Influence of ripening on pHstatleaching behaviour of heavy metals from dredged sediments

Abstract

The influence of oxidation and ripening on the kinetics of heavy metal release from anoxic sediments was investigated by means of porewater analysis and pH_stat leaching tests. During simulated land disposal of an anoxic river sediment, a considerable heavy metal (Cd, Zn and Ni) release into the porewater was observed after 25 days of exposure of the sediment to the air. The release kinetics of Cd and Zn during pH_stat leaching indicated that their release was related to the oxidation of Cd- and Zn-sulfides. It was not completely clear whether the release of Ni was due to the oxidation of Ni-sulfides or to the introduction of H⁺ (generated by the oxidation of sulfides or introduced by titration). The release of Cu seemed to be explained by the oxidation of both Fe-sulfides and Cu-sulfides. As such, pH_stat experiments provided an indirect way to assess the mineralogy of the anoxic sediment. During ripening of the sediment, the release rate of Zn and Cd in pH_stat leaching tests increased. For Ni, however, a similar release pattern was observed in the anoxic sediment and in the ripened sediments. Two weeks’ drying at 100 °C resulted in an increase in the leachability of Cu. Nevertheless, at pH 4 and 6, the thermally treated sediment displayed a slower leaching of Cd and Zn than the ripened sediment. Although thermal treatment is often used to simulate ageing in soils, it cannot be used to simulate ripening of anoxic sediments.