Automatic Preparation and Analysis of Microplastics in Soil

Significance of the Topic

Microplastics contamination is a major environmental concern, historically focused on marine ecosystems but increasingly recognized in terrestrial soils. Reliable methods for extracting and identifying microplastics from soil matrices are essential for monitoring pollution, evaluating ecological risks, and developing remediation strategies.

Objectives and Overview

This study demonstrates the automated preparation of soil samples for microplastic analysis using the Shimadzu MAP-100 device and subsequent Fourier Transform Infrared (FTIR) identification with the IRSpirit-TX spectrophotometer. Soil specimens from rice and vegetable fields were pretreated and analyzed to evaluate recovery efficiency and material characterization.

Methodology and Instrumentation

Soil pretreatment involved natural drying followed by sieving at 150 µm and 500 µm. Approximately 20 g of sieved soil was loaded into the MAP-100 reaction vessel strainer. The automatic protocol included:

• Oxidative digestion with 30 percent hydrogen peroxide for 3 days at 60 °C and 200 rpm stirring
• Density separation using a 5.3 mol per liter sodium iodide solution with 3 hours standing and 500 rpm stirring
• Three overflow cycles to isolate floating microplastics

Instrumentation:

• Shimadzu MAP-100 microplastic automatic preparation device
• Shimadzu IRSpirit-TX FTIR spectrophotometer with QATR-S diamond ATR attachment

FTIR measurement parameters:

• Resolution: 4 cm⁻¹
• Scans: 20
• Apodization: SqrTriangle
• Detector: TGS

Main Results and Discussion

Spiked polyethylene beads (1–1.2 mm) yielded average recoveries of 90 percent in the 500 µm fraction and 70 percent in the 150 µm fraction. Lower recoveries in the finer fraction were attributed to residual soil aggregates and incomplete separation during oxidation. Recovered candidate microplastics (300 µm–1 mm) were successfully identified as polyethylene via ATR-FTIR spectral matching. Additional signals indicated talc from agricultural films, cellulose, plant-derived materials, and inorganic silica, highlighting the presence of non-plastic particles.

Benefits and Practical Applications of the Method

Automated soil pretreatment offers:

• Reduced labor and improved reproducibility
• Enhanced safety by minimizing manual handling of chemicals
• Efficient recovery of microplastics across different soil types
• Integration with FTIR-enabled libraries for rapid polymer identification

This workflow supports standardized monitoring of microplastics in terrestrial environments for research, quality control, and regulatory compliance.

Future Trends and Potential Applications

Advances may include optimization for finer particle recovery, coupling with Raman spectroscopy for complementary analysis, and development of high-throughput automated platforms. Expanding spectral libraries and implementing machine learning algorithms will enhance identification accuracy and data processing speed. Broader adoption can facilitate large-scale soil pollution assessments and inform mitigation efforts.

Conclusion

The Shimadzu MAP-100 device effectively automates soil sample preparation for microplastic extraction after sieving, achieving high recovery rates for standard polyethylene beads. Subsequent FTIR analysis with IRSpirit-TX provided reliable polymer identification and distinction from natural organic and inorganic materials. This combined approach streamlines soil microplastic analysis, offering a robust solution for environmental monitoring and research.

References

1. Katsumi N., Okochi H. Accumulation of microcapsules derived from coated fertilizer in paddy fields. Chemosphere. 2021;267:129185.
2. Ikenoue T. Horizontal distribution of surface microplastic concentrations and water-column microplastic inventories in the Chukchi Sea, western Arctic Ocean. Science of the Total Environment. 2023;855:159564.
3. Ministry of the Environment. 2025 Environmental Technology Verification Program. March 2024.
4. Ministry of the Environment. River and Lake Microplastics Investigative Guidelines. Water Environment Management Division. March 2025.