Effect Of Humic Acid On The Sulfamethazine Adsorption By Multi-walled Carbon Nanotubes And Their Dispersion In Solution

The dispersion and stability of MWCNTs in solution is of great significance for evaluating the environmental behavior and ecological toxicology of MWCNTs.Natural organic matter?NOM?,is ubiquitous in the environment,have a huge impact on the environmental behavior of MWCNTs and contaminants.In order to explore the effect of NOM on the dispersion of MWCNTs and on the adsorption of antibiotics by MWCNTs,the Pristine,Carboxylated and Hydroxylated Multi-walled carbon nanotubes?P-MWCNT,C-MWCNT and H-MWCNT?were used in this work to investigate the adsorption of sulfamethazine and HA by MWCNTs under varied pH,ionic strength in solution.The effect of coexisting cations or HA on the SMZ adsorption by MWCNTs also were also investigated.The elemental contents,molecular weight and major structural carbons of HA were characterized by elemental analysis,high gel permeation chromatography?HGPC?and 13 C NMR,to reveal the effect of different properties of HA on the dispersion and stability of MWCNTs.The main conclusions in this study are as follow:SMZ adsorption by P-MWCNT and H-MWCNT can be depicted well by the pseudo-second-order kinetic model?R2 > 0.99?and Langmuir isotherm model?R2 > 0.99?.The SMZ adsorption by MWCNTs increased slowly with increasing pH from 2 to 7,then,decreased abruptly as pH values further increased.As ionic strength increased from 0.02 to 0.20 M?NaNO3?in solution,the adsorption capacity of SMZ by P-MWCNTs and H-MWCNT reduced 68% and 87%,respectively,which indicated electrostatic interactions were the main adsorption mechanisms.Cu²⁺ slightly decreased SMZ adsorption by 10% to 20% at solution pH of 2.3 and 4.9,due to the competing effect of Cu²⁺ with SMZ+ and SMZ±.But Cu²⁺ increased SMZ-adsorption by 20% to 60% at solution pH of 7.4 and 10.0,due to the facilitating effect of the complex formation of Cu²⁺-SMZ.HA adsorption by MWCNTs decreased as pH increased or ionic strength decreased.The main reason is that the changes of solution properties could alter the electrostatic interactions between HA and MWCNTs,influencing the adsorption of SMZ by MWCNTs,implying that electrostatic interactions were the main adsorption mechanisms.The adsorption of SMZ or HA by MWCNTs decreased with increasing surface oxygen content of MWCNTs.Two mechanisms could explain this phenomenon:?1?These Oxygen containing functional groups are usually hydrophilic,could reduce the hydrophobicity of the MWCNT surface,thus decrease the SMZ and HA adsorption by MWCNTs;?2?As the number of hydrophilic oxygen-containing functional groups increases,the aromatic content of MWCNTs decreased,weaken the ?-? interactions between HA and MWCNTs,indicated that the hydrophobic interaction and ?-? interaction were the main adsorption mechanisms.And ?-FTIR results also confirmed that ?-? interaction play an important role in the adsorption process.The presence of HA inhibited SMZ adsorption onto MWCNTs via competitive interactions.HA adsorbed on the MWCNTs surfaces also altered the electrostatic and hydrophobic interactions between SMZ and MWCNTs,decreasing the adsorption of SMZ.In ultrapure water solution,the concentrations of suspended MWCNTs increased with the increase of surface oxygen contents and solution pH;and decreased as ionic strength increased.The presence of HA enhanced significant the dispersion and stability of MWCNTs,this is because HA adsorbed on MWCNTs surface increased the electrostatic repulsion and steric hindrance between individual MWCNTs.And the suspended MWCNTs concentrations and adsorption capacity of HA by MWCNTs was positive correlation at lower solution ionic strength?0 40 mM?and pH from 3 to 7.But,at higher ionic strength,the double electrode layer of MWCNTs was destroyed by Na+,the MWCNTs precipitated rapidly.When the pH > 7,the adsorption of HA by MWCNTs was inherently lower,and the steric hindrance was weaker,electrostatic repulsion play a decisive role in MWCNTs dispersion.Thus,the suspended MWCNTs concentrations increased as the solution pH further increased.The dispersion of MWCNTs was induced by AHA and FHA.AHA showed better dispersion effect on MWCNTs based on the suspended concentrations,CCC of Na Cl and mean hydrodynamic diameter of MWCNTs.The differences of MWCNTs dispersion depended on HA with different chemical structures and properties.Compared with FHA,AHA has higher molecular weight and aromatic content,when being adsorbed on MWCNTs surface,leading to higher steric hindrance between individual MWCNTs,which also prove that steric hindrance is the main mechanism of HA dispersed MWCNTs.