ES2956480A2 - Procedure for the separation of microplastics from aqueous matrices - Google Patents

Abstract

Procedure for the separation of microplastics from aqueous matrices. The present invention relates to a procedure for the separation of microplastics from aqueous matrices based on the interaction of said microplastics with magnetic minerals containing iron, such as magnetite, which gives rise to the formation of an aggregate of the microplastics with the mineral. magnetic iron easily separable from water by application of a magnetic field. (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

Procedure for the separation of microplastics from aqueous matrices

field of invention

The present invention refers to a procedure for the separation of microplastics from aqueous matrices by their interaction with magnetic iron minerals, such as magnetite.

Background of the invention

The widespread presence of microplastics in the environment has become a global environmental concern and a growing problem as a result of the exponential increase in plastic production. This has not stopped growing globally since the development of the first synthetic polymers in the mid-20th century. Thus, in 2017 more than 348 million tons of plastics were produced in the world (PlasticsEurope, 2018). Among them, the main highlights are polypropylene (19.3%), high-density polyethylene (17.5%), low-density polyethylene (12.3%), polyvinyl chloride (10.2%), polyurethane (7.7%), polyethylene terephthalate (7.5%) and polystyrene (6.6%). In Europe there are about 60,000 companies working in the plastics sector, which generated about 350,000 million euros in 2017 (PlasticsEurope, 2018). Obviously, the high activity of this industry is a product of the great demand for plastic from different sectors such as packaging (39.9%), construction (19.7%), automotive (10%), electrical and electronic (6.2%) and agriculture (3.3%).

The main consequence of the high production of plastics is the generation of large quantities of these wastes that, to a large extent, end up in the environment, especially in the marine environment (GESAMP Report and Studies, 90 (2015), ISSN: 1020 4873 ). It is estimated that between 4.8 and 12.7 million tons of plastic waste enter the oceans each year, becoming a major environmental problem (Jambeck, JR et al., Science, 347 (2015) 768-771 ). This is mainly due to their high persistence, since it is estimated that they take between hundreds and thousands of years to degrade (Barnes, DKA et al., Philosophical Transactions of the Royal Society of London B: Biological Sciences 364 (2009)1985-1998 ). On the other hand, we add the fact that these materials can act as vectors for other persistent contaminants such as chlorinated biphenyls, polycyclic aromatic hydrocarbons and even metals. heavy, or allow the growth on its surface of pathogenic microorganisms or invasive species (GESAMP Report and Studies, 90 (2015), ISSN: 1020-4873). Among the different categories of plastic pollution, microplastics stand out, which have been found in practically all ecosystems, including continental and oceanic waters, sediments, air and soils (Grbic, J. et al., Environmental Sceince and Technology Letters, 6 (2019) 68-72). Consequently, microplastics are also present in organisms, mainly in aquatic ones (Cole, M. et al., Nature Scientific Reports, 4: 4528 (2014) 1-8), so there is significant concern about potential damage that its presence can exert both on humans and on fauna.

The first reference related to pollution produced by microplastics dates back to 1972 (Carpenter, E.J. et al., Science, 175 (1972) 1240-1241), although research in this field has mainly taken place in the last decade. In fact, the definition of microplastics was established in 2008 at the first international research conference on the presence, effects and fate of microplastic waste in the marine environment, organized by the National Oceanic and Atmospheric Administration (NOAA). In it, it was established that microplastics are all plastics smaller than 5 mm.

The separation of microplastics from different environmental matrices, mainly aqueous ones, constitutes an important challenge for the scientific community. In fact, there is currently no standardized method that allows microplastics to be separated from aqueous matrices. In the state of the art there are several methods used in the laboratory for the separation of microplastics, being able to distinguish classification and visual separation, filtration and flotation or separation by density (Crawford, CB et al., Microplastic Pollutants, (2017) 203-218; Miller, ME et al., Marine Pollution Bulletin, 123 (2017) 6-18). Visual classification and separation is based on the examination of the aqueous matrix by the human eye, either directly for the largest microplastics (between 1 and 5 mm) or by means of an optical or fluorescence microscope for the smallest ones ( <1mm). Possible microplastics are detected and classified based on their physical characteristics, such as their shape, size, color and lack of biological structure, and are subsequently confirmed using spectroscopic techniques. Filtration is the most frequently used method for removing microplastics from aqueous matrices. It is effective, although it requires high energy consumption and continuous cleaning of the chosen filtration system. In the case of flotation, the separation of microplastics is carried out by difference in densities using solutions of different types of salt, and by therefore, with different densities. One of the most used salts is NaCl (1.2 g mL-1), given its high availability, low cost and environmental nature, although others such as ZnCh (1.5-1.7 g mL-1) and NaI (1 .6-1.8 g mL-1) are also widely used (Van Cauwenberghe, L. et al., Marine Environmental Research, 111 (2015) 5-17). In all of the above cases, the methods for separating microplastics are oriented towards their analysis and, with the exception of filtration, they could not be used as a treatment method for their separation from the environment, as could be the case in a water purification plant. wastewater, in a water treatment plant or in a salt production plant from seawater. In addition, they have certain disadvantages. For example, visual sorting and separation is time-consuming, expensive, and not completely reliable as it is observer-dependent. For its part, filtration is very limited by the saturation of the filters, not only by the microplastics themselves but by the presence of other particles in the medium. In density separation, not all types of microplastics can be separated, and in addition, some of the chemical agents used are high cost or dangerous for the environment.

The newest method recently developed for the separation of microplastics is the one based on electrostatic separation. This technique takes advantage of the electrical charge accumulation properties of plastics to facilitate their separation and has allowed very promising results to be obtained with separation yields of up to 99% (Felsing, S. et al., Environmental Pollution, 234 (2018) 20 -28). In fact, most of the patents developed for the separation of microplastics, so far, are based on this principle (Ying, Z. et al., Chinese patent application CN108593401 A; Baoyuan, G. et al., Chinese patent application Chinese patent CN108407145 A; Cheng, Z. et al., Chinese patent application CN107553776 A; Ward, M., US patent US 8,944,253). However, it has some drawbacks such as the high cost of the required equipment, the long time required to carry out the separation of microplastics and the difficulty in scaling the system for the treatment of large volumes of water.

In a recently published work, the magnetic pathway is pointed out as a suitable mechanism for the separation of microplastics in aqueous matrices (Grbic, J. et al., Environmental Sceince and Technology Letters, 6 (2019) 68-72). In this study, clearly oriented to the separation of microplastics as a sampling method, extraction was carried out using synthetic iron nanoparticles. Commercial 25 nm iron nanoparticles were used that were modified with hexadecyltrimethoxysilane with the aim of making them hydrophobic and thus achieving interaction with microplastics. The method proved to be quite effective, with a recovery of 92% of microplastics of different nature in sizes ranging from 10 to 20 |jm. However, given the high cost of the nanoparticles used, their complex synthesis and the reduced possibility of regeneration and reuse, their potential application for water treatment is limited.

In view of this background, the development of a procedure that allows microplastics to be quickly and efficiently separated, regardless of their nature and size, from different aqueous matrices using low-cost and environmentally friendly materials would represent a crucial discovery. in this field, where there is currently no specific and viable technique for its separation from the aqueous medium.

Summary of the invention

Unexpectedly, the inventors have discovered a microplastic separation procedure with high separation rates, using low-cost and environmentally friendly materials using magnetic iron mineral particles. The procedure is universal in nature with respect to the chemical nature of the microplastic, that is, it works with all types of microplastics regardless of their chemical composition and said procedure also achieves high separation rates of microplastics regardless of their size. As shown in the examples, separation rates are greater than 95%. The process of the invention also achieves the separation of microplastics from aqueous matrices with different characteristics (for example, pH and salinity). The procedure of the present invention is particularly advantageous since it entails a significant reduction in the time necessary to achieve separation, compared to other methods included in the state of the art. The procedure of the present invention presents as an additional advantage the simplicity of the required equipment and the feasibility of its scaling for the treatment of large volumes of water.

Therefore, the present invention is related to a procedure for separating microplastics from an aqueous matrix that comprises microplastics, the procedure comprising:

a) provide an aqueous matrix comprising microplastics,

b) adding particles of a magnetic iron mineral to the aqueous matrix provided in step a) to form aggregates of the microplastics with the particles of the magnetic iron mineral, and

c) separate the microplastic aggregates with the magnetic iron mineral particles from the mixture obtained in step b) by applying a magnetic field.

Description of the figures

Figure 1: Photograph of the suspension of polystyrene microplastics in saline water (1.A), photograph of the suspension of polystyrene microplastics and magnetite particles in saline water (1.B), photograph of the suspension of polystyrene microplastics and magnetite particles in saline water under the influence of a magnet (1.C) and photograph of the same suspension in saline water once the magnet is removed (1.D).

Figure 2: Results obtained in the addition and separation experiments of polystyrene microplastics of different sizes in saline water. Error bars show standard deviations.

Figure 3: Photograph of the suspension of high-density polyethylene microplastics in saline water (1.A), photograph of the suspension of high-density polyethylene microplastics and magnetite particles in saline water (1.B), photograph of the suspension of high-density polyethylene microplastics and magnetite particles in saline water under the influence of a magnet (1.C) and photography of the same suspension in saline water once the magnet is removed (1.D).

Figure 4: Results obtained in the addition and separation experiments of high-density polyethylene microplastics of different sizes in saline water. Error bars show standard deviations.

Figure 5: Photograph of the suspension of polyethylene terephthalate microplastics in saline water (5.A), photograph of the suspension of polyethylene terephthalate microplastics and magnetite particles in saline water (5.B), photograph of the microplastic suspension of polyethylene terephthalate in saline water and magnetite particles in saline water under the influence of a magnet (5.C) and results obtained in the experiment of addition and separation of polyethylene terephthalate microplastics in saline water (5.D). Error bars show standard deviations.

Figure 6: Photograph of the suspension of polyvinyl chloride microplastics in saline water (6.A), photograph of the suspension of polyvinyl chloride microplastics and the particles of magnetite in saline water (6.B), photograph of the suspension of polyvinyl chloride microplastics in saline water and magnetite particles in saline water under the influence of a magnet (6.C) and results obtained in the addition experiment and separation of polyvinyl chloride microplastics in saline water (6.D). Error bars show standard deviations.

Figure 7: Photograph of the suspension of low-density polyethylene microplastics in saline water (7.A), photograph of the suspension of low-density polyethylene microplastics and magnetite particles in saline water (7.B), photograph of the suspension of low-density polyethylene microplastics in saline water and magnetite particles in saline water under the influence of a magnet (7.C) and results obtained in the experiment of addition and separation of low-density polyethylene microplastics in water saline (7.D). Error bars show standard deviations.

Figure 8: Photograph of the suspension of polypropylene microplastics in saline water (8.A), photograph of the suspension of polypropylene microplastics and magnetite particles in saline water (8.B), photograph of the suspension of polypropylene microplastics in saline water and magnetite particles in saline water under the influence of a magnet (8.C) and results obtained in the experiment of addition and separation of polypropylene microplastics in saline water (8.D). Error bars show standard deviations.

Figure 9: Photograph of the suspension of microplastic suspension of polyethylene terephthalate, high-density polyethylene, polyvinyl chloride, low-density polyethylene, polypropylene and polystyrene in saline water (9.A), photograph of the same suspension of microplastics and the magnetite particles in saline water (9.B), photograph of the same suspension of microplastics in saline water and the magnetite particles in saline water under the influence of a magnet (9.C) and results obtained in the addition experiment and separation of the same microplastic suspension in saline water (9.D). Error bars show standard deviations.

Figure 10: Photograph of the suspension of polycarbonate microplastics in saline water (10.A), photograph of the suspension of polycarbonate microplastics and magnetite particles in saline water (10.B), photograph of the suspension of polycarbonate microplastics in saline water and magnetite particles in saline water under the influence of a magnet (10.C) and results obtained in the addition and separation experiment of polycarbonate microplastics in saline water (10.D). Error bars show standard deviations.

Figure 11: Photograph of the suspension of polyurethane microplastics in saline water (10.A), photograph of the suspension of polyurethane microplastics and magnetite particles in saline water (10.B), photograph of the suspension of polyurethane microplastics in saline water and magnetite particles in saline water under the influence of a magnet (10.C) and results obtained in the experiment of addition and separation of polyurethane microplastics in saline water (10.D). Error bars show standard deviations.

Detailed description of the invention

The present invention relates to a procedure for the separation of microplastics from an aqueous matrix comprising microplastics, the procedure comprising: a) providing an aqueous matrix comprising microplastics,

b) adding particles of magnetic iron ore to the aqueous matrix provided in step a) to form aggregates of the microplastics with the particles of the magnetic iron ore, and

c) separate the microplastic aggregates with the magnetic iron mineral particles from the mixture obtained in step b) by applying a magnetic field.

In the context of the present invention, the term "microplastic" or "microplastics" refers to a plastic material with an average diameter of less than 5 mm, preferably with an average diameter in the range of 0.1 ^m to 5 mm, plus preferably in the range of 0.1 ^m to 1 mm, even more preferably in the range of 0.1 ^m to 500 ^m, even more preferably of 100 ^m to 250 ^m. The measurement of the size of the microplastics involves their prior separation from the aqueous matrix by vacuum filtration using a microporous membrane filter with a pore size not exceeding 0.1 ^m, and their subsequent drying either at room temperature or in stove at a temperature not exceeding 100 °C. The filter containing the microplastics is then analyzed by optical microscopy, for example using the conditions described in the examples. The length and width of the microplastics are measured and the size of the largest dimension (length) is assigned.

Microplastics can be made up of any plastic material, preferably synthetic organic polymers, such as polyethylene terephthalates (PET), polyethylenes (PE). including high-density polyethylenes (HDPE) and low-density polyethylenes (LDPE), polyvinyl chlorides (PVC), polypropylenes (PP), polystyrenes (PS), polycarbonates (PC) and polyurethanes (PU).

“Polyethylene terephthalate”, “polyethylene terephthalate” or “PET” is a polymer that can be obtained through a polycondensation reaction between terephthalic acid and ethylene glycol. Polyethylene terephthalate has the following structure.

“Polyethylene” or “PE” refers to a polymer that can be obtained by polymerization of ethylene. Polyethylene has the following structure.

“High-density polyethylene” or “HDPE” refers to a polyethylene whose density is in the range of 0.930 to 0.970 g mL-1. “Low density polyethylene” or “LDPE” refers to a polyethylene whose density is in the range of 0.917 to 0.930 g mL-1.

“Polyvinyl chloride” or “PVC” refers to a polymer that can be obtained by polymerization of vinyl chloride. Polyvinyl chloride has the following structure.

“Polypropylene” or “PP” refers to a polymer that can be obtained by polymerization of propylene. Polypropylene has the following structure.

“Polystyrene” or “PS” refers to a polymer that can be obtained by polymerization of styrene. Polystyrene has the following structure.

“Polycarbonate” or “PC” refers to a polymer that comprises functional groups linked by carbonate groups (-O-C=O)-O-), such as diallyldiethylene glycol polycarbonate, or the better known, bisphenol A polycarbonate, whose structure is shown below.

“Polyurethane” or “PU” refers to a polymer that can be obtained by condensation of polyols, among which ethylene glycol and propylene glycol stand out, and polyisocyanates, among which methylenediphenyl diisocyanate (MDI) and sodium diisocyanate stand out. toluene (TDI), which give rise to urethane groups (-NH-(C=O)-O-) in the polymer structure. There is a wide variety of polyurethanes due to the variety in polyisocyanates and polyols that can be used to form them.

In a preferred embodiment, the microplastic is selected from the group consisting of polyethylene terephthalate, high density polyethylene, polyvinyl chloride, low density polyethylene, polypropylene, polystyrene, polycarbonate, polyurethane and mixtures thereof. An advantage of the process of the invention is its universal character since the magnetic separation is effective regardless of the chemical nature of the microplastic.

The term "aqueous matrix" refers to a composition containing water as a major component, preferably containing at least 80% by weight of water relative to the total weight of the composition, more preferably at least 85%, even more preferably at least 90%, even more preferably at least 95%. The aqueous matrix may also contain other ingredients, either dissolved (for example salts such as NaCl) or in suspension. In particular, the aqueous matrix is selected from the group consisting of wastewater (including domestic, urban, industrial, agricultural and wastewater). livestock), sea water, river water, lake water, ground water, tap water or mixture thereof. Therefore, wastewater treatment plants, water purification stations, the agri-food industry (for example, fish farms), water sources for human consumption, water facilities for recreational purposes (for example, swimming pools), among others, are facilities of interest. for the application of the procedure of the present invention.

The procedure of the present invention allows the separation of microplastics regardless of the salinity of the water. It is effective both in fresh water, with low salt concentrations, less than 0.2 g/L, and in salt water, with a salinity of 35 g/L. Preferably, the aqueous matrix has a salinity in the range of 0.05 to 50 g L-1, more preferably 0.1 to 35 g L-1.

The term "salinity" refers to the concentration of inorganic salts in the aqueous matrix. The most frequent salts in aqueous matrices are salts of cations selected from the group consisting of lithium, sodium, potassium, calcium, magnesium, strontium and barium, with anions selected from the group consisting of fluoride, chloride, bromide, iodide, sulfate, nitrate, phosphate, carbonate and borate, such as NaCl, MgCh, Na ² CO ³ and Na ³ PO ⁴ , among others. Salinity can be determined using a conductivity meter, measuring electrical conductivity at 25 °C.

Step a) of the process of the invention is to provide an aqueous matrix comprising microplastics. The aqueous matrix can be provided continuously if the process is carried out continuously or it can be provided in batches if it is carried out in a batch process (or batchwise). The volume of the aqueous matrix provided in step a) is preferably 0.01 to 10,000 L.

In a preferred embodiment, the pH and/or temperature of the aqueous matrix provided in step a) is conditioned before performing step b) so that:

- the pH is in the range of 1 to 13, and/or

- the temperature is in the range of 5 to 95 °C.

Preferably, the pH of the aqueous matrix should be between 1 and 13, more preferably between 2 and 12, more preferably between 3 and 10, more preferably between 5 and 9, even more preferably between 6 and 8, most preferably about 7. The pH of the aqueous matrix can be adjusted by adding a base, such as sodium hydroxide, hydroxide potassium or sodium carbonate, among others, or an acid, such as nitric acid, hydrochloric acid or sulfuric acid, among others, until reaching a pH value within the range defined above. The pH can be determined using a pH meter.

Preferably, the temperature of the aqueous matrix should be between 5 and 95°C, more preferably between 15 and 35°C, even more preferably between 20 and 30°C, most preferably about 25°C. If the temperature of the aqueous matrix is lower, in the conditioning step the temperature can be increased by heating the aqueous matrix until a temperature within the defined range is reached. If the temperature of the aqueous matrix is higher, in the conditioning step of the aqueous matrix, the temperature can be reduced by cooling the aqueous matrix until it reaches a temperature within the defined range. Temperature can be measured using a thermometer.

In a preferred embodiment, the aqueous matrix is conditioned so that:

- the pH is in the range of 3 to 10, and/or

- the temperature is in the range of 15 to 35 °C.

In a particular embodiment, the aqueous matrix is conditioned so that:

- the pH is in the range of 5 to 9, and/or

- the temperature is in the range of 20 to 30 °C.

The next stage, stage b), is the addition of magnetic iron ore particles to the aqueous matrix provided in stage a) to form aggregates of the microplastics with the magnetic iron ore particles.

The term "magnetic iron mineral" refers to any solid, inorganic substance, of defined chemical composition, with ordered atomic spatial arrangement, comprising at least one species of iron and with ferromagnetic properties, that is, it is attracted by a magnet. Preferably, the magnetic iron mineral has a magnetic saturation of between 5 and 100 emu g-1 (between 5103 and 105 A m-1), more preferably between 30 and 90 emu g-1 (between 30103 and 90103 A m -1), even more preferably between 65 and 90 emu g-1 (between 65103 and 90103 A m-1), most preferably between 70 and 90 emu g-1 (between 70103 and 90103 A m-1). The magnetic properties of magnetic iron ore can be determined using a magnetometer with a SQUID ( superconducting quantum interference device ) sensor. The magnetization (M) of magnetic iron ore is measured as a function of the applied external magnetic field (H). in the range of -10000 to 10000 Oe (equivalent to -797700 to 797700 A m-1) at room temperature. The magnetic saturation value of the mineral is that value of M reached when any subsequent increase in H does not cause an increase in the magnetization of the mineral. Preferably, the iron ore has an iron content of at least 50% by weight relative to the total weight of the mineral, preferably at least 55% by weight, more preferably at least 60% by weight, even more preferably at least 50% by weight. less 65% by weight. The percentage of iron can be determined using a total reflection X-ray fluorescence (TXRF) spectrometer, using the procedure described in the examples.

Preferably the magnetic iron mineral comprises an iron species selected from the group consisting of Fe(II), Fe(III), metallic Fe and mixture thereof. Examples of magnetic iron minerals are magnetite (formed from ferrosodiferric oxide FeOFe ² O ³ , sometimes formulated as Fe ³ O ⁴ ), maghemite (y-Fe ² O ³ ), ilmenite (FeTiO ³ ) and pyrrhotite ( Fe ⁰ , ⁸ -iS), among others. In a preferred embodiment, the magnetic iron mineral is magnetite.

The use of magnetic iron minerals, particularly magnetite, is advantageous from an economic and environmental point of view since they are abundant materials, respectful of the environment and easy to separate from aqueous matrices.

Magnetic iron ore is in particle form. The term "particle" means that the magnetic iron ore is in the form of powder or granules with an average diameter of less than 5 mm, preferably whose average diameter is between 0.1 ^m and 5 mm, more preferably between 0. 1 ^m and 1 mm, even more preferably between 0.1 ^m and 500 ^m, even more preferably between 0.1 ^m and 100 ^m, most preferably between 50 μm and 100 ^m. The magnetic iron ore particles They can be obtained by grinding the mineral and subsequent sieving in the size ranges previously described. The size of the magnetic iron ore particles refers to the average diameter of said particles. For a more precise knowledge of the particle size, measurement techniques can be used. Optical microscopy.

The terms "microplastic aggregates with magnetic iron ore particles", "microplastic/magnetic iron ore aggregates" or "aggregates" refer to a group of particles formed by microplastics and magnetic iron ore particles due to to the interaction that occurs between the two.

The addition of the magnetic iron mineral particles to the aqueous matrix can be carried out continuously if the process is carried out continuously or it can be carried out discontinuously if it is carried out in a batch (or batch) process.

In a particular embodiment, the process of the invention is carried out discontinuously, that is, in batches.

In another particular embodiment, the process of the invention is carried out continuously.

In a preferred embodiment, the amount of magnetic iron mineral particles (by weight) added to the aqueous matrix in step b) is 0.1 to 20 the amount of microplastics (by weight) present in the aqueous matrix provided in step a), more preferably from 0.5 to 10, even more preferably from 1 to 10, most preferably from 2 to 7. To determine the amount of magnetic iron mineral to be added, an aliquot of the aqueous matrix can be taken. and determine the amount of microplastics contained in it. This involves the prior separation of the microplastics from the aqueous matrix by vacuum filtration using a microporous membrane filter with a pore size not exceeding 0.1 .^m, and its subsequent drying either at room temperature or in an oven at temperature not exceeding 100 °C. Next, the filter containing the microplastics is weighed on a precision scale and its mass is determined by the difference in weighing with the same filter before carrying out the microplastic separation process.

In a preferred embodiment, the mixture obtained in step b) is subjected to stirring before performing step c). Agitation favors the interaction of microplastics and magnetic iron mineral particles to form aggregates. The stirring can be carried out by any conventional method known to the person skilled in the art, such as, for example, mechanical, magnetic or electrostatic stirring, and more preferably, mechanical.

The process of the present invention is particularly advantageous over other state-of-the-art processes due to the significant reduction in the time required to achieve separation. As shown in the examples, the separation is practically immediate, requiring a contact time between the microplastic and the magnetic iron mineral particles of less than 1 min. Therefore, in a particular embodiment, the mixture obtained in step b) is kept in contact, preferably with stirring, for a time from 15 seconds to 2 min, more preferably from 15 seconds to 1 minute, most preferred about 30 seconds.

Once the aggregates are formed, they are separated from the aqueous matrix in step c) by applying a magnetic field.

Said magnetic field can be generated with a magnet or electromagnet, preferably by means of a magnet.

The procedure of the present invention presents as an additional advantage the simplicity of the required equipment and the feasibility of its scaling for the treatment of large volumes of water.

Once the aggregates are separated from the aqueous matrix, the magnetic iron mineral particles can be recovered from said aggregates and reused in the process according to the invention. Therefore, in a preferred embodiment, the magnetic iron mineral particles that form the aggregates with the microplastics separated in step c) are recovered for reuse. In a more preferred embodiment, the magnetic particles are recovered by heat treatment of the aggregates. Said thermal treatment can be carried out by any procedure known to a person skilled in the art, although preferably, recovery is achieved by combustion in an air atmosphere by heating the microplastics contained in the aggregates. To do this, the microplastic aggregate/magnetic iron mineral is heated to a temperature between 250 and 600 °C, preferably to a temperature between 300 and 500 °C. In particular, the heating is carried out at a rate between 0.5 and 20 °C min-1, more preferably at a speed of about 5 °C min-1. In a particular embodiment, it is heated to a temperature between 250 and 600 °C at a speed between 0.5 and 20 °C min-1, preferably at approximately 5 °C min-1. In another particular embodiment, it is heated to a temperature between 300 and 500 °C at a speed between 0.5 and 20 °C min-1, preferably at approximately 5 °C min-1. Generally, the heating start temperature is room temperature (20 25 °C). Preferably the heating is carried out by means of a thermal cycle.

The term “approximately” should be interpreted as the value referred to within ±10% of said value, preferably ±5% of said value.

On the other hand, magnetic iron minerals can be used as catalysts in advanced oxidation processes to degrade a wide variety of organic compounds (Munoz, M. et al., Applied Catalysis B: Environmental, 203 (2017) 106 173; Munoz, M. et al., Catalysis Today, 313 (2018) 142-147). Therefore, magnetic iron minerals can catalyze the degradation of microplastics contained in microplastic/magnetic iron mineral aggregates in an advanced oxidation process. Specifically, the advanced oxidation process could be of the Fenton type and its derivatives such as sono-Fenton, photo-Fenton or electro-Fenton.

Illustrative examples are described below that reveal the characteristics and advantages of the invention, however, they should not be interpreted as limiting the object of the invention as defined in the claims.

Examples

Materials and methods

Determination of particle size

In the present invention, the microplastics were added to the aqueous matrices to subsequently carry out their separation via magnetic means using the procedure described in the memory. The particle size range of the microplastics was previously determined by sieving them using sieves of different sizes (5 mm, 1 mm, 500 .^m, 250 .^m, 100 .^m, 50 .^m and 20 .^m ).

The measurement of the size of microplastics in real aqueous matrices involves their prior separation from the aqueous matrix by vacuum filtration using a microporous membrane filter with a pore size not exceeding 0.1 .^m, and their subsequent drying either at room temperature or in an oven at a temperature not exceeding 100 °C. Next, the filter containing the microplastics is analyzed using optical microscopy. In this case, a Nikonn Eclipse Ci-S/Ci-L vertical optical microscope was used, equipped with a DS-Fi2 digital camera and a DS-U3 control unit, with software for image measurement and processing (NIS-Elements L imaging). The length and width of the microplastics are measured and the size of the largest dimension (length) is assigned.

Determination of iron percentage

The iron content of the magnetic iron ore was determined using a total reflection X-ray fluorescence (TXRF) spectrometer, specifically the model S2 Picofox (Bruker), with a molybdenum source. The sample preparation procedure consisted of digestion in an open vessel using a solution of hydrochloric acid, nitric acid and hydrofluoric acid in a volume ratio of 6:2:0.5, at 105 °C for 1.5 h. The mineral concentration in the glass was 1000 mg L-1 and a total volume of 10 mL was used. If necessary, the magnetic iron ore was previously ground to a size less than 100 .^m. Once the mineral sample was digested, an internal gallium standard was added to the solution for subsequent quantification. A 3 ^L aliquot of the resulting solution was introduced into the spectrometer for analysis, which was carried out at 50 kV and 600 μA with an acquisition time of 500 s. The integration of the peak corresponding to iron in the spectrogram obtained allowed determining the percentage of iron by weight contained in the mineral.

X-ray diffraction

Powder X-ray diffraction (XRD) was used to carry out the identification and characterization of the crystalline phases present in the magnetic iron mineral. The samples were analyzed in a Panalytical X’Pert PRO theta/theta diffractometer with Cu tube (Ka radiation, 8.04 keV). The diffractometer has a secondary graphite monochromator and an Xe gas detector. A theta/2theta scan was performed with an interval in 20 of 10 to 80°, an angular increment of 0.04°, and an accumulation time of 4 s. Prior to analysis, the magnetic iron ore particles were ground, if necessary, to a size less than 100 .^m. For the identification of the crystalline phases present in the collected diffractograms, the High Score Plus software and the PDF-4 Full File crystallographic database of the ICDD (International Center for Diffraction Data) were used.

Determination of magnetic saturation

The magnetic properties of the magnetic iron mineral were measured using a Quantum Design MPMS XL-5 magnetometer with SQUID sensor (superconducting quantum interference devices). If necessary, the magnetic iron ore was previously ground to a smaller size. at 100 .^m. The magnetic moment (M) of the magnetic iron ore was measured as a function of the applied external magnetization field (H) in the range of -10000 to 10000 Oe (which is equivalent to -797700 to 797700 A m-1 in the SI) at room temperature. The magnetic saturation value is that value of M reached when any subsequent increase in H does not cause an increase in the magnetization of the material.

Example 1. Separation of polystyrene microplastics ( Type 6, PS) of a wide range of particle sizes from different aqueous matrices under different operating conditions.

Separation experiments of polystyrene microplastics of different particle sizes were carried out using aqueous matrices in which salinity and temperature were varied. The separation procedure used was carried out using a magnetic iron mineral, ground magnetite (in the range of 50 to 100 jm). The percentage of iron in the magnetite used is in the range of 68 - 78% by weight. The X-ray diffraction analysis of the magnetite confirmed its crystalline character, showing the only phase corresponding to magnetite (Fe ³ O ⁴ ). The magnetism analysis of this material gave a magnetic saturation value of 70 to 90 emu g-1 (70103 to 90103 A m-1).

The separation experiments were carried out in glass vials in which the aqueous matrix, polystyrene microplastics, and magnetic iron mineral particles were introduced. Both polystyrene microplastics and magnetic iron mineral particles remained suspended in the aqueous matrices. The size of polystyrene microplastics used was varied in the range of 1 |jm to 500 μm in the different experiments. Temperature, pH value and salinity were measured using a thermometer, a pH-meter and a conductivity meter, respectively. The conditioning of the aqueous matrix with respect to temperature, pH value and salinity was carried out as a prior step to the separation experiments. In the experiments that required it, the temperature of the aqueous matrix was increased using a thermostated bath. The pH value was adjusted to 7 using aqueous solutions of nitric acid (1 M) and sodium hydroxide (1 M). The salinity was varied using different concentrations of sodium chloride in the range of 0.1 to 35 g L-1. All experiments were performed in triplicate.

A volume of aqueous matrix of 10 mL and a quantity of polystyrene microplastics of 10 mg was used. 20 mg of magnetite particles were added to said suspension. Next, mechanical stirring of the suspension was carried out using a stirrer. of paddles for 30 s. Finally, the separation of the microplastic/magnetic iron mineral aggregates was carried out by applying a magnetic field generated with a magnet. Table 1 shows the size ranges of magnetite and microplastics studied, the amounts of magnetite and microplastics added, the operating conditions tested and the results obtained in the separation.

Table 1

Figure 1 shows a photograph of the suspension of polystyrene microplastics in saline water (1.A), a photograph of the suspension of polystyrene microplastics and magnetite particles in saline water (1.B), a photograph of the suspension of polystyrene microplastics and magnetite particles in saline water under the influence of a magnet (1.C) and a photograph of the same suspension in saline water once the magnet is removed (1.D). Likewise, Figure 2 shows the results obtained regarding the amount of polystyrene microplastics added to the aqueous matrix and the amount of them separated thanks to the interaction with the magnetite particles.

As can be seen, the separation of polystyrene microplastics was highly effective regardless of their size, reaching mass recovery values greater than 99.0%. It should be noted that no differences were observed in the recovery values of the microplastics when the temperature or salt concentration of the aqueous matrix was varied.

Regardless of the temperature, pH value or salt concentration in the aqueous matrix, iron leaching during the test was negligible (<0.1 mg L-1).

Example 2. Separation of high-density polyethylene ( Type 2, HDPE) microplastics of a wide range of particle sizes from different aqueous matrices under different operating conditions.

Separation experiments of high-density polyethylene microplastics of different particle sizes were carried out using aqueous matrices in which the salinity was varied. The separation procedure used was carried out using a magnetic iron mineral, ground magnetite (in the range of 50 and 100 ^m), as detailed in the description of the invention. The percentage of iron in the magnetite used is in the range of 68 - 78% by weight. The X-ray diffraction analysis of the magnetite confirmed its crystalline character, showing the only phase corresponding to magnetite (Fe ³ O ⁴ ). The magnetism analysis of this material gave a magnetic saturation value of 70 to 90 emu g-1 (70103 to 90103 A m-1).

The separation experiments were carried out in glass vials in which the aqueous matrix, high-density polyethylene microplastics, and magnetic iron mineral particles were introduced. Both high-density polyethylene microplastics and magnetic iron mineral particles remained suspended in the aqueous matrices. The size of high-density polyethylene microplastics used was varied in the range of 1 ^m to 500 ^m in the different experiments, as indicated in Table 2. The temperature, pH value and salinity were measured using a thermometer, a pH-meter and a conductivity meter, respectively. The conditioning of the aqueous matrix with respect to temperature, pH value and salinity was carried out as a prior step to the separation experiments. In the experiments that required it, the temperature of the aqueous matrix was increased using a thermostated bath. The pH value was adjusted to 7 using aqueous solutions of nitric acid (1 M) and sodium hydroxide (1 M). The salinity was varied using different concentrations of sodium chloride in the range of 0.1 to 35 g L-1. All experiments were performed in triplicate.

A volume of aqueous matrix of 10 mL and a quantity of high-density polyethylene microplastics of 10 mg were used. 70 mg of magnetite particles were added to said suspension. Next, mechanical stirring of the suspension was carried out using a paddle stirrer for 30 s. Finally, the separation of the microplastic/magnetic iron mineral aggregates was carried out by applying a magnetic field generated with a magnet. Table 2 shows the size ranges of magnetite and microplastics studied, the amounts of magnetite and microplastics added, the operating conditions tested and the results obtained in the separation.

Table 2

Figure 3 shows a photograph of the suspension of high-density polyethylene microplastics in saline water (3.A), a photograph of the suspension of high-density polyethylene microplastics and magnetite particles in saline water (3.B) , a photograph of the suspension of high-density polyethylene microplastics and magnetite particles in saline water under the influence of a magnet (3.C) and a photograph of the same suspension in saline water once the magnet has been removed (3.C). D). Likewise, Figure 4 shows the results obtained regarding the amount of high-density polyethylene microplastics added to the aqueous matrix and the amount of them separated thanks to the interaction with the magnetite particles.

As can be seen, the separation of high-density polyethylene microplastics was highly effective regardless of their size, reaching mass recovery values greater than 95%. It should be noted that no differences were observed in the recovery values of the microplastics when the salt concentration of the aqueous matrix was varied.

Regardless of the temperature, pH value or salt concentration in the aqueous matrix, iron leaching during the test was negligible (<0.1 mg L-1).

Example 3. Separation of polyethylene terephthalate microplastics ( Type 1, PET) from different aqueous matrices.

Separation experiments of polyethylene terephthalate microplastics were carried out using aqueous matrices in which salinity was varied. The separation procedure used was carried out using a magnetic iron mineral, ground magnetite (in the range of 50 and 100 ^m), as detailed in the description of the invention. The percentage of iron in the magnetite used is in the range of 68 - 78% by weight. The X-ray diffraction analysis of the magnetite confirmed its crystalline character, showing the only phase corresponding to magnetite (Fe ³ O ⁴ ). The magnetism analysis of this material gave a magnetic saturation value of 70 to 90 emu g-1 (70103 to 90103 A m-1).

The separation experiments were carried out in glass vials in which the aqueous matrix, polyethylene terephthalate microplastics, and magnetic iron mineral particles were introduced. Both polyethylene terephthalate microplastics and magnetic iron mineral particles remained in suspension in the aqueous matrices. The size of polyethylene terephthalate microplastics used was in the range of 100 to 250 ^m in the different experiments. Temperature, pH value and salinity were measured using a thermometer, a pH-meter and a conductivity meter, respectively. The conditioning of the aqueous matrix with respect to temperature, pH value and salinity was carried out as a prior step to the separation experiments. In the experiments that required it, the temperature of the aqueous matrix was increased using a thermostated bath. The pH value was adjusted to 7 using aqueous solutions of nitric acid (1 M) and sodium hydroxide (1 M). The salinity was varied using different concentrations of sodium chloride in the range of 0.1 to 35 g L-1. All experiments were performed in triplicate.

A volume of aqueous matrix of 10 mL and a quantity of polyethylene terephthalate microplastics of 10 mg were used. 20 mg of magnetic iron mineral particles were added to said suspension. Next, mechanical stirring of the suspension was carried out using a paddle stirrer for 30 s. Finally, the separation of the microplastic/magnetic iron mineral aggregates was carried out by applying a magnetic field generated with a magnet. Table 3 shows the size ranges of magnetite and microplastics studied, the amounts of magnetite and microplastics added, the operating conditions tested and the results obtained in the separation.

Table 3

Figure 5 shows a photograph of the suspension of polyethylene terephthalate microplastics in saline water (5.A), a photograph of the suspension of polyethylene terephthalate microplastics and magnetite particles in saline water (5.B), a photograph of the suspension of polyethylene terephthalate microplastics and magnetite particles in saline water under the influence of a magnet (5.C), and the results obtained with respect to the amount of polyethylene terephthalate microplastics added to the aqueous matrix and the amount of them separated thanks to the interaction with the magnetite particles (5.D).

As can be seen, the separation of polyethylene terephthalate microplastics was highly effective, reaching mass recovery values of 99.0%. It should be noted that no differences were observed in the recovery values of the microplastics when the salt concentration of the aqueous matrix was varied.

Regardless of the temperature, pH value or salt concentration in the aqueous matrix, iron leaching during the test was negligible (<0.1 mg L-1).

Example 4. Separation of polyvinyl chloride microplastics ( Type 3, PVC) from different aqueous matrices.

Separation experiments of polyvinyl chloride microplastics were carried out using aqueous matrices in which salinity was varied. The separation procedure used was carried out using a magnetic iron mineral, ground magnetite (in the range of 50 and 100 jm), as detailed in the description of the invention. The percentage of iron in the magnetite used is in the range of 68 - 78% by weight. The X-ray diffraction analysis of the magnetite confirmed its crystalline character, showing the only phase corresponding to magnetite (Fe ³ O ⁴ ). The magnetism analysis of this material gave a magnetic saturation value of 70 to 90 emu g-1 (70103 to 90103 A m-1).

The separation experiments were carried out in glass vials in which the aqueous matrix, polyvinyl chloride microplastics, and magnetic iron mineral particles were introduced. Both polyvinyl chloride microplastics and magnetic iron mineral particles remained suspended in the aqueous matrices. The size of polyvinyl chloride microplastics used was varied in the range of 100 to 250 μm in the different experiments. Temperature, pH value and salinity were measured using a thermometer, a pH-meter and a conductivity meter, respectively. The conditioning of the aqueous matrix with respect to temperature, pH value and salinity was carried out as a prior step to the separation experiments. In the experiments that required it, the temperature of the aqueous matrix was increased using a thermostated bath. The pH value was adjusted to 7 using aqueous solutions of nitric acid (1 M) and sodium hydroxide (1 M). The salinity was varied using different concentrations of sodium chloride in the range of 0.1 to 35 g L-1. All experiments were performed in triplicate.

A volume of aqueous matrix of 10 mL and a quantity of polyvinyl chloride microplastics of 10 mg were used. 20 mg of magnetic iron mineral particles were added to said suspension. Next, mechanical stirring of the suspension was carried out using a paddle stirrer for 30 s. Finally, the separation of the microplastic/magnetic iron mineral aggregates was carried out by applying a magnetic field generated with a magnet. Table 4 shows the size ranges of magnetite and microplastics studied, the amounts of magnetite and microplastics added, the operating conditions tested and the results obtained in the separation.

Table 4

Figure 6 shows a photograph of the suspension of polyvinyl chloride microplastics in saline water 6.A), a photograph of the suspension of polyvinyl chloride microplastics and magnetite in saline water (6.B), a photograph of the suspension of polyvinyl chloride microplastics and magnetite in saline water under the influence of a magnet (6.C), and the results obtained with regard to the amount of polyvinyl chloride microplastics added to the aqueous matrix and the amount of them separated thanks to the interaction with the magnetite particles (6.D).

As can be seen, the separation of polyvinyl chloride microplastics was highly effective, reaching mass recovery values of 99.0%. It should be noted that no differences were observed in the recovery values of the microplastics when the salt concentration of the aqueous matrix was varied.

Regardless of the temperature, pH value or salt concentration in the aqueous matrix, iron leaching during the test was negligible (<0.1 mg L-1).

Example 5. Separation of low-density polyethylene microplastics ( Type 4, LDPE) from different aqueous matrices.

Separation experiments of low-density polyethylene microplastics were carried out using aqueous matrices in which salinity was varied. The separation procedure used was carried out using a magnetic iron mineral, ground magnetite (in the range of 50 and 100 ^m), as detailed in the description of the invention. The percentage of iron in the magnetite used is in the range of 68 - 78% by weight. The X-ray diffraction analysis of the magnetite confirmed its crystalline character, showing the only phase corresponding to magnetite (Fe ³ O ⁴ ). The magnetism analysis of this material gave a magnetic saturation value of 70 to 90 emu g-1 (70103 to 90103 A m-1).

The separation experiments were carried out in glass vials in which the aqueous matrix, low-density polyethylene microplastics, and magnetic iron mineral particles were introduced. Both low-density polyethylene microplastics and magnetic iron mineral particles remained suspended in the aqueous matrices. The size of low-density polyethylene microplastics used was varied in the range of 100 to 250 ^m in the different experiments. Temperature, pH value and salinity were measured using a thermometer, a pH-meter and a conductivity meter, respectively. The conditioning of the aqueous matrix with respect to temperature, pH value and salinity was carried out as a prior step to the separation experiments. In the experiments that required it, the temperature of the aqueous matrix was 25 °C. The pH value was adjusted to 7 using aqueous solutions of nitric acid (1 M) and sodium hydroxide (1 M). The salinity was varied using different concentrations of sodium chloride in the range of 0.1 to 35 g L-1. All experiments were performed in triplicate.

A volume of aqueous matrix of 10 mL and a quantity of low-density polyethylene microplastics of 10 mg were used. 20 mg of magnetic iron mineral particles were added to said suspension. Next, mechanical stirring of the suspension was carried out using a paddle stirrer for 30 s. Finally, the separation of the microplastic/magnetic iron mineral aggregates was carried out by applying a magnetic field generated with a magnet. Table 5 shows the size ranges of magnetite and microplastics studied, the amounts of magnetite and microplastics added, the operating conditions tested and the results obtained in the separation.

Table 5

Figure 7 shows a photograph of the suspension of low-density polyethylene microplastics in saline water (7.A), a photograph of the suspension of low-density polyethylene microplastics and magnetite particles in saline water (7.B) , a photograph of the suspension of low-density polyethylene microplastics and magnetite particles in saline water under the influence of a magnet (7.C), and the results obtained with regard to the amount of low-density polyethylene microplastics density added to the aqueous matrix and the amount thereof separated thanks to the interaction with the magnetite particles (7.D).

As can be seen, the separation of low-density polyethylene microplastics was highly effective, reaching mass recovery values of 99.0%. It should be noted that no differences were observed in the recovery values of the microplastics when the salt concentration of the aqueous matrix was varied.

Regardless of the temperature, pH value or salt concentration in the aqueous matrix, iron leaching during the test was negligible (<0.1 mg L-1).

Example 6. Separation of polypropylene microplastics ( Type 5, PP) from different aqueous matrices.

Separation experiments of polypropylene microplastics were carried out using aqueous matrices in which the salinity was varied. The separation procedure used was carried out using a magnetic iron mineral, ground magnetite (in the range of 50 and 100 ^m), as detailed in the description of the invention. The percentage of iron in the magnetite used is in the range of 68 - 78% by weight. The X-ray diffraction analysis of the magnetite confirmed its crystalline character, showing the only phase corresponding to magnetite (Fe ³ O ⁴ ). The magnetism analysis of this material gave a magnetic saturation value of 70 to 90 emu g-1 (70103 to 90103 A m-1).

The separation experiments were carried out in glass vials in which the aqueous matrix, polypropylene microplastics, and magnetic iron mineral particles were introduced. Both polypropylene microplastics and magnetic iron mineral particles remained suspended in the aqueous matrices. The size of polypropylene microplastics used was varied in the range of 100 to 250 ^m in the different experiments. Temperature, pH value and salinity were measured using a thermometer, a pH-meter and a conductivity meter, respectively. The conditioning of the aqueous matrix with respect to temperature, pH value and salinity was carried out as a prior step to the separation experiments. In the experiments that required it, the temperature of the aqueous matrix was increased using a thermostated bath. The pH value was adjusted to 7 using aqueous solutions of nitric acid (1 M) and sodium hydroxide (1 M). The salinity was varied using different concentrations of sodium chloride in the range of 0.1 to 35 g L-1. All experiments were performed in triplicate.

A volume of aqueous matrix of 10 mL and a quantity of polypropylene microplastics of 10 mg was used. 30 mg of magnetic iron mineral particles were added to said suspension. Next, mechanical stirring of the suspension was carried out using a paddle stirrer for 30 s. Finally, the separation of microplastic aggregates/magnetic iron mineral was carried out by applying a magnetic field generated with a magnet. Table 6 shows the size ranges of magnetite and microplastics studied, the amounts of magnetite and microplastics added, the operating conditions tested and the results obtained in the separation.

Table 6

Figure 8 shows a photograph of the suspension of polypropylene microplastics in saline water (8.A), a photograph of the suspension of polypropylene microplastics and magnetite particles in saline water (8.B), a photograph of the suspension of polypropylene microplastics and magnetite particles in saline water under the influence of a magnet (8.C), and the results obtained with regard to the amount of polypropylene microplastics added to the aqueous matrix and the amount thereof separated thanks to the interaction with magnetite particles (8.D).

As can be seen, the separation of polypropylene microplastics was highly effective, reaching mass recovery values greater than 99.9%. It should be noted that no differences were observed in the recovery values of the microplastics when the salt concentration of the aqueous matrix was varied.

Regardless of the temperature, pH value or salt concentration in the aqueous matrix, iron leaching during the test was negligible (<0.1 mg L-1).

Example 7. Separation of a mixture of microplastics containing polystyrene ( Type 6), high-density polyethylene ( Type 2), polyethylene terephthalate ( Type 1), polyvinyl chloride ( Type 3), low-density polyethylene ( Type 4) and polypropylene ( Type 5) from different aqueous matrices.

Separation experiments were carried out on a mixture of microplastics containing polystyrene, high-density polyethylene, polyethylene terephthalate, polyvinyl chloride, low-density polyethylene and polypropylene, using aqueous matrices in which the salinity was varied. The separation procedure used was carried out using a magnetic iron mineral, ground magnetite (in the range of 50 and 100 jm), as detailed in the description of the invention. The percentage of iron in the magnetite used is in the range of 68 - 78% by weight. The X-ray diffraction analysis of the magnetite confirmed its crystalline character, showing the only phase corresponding to magnetite (Fe ³ O ⁴ ). The analysis of magnetism of this material gave a magnetic saturation value of 70 to 90 emu g-1 (70103 to 90103 A m-1).

The separation experiments were carried out in glass vials in which the aqueous matrix, the mixture of the aforementioned microplastics, and the magnetic iron mineral particles were introduced. Both microplastics and magnetic iron mineral particles remained suspended in the aqueous matrices. The size of microplastics used was varied in the range of 100 to 250 μm in the different experiments. Temperature, pH value and salinity were measured using a thermometer, a pH-meter and a conductivity meter, respectively. The conditioning of the aqueous matrix with respect to temperature, pH value and salinity was carried out as a prior step to the separation experiments. In the experiments that required it, the temperature of the aqueous matrix was increased using a thermostated bath. The pH value was adjusted to 7 using aqueous solutions of nitric acid (1 M) and sodium hydroxide (1 M). The salinity was varied using different concentrations of sodium chloride in the range of 0.1 to 35 g L-1. All experiments were performed in triplicate.

A volume of aqueous matrix of 10 mL and a total amount of microplastics of 10 mg were used, using the same amount by mass of each of the six aforementioned microplastics. 30 mg of magnetic iron mineral particles were added to said suspension. Next, mechanical stirring of the suspension was carried out using a paddle stirrer for 30 s. Finally, the separation of the microplastic/magnetic iron mineral aggregates was carried out by applying a magnetic field generated with a magnet. Table 7 shows the size ranges of magnetite and microplastics studied, the amounts of magnetite and microplastics added, the operating conditions tested and the results obtained in the separation.

Table 7

Figure 9 shows a photograph of the suspension of the microplastic mixture of polyethylene terephthalate, high-density polyethylene, polyvinyl chloride, low-density polyethylene, polypropylene and polystyrene in saline water (9.A), a photograph of the suspension of the same mixture of microplastics and magnetite particles in saline water (9.B), a photograph of the suspension of the same mixture of microplastics in saline water and magnetite particles in saline water under the influence of a magnet (9. C), and the results obtained regarding the amount of microplastics added to the aqueous matrix and the amount of them separated thanks to the interaction with the magnetite particles (9.D).

As can be seen, the separation of the microplastic mixture was highly effective, reaching mass recovery values of 99.0%. It should be noted that no differences were observed in the recovery values of the microplastics when the salt concentration of the aqueous matrix was varied.

Regardless of the temperature, pH value or salt concentration in the aqueous matrix, iron leaching during the test was negligible (<0.1 mg L-1).

Example 8. Separation of microplastics of a different nature from the previous ones ( Type 7, polycarbonate ( PC) and polyurethane ( PU)).

Separation experiments were carried out on microplastics of a different nature than those previously shown in examples 1 to 7, specifically polycarbonate and polyurethane microplastics using aqueous matrices in which the salinity was varied. The separation procedure used was carried out using a magnetic iron mineral, ground magnetite (in the range of 50 and 100 ^m), as detailed in the description of the invention. The percentage of iron in the magnetite used is in the range of 68 - 78% by weight. The X-ray diffraction analysis of the magnetite confirmed its crystalline character, showing the only phase corresponding to magnetite (Fe ³ O ⁴ ). The magnetism analysis of this material gave a magnetic saturation value of 70 to 90 emu g-1 (70103 to 90103 A m-1).

The separation experiments were carried out in glass vials in which the aqueous matrix, polycarbonate or polyurethane microplastics, and magnetic iron mineral particles were introduced. Both microplastics and magnetic iron mineral particles remained suspended in the aqueous matrices. He The size of microplastics used was varied in the range of 100 to 250 μm in the different experiments. Temperature, pH value and salinity were measured using a thermometer, a pH-meter and a conductivity meter, respectively. The conditioning of the aqueous matrix with respect to temperature, pH value and salinity was carried out as a prior step to the separation experiments. In the experiments that required it, the temperature of the aqueous matrix was increased using a thermostated bath. The pH value was adjusted to 7 using aqueous solutions of nitric acid (1 M) and sodium hydroxide (1 M). The salinity was varied using different concentrations of sodium chloride in the range of 0.1 to 35 g L-1. All experiments were performed in triplicate.

A volume of aqueous matrix of 10 mL and a quantity of polycarbonate or polyurethane microplastics of 10 mg was used. 20 mg of magnetic iron mineral particles were added to said suspension. Next, mechanical stirring of the suspension was carried out using a paddle stirrer for 30 s. Finally, the separation of the microplastic/magnetic iron mineral aggregates was carried out by applying a magnetic field generated with a magnet.

Table 8 shows the size ranges of magnetite and polycarbonate microplastics, the amounts of magnetite and polycarbonate microplastics added, the operating conditions tested and the results obtained in the separation.

Table 8

Figure 10 shows a photograph of the suspension of polycarbonate microplastics in saline water (10.A), a photograph of the suspension of polycarbonate microplastics and magnetite particles in saline water (10.B), a photograph of the suspension of polycarbonate microplastics and magnetite particles in saline water under the influence of a magnet (10.C), and the results obtained with respect to the amount of polycarbonate microplastics added to the aqueous matrix and the amount thereof separated thanks to the interaction with magnetite particles (10.D).

Table 9 shows the size ranges of magnetite and polyurethane microplastics, the quantities of magnetite and polyurethane microplastics added, the operating conditions tested and the results obtained in the separation.

Table 9

Figure 11 shows a photograph of the suspension of polyurethane microplastics in saline water (11.A), a photograph of the suspension of polyurethane microplastics and magnetite in saline water (11.B), a photograph of the suspension of microplastics of polyurethane and saline magnetite particles under the influence of a magnet (11.C), and the results obtained with respect to the amount of polyurethane microplastics added to the aqueous matrix and the amount of them separated thanks to the interaction with magnetite particles (11.D).

As can be seen, the separation of microplastics, both polycarbonate and polyurethane, was highly effective, reaching mass recovery values greater than 99.0%. It should be noted that no differences were observed in the recovery values of the microplastics when the salt concentration of the aqueous matrix was varied.

Regardless of the temperature, pH value or salt concentration in the aqueous matrix, iron leaching during the test was negligible (<0.1 mg L-1).

Claims (18)

1. Procedure for separating microplastics from an aqueous matrix comprising microplastics, the procedure comprising: a) providing an aqueous matrix comprising microplastics, where the aqueous matrix contains at least 80% by weight of water, b) adding particles of magnetic iron ore to the aqueous matrix provided in step a) to form aggregates of the microplastics with the particles of the magnetic iron ore, and c) separating the microplastic aggregates with the magnetic iron mineral particles from the mixture obtained in step b) by applying a magnetic field, 2. Method according to claim 1, wherein the magnetic iron mineral comprises a species of iron selected from the group consisting of Fe(II), Fe(III), metallic Fe and mixture thereof. 3. Method according to any of claims 1 or 2, wherein the magnetic iron mineral is magnetite. 4. Method according to any of claims 1 to 3, wherein the average diameter of the magnetic iron mineral particles is between 0.1 and 500 ^m. 5. Procedure according to any of claims 1 to 4, wherein the weight of magnetic iron mineral particles that is added to the aqueous matrix in step b) is from 0.1 to 20 the weight of microplastics present in the matrix. aqueous provided in step a). 6. Method according to claim 5, wherein the weight of magnetic iron mineral particles that is added to the aqueous matrix in step b) is 0.5 to 10 the weight of microplastics present in the aqueous matrix provided in step b). stage a). 7. Method according to any of claims 1 to 6, wherein the average diameter of the microplastics is in the range of 0.1 ^m to 5 mm. 8. Method according to any of claims 1 to 7, wherein the microplastics are selected from the group consisting of polyethylene terephthalate, high density polyethylene, polyvinyl chloride, low density polyethylene, polypropylene, polystyrene, polycarbonate, polyurethane and mixtures of the same. 9. Procedure according to any of claims 1 to 8, wherein the addition of the magnetic iron mineral particles to the aqueous matrix comprising microplastics of step b) is carried out discontinuously or continuously. 10. Procedure according to any of claims 1 to 9, wherein the mixture obtained in step b) is subjected to stirring before performing step c). 11. Method according to claim 10, wherein the stirring is mechanical, magnetic or electrostatic. 12. Procedure according to claim 11, wherein the stirring is mechanical. 13. Procedure according to any of claims 1 to 12, wherein the pH and/or temperature of the aqueous matrix comprising microplastics of step a) is conditioned before performing step b) so that: - the pH is in the range of 1 to 13, and/or - the temperature is in the range of 5 to 95 °C. 14. Procedure according to claim 13, wherein the pH and/or temperature of the aqueous matrix comprising microplastics of step a) is conditioned before performing step b) so that: - the pH is in the range of 3 to 10, and/or - the temperature is in the range of 15 to 35 °C. 15. Procedure according to any of claims 1 to 14, wherein the separation of the microplastic aggregates with the magnetic iron mineral particles from the aqueous matrix of step c) is carried out using magnets or electromagnets. 16. Method according to any of claims 1 to 15, wherein the magnetic iron mineral particles of the microplastic aggregates with the magnetic iron mineral particles separated in step c) are recovered for reuse. 17. Method according to claim 16, wherein the recovery is carried out by means of a heat treatment. 18. Procedure according to claim 17, wherein the heat treatment comprises heating the aggregates of the microplastics with the particles of the magnetic iron mineral separated in step c) in an air atmosphere to a temperature between 250 ° C and 600 ° C at a speed between 0.5 and 20 °C min-1.