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Title: Impacts of bridging complexation on the transport of surface-modified nanoparticles in saturated sand

item TORKZABAN, SAEED - University Of California
item WAN, JIAMIN - Lawrence Berkeley National Laboratory
item TOKUNAGA, TETSU - Lawrence Berkeley National Laboratory
item Bradford, Scott

Submitted to: Journal of Contaminant Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/21/2012
Publication Date: 5/28/2012
Citation: Torkzaban, S., Wan, J., Tokunaga, T.K., Bradford, S.A. 2012. Impacts of bridging complexation on the transport of surface-modified nanoparticles in saturated sand. Journal of Contaminant Hydrology. 136-137:86-95.

Interpretive Summary: Many natural and engineered particles in groundwater environments have negatively charged functional groups on their surfaces. The objective of this research was to investigate the interaction of these functional groups with clay in the presence of positively charged ions. Results indicated that calcium acted as a bridge between clay and virus sized particles to significantly enhance their retention in aquifers. This information will be of interest to scientists and engineers concerned with predicting the transport of viruses and engineered nanoparticles in soils and aquifers.

Technical Abstract: The transport of polyacrylic acid capped cadmium telluride (CdTe) quantum dots (QDs) and carboxylate-modified latex (CML) nanoparticles (NPs) was studied in packed columns at various electrolyte concentrations and cation types. The breakthrough curves (BTCs) of QDs and CML NPs in acid-treated Accusand showed significant amounts of increasing deposition with 0.5, 1, and 2 mM Ca2+, but only minute deposition at 50 and 100 mM Na+. Negligible QD and CML NP deposition occurred at 2 mM Ca2+ in columns packed with ultrapure quartz sand that was similar in size to the Accusand. These observations are not inconsistent with interpretations based on Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations of interaction energies. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis demonstrated that there were regions on the acid-treated Accusand covered with small amounts of clay that were absent on the ultrapure quartz sand. A salt cleaning method was therefore used to remove the clay from the acid-treated Accusand. The BTCs of QDs and CML NPs in this acid+salt treated Accusand exhibited much less deposition at any given Ca2+ concentration compared to those obtained from the acid-treated sand. SEM images showed that most of the QD deposited in acid-treated Accusand occurred on clay surfaces. Unlike our results with QD and CML NPs, negligible deposition of bare silica NPs occurred at 5 and 10 mM Ca2+ in acid-treated Accusand. The high deposition of QD and CML NPs was therefore attributed to cation bridging in which Ca2+ serves as a bridge between the cation exchange locations on the clay and carboxyl functional groups on the QD and CML NPs, which were absent on the bare silica NPs. Our results suggest that the transport of carboxylic ligand-modified NPs may be limited in subsurface environments because of the ubiquitous presence of clay and divalent cations.