Influence of oxidation on sequential chemical extraction of dredged river sludge

The influence of oxidation on the speciation of transition metal ions in dredged sludge obtained from sequential extraction was investigated. The transition metals cadmium, copper, lead and zinc became more extractable after oxidation. This shift is related with the oxidation of their metal sulfides. Although iron has a similar affinity for sulfides, iron was less extractable from oxidised sediments. The conversion of Fe(II) to Fe(III) and the formation of iron(III)oxide minerals could account for this observation. The results emphasise the need of working under strictly anoxic conditions when studying anaerobic sediments. Three different extraction schemes were compared. Two extraction schemes (Oakley and Wallman) were found to give comparable results. The extraction scheme of Tessier differs largely from the two other methods used. This is probably due to the more severe extraction conditions. The influence of drying of the sediment was also investigated Drying of sediments does not affect the speciation if the drying process is performed in anoxic conditions. The experiments indicate clearly that all possible precautions to avoid oxidation are prerequisite. Chemical analyses of sludge or sediment can be simplified considerably by anoxically drying the sample prior to analyses in anoxic conditions.