Loadings, Transformations and Remobilizations of Silver in a Hypoxia Urban Estuary

Pei-Yu Lin1, Chih-Ping Lee1, Cheng-Shiuan Lee2,3, Tsai-Luen Yu4, Liang-Saw Wen1, Kuo-Tung Jiann5 and Chuan-Chou Shen6, (1)National Taiwan University, Institute of Oceanography, Taipei, Taiwan, (2)National Taiwan University, Taipei City, Taiwan, (3)Stony Brook University, Stony Brook, United States, (4)National Taiwan University, Research Center for Future Earth, Taipei, Taiwan, (5)National Sun Yat-sen Univ., Kaohsiung, Taiwan, (6)National Taiwan University, High‐precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, Taipei, Taiwan
Abstract:
Silver (Ag) is a toxic element, which can be accumulated in organisms and cause lethal consequences. In recent years, silver nanoparticles have been engineered and extensively used, causing silver concentrations in nature water getting much higher than before and still rising.

As studies showed, silver can be removed to sediments at a fairly rapid rate in an estuarine system. However, little is known about the release of silver from sediments due to resuspension and estuarine turbidity maximum (ETM) formation. This study is focused on the transformation of dissolved, colloidal, and particulate silver during the process of sediment resuspension. Water samples were collected in November of 2017 from Danshuei estuary. Sampling is grouped in two parts－time series sampling at a fixed station for resuspension event, and at different geographic locations for spatial variation. Methods of in-situ filtration, cross flow filtration, ion-exchange, solid phase extraction, and chemical leaching were used. HR-ICPMS and GFAAS were used to measure silver concentrations in each isolated fractions.

The results indicated that the dissolved Ag exhibited a non-conservative estuarine removal mixing pattern, but also was influenced by sediment resuspensions. Moreover, 10-90% of dissolved silver fraction was in the colloidal phase, and there was a strong correlation between colloidal Ag and colloidal Fe. Ag concentrations in truly dissolved fractions (<1kDa) had a significant correlation with chloride concentrations, which was proved by mixing and thermodynamic speciation modelling (MINEQL+ Version 5.0). Dynamic changes and remobilizations were found in truly dissolved fractions, which were due to competition reactions of silver chloride complex formations, desorption from re-suspended particulate matter, and colloidal pumping during ETM and resuspension. Chemical leaching and elemental analysis also indicated that these re-suspended particles were mostly anthropogenic laced. It is clear that the behavior of Ag in an urban estuary is dynamic and much more complicated. Future studies should aim to quantify the effects of sediment resuspension and colloidal pumping in order to better modulate its environmental impact.

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