Comparative assessment of two sequential chemical extraction schemes for the fractionation of cadmium, chromium, lead and zinc is surface coastal sediments

摘要

Two existing sequential chemical extraction schemes. involving respectively five and six leaching steps with solutions of increasing dissolving power, were com- pared. The methods have been applied to surface sediment samples collected in a marine estuary zone potentially ex- posed to contamination arising from nearby industrial ac- tivities. A certified reference material (MURST-ISS-Al) .consisting of an Antarctic bottom sediment for which no information regarding phase dep~ndent concentration is available, was also analyzed. In order to evaluate the par- tition of metals among different geochemical forms, the concentrations of cadmium, chromium, lead and zinc were measured in the liquid extracts by Zeeman-corrected flame atomic absorption and by inductively coupled plasma-atomic emission spectrometry. The total metal concentrations were determined after strong acid attack, and the adequacy of this total digestion/dissolution tech- nique was verifi~,d by its application to the reference ma- terial. Comparison of total metal concentrations with the sum of concentrations associated with the individual phases was employed to assess possible analyte losses or contaminations. Precisions for both sequential procedures were comparab.le, but some inconsistencies in mass bal- ances were found in one of the samples for the distribu- tionof Zn in the soluble/exchangeable fractions and for Cd in the bound to carbonates form. In addition, the six steps procedure produced I.ower concentration values in the case of elements associated to the residual fraction.