THE SELECTIVITY OF A SEQUENTIAL EXTRACTION PROCEDURE FOR THE DETERMINATION OF IRON OXYHYDROXIDES AND IRON SULFIDES IN LAKE-SEDIMENTS

A popular sequential extraction procedure (Tessier et al. 1979) designed to extract metals partitioned in various sediment phases, was evaluated for its selectivity. Amorphous FeOOH, FeS, and FeS2 were added separately to natural lake sediments and sequentially extracted. The selectivity of the sequential procedure for the added solid phases was evaluated by determining the difference in the mass of Fe extracted from treated and control sediments. In the experiments where sulfide minerals were added, total S was measured in the residual solids in order to confirm selectivity of the method. Concentrations of total carbon remaining in the solid phase after each extraction step were also measured to determine the selectivity of the sequential procedure for carbon. The procedure was moderately selective for Fe added as FeOOH; a mean of 77 +/- 12% (p < 0.05) of the Fe added was extracted in the step designed to reduce Fe-Mn oxyhydroxides. In experiments where FeS was added, a mean of 69 +/- 11% (p < 0.05) of the Fe added as FeS was extracted in the fraction designed to oxidize sulfides and organic matter. Approximately 25% of the Fe added as FeS may have been extracted prematurely. Although less precise, total S analyses confirmed that much of the FeS was extracted in the oxidation step, yielding 104 +/- 87% (p < 0.05) of the S added as FeS. The procedure was highly selective for FeS2; 92 +/- 14% (p < 0.05) of the Fe added as pyrite was extracted in the sulfide extraction step. Extraction of 80 +/- 54% (p < 0.05) of S added as pyrite confirmed that FeS2 were selectively extracted in the sulfide extraction step. Carbon in the sediments was also selectively extracted in the oxidation step (77 +/- 2.4% of total C; p < 0.05). The applications and limitations of sequential extraction procedures as limnological research tools are discussed in light of our results.