Mobility and recalcitrance of organo–chromium(III) complexes Geoffrey J. Puzon a,b , Ranjeet K. Tokala b,c , Hua Zhang a , David Yonge b,c , Brent M. Peyton b,d , Luying Xun a,b, * a School of Molecular Biosciences, Washington State University, Pullman, WA 99164, United States b Center for Multiphase Environmental Research, Washington State University, Pullman, WA 99164, United States c Department of Civil and Environmental Engineering, Washington State University, Pullman, WA 99164, United States d Department of Chemical and Biological Engineering, Montana State University, Bozeman, MT 59717, United States Received 20 June 2007; received in revised form 5 September 2007; accepted 10 September 2007 Available online 23 October 2007 Abstract Hexavalent chromium [Cr(VI)] is a major industrial pollutant. Bioremediation of Cr(VI) to Cr(III) is a viable clean-up approach. However, Cr(VI) bioreduction also produces soluble organo–Cr(III) complexes, and little is known about their behavior in the environ- ment. When tested with soil columns, citrate–Cr(III) showed little sorption to soil; malate–Cr(III) had limited partitioning with soil; and histidine–Cr(III) exhibited significant interaction with soil. It appears that the mobility varies depending on the organic ligand. Further, Ralstonia eutropha JMP 134 and Pseudomonas aeruginosa pAO1 readily degraded malate, citrate, and histidine, but not the correspond- ing organo–Cr(III) complexes. The recalcitrance is not due to toxicity, but the complexes are likely to cause hindrance to enzymes, as malate dehydrogenase and amino acid oxidase could not use malate–Cr(III) and histidine–Cr(III), respectively. The data are in agree- ment with the reports of soluble organo–Cr(III) complexes in the environment. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Chromium; Organo–Cr(III) complex; Recalcitrant; Mobility; Soil 1. Introduction Chromium has multiple oxidation states, but hexavalent and trivalent are the most stable under physiological condi- tions (Barnhart, 1997). Chromium has been widely used in industrial processes such as stainless steel manufacturing, leather tanning, and wood treatment (Barnhart, 1997), con- sequently resulting in the release of chromium into the envi- ronment. The release of hexavalent chromium [Cr(VI)] has resulted in significant pollution because Cr(VI) forms oxya- nions, i.e. chromate, which are highly soluble and mobile in groundwater (Dragun, 1988). Soluble Cr(VI) is readily taken up by both prokaryotic and eukaryotic cells (Norseth, 1986; Arslan et al., 1987), and it is known to cause both carcinogenic and mutagenic problems (World Health Orga- nization, 1996). Conversely, trivalent chromium [Cr(III)] has very low solubility at neutral pH (<1 lM) and is gener- ally regarded as benign (James et al., 1997). To limit the toxic effects, various chemical and biological methods have been developed to reduce Cr(VI) to Cr(III). Bioremediation has been extensively studied for convert- ing Cr(VI) to Cr(III) (Barnhart, 1997). Numerous bacteria have been demonstrated to reduce Cr(VI); however, micro- bial reduction of Cr(VI) often produces both soluble Cr(III) (McLean and Beveridge, 2001; Middleton et al., 2003) and insoluble Cr(III) (Wielinga et al., 2001; Middleton et al., 2003). The soluble Cr(III) is likely complexed with organic ligands because both enzymatic (Assfalg et al., 2002; Puzon et al., 2002) and chemical (Puzon et al., 2005) reductions of Cr(VI) in the presence of cellular organics generate stable and soluble organo–Cr(III) complexes. Formation of org- ano–Cr(III) complexes from chromate reduction has been extensively documented in mammalian cells with the 0045-6535/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.09.010 * Corresponding author. Address: School of Molecular Biosciences, Washington State University, Abelson Hall 301, Pullman, WA 99164- 4234, United States. Tel.: +1 509 335 2787; fax: +1 509 335 1907. E-mail address: xun@mail.wsu.edu (L. Xun). www.elsevier.com/locate/chemosphere Available online at www.sciencedirect.com Chemosphere 70 (2008) 2054–2059