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Microplastics Case Study – The Smaller Picture, Part 2

Olivia A. Perez

Editor’s Note: Read Part 1 of the Case Study.

Case Study Objective

The objective of this case study was to analyze post-cycle laundry wastewater captured from a household washing machine for the presence or absence of microplastics. To maintain and preserve sample integrity, the wastewater was examined and processed in an ISO 14644 Class 5 Cleanroom facility, AKA an ultra-clean controlled environment where sterility and contamination control are critical.

Particle Isolation and Characterization

A representative aliquot of the wastewater, approximately 3 mL, was examined under a stereomicroscope and particulate was observed floating in the sample liquid. The observed particulate was characterized as large flakes, small flakes, and fibers.

particulate floating in wastewater
Figure 1a: Optical images of particulate observed floating in the wastewater aliquot.

To isolate the particulate, the wastewater was filtered—the entire contents of the sample aliquot were filtered through a 0.6 µm polycarbonate membrane. The sample container was rinsed with 0.2 µm of filtered deionized particle-free water (PFW) and the rinsate was added to the sample filter.

Side note – Prior to filtration, a blank filtration was performed on the PFW to ensure that no particles were coming from the system. The blank was found to be acceptable.

polycarbonate filter surface after wastewater was filtered
Figure 1b: Optical images of the polycarbonate filter surface after the wastewater was filtered.

The sample filter was viewed under a light microscope, and multiple particle types were observed including various colored fibers, numerous colorless fibers, dark particles, spherical orange particles, and residue. To limit the scope of the project, particle size was limited to 10 µm and larger. Analyzed particle size ranged from approximately 10 µm to 2370 µm. Representative particles from each particle type were removed from the filter surface using a tungsten needle for further characterization and sample preparation.

images of the representative particles found in wastewater
Figure 2a: Optical images of the representative particles.
images of the representative particles found in wastewater
Figure 2b: Optical images of the representative particles.

Sample Preparation and Analytical Findings

Given their properties, each representative particle was analyzed by micro-Fourier Transform Infrared spectroscopy (FTIR). FTIR is a technique that identifies what a material is made of by measuring the amount of infrared light that is absorbed. This method tells us which chemical groups are present and in return we obtain a spectrum – a pattern of peaks and valleys unique to the material, AKA a fingerprint, if you will.

In preparation for micro-FTIR analysis, a portion of each particle was pressed thin onto a potassium bromide crystal. Fourteen particles were analyzed and most of the particles were found to be consistent with cellulose. Of the analyzed particles, none were found to be plastic.

FTIR spectrum of the purple fiber
Figure 3a: FTIR spectrum of the purple fiber consistent with cellulose.
FTIR spectrum of the dark orange fiber
Figure 3b: FTIR spectrum of the dark orange fiber consistent with protein.
FTIR spectrum of the dark particle
Figure 3c: FTIR spectrum of the dark particle consistent silica/silicate with an unsaturated hydrocarbon, a carbonyl-containing component, and possibly some protein.
FTIR spectrum of residue 1
Figure 3d: FTIR spectrum of residue 1 consistent with silicone, protein, and possibly a fatty acid salt similar to calcium stearate.

Conclusion

Surprisingly, no polyester fibers were detected. Thus, more experiments are needed to understand the degradation of synthetic textiles and their impact on the environment, especially at the micrometer and nanometer scales.

See Table 1 for a complete summary of particle characteristics and analytical findings.

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