Performance And Mechanism Of Metal-organic Framework Materials For Removal Of Nickel Complexes

Advanced treatment of heavy metal complexes is a major challenge currently.Among metals,nickel（Ni）has a wide range of use but lower emission concentration limits.Ni and organic acid complexing agents（ethylenediaminetetraacetic acid:EDTA;citric acid:Citrate）form a stable metal complex,which makes the treatment of nickel containing wastewater more difficult.In order to solve this problem,this research is based on the porous adsorption capacity and metal-ligand double exchange function of metal-organic frameworks（MOFs）,this work is proposed that the iron-based MOFs material（MIL-101（Fe））with a large surface area was used to the advanced treatment of typical nickel-plated wastewater（Ni-EDTA and Ni-Citrate）.The effects of temperature,p H,material concentration,contaminant concentration and molar ratio of metal-complexing agent on the performance of MIL-101（Fe）materials for the removal of nickel containing wastewater were investigated.Expanding the new application of MOFs materials in the field of advanced treatment of heavy metal complex wastewater,and its possible mechanism provides new ideas for the advanced treatment of this type of wastewater.The main research contents are as follows:（1）In this project,MIL-101（Fe）material with uniform pore size and high specific surface area（1983 m²/g）was synthesized by hydrothermal method.The material showed good overall removal effect of Ni-EDTA,and its depth removal could be achieved in a short time,and reached discharge standard.Specifically:when the material dosage is 0.75 g/L,the concentration of Ni is 0.085 m M,the reaction temperature is 20℃,the removal efficiency of Ni can reach 99.8%,and the residual Ni concentration is less than 0.1 mg/L within 30 minutes.Combined with experiments and material characterization（including ATR-FTIR and XPS etc.）,the mechanism of Electrostatic adsorption synergistic metal ligand double exchange of MIL-101（Fe）in the Ni-EDTA removal process was revealed.On the one hand,42%of the Ni-EDTA in the system can be adsorbed on the material as a whole due to the positive charge nature of the surface of MIL-101（Fe）material;On the other hand,the terephthalic acid ligand in MIL-101（Fe）skeleton can be replaced by EDTA^4-ligand,and ligand exchange occurs,accompanied by metal Ni²⁺replacement of Fe³⁺in the skeleton,thereby leading to the efficient removal of Ni-EDTA.At the same time,MIL-101（Fe）showed a highly selective removal performance for Ni-EDTA.（2）Taking nickel citrate（Ni-Citrate）complex as the target pollutant,add MIL-101（Fe）material in the Ni-Citrate solution,to promote the rapid decomplexation of Ni-Citrate（reaction 30 min）and form a free Ni²⁺.Under the precipitation action of weak base（p H=10.5）,the efficient removal of Ni²⁺was realized,and the concentration of Ni in the wastewater was less than 0.1 mg/L.The possible mechanism of removing Ni-Citrate by MIL-101（Fe）was studied by investigating p H,material concentration,pollutant concentration and molar ratio of metal-complexing agent,etc.Through thermogravimetric analysis of the materials before and after Ni-Citrate treatment,combined with XPS and ATR-FTIR characterization,it is revealed that the possible mechanism of mechanism of MIL-101（Fe）promoting Ni-Citrate deactivation was Fe³⁺substitution of Ni²⁺in Ni-Citrate.Along with the exchange of Citrate^3-ligand and terephthalic acid ligand,MIL-101（Fe）-Citrate intermediate was formed,which inhibit the re-complexation of Citrate^3-with metal Ni²⁺.Thus,the efficient removal of Ni-Citrate can be realized by alkali precipitation.