Metal sequential extraction procedure optimized for heavily polluted and iron oxide rich sediments Original Research Article

A sequential extraction procedure has been optimized in relation with the characteristics of the sediments found in the Odiel Marshes Natural Park. They presented high percentages of iron oxides (up to 17%) and a total metal content, in the range of hundreds of μg g−1 and higher, for As, Cu, Pb and Zn. The metals studied included As, Cr, Cd, Cu, Fe, Hg, Mn, Ni, Pb and Zn. NH4OAc was superior to MgCl2 and NaOAc in relation to the metal determination using AAS. However, it was not possible to distinguish between exchangeable and bound-to-carbonate metals. For the evaluation of the sum of the metal concentration associated with both these phases, an optimal volume of 35 ml and a concentration of 1 M was selected for the extractant NH4OAc. No readsorption problems were observed for any metal except for Hg. Several repetitive extractions with NH2OH·HCl and H2O2–NH4OAc were necessary for the complete extraction of metals bound to oxides and organic matter, respectively. Percentages of iron oxide coated with organic matter were found insignificant for the low organic matter content sediments considered in the study. Finally, a comparison between the optimized and a low selectivity (LS) sequential extraction scheme for metal mobility assessment showed an important overestimation of As, Cr and Ni released by oxidant mechanism and an underestimation of As, Cd, Cr and Ni by ion exchangeable+acid–base and reductant mechanism using the LS scheme.