The effect of sequential extractions of suspended particulate matter on trace metal sorption and microbial cell stability.

Sequential extractions, according to a modified scheme proposed by Tessier et al. (Tessier A, Campbell PGC, Bisson M. Sequential extraction procedure for the speciation of trace metals. Anal Chem 1979;51:844-851), were performed on suspended particulate material (SPM) from the River Mersey, North-West England. The resulting solid-phase fractions were spiked with trace levels of Cd and Cu and their metal-binding properties were investigated as a function of pH. The results indicated that metal binding decreased as the material was successively extracted, i.e. the unextracted fraction bound the most metal, while the particles which had undergone all of the extractions bound the least metal. This effect was attributed to the loss of particle mass during the extractions and to the relative metal affinities of the newly exposed surfaces. The exposure of new potential binding sites was not an overriding influence on metal binding. The strongest binding of Cd appears to be to the nominal manganese oxyhydroxide phase, with no measurable binding of Cd by the residual mineral fraction. By contrast, the nominal iron and manganese oxyhydroxides, organic material and the residual mineral fraction all appear to affect Cu binding significantly. The effect of the extractions on the particles was also investigated by transmission electron microscopy. Micrographs indicated that the biological material in the sample had undergone significant alteration after treatment with the first and second extractants (acetate and hydroxylamine, respectively), i.e. before removal of the nominal organic fraction. These changes in biological material may affect metal binding, complicating the interpretation in terms of simple mineral and organic phases.