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Aggregation, sedimentation, dissolution and bioavailability of quantum dots in estuarine systems
Citation:
Xiao, Y., K. Ho, R. Burgess, AND M. Cashman. Aggregation, sedimentation, dissolution and bioavailability of quantum dots in estuarine systems. To be Presented at Society of Environmental Toxicology and Chemistry (SETAC) 37th Annual Meeting, Orlando, FL, November 06 - 10, 2016.
Impact/Purpose:
This research gives environmental managers information on the fate and effects of CdSe quantum dots with a ZnS shell. These quantum dots have a sedimentation rate of 4-10 mm/day in seawater. In addition, humic acid addition further slowed the settling rate. Natural sunlight increased the dissolution rate. Based on the settling and dissolution rate, the benthic zone in marine systems is the most probable long-term destination of CdSe QDs. The acute toxicity appears to be largely due to the Cd ions dissolved from the QD; however, research on the toxic effects of particulate QDs should be studied in other animal models where QDs may be lodged in critical tissues such as gills or filtering apparatus, and Cd ions may be released and delivered more directly to those tissues.
Description:
Due to increasing use in flat screen applications, solar cells, ink–jet printing, and medical devices, cadmium-based quantum dots (QDs) are among the fastest growing classes of engineered nanomaterial. These wide-ranging consumer product applications and end of use disposal issues assure that QDs will eventually enter the marine environment. In an effort to understand the fate and transport of CdSe QDs in estuarine systems, the aggregation, sedimentation, dissolution, and bioavailability of CdSe QDs in seawater was investigated. The size of CdSe QDs increased from 40-60 nm to >1 mm within one hour once introduced to seawater, and the diffusion-limited aggregation led to highly polydispersed aggregates with loose structures. As a result, the sedimentation rate of CdSe QD aggregates in seawater was measured to be 4-10 mm/day, which was slow considering their relatively large size. Humic acid (HA), as a model natural organic matter, further increased the size and polydispersity of CdSe QDs, and slowed their sedimentation accordingly. Given the effect of light on CdSe QDs, natural sunlight and light filters were employed to simulate the photic conditions at different water depths in an estuarine system. It was observed that light played a vital role in promoting the dissolution of CdSe QDs and the release of dissolved Cd. The ZnS shell surrounding the CdSe core also significantly hindered the degradation of CdSe QDs into their ionic components. With sufficient light, the presence of HA increased the dissolution of QDs, while with relatively scarce light, HA alone did not significantly change the dissolution of QDs. Our results demonstrate that the benthic zone in marine systems is the most probable long-term destination of CdSe QDs due to aggregation and sedimentation, despite being affected by slower transport processes than expected. In addition, the benthic community might be exposed to both particulate and ionic forms of CdSe QDs. The bioavailability of QDs to epibenthic organisms was evaluated by using mysid Americamysis bahia as a model organism, and the 7-day LC50s of particulate QDs and dissolved Cd was 290 and 23 ppt, respectively. For this organism, the acute toxicity appears to be largely a result of dissolved Cd from the QDs; however, research on the toxic effects of particulate QDs should be conducted with other animal models where QDs may be lodged in critical tissues such as gills or filtering apparatus, and Cd ions may be released and delivered more directly to those tissues.