Quantification of Microplastics in Soil and Sediment Using Dry-Ice-Assisted Fractionation

Significance of the topic

Terrestrial microplastics in soils and sediments are of growing concern because they affect soil health, ecosystem services and potential human exposure pathways (for example via agriculture). Robust, reproducible analytical workflows are needed to generate comparable data for monitoring, risk assessment and management. This application note introduces a practical extraction technique—dry-ice-assisted froth fractionation—coupled with automated infrared chemical imaging to improve isolation, identification and quantification of microplastics from complex solid matrices.

Objectives and overview of the study

The study aimed to develop and validate a fast, reproducible protocol for extracting and quantifying microplastics (1 µm to <1,000 µm) from soil and sediment. Key goals were to maximize recovery across a range of polymer densities, minimize contamination and sample loss, implement an internal particle standard for per-sample quality control, and demonstrate automated, repeatable identification and sizing using the Agilent 8700 LDIR chemical imaging system.

Methodology

Workflow summary:

• Freeze-dry 5 g aliquots of soil or sediment.
• Perform dry-ice-assisted froth (foam) fractionation: add ~3.5 g dry ice and 0.1% Triton X-100 to the sample in a glass vial with an overflow spout; sublimation generates vigorous bubbling that transports microplastics into the foam and solvent layers.
• Use a biphasic mixture of saturated CaCl2 solution and hexane to enable density separation across a wide polymer density range.
• Collect foam/solvent overflow, treat with 30% H2O2 at 55 °C and 120 rpm for 16 hours to remove coextracted organic fines, then filter through 10 µm filters.
• Perform secondary CaCl2-based density separation with sonication and centrifugation (repeated cycles) to purify particles.
• Transfer particles from polycarbonate (PC) filter to IR (Kevley Mirr IR) slides using an ethanol-assisted, controlled peel-transfer technique for LDIR analysis.

Quality control and spikes:

• Particle-based internal standard: green polyethylene (PE) beads (210–250 µm) spiked into every sample to monitor extraction and transfer; recoveries used for per-sample QC (acceptance window 50–150%).
• Spike-and-recovery tests: polypropylene (PP) fibers and fragments of polyethylene terephthalate (PET) and polytetrafluoroethylene (PTFE) were used at low, medium and high particle loads.
• Procedural blanks, field blanks and laboratory control samples (LCS) were included to monitor contamination and method consistency.

Instrumentation used

• Agilent 8700 LDIR chemical imaging system operating from 975 to 1800 cm–1 with Agilent Clarity software (v1.7.17). Analysis used the automated "particle analysis" workflow, fast scan/sweep modes, and a modified microplastics spectral library.
• Continuous nitrogen purge (15–20 L/min) and temperature-controlled laboratory (22 °C ± 0.5 °C) to stabilize conditions.
• Glass vial with overflow spout (inner tube 115 mm length, 27.7 mm diameter, spout opening 102 mm from base) used for froth fractionation; centrifugation and sonication equipment for density separation; polycarbonate filters and Kevley Mirr IR slides for particle capture and transfer.

Main results and discussion

Instrument repeatability and detection:

• The 8700 LDIR produced consistent localization, sizing and identification across repeated runs and across days. Example repeatability tests showed size and count stability for standard particles and environmental slides with low coefficients of variation (CVs typically 4–11%).

Internal standard performance:

• PE bead recoveries across 74 samples averaged 81% with ±12% variation; recoveries ranged from 48 to 100% depending on sample type (soil mean 84%, sediment 82%, LCS 58%, blanks 73%).
• The PE bead standard served as a process check (not a correction factor) to flag sample-specific losses during extraction and transfer.

Spike-and-recovery performance:

• Medium and high spike levels for PP and PET generally yielded recoveries within the acceptance band of 60–140% for both soil and sediment matrices.
• At low spike levels, PTFE recoveries frequently fell below the 60% threshold, indicating that small absolute particle losses disproportionately affect recovery estimates when only a few particles are present.
• Overall mean recoveries (soil): low level PET 64 ± 10%, PP 72 ± 10%, PTFE 55 ± 17%; medium: PET 84 ± 8%, PP 86 ± 8%, PTFE 72 ± 8%; high: PET 75 ± 4%, PP 76 ± 8%, PTFE 70 ± 4%. Sediment results showed comparable trends.
• PTFE discoloration during extraction complicated visual confirmation, making it less ideal as a color-based spike tracer.

Environmental application:

• The protocol was applied to soils (chromosol, dermosol, hydrosol) and freshwater sediments from Victoria, Australia. Soil concentrations ranged from ~4,360 to 102,000 microplastics per kg; sediments ranged from ~41,400 to 127,000 microplastics per kg, with many particles in the 10–50 µm fragment size class.
• Commonly detected polymers included acrylonitrile butadiene styrene and polyamide, and fragment-shaped particles dominated sediment samples.

Benefits and practical applications of the method

• Improved isolation efficiency: dry-ice-assisted froth fractionation provides vigorous, surfactant-stabilized foam that concentrates particles and enhances separation from dense solid matrices.
• Broad polymer coverage: the combined use of CaCl2 saturation and hexane supports recovery across a wide density range (low- to high-density polymers).
• Automated, reproducible analysis: the Agilent 8700 LDIR system automates particle location, spectral identification and sizing, reducing subjectivity and increasing throughput.
• Per-sample QC: inclusion of a particle-based ISTD enables routine, sample-specific verification of extraction and transfer performance.
• Compatibility with routine monitoring: the workflow is practical for laboratory adoption, enabling research and routine monitoring to produce defensible, repeatable datasets.

Future trends and potential applications

• Standardization and inter-laboratory comparisons: wider adoption of particle-based ISTDs and automated imaging/spectral workflows will support harmonization of reporting and comparability of datasets.
• Smaller size detection and library expansion: improving spectral libraries and instrument sensitivity for sub-10 µm particles will extend environmental relevance, particularly for biological exposure studies.
• Integration with high-throughput sample processing: combining froth fractionation with robotics and automated slide handling could increase sample throughput for large monitoring programs.
• Method refinements for dense and fragile polymers: alternative spike materials or modified transfer approaches may be required to improve recoveries for small, high-density or discolored polymers (e.g., PTFE).
• Application to risk assessment: consistent, quantitative datasets from soils and sediments will support exposure modeling and policy development related to land-applied biosolids, agricultural soils and sediment remediation.

Conclusion

The dry-ice-assisted froth fractionation protocol combined with Agilent 8700 LDIR imaging provides a practical, reproducible workflow for extraction, identification and quantification of microplastics in soils and sediments. The method demonstrated acceptable recoveries for common polymers at medium and high spike levels, strong instrument repeatability, and useful sample-specific QC using a particle-based internal standard. While small, dense particles (and low-count spikes) remain challenging, the approach offers a scalable foundation for monitoring and research applications.

Reference

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