Research advancements and future needs of microplastic analytics: microplastics in the shore sediment of the freshwater sources of the Indian Himalaya

Abstract

Microplastics (MPs) are commonly considered pieces of plastic particles that are between 1 μm and 5 mm in size. It is an emerging contaminant and highly pervasive in different environmental compartment. Likewise, MPs have received huge attention from the scientific community, local citizens, and the media. Irrespective of drastic increases in the publications related to MPs, the data quality needs improvement.

MPs studies in the remote regions of the globe are still lacking. The Indian Himalaya hosts large, glacier-fed perennial rivers and high-altitude mountainous lakes that serve the largest populated nations. Nevertheless, studies of MPs in the Indian Himalaya are scarce. In this thesis, MPs in the shore sediment of the freshwater sources including the Brahmaputra River, the Indus River, Pangong Lake, Tsomoriri Lake and Tsokar Lake were studied. Three sampling campaigns for shore sediment collection from the rivers and lakes of the Indian Himalaya were conducted during years 2018–2019. The samples of MPs were pretreated using density separation and digestion procedures.

MPs within size range 20–5000 μm were detected in all the studied sites of the Brahmaputra and Indus rivers. Among the examined rivers, the highest concentration of MPs was detected in the Brahmaputra River, a concentration of 3505 MP/kg dw. In the studied remote lakes including the Pangong, Tsomoriri and Tsokar, MPs were detected within the size range 100–5000 μm. Among studied lakes, the highest concentration of MPs was detected in the Tsomoriri Lake with concentration of 3800 MP/kg dw. However, the reported MPs concentration should not be considered as an ideal value for MPs concentration in the studied area because MPs concentration varies depending upon the method of sampling, pretreatment, and analysis.

Holistic understanding of MPs in freshwater sources is important. The intercomparison among MPs lacks reliability due to the lack of harmonisation of the utilised methods. Therefore, a means to move towards an understanding of the heterogeneity of MPs and the implications of different pretreatment methods on concentrations of MPs was assessed. Variation in MP concentrations was demonstrated in the lake samples by using different methods of digestion procedure including chemical degradation and/with enzymatic degradation. MPs concentration largely varies in samples pretreated with different pretreatment methods. Therefore, the study demonstrated the significance of different methods used in MPs handling on their reported concentrations and the heterogenous nature of MPs.

The need for time-efficient and reliable analytical technique for MP analysis is crucial, especially for environmental samples consisting of various materials. In this context, an assessment for identification of MPs using Raman microspectroscopy was aimed particularly to minimize the analysis duration. The automatic selection features of the Raman imaging microscope were explored using spiked MPs in aquatic and solid samples. Strict contamination control measures, recovery tests and procedural blanks were utilised.

The automatic particle selection features of the Raman imaging microscope significantly reduce the analysis duration compared to the pixel-based analysis. Therefore, Raman microspectroscopy has the potential to contribute to a reliable MPs identification. Nonetheless, the potential drawback of underestimation and overestimation due to the automatic selection features must be further improved in the future.

Based on this thesis, it can be concluded and recommended that MPs in the Indian Himalaya require more in-depth good quality studies. Furthermore, practical steps must be taken to avoid improper disposal of waste that is the most prominent source of MPs in the studied freshwater sources. In the broader aspect of MPs research, the importance of reporting sufficient details of MPs is crucial to a reliable intercomparison of data and to be able to assess the possible impacts of pollution. The quality control and quality assurance measures are necessary for reliable MPs data. Moreover, it is recommended to report both MPs concentration with and without blank correction.