Separation and Identification of Conventional Microplastics from Farmland Soils

Summary

Presented here is a method for extracting microplastics from soil and identifying their polymer types. The method has been optimized for execution, applicability, and cost-effectiveness. It lays a scientific foundation for standardizing the analytical method to identify microplastics in soils.

Abstract

Microplastics (MPs) pollution in the terrestrial environment has received increasing attention over the last decade, with increasing studies describing the numbers and types of MPs in different soil systems and their impacts on soil and crop health. However, different MPs extraction and analytical methods are used, limiting opportunities to compare results and generate reliable evidence for industry advice and policymakers. Here, we present a protocol that describes the methodology for sampling, separation, and chemical identification of conventional MPs from soil. The method is low-cost, and the materials are readily available. This enhances operational ease and may help with widespread adoption. The protocol provides detailed information on sample collection from the top 0-30 cm of soil using plastic-free utensils; simulation of different soil types through the use of various solid media (such as bentonite clay, silicon dioxide, and non-contaminated soil), with the addition of the same mass of polyethylene(PE)-MPs for subsequent quantification; density separation of plastic particles utilizing saturated sodium chloride (NaCl) solution and digestion of organic impurities in the supernatant using 4 M sodium hydroxide (NaOH) solution; quantification of particles using fluorescent microscopy after Nile Red staining; and polymer identification using micro Fourier-Transform Infrared Spectroscopy (μ-FTIR) or Laser-Direct Infrared (LDIR) spectroscopy. The MPs recovery rate ranged from 83% - 90% for the abovementioned media. This protocol presents an efficient method for soil MPs analysis that is optimized for feasibility, applicability, and cost-effectiveness. Moreover, the video accompanied can guide the process of analyzing the soil MPs step-by-step virtually. This study is dedicated to standardizing the methods for soil MPs analysis, enhancing the connectivity and comparability of measurements, and establishing a foundation for more standardized and scientific research.

Introduction

It is estimated that 4.8 to 12.7 million metric tons of plastic enter the ocean annually from terrestrial sources^1,2. These plastic particles gradually degrade into smaller fragments in response to ultraviolet irradiation, mechanical abrasion, and biodegradation^3,4. Microplastics (MPs) pollution, with plastic particles of diameter less than 5 mm, in the soil is becoming an increasing concern, particularly in terms of its potential effect on soil and crop health. It is primarily driven by the continual rise in plastic production and challenges surroundi....

Protocol

NOTE: The following solutions need to be prepared at ambient temperature prior to the extraction process: 1) Saturated NaCl solution (5.7 M) - dissolve 1 kg of NaCl in 3 L DI H₂O; 2) 4 M NaOH - dissolve 480 g NaOH in 3 L DI H₂O; 3) Nile Red (100 µg mL^-1) - dissolve 10 mg of Nile Red in 100 mL of appropriate solvent (e.g., methanol, acetone).

1. Soil sampling and preparation

1. Collect a representative soil sample using a five-point sampling method in a "W" shape manner across the study area (Figure 1). Use a 30 cm stainless-steel soil auger....

Results

To validate the recovery rates of this methodology, samples from three different solid matrices (silicon dioxide (SD), bentonite clay (BT), and soil) were analyzed in sets of three replicates. Samples were analyzed with and without the addition of 0.04% w/w white polyethylene (PE) microplastic (particle size range 40-48 µm). Soil samples were collected from Haidian District, Beijing, China (China Agricultural University West Campus), and soil was classified as umber soil. Additionally, three replicates of b.......

Discussion

The soil sampling strategy in the field, including approaches such as simple random sampling or systematic grid sampling, as well as the sampling area and depth, must be tailored to the specific research questions and clearly defined prior to the sample collection. Some studies have focused on the topsoil layer of 0-10 cm^34,37, whereas other collected soil samples with a depth of 0-40 cm³⁸. Since the size and abundance of MPs vary in diffe.......

Materials

Name	Company	Catalog Number	Comments
2-decimal balance	n/a	n/a	Standard 2-decimal balance
40 °C oven	n/a	n/a	Standard large fan-oven with temperature set to 40 °C
8700 LDIR	Agilent Technologies	n/a	LDIR used to identify particles
Aluminum container	n/a	n/a	Standard aluminum food take-away container
Aluminum foil	n/a	n/a	Standard heavy-duty aluminum foil
Axioplan 2	Zeiss	n/a	Fluorescence microscopy is used to observe microplastic particles in the fluorescent state
Bentonite clay	Sigma Aldrich	285234	Bentonite clay used for recovery tests
BX53	Olympus	n/a	Fluorescence microscopy is used to observe microplastic particles in the fluorescent state
Glass beaker (600 ml)	n/a	n/a	Standard glass beaker
Glass bottle (1 l)	n/a	n/a	Standard glass bottle
Glass magnetic stirrer bar	n/a	n/a	Standard glass coated magnetic stirrer bar
Glass measuring cylinder (500 ml)	n/a	n/a	Standard glass measuring cylinder
Glass pipette (10 ml)
Glass vacuum filtration device	Pyrex (purchased via Sigma Aldrich)	SLW5809/KIT	Glass filtration device with 500 ml funnel, porous plate, and 1 l collection beaker
LUMOS Alpha II	Bruker	n/a	FTIR used to analyze suspect microplastics.
Magnetic stirring plate	n/a	n/a	Standard magnetic stirring plate
MCE filter membrane	Jinteng company	JTMF0441/0442	White MCE membranes, 0.2 µm pore size, 50 mm diameter, with FTIR method
Nile Red	Fisher Scientific	10464311	Nile Red powder used to make stock solution of nile red dye using appropriate solvent (e.g. acetone)
PCTE filter membrane	Sterlitech Corporation	1270060	Black PCTE membranes, PVP-free, 0.2 µm pore size, 47 mm diameter, with LDIR method
Silicon dioxide	Sigma Aldrich	18649	Silicon dioxide used for recovery tests
Sodium chloride	Sigma Aldrich	S9888	Sodium chloride used for density separation
Sodium hydroxide	Fisher Scientific	10675692	Sodium hydroxide used for organic matter digestion
Soil auger	n/a	n/a	Length 30 cm; diameter 2 cm; material stainless steel
Ultra-high molecular weight polyethylene microplastic powder	Sigma Aldrich	434272	Polyethylene microplastic used to spike silicon dioxide, bentonite clay, and soil samples for recovery tests
Vacuum pump	Vacuubrand GmBH Co KG	ME 2C NT	Vacuum pump for vacuum filtration