| Adsorption is an effective technique for removal metal ions in water.During the progress, adsorbent is the key point. Therefore, it has theoretical and practical significance to research new-effective adsorbent and intensive study the adsorption low and mechanism. Zeolite Imidazole Frameworks, ZIFs,obtain the advantages of inorganic zeolite and MOFs: breath effect, stable hole structure, large specific surface area, flexible construction, thermostability and chemical stability. Therefore, ZIFs become the focus in application, such as adsorption separation, molecular probe, catalytic oxidation, membrane, etc.This paper study on the adsorptive progress, law and mechanism of heavy metal ions and rare metal ions onto ZIF-8 in water. Main research content and results are as follows.(1) Using 5 heavy metal ions (Zn2+, Cu2+, Pb2+, Cd2+ and Ni2+) as adsorbate,this part concentrate on adsorption variable (the solution pH, initial concentration of metal ion, the dosage of ZIF-8 and adsorption time) to research the adsorption capacity of ZIF-8. With high metal solution pH,functional groups have strong protonation, competitive ion H+will decrease,and chemical complex interaction became strong. So with this situation, ZIF-8 has higher adsorption capacity for metal ions.The equilibrium adsorption time of Zn2+, Cu2+, Pb2+, Cd2+ and Ni2+ are 5,25, 15, 45 and 10 h, respectively. And the saturated adsorption capacity are 249.6, 481.9, 1184.8, 812.8 and 247.2 mg·g-1, respectively. This result is better than traditional adsorbents, such as active carbon, zeolite, etc. The XRD and FTIR results of ZIF-8 and ZIF-8-ion powder reveal that crystal structure change after adsorption process. This reveals there are chemical adsorption except physical adsorption.(2) Correlating the experimental data with adsorption isotherm equations and kinetic equations, and analyzing ion exchang during the adsorption process, this part give the preliminary adsorption mechanism. The adsorption process get good correlation with Langmuir model. The KL order of five metal ions is Pb2+ > Zn2+ > Cu2+ > Cd2+ > Ni2+. The saturated adsorption capacity are close to the outcomes of correlation. That means experimental results agree with model.The kinetic fitting indicate that pseudo-second-order model can describe the adsorption process well. And the process is accompanied by chemisorption,and intra-particle diffusion is not the only control step in the adsorption process. The relationship between the initial adsorptive rate of different metal ions and the initial concentration of metal ions reveal the breath effect will generate when the metal ions concentration surpass a value. Before that value,there is the interaction between metal ions and active sites on the surface of ZIF-8.The intensity of ion exchange increase and then become flat with the increase of the heavy metal ion concentration during the adsorption process.When the initial concentration is the fixed value 300 mg·L-1, the order of ion exchange intensity is Pb2+>Cu2+ > Ni2+ > Cdz+.(3) This part study the adsorption capacitu of 5 rare metal ions (Ta5+,Nb5+, Ce3+, La3+ and Nd3+) onto ZIF-8, and research the influence of the different desorbents and the desorption temperature on desoption process.When the metal ion concentration below 20 mg·L-1, ZIF-8 can almost remove the metal ions completely. The saturated adsorption capacity of ZIF-8 for these 5 metal ions are 2-5 time of those of ZIF-90, and are also better than the traditional adsorbents. In the desorption experiments, two polar organic solution-acetonitrile and lethanol-were used for removing La3+ in the structure of La-ZIF-8. And the removal rate are 94.4% and 61.0%, respectively.In characterization by P-XRD, the change of diffraction generated by adsorption La3+ return to the state of ZIF-8 after the desorption progress. The adsorption of La3+onto ZIF-8 is reversible.Subjecting the ZIF-8 material to re-adsorption/desorption for 3 times resulted the stable structure of ZIF-8 under room temperature for desorption part.While for the desorption temperature 60 ?,the structure collapsed partly and the adsorption capacity reduced to 150 mg·g-1.(4) The quantum chemistry calculation (DFT-PBE-PAW) were adopted to calculate the adsorption of metal ions onto ZIF-8 to reveal the further adsorption mechanism. In the calculation, the 4-membered ring were adopted for simple the calculation work. The metal cations attract with the ? bond of imidazole ring and repel methyl C with positive potential. Therefore, for almost every metal cation, 2-methylimidazole rotale with the imidazole ring to cation and methyle group to outside. The framework of Zn2+ remain unchanged, and the degree of 2-methylimidazole rotale will different with different cations. Besides, some metal cation generate the new chemical bonds with the atom in the structure of 4-member ring. For Cu2+ and Pb2+, there is one bond generated between cation and sp3 C. And for Ta5+ and Nb5+, there are more bonds between cation and C/N in imidazole ring. The more bonds,the stronger adsorption force. The outcomes of calculation explain the structure distortion after adsorption and the interaction during the adsorption progress.(5)In this part, a series of catalyst Pd@ZIF-8 for different Pd/Zn2+ ratio were prepared and applied in the PROX of CO in hydrogen-rich gas. The XRD analysis indicate that Pd@ZIF-8 stay the same crystal structure with ZIF-8 to a great degree. The SEM analysis indicate that Pd@ZIF-8 stay the same morphology with ZIF-8 at low ratio of Pd/Zn2 ; the morphology change a little and the crystal degree decrease at high ratio of Pd/Zn2+. The TEM characterization reveal that the active component locate into ZIF-8 structure and also on the surface of ZIF-8. The preliminary test of catalytic activity show that CO can convert 98% with Pd@ZIF-8 under 150 ?. |
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