Comparison of metal release from recent and aged Fe-rich sediments

During oxidation, ripening and aging of contaminated river sediments, important changes in the speciation and binding of metals to the sediment matrix occur. The effect of ‘aging’ of contamination is usually studied in the laboratory with artificially spiked soils and sediments. An increased retention of heavy metals is often observed during aging of Fe-(hydr)oxides. However, little information is available concerning the importance of the time since disposal of contaminated Fe-rich sediments on land (by dredging or overbank flooding) with respect to heavy metal release. Metal release from recent and aged Fe-rich overbank sediments and land-disposed dredged sediments was studied by means of column and pHstat leaching tests and with single extractions with EDTA and acetic acid. In order to allow a more objective comparison between recent and aged sediments, the release of heavy metals (Cd, Zn, Ni and Cu), as well as As and Fe, during pHstat tests was described mathematically and operationally defined release constants were determined. Fe was the only element that allowed to make a clear distinction between aged and recent sediments, since Fe displayed a significant higher leachability from recent sediments than from aged sediments in all the leaching tests and extractions. The different release behaviour of Fe was nevertheless not systematically reflected in a higher heavy metal and As-mobility in recent sediments. Only during the column leaching test and acetic acid extraction, a higher release of Cd was observed from the recent sediments. These results indicate that sediment characteristics such as pH, clay, organic matter and Fe-(hydr)oxides-content are more significant in explaining heavy metal release from Fe-rich sediments than the age of the sediments.