Adsorptive Removal Of Cationic Metal Ions Over Modified Metal-organic Frameworks: Performances And Mechanisms

Heavy metal pollution poses an immediate threat to ecosystems and human health.Consequently,how to treat metal-containing wastewater production more efficiently,green and low-carbon is one of the key problems that need to be urgently solved at present.Adsorption technology has become increasingly important in the field of water pollutant treatment due to its cost-effective,low waste,energy-conserving and easy-recycling.Metal-organic frameworks（MOFs）with large surface areas,high porosity and controlled structures are considered as excellent metal cation adsorbents.However,MOFs are facing challenges such as poor water stability and limited adsorption sites in practical water purification applications.To tackle this need,this article prepared several modified MOFs by modulating the framework structures to introduce more active adsorption sites,for the adsorption and removal of harmful metal ions(Pb²⁺,Ba²⁺,Cd²⁺,Cs⁺,Sr²⁺,Eu³⁺and La³⁺)as well as the selective recovery of precious metal ions（Ag⁺）.The adsorption performance of adsorbents and the influence of various factors existed in the actual water during adsorption process were explored.The corresponding adsorption mechanism was obtained through a series of characterization analyses.Particularly,the fixed-bed continuous flow adsorption devices were used to evaluate the practicability of MOF adsorbents.The main findings and results of this thesis are as follows.（1）In order to improve the adsorption efficiency and capacity of the adsorbents,a post-synthesis method was used to modulate sulfonic acid functionalized SO₃H-MIL-101（Cr）.The as-prepared defective SS-SO₃H-MIL-101（Cr）-X（X represents the concentration of the modifier,X=2,3,and 4,respectively）with abundant oxygen vacancies were prepared successfully,in which the defect ratio could be regulated by the modulator concentrations.The lead ions(Pb²⁺)were selected as the target pollution to investigate the adsorption performance,behavior and the p H influence for adsorption capacity of SS-SO₃H-MIL-101（Cr）-X in aqueous solution.The results indicated that the modulated SS-SO₃H-MIL-101（Cr）-X outperformed the pristine SO₃H-MIL-101（Cr）in all the metal cations capture tests,in which the optimal SS-SO₃H-MIL-101（Cr）-3 exhibited satisfactory Pb²⁺adsorption kinetics（within 30 min）,adsorption capacity(189.6 mg·g^-1)and the excellent desorption recirculation performance（removal efficiencies>95%after 5 cycles）,as well as the desirable stability.Moreover,SS-SO₃H-MIL-101（Cr）-3demonstrated excellent sorption performances toward all the selected metal ions(Pb²⁺,Ba²⁺,Cd²⁺,Cs⁺,Sr²⁺,Eu³⁺,La³⁺,Ag⁺,10 mg·L^-1)with the nonspecifically capture efficiencies of>99%.Moreover,for the multiple heavy metals-containing wastewater(2 mg·L^-1),SS-SO₃H-MIL-101（Cr）-3 also exhibited remarkable performance in capturing multiple metal cations synchronously with a removal efficiency of>98%.The possible adsorption mechanism of SS-SO₃H-MIL-101（Cr）-X toward metal cations was explored systematically via both X-ray photoelectron spectroscopy（XPS）determination and density functional theory（DFT）calculation.（2）In order to prevent the occupation of active adsorption sites by coexisting cations,and achieve the selective adsorption and recovery toward precious metals,the amino-functionalized NH₂-MIL-125 was prepared via solvothermal method to capture Ag⁺,in which the adsorption performance and mechanisms were further explored.The results revealed that NH₂-MIL-125exhibited superior adsorption performance toward Ag⁺ions with uptake capacity of 192.5 mg·g^-1 within 60 min.The isotherm and kinetic data were exactly fitted to both Langmuir and Pseudo-second-order models,the adsorption process was spontaneous,exothermic and disordered.Furthermore,NH₂-MIL-125 exhibited a certain selectivity toward silver ions under cations(Cu²⁺,Zn²⁺and Fe³⁺)competitive conditions,in which the Ag⁺adsorption efficiency of NH₂-MIL-125 within the single Ag⁺,Ag/Cu,Ag/Zn and Ag/Fe binary solution,as well as the multivariate solutions were 75.32%,73.68%,75.09%,69.61%and 69.22%,respectively.The adsorptive selectivity was due to the more negative adsorption energy and the shorter bond distance.Being judged from both the experimental findings and the theoretical calculation results,it was concluded that the coordinative interactions between nitrogen or oxygen atoms of NH₂-MIL-125 and silver along with the electrostatic interactions were probable adsorption mechanisms.Based on the 3Rs（reduce,recycle and reuse）approach,the used NH₂-MIL-125saturated with Ag⁺ions could be either desorbed to release the Ag⁺for NH₂-MIL-125 re-generation or further calcinated into Ag/C/Ti O₂ photocatalyst to accomplish the resource recovery.Due to that the Ag NPs could heavily inhibited the recombination of the photo-generated e^–/h⁺pairs,the obtained Ag/C/Ti O₂ exhibited significantly degradation performance toward MB（methylene blue）,in which 96.5%MB could be degraded within 80 min.（3）To improve the practicality of MOF adsorbent and prevent the separation of powder adsorbent and accomplish the continuous flow purification,fixed-bed column was constructed to continuous adsorb heavy metal cations.Firstly,the powder SS-SO₃H-MIL-101（Cr）-3 was employed for various metal cations adsorption,in which the concentrations of all the selected metal ions in effluent were reduced from 5.0 mg·L^-1 to trace level(0.01～2.5μg·L^-1).Later,for the purpose of preventing the drawbacks of the pipeline blockage and secondary pollution result from the large adsorbent loss,NH₂-MIL-125 was immobilized on the cotton fiber to obtain NH₂-MIL-125@Cotton for Ag⁺continuous adsorption,in which the per unit（mg）NH₂-MIL-125 within the compound could treated 8.6 L 1 mg·L^-1 Ag⁺-containing wastewater continuously.This convenient approach maintained the excellent adsorption performance of NH₂-MIL-125,overcame the low mass transfer rate of traditional immobilized medium,maximized the utilization efficiency and the scope for practical applications of powder adsorbents.