Study On Aggregation And Deposition Characteristics Of Titanium Dioxide Nanoparticles In Aqueous Systems

With the rapid development of nanotechnology, nanomaterials are widely utilizedin industry and our everyday life. It is inevitable that large amount of nanoparticles willfind their way into the environment. As one of the most popular nanomaterials,nano-TiO₂may enter the environment both directly and indirectly, which will bringpotential harm to the environment and ecology. The toxicity character and exposurepossibility of nano-TiO₂are in close relationship with its behavior in water. So it is vitalto investigate the transport pattern of nano-TiO₂in aqueous systems.Aggregation and deposition are two key processes determining the mobility ofnano-TiO₂in water. This study is intended to investigate the particle character ofnano-TiO₂in water and the effect of chemistry conditions on the aggregation anddeposition characteristics of nano-TiO₂. Theoretic methods are used to quantitativelydescribe the transport ability of nano-TiO₂in order to have a deeper understanding ofthe characteristics of nano-TiO₂in aqueous systems. This study can also providereference for removal of nano-TiO₂by conventional water treatment processes.First, XRD, AFM and Nano-sizer were used to investigate the crystal form anddistribution chatracter of nano-TiO₂in water. The result showed that nano-TiO₂presented in water as aggregates. The aggregate size were about180⁶00nm,ultrasonication and addition of SDBS could enhance the dispersion of nano-TiO₂, butthe aggregates could not be seperated into primary particles.The point of zero charge of nano-TiO₂was5.2, when the pH value was far frompHpzc, the stability of nano-TiO₂was enhanced, it took more effort for the nano-TiO₂toaggregate. When pH was near pHpzc, strong attractive forces dominated, and when pHwas9, nano-TiO₂particles must overcome a high energy barrier （Umax=19.39kBT） tocollide with each other. The increase of IS could decrease the stability of nano-TiO₂bycompressing the electric double layer. When IS increased from0.001M to0.01M, Umaxdecreased from33.59kBT to14.88kBT, the further increase of IS to0.1M wouldeliminate the energy barrier. DLVO theory was in good consistent with the experimentresult when considering the effect of pH and ionic strength.The effect of humic acid, which is ubiquitous in the aqueous environment, was alsoinvestigated. When pH was5.5, HA was absorbed in the surface of nano-TiO₂, thesurface negative charge of nano-TiO₂was greater and steric force could also increasethe stability of nano-TiO₂. The absorbent of HA was enhenced when IS increased.Sedimentation experiment results showed that pH approaching pHpzcand increaseof ionic strength led to more rapid sedimentation of nano-TiO₂. The effect of HA wasassociated with pH. When pH was5.5, the sedimentation rate of nano-TiO₂was lowered with increase of HA concentration. When pH was9.0, however, such effect wasnegligible. When HA was considered, steric force was very important on the stability ofnano-TiO₂, modified DLVO theory should be applied to give a reasonable explanationof the effect of HA on the stability of nano-TiO₂.Column experiment was carried out to investigate the effect of pH, velocity andSDBS on the migragation ability of nano-TiO₂. The relusts showed that the departure ofpH from pHpzc, the increase of velocity and the presence of SDBS were combined toenhance the transport ability of nano-TiO₂. HA was also found to be very important onthe migragation ability of nano-TiO₂. Under condtion of pH was5.5, the absorbent ofHA on the surface of TiO₂could increase the migragation ability of nano-TiO₂greatly.When pH was9, however, the effect of HA on the transport of nano-TiO₂wasnegligible.At last, jar tests were carried out to stimulate the removal of nano-TiO₂byconventional water treatment processes. The result showed that, coagulation andsedimentation process could remove about45%of nono-TiO₂when PAC was utilized ascoagulate. Additional0.45μm membrane filtration could remove more than90percentof nano-TiO₂.Overall, this study clarified the effect of different factors on the stability ofnano-TiO₂, provided a quantitative way to describe the migration characteristic ofnano-TiO₂in water. References were also provided for water treatment processes toremove nano-TiO₂.