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

Nichapha Senamart; Krittanun Deekamwong; Jatuporn Wittayakun; Sanchai Prayoonpokarach; Narong Chanlek; Yingyot Poo-Arporn; Suttipong Wannapaiboon; Pinit Kidkhunthod; Sirinuch Loiha

doi:10.1039/d2ra03676b

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

RSC Adv. 2022 Sep 8;12(39):25578-25586. doi: 10.1039/d2ra03676b. eCollection 2022 Sep 5.

Authors

Nichapha Senamart¹, Krittanun Deekamwong², Jatuporn Wittayakun², Sanchai Prayoonpokarach², Narong Chanlek³, Yingyot Poo-Arporn³, Suttipong Wannapaiboon³, Pinit Kidkhunthod³, Sirinuch Loiha¹

Affiliations

¹ Center for Innovation in Chemistry (PERCH-CIC), Materials Chemistry Research Center, Department of Chemistry, Faculty of Science, Khon Kaen University Khon Kaen 40002 Thailand sirilo@kku.ac.th.
² School of Chemistry, Institute of Science, Suranaree University of Technology Nakhon Ratchasima 30000 Thailand.
³ Synchrotron Light Research Institute Nakhon Ratchasima 30000 Thailand.

Abstract

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⁺.