Comparison of metal release from recent and aged Fe-rich sediments Valérie Cappuyns ⁎ , Rudy Swennen Geologie, Katholieke Universiteit Leuven, Celestijnenlaan 200E, B-3001 Heverlee, Belgium Received 26 April 2005; received in revised form 26 June 2006; accepted 15 August 2006 Available online 27 September 2006 Abstract During oxidation, ripening and aging of contaminated river sediments, important changes in the speciation and binding of metals to the sediment matrix occur. The effect of ‘aging’ of contamination is usually studied in the laboratory with artificially spiked soils and sediments. An increased retention of heavy metals is often observed during aging of Fe-(hydr)oxides. However, little information is available concerning the importance of the time since disposal of contaminated Fe-rich sediments on land (by dredging or overbank flooding) with respect to heavy metal release. Metal release from recent and aged Fe-rich overbank sediments and land-disposed dredged sediments was studied by means of column and pH stat leaching tests and with single extractions with EDTA and acetic acid. In order to allow a more objective comparison between recent and aged sediments, the release of heavy metals (Cd, Zn, Ni and Cu), as well as As and Fe, during pH stat tests was described mathematically and operationally defined release constants were determined. Fe was the only element that allowed to make a clear distinction between aged and recent sediments, since Fe displayed a significant higher leachability from recent sediments than from aged sediments in all the leaching tests and extractions. The different release behaviour of Fe was nevertheless not systematically reflected in a higher heavy metal and As-mobility in recent sediments. Only during the column leaching test and acetic acid extraction, a higher release of Cd was observed from the recent sediments. These results indicate that sediment characteristics such as pH, clay, organic matter and Fe-(hydr)oxides-content are more significant in explaining heavy metal release from Fe-rich sediments than the age of the sediments. © 2006 Elsevier B.V. All rights reserved. Keywords: Aging; Contamination; Dredging; Overbank sediments 1. Introduction The binding of metals to soils is not completely reversible because, with time, part of the metals is immobilised, which means that they will not be released into the porewater. The influence of aging is usually assessed by comparing the ‘labile’ pool of heavy metals or As in fresh and aged spiked samples. The ‘labile’ pool of metals is operationally defined and can, for example, be estimated by means of single extractions (Houba et al., 1996), leaching tests or with isotopic dilution techniques (Nakhone and Young, 1993; Smolders et al., 1999). Knowledge of the ‘labile’ metal pool is necessary to make predictions about heavy metal leaching from soils. The short-term oxidation of anoxic sediments usually results in an increase in the ‘labile pool’ of Cd and Zn (Cappuyns et al., 2004a). On the long-term, no significant differences in heavy metal release during pH stat leaching tests were observed in CaCO 3 - and clay-rich sediments of different ages since disposal, varying from 10 to 70 years (Cappuyns et al., 2004a). Since an increased retention of heavy metals is often observed during aging of Fe-(hydr)oxides, the present study focuses on heavy metal (Zn, Cd, Ni and Cu) release from Fe-rich sediments. A term that is used to indicate the increased retention of metals with aging time is fixation. Fixation of metals takes place by the slow diffusion of metals into Fe-(hydr)oxides (Brümmer et al., 1988), hydrous oxides of Al and Mn (Trivedi and Axe, 2000), clay minerals (Ma and Uren, 1998) and by diffusion or coprecipitation in carbonates (Nakhone and Young, 1993). Ainsworth et al. (1994) suggested that Cd and Co are incor- porated into goethite during aging of hydrous iron oxides, while Geoderma 137 (2006) 242 – 251 www.elsevier.com/locate/geoderma ⁎ Corresponding author. Tel.: +32 16327807; fax: +32 16327981. E-mail addresses: Valerie.Cappuyns@iph.fgov.be (V. Cappuyns), Rudy.Swennen@geo.kuleuven.be (R. Swennen). 0016-7061/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2006.08.013