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Sediment Quality Assessment and Management: Insight and Progress Edited by M. Munawar© 2003 Ecovision World Monograph Series Aquatic Ecosystem Health & Management Society Decomposition of organotin compounds in dredged material from harbours by means of an electrochemical process H. Stichnothe*, W. Calmano Technical University Hamburg – Harburg, Dept. of Environmental Science and Technology, Eissendorfer Strasse 40, 21073 Hamburg, Germany. * Keywords: tributyltin (TBT), electrochemical sediment treatment Motivation Tributyltin (TBT) is used as an additive in antifouling paints, (Evans and Karpel, 1985), for the shipping industry to avoid micro-organism growth on the submerged surface of ships. Despite this economic benefit due to reduced fuel consumption, which also means a relief for the environment under the aspect of climate change, (Evans, 1999), intersex and imposex of snails (Umweltbundesamt, 1989; Porte et. al., 1998) are found to be a widespread phenomenon caused by the organotin compounds, especially TBT. Therefore organotin compounds belong to the most toxic substances for aquatic organisms, humans ever released into the environment (de Mora, 1996). The TBT problem is partially recognised in most parts of the world (Gui-Bin et. al., 2000; Chalaux et. al., 1998; Commission EU, 1988; Dobson and Cabridenc, 1990; Singh et. al., 1998), but there is still a lack of knowledge concerning the environmental behaviour of this substance. Moreover simple and cost effective detoxification techniques are missing that could relieve the environment until an alternative for the use of butyltin additives in ship paintings are found. To ensure the depth of rivers and navigation channels for harbour access, enormous amounts of TBT-contaminated sediments have to be dredged in Hamburg but also in other important harbours in Europe, e.g. Rotterdam, and can not be relocated to the sea. The contamination levels strongly depend on the origin of the sediments. Usually sediments from the navigation channel itself have relatively 258 low contamination compared to sediments close to dockyards. The aim of the electrochemical process is to detoxify sediment so that it can be relocated in the river and does not have to be dumped on land disposal sites. Therefore, mobility of heavy metals as well as an increase in concentration of other harmful substances like chlorinated organics have to be avoided. Furthermore the process must be integrated in the actual dredging operation. The new developed process described here, consists of the treatment of organotin-polluted dredged materials by means of electrochemically-activated water. Detoxification takes place in an electrolysis unit consisting of an undivided cell. The activated process water is capable of breaking up both the metal carbon compounds and the compounds of organic substances, so that even tin-organic compounds bound in such a matrix can be set free and converted into inorganic tin compounds (Stichnothe et. al., 2001). Electrochemical treatment Electrochemical oxidation of organics at PbO2, SnO2 and metal oxide anodes has been studied (Wabner and Grambow, 1985; Comninellis and Pulgarin, 1993).While metal oxide electrodes have been studied for chlorine electrolysis, PbO2 and SnO2 anodes have been investigated for electrochemical degradation of organics as a pre-treatment method of refractory water contents before discharge into biological/ mechanical wastewater treatment plants (Stucki et. al., 1991). More recently oxidation of organic pollutants at metal oxide anodes like TiO2RuO2 or TiO2IrO2 at simultaneous oxygen evolution gained interest, because these electrodes combine mechanical stability and hydroxyl radical formation. Hydroxyl radicals are the strongest oxidizing species in water systems. In contrast to PbO2 and SnO2 electrodes, where the hydroxyl radical yield is even higher but the mechanical stabilitylower,themetaloxideelectrodesarebetterapplicableformatricescontaining abrasive compounds like sand particles, common in sediments. From a mechanistical point of view electron transfer at these electrodes and/ or oxidation species formation occur due to different redox-couples, e.g. IrO2/ IrO3. Redox-mediator supported processes were initially mentioned by Lodowickes and Beck (1994) and described for IrO2 (Foti et. al., 1997). Foti postulated a mechanism for oxidation of organics on metal oxide (MOx) electrodes with simultaneous oxygen evolution described by the following equation: MOx + H2O Æ MOx(OH•) + H+ + eThese adsorbed radicals can form oxygen as well as oxidize organic compounds in the water phase depending on the adsorption strength on the metal oxide surface. 259 Additionally the following reaction takes place in the electrolysis cell: Hypo-chloride ions are also effective oxidation species, which support the degradation of organic compounds but can unfortunately lead to chlorinated organic substances. These unwanted substances have to be decomposed within the same process. Samples One sediment sample was collected from the harbour of Bremerhaven in Northern Germany...


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