Preparation Of Aluminum-Based Metal-Organic Framewoek Material With Tunable Structure And Pore Size And Study On Hg(Ⅱ) Adsorption In Water

Nowadays,the safety of drinking water has been perplexing people and attracted wide attention.Exposure to Hg（II）environment poses a great threat to human health,because its toxicity and carcinogenicity can lead to a series of fatal diseases.Therefore,the United States Environmental Protection Agency stipulates that the content of Hg（II）in drinking water is lower than that of 2ppb.Adsorption is one of the most effective methods because of its simple operation and high efficiency.Through simple physical and/or chemical interactions,the target contaminants are attached to the surface or internal cavity of the adsorbent to achieve efficient removal.However,the low mercury adsorption capacity of traditional adsorbents is difficult to meet people’s water needs.In order to achieve the ideal adsorption performance,it is the key to select/develop suitable adsorbents with higher mercury adsorption capacity.Metal-organic frameworks have attracted wide attention as a new type of mercury adsorbent because of their rich porosity,adjustable structure and surface function.Focusing on the theme of mercury pollution control,three kinds of metal-organic framework adsorbents with different structures and pore sizes,narrow-pore,large-pore and super-large pore NH₂-MIL-53（Al）（named NH₂-MIL-53（Al）-NP,NH₂-MIL53（Al）-LP and NH₂-MIL-53（Al）-NR）were prepared in this paper.The mercury removal performance of the three adsorbents was investigated and the corresponding mercury removal mechanism was analyzed.The efficient removal of Hg（Ⅱ）in water and the adsorption mechanism of Hg（Ⅱ）on the surface and interior of the three adsorbents were systematically studied and analyzed.In the first part,narrow-pore NH₂-MIL-53（Al）-NP mercury adsorbents were prepared,and their adsorption properties for mercury under different conditions were studied and the reasons were analyzed.First of all,we observed that the micro-morphology of narrow-pore NH₂-MIL-53（Al）-NP is mainly rhombic,and its internal pore size is an one-dimensional rhomboid pore structure（7.8（?）×19.52（?））,and its functional groups are amino groups and some uncoordinated carboxyl groups.The adsorption of Hg（Ⅱ）on NH₂-MIL-53（Al）-NP accords with the characteristics of chemical adsorption,and it is mainly monolayer adsorption,and the maximum fitting adsorption capacity is 112.75 mg/g.The adsorption process is endothermic and spontaneous reaction,which shows that the higher the temperature is,the more favorable the adsorption is.The initial p H of the solution has a great influence on the adsorption of Hg（Ⅱ）,and the adsorption capacity is obviously different in different acidic environments.The effect of metal ions(Na⁺,K⁺,Ca²⁺,Mg²⁺)on the adsorption of Hg（Ⅱ）is almost negligible,which may be attributed to the selectivity of amino to Hg（Ⅱ）.Hg（Ⅱ）is mainly coordinated with nitrogen atoms and carboxyl groups.In the second part,by adding surfactant（cetyltrimethylammonium bromide）,MOF crystal polymerization and directional assembly are restricted to form a planar aluminum oligomerization structure.2-aminoterephthalic acid substituted cetyltrimethylammonium bromide forms directional coordination with aluminum to assemble large-pore NH₂-MIL-53（Al）-LP with monolayer or few layers.Various characterization methods were used to realize the macro and micro analysis of NH₂-MIL-53（Al）-LP.The pore structure of NH₂-MIL-53（Al）-LP is ordered large pore（13.37（?）×16.39（?））.NH₂-MIL-53（Al）-LP has good water stability,and can quickly and efficiently adsorb and remove Hg（Ⅱ）from aqueous solution.Under the condition of p H=6.0,the theoretical maximum adsorption capacity of NH₂-MIL-53（Al）-LP for Hg（Ⅱ）is as high as 259.58 mg/g.The materials showed good regeneration and stability.In addition,it was found that the adsorption effect of Na⁺on Hg（Ⅱ）was significantly higher than that of other metal ions.The analysis of adsorption mechanism shows that in the pore of NH₂-MIL-53（Al）-LP,Hg（Ⅱ）mainly depends on the coordination with-NH₂,and nitrogen atoms are the main active sites for Hg（Ⅱ）adsorption.In the third part,the"directional attachment growth"mechanism of surface binding is triggered by the addition of sodium acetate.There is competition between acetate and 2-aminoterephthalic acid in binding aluminum centers,which makes the growth rate of[h 0 0],[0 l 0]and[h k 0]direction slower than that in[0 0 1]direction,resulting in the formation of rod crystals,that is,the formation of NH₂-MIL-53（Al）-NR.The one-dimensional diamond channels in MOF exist along the long axis of the rod,and these channels are very favorable for the separation of water pollutants.The NH₂-MIL-53（Al）-NR was qualitatively analyzed by characterization technique,in which the pore structure was super-large（14.03（?）×22.05（?））and the specific surface area was as high as 206.68 m²/g.In addition,NH₂-MIL-53（Al）-NR has strong stability and can adsorb and remove Hg（Ⅱ）in aqueous solution with large capacity.Under the condition of p H=3,the maximum adsorption capacity of Hg（Ⅱ）by NH₂-MIL-53（Al）-NR is 433.2 mg/g.It is also found that the addition of other metal ions has little effect on the adsorption of Hg（Ⅱ）,and the material performs well in reuse.The analysis of adsorption mechanism shows that a large number of nitrogen-containing oxygen-containing groups on the surface and pores of NH₂-MIL-53（Al）-NR are the main coordination sites of Hg（Ⅱ）.