SWELLABLE AND INSOLUBLE NANOFIBERS AND USE THEREOF IN THE TREATMENT OF ESSENTIALLY AQUEOUS EFFLUENTS

Abstract

Nanofibres are provided that are insoluble and swellable in an essentially aqueous effluent, a method for the preparation of these nanofibres and the use of these nanofibres for the extraction from an effluent of metals, in particular metal salts originating from heavy metals, of rare earths, alkali metals, alkaline earth metals or actinides, in the stable or unstable isotopic forms thereof.

Claims

1. Nanofibres that are insoluble and swellable in an essentially aqueous effluent, capable of being obtained by the process comprising the following steps: (a) obtaining the nanofibres by electrospinning or centrifugal spinning from a synthesis solution, said synthesis solution comprising: (i) at least one hydrophilic and water-soluble initial polymer selected from the group comprising: polyacrylic acid or the copolymers of polyacrylic acid; the anionic derivatives of polystyrene, such as polystyrene sulphonate or the copolymers of polystyrene sulphonate; the cationic derivatives of polystyrene, such as polystyrene trialkylbenzyl ammonium poly(4-vinylpyridine) or derivatives thereof, polyvinyl alcohol or hydrophilic derivatives thereof, polyvinylpyrrolidone, derivatives thereof, or copolymers thereof, or a mixture thereof; said hydrophilic and water-soluble initial polymer having a molecular weight from 110.sup.4 to 1.510.sup.6, in particular from 110.sup.4 to 510.sup.5, the hydrophilic and water-soluble initial polymer content in the synthesis solution being from 5 to 50 wt. %, preferably from 10 to 30 wt. %, relative to the synthesis solution, (ii) optionally at least one additive selected from: a cross-linking agent from 0.05 to 40 wt. %, in particular from 1 to 40 wt. %, particularly from 5 to 20 wt. %, in particular 15 wt. % relative to the hydrophilic and water-soluble initial polymer, a complexing molecule from 1-40 wt. %, in particular from 1 to 30 wt. % relative to the hydrophilic and water-soluble initial polymer, said complexing molecule being selected from: a calixarene, a crown ether, or a mixture thereof; (b) stabilizing the nanofibres obtained in step (a) in order to obtain nanofibres that are insoluble but swellable in an essentially aqueous effluent; said nanofibres having a cross-linking rate comprised between 5 and 40%, particularly from 5 to 20%, in particular 15%.

2. The insoluble and swellable nanofibres according to claim 1, characterized in that the stabilization of the nanofibres obtained by electrospinning or centrifugal spinning is carried out by a heat treatment or by radiation with ultra-violet rays of the VUV (Vacuum Ultra-Violet) or UV type or by visible radiation.

3. The insoluble and swellable nanofibres according to claim 1, characterized in that said cross-linking agent is selected from the group comprising: a diazide, in particular diazidostilbene, a diamine, in particular hexamethylenediamine, a hydrophilic polymer selected from the group comprising a polyethylene glycol, a polyhydroxyethyl methacrylate, polyvinylpyrrolidone, derivatives thereof or copolymers thereof, a (C.sub.1-C.sub.10) dibromoalkane, in particular 1,4-dibromobutane a dibromo-p-xylene, a (C.sub.1-C.sub.10) diiodoalkane in particular 1,4-diiodobutane, a (C.sub.1-C.sub.10) dichloroalkane in particular 1,4-dichlorobutane, and a calixarene of Formula I ##STR00011## in which: X.sub.1 and X.sub.2 each represent independently of each other H or a ##STR00012## group, in which R.sub.3 and R.sub.4 each represent, independently of each other, H or a (C.sub.1-C.sub.8) alkyl group, provided that X.sub.1 and X.sub.2 do not simultaneously represent H; L.sub.1, L.sub.2, L.sub.3 and L.sub.4 are spacer groups, selected independently of each other from the group consisting of a (C.sub.3-C.sub.10) cycloalkylenyl, O, NH, (CH.sub.2).sub.q, q being an integer from 1 to 12; Z.sub.1 and Z.sub.2 each represent, independently of each other, a functional group selected from an optionally protected amine, F, Cl, Br, I, OH, C(O)H, C(O)Hal, an aryl group or a substituted aryl group, such as a tosyl, a diazonium group, an aromatic heterocycle such as a pyrrolyl, furyl, thienyl, or pyridinyl group, an optionally protected sulphate or sulphonate group, n is an integer from 1 to 10; or a calixarene of Formula II ##STR00013## in which: R1 is selected from X(C.sub.2H.sub.4X).sub.m, or X(C.sub.2H.sub.4X).sub.p/2YX(C.sub.2H.sub.4/2, X being selected independently from O and/or N, m being equal to 3, 4, 5 or 6, p being equal to 2 or 4, Y being a (C.sub.3-C.sub.10) cycloalkylene or a (C.sub.6-C.sub.10) arylene; and R.sub.2 to R.sub.5 are selected independently of each other from H, or a (C.sub.1-C.sub.6) alkyl; L.sub.1 and L.sub.2 are spacer groups, selected independently of each other from a (C.sub.3-C.sub.10) cycloalkylene or a (C.sub.3-C.sub.10) arylene, (CH.sub.2).sub.q, q being an integer from 1 to 12; Z.sub.1, Z.sub.2 are grafting groups, selected independently of each other from F, Cl, Br, I, OHNH.sub.2, C(O)Hal, C(O)OH, an aryl group or a substituted aryl group, such as a tosyl, a diazonium group, an aromatic heterocycle such as a pyrrolyl, furyl, thienyl, or pyridinyl group, an optionally protected sulphate or sulphonate group.

4. The insoluble and swellable nanofibres according to claim 1, characterized in that they are obtained by the process comprising the following steps: (a) obtaining the nanofibres by electrospinning or centrifugal spinning from a synthesis solution, said solution comprising a cross-linking agent and a hydrophilic and water-soluble initial polymer as defined in claim 1 having a molecular weight from 110.sup.4 to 1.510.sup.6, in particular from 110.sup.4 to 510.sup.5; and (b) stabilizing the nanofibres obtained in step (a) by a heat treatment in order to obtain nanofibres that are insoluble and swellable in an essentially aqueous effluent.

5. The insoluble and swellable nanofibres according to claim 1, characterized in that they are obtained by the process comprising the following steps: (a) obtaining the nanofibres by electrospinning or centrifugal spinning from a synthesis solution of a hydrophilic and water-soluble initial polymer as defined in claim 1; and (b) stabilizing the nanofibres obtained in step (a) by VUV or UV radiation or by visible radiation in order to obtain nanofibres that are insoluble and swellable in an essentially aqueous effluent.

6. The insoluble and swellable nanofibres according to claim 4, characterized in that the abovementioned synthesis solution also comprises a calixarene of Formula I or of Formula II.

7. The insoluble and swellable nanofibres according to claim 1, characterized in that the abovementioned synthesis solution is a water/ethanol solution.

8. The insoluble and swellable nanofibres according to claim 1, characterized in that the diameter of said nanofibres is from 50 nm to 10 m, in particular from 100 nm to 5 m, more particularly from 100 nm to 1 m, even more particularly from 100 to 300 nm.

9. A membrane constituted by nanofibres that are insoluble but swellable in an essentially aqueous solvent according to claim 1.

10. Use of the nanofibres that are insoluble and swellable in an essentially aqueous effluent according to claim 1, or a membrane constituted by nanofibres that are insoluble but swellable in an essentially aqueous solvent, for treating an effluent containing metals, in particular metal salts originating from heavy metals, rare earths, alkali metals, alkaline earth metals, or actinides, in the stable or unstable isotopic forms thereof.

11. The use according to claim 10, for treating an effluent containing caesium, characterized in that the nanofibres are obtained from an organic synthesis solution comprising: polyacrylic acid or anionic derivatives thereof; and a calixarene of Formula I or Formula II or a non-crosslinking calixarene.

12. The use according to claim 10, for treating an effluent containing copper, characterized in that said nanofibres are obtained from an aqueous solution of polyacrylic acid or of co-polymers.

13. The use according to claim 10, for treating an effluent containing uranium, characterized in that said nanofibres are obtained from a synthesis solution comprising the cationic derivatives of polystyrene.

14. A process of extraction from an effluent of metals, in particular metal salts originating from heavy metals, rare earths, alkali metals, alkaline earth metals, or actinides, in the stable or unstable isotopic forms thereof, comprising the following steps: (i) placing insoluble but swellable nanofibres according to claim 1 or a membrane according to claim 9 in an essentially aqueous effluent for a sufficient time; and (ii) recovering the metals, in particular the metal salts originating from the heavy metals, rare earths, alkali metals, alkaline earth metals, or actinides, in the stable or unstable isotopic forms thereof complexed by said nanofibres or said membrane by placing said nanofibres or said membrane in a regeneration solution.

15. The process according to claim 14, characterized in that said regeneration solution is an acid solution.

Description

[0290] The present invention is further illustrated by the following figures and examples.

[0291] FIG. 1a: Diagram of the electrospinning device, adapted from Greiner and Wendorff, Angew. Chem. Int. Ed. 2007, 46, 5670-5703.

[0292] FIG. 1b: Diagram of the centrifugal spinning device, adapted from C. J. Luo et al. Chem. Soc. Rev., 2012, 41, 4708-4735.

[0293] FIG. 2: SEM image of polyacrylic acid (PAA) nanofibres with 1 wt. % hexamethylenediamine (HMDA) after a heat treatment at 145 C. for 35 minutes.

[0294] FIGS. 3a, 3b, 3c, 3d: SEM (scanning electron microscopy) images of PAA nanofibres after 1 min (3a), 2 min (3b), 3 min (3c), or 5 min (3d) of VUV treatment.

[0295] FIG. 4: Diagrammatic view of the treatment of effluents with ion-exchange resins or nanofibres.

[0296] FIGS. 5a, 5b and 5c: Comparison of performance between PAA nanofibres of the invention and Amberlite IRC 748I ion-exchange resin from SUPELCO. FIG. 5a relates to the kinetic study of the capture of copper (II) ions by nanofibres or by resins. FIG. 5b relates to the comparative study for an adsorption isotherm for a solution laden with copper (II) ions. FIG. 5c relates to the comparative study for an adsorption isotherm for a solution laden with strontium (II) ions.

[0297] FIGS. 5d and 5e show the results of the breakthrough studies of the PAA nanofibres of the invention, carried out with copper (II) ions in highly concentrated (FIG. 5d) and weakly concentrated (FIG. 5e) media.

[0298] FIG. 5f shows the result of the regenerability study of the nanofibres of the invention of PAA.

[0299] FIGS. 6a and 6b respectively show the SEM images of PAA nanofibres obtained by electrospinning (FIG. 6a) and by centrifugal spinning (FIG. 6b) before thermal cross-linking.

[0300] FIGS. 7a and 7b show respectively the SEM images of PAA nanofibres obtained by electrospinning (FIG. 7a) and by centrifugal spinning (FIG. 7b) after thermal cross-linking.

[0301] FIGS. 8a and 8b show respectively the SEM images of PAA nanofibres obtained by electrospinning (FIG. 8a) and by centrifugal spinning (FIG. 8b) after thermal cross-linking and after contact with water.

EXAMPLES

[0302] Materials and Methods

[0303] Electrospinning

[0304] Electrospinning is carried out using a traditional device (see the diagram in FIG. 1a adapted from Greiner and Wendorff), a solution of polymer is injected in a needle with an internal diameter varying between 0.1 and 1 mm. The syringe is placed in the syringe driver which is programmed for the chosen flow rate (in mL/h). The needle is connected to a high-voltage generator which furthermore gives it a role as an electrode. The counter electrode (connected to earth) is at a distance varying between 5 and 25 cm. The voltage applied between these two electrodes can vary between 0 and 100 kV depending on the power supply used. By using these very high voltages, very low currents are involved, from a few hundred nA to a few A.

[0305] The nanofibres produced by electrospinning are collected on a collector connected to the counter electrode and the needle at high voltage.

[0306] Closing the door of the chamber allows the high voltage supply to start. When the power supply starts, a first potentiostat makes it possible to limit the current, and the second, the voltage applied. For safety reasons, the current potentiostat is set to 25 A (maximum current) and the second potentiostat is varied in order to reach the desired voltage.

[0307] Centrifugal Spinning

[0308] Centrifugal spinning is carried out using a standard device (see the diagram in FIG. 1b adapted from C. J. Luo et al.) such as the FORCESPINNING device of the Cyclone L 1000 M/D type from FibeRio the productivity of which can reach 200 g/hour. The speed of rotation of the spinneret of this device is between 2,000 and 5,000 rpm.

[0309] Infra-Red (IR)

[0310] Infra-red analyses were carried out on a Bruker Vertex 70 FTIR device in the range 4,000-600 cm.sup.1. The spectrometer was used in ATR (Attenuated Total Reflectance) mode with a crystal of diamond type.

[0311] On reaching the diamond, the incident IR beam produces an evanescent wave which will be absorbed or changed by the sample (up to 3 m) then returned in the IR beam. The output IR beam is analysed by a DTGS (Deuterium TriGlycide Sulphate) spectrometer. The resolution is of the order of 2 cm.sup.1 and 64 scans are carried out in order to obtain a good signal/noise ratio.

[0312] Scanning Electron Microscopy (SEM)

[0313] The Scanning Electron Microscopy images were obtained on a JEOL JSM-5510LV. All the analyses were carried out at a low acceleration voltage (between 2 and 4 kV) as the fibres studied are insulators and the risk of damage is high for this type of material. The working distance is set to 8 mm and the acquisition is carried out in SEI mode. A tungsten filament is used as electron gun.

[0314] A SEM Zeiss Ultra 55 comprising a field emission gun makes it possible to produce images with an improved contrast and a greater magnification.

[0315] Processing the images was carried out with the ImageJ open-source software. Determination of the diameter of the nanofibres was carried out by a statistical study on a minimum of 100 fibres.

[0316] Copper Spectrophotometer

[0317] For detection of copper and calculation of the concentration of Cu.sup.2+, a Hanna Instruments spectrophotometer was used. Bicinchoninate is used as colorimetric reagent which emits at 562 nm when it is complexed with copper. The device makes it possible to detect concentrations ranging from 0 to 5 mg/l (0.02).

[0318] Atomic Absorption Spectroscopy (AAS)

[0319] Atomic absorption spectroscopy was carried out with a model iCE 3000 Series from Thermo Scientific. AAS is used for determining the concentration of caesium in a synthetic solution. A caesium lamp in emission mode is therefore used in order to obtain good accuracy.

[0320] Measurement of the Capacity to Capture Copper

[0321] The stabilized membrane is immersed in a beaker containing a given volume of synthetic solution. The stabilized membrane is brought into contact with a dilute soda solution (NaOH.sub.aq) (pH9-10) for 5 minutes in order to decompose the anhydride form in the case of heat treatment and obtain the carboxylate form; then with the synthetic solution of copper for 120 minutes (unless specified otherwise). Before each change of solution, the membrane is dried under vacuum for 30 minutes.

[0322] The initial and final copper concentrations of the solutions are measured with the spectrophotometer for copper. Zero is set with the sample without chelating agent. A sachet of bicinchoninate, then the solution to be analysed (dilute solution if exceeding 5 mg/L), are introduced into a cuvette and the detection is started.

[0323] The capacity of the material to capture copper (mg/g) (X) is determined by the following formula:

[00001]$X=\frac{C_i-C_{f}V}{W_{\mathrm{membrane}}}$

in which C.sub.i and C.sub.f correspond respectively to the initial mass concentration and to the final copper concentration (in mg/L),

[0324] V is the volume of the model solution (in L),

[0325] W.sub.membrane is the mass of the membrane (in g).

[0326] Measurement of the Capacity to Capture Caesium

[0327] The method described above for measuring the capacity to capture copper is applicable for measuring the capacity of a material to capture caesium. The stabilized membrane is brought into contact with the synthetic solution of the element to be studied for 30 minutes. The initial and final concentrations of Cs of the solutions are measured by atomic absorption spectrometry. The formula used for calculating the capacity for copper is valid for Cs.

Example 1. Extraction of Copper by Insoluble PAA Nanofibres Produced by Electrospinning and Stabilized by a Cross-Linking Agent

[0328] 1) Preparation of Polymer Solutions

[0329] Two types of polyacrylic acid (PAA) were used, their characteristics are as follows: [0330] PAA from Fluka: pure PAA (analytical standard), Mw=130,000 g.mol.sup.1, white powder [0331] PAA 35% H.sub.2O: PAA diluted to 35% in H.sub.2O, Mw=250,000 g.mol.sup.1, viscous colourless liquid.

[0332] The pK.sub.A of the PAA is 4.25.

[0333] The polyacrylic acid (PAA) is diluted to 20-25 wt. % in deionized water in order to obtain a viscous solution. Hexamethylene diamine (HMDA) is added as cross-linking agent at 15 wt. % relative to the mass of PAA.

[0334] 2) Manufacture of Nanofibres by Electrospinning

[0335] This solution is injected into an electrospinning device in order to produce soluble nanofibres. The voltage applied is situated between 15 and 20 kV at a distance between the solution and the collector of 10-20 cm. Under these conditions, PAA nanofibres with diameters of 100-130 nm are obtained. The nanofibres obtained have a smooth appearance and are well-defined. The nanofibres together form a white membrane with a diameter of 5 to 10 cm and from 30 to 150 mg.

[0336] 3) Stabilization of the Membranes by Heat Treatment

[0337] Heat treatment is carried out at 130-145 C. for 25-50 min. After stabilization the membranes are stable in aqueous media for several months.

[0338] Swelling of the membrane is noted when it is immersed in water. This phenomenon shows the hygroscopic character of the PAA which is a considerable advantage for the capture of the dissolved elements. The heat treatment has an impact on the morphology of the nanofibres (FIG. 2).

[0339] 4) Extraction of Cu.sup.2+

[0340] Synthetic solutions of Cu.sup.2+ of 5-500 mg/L are prepared from CuSO.sub.4.5H.sub.2O in deionized water. As the solutions of CuSO.sub.4 are slightly acidic, it is necessary to adjust the pH to 6 with an aqueous solution of sodium hydroxide.

[0341] The membrane of PAA nanofibres is immersed in the solution of Cu.sup.2+ (5-500 mg/L) and in less than a minute turns blue, which proves that the copper has become incorporated into the PAA nanofibres. After immersion for 2 h in the solution, the membrane is saturated. The final concentration of Cu.sup.2+ in said solution is measured with the Copper spectrophotometer and a capacity of 280 mg/g is obtained, which is much greater than those obtained with the nanofibres described in the prior art.

[0342] 5) Regeneration of the Membranes

[0343] A treatment in an acid medium (1M HCl) instantly causes the membrane to lose its colour and return to its white colouring. The regeneration is 100% effective.

[0344] It is possible to carry out several cycles of copper capture/release.

Example 2. Extraction of Copper by Insoluble PAA Nanofibres Produced by Electrospinning in an Alcoholic Medium and Stabilized by a Cross-Linking Agent

1) Preparation of the Synthesis Solution

[0345] PAA Fluka: pure PAA (analytical standard), Mw=130,000 g.mol1, white powder [0346] HMDA 15% [0347] Concentrated nitric acid HNO.sub.3

[0348] Polyacrylic acid, with a molecular weight (M.sub.W) ranging from 130,000 to is diluted to 10 wt. % in ethanol. A reflux assembly makes it possible to facilitate dissolution. The solution obtained is colourless and viscous. Nitric acid is added in order to obtain a pH of 2. Hexamethylene diamine (HMDA) is added as cross-linking agent at 15 wt. % relative to the mass of PAA.

2) Production of the Nanofibres by Electrospinning

[0349] This solution is injected into an electrospinning device in order to manufacture the nanofibres. The voltage applied is situated between 15 and 20 kV at a distance between the solution and the collector of 10-20 cm. Under these conditions, PAA nanofibres with a diameter of 150 nm are obtained. The nanofibres obtained have a smooth appearance and are well-defined. The nanofibres together form a white membrane from 5 to 10 cm diameter and from 30 to 150 mg.

3) Stabilization of the Membranes by Heat Treatment

[0350] Heat treatment is carried out at 130-145 C. for 25-50 min. After stabilization the membranes are stable in aqueous media for several months.

4) Extraction of Cu.SUP.2+

[0351] Synthetic solutions of Cu.sup.2+ of 5-500 mg/L are prepared from CuSO.sub.4.5H.sub.2O in deionized water. As the solutions of CuSO.sub.4 are slightly acidic, it is necessary to adjust the pH to 6 with an aqueous solution of sodium hydroxide.

[0352] The membrane of PAA nanofibres is immersed in the solution of Cu.sup.2+ (5-500 mg/L) and in less than a minute turns blue, which proves that the copper has become incorporated into the PAA nanofibres. After immersion for 2 h in the solution, the membrane is saturated. The final concentration of Cu.sup.2+ in said solution is measured with the Copper spectrophotometer and a capacity of 230 mg/g is obtained.

5) Regeneration of the Membranes

[0353] A treatment in an acid medium (1M HCl) instantly causes the membrane to lose its colour and return to its white colouring. The regeneration is 100% effective.

[0354] It is possible to carry out several cycles of copper capture/release

Example 3: Extraction of Copper by the Insoluble PAA Nanofibres Produced by Electrospinning and Stabilized by VUV Radiation

[0355] Manufacture of soluble nanofibres of PAA is the same as that described in Example 1.

[0356] Stabilization of the membrane is carried out by VUV radiation.

[0357] The membrane of nanofibres to be stabilized is placed in a VUV (Vacuum Ultra Violet) irradiation device at a distance of 4-8 cm from the source.

[0358] In order to prevent the formation of ozone, flushing with nitrogen (N.sub.2) is carried out for 15 minutes.

[0359] Irradiation is carried out at 172 nm under flushing with N.sub.2 for a duration varying between 1 and 5 minutes.

[0360] This physical treatment makes it possible to not alter the morphology of the nanofibres while allowing a cross-linking and therefore a stabilization in the medium of use (water, alcohols, etc.).

[0361] The spectroscopic (IR) and structural (SEM) analyses produce similar results before and after VUV treatment (for times varying from 1 to 5 minutes). FIGS. 3a, 3b, 3c, 3d respectively show the nanofibres after 1, 2, 3 and 5 minutes of VUV treatment. There is no notable change in morphology (shape, diameter) observed.

[0362] The method for the extraction of copper is the same as that described in Example 1. The performance of such a membrane for copper capture is 25 mg/g.

[0363] The regeneration of the membrane is implemented according to the same method as that described in Example 1 and also reaches 100% effectiveness.

Example 4: Extraction of Traces of Caesium by PAA Nanofibres Incorporating, by Solvent Impregnation, Calixarenes Selective for Caesium

[0364] 1) Preparation of the Mixed PolymerCalixarene Solutions

[0365] a) Preparation of the Polymer Solution

[0366] Polyacrylic acid, with molecular weight (Mw) ranging from 130,000 to 250,000 g/mol is diluted to 10 wt. % in ethanol. A reflux assembly makes it possible to facilitate dissolution. The solution obtained is colourless and viscous.

[0367] b) Synthesis and Treatment of Calixarene

[0368] The calixarene used is compound B described in the application WO 2013/124831.

[0369] The pure calixarene is then dissolved in tetrahydrofuran (THF).

[0370] c) PAACalixarene Mixtures

[0371] Various solutions can be prepared from the solution of PAA in ethanol and of calixarene in THF. By modifying the mixture the proportion of calixarene with respect to PAA is varied. Solutions can thus be prepared having as a relative proportion of calixarene from 10 to 70 wt. % relative to the PAA.

[0372] 2) Production of Nanofibres by Electrospinning

[0373] These solutions are respectively injected into an electrospinning device in order to produce soluble nanofibres. The voltage applied is comprised between 15 and 20 kV at a distance between the solution and the collector of 10-20 cm. Under these conditions, PAA nanofibres with diameters of 200-300 nm are obtained. The nanofibres obtained have a smooth appearance and they are well-defined. The nanofibres together form a white membrane with a diameter of 5 to 10 cm and from 30 to 150 mg.

[0374] 3) Stabilization of the Membranes by VUV Radiation

[0375] The membrane obtained is placed in a VUV (Vacuum Ultra Violet) irradiation device at a distance of 4-8 cm from the source. In order to prevent the formation of ozone, flushing with nitrogen (N.sub.2) is carried out for 15 minutes.

[0376] Irradiation is carried out under flushing with N.sub.2 for a duration varying between 1 and 5 minutes.

[0377] This physical treatment makes it possible to not alter the morphology of the nanofibres while allowing a cross-linking and therefore a stabilization in the medium of use (water, alcohols, etc.)

[0378] 4) Extraction of Traces of Caesium in a Medium Containing Interfering Ions

[0379] The media to be treated have a very low concentration of caesium in sea water (containing Na.sup.+ ions) which requires the use of a calixarene that is selective for caesium.

[0380] In order to carry out the extraction tests, the model solution used contains Na.sup.+ ions at 10.sup.1 M and Cs.sup.+ ions at 10.sup.4 M.

[0381] The membrane of PAA nanofibres containing calixarenes is introduced into a volume of this selectivity solution. The extraction time is generally 30 minutes.

[0382] The initial and final concentrations of caesium are measured by an atomic absorption spectrometer as described in the section Materials and methods. A 95% reduction in the concentration of caesium is measured.

[0383] By way of comparison, a membrane composed of PAA nanofibres (with no added calixarene) only allows a 6% reduction in caesium.

[0384] 5) Regeneration of the Membranes by Acid Treatment

[0385] A treatment of the membranes containing calixarenes having complexed caesium with an aqueous solution of HCl acid at 5M allows all of the trapped caesium to be released.

Example 5: Study of the Performances of Insoluble PAA Nanofibres

[0386] 5.1/ Kinetic Study of the Capture of Copper (II) Ions

[0387] A kinetic study was carried out in comparison with the ion exchange resin Amberlite IRC 748I.

[0388] The same solution of copper salt at 2 mg/l is placed respectively in two pillboxes of equal capacity. The volume of the solution is 50 ml. 250 ml of copper salt solution is prepared by introducing 157.5 mg of CuSO.sub.4 into a 250 ml measuring flask and making up to the mark with distilled water.

[0389] The following are placed respectively in these pillboxes: 127 mg of sorbent materials, namely the ion exchange resin (Amberlite IRC 748I resin from Supelco) and the membranes of the present invention produced from an initial synthesis solution containing 25% polyacrylic acid (PAA; molecular weight 2.510.sup.5) by weight relative to the synthesis solution, and 15% hexamethylenediamine by weight relative to the PAA as cross-linking agent.

[0390] The resin Amberlite IRC 748I from Supelco is designed to capture the divalent heavy metals such as copper, cobalt, nickel or zinc with amine-containing dicarboxylic groups.

[0391] In the two materials, it is the carboxylic groups that capture the metal salts.

[0392] Moderate magnetic stirring of 10 revolutions per second is immediately started after putting the sorbent materials into solution.

[0393] 1 ml of the solution of copper is taken respectively at 2, 5, 20, 50, 80 and 120 minutes, using a micropipette. The samples are analysed by atomic absorption.

[0394] The adjustment parameters of the device (ICE 3000 of Thermo Scientific) for the analysis are the following: [0395] Measurement mode: absorption [0396] Flame: air-acetylene [0397] Burner height: 7 mm [0398] Flow rate: 1.1 L/min.

[0399] The results of the analyses are given in FIG. 5a.

[0400] The results very clearly show that the performance of the nanofibres is superior to that of the ion exchange resins (IER). As early as 2 minutes after bringing the Cu solution into contact with the sorbent materials, the capture capacities of the nanofibres of the invention are very significantly greater than those measured with the IERs. These results evidently demonstrate that it is much quicker to totally impregnate a nanofibre with a diameter of 0.3 m than a bead of resin with a diameter of 500 m, despite all the efforts by the manufacturers of the IERs to improve the circulation of liquid inside the beads.

[0401] These results can be explained by the diffusion equation given below:

=I.sup.2/Ds,

in which represents the diffusion time, I represents the dimension of the materials and Ds is a diffusion coefficient.

[0402] The kinetics of capture by the beads of IER are limited by the kinetics of diffusion of the liquid within the beads (Ds), while the kinetics of capture by the nanofibres of the invention are driven by the distance to be covered (I). Given the dimensions of the nanofibres, this distance is very short and it therefore corresponds to short diffusion times.

[0403] Given the advantageous capture capacities of the nanofibres of the invention with respect to the IERs, it is therefore also possible, using the sorbent materials, to reduce the loads of materials in a column for the same effectiveness.

[0404] 5.2/ Adsorption Isotherm for a Solution Laden with Copper (II) Ions

[0405] In order to determine the properties of the PAA nanofibres described in Example 5.1, an adsorption isotherm was produced for the Copper (II) ions. The operating conditions are the following: [0406] pH: 3.8-4.6 [0407] adsorption temperature: 20 C. [0408] volume of the bed: 2 mL [0409] height of resin column: approximately 2.7 cm.

[0410] The maximum capacity (Q.sub.max) is determined as 208 mg of Copper (II)/g of nanofibres of the invention. By way of comparison and under the same conditions, the ion exchange resin Amberlite IRC748I, specialist product for the capture of metals, has a Q.sub.max of 79 mg of Copper (II)/g of resin. The nanofibres according to the invention therefore offer much higher capture performances (FIG. 5b).

[0411] Another study shows that the presence of a background salt (NaCl at 0.1 mol/L) does not modify the adsorption properties of the material.

[0412] 5.3/ Adsorption Isotherm for a Solution Laden with Strontium (II) Ions

[0413] Strontium (II) ions can be captured by the nanofibres of the invention described in Example 5.1. According to the absorption isotherm obtained, the maximum bearing capacity (Q.sub.max) reached is 244 mg of Strontium (II)/g for the nanofibres of the invention, while, under the same operating conditions, the ion exchange resin Amberlite IRC748I has a Q.sub.max of 38 mg of Strontium (II)/g of resin (FIG. 5c).

[0414] The operating conditions are the following: [0415] pH: 5 [0416] adsorption temperature: 20 C. [0417] volume of the bed: 1.9 mL [0418] column height: 1.5 cm.

[0419] 5.4/ Study of the Breakthrough of Copper (II) Ions

[0420] The studies of breakthrough of the PAA nanofibres described in Example 5.1 are carried out with Copper (II) ions in highly concentrated (1 g/L) and weakly concentrated (2 mg/L) media. The results (FIGS. 5d and 5e) show that the nanofibres of the invention comply with the standards of the current discharge legislation with values before breakthrough lower than 0.1 mg/L, while the standards recommend limit values for the concentration of Copper (II) ions between 0.5 and 2 mg/L (Circular DGS/SD 7 A n2004-45 of 5 Feb. 2004 relating to the control of the parameters for Lead, Copper and Nickel in water intended for human consumption).

[0421] The operating conditions for the study in a medium with a high concentration of Copper (II) ions are the following: [0422] pH: 4.6 [0423] concentration of Copper (II) ions=1.0 g/L [0424] temperature: 20 C. [0425] volume of the bed: 2.12 mL [0426] height of resin column: 2.7 cm.

[0427] The operating conditions for the study in a medium with a low concentration of Copper (II) ions are the following: [0428] concentration of Copper (II) ions=2.0 mg/L [0429] temperature: 20 C. [0430] volume of the bed: 2.30 mL [0431] height of resin column: 2.7 cm.

[0432] 5.5/ Bearing Capacity in the Presence of Interfering Substances

[0433] Depending on the effluent, different interfering substances can disturb the stage of adsorption of the Copper (II) ions by the nanofibres described in Example 5.1. Proportions of Sodium (I) (Nat) and Calcium (II) (Ca.sup.2+) ions were tested in order to determine the bearing capacity (Q.sub.e) of the nanofibres of the invention in a medium with interfering substances (Table 1).

TABLE-US-00001 TABLE 1 Interfering Bearing capacity Metal ion substance Ratio Q.sub.e (mg/g) Cu.sup.2+ 169 Cu.sup.2+ Na.sup.+ Cu.sup.2+/Na.sup.+ 1:10 169 Cu.sup.2+ Na.sup.+ Cu.sup.2+/Na.sup.+ 1:500 146 Cu.sup.2+ Ca.sup.2+ Cu.sup.2+/Ca.sup.2+ 1:1 158 Cu.sup.2+ Ca.sup.2+ Cu.sup.2+/Ca.sup.2+ 1:100 70

[0434] The operating conditions are the following: [0435] initial concentration of Copper (II) ions: 0.7 g/L [0436] adsorption temperature: 20 C. [0437] volume of the bed: 2 mL [0438] column height: 2.7 cm.

[0439] 5.6/ Bearing Capacity for Different Metal Ions

[0440] The bearing capacity (Q.sub.e) for several metal ions was determined with the following selectivity (dependent on the concentration conditions, pH, the presence of interfering substances, etc.): Ca.sup.2+<Cr.sup.3+<Eu.sup.3+<Fe.sup.2+<Zn.sup.2+<Ni.sup.2+<Co.sup.2+<Cd.sup.2+<Cu.sup.2+<Pb.sub.2+<Mg.sup.2+<Sr.sup.2+.

[0441] The operating conditions are the following: [0442] initial concentration of metal ions: approximately 1 g/L (20 g/L for Eu.sup.3+) [0443] adsorption temperature: 20 C. [0444] volume of the bed: 2 mL [0445] column height: 2.7 cm.

[0446] The results are given in Table 2 below.

TABLE-US-00002 TABLE 2 Bearing capacity v pH Q.sub.e (mg/g) Ca.sup.2+ 5.0 80 Cd.sup.2+ 5.0 186 Co.sup.2+ 5.0 183 Cr.sup.3+ 4.0 83 Cu.sup.2+ 5.0 188 Eu.sup.3+ * 105 Fe.sup.2+ 4.0 121 Mg.sup.2+ 5.5 215 Ni.sup.2+ 5.0 147 Pb.sup.2+ 4.0 195 Sr.sup.2 5.0 254 Zn.sup.2+ 5.0 124

[0447] 5.7/ Method for the Regeneration of the Nanofibres of the Invention

[0448] The use of 2 to 4 volumes of the bed of a solution of hydrochloric acid at 3.1% (corresponds to 1 mol/L) allows the regeneration of the PAA nanofibres described in Example 5.1 (90-100%). Several cycles of saturations/regenerations were carried out in order to demonstrate the maintenance of the performances of the material (FIG. 5f).

[0449] The operating conditions are the following: [0450] pH: 4.6 [0451] concentration of Copper (II) ions: 1 g/L [0452] adsorption temperature: 20 C. [0453] volume of the bed: 2.12 mL [0454] column height: 2.7 cm.

Example 6: Comparative Study of Nanofibres Obtained by Electrospinning or by Centrifugal Spinning

[0455] The synthesis solution for producing nanofibres by electrospinning or centrifugal spinning is a solution at pH=3.7 containing 25% polyacrylic acid (PAA; molecular weight 2.510.sup.5) by weight relative to the synthesis solution, and 15% hexamethylenediamine by weight relative to the PAA as cross-linking agent.

[0456] The nanofibres are produced respectively by the electrospinning technique and the centrifugal spinning technique according to the methods described in the section Materials and methods.

[0457] The two types of fibres produced are thermally cross-linked by heating at 145 C. for 25 minutes. The comparative results for the two types of nanofibres obtained by electrospinning and by centrifugal spinning are illustrated in FIGS. 6a, 6b, 7a, 7b, 8a and 8b.

[0458] The results show that the two types of nanofibres have similar structures and behave in the same way on contact with water.

[0459] Moreover, infra-red analysis on these two types of nanofibres confirms that the two materials are identical from a chemical point of view.

Example 7: Extraction of Copper by Insoluble PAA Nanofibres Produced by Centrifugal Spinning and Stabilized by Heat Treatment

[0460] The PAA nanofibres described in Example 6 and produced by centrifugal spinning are used to extract the Cu.sup.2+ from an aqueous solution.

[0461] A synthetic solution of Cu.sup.2+ at 2.16 mg/L was prepared from CuSO.sub.4.5H.sub.2O in deionized water.

[0462] 0.129 g of PAA nanofibres obtained by centrifugal spinning according to the method described previously are brought into contact with 50 mL of the solution of Cu.sup.2+. After two hours, the concentration of Cu.sup.2+ in the solution is 0.27 mg/L. That is to say, an 87% reduction in the concentration by simply bringing into contact.

[0463] By way of comparison, the same quantity of PAA nanofibres obtained by electrospinning is also brought into contact for 2 hours with 50 mL of the solution of Cu.sup.2+. The concentration of Cu.sup.2+ after the extraction is 0.23 mg/L. These results show that the performance of the nanofibres obtained by centrifugal spinning is equivalent to that of the nanofibres obtained by electrospinning.

[0464] The same experiment was carried out with ion exchange resins, after contact for 2 hours; the equilibrium concentration is 1.04 mg/L.

[0465] In another comparative experiment, a synthetic solution of Cu.sup.2+ at 5.2 mg/L was prepared from CuSO.sub.4.5H.sub.2O in deionized water.

[0466] 0.112 g of PAA nanofibres obtained by centrifugal spinning are brought into contact with 50 mL of the solution of Cu.sup.2+. After two hours, the concentration of Cu.sup.2+ of the solution is 0.69 mg/L, i.e. also a reduction in the concentration of the order of 87%.

[0467] The same experiment was carried out with 0.1036 g of the nanofibres of polyacrylonitrile obtained by centrifugal spinning. After contact for 2 hours, the concentration of Cu.sup.2+ in the solution is 4.78 mg/L, namely a reduction in the concentration of just 8%.

[0468] These comparative results show that only the use of an initial hydrophilic and water-soluble polymer makes it possible to achieve a desired reduction in the concentration of Cu.sup.2.

Example 8: Study Relating to the Choice of Molecular Weights for Optimizing the Stabilization of PAA Nanofibres

[0469] Two sources of polymers are used. A source of PAA with a molecular weight of 250,000 (Sigma Aldrich 41600235 wt. % in H.sub.2O) and a source of PAA polymer of 2,000 (Sigma Aldrich 53593150 wt. % in H.sub.2O).

[0470] These two sources of polymer are used to make a solution at 25% PAA relative to the total mass and 15% hexamethylenediamine by weight relative to the PAA as cross-linking agent.

[0471] In order to assess the relationship between the molecular weight of the PAA and the cross-linking rate, two thin films of PAA are made on gilded glass slides. The solution of PAA of mass 2,000 does not allow fibres to be produced.

[0472] For these two films a heat treatment at 200 C. for 20 min is carried out. These films are left to cool naturally then immersed for an hour in a solution of water at a pH between 8 and 9 by the addition of soda. These basic solutions are very solubilizing for the PAA.

[0473] After rinsing with distilled water and with ethanol only the PAA film of mass 250,000 is still present on the surface. On the other hand, the PAA film of mass 2,000 is no longer visible, as it has dissolved.

[0474] This example demonstrates that the effectiveness of the cross-linking for a given level of cross-linking agent is very varied. A polymer having a relatively high molecular weight makes it possible to have a better cross-linking effectiveness.