Particle size distribution of organometal(loid) compounds in freshwater sediments

Abstract

AbstractThe aim of this study was to determine to what extent particle size determines the occurrence of organometal(loid) compounds of the elements As, Sb, Sn and Te in freshwater sediments. In addition, the anthropogenic impact upon the distribution through differing usage of freshwater habitats was examined vis‐à‐vis flowing water, fish farms and a maturation pond for the bio‐treatment of wastewater. All habitats sampled were located in the basin of the river Ruhr, Germany. In addition to the detection of high concentrations of total metal(loid) content of As, Sb, Sn and Te in the maturation pond sediments, this habitat also possessed the highest concentration of organometal(loid) species. Interestingly, the concentration of monomethylated metal(loid)s was up to 100‐fold higher than those of higher methylated species of the same element. A maximum of 28 µg kg−1 MMAs, 18 µg kg−1 MMSb and 8 µg kg−1 MMSn per dry weight was detected. A similar tendency was noted for all other freshwater habitats tested. In contrast to methylated arsenic (arsenic containing pesticides are banned in Germany) and antimony species, there is no doubt that the alkyltin species detected, e.g. MBSn and DBSn, are of anthropogenic origin since biogenesis of these species does not occur. Alkyltins are, however, known to enter the environment in a continuous and diffuse manner via discharge to sewage and air. In samples from the maturation pond concentrations of up to 86 and 11 µg kg−1 per dry weight were detected for MBSn and DBSn, respectively. The detection of methylated arsenic and antimony species indicates that biotransformation of these elements is occurring in freshwater habitats. Irrespective of the usage and (anthropogenic) demands on the freshwater habitats tested, the highest concentration of organometal(loid) species was always detected in the sediment fractions that contained the highest concentration of humic substances and comprised up to 40% clays and silt particles (<63 µm). Owing to their high surface area to volume ratios these particles possess a high binding capacity for metal(loid) ions and are attractive microhabitats for microorganisms. The resulting microcosmos therefore has a high potential for the biomethylation of metal(loid)s. At this point in time it is not fully clear whether the high concentrations of organometal(loid) species detected in the clay/silt fraction are produced in situ by microbial biotransformation of bound metal(loid) ions or whether, as is the case for inorganic ions, the organometal(loid) species are translocated to this fraction and bound. Regardless of the mechanism, the accumulation of organometal(loid) species in clay/silt fraction means that these species are held in contact with the interstitial water and are therefore highly bioavailable, with potentially toxic consequences for aquatic organisms. Copyright © 2007 John Wiley & Sons, Ltd.