using Chaotic System in the SHVC Extension[END_REF], it was found to be roughly 15 -20% of the compressed media size. It should also be noted that the secret extraction can easily be parallelized by dealing with different sets of slices, tiles or pictures in parallel if needed.

Reducing the initial secret to a usable secret (divisible by k)

As with other (k, n)-threshold scheme, we want to divide the secret sequence into n shares, from which any k different shares per-mit to reconstitute the original secret. To ensure this, we use Reed-Solomon codes, standardly operating on Galois Field GF (2 p ), with p = Log2(n) + 1 to guarantee that k < n < 2 p . The RS(n, k) code takes as input p bits for each of the k data symbols to produce n coded symbols. Since the secret sequence length (x bits) is variable and unlikely not match systematically the Reed-Solomon symbols size, we need to systematize the reduction of the secret sequence to x bits, with x < x defined as x = k.p x/(k.p) . As represented in Fig. 4-2, the x bits constituting the secret, give k data sequences of x /k bits (e.g. x /(k * p) symbols in GF (2 p )).

Generating n sets of bits

The reason for using Reed-Solomon code, that would be true with any other Maximum-Distance Separable code (MDS code) or any perfect erasure code, is that it can correct up to n -k erasures, meaning that any k encoded symbols among the total of n encoded ones allow to reconstruct perfectly the original k data symbols. This family of codes has for generator matrix a (k × n) Vandermonde matrix, which encodes a message M (m0, . . . , m k-1 ) made up of k element symbols of Galois field GF (2 p ) into n symbols of GF (2 p ), leading us to obtain n sequences of x /k bits, as illustrated in Fig. 4-3 on the k sequences of x /k bits vertically ordered. Reed-Solomon encoding corresponds to the evaluation of a polynomial fm(a) = k-1 i=0 mi.a i and from the Lagrange Interpolation formula, we know that the polynomial fm(a) can be obtained from any k of the n points (since the polynomial fm(a) is of degree k -1) if and only if the n points are distinct (i.e. a1, ..., and an are distinct). An advantage of Reed-Solomon codes is that they have decoders more time-efficient than Lagrange interpolation, which is useful when wanting to decode the shares.

Create formalized shares

As already mentioned, this approach does not aim at reducing Cloud storage space but at ensuring privacy for the user, and as such we accept the cost of storing more than the original compressed file. In our method, we consequently generate n compressed files, that each differ by the values of their secret share. Having currently n sequences of x /k bits (as [START_REF] Thien | Secret image sharing[END_REF]), we need to expand these sequences to obtain for each a new sequence of x bits to substitute the cipherable bits in the original compressed file as shown in Fig. 4-4. For that, we propose to consider GF (256) and to first add a header containing the value of aj used for the j th share and the parameter k coded on p = 8 bits each. Then we concatenate the generated j th share on x /k bits, with also adding a cyclic-redundancy check (CRC) computed on previous bits to check for any error and prevent trying to decode with an erroneously transmitted share. Finally, we pad with pseudo-random generated bits to obtain the desired expanded size (i.e. from x /k bits to x bits) as shown in Fig. 6. It should be noted that in lieu of some padding bits, one could also add other information such as a media identifier (UUID), the value of p and the primitive polynomial of the Galois Field.

Create Compressed media shares

Remains now to generate the n different ciphered streams by replacing the initial cipherable bits by the x different shares. As shown in Fig. 4-5 we represent the creation of this j th compressed media share without forgetting to keep the last original bits if x < x. In Fig. 4, we illustrate application of Region-selective encryption where only tiles (containing people's faces) are ciphered by CSE [START_REF] Bergeron | Real-time selective encryption solution based on ROI for MPEG-A visual identity management AF[END_REF], but naturally the method is the same when the whole image or video stream are ciphered. We want to emphasize that this method permits us not to indicate which parts of the content are encrypted (even with Region selective Encryption), as the decoder will know it when comparing whether the cipherable bits are the same or not from two different media shares. Note also that by using CSE principle, the size of media shares is exactly the same as the original compressed file.

IMPLEMENTATION RESULTS

Our implementation is based on the reference software of MPEG-A VIMAF [START_REF] Bergeron | Content sensitive encryption: Break the CSE challenge[END_REF], which allows to automatically select the cipherable bits (approximately 15% to 25% of the compressed bitstream depending on the considered bitrate). First application was done with n = 7 files decodable by any compliant HEVC decoder, tested on seven complete reference video sequences (compressed with QP = 22) with operating on GF (256). For each, objective metrics were used to measure the ciphered videos quality: Peak Signal to Noise Ratio (PSNR), Structural Similarity (SSIM) with is strongly linked to the degree of perceptual encryption with the video quality levels [START_REF] Fezza | Visual security assessment of selective video encryption[END_REF] and Edge differential ratio (EDR) [START_REF] Zhang | Chaos-based partial encryption of SPIHT coded color images[END_REF] which measures deviation in the location of edge formation contributing pixels in the original image and its encrypted video frame (a value close to 1 corresponding to a high edge hiding capability). The PSNR, SSIM and EDR results for original (with lossless reconstruction) and Selective Secret Sharing schemes are provided in Table 2 for the seven reference video sequences and show the good performance. Visual results are given in Fig. 7 for the 1 st frame of Cactus video sequence reconstructed for each of the different 7 generated ciphered streams, as well as the reconstruction of the compressed bitstream when recovering k = 3 or more streams. It illustrates the results provided in Table 2 since the structural information of encrypted frame are completely hidden and become useless for the attacker. Furthermore, an example of visual result obtained on a HEIF image format (compressed in HEVC Intra mode) when restricting the privacy to a part of the media content (applied only on the 2 specific tiles containing faces) is presented with n = 4 shares in Fig. 8.

CONCLUSION

In this paper, we have described a reliable image and video secret sharing method relying on MPEG Content Sensitive Encryption standardised approach and based on Shamir's secret sharing principle, to allow for private compressed media files storage in the Cloud. This method permits to reinforce the Cloud storage providers security without having to manage secret keys. It works for full media ciphering but also for region only ciphering (for example by ciphering only human faces) by applying Region-selective encryption scheme. Compared to other solutions in the literature which mostly consider uncompressed images, this method allows to protect images and videos compressed by modern and widely used codecs, without changing the compression efficiency through the whole process. Furthermore, the objective analysis results presented demonstrate the effectiveness of the proposed Selective Secret Sharing methods, confirming the interest of such an encryption scheme in media Cloud storage applications. Further works will consider how this method can be used with reducing the data to be downloaded to decipher the protected media while in mobility, typically by recovering only one complete share and part of the others (the cipherable bits, i.e. approximately 20% of total bitrate) even with Region Encryption coding.

Fig. 2 :

 2 Fig. 2: Video Encryption Techniques.

Fig. 4 :

 4 Fig. 4: Overall creation of shares scheme

Fig. 5 :

 5 Fig. 5: Photos (and videos) back-up to Secured Cloud Storage.

Fig. 6 :

 6 Fig. 6: Formalized shares.

(a) 1 Fig. 7 :

 17 Fig. 7: Visual quality obtained for different ciphered versions of Cactus Sequence (a),(b),(c),(d),(e),(f), and (g) and the reconstructed video obtained with any 3 shares (h).

Table 1 :

 1 VLC codewords: Motion Vector Difference.

	Index Codeword MVD
	0	1	0
	1	010	1
	2	011	-1
	3	00100	2
	4	00101	-2
	5	00110	3
	...	...	...

Table 2 :

 2 PSNR, SSIM & EDR values of original/reconstructed HEVC video and the average of different selective video shares.

	Test Sequences	original	ciphered versions
	(QP=22)	PSNR SSIM	PSNR SSIM EDR
	PeopleOnStreet	42.8	0.96	11.2	0.17	0.92
	Kimono	43.2	0.97	9.9	0.22	0.85
	ParkScene	42.3	0.97	10.7	0.2	0.88
	Cactus	41.5	0.96	8.4	0.23	0.83
	BQTerrace	40.8	0.96	7.8	0.21	0.9
	BasketballDrive	41.5	0.98	10.1	0.23	0.87
	Vidyo1	45.2	0.99	11.3	0.17	0.85

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ACKNOWLEDGEMENT

The authors would like to express their respectful thanks to their former colleague Pierre-André Laurent for years of patient support.


Claudine El Khoueiry

Fabrice Giusti

Evan Lelong

Guilhem Arrachart

Bilal Nsouli

Iyad Karame

Stéphane Pellet-Rostaing

Claudine El Khoueiry

Thiourea resole polymers for recovery of noble element

Keywords: Adsorption, chelating resin, thiourea monomers, formo-phenolic resin, noble metals

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

Introduction

Precious metals (Au, Pd, Pt, Rh, Ru, etc.) are widely used in different sectors (electronics, jewelry, medicine and catalysis) due to their specific physical and chemical properties. Related to their increasing demand and the limitations of the primary nonrenewable sources, the recovery of precious elements from industrial waste has attracted the attention of many researchers from the perspective of both economics and environmental protection [START_REF] Bigum | Environmental impacts and resource losses of incinerating misplaced household special wastes (WEEE, batteries, ink cartridges and cables)[END_REF][START_REF] Das | Recovery of precious metals through biosorption-A review[END_REF][START_REF] Ding | Recovery of precious metals from electronic waste and spent catalysts : A review[END_REF][START_REF] Tofan | Chelating polymers with valuable sorption potential for development of precious metal recycling technologies[END_REF]. In addition, the industrial wastewater has been classified as a potential secondary source of noble ions whose concentration is typically much higher than the very low concentration found in the primary source [START_REF] Hasegawa | Selective recovery of gold, palladium, or platinum from acidic waste solution[END_REF][START_REF] Zhang | A review of current progress of recycling technologies for metals from waste electrical and electronic equipment[END_REF]. To recover precious elements from wastewater, industrial techniques including the pyrometallurgical and hydrometallurgical processes have been widely applied [START_REF] Cyganowski | Synthesis of Adsorbents with Anion Exchange and Chelating Properties for Separation and Recovery of Precious Metals -A Review[END_REF][START_REF] Das | Recovery of precious metals through biosorption-A review[END_REF]. The hydrometallurgical process has been used more frequently for reasons related to its advantages of being relatively inexpensive, requiring low temperature conditions, being efficient, readily available, adaptable, and presenting high metal extraction efficiencies [START_REF] Ding | Recovery of precious metals from electronic waste and spent catalysts : A review[END_REF][START_REF] Quinet | Recovery of precious metals from electronic scrap by hydrometallurgical processing routes[END_REF][START_REF] Tofan | Chelating polymers with valuable sorption potential for development of precious metal recycling technologies[END_REF]. Currently, there is a viable interest in the recovery of metals using the standard separation processes, such as precipitation, ion exchange, liquid-liquid extraction, membrane filtration,

Experimental Section

Materials and methods

All chemicals and solvent used for the synthesis were analytically pure from Sigma Aldrich, France and used without further purification. The metal solutions were prepared from chlorate salt of Noble elements purchased from Acros (Thermo Fisher Scientific) except for gold which were prepared from the ICP standard (in 1% HCl from ICP Science).

Characterization

Flash-chromatography runs were performed on silica cartridges using a Reveleris X-2 apparatus.

The 1 H and 13 C NMR spectra were carried out in DMSO as a deuterated solvent with a Bruker Advance 400 MHz instrument in order to follow the progress of the reactions. The 13 C Solidstate MAS NMR spectra were recorded at rotation speed of 12 KHz (4 mm outer diameter rotors) with a Bruker Advance 400 MHz. Mechanical grinding of the resins was performed at 50 Hz for 10 min using a Retsch mixer mill MM 200 with a Zr ball. Thermogravimetric Analysis (TGA) was performed with a Mettler Toledo instrument under air or nitrogen flow with a heating rate of 10 °C/min from 25 to 950 °C. It should be noted that an isothermal treatment of 30 minutes at 100 °C was performed under air to determine the moisture regain by the resins from storage. Fourier Transform Infrared (FTIR) absorption spectroscopy was carried out on a Perkin Elmer 100 spectrometer between 500 and 4000 cm -1 using an ATR crystal with resolution of 4 cm -1 . Elemental analyses were recorded with an Elementar Vario Micro Cube Instrument to determine the composition of the materials. Scanning Electron Microscopy (SEM) was performed using a Quanta 200 ESEM FEG instrument in order to study and observe the morphology of the resins. Metal ion concentrations before and after the sorption were determined using Inductively Coupled Plasma/Atomic Emission Spectroscopy (ICP/OES ICAP Pro spectrometer from Thermo Scientific). The wavelengths used for the Au, Pd, and Rh measurements were chosen to avoid spectral interference between the elements.

Synthesis

General procedure for the synthesis of the ligand

Substituted thioureas were synthesized via a known synthetic route, similar to procedure reported in the literature [START_REF] Alkherraz | Synthesis and Use of Thiourea Derivative (1-Phenyl-3-Benzoyl-2-Thiourea) for Extraction of Cadmium Ion[END_REF]. The isothiocyanate derivatives, (1 eq of benzoyl, benzyl, acetyl or ethyl), commercially available compounds, was added dropwise by a syringe pump to a solution of tyramine (1 eq) in dry toluene preheated at 90 °C for 30 min. The mixture was heated for 12 h and then cooled in an ice bath. The resulting product was precipitated and filtered with cold toluene. Then the pure solid was obtained after washing with ether and pentane. The 1 H and 13 C NMR characterizations are reported in the supporting note.

Resins synthesis

The resins were synthesized via alkaline polycondensation of an aldehyde, formaldehyde, with phenolic compounds according to the procedures described in the literature [START_REF] Arrachart | Design and Evaluation of Chelating Resins through EDTA-and DTPA-Modified Ligands[END_REF][START_REF] Cruz | Selective extraction and separation of germanium by catechol based resins[END_REF][START_REF] Oye Auke | Terephthalaldehyde-Phenolic Resins as a Solid-Phase Extraction System for the Recovery of Rare-Earth Elements[END_REF]. The phenolic thiourea precursors were dissolved in an alkaline solution using 1.5 eq of sodium hydroxide or 2 eq of 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) and 80 eq of water. After solubilization of the thiourea ligands by simple stirring at room temperature, formaldehyde was added and the mixture was stirred for 24 h at 60 °C. The reaction progress was monitored periodically by 1 H and 13 C DEPT 135 NMR measurement. The prepolymers obtained were heated in an oven in air at 130 °C for 72 h. After curing, the polymer was crushed by ball milling and washed with H2O and 1 mol/L HCl to remove unreacted compounds and the uncrosslinked oligomers. The resin was then dried at 80 °C overnight.

Desorption

The solution of gold(III) was prepared from a 10 g/L ICP standard (in 1% HCl from ICP Science) after dilution with deionized water. The solutions of palladium (Pd 2+ ), rhodium (Rh 3+ ), ruthenium (Ru 3+ ), and platinum (Pt 2+ ) were prepared from their chloride salts. Typical sorption experiments were carried out using 10 mg of resin equilibrated with 10 mL solutions of the metal ions. Batch contacts were generally run for 24 hours to ensure that equilibrium was achieved. After shaking the mixture, it was filtered through a 0.2 µm cellulose filter, and the filtrate was analyzed by ICP OES to determine the residual metal ion concentration. The sorption efficiency E (%) was calculated using Equation 1, where Ci is the initial metal ion concentration in aqueous solution (mg/L) and Cf is the residual metal ion concentration in aqueous solution after sorption (mg/L).

𝐸 = 𝐶 i -𝐶 f 𝐶 i × 100
Equation 1

The sorption capacity Qe (mg/g) was estimated from Equation 2, where V is the volume of the metal contact solution (mL) and m represents the mass of the resin (mg).

𝑄 e = (𝐶 i -𝐶 f ) × 𝑉 𝑚 Equation 2
The equilibrium of adsorption was described using Langmuir and Freundlich isotherms [START_REF] Ghosal | Determination of thermodynamic parameters from Langmuir isotherm constant-revisited[END_REF][START_REF] Limousin | Sorption isotherms : A review on physical bases, modeling and measurement[END_REF]. The two models are defined in Equation 3and Equation 4, respectively, where Ce is the equilibrium metal ion concentration in liquid phase (mg/L), Qe is the equilibrium metal ion concentration in solid phase (mg/g), Qmax represents the maximum amount of metal ions adsorbed per unit mass of adsorbent (mg/g), and K is the Langmuir or Freundlich constant (L/mg).

𝐶 e 𝑄 e = 1 𝑄𝑚𝑎𝑥 × 𝐶 e + 1 𝐾 × 𝑄𝑚𝑎𝑥 Equation 3 𝑙𝑛 𝑄 𝑒 = 𝑙𝑛 𝐾 + 1 𝑛 𝑙𝑛 𝐶 𝑒 Equation 4
The selectivity of the resin towards the element A in the presence of the element B is expressed by the separation factors SA/B obtained according to Equation 5, where KD is the distribution coefficient [START_REF] Cruz | Selective extraction and separation of germanium by catechol based resins[END_REF], represented in Equation 6:

𝑆 A/B = 𝐾 DA 𝐾 DB Equation 5 𝐾 D = ( 𝐶 i -𝐶 f 𝐶 i ) × 𝑉 𝑚 Equation 6
Finally, desorption experiments were carried out with a solution of 1 mol/L thiourea. The desorption efficiency DE (%) is represented in Equation 7, where Qp is the concentration of metal ions adsorbed by the resin and Qf represents the concentration of residual metal ion content in the resin after desorption.

𝐷𝐸 = 𝑄 P -𝑄 f 𝑄 p × 100

Equation 7

Adsorption kinetics

The kinetic experiment was conducted in the same way as the batch contacts, except that the supernatant aliquots were analyzed periodically by ICP-OES. The kinetic data were analyzed using pseudo-first order and pseudo-second order kinetic models [START_REF] Azizian | Kinetic models of sorption : A theoretical analysis[END_REF][START_REF] Simonin | On the comparison of pseudo-first order and pseudo-second order rate laws in the modeling of adsorption kinetics[END_REF][START_REF] Yuh-Shan | Citation review of Lagergren kinetic rate equation on adsorption reactions[END_REF]. The two models are defined by the Equation 8 and Equation 9, respectively:

log(𝑄 e -𝑄 t ) = log 𝑄 e - 𝑘 1 2.303 × 𝑡 Equation 8 𝑡 𝑄 t = 1 𝑄 e × 𝑡 + 1 𝑘 2 × 𝑄 e 2 Equation 9
where Qe is the equilibrium metal ion concentration in solid phase (mg/g), Qt is the equilibrium metal ion concentration in solid phase at time t (mg/g) and k1 (min -1 ) and k2 (g mg -1 min -1 ) represent the pseudo-first order and pseudo-second order equilibrium rate constant, respectively.

Results and Discussion

Ligand synthesis and characterizations

The chelating phenolic thiourea monomers were synthesized using tyramine such as amine precursor. The chelating ligand was obtained from nucleophilic addition reaction as shown in Figure 1. The monomers of the four compounds were synthesized from benzoyl/benzyl, acetyl/ethyl isothiocyanate, and tyramine in dry toluene heated at 90 °C. The monomers were purified after a simple washing in ether and pentane, except for C3 which was purified by chromatography (cyclohexane/ethyl acetate), with yields of 93%, 90%, 60% and 80% as shown in Figure 1. The 1 H and 13 C NMR, FT-IR spectra were used to confirm the structure of the compounds (see supporting note).

Resin synthesis and characterizations

Preliminary studies were carried out regarding the synthesis of the prepolymers. Resole resins were prepared using an excess of formaldehyde compared to the monomers in basic medium. As shown in Figure 2, the prepolymers were prepared using 1 equivalent mass (1 eq) of thiourea monomer (C1, C2, C3, or C4) with 2.5 eq of formaldehyde and 1.5 / 2 eq of base in water. Instead of NaOH,DBU (i.e.,1,. 0]undec-7-ene) was used as an alkaline agent for the benzoyl and acetyl derivatives, thus avoiding the hydrolysis of the monomer that occurs in the presence of hydroxide ions. The pre-polymerization progress was monitored by using 1 H and 13 C DEPT-135 NMR spectroscopy by following the consumption of CH aromatics (disappearance of a signal with chemical shift at 115 ppm) of the monomer and the formation of CH2 methylol (appearance of a signal with chemical shift at around 60 ppm) (Figure S1). After 12 h at 60 °C, no change was noticed related to the consumption of monomer but a problem in the precipitation was appeared related to the beginning of condensation. To prevent this phenomenon, the reaction was stopped after the conversion of 70%-80% of the thiourea monomers. The pre-polymer was then heated in an oven at 130 °C for 72 h. After washing and conditioning of the insoluble resins (H2O, HCl 1 mol/L, H2O), a decrease in the mass was detected; this could be attributed to the elimination of excess reagent and the partially soluble oligomeric species.

After curing and washing steps, insoluble, amorphous, infusible polymers (R1, R2, R3, and R4) were obtained. In order to obtain a homogeneous particle size and surface contact, each resin was crushed by ball milling. The IR spectra for the four monomers and the resins, highlight the presence of the thiourea precursor in the matrix of the resins R1-R4 (Figure 3). Table 1 shows the characteristic FT-IR spectral data for the phenolic thiourea monomer based on Figure 3, in agreement with the literature [START_REF] Alkherraz | Synthesis and Use of Thiourea Derivative (1-Phenyl-3-Benzoyl-2-Thiourea) for Extraction of Cadmium Ion[END_REF][START_REF] Binzet | Synthesis and Characterization of Novel Thiourea Derivatives and Their Nickel and Copper Complexes[END_REF][START_REF] Panicker | FT-IR, FT-Raman and abinitio studies of 1,3-diphenyl thiourea[END_REF]. The broad peaks at about 3350 cm -1 were attributed to N-H and O-H stretching vibrations, the presence of the thiourea was highlighted by the C=S stretching vibration observed at about 1200 and 800 cm -1 , in addition to the absorption band at about 1280 and 1500 cm -1 corresponding to the N-C=S and C-N fragments, respectively. For C1 and C3, the presence of the strong C=O stretching vibration band at about 1670 cm -1 is consistent with their structures. Elemental analysis of the resins was carried out, giving the elemental composition of the different resins and highlighting the presence of sulfur in the resin (Table S1 see supporting note). This result has been correlated with 13 C MAS solid state NMR, with the displacement of the aromatic carbon at 125 ppm, the phenolic carbon at 145 ppm, and the methylene bridge carbon at around 30 ppm (Figure 4). The thermal properties of the resins, thermal stability, and moisture regain were determined by thermogravimetric analysis (TGA) under air and nitrogen atmospheres (Figure 5). Endothermic peaks in the derivative thermogravimetric curves showed the thermal decomposition of the resins. For all polymers, there are two stages of decomposition. For R1, R3, and R4, the weight loss between 0 and 100 °C was related to the loss of water. Furthermore, from 160 to 250 °C, all the resins showed the beginning of polymer degradation. This degradation could be related to the oxidation of remaining methylols and methylene bridges. The second weight loss from around 500 °C is related to the decomposition of the polymeric matrix, giving CO and CO2, benzaldehyde, and char [START_REF] Arrambide | Extraction and recovery of rare earths by chelating phenolic copolymers bearing diglycolamic acid or diglycolamide moieties[END_REF][START_REF] Jackson | High temperature oxidative degradation of phenolformaldehyde polycondensates[END_REF][START_REF] Jiang | The pyrolysis mechanism of phenol formaldehyde resin[END_REF].

Batch contact

First, a study of the influence of pH value on the sorption properties was carried out. During the experiments, special attention was paid to check the precipitation of metals, but nothing was observed at the concentrations used, with batch contact when performed at pH below 7. The adsorption capacities of the different metal ions (Au 3+ , Pd 2+ , Rh 3+ ) did not show any significant variation with the change of pH between 0 and 6 (Figure S2 see supporting note). From this point, the strong acid-based desorption theory (Oye Auke et al., 2022) was rejected. For the following studies, a pH=4 was used. Preliminary screening tests were performed to evaluate the affinity of the resins towards the adsorption of noble elements (Au, Pd, Pt, Rh, and Ru). The experiment was carried out by contacting 10 mg of resin with 10 ml of metal ion solution of 20 mg/L concentration of each metal. The results indicate that the R1 and R3 resins based on, benzoyl and acetyl thiourea monomers, respectively, present a good affinity for the different noble elements in contrast to R2 and R4, which were based on benzyl and alkyl thiourea monomers, respectively (Figure 6 and Table S2 in the supporting note). This can be explained by the presence of the carbonyl (C=O), in combination with thiourea, that promotes the sorption of the noble metals considered in this study. Figure S3 (see supporting note) shows the SEM analysis for R1 and R3 before and after loading with Au.

Adsorption isotherm

Based on the previous results, the sorption capacity and the selectivity of the various noble elements, specifically Au, Pd, and Rh, by R1 and R3 were studied.

The adsorption isotherms are based on the relationship between the two parameters: the loading capacity of the resin (Qe) and the concentration of the metal in the aqueous phase (Ce) measured at equilibrium at constant temperature and pH. The adsorption isotherms of R1 and R3 towards Au(III) are shown in Figure 7(a). The initial concentration of metal ions was varied from 10 to 1200 mg/L, while other parameters were kept constant. concentrations and then a plateau was reached at higher concentrations (Figure 8 and Figure S4 (a)). This was expected to be consistent with the monolayer adsorption phenomenon and was verified according to the Langmuir isotherm, indicating an isothermal chemisorption mechanism. According to the results of the adsorption experiments, it was possible to determine the maximum loading capacity of the resin (Qmax), using the slopes and intercepts of the plots (Figure 8(b) and Figure S4(c)). As a result, the Qmax values of R1 for Au and Pd were estimated as 350 and 50 mg/g, respectively. Moreover, the results indicated that the Qmax of R3 for Au and Pd were 500 mg/g and 70 mg/g, respectively.

Compared to some results found in the literature that were obtained with other thiourea resins, R1 and R3 showed a higher capacity the recovery of gold [START_REF] Ertan | Separation of gold(III) ions from copper(II) and zinc(II) ions using thiourea-formaldehyde or urea-formaldehyde chelating resins[END_REF][START_REF] Ronka | Gold(III) ions sorption on sulfur-containing polymeric sorbent based on 2,2'-thiobisethanol dimethacrylate[END_REF] and palladium [START_REF] Birinci | Separation and recovery of palladium(II) from base metal ions by melamine-formaldehyde-thiourea (MFT) chelating resin[END_REF][START_REF] Muslu | Selective separation and concentration of Pd(II) from Fe(III), Co(II), Ni(II), and Cu(II) ions using thiourea-formaldehyde resin[END_REF]. The high capacity achieved by the resins used in this work, compared to the materials described in the literature could be explained by the presence of the carbonyl function, which has made the chelation of noble metals with thiourea more efficient. Indeed, the coordinating properties of the neutral oxygen donor of the amide groups with metal ions are well documented in the literature. This ability to coordinate with metal ions occurs through the carbonyl oxygen of the amide group [START_REF] Sigel | Coordinating properties of the amide bond. Stability and structure of metal ion complexes of peptides and related ligands[END_REF]. Due to the rapid increase at low concentration, it was concluded that the resins presented a high affinity towards Au(III) and Pd(II) ions. However, for Rh(III), R1 presented non-ideal multilayer adsorption, as it was verified that the adsorption law followed the Freundlich isotherm (Figure S4(b)). According to the experimental data, it was determined that the maximum capacity of R1 for Rh was found to be 30 mg/g, while the sorption of Rh by R3 showed a similarity to the sorption of Au and Pd with Qmax equal to 60 mg/g (Oye [START_REF] Oye Auke | Terephthalaldehyde-Phenolic Resins as a Solid-Phase Extraction System for the Recovery of Rare-Earth Elements[END_REF][START_REF] Sahoo | Chapter 7 -Adsorption processes for the removal of contaminants from wastewater : The perspective role of nanomaterials and nanotechnology[END_REF].

In terms of adsorption energy, considering the interactions of Au and Pd with the R1 or the R3 resin, it can be assumed that all adsorption sites are energetically equivalent. For Rh, the adsorption did not occur uniformly at the surface of the R1 adsorbent, whereas it was found to be homogeneous at the surface of the R3 material. Moreover, based on the results from this work, it can be established that the different adsorption mechanisms for the three soft noble elements, are influenced by the HSAB rules, the degree of oxidation, and the environment of the binding site. In consequence, both R1 and R3 resins have a good affinity for the three noble elements, in a decreasing order from highest to lowest capacity: Au>Pd>Rh.

Selectivity

Different experiments were carried out to study the selectivity of the resin towards the noble elements compared to the competitive elements (The experimental data are provided in Table S3 in the supporting note). Figure 8(a, b) shows that the resins have higher selectivity to the noble elements than the other competitive elements, usually found in the industrial waste, such as Fe, Ni, Cu, Al, and Zn. These elements are generally associated with secondary sources [START_REF] Tofan | Chelating polymers with valuable sorption potential for development of precious metal recycling technologies[END_REF].

Although, the competitive elements were present in high concentrations (Al: 1620, Fe: 2500, Zn: 970, Ni: 90, and Cu: 150 mg/L) the selectivity of the material was not affected (Figure 8(b)). The S factor of selectivity for the noble elements compared to competitive ones is higher than 500. Regarding the selectivity of noble elements, both resins showed the highest percentage of sorption for Au (Figure 8(c)). This difference does not seem to be linked to the hardness or softness character, since the three elements are classified as soft acids. The different oxidation state (Au(III), Pd(II) or Rh(III)), as well as the size and hydration of the metal ions could explain the observed differences in terms of sorption efficiency.

Kinetic studies

Adsorption kinetics provide information about the rate of adsorption and the time required to reach equilibrium. The adsorption kinetics of Au (Figure 9(a)), Pd, and Rh (Figure S5) on the R1 and R3 resins have been evaluated at room temperature. The kinetic data were fitted with pseudo-first order and pseudo-second order kinetic models, as discussed in the experimental section, to investigate the mechanism of each adsorption process. For Au, the model with the best correlation is presented in Figure 9(b); for Pd and Rh the kinetic models are presented in the supplementary data (Figure S5). Sorption experiments: 10 mL of Au (400 mg/L) at pH=4 contacted with 10 mg of resin R1 for 0-5 h at 25°C; 10 mL of Au (600 mg/L) at pH=4 contacted with 10 mg of resin R3 for 0-5 h at 25°C.

For the recovery of Au, it was found that the pseudo-second order model provided a better fit, with a high correlation coefficient of 0.990 (Table 2); the same observations were made for Pd (Table S4) for both resins R1 and R3. However, for Rh adsorption by R1 (Table S5), the pseudo-first order model was more suitable, consistent with previous studies [START_REF] Sahoo | Chapter 7 -Adsorption processes for the removal of contaminants from wastewater : The perspective role of nanomaterials and nanotechnology[END_REF]. The results showed that the recoveries of gold and palladium by both resins were achieved after 5 hours. Furthermore, R3 showed slow kinetics for rhodium.

In relation to the kinetic adsorption isotherms, the results defined that chemical sorption, or chemisorption, determines the sorption of Au and Pd, where the adsorption rate depends on the adsorption capacity and not on the concentration of adsorbates. However, for Rh adsorption by R1, the results suggested a physiosorption, where the adsorption rate was related to the difference between the saturation concentration and the amounts that are adsorbed with time [START_REF] Liu | Temperatureinduced adsorption and desorption of phosphate on poly(acrylic acid-co-N-[3-(dimethylamino)propyl]acrylamide) hydrogels in aqueous solutions[END_REF][START_REF] Vadivelan | Equilibrium, kinetics, mechanism, and process design for the sorption of methylene blue onto rice husk[END_REF]. The results for Rh by R3 were similar to the recoveries of Au and Pd, but since the complex formation reaction of Rh(III) complex is extremely slow, this should be taken with caution.

Desorption

The desorption (elution, stripping or back-extraction process), known as adsorbent regeneration, is critical. It allows adsorbents to be reused and regenerated while retaining sorption efficiency, cost reduction and lower environmental impact. Different reagents are used as desorption agents for noble elements. They include acids and chelating complexing agents such as cyanide, thioureas, and thiosulfate [START_REF] Guo | Review : Adsorbents for the recovery of precious metals from wastewater[END_REF]. The use of acid was eliminated based on the results of the effect of pH on the sorption efficiency.

Experiments using thiosulfate and thiourea as desorption agents was carried out for resin R1 (Table S6). The thiosulfate solution (2 mol/L) offered low desorption. However, the use of thiourea solution at different concentrations and pH values resulted in successful desorption. Three different solutions with 0.5 mol/L and 1 mol/L at pH 4 and pH 2 were examined. The solution prepared from 1 mol/L thiourea at pH 2 produced the best results, with a desorption efficiency of 99%.

The regeneration and reusability of the resin R1 was investigated for the Au(III) by adsorptiondesorption up to five cycles (Table S3 in the supporting note). After adsorption of Au(III) ions, the loaded resin was washed with a 1 M thiourea solution at pH 2. After each adsorptiondesorption cycle, the R1 resin was washed and dried to be reused for other adsorption cycles.

After five cycles, the recycled R1 had almost the same removal affinity as the original R1 resin (Table 3). 97.97 ± 0.75 98.01 ± 0.81 Sorption experiments: 10 mL of Au solution at 100 mg/L, pH=4, contacted with 10 mg of resin R1 for 24 h at 25 °C; Desorption experiments: 10 mL of 1 mol/L thiourea solution, pH=2, contacted with 10 mg of resin loaded R1-Au for 24 h at 25 °C.

Conclusion

Three new chelating sorbents (R1, R2 and R3) based on thiourea resole materials were obtained by a polymerization reaction between phenolic thiourea monomers (synthesized from tyramine derivatives) and formaldehyde under basic conditions. The three materials were engaged in sorption experiments with respect to noble elements. The resin R1 and R3 based on benzoyl (C1) and acetyl (C3) thiourea monomers have shown a high affinity for noble metals, with remarkable sorption capacities of 350 -500 mg/g for Au, 50 -70 mg/g for Pd, and 30 -60 mg/g for Rh. This behavior may be related to the combination of carbonyl and thiourea groups in such resins. In addition, these resins have presented a higher selectivity for noble metals than the competitive metals in industrial effluents, such as Zn, Cu, Ni, Fe, and Al. The adsorption mechanism towards Au and Pd could be well described by Langmuir monolayer chemical adsorption, while for Rh it was described by Langmuir and Freundlich isotherms. Preliminary studies indicated that noble metals could be stripped from the loaded resins using a 1 mol/L thiourea solution at pH= 2. As a result, thiourea resole materials are promising candidates for the recovery of noble elements, specifically gold.

Figure 1 :

 1 Figure 1: Synthesis of thiourea monomers.

Figure 2 :

 2 Figure 2: Illustration of the resins synthesis.

Figure 3 :

 3 Figure 3: FT-IR spectra for thiourea ligand and their appropriate polymer (resin).

Figure 4 :

 4 Figure 4: 13 C solid state NMR spectra of R1, R2, R3 and R4.

Figure 5 :

 5 Figure 5: TGA thermogram of resins recorded under air (a) and under nitrogen (b) atmosphere and corresponding derivatives curves under air (c) and nitrogen (d) atmosphere (The arrow indicates the endothermic direction in the derivative thermogravimetric curves).

Figure 6 :

 6 Figure 6: Affinity of the different polymers to noble metals. Sorption experiments: 10 mL of a mixture of Au, Pd, Pt, Rh, and Ru (20 mg/L of each elements at pH=4), contacted with 10 mg of resin R1, R2, R3 or R4 for 24 h at 25°C.

Figure 7 :

 7 Figure 7: a) Adsorption isotherms yielded for Au(III) on R1 or on R3; b) Plot of Langmuir isotherm equation for Au adsorption by R1 or by R3 resins. Sorption experiments: 10 mL of Au (10 to 1200 mg/L of elements at pH=4), contacted with 10 mg of resin R1 or R3 for 24 h at 25 °C.The adsorption data were fitted by Langmuir for Au(III) and Pd(II) and by Langmuir and Freundlich for Rh(III). For Au(III), the models with the best correlation are presented in Figure7(b). However, the adsorption data for Pd and Rh are represented in FigureS4(see supporting note).The sorption of Au and Pd for both R1 and R3 resins showed a rapid increase at low

Figure 8 :

 8 Figure 8: Selective sorption experiments : (a) 10 mL of a mixture of Au, Fe, Al, Zn, Cu and Ni at 20 mg/L of each elements at pH=4 contacted with 10 mg of resin R1 or R3 for 24 h at 25 °C; (b) 10 mL of a mixture of Pd (450 mg/L), Fe (2500 mg/L), Al (1620 mg/L), Zn (970 mg/L), Cu (6 mg/L), Ni (90 mg/L) at pH=1.2 contacted with 60 mg of resin R1 or R3 for 24 h at 25°C; (c) 10 mL of a mixture of Au, Pd, Pt, Rh, and Ru at 400 mg/L of each element at pH=3 contacted with 10 mg of resin R1 or R3 for 24 h at 25 °C.

Figure 9 :

 9 Figure 9:Adsorption Kinetics studies of Au by R1 and R3 (a) using pseudo-second order (b) and pseudo-first order (c). Sorption experiments: 10 mL of Au (400 mg/L) at pH=4 contacted with 10 mg of resin R1 for 0-5 h at 25°C; 10 mL of Au (600 mg/L) at pH=4 contacted with 10 mg of resin R3 for 0-5 h at 25°C.

Table 1 .

 1 Characteristic FT-IR spectral band position for ligand and polymer

	Wavenumber	Band assignment	Reference
	(cm -1 )
	3350	N-H ; O-H	Alkherraz et al., 2014 ; Binzet et al., 2013;Panicker et al., 2010
	3200	C-H	Alkherraz et al., 2014 ; Binzet et al., 2013;Panicker et al., 2010
	1670	C=O	Alkherraz et al., 2014 ; Binzet et al., 2013
	1500	C-N; C-C	Alkherraz et al., 2014 ; Binzet et al., 2013;Panicker et al., 2010
	1280	N-C=S	Alkherraz et al., 2014 ; Binzet et al., 2013;Panicker et al., 2010
	1200	C=S	Alkherraz et al., 2014 ; Binzet et al., 2013;Panicker et al., 2010
	800	C=S	Alkherraz et al., 2014; Panicker et al., 2010

Table 2 :

 2 Kinetic parameters for Au(III) uptake on R1 and R3.

	Kinetic Model	Resins	Qe (mg g -1 )	k1 (min -1 )	k2 (g mg -1 min -1 )	R 2
	Pseudo-First Order	R1 R3	230 330	6.0x10 -3 6.0x10 -3	--	0.933 0.777
	Pseudo-Second Order	R1 R3	333 556	--	4.1x10 -5 4x10 -5	0.991 0.975

Table 3 .

 3 Au sorption and desorption efficiencies for R1.

		Au sorption (%) Au recovery (%)
	Cycle 1	98.97 ± 0.99	99.47 ± 1.05
	Cycle 2	99.89 ± 1.10	98.89 ± 0.97
	Cycle 3	98.86 ± 0.96	98.78 ± 0.95
	Cycle 4	99.09 ± 0.99	98.50 ± 0.86
	Cycle 5

Scientific Research (CNRS-L) for co-financing the project and providing the doctoral fellowship. The authors are especially grateful to Beatrice BAUS-LAGARDE for the ICP analyses, Cyrielle REY for TGA analyses and to Sandra MAYNADIE for the solid state NMR studies.

Acknowledgments:

The authors would like to thank the French Embassy and the Lebanese National Council for

Supplementary notes:

Ligand characterizations; Figure S1 S1: Elemental analysis; Table S2 and Table S3: sorption data; Table S4 and Table S5: kinetic parameters. Table S6: Recovery of Au from R1 loaded resin; Table S7: sorption-desorption data.


archive ouverte pluridisciplinaire HAL, est destinée au dépôt

Nota bene: meeting postponed sine die after text sent to organizers at their request Sophie Duchesne, Centre Emile Durkheim (CNRS-Sciences Po Bordeaux). s.duchesne@sciencespobordeaux.fr

As my positions towards all forms of naturalization of nationalism are public [START_REF] Duchesne | Who's afraid of Banal Nationalism? A symposium[END_REF], I suppose I will not surprise by stating my strong disagreement with the central thesis of Robert Schertzer and Eric Taylor Woods' book -I even assume that this is the reason why I was invited to discuss this book. We know that our colleague Cynthia Miller-Idriss, the other discussant for this debate, is very fond of the book, which she describes in her endorsement as an "exceptional read". Knowing that my opinion is in the minority, I take the liberty of expressing it frankly.

I will start by summarising how I understood the book. I will then indicate why I think the authors do not demonstrate what they say they have proven. Then I will point out what the book produces, perhaps unintentionally, but surely, and seriously, namely to feed white nationalism, which seems to me very inappropriate in view of the climate crisis we are experiencing.

A thesis on the so-called deep roots of white nationalism I would summarise the thesis of the book as follows: the ethnic nationalism of the majority populations of Western countries -the white populations -is deeply rooted in their history. It is an integral part of the identity of these nations, and therefore the identity aspirations of these populations must be seriously considered.

This thesis is based on a comparative analysis of the tweets produced during the election campaign by Donald Trump, Marine Le Pen and the promoters of Brexit (5517 Trump tweets were coded, 2852 for Le Pen and 7375 for Brexit). The content of these tweets is put into perspective with the authors' historical syntheses of the myths and symbols affirmed and developed over the course of the long term, which form the basis of ethnic nationalism in these three countries. These syntheses are constructed with the help of an analytical grid nourished by the work of ethnosymbolist historians, to whom the authors frequently refer, first and foremost Anthony Smith and John Hutchinson (to whom the book is dedicated). The authors highlight the representations of the ethnic populations that were dominant at the time of national construction, regarding the characteristics of their people, their country, their religion and their ethos. They describe the persistence of these representations, rooted in the certainty of these white people moral and religious superiority, or even their genetic superiority; but they also show how they adapted over time. They thus underline the strong rejection of Islam and Muslims in the recent period. The analysis of the tweets shows that during the three campaigns mentioned, Trump, Le Pen and the 'leave' leaders are very much the vehicles for these representations -or, as the authors put it: "they taped into longstanding myths and symbols'.

Robert Schertzer and Eric Taylor Woods indeed argue that the proximity of the content of these tweets to the longstanding myths and symbols they have analysed is the sign of the deep identity of these white populations, end explains the mobilisation capacity of these messages and beyond, the electoral success of populist and nationalist leaders. Since this ethnic nationalism has deep roots, it is not going to disappear any time soon and they conclude that it must be taken into serious consideration.

A questionable puzzle setting

The demonstration proposed by the authors seems to me to suffer from several weaknesses, starting with the construction of the puzzle. Their aim is to make understandable the rise of ethnic nationalism in countries that embrace civic nationalism -or even originated it. To focus on the French and even British cases, which I know a little better, the presence of Marine Le Pen in the second round of the presidential election would have surprised observers, as would the result of the Brexit referendum, because the nationalism that these leaders defend, white nationalism, would be considered to have disappeared in these countries long ago 1 . In chapter 2, where they define the central concept of the study, ethnic nationalism, the authors note, however, that most scholars of contemporary nationalism have taken a stand against the distinction between civic and ethnic nationalisms. This distinction does not allow for the observation of the profound imbrication between the different registers of national preference. In other words, it is the category of analysis that is no longer used, not the facts that are denied. Contrary to the authors' suggested puzzle, many scholars have been observing the rise of 'hot' nationalism in Europe for years and did not wait until 2016 to take notice 2 . This is actually the rationale behind Billig's thesis of banal nationalism [START_REF] Billig | Banal nationalism[END_REF] -which the authors confuse with that of everyday nationalism. To understand the omnipresence of nationalism, it is necessary to look at it in all its manifestations and not just at its vehement forms. For the soft or 'cold' forms that we maintain and value are only the other side of the coin, and prepare the ground for new outpourings. In other words, there is no difference in nature between Le Pen's hot nationalism and Macron's patriotism. Both, like all politicians, naturalise the division of the world into nations, a division which has been the source of legitimacy for rulers on a worldwide scale since the founding of nations. After the Second World War, the horrors of the Holocaust and the hopes raised by the reconstruction and decolonisation have for a time moderated -or ''cooled, to remain in Billig's repertoire -nationalism. But it remained there, maintained by the multiplicity of flaggings, hardly visible but omnipresent. Visually and verbally, citizens are constantly reminded that they are nationals. These constant reminders construct a representation of the world that essentializes the nation, which is, though, as historians have shown, only a historical form of government that corresponded to a period in human history that is now over. This representation of the world, this ideology, has ended up becoming a universal ideology [START_REF] Billig | Nationalism as an International Ideology: Imagining the Nation, Others and the World of Nations[END_REF], with the formerly colonised countries not being lagging behind in terms of nationalism, even though they are the objective losers of the deeply, ontologically unequal and violent international system. Robert Schertzer and Eric Taylor Woods are right to consider that the pervasiveness of nationalism today is probably not analysed seriously enough. Their book has the advantage of pointing out the horrors and lies that nationalism maintains. But I think they are deeply wrong in trying to explain its omnipotence by the depth of its roots.

1 Moreover, the authors consider that the explanations proposed to account for these explosions of ethnic nationalism would be summed up by observing the concomitant increase in inequalities. This is also debatable.

2

I am thinking, for example, of the work (in alphabetical order) of Gil Delannoi, Xavier Crettiez, Guy Hermet, Vincent Martigny, Nonna Mayer, Pierre-André Taguieff, Anne-Marie Thiesse, Michel Winock, not to mention the many specialists of the Front/Rassemblement National. Some of them also analyse this nationalism over time, but most are careful to take into account the contemporary political situation and the state of the political scene.

An 'explanation' lacking in evidence

In fact, it is difficult to consider that showing how the tweets of our leaders carry nationalist and racist representations, which have been part of ideological repertoires for centuries, is enough to prove that ethnic nationalism is a matter of identity and thus cannot be ignored. I leave it to the historians who defend an ethnosymbolist conception of the genesis of nations to comment on the historical syntheses proposed by the authors, who bring together ideas expressed centuries apart, as if this proved that they were indeed longstanding myths, symbols and hence identity. This transfer of ideas to identity is hardly explained, except by recourse to a definition of nationalism proposed by Anthony Smith. To pose a definition does not prove anything.

It should be noted that the analysis of these tweets is very interesting. I regret that the pages devoted to it are so brief, and that no table or figure systematically highlights the results of an obviously very substantial coding, which represented a lot of work 3 . In the end, this coding is only used to provide the authors with the examples they use to illustrate their thesis. The three chapters devoted to the way in which these tweets draw on the ideological repertoires of white nationalism are reminiscent of the standardised nature of the ideological repertoire that accompanied the birth of nations -which Anne-Marie Thiesse has happily called "the identity checklist" (Thiesse, 2005) -and which contrasts with the claim to uniqueness and, in this case, to the superiority of the "people".

Of course, R. Schertzer and E. T. Woods are right: it is not enough to want people to stop being nationalists for them to refrain from it. But the job of social science, it seems to me, is to show what continues to build nationalism, rather than to help naturalise it further. The authors indicate several times that they do not question the modernity of nations, and therefore the historicity of nationalism, a thesis that is now considered historically valid. But the demonstration they propose does have the effect -voluntary or not, I do not want to assume their intentions -of essentializing nationalism anew. All the more so because the book is nicely written and easy to read. From my point of view, it will contribute directly to the reinforcement of nationalism, not to a better understanding, as the authors claim they want to do.

Don't look ahead! 4

Helping to sustain nationalism today -especially in its most aggressive, unjust form: white ethnic nationalism -seems to me particularly serious in the context of the climate crisis. Robert Schertzer and Eric Taylor Woods explain in the book how Western ethnic nationalism is being revived by recurring crises that challenge white identity in Western countries. Surprisingly, when talking about the crises we have been experiencing since the beginning of the century, they make very little mention of globalisation, which has caused governments to lose control of the economy to a large extent, and thus deeply alters the legitimacy of the (inter)nationalist model. White nationalist leaders can always tell their constituents that they want to regain control of their nation's fate: The (real) organisation of the world today no longer gives them the capacity to do so.

The nation is a historically situated system of world government. It has fostered the development of capitalism, democracy and a form of national wealth-sharing through public services and the welfare 3

The book links to the Havard dataverse where the authors have deposited the corpus and the codebooks, which they make available graciously to colleagues who request them. But the documents transmitted do not provide any further information on the frequency of the various codes and the possible cross-referencing.

4 This is (obviously) a reference to Adam McKay's "Don't look up!", released in 2021 on Netflix.

state; but that is no longer the case. The economic forces are largely playing out in a supranational mode. More seriously, the climate crisis urgently requires that human leaders be able to go beyond national preferences and consider the future of the planet in solidarity. It is quite astonishing to see a political science book published today by a major academic publisher that deals with the electoral consultations of the major powers, and in which the climate is never mentioned. If proof were needed, the certainty that the national model is outdated can be found in the urgency of the climate transition. Anything that slows down the awareness of the people, and particularly those of the dominant white populations who contribute in the first place to the degradation of a planet that belongs to all, is regrettable. Using the authority of the social sciences today to 'prove' the deep roots of white identity and thus contribute to the re-essentialisation of nationalism seems to me, at best, a rearguard struggle.

  Book review: The New nationalism in America and beyond. The deep root of ethnic nationalism in the digital age. Robert Schertzer and Eric Taylor Woods (OUP, 2022) Sophie Duchesne To cite this version: Sophie Duchesne. Book review: The New nationalism in America and beyond. The deep root of ethnic nationalism in the digital age. Robert Schertzer and Eric Taylor Woods (OUP, 2022). 2023. hal-04384645 Debate. The New nationalism in America and beyond. The deep root of ethnic nationalism in the digital age. Robert Schertzer and Eric Taylor Woods (OUP, 2022) London School of Economics (3 May Sumeet Valrani Lecture Theatre at 6pm).

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