Synthesis Of Novel Vinylimidazole/Dibromodecane/Vinylpyridine Polymers And Its Selective Adsorption And Electrochemical Detection Of Nitrate

As nitrate pollution is a serious threat to the ecological environment and human health,nitrate removal from water bodies has become a global concern.Among the nitrate treatment methods,one of the most effective and rapid methods is adsorption.However,most of the existing nitrate adsorbents suffer from low adsorption capacity,poor selectivity,cumbersome synthesis steps and toxic substances generated during the synthesis process.Therefore,it is necessary to develop a nitrate adsorbent with excellent nitrate adsorption capacity and selectivity,and at the same time with less synthesis monomer,shorter time,and environmentally friendly process.Based on this,a polymeric sorbent vinylimidazole/dibromodecane/vinylpyridine（VDV）was synthesized by rapid copolymerization method to improve the nitrate sorption capacity and selectivity of organic adsorbent.In addition,VDV was combined with chitosan and carbon multi-walled nanotubes to prepare a VDV/chitosan/carbon multi-walled nanotube（VDV/chitosan/MWNT）nitrate detection electrode to detect nitrate in water.The contents and results of this study are as follows:（1）The optimal molar ratio between the adsorbent materials vinylimidazole,dibromodecane and vinyl pyridine monomers was determined to be 1:1:1,and the materials were characterized and analyzed by scanning electron microscopy（SEM）,energy spectrum analysis（EDS）,Fourier transform infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）and simultaneous thermal analyzer（TGA）.The maximum adsorption capacity of VDV for nitrate nitrogen was up to 52.6 mg/g.Thermodynamically,the adsorption process was in accordance with the Freundlich model（correlation coefficients R~2 of 0.998,0.997 and 0.995 at temperatures of 15℃,25℃and 35℃,respectively）.In terms of kinetics,the adsorption process conformed to the quasi-primary and quasi-secondary kinetic models（R~2 are greater than 0.99）,indicating that the adsorption process involves chemical and multilayer adsorption;In terms of nitrate selectivity,for sulfate,when the sulfate and nitrate nitrogen concentrations were 350 mg/and 56.45 mg/L respectively(SO₄^2-/NO₃^-concentration ratio was 6.2),the selectivity(K_e/c)of VDV for nitrate still reached 2.6（greater than1）,indicating that the adsorbent is selective for nitrate;for bicarbonate,when the HCO₃^-/NO₃^-concentration ratio was less than or equal to 6.2,the adsorption amount of VDV did not change;for chloride,when the Cl^-/NO₃^-concentration ratio was less than 6.2,the adsorption capacity of VDV for nitrate slightly decreased and when its concentration ratio was equal to 6.2,the adsorption capacity decreased by 16.8%compared with the solution in which nitrate was present alone;In terms of VDV reusability,the adsorption capacity of nitrate by VDV remained stable at 91%of the first use of VDV after five cycles,indicating that VDV has stable reusability,where the optimal concentration of desorbent in sodium chloride is 0.3 M.In the column dynamic actual wastewater adsorption experiment,when the nitrate nitrogen concentration of secondary effluent from wastewater treatment plant was 20 mg/L,the VDV adsorption capacity of nitrate was 13.51 mg/g,and when the nitrate nitrogen concentration of industrial wastewater was 100 mg/L,the adsorption capacity was25.04 mg/g.The concentration of nitrate nitrogen after treatment was less than 10mg/L,which showed that VDV has good treatment effect on low concentration secondary effluent and high concentration industrial wastewater.Combining the material characterization and experimental results,the selective adsorption mechanism of VDV on nitrate mainly included the electrostatic effect of VDV on nitrate,ion exchange and hydrophobic effect between VDV and nitrate.（2）For nitrate detection,the VDV/chitosan/MWNT electrode was prepared using VDV as the active layer part for nitrate detection,combined with chitosan（VDV to chitosan mass ratio of 4:1）and carbon multi-walled nanotubes（concentration of 200mg/L）as the conduction layer,and the detection limits of this electrode were measured by cyclic voltammetry in the range of 1～50 mg/L with a correlation R~2 of0.994.The VDV/chitosan/MWNT electrode showed the lowest impedance compared to the glassy carbon electrode and the VDV/chitosan electrode;when optimizing the conditions,it was found that the VDV/chitosan/MWNT electrode showed a high electrical signal at p H=2 and the enrichment time of the electrode was stable at 5 min.The VDV/chitosan/MWNT electrode showed good linearity（R~2=0.994）in the detection range of 1 mg/L to 50 mg/L with the corresponding oxidation peak current of potassium ferricyanide.The VDV/chitosan/MWNT electrode showed good linearity in the detection range of 1 mg/L to 50 mg/L with the corresponding oxidation peak current of potassium ferricyanide;the VDV/chitosan/MWNT electrode showed good linearity in the detection range of 1 mg/L to 50 mg/L with the corresponding oxidation peak current of potassium ferricyanide.The coexistence ion experiment with the electrode revealed that SO₄^2-interfered more with the electrical signal generated by the detection electrode in the binary phosphate buffer solution with both interfering ion/nitrate concentration of 50 mg/L,while HCO₃^-and Cl^-had no effect on the electrical signal of this electrode.