Structural elucidation of hexavalent Cr adsorbed on surfaces and bulks of Fe(3)O(4) and α-FeOOH

Magnetite (Fe(3)O(4)) and goethite (α-FeOOH) were synthesized via a hydrothermal approach and utilized as adsorbents for Cr(6+) removal in an aqueous medium. The typical crystal structures of the synthesized Fe(3)O(4) and α-FeOOH were confirmed by XRD and TEM. Fe(3)O(4) in a spherical shape with a surface area of 32 m(2) g(−1) was established. While α-FeOOH had a rod-like form with a larger surface area of 84 m(2) g(−1). Cr(6+) 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. Fe(3)O(4) 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 Cr(3+) species of Fe((1−x))Cr(x)OOH and Fe((3−x))Cr(x)O(4) by Cr(6+) reduction and migration processes into the bulk structure. Thus, the existence of stable Cr-species in Fe(3)O(4) structure strongly affected Cr-adsorption ability rather than the surface area of the adsorbent. However, the precipitated Cr(2)O(3) and HCrO(4)(−) molecules electrostatically adsorbed on the outer surface of α-FeOOH without bulk transformation. The presence of physisorbed FeO–HCrO(4) species on α-FeOOH led to low reducibility and adsorption capability of Cr(6+).