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+ .