Alexandre Poirier

Korin Ozkaya

Julie Gredziak

Delphine Talbot

Dr Niki Baccile
email: niki.baccile@sorbonne-universite.fr

Heavy metal removal from water using the metallogelation properties of a new glycolipid biosurfactant

Water pollution by heavy metals is a problem of both western and developing countries.

Heavy metal pollution can be associated to human activity, such as wastewaters from processing of ore mining, but also to simple contamination from metal rich soils. Whichever the case, chemical and physical methods are generally employed to depollute water. Since most chemicals are themselves polluting agents, there is an increasing interest in finding biobased and biodegradable alternative chemicals, both efficient in removing metals and benign to the environment. Biosurfactants are green chemicals produced by fermentation of yeasts and bacteria and with a good environmental score. Among many applications, this class of compounds has been used to remove heavy metals from contaminated soils. Within this framework, we propose a new mechanism of depolluting water using a glucolipid biosurfactant, G-C18:1, composed of glucose (G) and a C18:1 fatty acid (oleic acid). This compound is able to form a metallogel by complexing cations in water, thus trapping heavy metals (Cu 2+ , Ni 2+ , Cr 2+ and Co 2+ ) in the gel phase. This mechanism allows to remove up to 95% for cobalt and 88 ± 10%, 80 ± 3% and 59 ± 6% for Cu 2+ , Ni 2+ and Cr 2+ , respectively. A dedicated structural study shows that this is possible because positively charged species induce gelation of G-C18:1 through a micelle-to-wormlike phase transition, most likely driven by a charge neutralization process. This work shows that wise control of the nanoscale properties of green chemicals can strongly benefit to develop a sustainable future.

Introduction

Wastewater remediation is a crucial challenge worldwide, may it occur in rich or developing countries. Different methods are currently employed together, mainly combining physical processes, like membranes filtration and decantation, associated to a chemical process, involving flocculation. [START_REF] Lee | A review on application of flocculants in wastewater treatment[END_REF] In the latter, colloids are coagulated by charge screening and flocculation occurs by inter-colloidal bridging in flocs, which can sediment or filtered out.

In the last decades, metal salts were widely used as flocculants, however of intermediate efficiency, for their low cost compared to more efficient synthetic polymers [START_REF] Sarika | Treatment of olive mill effluents: Part II. Complete removal of solids by direct flocculation with poly-electrolytes[END_REF] or bio-flocculants. [START_REF] Othmani | Toward green technology: a review on some efficient model plant-based coagulants/flocculants for freshwater and wastewater remediation[END_REF] However, metal salt flocculants have been limited along the years due to the large amounts of toxic metal released in the nature.

Aluminium, a well-known flocculant, is known to affect living organisms, [START_REF] Ward | Influence of aqueous aluminium on the immune system of the freshwater crayfish Pacifasticus leniusculus[END_REF] especially acting as neurotoxic agent for human brain and which could contribute to the Alzheimer disease. [START_REF] Baes | Aluminum complexing enhances amyloid β protein penetration of blood-brain barrier[END_REF] Depollution from heavy metals is itself an extensively studied problem [START_REF] Fu | Removal of heavy metal ions from wastewaters: A review[END_REF] and one can distinguish four methods based on: adsorption by porous materials, membrane filtration, electric separation and chemical flocculation. The latter is well-mastered, but it employs large amounts of non-biodegradable chemicals. [START_REF] Qasem | Removal of heavy metal ions from wastewater: a comprehensive and critical review[END_REF] Nowadays, other potential chemicals are tested for heavy metal depollution, mostly based on plant mixture or living plants, [START_REF] Amari | Cactus material-based adsorbents for the removal of heavy metals and dyes: A review[END_REF][START_REF] Nedjimi | Phytoremediation: a sustainable environmental technology for heavy metals decontamination[END_REF] hydroxyapatite bones derivatives [START_REF] Brazdis | Recent progress in the application of hydroxyapatite for the adsorption of heavy metals from water matrices[END_REF] or even fly ash, a known by-product of power production. [START_REF] Gupta | Fly ash-based geopolymers: an emerging sustainable solution for heavy metal remediation from aqueous medium[END_REF] In this context, low molecular weight (LMW) compounds, like surfactants, are well-known flocculants, employed since long time. [START_REF] Jian-Xiao | Interaction mechanisms between anionic surfactant micelles and different metal ions in aqueous solutions[END_REF][START_REF] Paton | Effect of pH on the Flocculation of SDS micelles by Al3+[END_REF][START_REF] Somasundaran | Precipitation-Redissolution Phenomena in Sulfonate-AlCl3 Solutions[END_REF] However, the environmental concern of surfactants is itself an issue, and other LMW amphiphiles have been looked at as possible substituents for petrochemical compounds. Peptides amphiphiles [START_REF] Abdellatif | Efficient and eco-friendly lowmolecular-weight gelators based on L-phenylalanine as promising remediation tool for oil pollution[END_REF][START_REF] Bhattacharya | First report of phase selective gelation of oil from oil/water mixtures. Possible implications toward containing oil spills[END_REF]Suzuki et al., 2006) and, more importantly, microbial biosurfactants, like rhamnolipids, sophorolipids or surfactin, (Q. [START_REF] Chen | Removal of Pb and Hg from marine intertidal sediment by using rhamnolipid biosurfactant produced by a Pseudomonas aeruginosa strain[END_REF][START_REF] Hari | Rhamnolipid-Metal Ions (CrVI and PbII) Complexes: Spectrophotometric, Conductometric, and Surface Tension Measurement Studies[END_REF][START_REF] Malkapuram | A review on recent advances in the application of biosurfactants in wastewater treatment[END_REF][START_REF] Mishra | Biosurfactant is a powerful tool for the bioremediation of heavy metals from contaminated soils[END_REF][START_REF] Mulligan | Sustainable Remediation of Contaminated Soil Using Biosurfactants[END_REF][START_REF] Singh | Efficiency of lipopeptide biosurfactants in removal of petroleum hydrocarbons and heavy metals from contaminated soil[END_REF] have been shown to have interesting depollution properties against both organic and heavy metal pollutants. However, in the latter case, the essential work, which has started more than thirty years ago, has been carried out on soil pollution, (Q. [START_REF] Chen | Removal of Pb and Hg from marine intertidal sediment by using rhamnolipid biosurfactant produced by a Pseudomonas aeruginosa strain[END_REF][START_REF] Kim | Removal of Lead from Contaminated Water and Clay Soil Using a Biosurfactant[END_REF][START_REF] Malkapuram | A review on recent advances in the application of biosurfactants in wastewater treatment[END_REF][START_REF] Miller | Biosurfactant-facilitated Remediation of Metal-contaminated Soils[END_REF][START_REF] Mishra | Biosurfactant is a powerful tool for the bioremediation of heavy metals from contaminated soils[END_REF][START_REF] Mulligan | Surfactant-enhanced remediation of contaminated soil: A review[END_REF][START_REF] Mulligan | Environmental applications for biosurfactants[END_REF][START_REF] Mulligan | Recent advances in the environmental applications of biosurfactants[END_REF], 2021;Catherine N. Mulligan, Yong, & Gibbs, 1999;Catherine N. Mulligan, Yong, Gibbs, et al., 1999;[START_REF] Mulligan | Remediation of a heavy metal-contaminated soil by a rhamnolipid foam[END_REF][START_REF] Prakash | Bio-electrokinetic remediation of crude oil contaminated soil enhanced by bacterial biosurfactant[END_REF][START_REF] Sonowal | Biosurfactant-assisted phytoremediation of potentially toxic elements in soil: Green technology for meeting the United Nations Sustainable Development Goals[END_REF] with relatively less contributions to water depollution. Despite such large engineering efforts, including the development of performing extraction processes like flotation [START_REF] Dhar | Toward green flotation: Interaction of a sophorolipid biosurfactant with a copper sulfide[END_REF]Dhar, Thornhill, et al., 2021;[START_REF] Malkapuram | A review on recent advances in the application of biosurfactants in wastewater treatment[END_REF] or electrokinetic remediation, [START_REF] Tang | Heavy metal removal effectiveness, flow direction and speciation variations in the sludge during the biosurfactant-enhanced electrokinetic remediation[END_REF] little is still known about the fundamental interaction mechanisms of between heavy metals and biosurfactants, the latter mainly concerning rhamnolipids. [START_REF] Dahrazma | Effects of additives on the structure of rhamnolipid (biosurfactant): A small-angle neutron scattering (SANS) study[END_REF]Ochoa-Loza et al., 2001a) In this work, we present the heavy metal removal ability of a new glycolipid biosurfactant, G-C18:1 (Figure 1), consisting in a glucose group covalently linked to a C18:1 cis fatty acid. This compound, prepared by fermentation of S. bombicola ΔugtB1 in the presence of glucose and vegetable oils, like rapeseed oil, [START_REF] Saerens | Cloning and functional characterization of the UDP-glucosyltransferase UgtB1 involved in sophorolipid production by Candida bombicola and creation of a glucolipid-producing yeast strain[END_REF] is much less known and studied than other biosurfactants, like sophorolipids or rhamnolipids. Nevertheless, recent results

show that G-C18:1 strongly interacts with alkaline earth and transition metal ions, which trigger gelation through micelle-to-wormlike or micelle-to-fiber phase transitions. [START_REF] Poirier | Shear recovery and temperature stability of Ca2+ and Ag+ glycolipid fibrillar metallogels with unusual βsheet-like domains[END_REF][START_REF] Poirier | Ca2+ and Ag+ orient low-molecular weight amphiphile self-assembly into fibrillar hydrogels with unusual β-sheet-like structure[END_REF]Poirier, Griel, Perez, & Baccile, 2022;[START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] The structure of the micelles and fibers, as well as the conditions under which they are formed and elastic properties of the gels have been thoroughly studied for selected metal ions by mean of cryogenic transmission electron microscopy, rheology and in situ small angle X-ray scattering. [START_REF] Poirier | Shear recovery and temperature stability of Ca2+ and Ag+ glycolipid fibrillar metallogels with unusual βsheet-like domains[END_REF][START_REF] Poirier | Ca2+ and Ag+ orient low-molecular weight amphiphile self-assembly into fibrillar hydrogels with unusual β-sheet-like structure[END_REF]Poirier, Griel, Perez, & Baccile, 2022) On the other hand, the interactions between G-C18:1 and metal ions have been probed by isothermal titration calorimetry and solid state nuclear magnetic resonance. [START_REF] Baccile | Chameleonic Amphiphile: the Unique Multiple Self-Assembly Properties of a Natural Glycolipid in Excess of Water[END_REF]Poirier, Griel, Perez, & Baccile, 2022) These data are unique in the panorama of biosurfactant science, where their interactions with metal ions have only been rarely studied [START_REF] Dahrazma | Effects of additives on the structure of rhamnolipid (biosurfactant): A small-angle neutron scattering (SANS) study[END_REF][START_REF] Miller | Biosurfactant-facilitated Remediation of Metal-contaminated Soils[END_REF] and a number of questions are still open.

By controlling the effect of cations on the self-assembled structure and macroscopic properties of G-C18:1, we explore in this work the removal of Cu 2+ , Ni 2+ , Cr 2+ and Co 2+ , all well-known groundwater pollutants, (Https://Www.Epa.Gov/Ground-Water-and-Drinking-Water/National-Primary-Drinking-Water-Regulations#Inorganic, n.d.; [START_REF] Vetrimurugan | Human exposure risk to heavy metals through groundwater used for drinking in an intensively irrigated river delta[END_REF] through metallogelation. This approach, unprecedented for biosurfactants, is controlled by tuning the [ion]/[G-C18:1] molar ratio and pH. [START_REF] Poirier | Shear recovery and temperature stability of Ca2+ and Ag+ glycolipid fibrillar metallogels with unusual βsheet-like domains[END_REF][START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] In the process proposed here, complementary to other existing separation processes, like flotation [START_REF] Dhar | Toward green flotation: Interaction of a sophorolipid biosurfactant with a copper sulfide[END_REF]Dhar, Thornhill, et al., 2021;[START_REF] Malkapuram | A review on recent advances in the application of biosurfactants in wastewater treatment[END_REF] or electrokinetic remediation [START_REF] Prakash | Bio-electrokinetic remediation of crude oil contaminated soil enhanced by bacterial biosurfactant[END_REF][START_REF] Tang | Heavy metal removal effectiveness, flow direction and speciation variations in the sludge during the biosurfactant-enhanced electrokinetic remediation[END_REF], the gel is easily concentrated by centrifugation and removed. UV-Vis and atomic absorption spectroscopy are used to evaluate the molar fraction of the cations in the supernatant.

Compared to the use of other biosurfactants for depollution purposes, G-C18:1 is certainly a newcomer. However, its unexpectedly strong interactions with heavy metals make this new molecule an interesting competitor with respect to all other commercial biosurfactants, like sophorolipids, rhamnolipid and surfactin, and in this regard it is certainly interesting to explore its application in the field of environmental remediation.

Material and methods

Chemicals.

The monounsaturated glucolipid G-C18:1 (Mw = 460 g.mol -1 ) contains a β-Dglucose unit covalently linked to oleic acid. The molecule is obtained by fermentation from the yeast Starmerella bombicola ΔugtB1 according the protocol given before. [START_REF] Baccile | Self-Assembly Mechanism of pH-Responsive Glycolipids: Micelles, Fibers, Vesicles, and Bilayers[END_REF][START_REF] Saerens | Cloning and functional characterization of the UDP-glucosyltransferase UgtB1 involved in sophorolipid production by Candida bombicola and creation of a glucolipid-producing yeast strain[END_REF] The compound is purchased from the Bio Base Europe Pilot Plant, Gent, Belgium, lot N° APS F06/F07, Inv96/98/99 and used as such.

According to the specification sheet provided by the producer, the batch (99.4% dry matter) is composed of 99.5% of G-C18:1, according to HPLC-ELSD chromatography data. NMR analysis of the same compound (different batch) was performed elsewhere. [START_REF] Baccile | PH-Driven Self-Assembly of Acidic Microbial Glycolipids[END_REF] Heavy metal chlorides, CuCl2, CrCl2, and NiCl2, were purchased from Sigma Aldrich (Saint-Quentin-Fallavier, France), CoCl2 came from Fluka (Saint-Quentin-Fallavier, France). For atomic absorption spectroscopy, standard solutions are purchased from Supelco (Saint-Quentin-Fallavier, France) and Sigma Aldrich (Saint-Quentin-Fallavier, France).

Solutions preparation. G-C18:1 is dissolved in milli-Q water at 1 wt% by stirring and sonication, the pH is adjusted to 8 by adding few µL of concentrated NaOH. At pH 8, the G-C18:1 is clear. Cu 2+ , Co 2+ and Cr 2+ solutions are prepared at concentration of 1 M, Ni 2+ solution is prepared at 0.5 M in milli-Q water. A few µL of each cation solution are added to the G-C18:1 solution so to reach the targeted molar ratio [ion]/[G-C18:1] shown in Table 1.

The metallogels form at [ion]/[G-C18:1]= 0.5 for Cu +2 , 0.6 for Cr +2 , and 1.0 for Co +2 and Ni +2 , with an approximation on the ratio of ± 0.1. However, to improve metallogel formation, pH is adjusted for Co 2+ , Cr 2+ and Ni 2+ systems only and according to the values given in Table 1. To remove the metallogel phase entrapping the heavy metals from the supernatant, the solutions (1.5 mL) are centrifuged at 13400 rpm during 5 min using an Eppendorf Minispin bench centrifuge.

Table 1. Molar ratio (± 0.1) and pH conditions employed to prepare G-C18:1 metallogels.

CuCl2

CoCl2 CrCl2 NiCl2

Replica UV-Vis spectrometry. The spectra from 300 to 800 nm are collected on a Uvikon XL spectrophotometer (SECOMAM) and its associated software. A 1.5 mL cuvette in plastic (Brand) is used as sample holder. Before each measurement, a few µL of the cation solution are mixed in the surfactant solution, which is then centrifuged. The absorbance is measured on the supernatant. Cation solutions are added in the same supernatant and the same protocol occurs over and over until high molar ratio [ion]/[G-C18:1] ratio are achieved. Since the actual [ion]/[G-C18:1] ratio is not known with precision, due to the content removed by centrifugation, plots report the calculated ratio. For all metal ions except Cu 2+ , the pH of the supernatant is artificially increase to trigger gel formation.

1 2 3 1 2 3 1 2 3 1 2 3 [ion]/[G-C18:1] 0.4 0.6 0.5 1.0 1.1 1.0 0.6 0.6 0.
Atomic absorption spectrometry. The supernatants are analyzed via Atomic Absorption Spectroscopy (AAS) using a PerkinElmer PinAAcle 500 apparatus. The calibration curves are obtained for Cu 2+ , Co 2+ , Cr 2+ and Ni 2+ by diluting commercial standard solutions at 0.5, 1, 1.5, 2.0 and 2.5 mg/L in a 1.5% HNO3 solution. The samples are prepared by diluting the supernatants in a HNO3 solution at 1.5%.

Discussion

In a series of previous works, it was shown how the glycolipid biosurfactant G-C18:1 fibrillates into hydrogels by adding a source of alkaline earth and transition metal ions to its micellar solution at pH> 7 (Figure 1). [START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] Specific ions like Ca 2+ , Ag + , Mn 2+ , Cr 2+ Fe 3+ or Zn 2+ tend to form gels or aggregates with a fibrillar morphology [START_REF] Poirier | Ca2+ and Ag+ orient low-molecular weight amphiphile self-assembly into fibrillar hydrogels with unusual β-sheet-like structure[END_REF]Poirier, Griel, Perez, & Baccile, 2022;[START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] while Co 2+ , Cu 2+ , Fe 2+ , Al 3+ and Ni 2+ drive the assembly towards an intertwined wormlike micellar network. (Poirier, Griel, Perez, & Baccile, 2022;[START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] According to the present knowledge, fibers are driven by possible metal-ligand interactions, while wormlike micelles rather form from a charge-screening mechanism, presumably via the interaction between the metal ion and the COO -of G-C18:1. [START_REF] Baccile | Chameleonic Amphiphile: the Unique Multiple Self-Assembly Properties of a Natural Glycolipid in Excess of Water[END_REF][START_REF] Poirier | Ca2+ and Ag+ orient low-molecular weight amphiphile self-assembly into fibrillar hydrogels with unusual β-sheet-like structure[END_REF] The structure of Ca 2+ and Ag + driven fibrous gels are reported in detail elsewhere, [START_REF] Poirier | Ca2+ and Ag+ orient low-molecular weight amphiphile self-assembly into fibrillar hydrogels with unusual β-sheet-like structure[END_REF] Water-Regulations#Inorganic, n.d.; [START_REF] Vetrimurugan | Human exposure risk to heavy metals through groundwater used for drinking in an intensively irrigated river delta[END_REF] and their interaction with G-C18:1 during the fibrillation process could be an effective way to remove the complex through a physical method. In the following, we have opted for centrifugation, which, at a laboratory scale is an easy and effective way to separate the flocculate, here supposed to be a

Glycolipid biosurfactant Metal ions

Al 3+ Ag + Fe 2+

Metallogels

Ca 2+

Self-assembly

Metal ions glycolipid Fibrous gel network of wormlike micelles or fibers, depending on the cation. However, other methods, like flotation [START_REF] Dhar | Toward green flotation: Interaction of a sophorolipid biosurfactant with a copper sulfide[END_REF]Dhar, Thornhill, et al., 2021;[START_REF] Malkapuram | A review on recent advances in the application of biosurfactants in wastewater treatment[END_REF], electrokinetic remediation [START_REF] Tang | Heavy metal removal effectiveness, flow direction and speciation variations in the sludge during the biosurfactant-enhanced electrokinetic remediation[END_REF] or filtration could easily be employed.

Figure 2 shows the absorbance spectra of Cu 2+ and Ni 2+ , added to a micellar solution of G-C18:1 at pH 8. After each addition of the metal ion solution, the mixture containing the cation and G-C18:1 is systematically centrifuged, with no remarkable effect. However, at an where the cation displays the maximum of absorption (Figure 2c,d). Centrifugation between each injection of the metal ion solution guarantees the removal of the large aggregates, but neither ions nor any stable self-assembled form of G-C18:1 (e.g., micelles, individual fibers).

It is then necessary to discriminate the UV-Vis absorbance peak attributed to the cation absorption and the wavelength-dependency (λ -4 ) background associated to the Rayleigh scattering produced by the self-assembled structures (wormlike micelles (Poirier, Griel, Perez, & Baccile, 2022;[START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF]), which are reasonably considered here to be at least ten times smaller than the wavelength. To do so, we perform a fit of the Rayleigh scattering contribution to the total absorbance using the simplified expression A= cλ -4 +b, with A being the experimental absorbance in the UV-Vis range.

Considering the present qualitative approach, the classical optical and size parameters, which are not exploited here, are integrated in the adjustable constant c, with b being the baseline.

This is illustrated by a typical fit curve (black dash line) in Figure 2a,b.

For Cu 2+ system, the Rayleigh scattering dominates the signal up to a molar ratio of about 0.1, while absorption due to copper (λ= 670 nm) dominates above about a molar ratio of 0.15. The absorption increases linearly up to a molar ratio close to 0.5, at which gelation occurs (Figure 2a,c and Figure 3a). The gel, constituted by entangled wormlike micelles, [START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] is easily removed by centrifugation of the vials. The final supernatant is transparent by the eye, confirmed by UV-Vis spectroscopy, which shows a flat spectrum in the visible range (Figure 2a, solid black line at [Cu 2+ ]/[G-C18:1]= 0.57). According to UV-Vis, removal of Cu 2+ is quantitative. However, a precise residual content of copper will be evaluated by mean of atomic absorption, presented below. former being unstable and easily oxidizing into Cr 3+ . [START_REF] Baes | Aluminum complexing enhances amyloid β protein penetration of blood-brain barrier[END_REF] The presence of Cr 3+ is indicated by the poorly defined, grey color of the gel (Figure 3b), as also reported before, [START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] but also by the absorption peaks at λ= 420 nm and λ= 577 nm in the corresponding UV-Vis spectra in Figure S 1a. These values are intermediate between the expected absorption peaks of carboxylate-complexed aqueous solutions of Cr 2+ (λ= 410 nm, λ= 468 nm) and Cr 3+ (λ= 432 nm, λ= 578 nm).

( [START_REF] Vargas-Vasquez | Characterization of Cr(II) and Cr(III) acetate aqueous solutions using UV-vis Spectrophotometry and 1H NMR[END_REF] pH has a strong impact on both the cations' chemistry and G-C18:1 and these are discussed separately below. As a general trend, metallogels of G-C18:1 form in the region of pH neutrality and above. This is most likely explained by the higher abundancy of negatively charged carboxylate groups, which are supposed to interact with cations. Metal-induced gelation is well-known for polymers [START_REF] Cheng | Triple Stimuli-Responsive N-Isopropylacrylamide Copolymer toward Metal Ion Recognition and Adsorption via a Thermally Induced Sol-Gel Transition[END_REF][START_REF] Diener | Rigid, Fibrillar Quaternary Structures Induced by Divalent Ions in a Carboxylated Linear Polysaccharide[END_REF] but also many low-molecular weight (LMW) amphiphiles, (L. Chen et al., 2013;[START_REF] Greenfield | Tunable mechanics of peptide nanofiber gels[END_REF][START_REF] Shi | Calcium ions to cross-link supramolecular nanofibers to tune the elasticity of hydrogels over orders of magnitude[END_REF][START_REF] Westcott | Metallo-gels and organo-gels with tripodal cyclotriveratrylene-type and 1,3,5-substituted benzene-type ligands[END_REF][START_REF] Xie | Calcium-Ion-Triggered Co-assembly of Peptide and Polysaccharide into a Hybrid Hydrogel for Drug Delivery[END_REF][START_REF] Zhou | Self-assembly of pH and calcium dual-responsive peptide-amphiphilic hydrogel[END_REF] including anionic surfactants. [START_REF] Qiao | Metal-driven viscoelastic wormlike micelle in anionic/zwitterionic surfactant systems and template-directed synthesis of dendritic silver nanostructures[END_REF][START_REF] Vasilescu | Fluorescence study on the size and shape of sodium dodecyl sulphate-aluminium salt micelles[END_REF] The difference between hydrogels obtained by LMW amphiphiles and surfactants is generally structural, as the former often assemble into fibers, while the latter into wormlike micelles. Such effects are exploited in chemical depollution methods, which employ surfactants as flocculants to remove heavy metals from wastewater. [START_REF] Paton | Effect of pH on the Flocculation of SDS micelles by Al3+[END_REF][START_REF] Somasundaran | Precipitation-Redissolution Phenomena in Sulfonate-AlCl3 Solutions[END_REF][START_REF] Talens-Alesson | Behaviour of SDS micelles bound to mixtures of divalent and trivalent cations during ultrafiltration[END_REF] Interestingly, G-C18:1 displays both characters according to the type of cation used. [START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] However, according to SAXS arguments presented elsewhere, [START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] the cations studied in this work drive the formation of wormlike gels. At pH above neutrality, G-C18:1 forms a stable micellar phase in water, where micelles have a spheroidal morphology. [START_REF] Baccile | Self-Assembly Mechanism of pH-Responsive Glycolipids: Micelles, Fibers, Vesicles, and Bilayers[END_REF][START_REF] Baccile | PH-Driven Self-Assembly of Acidic Microbial Glycolipids[END_REF]) Ni 2+ , Cr 2+ and Co 2+ , among others, induce gelation by driving a sphere-to-wormlike transition. Sphere-to-cylinder and sphere-towormlike transitions induced by metal salts are well-known in surfactant science, (R. . G. [START_REF] Alargova | Micellization and interfacial properties of alkyloxyethylene sulfate surfactants in the presence of multivalent counterions[END_REF][START_REF] Alargova | Growth of giant rodlike micelles of ionic surfactant in the presence of Al3+ counterions[END_REF][START_REF] Alargova | Growth of rod-like micelles in anionic surfactant solutions in the presence of Ca2+ counterions[END_REF][START_REF] Angelescu | Some observations on the effect of the trivalent counterion Al3+ to the self-assembly of sodium dodecyl sulphate in water. Colloids and Surfaces A: Physicochem[END_REF][START_REF] Jensen | Monitoring the Transition from Spherical to Polymer-like Surfactant Micelles Using Small-Angle X-Ray Scattering[END_REF][START_REF] Vasilescu | Fluorescence study on the size and shape of sodium dodecyl sulphate-aluminium salt micelles[END_REF] and they are generally attributed to the charge-screening effect of ions, which reduce repulsive interactions between negatively-charged headgroups. According to the thermodynamic theory of surfactant self-assembly, [START_REF] Israelachvili | Theory of self-assembly of hydrocarbon amphiphiles into micelles and bilayers[END_REF][START_REF] Tanford | The hydrophobic effect: Formation of micelles and biological membranes[END_REF] smaller area-perheadgroup favors transitions from high to low curvature morphologies, like cylinders and worms. Wormlike gels are also well-known, [START_REF] Raghavan | The conundrum of gel formation by molecular nanofibers, wormlike micelles, and filamentous proteins: gelation without cross-links?[END_REF] although cationinduced viscoelasticity in anionic surfactants is less common, but possible. [START_REF] Qiao | Metal-driven viscoelastic wormlike micelle in anionic/zwitterionic surfactant systems and template-directed synthesis of dendritic silver nanostructures[END_REF][START_REF] Vasilescu | Fluorescence study on the size and shape of sodium dodecyl sulphate-aluminium salt micelles[END_REF] Most of the studies reporting cation-induced morphological transitions in surfactants and LMW amphiphiles generally consider the ion in its free M z+ form. Broadly speaking this is imprecise.

The effect of pH on metal ions is well-known in inorganic chemistry and it is responsible for the precipitation of metal oxides. pH determines the form of the ion, whether free (M z+ ), although generally hydrated by water, or complexed as hydroxo species (M(OH)y (z-y)+ ). [START_REF] Baes | Aluminum complexing enhances amyloid β protein penetration of blood-brain barrier[END_REF]) Speciation is particularly important for transition metal compared to alkaline earth cations. The former generally display a large number of species, depending on the valence of the metal, its polarizability and electronegative character. If speciation of metal in water should be taken into account when discussing the metal-surfactant interactions, this is rarely the case. We have identified a limited number of works, [START_REF] Jian-Xiao | Interaction mechanisms between anionic surfactant micelles and different metal ions in aqueous solutions[END_REF][START_REF] Paton | Effect of pH on the Flocculation of SDS micelles by Al3+[END_REF][START_REF] Somasundaran | Precipitation-Redissolution Phenomena in Sulfonate-AlCl3 Solutions[END_REF] compared to the majority, where speciation is taken into account (refer to Table S 1 in Ref. [START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] for a more extended literature survey).

Understanding the interactions between G-C18:1 and metal ions is then very challenging on the basis of previous literature. The chemical nature of G-C18:1 also adds another source of complexity.

Most literature studies involve anionic surfactants, generally derivatives of sulfates, which do not chelate metals. On the contrary, G-C18:1 has a carboxylate group, which is known to be a good ligand for metals. [START_REF] Ringbom | Complexation in Analytical Chemistry[END_REF] For this reason, a competition must exist between carboxylates and hydroxyl groups, since pH controls both the amount of carboxylates in solution and the type and relative content of metal hydroxo species.

Furthermore, one cannot exclude a charge-screening effect by M(OH)y (z-y)+ complexes themselves, instead of free metal ions, as also hypothesized by others. [START_REF] Jian-Xiao | Interaction mechanisms between anionic surfactant micelles and different metal ions in aqueous solutions[END_REF] It goes without saying that the overall complexity of the system goes far beyond the scope of this work.

We have however attempted to propose a general rationale with some exceptions. [START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] Those cations which exist as free ions (M z+ ) in the pH region above 7 (e.g., Ag + , Ca 2+ , Mn 2+ ) tend to form strong metal-ligand complexes with G-C18:1 and their mutual interaction drives the formation of fibers. [START_REF] Poirier | Ca2+ and Ag+ orient low-molecular weight amphiphile self-assembly into fibrillar hydrogels with unusual β-sheet-like structure[END_REF] On the other hand, those cations, which have a more complex speciation (M(OH)y (z-y)+ ) tend to form micellar gels, probably driven by electrostatic screening between coexisting forms of M(OH)y (z-y)+ and negatively-charged G-C18:1 micelles. This seems to be the case of Ni 2+ , Cr 2+ , Co 2+ and Cu 2+ . However, exceptions exist, as

shown by the gelation of the G-C18:1 at pH 5 containing Cu 2+ . Complexation phenomena between Cu 2+ and carboxylic acids, or even glucose, are not excluded.

The atomic absorption spectroscopy (AAS) allows a quantitative analysis of the cations concentration in the supernatants, after centrifugation of the hydrogels. Experiments are performed in triplicate. Preparation of each sample is given in Table 1, while the efficiency of cation removal is given in Figure 4. The higher efficiency is obtained with Co 2+ where 95 ± 3 % of cation is removed from water and integrated in the gel phase. Cu 2+ and Ni 2+ are also removed with a very good efficiency of about 88 ± 10 % and 80% ± 3 % for each cation. Cr 2+ exhibit the lower efficiency with only 59 ± 6 % removed, probably due to the presence of Cr 3+

species.

The heavy metal ion removal efficiency found for several ions, like Co 2+ or Ni 2+ , is impressively high, especially if one considers that nearly no optimization has been performed, as otherwise found for other systems, [START_REF] De Franҫa | Production of a biosurfactant by Bacillus subtilis ICA56 aiming bioremediation of impacted soils[END_REF][START_REF] Talbot | pH-sensitive magnetic alginate/γ-Fe2O3 nanoparticles for adsorption/desorption of a cationic dye from water[END_REF] and the mixing protocol is relatively simple. The comparison to other biosurfactants is not straightforward, as most work has been carried out on soil depollution, (Q. [START_REF] Chen | Removal of Pb and Hg from marine intertidal sediment by using rhamnolipid biosurfactant produced by a Pseudomonas aeruginosa strain[END_REF][START_REF] Kim | Removal of Lead from Contaminated Water and Clay Soil Using a Biosurfactant[END_REF][START_REF] Malkapuram | A review on recent advances in the application of biosurfactants in wastewater treatment[END_REF][START_REF] Miller | Biosurfactant-facilitated Remediation of Metal-contaminated Soils[END_REF][START_REF] Mishra | Biosurfactant is a powerful tool for the bioremediation of heavy metals from contaminated soils[END_REF]; C. N. [START_REF] Mulligan | Surfactant-enhanced remediation of contaminated soil: A review[END_REF][START_REF] Mulligan | Environmental applications for biosurfactants[END_REF][START_REF] Mulligan | Recent advances in the environmental applications of biosurfactants[END_REF], 2021;Catherine N. Mulligan, Yong, & Gibbs, 1999;Catherine N. Mulligan, Yong, Gibbs, et al., 1999; Catherine N. [START_REF] Mulligan | Remediation of a heavy metal-contaminated soil by a rhamnolipid foam[END_REF]Ochoa-Loza et al., 2001b;[START_REF] Sonowal | Biosurfactant-assisted phytoremediation of potentially toxic elements in soil: Green technology for meeting the United Nations Sustainable Development Goals[END_REF] rather than aqueous solutions. [START_REF] Ghaith | Removal of Cd++ from Contaminated Water Using Bio-surfactant Modified Ground Grass as a Biosorbent[END_REF][START_REF] Huang | Biosorption of Cd(II)/Pb(II) from aqueous solution by biosurfactant-producing bacteria: Isotherm kinetic characteristic and mechanism studies[END_REF][START_REF] Kim | Removal of Lead from Contaminated Water and Clay Soil Using a Biosurfactant[END_REF] In the latter case, the biosurfactants (generally rhamnolipids) are not employed alone but in association with biomass, like dry ground grass or bacteria, or the nature of the biosurfactant itself is not clarified. [START_REF] Kim | Removal of Lead from Contaminated Water and Clay Soil Using a Biosurfactant[END_REF] The unique metallogelation mechanism of G-C18:1 in the presence of cations [START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] displays a high cation removal efficiency compared to other biosurfactants. In the case of water purification, it was reported a removal efficiency of lead in the order of 80%, although the type of biosurfactant used was not clear. [START_REF] Kim | Removal of Lead from Contaminated Water and Clay Soil Using a Biosurfactant[END_REF] An interesting study by

Ochoa-Loza provides the binding constants between monorhamnolipids and a number of metal ions, showing that the best affinity occurs with Al 3+ and Cu 2+ , without however quantifying the removal efficiency from contaminated water solutions. In terms of soil washing, a recent review [START_REF] Mishra | Biosurfactant is a powerful tool for the bioremediation of heavy metals from contaminated soils[END_REF] lists the efficiency of heavy metal removal for a number of biosurfactants, including rhamnolipids, sophorolipids, surfactin and other unspecified molecules. Although difficult to compare in a direct manner, the water cleaning process using metallogelation from G-C18:1 with Co 2+ and Cu 2+ still ranges among the highest standards with respect to all metals reported in Table 3 of Ref. [START_REF] Mishra | Biosurfactant is a powerful tool for the bioremediation of heavy metals from contaminated soils[END_REF].

Specifically, the high efficiency values obtained with Co 2+ (95 ± 3 %) and Cu 2+ (88 ± 10 %)

as well as less impressive values found for Ni 2+ (80 ± 3 %) and Cr 2+ (59 ± 6 %), outperform the effect of rhamnolipid, sophorolipid and surfactin on these same cations, which were shown to have efficiencies ranging between 1.68 % (surfactin/chromium) and 74%

(rhamnolipid/copper). [START_REF] Mishra | Biosurfactant is a powerful tool for the bioremediation of heavy metals from contaminated soils[END_REF] To the best of our knowledge, the glucolipid presented here seems to be one of the most performing low molecular weight bio-flocculants.

Although never tested in depollution of soils, one does not expect sophorolipids, rhamnolipid or surfactin to have such high efficiencies in water treatment. In fact, previous work focusing on the impact of free metal ions on the self-assembly of these molecules did not demonstrate any major impact with noticeable macroscopic behavior in terms of ion trapping. [START_REF] Baccile | Surface charge of acidic sophorolipid micelles: effect of base and time[END_REF][START_REF] Chen | Influence of calcium ions on rhamnolipid and rhamnolipid/anionic surfactant adsorption and selfassembly[END_REF][START_REF] Dahrazma | Effects of additives on the structure of rhamnolipid (biosurfactant): A small-angle neutron scattering (SANS) study[END_REF][START_REF] Shen | Surfactin structures at interfaces and in solution: The effect of pH and cations[END_REF] The present results are explained by the atypical, bolaform and functional, molecular structure of G-C18:1 but also by the control and fine tuning of its unique phase behavior. [START_REF] Baccile | Chameleonic Amphiphile: the Unique Multiple Self-Assembly Properties of a Natural Glycolipid in Excess of Water[END_REF][START_REF] Baccile | Self-Assembly Mechanism of pH-Responsive Glycolipids: Micelles, Fibers, Vesicles, and Bilayers[END_REF][START_REF] Baccile | PH-Driven Self-Assembly of Acidic Microbial Glycolipids[END_REF][START_REF] Poirier | Shear recovery and temperature stability of Ca2+ and Ag+ glycolipid fibrillar metallogels with unusual βsheet-like domains[END_REF][START_REF] Poirier | Ca2+ and Ag+ orient low-molecular weight amphiphile self-assembly into fibrillar hydrogels with unusual β-sheet-like structure[END_REF]Poirier, Griel, Perez, & Baccile, 2022;[START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF] Ni

Conclusions

The microbial biosurfactant, glucolipid G-C18:1, forms metallogels with a broad set of heavy metals. The gel phase is employed here as heavy metal adsorbent intended to clean the water phase. G-C18:1 is tested with four well-known heavy metal pollutants of industrial waste and ground waters, Cu 2+ , Ni 2+ , Cr 2+ and Co 2+ . By increasing the cation-to-biosurfactant ratio up to an optimum value, generally at charge compensation, and adjusting the value of pH between 7 and 8 for some of them, G-C18:1 strongly interacts with all metals in the form of hydroxo species, which are entrapped in the gel phase. The mechanism behind metallogelation, reported elsewhere, involves a micelle-to-wormlike morphological transition driven by the interaction between the carboxylate group of G-C18:1 and the charged metal species.

Centrifugation is eventually used as a practical laboratory-scale process to separate the gel from water. Qualitative observations by the naked eye show nearly complete removal of the heavy metals. This is confirmed by UV-Vis spectrophotometry experiments, which, within the sensitivity of the instrument, show no trace of the heavy metals in the supernatant.

Precise quantification has been performed with atomic absorption spectrometry, which reveals the high efficiency in terms of flocculant behavior of G-C18:1 towards selected heavy metals.

We find that the efficiency removal is as high as 95 ± 3% for Co 2+ , 80 ± 3% for Ni 2+ , 88 ± 10% for Cu 2+ and 59 ± 6% for Cr 2+ . Due to the little work performed on the depollution of water by biosurfactants, these values cannot benefit direct comparison. Nevertheless, in comparison to the extensive work performed on the removal of heavy metals in soil, these values are in the top range and show promising application of G-C18:1 also in soil cleaning.

This study confirms the strong interest of biosurfactants as remediation green chemicals. Moreover, considering the very high efficiency of G-C18:1 for heavy metal removal, it also shows the strong interest in studying this compound further in this field.

Figure 1 -

 1 Figure 1 -Process of metallogel formation from glycolipid G-C18:1 biosurfactant in water.(Poirier, Griel, Perez, & Baccile, 2022;[START_REF] Poirier | Metallogels from Glycolipid Biosurfactant[END_REF]

  appropriate molar ratio of[Cu 2+ ]/[G-C18:1] of about 0.5, gelation occurs and syneresis can be easily induced by centrifugation. A colorless supernatant is then collected and further analyzed. Similar observations (appropriate [ion]/[G-C18:1] are given in Table1) are obtained for Co 2+ , Cr 2+ and Ni 2+ , after increasing the pH between pH 7.0 and 8.5. UV-Vis spectrometry is used to follow the above process and to evaluate the cationic content in the supernatant, after each cation addition and centrifugation step.

Figure 2 -

 2 Figure 2 -Absorbance spectra for a) Cu 2+ and b) Ni 2+ -containing G-C18:1 solutions (CG-C18:1= 1 wt%) at different molar ratios, indicated in the caption. The vertical red line shows the wavelength targeted to follow the absorbance evolution, c) 670 nm for Cu 2+ and d) 395 nm for Ni 2+ addition. The pH is adjusted at 8.5 before the last centrifugation.

Figure

  Figure 3 -a) Illustration of the Cu 2+ solubility in a G-C18:1 solution (CG-C18:1 = 3 wt%) at a molar ratio 0.3 and the Cu 2+ -containing G-C18:1 hydrogel obtained at [Cu 2+ ]/[G-C18:1]= 0.5. b) Picture of G-C18:1 solution (CG-C18:1 = 3 wt%) obtained with Ni 2+ , Cr 2+ and Co 2+ addition before and after basification.

Figure 4 -

 4 Figure 4 -Residual concentration of heavy metals, measured by atomic absorption spectrometry (AAS), in the supernatant after metallogelation and centrifugation. Composition values are indicated Figure S 1. The molar fraction removed is obtained by ratio of the effective ion concentration measured by AAS experiment [ion]AAS in the supernatant over the ion concentration initially added to the glycolipid solution, [ion]added.

Figure

  Figure S 1 -Absorbance spectra for a) Cr 2+ and b) Co 2+ containing G-C18:1 solutions (CG-C18:1=1 wt%) at different [ion]/[G-C18:1] molar ratio, indicated in the caption. The vertical red line show the wavelength targeted to follow the absorbance evolution, c) λ= 420 nm for Cr 2+ and d) λ= 520 nm for Co 2+ addition.

  b s o r p ti o n + a g g r e g a te s c a tt e r

Acknowledgements

We thank Dr. S. Roelants at Gent University and Bio Base Europe Pilot Plant, Gent, Belgium for dealing with and shipping the G-C18:1 glycolipid. Authors kindly acknowledge the French ANR, Project N° SELFAMPHI -19-CE43-0012-01.

All authors contributed to and approved the final draft of the manuscript.

Ethics statement

No human or animal subjects were used in this research.

Supporting information