Quantification and characterization of microplastics in coastal environments: Insights from laser direct infrared imaging

The study identified and quantified nine plastic polymers frequently detected in the environment by collecting sediment and seawater samples from coastal areas in Auckland, New Zealand. Polymer types, size distributions, and number of microplastics (MPs) were analyzed using a laser direct infrared (LDIR) imaging technique. Compared to conventional spectroscopic or microscopic methods, LDIR enabled capturing and quantifying MPs in much lower size ranges (20–5000 μm). The results demonstrated the widespread occurrence of MPs in the Auckland coastal environment, with polyethylene terephthalate (PET) being the most frequently detected plastic polymer. MP contamination levels ranged from 13 to 83 particles per liter of coastal water and from 1200 to 3400 particles/kg of dry sand in beach sediments. Six additional locations were investigated to assess the contribution of MPs from stormwater drains to the coastal environment. The total count of identified MPs extracted from sediments near stormwater drains reached a maximum of 18,000 particles/kg of dry sand, representing an order of magnitude increase compared to MP levels found in beach sediments at the same location. In contrast to the prevalence of PET and polyamide observed in beach sediments and coastal waters, polyurethane and polyethylene emerged as the predominant plastic polymers in the vicinity of stormwater drain sediments, implying that the variation could potentially stem from distinct sources of plastics. This significant disparity in quality and quantity underscored the potential link between urban runoff and MP pollution in marine ecosystems. A sample preparation method using 100 g sediment samples was developed and used to assess and compare MPs detection in sediment samples. The commonly used 5 g sample method showed higher extraction efficiency and better detection of the most abundant MPs, but the new 100 g method enabled the detection of previously missed, less abundant plastics.