Organotin compounds in marine sediments near sewage outfall areas in Malta

Abstract

Tributyltin (TBT) has been widely used in antifouling marine paints. This study involved the determination of tributyltin and its degradation products dibutyltin (DBT) and monobutyltin (MBT) in marine sediments found near sewage outfalls around Malta. The organotin species which were analysed and identified included also the methylbutyltins mainly BuSnMe3, Bu2SnMe2, and Bu3SnMe. Inorganic tin(iv) is also generally found in the marine sediments analysed. A total of nineteen samples were taken from four different areas around Malta. A van Venn grab sampler was used for sediment collection and the four areas analyzed were Ċumnija, Ġnejna, St George's Bay and Xgħajra. Xgħajra is situated close to the major sewage outfall in Malta; Ċumnija receives sewage from the northern part of Malta while St George's Bay is surrounded by hotels. Gnejna Bay was chosen as the 'clean site' since it is free from municipal sewage outfall and does not receive sewage-contaminated water by surface water current transport. Granulometry was also performed on the sediments analysed and it was found that the sediments were mostly gravely sand type. The procedure for analysis involved organotin extraction from sediment by acetic acid and treatment of the extract with sodium tetraethylboroate in the presence of isooctane. Separation of the organic layer and determination of the organotins TBT, DBT, MBT, and Sn(iv) in the ethylated forms was carried out using gas chromatography coupled with flame photometric detection. The methylbutyltins were also recovered and determined in the analytical procedure. TBT concentrations ranged from 7 to 878 ngSn/g, DBT ranged from 8 to 304 ngSn/g and MBT from 48 to 563 ngSn/g. The highest level of TBT concentration (878ngSn/g) was found in the Xghajra area which represents the site which is mostly affected by the sewage outfall. Gnejna was the one with least butyltin contamination in it. Methylbutyltins were found to be present in all samples analysed. Gas chromatography - mass spectrometry was used to confinn qualitatively the presence of organotins in the most contaminated sample. TBT contamination in sediments is likely to decrease in the future in view of the ban on TBT use in 2008. Unfortunately these compounds have long lifetimes in marine sediments and their effects may still be felt for several years.