Adsorption Of Perfluorooctanoic Acid On Surface Grafted Carbon Nanotubes

As a new kind of carbon nanomaterials,carbon nanotubes?CNTs?have a wide range of applications in the fields of biology,medicine,material science,environmental protection and energy because of its unique structure and phys-chemical properties.CNTs have many advantages,such as larger surface area and controllable pore size distribution.It overcomes the defects of low adsorption site,slow adsorption speed and weak perforation ability of conventional carbon based adsorbents,and it is becoming more and more widely used in the field of adsorption.However,the poor dispersion and recyclability of CNTs in water limits its application in environmental pollution treatment.In this study,perfluorooctanic acid?PFOA?was used as adsorbate and different grafted multi-wall carbon nanotubes?MWCNTs?were used as adsorbents.Adsorption kinetics and isotherms of PFOA on adsorbents were investigated by using radioactive labeled PFOA(¹⁴C-PFOA)to quantify the trace PFOA.The effects of environmental factors?ionic strength,ionic species,electrolyte pH values and humic acids?on adsorption behavior were also investigated.At the same time,the microstructure and surface chemical properties of grafted MWCNTs were characterized by means of TEM,XRD,XPS,BET and FTIR,and the adsorption mechanisms of the grafted MWCNTs on PFOA were determined by combining adsorption and desorption process.The main results of the study are as follows:The MWCNTs were characterized by using specific surface area measurement system,element analysis,surface acidity,Zeta potential,XRD,TEM,FTIR and XPS before and after treated by strong acid?HNO₃,H₂SO₄ and HNO₃ mixture?,strong alkali?KOH?,strong oxidize?H₂O₂,NaClO??OCNTs?and hybridized metal nanoparticles?nano iron,nano iron oxide,nano copper,nano copper oxide,nano zinc and nano zinc oxide??MHCNTs?.The oxygen content of treated MWCNTs was obviously increased and the dispersion in the aqueous solution was better.It is proved that the surface of MWCNTs had been grafted with some hydroxyl,carboxyl and other oxygen containing functional groups,which leads to the enhancement of the hydrophilicity of the grafted MWCNTs.The results show that the specific surface area,pore volume and pore size of the MHCNTs were reduced after the hybridization,which is mainly caused by the fact that metal nanoparticles loaded on MWCNTs and blocked some surface channels and defects of MWCNTs.The adsorption kinetics,isothermal adsorption-desorption process of PFOA on the pristine MWCNTs,OCNTs and MHCNTs were studied by batch technique and radioisotope tracing(¹⁴C-PFOA).Various kinetic models were used to fit the adsorption kinetics results.It was found that the adsorption rate of PFOA on the adsorbents was influenced by various factors,mainly controlled by the internal diffusion process.The adsorption of PFOA by modified carbon nanotubes conformed to the pseudo-second-order model and reached the adsorption equilibrium within 360 to 720 min.The isothermal adsorption results showed that the adsorption capacity of PFOA on oxidationgrafted MWCNTs were lower than that of MWCNTs,while the adsorption of PFOA on the MHCNTs were higher than on the control.For example,under the condition of low PFOA equilibrium concentration(20?g L^-1),the adsorption capacity of PFOA on nano copper grafted MWCNTs is 17.99-times that of MWCNTs.This was because of electrostatic and hydrophobic interactions between PFOA and MHCNTs.In addition,PFOA formed inner-sphere complexes with metal nano-particles via ligand exchange.The results of isothermal adsorption-desorption showed that the adsorption of PFOA on MHCNTs was irreversible.The adsorption mechanisms of PFOA on MHCNTs were deeply analyzed by using XPS.Metal nanoparticles carry large amounts of-OH on the surface,and they are protonated by hydrolysis reaction in the solution,resulting in a large number of positive charges on the surface of the metal nanoparticles.At the same time,due to PFOA was negative charged under the neutral condition,and combined with protonated-OH through electrostatic force to form a combination bond,so that a large number of PFOA molecules were firmly fixed on the surface of the metal nanoparticles.After that,the hydrophobic tail of PFOA^–combined on metal nanoparticles surface adsorbed the free PFOA^–molecule in the solution by hydrophobic interaction,and further increase the adsorption amount of PFOA.It must be pointed out that a part of water molecules were catalytic decomposed into H and OH by CuO,and then the p orbital of the O atoms coming from OH combined with the d orbital of the Cu atoms,making the OH tightly bind to the CuO surface.The alteration of PFOA adsorption capacity by increasing ionic strength was attributed to the aggregation degree of MWCNTs,electrostatic shielding and/or salting out effect.The presence of Ca²⁺increased the adsorption,owing to not only its higher electrostatic shielding ability than Na⁺but also forming bridge between PFOA and MHCNTs.PFOA adsorption on MHCNTs strongly depended on pH value in the medium,that is,with the increasing p H value of electrolyte,the adsorption capacity of PFOA on MHCNTs gradually decreased.Humic acid mainly inhibited the adsorption capacity of PFOA through competitive adsorption.This study used metal nanoparticles to hybridize carbon nanotubes,which can greatly improve the adsorption capacity of trace PFOA.It provides a theoretical basis for the application of grafted CNTs to remove trace PFOA in surface water and tap water,and promotes the application and development of carbon nanotubes in the field of water protection.