TY - JOUR

T1 - Settling of buoyant microplastic in estuaries

T2 - The importance of flocculation

AU - Laursen, Simon Nyboe

AU - Fruergaard, Mikkel

AU - Dodhia, Maya Shakunt

AU - Posth, Nicole Rita

AU - Rasmussen, Maja Bar

AU - Larsen, Marianne Nylandsted

AU - Shilla, Dativa

AU - Shilla, Daniel

AU - Kilawe, Jane James

AU - Kizenga, Hellen Joseph

AU - Andersen, Thorbjørn Joest

N1 - Funding Information: We would like to thank the Head of Chemistry Department, Dr. James Mgaya and the Dean, Faculty of Science, Dr. David Kacholi from Dar es-Salaam University College of Education (DUCE-UDSM) for providing logistic support and laboratory facilities during the research campaign. We would also like to thank Mr. Nzobela Emilian Ndunguru for driving us around the city of Dar es-Salaam and Mette Bjørn and Søs Marianne Ludvigsen for assistance in the laboratory and Tonci B. Zunic for aid and advice in interpreting the XRD results. Three anonymous reviewers are thanked for providing constructive comments that improved the final version of the paper. This work was supported by grants to the project TRACE (project no. 2-2020 ) by Geocenter Denmark , by the VILLUM FONDEN (grant 15397 , Cycling in the Plastisphere: the biogeochemical fate of marine microplastics), by the VILLUM FONDEN (project no. 42069 ), and the Independent Research Fund Denmark (grant 0217-00004B ). Funding Information: We would like to thank the Head of Chemistry Department, Dr. James Mgaya and the Dean, Faculty of Science, Dr. David Kacholi from Dar es-Salaam University College of Education (DUCE-UDSM) for providing logistic support and laboratory facilities during the research campaign. We would also like to thank Mr. Nzobela Emilian Ndunguru for driving us around the city of Dar es-Salaam and Mette Bjørn and Søs Marianne Ludvigsen for assistance in the laboratory and Tonci B. Zunic for aid and advice in interpreting the XRD results. Three anonymous reviewers are thanked for providing constructive comments that improved the final version of the paper. This work was supported by grants to the project TRACE (project no. 2-2020) by Geocenter Denmark, by the VILLUM FONDEN (grant 15397, Cycling in the Plastisphere: the biogeochemical fate of marine microplastics), by the VILLUM FONDEN (project no. 42069), and the Independent Research Fund Denmark (grant 0217-00004B). Publisher Copyright: © 2023 The Authors

PY - 2023

Y1 - 2023

N2 - Rivers and estuaries are regarded as major pathways of microplastic (MP) transport from terrestrial areas to marine ecosystems. Despite this knowledge on the transport dynamics and fate of MP in freshwater riverine and brackish estuarine waters is limited. Via ex situ settling experiments emulating the Msimbazi River and Estuary in Dar es-Salaam, Tanzania, we demonstrate that flocculation and subsequent settling of positively buoyant MP and fine-grained suspended sediment in riverine and estuarine waters are important for the environmental fate of the plastic particles. Our results show that settling velocities of MP and fine-grained sediment in estuarine water were between five and 21 times larger than in freshwater, explained by the increase in ionic strength that occurs when particles enter saline water. This confirms the concept of increased flocculation and settling of fine-grained particles as they are transported from freshwater to estuarine and marine waters. The implication is that land-based sources of small positively buoyant high-density polyethylene (HDPE) MP transported by rivers will tend to settle and accumulate in estuarine environments and thereby lead to a decrease in the overall load of MPs delivered to the wider marine environment. Thereby our results support the notion of estuaries as MP traps and that flocculation explains the trapping of large quantities of MP debris. Based on these findings we recommend that the interaction of MP with fine-grained sediment should be taken into account when transport models of this pollutant are established.

AB - Rivers and estuaries are regarded as major pathways of microplastic (MP) transport from terrestrial areas to marine ecosystems. Despite this knowledge on the transport dynamics and fate of MP in freshwater riverine and brackish estuarine waters is limited. Via ex situ settling experiments emulating the Msimbazi River and Estuary in Dar es-Salaam, Tanzania, we demonstrate that flocculation and subsequent settling of positively buoyant MP and fine-grained suspended sediment in riverine and estuarine waters are important for the environmental fate of the plastic particles. Our results show that settling velocities of MP and fine-grained sediment in estuarine water were between five and 21 times larger than in freshwater, explained by the increase in ionic strength that occurs when particles enter saline water. This confirms the concept of increased flocculation and settling of fine-grained particles as they are transported from freshwater to estuarine and marine waters. The implication is that land-based sources of small positively buoyant high-density polyethylene (HDPE) MP transported by rivers will tend to settle and accumulate in estuarine environments and thereby lead to a decrease in the overall load of MPs delivered to the wider marine environment. Thereby our results support the notion of estuaries as MP traps and that flocculation explains the trapping of large quantities of MP debris. Based on these findings we recommend that the interaction of MP with fine-grained sediment should be taken into account when transport models of this pollutant are established.

KW - Buoyant microplastic

KW - Estuary

KW - Flocculation

KW - Settling experiments

KW - Urban catchment

U2 - 10.1016/j.scitotenv.2023.163976

DO - 10.1016/j.scitotenv.2023.163976

M3 - Journal article

C2 - 37160184

AN - SCOPUS:85158064362

VL - 886

JO - Science of the Total Environment

JF - Science of the Total Environment

SN - 0048-9697

M1 - 163976

ER -