Aggregation And Deposition Of Multi-walled Carbon Nanotubes And Their Migration In Micromodels

Carbon nanotubes?CNTs?have unique physical and chemical properties,and have been widely used in material science,electronics,medicine and other fields.However,a large number of studies have proved the ecological toxicity of CNTs on microorganisms,animals and plants.Therefore,it is necessary to understand the environmental processes of CNTs in nature,especially the aggregation and deposition behaviors,which is of great significance for the safe use of CNTs.In this paper,multi-walled carbon nanotubes?MWCNTs?were selected as the research object and bovine serum albumin?BSA?was absorbed onto the surface of MWCNTs.The aggregation,deposition and pore migration of MWCNTs were studied,and the effects of electrolytes were investigated.The aggregation and deposition of MWCNTs were studied by use of dynamic light scattering?DLS?and quartz crystal microbalance?QCM?respectively,and the migration of MWCNTs in the porous media was studied using the micromodel.Compared with the conventional packed column experiments,the micromodel experiments can directly observe the distribution of MWCNTs with time and space in the porous media.The research shows the aggregation of protein absorbed MWCNTs is consistent with the classical DLVO theory.The aggregation behavior of MWCNTs is divided into reaction-limited and diffusion-limited aggregation stage.The critical coagulation concentration?CCC?of MWCNTs is 175 mM in NaCl and 2.7 mM in CaCl2 solution,respectively.The gravitational deposition of MWCNTs is affected by the aggregation of MWCNTs.Favorable gravitational deposition is likely to achieve only when MWCNTs reaches the fastest aggregation.The electrolyte needed for the favorable gravitational deposition of MWCNTs is 400 mM in NaCl and 4 mM in CaCl2.The deposition of MWCNTs on the silica surface also accords with the DLVO theory.The deposition behavior of MWCNTS is divided into unfavorable deposition stage and favorable deposition stage.The critical deposition concentration?CDC?of MWCNTs is 100 mM in NaCl and 0.9 mM in CaCl2 solution,respectively.The deposition of MWCNTs on the alumina surface is always dominated by the favorable deposition regime.The presence of electrolytes can promote the aggregation and deposition of MWCNTs,indicating that electrostatic interaction is the main control mechanism of the aggregation and deposition of MWCNTs.The migration experiments of MWCNTs in the micromodel suggest that the increase of electrolyte concentration can enhance the deposition of MWCNTs in the micromodel and weaken the fluidity of MWCNTs.In the absence of electrolyte,the deposition of MWCNTs is also observed,which indicates that the fluid motion transport mechanism affects the migration of MWCNTs in the micromodel.The aggregation and deposition affects the pore deposition of MWCNTs.In addition to electrostatic interactions,different flow rates at different locations and the unique structure of MWCNTs also lead to the formation of MWCNT aggregates.The deposition distribution of MWCNTs in the micromodel is relatively uniform,and MWCNTs tend to deposit in the narrow pores,indicating that the physical strain is the main reason for the deposition of MWCNTs in the porous medium.The effect of bivalent Ca²⁺ on the aggregation,deposition and pore deposition behavior of MWCNTs is always greater than that of monovalent Na+.Except that Ca²⁺carries more charge,Ca²⁺ can undergo specific interaction with special groups on the surface of MWCNTs,BSA and silica,which is also an important reason for the greater effect of Ca²⁺.Because of this specific interaction,the rigidity of the deposition layer of MWCNTs on the interface increases rapidly when Ca²⁺concentration increases.This study proves that the electrolyte can affect the environmental processes of CNTs,and provide an important basis for predicting the environmental behavior and assessing the risks of CNTs.