Aggregation and transport of nano-TiO2 in saturated porous media: Effects of pH, surfactants and flow velocity

Transport of manufactured nano-TiO2 in saturated porous media was investigated as a function of morphology characteristics, pH of solutions, flow velocity, and the presence of anionic and non-ionic surfactants in different concentrations. Surfactants enhanced the transport of nano-TiO2 in saturated porous media while a pH approaching the point of zero charge of nano-TiO2 limited their transport. The deposition process, a retention mechanism of nano-TiO2 in saturated porous media was impacted by surfactant and pH. In Dispersion 1 systems (pH 7), the size of the nano-TiO2 aggregates was directly related to the presence of surfactants. The presence of non-ionic surfactant (Triton X-100) induced a size reduction of nano-TiO2 aggregates that was dependent on the critical micelle concentration. In Dispersion 2 systems (pH 9), the stability provided by the pH had a significant effect on the size of nano-TiO2 aggregates; the addition of surfactants did impact the size of the nano-TiO2 aggregates but in less significance as compared to Dispersion 1 systems. The electrostatic and steric repulsion forces in connection with the size of nano-TiO2 aggregates and flow velocity impacted the single-collector efficiency and attachment efficiency which dictated the maximum transport distance of nano-TiO2 for the Dispersion 1 and Dispersion 2 systems. By doubling the flow velocity at pH 9, the No Surfactant, 50% CMC Triton X-100, 100% CMC Triton X-100 and 100% CMC SDBS dispersion systems allowed nano-TiO2 to attain maximum transport distances of 0.898, 2.17, 2.29 and 1.12 m, respectively. Secondary energy minima played a critical role in the deposition mechanisms of nano-TiO2. Nano-TiO2 deposited in the secondary energy wells may be released because of changes in solution chemistry. The deposition of nano-TiO2 in primary and secondary energy minima, the reversibility of their deposition should be characterized to analyze the transport of nanoparticles in porous media. This is necessary to assess the risk of nanoparticles to the environment and public health. (C) 2010 Elsevier Ltd. All rights reserved.