Bioleaching strategies applied to sediments contaminated with metals: Current knowledge and biotechnological potential for remediation of dredged materials

In all the aquatic ecosystems, metal contaminants are not persistently stored in the sediment but may easily enter the food web or spread in the environment due to changes in the environmental conditions (Bortone et al., 2004; Agius and Porebski, 2008; Förstner and Salomons, 2010). The management of contaminated aquatic sediments as dredged material is a modern-day issue of significant concern. Industrial/commercial ports, rivers, channels, lakes, and estuaries need to be dredged periodically to ensure navigational depth and a good capacity of drainage and flood prevention. Due to the burden of the anthropogenic activities usually allocated in these environments and the low hydrographic regime, the concentration of metal contaminants in the dredged materials is often high. Currently, the main management options for these materials are landfill disposal and confined aquatic disposal—two solutions with high costs and low environmental sustainability (Bortone et al., 2004; Agius and Porebski, 2008). In this context, bioleaching has been thought to have a potential to match the need of environment-friendly 132and cost-effective management options for dredged aquatic sediments contaminated with metals (White et al., 1998; Blais et al., 2001; Chen and Lin, 2004; Tabak et al., 2005). Because of the non-degradable nature of metal pollutants, the remediation strategies aimed at coping with them can be only aimed either (1) at increasing the solubility of metals to facilitate their removal (i.e., mobilization) or (2) at increasing their stability to reduce their bioavailability (i.e., immobilization; Fig. 1). Ideally, dredged sediments can decontaminated by bioleaching-based strategies, where the extraction of metals and semi-metals is catalyzed by bacteria (and/or archaea) that are able to oxidize inorganic sulfur compounds and/or Fe(II). Once cleanzed, the material could well suit the building industry or could be used for beach refill as well as for numerous other applications (Lee, 2000; Ahlf and Förstner, 2001; Barth et al., 2001; Siham et al., 2008). The possibility to exploit such Fe/S oxidizing strains for the reclamation of aquatic sediment arises in the early 50’s from the implementation by Kennecott Copper Corporation of an industrial scale process of copper extraction from mine dumps that was (and still is) mediated by Acidithiobacillus ferrooxidans. Subsequently, that technology has been improved and further mining applications have been implemented. Today, minerals/ores containing Cu, Au, and Co are processed on industrial scale. Promising results have also been obtained with sulfide ores bearing Ni, Zn, Mo, Ga, Pb, and metals in the Pt group (Ehrlich, 2001; Lee and Pandey, 2012). Figure 1 Abiotic and biotic influences on processes leading to either mobilization or immobilization of metal contaminants in the sediment.