Tributyltin (TBT) is used as an additive in antifouling paints by the shipping industry to avoid micro-organism growth on the submerged surface of ships. The economic benefit of a reduced fuel consumption is, however, set off by the widespread phenomenon of intersex and imposex of snails caused by organotin compounds, especially TBT. As a result of their hormone-like activity, organotin compounds belong to the most toxic substances for aquatic organisms and humans alike which have ever been released into the environment. Moreover, there is still a lack of knowledge concerning the environmental behaviour of these substances. Organotin compounds, especially TBT, have been ubiquitously distributed in the marine environment. TBT has endocrine properties, but also produces other unknown antibiotic activities. TBT and other butyltin compounds are persistent, bioaccumulating in the marine environment and embedded in fish. The effect on human health of consuming contaminated fish over a period of time is still mostly unknown. TBT is mentioned as priority hazardous substance in the Water Framework Directive (WFD). To ensure the depth of rivers and navigation channels for harbour access, enormous amounts of TBT-contaminated sediments have to be dredged in the most important harbours in Europe, e.g. Rotterdam, and cannot be relocated to the sea. The contamination levels strongly depend on the origin of the sediments. Moreover, simple and cost-effective detoxification techniques are missing, that could relieve the environment. Even if the use of TBT-containing antifouling paints has been banned since 2003, the problem of sediment contamination still remains and endangers the quality of water.
- Detoxify and subsequently safely dispose off the dredged materials from marine waters by means of a simple process, which can be integrated in the actual dredging operation.
- Carry out tests in actual harbour operations to bring this technology to the level of marketability.
- Achieve an estimated cost target of 15 EURO per ton of treated dredged material for the safe disposal.
The project was successful. During the project four different sediment materials were investigated. The range of TBT pollution in the treated sediments or dredged material, respectively, varied from extremely (96000 µg kg-1) to slightly (300 µg kg-1) contaminated. In all cases it was possible to reduce the TBT-concentration below the target value of 100 µg kg-1. Furthermore, PAHs in the sediments were reduced by 50 to 90% at the same time, depending on the origin of the samples. Heavy metal concentrations and mobility was also investigated. Both were not affected by the electrochemical treatment. In order to ensure that unknown by-products, which could possibly be formed during the electrochemical treatment, do not harm the environment, several biotests were performed. Depending on the origin of the sediment, a 3-day storage or a post washing step of the treated material was necessary to ensure the compliance with ecotoxicological criteria for relocation. In addition, a life-cycle analysis was carried out to identify the impacts of the sediment treatment on a local as well as global scale. To assess the process with regard to sustainability, it has to be said, that the electrochemical process - like all other end-of-pipe techniques - could just be an interim solution for mistakes made in the past. The project results could in the future be used to define technically feasible limit values, which could be integrated in the EU water directive or could contribute to the implementation of comprehensive concepts for dredged material treatment. Technical details The newly developed process consists of the treatment of organotin-polluted dredged materials by means of electrochemically activated water. The dredged material is suspended and the coarse material removed prior to the electrochemical treatment. Detoxification takes place in an electrolysis unit consisting of undivided cells. The activated process water is capable of breaking up both the metal carbon compounds and compounds of organic substances, so that the tin-organic compounds bound in such a matrix can be set free and converted into non-toxic inorganic tin compounds. The process is applied to four different sediments from different origins and their level of TBT contamination varies from extremely to slightly contaminated. In every case TBT decomposition was successful. The following figures reveal two exemplary results:
An extremely contaminated sediment
- before treatment: 96.000 µg/kg
- after treatment: 100 µg/kg
A moderately contaminated fine sand
- before treatment: 600 µg/kg
- after treatment: 30 µg/kg