Removal of PAHs from laboratory columns simulating the humus upper layer of vertical flow constructed wetlands N. Cottin * , G. Merlin Laboratoire d’Optimisation de la Conception et Ingénierie de l’Environnement (LOCIE), POLYTECH’SAVOIE-Université de Savoie, Campus Scientifique, 73376 Le Bourget du Lac, France article info Article history: Received 31 January 2008 Received in revised form 25 June 2008 Accepted 27 June 2008 Available online 3 August 2008 Keywords: Polycyclic aromatic hydrocarbons Degradation Column studies Metabolites Constructed wetlands abstract Removal of three polycyclic aromatic hydrocarbons or PAHs (fluoranthene, pyrene and benzo(k)fluo- ranthene) from two types of PAH-contaminated effluents was investigated using four laboratory columns filled with two different organic media: a green compost and a layer coming from the first stage of ver- tical flow constructed wetlands. Synthetic runoff polluted by polycyclic aromatic hydrocarbons were fed through the columns during a period of two months. After a period of hydrodynamic stabilisation, the results showed a great adsorption of PAHs (>95%) on the solid media due to their large adsorption capac- ities. Leaching of these compounds by water was monitored. The concentrations of PAHs in leaching sam- ples indicated that PAHs were strongly adsorbed on organic substrates and that lixiviation was limited. Fluoranthene metabolites were also investigated. Accumulation of metabolites was transitory and located in the first few cm of the media, as was observed for PAH concentrations. A toxicity test of leach- ates based on the inhibition of the bioluminescence of luminescent bacteria Vibrio fischeri indicated a low inhibition which can be enhanced by metal traces. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Road runoff has been reported as one of the most frequent causes of surface water pollution and a major source of contami- nants. These contaminants include oil and tar products, dioxins and oxygenated compounds (Stotz, 1987; Alistair et al., 1995). They can cause surface oil films and increase the concentrations of sus- pended solids, trace metals and hydrocarbons. Sources of PAHs in urban runoff include asphalt leaching, particles from tire abrasion, automobile exhausts, other combustion processes and lubricating oil spills. Additionally, relative high concentrations of hydrocarbons and metal traces have been related. Measurements of 16 PAH con- centrations in water and sediments of the southern drainage basin of Annecy Lake (France) showed that road runoff is likely to be the main source of PAH contamination (Naffrechoux et al., 2000). A ma- jor part of the roadside PAH contamination has been attributed to the spread of asphalt particles abraded from the road. Sadler et al. (1999) found significant PAH concentrations in the soil beneath the asphalt cover of an electric trolley bus depot. These PAHs pres- ent in road runoff after each rain event contaminate the aquatic environment, specially sediments. These contaminated sediments can be toxic to aquatic organisms (Clément et al., 2005) and some benthic macroinvertebrate communities are affected by these pollutants (Maltby et al., 1995). Changes in European legislation require European countries to keep their rivers in good ecological conditions. Consequently, con- structed wetlands designed to treat runoff have been developed and several studies have been conducted to evaluate performances and pollutant removal processes (Hares and Ward, 2004; Pontier et al., 2004; Revitt et al., 2004). The concept of constructed wetlands for the purification of wastewaters has received a great deal of attention because they are easy to use, require low maintenance and have low construc- tion costs (Machate et al., 1997). They may be an interesting alter- native for the treatment of micropollutants such as PAHs. Many of these systems use horizontal subsurface flow, but an interesting alternative is vertical flow constructed wetlands (VFCWs) for treating runoff. VFCWs are the most common constructed wetland found in France to treat domestic wastewaters and directly take in raw sewage at the first stage (Molle et al., 2005). In the VFCWs, there is a sequential loading of effluent on the surface with the creation of a layer of organic matter on the top, which develops during the wet- land’s ageing. Organic matter accumulation comes from external sources such as organic compounds in wastewater and in situ sources such as plants and microorganism residues (Tanner et al., 1998; Nguyen, 2000). The degradation mechanisms in this organic upper layer are similar to those observed in organic soils. This organic upper layer plays an important role in the regulation of pollution, with both purification and storage capabilities. Retention by the soil and resis- tance to degradation are the two fundamental criteria for the persis- tence of pollutants in the environment. 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.06.060 * Corresponding author. Tel.: +33 04 79 75 86 70; fax: +33 04 79 75 88 90. E-mail address: nathalie.cottin@univ-savoie.fr (N. Cottin). Chemosphere 73 (2008) 711–716 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere