Formation of microscopic particles during the degradation of different polymers
Introduction
It has recently been shown that indicators of anthropogenic activity in lake sediments include a complex combination of materials, including plastics, and that these indicators differ greatly from Holocene signatures (Waters et al., 2016). As such, the environmental degradation of different plastic materials is of great interest. Conventional understanding regarding plastic degradation processes is derived from laboratory studies that often investigate a single mechanism of degradation such as photo-, thermal-, or bio-degradation (Lambert et al., 2013b). These studies have tended to focus on thin films and characterise degradation in terms of weight loss, changes in tensile strength, breakdown of molecular structure, and the identification of specific microbial strains to utilize specific polymer types. Presently, little is known about the formation of microscopic particles during the degradation of different plastic materials. This is an important issue given the current interest in the environmental occurrence and effects of microplastics that are generally classified as having a diameter less than 5 mm (GESAMP, 2015). In addition, the formation of particles below the threshold of visible detection, such as those at the nanoscale, during plastic degradation is also considered highly likely (Andrady, 2011, Lambert et al., 2014, Mattsson et al., 2015).
Analytical methods for particle characterisation include Coulter Counter and nanoparticle tracking analysis (NTA). Coulter Counter is a conductivity based technique that is able to measure particles ranging from 0.4 to 1200 μm diameter depending on the size of aperture used (Rhyner, 2011). The sample of interest is diluted in a weak electrolyte solution and is then passed through the aperture concurrent with an electrical current. The voltage applied across the aperture is known as the sensing area and as particles pass through the aperture they displace their own volume of electrolyte and alter the impedance (Demeule et al., 2010, Narhi et al., 2009). This change in impedance produces a pulse that is directly proportional to the volume of the particle and this converted signal enables the instrument to calculate particle size and size distribution (Demeule et al., 2010, Narhi et al., 2009).
NTA visualizes, measures, and characterises particles in the 30–2000 nm size range through the use of a laser beam to illuminate particles (Filipe et al., 2010). The NTA technique is based on the tracking of single particles and the software relates the rate of Brownian motion to particle size. The scattered light is captured using a digital camera and the motion of each particle is tracked from frame to frame by the NTA software (Kramberger et al., 2012). The rate of particle movement is related to a sphere equivalent hydrodynamic radius as calculated through the Stokes-Einstein equation and particle size is observed on a particle-by-particle basis (La Rocca et al., 2014). The principles of NTA are further described in Hole et al., 2013, Gillespie et al., 2011, and Gallego-Urrea et al. (2010). The NTA platform has previously been used to investigate the particle formation during the degradation of natural rubber latex (Lambert et al., 2013a) and polystyrene (Lambert and Wagner, 2016).
The size distribution of particles in aquatic environments is an important property that influences many natural processes, such as the scattering and absorption of light in the water column, the exchange of substances between solid and liquid phases, and the transportation of substances to sediments, as well as their interactions with biological processes (Jonasz, 1983, Reynolds et al., 2010). A particle size distribution (PSD) can be defined as the average number of particles within a given size classification (Reynolds et al., 2010). In addition, modelling the size distribution of particles is often useful when highly variable particle populations are expected as they can provide a more complete description of PSD properties (Hwang et al., 2002). Therefore, an investigation of PSD of plastic particles formed during degradation processes is of particular interest, and representing the particle concentration data as a mathematical function may provide an opportunity to search for materials with similar degradation profiles.
In this study, we aimed to investigate the formation of microscopic particles during the degradation of different plastic materials, under aqueous conditions. To do this, a weathering chamber was used to accelerate the degradation process, and seven plastic materials were compared that represented five different polymer types. The materials selected consisted of two plastic pellets (polyethylene (PE) and polypropylene (PP)), and five consumer plastics including two types of PP packaging materials, a polystyrene (PS) coffee-to-go lid, a polyethylene terephthalate (PET) water bottle, and a polylactic acid (PLA) beverage cup. The main objective was to characterise the formation of particles from 30 nm to 60 μm in diameter and to explore the effects of polymer type on particle formation. A second objective was to further examine the particle distribution data generated by NTA within the context of commonly used PSD modelling approaches.
Section snippets
Test materials and their preparation
The plastic pellets used in this work were purchased from a commercial source and the consumer plastics were sourced from commercial retailors in Darmstadt, Germany (supporting information (SI) Table S1). The polymer type of the consumer plastics were identified through their recycling code, and the identification of all materials was further confirmed using ATR-FTIR spectroscopy (Attenuated Total Reflection-Fourier Transform Infra-Red, PerkinElmer UATR Two; the individual spectra are available
Analysis of laboratory blank samples
A laboratory blank consisting of demineralised water was created on the day of each sample collection to document any contamination that may result from sample handling and analytical processes (SI Fig. S2). In the largest size class (2–60 μm) mean particle concentrations ranged from a low of 425 particles ml−1 on day zero to a high of 10,164 particles ml−1 on day 14. In the 0.6–18 μm size class mean particle concentrations ranged from 5907 particles ml−1 on day 28–43,107 particles ml−1 on day
Discussion
The present study aimed to investigate the formation of microscopic particles during the degradation of different plastic materials under accelerated weathering conditions in the laboratory. All plastics generated increasing numbers of particles over time, with PS and PLA often generating higher particle concentrations than other polymers. In addition, the replicate measurements become increasingly variable as time progressed. However, as evidenced by PCA, PLA clusters individually particularly
Acknowledgments
This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 660306. We would also to thank Dr Robert Tampé at the Institute of Biochemistry, Goethe University Frankfurt for access to the NanoSight.
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