Performance and mechanism of chromate (VI) adsorption by δ-FeOOH-coated maghemite (γ-Fe2O3) nanoparticles

Earlier studies have demonstrated the effectiveness of γ-Fe2O3 nanoparticles on Cr(VI) removal from wastewater. In this study, δ-FeOOH was used as a surface coating material on the γ-Fe2O3 nanoparticles to enhance Cr(VI) adsorption. Physical characterizations of the synthesized materials were preformed using TEM, XRD and VSM. TEM images showed the multidispersed circular particles with a dimension of 15 nm and VSM patterns indicated the decreased magnetic properties with the surface-coating. The δ-FeOOH-coated γ-Fe2O3 nanoparticles with different mass ratio of δ-FeOOH to γ-Fe2O3 were tested for the adsorption of Cr(VI) and the primary results showed that the optimal ratio was 1.0. Equilibrium studies showed that Cr(VI) adsorption data follow Langmuir model. The Cr(VI) adsorption capacity of the δ-FeOOH-coated γ-Fe2O3, determined to be 25.8 mg/g, is higher than that of pure γ-Fe2O3 (19.4 mg/g). The high stability and recovery capacity of the δ-FeOOH-coated γ-Fe2O3 were verified from regeneration studies and XRD investigations. Moreover, FTIR and XPS results suggest that the Cr(VI) adsorption onto the δ-FeOOH-coated γ-Fe2O3 be mainly controlled by outer-sphere complexation.