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Transport and Retention of TiO2 Rutile Nanoparticles in Saturated Porous Media at Low-Ionic-Strength Conditions: Measurements and Mechanisms
Citation:
Chen, G., X. Liu, AND C. SU. Transport and Retention of TiO2 Rutile Nanoparticles in Saturated Porous Media at Low-Ionic-Strength Conditions: Measurements and Mechanisms. LANGMUIR. ACS Publications, Washington, DC, 27(9):5393-5402, (2011).
Impact/Purpose:
Journal article for Langmuir
Description:
The mechanisms governing the transport and retention kinetics of titanium dioxide (TiO2, rutile) nanoparticle (NP) aggregates were investigated in saturated porous media. Experiments were carried out under a range of well-controlled ionic strength (from DI water up to 1 mM) and ion valence (NaCl vs. CaCl2) comparable to the low end of environmentally relevant solution chemistry conditions. Solution chemistry was found to have a marked effect on the electrokinetic properties of NP aggregates and the sand – and on the resulting extent of NP aggregate transport and retention in the porous media. Comparable transport and retention patterns were observed for NP aggregates in both NaCl and CaCl2 solutions, but at much lower ionic strength with CaCl2. Transport experimental results showed temporal and spatial variations of NP aggregate deposition in the column. Specifically, the breakthrough curves displayed a transition from blocking to ripening shapes, while the NP retention profiles exhibited a shift of the maximum NP retention segment from the end towards the entrance of the column gradually with increasing ionic strength. Additionally, the deposition rates of the NP aggregates in both KCl and CaCl2 solutions increased with ionic strength, a trend consistent with traditional Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Upon close examination of the results, it was found that characteristics of the obtained transport breakthrough curves well followed the general trends predicted by the DLVO interaction energy calculations. However, the obtained NP retention profiles were found to deviate severely from the theory. We propose that a NP aggregate re-conformation through collision between NP aggregates and sand grains reduced the repulsive interaction energies for NP – NP and NP – sand surfaces, consequently accelerating NP deposition with transport distance and facilitating approaching NPs deposition onto NPs that had been already deposited. It is further suggested that TiO2 NP transport and retention is determined by the combined influence of NP aggregate re-conformation associated with solution chemistry and travel distance and DLVO interactions of the system. This research was funded by the National Nanotechnology Initiative through the U.S. Environmental Protection Agency (EPA). This article has not been subjected to internal policy review of the U.S. EPA. Therefore, the research results do not necessarily reflect the views of the agency or its policy. The authors acknowledge Stephanie Burrage for the laboratory assistance (funded by the U.S. EPA Environmental Research Apprenticeship Program (ERAP) at the Robert S. Kerr Environmental Research Center).
