Estimated solid phase distribution of metals released in the acid extractable and reducible steps of a sequential extraction

The distribution of metals as determined by means of sequential extraction does not necessarily reflect their association with discrete sediment phases, but is operationally defined by the method of extraction. Sequential extraction was performed on a sediment as such and after extraction with NH4OAc-EDTA to remove sorbed and complexed metal forms. From the data, we estimated a presumed solid phase distribution of the metals released in the acid extractable and reducible fractions. The metal fractions were subdivided in (1) sorbed, (2) carbonate associated and (3) ironoxihydroxide associated metals. Metals, associated with ironoxihydroxides, but extracted in the acid extractable fraction, were negligible. An important portion (10 to 25%) of the reducible fraction was estimated to consist of metals associated with carbonates, not released during the previous acid extractable step. In both sequential fractions, a significant portion (at least 10%) of the metals was identified as sorbed, rather than specifically associated with carbonates or iron-manganese oxihydroxides.