WO2022015147A1 - Polymeric magnetic nanofibres with strontium hexaferrite nanoparticles for arsenic adsorption - Google Patents

Abstract

The present invention relates to a nanostructured composite of polyvinyl alcohol (PVA) nanofibres with homogeneously distributed strontium hexaferrite nanoparticles (SrM-NPs) and their application to remove arsenic mixed in aqueous media. The magnetic PVA nanofibres exhibit a large surface area, magnetic shape anisotropy along the longitudinal axis with new modes of magnetisation reversal and a distinct arsenic adsorption capacity, controlling the density of SrM-NPs below the percolation limit. The dispersed system of SrM-NPs in the polymer matrix promotes excellent physico-chemical and magnetic properties, i.e. thermal stability, high values of remanence magnetisation normalised with respect to saturation magnetisation, Mr/Ms, also of the coercive field, Hc, a reduced value of the switching field distribution, SFD, and a high energy product, (BH)max. This invention may be of scientific interest and have a wide range of technological applications. It also implies an interesting commercialisation potential, since magnetic nanofibres can be produced at relatively low costs using a continuous electrospinning process. The invention is intended for use in arsenic adsorption and fixation processes. Therefore, the physico-chemical properties exhibited by this nanostructured composite of magnetic nanofibres make it an economical and ideal invention for potential applications in environmental remediation in places where arsenic contamination is a serious problem.

Description

POLYMERIC MAGNETIC NANOFIBERS WITH STRONTIUM HEXAFERRITE NANOPARTICLES FOR THE ADSORPTION OF

ARSENIC

TECHNICAL FIELD OF THE INVENTION.

The present invention relates to a material for environmental sanitation, specifically a nanostructured compound based on polyvinyl alcohol nanofibers with embedded magnetic nanoparticles of strontium hexaferrite (SrFe-120-19), effective for adsorbing arsenic mixed in water contaminated with this metalloid.

OBJECTIVES OF THE INVENTION. i). Develop a composite of polymeric magnetic nanofibers with strontium hexaferrite nanoparticles, SrFe-120-19, capable of removing arsenic to help clean up wastewater and water used in food production in the agricultural industry. ii). Contribute to the effort to guarantee the supply of clean and appropriate water for human consumption by controlling high levels of arsenic, recognized as one of the main chemical contaminants that cause adverse health effects.

BACKGROUND.

For several decades, the presence and constant increase of pollutants in the environment has drawn the attention of some governments, environmental groups and researchers in various disciplines of natural sciences around the world, due to the discussion on health-related issues. with this problem; and because of the danger that global warming and climate change represent for all living beings on the planet. This is due, on the one hand, to the average increase in temperature due to the excessive use of fossil fuels that pollute and cause the greenhouse effect, but also due to the presence and continuous increase of a wide spectrum of pollutants in the environment. Among them stands out the concentration of heavy metal ions that are discharged into the air, rivers, lakes and other water reservoirs. Certainly this has a negative effect on all forms of life on Earth and will absolutely affect our children, grandchildren and future generations, who will suffer the consequences of what we fail to do, today, with the care and responsible management of resources. nature and the environment.

Water has been the main stage for the evolution of life and is its essential ingredient, but we underestimate this fact and do not correctly assess its importance in the integral and healthy balance of all ecosystems. It is evident that we are putting at risk and compromising the existence and future of the human species and of all living beings on our planet, as a result of the deforestation of tropical forests and jungles, atmospheric pollution, contamination of fresh water reserves, contamination of oceans and farmland with chemical fertilizers, pesticides, radioactive materials, heavy metals such as: cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), mercury (Hg), zinc (Zn ), etc.; and the metalloids arsenic (As), antimony (Sb) and polonium (Po), which are also considered in this category. Heavy metals accumulate in living cells of biological systems through food chains and are potentially toxic, non-degradable and tend to accumulate in the environment. Although it is true that there is a natural component in the environmental pollution generated by forest fires, volcanic activity and the accumulation of pollutants in the deep underground aquifers, due to the transport of minerals mixed with the water, since they can contain high concentrations of arsenic and fluorine in aqueous solution, there is also no doubt that a great contribution to global environmental pollution is of anthropogenic origin. This is linked, among other factors, to poverty and accelerated population growth, since the environmental impact is obvious: the larger the population, the more natural resources are consumed, more garbage is generated, and more damage is caused to the planet. The incidence of deliberate fires in jungles and forests multiplies every year to give way to livestock and monocultures, which have great demand in the population worldwide and, it is an unquestionable fact that industrial activity and mining generate many pollutants in the processes of extraction and smelting of metals, petrochemicals, the manufacture of electric batteries, the semiconductor industry for manufacture of computers, lasers and many other electronic devices, etc. In such a way that the amount of industrial waste containing arsenic and other contaminants has been increasing gradually over time. Therefore, the useful reserves of clean water to produce enough grains and other foods necessary for human consumption and also to reduce malnutrition in many poor countries are becoming more limited every day.

Given this scenario, it is necessary to promote and propose viable and efficient solutions that reverse the problem in the short term, with an ecological vision and awareness to learn to responsibly care for the quality of water reservoirs and the environment, for example, reducing the emissions of polluting gases such as CO2, CO, CPU, O3, SO2, NOx, chlorofluorocarbons (CFC), etc., to avoid acid rain and the greenhouse effect, effective control of biologically-derived pollutants; and also reduce human exposure to arsenic, analyzing water supply sources and clearly identifying those with concentrations above 10 pg/L of this metalloid [1-2]. We must commit ourselves to protect and preserve the freshwater reserves on our planet by applying scientific knowledge, to guarantee its purity and abundance through the application of hydraulic technology and physical or chemical processes for water decontamination, for its rational use in agriculture, livestock and also for human consumption. But in addition, we must be aware and capable of ensuring the supply and sufficient supply of clean water for the next generations of inhabitants on our planet. Arsenic is a chemical element that presents allotropy and is widely distributed throughout the earth's crust, mostly in the form of arsenic sulfide or metallic arsenates and arsenides. It is relatively inert at ordinary temperatures, but when heated in air it burns with a bluish flame, producing white clouds with the aliaceous odor of trioxide. solid arsenic (4 As +3 O2 ® 2 AS2O3). Currently its presence in water, both for human consumption and for agriculture, represents a serious public health problem in our country and in many other nations of the world, but in general it is a harmful and dangerous element for all living beings. .

In order to try to solve this problem, various technologies for removing arsenic from water have been developed over the years [3-7], some conventional and others more sophisticated that involve the application of knowledge in the areas of physics and chemistry. The processes to remove arsenic from wastewater and water for human consumption use techniques that, for example, include: (a) technologies based on commercial absorbents or natural materials to remove arsenic, through filtration, using vegetable membranes and other industrial waste materials such as slag metallurgical, stuffed with wires, nails or iron rods, etc.; and also with remains of the food industry and agriculture [8]; (b) coagulation and flocculation methods [9-10]; (c) ion exchange and microfiltration [11-12]; and (d) removal by oxidation techniques, with activated carbon and adsorption [13]. In addition, there are many studies with nanocomposites based on graphene oxide and nanoparticles (NPs) of some metal oxides such as oxides of: iron (hematite, magnetite, etc.), manganese, zinc, titanium, aluminum, magnesium, etc., the which show to be effective in the adsorption of heavy metals [14] The adsorption technique also includes nanomaterials based on carbon nanotubes and magnetic NPs of hematite (a-Fe2C>3), polymeric nanocomposites with magnetic NPs of magnetite, FesCVPVA , deposited inside and on the surface of nanofibers with magnetite agglomerates, reported for different concentrations [15]; and other polymer supported iron oxides. Some silica compounds containing iron (III) oxide adsorbents and polymeric materials such as chitosan and its derivatives have also been tested to remove arsenic from aqueous media [16-17]. Filaments of magnetic materials have also been developed, with which filters can be manufactured for the removal of heavy metals, as This is the case of the invention of a barium hexaferrite filament filter [18]. In this invention, magnetite NPs are used, which have a weak ferrimagnetic behavior, which must first be dispersed in the contaminated water to adsorb heavy metals, then through the continuous flow of water through the filter, the NPs are attracted and trapped by the barium hexaferrite membrane. This barium hexaferrite filament filter is efficient to remove arsenic ions adsorbed by magnetite NPs, when it manages to attract all the arsenic-loaded NPs from the contaminated aqueous medium. Otherwise, another additional problem can be generated for the environment if the magnetite NPs are not fully attracted and trapped by said filter.

In this category of nanostructured materials, the invention of a new composite of polyvinyl alcohol (PVA) magnetic nanofibers with SrFei20i9 nanoparticles (also denoted by SrM-NPs), homogeneously distributed in the polymer matrix, complements nanostructured magnetic systems for the remediation of water contaminated with heavy metals, due to its capacity to adsorb As. With the advantage that it works without the need to disperse other types of naked NPs, as a primary adsorbent, which can make it difficult to locate and extract of the aqueous medium. It should be noted that this nanostructured compound is attractive for its application, since the removal procedure for arsenic dissolved in water is simple, no toxic sludge is produced after its use, and both the adsorbate ions and the adsorbent materials of the magnetic polymer can be recycled properly. Other properties and advantages of polymeric magnetic nanofibers are related to their physical stability, low production cost, optimal control of the size of SrM-NPs and homogeneous distribution on the inner surface of the polymer matrix. Therefore, this invention allows the tailor-made manufacture of a polymeric magnetic nanocomposite, with optimal physical-chemical characteristics, for the efficient removal of arsenic in aqueous media.

TECHNICAL PROBLEM TO SOLVE. To meet the challenge and address the complex problem of contamination of the various aquifers with heavy metals and in view of the global demand of the population to guarantee water free of organic and inorganic contaminants, to prevent the transmission of diseases and general exposure to chemical substances. dangerous, several conventional and emerging technologies have been developed over time to attack this problem. The World Health Organization (WHO) has recommended in the Guidelines for the quality of drinking water, published in 1984, a provisional maximum reference value of 10 pg/L (0.01 mg/L) for arsenic in water for human consumption, based on concern about its carcinogenicity (IPCS, 2001); (WHO, 2003). It is widely documented that arsenic and other heavy metals have cumulative effects on biological systems and, in particular, arsenic is one of the most toxic chemical elements, since its presence increases the risk of acquiring skin, kidney, bladder and breast cancer. lung. However, much debate remains about the mechanism of carcinogenic action and about the shape of the dose-response curve for low intakes. Undoubtedly, the largest source of arsenic contamination in the environment is due to the mining of gold, silver, copper and other mineral deposits. This metalloid is distributed in large concentrations in the air, soil, surface and underground aquifers, finding its way to living beings by direct inhalation or through the contamination of food products, the oral route being the most important due to the consumption of food and drinks.

Even though various technologies, many of them conventional, have been developed to control high levels of arsenic in wastewater and other types of aquifers, the problem persists today. But among the less conventional technologies, based on adsorption methods, various nanostructured materials have been developed, effective as adsorbents of heavy metals, such as: carbon nanotubes, graphene nanocomposites (Jinyue et al., 2019), filaments of materials based on barium hexaferrite (Patent KR101433332B1), polymeric nanocomposites with nanoparticles magnetite magnetic particles, FeaC PVA, deposited inside and on the surface of nanofibers with magnetite agglomerates, reported for different concentrations (Wang et al., 2010); and now this new nanostructured compound of PVA nanofibers with SrM-NPs, homogeneously distributed on the inner surface of the polymeric matrix, is added. Practically all the techniques for treating contaminated water, except adsorption, present the problem that during the removal process of arsenic or other heavy metals, large volumes of highly polluting sludge are produced, which need to be treated before being returned again. to the environment. When isolated magnetic particles are used, already charged with polluting ions, they disperse in the water, making their recovery more complicated. So in this sense, the operational and maintenance costs of wastewater treatment plants contaminated with heavy metals can be very high. On the other hand, when organic materials such as sugar cane bagasse, wood sawdust, or other waste of plant origin and industry by-products are used to trap heavy metals, even when they turn out to be effective and inexpensive, they demand large volumes of material and excessive amounts of oxygen, causing this chemical element dissolved in the water to be reduced, which affects the biota due to the deterioration in the dynamics and regulation of the metabolic processes of aquatic ecosystems. It is worth mentioning that dissolved oxygen in water is a critical parameter used to measure the degree of contamination present by organic matter. Other natural materials such as bentonite, zeolite, sepiolite, although they have a high surface area to volume ratio, are not as efficient in removing heavy metal ions in polluted water [19]. The use of NPs based on graphene oxide and bare metal oxides in aqueous media, even when shown to be effective in the adsorption of heavy metals, most of the time they present difficulties in separating them from aqueous solutions; and they also have the disadvantage of being prone to form aggregates, decreasing the surface area, which also reduces their capacity and adsorption selectivity. In the same way, the application of granular activated carbon as an absorbent for heavy metals is also very expensive.

The great advantage of the processes for the removal of arsenic and other heavy metals through the application of nanostructured magnetic compounds in the form of nanofibers, over the aforementioned techniques, lies in the fact that they are effective in adsorbing heavy metals. It is also relatively easy to separate these from contaminated aqueous media, for example by applying external magnetic fields or by fabricating magnetic filters. In the case of the method of filtration by a membrane of nanofibers of BaFei2Üi9 (invention KR101433332B1), the procedure is carried out in two stages: first dispersing particles of magnetite, Fe3Ü4, in the aqueous medium to adsorb the contaminant; and later these are trapped using the magnetic filter. The typical magnetic characteristics of this nanostructured filter are: Fie = 2,568 kOe; M _s = 71.96 emu g ^_1 ; M _r /M _s = 0.5. By this method, no polluting sludge is produced at the end of the decontamination process and the production costs of these nanostructured compounds are relatively low. However, the particles of magnetite, Fe3Ü4, in an aqueous medium experience forces of attraction of various physical and chemical natures, since in addition to the magnetic force, they are subject to electrostatic, surface tension and van der Waals forces; and although these last three are very small compared to the magnetic force, they also influence the formation of aggregates of magnetite particles, reducing the effective adsorption surface. In addition, many times there is also the drawback of not being able to recover 100% of the particles loaded with the heavy metal. Therefore, when it is desired to remove chemical elements from water, such as arsenic, it is necessary to resort to more direct adsorption methods using nanostructured materials and more sophisticated technologies, through which it is technically feasible to reduce the arsenic concentration to 5 pg/L. , or less.

Taking this scenario into account as a frame of reference and to contribute to the control or elimination of this contaminant, the invention of this A new nanostructured magnetic polymeric compound, capable of adsorbing arsenic dissolved in water, emerges as a promising option among the schemes currently practiced in the treatment of water contaminated with this metalloid. In this way, the composite of magnetic nanofibers has important advantages in that: a). It is not necessary to disperse nanoparticles of another adsorbent material to trap arsenic dissolved in the aqueous medium, since nanofibers with SrM-NPs, of nanometric size (~4 nm) homogeneously distributed in the polymeric matrix adsorb arsenic by themselves and exhibit better magnetic properties than the barium hexaferrite filter described in patent KR101433332B1, which has limitations inherent to the adsorption method as a secondary contaminant is produced when it is not possible to trap all the magnetite NPs loaded with arsenic dispersed in the aqueous medium. However, the arsenic adsorption efficiency of PVA magnetic nanofibers, with a relatively low density of SrM-NPs, is similar to the adsorption efficiency reported in said patent. b). The magnetic properties of the nanofibers can be modulated by controlling the density of SrM-NPs, below the percolation limit, offering great advantages such as the high quadrature values of the magnetic hysteresis, reflected in a normalized value of the magnetization, Mr/ Ms, close to 1.0, high value of its coercive force, H _c , and of the maximum energy product, (BH)max; and also high magnetic anisotropy of shape, among other important characteristics.

Thus, this invention represents a technological advance in the development of nanostructured magnetic systems for the remediation of water contaminated with heavy metals, by virtue of its As adsorption capacity.

BRIEF DESCRIPTION OF THE INVENTION.

To contribute to the efficient solution of the problem of contamination by one of the most toxic heavy metals on Earth, such as arsenic, a PVA polymeric nanofiber composite has been developed that contains magnetic nanoparticles of SrFei2Üi9 homogeneously distributed at the surface level inside. In such a way that to improve the method of adsorption of arsenic dissolved in water, specific amounts of nanometric-sized SrM-NPs are used and properly distributed in the polymeric matrix, to apply this new material in the treatment of wastewater and also reduce the high levels of arsenic contamination in the aquifers.

Basically, for the elaboration of these magnetic nanofibers, two important techniques are used, which are referred to below: First, the Pechini technique [20] is described, to synthesize magnetic powders of strontium hexaferrite, SrFei20i9, and obtain fine particles chemically homogeneous, which receive an ultrasonic treatment. Next, the electrospinning technique by means of which magnetic nanofibers are manufactured is explained, incorporating SrM-NPs in a PVA precursor polymeric solution.

1. Pechini technique. For the synthesis of magnetic ceramics and many other materials, numerous wet methods have been investigated as an alternative to the solid state reaction technique. These include: polymerization with carboxylic acids, coprecipitation, sol-gel processing, microemulsion, electrochemical synthesis, self-propagating reactions, and hydrothermal synthesis, which have been found effective in the preparation of magnetic materials. The process invented by Maggio Pechini in 1967, object of this description, assumes as a starting point an aqueous solution of appropriate oxides, or salts mixed with a hydroxycarboxylic acid (citric acid; lactic acid; or tartaric acid); in this medium the metal ions have many possibilities to form coordination compounds with the carboxylic acid, for example, citric acid is a chelating agent that associates with trivalent or divalent metal cations (they can be iron Fe ⁺³ or Sr ⁺² ) to form stable and soluble complexes. Then, the addition of a polyhydroxyalcohol (ethylene glycol, propylene glycol, glycerin) facilitates polyesterification at low temperatures with the free carboxylate groups, to form a homogeneous and soluble sol, which, when combined with another system of the same nature, facilitates good contact, almost at the molecular level, of the precursor ions of solid phases. When this mixture is heated to slightly higher temperatures to remove excess solvents, a solid intermediate polymeric resin is formed with precursor ions in the matrix. Finally, when this resin is calcined at higher temperatures, the thermal degradation of the polymer occurs, the organic residues are removed and the compounds with the calculated stoichiometry are formed. Due to the homogeneity of the ion distribution, it is possible to obtain particles of nanometric dimensions with the phase of the desired product, through the diffusion of cations at temperatures below 1000 ^Q C. When the product is a magnetic material such as hexaferrites, the intrinsic properties are normally improved through this synthetic route. For many ceramic materials, the stoichiometry, the synthesis method and the thermal treatments involved are decisive to control the grain size, the crystalline texture and, in general, the microstructure of the porous and dense solids. Therefore, the Pechini technique allows, on the one hand, to produce phases with high purity, regulate porosity and achieve a uniform growth of chemically homogeneous fine particles with great thermal stability and, on the other hand, contributes to improve the magnetic properties and of the ferrites by facilitating the control of the chemical composition with lower energy consumption, since the sintering temperatures by the wet method are lower compared to the ceramic method, which is the traditional method for the synthesis of these materials.

2. Electrospinning technique. The electrospinning technique (referred to in the literature as electrospinning in English) is a method that represents a great technological advance, because since its invention in the laboratory it was designed to be applied at an industrial level due to the advantages it offers in the production of nanofibers at a large scale. scale, easily and at low cost. It is an effective and simple technique to produce ultra-thin fibers from a polymer solution, where electrostatic forces are used to obtain filaments with diameters that can be controlled in the range from micrometers to a few nanometers. On the other hand, the technique offers the advantage of being able to incorporate various types of nanoparticles into polymeric nanofibers [21]. It was A. Formhals [22] who invented and patented in 1934 an electrostatic apparatus for the electrospinning of plastics, applying an intense electric field to a polymeric solution to form thin fibers that were attracted to a moving electrode of negative polarity. Nanofibers can be considered as one-dimensional objects with a diameter of less than one micron and a very high surface area to volume ratio, which have extraordinary mechanical, physical and chemical properties. The basic characteristics of the electrospinning process consist of applying a high voltage between a steel capillary (anode), through which a polymeric solution is propelled, and the collector (cathode), which serves to accumulate the electrospun fibers. The evolution of the intrinsic characteristics of the resulting nanofibers, such as: diameter, morphology, distribution of nanoparticles dispersed within the fiber, etc., are governed by self-assembly processes induced by electrostatic Coulomb interactions between charged elements. , present in the drop of polymer solution that is pumped through the capillary; and that it is first retained by its surface tension. By applying a high voltage between the anode and cathode, positive charge is induced on the fluid surface and the droplet shape begins to distort. When the voltage is increased above a critical value, the surface tension force acting on the drop is overcome and an initial beam of polymeric fluid is produced that acquires the shape of a cone, referred to as the Taylor cone [23], with a surface charge distribution; and that is when the fluid moves towards the collector plate getting thinner rapidly with distance. Due to the repulsive forces, the ions present and the charged nanoparticles, in the heterogeneous polymeric mixtures, move radially towards the surface of the tiny fluid jet, producing a rapid evaporation of the solvent near the cathode to convert the beam into solid nanofibers, via a shake instability, where the fiber polymerizes and the incorporated nanoparticles are arrested and fixed on the inner surface of the polymeric matrix, finally depositing in the collector. Thus, a possibility to produce magnetic nanofibers in a simple way is offered by the electrospinning technique, incorporating magnetic nanoparticles synthesized by the Pechini technique in the precursor polymer mixture of the nanofibers, for which the SrM-NPs are previously sonicated to deagglomerate and achieve a small particle size (~4 nm), then they are added to a PVA solution and homogeneously dispersed in the polymeric matrix during their manufacture. Applying this technique, it is possible to control the magnetic properties of the compound, by varying the density of SrM-NPs, it is possible to modify the physical-chemical properties of the polymer and, with this, the interaction between the NPs and the PVA polymeric matrix. . From studies carried out using thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA), it has been observed that embedded strontium hexaferrite nanoparticles play a protective role by delaying the thermal degradation of the PVA matrix with respect to PVA without SrM- NPs. Therefore, the nanoparticles in the polymeric matrix have a thermoprotective effect on the polymer degradation temperature, in analogy with a PVA/Fe2Ü3 polymeric compound [24]. On the other hand, the magnetic characteristics achieved with this new nanostructured material reveal a from the highest quadrature hysteresis and magnetic energy product values reported in the literature for SrM-NPs, i.e., H _c = 6.66 kOe, M _r /M _s , = 0.81 , (BH)max = 5.26 MGOe; for polymeric nanofibers with a high magnetic anisotropy of shape fabricated with a low concentration of SrM-NPs.

In this way, the systematic investigation of the properties exhibited by the magnetic nanofibers with embedded SrM-NPs, together with the low cost of the chemical ingredients and the ease of manufacturing, contributed to conceive and develop a new nanostructured magnetic material with an excellent capacity for adsorption and removal of arsenic in aqueous media.

BRIEF DESCRIPTION OF THE FIGURES. Figure 1 shows patterns XRD refined by the Rietveld method nanoparticle strontium hexaferrite SRM-NPs prepared by the Pechini method: (a) after sintering at 900 C ^Q; (b) The same SrM-NPs after being sonicated in ethanol for two hours.

Figure 2.- Shows TEM images of SrM-NPs powders prepared by the Pechini method (a), of agglomerates made up of nanoparticles of different sizes (b); and size distribution of strontium hexaferrite nanoparticles in sintered powders at 900 ^Q C (c). TEM micrograph of PVA nanofibers with sonicated SrM-NPs (d); TEM micrograph of a PVA nanofiber with small-diameter (~4 nm) SrM-NPs uniformly distributed on the inner surface (e); and size distribution of SrM-NPs embedded in the PVA nanofiber matrix (f).

Figure 3.- Shows a TEM micrograph of the nanostructured composite of PVA nanofibers with small-diameter strontium hexaferrite magnetic nanoparticles embedded in the polymeric matrix (a); diameter distribution of magnetic nanofibers (b).

Figure 4.- Shows the hysteresis curves of the normalized magnetization as a function of the applied magnetic field. The segmented curve corresponds to the powders of SrM-NPs sintered at 900 ^Q C and the solid curve corresponds to the PVA magnetic nanofibers made with a precursor sample with 20% SrM-NPs.

Figure 5.- Shows the behavior of the energy product (BFI)max as a function of the normalized mass x of SrM-NPs for the manufactured magnetic nanofibers. The maximum value of (BFI)max is reached for x = 0.527, corresponding to the percolation limit. Figure 6.- Shows the removal efficiency of As as a function of time for pure PVA nanofibers, for naked SrM-NPs; and for PVA nanofibers made with two different concentrations of sonicated SrM-NPs. The segmented lines only serve to guide the eye.

Figure 7.- Shows the graphic sequence of the magnetic extraction of the PVA nanofibers with SrM-NPs, used to remove arsenic from the aqueous medium: (a) magnetic attraction of the nanofibers immersed in a contaminated aqueous medium once the process of arsenic adsorption using a permanent magnet, (b) the magnetic nanofibers at the edge of the beaker adhered to the permanent magnet; and (c) the arsenic-loaded nanofibers outside the decontaminated aqueous medium.

DETAILED DESCRIPTION OF THE INVENTION.

The present invention consists in obtaining a nanostructured composite of polymeric magnetic nanofibers, which is manufactured using the electrospirming technique, with the ability to adsorb arsenic in aqueous media. The procedure for the preparation of this nanofiber composite is described through the development of the following activities:

1. Very small strontium hexaferrite nanoparticles, SrFei2Üi9, are produced by means of ultrasonic cavitation mechanical effects, deagglomerating and reducing in size the fine hexaferrite powders obtained by the Pechini technique, until obtaining NPs with an average diameter ~4nm.

2. A heterogeneous precursor mixture is prepared by incorporating nano-sized SrM-NPs into a PVA solution; and then the magnetic nanoparticles are homogeneously dispersed in the polymer solution by means of ultrasonic cavitation.

3. The electrospirming technique is applied for the fabrication of PVA polymeric magnetic nanofibers with strontium hexaferrite NPs, SrFei20i9, with an average diameter of ~4 nm, optimally distributed on the inner surface of the PVA matrix with a uniform density , for obtain the best physical-chemical properties of the nanostructured polymeric compound.

IT'S COMPONENTS.

The substances used both for the preparation of magnetic strontium hexaferrite nanoparticles and for the electrospinning of PVA polymeric nanofibers containing embedded SrFei2Üi9 nanoparticles, comprising: citric acid, ethylene glycol, ferric nitrate nonahydrate, strontium nitrate, ethanol, polyvinyl alcohol, methanol; and deionized water, are analytical grade reagents.

THE CHARACTERISTICS OF EACH COMPONENT.

Citric acid, C6Hs07, is a weak organic tricarboxylic acid, present in most fruits, it is an important metabolite in all animals and plants. Citric acid readily forms citrate complexes with metal cations. It is found as colorless, odorless crystals with an acid taste. It is highly soluble in water. It is found naturally in many fruits and vegetables, with the highest amounts found in citrus fruits such as oranges, lemons, and limes. It is an important metabolic intermediate in the biochemical cycle of citric acid and is present in all living things. It is mainly produced by the microbial fermentation of carbohydrates such as molasses, cane sugar, beets, etc. It has many uses in the food industry as a flavoring agent, pH modifier, and preservative. It is also used as an anticoagulant and antioxidant. Citric acid solutions are safe for human consumption. However, concentrated solutions or pure citric acid can be irritating and corrosive, burning eyes and skin on contact. Its inhalation can irritate the nose, throat and mucous membranes.

Ethylene glycol, C2H6O2, or 1, 2-ethanediol, organic chemical compound, clear, colorless, odorless, sweet-tasting liquid. It is hygroscopic and completely miscible with many polar solvents such as water, alcohols, glycol esters and acetone. Low solubility in nonpolar solvents such as benzene, toluene, and chloroform. Difficult to crystallize, when cooled it forms a highly viscous mass which solidifies to produce a glass-like substance. It is easily oxidized by oxygen, nitric acid and other oxidizing agents to form aldehydes and carboxylic acids. It is mainly used as an antifreeze in automobile radiators as a heat diffuser, mixed with water for commercial aircraft de-icing and anti-icing procedures, manufacture of polyester compounds. It is also an ingredient in photo developing fluids, hydraulic brake fluids. It is widely used as a raw material for the manufacture of polyester fibers. It affects the chemistry of the organism, increasing the amount of acid, which produces metabolic problems. Ingestion in very high amounts can cause death, in low amounts it can cause nausea, convulsions, disorientation and heart and kidney problems. It can cause deafness, blindness, and can leave major brain problems. In addition to anaerobic biodegradation, it can also release relatively toxic products such as acetaldehyde, ethanol, acetate, and methane.

Ferric nitrate nonahydrate, Fe(N03)3 ^« 9H20, is a salt and, being a deliquescent compound, it is commonly found in its nonahydrate form in which it forms colorless to pale violet crystals. Ferric nitrate is the preferred catalyst for the synthesis of sodium amide in ammonia. Ferric nitrate solutions are used in jewelry and blacksmithing to engrave silver and its alloys; also in the synthesis of magnetic materials.

Strontium nitrate, Sr(N03)2, is an inorganic compound, easily dissolved in water, liquid ammonia, slightly soluble in anhydrous alcohol and acetone. Strontium nitrate is typically generated by the reaction of nitric acid with strontium carbonate. Used to produce an intense red flame in fireworks, flares, shipping, railway, and airfield signal lamps. The oxidizing properties of this salt are advantageous in such applications. It is also used as an aluminiferous agent in the manufacture of television tubes and optical glass, as well as in the manufacture of permanent magnets and in medicine.

Ethanol, C2H5OH, is an organic chemical compound of the class of alcohols, transparent and volatile liquid, colorless, with a characteristic odor and a burning taste. It is flammable and produces a smokeless blue flame. It is miscible with water and most organic solvents such as acetic acid, acetone, benzene, carbon tetrachloride, chloroform, and ether. Ethanol can be widely found in nature because it is part of the metabolic process of yeast such as Saccharomyces cerevisiae, it is also present in ripe fruit; and it is also produced by some plants through anaerobiosis. It can be produced by yeast using fermentation of sugars found in grains such as corn, sorghum, and barley, as well as potato, rice, and sugar cane skins, or by organic synthesis. It is used in medicine as an antiseptic. It kills organisms by denaturing their proteins and dissolving their lipids, and is effective against most bacteria, fungi, and many viruses. Ethyl alcohol is highly flammable, it will be easily ignited by heat, sparks or flames. Vapors may form explosive mixtures with air. It is toxic when ingested in large amounts or in high concentrations.

Polyvinyl alcohol, [CH2CH(OH)]n, (MW = 75,000), is a high molecular weight, water-soluble synthetic polymer. This alcohol can form films with emulsifying and adhesive capacity, which can withstand strong tensions. In addition to being a flexible material, it is hygroscopic and very soluble in water, its properties being affected by the degree of hydration. It is soluble in ethanol, but insoluble in other organic solvents. Insoluble in petroleum solvents, practically insoluble in vegetable and animal oils, esters, ethers and acetone. The fibers of this alcohol have a water absorption capacity 30% higher than that of other fibers. It is material for the synthesis of other polymers such as sodium nitrate. polyvinyl, an ester of nitric acid and polyvinyl alcohol. It is used in some propellants and castable explosives. This alcohol can be used in the manufacture of sheets or films that are barriers to oxygen and aromas. This has allowed it to be used in food packaging, this being its main use since more than 30% is used for this purpose. When heated above 200 ^QC , it decomposes and releases fumes that are irritating to the eyes, nose, and throat.

Methanol, CH3OH, is an organic chemical compound belonging to the family of alcohols. It is the simplest alcohol of the homologous series of alcohols. It is a colorless, transparent, volatile liquid with an aroma and flavor similar to those of ethanol; and is completely soluble in water. It is quite flammable and, like ethyl alcohol, has disinfectant and antiseptic properties. The ancient Egyptians obtained methanol by pyrolysis of wood. To this day, the most precise reference to synthesize methanol is through a catalytic process from carbon monoxide and hydrogen. Methanol has countless uses and applications, it is used as fuel, organic solvent for natural essences and resins, in the synthesis of dyes and methylated products; as well as in the manufacture of plastics, glues and varnishes. It is also used as antifreeze, fuel and anti-knock in vehicles; and as raw material to obtain formaldehyde. It is quite toxic and when this happens it becomes a chronic condition where several injuries are generated, brain and respiratory. It can cause chronic bronchitis and changes in the mucosa of the upper respiratory tract.

Deionized or demineralized water is one from which cations such as sodium, calcium, iron, copper and others have been extracted; and anions such as carbonate, fluoride, chloride, etc., by an ion exchange process.

Deionization is a process that uses specially manufactured ion exchange resins that remove ionized salts from the water. Water deionized can easily change its pH when stored because it absorbs atmospheric CO2, it is very aggressive with metals, even stainless steel; therefore plastic or glass should be used for storage and handling. Its most common use is in automotive batteries, but it is also used in analytical chemistry, materials synthesis, and experimental laboratories.

EQUIPMENT AND CHARACTERIZATION TECHNIQUES.

The main equipment used in the preparation of the polymeric composite of nanofibers with SrM-NPs and the characterization techniques applied to evaluate the magnetostructural properties of hexaferrite nanoparticles, as well as magnetic nanofibers and their capacity to adsorb arsenic dissolved in water are the following:

The X-ray diffraction analysis was carried out using a Siemens D5000 diffractometer, which uses a cobalt source (l = 1.7890 Á), the structural parameters and the phase quantification of the hexaferrite powders were refined with the analysis of Rietveld, incorporating the MAUD program [25]. To investigate the morphological characteristics and size distribution of nanofibers and nanoparticles, a scanning electron microscope SEM, JEOL, model JSM-7600F; and a transmission electron microscope TEM, Hitachi S-570, operated at 100 kV. Uranyl acetate was used to improve the contrast and to be able to observe the magnetic NPs within the nanofibers. Here, X-Ray Energy Dispersive Spectrometry (XEDS) was also performed on the SrM-NPs powders to identify their elemental composition and quantify their purity. For the analysis of the magnetic properties, a vibrating sample magnetometer LDJ-9600 was used; all hysteresis loops were obtained at room temperature. To evaluate the effect of embedded magnetic NPs on the thermal properties of nanofibers, thermogravimetric and differential scanning calorimetry (TGA/DSC) analyzes were performed with a thermogravimetric analyzer, TA Instruments DSC Q200, in a nitrogen atmosphere by performing temperature in the range from room temperature to 1000 ^Q C, with a heating ramp of 10 °C/min. For the arsenic adsorption tests with the magnetic nanofibers, a PerkinElmer Optima 8300 ICP-OES spectrometer was used. The average absorbance was used to calculate the arsenic removal efficiency in an aqueous medium.

HOW THE NANOSTRUCTURED COMPOSITE IS ASSEMBLED.

To incorporate SrM-NPs into the polymeric matrix of the magnetic nanofibers, the following procedure is carried out, which includes the following steps: first, strontium hexaferrite powders are synthesized with the M phase, by the Pechini method to obtain, as a starting point, High purity homogeneous powders with high crystallinity and reduced nanoparticle size (~78 ± 20 nm). Then, an ultrasonic treatment is applied to these powders to deagglomerate and obtain SrFei20i9 nanoparticles with an average diameter of ~4 nm. Then, the preparation of a polymeric precursor mixture is carried out, adjusting the exact proportions of SrM-NPs and the PVA solution, to control the density and homogeneous distribution of NPs in the manufacture of the magnetic compound through the electrospinning technique. Through this procedure, a composite of nanostructured polymeric magnetic nanofibers with SrFei20i9 nanoparticles is obtained, with excellent physical-chemical properties, capable of removing arsenic from aqueous media.

BEST WAY TO CARRY OUT THE INVENTION.

This invention offers the possibility of being able to control the magnetic properties of the nanostructured compound, varying the density of SrM-NPs in the PVA polymeric matrix, within the percolation limit of the dispersed system of magnetic NPs. Therefore, to explain the development and illustrate the scope of the invention, the manufacture of polymeric magnetic nanofibers through a method where the concentration of SrM-NPs is varied is described in detail below. Essentially, the procedure consists of three stages, i.e. 1. Synthesis of strontium hexaferrite nanoparticles, SrFei20i9, (SrM-NPs), comprising the steps:

(to). Obtaining powders of a precursor resin of strontium hexaferrite nanoparticles by the Pechini method, using ferric nitrate, strontium nitrate, citric acid, ethylene glycol and deionized water.

(b). Thermal treatment to obtain fine powders of strontium hexaferrite type M, SrFei20i9.

(c) Ultrasonic treatment to deagglomerate, disperse and fragment SrFei2Üi9 particles to obtain smaller diameter NPs.

2. Preparation of a heterogeneous precursor mixture of PVA with the incorporation of nanometric-sized SrM-NPs, comprising the steps:

(d). Preparation of a base polymeric solution, composed of a polymer and a solvent.

(and). Preparation of a precursor heterogeneous mixture of polymeric magnetic nanofibers, with the base polymeric solution and nanoparticles of strontium hexaferrite, SrFei20i9.

3. Fabrication of PVA polymeric magnetic nanofibers with SrM-NPs by the electrospirming technique, comprising the steps:

(F). Preparation of a composite of nanostructured polymeric nanofibers with SrFei2Üi9 nanoparticles inside, using the electrospinning process.

1. Synthesis of strontium hexaferrite nanoparticles, SrFei2Üi9 (SrM-NPs). In step (a), for the proportional preparation of 1.0 g of strontium hexaferrite nanoparticles, anhydrous citric acid, AC, (ObHdOz), 1.085 g, and ethylene glycol, EG, (C2FI6O2), 0.5 mL, were used. Citric acid functions as a complexing agent and ethylene glycol as a polymerizing agent. The precursor compounds of ions A and B are soluble salts (nitrates): ferric nitrate (Fe(N03)3*9H20), 4.566 g; and strontium nitrate (Sr(N03)2), 0.199 g, which are first mixed for 30 minutes under constant stirring, together with AC, in 70 mL of deionized water at room temperature. Then the aqueous solution is heated between 50°C and 60°C, preferably 60°C, stirring constantly for 1 hour to promote the formation of complexes (chelates) with the metal cations A and B. Once the metal citrate is formed, add the polyalcohol (EG), 0.5 mL, and continue stirring the mixture for 20 minutes, keeping the temperature at 60°C; which contributes to the formation of an organic ester or esterification with the unbound carboxylate groups. The existence of two or more soluble metal complexes supposes an intimate mixture of the precursors in a homogeneous medium (the organic matrix). Polymerization is promoted by heating the mixture, continuing to stir and increasing the temperature to 80°C or 100°C, preferably 80°C, to evaporate all the solvent (H2O), resulting in a polymeric resin through reaction. polyesterification reaction, where the precursor ions A and B of the magnetic compound are homogeneously distributed at the molecular level. It is then allowed to cool to room temperature. This polymeric resin, made of iron citrate and strontium citrate, obtained in the presence of EG, is pulverized and then calcined at a temperature between 250°C and 400°C, preferably 250°C, for 2 hours. It is allowed to cool to room temperature, it is pulverized and then calcined again at 400°C for another 2 hours, which serves to remove the organic matrix, obtaining a powder of the strontium hexaferrite precursor resin.

In step (b), the precursor powder at room temperature is ground again and sintered at a temperature between 800°C and 1000°C, preferably 900°C, for 2 hours, giving way to obtaining nanoparticle powders of strontium hexaferrite, SrFei20i9, with the M phase. Thus, nanometer-scale strontium hexaferrite nanoparticles were synthesized using the standard method of Pechini. Fig. 1(a) shows the X-ray diffraction pattern for hexaferrite powders calcined at 900 ^QC . The structural parameters were refined by the Rietveld method, revealing that SrFei20i9 nanoparticles crystallized in the M phase and resulted high purity. The presence of only 1.8% by weight of iron oxide (hematite) was determined in the manufactured strontium hexaferrite. The crystallite size calculated with the Rietveld method was 76 ± 2 nm. In step (c), strontium hexaferrite powders, SrFei20i9, were placed in 5 mL of ethanol (C2H6O) and subjected to ultrasonic treatment for 2 hours using a Branson 2510 sonicator operated at 40 kHz, to deagglomerate, disperse and further fragment the hexaferrite particles. They soon dried in the air. Fig. 1(b) shows the X-ray diffraction pattern for the synthesized strontium hexaferrite powders at 900 ^Q C after ultrasonic treatment. In both cases, the Rietveld refinement method was applied to verify the presence of the magnetic phase M. In Fig. 1 (b) the broadening in the diffraction maxima can be seen, attributed to the reduction in crystallite size ( 3.4 nm), due to the sonication effect.

2. Preparation of a heterogeneous precursor mixture of PVA with the incorporation of nanometric-sized SrM-NPs.

In step (d), a PVA-based polymeric solution (MW = 75,000) is prepared in deionized water at 7.4% by weight and heated to a temperature of 80 ^Q C with continuous stirring until a homogeneous translucent solution free of lumps is obtained. ; then allowed to cool to room temperature.

In step (e), a series of 5 heterogeneous precursor mixtures are then prepared, each with 3 mL of the 7.4% by weight PVA solution, adding in each individual volume a specific mass, m, of hexaferrite nanoparticles of strontium, SrM-NPs, previously sonicated (~4 nm in diameter), where

SrM-NPs = m; for m = 0.10 g, 0.15 g, 0.20 g, 0.25 g, 0.30 g

(1) continuously stirring until combined and each up to 5 mL with the PVA solution, obtaining 5 precursor mixtures: M1, M2, M3, M4, M5, with different densities of magnetic NPs. Each of these 5 mixtures of the PVA solution with hexaferrite powders was named in reference to the amount of SrM-NPs added: 10% SrM-NPs, 15% SrM-NPs, 20% SrM-NPs, 25% SrM-NPs; and 30% SrM-NPs respectively. Where the percentages are referred to a mass of 1.0 g of SrM-NPs (normalized mass x = m/1.0 g). To homogeneously disperse the magnetic nanoparticles in the PVA polymer solution, each precursor mixture to be electrospun is previously subjected to a low-frequency sonication process (40 kHz) for one hour.

3. Fabrication of PVA polymeric magnetic nanofibers with SrM-NPs by the electrospirming technique.

In step (f), the precursor sample in turn is then transferred to a plastic syringe, fitted with a 0.15 mm internal diameter stainless steel needle, and installed in the NE-300 New Era infusion pump, INC, MA, USA to perform electrospinning.

The fabrication of the PVA polymeric nanofibers with strontium hexaferrite NPs was carried out using an electrospirming system, self-constructed in the laboratory, equipped with a high voltage source from 0.0 to 30 kV and an infusion pump. The flow rate and distance between the needle and the collector are adjustable and the system has a protective acrylic cabinet, to minimize variations in environmental conditions, with a support for the infusion pump and the high voltage source. The syringe needle is connected to the positive electrode and the negative electrode is connected to an aluminum plate covered with aluminum foil, which functions as a stationary collector. The distance between the tip of the needle and the collector was set at 5.3 cm and the precursor mixture was released at a rate of 0.5 mL/h. The voltage was set at 25 kV.

Once all the nanofibers were manufactured, for the different SrM-NPs/polymer proportions, the morphological and structural characteristics they exhibit were analyzed, their magnetic properties were investigated and analyzed, studies were carried out using thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA). ). From these studies it was observed that the embedded strontium hexaferrite nanoparticles play a protective role by delaying the thermal degradation of the PVA matrix for more than 150 ^Q C relative to PVA without SrM-NPs. Therefore, the nanoparticles in the polymeric matrix have a thermoprotective effect on the polymer degradation temperature, in agreement for a PVA/Fe ₂ 0 ₃ polymeric compound (Guo et al., 2010).

The morphological characteristics of the nanoparticles and nanofibers were analyzed using SEM and TEM electron microscopy in imaging and electron diffraction modes. Fig. 2 (a) shows the TEM micrograph of powders strontium hexaferrite with a morphology plates, after being sintered at 900 ^Q C for the Pechini method. The TEM micrograph of Fig. 2(b) shows agglomerates of particles whose dimensions reach several tens of nanometers. In relation to these agglomerates, Fig. 2(c) shows the particle size distribution, obtained from the measurement of 100 particles from various TEM micrographs. The average diameter of the hexaferrite particles is 78 nm, with a standard deviation of 1.8 nm. On the other hand, Fig. 2 (d) shows the TEM micrograph of nanofibers fabricated by electrospinning, with PVA and SrM-NPs of previously sonicated strontium hexaferrite. Here you can see the magnetic nanoparticles, homogeneously distributed on the inner surface, along the longitudinal axis of the nanofibers. The TEM micrograph of Fig. 2 (e) shows an amplification of a nanofiber, where the magnetic nanoparticles arrested in the polymer matrix can be seen, with a significant reduction in size after being sonicated. The nanoparticles appear perfectly separated from each other with an average diameter of ~3.4 nm and a standard deviation of 0.79 nm. According to the distribution curve shown in Fig. 2(f); the measured diameters are in the range from 2.3 nm to 7.0 nm. Fig. 3(a) shows the TEM micrograph of the PVA nanostructured compound with SrM-NPs obtained by electrospinning, with the nanoparticles in the dispersion phase. Nanofiber diameters range from 97 nm to 269 nm, with an average diameter of 165 nm and a standard deviation of 39 nm. The diameter distribution is shown in Fig. 3(b). Magnetic parameters were obtained from hysteresis loops, at room temperature, using vibrating sample magnetometry (VSM). Fig. 4 shows two hysteresis curves of the normalized magnetization as a function of the magnetic field, one corresponds to the powders of SrM-NPs sintered at 900 ^Q C and the solid curve corresponds to the PVA nanofibers obtained with a precursor sample with 20 %SrM-NPs. The saturation magnetization value, M _s , of the strontium hexaferrite powders was 72.2 emu/g, for an applied field of 14 kOe; the normalized value of the remanence magnetization, M _r , with respect to the saturation magnetization, M _s , (M _r /M _s ) = 63%; the maximum energy product value, (BH)max = 3.01 MGOe; and its coercivity, H _c = 6.22 kOe. When the density of magnetic NPs in the precursor mixture is low (for example: mass, m = 0.10 g, or normalized mass x = 0.10), the nanostructured system is very dilute, with very little magnetic material dispersed in the polymeric matrix. Therefore it is to be expected that small values of magnetization will be generated. On the other hand, when the density of magnetic nanoparticles is high (for example: x > 0.30), a greater increase in the magnetization of the nanostructured magnetic system is anticipated when approaching the percolation limit, due to the increase of magnetic material in the precursor mixture. . Electrospinning of magnetic nanofibers with a higher amount of SrM-NPs (x > 0.45), was not technically possible in this electrospinning system due to the high density of SrM-NPs present. Table I shows the results of the magnetic characterization of the nanofibers made with the precursor samples M1, M2, M3, M4, M5 of the present invention.

Table I. Results of the magnetic characterization of the nanostructured composite of magnetic nanofibers for different amounts of SrM-NPs mixed in the PVA solution.

From this table of results it can be deduced that the SrM-NPs/polymer ratio, in the preparation of the precursor mixture of the nanostructured compound, plays an important role in the magnetic properties of the manufactured nanofibers. This parameter affects the packing configuration of the nanoparticles, inducing a transition from the diluted state of SrM-NPs in the polymeric matrix, that is, from the dispersion phase, to a morphology where the nanocomposite can present agglomerates, when the limit is reached. of percolation. It is also observed, as previously anticipated, that in this diluted system the normalized magnetization (M _r /M _s ), the coercive force, H _c , and the energy product, (BH)max, increase with increasing concentration of nanoparticles. from 10% SrM-NPs to 30% SrM-NPs. In relation to this, the last row of Table I highlights the optimal values of the magnetic parameters of the nanofibers produced with the precursor sample M5 [0.30 g (SrM-NPs)], which are better than those obtained for the samples precursors M1, M2, M3, M4; and even better than those corresponding to the naked SrFei20i9 strontium hexaferrite NPs.

The method used to study the magnetic interactions between the NPs is based on obtaining the differential susceptibility dM/dH of the hysteresis demagnetization curve, for each concentration of SrM-NPs in the polymeric matrix. This method is known as Switching Field Distribution, abbreviated SFD (Switching Field Distribution). These micromagnetic characteristic curves are used to evaluate the extension of the distribution of the field in which the inversion of the magnetization occurs, of the system of particles interacting with each other; and it is useful to estimate the intensity and type of interaction. In a dilute system composed of not completely identical NPs, the coercive field of a particle is independent of that of the other particles. Therefore the system is characterized by having a certain distribution of coercive fields, called intrinsic switching field distribution (iSFD); having access to this distribution through the derivative of the demagnetization curve of the hysteresis loop with respect to the applied field. When the density increases and the nanoparticles are very close to each other, without being in contact, the field produced by each particle begins to interfere with that of neighboring particles. In this system the extent of the coercive field distribution, SFD, increases because the dipole interaction becomes very intense; and so a larger magnetic field is required to switch the magnetization of the entire set of particles. In this case, the total width increases by half the maximum value, abbreviated FWFIM (Full Width at Half Maximum), which is a measure of the extension of the commutation field. For particle systems with the same iSFD, the relative changes in their FWFIM give a measure of the magnetostatic interactions.

As shown, from the experimental values indicated in Table I, this invention of polymeric nanofibers with SrM-NPs exhibits excellent magnetic properties for 30% SrM-NPs, that is, for this density of magnetic nanoparticles, high values of quadrature of magnetic hysteresis, (M _r /M _s )= 81%; Fie = 6.66 kOe; and (BFI)max = 5.26 MGOe, which makes them easily recoverable from aqueous media using an electromagnet or a permanent magnet when used to adsorb arsenic. Eventually, these magnetic nanofibers could be applied in a filter. Another important characteristic of this magnetic nanocomposite is that it presents a reduced value for the switching field distribution, SFDFWHM = 2.68 kOe, whose maximum value SFDcenter is centered on a high value of the external magnetic field, SFDcenter = 7.122 kOe. Comparing the magnetic hysteresis quadrature values of this invention for 30% SrM-NPs, with those reported for randomly oriented SrFei20i9 nanowires [26], the nanowires exhibit lower hysteresis quadrature with (Mr/Ms) = 0.57 and FH _C = 521 kA/m (6.54 kOe), since these values hardly are very similar to those of the strontium hexaferrite NPs synthesized by the Pechini method.

Fig. 5 shows the behavior of the maximum magnetic energy (BH)max vs. the normalized mass, x, of SrM-NPs in the polymeric matrix of the nanofibers, which was adjusted according to the following equation:

(BH)max (x) = 2.30 + 13.85x - 13.14x ²

(two)

This equation describes a parabola in which the (BH)max of the SrM-NPs powders prepared by the Pechini method corresponds to x = 1.0; and according to this adjustment the maximum value of (BH)max of the nanostructured compound would be obtained for x = 0.527 (percolation limit). Almost doubling the value of (BH)max for the bare powders of SrM-NPs. According to this model, above this value the energy product decreases as the concentration of SrM-NPs in the polymeric matrix increases due to an increase in demagnetization effects; and because when the percolation limit is reached, the order that produces the magnetic anisotropy decreases. The quadratic trend of this equation is phenomenological, in agreement with the experimental results obtained. The demagnetizing field is defined in terms of the shape of the magnet and the distribution of its magnetostatic charges. Then, a nanostructured magnet like the one of the present invention offers the possibility of being able to modulate the demagnetizing field through the concentration of magnetic NPs and consequently its energy product (BH)max. Thus, an increase in (BH)max is anticipated for an anisotropic configuration of magnetic NPs within the magnet. When the PVA/SrFei20i9 nanostructured system is very dilute (x < 0.1) the magnetization decreases and the energy product cannot be exactly described by Eq. (2). As confirmed by the empty circle in Fig. 5 for m = 0.1 g in the precursor sample M1. However, for the following compositions, before the percolation limit (x = 0.527), the parabolic behavior for (BH)max fits very well to the curve described by Eq. (2), including the (BH)max value of the SrM-NPs powders prepared by the Pechini method (x = 1.0).

Based on these experimental results, it is shown that the nanoparticles, in the dispersion phase, are distributed along the longitudinal axis of the PVA nanofibers, due to the strong interaction with the electric field during their fabrication by electrospirming. In addition, one of the causes that intervene for the increase in the quadrature of the magnetic hysteresis, obtained for 30% SrM-NPs in the manufacture of polymeric nanofibers, is due to the anisotropy created. The ratio of length to diameter generates a magnetic anisotropy of shape along the longitudinal axis of the nanofibers, for low densities of SrFei20i9 magnetic nanoparticles embedded in the PVA polymeric matrix, modifying inversion modes in an unusual and interesting way. of magnetization. It should be noted that this type of anisotropy is not possible for the bulge. When magnetic materials are reduced in size to a nanometric scale and distributed homogeneously in a polymeric matrix as in this nanostructured system with SrM-NPs, the magnetic interactions between the particles reveal a different magnetization behavior than that of the magnetic material in the sample. volume. Undoubtedly, the ability to configure magnetic NPs in a polymeric matrix makes this invented system an interesting model to investigate the role of the interactions between the embedded particles, in the magnetic properties, when the concentration of NPs varies below the percolation threshold; therefore, the possibility of modulating the magnetic characteristics of the polymeric nanocomposite by varying the density of SrM-NPs in the polymer is evident. In this way, the induced properties allow the custom design of a PVA polymeric magnetic nanocomposite with different physical-chemical properties, useful for removing arsenic, with optimal values of its magnetic properties for 30% SrM-NPs in the precursor mixture, such as specified in the last row of Table I.

THE WAY IT WORKS. The invention contemplates a procedure to remove arsenic from water contaminated with this metalloid, separating the ions by adsorption using polymeric magnetic nanofibers as adsorbent. Once the PVA magnetic nanofibers with SrM-NPs are obtained by means of the electrospinning technique, they are subjected to a chemical cross-linking procedure, to reinforce the polymeric chain of the nanostructured compound. Thus, the cross-linking It is carried out by immersing the nanofibers in methanol for 24 hours at room temperature, then drying them in the air.

Arsenic (V) adsorption efficiency tests were performed at room temperature with pure PVA nanofibers, bare strontium hexaferrite nanoparticles (~4 nm in diameter); and PVA magnetic nanofibers for 10% SrM-NPs and 20% SrM-NPs. Measurements were carried out using twelve working solutions, each with the same initial concentration of arsenic (V) of 3.26 mg/L. The PerkinElmer Optima spectrometer was used to measure the average absorbance and then calculate the removal efficiency of arsenic in water with the equation:

Removal efficiency = [(C ₀ - Ce) / C ₀ ] (100)

(3) where C ₀ is the initial concentration of As (V), and Ce is the equilibrium concentration in mg/L. Each adsorbent sample was tested three times and in each test, 0.2 g of nanofibers or naked SrM-NPs were separately introduced into the working solution. Adsorption sampling for naked SrM-NPs was performed for 2.5, 5, 7.5 and 10 minutes, since the mixture of magnetic NPs in the working solution was kept under stirring for 1 minute, and then 1.5 minutes passed without stirring to attract the NPs to the bottom of the beaker with the help of an external magnet to take the first 1 mL sample with a micrometer. Then the mixture was stirred again for another minute and the procedure was repeated to concentrate the NPs at the bottom with the help of the magnet to proceed with the taking of the second 1 mL sample; Y so on until the last sample is taken at 10 minutes. Each sample was placed in a 25 ml volumetric flask and labeled with the sample number and time for evaluation. For the sampling of As (V) adsorption with PVA nanofibers, pure or magnetic, the working solution was maintained all the time with constant agitation with the nanofibers, taking samples of 1 ml_ of the solution at different times, that is,

Sampling time for pure PVA nanofibers: 1, 3, 5, 7 and 10 minutes. Sampling time for magnetic nanofibers: 1, 2, 3, 5, 7 and 10 minutes.

The sample labeled "reference sample" to 0 minutes corresponds to a 1mL sample of the working solution with the initial concentration of arsenic (V), C ₀ = 3.26 mg / L.

Fig. 6 shows the experimental results for As adsorption efficiency as a function of time using bare SrM-NPs, pure PVA nanofibers, and magnetic PVA nanofibers made of 10% SrM-NPs and 20% SrM-NPs. Pure PVA nanofibers exhibit poor adsorption efficiency, but once the nanostructured composite is assembled with a small amount of strontium hexaferrite NPs (x = 0.1), the adsorption efficiency improves somewhat over PVA nanofibers. pure PVAs. It is interesting to observe that as the amount of magnetic NPs in the polymeric matrix increases, the adsorption efficiency also increases proportionally, since in Fig. 6 it can be seen that the PVA nanofiber compound with only a minimum concentration of 20% SrM-NPs (x = 0.2) adsorbed more than 60% of As in the first 3 minutes and 92% at 10 minutes. But in addition, the adsorption kinetics induced for this concentration of NPs, which is below 50% of the percolation threshold (x = 0.52), is equal to the adsorption efficiency of the naked SrFei2Üi9 nanoparticles, which also adsorb more than 60% As in the first 3 minutes, in agreement with a previous study by Patel et al., using barium hexaferrite NPs [27]. After 10 minutes, the naked SrFei2Üi9 NPs reach an As removal efficiency of 98%. These experimental results show the adsorption capacity of As with this magnetic compound. nanostructured when the PVA/SrFei20i9 system remains within the percolation limit of the dispersed system of magnetic NPs, that is, for values of the concentration of SrM-NPs less than 0.52, shown in Table I. The behavior observed in the experiments of Adsorption is related to the length to diameter ratio of the nanofibers, which exhibit a large surface area for arsenic adsorption. In addition, the interaction between the embedded magnetic NPs and the polymer matrix modifies the physical and chemical properties of the composite material depending on the concentration of NPs, which improve the crosslinking of the polymer chain and considerably promote the adsorption capacity of arsenic ions. . In particular, it should be noted that the density of NPs induces a high magnetic anisotropy in the form exhibited by the nanofibers, which is not present in the volume of the magnetic material. Thus, this nanostructured magnet has a larger adsorption surface as the strontium hexaferrite nanoparticles are confined within the polymeric matrix, without being in direct contact with the As ions, but interacting with them at a distance to improve adsorption kinetics.

Finally, Figure 7 shows the graphic sequence of the extraction of PVA nanofibers with SrM-NPs, used to remove arsenic from the aqueous medium using a permanent magnet: (a) magnetic attraction of the nanofibers immersed in a contaminated aqueous medium once the arsenic adsorption process is complete, (b) the magnetic nanofibers on the edge of the beaker adhered to the permanent magnet; and (c) the arsenic-loaded nanofibers outside the decontaminated aqueous medium.

With these experimental results, it is shown that the composite of polymeric magnetic nanofibers with nanoparticles of strontium hexaferrite manufactured is efficient to adsorb arsenic in an aqueous medium contaminated with this heavy metal, since for a minimum concentration of only 20% SrM- NPs (x = 0.2) adsorbed more than 60% of As in the first 3 minutes and 92% at 10 minutes; and unlike the use of naked magnetic NPs to adsorb arsenic, magnetic nanofibers have the advantage of being more easily recovered from the aqueous medium than isolated NPs, once loaded with the contaminating adsorbate.

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Claims

CLAIMS Having described the invention as above, what is contained in the following claims is claimed as property:

1. A method for manufacturing a composite of polymeric magnetic nanofibers with nanoparticles of strontium hexaferrite, SrFei20i9, distributed at the surface level inside, with exceptional magnetic characteristics, comprising the following steps: a). Ultrasonic cavitation treatment of fine powders of strontium hexaferrite type M, SrFei20i9. b). Preparation of a base polymeric solution, composed of a polymer and a solvent. c). Preparation of heterogeneous precursor mixtures of polymeric magnetic nanofibers, with the base polymeric solution and the sonicated SrFei2Üi9 strontium hexaferrite nanoparticles. d). Sonication of heterogeneous precursor mixtures of PVA magnetic nanofibers. and). Fabrication of a composite of PVA polymeric magnetic nanofibers with SrFei2Üi9 nanoparticles inside using the electrospinning process.

2. The method of claim 1, in which 5 mL of ethanol are proportionally mixed for each gram of fine powders of strontium hexaferrite type M, SrFei20i9, then subjected to an ultrasonic cavitation treatment to deagglomerate and reduce the size of the particles, until obtaining SrM-NPs with a nanometric size of ~4 nm in diameter.

3. The method of claim 1, wherein the polymer is polyvinyl alcohol (PVA).

4. The method of claim 1, wherein the solvent is deionized water.

5. The method of claim 1, in which a 7.4% by weight PVA-based polymeric solution is prepared, heated at 80 ^Q C with continuous stirring until a lump-free homogeneous translucent solution is obtained, then allowed to cool to room temperature.

6. The method of claim 1, in which individual heterogeneous precursor mixtures with different densities of

NPs in the PVA solution at 7.4% by weight, where the compositional ratio is fixed by adding to each of them a mass, m, of SrM-NPs of ~4 nm in diameter, included in the range of values

0.0g < m < 0.40g; and bringing each precursor mixture up to 5 mL with the PVA solution.

7. The method of claim 1, in which the heterogeneous precursor mixtures are subjected to ultrasonic treatment.

8. The method of claim 1, wherein each heterogeneous precursor mixture of PVA magnetic nanofibers is electrospun.

9. A composite of PVA polymeric magnetic nanofibers with nanoparticles of strontium hexaferrite, SrFei20i9, homogeneously distributed at the surface level inside, produced by the method of any of claims 1 to 8, with differentiated magnetic parameters, when the mass of SrM-NPs in the heterogeneous precursor mixtures is comprised in the range of values 0.10 g < m <

0.30 g, that is:

Normalized magnetization, 63% < M _r /M _s £ 81%.

Coercive force, 6.22 kOe £ H _c £ 6.66 kOe.

Maximum energy product, 3.32 MGOe < (BFI)max £ 5.26 MGOe. Switching field distribution, 3.22 kOe > SFDFWHM ³ 2.68 kOe.

10. A process for removing heavy metals from contaminated water, separating heavy metal ions by adsorption, comprising: a). The polymeric magnetic nanofiber composite of PVA with strontium hexaferrite nanoparticles, SrFei20i9, of claim 9. b) Cross-linking of the polymeric magnetic nanofiber composite using an alcohol. c) Adsorption of heavy metal ions, adding polymeric magnetic nanofibers intertwined in the aqueous medium contaminated with said metals. d) Magnetic separation of the nanostructured compound, of intertwined polymeric magnetic nanofibers, loaded with heavy metal ions adsorbed from the contaminated aqueous medium.

11. The method of claim 10, wherein the mixture of heavy metals with the aqueous medium includes at least one selected from the group comprising: arsenic (As), cadmium (Cd), mercury (Hg), antimony (Sb), bismuth (Bi); and polonium (Po).

12. The method of claim 10, wherein the alcohol is methanol.

13 The method of claim 10, in which the nanofibers of the PVA magnetic nanofiber composite are crosslinked, with nanoparticles of strontium hexaferrite, immersing them in methanol for 24 hours at room temperature, then they are extracted and dry.

14. The method of claim 10, in which the crosslinked polymeric magnetic nanofibers are introduced into the aqueous medium contaminated with arsenic.

15. The method of claims 10 to 14, in which magnetic nanofibers are extracted from the aqueous medium, loaded with arsenic ions using an external magnetic field.