Lability of polycyclic aromatic hydrocarbons in the rhizosphere Naressa Cofield a , M. Katherine Banks a, * , A. Paul Schwab b a School of Civil Engineering, Purdue University, West Lafayette, IN 47907, United States b Department of Agronomy, Purdue University, West Lafayette, IN 47907, United States Received 20 December 2006; received in revised form 16 July 2007; accepted 26 July 2007 Available online 27 September 2007 Abstract Remediation of soils containing high concentrations of polycyclic aromatic hydrocarbons (PAHs) seldom results in complete removal of contaminants, but residual toxicity often is reduced. In this study, soil from a former manufactured gas plant site was treated for 12 months by phytoremediation and then tested for total PAHs, Tenax-TA extractable (‘‘labile’’) PAHs, aqueous soluble PAHs (PAH wp ) , and biotoxicity assessed by earthworms survival, nematode mortality, emergence of lettuce seedlings, and microbial respira- tion. Prior to phytoremediation, the soil had toxic impacts on all bioassays (except the nematodes), and 12 months of remediation decreased this response. Change in labile PAHs was a predictor for change in total PAH for 3- and 4-ring compounds but not for the 5- and 6-ring. Decreases in labile PAHs were correlated (r 2 P 0.80) with toxicity in the bioassays except microbial respiration. PAH wp was correlated only with nematode toxicity prior to remediation but with none of the tests after remediation. Total PAHs were not correlated with any of the bioassay tests. Tenax-TA appears to have potential for predicting residual toxicity in remediated soils and is superior to total concentrations for that application. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Polycyclic aromatic hydrocarbons; Soil; Plants; Rhizosphere; Phytoremediation; Adsorption; Toxicity; Bioavailability; Lability; Bioacces- sibility 1. Introduction Phytoremediation has received attention over the past decade as a viable method for the reclamation of sites con- taminated with polycyclic aromatic hydrocarbons (PAHs) (Liu et al., 2001; Ke et al., 2003; Chen and Banks, 2004; Gentry et al., 2004; Sun et al., 2004). Remediation efficacy for the most available fraction of contaminants in soils has become a recent focus (Tang et al., 2002; Ghosh et al., 2003). Lability refers to the fraction of contaminants that is most chemically active, e.g., most soluble or most easily extracted. Bioavailability refers to the portion of the con- taminant that can be assimilated or otherwise exposed to a target organism. Organic compounds may experience a time-dependent sequestration in soil that results in a decline in lability and bioavailability without a parallel decline in the total concentration as determined by vigor- ous extraction with organic solvents (Morrison et al., 2000). The bioavailable fraction of PAHs in n-butanol was correlated to the following PAH properties: Henry’s law constant (Beck et al., 1996), octanol–water partition coeffi- cient, log K ow (Zhao and Pignatello, 2004), organic carbon– water partition coefficient, log K oc (Klimkowicz-Pawlas and Maliszewska-Kordybach, 2003) and molecular surface area (Karickhoff et al., 1979; Maliszewska-Kordybach, 1998). The n-butanol PAH fraction also was measured in a mod- erately contaminated soil (Oleszczuk and Baran, 2006) and was found to be slightly less in the rhizosphere of plants than in unvegetated soil. Calculation of phase partitioning may assist with the estimation of contaminant availability in soil. The primary soil property controlling non-ionic hydrophobic compound sorption to soil organic matter is the organic carbon:water partition coefficient, K oc (Gustafson et al., 1997). Because 0045-6535/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.07.057 * Corresponding author. Tel.: +1 765 496 3424; fax: +1 765 496 3449. E-mail address: kbanks@ecn.purdue.edu (M.K. Banks). www.elsevier.com/locate/chemosphere Available online at www.sciencedirect.com Chemosphere 70 (2008) 1644–1652