Method for quantifying microplastics present in an unconsolidated solid matrix further comprising fresh organic matter

Abstract

The present invention relates to a method for quantifying microplastics in a sample comprising an unconsolidated solid matrix and fresh organic matter. The sample is heated in an inert atmosphere between an initial temperature (T0) ranging from 340°C to 360°C and a final temperature (TF) of at least 550°C, the temperature sequence comprising first and second isothermal stages (P1, P2) at the initial temperature (T0) and at an intermediate temperature (TI) ranging from 380°C to 400°C, respectively, and a curve of the amount of hydrocarbon compounds released by the sample as a function of temperature is measured. An amount of hydrocarbon compounds released by the microplastics in the sample is determined by integrating the curve measured from the intermediate temperature (TI) up to a final integration temperature ranging from the intermediate temperature (TI) to the final temperature (TF), and a quantification of the microplastics in the sample is deduced therefrom.

Description

[0001] METHOD FOR QUANTIFYING MICROPLASTICS PRESENT IN A NON-CONSOLIDATED SOLID MATRIX ALSO CONTAINING FRESH ORGANIC MATTER

[0002] technical field

[0003] The present invention relates to the quantification of microplastics present in an unconsolidated solid matrix. In particular, the present invention may be relevant to the monitoring of plastic pollution in the environment, as well as the qualification of processes for the treatment and / or disposal of microplastics.

[0004] The presence of micropollutants, such as microplastics (MP), is now recognized across all environments, from the highest mountain ranges, lakes, rivers, and down to the deepest abyssal plains in the oceans.

[0005] In particular, microplastics can be found in the environment in a microscopic form invisible to the human eye but still generate significant pollution for fauna and flora.

[0006] Furthermore, these microplastics can transform in the environment, through physical or chemical phenomena, and thus create compounds that are called "neoformed".

[0007] Thus, the problem of plastic pollution and more specifically microplastics (MP) in the environment is a leading societal issue, as well as a major economic and environmental challenge.

[0008] Methods for detecting and quantifying plastics in the environment have progressed over the last ten years. However, the scientific community agrees on the need to implement standardized and harmonized methods for identifying and quantifying plastics in the environment, methods which require significant pre-treatment in terms of time and cost, and which also impact the quality of the analyses.

[0009] In particular, there is a real challenge in classifying sediments from rivers, ports, beaches, landfills, sewage treatment plant sludge and Stormwater Treatment Unit (UTEP), dry residues from laundries, dye works, etc.

[0010] Previous technique

[0011] In general, there are known methods for characterizing plastics and polymers that can be used to identify their presence in a sample: for example, differential scanning calorimetry, thermogravimetric analysis, pyrolysis coupled with gas chromatography-mass spectrometry (Py-GC-MS), or Raman spectroscopy. However, these methods vary in complexity and time.

[0012] For example, Fourier transform infrared microspectroscopy (pFTIR, for micro Fourier Transform Interferometer) is a technique that allows the identification of unknown chemical species by characterizing their composition, size, and quantity. However, this method cannot be used for the quantification and differentiation of microplastics smaller than approximately 20 µm (the detection threshold of the pFTIR method).

[0013] We are also familiar with patent application WO 2022 / 243080 A1, which concerns a thermal analysis for characterizing and quantifying the plastic content in samples of a porous medium such as sand. More specifically, this method is based on measuring the quantities of hydrocarbon compounds (HC), carbon monoxide (CO), and / or carbon dioxide (CO2) released over time by a sample subjected to a heating sequence in an inert atmosphere followed by a heating sequence in an oxidizing atmosphere, applied to solid samples. A database of reference samples, previously prepared from different mineral matrices and several types of polymers (e.g., PE, PP, PE100, PA6, PA11, PFA, and PET), distributed in predetermined concentrations, is established beforehand.Parameters derived from the thermal analysis results for the sample under analysis are compared to those from the reference sample database for the identification and differentiation of polymer families. This approach therefore requires that the type(s) of microplastic(s) present in the sample be included in the reference sample database and involves a step to identify these different polymer types. Specifically, the identification of the microplastic type can be performed by identifying the peak temperatures in the measured HC, CO, and / or CO2 curves and by cross-referencing them with the peak temperatures of the corresponding curves measured for the plurality of reference samples. Quantification for each polymer type is then performed by integrating the areas of the identified peaks.However, the quantification method described in this document can be difficult to apply when organic matter is present in the sample, as it is then difficult to distinguish the peaks associated with microplastics from those associated with organic matter. Therefore, it may be necessary to first implement a protocol for removing the organic matter present in the sample, for example, by cleaning with hydrogen peroxide. The present invention overcomes these drawbacks. In particular, the present invention allows, from a thermal analysis comprising a single heating sequence under an inert atmosphere (pyrolysis), a rapid, simple, and reliable quantification of microplastics in a sample of an unconsolidated solid matrix also containing fresh organic matter.Furthermore, the present invention does not require prior pretreatment, in particular the removal of fresh organic matter, and requires only a sample of a few mg.

[0014] Summary of the invention

[0015] The present invention relates to a method for quantifying microplastics present in a sample of an unconsolidated solid matrix further comprising fresh organic matter. The method comprises performing at least the following steps on the sample:

[0016] A) said sample is heated in an inert atmosphere according to a sequence of temperatures having an initial temperature (T0) between 340°C and 360°C and a final temperature (TF) greater than or equal to 550°C, said sequence of temperatures comprising at least a first isothermal stage (P1) at said initial temperature (T0) and a second isothermal stage (P2) at an intermediate temperature (T1) between 380°C and 400°C, said first and / or second isothermal stages having a duration of between 3 and 7 minutes, and at least a representative quantity of hydrocarbon compounds released during said heating in an inert atmosphere is continuously measured;

[0017] B) From a curve of the evolution as a function of temperature of said representative quantity of hydrocarbon compounds released during said heating in an inert atmosphere by said sample, a quantity of hydrocarbon compounds released by said microplastics of said sample is determined from a mass of said sample and an integration of said curve between said intermediate temperature (Tl) and a final integration temperature of said curve, said final integration temperature being between said intermediate temperature (Tl) and said final temperature (TF) of said temperature sequence.

[0018] C) a mass percentage of microplastics present in said unconsolidated solid matrix sample is deduced by means of a predetermined correspondence law linking a quantity of hydrocarbon compounds released by microplastics from a sample to a mass percentage of microplastics from a sample.

[0019] According to one embodiment of the invention, said predetermined correspondence law relating a quantity of hydrocarbon compounds released by microplastics from a QHCP sample to a mass percentage of microplastics %FP in said sample can be defined by a formula of the type: %FP = a. QHCP where a is a coefficient between 200 and 600.

[0020] According to one implementation of the invention, prior at least to step C), starting from a plurality of reference samples for which a mass percentage of microplastics is predetermined, the following steps can be carried out:

[0021] - for each of said reference samples of said plurality of reference samples, steps A) and B) are applied so as to determine a quantity of hydrocarbon compounds released by said microplastics for each of said reference samples of said plurality of reference samples;

[0022] - said correspondence law relating a quantity of hydrocarbon compounds released by microplastics of a sample to a mass percentage of microplastics of a sample is determined by means of a linear regression method applied to said determined quantities of hydrocarbon compounds released by said microplastics for each of said reference samples of said plurality of reference samples and to said predetermined mass percentages of microplastics of said reference samples of said plurality of reference samples.

[0023] According to one embodiment of the invention, said microplastics may correspond to PET and / or PA, and in which said coefficient a is between 360 and 390, and is preferably 372.

[0024] According to one implementation of the invention, at least one preliminary step of step C) can be carried out in the following manner:

[0025] - from said curve of the evolution as a function of temperature of said representative quantity of hydrocarbon compounds released by said sample during said heating in an inert atmosphere, at least one parameter is determined and said at least one parameter determined is compared to at least one reference parameter from a reference database relating to a plurality of types of microplastics;

[0026] - from said comparison, the presence or absence of at least one type of microplastics in said sample is characterized.

[0027] According to one embodiment of the invention, said parameter may be a temperature of a peak of said curve of the evolution as a function of temperature of said representative quantity of hydrocarbon compounds released by said sample during said heating in an inert atmosphere.

[0028] According to one embodiment of the invention, said reference database can be constructed in the following manner:

[0029] I) We define a plurality of types of microplastics, preferably at least polyethylene terephthalate, polyethylene, polyamide and / or perfluoroalkoxy;

[0030] II) for each of the said types of microplastics defined, step A) is applied to a sample comprising said type of microplastics, and at least one reference parameter for said type of microplastics is determined from a curve of the evolution as a function of temperature of the representative quantity of hydrocarbon compounds released during heating in an inert atmosphere by said sample of said type of microplastics;

[0031] III) for each of the said types of microplastics defined, at least one reference parameter is added to said reference database.

[0032] According to one embodiment of the invention, said initial temperature (TO) may be 350°C and / or said final temperature (TF) may be 550°C and / or said intermediate temperature (Tl) may be 390°C and / or said final integration temperature (TF') may be 550°C, and / or said predetermined duration of said first and / or second isothermal stages may be 5 minutes.

[0033] According to one embodiment of the invention, in step B), the quantity of hydrocarbon compounds released by the microplastics of the QHCP sample can be determined according to a formula of the type: where C(T) is said curve of the evolution as a function of temperature of said representative quantity of hydrocarbon compounds released during said heating in an inert atmosphere by said sample, mech is said mass of said sample, TI is said intermediate temperature, TF' is said final integration temperature.

[0034] The invention further relates to a system for implementing the process as described above, comprising at least one pyrolysis furnace in an inert atmosphere and means for measuring hydrocarbon compounds.

[0035] Other features and advantages of the process according to the invention will become apparent from the following description of non-limiting examples of embodiments, with reference to the attached figures described below. List of figures

[0036] Figure 1 illustrates an example of implementation of the temperature sequence according to the invention.

[0037] Figure 2 presents an example of a curve showing the evolution as a function of temperature of the representative quantity of hydrocarbon compounds released at the end of step 1) of the process according to the invention applied to a first sample, as well as a curve representing the sequence of temperatures used for this implementation of step 1) of the process according to the invention.

[0038] Figure 3 presents an example of a curve showing the evolution as a function of temperature of the representative quantity of hydrocarbon compounds released at the end of step 1) of the process according to the invention applied to a second sample, as well as a curve representing the sequence of temperatures used for this implementation of step 1) of the process according to the invention.

[0039] Figure 4 shows a curve of the evolution as a function of temperature of the representative quantity of compounds released at the end of step 1) of the process according to the invention applied to a third sample.

[0040] Description of the implementation methods

[0041] The invention relates to a method for quantifying microplastics present in a sample of unconsolidated solid matrix, the sample further comprising fresh organic matter.

[0042] In other words, the method for quantifying microplastics present in an unconsolidated solid matrix sample according to the invention is particularly suitable when the unconsolidated solid matrix sample also contains fresh organic matter. Indeed, the method according to the invention allows for the separate quantification of the mass concentrations of microplastics contained in a sample, without requiring a prior protocol for the removal of fresh organic matter.

[0043] Microplastics are defined as any particle comprising one or more polymers and ranging in size from 1 µm to 5 mm. For example, microplastics according to the invention may include particles of various types of polymers such as polyethylene terephthalate (PET), polyethylene (PE), polyamide (PA), perfluoroalkoxy (PFA), and / or polypropylene (PP), among others. Microplastics may be in various forms, such as fibers, films, powder, or pellets. Microplastics according to the invention may, in particular, include chemical textile microfibers, i.e., particles from woven, non-woven, or knitted materials (such as clothing or linen) composed of synthetic fibers (i.e., derived from hydrocarbon products, such as PET, PA, etc.).) and / or artificial fibers (such as viscose) resulting from the chemical transformation (i.e. a transformation changing the nature of the fiber) of a natural material (cellulose, wood, plant etc.).

[0044] Unconsolidated solid matrix refers to a material composed of uncemented solid particles. The unconsolidated solid matrix itself can be of any type and be composed of several different materials, such as quartz particles, clay particles, etc. Fresh organic matter refers to debris of plant origin (plant residues, exudates), animal origin (excrement, carcasses), fungal origin, and / or microbial origin. Thus, fresh organic matter is considered less transformed compared to sedimentary organic matter contained in parent rocks, which has undergone more significant transformation due to increased temperature and pressure during sediment burial. For example, fresh organic matter can include debris from plant plankton, phytoplankton, zooplankton, algae, lignin, or fecal matter, among other things.

[0045] According to one embodiment of the invention, the sample of the unconsolidated solid matrix comprising microplastics and fresh organic matter can be chosen from the following non-exhaustive list: a sediment sample from a river, a port, a beach, a landfill, sludge from STEP (wastewater treatment plant) and IITEP (drinking water treatment unit), or solid residues from a textile processing device.

[0046] The term "textile processing device" refers in particular to a textile washing device, for example, an individual washing machine (or washing machine), for domestic or commercial use, a set of washing machines (for example, in laundries), an industrial laundry (for example, a laundry facility), etc. However, a textile processing device according to the invention generally includes any device that brings a textile into contact with a liquid, the liquid then being separated from the textile, such as a device for dyeing a textile, or a device for waterproofing a textile.

[0047] The process according to the invention requires a sample of the unconsolidated solid matrix comprising microplastics and fresh organic matter. The sample to be analyzed may result from a sediment sample taken from a riverbed, a port, a beach, sewage treatment plant sludge, etc.

[0048] The sample can also be obtained from a liquid effluent that has been filtered to obtain its dry residue, that is, solid particles (microfibers) that are not interconnected. The liquid effluent can be taken from the outlet of the textile processing device's drain line, for example, after each wash in the case of a domestic washing machine, or hourly in an industrial laundry or dry cleaner. According to one embodiment of the invention, the process according to the invention may include a preliminary step in which the liquid effluent from the textile processing device is filtered by means of filtration means in order to collect a residue of filter particles (fibers) from the liquid effluent. In one embodiment, the filtration means comprise a membrane filter, preferably made of glass (an inert material).Advantageously, the membrane filter can be made of a material that does not generate particles that could interfere with the quantification of microplastics according to the invention. Most preferably, fiberglass membrane filters can be used. Indeed, these filters are inert with respect to the process according to the invention (they neither contain nor produce microplastics) and can therefore be introduced, along with their residue, directly into a device suitable for carrying out the process according to the invention.

[0049] The process according to the invention can advantageously, but not exclusively, be implemented using the ROCK-EVAL® device (IFP Energies nouvelles, France), as described in patents FR 2227797 (US 3953171) and FR 2472754 (US 4352673). Indeed, the ROCK-EVAL® device comprises at least:

[0050] - a pyrolysis oven in an inert atmosphere,

[0051] - means of measuring hydrocarbon compounds (HC), for example in the form of a flame ionization detector (FID).

[0052] The process can alternatively be implemented using any furnace allowing heating in an inert atmosphere, cooperating with one or more devices for measuring hydrocarbon compounds.

[0053] The method according to the invention can also be implemented using analytical means. These analytical means may include computer-based means such as a computer, a processor, or a calculator.

[0054] The invention also relates to a system for implementing the process as described below, comprising at least one pyrolysis furnace in an inert atmosphere and means for measuring hydrocarbon compounds.

[0055] The process according to the invention may include the following steps:

[0056] 1) Heating sequence under an inert atmosphere (pyrolysis) 2) Determination of the quantity of hydrocarbon compounds released

[0057] 3) Quantification of microplastics

[0058] The steps of the process according to the invention are described below.

[0059] 1) Heating sequence under an inert atmosphere (pyrolysis)

[0060] During this step, the sample is heated under an inert atmosphere (such as under a flow of nitrogen, argon, or helium) according to a temperature sequence where the initial temperature (hereafter denoted T0) is between 340°C and 360°C, and preferably 350°C, and the final temperature (hereafter denoted TF) is greater than or equal to 550°C, and preferably 550°C. Furthermore, the temperature sequence according to the invention includes at least two isothermal plateaus (i.e., periods of holding the temperature at a constant temperature): a first plateau at the initial temperature and a second isothermal plateau at an intermediate temperature (hereafter denoted Tl) between 380°C and 400°C, and preferably 390°C. The first and second isothermal stages according to the invention have a predetermined duration of between 3 and 7 minutes, and are preferably worth 5 minutes.

[0061] Indeed, the Applicant has observed, through numerous tests carried out on a variety of unconsolidated solid matrix samples containing microplastics and fresh organic matter, that heating to a constant temperature between 340°C and 360°C, corresponding to the first isothermal stage according to the invention, allows for the complete release of organic compounds from the fresh organic matter contained in unconsolidated solid matrix samples comprising both microplastics and fresh organic matter. Thus, the heating sequence according to the invention does not require starting the heating of the sample from lower temperatures, contrary to what is taught in the prior art (temperature between 200°C and 300°C in the prior art based on the aforementioned thermal analysis). Therefore, compared to the prior art, the process according to the invention is simpler and more efficient.Heating to a constant temperature between 380°C and 400°C, corresponding to the second isothermal stage according to the invention, followed by a temperature increase to a final temperature of at least 550°C, allows for the complete release of organic compounds from microplastics contained in unconsolidated solid matrix samples comprising both microplastics and fresh organic matter. Furthermore, the second isothermal stage according to the invention allows for better temporal separation between the release of hydrocarbon compounds from the fresh organic matter and the release of those from the microplastics. The preferred final temperature of 550°C allows for the release of microplastics that are predominantly found in the environment, such as PP, PET, PE, PA, and PFA, among others.Thus, the preferred final temperature of the temperature sequence according to the invention allows for efficient implementation (in terms of duration and energy consumption) of the process according to the invention.

[0062] In other words, the Applicant was able to demonstrate, through numerous tests carried out on a plurality of unconsolidated solid matrix samples comprising microplastics and fresh organic matter, that fresh organic matter is released at temperatures lower than the temperature at which microplastics begin to be released, which occurs at the temperature of the second isothermal stage according to the invention, and that, conversely, microplastics, regardless of their type and shape, are released between the temperature of the second isothermal stage according to the invention and the preferred final temperature according to the invention.It is quite clear that the final temperature of the process according to the invention can be set at any value above 550°C, but that, for the efficiency in duration and energy consumption of the process according to the invention, it is advantageous that the final temperature of the temperature sequence according to the invention be equal to the preferred temperature of 550°C.

[0063] According to one embodiment of the invention, the temperature sequence may further include a third isothermal step at the final temperature of the temperature sequence according to the invention. Such an additional isothermal step may allow, if necessary, the cracking of compounds with a cracking temperature close to the final temperature TF of the temperature sequence under an inert atmosphere according to the invention.

[0064] According to one embodiment of the invention, the temperature sequence according to the invention may comprise at least two thermal gradients ranging from 1°C / min to 25°C / min, and preferably having a value of 25°C / min. According to an embodiment in which the temperature sequence comprises only the first and second isothermal stages according to the invention, and optionally the third isothermal stage as defined above, the first thermal gradient according to the invention may connect the first isothermal stage (at the initial temperature) according to the invention to the second isothermal stage (at the intermediate temperature) according to the invention, and the second thermal gradient according to the invention may connect the second isothermal stage (at the intermediate temperature) according to the invention to the final temperature (optionally maintained by a third isothermal stage) according to the invention.The aforementioned thermal gradient values ​​constitute compromises allowing the thermal cracking of organic compounds, while limiting the implementation time of the process according to the invention.

[0065] Figure 1 illustrates an implementation of the temperature sequence of the process according to the invention, comprising two isothermal stages P1, P2 at initial temperatures T0 and intermediate T1, and a first thermal gradient G1 to link the first isothermal stage P1 to the second isothermal stage P2, and a second thermal gradient G2 to link the second isothermal stage P2 to the final temperature TF of the process according to the invention.

[0066] According to one embodiment of the invention, the first and second isothermal stages according to the invention, and any other isothermal stage of the temperature sequence according to the invention, may have a different duration from one isothermal stage to another.

[0067] According to one embodiment of the invention, the first and second thermal gradients according to the invention, and any other thermal gradient in the temperature sequence according to the invention, may have a different value from one thermal gradient to another.

[0068] According to the invention, a representative quantity of hydrocarbon compounds (HC) contained in an effluent resulting from said heating is continuously measured (i.e., continuously over time). In other words, during this sequence, the representative quantity of HC released by the sample through thermal cracking of the organic matter and thermal decomposition of the microplastics contained in the sample can be continuously measured. The measurement of the representative quantity of hydrocarbon compounds can be carried out using a flame ionization detector (FID). Note that such sensors measure an HC flux and provide values ​​measured in millivolts (mV). Conventionally, the quantity of HC can be determined by calculating the area under the curve measured (possibly between predefined temperatures) by these sensors and dividing this area by the mass in mg of the sample.Alternatively, other means of measuring the quantity of HC can be used.

[0069] According to one embodiment of the invention, the temperature sequence under an inert atmosphere according to the invention may be preceded by a heating phase of the pyrolysis furnace, which may take the form of a thermal gradient from 1 to 25°C / min, preferably 25°C / min, or any other form of heating curve for the pyrolysis furnace. This preliminary heating phase of the pyrolysis furnace allows the furnace to reach the initial temperature of the temperature sequence under an inert atmosphere according to the invention. This preliminary phase can help initiate the thermal cracking of compounds whose cracking temperature is lower than the initial temperature of the temperature sequence under an inert atmosphere according to the invention, corresponding to those of fresh organic matter.

[0070] According to one embodiment of the invention, the temperature sequence under an inert atmosphere according to the invention can be followed by a phase of lowering the temperature of the pyrolysis furnace, which can be in the form of a thermal gradient, for example, between -1 and -25°C / min, preferably between -20 and -25°C / min, or any other form of temperature decrease curve for the pyrolysis furnace. This final phase of lowering the temperature of the pyrolysis furnace allows, if necessary, the completion of the thermal cracking of the compounds associated with the final temperature of the temperature sequence under an inert atmosphere according to the invention.

[0071] According to the invention, at the end of this step, a curve is obtained representing the amount of HC released over time during the pyrolysis phase, hereafter denoted C(T). It is readily apparent to a person skilled in the art to transform a curve representing the amount of HC released over time into a curve representing the amount of HC released as a function of temperature, since the temperature sequence (evolution of temperature as a function of time T(t)) is known.

[0072] 2) Determination of the quantity of hydrocarbon compounds released

[0073] During this step, from a curve of the evolution as a function of temperature of the representative quantity of hydrocarbon compounds released during heating in an inert atmosphere by said sample and a mass of the sample, a quantity of hydrocarbon compounds released by the microplastics of the sample is determined from the mass of the sample and an integration of said curve between said intermediate temperature (Tl) and a final integration temperature of said curve, said final integration temperature being between said intermediate temperature (Tl) and said final temperature (TF) of said temperature sequence.

[0074] According to one embodiment of the invention, from the C(T) curve of the evolution as a function of temperature of the representative quantity of hydrocarbon compounds released by the sample during heating in an inert atmosphere, a quantity of hydrocarbon compounds released by the microplastics of the QHCP sample is determined according to a formula of the type Or :

[0075] - mech is the mass of the sample; - TI is said intermediate temperature, between 380 and 400°C, and preferably 390°C;

[0076] - TF' is the final integration temperature (more precisely, "final integration temperature of the C(T) curve") between the intermediate temperature Tl and the final temperature TF of the temperature sequence according to the invention. Preferably, the final integration temperature may be 550°C.

[0077] In other words, the C(T) curve is integrated, weighted by the mass of the sample, between the temperatures identified as characteristic of the release of organic compounds from microplastics. Indeed, as discussed in the previous step, the Applicant was able to observe, through numerous tests carried out on a plurality of unconsolidated solid matrix samples containing fresh organic matter and microplastics, that the fresh organic matter present in a sample releases organic compounds at a temperature lower than the intermediate temperature Tl according to the invention, and that the microplastics present in a sample release organic compounds between the intermediate temperature Tl according to the invention and the final release temperature of the organic compounds of the microplastics, which is approximately 550°C.It is clear that the final integration temperature TF' can be chosen lower than the final release temperature of the organic compounds in microplastics, which is 550°C, if the C(T) curve has a peak ending at a temperature below 550°C. The final integration temperature TF' can also be chosen higher than the final release temperature of the organic compounds in microplastics, which is 550°C, as this does not affect the estimation of the quantity of microplastics in a sample containing fresh organic matter (no organic compounds are released from the fresh organic matter beyond the intermediate temperature according to the invention, and therefore beyond 550°C).

[0078] This is illustrated in Figure 2 (respectively Figure 3), which shows an example of a temperature-dependent curve for the representative quantity of hydrocarbon compounds relative to the mass of the QHC-M sample (unit in mV / mg) released at the end of step 1) applied to a sample consisting of 60 mg of sand, 0.5 mg of plankton, and 0.5 mg of PE (respectively PET). Figure 2 (respectively Figure 3) also shows the temperature sequence ST applied during step 1), characterized by a first isothermal plateau at an initial temperature of 350°C, a second isothermal plateau at an intermediate temperature of 390°C, and a final temperature of 650°C followed by a temperature reduction phase. Peak P1 (respectively PT) corresponds to hydrocarbon compounds released by plankton, and peak P2 (respectively P2') corresponds to hydrocarbon compounds released by PE (respectively PET).These figures show that the final release temperature of HC compounds from plankton is significantly lower than the intermediate temperature according to the invention, and that the initial release temperature of HC compounds from PE and / or PET is higher than the intermediate temperature according to the invention. Thus, it is clear from this example that the intermediate temperature according to the invention is a temperature that allows for the differentiation of hydrocarbon compounds from microplastics from those from fresh organic matter. Furthermore, it can be observed that the final release temperature of PE and / or PET is significantly lower than the preferred final integration temperature of the temperature sequence according to the invention.

[0079] 3) Quantification of microplastics

[0080] During this step, from the quantities of hydrocarbon compounds released by the microplastics in the sample determined in step 2), a mass percentage of microplastics present in the sample is determined by means of a predetermined correspondence law linking a quantity of hydrocarbon compounds released by microplastics in a sample to a mass percentage of microplastics in a sample.

[0081] According to a first embodiment of the invention, prior at least to step 3), the correspondence law linking a quantity of hydrocarbon compounds released by microplastics in a sample to a mass percentage of microplastics in a sample can be determined as follows: from a plurality of reference samples for which a mass percentage of microplastics (possibly of one or more predetermined types of microplastics) is predetermined, the following steps are carried out:

[0082] - for each of the reference samples of the plurality of reference samples, steps 1) and 2) are applied in such a way as to determine a quantity of hydrocarbon compounds released by microplastics for each of the reference samples of the plurality of reference samples;

[0083] - we determine the correspondence law linking a quantity of hydrocarbon compounds released by microplastics from a sample to a mass percentage of microplastics from a sample by means of a linear regression method applied to the determined quantities of hydrocarbon compounds released by microplastics for each of the reference samples of the plurality of reference samples and to the predetermined mass percentages of microplastics of these reference samples of the plurality of reference samples.

[0084] According to a second embodiment of the invention, the correspondence law linking a quantity of hydrocarbon compounds released by microplastics in a sample to a mass percentage of microplastics present in the sample %FP is defined by a formula of the type: %FP = a. QHCP where a is between 200 and 600. The values ​​of the coefficient a were established by the Applicant, by applying the first embodiment of the invention described above to a plurality of reference samples corresponding to mixtures in various proportions of microplastics and an inert material (not releasing hydrocarbon compounds) such as sand.

[0085] According to a third embodiment, in which the microplastics present in the sample to be analyzed correspond to polyester (PET) and / or polyamide (PA), the coefficient 'a' can be between 360 and 380, and preferably 372. The values ​​of the coefficient 'a' were established by the Applicant, by applying the first embodiment of the invention described above to a plurality of reference samples corresponding to mixtures in various proportions of PET and / or PA type microplastics and an inert material (not releasing hydrocarbon compounds) such as sand. This correspondence law can be advantageously used to quantify microplastics present, for example, in a residue from a textile processing device, since for this type of sample, the predominant plastic microfibers are PET and PA.

[0086] According to one embodiment of the invention, a preliminary step of identifying at least one type of microplastics present in the sample to be analyzed can be carried out. This preliminary step can be carried out as follows:

[0087] - from the curve of the evolution as a function of temperature of the representative quantity of hydrocarbon compounds released by the sample during heating in an inert atmosphere, at least one parameter is determined and said at least one parameter determined is compared to at least one reference parameter from a database relating to a plurality of types of microplastics;

[0088] - from said comparison, the presence or absence of at least one type of microplastics in said sample is characterized.

[0089] Advantageously, the parameter can be the temperature of a peak on the temperature-dependent curve of the representative quantity of hydrocarbon compounds released by the sample during heating in an inert atmosphere. Advantageously, the reference database can be constructed as follows:

[0090] I) We define a plurality of types of microplastics, preferably at least polyethylene terephthalate, polyethylene, polyamide and / or perfluoroalkoxy;

[0091] II) for each of the defined types of microplastics, step 1) of the process according to the invention is applied to a sample of said type of microplastics, and at least one reference parameter of the reference database for said type of microplastics is determined from a curve of the evolution as a function of temperature of the representative quantity of hydrocarbon compounds released during heating in an inert atmosphere by said sample of said type of microplastics;

[0092] III) for each type of microplastics defined, at least one reference parameter determined in said reference database is added.

[0093] The preliminary step of identifying at least one type of microplastic present in the sample to be analyzed is particularly advantageous when implementing the first and third variants mentioned above, as it allows for the identification of the type(s) of microplastic present in the sample. When this preliminary step is combined with the first variant described above, this combination can lead to the determination of a relevant correspondence law for the type(s) of microplastic present in the sample. The correspondence law thus determined can then be used to create a (second) database associating a correspondence law with each type of microplastic present in a sample.In the case of the implementation of the third variant described above, this preliminary step can make it possible to identify in advance that the microplastics present in the sample to be analyzed correspond to polyester (PET) and / or polyamide (PA), and thus support the implementation of the third variant according to the invention.

[0094] Thus, the present invention makes it possible, from a quantity of hydrocarbon compounds released during a heating sequence in an inert atmosphere with predefined minimum and maximum temperatures, to determine the mass percentage of microplastics present in an unconsolidated solid matrix also containing fresh organic matter. These results are obtained from a single heating sequence (in this case, under an inert atmosphere), the minimum and maximum temperatures of which are defined to optimize energy consumption and the implementation time of the process according to the invention. Furthermore, unlike the prior art, the process according to the invention does not require a preliminary step to remove the organic matter present in the sample to be analyzed (for example, with hydrogen peroxide).Thus, the time required to quantify microplastics using the method according to the invention is reduced by a factor of 5 to 10 compared to the prior art, and the number of analyses is halved. Such a method can therefore enable real-time analysis, as is required, for example, on landfill remediation sites.

[0095] Examples

[0096] The characteristics and advantages of the process according to the invention will become clearer upon reading the application example below.

[0097] The method according to the invention was applied to a sediment sample taken from a riverbed. The analyses were performed in triplicate to verify the repeatability of the result of the method according to the invention.

[0098] The sample was subjected to a heating sequence under an inert atmosphere comprising a first isothermal stage of 5 minutes at 350°C, a second isothermal stage at a temperature of 390°C also of 5 minutes, and a final temperature of 650°C.

[0099] Figure 4 shows the curves of the evolution as a function of temperature T of the representative quantity of hydrocarbon compounds relative to the mass of the QHC-M sample (unit in mV / mg) released at the end of step 1 applied to one of the three triplicates.

[0100] Step 3) of the process according to the invention was applied with a final integration temperature TF' of 650°C, and according to the third variant described above for step 3). The implementation of the process according to the invention thus made it possible to determine that the sediment sample taken from the bed of a river contains a mass percentage of microplastics %FP of 64.8%.

Claims

Demands 1. A method for quantifying microplastics present in a sample of an unconsolidated solid matrix further comprising fresh organic matter, characterized in that at least the following steps are carried out for said sample: A) said sample is heated in an inert atmosphere according to a sequence of temperatures having an initial temperature (T0) between 340°C and 360°C and a final temperature (TF) greater than or equal to 550°C, said sequence of temperatures comprising at least a first isothermal stage (P1) at said initial temperature (TO) and a second isothermal stage (P2) at an intermediate temperature (Tl) between 380°C and 400°C, said first and / or second isothermal stages having a duration of between 3 and 7 minutes, and at least a representative quantity of hydrocarbon compounds released during said heating in an inert atmosphere is continuously measured; B) From a curve of the evolution as a function of temperature of said representative quantity of hydrocarbon compounds released during said heating in an inert atmosphere by said sample, a quantity of hydrocarbon compounds released by said microplastics of said sample is determined from a mass of said sample and an integration of said curve between said intermediate temperature (Tl) and a final integration temperature of said curve, said final integration temperature being between said intermediate temperature (Tl) and said final temperature (TF) of said temperature sequence. C) a mass percentage of microplastics present in said unconsolidated solid matrix sample is deduced by means of a predetermined correspondence law linking a quantity of hydrocarbon compounds released by microplastics from a sample to a mass percentage of microplastics from a sample.

2. A method according to claim 1, wherein said predetermined correspondence law relating a quantity of hydrocarbon compounds released by microplastics from a sample QHCP to a mass percentage of microplastics %FP in said sample is defined by a formula of the type: %FP = a. QHCP, where a is a coefficient between 200 and 600.

3. A method according to claim 1, wherein, prior to at least step C), starting from a plurality of reference samples for which a mass percentage of microplastics is predetermined, the following steps are carried out: - for each of said reference samples of said plurality of reference samples, steps A) and B) are applied so as to determine a quantity of hydrocarbon compounds released by said microplastics for each of said reference samples of said plurality of reference samples; - said correspondence law relating a quantity of hydrocarbon compounds released by microplastics of a sample to a mass percentage of microplastics of a sample is determined by means of a linear regression method applied to said determined quantities of hydrocarbon compounds released by said microplastics for each of said reference samples of said plurality of reference samples and to said predetermined mass percentages of microplastics of said reference samples of said plurality of reference samples.

4. A method according to claim 2, wherein said microplastics correspond to PET and / or PA, and wherein said coefficient a is between 360 and 390, and preferably equals 372.

5. A method according to claim 3 or 4, wherein at least one preliminary step of step C) is carried out in the following manner: - from said curve of the evolution as a function of temperature of said representative quantity of hydrocarbon compounds released by said sample during said heating in an inert atmosphere, at least one parameter is determined and said at least one parameter determined is compared to at least one reference parameter from a reference database relating to a plurality of types of microplastics; - from said comparison, the presence or absence of at least one type of microplastics in said sample is characterized.

6. Method according to claim 5, wherein said parameter is a temperature of a peak of said curve of the evolution as a function of temperature of said representative quantity of hydrocarbon compounds released by said sample during said heating in an inert atmosphere.

7. A method according to claim 5 or 6, wherein said reference database is constructed in the following manner: I) We define a plurality of types of microplastics, preferably at least polyethylene terephthalate, polyethylene, polyamide and / or perfluoroalkoxy; II) For each of the aforementioned defined types of microplastics, step A) is applied to a sample comprising said type of microplastic, and at least one reference parameter for said type of microplastic is determined from a curve of the evolution as a function of temperature of the representative quantity of hydrocarbon compounds released during heating in an inert atmosphere by said sample of said type of microplastics; III) for each of the said types of microplastics defined, at least one reference parameter is added to said reference database.

8. A method according to any one of the preceding claims, wherein said initial temperature (T0) is 350°C and / or said final temperature (TF) is 550°C and / or said intermediate temperature (Tl) is 390°C and / or said final integration temperature (TF') is 550°C, and / or said predetermined duration of said first and / or second isothermal stages is 5 minutes.

9. A method according to any one of the preceding claims, wherein, in step B), the quantity of hydrocarbon compounds released by said microplastics from said QHCP sample is determined according to a formula of the type: where C(T) is said curve of the evolution as a function of temperature of said representative quantity of hydrocarbon compounds released during said heating in an inert atmosphere by said sample, mech is said mass of said sample, TI is said intermediate temperature, TF' is said final integration temperature.

10. System for implementing the process according to any one of the preceding claims comprising at least one pyrolysis furnace in an inert atmosphere and means for measuring hydrocarbon compounds.

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