Cr(VI) reduction by activated carbon and non-living macrophytes roots as assessed by Kβ spectroscopy

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Abstract

In this work, the behavior of cationic-exchange resin, activated carbon, and non-living aquatic macrophytes biomasses on the Cr(VI) and Cr(III) uptake and Cr(VI) reduction was investigated. The high-resolution X-ray fluorescence (HR-XRF) technique was used to study the adsorption process, as well as to study Cr(VI) reduction and removal from metal solutions. Batch Cr ions sorption experiments at pH 3.5 were carried out in order to speciate 3d-transition metal onto the surface of these types of adsorbents by a Kβ spectra analysis. Cr-Kβ satellite lines have been characterized for all Kβ spectra of Cr ions onto treated samples and reference material. Based on their energy position and intensity of Cr-Kβ satellite lines as well as their related to reference material shift energy, activated carbon and non-living aquatic macrophytes roots were found to act mainly as good adsorbents, first reducing Cr(VI) to Cr(III) and then followed by a Cr(III) adsorption. Although cationic-exchange resin was treated with Cr(VI) solution, no evidence of any Cr-Kβ spectral satellite lines was shown in it, suggesting that Cr(VI) was not removed in a cationic-exchange process. Evidence of reduction of hexavalent chromium by adsorbent materials was assessed by Kβ spectral lines analysis.

Introduction

Environmental pollution by the introduction of heavy metals into bodies of water is of growing concern because of health risks on living organisms and humans. Hexavalent and trivalent chromium are released into the environment from a number of different industrial activities such as iron and steel manufactory, tannery, chromium plating and other anthropogenic sources [1]. Usually, conventional treatment methods are applied in industries for Cr(VI) removal by its reduction to Cr(III) with reducing agents, followed by a precipitation of a hydroxide form of Cr(III).

Many treatment technologies have been proposed to solve this problem and replace the high cost, conventional treatment process. Among these technologies, adsorption that is one most frequently used method has gained increased creditability during the last decades [2], [3]. There are many types of adsorbents that have been studied for the metal ions adsorption from aqueous solutions, such as activated carbon [4], sawdust [5], [6], chitosan [7], chelating resins [8], [9], clay mineral [10], [11], non-living aquatic plants [12], [13], [14] and wetland plants [15], [16], [17], [18], [19].

In order to gain information about these chemically important systems, X-ray emission spectroscopy could be an adequate tool in the study of the 3d metal removal process such as chromium ions in different kind of materials. A recent review of Kα and Kβ spectroscopy has shown the chemical sensitivity of the fluorescence transitions that could be applied to study 3d metal transition complexes [20]. The Kα lines are the strongest fluorescence transitions that are the contribution of X-ray transitions from 2p to 1s electronic state. The X-ray fluorescence transitions denoted as Kβ satellite lines are weaker than Kα ones. These latter X-ray lines are fluorescence transitions from orbitals higher than the metal 3p shell that are often dominated by the oxidation state and chemical environment.

When a synchrotron radiation-based X-ray emission spectroscopy is used to speciate 3d-transition metal, the metal Kβ spectra could be enhanced in order to measure the Kβ transition energies and intensities. In addition, in order to be able to separate spectral features within the Kβ group and detect changes of the spectral shape due to the chemical environment, it is necessary to achieve a resolving power of order EE > 5000, which currently can only be obtained with a spectrometer based on perfect-crystal Bragg optics [20].

In this work, as an essential aid to understanding the Cr(VI) reduction and Cr ions sorption in Amberlite IR 120 cationic-exchange resin, activated carbon, and non-living aquatic macrophytes biomasses, a combination of the high-resolution X-ray fluorescence (HR-XRF) technique and a high flux, 6.1-keV X-ray monochromatic beam was used to enhance the spectral satellite lines that are chemically dependent. A hypothesis was proposed for Cr(VI) reduction as a function of an adsorption process, and it was proved by Kβ spectroscopy results.

Section snippets

Adsorbents and chemicals

For the study of hexavalent chromium reduction during the removal process, three kinds of materials were tested: a cationic-exchange resin, an activated carbon, and two non-living aquatic macrophytes. The Amberlite® IR 120 in the Na+ form (CAS # 9002-23-7) was used as a strong acidic cation-exchange resin. It is a gel type strongly acidic exchange resin with the sulfonic groups (SO3–Na) on styrene–divinylbenzene copolymeric backbone, produced by Rohm and Hass Company. It has the advantages of

Speciation of chromium

From a qualitative analysis point of view, the results from the EDTA colorimetric method-based Cr(III) chemical extraction from Cr(VI) solution have shown that there is no presence of Cr(III) species during the course of the adsorption experiments (data not shown), suggesting that the reduction of Cr(VI) to Cr(III) in pH 3.0 was not found [23].

In addition, the analysis of the intensity of the area of Cr-Kβ2,5 satellite line in all Kβ spectra (data shown in Section 3.6), showed a great amount of

Conclusion

The comparison of the high energy region of the Cr-Kβ spectra obtained from Cr(VI) uptake experiments by non-living biomass and activated carbon has shown that there is no evidence of hexavalent oxidation state in all samples, but only a good resemblance with that of Cr(III)-Kβ spectra. The Cr(VI) reduction to Cr(III) was identified to occur in non-living biomass and activated carbon. Experimental data for the cationic-exchange resin have shown a high performance to remove the Cr(III) from an

Acknowledgments

F.R. Espinoza-Quiñones thanks the Brazilian Research Supporting Council (CNPq) for financial support under Project #476724/2007-4. We also thank the Brazilian Light Synchrotron Laboratory (LNLS) for the partial financing of this study (Project #XRD1-8113). A.S.C thanks the Araucaria Research Supporting Agency for awarding the scholarship.

References (27)

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