Abstract
We investigate the physical- and photodegradation of microplastic (MP) particles and attempt to understand which MP physicochemical properties exert the most significant influence on these processes. Polystyrene (PS) and polyethylene (PE) MPs are used to study the effect of polymer composition on degradation, whilst particles of ~10 µm and ~200 µm are used to investigate the role of particle size on degradation. The 10 µm MPs are synthesised at this size range as model spherical reference materials. The 200 µm MPs are produced by cryogenically milling pristine low density PE (LDPE) and general purpose PS (GPPS) pellets. The physicochemical properties of all four MPs were comprehensively characterised prior to use and methods developed for monitoring changes in these properties during the two different degradation studies. Scanning electron microscopy is used to provide information about MP particle size, shape and morphology. Pyrolysis GC-MS is used to identify the main chemical additives present in each of the test MPs. ATR-FTIR was used to verify the polymer composition of each material and provide a reference point for studying photochemical transformation.
Physical degradation is studied by placing the MPs in conical flasks containing sterilised seawater and reference sediment. Samples are placed on an orbital shaker (150 rpm), in the dark for up to 56 days. Prior to analysis, the MP fraction is isolated using density separation. The reference sediment is pre-treated with the density separation method to minimise the presence of inorganic particulates in the MP fraction. Changes in the average particle size distribution are determined using optical light microscopy and manual counting (200 µm particles) or by Coulter Counter (10 µm particles). Photodegradation studies are conducted by placing the MPs in custom-made quartz tubes with sterilised seawater. Samples are fixed on a rocking table located directly beneath an Atlas SUNTEST CPS+ instrumented fitted with a xenon lamp and sunlight filter (UV and visible; 280-800 nm), and exposed continuously at 600 W/m2 for up to 56 days. Following isolation by filtration, the MPs are analysed by ATR-FTIR in order to study changes in the surface chemistry resulting from photochemical transformation. In both degradation studies, pyrolysis GC-MS is used to investigate the degree of additive leaching through studying changes in the ratio between additive peaks and those of the main polymer matrix.
Physical degradation is studied by placing the MPs in conical flasks containing sterilised seawater and reference sediment. Samples are placed on an orbital shaker (150 rpm), in the dark for up to 56 days. Prior to analysis, the MP fraction is isolated using density separation. The reference sediment is pre-treated with the density separation method to minimise the presence of inorganic particulates in the MP fraction. Changes in the average particle size distribution are determined using optical light microscopy and manual counting (200 µm particles) or by Coulter Counter (10 µm particles). Photodegradation studies are conducted by placing the MPs in custom-made quartz tubes with sterilised seawater. Samples are fixed on a rocking table located directly beneath an Atlas SUNTEST CPS+ instrumented fitted with a xenon lamp and sunlight filter (UV and visible; 280-800 nm), and exposed continuously at 600 W/m2 for up to 56 days. Following isolation by filtration, the MPs are analysed by ATR-FTIR in order to study changes in the surface chemistry resulting from photochemical transformation. In both degradation studies, pyrolysis GC-MS is used to investigate the degree of additive leaching through studying changes in the ratio between additive peaks and those of the main polymer matrix.
