The Adsorptive Removal Of Pb And Cd From Water By Two Kinds Of Iron- Based MOFs

Among all of water environment problems in nowadays saciety,pollution caused by heavy metals is always one of the most serious problems because heavy metals are hardly biodegradable which means heavy metals tend to accumulate in environment and eventually lead to unreversed pollution.Lead(Pb)and cadmium(Cd)are two kinds of common heavy metals with serious toxicity.Within the research of heavy metal removal from water,the adsorption methods using nanomaterials has drawn more and more attentions thanks to the development of nanotechnology.Metal-organic frameworks(MOF),which have highly porous structure,large surface area,and adjustable structure,are promising nanomaterials in adsorption applications.This research synthesized two kinds of iron-based MOF,MIL-100(Fe)and Fe-BTC,and then studied their adsorption abilities for Pb and Cd with micro and trace concentration,followed by the evaluation of adsorption processes with theoretical models,accompanied with the exploration of adsorption mechanism.These two materials' removal ability for Pb was much better than Cd in both adsorption systems with micro metal concentration and trace metal concentration.In the adsorption system with micro metal concentration,Fe-BTC showed generally higher removal percentages than MIL-100(Fe)for Pb and Cd under different p H conditons,whereas MIL-100(Fe)with higher porosity structure had faster adsorption equilibrium processes.In the adsorption system with trace metal concentration,MIL-100(Fe)showed more effictive performance.The kinetic research revealed that the pseudo-second order model and the Weber-Morris internal diffusional model could describe the adsorption process.Meanwhile,the phenomenon of multilayer adsorption was observed in adsorption isotherms,which could be well fitted by Langmuir-Freundlich model and FreundlichBET model.In another aspect,the Dubinin-Radushkevich isotherm model emphasized the domination of chemical adsorption with the analysis of adsorption potential.Besides,the study of thermodynamics revealed that the adsorption processes are endothermic,entropy-increasing and spontaneous at different temperatures.As for the adsorption system with trace metal concentration,this research also studied how the removal percentages varied with the influence of coexisting chemicals,which showed that hardness(Ca2+ and Mg2+)of water could lower the removal percentages of Pb and Cd,whereas the alkalinity of water raised the removal percentages of Pb and Cd.Besides,HA caused the apparent decrease of removal percentages of Pb,but hardly influenced the removal percentages of Cd.The experiments results and the theory analysis revealed that the ion-exchange interaction and the electrostatic interaction account for the adsorption mechanism in this research,which means the adsorption of Pb and Cd by these two materials are the combined effects of physical adsorption and chemical adsorption.Furthermore,it is reasonable to explain the multilayer adsorption phenomenon by the Stern electric double layer theory,the BDM theory and the inner-/outer-sphere complexation model.In addition,the bad removal performance of Cd is attributes to lower solubility of Cadmium salts,larger hydration energy of Cd2+ and weaker ion-exchange ability of Cd2+.This research contributes to the research field of heavy metals removal by metal-organic frameworks,and is supposed to be a theory reference for other similar researches.