Commonly, crop producers use plastic mulch films to conserve water, to suppress weeds, and to prevent contamination from atmospheric agents. However, plastic mulch films are difficult to recycle and there is an added risk of what is termed ‘microplastics derived from mulch film’ (otherwise known as MMF) polluting the wider environment.
A large amount of microplastics can be contained in farmland soil. These can then be passed through water to rivers, which in turn are likely to carry them out to oceans. Once situated within the seas, microplastics can harm marine organisms and then reach the human body through the food chain.
Raman spectroscopy has been used by researchers at Guangxi University and the Chinese Academy of Tropical Agricultural Sciences in order to determine the portion of microplastics in existence which derive from agricultural mulch films. The teams of researchers analyzed soil and sediment collected from different sites around four main rivers in the Hainan Province of China, the Nandu, Wenlan, Zhubi and Changhua using a Renishaw inViaTM confocal Raman microscope.
First, the team extracted the microplastics by digesting the samples with hydrogen peroxide. Next, the solution was precipitated with potassium formate and the resulting supernatant filtered. Finally, the remaining residues were inspected with a stereomicroscope, microplastic fragments identified, and then carefully transferred to transparent quartz slides.
Microplastics Derived from Mulch Film Detection
Typically, mulch films consist of additives and a core polyethylene matrix. Raman spectra from polyethylene, a mulch film standard, and suspected MMF samples were collected by the researchers. A combination of visible (532 nm) and UV (325 nm) excitation, and analyzed microplastics were used, ranging from 1 µm to 5000 µm in diameter. Peaks at 1130 cm-1, 1440 cm-1, 1296 cm-1, and from 2800 cm-1 to 2900 cm-1 were detected in spectra from all three sample types. This confirmed the researchers’ hypothesis that the suspected MMFs were derived from mulch film.
Figure 1. Raman spectra from a typical suspected MMF sample, a mulch film standard sample, and polyethylene. Image Credit: Renishaw plc – Spectroscopy
The researchers also looked for similarities in the surface composition of both the mulch film samples and the MMF. From the peaks in the Raman spectra, they could identify chemical groups.
Table 1 shows the surface composition of suspected MMF samples from the four river estuaries, and the mulch film standard sample. All suspected MMF samples were similar to the mulch film standard.
Table 1. Surface composition data from suspected MMF samples from estuary sites and a mulch film standard sample. Standard deviations were derived from duplicate measurements. Source: Renishaw plc – Spectroscopy
Microplastic Abundance
The abundance of microplastics (including MMFs) was assessed by the researchers at farmland sites and estuaries by use of Raman spectroscopy. At farmland sites, it was found that the MMF content ranged between 1240 to 2860 items per kg of soil, which accounted for around half of the total microplastics. The abundance of MMFs at estuary sites ranged from 38 to 82 items per kg of sediment, which accounted for approximately 10% of the total microplastic content.
Figure 2. Abundance of total microplastics and the portion made up of MMFs at farmland and estuary sites around the four investigated rivers. Image Credit: Renishaw plc – Spectroscopy
Microplastics in Sediment and Soil
The researchers in this study demonstrated that Raman spectroscopy is a valuable technique when it comes to microplastics research. It was used in analysis of both farmland soil and river sediments to identify a high abundance of MMFs in the total microplastic content, and revealed that MMFs are a neglected contributor to microplastic pollution.
Note
This article has been adapted and key elements extracted from the piece 'Occurrences and characteristics of full-size microplastics derived from mulch films in major exorheic rivers of Hainan, China', by Meng Jiao, Beibei Liu, Lin Wu, Lu Ren, Luya Wang, Wending Ma and Ruilong Li, published in Research Square, 10, 2021. Follow this link for the original piece to learn more: https://www.researchsquare.com/article/rs-991254/v1
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The original article is copyright © 2021 Jiao, Liu, Wu, Ren, Wang, Ma and Li. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
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