Adsorption Of Contaminants On Carbon Nanomaterials And Its Transport In Saturated Porous Media

With the rapid development of industry,heavy metal pollution and dye pollution have become increasingly serious,posing a huge hidden danger to environment and human health.Therefore,how to remove heavy metals and dye in the environment has attracted more and more attention from researchers.For their large specific surface area and pore structure,carbon nanotubes,are widely used in the field of adsorption.With the rapid development of technology,the application of nanomaterials is more and more extensive.Therefore,nanomaterials will inevitably enter the environment.For their nanometer scale,nanomaterials have potential environmental toxicity,so understanding the transport of nanomaterials in porous media is critical to assesse its impact on the environment.In this study,methylene blue,Cd(II)and Cr(VI)were performed as pollutants and carbon nanotubes modified by different methods as adsorbents,the adsorption of methylene blue,Cd(II)and Cr(VI)on modified carbon nanotubes and carbon nanotubes were investigated by batch technique,and the effects of temperature,pH,and ionic strength on the adsorption were also probed.The indoor simulation experiment was adopted to investigate the transport of two types of nanomaterials(carbon nanotubes and nitrogen-doped graphene)in saturated porous media.This study is of great significance to evaluate the transport and fate of carbon nanotubes and nitrogen-doped graphene in the natural environment.The main results are as follows:1.Compared with the pristine carbon nanotubes,carboxyl modification and amino modification removed impurities on the carbon nanotubes,and increased the type and number of functional groups and adsorb sites on the carbon nanotubes,thereby improved the dispersion properties of carbon nanotubes.The surface of the metal-modified carbon nanotubes does have metal and metal oxide,and the surface of the metal-modified carbon nanotube a great of oxygen-containing functional group such as carboxyl group and hydroxyl group.The Zeta potentials of carbon nanotubes,nitrogen-doped graphene,quartz sand and sandy soil were all negatively charged,and the Zeta potential decreased with increasing ionic strength and pH.2.The pseudo-second-order equation was more suitable for the adsorption kinetics than the pseudo-first-order equation.The adsorption equilibrium reached in 2 h.The adsorption isotherms of the three types of carbon nanotubes conformed to the Freundlich model.The adsorption capacity of the carbon nanotubes increased with increasing pH and ionic strength in the medium.However,the adsorption capacity of the carbon nanotubes decreased as the temperature increased.These results indicate that the adsorption mechanisms mainly include hydrophobicity interaction,electrostatic interaction,and complexation between copper and methylene blue.3.The adsorption equilibrium of Cd(II)and Cr(VI)reached in 90 min.Compared with prinstine carbon nanotubes,the adsorption capacity of modified carbon nanotubes was larger.The adsorption isotherms of the three types of carbon nanotubes conformed to the Freundlich model and Langmuir model.The adsorption capacity of Cd(II)on carbon nanotubes increased with increasing pH,but decreased with ionic strength increasing in the medium.The adsorption capacity of Cr(VI)on carbon nanotubes showed the opposite trend.It decreased with increasing pH but increased with ionic strength increasing in the medium.The adsorption mechanisms mainly contain physical adsorption,hydrophobicity interaction,electrostatic interaction,and chemical reaction of metal ions with functional groups on the surface of carbon nanotubes.4.The transport of carboxyl modified carbon nanotubes was the largest,and followed by the prinstine carbon nanotubes and the amino carbon nanotubes,and the transport of metal-modified carbon nanotubes was the lowest.The transport of the iron modified carbon nanotubes was not sensitive to solution pH,but the transport of the other four carbon nanotubes decreased as the pH increasing.The transport of carbon nanotubes and nitrogen-doped graphene depended on solution ionic strength.The transport decreased with the increasing of ionic strength.The transport of nanomaterials increased with the increasing of sand grain size,and the results could be expounded by the colloid filtration theory.The transport of nanomaterials was larger at the lower temperature tested because of the reduction of repulsively electrostatic forces between sand and NG as temperature increased.In addition,Brownian movement of nanoparticles increases as temperature increasing,so the collision probability between NG and sand grain would also increase,promoting the deposition of NG onto the sand surface.Compared with quartz sand,natural soil was not conducive to the transport of carbon nanotubes.