| Arsenic and chromium are ubiquitous elements in nature.In natural water,arsenic exists mainly in the form of As(Ⅲ)and As(Ⅴ),while chromium is primarily present in the form of Cr(Ⅵ))and Cr(Ⅲ).As(Ⅲ)is considered much more toxic than As(Ⅴ),and Cr(Ⅵ))is much more toxic than Cr(Ⅲ).Arsenic and chromium are highly toxic and carcinogenic,both of which are listed in the priority pollutant list in China and need priority monitoring and control.Due to its abundance in nature,high surface area and lager number of surface active site,iron(hydr)oxide plays an important role in affecting the migration and transformation of contaminants in aquatic environments.However,iron(hydr)oxide is rarely found as a pure phase and instead often contains foreign ions.Al is the third most abundant element in the Earth’s crust,and it is often found in iron(hydr)oxide.Therefore,iron(hydr)oxide usually contains a certain amount of aluminum(hydr)oxide.In this study,both As(Ⅲ)and Cr(Ⅵ))were used as target contaminants.Fe-Al bimetal oxides and Al-containing ferrihydrite were prepared by chemical coprecipitation method to remove As(Ⅲ)and Cr(Ⅵ))from wastewater.Batch experiments were carried out to explore the effect of Fe/Al molar ratio and initial pH on the removal of As(Ⅲ)and Cr(Ⅵ)).X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),specific surface area analyzer(BET),Fourier transform infrared spectroscopy(FTIR),and X-ray photoelectron spectroscopy(XPS)were used to characterize the crystalline structure,morphology,specific surface area,pore structure,functional groups,element valence state of the samples before and after reaction,respectively.Finally,the removal mechanism of As(Ⅲ)by Fe-Al bimetal oxides and As(Ⅲ)and Cr(Ⅵ))simultaneous removal mechanism by Al-containing ferrihydrite were proposed,respectively.The main conclusions of the study are as follows:(1)Fe-Al bimetal oxide is a mesoporous material with high specific surface area,and it can remove As(Ⅲ)efficiently in a wide pH range.When the Fe-Al bimetal oxide dosage is3.0 g/L,the removal efficiency of 10 mg/L As(Ⅲ)can reach 99% within 90 min in the pH range of 3.0-11.0.The removal efficiency of As(Ⅲ)by Fe-Al bimetal oxide was high within5 cycles under the neutral condition.Fe-Al bimetal oxide performed much better than their single metal oxide for As(Ⅲ)removal.(2)The removal of As(Ⅲ)from wastewater by Fe-Al bimetal oxide was proposed as the following three steps.First,As(Ⅲ)was adsorbed onto the surface of Fe-Al bimetal oxides due to its complexation with surface-OH of Fe-Al bimetal oxides.Second,part of As(Ⅲ)in aqueous solution and on the surface of Fe-Al bimetal oxides was oxidized to As(Ⅴ)by dissolved oxygen.Finally,the generated As(Ⅴ)can be adsorbed onto the surface of Fe-Al bimetal oxides.(3)Al-containing ferrihydrite is a poorly crystalline material with a lager surface area,and it can quickly remove As(Ⅲ)and Cr(Ⅵ))under acidic and neutral conditions.With the increase of Al molar percentage in ferrihydrites from 0 to 30,the adsorption capacity of As(Ⅲ)decreased,whereas it increased for Cr(Ⅵ)).On the other hand,with the increase of pH value from 3.0 to 11.0,the decreasing rate of As(Ⅲ)was accelerated first,then slowed down,whereas the Cr(Ⅵ))decreasing rate slowed down dramatically.The redox conversion between As(Ⅲ)and Cr(Ⅵ))occurred after the two contaminants were coadsorbed onto the surface of Al-containing ferrihydrite.(4)The simultaneous removal of As(Ⅲ)and Cr(Ⅵ))by Al-containing ferrihydrite includes coadsorption and redox conversion behaviors.First,both As(Ⅲ)and Cr(Ⅵ))can be adsorbed onto the surface of Al-containing ferrihydrite due to electrostatic interaction and inner-sphere complexation.The electron may transfer from the As atoms,pass through the solid phase of Al-containing ferrihydrite,and finally reach the Cr atoms,thus leading to the oxidation of As(Ⅲ)to As(Ⅴ)and the reduction of Cr(Ⅵ))to Cr(Ⅲ).The redox conversion between As(Ⅲ)and Cr(Ⅵ))can greatly lower the environmental hazard of the two contaminants. |
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