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

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

Nichapha Senamart  1 , Krittanun Deekamwong  2 , Jatuporn Wittayakun  2 , Sanchai Prayoonpokarach  2 , Narong Chanlek  3 , Yingyot Poo-Arporn  3 , Suttipong Wannapaiboon  3 , Pinit Kidkhunthod  3 , Sirinuch Loiha  1
Affiliations

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

Nichapha Senamart et al. RSC Adv. .

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+.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. XRD patterns of as-synthesized and Cr-adsorbed (a) Fe3O4 and (b) α-FeOOH.
Fig. 2
Fig. 2. TEM images of (a) α-FeOOH and (b) Fe3O4.
Fig. 3
Fig. 3. Effect of pH on Cr6+ uptake by the Fe3O4 and α-FeOOH adsorption condition: Cr6+ initial concentration = 1 ppm, adsorbent dose = 0.4 g L−1, time = 30 min.
Fig. 4
Fig. 4. Reusability for Cr6+ removal of Fe3O4 and α-FeOOH adsorbents with Cr6+ initial concentration = 1 ppm, adsorbent dose = 0.4 g L−1, pH = 3, time = 30 min.
Fig. 5
Fig. 5. Fe 2p XPS spectra before and after Cr6+ adsorption of (a) Fe3O4 and (b) α-FeOOH and O 1s spectra before and after Cr6+ adsorption of (c) Fe3O4 and (d) α-FeOOH.
Fig. 6
Fig. 6. Cr 2p XPS spectra of Cr@Fe3O4 and Cr@FeOOH.
Fig. 7
Fig. 7. Fe K-edge XANES spectra of bare and Cr-adsorbed (a) Fe3O4 and (b) α-FeOOH and (c) Fe L3-edge XANES spectra of Cr@Fe3O4 and Cr@FeOOH comparing with the iron standards.
Fig. 8
Fig. 8. Cr K-edge XANES spectra of Cr@Fe3O4 and Cr@FeOOH comparing with Cr-standards.
Fig. 9
Fig. 9. O K-edge XANES spectra (a) and their derivatives (b) of bare and Cr-adsorbed Fe3O4 and α-FeOOH.
Fig. 10
Fig. 10. Fourier transforms of the k3 (χ) weighted in Cr K-edge EXAFS data collected from (a) Cr@Fe3O4 and (b) Cr@FeOOH.
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