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Title: Release of quantum dot nanoparticles in porous media: Role of cation exchange and aging time

Author
item TORKZABAN, SAEED - Commonwealth Scientific And Industrial Research Organisation (CSIRO)
item Bradford, Scott
item WAN, JIAMIN - Lawrence Berkeley National Laboratory
item TOKUNAGA, TETSU - Lawrence Berkeley National Laboratory
item MASOUDIH, ARASH - Catholic University - Washington, Dc

Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2013
Publication Date: 9/16/2013
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2435.pdf
Citation: Torkzaban, S., Bradford, S.A., Wan, J., Tokunaga, T., Masoudih, A. 2013. Release of quantum dot nanoparticles in porous media: Role of cation exchange and aging time. Journal of Environmental Science and Technology. 47(20):11528-11536.

Interpretive Summary: There is concern that engineered nanoparticles (NPs), such as quantum dots, will eventually be released into the environment and pose a risk to ecosystem health. The objective of this study was to investigate the influence of changes in solution chemistry on the release of a representative nanoparticle (quantum dots) deposited in sand to flowing water. NP release was found to be sensitive to the solution chemistry composition and ionic strength. In particular, release of quantum dots was enhanced when adsorbed calcium was replaced by sodium on the sand surface, and then the ionic strength was reduced. The release amount was also dependent on the time that the quantum dots were exposed to calcium ions. A mathematical model was successfully developed and used to describe the observed release behavior. This information will be of interest to scientists and engineers concerned with predicting the fate of other nanoparticles, include pathogenic viruses, in soils and groundwater.

Technical Abstract: Understanding the fate and transport of engineered nanoparticles (ENPs) in subsurface environments is required for developing the best strategy for waste management and disposal of these materials. In this study, the deposition and release of quantum dot (QD) nanoparticles were studied in saturated sand columns. The QDs were first deposited in columns using 100 mM NaCl or 2 mM CaC12 solutions. Deposited QDs were then contacted with deionized (DI) water and/or varying Na+ concentrations to induce release. QDs deposited in 100 mM Na+ were easily reversible when the column was rinsed with DI water. Conversely, QDs deposited in the presence of Ca2+ exhibited resistance to release with DI water. However, significant release occurred when the columns were flushed with NaCl solutions. This release behavior was explained by cation exchange (Ca2+ in exchange sites were replaced by Na+) which resulted in the breakdown of calcium bridging. We also studied the effect of aging time on the QD release. As the aging time increased, smaller amounts of QDs were released following cation exchange. However, deposited QDs were subsequently released when the column was flushed with DI water. The release behavior was modeled using a single first-order kinetic release process and changes in the maximum solid phase concentration of deposited QDs with transition in solution chemistry. The results of this study demonstrate that the presence of carboxyl groups on ENPs and divalent ions in the solution plays a key role in controlling ENP mobility in the subsurface environment.