| It is very important to improve the removal performance in view of the construction of fine structure in adsorbent.Compared with traditional materials,metal organic frameworks(MOFs)are widely used in the resource recovery and treatment of heavy metals in wastewater because of their tailorability and rich structure.It is a key problem of great value and urgent need to build a suitable coordination environment in MOFs-based materials to improve the removal performance of arsenic and antimony in water.In this thesis,by changing the position of functional groups in ligands,the types of ligands,the types of metal coordination centers and the types of adsorption objects,a series of MOFs-based adsorbents are designed and prepared to explore the influence of material coordination environment on the adsorption properties of arsenic and antimony,and the adsorption mechanism is deeply explained combined with theoretical calculation.The main research contents are as follows:1.Three La-MOFs named La-p-PTAs,La-m-PTAs and La-o-PTAs are synthesized by employing p-phthalic acid(p-PTA),m-phthalic acid(m-PTA)or o-phthalic acid(o-PTA)as the organic ligands and La(III)as metal centers to explore the effect of the position difference of functional groups in MOFs adsorbent on the adsorption performance of As(III).Scanning electron microscope images show that La-p-PTAs,La-m-PTAs and La-o-PTAs obtained different morphologies.XRD riveted refinement,thermogravimetry and electron paramagnetic resonance jointly demonstrate that La-o-PTAs obtains more edge adsorption sites and higher oxygen defects.Batch experiments exhibit that rational change of-COOH substituent locations from La-p-PTAs to La-o-PTAs results in better adsorption capacity for As(III)reaching 233.2 mg/g,which is 51.7%higher than that of La-p-PTAs,and the adsorption kinetics reaches the adsorption equilibrium within 30 minutes.Density functional theory(DFT)calculations reveal that the lower adsorption energy,higher charge transfer,and stronger d-p orbital hybridization generate superior As(III)adsorption performance for La-o-PTAs.2.Based on the steric hindrance effect,La-p-PTAs,La-m-PTAs,La-o-PTAs with two other adsorbents named La-MGs and La-MCs,which is constructed with employingN,N’-dicarboxymethyl-2-methylbenzimidazole(MG)and2-methylbenzimidazole(MC)as organic ligands and La(III)as metal center,is conducted to further explore the effect of ligand effect(including the position and type of functional groups)on the adsorption performance of Sb(Ⅴ).Rational change of–COOH substituent locations from La-p-PTAs to La-m-PTAs results in larger BET surface area and bigger pore volume,improving the accessibility of adsorption sites,and makes its adsorption capacity of Sb(Ⅴ)reaching 798.6 mg/g,which is 1.33 times and 1.18 times that of La-p-PTAs and La-o-PTAs.In addition,both experimental results and DFT calculations show that the introduction of imidazole ring contributes to the exposure of edge adsorption sites and charge transfer,reduces the adsorption energy of La-MGs for Sb(Ⅴ),improves the hybridization between La 5d,6s orbits and O 2p orbital in Sb(Ⅴ),and further optimizes its adsorption performance for Sb(Ⅴ)reaching 896.5 mg/g,which is the highest value reported in the literature.3.To depict the impact of metal center effect on heavy metals removal performance in MOFs-based adsorbents,Mg without d orbital electron and displaying bivalent,La with one-d orbital electron and displaying trivalent,and Zr possessing two-d orbital electron and tetravalent are chosen as coordination centers to construct three MOFs-based adsorbents(i.e.,Mg-MGs,La-MGs,and La-MGs)via the ultrasound-assisted solvent approach.The obtained La-MGs with abundant edge adsorption sites achieve Sb(Ⅴ)adsorption capacity of 897.6 mg/g,which is about 1.3and 4.5 times above average than those of Zr-MGs and Mg-MGs,respectively.On account of higher BET surface area and larger pore volume,the sites utilization efficiency of La-MGs(92.1%)is much better than Zr-MGs(75.0%)and Mg-MGs(20.4%).Furthermore,density functional theory(DFT)calculations reveal that La-MGs is more active than Mg-MGs and Zr-MGs,owing to the lower adsorption energy,higher charge transfer and stronger bonding interaction,which will accelerate the reaction kinetics and promote the Sb(Ⅴ)removal performance.The experimental results in practical water indicate that La-MGs effectively capture antimony at low concentration,reaching drinking water standard in samples from Ganjiang River,which indicates a good practical application prospect for antimony adsorption at low concentration.4.Based on the above two systems,it is found that the difference of coordination environment of adsorption sites will affect the adsorption performance.In order to systematically explore this difference,La-MGs with the best performance is selected as the adsorption material,and Sb(Ⅴ),P(Ⅴ),As(III),As(Ⅴ)and Sb(III)with similar structure are selected as the removal objects to explore the influence of the orbital orientation of coordination oxygen atoms in the adsorption object on the adsorption performance.When served for Sb(Ⅴ)removal,the adsorption energy is-2.42 e V,which is much lower than that of the other three pollutants(P(Ⅴ),As(III),As(Ⅴ)and Sb(III)).Batch adsorption experiments exhibit that La-MGs possessed a larger capacity and a faster rate for the uptake of Sb(Ⅴ)compared to other four objects.In addition,the site utilization rate is as high as 92.0%for Sb(Ⅴ)removal,which is 9.8times higher than that of Sb(III).Theoretical calculation shows that the O 2px orbitals of octahedral Sb(Ⅴ)shift to the fermi level and forms a strong bond with the La 5dx~2orbital nearby during the adsorption process,which increases the amount of charge transfer of La-MGs-Sb(Ⅴ)and reduces the adsorption energy of La-MGs for Sb(Ⅴ),thus greatly improving its removal performance of Sb(Ⅴ). |
