Characterization of how contaminants arise in a dredged marine sediment and analysis of the effect of natural weathering
Graphical abstract
Introduction
Careful management of the dredged sediment is necessary for the good operation of ports and navigable waterways. In 2010, about 18.6 million tons of sediment (dry weight) were dredged in mainland France and France's overseas territories, 33.56 million tons in 2009, and 23.2 million tons in 2008 (Le Guyader, 2011, Le Guyader, 2013). According to European legislation, once the sediments are extracted from their natural environment, they are considered as waste and thus must be consequently managed according to the legislation in force (Commission Decision of 3 May 2000 Replacing Decision 94/3/EC Establishing a List of Wastes, 2002; JORF, 2007).
These marine sediments are mainly composed of mineral species (quartz, silicates, carbonates, iron and manganese oxyhydroxides and sulfides, etc.) and organic matter. As they accumulate by the deposit of solid and colloidal matter, marine sediments are also the final “reservoir” for numerous contaminants coming from industrial and port activities, urban effluent, nautical activities or deposited in the form of aerosols. In this way, they integrate and amplify the contaminant concentrations (DelValls et al., 1998). Inorganic pollutants such as copper, zinc, lead, chromium, mercury, and arsenic, are a particular issue due to their ubiquity and persistence in the environment (Frémion et al., 2017, Jones and Turki, 1997, Caplat et al., 2005, Eek et al., 2008, Lions et al., 2010, Chatain et al., 2013a). A large number of organic pollutants also accumulate in the marine sediments such as PCBs, PAHs, residues of medicines and endocrine disruptors, as well as organometallic compounds e.g. organotin compounds principally coming from antifouling paints used to prevent aquatic organisms from attaching to boats. These compounds, frequently encountered at various concentrations in the dredged marine sediments, are considered dangerous for the environment as well as for human health (Dı́ez et al., 2002, Mamindy-Pajany et al., 2012, Casado-Martínez et al., 2009, Ribecco et al., 2011, Staniszewska et al., 2011).
To set up a long-lasting management plan for the dredged sediment, it is necessary to have more precise knowledge and more than just the degree of contamination. Therefore, it is essential to determine how the contaminants are distributed in the sedimentary matrix. Furthermore, these sediments are renowned for being particularly reactive materials following their abrupt change in surroundings after dredging; for example, when the saturation in water greatly diminishes, the redox potential increases significantly due to an increased availability of molecular oxygen. Wind and rain also induce cycles of lixiviation and drying favoring the formation of secondary mineral phases likely to control the mobility of dissolved metallic elements (Tack et al., 2006). Following these changes, these metals can be released into the lixiviation water and thus contaminate the soil if the effluent is not controlled (Caille et al., 2003, Chatain et al., 2013b, Stephens et al., 2001). Actually, the paper aim in characterizing the contaminant mineralogical speciation and their geochemical evolution during weathering under natural conditions. From this in-depth characterization, adapted management solutions could be proposed and evaluated.
Section snippets
Sampling, storage and treatment
The two sediments in this study come from the same sampling point, from a port in the south of France, submitted to an extensive anthropic activity during many centuries and contaminated by industrial activity. They were collected by dredging the 50–80 first centimeters of the harbor seabed with a mechanical shovel. The unweathered sediment was immediately put in 50 L opaque containers under a 10 cm layer of sea water to preserve the anoxic conditions and stored in a dark cold room at 4 °C.
Physicochemical parameters
Physicochemical parameters measurements were performed on both sediments. The natural pH, ORP (oxidation-reduction potential, which was afterward utilized to determine the Eh by using the standard reference electrode potential, SHE of 204 mV), and conductivity were measured on a sediment slurry with a water-sediment ratio of 1:2; after a contact time of 48 h using an Accumet® combined glass electrode. The redox potential of the sediment (ORP) was measured using a portable multi-meter (VWR
Physicochemical characterization
The values for the principal physicochemical parameters measured for the unweathered and naturally-weathered sediments are given in Table 1. As the samples come from the same sampling point they have some common characteristics for certain parameters, notably the density of 2.5 g/cm3 which is close to that of quartz (2.65 g/cm3) or carbonates (2.6–2.8 g/cm3) and a natural pH of 7.3 for both sediments. However, the bioremediation treatment and the method of conservation for the weathered sediment,
Discussion
Although the two sediments studied come from the same sampling campaign, the conditions in which they were kept have significantly influenced their physicochemical characteristics. First of all, the impact of storage in the open air after bioremediation treatment influences their redox state which increases as the saturation in water decreases. In fact, oxygen diffuses into the sediment to reach the phases sensitive to oxidation, such as the sulfides and organic matter. In the presence of
Conclusion
The characterization of dredged sediment is essentially done in order to choose and ensure the right management strategy. Both sediments studied possess a mineralogical composition typical of marine sediments. They are essentially constituted of quartz, aluminosilicates, evaporites (halite) and carbonates (37 and 52 wt% in the unweathered and naturally-weathered sediments, respectively). The two sediments contain framboïdal pyrite which shows significant signs of oxidation. In the weathered
Acknowledgments
The authors are grateful to the EEDEMS platform (French research network on waste and polluted materials management) for experimental support. The authors also acknowledge the Research and Service Unit in Mineral Technology (URSTM), University of Quebec in Abitibi-Temiscamingue (UQAT) for their experimental support.
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