Journal of Hazardous Materials 180 (2010) 759–763 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Short communication A reinvestigation of EXAFS and EPR spectroscopic measurements of chromium(VI) reduction by coir pith Parinda Suksabye a , Niramon Worasith b , Paitip Thiravetyan b , Akira Nakajima c , Bernard A. Goodman d,∗ a Department of Environmental Science, Faculty of Science and Technology, Suan Dusit Rajabhat University, 10300, Thailand b Division of Biotechnology, School of Bioresources and Technology King Mongkut’s University of Technology, Thonburi, Bangkhuntien, Bangkok, Thailand c Division of Chemistry, Department of Medical Science, Faculty of Medicine, University of Miyazaki, Kiyotake, Miyazaki 8891692, Japan d Health and Environment Department, Unit of Environmental Resources & Technologies, Austrian Institute of Technology, Seibersdorf A-2444, Austria article info Article history: Received 4 September 2009 Received in revised form 17 March 2010 Accepted 19 March 2010 Available online 25 March 2010 Keywords: Coir pith Chromium Reduction EPR EXAFS abstract New measurements using extended X-ray absorption fine structure (EXAFS) and electron paramagnetic resonance (EPR) techniques are consistent with Cr in the Cr(III) oxidation state as the main product from the adsorption of Cr(VI) on coir pith. These confirm the previous X-ray measurements, but differ from the results of previous EPR studies, which indicated that Cr(V) was the main form of Cr. The reason for this discrepancy is the presence of a broad signal from Cr(III) in an unsymmetrical environment that was missed previously; the Cr(V) component is in fact only a minor reaction product. As a result of this problem with spectral acquisition and interpretation, some simple recommendations are presented for conducting EPR investigations on natural systems. © 2010 Published by Elsevier B.V. 1. Introduction The production of low cost adsorbents for the clean-up of contaminated waters is of considerable importance for the improvement of environmental quality in developing countries. One product that is currently showing potential for use in the removal of environmental pollutants is coir pith, an agro-industrial waste product that is derived from coconut husk. Examples have been published recently demonstrating the use of coir pith in its natural form [1–5] or after chemical modification to improve its adsorption capacity or specificity [4,6–9]. In previous studies, the mechanisms of Cr(VI) adsorption by coir pith were investigated [2,3,5], and the adsorbed species char- acterized by X-ray absorption near edge structure (XANES) [2] and electron paramagnetic resonance (EPR) spectroscopy [3,5]. Although both spectroscopic techniques indicated that Cr(VI) adsorption involved reduction of Cr(VI) to lower oxidation states, there were fundamental differences in the conclusions as to the identities of the final forms of the metal. The most intense signal that was seen in the EPR spectra was associated with Cr(V), whereas XANES spectra indicated that Cr(III) was the major form of the metal, and no evidence was observed for the presence of Cr(V) [2]. ∗ Corresponding author. Tel.: +43 50550 3434; fax: +43 50550 3520. E-mail address: bernard a goodman@yahoo.com (B.A. Goodman). Furthermore, desorption studies [5] indicated that Cr(III) accounted for ∼75% of the Cr adsorbed on coir pith, although these results suggested that some Cr(V) and Cr(VI) forms were also present. In order to address the discrepancy in the conclusions from these reports, further measurements have now been performed using the EXAFS (extended X-ray absorption fine structure) and EPR techniques, and these are reported in the present communication. 2. Materials and methods Na[Cr(O)(ehba) 2 ]·H 2 O and the products of Cr(VI) adsorption on coir pith at pH 2 and 10 were described by Suksabye et al. [5]. Coir pith (<75 m) was treated with Cr(VI) solution (2%, w/v) at pH 2 or 10 for 18 h, then dried for 1 h at 105 ◦ C, and ground with an agate mortar before use for the spectroscopic measurements. Addi- tional samples were prepared using mixed Cr(VI)/Al(III) solutions with Cr:Al ratios between 1:1 and 1:10. Cr 2 O 3 enriched to >99% in the 53 Cr isotope was obtained from AERE, Harwell, UK. This was dissolved in strong alkali, precipitated as [Cr(OH) 6 ] 3- , then redis- solved in distilled water and oxidized to Cr(VI) using H 2 O 2 . The preparation of 53 Cr/coir pith was performed as described above for the natural abundance isotopes. For EXAFS measurements, samples were loaded into sample plates and secured with kapton tape for analysis at the National Synchrotron Research Center, Thailand on beamline 8. Transmis- sion XAS data for Cr were collected with K edges and an energy of 0304-3894/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.jhazmat.2010.03.090