The Transport Mechanism Of Carbon Quantum Dot Doped Titanium Dioxide（C-TiO₂）in Packed Columns And 3D Primted Micromodels

As a new nanoparticle,carbon quantum dots doped titanium dioxide（C-TiO2）has a high catalytic efficiency and the ability to degrade pollutants.Hence,C-TiO2 may have a broad application prospect in the future.The environmental risks of nanoparticles are closely related to their stability and mobility in aqueous phase.Therefore,in order to predict the environmental risks of C-TiO2,it is of great significance to study the retention and transpoort of C-TiO2 in environmental media（earth’s surface,groundwater and soil）.In this study,C-TiO2 is prepared by traditional hydrothermal method and its physical and chemical structure are characterized.The aggragation behavior and the transport behavior in porous media（packed column and micromodel）are studied.The colloidal stability of C-TiO2 is explored by studying the relationship between the hydrodynamic particle size of C-TiO2 and the ionic strength using dynamic light scattering.The aggregation experiments show that the hydrodynamic particle size and hydrodynamic particle size growth rate of C-TiO2 increase with the increased ionic strength.The critical coagulation concentration（CCC）of C-TiO2 is 95.9 mM for NaCl and 1.1 mM for CaCl2,respectively.The CCCCaCl2 is low,because Ca2+ not only compress the electric double layer,but also have a bridging effect.These results indicate that the electrolyte can promote the aggregation of C-TiO2,and the promotion effect of ions with different valence states is different.The transport behavior of C-TiO2 is explored by studying the breakthrough of CTiO2 in the saturated packed column filled with quartz sand with three different sizes.The effects of ionic strength,dissolved organic matter（DOM）,pH,flow rate,initial concentration of particle and sand grain size on the transport behavior of C-TiO2 are systematically studied in the column experiment.The traspoort capacity of C-TiO2 increases with the increased sand grain size,DOM concentration,pH,initial concentration of particle and flow rate,and decreases with the increased ionic strength.The transport mechanism of C-TiO2 is analyzed by using colloidal filtration theory and Derjaguin-Landau-Verwey-Overbeek（DLVO）theory.The promotion efficiency of DOM ranks as humic acid（HA）>alginate（Alg）>bovine serum albumin（BSA）,which is in the same order as their ability to change surface charges.The results show that the physical and chemical factors in nature can affect the transport behavior of C-TiO2.The micromodels are prepared via 3D printing to further reveal the deposition mechanisms and spatial/temporal distribution of C-TiO2 in porous space.C-TiO2 mainly attaches to the upstream region of collectors because of interception.In addition,the diameter of C-TiO2 is less than 1 μm,indicating that Brownian diffusion also plays a role in the transport of C-TiO2 to the the collector surface.The collector ripening is observed after long-time deposition.The existence of DOM caused visible decrease of C-TiO2 deposition in the pore network.HA caused the most remarkable reduce of deposition in the three types of DOM,which is consistent with the column experiment results.The results show that the colloid filtration theory and DLVO theory can explain the pore retention phenomenon of C-TiO2,and 3D printed micromodel is a feasible new tool to study the transport of nanoparticle.In this study,the aggregation and transport behavior of C-TiO2 are systematically studied.It is found that many environmental factors affect its behavior in the environment.The combination of column experiment and micromodel experiment provides knowledge for understanding its transport behavior and mechanism,and provides theoretical help for understanding its environmental fate and toxicity risk to the nature and human beings.