Microplastic Pollution in The Topsoil of Hot And Dried Areas, Human Exposure and Source Assessment, Aghili Plain, Iran

Although the distribution of microplastic (MPs) has been studied in different environmental compartments during the last decade, there is still a knowledge gap in their distribution and abundances in soil. This study aimed to investigate the abundance, distribution, and type of MPs in the soil of Aghili plain as a hot and dry area in southwest of Iran. In this study, composite soil samples (n=102) were collected from the residential and agricultural areas in Aghili plain, Iran. A combination of visual observations, Scanning Electron Microscopy (SEM) – Energy-dispersive X-ray spectroscopy (EDS), and Raman micro -spectroscopy was conducted to quantify and characterize MPs in soil samples. The intakes of MPs in adults and children were estimated through two exposure EPA scenarios. The total MPs loading in the studies soil was 11.93 ± 0.9 items in Kg -1 of surface soil in Aghili plain. The MPs had various morphology (ber, pellet, fragment, and spherule shapes), colors (white-transparent, yellow-orange, red-pink, blue-green and black-grey colors), and sizes (<100 µm up to 1000 ≤ µm). Black-grey bers in size less than 100 μm were dominant MPs in soil samples using a binocular microscope. Trace amounts of C, N, O, Na, P, Si revealed by EDS suggested the organic and inorganic contaminations on the surface of identied MPs. Intake of MPs per day/year through ingestion of polluted soil was calculated. Mean normal and acute exposure was estimated at 0.435 and 0.871 MPs per year through ingestion by children and adults, respectively.


Introduction
Microplastics (MPs) generally are categorized as particles with nano (<100 nm in one dimension) and micro (100 nm to 5 mm) sizes, which can be uptake by many organisms due to their small size and ubiquitous distribution in Aghili plain is located in Khuzestan Province, southwest of Iran. Since 2004, the study area has been experiencing heavy dust storm events, the so-called "Middle Eastern Dust (MED) event" (Shepherd et al., 2016), which lead to considerable air dust pollution and health effects on residents. Furthermore, Aghili plain is one of the most important agricultural centers in the north of Khuzestan province. It is irrigated by Karun and Shoor rivers as two of the main rivers in Iran. Rivers pollution by several anthropogenic sources and local pollution sources release different environmental pollutants, including MPs, into the soil with hot and dry climate. According to our best knowledge, the abundance of MPs in Iranian plain soil has not yet been studied. This study aimed to investigate the frequency, characterization, and distribution of MPs contamination in the soil of Aghili plain in Iran.

Microplastic intake via ingestion
The number of Microplastic intakes via ingestion was calculated based on a normal exposure scenario of USEPA with a mean particle ingestion rate of 100 and 200 mg day −1 for adults and children (1-6 years old), respectively (EPA, 2008). The acute exposure scenario suggested 1 g of particle ingestion per outdoor day for children. Also, based on the guidance values to calculate the intake of microplastics in occupational exposure, it is suggested that 330 mg day−1 of ingested soil for outdoor workers (29) and 200 days per year is considered the mean value of working days.
QA/QC control During this research, different control procedures were implemented. All sampling instruments and containers were washed with deionized water before sampling. Soil samples were taken with a metallic pan and kept in the aluminum foil. All the experiments were conducted under a clean laboratory environment. So, all working surfaces cleaned with ethanol, all reagents and distilled water ltered through chm lter papers (F2024 with a pore size of 2μ), all glassware was cleaned with distilled water to prevent plastic and ber cross contamination. Moreover, all windows and doors keep closed to avoid the impact of air turbulence and airborne and MPs pollution during the lab workings. Cotton laboratory coats, face masks and single-use latex gloves were used. Simultaneously, three replicates of blanks were conducted to check the possible suspended airborne contamination.
Statistical analysis SPSS software (version 16.0) was applied to analyze all data and expressed as mean ± standard deviation of the mean (SDM).

Characteristics of the MPs in soil
The average mean of MPs was identi ed as 11.93 ± 0.9 items in Kg −1 of surface soil in the Aghili plain, Iran.
Moreover, different ber, pellet, fragment, and Spherule shapes were identi ed MPs within various colors and diameters ( Figure.  MPs abundance were observed at site 9 (N =22 items Kg −1 ) and site12 (N= 1 items kg −1 ), respectively. The total number of items Kg −1 per sites were observed as the following trend: S9 (22) > S6 (21) > S3(19) > S11 (16) > S2 and S4 (15) > S1(14) > S14 (12) >S13, S7 and S8 (9) > S5 (3) > S10 (2) > S12 (1). Yan and et al.  to be ingested was MPs less than 100 µm. The daily and annually intake of MPs by adults and children was estimated in normal and acute exposure scenarios. The results con rmed the probability of the health risks due to MPs exposure. However, additional research is required to assess the rates of MPs exposure, ingestion toxicokinetic information, and adherence to skin. It was revealed that the human intake of MPs by ingesting soil contaminated particles must be considered a signi cant health issue in cities.

Declarations
Ethical Approval: All Author con rm that the manuscript is the authors' own original work, which has not been previously published elsewhere, and re ects the authors' own research and analysis in a truthful and complete manner. The paper properly credits the meaningful contributions of co-authors and co-rsearchers. The results are appropriately placed in the context of prior and existing research. All sources used are properly disclosed. All authors have been personally and actively involved in substantial work leading to the paper, and will take public responsibility for its content.   Four MPs samples spectra analyzed by Raman spectrometry