Adsorption Behavior And Mechanism Of Ionic Liquid Bonded Silica Gel On Heavy Metal Ions

With the development of industry,people are more active on mining,smelting,processing and commercial manufacturing of heavy metal.The variety kinds of heavy metals produced by activities has greatly increased,which lead to heavy metals pollution affecting the environment.Soil erosion and water pollution have occurred which cause a great threat to human health.Therefore,it has significant meaning to investigate the treatment of heavy metal pollution.There are many methods for the treatment of heavy metals.Among them,the adsorption method has been widely studied because of its low cost,simple operation,multiple types of adsorbents,and good selectivity.Most of the current adsorbents are cationic adsorbents.Therefore,in order to seek for adsorbents with high toxicity and selectivity for highly toxic heavy metal anions,a series of studies were conducted in this paper.The imidazole ion-liquid bonded silica gel was synthesized and characterized.The effect of imidazolium ion-liquid bonded silica gel structure on the adsorption performance of heavy metal ions such as Cr（Ⅵ）and permanganate was investigated from the molecular perspective.The main research and results are as follows:（1）Selection of anions and synthesis、characterization ionic liquid bonded silica ge.The stability of six imidazole ionic liquids containing different anions was investigated from the molecular perspective.The structure of six ionic liquid molecules was optimized by using density functional theory at the B3LYP/6-311+G（d,p）level.Then the stability of ionic liquids with different anions were analyzed from the aspects of geometry,reduced density gradient（RDG）and interaction energy.The results show that there is a hydrogen bonding interaction between the anion and the imidazolium cation,and the order of stability between the anion and the imidazole ring was hexafluorophosphate<tetrafluoroborate<chloride.Therefore,the imidazolium ionic liquid was selected to bond with silica gel and the modified silica gel was characterized.The characterization results show that the bonding amounts of methyl imidazole and ethyl imidazole bonded silica were approximately 2.89μmol/m²and 3.10μmol/m²,respectively.（2）Study on the adsorption of ionic liquid modified silica gel on dichromate.silica gel,activated silica gel,chloropropyl silane silica gel,1-methylimidazolium ion liquid bonded silica gel（SilprMmimCl）and 1-ethylimidazolium ion liquid bonded silica gel（SilprEmimCl）were tested for adsorption.The results show that only SilprMmimCl and SilprEmimCl have adsorption effect on Cr（Ⅵ）,which shows that the adsorption site of ionic liquid-bonded silica gel is mainly on the imidazole ring.In addition,by static adsorption experiments,when the pH was 5.6 and initial concentration of Cr was 200μg/mL.The Cr（Ⅵ）adsorption capacities of SilprMmimCl and SilprEmimCl are 24.03 mg/g and 16.52 mg/g,respectively.The desorption experiments were measured with sodium chloride.The desorption rates of SiprMmimCl and SiprEmimCl were 78.58%and 77.35%,respectively.To investigate the mechanism of adsorption on molecular scale,an approximation model of SilprMmimCl and SilprEmimCl have been modeled with[Mmim]Cl,[Emim]Cl,[Mmim][Cr₂O₇]and[Emim][Cr₂O₇].The structures of related models were optimized at the B3LYP/6-311+G（d,p）level.The weak interaction between cations and anions of models,such as[Mmim]⁺,[Emim]⁺,Cl^-,[Cr₂O₇]^2-,was analyzed by vibration analysis and bond length analysis;Atoms In Molecules（AIM）theory was used to study the features of bonds at bond critical points and the variations of bond types.Simultaneously,noncovalent interactions were characterized and visualized by analysis of reduced density gradient（RDG）.Finally,by comparing the intra-molecular interaction of these four complexes,the stability of these four complexes follow the order of[Mmim]⁺[Cr₂O₇]^2->[Emim]⁺[Cr₂O₇]^2->[Mmim]Cl>[Emim]Cl.which demonstrates why SilprMmimCl and SilprEmimCl have a good adsorption capacity for dichromate and the adsorption capacity of SilprMmimCl is slightly less than that of SilprEmimCl.（3）Study on the adsorption of different forms of hexavalent chromium on ionic liquid modified silica gel.The adsorption behavior of hexavalent chromium with SilprMmimCl and SilprEmimCl were studied at different pH.It was found that the adsorption capacity of the adsorbent was the highest around pH=3,and the adsorption of hexavalent chromium by SilprMmimCl was greater than that of SilprEmimCl at same pH value.In addition,[Methyl]⁺[H₂CrO₄],[Ethyl]⁺[H₂CrO₄],[Methyl]⁺[HCrO₄]^-,[Ethyl]⁺[HCrO₄]^-,[Methyl]⁺[Cr₂O₇]^2-and[Ethyl]⁺[Cr₂O₇]^2-were simulated under gas phase conditions.From the aspects of geometric structure,electrostatic potential,RDG analysis and interaction energy analysis,it is found that the interaction between imidazole ringed and H₂CrO₄,HCrO₄^-,and Cr₂O₇^2-were hydrogen bonds.It can be speculated that the order of the interaction strength between hexavalent chromium with the imidazole ring may be Cr₂O₇^2->HCrO₄^->H₂CrO₄.Then,[Methyl]⁺[H₂CrO₄],[Ethyl]⁺[H₂CrO₄],[Methyl]⁺[HCrO₄]^-,[Ethyl]⁺[HCrO₄]^-,[Methyl]⁺[Cr₂O₇]^2-,[Ethyl]⁺[Cr₂O₇]^2-,[Methyl]⁺[CrO₄]^2-,and[Ethyl]⁺[CrO₄]^2-were simulated and analyzed under implicit solvent model.It is found that the adsorption of hexavalent chromium on methyl imidazole is more stable than that of ethyl imidazole.Moreover,other interaction of cation-anion were van der Waals forces besides the hydrogen bonding between cation-anion of[Methyl]⁺[CrO₄]^2-and[Ethyl]⁺[CrO₄]^2-.The order of intramolecular stability is as follows:[Methyl]⁺[CrO₄]^2->[Methyl]⁺[Cr₂O₇]^2->[Methyl]⁺[HCrO₄]^->[Methyl]⁺[H₂CrO₄],[Ethyl]⁺[CrO₄]^2->[Ethyl]⁺[Cr₂O₇]^2->[Ethyl]⁺[HCrO₄]^->[Ethyl]⁺[H₂CrO₄].The difference between the gas phase model and the implicit solvent model indicates that the solvent will have a greater influence on the interaction of the molecule.It may be one of the reason why the adsorption of hexavalent chromium by SilprMmimCl and SilprEmimCl is not particularly large.（4）Study on the adsorption of permanganate on ionic liquid modified silica gel.The analysis was made from the aspects of geometric structure,bond length,vibration frequency,RDG analysis,and AIM analysis.The results show that the interaction between permanganate and imidazole rings is hydrogen bonding.The electrostatic potential map shows that the minimum value of the electrostatic potential of the permanganate is smaller than the minimum value of the electrostatic potential of the chlorine atom,which indicates that permanganate is difficult to replace chloride ions.The interaction energy shows that the interaction energy in[Mmim]⁺[MnO₄]^-,[Emim]⁺[MnO₄]^-molecules is less than that in[Mmim]Cl and[Emim]Cl molecules.In summary,theoretical calculations show that permanganate is difficult to be adsorbed by the adsorbent.