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. 2021 Mar 11;13(6):871.
doi: 10.3390/polym13060871.

Optical Monitoring of Microplastics Filtrated from Wastewater Sludge and Suspended in Ethanol

Benjamin O Asamoah  1 , Pauliina Salmi  2 , Jukka Räty  3 , Kalle Ryymin  4 , Julia Talvitie  5 , Anna K Karjalainen  4 , Jussi V K Kukkonen  6 , Matthieu Roussey  1 , Kai-Erik Peiponen  1
Affiliations

Optical Monitoring of Microplastics Filtrated from Wastewater Sludge and Suspended in Ethanol

Benjamin O Asamoah et al. Polymers (Basel). .

Abstract

The abundance of microplastics (MPs) in the atmosphere, on land, and especially in water bodies is well acknowledged. In this study, we establish an optical method based on three different techniques, namely, specular reflection to probe the medium, transmission spectroscopy measurements for the detection and identification, and a speckle pattern for monitoring the sedimentation of MPs filtrated from wastewater sludge and suspended in ethanol. We used first Raman measurements to estimate the presence and types of different MPs in wastewater sludge samples. We also used microscopy to identify the shapes of the main MPs. This allowed us to create a teaching set of samples to be characterized with our optical method. With the developed method, we clearly show that MPs from common plastics, such as polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), and polyethylene (PE), are present in wastewater sludge and can be identified. Additionally, the results also indicate that the density of the plastics, which influences the sedimentation, is an essential parameter to consider in optical detection of microplastics in complex natural environments. All of the methods are in good agreement, thus validating the optics-based solution.

Keywords: Raman spectroscopy; laser speckle pattern; microplastics; sedimentation; sludge; transmittance; wastewater.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Micrograph of microplastics (MPs) during preparation (left) with different shapes and sizes and those identified in the measured sample (middle). In the white circle, a hook-shaped MP is highlighted, and the other black spots are air bubbles and unidentified organic materials (also in the right image).
Figure 2
Figure 2
(a) Speckle pattern of samples on a rough glass surface for samples of ethanol (left) and MPs filtrated and preserved in ethanol (FEMPs) (right). The recorded static images (patterns) are very similar to the naked eye for both samples. (b) Basic optical experimental setup for recording the beam profile after propagation through the FEMP sample.
Figure 3
Figure 3
Specular reflection recorded with the diffractive optical element (DOE)-based sensor for the smooth glass (SG) disk base only for the pure ethanol (Ethanol) and FEMP (MPs preserved in ethanol) samples.
Figure 4
Figure 4
Transmission spectra of (a) pure ethanol (black curve) and the FEMP (MPs preserved in ethanol, red curve) samples and (b) differences between the two spectra (ΔT).
Figure 5
Figure 5
Sedimentation: (a) Measured transmittance (red dots) of the FEMP (MPs preserved in ethanol) samples with time at λ = 800 nm. The red curve is an exponential fit of the data. (b) Calculated speckle contrast from the measured speckle pattern using the diffractive optical element (DOE)-based sensor with a rough glass disk base and 700 μL volume of the samples. (c) Recorded patterns of the light beam transmitted through (i) pure ethanol and FEMPs after shaking at (ii) 0, (iii) 60, and (iv) 220 s with the charge-coupled device (CCD) camera.
See this image and copyright information in PMC

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