Chromium sorption and Cr(VI) reduction to Cr(III) by grape stalks and yohimbe bark
Introduction
Chromium is usually encountered in both hexavalent and trivalent forms in aqueous solution. The hexavalent form of chromium is of particular concern because of its great toxicity. Several wastewaters such as those produced during dyes and pigments production, film and photography, galvanometry, metal cleaning, plating and electroplating, leather and mining may contain undesirable amounts of chromium(VI) ions (Kumar et al., 2007, Landis and Yo, 2003).
Among available conventional processes used to remove hexavalent chromium, the most commonly used are: (a) reduction to trivalent chromium followed by precipitation as chromium hydroxide, (b) removal by ion exchange and (c) removal by adsorption. Most of these methods are costly due to operational, treatment and sludge disposal costs.
Recently, low cost and easily available sorbents as extracted substrate from wheat bran (Dupont and Guillon, 2003), soya cake (Daneshvar et al., 2002) and condensed tannin gel (Nakano et al., 2001) have been investigated for chromium removal.
Almost most of the treatment processes to remove chromium from wastewater have to ensure the removal of both Cr(VI) and Cr(III), therefore, the objective of this paper is to examine the use of grape stalks and yohimbe bark wastes for the removal of chromium in both oxidation states. The ability of the sorbent to reduce Cr(VI) to Cr(III) and the mechanisms that govern Cr(VI) and Cr(III) sorption on grape stalks and yohimbe bark wastes have also been investigated.
Section snippets
Materials and reagents
Grape stalks and yohimbe bark wastes were kindly supplied by a wine producer and a pharmaceutical company dealing with the alkaloid yohimbine extraction, from the Catalonia region, Spain, respectively. Grape stalks were rinsed three times with abundant cold water and then, dried in an oven at 105 °C until constant weight. Yohimbe bark wastes were used directly without any treatment. The wastes were cut and sieved for a particle size of 1.0–1.5 mm. Hexavalent and trivalent chromium solutions were
Cr(VI) sorption
The variation of total chromium and hexavalent chromium concentration with time due to chromium adsorption on grape stalks and yohimbe bark from an initial Cr(VI) solution of around 100 mg/L is shown in Fig. 1a and b, respectively. In the same figures, the concentration of Cr(III) in the solution, obtained from the difference between the total chromium and hexavalent chromium, as a function of time has also been plotted.
As can be seen in the figures, Cr(III) appears in solution few seconds after
Conclusions
Based on the results obtained, grape stalks and yohimbe bark can be a suitable sorbent for the removal of both Cr(VI) and Cr(III) from aqueous acidic media at pH 2–3. Grape stalks resulted to be the most efficient sorbent for either initially Cr(VI) and Cr(III) solutions. Although for Cr(VI) sorption two mechanisms adsorption and reduction to trivalent chromium have been postulated, more development is required to determine the kinetics of Cr(VI) sorption and reduction to Cr(III) and to
Acknowledgements
Thanks are due to Mr. Gianluca Silesu Erasmus student from the Università di Cagliari for their help in the experimental work and to Dr. L. Dupont (GRECI, Université de Reims-Champagne Ardenne, Reims, France) and Mr. J. Lambert (Laboratoire de Chimie-physique et Microbiologie pour l’Environnement: Université Henri Poincaré, Nancy, France) for the XPS analysis performance and spectra interpretation. This work has been supported by Ministerio de Educación y Ciencia Project
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