Effects Of Coexistent Divalent Heavy Metal Cations On The Synthesis And Characteristics Of Magnetite

Due to the scarcity of groundwater resource and increasing consumption of water,our country is facing a serious crisis of groundwater,which is aggravated by serious groundwater pollution.Among all kinds of groundwater contaminants,heavy metals have attracted widespread attention for several decades due to their high toxicity,strong concealment and strong transport property.Various remediation methods have been developed to remediate heavy metal pollution in groundwater.Since there is a close relationship between the iron(Fe)cycling and the environmental behaviors of heavy metals,Fe-containing substances are widely used in various remediation methods.However,these remediation methods suffer from high cost,poor remediation effect and long-term instability.Thus,the remediation of groundwater contaminated by heavy metals is still a great challenge,and new remediation methods need to be developed.Recent studies have demonstrated that in situ formation of magnetite might be an effective method to more permanently immobilize metal(loid)s.However,how the co-existence of additional heavy metals affects the synthesis of magnetite under environmentally relevant conditions and how it affects the associated heavy metal removal efficiency have remained unclear.Here,simulation experiments were carried out in a simplified groundwater environment The effects of three representative heavy metals including Cu2+,Cd2+and Pb2+on the formation of magnetite and the adsorption capacity of mineral products were explored by X-ray diffraction(XRD),Mossbauer spectroscopy,scanning electron microscope(SEM),specific surface area measurement and inductively coupled plasma optical emission spectrometry(ICP-OES).This study can improve our understanding the ways of Fe mineral transformation and the influence factors in groundwater environment,and also provide ideas for improving the content of magnetite formation and the remediation effect of in situ formation of magnetite.The main results are as follows:1.The ways of immobilizing heavy metals by Fe mineralsMany studies have shown that Fe minerals can remove heavy metal ions from aqueous solution by lattice substitution and surface adsorption.Here,the ways of immobilizing heavy metals by in situ formation of magnetite were explored by XRD,Mossbauer spectroscopy and ICP-OES.Under near-neutral pH condition,mineral samples were synthesized under the condition of Fe:Me=100:l and 10:1(Me=Cu,Cd or Pb)by coprecipitation method.The fresh minerals were obtained by by washing the synthesized mineral precipitation.The crystalline Fe minerals were obtained by removing the amorphous minerals and surface adsorbed ions from mineral precipitates by the isolation including acid wash,DI water wash and magnetic separation steps.The element compositions of crystalline Fe minerals synthesized under different conditions were compared and analyzed by chemical digestion and ICP-OES analysis.The results showed that Cu2+,Cd2+and Pb2+were incorporated into the structure of Fe minerals,and the content was positively correlated with the concentration of heavy metals in the solution.The hyperfine magnetic field distributions of fresh Fe minerals provided another line of evidence for structural incorporation.Due to the similar atomic radius,Cu2+and Cd2+could enter the lattice of Fe minerals,while Pb2+would enter the structure by incorporation or adsorption at the defects of Fe minerals.Based on the comparative analysis of elemental composition of fresh Fe minerals and crystalline Fe minerals,the Fe:Cu(Pb)ratio of Fe minerals changed greatly after isolation,indicating that Cu2+or Pb2+could be immoblized on the surface and lattice of Fe minerals,and mainly adsorbed on the surface.However,the small change of Fe:Cd ratio indicated that Cd2+was mainly immoblized in the Fe mineral lattice.2.Effects of heavy metals on the composition and morphology of Fe mineralsThe synthesized mineral products were characterized by Mossbauer spectroscopy,XRD,SEM-EDS,magnetic susceptibility and specific surface area measurement.The results demonstrated that neutral pH resulted in poor crystallinity of Fe minrerals and low magnetite content.Based on the comparative analysis of the phase composition of Fe minerals synthesized with and without heavy metals,the results showed that heavy metal ions inhibited the formation of crystalline Fe minerals by incorporating the structure of Fe minerals.The mineralogy and morphology of Fe minerals were regulated by the surface affinity of heavy metals.The co-existence of divalent heavy metal cations that had higher surface affinity than Fe2+inhibited the growth of more crystalline Fe minerals,including magnetite.On the countrary,heavy metal ions whose surface affinity is lower than Fe2+had no obvious effect.3.Effects of heavy metals on the adsorption capacity of Fe mineralsThe adsorption capacity of the crystalline Fe minerals for heavy metals were determined based on adsorption isotherms.Isotherm experiments were used to explore the effects of heavy metals on the adsorption capacity of Fe minerals based on ICP-OES analysis.The results showed that the coexistence of heavy metal ions reduced the adsorption capacity of Fe minerals.There were two possible reasons for the decrease of adsorption capacity.First,the heavy metals with strong surface adsorption capacity decreased the magnetite content of Fe minerals,which reduced the adsorption capacity of Fe minerals.Second,heavy metals entering the lattice of Fe minerals increased the pHpzc of Fe minerals,which also reduced the adsorption capacity of Fe minerals.