Transport and Retention of TiO2 Rutile Nanoparticles in Saturated Porous Media under Low-Ionic-Strength Conditions: Measurements and Mechanisms

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• 作者：Gexin Chen ; Xuyang Liu ; Chunming Su
• 刊名：Langmuir
• 出版年：2011
• 出版时间：May 3, 2011
• 年：2011
• 卷：27
• 期：9
• 页码：5393-5402
• 全文大小：923K
• 年卷期：v.27,no.9(May 3, 2011)
• ISSN：1520-5827

文摘

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, and the NP retention profiles exhibited a shift of the maximum NP retention segment from the end toward 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鈭扡andau鈭扸erwey鈭扥verbeek (DLVO) theory. Upon close examination of the results, it was found that the characteristics of the obtained transport breakthrough curves closely 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 reconformation through collision between NP aggregates and sand grains reduced the repulsive interaction energies of NP鈭扤P and NP鈭抯and surfaces, consequently accelerating NP deposition with transport distance and facilitating approaching NP deposition onto NPs that had already been deposited. It is further suggested that TiO₂ NP transport and retention are determined by the combined influence of NP aggregate reconformation associated with solution chemistry, travel distance, and DLVO interactions of the system.