Transport Of Nano-TiO₂ In Soil And The CO-Transport Mechanisms Of Pb With Nano-TiO₂

Based on the rapid development of nanotechnology and gradually understanding of nano-materials ecotoxicity by people, the environmental behavior of nano-materials containing migration, transformation, diffusion and so on has become a hot research topic. Because of the small size and large specific surface area, nano-materials generally exhibit strong moving ability, moreover, it can strongly interactions with heavy metals. If nano-materials act as a carrier for the transport of heavy metals, the environmental risk of nano-materials itself and heavy metals will greatly be increased. However, up till now there has been little investigation concerning the co-transport behavior of heavy metals with nano-materials in natural soil.In this thesis, we chosed TiO2 nanoparticles (nTiO2) and Pb as research objects. The effect of surfactants on the stability and mobility of nTiO2 was comprehensive investigated, deposition mechanisms of nTiO2 on soil were explored through the two site kinetic model and XDLVO/DLVO theory. By analyzing Pb2+ concentration and form in effluent associated with the release of soil colloid and the adsorption behavior of Pb2+ on soil and nTiO2, the effect mechanism of fulvic acid and ionic strength on the co-transport of nTiO2 and Pb2+ in soil was revealed.The principal conculsions were obtained as follows:The effect mechanism of soil particles on the stability of nTiO2 both in the absence and in the presence of surfactants was systematically investigated in this study. Results showed that soil particles significanlty destabilize nTiO2 in aqueous. In the absence of surfactnats, the adsorption of nTiO2 on large soil particles was mainly responsible for the destabilization and sedimentation of nTiO2.In the presence of surfactants, surfactants significantly enhanced the destabilization effect of soil on the stability of nTiO2. On the one hand, surfactants enhanced the sedimentation rate of soil which led to the enhanced destabilization of nTiO2.On the other hand, the large adsorption of surfactants on soil was also responsible for the co-deposition of surfactant and nTiO2 on soil.Compared the breakthrough curves of surfactant-nTiO2 with bare nTiO2 in soil, results showed that nonionic surfactant (TX-100), anionic surfactant (SDBS) and cationic surfactant (CTAB) were all inhibited the migration nTiO2 in soil. The inhibition order from strong to weak was CTAB>SDBS>TX-100。The effluent recovery of nTiO2 was decreased from 54.3% to 39.2%～43.8% (TX-100)、34.1%～36.9%(SDBS)、 0.3%～2.1% (CTAB), respectively.The transport ability of surfactants in soil was relatively poor, the transport ability of surfactants in soil had an order of SDBS>TX-100> CTAB, and the effluent recoveries was 0～44%(SDBS).0～19% (TX-100)、0%(CTAB), respectively, indicating that the strong retention of surfactants on soil was importantly responsible for the decrease of mobility of nTiO2 in soil.Results from adsorption experiments of Pb on soil and nTO2 demonstrated that Ionic strength, FA and soil solution would decrease the adsorption of Pb on soil and nTiO2. Although the adsorption (79.69 mg L-1) of Pb on nTiO2 was significantly decreased in the presence of soil solution, it was still much more than the adsorption of Pb on soil (5.59 mg g-1), providing the possibility of co-transport of Pb with nTiO2 in soil.Common environmental electrolyte (NaCl) and FA on the co-transport of nTiO2 and Pb were investigated in this study. Results showed that the stable plateau concentration (C/Co) of nTiO2 decreased from 0.85 to 0.42 when the IS increased from 0 to 0.5 mM, while it increased to 0.95 in the presence of 10 mg L-1 FA. Model predicted that the maximum transport distance of nTiO2 in control experiment was 425 cm, and it decreased to 79.6 cm when the system contained 0.5 mM NaCl, however, it significantly increased to 1346.7 cm when the influent in presence of 10 mg L-1 FA.In the absence of nTiO2, negligible breakthrough of Pb was observed in the bottom of soil column, only 0.4% of the initial concentration (5.1 μg L-1). However, when nTiO2 was present in the influent, significant amount was detected in the effluent, reached 11.9% of the initial concentration (156.8 μg L-1). Moreover, only very low concentrations of Pb were detected in a soluble form, demonstrating that combined state was principally responsible for Pb transport in soil. Pb breakthrough was 7.0% (63.3μg L-1) when the IS of influent nTiO2 suspension increased to 0.5 mM and it increased greatly to 22.8%(179.6 μg L-1) when the influent contained 10 mg L-1 FA.