Importance of different types of marine particles for the scavenging of heavy metals in the deep-sea bottom water

Abstract

In experiments of 7 days duration using voltammetric and radiotracer measurement techniques, the role of different particle types in the sorption of dissolved metal species in a disturbed deep-sea bottom seawater system were investigated. Resuspension of oxic to suboxic surface sediment into the bottom water in the deep sea (either by natural events or industrial activities like Mn nodule mining) has been shown to be followed quickly by scavenging of dissolved heavy metals, e.g. released from interstitial water, on the resuspended particles. Compared to other deep-sea particles (like clay minerals, calcite and apatite), Mn and Fe oxides and oxyhydroxides were found to be by far the most important phases in scavenging many dissolved heavy metals. Only Pb was sorbed strongly on all particles used, with highest affinity to carbonate fluorapatite. Caesium + was significantly scavenged only by clay minerals like illite. The sorption experiments support a simple electrostatic model: Hydrated cations and labile cationic chloro-complexes in seawater like Mn 2+, MnCl +, Co 2+, Ni 2+, Cu 2+, Zn 2+, Ba 2+, and PbCl +, are preferentially adsorbed or ion-exchanged on the negatively charged surfaces of Mn oxides. In contrast, oxyanions and neutrally or negatively charged complexes like HVO 4 2−, MoO 4 2−, HAsO 4 2−, UO 2(CO 3) 2 2−, and PbCO 3 0 associate with neutral to slightly positive amphoteric Fe oxyhydroxide particles. Metals forming strong chloro-complexes in seawater like Cd (CdCl 2 0), are less readily sorbed by oxides than others. A comparison of the results of voltammetric and radiotracer techniques revealed that after fast sorption within the first hour, isotopic exchange dominated reactions on MnO 2-rich particles in the following days. This was especially pronounced for Mn and Co which are bound to the Mn oxide surface via a redox transformation.