EFFECTIVENESS OF A COMMONLY USED SEQUENTIAL EXTRACTION TECHNIQUE IN DETERMINING THE SPECIATION OF CADMIUM IN SOILS

Various cadmium-sorbed and cadmium-coprecipitated "soil phases" were prepared and sequentially extracted using the five-step method of Tessier et al. Reagent selectivity was assessed by performing sequential extractions of individual cadmium-spiked phases, and the extent of cadmium redistribution between phases during the course of the extractions was gauged by sequentially extracting a range of "synthetic" soils and a real soil into which cadmium-spiked phases had been mixed. Reagent selectivity was in some cases very good. The main shortcoming in reagent selectivity was that a portion of the cadmium sorbed on humic acid and clay remained associated with those substrates until step 3 of the sequential extractions. Release during this step would lead to the incorrect classification of the formerly sorbed cadmium as "bound to easily reducible manganese oxides and/or amorphous iron oxides". Redistribution of the cadmium liberated from each contaminated phase of the synthetic soil and real soil samples during the course of each sequential extraction was minimal. However, in some cases the pH of the material being extracted markedly influenced the relative proportions of cadmium liberated during steps 1 and 2 of the extractions.