Automatic Preparation and Analysis of Microplastics in Soil

Importance of the Topic

Microplastics in soil pose a growing environmental concern due to their persistence and potential to affect ecosystems and human health. While research has largely focused on aquatic pollution, recent studies highlight the widespread occurrence of microplastics on land surfaces and agricultural fields. Reliable, reproducible methods for isolating and identifying soil microplastics are essential to assess contamination levels, guide remediation efforts, and inform policy.

Objectives and Study Overview

This application note evaluates an automated workflow for the pretreatment and analysis of microplastics in agricultural soil. Soil samples from rice and vegetable fields were processed using an automated preparation device (MAP-100) and characterized by Fourier transform infrared spectroscopy (FTIR) to assess recovery rates and identify polymer types in field-collected particles.

Methodology

• Soil sampling from rice and vegetable fields followed by natural drying.
• Sieving through 500 µm and 150 µm meshes to separate size fractions.
• Pretreatment using the MAP-100 device: oxidative digestion with 30% hydrogen peroxide (3 days at ~60 °C, 200 rpm stirring) and density separation with 5.3 mol/L sodium iodide (500 rpm stirring, 3 h standing, three overflows).
• Spiking samples with ten 1–1.2 mm polyethylene beads to evaluate recovery efficiency.

Used Instrumentation

• MAP-100 Microplastic Automatic Preparation Device (Shimadzu).
• IRSpirit-TX FTIR Spectrophotometer equipped with QATR-S diamond ATR accessory.
• LabSolutions IR software for spectral library searching and data management.

Main Results and Discussion

Recovery experiments demonstrated average capture rates of 90% for particles retained on the 500 µm sieve and 70% for those on the 150 µm sieve. The lower recovery at the smaller size was attributed to particle encapsulation by residual soil aggregates and incomplete separation. ATR-FTIR analysis confirmed the presence of polyethylene in all candidate microplastics, with talc additives detected in some field-derived samples. Non-polymeric residues such as cellulose, lectin, and silica were also identified, underscoring the need for optimized digestion and separation protocols.

Benefits and Practical Applications

Automated pretreatment with the MAP-100 streamlines labor-intensive steps, enhances reproducibility, and improves safety by minimizing manual handling of reagents. Coupled with FTIR analysis, this workflow supports standardized monitoring of soil microplastics for environmental assessment and compliance testing.

Future Trends and Opportunities

Advancements may include integration of micro-FTIR imaging, coupling with Raman spectroscopy or pyrolysis-GC/MS for detailed polymer characterization, and refinement of density separation media to target a broader size range. Expanded spectral libraries and machine-learning algorithms could further improve identification accuracy. Scaling automation for high-throughput screening in diverse environmental matrices offers promising research avenues.

Conclusion

The study demonstrates that the MAP-100 device, in combination with ATR-FTIR, provides a robust approach for isolating and identifying microplastics in soil samples from agricultural fields. This automated protocol achieves high recovery rates for standard polyethylene beads and enables compositional analysis of environmental particles, laying the groundwork for broader soil microplastic monitoring programs.

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. Sci Total Environ. 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, Environmental Management Bureau; March 2025.