Effects Of Citrate And Aluminum Ion On The Immobilization Of Heavy Metals By Iron-bearing Minerals And Their Mechanism Research

In recent years,with the rapid development of industry and mining,large amounts of industrial and mining wastewater have been discharged into the environment.The wastewater contains large amounts of hexavalent chromium(Cr(Ⅵ))and pentavalent antimony(Sb(Ⅴ)),and the pollution of the water and soil environments has caused widespread concern worldwide.Due to the high adsorption capacity of iron-bearing minerals,it becomes a research hotspot in heavy metal pollution treatment.The natural environment is complex,and organic acids and metal ions are common exist in water and soil.However,the process of iron-bearing minerals immobilizing heavy metals will be affected by organic acids and metal ions,and it is of far-reaching environmental significance to study the influences of them on iron-bearing minerals immobilizing heavy metals.In this study,the effects of structural Fe of nontronite and aqueous Fe(II)on the removal of Cr(Ⅵ)in the presence of organic acids and the effect of Al(Ⅲ)on the immobilization of Sb(Ⅴ)by ferrihydrite were studied.The effects of citrate and aqueous Fe(II)on the removal of Cr(Ⅵ)by nontronite in different systems and p H values were studied.The effect of Al(Ⅲ)on the immobilization of Sb(Ⅴ)on ferrihydrite and the effect of Sb(Ⅴ)on the substitution of Al(Ⅲ)for ferrihydrite were studied to understand the interaction among Al(Ⅲ),Sb(Ⅴ)and ferrihydrite.Scanning electron microscopy,Fourier transform infrared spectroscopy,transmission electron microscopy,X-ray diffraction,Raman spectroscopy and X-ray photoelectron spectroscopy were used to characterize the structure,morphology,surface composition and removal mechanism of the samples.The main conclusions are as follows:(1)Citrate has no effect on the removal of Cr(Ⅵ),while citrate can improve the removal effect of nontronite.Citrate plays two main roles in this process,one is to combine with nontronite as a reducing agent,the other is to promote the adsorption and reduction of more Cr(Ⅵ)by nontronite.(2)The removal of Cr(Ⅵ)by nontronite is mainly by adsorption and reduction.Citrate can dissolve the iron oxide on the surface of nontronite and the Fe-Cr precipitate produced in the reaction process.This process can accelerate the iron redox cycle between Fe(II)and Fe(Ⅲ)in the structure of nontronite and improve the removal rate of Cr(Ⅵ).In addition,citrate can combine with aqueous Fe(II)to form Fe-citrate complex to reduce Cr(Ⅵ).The aqueous Fe(II)and structural Fe of nontronite are competitive to citrate.(3)Co-precipitated Al(Ⅲ)in ferrihydrite reduces the extractable Sb(Ⅴ)in ferrihydrite under neutral and alkaline conditions.During aging proceeds,the ability of ferrihydrite to immobilize Sb(Ⅴ)will increasing.Co-precipitated Al(Ⅲ)significantly inhibited the transformation rate of ferrihydrite,while Sb(Ⅴ)did not have a large effect on the transformation rate of ferrihydrite.When Al(Ⅲ)and Sb(Ⅴ)exist together on ferrihydrite,only hematite is the final product,which can be inferred that Al(Ⅲ)plays a decisive role in the species of transformation product of ferrihydrite.(4)Sb(Ⅴ)promotes the transformation of Al-coprecipitated ferrihydrite,which can be explained by the competitive substitution of Al(Ⅲ)and Sb(Ⅴ)for the lattice Fe(Ⅲ)of ferrihydrite.This competitive process reduces the substitution of Al(Ⅲ)for ferrihydrite,thus weakening the inhibition ability of Al(Ⅲ).Besides,the presence of Sb(Ⅴ)can modify the point of zero charge that is changed by Al(Ⅲ),which can weaken the effect of Al(Ⅲ)on the aggregation and nucleation and thereby shorten the transformation period of ferrihydrite to hematite.