Sequential leaching of Peru Basin surface sediment for the assessment of aged and fresh heavy metal associations and mobility

Sequential leaching experiments were carried out using the upper 25 cm of oxic to suboxic sediment from the Peru Basin and ferromanganese nodules in order to assess potential mobility and the associations of heavy metals in the mineral phases. The general leaching pattern confirmed that in the nodules, as well as in the upper dark-brown sediment layer, significant proportions of heavy metals like Mn, Co, Ni, Zn, Cu, and Cd are associated with MnO2 and would be released under reducing conditions. In the underlying Mn-poor layers, Mn oxide-hosted metals are minor. To distinguish the leaching behaviour of aged and fresh sorbents, radiotracers were adsorbed on the solids before leaching. The dominant role of the Mn oxide phase for scavenging dissolved heavy metals was confirmed for the upper sediment layer and the nodules for sorption equilibration times of 5 h and 7 days, respectively. The sorption process was rapid, freshly sorbed metals were leached mostly in the same fractions as those in aged sorbents, and little difference was found for sorption times of 5 h and 7 days. In contrast, in the Mn-poor sediment layers a significant amount of adsorbed radiotracers was easily remobilised by simple ion exchange (especially Cs and Cd), and an “ageing” effect (shift to less easily leachable fractions) was observed after 7 days. When surface sediment is resuspended into the bottom water, the Mn oxide-rich surface layer and nodule particles represent a much more effective heavy-metal scavenger under oxic conditions compared to other Mn-poor sediment phases. However, at the same time the Mn-rich particles are a stronger potential source of toxic heavy metals in a shift to more reducing conditions.