Research On Aggregation And Deposition Characteristics Of Titanium Dioxide Nanoparticles In Aqueous Systems Under Different Scales

In 2015, nano-products valuing one trillion dollars is entering the market. Nano-products will enter the aquatic environments and water treatment system in the processes of production, consumption and discard. The aggregation and deposition of nanoparticles in aqueous solutions will change the morphology of nanoparticles significantly and reduce the content of nanoparticle exposed in the environments. Therefore, it is essential to study the aggregation and deposi tion processes, and reveal the aggregation and deposition mechanisms in the environments solutions and water treatment systems, to ensure the nanotechnology being a useful tool rather than a burden for the environments.Due to the limitations of experimental methods in the spatial and temporal scale, it is almost impossible to investigate the dynamic change processes of aggregation and deposition of nanoparticles（especially for small nanoparticle with a few nanometers） in the atomic level and the nanosecond or microsecond level. In the study, Brownian dynamics simulation, molecular dynamics simulation and experimental methods were used to investigate aggregation and deposition processes of nanoparticles under different scales from the views of dynamical fea tures, solution chemistry conditions and the interface microstructure.At the nanoscale, based on the Brownian dynamic simulation, the number of the aggregates and the size of the aggregates were investigated by using the DLVO theory to calculate the particle-particle interaction energy. The dynamic aggregation process of the TiO₂ nanoparticles under different particle size, p H conditions and ionic strength conditions were discussed. The number of aggregates reached a peak at the initial stage of the aggregation process. Next, the small aggregates attracted each other and formed a new larger aggregates. The larger aggregates can also break up to small aggregates in the aggregation process. The aggregation extent of TiO₂ nanoparticles were influenced strongly by the particle size. For the number and size of the aggregates, the ionic strength has a stronger influence than p H. Based on the experimental methods, the influence mechanisms of HA and SDBS on the aggregation of TiO₂ nanoparticle were discussed. Both the HA and SDBS can impede the aggregation process of TiO₂ nanoparticles. The interaction process can be divided into three steps: approach, aggregate and strongly interact.At the nanoscale, based on the Brownian dynamic simulation, the number of the deposits and the structure of the deposits were investigated by using the DLVO theory to calculate the interaction energy of the particle-surface. The dynamic deposition process of the TiO₂ nanoparticles under different particle size, p H conditions and ionic strength conditions were discussed. The TiO₂ nanoparticles or aggregates approached the surface of SiO₂, and then deposit on the surface of SiO₂. The deposits can be the single TiO₂ nanoparticles or the small aggregates, that is, formed the monolayer, double layer or multilayer deposition. The small aggregates can be tiled or vertical on the surface of SiO₂. The influence extent of particle size, p H and ionic strength on the deposition process of TiO₂ can be sort as below: ionic strength > particle size > p H. Based on the experimental methods, the HA on the deposition of TiO₂ nanoparticles and the sedimentation characteristics of TiO₂ nanoparticles in aqueous solutions were discussed with the DLVO theory and deposition theory. The results of column experiment showed that the HA can enhance the mobility of TiO₂ nanoparticles in porous medium. The adsorption efficiency a increased with the increasing of p H and decreased with the increasing of ionic strength, on the contrary of the largest migration distance maxL. In general, the influence mechanisms of HA on the mobility of TiO₂ nanoparticles in porous medium is dominated by the adsorption behavior between humic acid and the surface of TiO₂ nanoparticles and depositional medium. The results of sedimentation experiments showed that the TiO₂ nanoparticles are easy to sediment with the increasement of ionic strength and near to the p Hpzc. The DLVO theory were utilized to calculates the interaction energy between the TiO₂ nanoparticles under different p H and ionic strength conditions, which is consistent with the results of experiments. The humic acid can impede the sedimentation of the TiO₂ nanoparticles.Based on the molecular dynamic simulation, the micro interfacial interaction of homoaggregation between the TiO₂ nanoparticles, heteroaggregation between the TiO₂ nanoparticles and TNB, the deposition of the TiO₂ nanoparticle on the surface of SiO₂ were discussed. The 2 nm TiO₂ nanoparticles formed particle-particle bonding due to higher surface diffusion and strong particle-particle interaction. However, a large amount of water molecules absorbed on the surface of 3 nm and 4 nm TiO₂ nanoparticles impeded the direct bonding between particles. Water molecules and cations mediated the interaction between TiO₂ nanoparticle and TNB. In vacumm, the –COO- of TNB strongly attracted the OTiO₂ of TiO₂ nanoparticles by electrostatic interaction. In aqueous solution, the TNB formed the hydrogen bonds with TiO₂ nanoparticle through the attraction between Hphenol, HCOH and OTiO₂, with a horseshoe configuration. In the presence of cations, the TNB formed the hydrogen bonds with TiO₂ nanoparticle through the attraction between HCOH and OTiO₂, with a hinge configuration. The water layers on the surface of SiO₂ impeded the direct bonding between TiO₂ nanoparticle and SiO₂. However, the mediation of cation on the interaction between TiO₂ nanoparticle and SiO₂ is insignificant.