Study On Adsorption Characteristics Of Cr(Ⅵ) By Ionic Liquid Modified Montmorillonite

In this paper,calcium montmorillonite was used as raw material to be organically modified by 1-hexadecyl 3-methyl imidazole(C16mimCl)and cetylpyridine chloride(CPB)in two ionic liquids,and then hexavalent chromium(Cr(Ⅵ))in the solution was adsorbed by modified montmorillonite.In this paper,the loading characteristics of the two ionic liquids on montmorillonite and the change rule of montmorillonite structure after loading are studied firstly,and then the adsorption mechanism of Cr(Ⅵ)by ionic liquid modified montmorillonite(ILs-Mt)is illustrated.The study showed that with the increase of the amount of modifier,the modified dose of montmorillonite load also increased.When the amount of C16mimCl and CPB respectively exceeded 3.0 CEC,the load tended to be stable.pH had little effect on the load of the ionic liquid on montmorillonite.The load first increased and then decreased slightly,and the maximum adsorption value appeared at pH=8.The effect of CaCl2 concentration on the modification of CEC ionic liquids was studied.Studies have shown that ionic liquids in montmorillonite interlayer of isothermal adsorption process in line with the Langmuir model,25℃ when fitting the montmorillonite saturated adsorption amount of C16mimCl and CPB tendency were 2.67 and 2.85 mmol/g,and adsorption capacity decreases with the increase of temperature;The adsorption process of montmorillonite on C16mimCl and CPB conforms to the quasi-second-order kinetic model.Thermodynamic studies have shown that modified process is spontaneous exothermic process of entropy,ΔH smaller value shows that the process could be hydrogen bonding,ion exchange,hydrophobic bonding force,van der Waals force and other forces.XRD,FTIR,DTA,Zeta potential,BET and SEM characterization results indicated that C16mimCl/CPB was successfully embedded into the montmorillonite layer or adsorbed to its surface.With the increase of the usage of C16mimCl and CPB,the distance between montmoronite layers increased from the original 1.53nm to 4.13 nm and 4.16 nm,respectively.The hydrophobicity gradually increased,the specific surface area and pore volume decreased,the pore diameter increased,and the Zeta potential increased(when pH=6,the Zeta potential of C16mimCl-Mt and CPB-Mt reached the maximum value,65.63 eV and 55.36 eV,respectively).SEM characterization showed that the natural montmorillonite was in the form of flat sheet and the modified montmorillonite was in the form of crimp and crease.The modified montmorillonite models with different ILs loads of 0.5 CEC,1.0 CEC and 3.0 CEC were constructed by molecular dynamics simulation.From the structure simulation diagram,it can be intuitively observed that the spatial height of interlayer domain and the change of interlayer substance,and the d(001)value calculated by molecular simulation is basically consistent with the measured value.Two ILs-Mt(C16mimCl-Mt and CPB-Mt)were optimized.Two kinds of ILs-Mt were used to adsorb Cr(Ⅵ)in solution.The results showed that with the increase of the dosage of c16mimcl-mt,the unit adsorption amount of Cr(Ⅵ)first increased and then decreased.When the dosage was 3 g/L,the maximum unit adsorption amount 41.76 mg/g appeared.However,the unit adsorption capacity of cpb-mt for Cr(Ⅵ)gradually decreased from 47.83 mg/g to 22.02 mg/g.The adsorption of modified montmorillonite on Cr(VI)was highly dependent on the pH value.As the pH value increased,the adsorption values of c16mimcl-mt and cpb-mt on Cr(VI)decreased from 38.28 mg/g and 38.47 mg/g to 13.72 mg/g and 13.44 mg/g,respectively.Cl-in solution can inhibit anion exchange.C16mimC1-Mt,the CPB-Mt under 25℃ and 40℃and 55℃ of Cr(Ⅵ)data fitting all conform to the Langmuir adsorption isothermal adsorption model,the maximum adsorption capacity were 39.95,42.70,46.17 mg/g and 43.84,46.49,51.02 mg/g.The results showed that the adsorption capacity of cpb-mt on Cr(Ⅵ)was higher,mainly due to the higher load of CPB on montmorillonite and the higher Zeta potential of CPB-Mt.The whole adsorption process conforms to the quasi-second order kinetic model and is a spontaneous entropy increasing heat absorption process.A large amount of Cr(Ⅲ)was found in the XPS narrow sweep after adsorption of modified montmorillonite,which may be due to the reduction of Cr(Ⅵ)and Fe2+.In summary,the adsorption mechanism of ILs-Mt for Cr(Ⅵ)mainly includes electrostatic attraction,anion exchange and reduction.