SYNTHESIS OF COMPOSITE BEADS COATED WITH LAYERED MANGANESE OXIDE AND USE OF SUCH BEADS TO REMOVE TOXIC ELEMENTS CONTAINED IN FLUIDS

Abstract

The present invention relates to composite beads coated with H-Birnessite layered manganese oxide with a sheet nanostructure, the average thickness of the sheets being between 1 and 50 nm, and the length L of the sheets being between 0.2 μm and 3 μm, as well as the manufacturing method thereof and a cell for producing said composite beads. The present invention also relates to the use of such beads in treatments for decontaminating fluids containing toxic elements such as heavy metals or organic pollutants.

Claims

1. A composite bead (1) comprising a support particle (10) made of conductive material, said support particle (10) having a three-dimensional shape, and a continuous and nanostructured coating (11) covering said support particle (10), said coating (11) consisting of a layered manganese oxide belonging to the phyllomanganates family and having a nanostructure in the form of sheets of average thickness e and length L, said bead being characterized in that: the layered manganese oxide constituting the coating (11) is H-Birnessite with sheets whose average thickness e is between 1 and 50 nm, and is preferably of the order of nm and whose length L of the sheets of said coating (11) is between 0.2 μm and 3 μm, and preferably of the order of 0.5 μm.

2. The composite bead (1) according to claim 1, wherein the conductive material of said support particle (10) is selected from the group consisting of glasses covered with a semiconductor, the semiconductors, stainless steels, noble metals and mixtures thereof.

3. The composite bead (1) according to any of claims 1 and 2, whereby said support particles (10) have a spherical shape with diameter D, which is preferably between 0.3 mm and 2 mm.

4. The use of beads (10) as defined in any one of claims 1 to 3, in a method for decontaminating a fluid containing at least one toxic element.

5. The use according to claim 4, whereby the fluid to be decontaminated is an aqueous solution containing an organic pollutant selected from the group consisting of detergents, drug residues, pesticides, organic dyes, formaldehyde, and aminoalkylphosphonic acids and/or at least one metallic toxic element selected from the group consisting of lead, copper, cadmium, nickel, arsenic, manganese, iron, and mixtures thereof.

6. The use according to claim 4, whereby the fluid to be decontaminated is gaseous and the toxic element is an organic pollutant selected from the group consisting of glyphosate and formaldehyde.

7. A cell suitable for electrodepositing a coating (11) of layered manganese oxide on support particles (10) made of conductive material and three-dimensional in shape, said electrochemical cell (2) including a compartment (20) for receiving an electrolytic solution (21), wherein a reference electrode (22), a working electrode (23) acting as an anode, and a counter electrode (24) acting as a cathode are arranged, said electrochemical cell (2) being characterized in that said working electrode (23) consists of a conductive substrate on which said support particles (10) are arranged, said conductive substrate constituting the bottom of said electrochemical cell (2), and said counter electrode (24) consists of a carbonaceous material covering said compartment (20), said conductive substrate and said counter electrode (24) being arranged facing each other while being separated by a distance equal to or less than 1 cm, and in that said electrochemical cell (2) further includes an outer tank (25) intended to be in fluid communication with said compartment (20) by means of pumps (26), the volume of said outer tank (25) representing at least three times that of said compartment (20), said pumps (20) being able to circulate said electrolytic solution (21) in said electrochemical cell (2).

8. A method for manufacturing composite beads (1) as defined according to any one of claims 1 to 3, by electrodeposition of a coating (11) of H-Birnessite layered manganese oxide on support particles (10) made of conductive material and three-dimensional in shape, said method comprising the following steps: A) providing an electrochemical cell (2) as defined according to claim 11; B) filling said compartment (20) of said electrochemical cell (2) with an aqueous electrolytic solution (21) containing dissolved oxygen and soluble Mn (II) ions; C) circulating using the pumps the electrolytic solution (21) between said compartment and said tank (25) at a flow rate between 0.5 mL/minute and 3 mL/minute, and preferably between 1 mL/minute and 3 mL/minute; D) applying a potential E between the working electrode (23) and the reference electrode (22) of between 0.8 V and 1.1 V, so as to electrodeposit the Mn(II) ions on the surface of the support particles (10) according to equation (1):
(x+y)Mn(II)+zH.sub.2O.fwdarw.Mn(III).sub.xMn(IV).sub.yOz+2zH.sup.++n e−  (1) with Mn(III)xMn(IV)yOz constituting a layered manganese oxide having a sheet nano structure, and n, x, y and z defining natural integers such that
n=2z−2(x+y) and z=(3x+4y)/2.

9. The method according to claim 8, whereby the pH of the electrolytic solution (21) is at most 8, preferably between 2 and 6, and better still between 5 and 6.

10. The method according to any one of claims 8 and 9, whereby the electrolytic solution (21) comprises: manganese sulfate MnSO.sub.4 the concentration of which is between 10.sup.−4 M and 5×10.sup.−3 M, preferably between 10.sup.−3 M and 5.Math.10.sup.−3 M, and better still of the order of 1.6.Math.10.sup.−3 M, and a support electrolyte consisting of sodium sulfate Na.sub.2SO.sub.4, the concentration of which is preferably of the order of 0.4 M.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0059] Further advantages and particularities of the present invention will become apparent from the following description, given as a non-limiting example and made with reference to the attached figures and examples:

[0060] FIG. 1 shows schematic representations of a triclinic Birnesitic structure (left part of [FIG. 1]) and of a hexagonal Birnessite structure (right part of [FIG. 1]);

[0061] FIG. 2 schematically represents the deposit of H-Birnessite (Mn.sub.7O.sub.13,5H.sub.2O) at the anode during step C of the method according to the invention;

[0062] FIG. 3 shows a schematic top view of an electrochemical cell according to the invention;

[0063] FIG. 4 shows a schematic view in longitudinal section along the longitudinal axis x of the electrochemical cell shown in FIG. 3;

[0064] FIG. 5 shows a schematic view in lateral section along the lateral axis x of the electrochemical cell shown in [FIG. 3] and [FIG. 4];

[0065] FIG. 6 shows a photograph taken with an optical microscope of a stainless steel plate, which is coated with H-Birnessite by electrodeposition in a conventional electrochemical cell in accordance with a method of the prior art developed by the Applicant, as well as a photograph taken with a scanning electron microscope (magnification X3000, with an enlarged area at magnification X20,000) showing that this layered oxide has a sheet structure;

[0066] FIG. 7 shows a photograph of stainless steel beads coated with layered manganese oxide in accordance with the method according to the invention: these photographs show that the degree of coverage of the beads by manganese oxide is at least 80%;

[0067] FIG. 8 is a photograph of composite beads according to the invention entirely coated with layered manganese oxide electrodeposited in accordance with the method according to the invention;

[0068] FIG. 9 shows two photographs taken with a scanning electron microscope (X32 and X3000) of a bare stainless steel bead (with a diameter of 2 mm) prior to the deposition of layered manganese oxide according to the method according to the invention;

[0069] FIG. 10 shows a first photograph taken with a scanning electron microscope (X32) of a stainless steel bead (with a diameter of 2 mm) coated with layered manganese oxide in accordance with the method according to the invention (left photograph), as well as three photographs taken with an electron microscope of this bead at different magnifications (respectively X1000, x3000 and X20,000 from left to right);

[0070] FIG. 11 is the Raman spectrum of an H-Birnessite film electrodeposited on a SnO.sub.2-covered glass plate doped with fluorine, sold by SOLEMS (120 Ω/cm.sup.2), under the conditions developed in the prior work of the applicant.sup.[5]. The bands present on this spectrum are characteristic of H-Birnessite according to the scientific literature.sup.[8].

[0071] FIG. 12 shows the Raman spectrum (curve B) of a stainless steel bead coated with layered manganese oxide in accordance with the method according to the invention, in comparison with the spectrum of an uncoated stainless steel bead (curve A);

[0072] [FIG. 1], [FIG. 2] and [FIG. 6] to [FIG. 8] have been described in the descriptive part relating to the preceding prior art, and [FIG. 7] to [FIG. 12] are described in greater detail in the examples which follow, which illustrate the invention without limiting the scope thereof.

DETAILED DESCRIPTION OF THE INVENTION

[0073] [FIG. 3] to [FIG. 5] schematically illustrate an example electrochemical cell 2 according to the invention which comprises a compartment 20 intended to receive an electrolytic solution 21, and wherein a reference electrode 22, a working electrode 23 acting as anode, and a counter electrode 24 acting as cathode are arranged.

[0074] In the electrochemical cell according to the invention 2, the space between the counter electrode 24 (acting as cathode) and the working electrode 23 (anode) constitutes the compartment 20 intended to contain the electrolytic solution 21.

[0075] The working electrode 23 (anode) is made up of a conductive substrate made of SnO.sub.2 doped with fluorine (having a contact surface of 2.25 cm.sup.2) which is sold by SOLEMS (120 Ω/cm.sup.2), and support beads 10 which are arranged on the substrate.

[0076] Moreover, as reference electrode 22, an Ag/AgCl/0.1 M NaCl silver chloride electrode (3 mm in diameter) whose potential is Eref=0.29 V/SHE is used.

[0077] As regards the counter electrode 24, it is a counter electrode 24 of large surface area advantageously consisting of a carbon-doped polyimide film (kapton). It is for example possible to use the material sold by the company Goodfellow (sheet resistance 370 Ω/cm.sup.2)

[0078] The volume of the compartment 20 is very small (less than 2 mL) because the electrodes 23, 24 are parallel and separated from one another by a distance of less than 1 cm (in particular of the order of a few millimeters). Thus, the thickness of the electrolytic solution 21 (not visible in FIGS. 3 to 5) located above the conductive substrate constituting the working electrode 23 is less than or equal to 1 or 2 mm, and the stainless steel beads 10 are very close to the electrodes 23, 24.

[0079] Furthermore, [FIG. 4] shows more particularly that in the electrochemical cell 2, the electrolytic solution 21 in continuous flow between the compartment 20 and an outer tank 25, containing approximately 15 mL of electrolytic solution and in fluid communication with this compartment 20 by means of pumps 26 (and more particularly micro-pumps) capable of circulating the electrolytic solution 21 in the electrochemical cell 2 at a flow rate of the order of 1 mL/minute. This allows a low but continuous renewal of the electrolytic solution 21.

EXAMPLES

First Example Embodiment (Comparative): Electrodeposition of Layered Manganese Oxide on a Stainless Steel Plate

[0080] In a conventional electrochemical cell, the deposition of layered manganese oxide is carried out on a Z2CND17-12 stainless steel plate according to the AFNOR NF A 35573 standard.

[0081] In this cell, use is made, as a reference electrode 22, of a Hg/Hg2SO4/K2SO4 mercurous sulfate electrode. The potential at 25° C. of the saturated Hg/Hg2SO4/K2SO4 electrode with respect to the standard hydrogen electrode is: E=0.6513 V/SHE. It is however also possible to work with a saturated calomel electrode (SCE: the potential of the SCE at 25° C. relative to the standard hydrogen electrode is E=0.2412 V at 25° C.).

[0082] The duration of the test is of the order of 1 hour.

[0083] FIG. 6 shows a photograph taken with an optical microscope of a stainless steel plate, which is coated with H-Birnessite by electrodeposition in a conventional electrochemical cell in accordance with a method of the prior art developed by the Applicant, as well as a photograph taken with a scanning electron microscope (magnification X3000, with an enlarged area at magnification X20,000) showing that this layered oxide has a sheet structure.

Second Example Embodiment: Electrodeposition of Layered Manganese Oxide on Stainless Steel Beads (with a Diameter of 2 mm) in Accordance with the Method According to the Invention

[0084] Use is made, as support particles 10, of Z2CND17-12 stainless steel beads according to the AFNOR NF A 35573 standard (with a diameter of 2 mm), which are deposited on the conductive substrate 20 of the working electrode 23, in the electrochemical cell 2 according to the invention shown in [FIG. 3] to [FIG. 5]. As regards the electrolytic solution 21, it has not been degassed and comprises manganese sulfate MnSO4 at a rate of 1.6.Math.10.sup.−3M, and sodium sulphate Na.sub.2SO.sub.4 at a rate of 0.4M. The free pH is between 5 and 6.

[0085] The electrolytic solution is circulated between the compartment 20 and the tank using the pumps 26, at an adjustable flow rate of the order of 1 mL/minute (step C of the method according to the invention) and a potential E is applied between the working electrode (23) and the reference electrode 22 of 0.9 V (step D of the method according to the invention). This leads to electrodepositing the Mn(II) ions on the surface of the support particles 10 according to the equation (1) indicated above, forming a layered manganese oxide coating 11 having a sheet nanostructure.

[0086] The continuous renewal of the electrolytic solution 21 in the electrochemical cell 2 makes it possible to avoid depletion of the concentration of Mn(II) ions in the electrolytic solution 21, such that the concentration of Mn(II) ions in the compartment is at least equal to 80% of the value of the concentration of Mn(II) ions in the outer tank 25.

[0087] In the present test, the duration is typically of the order of 2 to 20 hours, and typically in order to avoid depletion of the electrolytic solution, while making it possible to ensure a homogeneous deposition of layered manganese oxide.

[0088] FIG. 8 is a photograph of composite beads according to the invention entirely coated with layered manganese oxide and [FIG. 10] shows a first photograph taken with a scanning electron microscope (X32) of a stainless steel bead (with a diameter of 2 mm) coated with layered manganese oxide in accordance with the method according to the invention (left photograph).

[0089] FIG. 9 shows photographs taken with a scanning electron microscope (X32 and X3000) of a bare stainless steel bead (with a diameter of 2 mm) prior to the deposition of layered manganese oxide according to the method according to the invention; FIG. 9 clearly shows that the surface of the stainless steel bead is perfectly smooth without any particular nanostructure.

Characterization by Scanning Electron Microscopy (SEM)

[0090] The coating 11 of layered manganese oxide was characterized by scanning electron microscopy (SEM), as illustrated by the three SEM photographs of [FIG. 10] carried out at different magnifications (respectively X1000, A3000 and X20,000 from left to right). These SEM photographs show that the deposition of layered manganese oxide is very homogeneous and very nanostructured, and resembles that obtained on a stainless steel plate in example 1: The sheet structure shown in [FIG. 6] (example 1, deposition on a stainless steel plate) is very similar to that shown by the SEM photograph of [FIG. 10] (deposition on stainless steel beads) obtained with a magnification X20,000).

[0091] In the present example, the sizes of the sheets (length L and thickness e) of the manganese oxide coating are ten times smaller than those obtained on stainless steel plates (cf. comparative example 1), but this difference in size is due to the experimental conditions.

Characterization by RAMAN Spectroscopy

[0092] The coating 11 of layered manganese oxide was also characterized by RAMAN spectroscopy, as illustrated by FIG. 12.

[0093] Raman spectroscopy makes it possible to identify the nature of the compounds via the presence of characteristic bands. Thus, for the H-birnessite, the characteristic bands correspond to the three bands circled and located respectively at 500 cm.sup.−1, 572 cm.sup.−1, 646 cm.sup.−1 according to one of the main references of the literature indicated below (measurements on birnessite powders). FIG. 11 shows that these same bands are obtained for films synthesized by electrochemistry identified as being H-Birnessite according to the values from the scientific literature.sup.[8].

[0094] In comparison with FIG. 11, FIG. 12 together shows the spectrum of a stainless steel bead without deposition (substrate alone, lower curve A) with the spectrum of a composite bead according to the invention covered with a deposition synthesized by electrochemistry (upper curve B). As can be seen, the characteristic bands of the electrodeposited coating on the beads are located exactly at the same values as those of the films, thus confirming that it is indeed H-Birnessite.

LIST OF REFERENCES

[0095] [1] “In situ grazing-incidence X-ray diffraction during electrodeposition of birnessite thin films: Identification of solid precursors.” M. Ndjeri, S. Peulon, M. L. Schlegel, A. Chaussé, Electrochemistry Communication, 13 (2011) 491-494. [0096] [2] “In situ XANES measurements during electrodeposition of thin film: Example of birnessite, a promising material for environmental applications” A. Pensel, S. Peulon, Electrochimica Acta 281 (2018) 738-745. [0097] [3] “Etudes fondamentales de diverses interfaces, solide-liquide et liquide-liquide, pour des applications environnementales” S. Peulon, Soutenance de HDR, Université d′Evry, Septembre 23, 2015. [0098] [4] “Développement d'une cellule electrochimique permettant la caractérisation in situ par diffraction de rayons X rasants: étude de l'électrodépôt de la bimessite.” M. Ndjeri, S. Peulon, M. L. Schlegel, A. Chaussé, Journées d'Electrochimie JE' 11, Grenoble, France, Jul. 4-8, 2011. [0099] [5] “Couplage électrochimie-spectroscopie EXAFS/XANES (XAS): Etude in situ et en temps réel de l'électrodépôt de birnessite” A. Pensel, S. Peulon, A. Chaussé, JE'20 1 7, Journées d′Electrochimie 2017, Bordeaux, France. [0100] [6] “Coating techniques for glass beads as filter media for removal of manganese from water” Peter Rose, Simon Hager, Karl Glas, Dirk Rehmann and Thomas Hofmann, Water Science & Technology: Water Supply, 17.1, 2017, 95-106. [0101] [7] EP 1698395 (A1) European patent application: An adsorptive-filtration media for the capture of waterborne or airbone constituents. [0102] [8] “Raman spectra of birnessite manganese dioxides. Solid State Ionics”, Julien, C., Massot, M., Baddour-Hadjean, R., Franger, S., Bach, S. & Pereira-Ramos, J. P. (2003). 159(3-4), 345-356.