Efficient removal of chromate at wide range of pH values by β-FeC2O4·2H2O with an amplified Kirkendall effect

Abstract

In this study, we investigated the chromate (Cr(VI)) removal from the simulated wastewater by β-FeC2O4·2H2O, the maximum Cr(VI) removal capacity of β-FeC2O4·2H2O was 93.6 mg/g at the initial pH values of 3.0–10.0 and electron utilization efficiencies approached nearly 100%. The morphology and chemical compositions of fresh-prepared and Cr(VI)-reacted β-FeC2O4·2H2O were carefully examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), 57Fe Mössbauer spectrum, energy dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopic (XPS) with depth profiles analysis. The results demonstrated that the Cr(VI) was efficiently removed by β-FeC2O4·2H2O due to the amplified Kirkendall effect. The Cr(VI) was reduced to Cr(III) by Fe(II) when it adsorbed on β-FeC2O4·2H2O. The in situ generated Cr(III) was then co-precipitated with Fe(III) to form Cr(III)/Fe(III) oxyhydroxides layer on the surface of β-FeC2O4·2H2O. Meanwhile, soluble [Fe(III)-(C2O42-)2]- was generated and preferred to diffuse into aqueous solution. Therefore, the channel was formed for outward diffusion of [Fe(III)-(C2O42-)2]-. With assistance of the channel, surface absorbed Cr(VI) could move across the oxide layer facilely and get reduced at inner part. The higher diffusivity of [Fe(III)-(C2O42-)2]- than Cr(VI) drave marked hollow-out phenomenon of the Cr(VI)-reacted β-FeC2O4·2H2O. This mutual mass diffusion process would offset the negative effects of incrassated iron oxide shell, realizing efficient Cr(VI) removal of β-FeC2O4·2H2O.