A petrographic investigation of two sequential extraction techniques applied to anaerobic canal bed mud

The application of sequential extraction procedures to determine metal spec iation in sediments is fraught with uncertainty regarding what is actually dissolving or re-precipitating at each stage. In order to choose an appropr iate scheme for the investigation of contaminated anaerobic mud two differe nt sequential extraction procedures (Kersten and Forstner, 1986; Quevauvill er, 1998) were investigated using a Cryogenic SEM (CryoSEM) technique coupl ed with energy dispersive X-ray analysis (EDXA). This enabled assessment of the degree of reagent selectivity and any re-precipitation associated with the respective methods. Analysis of the non-leached sediment revealed the most abundant authigenic minerals in order of decreasing abundance to be Fe 2+-phosphate vivianite (Fe-3(PO4)(2). 8H(2)O), mixed Fe, Zn, Cu sulphides, pyrite and calcite. After each stage of the sequential extraction the sedim ent residue was examined using CryoSEM. After extraction of the exchangeabl e fraction no obvious evidence of mineral dissolution was observed. Calcite was not completely dissolved during the carbonate extraction stage of eith er procedure. Vivianite began to dissolve in the carbonate extraction stage of both procedures and was completely dissolved by oxide extraction stage of both procedures. The sediment leached by acidified ammonium oxalate, con tained abundant Fe oxalate crystals, suggesting that a large proportion of the Fe released from the vivianite has been re-precipitated. The Fe oxalate was then dissolved with the subsequent sulphide fraction. The technique us ed to extract the sulphide and organic fraction is the same in both schemes and no sulphide or metal rich organic matter was found in either residue.