Structural elucidation of hexavalent Cr adsorbed on surfaces and bulks of Fe₃O₄ and α-FeOOH

Magnetite (Fe₃O₄) and goethite (α-FeOOH) were synthesized via a hydrothermal approach and utilized as adsorbents for Cr⁶⁺ removal in an aqueous medium. The typical crystal structures of the synthesized Fe₃O₄ and α-FeOOH were confirmed by XRD and TEM. Fe₃O₄ in a spherical shape with a surface area of 32 m² g^-1 was established. While α-FeOOH had a rod-like form with a larger surface area of 84 m² g^-1. Cr⁶⁺ removal in an aqueous solution was studied in various conditions to evaluate thermodynamic and kinetic parameters. The adsorption isotherms on both adsorbents fit the Langmuir model indicating monolayer adsorption. Fe₃O₄ showed a better adsorption ability than α-FeOOH even though it had a lower surface area. XAS and XPS analysis strongly evidenced the production of stable Cr³⁺ species of Fe_(1-x)Cr _x OOH and Fe_(3-x)Cr _x O₄ by Cr⁶⁺ reduction and migration processes into the bulk structure. Thus, the existence of stable Cr-species in Fe₃O₄ structure strongly affected Cr-adsorption ability rather than the surface area of the adsorbent. However, the precipitated Cr₂O₃ and HCrO₄ ^- molecules electrostatically adsorbed on the outer surface of α-FeOOH without bulk transformation. The presence of physisorbed FeO-HCrO₄ species on α-FeOOH led to low reducibility and adsorption capability of Cr⁶⁺.