Introduction
In the absence of natural mechanisms of decomposition of microplastic, the scale of its accumulation in the environment transforms in the rank of a planetary environmental problem. Currently, the leadership in the study of microplastic pollution of the oceans belongs to foreign researchers whose work is devoted to sample collection and techniques of sample processing (Hidalgo-Ruz et al., 2012; Imhof et al., 2012; Claessens et al., 2013; Laboratory ..., 2015 ; Losh, 2015; Löder, Gerdts, 2015; Besley et al., 2016; Kedzierski et al., 2016; Mahat, 2017), microplastic pollution assessment of different ecological zones and areas of the World Ocean. Domestic scientists are just starting these researches (Зобков, Есюкова, 2018, etc.). The first observations of the distribution of microplastic were carried out on the beaches of the Baltic and Black Seas, in the Sea of Japan (Блиновская, Якименко, 2018; Есюкова, Чубаренко, 2018; Зобков, Есюкова, 2018; Сибирцова, 2018; Sibirtsova, 2016; Esiukova, 2017).
One of the poorly studied aspects of microplastic pollution of the coastal sea zone both in foreign and domestic works is the role of the granulometric composition (granocomposition) of beach and bottom sediments in the accumulation, movement, and spatial-temporal distribution of microplastics. Thus, the partial analysis of the granocomposition of the beaches of the Baltic Sea performed in the article (Есюкова, Чубаренко, 2018), did not allow to establish any clear connection with the content of microplastics. At the same time, the authors of this work noted the need for a comprehensive analysis of the composition of sediments in the future. However, field studies on the beach of Sevastopol performed by one of the authors of this work, revealed qualitative differences between microplastic particles found in different layers of beach sediments with a thickness of 5 cm during their layer-by-layer selection (Сибирцова, 2018; Sibirtsova, 2016). Thus, the study of the behavior of microplastic particles in coastal-marine sediments of various composition is of particular interest.
As is known, the hydrodynamic factors (wave and storm activity) have a decisive effect on the formation of the thickness of beach-bottom sediments, and with them the redistribution of microplastic particles within the surface and underwater parts of the coastal sea zone. A significant contribution to the entry and differentiation of microplastics within the coastal zone is also made by human activity. That is, the process of entry and migration of microplastic in beach and bottom sediments can be considered as the result of hydrodynamic activity and anthropogenic load on the coastal sea zone.
In this regard, the authors proceed from the assumption that the distribution of microplastics in the thickness of beach-bottom sediments and along the profile of the coastal zone of the sea is closely related to their granular composition. To test this hypothesis, a method is proposed for a conjugate analysis of the granocomposition of sediments and their contamination by microplastic.
The purpose of the work is to present a new approach to the assessment of microplastic pollution of beach and bottom sediments of the sea coastal zone, based on an adapted procedure for their full granulometric analysis.
Materials
The procedure for conducting of granulometric analysis is based on the main provisions of the existing interstate standards (ГОСТ, 2000, 2015), amended with taking into account the possible presence of microplastic in the coastal marine sediments. Sampling was carried out in five areas of the coastal zone of the Sevastopol region (Fig. 1), differing in recreational load and local conditions (hydrodynamic regime, type and width of the beach, coastal zone profile, etc.). Sampling points 2, 4, 5 are located in bays with a weakened wave regime, point 1 is on the open coast, point 3 is on the estuary site. The widest are the beaches at points 1 and 5, the smallest - 2, 3 and 4.
Fig. 1. Sampling areas. 1 – beach "Uchkuevka", 2 – beach "Konstantinovsky", 3 – mouth of the Black River, 4 – beach "Solnechny", 5 – beach "Omega"
Original research
Granulometric analysis is one of the stages of the method of complex study of granulation composition and microplastic pollution of beach and bottom sediments of the coastal zone of the sea, including sampling, granulometric analysis, evaluation of microplastic pollution and discussed in detail in the work (Сибирцева, Агаркова-Лях, 2019). The standard procedure of the granulometric analysis of sediments, carried out at the secoWhile using the nd stage, is adapted to the conditions of their possible contamination with microplastics, making adjustments to the drying temperature, the intensity of mechanical action, the use of purified (distilled) water, etc. The sieve method with washing and without washing with water is used during the grananalysis. This method allows to detect visible to the naked eye particles of micro-and mesoplastyc with sizes from 0.25-5 mm to 5-25 mm, respectively. The main differences between the adapted grananalysis procedure and the standard one (ГОСТ, 2015) are presented in table. 1.
Table 1. Differences of standard grananalysis from adapted to study microplastics contamination
| Characteristics and procedures | GOST 12536-2014 | Adaptation of the method for the study of microplastic contamination |
| Quartering and medium sampling | yes | no |
| Minimum sample weight | From 5- to 2000 g | From 1500 to 4500 g |
| Grinding in preparation and sifting of samples | yes | no |
| Drying temperature |
(105 ± 5) °С
(for mineral samples) (70 ± 5) °С (for organo-mineral samples) |
(50-6- ± 5) °С
(for all samples)
|
| Sample washing | tap or filtered rain (river) water | distilled water |
| Elutriation of particle less than 0.1 mm in size | under running water | in a container of water |
| Evaporation procedure in a sand bath after washing with water | yes | no |
The following describes the sequence of actions when carrying out the grananalysis of beach and bottom sediments of coastal zone.
Required equipment: sieve set with mesh size 10.0; 5.0; 2.0; 1.0; 0.5; 0.25 and 0.1 mm, lid and tray; laboratory electronic scales; desiccator; porcelain or metal cups; glass container (such as a glass cylinder); glass or wooden sticks; rubber syringe; containers for storing samples and fractions; labels; pencil. In order to avoid the risk of external contamination of samples with microplastic particles, the use of plastic utensils, tools and clothing made of artificial materials is excluded during laboratory work.
In the presence of restrictions at the researcher for minimization of efforts for delivery and processing of samples the standard procedure of grananalysis assumes their quartering and selection of average sample. In our opinion, it is desirable to exclude the quartering procedure in the study of microplastic contamination, because the reduction of the sample weight reduces the probability of obtaining an objective picture of the quantitative content of microplastic particles in the sediments.
The sieve method without washing with water is used for grananalysis of samples with a predominant particle size from 0.5 to 10.0 mm.
The sample taken may be dry or wet. The wet sample is firstly dried and then weighed. The dry sample is immediately weighed.
Drying of the sample. The sample is placed in a pre-weighed porcelain or metal cup and dried in a desiccator at a temperature of (50-60 ± 5) °C (Hidalgo-Ruz et al., 2012; Laboratory..., 2015; Löder, Gerdts, 2015) to air-dry state.
Weighing the dry sample. The already dry or dried sample is weighed together with a porcelain or metal cup. The mass of the cup is taken from the obtained value, and so the weight of the dry sample is obtained. The result is recorded in the calculation table.
Sifting of the sample through a set of sieves. To divide the sample into fractions, sieves with a cell size of 10.0; 5.0; 2.0; 1.0 and 0.5 mm are used. The sieves are mounted in a column, placing from the tray in order of increasing the size of the cells from 0.5 to 10.0 mm. A sample is poured onto the upper sieve in several stages and closed with a lid. The samples are sifted manually (by light strokes of the palms of the sides of the sieves) or by mechanized method. For better extraction of microplastic from organic material (algae fragments, pieces of wood, etc.), the latter is thoroughly washed with distilled water in a separate container, then water is drained from the container through a sieve with a mesh size of 0.5 mm. The material staying on a sieve is transferred using a rubber bulb to a pre-weighed cup, dried in a desiccator, weighed and the value is fixed in the table, poured to the rest of the sample on the upper sieve.
The completeness of sifting fractions through each sieve is checked by shaking it over a sheet of paper. If at the same time particles fall out on the sheet, then they are poured out on the next sieve. Sifting is continued until no particles will fall on the paper or until the amount of sand passing through each sieve for 1 min, will not be less than 0.1 % of the total mass of the sample.
Weighing the fractions and the calculation of the mass loss of the sample. Each fraction, delayed after sieving on a sieve, as well as passed into the pallet, is transferred from the upper sieve to pre-weighed porcelain cups and weighed separately. The obtained values are entered in the calculation table.
After sifting, the weights of all fractions are summed. Usually during screening there is a slight loss of sample mass relative to its initial amount. If the received sum of weights of all fractions more than on 1% exceeds weight of the dry sample taken for the analysis, then sifting is repeated. The loss of the sample during its sieving (npr, r) is spread over all selected fractions in proportion to their mass (except for samples in which fractions of more than 2 mm are represented by single inclusions of debris material. In this case, these fractions are not taken into account when calculating the mass loss of the sample). The calculation formula for each fraction is as follows:
Мfr = mfr + (пpr ∙ mfr) ∕ mpr, [1]
where Мfr – the fraction weight, with taking into account the loss in sample weigh, g; mpr – the fraction weight after sifting, g.
The calculation results are fixed in the table.
Calculation of the proportion of each fraction in the sample. After adjusting the mass of each fraction, its share (F, %) in the sample is calculated by the formula:
F = (Мfr ∙ 100%) ∕ ms, [2]
where ms – the mass of a dry sample taken for analysis, g.
The results of the calculation of the granule composition should be determined with an error of 0.1 %. The results of the calculations are recorded in the table.
Packaging and packing of fractions. The material sifted by fractions is placed in prepared dry containers, labelled, signed and left in the laboratory until the stage of analysis of their microplastic contamination.
The sieve method with water washing is used for granulometric analysis of samples with a high particle content of less than 0.5 mm.
Drying and weighing of the sample is carried out similarly to the procedures described above for the sieve method without washing with water.
The mass of particles smaller than 0.1 mm (Cg, r) is determined by the difference between the mass of the dry sample taken for analysis (ms) and its mass after washing with water and drying (ml). The content of clay fraction is calculated by the formula [3] with rounding to 0.1.
Cg = ms - ml [3].
Sifting the sample through a set of sieves. To divide the sample into fractions, the sieves with the cells size of 10.0; 5.0; 2.0; 1.0; 0.5; 0.25 and 0.1 mm are used. Sieves are mounted in the column, placing from the tray in order to increase the size of the holes from 0.1 to 10.0 mm. Then the steps described above for the sieve method without washing with water are performed.
Weighing of the fractions and calculation the mass loss of the sample, the calculation of the proportion of each fraction in the sample, packaging and packing of fractions is carried out similarly to the previously described manipulations for the sieve method without washing with water.
Conclusion
A new approach to the study of the behavior of microplastics in beach-bottom sediments of the coastal zone of the sea, based on the grananalysis, is presented. A sieve method with washing and without washing with water, adapted to the conditions of their possible contamination with microplastics, is proposed for the complete granulation of sediments.
Verification of the proposed method of conjugate analysis of sediment granulation and microplastic contamination will confirm or deny the relationship between the qualitative and quantitative characteristics of microplastic contamination and the granulometric composition of coastal and marine sediments.
In the case of establishing of correlations between granulation and microplastic content, it will be possible to use the former as an indicator of microplastic pollution of beach-bottom sediments. Currently, this method is being tested in the laboratory, during which the methods of material processing will be worked out and improved.
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