Fate and control of road dust associated microplastics in stormwater
Creators: name: Monira, Sirajum
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400513
subject: Water resources engineering
classificationCode: 400599
subject: Civil engineering not elsewhere classified

description: Microplastic (MP), the tiny plastic debris, which is smaller than 5 mm in size, has become a major emerging class of pollutants representing significant eco-toxicological risks for ecosystems and marine environments. Among the various sources, road dust is the major source of MP pollution in the marine environment. Road dust-associated MPs are mainly derived from the wear of vehicle tyres, road marking paints, polymer-modified bitumen and the crushed plastics on the surface of the road. Due to the low density and small particle size, road dust-associated MPs can be easily released into local waterways during runoff events, which ultimately threaten the water environment.

The aim of this study was to provide a comprehensive understanding of the fate and control of MPs in stormwater. The specific objectives were to (i) identify, classify and quantify MPs in road dust and stormwater, and (ii) assess the performance of the chemical coagulation process for MPs removal from stormwater. In this study, road dust and stormwater samples were collected under various land use conditions such as residential and industrial areas in Melbourne metropolitan city, Australia. Visual identification and quantification of MPs were conducted by a light-emitting diode microscope. Fourier-transform infrared and scanning electron microscopy were used to identify the polymer types and morphologies of the MP particles, respectively. The main polymeric materials detected in both road dust and stormwater samples were low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), polyester and polyamide. The dominant shape of MP was fibre and accounted for 53% and 74% in road dust and stormwater samples, respectively. A significantly higher level of MPs was detected in road dust samples from the industrial (1130 particles/kg of dust) than the residential area (520 particles/kg of dust). The study also found an average of 26 particles/L and 17 particles/L in stormwater samples from industrial and residential sites, respectively. Most of the MP size was less than 2 mm in both road dust and stormwater samples.

Effective removal of MPs from stormwater using conventional stormwater management processes (e.g., constructed wetland) remains a challenge. Therefore, this study has investigated the performance of the coagulation process for the removal of different types of MPs such as LDPE, HDPE and PP from synthetic stormwater. The effect of different types of coagulants such as alum and polyacrylamide (PAM), dosages of coagulants, solution pH and weathering conditions was investigated for MPs removal. A combination of alum and PAM performed better than standalone coagulant. Combined alum (50-200 mg/L) and PAM (5-20 mg/L) coagulant could remove 60% of LDPE, 54% of HDPE and 79% of PP. The highest removal for the combined coagulant was due to a strong hydrophobic interaction between positively charged alum and negatively charged MPs which further enhanced the interaction by anionic PAM. It is also found that coagulation was highly effective to remove weathered MP particles as the removal efficiency of LDPE, HDPE and PP was 92%, 84% and 96%, respectively. This was likely due to the changes of physiochemical properties and surface morphology of MPs by weathering process which increased their interaction with flocs and provided a high removal efficiency of MP.

Overall, this study has demonstrated the occurrence of road dust-associated MPs in stormwater, and the chemical coagulation process could be a reliable method for MPs removal from stormwater. Therefore, the treated water can be used either for households, sanitation, industrial and agricultural purposes, or alternatively, for safe discharge into open waterways which eventually ends up in the water environment.