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Surface transformation can affect the stability, reactivity, and toxicity of titanium dioxide (TiO2) nanoparticles (NPs) in water environments. Herein, we investigated the release kinetics of Si impurity frequently introduced during NP synthesis and the resulting effect on TiO2 NP transformation in aqueous solutions. The release of Si increased from 2 h to 19 d at three pHs with the order: pH 11.2 >= pH 2.4 > pH 8.2. The Si release process followed parabolic kinetics which is similar to diffusion controlled dissolution of minerals, and the release magnitude followed the order: 10 x 40 nm rutile > 50 nm anatase > 30 x 40 nm rutile. FTIR data indicated preferential dissolving of less polymerized Si species on NP surface. Surface potential and particle size of TiO2 NPs remained almost constant during the 42-day monitoring, implying the unaffected stability and transport of these NPs by the incongruent dissolution of impurities. (C) 2013 Elsevier Ltd. All rights reserved.