Adsorption And Reuse Of Heavy Metal Ions With Magnetic Nanoparticles

Heavy metal ions in water pose a great threat on human health.It is of great emergency to reduce the content of heavy metal ions in water.Separating heavy metal ions from water with magnetic nano-adsorbents is an effective way to deal with heavy metal pollution.However,magnetic nanoparticles(e.g.Fe3O4 colloidal nanocrystal clusters)could be etched into ions and dissolved when exposed to acidic waster water or acidic desorption conditions,not benefiting for cycle use.Previous studies reveal that encapsulating magnetic nanoparticles with silica shells can improve their resistance to acidic environment.However,there still exists some problems for the application of Fe3O4@SiO2 nanoparticles in the removal of heavy metal ions.Firstly,thicker silica shells are needed to improve the acid resistance of Fe3O4#SiO2,which will reduce magnetic separation efficiency of the adsorbents.Secondly,traditional way to deal with the adsorbed metal ions(desorption followed by purification for reuse)is relatively complicated and costly.Facing these problems,a series of researches had been conducted as follows:Part 1:Fe3O4@SiO2 core-shell structure was chosen as the research object to study the factors that could influence the permeability of silica shells.We found the permeability of silica shells is tightly related to their aging degrees.By controlling the sol-gel reaction time,we could tune the aging degree of the silica shell to reduce its permeability without changing its thickness,realizing protection of nanoparticles with thin but low-permeability silica shells.Part 2:Amino fiunctionalized nanopartices(Fe3O4@SiO2 and Fe3O4#SiO2@mSiO2)were used as the adsorbents to remove heavy metal ions from waste water.It is found that the stability under acidic desorption conditions and thereby cycle adsorption performance of Fe3O4@SiO2 based adsorbents was greatly improved by tunning the aging degree of the silica shells.In addition,by coating a layer of mesoporous silica on the surface of Fe3O4@SiO2(denoted as Fe3O4#SiO2@mSiO2)where the middle layer of silica is low-permeability,the adsorbents attained higher specific surface area and thereby larger adsorption capacity(increase from 15.26 mg/g to 23.51 mg/g)apart from strong acidic resistance.Part 3:Adsorbents with adsorbed metal ions(Fe3O4@SiO2@mSiO2-M and Fe3O4@mSiO2-M,M=Co2+ and Ni2+)were reduced and then used in photothermal catalytic and thermocatalytic CO2 hydrogenation reactions,realizing simple and effective reuse of wasted heavy metal ions.The main products in photothermal catalytic and thermocatalytic reactions are both CO,with a maximum production rate of 2.6 mmol·gcat-1·h-1 and 81.8 mmol·gcat-1·h-1,respectively.Notably,the metal ions were reduced into subnanometer particles which were uniformly distributed in the mesoporous silica shell.In addition,it is found that the inner core plays an important role in the catalytic process.On the one hand,the core has catalytic activity in the reactions itself;on the other hand,Fe3O4 with excellent light harvesting ability has improved the overall light absorption performance of the catalyst.The energy of incident light was effectively harvested and transformed into thermal energy by Fe3O4 core which has greatly promoted the catalytic reactions.As a result,the CO production rate was greatly increased(from 0 to 0.31 mmol·gcat·h-1).