Fabrication And Performance Of Electrode Materials For Capacitive Removal Of Heavy Metal Ions

Heavy metal ions have become the most serious water pollution problem,which is a serious threat to human health or water environment.The efficient removal of heavy metal pollutants from complex waters has become a difficult spot.For the removal of a variety of heavy metal pollutants,many workers have made some research methods.However,these methods require extensive pretreatment steps and chemical addition,even the adsorbent is so unstable in the solution that it is difficult to completely remove heavy metal ions from the solution.Capacitive deionization（CDI）is an environmentally friendly,low-energy water treatment technology,which has been paid more and more attention.The electrode materials are very important for deionized capacitors.The carbon-based materials mainly capture charged ions in solution by double-layer mechanism（EDL）.Although the functional modification of carbon materials has some improvement in deionization,its wide application is limited because of its low capacitance.The addition of Faraday material can enhance the deionization performance.Although some Faraday electrodes have high deionization performance,one-component Faraday materials are easily limited by their conductivity and specific surface area,and repeated adsorption and desorption of ions can easily lead to volume expansion.As a result,the electrode has serious attenuation,poor stability and low removal efficiency.The composite of Faraday material and carbon-based material can improve the electrical conductivity and specific surface area,so that there is enough layered space to store more ions,which can effectively improve the removal efficiency and stability of the electrode.Therefore,the work is to design a kind of Faraday and carbon matrix composites with high efficiency capacitance removal ability for heavy metal ions,and study the removal mechanism of heavy metal ions.The main contents are as follows:（1）The metal oxides of iron have high deionization capacity,but the repeated adsorption and desorption of ions can easily lead to volume expansion and poor stability.Fe-N-C materials have good electrochemical stability.This is because the introduction of Fe can adjust the electronic structure and change its physical and chemical properties.The Fe-N-C material are synthesized by using iron tellurium（Fe Te）trapped ZIF-8coated with polydopamine as the precursor.Then the precursor was carbonized in nitrogen atmosphere.We used the Fe-N-C material as cathodes and explored its ability of desalting with different voltage and concentration from salt water via the capacitive deionization system.The salt adsorption capacity of the Fe-N-C electrode reached 41mg/g.On the other hand,we also studied the removal ability of heavy metal ions by Fe-N-C electrode in the mixed solution of Na Cl and various heavy metal ions.In addition,we further explored the ability of the electrode to remove heavy metal ions in mixed heavy metal ion solution.The electrode also showed the high capacitive removal efficiency（90%～98%）of heavy metal ions from complex water systems.At the same time,we also studied the adsorption mechanism of heavy metal ion by XRD,XPS and in-situ Raman.The results show that the adsorption of sodium and heavy metal ions is mainly through electro-adsorption.The removal of heavy metal ions is attributed to the fact that trace iron can provide charge transfer to promote the adsorption of heavy metal ions on the adsorption site.（2）Although Fe-N-C electrode has a certain removal efficiency of heavy metalions,the removal of various heavy metal ions by Fe-N-C electrode is still limited.Therefore,how to further improve the ability of Fe-N-C electrode to remove heavy metal ions is the purpose of this chapter.Sulfur-based materials have been effectively applied to the removal of heavy metal ions.This is because the Lewis base in sulfur-based materials has a high affinity for heavy metal ions.The Fe-S-N-C materials were synthesized by using melamine and thiocyanic acid to form nanotubes,followed by the growth of ZIF-8 and coated polyaniline on its surface to adsorb iron source,and then carbonized at high temperature.Herein,we used the Fe-S-N-C material as cathodes and studied its ability to desalting with different voltage and concentration from salt water through an asymmetric capacitive deionization system.The salt adsorption capacity of the Fe-S-N-C electrode reached 93.5 mg/g.At the same time,we also studied the adsorption mechanism of Na⁺ion by XRD and in-situ Raman.The synergistic adsorption mechanism of Fe-S-N-C composites to sodium ions is mainly through the reversible transition the Na₂S₅ to realize the trapping and detachment of sodium ions.On the other hand,we also studied the removal ability of various heavy metal ions by Fe-S-N-C electrode and the removal ability of heavy metal ions in the mixed solution of Na Cl and various heavy metal ions.In addition,we further explored the ability of the electrode to remove heavy metal ions in mixed heavy metal ion solution.the electrodes also show an efficient ability to remove heavy metal ions from complex water systems.The removal efficiency can reach 99%to 100%.It has been demonstrated that the doping S can not only enhance the activity of Fe-N sites and improve the removal ability of heavy metal ions,but also combine with heavy metal ions by forming covalent bonds of S-clusters on Lewis bases.（3）In the previous paper,the capacitance removal efficiency of heavy metal ions in Fe-S-N-C materials was improved by introducing S doping,but the regeneration process of the electrode was complex,and the adsorption and desorption of heavy metal ions could not be regulated by electrochemistry.Therefore,how to design an electrode material which not only has high efficiency capacitance removal ability for heavy metal ions,but also can regulate the intercalation and removal of heavy metal ions by electrochemistry is the purpose of this chapter.The layered spherical W₁₈O₄₉/graphene material were prepared by compounding ammonium tungstate,graphene oxide and dopamine hydrochloride,and then carbonizing at high temperature.We used the W₁₈O₄₉/graphene material as cathodes and explored its ability of desalting with different voltage and concentration from salt water via the hybrid capacitive deionization system.The salt adsorption capacity of the W₁₈O₄₉/graphene electrode reached 78 mg/g.On the other hand,we also studied the removal ability of various heavy metal ions by W₁₈O₄₉/graphene electrode in the mixed solution of Na Cl and various heavy metal ions.In addition,we further explored the ability of the electrode to remove heavy metal ions in mixed heavy metal ion solution.The electrode also shows the high capacitive removal efficiency of heavy metal ions from complex water systems.The removal efficiency can reach 99%to 100%.At the same time,we also studied the adsorption mechanism of heavy metal ion by XRD,XPS and in-situ Raman.The results show that the intercalation and detachment of ions can be regulated by electrochemistry.The adsorption of sodium ions on W₁₈O₄₉/graphene electrode is mainly through electro-adsorption,while the adsorption of heavy metal ions is mainly through the coupling mechanism of electro-adsorption and electro-reaction.（4）All kinds of heavy metals exist in wastewater.How to selectively remove a specific heavy metal ion from wastewater is a great challenge.The lead ion is one of the most common heavy metal pollutants.Whether lead ions can be selectively adsorbed by electrochemical adsorption in wastewater with a variety of heavy metal ions is the focus of this chapter.The MoO₂/C materials were prepared by compounding ammonium molybdate and dopamine hydrochloride and then carbonizing at high temperature.Here,selective capacitive removal（SCR）of lead ions from wastewater over molybdenum dioxide/carbon（MoO₂/C）electrodes has been originally developed via an asymmetric capacitive deionization（CDI）method.The MoO₂/C spheres as cathodes can effectively reduce the concentration of Pb²⁺from 50 mg/L to<0.21μg/L.Moreover,the SCR efficiency of lead ions over redox-active MoO₂/C electrodes of>99%.Besides,the electrodes present high regeneration performance and high SCR efficiency for lead ions in the mixture solutions of Na Cl and Pb（NO₃）₂.At the same time,we also studied the adsorption mechanism of lead ion by XRD,XPS,in-situ Raman.It demonstrated that the tetrahedral structure of[MoO₄]lattice is more favorable for the intercalation of lead ions.Meanwhile,the in-situ Raman spectroscopy test further discovered that the transition of the crystal interface between[MoO₆]lattice clusters and[MoO₄]lattice clusters could be electrochemically controlled during the SCR process.