Binding of polycyclic aromatic hydrocarbons by humic acids formed during composting Ce ´ sar Plaza a, * , Baoshan Xing b , Jose ´ M. Ferna ´ ndez a , Nicola Senesi c , Alfredo Polo a a Centro de Ciencias Medioambientales, Consejo Superior de Investigaciones Cientı ´ficas, Serrano 115 dpdo., 28006 Madrid, Spain b Department of Plant, Soil and Insect Sciences, University of Massachusetts, Stockbridge Hall, Amherst, MA 01003, USA c Dipartimento di Biologia e Chimica Agroforestale ed Ambientale, University of Bari, Via Amendola 165/A, 70126 Bari, Italy Composting of organic materials decreases the binding affinity of the humic acid fraction for polycyclic aromatic hydrocarbons. article info Article history: Received 13 May 2008 Received in revised form 20 June 2008 Accepted 3 July 2008 Keywords: Bioremediation Composting Fluorescence spectroscopy Humic acids Organic amendment Polycyclic aromatic hydrocarbons Soil organic matter abstract Binding of two model polycyclic aromatic hydrocarbons (PAHs), phenanthrene and pyrene, by humic acids (HAs) isolated from an organic substrate at different stages of composting and a soil was investi- gated using a batch fluorescence quenching method and the modified Freundlich model. With respect to soil HA, the organic substrate HA fractions were characterized by larger binding affinities for both phenanthrene and pyrene. Further, isotherm deviation from linearity was larger for soil HA than for organic substrate HAs, indicating a larger heterogeneity of binding sites in the former. The composting process decreased the binding affinity and increased the heterogeneity of binding sites of HAs. The changes undergone by the HA fraction during composting may be expected to contribute to facilitate microbial accessibility to PAHs. The results obtained also suggest that bioremediation of PAH-contami- nated soils with matured compost, rather than with fresh organic amendments, may result in faster and more effective cleanup. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Polycyclic aromatic hydrocarbons are a heterogeneous group of environmental contaminants with known harmful effects on humans and wildlife (ATSDR, 1995; Luch, 2005). These compounds are widely distributed in nature, originating from the incomplete combustion of organic materials during both anthropogenic and natural processes, such as burning of fossil fuels, waste incineration, petroleum refining, forest fires, and volcanic eruptions (Douben, 2003). The environmental release of PAHs in combination with transport phenomena may result in soil contamination (Garcı ´a- Falco ´ n et al., 2006; Heywood et al., 2006). Furthermore, PAHs can become associated with land-applied organic wastes (e.g., biosolids) and be increasingly accumulated in amended soils (Overcash et al., 2005). For these reasons, effective and viable treatments of organic wastes and soils contaminated with PAHs have become a major focus of recent research. Composting, defined as the controlled biological decomposition and stabilization of organic matter into a humus-like product called compost, is a common way of reclaiming organic wastes for agricultural use (De Bertoldi et al., 1996; Epstein, 1996). Nowadays, composting is also viewed as a cost-effective option for treating organic wastes and soils contaminated with toxic organic compounds, such as PAHs. In particular, bioremediation by means of the composting process relies on mixing bulking agents and organic amendments with the contaminated matrix, wherein as composting proceeds, active microorganisms transform and degrade PAHs into innocuous compounds (Semple et al., 2001; Singh and Ward, 2004). Similar to the composting process, the in situ application of compost to PAH-contaminated soils has a great potential for bioremediation purposes by enhancing indigenous microbial activity, adding exogenous PAH-degrading microbial strains, and facilitating plant growth and establishment (Semple et al., 2001; Singh and Ward, 2004). The success of the composting process and use of compost in bioremediation of contaminated wastes and soils depend on a number of physical, chemical, and biological factors that deter- mine the microbial accessibility to the target molecules (Semple et al., 2001; Wick et al., 2007). Among these, the content and nature of waste or soil organic matter, which is largely composed of humic acids (HAs), are believed to play the most important role (Cornelissen et al., 2005; Tang et al., 2008; Thorn et al., 2008). In particular, binding of PAHs by HAs can limit their availability to degrading microorganisms, thus increasing their persistence in the * Corresponding author. Tel./fax: þ34 91 4115301. E-mail address: c.plaza@ccma.csic.es (C. Plaza). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol 0269-7491/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2008.07.016 Environmental Pollution 157 (2009) 257–263