METHOD FOR MANUFACTURING A MEMBRANE WITH HIGH PERCOLATION POWER

Abstract

A method for manufacturing a membrane, which includes at least the following steps of: preparing a mixture that contains at least an aqueous solution of a cationic polymer whose pH is between 5 and 8, the cationic polymer having positively-charged groups in this aqueous solution, and an aqueous solution of an anionic polymer, the anionic polymer having negatively-charged groups in this aqueous solution; stirring the mixture; leaving the mixture to mature to cause the ionic interaction between positively-charged groups of the cationic polymer and negatively-charged groups of the anionic polymer, until obtaining within the mixture a membrane in the form of a hydrogel; adding at least one crosslinking agent so as to crosslink the membrane; drying the crosslinked membrane obtained upon completion of the previous step. This membrane is used for the treatment of liquid or gaseous effluents, as well as an antimicrobial support or for heterogeneous catalysis.

Claims

1. A method for manufacturing a membrane, wherein it comprises at least the following steps of: a) preparing a mixture that contains at least: an aqueous solution of a cationic polymer whose pH is comprised between 5 and 8, said cationic polymer having positively-charged groups in this aqueous solution; an aqueous solution of an anionic polymer, said anionic polymer having negatively-charged groups in this aqueous solution; b) stirring the mixture; c) leaving the mixture to mature to cause the ionic interaction between positively-charged groups of the cationic polymer and negatively-charged groups of the anionic polymer, until obtaining within the mixture a membrane in the form of a hydrogel; d) adding at least one crosslinking agent so as to crosslink the membrane; e) drying the crosslinked membrane obtained upon completion of step d).

2. The method for manufacturing a membrane according to claim 1, wherein the cationic polymer is selected amongst polyethylenimine, polyallylamine hydrochloride, chitosans and proteins.

3. The method for manufacturing a membrane according to claim 1, wherein the anionic polymer is selected amongst polyacrylic acid, pectin, carrageenan, alginate and polystyrene sulfonate.

4. The method for manufacturing a membrane according to claim 1, wherein the mixture of step a) comprises, in weight percent expressed with respect to the weight of said mixture: between 0.2% and 0.5% the cationic polymer; between 0.6% and 1.5% of the anionic polymer.

5. The method for manufacturing a membrane according to claim 1, wherein at step a), at least one solid compound selected amongst activated charcoal, silica and clay is added to the mixture.

6. The method for manufacturing a membrane according to claim 1, wherein the crosslinking agent is glutaraldehyde.

7. A method comprising treating liquid or gaseous effluents with the membrane obtained according to the manufacturing method according to claim 1.

8. A method comprising supporting a heterogeneous catalysis with the membrane obtained according to the manufacturing method according to claim 1.

9. A method comprising applying the membrane obtained according to the manufacturing method according to claim 1 as an antimicrobial support.

Description

DESCRIPTION OF THE FIGURES

[0071] FIG. 1 represents a graph of the evolutions of the fixing capacity denoted «q.sub.eq» of chromium(VI) (hereinafter abbreviated «Cr(VI)») and of the total chromium (hereinafter «Cr(total)») as a function of the residual concentration C.sub.eq of Cr(VI) and Cr(total) respectively after experiments of adsorption of chromium ions on a membrane obtained according to a 1.sup.st embodiment of the manufacturing method according to the invention.

[0072] FIG. 2 represents a captured photo of a membrane obtained according to a 2.sup.nd embodiment of the manufacturing method according to the invention.

[0073] FIG. 3 is a graph representing the evolutions of relative residual concentration of palladium denoted «Ct/C0» for 3 membranes with different thicknesses and which have been obtained according to this 2.sup.nd embodiment of the manufacturing method according to the invention as a function of the 3-nitrophenol (hereinafter abbreviated «3-NP») hydrogenation reaction time.

[0074] FIG. 4 is a graph of the model of the kinetic profiles by the pseudo-first order equation (In(Ct/C.sub.0)) as a function of the reaction time plotted based on the reported relative residual concentrations of palladium.

[0075] FIG. 5 represents a graph of the breakthrough curves obtained with other 3-NP hydrogenation reaction experiments.

[0076] FIG. 6 represents a microphotograph of a membrane produced according to the invention illustrating the macroporosity of the material (scanning electron microscope).

[0077] FIG. 7 represents a captured photo illustrating the high percolation power (by gravity drainage) of a membrane produced according to the invention when a liquid is flowing.

[0078] FIG. 8 represents a captured photo of a membrane produced according to the invention (plate, 20×10 cm).

[0079] FIG. 9 represents a captured photo of the inner macroporosity of the membranes produced according to the invention (in section, after die cutting).

EXPERIMENTAL PART

[0080] 1.sup.st Series of Experiments:

[0081] 15 g of a PEI solution with a mass content of 50% have been diluted in 250 g of demineralized water. The pH of this solution has been set to a value of 6.5 with nitric acid. Thus, an aqueous PEI solution with a mass content of 3% has been obtained. 40 g of alginate have been diluted with 960 g of demineralized water so as to obtain an aqueous alginate solution with a mass content of 4%. 132 g of the alginate solution obtained in this manner have been mixed with 368 g of demineralized water.

[0082] The whole has been stirred, and then 35 mL of the PEI solution have been added (namely 5 mL every 10 seconds, this operation having been repeated 7 times).

[0083] The mixture has been stirred for one minute.

[0084] The mixture has been poured into a polypropylene box while avoiding the formation of bubbles and the whole has been left at room temperature for 24 hours. A membrane has been obtained through a gelling reaction of the alginate with PEI.

[0085] The membrane has been washed 5 times with demineralized water.

[0086] The washed membrane has been put in suspension in 300 mL of demineralized water to which 4 mL of an aqueous glutaraldehyde solution with a mass content of 50% have been added so as to achieve crosslinking of the membrane.

[0087] Afterwards, the membrane has been subjected to a moderate stirring of 30 «back-and-forth» movements per minute for 24 hours.

[0088] The membrane has been rinsed (6 times) with 300 mL of demineralized water, and then dried at room temperature for 2 days.

[0089] Characterizations of the Obtained Membrane:

[0090] The membrane obtained in this manner had the following characteristics: [0091] a porosity (measured with a pycnometer) of 93.4%; [0092] a stability to attrition of 94%; [0093] a water flux (in natural percolation) of 33.6 mL/(cm.sup.2.Math.min); [0094] a point of zero charge pH (hereinafter abbreviated as «pH.sub.PZC») of 5.7.

[0095] The 93.4% value reflects a high porosity of the membrane which is quite adequate to ensure good natural percolation performances, as shown by the filtering flux (filtering surface speed in the range of 20 m/h).

[0096] The stability of the membrane has been determined by subjecting a sample of the membrane in the form of a disk with a diameter of 25 mm immersed in 20 mL of water to stirring at 150 rpm for 72 hours. Then, the membrane has been dried and weighted. Stability is the percentage of remaining membrane upon completion of this stirring with respect to the initial membrane mass. The 94% value reflects a very good stability of the membrane to attrition and a maintenance of its integrity when it is subjected to a strong stirring in water.

[0097] The water flux has been determined by measuring the time required for the passage of 100 mL of water throughout a membrane sample with a surface of 4.64 cm.sup.2, and that being so at 20° C. and at a pressure of 0.006 bar. The value of the water flux (in natural percolation) of 33.6 mL/(cm.sup.2.Math.min) reflects excellent percolation properties of the membrane. FIG. 7 illustrate the natural flow by gravity drainage throughout the macroporosity of the membranes with high percolation power.

[0098] Furthermore, the membrane obtained in this manner has been subjected to sorption experiments with a solution containing Cr(VI) ions in order to characterize its adsorption properties.

[0099] For this purpose, the device used for these experiments consisted of a device operating continuously for the recirculation of solutions containing metallic ions which comprised: [0100] a peristaltic pump commercialized by the company Ismatec under the commercial name ISM404B; [0101] a support configured to contain the membrane and enable the circulation of the solution of metallic ions throughout said membrane; [0102] a reservoir containing the solution of metallic ions equipped with a magnetic stirrer commercialized by the company Thermo Scientific under the commercial name Variomag® Poly 15 to stir the solution; [0103] a device for the loop circulation of the solution of metallic ions and the passage thereof throughout the membrane.

[0104] Throughout these experiments, the solution of Cr(VI) ions has been put to circulate in loop in the device with a pumping rate of 15 or 30 mL/minute.

[0105] Throughout these experiments, Cr(III) ions have appeared by reduction of the Cr(VI) ions in situ on the membrane.

[0106] The concentration of Cr(VI) has been determined with a UV spectrophotometer commercialized by the company Shimadzu under the commercial name UV-1650PC at a wavelength of 540 nm by means of a colorimetric method with diphenylcarbazone.

[0107] The total concentration of Cr (namely the sum of the Cr(VI) and Cr(III) ions) has been determined by atomic emission spectrometry with induced plasma with a spectrometer commercialized by the company Horiba under the commercial name Activa.

[0108] The concentration Cr(III) has been determined by subtraction of the concentration of Cr(VI) from the concentration of Cr(total).

[0109] The membrane has also been characterized by: [0110] scanning electron microscopy with a microscope commercialized by the company Thermo Fisher Scientific under the commercial name Quanta™ FEG 200, so as to demonstrate the high porosity of the membrane; [0111] energy dispersive analysis so as to reveal the dense and homogeneous distribution of metallic ions (chromium) within the membrane.

[0112] Photos Captured by Scanning Electron Microscopy:

[0113] The photos captured by scanning electron microscopy have shown that the structure of the membrane obtained in this manner upon completion of the manufacturing method according to the invention was porous. More specifically, the observed porosity was irregular in terms of cell geometry but evenly distributed in the space and in terms of average size whose order of magnitude was between 100 and 200 μm. This is illustrated in FIG. 6.

[0114] After the membrane (a 30 mg sample) has been subjected to Cr(VI) sorption upon completion of a recirculation in the experimentation device as detailed hereinabove with an aqueous solution that contained Cr(VI) ions at a concentration of 200 mg/L at a pH of 2 for a duration of 96 hours with a recirculation feed rate of 15 mL/minute and at a temperature of 20° C., the photos captured by scanning electron microscopy have shown that the structure of the membrane was slightly more compact and always porous. This compression of the membrane could be explained by the liquid stream that has crossed the membrane and also by the reaction of Cr(VI) with the functional groups of the membrane during the adsorption thereof on the membrane such that Cr(III) ions have been formed in situ. Indeed, the partial reduction of Cr(VI) in situ could contribute to modifying the apparent structure of the membrane by causing oxidation thereof.

[0115] Energy Dispersive Analysis Spectrum:

[0116] The energy dispersive analysis spectrum after this sorption experiment has shown the homogeneous presence of chromium ions at the surface of the membrane. This confirms the adsorbent properties of the membrane obtained with the manufacturing method according to the invention.

[0117] Adsorption Isotherm (at Room Temperature) of Cr(VI) by the Membrane

[0118] The adsorption isotherm has been determined with the above-described device by making 50 mL of Cr(VI) solutions at a pH of 2 and at initial concentrations comprised between 20 and 300 mg/L circulate in loop at 20° C. and continuously for 96 hours. The circulation flow rate was 15 mL/minute.

[0119] Once equilibrium is reached, the filtrate recovered upon completion of the experiment has been analyzed in order to determine: [0120] the residual concentration C.sub.eq of Cr(VI), and [0121] the residual concentration C.sub.eq of Cr(total),

[0122] with the techniques as detailed hereinabove.

[0123] Through a material balance, the amounts of Cr(VI) and of Cr(total) that have been fixed on the membrane, as well as the corresponding fixing capacities (q.sub.eq) have been deduced.

[0124] FIG. 1 is a graph of the evolutions of the fixing capacity q.sub.eq of Cr(VI) and of Cr(total) as a function of the residual concentration C.sub.eq of Cr(VI) and Cr(total), respectively. These plots allow obtaining the adsorption isotherms of Cr(VI) and of Cr(total) and deducing the maximum adsorption capacities (q.sub.max) of Cr(VI) and of Cr(total), as well as the affinity of the adsorbent for the solute (adsorbate) which is proportional to the slope at the origin of the curve.

[0125] In light of FIG. 1, the maximum adsorption capacity exceeds 300 mg of Cr(VI)/g. This maximum fixing capacity is very high (amounting to more than 6 mmol Cr(VI)/g of adsorbent). The slope at the origin for Cr(VI) is almost vertical. This proves the strong affinity of the membrane obtained with the manufacturing method according to the invention for chromate ions. The slope at the origin for Cr(total) is lower. This is to be related to mechanisms of reduction of Cr(VI) in situ on the membrane in acid medium.

[0126] Thus, these experiments of adsorption of chromium by a membrane obtained according to the manufacturing method according to the invention reflect its excellent adsorbent properties and therefore its potential for use thereof in the treatment of liquid effluents that contain Cr(VI) ions in particular.

[0127] 2.sup.nd Series of Experiments:

[0128] A volume of 100 mL of an alginate solution at 4 weight % has been diluted with 400 mL of demineralized water so as to obtain a 1.sup.st solution.

[0129] A volume of 35 mL of a PEI solution at 4 weight % whose pH has been set to 6.5 with nitric acid has been progressively added under stirring to the 1.sup.st solution (namely 5 mL every 10 seconds, this operation having been repeated 7 times).

[0130] After adding PEI, the mixture has been poured into a polypropylene box while avoiding the formation of bubbles and the whole has been left at room temperature for 24 hours. A membrane has been obtained through a gelling reaction of the alginate with PEI.

[0131] The obtained membrane has been washed 5 times with demineralized water in order to eliminate the free reagents.

[0132] 300 mL of demineralized water have been added to the washed membrane, and then 2.5 mL of an aqueous glutaraldehyde solution with a mass content of 50% have been added so as to enhance crosslinking of the membrane.

[0133] Afterwards, the membrane has been subjected to a moderate stirring of 30 «back-and-forth» movements/minute for 24 hours.

[0134] The membrane has been washed 4 times with demineralized water, and then dried at room temperature for 2 days.

[0135] Characterizations of the Obtained Membrane:

[0136] The membrane obtained in this manner had the following characteristics: [0137] a porosity (measured with a pycnometer) of 70.93%; [0138] a stability to attrition of 97.0%; [0139] a water flux of 24.8 mL/(cm.sup.2.Math.min); [0140] a pH.sub.PZC of 6.29; [0141] a density of 0.0637 g/cm.sup.3 (which reveals a high macroporosity of the membrane).

[0142] The stability has been determined in the same manner as with the 1.sup.st series of experiments. The 97% value reflects a very good stability of the membrane to attrition and a maintenance of its integrity when it is subjected to a strong stirring in water.

[0143] The water flux has been determined in the same manner as with the 1.sup.st series of experiments. The value of 24.8 mL/(cm.sup.2.Math.min) reflects excellent percolation properties of the membrane.

[0144] FIG. 2 represents a photo of a sample of this absorbent membrane 1 which has been obtained in this manner. The sample measures 55 mm in length and has a diameter of 25 mm.

[0145] Use of the Membrane as a Catalysis Support for the Hydrogenation of 3-Nitrophenol Catalyzed with Palladium

[0146] Immobilization of Pd(II):

[0147] The membrane has been cut into disks with a diameter of 25 mm.

[0148] Afterwards, a disk (with a dry weight of 250 mg) has been disposed in the support configured to contain the membrane of the device described in the 1.sup.st series of experiments so as to make a fixed-bed column.

[0149] One liter of a Palladium(II) solution (hereinafter abbreviated «Pd(II)») with a variable concentration, comprised between 10 and 50 mg/L, whose pH has been set to 1 with sulfuric acid has been put to circulate in loop within this device for 24 hours with a flow rate of 30 mL/min. The optimum conditions of palladium fixation on the membrane (in other words «the best maximum use efficiency of palladium») have been obtained when its concentration was 28 mg Pd/L.

[0150] After fixation of palladium, the column has been rinsed 4 times with demineralized water at a pH of 1. The membrane has not been dried before proceeding with the reduction of the metal.

[0151] The maximum fixing capacity of palladium reached with the membrane throughout these crosslinkings amounted to 224 mg Pd/g. This fixing capacity is much higher than that of the membranes known from the prior art which are used for catalytic experiments and which contain about 8.8 weight % of palladium.

[0152] Reduction of Pd(II) into Pd(0):

[0153] The reduction of Pd(II) immobilized on the membrane has been carried out by hydrazine hydrate (of chemical formula: N.sub.2H.sub.4.H.sub.2O) at a concentration of 0.03 mol/L in 200 mL of an alkaline solution (at a concentration of 0.5 mmol/L of NaOH) under stirring at 60° C. for 5 hours.

[0154] A final rinsing (4 successive cycles) has been carried out in order to eliminate all traces of free reagents.

[0155] A membrane on which palladium has been adsorbed was obtained. This membrane is abbreviated hereinafter as «the catalytic membrane».

[0156] Photos Captured by Scanning Electron Microscopy and Coupling with Energy Dispersive Analysis:

[0157] The photos captured by scanning electron microscopy and the energy dispersive analysis (by semi-quantification) have shown that the structure of the membrane obtained in this manner was macroporous, relatively homogeneous at the surface and across the section thereof.

[0158] Observation with Transmission Electron Microscopy

[0159] The observations with transmission electron microscopy have shown a homogeneous distribution of the palladium nanoparticles at the surface of the membrane after the reduction of Pd(II) by hydrazine hydrate.

[0160] After reduction of the metal, the size of the palladium nanoparticles ranged between 4.5 and 10.5 nm.

[0161] Test of the Catalytic Properties of the Catalytic Membrane

[0162] Hydrogenation of 3-NP on the Membrane (with Recirculation)

[0163] The operative procedure used to test the catalytic properties of the catalytic membrane has been implemented with the recirculation device described hereinabove in the 1.sup.st series of experiments.

[0164] The catalytic membrane has been fed in loop recirculation for 12 minutes with 100 mL of a 3-NP solution at 50 mg 3-NP/L whose pH has been set to 2.84 in the presence of formic acid at a concentration of 0.2 weight %. The concentration of formic acid has been set in molar excess relative to 3-NP (formic acid/3-NP molar ratio of 160/1). The recirculation flow rate was 50 mL/min.

[0165] The concentration of 3-NP has been measured by spectrophotometry at 332 nm. For this purpose, the collected samples have been acidified with 20 μL of a solution at 5 weight % of sulfuric acid prior to the spectrophotometry analysis.

[0166] 3 experiments have been carried out with 3 catalytic membranes with different thicknesses: [0167] 1.sup.st catalytic membrane with a thickness of 0.58 cm and with a mass of 175 mg over which 26.1 mg of palladium have been immobilized; [0168] 2.sup.nd catalytic membrane with a thickness of 0.85 cm and with a mass of 255 mg over which 27.2 mg of palladium have been immobilized; [0169] 3.sup.rd catalytic membrane with a thickness of 1.06 cm and with a mass of 313 mg over which 27.7 mg of palladium have been immobilized.

[0170] FIG. 3 is a graph representing the evolutions of the relative residual concentration of palladium denoted «(C.sub.t/C.sub.0)» as a function of the hydrogenation reaction time, for the 3 tested membranes: [0171] 1.sup.st curve denoted «A» for the 1.sup.st catalytic membrane; [0172] 2.sup.nd curve denoted «B» for the 2.sup.nd catalytic membrane; [0173] 3.sup.rd curve denoted «C» for the 3.sup.rd catalytic membrane.

[0174] Considering the similar aspect of the 3 curves A to C, it is concluded that the thickness of the catalytic membrane does not affect the kinetic profile of the reaction of hydrogenation of 3-NP catalyzed by palladium.

[0175] FIG. 4 is a graph of the model of the kinetic profiles by the pseudo-first order equation (In(Ct/C.sub.0)) as a function of the reaction time for the 3 tested catalytic membranes: [0176] 1.sup.st curve denoted «A» for the 1.sup.st catalytic membrane; [0177] 2.sup.nd curve denoted «B» for the 2.sup.nd catalytic membrane; [0178] 3.sup.rd curve denoted «C» for the 3.sup.rd catalytic membrane.

[0179] This model has shown a limited variation of the 1st order kinetic coefficient. Indeed, this coefficient amounted to: [0180] 0.0061 s.sup.−1 for the 1.sup.st catalytic membrane; [0181] 0.0068 s.sup.−1 for the 2.sup.nd catalytic membrane; [0182] 0.0083 s.sup.−1 for the 3.sup.rd catalytic membrane.

[0183] These experiments show that the palladium nanoparticles remain available and the high percolation power of the membranes obtained with the manufacturing method according to the invention allows preserving a good accessibility which is independent of their thickness.

[0184] Furthermore, the analysis of these data has allowed calculating the rotation frequency value which is in the range of 0.1 mmol of substrate/(mmol Pd.Math.minute).

[0185] Finally, assessment of the hydrogenation performance over thirty cycles with these 3 catalytic membranes has shown a very limited reduction of the hydrogenation rate: namely a decrease by less than 15%. A simple rinsing with demineralized water allows a regeneration of the membrane obtained with the manufacturing method according to the invention.

[0186] Hydrogenation of 3-Nitrophenol on the Membrane (Without Recirculation)—Effect of the Regeneration of the Support

[0187] The effects of the feed rate of 3-NP and of the regeneration of the membrane obtained according to the manufacturing method according to the invention have been studied.

[0188] For this purpose, increasing volumes (up to 120 mL) of a 3-NP solution at a concentration of 200 mg 3-NP/L at a pH of 2.7 with a circulation flow rate of 20 or 30 mL/minute have circulated (one single passage) throughout a sample of the membrane (27.2 mg).

[0189] Furthermore, the catalytic membrane has been fed into this 3-NP solution at these circulation flow rates of 20 or 30 mL/minute by while regenerating it (by simple rinsing with demineralized water using a volume corresponding to about 9 times the volume occupied by said membrane) when the volume of 3-NP that has circulated throughout the catalytic membrane has reached the values of 40 mL and 80 mL.

[0190] FIG. 5 represents a graph of the breakthrough curves obtained with these experiments. This consists of the evolution of the residual concentration of 3-NP as a function of the volume of the 3-NP solution that has passed throughout the catalytic membrane when: [0191] the feed rate was 20 mL/minute (curve «A»); [0192] the feed rate was 30 mL/minute (curve «B»); [0193] the feed rate was 20 mL/minute and the membrane has undergone a regeneration after passage of 40 mL and 80 mL of the 3-NP solution (curve «C»); [0194] the feed rate was 30 mL/minute and the membrane has undergone a regeneration after passage of 40 mL and 80 mL of the 3-NP solution (curve «D»).

[0195] The breakthrough curves reveal a progressive increase in the residual concentration of 3-NP as a function of the volume of the 3-NP solution that has passed throughout the catalytic membrane. The increase of the circulation flow rate increases the slope of the breakthrough curve (because of an insufficient stay-time in the catalytic membrane).

[0196] The interruption of feeding of the catalytic membrane and the regeneration thereof by means of demineralized water induces a break-up in the breakthrough curves and a partial resumption of its catalytic effectiveness.

[0197] This catalytic reaction of 3-NP hydrogenation clearly illustrates the possibility of using the membranes with high percolation power obtained with the manufacturing method according to the invention for the immobilization of catalytic metals and the synthesis of catalytic supports to be used in a dynamic operating mode with high filtering rate, with confinement of nanoparticles. Furthermore, as explained hereinabove, these membranes have the advantage of allowing an easy recovery of the catalysts at the end of their service life, for example by thermal degradation of the membranes. Thus, the precious metals that form the catalysts are recycled.