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Identification of microplastics in wastewater samples by means of polarized light optical microscopy

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Many reports state the potential hazards of microplastics (MPs) and their implications to wildlife and human health. The presence of MP in the aquatic environment is related to several origins but particularly associated to their occurrence in wastewater effluents. The determination of MP in these complex samples is a challenge. Current analytical procedures for MP monitoring are based on separation and counting by visual observation or mediated with some type of microscopy with further identification by techniques such as Raman or Fourier-transform infrared (FTIR) spectroscopy. In this work, a simple alternative for the separation, counting and identification of MP in wastewater samples is reported. The presented sample preparation technique with further polarized light optical microscopy (PLOM) observation positively identified the vast majority of MP particles occurring in wastewater samples of Montevideo, Uruguay, in the 70–600 μm range. MPs with different shapes and chemical composition were identified by PLOM and confirmed by confocal Raman microscopy. Rapid identification of polyethylene (PE), polypropylene (PP) and polyethylene terephthalate (PET) were evidenced. A major limitation was found in the identification of MP from non-birefringent polymers such as PVC (polyvinylchloride). The proposed procedure for MP analysis in wastewater is easy to be implemented at any analytical laboratory. A pilot monitoring of Montevideo WWTP effluents was carried out over 3-month period identifying MP from different chemical identities in the range 5.3–8.2 × 103 MP items/m3.

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Authors wish to thank WWTP staff at Punta Carretas, Intendencia de Montevideo.


This work was financially supported by Agencia Nacional de Investigacion e Innovacion (ANII) by Project FCE_2016_1_127100. Sistema Nacional de Investigadores (SNI, ANII) and PEDECIBA Química are acknowledged for financial support.

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Correspondence to Andrés Pérez-Parada.

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Figure S1

Examples of OAPs-C microplastics from wastewater under PLOM. Different examples such as fibres, fragments and films are shown. The particles are shown under unpolarised light (particle on the left, pink background) and polarized (particle on the right, blue background). Under polarized light we can observe the phenomenon of birefringence that allows the primary identification of microplastics. (JPG 89 kb)

Figure S2

Examples of OAPs-W microplastics from wastewater under PLOM. Different examples such as fibres, fragments and films are shown. The particles are shown under unpolarised light (particle on the left, pink background) and polarized (particle on the right, blue background). Under polarized light differences to OAPs-C are exhibited. (JPG 988 kb)

Fig S3

SEM/EDX analysis of MPs in wastewater. Composition of selected particles (film, fibre and hexagon (W%: weight percent and A%: atomic percent). (JPG 1265 kb)

Fig S4

toilet paper cellulose under PLOM. (JPG 40 kb)

Table S1

(DOCX 11 kb)

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Sierra, I., Chialanza, M.R., Faccio, R. et al. Identification of microplastics in wastewater samples by means of polarized light optical microscopy. Environ Sci Pollut Res (2019).

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  • Microplastics
  • Wastewater
  • Water pollution
  • Polarized light optical microscopy
  • Raman microscopy