Application of high resolution X-ray emission spectroscopy on the study of Cr ion adsorption by activated carbon

Abstract

In this work granular activated carbon has been chosen as an absorbent in order to investigate the Cr(VI) reduced by adsorption experiments. Several batch chromium-sorption experiments were carried out using 0.25 g of granular activated carbon in 50 mL aqueous solution containing approximately 70 and 140 mg L⁻¹ of Cr(VI) and Cr(III), respectively. Cr–Kβ fluorescence spectra of Cr adsorbed in a carbon matrix and Cr reference materials were measured using a high-resolution Johann-type spectrometer. Based on evidence from the Cr–Kb satellite lines, the Cr(VI) reduction process has actually happened during metal adsorption by the activated carbon.

Introduction

Environmental pollution with metals is a global problem, and the development of new technologies for metal removal from waste waters is therefore of significant interest. Chromium is introduced into ecosystems as a result of different industrial activities such as iron and steel manufacturing, tannery, chromium plating and other anthropogenic sources, and represents a severe environmental hazard. The concentration and toxicity of soluble chromium and its mobility in aquatic ecosystem depends on its oxidation state (Soares et al., 2008). Although chromium as a trivalent form is an essential element for humans at low dose, the hexavalent one is recognized as a carcinogenic and mutagenic agent (Brauer and Wetterhahn, 1991).

Significant efforts have been directed toward the use of methods for the removal of metal ions from industrial effluents, including chemical precipitation, solvent extraction, oxidation, reduction, dialysis/electrodialysis, electrolytic extraction, reverse osmosis, ion-exchange, evaporation, cementation, dilution, adsorption, filtration, flotation, air stripping, steam stripping, flocculation, sedimentation, soil flushing/washing chelation, etc. (Hankins et al., 2006, Hasany and Ahmad, 2006, Pascal et al., 2007, Polat and Erdogan, 2007, Wang and Wai, 2007, Qiu et al., 2009). Among these, adsorption has evolved as the front line of defense and especially for those metal ions that cannot be removed by other techniques (Mohan and Singh, 2002). The adsorption process is governed by Van der Waals forces that exist between molecules. The highly porous nature of a good adsorbent provides a large surface area for contaminants to collect. Molecules at the surface of a solid attract other molecules, allowing improved separation of components of the flow system. Among the great variety of adsorbents, the active carbon is very effective in removing pollutants (Sarin and Pant, 2006, Acharya et al., 2009).

Nowadays, valence-to-core X-ray emission spectroscopy has allowed the retrieval of information about the chemical and physical properties of materials without introducing alterations on the results, for example as seen in several studies about the application of chemical techniques on plant-based hexavalent chromium reduction (Lytle et al., 1998).

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 that are often dominated by the oxidation state and chemical environment. In an experimental setup that employs a solid state detector to record the K fluorescence emission (e.g. for Ge the resolving power is E/ΔE∼40), the Kα and Kβ groups can be separated from each other but no fine structure can be resolved. In addition, in order to be able to separate spectral features within the Kα and Kβ groups and detect changes of the spectral shape due to the chemical environment, it is necessary to achieve a resolving power of order E/ΔE>5000, which currently can only be obtained with a spectrometer based on the perfect-crystal Bragg optics (Glatzel and Bergmann, 2005).

The aim of the present work is to apply the high resolution X-ray emission spectroscopy in order to assess the metal oxidation state reduction by activated carbon. Therefore, the study of Cr-Kβ spectra of Cr adsorbed-activated carbon can allow clarifying the main responsible process for the hexavalent chromium reduction.