Simulation Experiment Study On Polonium Purification And Detection From Radioactive Wastewater

Polonium is an important radionuclide produced in dealing with spent fuel,mining uranium resources and nuclear accidents of lead-bismuth cooled reactors.It is extreme toxic and radioactive.Its purification and monitoring is one of the main considers for the sustainable development of nuclear energy.In this work,non-radioactive tellurium was used to substitute radioactive and toxic polonium.The adsorption behavior and mechanism of Te(?)by silver loaded stainless steel mesh and graphene oxide were studied.Based on that,graphene oxide was chose as precursor and vitamin C(Vc),sodium hydrogen sulfite(NaHSO3)and ethylenediamine(EDA)were used as reducing agents.Silver nanoparticles/graphene oxide hydrogels were fabricated by reduction-induced self-assembly method.The hydrogel prepared by EDA reduction was determined as surface-enhanced Raman scattering(SERS)substrate,basing on which the detection of trace Te(?)in aqueous solution was studied.The conclusions are listed as following:The effects of copper electroplating current density,contact time and the concentration of coexisting iodine ions on the Te(?)accumulation on silver loaded stainless steel meshes(SNCAs)were investigated.The result showed that the copper electroplating current density has a significant positive effect on the adsorption of Te(?),which resulted from the increase of silver coverage with increasing copper electroplating current density.The Te(?)accumulation by SNCAs deposited silver for 2.5 hours(SNCA-2.5)and 0.5 hours(SNCA-0.5)can be obviously divided into two phases,rapid rising at the early stage and steady state at the later one.However,the percentage of Te(?)accumulation on the stainless steel mesh deposited silver for 1.5 hours(SNCA-1.5)increased with a constant rate,and reached up to 63±0.1%after contacting for 64 h.The time-dependent variation of Ag surface areas was the main reason for the experimental phenomena.The Te(?)adsorption by SNCA first decreased with the increasing coexisting iodine ions(?-)and then increased,which were the results of the competitive adsorption between ?-and Te(?)species and the variation of contacting area between Ag and Te(?).Microstructure and compositional analyses indicated that tellurium accumulated on the surface of silver layer mainly exists in the form of tetravalent oxides and compound Ag2Te,confirming that the spontaneous deposition of Te(?)and the chemical reaction of Te(?)with Ag under acidic condition were two main mechanisms involved in tellurium(?)adsorption by SNCAs.The effects of contacting time,pH and concentration of graphene oxide on Te(?)adsorption by graphene oxide were studied.The results represent that with the the increase of contact time,the binding sites on graphene oxide decreased,so the adsorption of Te(?)was increased at first and then stopped.The pH of aqueous solution had a powerful effect on Te(?)species.The Te(?)accumulation ascended at first and then descend due to the influence of Te(?)species and the affinity of functional groups to them,and it reached maximum at about 4.7.With the increasing concentration of graphene oxide,the Te(?)accumulation raised first and then kept constant.The experimental phenomenon resulted from the increase of adsorption sites and the agglomeration.Based on the experiments mentioned above,the kinetics and isotherms of Te(?)adsorption by graphene oxide were examined.The results suggested that the kinetic data of Te(?)adsorption by graphene oxide highly conformed to pseudo-second-order kinetic equation and the adsorption equilibrium data accorded with Sips and Freundlich equations.It illustrated that the binding sites on the surface of graphene oxide were heterogeneous and the Te(?)accumulation was non-monolayer coverage.For the progress of Te(?)adsorption,the rate-limiting step at early stage,transformed from intraparticle diffusion to membrane diffusion gradually.It can be concluded that Te(?)adsorption by graphene oxide is a complicated process involved various interactions.Microstructure and compositional analyses indicated that the chemical state of Te(?)was not changed in Te(?)adsorption by graphene oxide,and the adsorption mechanisms involved the interactions of epoxy groups,carboxyl groups and the benzene ring structure to Te(?).By means of surface enhanced Raman spectroscopy(SERS),an experimental study on the detection of trace Te(?)in aqueous solution was carried out.We found that the Raman peaks of Te-O at 584.9 cm-1 and Te=O=Te at 642.9 cm-1 were significantly enhanced on the EDA reduction-induced self-assembly silver nanoparticles/graphene oxide hydrogel.In contrast,the Raman signals on the hydrogels prepared by NaHSO3 and Vc reduction were not obviously strengthened.Therefore,the hydrogels fabricated by EDA were selected as substrate materials,and the effect of pH and Te(?)concentration on the Raman signals of the substrate after adsorption were further examined.The experimental results indicated that the pH=5.5 and the peak at 584.9 cm-1 were determined as the detection condition and indicative sign.In the concentration range of 7.84×10-9?7.84×10-5 M,the intensity of the signal descended with the decrease of Te(?)concentration and the detection limit was about 100 nM.In summary,silver is a strong adsorbent for Po(?)and graphene oxide is a favorable matrix with good adsorptivity for trace Po(?).This work is expected to provide potential new effective purification and detection methods for Po in radioactive wastewater.