Chemical Fractionation of Heavy Metals in Anoxic Estuarine and Coastal Sediments

Chemical extraction experiments for estimating characteristic particulate binding forms of heavy metals in anoxic marine and freshwater sediments were carried out under both the presence and absence of atmospheric oxygen during the analytical procedure. Storage methods including aeration of anoxic sediments do not preserve the initial fractionation patterns of heavy metals and may both increase (Ni, Pb, Cu, Zn, Cd) and decrease (Fe, Mn) their mobility. Elutriate tests of methanogenic freshwater sediments from Hamburg harbour basins show that metals with increased carbonatic fractions (Fe, Hn) and those with predominantly sulphidic/organic fractions (Zn, Cu, Cd) are particularly sensitive to sample aeration. Transformations of oxidizable Ni (11%), Zn (62%), and Cd (67%) predominantly lead to easily reducible associations. Drying of the sediments decreases the proportion of the sulphidic/organic fraction of the metals to a stronger degree. Oxidized Zn and Cd is then found even in the least stable exchangeable fraction. If pretreatment of sediment samples is done adequately the chemical leaching approach may provide information on diagenetic effects and reaction kinetics, which influence the mobility of contaminants in highly complex matrices.